THE JOURNAL OF CHEMICAL PHYSICS 134, 194501 (2011)
Photolysis of pure solid O3 and O2 films at 193 nm
U. Raut,a) M. J. Loeffler,b) M. Famá,c) and R. A. Baragiolad)
Laboratory of Atomic and Surface Physics, University of Virginia, Charlottesville, Virginia 22904, USA
(Received 31 May 2010; accepted 19 April 2011; published online 17 May 2011)
We studied quantitatively the photochemistry of solid O3 and O2 films at 193 nm and 22 K with infrared spectroscopy and microgravimetry. Photolysis of pure ozone destroyed O3 , but a small amount
of ozone remained in the film at high fluence. Photolysis of pure O2 produced O3 in an amount that
increased with photon fluence to a stationary level. For both O2 and O3 films, the O3 :O2 ratio at large
fluences is ∼0.07, about two orders of magnitude larger than those obtained in gas phase photolysis.
This enhancement is attributed to the increased photodissociation of O2 due to photoabsorption by
O2 dimers, a process significant at solid-state densities. We obtain initial quantum yield for ozone
synthesis from solid oxygen, (O3 ) = 0.24 ± 0.06, and quantum yields for destruction of O3 and O2
in their parent solids, (−O3 ) = 1.0 ± 0.2 and (−O2 ) = 0.36 ± 0.1. Combined with known photoabsorption cross sections, we estimate probabilities for geminate recombination of 0.5 ± 0.1 for O3
fragments and 0.88 ± 0.03 for oxygen atoms from O2 dissociation. Using a single parameter kinetic
model, we deduce the ratio of reaction cross sections for an O atom with O2 vs. O3 to be 0.1–0.2. The
general good agreement of the model with the data suggests the validity of the central assumption of
efficient energy and spin relaxation of photofragments in the solid prior to their reactions with other
species. © 2011 American Institute of Physics. [doi:10.1063/1.3589201]
I. INTRODUCTION
For more than a century, photoabsorption of UV light by
ozone has received great attention because of its importance
in the protection to life by the ozone layer in the outer atmosphere and in air pollution at ground level. In the last few
decades, studies of the photolysis of solid ozone have been
made to understand specific solid-state effects1, 2 and to explain the observation of condensed ozone on the surfaces of
cold satellites3 of the outer planets. The proposal that radiolysis of H2 O and O2 ices is the source of the condensed ozone in
planetary ices has been tested in experiments where thin films
were irradiated with energetic particles or photons. Bahr et al.
found4 that radiolysis of pure water ice does not produce any
significant amounts of ozone, but Teolis et al.5 discovered that
ozone can indeed be produced if water was deposited during
irradiation, which enhanced accumulation of radiolytic oxygen. In contrast, ozone is readily produced when O2 (Ref. 6)
and O2 + H2 O films (Refs. 7 and 8) are irradiated with energetic particles (H+ , e− ) and ultraviolet photons.9 In addition
to planetary applications, there are predictions by astrochemical models that condensed O2 and O3 should be present in the
icy grain mantles in dense molecular clouds.10–12 The presence of photosensitive molecules such as O3 on interstellar
grains, which can yield reactive fragments by energetic radiation, could be crucial to chemistry occurring in dark molecular clouds due to a background ultraviolet flux from the decay
of hydrogen excited by cosmic rays.
The vast majority of the work on photolysis of oxygen and ozone has been done in the gas phase, motivated
a) Electronic mail: [email protected].
b) Electronic mail: [email protected].
c) Electronic mail: [email protected].
d) Electronic mail: [email protected].
0021-9606/2011/134(19)/194501/7/$30.00
by the need to understand the atmospheric ozone problem.
Such studies have revealed the existence of multiple photolysis channels with different energy thresholds and involving
many excited atomic and molecular states.13 By comparison,
the photolysis of ozone and oxygen solids has barely been explored, mostly using photons in the Hartley band, above 200
nm.1, 14, 15 Still unanswered is the question of the photolysis
of O2 in the region of Schumann-Runge bands and continuum, where photoabsorption cross sections differ by orders
of magnitude between the gas and the solid. Here we present
results of experiments of the photolysis of optically thin
(<100 nm) solid films of pure ozone and oxygen at 22 K
by 193 nm photons and a simple photochemical model from
which we derive initial quantum yields, effective dissociation
cross sections and probabilities for geminate recombination
and reaction involving O atoms.
