JOURNAL OF CHEMICAL PHYSICS
VOLUME 121, NUMBER 17
1 NOVEMBER 2004
Kinetics of electron-induced decomposition of CF2 Cl2 coadsorbed
with water „ice…: A comparison with CCl4
N. S. Faradzhev
Department of Physics and Astronomy, and Laboratory for Surface Modification, Rutgers,
The State University of New Jersey, Piscataway, New Jersey 08854-8019
C. C. Perry
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218
D. O. Kusmierek
Department of Physics and Astronomy, and Laboratory for Surface Modification, Rutgers,
The State University of New Jersey, Piscataway, New Jersey 08854-8019
D. H. Fairbrother
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218
T. E. Madeya)
Department of Physics and Astronomy, and Laboratory for Surface Modification, Rutgers,
The State University of New Jersey, Piscataway, New Jersey 08854-8019
共Received 31 March 2004; accepted 30 July 2004兲
The kinetics of decomposition and subsequent chemistry of adsorbed CF2 Cl2 , activated by
low-energy electron irradiation, have been examined and compared with CCl4 . These molecules
have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal
surfaces 共Ru; Au兲 at low temperatures 共25 K; 100 K兲. The studies have been performed with
temperature programmed desorption 共TPD兲, reflection absorption infrared spectroscopy 共RAIRS兲,
and x-ray photoelectron spectroscopy 共XPS兲. TPD data reveal the efficient decomposition of both
halocarbon molecules under electron bombardment, which proceeds via dissociative electron
attachment 共DEA兲 of low-energy secondary electrons. The rates of CF2 Cl2 and CCl4 dissociation
increase in an H2 O (D2 O) environment 共2–3⫻兲, but the increase is smaller than that reported in
recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with
H2 O, using 180 eV incident electrons, are measured 共using TPD兲 to be 1.0⫾0.2⫻10⫺15 cm2 for
CF2 Cl2 and 2.5⫾0.2⫻10⫺15 cm2 for CCl4 . RAIRS and XPS studies confirm the decomposition of
halocarbon molecules codeposited with water molecules, and provide insights into the irradiation
products. Electron-induced generation of Cl⫺ and F⫺ anions in the halocarbon/water films and
production of H3 O⫹ , CO2 , and intermediate compounds COF2 共for CF2 Cl2 ) and COCl2 , C2 Cl4 共for
CCl4 ) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and
the decomposition kinetics are similar to those observed in our recent experiments involving x-ray
photons as the source of ionizing irradiation. © 2004 American Institute of Physics.
关DOI: 10.1063/1.1796551兴
In addition to solar photons, other ionizing radiation 共energetic subatomic particles兲 are also present in the upper
earth’s atmosphere; the primary source is associated with
cosmic rays.3,4 The interaction of cosmic rays with matter
共e.g., polar stratospheric cloud particles, or microscopic water droplets兲 results in a cascade of low-energy secondary
electrons that can influence the chemistry of atmospheric halocarbons. However, the extent to which low-energy electrons contribute to halocarbon decomposition in the stratosphere is a matter of debate.5–9 CF2 Cl2 and CCl4 have
positive electron affinities of ⬃0.4 eV 共Ref. 10兲 and ⬃1 eV,11
respectively, and are known for their ability to capture lowenergy electrons efficiently. In the process of DEA 共dissociative electron attachment兲, the molecule captures a lowenergy electron forming a negative ion resonance 共NIR兲,
which leads to dissociation into neutral and negatively
charged fragments:
I. INTRODUCTION
There are myriad ways in which halomethanes and chlorofluorocarbons 共CFCs兲 impact the environment and human
health, and there is much interest in the effect of irradiation
on the chemistry and remediation of these compounds. Due
to its widespread use in industry, environmental contamination by the halomethane CCl4 is of great concern: even low
doses of exposure to CCl4 , a suspected human carcinogen,
can cause adverse health effects.1 Substantial amounts of
CFCs, including CF2 Cl2 共CFC-12兲, have also been emitted
into the stratosphere. Solar photon-induced dissociation of
CF2 Cl2 results in the production of Cl atoms, which in turn
react to destroy ozone.2
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0021-9606/2004/121(17)/8547/15/$22.00
8547
© 2004 American Institute of Physics
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8548
Faradzhev et al.
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
TABLE I. Comparison of the electron interaction gas-phase cross sections ␴ of CF2 Cl2 and CCl4 .
␴ 共units of 10⫺15 cm2兲
CF2 Cl2
Total electron-scattering ␴ in
electron transmission experiments
⬃100 eV
2.7
Ref. 64
⬃1 eV
Total electron-impact ionization ␴
␴ DEA
⬃100 eV
1.2
⬃0 eV
⬃0.4
AB⫹e ⫺ → 共 AB ⫺ 兲 * →A⫹B ⫺ .
For both CF2 Cl2 and CCl4 , the dissociation can be initiated
by ⬃0 eV electrons; the cross sections for gas-phase DEA of
⬃0 eV electrons are ⬃4⫻10⫺16 cm2 共Refs. 10 and 12兲 and
1.3⫻10⫺14 cm2 , 13 respectively 共ref. Table I兲.
DEA of CF2 Cl2 and CCl4 have been investigated in both
the gas10 and condensed phase.14,15 In the condensed phase,
effects such as polarization of the substrate, intermolecular
interactions, and electron mobility may influence DEA rates
and reaction pathways.14,15 For example, in the case of adsorption on a metal surface, the substrate can 共1兲 act as a
source of low-energy electrons; 共2兲 modify the energy of the
intermediate states by the polarization energy; and 共3兲 influence the probability that an electron will autodetach from a
NIR.16
Electron bombardment of CF2 Cl2 coadsorbed with polar
molecules 共e.g., NH3 , H2 O) has also been observed to result
in an increase in the anion ESD 共electron stimulated desorption兲 yield and an increased charge trapping coefficient.17,18
Lu and Madey discovered that coadsorption of CF2 Cl2 and
H2 O on a Ru共0001兲 substrate leads to an enhancement of
detected Cl⫺ ion yields of ⬃100⫻ more than the yield from
CF2 Cl2 adsorbed alone.18 Lu and Sanche also reported a
huge increase in the electron trapping coefficient of CF2 Cl2
when Freon was coadsorbed with H2 O. 17 The phenomenon
was assigned to an increase of the DEA cross section: ␴
⫽1.3⫻10⫺14 cm2 vs ␴ ⫽1.4⫻10⫺15 cm2 without coadsorption of H2 O. The enhancements were attributed to an
electron-trapping mechanism by the polar molecule, which
increases the probability of DEA.
Past studies have focused primarily on anion desorption
yields and charge trapping in the irradiated films. In very
recent studies, the effects of electron and x-ray irradiation of
CCl4 /H2 O 共ice兲 films19,20 and the effect of x-ray irradiation
on CF2 Cl2 /H2 O 共ice兲 films21 have been investigated.
Radiation-induced chemical reactions were found to accompany the dissociation of parent molecules. For the CCl4 /H2 O
共ice兲 films, the production of COCl2 , C2 Cl4 , and CO2 as
well as H3 O⫹ and Cl⫺ ions occurred in the films, while
CO2 , CO, and HCl were identified as neutral gas-phase
products. CO2 was found to be the dominant carbon containing species for dilute films (CCl4 :H2 O⬍0.1), while carboncarbon coupling reactions became more prevalent for films
with higher CCl4 concentrations. In the x-ray irradiation
studies of CF2 Cl2 /H2 O 共ice兲 the production of COF2 and
Ref. 67
CCl4
3.7
Ref. 64
7
Ref. 68
⬃1
Ref. 69
⬃13
Ref. 13
Refs. 10, 12
CO2 is observed in the films, along with solvated Cl⫺ , F⫺ ,
and H3 O⫹ ions. The product distribution was also found to
depend on the film’s initial chemical composition. COF2 and
CO2 production was favored in dilute films, while carboncarbon coupling reactions resulted in thermally stable partially halogenated polymeric CClx Fy species in more concentrated films.
In the present work, we measure directly electron beaminduced changes in the composition of CF2 Cl2 /H2 O and
CCl4 /H2 O films condensed on metal surfaces at T⫽100 K,
using several surface spectroscopic methods. The main goal
is to compare the effect of H2 O coadsorption on the electroninduced dissociation of CF2 Cl2 and CCl4 with previous studies of anion ESD yield enhanced by H2 O. A second goal is to
compare electron beam-induced effects with x-ray induced
processes identified previously in CF2 Cl2 /H2 O 共ice兲 films,21
in an attempt to provide further insights into the role of H2 O
on the dynamics of halocarbon decomposition and the subsequent chemistry of dissociation fragments. Low-energy
electron beams are used as radiation sources, for comparison
with x-ray induced processes.
TPD 共temperature programmed desorption兲 data indicate
decomposition of both halocarbon molecules during irradiation by electrons and demonstrate that codeposition with
H2 O affects the decomposition rates of both CCl4 and
CF2 Cl2 where the magnitude of the increase is two to three
times. X-ray photoelectron spectroscopy 共XPS兲 and
reflection-absorption infrared spectroscopy 共RAIRS兲 are
used to detect the production of H3 O⫹ , Cl⫺ , F⫺ , and CO2 ,
under the influence of both x-ray and electron irradiation. In
addition, COF2 and COCl2 are identified as intermediates in
the electron and x-ray induced degradation of CF2 Cl2 and
CCl4 , respectively. Quantitative measurements of cross sections confirm that electron-induced decomposition rates of
CF2 Cl2 and CCl4 are increased upon coadsorption with H2 O,
but the magnitudes of the enhancements are considerably
less than those reported in previous ESD and charge-trapping
studies of CF2 Cl2 . 17,18 Possible reasons are discussed. The
dominant role of low-energy secondary electrons in
radiation-induced chemistry of halocarbons in H2 O films is
strongly implied.
II. EXPERIMENT
The experiments are performed under ultrahigh vacuum
共UHV兲 conditions in three different experimental systems.
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J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
Hereafter they are referred to as apparatus 共1兲 temperatureprogrammed desorption 共TPD兲, 共2兲 XPS, and 共3兲 RAIRS.
