JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 2
8 JANUARY 2004
Negative ion formation in electron-stimulated desorption of CF2 Cl2
coadsorbed with polar NH3 on Ru„0001…
S. Solovev,a) D. O. Kusmierek, and T. E. Madeyb)
Department of Physics and Astronomy and Laboratory for Surface Modification,
Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854-8019
共Received 26 March 2003; accepted 8 October 2003兲
Photon-induced dissociation of CF2 Cl2 共freon-12兲 in the stratosphere contributes substantially to
atmospheric ozone depletion. We report recent results on dissociation and negative ion formation in
electron-stimulated desorption 共ESD兲 of CF2 Cl2 on Ru共0001兲, when CF2 Cl2 is coadsorbed with a
polar molecule (NH3 ), for electron energies ranging from 50 to 300 eV. Two different time-of-flight
methods are used in this investigation: 共a兲 an ESD ion angular distribution detector with wide
collection angle and 共b兲 a quadrupole mass spectrometer with narrow collection angle and high mass
⫺
resolution. Many negative ESD fragments are seen (F⫺ ,Cl⫺ ,FCl⫺ ,CF⫺ ,F⫺
2 , and Cl2 ), whose
intensities depend on the surface preparation. Using both detectors we observe a giant enhancement
of Cl⫺ and F⫺ yields for ESD of CF2 Cl2 coadsorbed with ⬃1 ML of NH3 ; this enhancement
(⬎103 for Cl⫺ ) is specific to certain ions, and is attributed to an increased probability of
dissociative electron attachment due to ‘‘trapped’’ low-energy secondary electrons, i.e., precursor
states of the solvated electron in NH3 . In further studies, the influence of polar NH3 spacer layers
共1–10 ML兲 on ESD of top-layer CF2 Cl2 is determined, and compared with thick films of condensed
CF2 Cl2 . The magnitudes and energy dependences of the Cl⫺ yields are different in these cases, due
to several contributing factors. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1630296兴
polar stratospheric cloud 共PSC兲 ice particles may be affected
by trapped electrons with energies near 0 eV. The Cl⫺ released may react to destroy ozone on PSC ice surfaces,12–15
or be converted to Cl, which can destroy gaseous ozone. In a
series of papers,9–11 they have shown that there is a striking
enhancement of the electron-stimulated desorption 共ESD兲
yield of Cl⫺ desorbed from a surface containing CF2 Cl2
coadsorbed with H2 O, as well as other polar molecules
(NH3 , CH3 OH). The Cl⫺ yield is orders of magnitude
higher than in the absence of the coadsorbed polar molecules. Recently, Langer et al.16 reported that coadsorption
of CF2 Cl2 with ammonia did not exhibit enhanced DEA in
the electron energy range 0–20 eV, although their experimental conditions were different from those in Refs. 9–11.
Moreover, in a separate experiment, Lu and Sanche have
recently reported that deposition of CF2 Cl2 onto films of
condensed polar molecules (H2 O and NH3 ) greatly enhances
the charge trapping probability of ⬃0 eV electrons.17 Their
results indicate that the enhancement is due to the transfer of
electrons to CF2 Cl2 that are temporarily localized at precursors to the fully solvated state. Lu and Sanche18 have also
proposed that dissociation of chlorofluorocarbons by capture
of electrons produced by cosmic rays and localized in PSC
ice may play a significant role in causing the polar ozone
hole.
The present work focuses on the system for which the
greatest Cl⫺ enhancement is reported, CF2 Cl2 coadsorbed
with NH3 on a metal surface, Ru共0001兲.11 We expand on the
previous studies to provide new insights into negative ion
formation, and re-examine the enhancement of negative ion
yields by coadsorbates. Below, we describe improvements in
I. INTRODUCTION
There is much current interest in the study of chlorofluorocarbons 共CFCs兲, including the effects of radiation on their
dissociation. Man-made CFCs are believed to play a key role
in the destruction of the ozone layer in the Earth’s upper
atmosphere;1 Cl atoms are released from CFCs such as
CF2 Cl2 共Freon-12兲 via solar photon-induced dissociation in
the stratosphere, and react to destroy ozone (O3 ). The ozone
hole over Antarctica has been increasing in size recently,2
and the product concentration of an ozone reaction product
共OCl兲 in the Antarctic vortex is several hundred times greater
than the concentration in the general stratosphere.3,4
It is well documented that dissociation of gaseous and
condensed phase CFC molecules induced by collisions with
very low energy electrons is an extremely efficient
process.5– 8 Negative ion formation occurs via dissociative
electron attachment 共DEA兲, e.g.,
e⫺ ⫹CF2 Cl2 →Cl⫺ ⫹CF2 Cl,
共1a兲
e⫺ ⫹CF2 Cl2 →F⫺ ⫹CF2 Cl.
共1b兲
The cross section for DEA of CF2 Cl2 and other CFCs is
extremely large at electron energies close to 0 eV.5– 8
Whereas electron-induced dissociation processes are
generally believed to be of little importance for gaseous
CFCs in the stratosphere, Lu and Madey recently
proposed9–11 that DEA of CFCs adsorbed on the surfaces of
a兲
Permanent address: A. F. Ioffe Physico-Technical Institute, St. Petersburg,
Russia.
b兲
Author to whom all correspondence should be addressed. Electronic mail:
[email protected]
0021-9606/2004/120(2)/968/11/$22.00
968
© 2004 American Institute of Physics
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J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
Negative ion formation in electron-stimulated desorption
969
negative ion detection, and provide low-noise highresolution electron-stimulated desorption 共ESD兲 mass spectra
that reveal details not seen in the earlier low-resolution timeof-flight 共TOF兲 mass spectra. We remove ambiguities in detection of 35Cl⫺ , 37Cl⫺ , and 38F⫺
2 and confirm unambiguously that the Cl⫺ ESD yield is enhanced by over three
orders of magnitude upon coadsorption with NH3 . We increase the range of parameter space (CF2 Cl2 coverage, thickness of NH3 spacer layers, and electron energy兲 to provide a
more clear understanding of the factors influencing Cl⫺ enhancement.
II. EXPERIMENTAL SETUP
The experimental setup has been described
previously.9,19 Here we repeat only some important experimental details and methods. The experiments are performed
in an ultrahigh vacuum chamber equipped with instrumentation for Auger electron spectroscopy 共AES兲, temperature
programmed desoprtion 共TPD兲, electron-stimulated desorption ion angular distribution 共ESDIAD兲 with time-of-flight
共TOF兲 capability for mass- and angle-resolved ion detection,
and a quadrupole mass spectrometer 共QMS兲 with TOF capability. The QMS can be used as a residual gas analyzer, and
for mass-resolved detection of negative and positive ions in
electron-stimulated desorption 共ESD兲 experiments.