II. EXPERIMENTAL SETUP
Experiments were performed in an ultrahigh vacuum
chamber (base pressure ∼10−9 Torr). Ozone was prepared by
applying a Tesla coil discharge to the glass manifold pressurized with 100–150 Torr of O2 . Ozone formed from the
discharge was condensed to the liquid phase by immersing
the manifold in liquid N2 . After 1–2 min of discharge, we
removed the remaining O2 by pumping the manifold and admitted sublimating ozone molecules into the chamber through
a Teflon valve.
Ozone molecules from the glass manifold were directed
towards a gold-coated quartz crystal microbalance (QCM)
(Ref. 16) cooled to 53 K. We chose this temperature to
avoid condensation of trace O2 from the glass manifold or
O2 produced from ozone reacting with any surface inside the
134, 194501-1
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194501-2
Raut et al.
vacuum chamber. The microbalance measures the areal mass
of the condensed film, which is converted into column density (molecules cm−2 ) by dividing by the molecular mass. We
deposited films with column densities between 155 × 1015
and 160 × 1015 O3 cm−2 at a rate that varied between 0.15
× 1015 and 0.5 × 1015 O3 cm−2 s−1 . Using the density of
1.65 g cm−3 for solid ozone,17, 18 we obtain film thicknesses
of ∼75–77 nm. After deposition at 53 K, the ozone films were
heated to 55 K to ensure complete crystallization and cooled
to 22 K prior to UV exposure. To study photolysis of solid
O2 , we deposited films containing 230 × 1015 –240 × 1015
O2 cm−2 at 22 K from a microcapillary-array doser at the
rate of 0.3 × 1015 O2 cm−2 s−1 . These films have a thickness
of 80–84 nm, derived using a density of 1.53 g cm−3 for solid
oxygen.19
The films were irradiated with 193 nm pulses produced
by ArF excimer laser at normal incidence. The laser beam was
defocused using a MgF2 lens (f = 50 cm) to cover a rectangular spot ∼25 mm × 75 mm at the target, larger than the active
diameter of the microbalance (6 mm) to avoid multi-photon
excitations or heating during each laser pulse. To measure the
photon fluence reaching the target, we replaced the QCM with
an Ophir power meter, which measured constant beam intensity across the area of the QCM. The laser delivered pulses
of 0.72 mJ cm−2 uniformly across the films at a rate that was
varied from 1 to 3 Hz during the experiment. The energy per
pulse was constant within 3% during the experiments. The
results did not depend on the value of the fluence per pulse,
within errors. The fluence of the laser pulse attenuates as it
traverses the film due to absorption from ozone and oxygen
molecules. The gold reflects 18% of the light that reaches the
substrate, following attenuation at 193 nm.20 The total fluence
is the sum of the incident and reflected values.
The amount of ozone in the film was measured using
specular reflectance infrared spectroscopy (35o incidence) at
2 cm−1 resolution with a Thermo Nicolet Nexus 670 Fourier
transform infrared spectrometer. The O3 column density η3
was deduced from the area of the 9.5 and 4.74 μm absorption bands due to the υ 3 asymmetric O–O and υ 1 +
υ 3 symmetric-asymmetric stretch combination vibrations, respectively (Fig. 1). The infrared spectra are expressed in optical depth units, −ln(R/R0 ), where R/R0 is the ratio of the
reflectance of the sample to the reflectance of the gold substrate.
The area of the O3 band divided by an effective band
strength A* gives the ozone column density. The qualifier “effective” is used to emphasize that A* is not a constant property of a molecule, but rather depends on the thickness and
the optical properties of the film.21 Though the film thickness only changes slightly (∼10%), the refractive index of the
films, specially in the case of the pure ozone, changes significantly during 193 nm irradiation due to amorphization of the
crystalline phase and the decrease in O3 concentration in the
film.18
For pure ozone, we obtain initial effective band strengths
for the υ 3 and υ 1 + υ 3 bands by dividing the respective band
areas by the initial column density measured by the microbalance following deposition. The values are Ai * = 3.6 ± 0.4
and 0.47 ± 0.08 in units of 10−18 cm mol−1 for the υ 3 and the
J. Chem. Phys. 134, 194501 (2011)
(a)
(b)
FIG. 1. Optical depth in the regions of the υ 3 (9.6 μm) and υ 1 + υ 3
(4.74 μm) ozone absorptions for solid ozone at 22 K, showing a decrease
in band area following exposure to 193 nm photons to the indicated fluences
in units of 1016 photons cm−2 . The spectra have been displaced vertically for
clarity.
weaker υ 1 + υ 3 bands, respectively. As the film is irradiated,
the band shape and area change as ozone becomes diluted
in O2 .