A. Analytical chambers
Kinetics of decomposition of CF2 Cl2 co-adsorbed with water
8549
ter, deuterium oxide, and carbon tetrachloride are subjected
to purification procedures, which involve several freezepump-thaw cycles. For XPS, RAIRS, and some TPD
halocarbon/water mixture experiments, the gases are first
intermixed in a separate manifold, where the
CF2 Cl2 /H2 O(D2 O) and CCl4 /H2 O(D2 O) mixtures are allowed to stabilize for several minutes prior to dosing. Adjusting the partial pressures of the mixing components is used to
vary halocarbon/water ratios. In all set-ups the purity and the
concentration of gases introduced into the chamber are monitored by MS. Details on mixture preparation are also reported elsewhere.21
In the TPD system 共chamber 1兲 neat water and neat halocarbons 共or water/halocarbon mixtures兲 are deposited onto
the cold surface at 25 K via separate dosers capped with
microcapillary arrays. The molecular beam is incident normal to the surface. The water films grown under these conditions are nonporous.24
In the XPS system 共chamber 2兲 gases are dosed from
background at ⬃2⫻10⫺7 Torr via a UHV-compatible leak
valve. Similarly, in the RAIRS system 共chamber 3兲 background dosing at ⬃1⫻10⫺6 Torr is used for deposition of
gases on the surface. In the RAIRS and XPS systems, the
water ice layer grown at 100 K has microporous structure.24
The TPD chamber with a base pressure of ⬃5
⫻10⫺11 Torr also houses apparatus for Auger electron spectroscopy 共AES兲, low-energy electron diffraction 共LEED兲,
and electron-stimulated desorption-ion angular distribution
共ESDIAD兲 with time-of-flight capability for mass and angleresolved ion detection. The substrate is a Ru共0001兲 single
crystal mounted via Ta leads on a copper holder of an XYZrotary manipulator. The holder design allows for the cooling
of the crystal to 25 K and for heating to 1600 K by electron
bombardment. A UTI 共model 100C兲 quadrupole mass spectrometer 共QMS兲 is used for TPD; the sample is heated from
the rear at a rate of ⬇1 K/s via thermal radiation from a hot
W filament. In order to prevent electrons from the filament of
the QMS ionizer from bombarding the Ru surface, the
sample is negatively biased at ⫺200 V. In the present study
we use TPD predominantly to identify coverages, reaction
products and desorption temperatures, without detailed reaction kinetic analysis. The sample is cleaned by occasional
Ar⫹ sputtering and heating in oxygen, followed by annealing
in vacuum. Surface cleanliness and structure are monitored
by AES and LEED. Further details of the apparatus have
been reported recently.21–23
The second experimental setup, the XPS system, is
equipped with a dual anode x-ray source, a Physical Electronics 10–360 multichannel hemispherical analyzer and a
Balzers Prisma QMS for gas analysis. A base pressure of
⬇2⫻10⫺9 Torr is maintained in the chamber during experiments. The sample is a polycrystalline Au 共Goodfellow,
99.95%兲 square platelet (1⫻1⫻0.2 cm3 ) mounted on a manipulator that allows cooling to ⬇100 K. The gold surface is
cleaned by Ar⫹ sputtering, and its chemical composition is
checked by XPS.
The RAIRS arrangement is an integrated two-chamber
design with a typical base pressure of ⬇5⫻10⫺9 Torr,
which houses the apparatus for RAIRS, mass spectroscopy
共MS兲, and XPS. RAIR spectra are recorded with a Mattson
infinity series Fourier transform infrared spectrometer
equipped with external beam capabilities and a narrow-band
mercury-cadmium-telluride 共MCT兲 detector 共700– 4000
cm⫺1兲. All spectra are taken with a resolution of 4 cm⫺1 by
summing 500 scans and are referenced to the clean substrate
surface prior to film deposition. The substrate is a reflective
polycrystalline Au 共99.99%, Accumet兲 foil 共rectangular: surface area ⬇7 cm2兲 mounted on a copper sample holder; the
sample is cooled to ⬇100 K. The details of this setup have
been reported recently.19,21
In all set-ups the temperature is monitored by a chromelalumel thermocouple attached directly to the rear side of a
sample.
After deposition the composition and thickness of the
adsorbed film are monitored by XPS 共in both the RAIRS and
XPS setups兲 or by TPD 共in the TPD setup兲. Using XPS, the
concentration of a film is determined using the ratio of Cl 2p
and O 1s areas 共for both CF2 Cl2 and CCl4 ), and the ratio of
F 1s and O 1s areas 共for CF2 Cl2 only, as an additional estimate兲 of the initial film taking into account stoichiometry
and elemental sensitivity factors. The film thickness is determined by the attenuation of the Au 4 f signal after gas deposition using the electron inelastic mean free path 共IMFP兲 in
Freon25 and water.25,26 The second approach, where we estimate the ratios of the Au 4 f peak area to the C 1s, F 1s, Cl
2 p, and O 1s peak areas, gives a similar estimation of film
thickness, within ⬇25%. In TPD experiments, the coverage
is expressed in monolayer 共ML兲 units. 1 ML CCl4 共Ref. 27兲
and 1 ML CF2 Cl2 共Ref. 28兲 on Ru are the coverages corresponding to the saturation of the appropriate monolayer peak
in the thermal-desorption spectra. 1 ML of H2 O (D2 O) is
associated with the formation of a hydrogen-bonded bilayer
on Ru共0001兲.29 The calculation of partial concentrations of
components in a film is based on the monolayer definition.
The typical film thickness is ⬎30 nm in RAIRS studies,
⬇6.5 nm in XPS experiments, and from fractional monolayer to several monolayers in TPD measurements 关i.e., up to
⬃2.5 nm, taking into account the hexagonally ordered structure of water on Ru共0001兲 共Ref. 29兲兴.
B. Film preparation
D. Irradiation of the films
In all experiments high-purity CF2 Cl2 共Matheson;
99.995%兲, CCl4 共Aldrich; 99.9%兲, H2 O 共Millipore; deionized兲, and D2 O 共Aldrich; 99.9%兲 are stored in separate
vacuum vessels attached to a gas manifold. Prior to use, wa-
For experiments dealing with electron beam damage of
the adsorbed layer the QMS filament is used in chamber 1 as
a source of defocused electrons with a broad and nearly uniform 共in the vicinity of the sample surface兲 spatial distribu-
C. Film composition and coverage estimation
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8550
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
Faradzhev et al.
FIG. 1. Thermal desorption spectra for CF2 Cl2 coadsorbed with water on Ru共0001兲 at 25 K. Spectra are measured with a heating rate of ⬇1 K/s: 共a兲 1 ML
of CF2 Cl2 on top of ⬇10 ML H2 O; also shown by a dashed line is a spectrum for 1 ML of CF2 Cl2 on Ru共0001兲; 共b兲 1.5 ML of CF2 Cl2 buried under ⬇10
ML of H2 O; desorption occurs during the ‘‘amorphous-to-crystalline’’ phase transition of water with a maximum rate at ⬃155 K; 共c兲 dilute 共⬃4%兲 mixture
of CF2 Cl2 and heavy water (D2 O) with a total film thickness of ⬇8 ML; also shown by a dashed line is a CF2 Cl2 desorption spectrum for a film of the same
concentration but lower thickness 共⬇4 ML兲; 共d兲 concentrated 共⬃20%兲 mixture of CF2 Cl2 and heavy water (D2 O) with a total film thickness of ⬇6.5 ML.
tion. The same approach has been used recently in Refs. 23
and 30. During irradiation, the sample is biased at ⫹105 V,
so that 180 eV electrons from the filament (I⬇1.5 ␮ A,E
⫽75 eV) bombard the surface. In chambers 2 and 3 a lowenergy flood gun 共Specs 15/40兲 is used, operating at 0.1 mA
emission current and 10 eV extraction voltage. The sample is
biased at ⫹200 V, and the kinetic energy of incident electrons is 210 eV. For x-ray irradiation experiments performed
in the XPS chamber, an Al K ␣ 共1486.6 eV兲 anode 共300 W,
15 kV兲 is used as a source of x-rays.
III. RESULTS
This section focuses on the kinetics of electron beam
induced CF2 Cl2 and CCl4 decomposition, and a comparison
of their rates of dissociation in a water matrix. The data
demonstrate the similarity of processes caused in halocarbon/
water layers by two types of ionizing radiation: high-energy
photons and ⬃200 eV electrons.
A. Thermal desorption of Freon and water
from Ru„0001…
Figure 1 shows the thermal desorption spectra measured
for CF2 Cl2 coadsorbed with H2 O(D2 O) on Ru共0001兲 at 25
K. CF2 Cl⫹ 共85 amu兲 and H2 O⫹ (18 amu)/D2 O⫹ 共20 amu兲
fragments are detected, which have the highest intensity in
corresponding gas-phase mass spectra.31 The H2 O (D2 O)
films are deposited at normal incidence at 25 K. Under these
deposition conditions, the films are expected to be
nonporous.24 The ice films grown under normal incidence in
the temperature range 20–140 K are reported to be flat and
homogeneous, with a constant density.24,32
Figure 1共a兲 shows the spectra obtained for 1 ML CF2 Cl2
on top of 10 ML water ice 共solid lines兲. The maximum desorption rate of H2 O is observed at ⬇160 K. The H2 O spectrum also exhibits a distinct feature at ⬇155 K, which is
attributed to an irreversible structural transition from amorphous solid ice to a crystalline phase.33 This spectrum is
indistinguishable from that measured for water on Ru共0001兲
without the Freon overlayer.21 CF2 Cl2 starts to desorb from
the solid water film prior to the onset of H2 O desorption,
reaching its maximum desorption rate at ⬇110 K 关see solid
line for CF2 Cl2 in Fig. 1共a兲兴. This temperature is lower than
the peak temperature 共⬇140 K兲 for desorption of CF2 Cl2
molecules in direct contact with the Ru crystal in the absence
of H2 O 共shown by a dashed line兲, and nearly coincides with
the temperature of multilayer Freon desorption.28 For 1.5 ML
of CF2 Cl2 deposited on Ru共0001兲 and then covered by 10
ML of H2 O, a dramatic change in the desorption of CF2 Cl2
is observed. Figure 1共b兲 illustrates that Freon desorption is
blocked by the amorphous solid ice overlayer until the onset
of water crystallization. At this point, abrupt desorption of
Freon over a narrow temperature range is observed, coincid-
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J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
Kinetics of decomposition of CF2 Cl2 co-adsorbed with water
8551
FIG. 2. Thermal desorption spectra measured before 共dashed lines兲 and after 共solid lines兲 180 eV electron irradiation of 0.2 ML halocarbon 共a, b, c—CF2 Cl2 ;
d, e, f—CCl4 ) adsorbed on top of 5 ML D2 O ice at 25 K. In all cases, electron exposure was kept at 1⫻1015 cm⫺2 . Shown are fragments CF2 Cl⫹ 共85 amu兲,
⫹
⫹
⫹
CCl⫹ /COF⫹ 共47 amu兲, and C2 F⫹
4 共100 amu兲 for CF2 Cl2 /D2 O, and CCl3 共117 amu兲, COCl 共63 amu兲, and C2 Cl4 共164 amu兲 for CCl4 /D2 O. The spectra
demonstrate decomposition of CF2 Cl2 and CCl4 under electron bombardment 共a, d兲 and production of COF2 共b兲, COCl2 共e兲, and C2 Cl4 共f兲 under electron
bombardment of the adsorbed layer.
ing with the occurrence of the water-phase transition.