The sample is a Ru共0001兲 crystal. It can be cooled to 25
K with a closed-cycle helium refrigerator and heated to 1600
K by electron bombardment. The surface is cleaned by heating in oxygen up to 1500 K, followed by annealing in
vacuum. Surface cleanliness is checked by AES. All gases
(CF2 Cl2 , Xe, O2 , NH3 ) are deposited directly onto the
Ru共0001兲 surface held at 25 K via leak valves and directional
gas dosers 共normal incidence deposition兲 and the gas purity
is checked by the QMS. The gas pressure is measured by an
uncalibrated Bayard–Alpert ionization gauge.
The coverages of all gases are determined from TPD
spectra obtained using the QMS; the sample is heated by
radiation from a hot W filament. One monolayer 共ML兲 of
deposited CF2 Cl2 is defined as the coverage corresponding to
the saturation of the monolayer peak in thermal-desorption
spectra.19 In the present experiments, 1 ML corresponds to
an exposure of 0.09 L (1 L⫽1⫻10⫺6 torr s⫽1.33⫻10⫺4
Pas兲. The monolayer exposure is low because the direct flux
of molecules onto the surface is greater than the random flux
detected by the Bayard–Alpert gauge.
The electron beam energy for ESD measurements is adjustable from 20–350 eV. The electron gun is typically
pulsed at a repetition rate of 10 kHz and a pulse width of 100
ns. The beam size is about 1 mm2 and the average electron
flux for ESD measurements is ⬍3⫻10⫺10 A. To minimize
electron beam damage, the usual beam exposure time is less
than 1 min. For many measurements 共unless otherwise specified兲, the sample bias is ⫺30 V and the electron gun filament
potential is ⫺200 V, resulting in an incident electron beam
energy of 170 eV.
Negative ion ESD-signals are measured in one of two
ways, using TOF-ESDIAD9,19 or the QMS detector modified
for negative ion detection 共see Sec. II A below for details兲. A
FIG. 1. Time-of-flight 共TOF兲 spectra of negative ions desorbed from the
Ru共0001兲 surface covered by 1 ML of CF2 Cl2 and coadsorbed with 1.5 ML
of NH3 . 共a兲 ESDIAD and 共b兲 QMS. The bias sample voltage for ESDIAD is
U s ⫽⫺100 V, and for QMS U s ⫽⫺30 V. The incident electron flux is
⬃10⫺9 A/cm2 . The time of measurement is ⬃60 s. The electron energy
E e ⫽E gun⫺U s where E gun is the electron gun filament potential. In these
measurements, E e ⫽170 eV, unless otherwise stated. Lines in 共a兲 and in all
subsequent figures with lines through data points, are merely guides to the
eye.
comparison of the two methods is shown in Fig. 1. Whereas
TOF-ESDIAD has high sensitivity and a large acceptance
angle, its mass resolution is much lower than that of the
QMS, and additional ions are seen in the QMS signal 关e.g.,
mass 35 and 37 Cl⫺ signals are completely resolved in Fig.
1共b兲兴.
A. Apparatus for negative ion detection,
using the quadrupole mass spectrometer
The experimental setup used for negative ion detection
in ESD experiments is shown in Fig. 2. We modified our
QMS 共a UT1 Model 100C Mass Analyzer兲 to allow for negative ion detection. The components include a power supply,
which provides ⫹3 kV needed to run the channeltron, a
pulse amplifier and discriminator, which control the external
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970
J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
FIG. 2. Schematic diagram of QMS apparatus used for detection of negative
ions. E-gun: Kimball Physics model ELG-2 electron gun; QMS: UTI quadrupole mass-spectrometer model 100C; channeltron: Galileo Electron Optics
Corp.; amplifier: high sensitivity low noise pulse preamplifier F100-T; measurement system: Keithley 500 Series Measurement and control system,
sample: Ru共0001兲 single crystal.
pulse counting electronics, and various resistor- and
capacitor-networks to set potentials and provide the necessary biases.
The input of the QMS channeltron is grounded via a 30
M⍀ resistor. The net positive potential on the input can be
changed to increase sensitivity while high voltage is applied
to the output of the channeltron. The pulse amplifier is isolated from the high voltage supply by a 500 pF capacitor
共C1兲. The amplifier must be able to withstand applied voltages ranging up to ⫹5 kV. A Kimball Physics model ELG-2
electron gun is used to irradiate the sample with electrons, to
cause ESD of negative ions. There is no flux of electrons
from the gun when a ⫺25 V potential is applied to the G2
grid in the electron source triode assembly. By applying a
100 ns positive pulse to the grid, the gun is ‘‘opened’’ and a
small bunch of electrons bombards the sample.
The detection of the ion signal is delayed a specified
time after electron irradiation of the sample has begun, in
order to exclude a large part of the background caused by
secondary electrons. This is done using a time-of-flight pulsing technique, based upon the observation that the ‘‘fast’’
electrons reach the input of the QMS much faster than the
slow, massive ions. As a result, the signal consists of groups
of pulses, each corresponding to a different ion of a specific
mass.
The time-of-flight separation of the negative ion signal is
accomplished by pulsing the PIM2 pulse counter, which resides in the Keithley 500 Series Measurement and Control
System. The timing diagrams for these pulses are shown in
Fig. 3. The ion selection pulse G, ‘‘gate,’’ is applied to the
gate input of the PIM2 counter. The counter begins to count
pulses only when it coincides in time with ‘‘gate’’ G. By
changing the delay time ␶ d and the width of the gate pulse ␶ g
共Fig. 3兲, a specific group of negative ions can be selected for
Solovev, Kusmierek, and Madey
FIG. 3. Schematic timing diagrams for time-resolved QMS negative ion
detection system 共Fig. 2兲. See text for discussion.
detection. Taking into account the delay in the electronic
circuit, the usual delay time is in the range of 10– 40 ␮s. The
full mass range of the negative ions can also be detected, by
increasing the width of the ␶ g pulse to 40– 80 ␮s. In this
case, however, the tail of the low-energy ion signal together
with slow electrons cause an increase in the background signal. For low-level signals, this background may be as high as
30–50% of the total signal, and it very difficult to subtract
out. It is therefore necessary to optimize the width ␶ g and the
time delay ␶ d of the G gate in order to achieve the best
compromise between background level and sensitivity.