To obtain the effective band strengths Af * at the end of irradiation, we thermally desorbed the photolyzed ozone and
oxygen films and measured the mass change with the microbalance. In Fig. 2 we compare the thermal desorption spectra of ozone and oxygen films, as deposited and photolyzed,
while heating at 1 K min−1 . The desorption rates are obtained
FIG. 2. Thermal desorption spectra at 1 K min−1 of (a) an unirradiated
ozone film (154 × 1015 O3 cm−2 ), (b) a photolyzed ozone film (156 × 1015
O3 cm−2 ), (c) an unirradiated O2 film (230 × 1015 O2 cm−2 ) and (d) a photolyzed O2 film (238 × 1015 O2 cm−2 ). The desorption rates were obtained
from the time derivative of the mass loss measured with the microbalance.
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194501-3
Photolysis of pure solid O2 and O3 at 193 nm
from the time derivative of the mass loss measured with the
microbalance. The desorption of the unirradiated ozone film
peaks at 64 K while that of photolyzed O3 has two peaks, one
at 32 K and a smaller one at ∼58 K. The desorption of unirradiated O2 peaks at 32 K, while the photolyzed O2 film also
shows an additional smaller peak at 58 K.
We get Af * = (5.1 ± 1.2) × 10−18 and (0.52 ± 0.15)
× 10−18 cm mol−1 by dividing the area of the υ 3 and υ 1 + υ 3
bands at large fluences by the residual O3 column density obtained from the microbalance measurement of the small desorption between 55 and 60 K in the photolyzed films. The
column densities of ozone during irradiation are obtained by
dividing the band areas of υ 3 and υ 1 + υ 3 bands with the linearly interpolated A*. Since A* for the υ 1 + υ 3 band varies
only ∼10% with the amount of O3 in the film, we use this
band to quantify the experiments on ozone films. In addition,
we observe a small but systematic (∼10%) increase in υ 3
band area for fluences <5 × 1016 cm2 . This increase is likely
due to variations in the index of refraction caused by effects
such as amorphization or compaction. We find that the υ 1 +
υ 3 band is free from such interference. For oxygen films, the
range of ozone values and therefore the error in A* is small. In
this case, we use the stronger υ 3 band as a measure of ozone
column density. Also, the υ 1 + υ 3 band is too weak to be
detected at very low fluence. The errors in A* are estimated
from dispersion in band areas obtained from several experiments, and translate into an error of at most ±15% in the O3
column densities.
III. RESULTS
All photolysis experiments were performed at 22 K.
Figure 1 shows changes in the υ 3 and υ 1 + υ 3 bands of a
∼75 nm, initially pure, ozone film during photolysis. The
band area decreases and its position shifts to lower wavenumber with increasing photon fluence. The shift is more prominent in the υ 3 band. Figure 3 shows that η3 (F) decreases
nearly exponentially with fluence F to a stationary level that
is 10% of η3 (0).
Figure 4 shows the appearance and growth of the υ 3
ozone absorption band during photolysis of solid O2 . For large
fluences, the bands measured on initially pure O2 and O3 films
are essentially the same. In Fig. 5 we show a log-log plot of
the fluence dependence of the production of O3 in irradiated
O2 films.
J. Chem. Phys. 134, 194501 (2011)
FIG. 3. Decrease in the O3 column density estimated from the υ 1 + υ 3 absorption band as the ozone film is irradiated with 193 nm light at 22 K. The
thick marker placed on the vertical axis indicates the initial amount of ozone
(156 × 1015 O3 cm−2 ) in the film. A residual amount of O3 (10% of the
initial value) remains in the film at the end of the photolysis. The solid line is
the fit to a model described in the text.
due to closely spaced rovibrational energy levels. Thus, the
results of measurements depend on the energy width of the
exciting light, which may explain the range of reported cross
sections:23 1.2 × 10−22 – 200 × 10−22 cm2 . Because of the
polarization of the medium and the intermolecular interactions, the energy levels in the solid are shifted and broadened compared with the gas phase, as seen in ultraviolet
photoabsorption1, 24—26 and photoelectron emission27 studies,
with cross sections varying smoothly with photon energy. The
IV. DISCUSSION
To analyze our experiments we start by summarizing results of gas phase experiments and discuss differences between experiments done in the solid and with free molecules.