Figure 1共c兲 illustrates thermal desorption spectra observed for CF2 Cl2 molecules inside a D2 O matrix, following
deposition of a CF2 Cl2 /D2 O mixture. The solid line shows
the spectra for a dilute mixture consisting of 0.35 ML
CF2 Cl2 and 8 ML D2 O 共corresponding concentration is
⬃4%兲. The dashed line indicates the CF2 Cl2 spectrum for a
thinner film 共⬇4 ML兲 of the same concentration. Both Freon
spectra exhibit two distinct peaks at ⬇110 K and ⬇165 K
共⬇160 K for 4 ML film兲, and a small shoulder at higher
temperatures. Comparison of the spectra shown in Figs. 1共a兲
and 1共c兲 suggests that the low temperature peak corresponds
to desorption of molecules from the ice surface. The high
temperature features are attributed to the escape of molecules
from the bulk of the ice matrix. The CF2 Cl2 desorption peak
at 165 K correlates with the ASW 共amorphous solid water兲to-crystalline phase transition, which for D2 O occurs at
⬇165 K.33 This CF2 Cl2 peak exhibits a smooth leading edge
until water crystallization 关Fig. 1共c兲兴, followed by an abrupt
drop and shoulder similar to that observed in Fig. 1共b兲. The
data obtained for a concentrated film, associated with a mixture of CF2 Cl2 and H2 O in a 1:4 ratio 共the corresponding
Freon concentration is ⬃20%; total film thickness ⬇6.5
ML兲, are shown in Fig. 1共d兲. They indicate the disappearance
of the phase transition shoulder for water and a broadening
of the high temperature Freon desorption peak towards lower
desorption temperatures. A discussion of these results follows in Sec. IV.
B. Postirradiation TPD for halocarbons adsorbed
on water layer
TPD spectra measured before and after electron irradiation 共180 eV兲 of 0.2 ML CF2 Cl2 adsorbed on top of 5 ML
D2 O are shown in Figs. 2共a兲–2共c兲 and corresponding spectra
for 0.2 ML CCl4 are shown in Figs. 2共d兲–2共f兲. The data show
that electron irradiation induces chemical reactions in the
films that do not occur in the absence of irradiation. Comparison of spectra for undamaged and irradiated films detected for mass 85 (CF2 Cl⫹ , the major fragment in the Freon
cracking pattern兲 indicates a noticeable decrease of the Freon
signal after exposure to ⬇1⫻1015 electrons cm⫺2 关Fig.
2共a兲兴. Freon desorption temperatures before and after irradiation remain unchanged 共⬇110 K兲. The origin of a new high
temperature shoulder at 120–125 K 关Fig. 2共a兲兴, revealed in a
postirradiated TPD spectrum, is not clear.
In contrast to the CF2 Cl⫹ signal, the intensity of the
⫹
CCl signal 共mass 47, a minor fragment of Freon兲 increases
after irradiation, with the line shape of the TPD curve and the
desorption temperature 共⬇110 K兲 remaining unchanged 关Fig.
2共b兲兴. We assign this increase to a contribution from the
COF⫹ fragment 共also mass 47兲, the major fragment of COF2
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8552
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
Faradzhev et al.
FIG. 3. 共a兲 Electron irradiation time dependences of the following IR band areas: 1146 and 1090 cm⫺1 关associated with ␯ s(CuF) and ␯ as(CuF) stretch
modes of CF2 Cl2 ]; 1936 and 1906 cm⫺1 关 ␯ 1 (CO) stretch mode and 2 ␯ 2 (CuF) Fermi Resonance of COF2 ]; and 2339 cm⫺1 关 ␯ (CvO) stretch mode of
CO2 ]. The lines drawn through the data points are guides to the eye. The data are measured for a thick CF2 Cl2 /H2 O 共ice兲 film 共⬃30 nm兲 共ratio ⬇0.2兲, which
has been adsorbed on polycrystalline Au at ⬃100 K. Incident electron energy is 210 eV. 共b兲 Dissociation of CF2 Cl2 solvated in water and variation of COF2
⫹
⫹
observed under electron bombardment: Areas of TPD peaks for fragments 85 amu (CF35
2 Cl ) and 47 amu (COF ) as a function of electron exposure. Each
point of this plot represents an integrated area of the corresponding TPD peak (CF2 Cl2 and COF2 ) normalized to the area of the Freon peak of the undamaged
共nonirradiated兲 film. The Freon curve (CF2 Cl⫹ m/q⫽85) incorporates extra points derived from multiple experiments. The line drawn through the CF2 Cl2 data
is an exponential fit, and the one drawn through the COF2 points is a guide to the eye. The data are measured for 1.5 ML of CF2 Cl2 in H2 O matrix 共6 ML兲
adsorbed on Ru共0001兲 at 25 K; incident electron energy is 180 eV. The area of the TPD peak for COF⫹ fragment is corrected for the cracking fraction from
Freon.
共carbonyl difluoride兲31 共see additional evidence for COF2 in
Sec. III C, below兲. We also observe a much smaller than
expected decrease of the CFCl⫹ signal 共mass 66 fragment of
Freon; not shown in Fig. 2兲 due to a contribution from COF⫹
2
共mass 66兲, the parent fragment of carbonyl difluoride, COF2 .
These TPD results provide strong evidence for electroninduced reactions at the water-Freon interface, in accordance
with our previous observations for CF2 Cl2 /H2 O films with
x-rays as the initial source of ionizing radiation.21 In
CF2 Cl2 /H2 O films, no formation of C2 Cl4 or hexachloroethane C2 Cl6 共mass 164 C2 Cl⫹
4 fragment兲 nor of their fluorinated analogs, tetrafluoroethane C2 F4 共mass 100 C2 F⫹
4 fragment兲 and hexafluoroethane C2 F6 , 共mass 119 C2 F⫹
5
fragment兲, is observed in TPD spectra measured before and
after electron irradiation 关ref. Fig. 2共c兲 for mass 100 C2 F⫹
4
fragment兴.
For 0.2 ML CCl4 adsorbed on a D2 O ice surface, the
postirradiated TPD signal 关Fig. 2共d兲兴 of mass 117 (CCl⫹
3 , the
major fragment in the carbon tetrachloride cracking pattern兲
decreases even more rapidly than that observed for Freon for
a corresponding e-beam exposure. Desorption of several species after irradiation is also detected 关Figs. 2共e兲–2共f兲兴. We
identify them to be COCl2 共carbonyl chloride, or phosgene兲
and C2 Cl4 , based on the TPD signals for mass 63 (COCl⫹ ,
the major fragment of phosgene兲 and mass 164 (C2 Cl⫹
4 , a
strong fragment of C2 Cl4 ) 关Figs. 2共e兲 and 2共f兲兴. These data
are consistent with our previous RAIRS studies.19,20 The desorption temperature of phosgene from the ice surface is
lower 共⬃130 K兲, and that for C2 Cl4 is higher 共⬃145 K兲, than
the desorption temperature of CCl4 共⬃135 K兲. Formation of
C2 Cl6 is ruled out, because we detect no signal for one of the
expected fragments of C2 Cl6 , 31,34 C2 Cl⫹
5 共mass 199, not
shown兲, unless its concentration is below the detectable level
of the signal for heavy ions of our spectrometer. Thus, the
data in Fig. 2 indicate that electron-induced decomposition
of halocarbons adsorbed on an ice layer is accompanied by
chemical reactions. The reactions occur prior to heating the
sample in TPD experiments, as indicated by data reported in
Sec. III C.
C. Kinetics of halocarbon decomposition:
TPD and RAIRS results
Our present and previous studies19–21,35 indicate that
electron-induced reactions between water and halocarbons
take place not only on the surface but also in the bulk of the
water ice film. Recently, production of both phosgene and
tetrachloroethylene as a result of electron-induced reactions
has been observed in RAIRS and XPS experiments for
mixed CCl4 /H2 O films.19,20 In the present RAIRS study of
CF2 Cl2 /H2 O mixed films irradiated by electrons and carried
out under the same experimental conditions 共⬃30 nm films
on polycrystalline Au substrate at ⬇100 K; incident electron
energy 210 eV兲 we also observe dissociation of both Freon
and water, and detect production of CO2 , COF2 , and H3 O⫹
species. No evidence for the production of carbonyl chloride
fluoride COFCl or phosgene COCl2 is observed. The major
bands observed and the evolution of the spectra during electron bombardment are substantially similar to those detected
under x-ray irradiation reported recently.21,35
In Fig. 3 we compare the kinetics of CF2 Cl2 decomposition and production of CO2 and COF2 observed by RAIRS
and TPD during electron bombardment. The data points
shown in Fig. 3共a兲 represent the integrated areas of the IR
bands for Freon 关 ␯ (CuF) symmetric and asymmetric
stretches at 1146 and 1090 cm⫺1兴, carbonyl difluoride
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J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
关 ␯ 1 (CvO) stretch and a Fermi resonance 2 ␯ 1 (CuF) at
1936 cm⫺1 and 1906 cm⫺1, respectively兴, and carbon dioxide
关 ␯ (CvO) peak at 2339 cm⫺1兴. They are measured for ⬎30
nm films consisting of mixed CF2 Cl2 /H2 O in the ratio ⬇0.2
关corresponding spectra are shown in Fig. 3共a兲兴. All intensities
are normalized to the initial area of the Freon peaks.
Figure 3共b兲 shows the kinetics of Freon decomposition
and formation of COF2 as a function of electron exposure
derived from TPD data. The data in Fig. 3共b兲 are obtained for
mixed Freon/water films of similar composition 共1:4 ratio of
CF2 Cl2 /H2 O in the mixture兲, which is thinner 共6 ML; ⬃2
nm兲 than films used in RAIRS studies. In this figure, CF2 Cl2
and COF2 curves are derived from the CF2 Cl⫹ (m/q⫽85)
and CCl⫹ /COF⫹ (m/q⫽47) signals. Specifically, the COF2
curve is obtained by subtracting the CCl⫹ contribution from
the CCl⫹ /COF⫹ (m/q⫽47) total TPD signal. The CCl⫹
contribution as a function of electron irradiation is estimated
by multiplying the CF2 Cl⫹ (m/q⫽85) signal by the
CCl⫹ :CF2 Cl⫹ ratio obtained from the Freon cracking pattern
detected for the undamaged film. However, a quantitative
estimate of COF2 in the CF2 Cl2 /H2 O film during irradiation
cannot be determined directly using our TPD data without
additional calibration.
Qualitatively, the results of both techniques 共TPD and
RAIRS兲 are in agreement demonstrating dissociation of
CF2 Cl2 under electron bombardment and formation of COF2
followed by its subsequent dissociation under the influence
of electron-beam irradiation. Although the qualitative variations in the film’s chemical composition are similar, a comparison of the curves in Fig. 3 shows that they are quantitatively different; this is discussed in Sec. IV.