It is interesting to note that the quadrupole mass filter
can be used as a ‘‘magnifier’’ of the time difference between
two sets of negative ions with different energies. This is a
consequence of the longer flight path from sample to detector
for the QMS, and can be illustrated in Figs. 4共a兲, and 4共b兲
where the normalized Cl⫺ ESDIAD 共a兲 and QMS-TOF 共b兲
time-of-flight spectra from two different surfaces are shown.
Spectra 共1兲 represent the signal received for 1 ML of CF2 Cl2
on Ru共0001兲 and spectra 共2兲 for 1 ML CF2 Cl2 coadsorbed
with 1 ML NH3 . The adsorption of NH3 reduces the surface
work function by ⬃2.3 eV and thus causes a decrease of the
accelerating potential for desorbing negative ions. As a result, the time-of-flight Cl⫺ spectra from the NH3 -dosed surface shift to longer times. The time difference between
QMS-TOF spectra in Fig. 4共b兲 is increased by a factor of ten
with respect to the ESDIAD spectra in Fig. 4共a兲 because of
the longer flight path through the QMS in 4共b兲.
In order not to overload the channeltron with a large
background signal caused by secondary electrons, a 200 ns
negative pulse 共G3兲 is applied to the grid of the QMS at the
same time as the electron gun pulse 共G2兲. This pulse blocks
the input of the spectrometer during the time of electron
irradiation. All pulses 共usually 10 kHz pulses are used兲 are
synchronized using transistor–transistor logic pulses generated by the DG-535 Pulse Generator.
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J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
Negative ion formation in electron-stimulated desorption
971
FIG. 4. Effect of coadsorption on normalized TOF 35Cl⫺ Spectra. 共a兲 ESDIAD: 共1兲 1 ML of CF2 Cl2 and 共2兲 1 ML of CF2 Cl2 coadsorbed with 1 ML
of NH3 . 共b兲 QMS: 共1兲 1 ML of CF2 Cl2 and 共2兲 1 ML of CF2 Cl2 coadsorbed
with 1 ML of NH3 .
III. EXPERIMENTAL RESULTS
A. Effect of surface preparation on ESD yields
and anion fragmentation patters from adsorbed
CF2 Cl2
The ESD mass spectra of CF2 Cl2 are strongly dependent
on the surface upon which CF2 Cl2 is deposited. ESD of various surfaces leads to the observation of negative ions that are
not reported in gas phase mass spectra, as well as different
ratios between ion yields. The ESD mass spectra in the mass
region 10– 80 amu for CF2 Cl2 molecules adsorbed on differently prepared surfaces are shown in Figs. 5共a兲–5共d兲. As can
be seen, the spectra differ greatly and include many CF2 Cl2
molecular fragments. In all cases, a high yield of F⫺ ions is
observed. The intensities of other fragments, such as Cl⫺ ,
⫺
ClF⫺ , CF⫺ , F⫺
2 , and Cl2 , are much smaller and depend on
CF2 Cl2 coverage and adsorption conditions.
On clean Ru共0001兲 at low concentration of adsorbed
CF2 Cl2 , F⫺ is the dominant ion fraction and F⫺
2 is the second most abundant ion fraction 关Fig. 5共a兲兴. F⫺
2 ions as well
as large intensities of F⫺ ions have been reported in ESD of
metal/fluorine systems.20,21 The deposition of NH3 onto a
CF2 Cl2 layer preadsorbed on Ru共0001兲 关Fig. 5共b兲兴 results in
the observation of a large enhancement of F⫺ and Cl⫺ ion
yields, in agreement with previous results.10,11 In addition,
the ClF⫺ ion yield increases by about a factor of ten. A new
FIG. 5. ESD negative ion mass spectra 共QMS兲 from 共a兲 0.3 ML CF2 Cl2 , 共b兲
0.3 ML CF2 Cl2 coadsorbed with 0.4 ML NH3 , 共c兲 1 ML Xe⫹0.3 ML
CF2 Cl2 共Xe spacer layer兲, and 共d兲 10 ML CF2 Cl2 on Ru共0001兲.
ion, Cl⫺
2 , also appears 关Fig 5共b兲兴. In order to demonstrate
that the large enhancement observed for coadsorption with
polar NH3 is unusual behavior, a monolayer of nonpolar Xe
is preadsorbed onto the Ru共0001兲 surface, prior to deposition
of CF2 Cl2 关Fig. 5共c兲兴. Large differences are observed. The
Cl⫺
2 yield, although still detected, decreases greatly compared with the Cl⫺
2 yield upon coadsorption with NH3 关Fig.
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972
J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
Solovev, Kusmierek, and Madey
TABLE I. Relative intensities of negative ions. The gaseous CF2 Cl2 data of Illenberger et al.6 of column 1 are
normalized to a value of 100 for the most intense peak, Cl⫺ ; electron energy ⬃0 – 10 eV. Column 2 corresponds to mass spectral peak height data of Fig. 5共d兲, column 3 to Fig. 5共a兲, column 4 to Fig. 5共b兲, and column
5 to Fig. 5共c兲. The data of columns 2 to 5 are normalized to a value of 100 for the most intense peak (F⫺ ) from
a thick condensed CF2 Cl2 film 共column 2兲; electron energy for columns 2 to 5 is 170 eV.
Ion
F⫺
CF⫺
Cl⫺
F⫺
2
ClF⫺
Cl⫺
2
1
2
3
4
5
Gaseous
CF2 Cl2 6
Condensed
Phase CF2 Cl2
0.3 ML CF2 Cl2
on clean
Ru共0001兲
0.3 ML
CF2 Cl2 ⫹
0.4 ML NH3
1 ML Xe⫹
0.3 ML CF2 Cl2
13.8
100
100
⬍2
38
2.25
2.1
3
5.5
150
0.03
0.15
1.8
0.21
430
0.15
29
2.2
3.4
1.5
170
0.21
2.3
0.81
0.85
0.2
5共b兲兴. As compared with the yield of ESD fragments from
Fig. 5共a兲 共0.3 ML CF2 Cl2 ) the Cl⫺ yield in Fig. 5共c兲 共1 ML
Xe⫹0.3 ML CF2 Cl2 ) increases 共as seen also in Ref. 9兲 while
the F⫺ yield does not change significantly. The relative
yields of all of these ions are listed in Table I. The ESD
yields tabulated in Table I are normalized to the ESD mass
spectrum for a condensed CF2 Cl2 film 共column 2兲 关Fig.