In the gas phase, photoabsorption of a 193.3 nm (6.41 eV)
photon dissociates O2 into two ground state O (3 P) atoms.22
O2 X 3 g − + hυ → 2O(3 P).
(1)
The absorption by a free molecule is in the SchumannRunge region, where the cross section varies over two orders
of magnitude across wavelength intervals as small as 0.1 nm,
FIG. 4. Optical depth in the region of the υ 3 ozone absorption for a solid O2
film (∼238 × 1015 O2 cm−2 ) irradiated at 22 K, showing the increase in the
band area with photon fluence, in units of 1016 photons cm−2 . The top curve
is for initially pure ozone film irradiated to 2317 × 1016 photons cm−2 . Its
similarity to the spectrum of the pure O2 irradiated to highest fluence (928
× 1016 photons cm−2 ) confirms that the initially pure O2 or O3 films approach the similar steady state concentrations at high fluences. The spectra
have been displaced vertically for clarity.
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194501-4
Raut et al.
J. Chem. Phys. 134, 194501 (2011)
FIG. 5. Fluence dependence of O3 production from irradiation of solid O2
films (238 × 1015 () – 240() × 1015 O2 cm−2 ) with 193 nm light at
22 K. The solid lines are the fits obtained using the analytical model for different R23 . The line labeled L indicates the slope of an initial linear fluence
dependence.
photoabsorption cross section of solid O2 at 193 nm is σ 2
= 4.2 × 10−19 cm2 , orders of magnitude larger than for the
free molecule.26
For the free ozone molecule, photoabsorption at 193 nm
leads to O (3 P) and electronically excited O (1 D) (1.97 eV) in
roughly equal proportions,28, 29 with a cross section σ 3 = 4.3
× 10−19 cm2 .
O3 (X 1 A1 ) + hυ → O(1 D) + O2 a 1 g , b1 g + ,
(2)
O3 (X 1 A1 ) + hυ → O(3 P) + O2 X 3 g − ,
(3)
where the O2 in (3) may be vibrationally hot. Dissociation
into three O atoms29 or production of O (1 S) (Ref. 30) has
yields two orders of magnitude smaller than those for O
(1 D) or O (3 P) production. In the solid, σ 3 = (2.50 ± 0.05)
× 10−18 cm2 , obtained by extrapolating measurements of
Sedlacek and Wight1 from 200 to 193 nm. This value is 5.8
times larger than for gas phase ozone.
In the gas phase, the main reactions of the oxygen radicals are 2, 22
O(1 D) + O2 X 3 g − + M → O3 (X 1 A1 ) + M,
(4)
O(3 P) + O2 X 3 g − + M → O3 (X 1 A1 ) + M,
O(1 D) + O3 (X 1 A1 ) → 2O2 X 3 g − ,
O(3 P) + O3 (X 1 A1 ) → 2O2 X 3 g − ,
O2 X 3 g − , v ∗ + O2 → O3 (X 1 A1 ) + O(3 P).
of inter-molecular interactions that produce many ways to dissipate kinetic, vibrational and electronic energy. Prominent is
the cage effect, where dissociation products can bounce back
from neighboring molecules (the cage) and recombine (geminate recombination), thereby lowering the effective dissociation cross section.
The high molecular density in the solid state has multiple implications. For instance, a photon may actually interact
with a pair of O2 molecules or dimer, a process that causes absorption in the visible region in the Earth’s atmosphere and on
planetary ices31 and also causes ultraviolet absorption at photon energies between 4.5 and 7 eV in O2 at high pressures.32
Photoabsorption by the (O2 )2 dimer is much more efficient
than the dipole-forbidden transitions of the isolated molecule,
and is likely the reason for the larger photoabsorption cross
section of the solid at 193 nm, mentioned above.
The transformation of electronic energy into kinetic energy during dissociation is different from the gas phase since
the recoiling products in the solid interact with surrounding
molecules, setting them in motion as well. This sharing of
kinetic energy implies that the energy of a given dissociation product, say, an O atom, is lower than in the gas phase
and cannot be derived simply from the photon energy and
the bond energy of the molecule. In the following, we will
assume that reaction products dissipate their energy (vibrational, electronic and kinetic) before they react, because of
many body interactions in the solid, including multi-phonon
relaxation processes and multiple crossing of potential energy
surfaces. We also assume that spin, which is important in the
gas phase, does not play a major role in the solid, since collisions are known to efficiently quench O (1 D) to O (3 P).22
We will present below a simple photochemical model based
on those heuristic assumptions, and show that it describes the
data well.