A comparison of the electron-beam induced decomposition of CCl4 and CF2 Cl2 is shown in Fig. 4. The semilogarithmic plots in Fig. 4 are the areas of corresponding TPD
spectra, similar to those presented in Fig. 3共b兲, versus electron exposure. The data are measured at an incident electron
energy of 180 eV for two different coverages of halocarbons
adsorbed at 25 K either on Ru共0001兲 or on 5 ML D2 O. These
dependencies are normalized to the values detected for an
undamaged film. The cross sections for halocarbon decomposition are indicated in Fig. 4, and corresponding solid lines
are exponential fits assuming first-order decay.22,36 The plot
shows an increase of decomposition rate for both halocarbons on water ice compared to adsorption on Ru共0001兲 as
indicated by the different slopes for 1 ML CF2 Cl2 关Fig. 4共a兲兴
and 1 ML CCl4 关Fig. 4共b兲兴 adsorbed on Ru共0001兲 and on the
ice surface. Discussion follows in Sec. IV.
D. Decomposition of Freon under high-energy photon
and electron irradiation
Figure 5 illustrates the evolution of the Cl 2p XPS regions of a dilute CF2 Cl2 /H2 O film 共Freon:water ratio ⬇0.06;
thickness 6.5 nm兲 as a function of 共a兲 x-ray irradiation time
and 共b兲 exposure to 210 eV electrons. The curves indicated in
Figure 5 as ‘‘initial’’ correspond to ⬇1.5 min of irradiation
by x-rays, the time needed for collection of the photoelectron
signal in the Cl 2p region. The initial spectra contain the
CuCl (2p 3/2/2p 1/2) doublet associated with Freon 共the 2 p 3/2
Kinetics of decomposition of CF2 Cl2 co-adsorbed with water
8553
FIG. 4. Dynamics of decomposition of halocarbons adsorbed on Ru共0001兲
and a thick 共5 ML兲 D2 O film at 25 K, observed during irradiation by 180 eV
electrons. Each point represents the integrated area of a TPD spectrum for
either CF2 Cl2 共a兲 or CCl4 共b兲. Halocarbon thickness: 0.25 ML on D2 O 共filled
squares兲 1 ML on D2 O 共open squares兲 and 1 ML on Ru 共filled circles兲. Also
shown in 共a兲 are points 共marked by open triangles兲 obtained for dilute mixture of CF2 Cl2 and H2 O 共1:4兲. The lines drawn through the data points are
exponential fits 共see text for details兲.
peak is located at ⬇201 eV兲. Both x-ray and electron irradiations lead to a broadening of the spectra to lower binding
energies. A new (2p 3/2/2p 1/2) doublet with a 2p 3/2 peak at
⬇198 eV is evident in both sets of spectra 关i.e., 共a兲 and 共b兲兴.
This feature is clearly seen after 25 min of x-ray irradiation
关Fig. 5共a兲兴 and 5⫻1016 electrons cm⫺2 关Fig. 5共b兲兴. Its formation is consistent with the production and accumulation of
chloride anions in the film, as observed recently in our x-ray
irradiation study.21
In spite of different sources of primary ionizing irradiation 共high-energy photons vs ⬇200 eV electrons兲 the effects
are quite similar. A visual comparison of the spectra in Figs.
5共a兲 and 5共b兲 indicates that exposure to 5⫻1014 cm⫺2 incident electrons and 25 min of x-ray irradiation result in comparable concentrations of Cl anions produced and CuCl
bonds broken. The measured drain current through the
sample used as a rough assessment of electron fluence generated in the film as a result of x-ray irradiation of a Mg or Al
anode, indicates that the effective electron exposure in Fig.
5共a兲 共i.e., a value corresponding to x-ray irradiation time兲 is
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8554
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
Faradzhev et al.
FIG. 5. Dynamics of Cl 2p regions
observed in XPS during prolonged
x-ray 共a兲 and electron beam 共b兲 irradiation of a CF2 Cl2 and H2 O mixture
共ratio ⬃0.06兲. The film of ⬇6.5 nm
thickness is deposited onto a polycrystalline gold substrate. The estimated
electron exposures following x-ray irradiation was determined by visual
comparison with electron-beam data
and by measurement of the drain current. In both cases, the initial spectrum
includes damage due to ⬇1.5 min of
x-ray exposure during measurement.
All spectra are baseline corrected. The
fits and the resultant envelopes are displaced downward from the raw data
for clarity.
comparable 共within a factor of 2兲 to the corresponding
electron-beam exposure in Fig. 5共b兲.
The production of ions is very rapid during the initial
stages of irradiation 共the 198 eV doublet can already be seen
in the initial spectra, i.e., after 1.5 min of x-ray exposure兲.
However, the rate of the process decreases at higher electron
exposures 关Fig. 5共b兲兴 or longer x-ray irradiation times 关Fig.
5共a兲兴. Figure 6 illustrates the dependence of Cl⫺ production
on the effective electron exposure, shown as solid symbols.
The fraction of Cl⫺ is plotted 共i.e., the ratio of Cl⫺ to the
total Cl in the CF2 Cl2 /H2 O films兲 for films with different
concentrations but similar thickness 共⬇6.5 nm兲. X-ray irradiation experiments are indicated by hollow symbols with
horizontal error bars representing the uncertainty in the effective electron exposure 共i.e., the difference between the
exposures calculated from the drain current and the exposures estimated by comparison of XPS spectra after x-ray
and electron irradiation兲. The plot indicates that the fraction
of chlorine which transforms to chloride ions at initial exposures depends on the concentration of the film, being higher
for more dilute Freon/water films and vice versa. This has
also been observed in our x-ray study of Freon/water films.21
The similarity of the plots for x-ray and electron exposures
indicates that chloride ion formation is nearly independent of
the primary radiation source.
The evolution of the F 1s XPS region during x-ray21 and
electron irradiation 共not shown here兲 also indicates production of F⫺ ions solvated in the ice film, as seen from the
appearance and growth of a new peak at ⬇686 eV associated
with F⫺ , along with a decrease of the peak at ⬇688 eV
associated with the CuF bonds. The kinetics of fluoride and
chloride ion production differ markedly:21,35 F⫺ forms more
slowly at initial exposures (⭐3⫻1015 cm⫺2 ) than Cl⫺ , but
for higher exposures, F⫺ forms at a nearly constant rate,
while the growth rate of the Cl⫺ ion concentration in the film
decreases. As a result, after an exposure of ⬃4
⫻1016 cm⫺2 most of the fluorine species in the film are in
the form of F⫺ , whereas only about 60–70% of the original
Cl atoms have become Cl⫺ .
E. Comparison of the rates of dissociation
for two halocarbons
FIG. 6. Dependences of Cl anion production 共measured from the Cl 2p XPS
region兲 as a function of effective electron exposure measured for
CF2 Cl2 /H2 O films 共thickness ⬇6.5 nm deposited on Au at 100 K兲 with
various concentrations 共indicated on the right兲. Two major data sets are
presented: one obtained during x-ray irradiation 共hollow symbols兲 and another detected during irradiation by 210 eV electrons 共solid symbols兲. Horizontal error bars indicate uncertainty in the effective electron exposure determined for x-ray irradiation experiments 共see Sec. III E for details兲.
The radiation-induced decomposition of CF2 Cl2 and
CCl4 in water matrices has been compared using XPS during
irradiation by 210 eV electrons of dilute mixtures of the two
halocarbons in ice 共halocarbon/water ratio ⬇0.06; total film
thickness ⬇6.5 nm兲. Figure 7共a兲 shows the evolution of the
C 1s spectral region measured for CF2 Cl2 /H2 O films, and
Fig. 7共b兲 represents similar spectra of the C 1s region measured for CCl4 /H2 O films as a function of electron exposure.
The initial spectra reveal a peak in the C 1s region at
⬇293 eV 关Fig. 7共a兲兴 attributed to CF2 Cl2 共Ref. 21兲 and a
peak in the C 1s region at ⬇290 eV 关Fig. 7共b兲兴 attributed to
CCl4 . 20 For both halocarbons a similar transformation of XP
spectra during electron irradiation is observed. The C 1s
XPS regions 关Figs. 7共a兲 and 7共b兲兴 show electron-induced decomposition of both halocarbons, as indicated by the decrease of CF2 Cl2 and CCl4 features 共peaks at ⬇293 eV and
⬇290 eV in Figs. 7共a兲 and 7共b兲 respectively兲. The features at
⬇285 eV are assigned to adventitious carbon
contamination.20,21 The new peaks at ⬇291 eV in both fig-
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J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
FIG. 7. Transformation of C 1s XPS regions of CF2 Cl2 共a兲 and CCl4 共b兲 in
a H2 O matrix as a function of electron exposure 共labeled in plots兲. The films
with a halocarbon/water ratios of ⬇0.06 and thicknesses of ⬇6.5 nm were
deposited on a Au substrate at ⬃100 K. Incident electron energy is 210 eV.
Spectra are baseline corrected. The fits and the resultant envelopes are displaced downward from the raw data for clarity.
ures are attributed to the production of CO2 in dilute
halocarbon/water films.20,21 This is confirmed by a separate
XPS experiment where CO2 was adsorbed directly onto an
ice surface. The production of CO2 during electron beam
irradiation of dilute CCl4 /H2 O or CF2 Cl2 /H2 O films is also
in accordance with RAIRS data. The broad features at lower
binding energies 共between 285 eV and 290 eV兲 are attributed
to the formation of dehalogenated species 关indicated as
CFx Cly in Fig. 7共a兲 and as CCly in Fig. 7共b兲兴.20,21 The corresponding peak positions are expected to be shifted from the
parent halocarbon peak by ⬇1.5–1.8 eV per removed chlorine atom 共for CF2 Cl2 or CCl4 ), 34,37 and by ⬃2 eV per removed fluorine atom 共for CF2 Cl2 ). 38 The peaks observed at
the lowest binding energy ⬇285 eV 共see Fig. 7兲 are consistent with the presence of a carbonaceous graphitic
overlayer.20,21,38 The contribution of this feature to the C 1s
XP spectrum is observed to increase for higher concentrations of halocarbon in the ice films.20,21 Experimental and ab
initio studies predict that COF2 and COCl2 C 1s binding
energies are shifted higher 共⬇⫹1.9 eV兲 and lower 共⬇⫺1.0
eV兲, respectively, with respect to the ⬇291 eV CO2 peak.39
Thus, one might expect that in the C 1s spectral region, the
COX2 (X⫽Cl,F) peaks will be situated between the parent
halocarbon and CO2 peaks. However, the overlapping peaks
for the halocarbon, carbon dioxide, and carbonyl dihalide
molecules prevent the unambiguous identification of these
species.