5共d兲兴; the F⫺ intensity for this film is set to 100. Reference
data for negative ion formation in gas-phase CF2 Cl2 under
electron irradiation are given in the first column of Table I
for comparison.6 共Note that data for negative ion formation
in gas-phase CF2 Cl2 was obtained using an electron monochromator, which produces a beam of nearly monoenergetic
electrons 共0–10 eV兲. The numbers indicate the relative energy integrated intensities of the negative ion fragments兲. As
can be seen in comparing gas-phase and condensed phase
⫺
data, the main differences are that 共i兲 F⫺
2 and CF ions are
not observed in gas-phase fragmentation of CF2 Cl2 , and 共ii兲
the Cl⫺ ion yield exceeds the F⫺ yield in the gas phase,
while the opposite is true in the condensed phase.
The Cl⫺ ion relative yields 共illustrating Cl⫺ enhancement兲 for NH3 coadsorption studied using the QMS are
shown in Fig. 7共a兲. A maximum enhancement of the Cl⫺ ion
yield of nearly three orders of magnitude is observed for low
precoverage 共0.2 ML兲 of CF2 Cl2 ; this maximum occurs
when approximately 1.5 ML of NH3 is adsorbed onto the
CF2 Cl2 layer. For higher precoverages of CF2 Cl2 , the enhancement is nearly one-hundred-fold. These results are in
substantial agreement with those presented in previous
studies,10 in which giant enhancements of Cl⫺ are reported
for ESD of fractional CF2 Cl2 monolayers coadsorbed with
NH3 . However, the value of maximum enhancement that we
observe using the QMS is smaller than the ESDIAD detected
enhancement reported previously.11 In order to probe the reason for differences between present and previous results, additional ESDIAD experiments were made under similar conditions as the QMS experiments. The results of these
experiments are shown in Fig. 7共b兲. The values of enhancement are indeed slightly bigger for ESDIAD measurements
than those for the QMS; the difference is most evident for 1
ML of CF2 Cl2 precoverage. The reasons for these differences
are believed to be technical, including increased resolution
B. Variation of negative ion yields with CF2 Cl2
coverage, and influence of coadsorbed NH3 :
The giant enhancement effect
The F⫺ and Cl⫺ ion yields from adsorbed CF2 Cl2 molecules as a function of CF2 Cl2 coverage have been reported
previously using TOF-ESDIAD.10 In this work, we carried
out measurements of Cl⫺ ESD using the higher mass resolution QMS detector, in order to see if new information is
obtained. The higher resolution of the QMS detector allows
⫺
(m/e
us to differentiate between F⫺
2 (m/e⫽38) and Cl
⫽35, 37兲 negative ions. This is especially important at low
concentrations of CF2 Cl2 where the F⫺
2 yield is predominant
关cf. Fig. 5共a兲兴. In Fig. 6 the dependence of the Cl⫺ and F⫺
2
ion yields on CF2 Cl2 coverage is shown: the Cl⫺ yield initially increases, reaches a maximum at 2 ML of CF2 Cl2 and
then decreases, while F⫺
2 reaches a maximum at ⬃1 ML.
The behavior of the Cl⫺ curve is similar to the data reported
by Lu et al.19 The inset in Fig. 6 is a plot of Cl⫺ and F⫺
2
yields versus CF2 Cl2 coverage for low concentrations of
CF2 Cl2 .
FIG. 6. Cl⫺ and F⫺
2 intensities as a function of CF2 Cl2 coverage as measured using the QMS. Inset: the same data for low CF2 Cl2 coverages.
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J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
Negative ion formation in electron-stimulated desorption
973
FIG. 9. Dependence of Cl⫺ yield on incident electron energy under different
conditions. Data for each point is normalized to the value of Cl⫺ yield from
1 ML of CF2 Cl2 on Ru共0001兲 共QMS兲. 共a兲 1 ML of CF2 Cl2 ⫹1 ML NH3 , 共b兲
1 ML of CF2 Cl2 on a 10 ML NH3 spacer layer, and 共c兲 10 ML CF2 Cl2 .
FIG. 7. Relative Cl⫺ yield for various CF2 Cl2 precoverages as a function of
NH3 coverage. The data in each curve are normalized to the initial value at
zero NH3 coverage. 共a兲 QMS and 共b兲 ESDIAD.
comparison. As can be seen, there is only small enhancement
of F⫺
2 for coverages of NH3 up to 2 ML of NH3 ; above 2
ML of NH3 , the F⫺
2 intensity diminishes with NH3 coverage,
in a manner similar to the Cl⫺ ion yield. The observed decrease of ion yields for high NH3 coverages agrees with
previous findings:10 the attenuation of ion signals due to passage through condensed overlayer films is caused by elastic
and inelastic scattering of the desorbed ions in the
overlayer.22 The different trends for the Cl⫺ and F⫺
2 ion enhancement with coadsorption of NH3 molecules are related
to the different processes that lead to ion formation. See Sec.
IV B for discussion.
C. Role of secondary electrons
using the QMS, different background corrections, and differing collection angles for QMS and ESDIAD.
In Fig. 8, the F⫺
2 relative yield, as measured by the QMS
detector, is plotted versus NH3 coverage for 1 ML of CF2 Cl2
molecules preadsorbed onto Ru共0001兲. The Cl⫺ relative
yield measured under the same conditions is presented for
⫺
FIG. 8. Comparison of relative Cl⫺ and F⫺
2 yields from 1 ML CF2 Cl2 as a
function of NH3 coverage. The data in each curve are normalized to the
initial value at zero NH3 coverage 共QMS兲.
The role of secondary electrons in negative ion enhancement has been proposed previously.10,11 Two recent experiments support this hypothesis, although the range of incident
energies was limited.17,23 One of the ways to investigate the
influence of secondary electrons on negative ion formation is
to measure the negative ion yields for different energies of
incident electrons. In the energy range 0–200 eV, the secondary emission coefficient ␴ has a very strong dependence on
the primary electron energy;24 for most metals, ␴ is very
small for energies ⬍30 eV and increases monotonically to a
maximum in the range ⬃150 to 800 eV for different materials.
The dependence of the Cl⫺ yield on incident electron
energy is shown in Fig. 9. Curve 9共a兲 is a plot of Cl⫺ yield
for 1 ML of CF2Cl2 subsequently dosed with 1 ML of NH3.
Each point in Fig. 9共a兲 is divided by the corresponding Cl⫺
yield from 1 ML CF2Cl2 with no coadsorbates. Curve 9共a兲
shows a strong dependence of Cl⫺ on incident electron energy. The relative Cl⫺ yield increases by more than an order
of magnitude between 50 and 100 eV electron energy; at 200
eV it reaches a maximum and then decreases slightly with
increasing electron energy. This behavior is reminiscent of
the strong energy dependence of secondary electron yield
from metals. Consistent with earlier suggestions,9,10,17 curve
9共a兲 supports the proposal that the interaction between secondary electrons and polar molecules such as NH3, plays a
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974
J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
FIG. 10. Effect of spacer layers on Cl⫺ yield as a function of spacer layer
thickness 共ML兲. Electron energy, E e ⫽170 eV. 共a兲 0.3 ML CF2 Cl2 on NH3 ,
共b兲 1 ML CF2 Cl2 on NH3 , and 共c兲 1 ML CF2 Cl2 on CF2 Cl2 .
major role in enhanced ESD of Cl⫺ from CF2Cl2. Curves
9共b兲 and 9共c兲 display the influence of spacer layers and are
discussed in Sec. IV C.
Solovev, Kusmierek, and Madey
causes a maximum enhancement in the Cl⫺ yield of ⬃100 to
⬃1000 共larger enhancement for lower CF2 Cl2 coverage, for
which intermolecular CF2 Cl2 interactions are reduced兲. The
Cl⫺ yield decreases with increasing NH3 spacer thickness;
similar behavior was reported earlier for CF2 Cl2 on an H2 O
spacer layer.11
The effect of CF2 Cl2 ‘‘spacer layer’’ thickness on Cl⫺
yield, shown in Fig. 10共c兲, is simply a different way of plotting Cl⫺ yield versus CF2 Cl2 coverage, as shown in Fig. 6.
The Cl⫺ yield passes through a maximum at a spacer layer
thickness of ⬃1 ML CF2 Cl2 共i.e., 1 ML of CF2 Cl2 on top of
a 1 ML CF2 Cl2 spacer兲.
Finally, the effect of electron beam energy on ESD of
Cl⫺ from 1 ML of CF2 Cl2 on thick layers of NH3 and
CF2 Cl2 is shown in Figs. 9共b兲, and 9共c兲. In striking contrast
to the case of a single monolayer of CF2 Cl2 ⫹NH3 , there is
little or no electron energy dependence for the thick layers.
This is addressed in the discussion.
IV. DISCUSSION
D. Influence of thick films on negative ion yields
The presence of an intermediate atomic or molecular
‘‘spacer layer’’ between the metal substrate and an adsorbed
molecular monolayer 共e.g., CF2Cl2兲 is known to affect ESD
ion yields via a variety of possible mechanisms.25 For example, the increased distance between metal and adsorbate
means reduced coupling between the excited molecular wave
functions and the filled and unfilled states of the metal.
Spacer layers can also influence image potential and polarization effects, excited state lifetimes, and charge transfer
and energy transfer processes.25,26 Quenching of electronic
excitations can also be due to dipolar coupling of the excited
state with the metal substrate.27
In the present experiments, we have examined the effects
of spacer layers of NH3 on ion yields from 0.3 ML of postadsorbed CF2Cl2 关Fig. 10共a兲兴 and 1 ML of post-adsorbed
CF2 Cl2 关Fig. 10共b兲兴; the Cl⫺ yields are not normalized. The
incident electron energy is 170 eV. The two different CF2 Cl2
coverages are chosen to test the effect of lateral intermolecular CF2 Cl2 interactions on Cl⫺ ion yield 共reduced interactions at lower coverage10,11兲. A greater density of CF2 Cl2
molecules leads to a higher probability of deexcitation processes, including collisions of molecules in excited states and
charge transfer between neighboring molecules. Also shown
in Fig. 10共c兲 is the effect of increasing the thickness of the
CF2 Cl2 film 共in effect, a ‘‘spacer layer’’ of CF2 Cl2 beneath
the CF2 Cl2 monolayer兲. Thus, we have chosen as spacer layers a highly polar molecule (NH3 ) and a molecule with a
small permanent dipole moment (CF2 Cl2 ). In all cases, the
spacer layers were dosed onto Ru共0001兲 at 25 K prior to
deposition of the top layer of CF2 Cl2 . Each data point represents a separate experiment, with a new spacer layer and
CF2 Cl2 exposure. NH3 and CF2 Cl2 are most probably amorphous films.
Consider the influence of NH3 layers. In both Figs. 10共a兲
and 10共b兲, the Cl⫺ yield exhibits a pronounced maximum at
⬃1 ML NH3 coverage. This is another manifestation of the
giant enhancement effect, whereby coadsorption with NH3
A. Background: Interaction of low energy electrons
with gaseous and condensed CF2 Cl2
The electronic properties of CF2 Cl2 and electron interactions with gaseous CF2 Cl2 have been summarized in a
comprehensive review by Christophorou, Olthoff, and
Wang.7 The CF2 Cl2 molecule is quasitetrahedral with C2 v
symmetry. Its lowest vertical ionization energy is 12.26 eV,
corresponding to ionization of the outer 4b 2 共C–Cl ␴兲 orbital. CF2 Cl2 has a dipole moment of 1.835⫻10⫺30 Cm
共0.55D兲, and a static polarizability of ⬃64⫻10⫺25 cm3 .
CF2 Cl2 is an electronegative species, and there have
been many studies of electron attachment processes.7 The
adiabatic electron affinity 共EA兲 of CF2 Cl2 is measured to be
0.4⫾0.3 eV, 28 and quantum mechanical calculations also
give positive values of EA equal to 0.4 eV 共Ref. 29兲 and 0.67
eV 共Ref. 30兲. 共N.B. A positive value of EA implies a bound
electron state, which has a negative energy with respect to
the zero energy state corresponding to an electron at rest at
infinity兲. There are at least five negative ion resonant states
whose energies lie above the zero energy level; their average
energies are 0.9, 2.5, 3.5, 6.2, and 8.9 eV.
In collisions of low energy electrons with CF2 Cl2 , dissociation can occur as a result of dissociative electron attachment 共DEA兲 for electron energies ⭐15 eV, and dipolar dissociation 共DD兲 for electron energies ⭓15 eV. In the DEA
process, a target molecule captures a low energy electron
共0–15 eV兲 in the Franck–Condon region to form a transient
negative ion state; if the lifetime of this state is comparable
to the vibrational period 共e.g., ⬎10-14 s) the transient negative ion can dissociate into neutral and anionic fragments.21
The majority of DEA reactions below ⬃1 eV lead to dissociation of CF2 Cl2 to produce Cl⫺ . 7 Since the CF2 Cl–Cl
bond dissociation energy is smaller than the electron affinity
of Cl 共3.61 eV兲 the DEA reaction
⫺
e⫺ ⫹CF2 Cl2 →CF2 Cl*
2 →CF2 Cl⫹Cl
is exoergic by ⬃0.28 eV. The energy position of this lowest
negative ion state makes the DEA process highly tempera-
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J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
ture dependent.8 In the temperature range from 205 to 777 K,
the total electron attachment rate constant increases by more
than a factor of 300 as T increases.
Christophorou et al.7 have replotted the negative ion
data of Illenberger et al.6 to illustrate the energy dependence
of fragment ion formation. The resonance below 1eV is
mainly due to Cl⫺ with a small contribution from Cl⫺
2 .
Many fragment anions contribute to a second broad maximum at ⬃3.5 eV. As indicated in Table I, the fragment anions observed for gaseous CF2 Cl2 are Cl⫺ 共dominant兲, F⫺ ,
FCl⫺ , Cl⫺
2 .
There have been a few reports concerning negative ion
desorption from condensed phase CF2 Cl2 . In experiments by
Illenburger,5 the ESD Cl⫺ yield exhibits three peaks around
electron energies of 1, 2.5, and 7 eV. The intense peak at 7
eV is not observed in the gas phase, and has been attributed
to a negative ion resonance associated with the first electronically excited state. More recent results by Meinke31 and
Hedhili et al.32 confirm the Cl⫺ peak at 7 eV for ESD of
condensed CF2 Cl2 films, and all three investigations agree
that the F⫺ desorption yield versus electron energy has a
peak at ⬃3 – 4 eV. There is also agreement that the total
detected F⫺ yield for low energy electron excitation considerably exceeds the Cl⫺ yield, in contrast to the gas phase.
This is due to the low kinetic energies of Cl⫺ ions: the majority of the Cl⫺ ions remain trapped on the surface since
they do not have sufficient kinetic energies to overcome the
polarization barrier and desorb. In contrast to the investigations of Illenburger,5 for which a significant Cl⫺ signal is
reported for electron energies ⬍1 eV, Meinke,31 Hedhili
et al.,32 and Langer et al.16 did not detect significant Cl⫺
yields for electron energies ⬍1 eV; whether this is due partially to electron /ion optical effects at such low energies, or
mainly to trapping of Cl⫺ formed via DEA is not clear.
B. Anion fragmentation patterns for adsorbed CF2 Cl2
It is often observed in ESD of positive and negative ions
from fractional molecular monolayers that the ion fragmentation patterns are quite simple: low mass atomic ions (H⫾ ,
O⫾ , F⫾ , etc.兲 are dominant and molecular ions 共except for
OH⫹ , CO⫹ ) are seldom seen. The reason is that massive,
low velocity ions have longer lifetimes near the surface
where de-excitation pathways exist: only low-mass, highvelocity ions survive to desorb, while high-mass, lowvelocity ions are quenched 共neutralized兲. Also, it is often
sufficient to break a single bond to initiate desorption of
atomic ions, whereas multiple bond-breaking processes are
often necessary for molecular fragments. The CF2 Cl2 ion
yields do not follow this behavior. Whereas ESD from a 10
ML film gives mainly F⫺ and Cl⫺ 共mass 35, 37兲 and traces
of ClF⫺ and Cl⫺
2 关Fig. 5共d兲兴, Figs. 5共a兲–5共c兲 show considerably more complexity. For 0.3 ML CF2 Cl2 关Fig. 5共a兲兴 the
second most abundant ion is F⫺
2 , and this signal far exceeds
the tiny Cl⫺ signal. Traces of ClF⫺ are also seen. Upon
coadsorption of 0.3 ML CF2 Cl2 with NH3 关Fig. 5共b兲兴 or Xe
关Fig. 5共c兲兴 the Cl⫺ signals increase substantially, and other
⫺
fragments also persist 共notably F⫺
2 and ClF ).
Consider the most-likely bonding configuration for qua-
Negative ion formation in electron-stimulated desorption
975
sitetrahedral CF2 Cl2 to Ru共0001兲. To maximize coordination
with the substrate, one would expect three atoms 共Cl, Cl, F兲
or 共Cl, F, F兲 in contact with the surface and one 共Cl or F兲
pointing away 共this geometry is consistent with ESDIAD
observations19 of single F⫺ , Cl⫺ beams normal to the surface at a coverage of ⬃1 ML). To form, for example, F⫺
2
from a molecule adsorbed in this way, it is necessary to break
two C–F bonds and to form a F2 species in a configuration
that is sufficiently repulsive for desorption to occur.
⫺
⫺
Formation of Cl⫺
2 , CF , and ClF also requires complex molecular rearrangements on the excited state potential
curves. One might expect that ligands in direct contact with
the substrate would be effectively neutralized or quenched
upon electron excitation, in contrast to ligands pointing away
from the surface. A detailed understanding of the complexities of the mass spectra of Figs. 5共a兲–5共d兲, and of the different branching ratios for various surface conditions, is beyond
our current understanding.
C. Influence of spacer layers on negative ion yields
In the hope of gaining a better understanding of the processes involved in the giant enhancement of Cl⫺ ions, we
have examined the effects of polar NH3 spacer layers on Cl⫺
yields under various conditions.
Figure 10 shows the effects of the thickness of various
spacer layers on the Cl⫺ yield. For NH3 spacer layers, the
detected Cl⫺ yield exhibits a maximum for ⬃1 ML NH3 ,
which is another manifestation of the giant enhancement effect. A comparison of Figs. 7, 8, 10共a兲, and 10共b兲 indicates
that adsorption of CF2 Cl2 prior to or subsequent to the deposition of ⬃1 ML of NH3 has little effect; the enhancement is
still observed. Figure 11 confirms that for coadsorption of
CF2 Cl2 and NH3 , the order of dosing (NH3 dosed before or
after CF2 Cl2 ) does not affect Cl⫺ yield significantly for NH3
coverages ⬍2 ML. Figures 11共a兲 and 11共b兲 depict the effect
of deposition sequence on the Cl⫺ yield. In Fig. 11共a兲, data
for ESD of Cl⫺ ions from 1 ML of CF2 Cl2 adsorbed on a
NH3 layer are shown as a function of the thickness of the
NH3 spacer layer. The relative Cl⫺ yield for CF2 Cl2 precoverage is presented in Fig. 11共b兲, for comparison. For both
curves, a maximum enhancement of approximately 100⫻ is
observed. These results indicate that the enhancement effect
is associated with proximity to the metal surface; the effect
decreases as CF2 Cl2 is moved further from the surface by a
spacer layer. The trend for Cl⫺ enhancement in the coverage
range of 0–2 ML of NH3 is the same for both curves, but
above 2 ML of NH3 , the trend is very different. For the case
when CF2 Cl2 molecules are adsorbed first 关Fig. 11共b兲兴, the
Cl⫺ relative yield falls sharply and reaches a negligible value
at 5 ML of NH3 . The attenuation of the Cl⫺ yield is due to
elastic and inelastic scattering of the desorbed ions in the
NH3 overlayer. However, when CF2 Cl2 is adsorbed on top of
the NH3 layers 关Fig. 11共a兲兴, the Cl⫺ relative yield decreases
very slowly and at 5 ML of NH3 reaches a magnitude of
approximately 10. Similar results have been reported for Cl⫺
yield with H2 O precoverage.11 The Cl⫺ yield reaches a maximum at H2 O coverage of ⬃1 ML, and then decreases with
thicker ice films. The decrease may be attributed to scattering
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976
J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
FIG. 11. Effect of deposition sequence on relative Cl⫺ yield as function of
NH3 coverage. Data in each set are normalized to the value of Cl⫺ yield
from 1 ML CF2 Cl2 on Ru共0001兲. 共a兲 NH3 precovered Ru共0001兲 surface and
共b兲 1 ML of CF2 Cl2 precovered Ru共0001兲 surface.
and trapping of the low-energy secondary electrons, originating from the substrate 关Ru共0001兲兴, in the NH3 or H2 O film.
The exponential attenuation of the Cl⫺ signal for NH3
over-layer thicknesses⬎2 ML indicated in Figs. 7 and 11 has
also been observed for ESD F⫺ ions from 1 ML PF3 on Ru
covered by H2 O films. The attenuation is attributed mainly to
elastic scattering, with an attenuation cross section of ⬃10
⫻10⫺16 cm2 ; the attenuation length (1/e) is ⬃0.9 ML
H2 O. 33 Based on the data of Figs. 7 and 11, the attenuation
length (1/e) for the Cl⫺ signal for NH3 layers is 0.5
⫾0.2 ML NH3 , implying a similar mechanism and similar
attenuation cross section.
The Cl⫺ yield 关Fig. 10共c兲兴 also exhibits a maximum for
⬃1 ML of CF2 Cl2 spacer layer thickness 关this corresponds
to a total film thickness of ⬃2 ML of CF2 Cl2 on Ru共0001兲兴.
The maximum is a result of several factors. During the initial
deposition of CF2 Cl2 , the Cl⫺ yield increases with the number of molecules available for dissociation. Also, with increasing distance between the top of the CF2 Cl2 film and
metal, the image potential between the Cl⫺ ion and the metal
decreases, reducing the potential barrier that the Cl⫺ ion
must overcome in order to desorb. The probability of neutralization of Cl⫺ ions also decreases with increasing spacing
between the top of the CF2 Cl2 film and the metal. For coverages ⬎2 ML however, the probability for the low-energy
secondary electrons responsible for initiating the dissociation
of CF2 Cl2 to penetrate to the top of the CF2 Cl2 layer from
which the Cl⫺ escape decreases exponentially, leading to a
decrease in the detected Cl⫺ yield. Similar results have been
Solovev, Kusmierek, and Madey
obtained for CCl4 adsorbed on Ru共0001兲.34 The Cl⫺ yield
increases linearly with increasing CCl4 coverage up to 1 ML,
maximizes around 2–3 ML, and then decreases.
The maximum of the Cl⫺ yield as a function of spacer
layer thickness for ⬃1 ML 共Fig. 10兲 of spacer layer may also
be due partly to the varying contribution of DD 共dipolar
dissociation兲 and DA 共dissociative attachment兲 processes to
the total Cl⫺ yield. Sambe et al.25 investigated the effect of
the image potential on ESD of O⫺ from O2 condensed on Ar
films grown on Pt. The effect of the image potential was
varied by changing the thickness of the Ar spacer layer, i.e.,
changing the distance between the metal and the ion. It was
observed that the contribution of DD processes to the overall
O⫺ yield exhibits a sharp peak for ⬃1 ML of spacer layer
thickness. The contribution of DA processes, however, increases gradually with increasing layer thickness 共decreasing
image potential兲.
The detected Cl⫺ yield of Fig. 9共a兲, in which 1 ML NH3
is coadsorbed with 1 ML CF2 Cl2 on Ru共0001兲, exhibits a
maximum as a function of incident electron energy. For this
system, most secondary electrons originate in the Ru共0001兲
substrate. We have also investigated the effect of incident
electron beam energy on ESD of Cl⫺ from 1 ML CF2 Cl2 on
thick layers of NH3 and CF2 Cl2 ; for these cases low-energy
secondaries originate from both the substrate and the film.
The data from Figs. 9共b兲 and 9共c兲 indicate that there is no
energy dependence for the 10 ML thick NH3 and CF2 Cl2
films. This appears to indicate that low-energy secondary
electrons originating from the thick layers of NH3 and
CF2 Cl2 共as manifested by their energy dependence兲 are not
major contributors to the Cl⫺ yield. This is unexpected: If
low-energy secondary electrons (⬃0 eV) are primarily responsible for DEA of CF2 Cl2 leading to Cl⫺ then one would
expect the Cl⫺ yield to vary with incident electron energy in
a manner similar to the total secondary electron yield 共the
secondary electron yield as a function of incident electron
energy exhibits a maximum at a few hundred electron volts
for most materials24兲. The apparent lack of electron energy
dependence merits further study.
D. Mechanisms for giant enhancement
Giant enhancements of Cl⫺ and F⫺ ESD yields from
adsorbed CF2 Cl2 upon coadsorption with polar molecules
(H2 O and NH3 ), first measured using an ESDIAD
detector,9–11,23 have now been reproduced and verified using
a QMS 共quadrupole mass spectrometer兲. By measuring
buildup of surface charge, Lu and Sanche17,18 have also reported a giant enhancement of absolute cross sections for
DEA of adsorbed CF2 Cl2 by ⬃0 eV electrons, for the case of
a fractional monolayer of CF2 Cl2 deposited on a film of polar molecules (⬃5 ML of NH3 or H2 O), which is in turn
supported on a ⬃5 ML film of Kr deposited on a metal substrate. These diverse experiments can be interpreted to indicate that coadsorption of CF2 Cl2 with polar molecules substantially affects the electron-induced decomposition of
CF2 Cl2 . However, in the charge trapping experiments, the
possibility that electrons are trapped on caged CF2 Cl⫺
2 anions rather than Cl⫺ cannot be eliminated.35
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J. Chem. Phys., Vol. 120, No. 2, 8 January 2004
The enhancement of Cl⫺ and F⫺ ESD ion yields from
adsorbed CF2 Cl2 upon coadsorption with other molecules
exhibits three main characteristics: 共1兲 for identical CF2 Cl2
precoverage, the Cl⫺ ion yield exhibits a much larger enhancement than the F⫺ yield; 共2兲 the largest enhancement
occurs for low CF2 Cl2 coverages; and 共3兲 coadsorption with
NH3 共and H2 O) leads to great enhancements, while coadsorption with nonpolar molecules leads to much smaller enhancements. Also, while not shown in the present experiments, it is interesting to consider why coadsorption with
NH3 leads to greater enhancement than coadsorption with
H2 O, as has been reported earlier,9–11,23 and recently
verified.17 Analyses of these characteristics and possible explanations have been reported. The interested reader is referred to Refs. 9–11, 22, 23, and 36 –39.
The proposal that the giant enhancement could be affected by increasing the lifetime of the low-energy electrons,
i.e., decreasing the rate at which the secondary electrons tunnel back to the metal,10,23 is still favored. The greatest enhancements are detected for coadsorption with polar molecules, NH3 and H2 O, both known to be very efficient
electron solvents. Upon injection of secondary electrons
originating in the Ru共0001兲 metal sample into the water or
ammonia molecular layer, some of the low-energy electrons,
which have a maximum yield at 0–2 eV and high energy tail
extending to 10 eV, may become ‘‘trapped’’ by the polar
molecules and therefore have increased lifetimes locally.
When a CF2 Cl2 (EA⫽⫹0.4 eV) molecule is present on the
surface, the trapped electron can propagate through the polar
molecular film to the molecule, forming a vibrationally excited intermediate state, CF2 Cl2 * ⫺ , which can then dissociate, forming Cl⫺ or F⫺ ions and the corresponding radicals.
It was argued that only unbound electrons with energies
⬃0 eV could contribute to the desorption of Cl⫺ ions.10,23 In
consideration of the energetics of the DEA process, it is evident that fully solvated electrons cannot be responsible for
the great enhancements.10,17 The electron energy threshold
E min for the DEA of CF2 Cl2 leading to Cl⫺ is equal to the
bond dissociation energy of CF2 Cl–Cl minus the electron
affinity of Cl. The value of E min is ⬃⫺1.4 eV in the condensed phase (E min is ⬃⫺0.4 eV in the gas phase, and polarization effects due to the adsorption of CF2 Cl2 on the film
leads to a polarization potential of ⬃1.0 eV). However, the
binding energy of fully solvated electrons in bulk H2 O is 3.2
eV; thus, despite their long lifetimes, fully solvated electrons
cannot contribute to DEA of CF2 Cl2 .
Recently, the mechanism for the giant enhancement due
to coadsorption with polar molecules has been attributed to
the transfer of near 0 eV electrons that are temporarily localized as precursors of the fully solvated state in polar molecular ices.17 In femtosecond-time resolved studies of the dynamics of electron trapping and solvation in polar media, it
has been observed that solvation is 共at minimum兲 a two step
process; before solvation, the electron is believed to reside in
a weak ‘‘pre-existing trap,’’ with a lifetime on the scale of
picoseconds, which is a precursor of the fully solvated
state.40 The exact nature of the precursor is not clear; it is
believed to be a transient state in an energy above or near the
vacuum level, i.e., an excited state of the solvated electron
Negative ion formation in electron-stimulated desorption
977
共‘‘hot solvated electron’’兲. Electrons in this state are believed
to be very mobile, and therefore very reactive, and will rapidly react with molecules that have a positive electron affinity. The dynamics of localization and solvation of photoinjected electrons in amorphous solid H2 O and D2 O films from
a copper substrate are also believed to occur in a sequence of
steps: 共1兲 quasi-instantaneous electron injection into the ice
layer (⬃10 fs); 共2兲 subsequent localization (⬃100 fs); and
共3兲 solvation caused be rearrangement of the surrounding
water dipoles (⬃ps). 40
In summary, we observe giant enhancements of negative
ion yields if the target molecule and coadsorbate display certain characteristics:
共1兲 Target molecule has an EA⬎0 eV; EA(CF2 Cl2 )
⫽⫹0.4 eV.
共2兲 Target molecule has a very low-energy negative ion
resonance (⬃0 eV in the case of CF2 Cl2 leading to
Cl⫺ ); the cross section for electron trapping increases
with decreasing electron energy.
共3兲 Constituents of the target molecules are atoms with
large, positive EA 关 EA(Cl)⫽3.6 eV, EA(F)⫽3.4 eV].
共4兲 Coadsorbate molecules are polar; the molecules 共either
singly or in clusters兲 are able to trap low-energy
electrons,41 increasing their lifetime, but at the same time
allowing a low-energy electron sufficient mobility to
tunnel efficiently to the target molecule.
The ‘‘trapped’’ electron mechanism, where the lifetime of
low-energy electrons is increased due to the presence of
coadsorbed polar molecules, is able, qualitatively at least, to
account for the main characteristics of the giant enhancements.
V. CONCLUSION AND SUMMARY
We have measured ESD negative ion mass spectra of
CF2 Cl2 coadsorbed with polar NH3 . These spectra reveal
ions that have not been observed in gas-phase spectra, as
well as ratios between ion yields that differ from those in the
gas-phase. The mass resolution of the QMS has allowed us
to remove ambiguities in detection of 35Cl⫺ , 37Cl⫺ , and
38 ⫺
F2 . We have verified the giant enhancement of Cl⫺ and
⫺
F yields for ESD of CF2 Cl2 adsorbed with 1 ML NH3 using
two time-of-flight methods, ESDIAD 共detector with wide
collection angle兲 and QMS 共narrow collection angle兲. The
role of secondary electrons in the giant enhancement mechanism has been demonstrated by showing that the Cl⫺ yield
scales with the secondary yield for 1 ML CF2 Cl2 coadsorbed
with 1 ML NH3 on Ru共0001兲.
Thus far, we have investigated the giant enhancement of
negative ion yields from CF2 Cl2 , coadsorbed with a very
limited number of polar molecules: NH3 , H2 O, and CH3 OH.
In the near future we plan to increase the scope of our investigation by performing experiments with various target molecules and coadsorbates, in order to better understand the
mechanisms of giant enhancement of negative ion yields.
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