With this simplification, we consider two initial processes
O2 + hυ → 2O,
(9)
O3 + hυ → O + O2 ,
(10)
and three subsequent reactions
O + O → O2 ,
(11)
O + O2 → O3 ,
(12)
O + O3 → 2O2 .
(13)
(5)
(6)
(7)
(8)
In (8) v* denotes high vibrational excitation (>4.2 eV) and M
is a third body needed to conserve energy and momentum in
the association reactions.
In the solid state, the picture is modified not only because of possibly different branching ratios, but also because
Here, conservation of energy and momentum in reactions (11)
and (12) is allowed by the presence of the lattice.
A. Initial cross sections and quantum yields in solid
ozone
For low fluences, the variation of the ozone column density η3 with F is given by
dη3
= −m 3 (1 − α3 )σ3 η3 ,
dF
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194501-5
Photolysis of pure solid O2 and O3 at 193 nm
J. Chem. Phys. 134, 194501 (2011)
B. Initial cross sections and quantum yields
in solid O2
The O atoms resulting from photoabsorption and subsequent dissociation can either recombine to reform O2 or react with additional O2 to form two ozone molecules: −3 O2
→ 2 O3 . The initial photodissociation of O2 is described by
dη2
(16)
= −m 2 (1 − α2 ) σ2 η2 ,
dF
where m2 is the number of O2 molecules that are destroyed
per net dissociation event. Since the number of oxygen atoms
in the matrix is conserved, Eq. (16) is integrated to
3η3
= −m 2 (1 − α2 )σ2 F.
(17)
ln 1 −
2η2 (0)
FIG. 6. Top: initial O3 destruction in a 156 × 1015 O3 cm−2 film irradiated
with 0.72 mJ cm−2 (●) and 154 × 1015 O3 cm−2 film with 0.41 mJ cm−2
() laser pulses. Bottom: initial O3 synthesis in several O2 films (230 × 1015
– 240 × 1015 O2 cm−2 ) when irradiated with 0.72 mJ cm−2 laser pulses.
The significance of the slope of the solid lines of both panels is discussed in
the text.
which integrates to
η3
ln
= − m 3 (1 − α3 )σ3 F.
η3 (0)
(15)
Here σ 3 is the photoabsorption cross section and α 3 is the
probability of geminate recombination of O + O2 back to
O3 . The factor m3 takes into account the additional ozone
molecule destroyed in a subsequent reaction by the O photodissociation fragment. The product m3 (1−α 3 ) gives the initial quantum yield (−O3 ), or number of ozone molecules
destroyed per absorbed photon and σ 3 ´ = (1−α 3 )σ 3 is the net
dissociation cross section in the solid.
The top panel in Fig. 6 shows a fit of Eq. (15) to the experimental η3 (F) for two values of fluence per laser pulse. The
fit gives a slope m3 (1−α 3 )σ 3 = (−O3 ) σ 3 = (2.4 ± 0.4)
× 10−18 cm2 , which is nearly equal to the photoabsorption
cross section,1 σ 3 = (2.50 ± 0.05) × 10−18 cm2 . Thus, the initial quantum yield for O3 photodestruction is (−O3 ) = 1.0
± 0.2, not too different from the value of 1.5 ± 0.2 reported
for 308 nm photons on solid ozone, on the higher wavelength
edge of the Hartley band.1 Since, according to (13), another
ozone molecule is destroyed after each dissociation event, m3
= 2, the value of (−O3 ) for the gas phase, where there is no
cage effect. Therefore, the probability for geminate recombination of O2 and O is α 3 = 0.5 ± 0.1.
The bottom panel in Fig. 6 shows a fit of Eq. (17) to experimental results at low fluences, which gives an average slope
of m2 (1−α 2 )σ 2 = (1.5 ± 0.3) ×10−19 cm2 . Since σ 2 = 4.2
×10−19 cm2 (Refs. 25–27), with an estimated uncertainty of
25%, the quantum yield for O2 destruction is (−O2 ) = 0.36
± 0.10. Furthermore, m2 = 3, because 2 O escaping recombination after O2 dissociation react with 2 additional O2 , producing 2 O3 . Thus, the probability of geminate recombination
is α 2 = 0.88 ± 0.03, nearly twice the value for ozone. Since
each dissociation produces two ozone molecules, (O3 )
= (2/3) (−O2 ) = 0.24 ± 0.06 for ozone synthesis.