Figure 8 compares the decomposition kinetics of both
halocarbons at low exposures (⭐7⫻1015 cm⫺2 ). The kinetics are estimated from the variation in the areas of the
CF2 Cl2 and CCl4 components in the C 1s XPS region
共shown by solid lines in Fig. 7兲. The plot contains data from
two experiments: electron bombardment and x-ray irradiation 共the time of irradiation is converted into an effective
electron exposure, as described above in Sec. III E兲. Al-
Kinetics of decomposition of CF2 Cl2 co-adsorbed with water
8555
FIG. 8. Plot of the CF2 Cl2 and CCl4 areas in the C 1s XPS region as a
function of electron exposure. The halocarbons are separately coadsorbed in
the water matrix 共dilute film with halocarbon/water ratio ⬇0.06; film thicknesses ⬇6.5 nm兲 on a Au substrate at ⬃100 K. The data are normalized to
the corresponding total initial signal of carbon in the C 1s region. The line
drawn is an exponential fit assuming first-order decay kinetics. 共The corresponding cross section for CF2 Cl2 halocarbon decomposition is ⬃7
⫻10⫺16 cm2 .)
though accurate quantitative measurements are limited by the
uncertainty in the contribution of various carbonaceous species, the plot qualitatively indicates similar dissociation rates
within the same order of magnitude for both halocarbons in a
water ice matrix 共the corresponding cross section is ⬃1
⫻10⫺15 cm2 ; compare Fig. 4兲.
IV. DISCUSSION
In the absence of nonthermal activation, Freon is inert
with condensed water films. However, when the films are
exposed to low-energy electrons 共or x-rays, as discussed
previously21兲 a rich chemistry occurs. The following text details the role of the water ice matrix in the electron-induced
kinetics of decomposition and the reaction pathways of
CF2 Cl2 , in comparison with CCl4 . An important conclusion
is that adsorption of CF2 Cl2 on a water ice surface does not
lead to a substantial 共order-of-magnitude兲 increase in the
cross section for electron-induced dissociation as determined
via direct TPD measurements of reactant concentration vs
electron-beam exposure. This observation is in contrast to
recent reports of enhanced cross section based on measurements of ESD anion yields and charge-trapping
coefficients.17,18 We discuss this difference and the validity
of the comparison in detail, in Sec. IV E 1, below.
A. Adsorption and interaction of halocarbons
with water ice and metal surfaces
The low halocarbon desorption temperatures seen in our
TPD data indicate a weak interaction of the halocarbon molecules with the Ru surface, consistent with a physisorbed
state.27 For CF2 Cl2 deposited on a H2 O (D2 O) surface, the
peak desorption temperature 关solid curve in Fig. 1共a兲 and
dashed curve in Fig. 2共a兲兴 shifts to a lower value vs CF2 Cl2
on Ru 关dashed curve in Fig. 1共a兲兴. These lower desorption
temperatures indicate that the binding of Freon to a water ice
surface is weaker than to the metal substrate. Similarly, we
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8556
Faradzhev et al.
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
observe a decrease of the desorption temperature from ⬇165
K to ⬇135 K for CCl4 adsorbed on a D2 O ice surface
关dashed curve in Fig. 2共d兲兴 versus Ru共0001兲.27
For CF2 Cl2 adsorbed on Ru共0001兲 under a thick H2 O
overlayer 关Fig. 1共b兲兴, the desorption temperature increases
from 145 K to 155 K. The increase in temperature does not
imply a change of adsorption energy, but rather the suppression of Freon desorption due to the water overlayer. This
overlayer, grown at 25 K using directional dosing with normal incidence, is expected to be a pore-free dense ASW film
with a flat surface24,32 共in contrast, the background deposited
water at ⬃100 K in our XPS and IR experiments has an
amorphous, microporous structure40,41兲. For CF2 Cl2 adsorbed under a thick H2 O overlayer, desorption of Freon is
not possible until the onset of H2 O crystallization. This
blocking effect has recently been observed for the thermal
desorption of a CCl4 film through H2 O and D2 O overlayers42
and also reported for methyl chloride CD3 Cl under H2 O
ice.43 In accordance with these studies, we explain the volcanolike desorption of CF2 Cl2 as the result of the release of
Freon molecules through connected pathways in the ice film,
which are formed in the water overlayer by defects 共cracks,
fissures, grain boundaries, etc.兲 during the amorphous-tocrystalline phase transition. The additional high temperature
shoulder in the CF2 Cl2 curve 关Fig. 1共b兲兴 is believed to be
caused by Freon trapped in crystalline ice, which desorbs in
coincidence with the ice sublimation.
The mixed Freon/water film 关Fig. 1共c兲兴 measurements
exhibit desorption of CF2 Cl2 both from the D2 O ice surface
共⬇110 K兲 and from the bulk of the film 共⬇165 K兲. The
release of the majority of the molecules from the bulk is
blocked by thermally stable ASW, similar to the case of
Freon under a water overlayer 关Fig. 1共b兲兴. However, a small
fraction of molecules trapped in the topmost surface layers of
the ice film is able to escape before the water-phase transition, as D2 O molecules desorb. This is indicated by the relatively broad leading edge of the desorption profile in Fig.
1共c兲 as compared to the sharp peak of the corresponding
curve in Fig. 1共b兲. Variations in the film thickness 共the concentration of dosing mixture was conserved in the course of
experiments兲 leads to a proportional change in the total area
of the CF2 Cl2 and D2 O TPD curves, while the area of the
Freon surface peak at ⬇100 K remains nearly constant
共within ⬇20% accuracy兲 关compare solid and dashed lines in
Fig. 1共c兲兴. This implies that during thermal annealing segregation of CF2 Cl2 to the surface of D2 O ice does not occur,
and furthermore that the statistical distribution of CF2 Cl2
molecules in thin and thick films is homogeneous along the
surface normal.
B. Reactions induced by ionizing radiation
in halocarbonÕwater ice layer
Our TPD and RAIRS experiments indicate that in the
absence of electron or x-ray bombardment of the film, there
is no reaction between Freon and water for either CF2 Cl2 on
the D2 O surface or for Freon inside a water matrix, irrespective of the substrate 共Ru single crystal and Au polycrystal兲,
substrate temperature 共25 K or 100 K兲 or film thickness
共from one to several tens nm兲.
Our recent studies19–21,35 along with the data of
the present XPS, RAIRS, and TPD measurements show that
both irradiation sources 共electrons and x-rays兲 lead to similar
modifications of CCl4 /H2 O 共ice兲 and CF2 Cl2 /H2 O
共ice兲 films. Here we generalize the reaction pathways in
Scheme 1.
Scheme 1. Electron-stimulated reactions in halocarbon/water 共ice兲 films:
CCl4 /H2 O (y⫽0) and CF2 Cl2 /H2 O (y⫽2). Here, X denotes a halogen
atom: F for CF2 Cl2 and Cl for CCl4 .
For both halocarbons, DEA mediated by low-energy secondary electrons 共near 0 eV兲 is the first step in the processes
observed. It results in formation of a free radical
•CFy Cl3⫺y 共where y⫽0 for CCl4 , and 2 for CF2 Cl2 ) and
chloride ion Cl⫺ 共for CF2 Cl2 the CuCl bond cleavage is the
dominant process of its decomposition resulting in preferred
production of Cl⫺ ). The reaction between •CFy Cl3⫺y and
hydroxyl radical •OH 共generated by electron-stimulated dissociation of H2 O) results in formation of carbonyl dihalide
COFy Cl2⫺y and the hydronium cation H3 O⫹ in balance with
Cl⫺ . The COFy Cl2⫺y molecule undergoes further electronstimulated decomposition to yield halide ions X ⫺ 共where X is
Cl for CCl4 and F for CF2 Cl2 ) and carbon dioxide CO2 as
the final stable carbon-containing species. The halide ions
produced at different steps are solvated in water ice.
Electron-stimulated desorption of CO and CO2 is not a significant reaction pathway except for prolonged electron irradiation times. For concentrated halocarbon films 共halocarbon:water ratio ⬎0.3兲 carbon-carbon coupling reactions
become increasing prevalent 共not shown in Scheme 1兲. The
interested reader is referred to Refs. 19–21 and 35 for details.
C. Role of secondary electrons
in halocarbon decomposition
An important observation in the present study is that
reaction products initiated by either x-rays or electrons are
similar, as seen by comparing the XPS and RAIRS spectra
共Figs. 3, 5, and 7兲 for a Freon/water layer under electron
bombardment and similar data obtained recently under x-ray
irradiation.21 The reason for this similarity is the crucial role
of low-energy secondary electrons in halocarbon decomposition. The secondary electrons have energies in the range
from zero to several tens eV 共Ref. 44兲 共e.g., recent data for
Ru共0001兲 gives the maximum in the secondary electron energy distribution between 10 and 20 eV for ⬃200 eV electrons incident45兲.
Low-energy electrons may cause molecular decomposition via DEA for electron energies ⱗ15 eV or dipolar dissociation 共DD兲 for electron energies ⲏ15 eV. CF2 Cl2 and CCl4
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J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
are known to readily capture near zero eV electrons 共both in
the gas and condensed phase兲, forming NIRs with a lifetime
of several picoseconds,46 followed by dissociation into a
radical and anion 共DEA mechanism兲.10,14 The DEA process
is believed to be responsible for carbon-halogen bond cleavage in the radiolysis of organohalides.47 The interaction of
low-energy electrons 共⬃0 eV兲 with gas phase CF2 Cl2 leads
predominantly to CuCl bond cleavage and production of
Cl⫺ and a trihalomethyl radical •CF2 Cl. 12,16,48 In this lowenergy regime the rate of electron stimulated CuCl bond
scission dominates over CuF bond cleavage by about one
order of magnitude.10
Although DD of condensed CF2 Cl2 may also be initiated
by ⬃200 eV electrons, we suggest that in our experiments
DEA, activated by low-energy secondary electrons, is the
major mechanism of halocarbon decomposition.35 This is
based on the following experimental facts: 共1兲 both x-rays
and electrons lead to decomposition of halocarbons and produce the same chemistry; 共2兲 anions (Cl⫺ and F⫺ ) are efficiently produced 共and accumulated兲 in the film; 共3兲 bond
breaking in the CF2 Cl2 molecule is selective towards CuCl
bond cleavage, which is clearly seen from the different initial
rates of chloride and fluoride anions formation. Although
x-ray radiation can also dissociate molecules by creating core
holes, the cross section for such processes is much smaller
than the typical cross section for DEA by low-energy
electrons.48 Thus we conclude that DEA is the dominant initial bond breaking process in all of the films studied in this
investigation.
D. Reaction kinetics in CF2 Cl2 ÕH2 O film and stability
of chloride anions
1. Comparison of Freon decomposition kinetics
observed by various techniques
The decomposition of CF2 Cl2 and production of COF2
and CO2 under electron bombardment of a condensed Freon/
water mixture observed by RAIRS for ⬎30 nm films and by
TPD for ⬃2.5 nm films are presented in Fig. 3. Although
there are quantitative differences in the two data sets, there
are qualitative similarities in the curves for CF2 Cl2 . The
concentration of COF2 in the film as measured by RAIRS
and TPD 共Fig. 3兲 is observed to pass through a maximum
and then decrease at longer electron-beam exposures. Similarly, the quantitative differences between Figs. 3共a兲 and 3共b兲
are attributed to effects of film thickness: the limited IMFP
of electrons in matter leads to the gradual attenuation of the
electron flux as a function of distance below the surface. The
thickness of the Freon/water layer used in the TPD experiments is comparable to IMFP of the bombarding electrons
共⬃1.5 nm兲. We expect, therefore, that a relatively substantial
共⬃50%兲 fraction of the primary electrons pass through the
film without energy loss, reach the metal/film interface, and
generate low-energy secondary electrons from the metal substrate. Due to energy losses in the condensed ⬎30 nm film
共in RAIRS experiments兲, incident electrons are not able to
reach the substrate with their full kinetic energy. In contrast
to thin-film TPD experiments 关Fig. 3共b兲兴, our RAIRS measurements 关Fig. 3共a兲兴 are therefore characterized by a nonuniform depth distribution of the incident electron flux that
Kinetics of decomposition of CF2 Cl2 co-adsorbed with water
8557
may lead to deviations from the expected first-order halocarbon decomposition kinetics. Additional RAIRS experiments
for lower electron energy of 30 eV 共electron current, mixture
concentration, and film thickness were kept constant兲 reveal
only small changes in Freon decomposition kinetics. This is
due to the weak dependence of electron IMFP in water in this
energy range 共the corrected experimental electron attenuation
length in H2 O ice is ⬃0.7 nm at 20 eV and ⬃1.4 nm at 180
eV,49 less than an earlier report of ⬃1.5 nm at 20 eV;50 see
also Refs. 26 and 51兲.
The main significance of the present comparison is that
both data sets for different film thicknesses 共⬃2.5 nm versus
⬎30 nm兲, different film growth conditions 共adsorption
temperature: 25 K vs 100 K; film growth: deposition using
normally oriented doser versus gas-phase deposition兲, and
different techniques 共TPD versus RAIRS兲 exhibit qualitatively similar kinetics for Freon decomposition and product
formation. The decomposition kinetics is seen to depend on
the secondary electron fluence, rather than primary excitation
source or growth conditions.
2. Influence of film concentration on the stability
of chloride anions: A simple statistical model
The dependence of Cl⫺ production in the Freon/water
layer as a function of effective electron exposure and Freon
concentration is shown in Fig. 6. The plot contains data obtained for films with similar thicknesses and includes results
from both x-ray irradiation and electron bombardment experiments. The curves indicate a substantial increase in the
fraction of stable anions present in the film during irradiation
with decreasing halocarbon/water ratios 共cf. Fig. 6 for Cl⫺ ;
qualitatively similar behavior is observed for F⫺ ). This has
also been reported recently for halouracil/water sandwiches
under x-ray and electron exposure.52
Recent RAIRS data21 and the present TPD data 共Fig. 4兲
indicate that the rate of CF2 Cl2 decomposition, which occurs
predominantly via DEA, does not depend strongly on the
concentration of Freon in the Freon/water film. However, the
fraction of chlorine converted to stable chloride anions
changes systematically with Freon concentration, especially
for dilute films 关see Fig. 6; also Fig. 10共c兲 in Ref. 21兴. Perry
et al.21 have proposed that the concentration of stable Cl⫺
ions formed as a function of the film’s initial chemical composition can largely be correlated with the fate of the
•CF2 Cl intermediate produced as a result of the initial
CuCl bond cleavage event 共Scheme 1兲. In dilute films Cl⫺
ions are generated by the initial DEA process (CF2 Cl2 ⫹e ⫺
→Cl⫺ ⫹•CF2 Cl) and also during the subsequent formation
of COF2 共Scheme 1兲. However, in CF2 Cl2 rich films, although Cl⫺ ions are produced during the initial DEA process
the •CF2 Cl intermediate participates in carbon-carbon coupling reactions leading to the production of a CFx Cly overlayer with no production of the second Cl⫺ anion. Thus, a
qualitative interpretation for the variation in the Cl⫺ yield
with respect to film composition can be derived by considering the fate of the •CF2 Cl intermediate.
The trends in Fig. 6, however, imply that the fate of the
•CF2 Cl intermediate is not the only factor responsible for the
total number of Cl⫺ ions in the film. Specifically, since
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8558
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
Faradzhev et al.
FIG. 9. The calculated probabilities P
for Freon molecule to occupy a certain
site in a water ice cage as a function of
CF2 Cl2 :H2 O ratio r. Shown by solid
lines are the probabilities for Freon to
have only water molecules at six nearest neighbor sites ( P 1 , illustrated by
the sketch on the top right兲 or to have
a second Freon molecule among nearest neighbors with the other five sites
occupied by water ( P 2 ; sketch on the
bottom right兲. P 12 includes both cases
共dash line兲. The probabilities are normalized to the number of Freon molecules per unit volume 共i.e., divided
by p兲. Symbols on the plot indicate the
experimental points 共taken from Cl 2p
XPS data兲 of the fraction of chlorine converted to chloride anion after an electron exposure of ⬃0.5
⫻1015 cm⫺2 共hollow circles; taken
from Fig. 6兲 and after ⬃20 min of
x-ray exposure 关hollow triangles;
taken from Fig. 10共c兲 in Ref. 21兴. For
each ratio r the corresponding experimental point is obtained at the ‘‘knee’’
of the plot of fraction of Cl⫺ produced
on electron/x-ray exposure.
the dissociation of Freon is initiated exclusively via DEA to
form •CF2 Cl⫹Cl⫺ , one would expect the lower bound for
the value of the Cl⫺ /Cl(tot) fraction to be 0.5. However, Fig.
7 indicates that the Cl⫺ /Cl(tot) fraction is ⬍0.5 for concentrated films, implying that some of the Cl⫺ ions formed via
DEA are converted to other species in the film, which cannot
be identified by XPS and IR. One possibility is that in concentrated films, other reactions 共e.g., Cl2 formation during
irradiation of condensed pure CF2 Cl2 films53兲 may play an
important role due to the proximity of neighboring Freon
molecules.
Additional insight can be also be gained by considering
the potentially stabilizing 共solvating兲 effect of water. In this
capacity water, known to be an efficient solvent of charged
particles,54 –59 may provide stabilization for anions, preventing recombination and/or subsequent reactions. Such an effect would be consistent with the increase in Cl⫺ production
observed in dilute films where Freon molecules are surrounded predominantly by H2 O.
A simple statistical model is derived that incorporates
the stabilizing influence of water molecules on Cl⫺ ions and
is shown to reproduce the experimental trends in the
Cl⫺ /Cl(tot) yield. The model proceeds from the assumption of
a random distribution of CF2 Cl2 molecules in the Freon/
water film. Within this limit, we test how the local water
environment influences anion stability and in particular, the
correlation between the degree of isolation of a Freon molecule and the formation of a stable 共i.e., long-lived兲 chloride
anion. The random distribution implies a certain probability
to find a Freon molecule surrounded exclusively by water
molecules. The probability that a single site 共in the bulk of
the film兲 is occupied by a halocarbon molecule depends on
its concentration p, which is defined as the number of Freon
molecules as a fraction of the total number of molecules
共water and Freon兲 in the mixture. The probability that a wa-
ter molecule occupies a certain site is (1⫺ p). Therefore the
probability, P 1 , that a single site is occupied by a Freon
molecule having only water molecules at n nearest neighbor
sites is p•(1⫺p) n . The probability, P 2 , that a single site is
occupied by a Freon molecule having another Freon molecule among one of its n nearest neighbors is n• p 2 •(1
⫺p) n⫺1 , where n arises due to n possible degenerate sites
for the second Freon molecule.
In Fig. 9 we compare these probabilities 共representing a
random distribution of Freon molecules in a water matrix兲
with the experimentally observed fraction of stable chloride
anions produced as a function of the initial Freon/water ratio
in the mixed film 共includes both x-ray and electron irradiation experiments兲. P 1 (p), P 2 (p), and P 12(p) ( P 12 is the
sum of P 1 and P 2 ) are calculated for n⫽6, which is the
choice of n based on ab initio calculations for low-energy
configurations of Cl⫺ (H2 O) n complexes.56,57 Both experimental data points and calculated probabilities are normalized to the total amount of chlorine in the bulk ( P 1 , P 2 , and
P 12 are divided by p; experimental points are proportional to
the fraction of Cl⫺ ions measured within the Cl 2p region兲.
The relation between the ratio r of CF2 Cl2 to H2 O and the
concentration of CF2 Cl2 in the film is given by the expression: r⫽p/(1⫺p).
Figure 9 clearly indicates that for dilute films, the fraction of irradiation-generated chloride anions that are stabilized in the film correlates with the fraction of isolated Freon
molecules, i.e., the molecules surrounded only by H2 O 共compare experimental data and calculated probability P 1 ). The
variation of n in the range up to 8 results in a qualitatively
similar relationship between the experimental results and the
calculated dependencies. This implies that the presence of
surrounding H2 O molecules is important for the stabilization
of chloride anions, whereas the exact number of surrounding
H2 O molecules is less important. The quantitative differ-
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Kinetics of decomposition of CF2 Cl2 co-adsorbed with water
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
8559
TABLE II. Comparison of the electron interaction condensed-phase cross sections for CF2 Cl2 and CCl4 .
␴ 共units of 10⫺15 cm2兲
CF2 Cl2
Total dissociation cross-section:
⫺1 ML on Ru共0001兲
⫺1 ML on 5 ML D2 O
⫺0.25 ML on 5 ML D2 O
⫺0.1 ML on 5 ML D2 O
⬃200
⬃200
⬃200
⬃200
eV
eV
eV
eV
0.36
0.82
0.95
1.2
DEA ␴ measured by F⫺ ESD yield:
⫺1 ML on Ru共0001兲
⬃200 eV
0.92
DEA ␴ measured in charge
trapping experiments:
⫺0.1 ML on 5 ML H2 O/Kr 共ice兲
⫺0.1 ML on Kr 共ice兲
⬃0 eV
⬃0 eV
ences between the experimentally observed concentration of
Cl⫺ anions and our model may arise from various sources
including 共1兲 experimental uncertainty; 共2兲 an inhomogeneous distribution of Freon and H2 O in the film; 共3兲 the
interaction of chloride anions with other reaction products;
and 共4兲 different mechanisms for production of the first and
second chloride anions from a single parent CF2 Cl2 molecule. Results from this analysis do however suggest that the
fate of chlorine is determined by a combination of the reaction pathways outlined in Scheme 1, coupled with the longterm stability of Cl⫺ anions produced, the latter correlated
with the availability and local concentration of surrounding
H2 O molecules.
E. Kinetics of CF2 Cl2 and CCl4 decomposition
1. Role of H2O coadsorption
To ascertain the effect of H2 O on the kinetics of decomposition of the halocarbon molecules, TPD data were obtained for thin films 共⬃2.5 nm兲, which ensures a more uniform distribution of secondary electrons than in the case of
thicker films 共⬎30 nm兲. Previous results revealed an enhancement in ESD yields of Cl⫺ ions for CF2 Cl2 coadsorbed
with H2 O60,61 and an increase in the charge-trapping coefficient for CF2 Cl2 adsorbed on thick water films.17 These enhancements have been attributed to an increase of DEA cross
sections for CF2 Cl2 when coadsorbed with H2 O, as a result
of electron transfer from precursor states of the solvated
electron in ice to the CF2 Cl2 molecule, resulting in its decomposition into a neutral fragment and the Cl⫺ ion. Based
on these previous studies, we might expect that ‘‘wet’’ halocarbons 共i.e., halocarbons coadsorbed with water兲 should decompose much faster than ‘‘dry’’ ones 共i.e., neat halocarbons兲.
In Fig. 4 the normalized TPD peak areas for CF2 Cl⫹ and
⫹
CCl3 共corresponding to the dominant fragmentation ions for
CF2 Cl2 and CCl4 , respectively兲 are plotted as a function of
electron irradiation. The slopes of straight lines in the semilogarithmic plots are proportional to the cross section for
dissociation of the CF2 Cl2 and CCl4 molecules, assuming
first-order kinetics. The increased slope for 1 ML CF2 Cl2
关Fig. 4共a兲兴 and 1 ML CCl4 关Fig. 4共b兲兴 adsorbed on the water
13
1.4
Present
Present
Present
Present
CCl4
study
study
study
study
0.4
1.5
2.6
Present study
Present study
Present study
4.7
Ref. 65
Ref. 70
Ref. 17
Ref. 17
ice surface versus Ru共0001兲 implies an increase of the decomposition rate for both halocarbons (CF2 Cl2 and CCl4 )
adsorbed on ice. A quantitative estimation shows that the
cross sections for decomposition of Freon and CCl4 increase
on water ice films by a factor of ⬃2 and ⬃4, respectively
共Table II兲. A detailed study62 indicates that this increase cannot be attributed to a smaller probability of de-excitation
processes by increasing the distance between the Freon layer
and the metal surface. Overall, the decomposition cross section of CF2 Cl2 is smaller than that of CCl4 .
The initial coverage 共up to 1 ML兲 of both CF2 Cl2 and
CCl4 on the thick D2 O 共ice兲 surface, appears to weakly affect
the decomposition cross section: lower coverages appear to
result in greater cross sections, though the differences observed are quite small. For instance, from Fig. 4共b兲, the cross
section for electron-induced decomposition of 0.25 ML CCl4
is only ⬇1.7 times greater than for 1 ML CCl4 共Table II兲. For
Freon 关Fig. 4共a兲兴, the effect is even less pronounced, but the
general trend is similar. This may reflect the special role of
halocarbon-halocarbon intermolecular interactions as an additional channel for dissipation of an excited state 共e.g.,
⫺
CF2 Cl⫺
2 or CCl4 ). Additionally, as discussed above, the
fraction of solvated ions increases for more dilute films 共a
smaller initial coverage of a halocarbon on a water ice surface is equivalent to a more dilute mixed film兲. These solvated ions might have a smaller probability of recombining
with dissociated parent fragments, increasing the overall dissociation cross section. On the whole, the TPD results are in
agreement with the cross sections calculated from the attenuation of the corresponding peaks for parent CF2 Cl2 and CCl4
molecules in C 1s XP region 共taking into account the larger
error bars for XPS measurements; see Fig. 8兲.
The ability of water to trap near zero eV electrons might
be one of the factors responsible for an increase in the rate of
halocarbon decomposition in dilute Freon/water films. Fully
solvated electrons are trapped with energies below 0 eV and
cannot cause DEA of halocarbon molecules, but the water
media may slow down scattered and secondary electrons and
shift the energy distribution of secondary electrons towards
lower energies, closer to the resonance energy. This may increase the number of carriers participating in resonant coupling to Freon molecules, increasing the DEA cross sections.
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8560
Faradzhev et al.
J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
While the current results imply similar trends reported in
previous ESD experiments for anion desorption,18,60 the
magnitudes of enhancement differ greatly. ESD experiments
indicate that the detected yield of Cl⫺ ions increased by a
factor of ⬃100 for 0.3 ML CF2 Cl2 upon the coadsorption of
⬃1 ML H2 O, and that the Cl⫺ yield enhancement for an
initial coverage of 0.3 ML 共coadsorbed with ⬃1 ML H2 O)
was ⬃10 times greater than for 1 ML initial coverage of
CF2 Cl2 alone.18,60
A comparison of the anion ESD yield experiments18,60 as
well as the charge trapping experiments17 with the present
work is not straightforward. The ESD of Cl⫺ ions is a
multiple-step process involving both the dissociation of the
parent CF2 Cl2 molecule and desorption of the Cl⫺ ions. In
order for an ion to desorb, it must be stable for a sufficient
amount of time after formation and must posses enough
translational kinetic energy to desorb from the surface. Typically, only a very small fraction of ions formed actually desorbs from a surface (⬃10⫺7 ). 14,15,63 Moreover, the presence
of an H2 O layer may influence the anion desorption lifetime,
increasing the fraction of ions that survive to desorb.55 It
should be noted that ⬃200 eV incident electrons are used in
the present TPD/XPS/RAIRS experiments and the previous
ESD anion yield experiments,18 while near ⬃0 eV electrons
were used by Lu and Sanche.17 As was mentioned in Sec. I,
Lu and Sanche observed a giant enhancement of the charge
trapping coefficient of fractional monolayers of CF2 Cl2 deposited on an H2 O ice layer versus a Kr 共ice兲 layer.17 For ⬃0
eV incident electrons, the charge trapping coefficient, assumed to be approximately equal to the DEA cross section,
increased by ⬃10 times for Freon adsorbed on H2 O. 17 In
these experiments, however, the authors are not able to distinguish conclusively between a trapped Cl⫺ fragment and
stabilized CF2 Cl⫺
2 . The increase in the charge trapping
might, for instance, indicate an increase of the electron
attachment process and longer lifetime of CF2 Cl⫺
2 , but not
necessarily indicate an enhancement of CF2 Cl2 decomposition rate and Cl⫺ formation. This is an important open issue
that requires further study.
2. Comparison of CF2Cl2 and CCl4 decomposition
The TPD data of Fig. 4 indicate similar cross sections for
electron-induced decomposition of CF2 Cl2 and CCl4 on a
water ice surface 共for 0.25 ML, the values are 0.95
⫻10⫺5 cm2 and 2.6⫻10⫺15 cm2 , respectively; see Table II兲.
In Fig. 8, the integrated XP intensity of the C 1s spectra for
films containing either CF2 Cl2 关Fig. 7共a兲兴 or CCl4 关Fig. 7共b兲兴
are plotted as a function of electron irradiation. The similar
fits to decay profiles imply similar cross sections of ⬃1
⫻10⫺15 cm2 for both halocarbons. Interestingly, the total
gas-phase electron-scattering cross sections for the two molecules are also similar 共see Table I兲, as recently observed in a
study of electron transmission through halocarbons: for 100
eV electrons the reported cross sections are 3.7⫻10⫺15 cm2
for CCl4 and 2.7⫻10⫺15 cm2 for CCl2 F2 . 64 Since condensation can dramatically influence the DEA cross section,65 it is
also necessary to look at cross sections for the halocarbons in
the condensed phase. For the case of 0.1 ML of CF2 Cl2 and
CCl4 on Kr, the charge trapping cross sections for ⬃0 eV
electrons are ⬃1.4⫻10⫺15 cm2 共Ref. 17兲 and ⬃5
⫻10⫺15 cm2 . 65 In all of these measurements, the cross section for electron scattering, charge trapping, and decomposition is always greater for CCl4 . It has been noted that the
chemical composition of the molecules greatly influences the
DEA cross section, increasing substantially with the number
of Cl atoms.65,66
V. CONCLUSIONS
At 25 K both H2 O and D2 O, in the form of ASW adsorbed on top of CF2 Cl2 on a Ru surface, impede CF2 Cl2
thermal desorption from Ru共0001兲 until the onset of water
crystallization 共⬇155 K for H2 O and ⬇165 K for D2 O). In
contrast, thermal desorption of CF2 Cl2 from the water ice
surface occurs at ⬇110 K, well before the ASW-tocrystalline phase transition temperature, indicating weak
binding of CF2 Cl2 to the ice surface.
In the absence of ionizing radiation, neither CF2 Cl2 nor
CCl4 reacts with condensed H2 O. The interaction of ionizing
radiation 共⬃200 eV electrons or Al K ␣ x-rays兲 with molecular CF2 Cl2 or CCl4 coadsorbed with water at low temperatures 共25 K–100 K兲 leads to decomposition of the halocarbons. Damage fragments and products of subsequent
chemical reactions detected in the films include solvated Cl⫺
and F⫺ anions, H3 O⫹ ions, COF2 共for CF2 Cl2 ), COCl2 共for
CCl4 ), C2 Cl4 共for CCl4 ), and CO2 . The kinetics of halocarbon decomposition and reaction products are qualitatively
independent of the source of radiation 共hundred eV electrons
versus high-energy photons兲.
The initial radiation-induced process in the adsorbed halocarbons is CuCl bond cleavage, which dominates over
CuF bond breaking for CF2 Cl2 . The decomposition of
CF2 Cl2 and CCl4 is believed to proceed via DEA of lowenergy secondary electrons generated by radiation in condensed matter 共both in the metal substrate and in the bulk of
the ice film兲. The present results of direct measurements 共using TPD兲 of total dissociation cross sections for CF2 Cl2 and
CCl4 adsorbed alone and coadsorbed with water on a metal
surface are compared with recent ESD and charge-trapping
experiments that provided indirect measurements of DEA
cross sections. We have shown that the cross sections for the
electron-induced decomposition of CF2 Cl2 and CCl4 are
similar on the metal surface and increase by factors of 2– 4
for both molecules in a water 共ice兲 environment. The highest
measured decomposition cross sections for 180 eV incident
electrons have been observed for fractional monolayer halocarbon coverage on a water ice surface and for very dilute
halocarbon/water mixtures: 1.0⫾0.2⫻10⫺15 cm2 for CF2 Cl2
and 2.5⫾0.2⫻10⫺15 cm2 for CCl4 . The increase is considerably smaller than inferred from previous indirect
measurements,17,18,60 where the measured magnitudes might
be influenced by other effects, such as the anion desorption
lifetime 共in anion ESD yield experiments18,60兲 or by the possible stabilization of CF2 Cl⫺
2 species 共in charge trapping
experiments17兲.
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J. Chem. Phys., Vol. 121, No. 17, 1 November 2004
ACKNOWLEDGMENTS
The Rutgers University work has been supported by the
National Science Foundation 共NSF兲, Grants Nos. CHE
0075995 and CHE 0315209; the studies carried out at Johns
Hopkins University have also been supported by NSF, Grant
No. CHE-0089168, as part of the Collaborative Research
Activities in Environmental Molecular Science in Environmental Redox-Mediated Dehalogenation Chemistry at the
Johns Hopkins University.
I. Hua and M. R. Hoffman, Environ. Sci. Technol. 30, 864 共1996兲.
M. J. Molina and F. S. Rowland, Nature 共London兲 249, 810 共1974兲.
3
E. J. Hall, Radiation and Life 共Pergamon, New York, 1984兲.
4
T. E. Madey, R. E. Johnson, and T. M. Orlando, Surf. Sci. 500, 838
共2002兲.
5
Q.-B. Lu and L. Sanche, Phys. Rev. Lett. 87, 078501 共2001兲.
6
N. R. P. Harris, J. C. Farman, and D. W. Fahey, Phys. Rev. Lett. 89,
219801 共2002兲.
7
Q.-B. Lu and L. Sanche, Phys. Rev. Lett. 89, 219802 共2002兲.
8
P. K. Patra and M. S. Santhanam, Phys. Rev. Lett. 89, 219803 共2002兲.
9
R. Müller, Phys. Rev. Lett. 91, 058502 共2003兲.
10
L. G. Christophorou, J. K. Olthoff, and Y. Wang, J. Phys. Chem. Ref. Data
26, 1205 共1997兲.
11
P. O. Staneke, G. Groothuis, S. Ingemann, and N. M. M. Nibbering, Int. J.
Mass Spectrom. Ion Processes 142, 83 共1995兲.
12
A. Kiendler, S. Matejcik, J. D. Skalny, A. Stamatovic, and T. D. Märk, J.
Phys. B 29, 6217 共1996兲.
13
S. C. Chu and P. D. Burrow, Chem. Phys. Lett. 172, 17 共1990兲.
14
St. J. Dixon-Warren, E. T. Jensen, and J. C. Polanyi, J. Chem. Phys. 98,
5938 共1993兲.
15
P. Rowntree, L. Sanche, L. Parenteau, M. Meinke, F. Weik, and E. Illenberger, J. Chem. Phys. 101, 4248 共1994兲.
16
O. Ingolfsson, F. Weik, and E. Illenberger, Int. J. Mass Spectrom. Ion
Processes 155, 1 共1996兲.
17
Q.-B. Lu and L. Sanche, Phys. Rev. B 63, 153403 共2001兲.
18
Q.-B. Lu and T. E. Madey, Surf. Sci. 451, 238 共2000兲.
19
A. J. Wagner, C. Vecitis, and D. H. Fairbrother, J. Phys. Chem. B 106,
4432 共2002兲.
20
A. J. Wagner, C. Vecitis, G. M. Wolfe, C. C. Perry, and D. H. Fairbrother,
Phys. Chem. Chem. Phys. 4, 3806 共2002兲.
21
C. C. Perry, G. M. Wolfe, A. J. Wagner, J. T. Torres, N. S. Faradzhev, T. E.
Madey, and D. H. Fairbrother, J. Phys. Chem. B 107, 12740 共2003兲.
22
N. S. Faradzhev, D. O. Kusmierek, B. V. Yakshinskiy, and T. E. Madey,
Low Temp. Phys. 29, 286 共2003兲.
23
N. S. Faradzhev, D. O. Kusmierek, B. V. Yakshinskiy, S. M. Solovev, and
T. E. Madey, Surf. Sci. 528, 20 共2003兲.
24
K. P. Stevenson, G. A. Kimmel, Z. Dohnálek, R. S. Smith, and B. D. Kay,
Science 283, 1505 共1999兲.
25
S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal. 17, 911
共1991兲.
26
D. Emfietzoglou and M. Moscovitch, Nucl. Instrum. Methods Phys. Res.
B 193, 71 共2002兲.
27
N. J. Sack, L. Nair, and T. E. Madey, Surf. Sci. 310, 63 共1994兲.
28
Q.-B. Lu, Z. Ma, and T. E. Madey, Phys. Rev. B 58, 16446 共1998兲.
29
D. L. Doering and T. E. Madey, Surf. Sci. 123, 305 共1982兲.
30
K. H. Junker and J. M. White, Surf. Sci. 382, 67 共1997兲.
31
IR and Mass Spectra NIST Chemistry WebBook, NIST Standard Reference
Database Number 69, edited by P. J. Linstrom and W. G. Mallard 共National Institute of Standards and Technology, Gaithersburg, MD, 2001兲
共http://webbook.nist.gov兲.
32
M. S. Westley, G. A. Baratta, and R. A. Baragiola, J. Chem. Phys. 108,
3321 共1998兲.
33
R. J. Speedy, J. Phys. Chem. 96, 2322 共1992兲.
1
2
Kinetics of decomposition of CF2 Cl2 co-adsorbed with water
8561
34
K. Adib, N. Camillone, III, J. P. Fitts, K. T. Rim, G. W. Flynn, S. A. Joyce,
and R. M. Osgood, Jr., Surf. Sci. 497, 127 共2002兲; K. Adib, D. R. Mullins,
G. Totir, N. Camillone, III, J. P. Fitts, K. T. Rim, G. W. Flynn, and R. M.
Osgood, Jr., ibid. 524, 113 共2003兲.
35
C. C. Perry, N. S. Faradzhev, D. H. Fairbrother, and T. E. Madey, Int. Rev.
Phys. Chem. 共in press兲.
36
T. E. Madey and J. T. Yates, Jr., J. Vac. Sci. Technol. 8, 525 共1971兲.
37
Electron Spectroscopy, Theory, Techniques and Applications, edited by A.
Dilks, C. R. Brundle, and A. D. Baker 共Academic, London, 1981兲, Vol. 4.
38
J. P. Chang, H. W. Krautter, W. Zhu, R. L. Opila, and C. S. Pai, J. Vac. Sci.
Technol. A 17, 2969 共1999兲.
39
P. H. Guadagnini, A. E. Oliveira, R. E. Bruns, and B. de Barros Neta, J.
Am. Chem. Soc. 119, 4224 共1997兲.
40
L. Delzeit, B. Rowland, and J. P. Devlin, J. Phys. Chem. 97, 10312 共1993兲.
41
V. P. Zhdanov and P. R. Norton, Surf. Sci. 449, L228 共2000兲; V. P.
Zhdanov and P. R. Norton, ibid. 459, 245 共2000兲.
42
R. S. Smith, C. Huang, E. K. L. Wong, and B. D. Kay, Phys. Rev. Lett. 79,
909 共1997兲.
43
Y. Lilach and M. Asscher, J. Chem. Phys. 117, 6730 共2002兲.
44
V. Cobut, Y. Frongillo, J. P. Patau, T. Goulet, M.-J. Fraser, and J.-P. JayGerin, Radiat. Phys. Chem. 51, 229 共1998兲.
45
J. J. Czyzewski, S. Kaszczyszyn, and J. Krajniak, Appl. Surf. Sci. 177, 42
共2001兲.
46
R. A. Popple, C. D. Finch, K. A. Smith, and F. B. Dunning, J. Chem. Phys.
104, 8485 共1996兲.
47
R. E. Buehler, in Radiation Chemistry of the Carbon-Halogen Bond, The
Chemistry of the Carbon-Halogen Bond, Part 2, edited by S. Patai 共Wiley,
Chichester, 1973兲, pp. 795– 864.
48
E. Illenberger, H. U. Scheunemann, and H. Baumgaertel, Chem. Phys. 37,
21 共1979兲.
49
Q. Wang, R. Stockbauer, R. L. Kurtz, and T. E. Madey 共unpublished兲.
50
R. L. Kurtz, N. Usuki, R. Stockbauer, and T. E. Madey, J. Electron Spectrosc. Relat. Phenom. 40, 35 共1986兲.
51
M. Michaud and L. Sanche, Radiat. Res. 159, 3 共2003兲.
52
D. V. Klyachko, M. A. Huels, and L. Sanche, Radiat. Res. 151, 177
共1999兲.
53
M. N. Hedhili, M. Lachgar, Y. Le Coat, R. Azria, M. Tronc, Q.-B. Lu, and
T. E. Madey, J. Chem. Phys. 114, 1844 共2001兲.
54
D. J. Tobias, P. Jungwirth, and M. Parrinello, J. Chem. Phys. 114, 7036
共2001兲.
55
M. Akbulut, T. E. Madey, and P. Nordlander, J. Chem. Phys. 106, 2801
共1997兲.
56
G. Markovich, S. Pollack, R. Giniger, and O. Cheshnovsky, J. Chem.
Phys. 101, 9344 共1994兲.
57
W. L. Jorgensen and D. L. Severance, J. Chem. Phys. 99, 4233 共1993兲.
58
W. H. Robertson, G. H. Weddle, J. A. Kelley, and M. A. Johnson, J. Phys.
Chem. A 106, 1205 共2002兲.
59
J. P. Devlin, N. Uras, J. Sadlej, and V. Buch, Nature 共London兲 417, 269
共2002兲.
60
Q.-B. Lu and T. E. Madey, J. Chem. Phys. 111, 2861 共1999兲.
61
Q.-B. Lu and T. E. Madey, Phys. Rev. Lett. 82, 4122 共1999兲.
62
N. S. Faradzhev, D. O. Kusmierek, and T. E. Madey 共unpublished兲.
63
S. J. Dixon-Warren, E. T. Jensen, and J. C. Polanyi, Phys. Rev. Lett. 67,
2395 共1991兲.
64
Y. Kondo and T. Yamada, Proceedings of 51st Annual Gaseous Electronics
Conference and 4th International Conference on Reactive Plasmas,
OWP6.14 共Maui, Hawaii, 1998兲.
65
A. D. Bass, J. Gamache, P. Ayotte, and L. Sanche, J. Chem. Phys. 104,
4258 共1996兲.
66
J. Langer, S. Matt, M. Meinke, P. Tegeder, A. Stamatovic, and E. Illenberger, J. Chem. Phys. 113, 11063 共2000兲.
67
K. K. Irikura, M. A. Ali, and Y.-K. Kim, Int. J. Mass. Spectrom. 222, 189
共2003兲.
68
C. Szmytkowski, A. M. Krzysztofowicz, P. Janicki, and L. Rosenthal,
Chem. Phys. Lett. 199, 191 共1992兲.
69
M. Bart, P. W. Harland, J. E. Hudson, C. Vallance, Phys. Chem. Chem.
Phys. 3, 800 共2001兲.
70
Q.-B. Lu and T. E. Madey, Surf. Rev. Lett. 6, 313 共1999兲.
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