C. Ozone/oxygen ratios at high fluences
The saturation η3 :η2 for O3 photolysis at high fluence is
0.070 ± 0.030, with little variation among experiments. For
experiments with O2 films, the ratio ranged from 0.05 to 0.08
in different O2 experiments. It is possible that photolysis of
O2 films is affected by trace impurities present in the ultrahigh vacuum environment. The saturation η3 :η2 in the solid
phase is nearly two orders of magnitude larger than gas phase
values, due mostly to the much larger O2 photodissociation
cross section in the solid and, to a lesser extent, to larger probability for geminate recombination (absent in the gas phase)
of dissociation fragments from O2 vs. O3 . The enhancement
in photoabsorption cross section due to O2 –O2 dimer interaction, possible at high densities, is responsible for higher η3 :η2 .
D. Rate equations and fluence dependence
Modeling ozone production and destruction on the basis of the Chapman equations is difficult because of the existence of two widely different time scales:33 those for reactions
of atomic O atoms and for the buildup of radiation products
in the film. We circumvent this problem by assuming that O
atoms are consumed by reactions as soon as they are formed.
This means that they do not accumulate in the film to a significant level, which should be a good assumption based on
previous discussions.7 Thus, the evolution of the O3 and O2 ,
determined by a competition between creation and destruction rates, is given by
η3
η2
dη3
= −σ3 η3 − σ3 η3
+ σ3 η3 R23
dF
η∗
η∗
η3
η2
+ 2σ2 η2 R23 ,
−2σ2 η2
η∗
η∗
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194501-6
Raut et al.
J. Chem. Phys. 134, 194501 (2011)
TABLE I. Comparison of different parameters obtained from 193 nm photolysis of solid O2 and O3 film with gas phase values.
σ 2 (10−20 cm2 )
σ 3 (10−18 cm2 )
α3
α2
(−O3 )
(−O2 )
(O3 )
η3 :η2 (large F)
R23
Solid
Gas (from literature)
5 ± 1.5a
1.2 ± 0.2c
0.5 ± 0.1
0.88 ± 0.03
1.0 ± 0.2
0.36 ± 0.10
0.24 ± 0.06
0.05 – 0.08 (O2 )
0.070 ± 0.03 (O3 )
0.2 – 0.22 (O3 )
0.1 – 0.18 (O2 )
0.012 – 2b
0.43d
phase. The enhanced photoabsorption increases the number
of O that can react with O2 to form O3 , leading to the higher
η3 :η2 ratio in the solid, at the end of irradiation. This is likely
also the reason for the observed 50-fold increase in the final
η3 :η2 ratio observed in photolysis of solid O2 at 210 nm compared to 250 nm.15
V. ASTROPHYSICAL IMPLICATIONS
0.001 – 0.003 (50–800 Torr)e
σ 2 required to get σ 2 is obtained from Refs. 25 and 26
Reference 23.
c
σ 3 required to get σ 3 is obtained from Ref. 1.
d
Reference 38.
e
Liu et al. (Ref. 23).
a
b
where σ i ´ = σ i (1−α i ) are the net dissociation cross sections.
The first term accounts for reduction in O3 column density
due to photoabsorption followed by dissociation, which produces O and O2 fragments; the other terms describe reactions
of O atoms with additional O2 or O3 molecules that occur
according to their relative abundance in the films and their
respective reaction cross sections. The second and the third
terms are for O formed from photodissociation of O3 ; the last
two are for the two O atoms from photodissociation of O2 .
R23 is the ratio of reaction cross sections of O with O2 and
O3 , and η = R23 η2 + η3 .
Equation (18) has only one unknown, R23 , and can be
used to fit the numerical derivatives of the fluence dependences of O3 column density. The fit is subjected to the following constraints: dη3 /dF = 2σ 2 ´η2 (0) or – 2σ 3 ´η3 (0) or for
O2 and O3 , respectively, at low F, within errors, dη3 /dF = 0
F 3
at saturation fluences and 0 dη
d F = η3 (F) − η3 (0), known
dF
accurately from the microbalance data. The fit to the O3 photolysis data shown in Fig. 3 gives R23 = 0.21. This implies
that O atoms are five times less likely to react when they encounter an O2 vs. an O3 molecule. The fit applied to different
O2 photolysis experiments give R23 in the range of 0.1–0.18, a
variation that we mentioned above for the η3 :η2 values at high
fluence. The results of some fits are shown in Fig. 5. The range
of R23 values may indicate limitations in the model, such as
the assumption of energy and spin relaxation, but it is also
possibly connected with effects of trace amounts of impurities
from deposition of background gas even at ultrahigh vacuum
pressures, which could also account for the variations in the
saturation η3 :η2 values. Among the results reported here, the
saturation η3 :η2 are the most reliable since they come directly
from the measurements. The coefficients α 2 and α 3 are obtained from initial slopes and rely on photoabsorption cross
sections measured elsewhere, but they are less dependent on
the model than R23 .
Table I summarizes the results of these experiments and
compares them with gas phase values. The remarkably higher
photoabsorption cross section of solid O2 at 193 nm is due
to the contribution of O2 dimer absorption, absent in the gas
The prospect of the presence of reactive species such as
ozone in low temperature environments such as the molecular
clouds is interesting because radiolytic O atoms can start multiple routes of molecular synthesis in the interstellar medium.
An estimate of the solid O2 abundance in molecular clouds,12
based on observations of the weak O2 transition at 6.45 μm
and the profile of the 4.67 μm CO absorption feature, gave
upper limits of 6 × 1017 –12 × 1017 O2 cm−2 , which only
accounts for ∼6% of total oxygen budget in the interstellar
medium (ISM). These O2 condensed on the grains can be
partially converted into ozone by energetic cosmic rays34 or
the UV radiation they generate by exciting the background
hydrogen gas.35 Though, so far, results of the observations
have been negative on the presence of O3 on the grains, it
is possible that the 9.6 μm ozone absorption is masked by a
broad absorption feature at ∼10 μm due to silicates10 or other
materials.36 Photodissociation of ozone produces O atoms
that could react with other molecules, such as CO to produce
CO2 , possibly assisted by the localized energy deposition. If
this process is relevant, it could help to resolve the problem of
accounting for the observed abundance of solid CO2 in interstellar clouds.37
ACKNOWLEDGMENTS
This work was supported by National Aeronautics and
Space Administration (NASA) Grant Nos. NNX07AL48G
(Cosmochemistry) and NNX08AI44G (Origins of the Solar
System).
1 A.
J. Sedlacek and C. A. Wight, J Phys. Chem. 93(2), 509 (1989).
B. Alexander and A. W. Charles, J. Chem. Phys. 101(1), 292
(1994).
3 K. S. Noll, R. E. Johnson, A. L. Lane, D. L. Domingue, and H. A. Weaver,
Science 273(5273), 341 (1996); K. S. Noll, T. L. Roush, D. P. Cruikshank,
R. E. Johnson, and Y. J. Pendleton, Nature (London) 388, 45 (1997).
4 D. A. Bahr, M. Fama, R. A. Vidal, and R. A. Baragiola, J. Geophys. Res.
106, 33285 (2001); G. P. Nikolay, G. K. Alexander, and A. K. Greg, J.
Chem. Phys.125(12), 124702 (2006).
5 B. D. Teolis, M. J. Loeffler, U. Raut, M. Fama, and R. A. Baragiola, Astrophys. J. Lett. 644(2), L141 (2006).
6 M. Famá, D. A. Bahr, B. D. Teolis, and R. A. Baragiola, Nucl. Instrum.
Methods Phys. Res. B 193, 775 (2002); S. Lacombe, F. Cemic, K. Jacobi,
M. N. Hedhili, Y. Le Coat, R. Azria, and M. Tronc, Phys. Rev. Lett. 79,
1146 (1997).
7 R. A. Baragiola, C. L. Atteberry, D. A. Bahr, and M. M. Jakas, Nucl. Instrum. Methods Phys. Res. B 157(1–4), 233 (1999).
8 P. D. Cooper, M. H. Moore, and R. L. Hudson, Icarus 194(1), 379
(2008); C. J. Bennett and R. I. Kaiser, Astrophys. J. 635, 1362
(2005).
9 P. A. Gerakines, W. A. Schutte, and P. Ehrenfreund, Astron. Astrophys.
312, 289 (1996).
10 P. Ehrenfreund, M. Burgdorf, L. D’Hendecourt, and J. M. Greenberg, Adv.
Space Res. 13, 465 (1993).
2 V.
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
194501-7
11 A.
Photolysis of pure solid O2 and O3 at 193 nm
G. G. M. Tielens and W. Hagen, Astron. Astrophys. 114, 245 (1982);
P. Ehrenfreund, R. Breukers, L. D’Hendecourt, and J. M. Greenberg, ibid.
260, 431 (1992).
12 B. Vandenbussche, P. Ehrenfreund, A. C. A. Boogert, E. F. van Dishoeck,
W. A. Schutte, P. A. Gerakines, J. Chiar, A. G. G. M. Tielens, J. Keane,
D. C. B. Whittet, M. Breitfellner, and M. Burgdorf, Astron. Astrophys. 346,
L57 (1999).
13 D. H. Parker, Acc. Chem. Res. 33(8), 563 (2000); T. G. Slanger and
R. A. Copeland, Chem. Rev. 103(12), 4731 (2003); Y. Matsumi and
M. Kawasaki, ibid. 103(12), 4767 (2003).
14 H. Chaabouni, L. Schriver-Mazzuoli, and A. Schriver, Low Temp. Phys.
26(9), 712 (2000).
15 M. J. Dyer, C. G. Bressler, and R. A. Copeland, Chem. Phys. Lett. 266(5–
6), 548 (1997).
16 N. J. Sack and R. A. Baragiola, Phys. Rev. B 48(14), 9973 (1993).
17 A. G. Streng and A. V. Grosse, J. Am. Chem. Soc. 81(4), 805 (1959).
18 B. D. Teolis, M. Famá, and R. A. Baragiola, J. Chem. Phys. 127, 4507
(2007).
19 C. S. Barrett and L. Meyer, Phys. Rev. 160(3), 694 (1967).
20 L. R. Canfield, G. Hass, and W. R. Hunter, J. Phys. 25(1–2), 124 (1964).
21 B. D. Teolis, M. J. Loeffler, U. Raut, M. Famá, and R. A. Baragiola, Icarus
190(1), 274 (2007).
22 H. Okabe, Photochemistry of Small Molecules (Wiley, New York, 1978).
23 H. T. Liou, S. F. Chiou, and K. L. Huang, Ozone: Sci. Eng. 19(3), 273
(1997); Y. Niwa, A. Matsuzaki, S. Nishio, H. Sato, and I. Tanaka, J. Phys.
Chem. A 101(4), 668 (1997); K. Yoshino, D. E. Freeman, J. R. Esmond,
and W. H. Parkinson, Planet. Space Sci. 31(3), 339 (1983).
24 S. R. Scharber, Jr. and S. E. Webber, J. Chem. Phys. 55(8), 3977 (1971).
J. Chem. Phys. 134, 194501 (2011)
25 N.
J. Mason, A. Dawes, P. D. Holtom, R. J. Mukerji, M. P. Davis,
B. Sivaraman, R. I. Kaiser, S. V. Hoffmann, and D. A. Shaw, Faraday Discuss. 133, 311 (2006).
26 H.-C. Lu, H.-K. Chen, B.-M. Cheng, and J. F. Ogilvie, Spectrochim. Acta,
Part A 71(4), 1485 (2008).
27 F.-j. Himpsel, N. Schwentner, and E. E. Koch, Phys. Status Solidi B 71(2),
615 (1975).
28 A. T. Andrew, L. V. Ghanshyam, G. Tomasz, E. T. John, and A. R.
Ravishankara, J. Chem. Phys. 95(5), 3244 (1991); S. Domenico, Y.
Xueming, D. C. James, and G. S. Arthur, J. Chem. Phys. 102(15), 6067
(1995).
29 S. Nishida, F. Taketani, K. Takahashi, and Y. Matsumi, J. Phys. Chem. A
108(14), 2710 (2004).
30 K. Takahashi, T. Nakayama, and Y. Matsumi, J. Phys. Chem A 107(44),
9368 (2003).
31 R. A. Vidal, D. Bahr, R. A. Baragiola, and M. Peters, Science 276(5320),
1839 (1997).
32 Y. Akahama and H. Kawamura, Chem. Phys. Lett. 392(4–6), 476 (2004).
33 R. M. Velasco, F. J. Uribe, and E. Pérez-Chavela, J. Math. Chem. 44(2),
529 (2008).
34 A. G. G. M. Tielens, The Physics and Chemistry of the Interstellar Medium
(Cambridge University Press, Cambridge, 2005).
35 S. S. Prasad and S. P. Tarafdar, Astrophys. J. 267, 603 (1983).
36 J. H. Lacy, H. Faraji, S. A. Sandford, and L. J. Allamandola, Astrophys. J.
Lett. 501(1), L105 (1998).
37 V. Mennella, M. E. Palumbo, and G. A. Baratta, Astrophys. J. 615, 1073
(2004).
38 L. T. Molina and M. J. Molina, J. Geophys. Res. 91(D13), 14501 (1986).
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp