THE JOURNAL OF CHEMICAL PHYSICS 130, 134704 共2009兲
Formation, trapping, and ejection of radiolytic O2 from ion-irradiated water
ice studied by sputter depth profiling
B. D. Teolis,a兲 J. Shi, and R. A. Baragiolab兲
Laboratory for Atomic and Surface Physics, University of Virginia, Charlottesville, Virginia 22904, USA
共Received 11 January 2009; accepted 10 February 2009; published online 2 April 2009兲
We report experimental studies of 100 keV Ar+ ion irradiation of ice leading to the formation of
molecular oxygen and its trapping and ejection from the surface, at temperatures between 80 and
150 K. The use of a mass spectrometer and a quartz-crystal microbalance and sputter depth profiling
at 20 K with low energy Ar ions allowed us to obtain a consistent picture of the complex radiolytic
mechanism. We show that the dependence of O2 sputtering on ion fluence is mainly due to the
buildup of trapped O2 near the surface. A small proportion of the O2 is ejected above 130 K
immediately upon creation from a precursor such as OH or H2O2. The distribution of trapped
oxygen peaks at or near the surface and is shallower than the ion range. Measurements of sputtering
of H2 help to elucidate the role of this molecule in the process of O2 formation: out-diffusion leading
to oxygen enrichment near the surface. The competing phenomena of OH diffusion away from the
ion track and hydrogen escape from the ice and their temperature dependence are used to explain the
finding of opposite temperature dependencies of O2 and H2O2 synthesis. Based on the new data and
understanding, we discuss the application of our findings to ices in the outer solar system and
interstellar space. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3091998兴
INTRODUCTION
The creation of new molecules through the interaction of
ionizing radiation with water has broad significance in astrophysics, where icy surfaces of satellites, rings, and interstellar grains are irradiated by cosmic rays, x rays, UV photons,
electrons, and energetic ions.1,2 Studies of radiation chemistry in ice date back at least half a century and have used a
wide array of techniques such as electron spin resonance,
optical absorption spectroscopy, near-edge x-ray absorption
fine structure spectroscopy, luminescence spectroscopy, mass
spectrometry, and direct chemical analysis of melted
samples. These techniques have been used to characterize the
formation and trapping of water dissociation fragments 共i.e.,
H, O, and OH兲 and newly created molecules 共e.g., HO2,
HO3, H2O2, H2, O2, and O3兲 in irradiated ice. Evidence for
the ejection of stable radiolytic species from ice during irradiation comes from the discovery that, in addition to water
molecules, H2 and O2 are sputtered from ice irradiated with
ions,3–8 photons,9,10 and electrons.11–19 The astrophysical significance of this phenomenon was highlighted by the detection of tenuous oxygen atmospheres at the Jovian satellites
Europa20 and Ganymede21 and at Saturn’s rings.22 Several
experimental studies have therefore attempted to elucidate
the process whereby O2 is produced and ejected from irradiated ice,6,7,12–14,17–19,23 but while considerable progress has
been made, a consensus has yet to emerge.
A key piece of evidence is the observation in ion6,7,23
and electron-irradiated12,13,18,19 ice of a fluence dependence
in the O2 desorption yield 共i.e., the number of molecules
a兲
Present address: Southwest Research Institute, San Antonio, TX.
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
b兲
0021-9606/2009/130共13兲/134704/9/$25.00
ejected, or “sputtered,” per incident projectile兲, which is initially negligible but increases asymptotically during irradiation to a temperature-dependent steady-state value. The fluence dependence indicates a multistep process in which the
initially arriving projectiles produce trapped radiochemical
species, precursors that enable the ejection of O2 by later
projectiles, but their identification has until recently remained a crucial unresolved problem. Based on their experiments using 1.5 MeV Ne+ projectiles, Boring et al.6 and
Reimann et al.7 originally proposed that the precursor species is the O2 molecule itself, initially produced as a trapped
species in the ice and then ejected by later ions. In fact, the
production and trapping of O2 in irradiated ice have been
demonstrated in multiple experiments.18,19,24–31 These observations lead to the suggestion that absorption bands of condensed O2 in the reflectance spectra of Ganymede,32 Europa,
and Callisto33 are due to high concentrations of radiolytic
oxygen in the irradiated surface ice of these satellites.34
However, this hypothesis was called into question by Vidal
et al.35 and Baragiola and Bahr,36 who found rapid outdiffusion of O2 from unirradiated vapor-deposited O2 – H2O
ice mixtures at the reported Ganymede surface temperatures
共90– 152 K37兲. These authors therefore proposed that sufficiently high concentrations of radiolytic O2 cannot be
trapped in ice at these temperatures. A low concentration of
radiolytic oxygen is also implied in the work of Orlando and
co-workers,12,13 who did not consider trapped oxygen to be a
possible precursor species in their studies of O2 production
from ice by low-energy 共5 – 100 eV兲 electrons. Rather, they
interpreted the initial linear fluence dependence in the desorption yield to imply that the precursor must be another
stable species12,13 such as HO2 or H2O2.13,38 In contrast to
these authors, Johnson et al.14 suggested that a stable precur-
130, 134704-1
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134704-2
Teolis, Shi, and Baragiola
sor requires two incident projectiles to form, thus implying
an initially nonlinear fluence dependence, and concluded that
the precursor is a radical 共e.g., OH or O兲. Meanwhile, Petrik
et al.15,16,18,19 studied the fluence dependence of the H2 and
O2 desorption yields from vapor-deposited ice films using
low-energy 共100 and 87 eV兲 electrons. Based on experiments
showing that ice films predosed with OH and H2O2 still exhibit an initially negligible O2 yield, but an enhanced yield at
nonzero fluences, these authors hypothesize that OH, H2O2,
and HO2 are involved in intermediate stages of the O2 formation process.
Although the low-energy electron studies have not regarded trapped O2 as a precursor to O2 emission, this assumption is difficult to reconcile with the recent thermal desorption results for ion23,26,31 and electron-irradiated19,27,28
ice suggesting the production and retention of trapped O2 up
to ⬃150 K. If present, trapped oxygen will contribute to the
O2 desorption yield in both electron and ion-irradiated ice
since, in both cases, sputtering will eventually erode a thickness of ice sufficient to expose and eject O2 trapped below
the surface. But how does the contribution of trapped O2
compare with that of other precursor species? In the case of
ion irradiation, our recent experimental studies23,29,31 have
begun the process of addressing this question.
As in recent experiments with low-energy
electrons,18,19,39 our studies with 100 keV Ar+ showed a dependence of the O2 desorption yield on the irradiation and
thermal history of the ice, with the fluence dependence exhibiting complex transients after previously irradiated ices
were either 共i兲 temporarily heated or 共ii兲 capped with fresh
ice overlayers.23 The dependence on history suggests chemical reactions and/or diffusion involving O2 precursors. Out
of the five precursors previously proposed in the literature
共H2O2, HO2, OH, O, and trapped O2兲, H2O2 and HO2 are
ruled out by infrared spectroscopy measurements,40–42 which
detect neither HO2 nor sufficient H2O2 to account for the O2
ejected from the ice surface.23,43 The infrared measurements
do not exclude O or OH as possible precursors, since O
atoms do not absorb infrared light and the infrared signature
of OH would be masked by that of the H2O molecules. However, other experimental evidence on ion-irradiated ices
points strongly to trapped O2 as the precursor principally
responsible for the O2 fluence dependence. First, our observation, at temperatures 艌130 K, of continuous out-diffusion
of intact O2 molecules from ice after irradiation by 100 keV
Ar+ suggests the synthesis of transiently trapped O2 that thermally detraps and gradually escapes the ice by diffusion.23
Second, we demonstrated that the burial of trapped radiation
products by simultaneous H2O condensation onto the ice surface during irradiation causes a reduction in the O2 sputtering yield and a related enhancement in the amount of O2
trapped in the ice.31 In fact, ultraviolet and infrared reflectance measurements showed the enhancement in the trapped
O2 concentration to be sufficient for the generation of
ozone,31 thus demonstrating for the first time the synthesis of
O3 by radiolysis of pure water ice—a process believed to
take place on Ganymede44 and the Saturnian satellites Dione
and Rhea.45 Finally, in related experiments where water ice
containing a near-surface concentration of 22% trapped O2
J. Chem. Phys. 130, 134704 共2009兲
was produced by 50 keV H+ irradiation of solid H2O2 at
17 K, we observed the weak 1550 cm−1 infrared absorption
band of O2.29 The band is temperature sensitive, disappearing rapidly on warming of the ice, but the corresponding O2
was retained 共as confirmed by mass spectrometry and a
quartz-crystal microbalance兲 up to ⬃155 K, thus providing
additional confirmation that ion-irradiated ices are capable of
storing concentrations of trapped O2 that far exceed that of
other possible precursor species.
Unlike low-energy electrons, irradiation of ice by highenergy ions produces many damage and defect sites17 that
may act as trapping sites for O2.23,29 Moreover, a high density of radiation products in ion tracks react chemically as
easily in the bulk ice as at surfaces, which contrasts with
low-energy electron irradiation.15–19 Hence, compared to
high-energy ions, the role of trapped O2 as a precursor to O2
desorption could be very different for low-energy electrons,
but whether trapped oxygen can be altogether neglected is
uncertain. Petrik et al.19 recently addressed this question
using 87 eV electrons by showing that, for a fluence of
⬃2.8⫻ 1015 electrons/ cm2, the total amount of O2 desorbed
over the irradiation period is much greater than the amount
trapped in the ice at the end of irradiation. However, since
this result gives no information on the fraction of ejected O2
that is first trapped in the ice 共i.e., during irradiation兲, the role
of trapped oxygen in the low-energy electron experiments
remains unclear.
In this work we use the sputter depth profiling technique
introduced in our previous study23 to yield a fresh perspective on the problem of O2 radiolysis in ice. Using 100 keV
Ar+, we measured the fluence and temperature dependence of
共i兲 the trapped O2 concentration versus depth below the ice
surface and 共ii兲 the O2 desorption yield. The results enable us
to illustrate quantitatively the relationship between oxygen
trapping and ejection. We also reevaluate our previous
hypothesis23 that cooling the ice to low temperatures suppresses radiation-induced diffusion of trapped O2 by
100 keV Ar+ and, instead, find that the diffusion is reduced
by lowering the ion energy 共rather than the sample temperature兲. We therefore perform sputter depth profiling measurements with 4.5 keV Ar+ projectiles. Our findings distinguish
for the first time the role of species such as OH and H2O2 as
compared to trapped oxygen in determining the O2 desorption yield, and provide new insight into the processes responsible for the generation, trapping, and sputtering of radiolytic
O2 in ion-irradiated ice.
EQUIPMENT, PROCEDURES, AND SPUTTER DEPTH
PROFILING
We performed all measurements on thin water ice films
vapor deposited onto a gold-coated quartz-crystal microbalance in a cryopumped ultrahigh vacuum chamber 共minimum
pressure ⬃2 ⫻ 10−10 Torr兲. The microbalance was mounted
to a rotatable cryostat, and ice samples could therefore be
faced toward different instruments for processing and analysis as needed. Using a microcapillary doser to produce a
collimated water vapor flux directed normally at the gold
substrate at 100⫾ 0.2 K, we deposited ⬃500 ML 共monolayer兲 films at a rate of ⬃2.5 ML/ s, i.e., conditions which
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134704-3
J. Chem. Phys. 130, 134704 共2009兲
O2 radiolysis in ice
produce amorphous ice with small porosity46 共1 ML is defined here as 1015 molecules/ cm2兲. From the shift of the
resonance frequency of the microbalance, we determined the
evolution in the mass column density of the films to an accuracy of 0.1 ML during vapor deposition and during ion
irradiation to measure the erosion of the films by sputtering.
After deposition we produced trapped O2 by irradiating the
films at 80– 150 K with a 100 keV Ar+ beam from a
20– 300 keV ion accelerator at 45° with respect to the film
surface. During irradiation we monitored the resulting ejection of O2 and H2 molecules with a Hiden EQS-300 quadrupole mass spectrometer positioned normally to the sample
surface. The sputtered species are predominantly neutral, and
we therefore operated the mass spectrometer with the electron impact ionizer turned on. The ion beam was rastered to
ensure uniform irradiation of the films over the sensitive region of the microbalance 共an ⬃5 mm diameter spot at its
center兲, and the ion flux was measured with a Faraday cup
and kept constant at ⬃1.5⫻ 1012 ions/ cm2 s during irradiation. The projectiles penetrated ⬃500 ML below the film
surface on average,47 and therefore did not reach the substrate. We also note that any micropores remaining in the ice
from deposition are destroyed by warming 共for experiments
⬎100 K兲 and irradiation.48,49
Following the initial irradiation, we rotated the sample
30° from the mass spectrometer toward a 0.01– 5 keV NTI
Nonsequitur Model 1401 ion gun. Sputter depth profiling of
the trapped O2 concentration was then performed by irradiating the ice with a collimated 4.5 keV Ar+ beam at a constant rate of ⬃2 ⫻ 1012 ions/ cm2 s while measuring 共i兲 the
film thickness lost by sputtering with the microbalance and
共ii兲 the sputtering of trapped O2 with the mass spectrometer.
The 4.5 keV beam was scanned over a 5 mm spot at the
center of the microbalance, i.e., the same spot previously
processed at 100 keV. Irradiation was carried out at an oblique incidence angle of 60° with respect to the surface normal with 4.5 keV 共rather than 100 keV兲 ions to limit the
penetration of the projectiles below the ice surface 关ion range
is ⬃60 ML 共Ref. 47兲兴 and, as stated earlier, to minimize the
redistribution of trapped O2 during the measurement. Since
oxygen production increases with temperature, the depth
profile measurements were performed with the ice cooled to
20 K to minimize synthesis of additional trapped O2 by the
4.5 keV Ar+ beam.
The mass spectrometer signals Iw and IO2 at 18 and
32 amu are proportional to the sputtering yields Y w and Y O2
for H2O and O2, i.e., Y w = ␣wIw and Y O2 = ␣O2IO2. To obtain
the sensitivity factors ␣w and ␣O2, we incorporate into our
analysis the microbalance measurement of S: the mass loss
rate per unit area during sputtering of the film. Since O2 and
H2O account for nearly all the mass loss,3,6,23,50 we can express S as a linear combination of the sputtering yields for
H2O and O2 as follows:23
S共F兲 ⬇ ␸ M wY w + ␸ M O2Y O2共F兲
= M w␸␣wIw + M O2␸␣O2IO2共F兲,
共1兲
where M w and M O2 denote the molecular masses, ␸ is the
irradiation flux, and F is the fluence. We therefore obtain ␣i
FIG. 1. 共Color online兲 共a兲 The O2 sputtering yield during irradiation of
several ice films at 20 K by 4.5 keV Ar+ plotted vs monolayers sputtered.
The films were first irradiated with 100 keV Ar+ at 130 K to a fluence of
⬃2 ⫻ 1015 ions/ cm2, then cooled to 20 K and capped with fresh ice overlayers of the indicated thickness in ML. Also shown is the O2 yield without
prior irradiation at 130 K 共lower curve兲. 共b兲 The fractional O2 concentration
vs depth calculated from 共a兲 using Eqs. 共2兲 and 共3兲 共see text兲.
by fitting the fluence dependent right hand side of Eq. 共1兲 to
the measured S共F兲, and then multiply by the mass spectrometer signals Ii to determine the H2O and O2 sputtering yields.
In general, Y O2 depends on fluence, but we measure Y w to be
constant, as in previous reports.6,7,14,23 Y w ⬃ 70 共⬃40兲 H2O
molecules per ion for 100 共4.5兲 keV Ar+ at 45° 共60°兲
incidence.
Since in sputtering, the yields Y i are proportional to the
fractional surface concentrations Ci, then
CO2 =
NO2
Y O2
=
,
Nw + NO2 Y w + Y O2
共2兲
where we consider that the molecular densities of H2O and
O2 at the surface, Ni, are much larger than those of other
trapped species.23,26,29,31 As the ice is eroded by sputtering,
the O2 yield evolves as different oxygen concentrations are
exposed at the surface. We obtain the O2 concentration originally present in the ice versus depth sputtered x expressed in
monolayers, i.e., in units of 1015 molecules/ cm2, calculated
by integrating over S, the mass per cm2 lost by sputtering:
x共in ML兲
= 10−15 cm2
冕
S
0
dS⬘
. 共3兲
M O2CO2共S⬘兲 + M w关1 − CO2共S⬘兲兴
Here, we divide S by the weighted average M O2CO2 + M w共1
− CO2兲 of the H2O and O2 molecular masses to convert mass
to number of molecules.
In Fig. 1 we apply the depth profiling technique to several ice films in which trapped O2 was first generated at
130 K by 100 keV Ar+ irradiation to fluences sufficient for
steady-state O2 emission 共⬃2 ⫻ 1015 ions/ cm2兲. To minimize
alteration of the depth profile by diffusion, we cooled the
films from 130 to 20 K as fast as possible, usually over a
few minutes. In the figure we also show the effect of ice
overlayers on the results and compare to the case of a fresh
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134704-4
J. Chem. Phys. 130, 134704 共2009兲
Teolis, Shi, and Baragiola
FIG. 2. 共Color online兲 共a兲 The O2 sputtering yield during irradiation of
an ice film at 20 K by 100 keV Ar+ plotted vs sputtered depth. The film
was first irradiated with 100 keV Ar+ at 130 K to a fluence of
⬃2 ⫻ 1015 ions/ cm2, then cooled to 20 K and capped with a 216 ML overlayer. 共b兲 Experiment of 共a兲 without an overlayer.
ice, i.e., without prior irradiation at 130 K. As shown in Fig.
1共a兲, the O2 sputtering yield from a fresh ice film exhibits
typical behavior during sputtering, rising asymptotically to a
steady-state value, at which all ejected O2 is just due to radiolysis by the 4.5 keV Ar+ beam. However, the O2 emission
evolves differently for a previously irradiated ice, transiently
exceeding and then returning to the fresh ice steady-state
value 关Fig. 1共a兲兴. The overshoot is caused by the added contribution of trapped O2 from prior irradiation at 130 K to that
synthesized directly by the 4.5 keV Ar+ beam. We assume
that the O2 concentrations produced by the 100 and 4.5 keV
ions are additive. Thus, to calculate the O2 depth profile produced only by the previous 100 keV irradiation, we remove
the concentration of O2 produced by the 4.5 keV ions. The
depth profile in Fig. 1共b兲 shows the generation of a subsurface layer of trapped oxygen by the 100 keV irradiation, with
the O2 concentration rising with depth below the surface,
then declining beyond ⬃120 ML. The burial of the trapped
O2 by ice overlayers deposited at 20 K is also reflected in the
depth profile measurement. In this case, we do not detect the
subsurface O2 layer until most of the overlayer is removed
by sputtering 关Fig. 1共b兲兴, which indicates minimal distortion
of the depth profile by radiation-induced diffusion.
In contrast, when the experiment of Fig. 1 is repeated but
using a 100 keV Ar beam at 45° incidence to measure the
depth profile, the results show the expected distortion by
diffusion. This can be seen in Fig. 2, where we compare
results with and without overlayer deposition at 20 K. With
216 ML of fresh ice deposited atop the film, the excess O2 is
seen to escape the ice well before removal of the overlayer
by sputtering, thus demonstrating that radiation-induced dif-
FIG. 3. 共Color online兲 The O2 fractional concentration vs depth in fresh ices
irradiated with 100 keV Ar+ at 130 K with fluence as a parameter: 1.5, 3.5,
5, 7, and 20⫻ 1014 Ar+ / cm2, from lowest to highest concentration. Note that
the O2 buildup is not linear with fluence.
fusion transports buried O2 through the ice overlayer during
irradiation. The shape of the transient O2 yield is drastically
distorted by the overlayer 共Fig. 2兲 due to the combined effect
of simultaneous sputtering and O2 transport to the film surface. We note the contrast with our previous study,23 where
we performed the overlayer experiment of Fig. 2, but with
the initial irradiation carried out at 20 K rather than 130 K.
In that work, we concluded that radiation-induced diffusion
is minor at 20 K, based on the lack of an overshoot in the O2
yield. However, the result of Fig. 2 points to another explanation: that irradiation at 20 K does not generate sufficient
trapped O2 to produce an overshoot in the O2 sputtering yield
after overlayer deposition 共see later discussion on the temperature dependence of O2 production兲. Our new results
therefore indicate that suppression of radiation-induced diffusion is accomplished by reducing the projectile energy
rather than the ice temperature. The effect of diffusion on the
measurement is seen on comparison of Fig. 1共b兲 to the preliminary depth profile measurement performed using
100 keV Ar+ in our earlier work.23
RESULTS
Fluence dependence
Our experimental technique has allowed us to investigate the dependence of the trapped oxygen distribution on
two important parameters: the ice temperature and the irradiation fluence 共Table I兲. For the measurement of the fluence
dependence we chose a temperature of 130 K, and irradiated
several ice films to different fluences of 100 keV Ar+ ions
before cooling each film to 20 K for depth profiling. As
shown in Fig. 3, trapped O2 accumulates 共nonlinearly兲 during
TABLE I. Experiments performed.
O2 depth profile
O2 sputtering yield
H2 sputtering yield
Temperature dependence
Fluence dependence
80– 150 K at 2 ⫻ 1015 Ar+ / cm2
80– 150 K
80– 150 K
0 – 2 ⫻ 1015 Ar+ / cm2 at 130 K
0 – 2 ⫻ 1015 Ar+ / cm2
0 – 2 ⫻ 1015 Ar+ / cm2
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134704-5
J. Chem. Phys. 130, 134704 共2009兲
O2 radiolysis in ice
FIG. 4. 共Color online兲 The O2 column density 共䉱兲, subsurface peak concentration 共䊏兲, and twice the surface concentration 共䊊兲 vs fluence during
irradiation of a fresh ice at 130 K, as determined from the data of Fig. 3. The
line shows the fluence dependence of the O2 sputtering yield for
comparison.
irradiation up to a saturation above a fluence of
⬃1015 Ar+ / cm2 when the rates of O2 production, destruction, and ejection from the surface equilibrate. The relationship of the O2 sputtering yield to the amount of trapped
oxygen is seen on comparison of their fluence dependences.
As shown in Fig. 4, O2 sputtering has the same fluence dependence as the surface O2 concentration, an additional indication that the ejected O2 is first present as a trapped species in the ice. Concurrent diffusion and surface removal of
trapped oxygen produces the depletion layer23 seen in Figs. 1
and 3, i.e., the region near the surface where the O2 concentration is suppressed. The diffusion is both thermal and radiation induced:23 An O2 traps at a defect site for a finite time
and then detraps due to thermal agitation or excitation by a
projectile ion. The mobilized O2 then travels through preferred pathways 共e.g., a transiently heated ion track兲 and
eventually 共i兲 retraps at another defect site or 共ii兲 reaches the
ice surface and desorbs.
An interesting finding is the unusually low O2 column
density at a fluence of 1.5⫻ 1014 Ar+ / cm2 共Fig. 4兲, which
results from the apparent lack of O2 below the topmost
monolayer 共Fig. 3兲. The requirement of an incubation fluence
suggests that the buildup of another precursor species is required for the production of trapped oxygen deeper in the
ice. As we discuss below, our measurements of the temperature dependence of the oxygen depth profile and the O2 and
H2 emissions provide valuable insight into the identity of the
precursor species and the process of its conversion to trapped
O 2.
FIG. 5. 共Color online兲 The fractional O2 concentration vs depth 共in ML兲
after 100 keV, 2 ⫻ 1015 Ar+ / cm2 irradiation of fresh ices with the temperature as a parameter. Bottom panel: 80– 120 K; top panel: 130– 150 K. The
dashed line is the depth distribution of energy deposited by the projectiles
Ref. 47, which is shown for comparison.
depletion 共Fig. 5兲 as well as the O2 outgassing previously
observed.23 The results also show that trapped O2 is synthesized most efficiently close to the surface, particularly at low
temperatures. From 80 to 120 K, for instance, most trapped
O2 is found within 100–200 ML of the ice surface, which is
much shallower than the distribution of energy deposition
共see Fig. 5兲 and the penetration depth 关⬃500 ML 共Ref. 47兲兴
of the projectiles. Preferential O2 generation near the surface
is consistent with the out-diffusion of hydrogen 共in the form
of H2兲 from the near-surface region23 which changes the stoichiometry of the ice, enhancing the formation14 of O2 or
oxygen-rich precursors. Hydrogen generated deeper in the
ice is more likely to trap51 or react rather than escape.
Hydrogen loss
To examine the preferential depletion of hydrogen at low
fluence more closely, we also measured the sputtering yield
for H2 from a fresh ice. Figure 6 shows the H2 sputtering
yield versus fluence for ice films irradiated with 100 keV Ar+
at different temperatures. The mass of the H2 molecule is too
low for the yield to be determined by comparison of the mass
spectrometer and microbalance data 共as done for O2, see ear-
Temperature dependence
As shown in Fig. 5 the O2 depth profile at saturation
fluence 共⬃2 ⫻ 1015 Ar+ / cm2兲 increases strongly with irradiation temperature, as seen in the larger peak concentration
from 80 to 120 K, and a wider distribution from 130 to 150
K. The widening above ⬃130 K is explained by the onset of
thermal diffusion of trapped O2 at these temperatures,23
which transports oxygen produced close to the surface
deeper into the ice. At 130 K and higher temperatures, thermal diffusion also produces the pronounced near-surface O2
FIG. 6. 共Color online兲 共a兲 The H2 sputtering yield vs fluence during
100 keV Ar+ irradiation of fresh ices with temperature as a parameter: 80,
100, 110, 120, 130, 140, and 150 K, from bottom to top, at zero fluence. 共b兲
The initial and saturation H2 yields from 共a兲, and the saturation O2 yield
from Fig. 7, vs temperature.
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134704-6
J. Chem. Phys. 130, 134704 共2009兲
Teolis, Shi, and Baragiola
lier兲. Instead, we multiply the H2 signal by a constant sensitivity factor that takes into account that, on average, the H2
yield is twice the O2 yield at saturation fluence over the
measured temperature range 共since the stoichiometry of the
ice is maintained after saturation of the yields兲. The ratio of
the sensitivity factors agrees, within 10%, with those derived
from relative ionization cross sections in the ion source of
the mass spectrometer. Contrary to the O2 yield, the H2 yield
is highest at zero fluence, then declines to a saturation value
as oxygen-rich species accumulate and react with a fraction
of the hydrogen before it can escape. A similar result was
obtained by Reimann et al.7 using MeV Ne+ as projectiles.
The saturation fluence for the H2 yield is similar to that of
the O2 concentration 共Figs. 3 and 4兲, which suggests a reaction of hydrogen with the accumulating O2 and other
oxygen-rich species.
Finally, we note that the column density ␩O2 of trapped
O2 at saturation is 21 the net oxygen atom enrichment of the
ice, minus the oxygen incorporated into species other than
O2. This is given 共as derived from stochiometric considerations兲 by
␩O2 =
1
2
冕
⬁
0
关Y H2共F兲 − 2Y O2共F兲兴dF − 兺
i
2AO,i − AH,i
␩i ,
4
共4兲
where Y H2 denotes the H2 yield, ␩i the saturation column
density of species i other than O2, and AO,i and AH,i the
number of O and H atoms they contain per molecule. Note
that if sputtering is stoichiometric 共at high fluences兲, the integrand becomes zero. Above 80 K, we can neglect the second term, since the column densities of other radiolytic species such as H2O2 and OH 关weighted by 共2AO,i − AH,i兲 / 4 = 21
and 41 , respectively兴 are much less than ␩O2 in this temperature range 共see below兲. The resulting values for ␩O2 are in
agreement with the O2 column density calculated from the
integration over the depth profiles 共as shown in Fig. 8兲 and
that determined from thermal desorption measurements,31
thus providing additional verification of the measurements.
Fluence dependence of O2 sputtering
at high temperatures
During the experiments of Fig. 6, the O2 sputtering yield
showed the expected increase with fluence, but also
exhibited an intriguing behavior at temperatures above
130 K 共Fig. 7兲. At these high temperatures we measured two
separate increases in the yield: one below a fluence of
⬃3 ⫻ 1013 Ar+ / cm2 and the other between ⬃1014 and
1015 Ar+ / cm2. We note that the threshold fluence of
1014 Ar+ / cm2 for the second increase is similar to the incubation fluence measured for O2 buildup in 130 K ice 共Figs. 3
and 4兲. A common explanation for the results is the accumulation of a precursor species below the threshold fluence
which, upon further irradiation, produces trapped O2. With
the onset of thermal diffusion at 140– 150 K, the threshold
phenomenon is manifested in the O2 yield, as the accumulating O2 simultaneously diffuses to the ice surface and escapes. Additionally, in this temperature range, oxygen pro-
FIG. 7. 共Color online兲 The O2 sputtering yield vs fluence during 100 keV
Ar+ irradiation of fresh ices with temperature as a parameter. The
inset shows the O2 yield at 140 and 150 K at low fluence
共0 – 2 ⫻ 1014 Ar+ / cm2兲.
duced in sufficient proximity to the ice surface 共e.g., within a
few monolayers兲 can diffuse out of the ice immediately, thus
producing the initial increase in the O2 yield. This process is
analogous to that invoked by models12–14,18,19 in which the
fluence dependence of the O2 yield tracks that of the precursor concentration. However, we point out that the initial yield
increase seen here is only ⬃1 / 8 that of the second increase
at both 140 and 150 K, which suggests that most O2 ejected
after the second increase comes from oxygen initially
trapped in the ice, rather than that produced directly from the
precursors.
We can obtain an upper limit for the saturation column
density of the precursor at 140 and 150 K by assuming that,
instead of trapped O2, only the precursor is produced in the
initial step 共⬍3 ⫻ 1013 Ar+ / cm2兲. By comparing the net oxygen enrichment during irradiation to the elemental composition of the precursor, we can equate the column density of a
precursor i to the integral of 2关Y H2共F兲 − 2Y O2共F兲兴 / 共2AO,i
− AH,i兲, i.e., below 3 ⫻ 1013 Ar+ / cm2. The calculation gives
upper limits of 6.5⫻ 1015 cm−2 for H2O2 and 1.3
⫻ 1016 cm−2 for OH and O at 140 K and the corresponding
values of 6 ⫻ 1015 and 1.2⫻ 1016 cm−2 at 150 K. Here, we
rule out HO2 as a precursor based on the lack of detection by
infrared spectroscopy.42
DISCUSSION
By considering the important role of hydrogen outdiffusion in the synthesis of trapped O2, we can account for
the temperature dependence of the synthesis of all three
stable radiation products: O2, H2, and H2O2, and identify the
potential O2 precursor species. We start by examining the
possible reaction pathways leading to the formation of O2: a
process that begins with the dissociation of water by the
projectile ion or secondary electrons through ionization
H2O+ + e− + H2O → H3O+ + OH + e− ,
共5兲
e− + H2O → OH− + H,
共6兲
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134704-7
J. Chem. Phys. 130, 134704 共2009兲
O2 radiolysis in ice
OH− + H3O+ → 2H2O,
共7兲
or direct excitation
H2O* → OH + H,
共8兲
H 2O * → O + H 2 ,
共9兲
H2O* → O + 2H.
共10兲
One O2 formation mechanism is by reactions involving O
atoms14 from Eqs. 共9兲 and 共10兲, e.g.,
O + O → O2 .
共11兲
However, since reactions 共9兲 and 共10兲 account for only 22%
of dissociations in the gas phase,52 and reaction 共9兲 may be
hindered in the solid state by the recombination of O with H2
due to the cage effect,42 we assume here that O atoms are
much less abundant than H and OH produced in reactions
共5兲–共8兲. We thus consider only reactions involving the latter.
H atoms can either reform water by recombination with OH:
OH + H → H2O
共12兲
or associate to form H2:
H + H → H2 ,
共13兲
with the product H2 either escaping from the ice by diffusion
to the surface or reacting with OH to reform water and H:
OH + H2 → H2O + H.
共14兲
Diffusion is minimal at low temperature 共e.g., 20 K兲, and
therefore, most hydroxyl radicals not consumed by reactions
共12兲 and 共14兲 with hydrogen will remain concentrated close
to the ion track, resulting in a high likelihood for H2O2 formation from OH:
OH + OH → H2O2 .
共15兲
With increasing temperature, OH diffusion away from the
ion track reduces the H2O2 creation cross section,42 which
helps to suppress the saturation peroxide column density, as
shown in Fig. 8.
At intermediate temperatures 共i.e., ⬃80– 100 K兲, OH
will be mostly consumed by reactions 共12兲 and 共14兲 with
hydrogen after escape from the track, but hydrogen outdiffusion will leave some excess OH near the ice surface.
The remaining OH is either 共i兲 trapped and then converted to
O2 by dissociation by another projectile ion X:
X + OH → OH* → O + H,
共16兲
followed by reaction 共11兲 between the product O atoms, or
共ii兲 converted to O2 by reacting with trapped peroxide:
OH + H2O2 → H2O + HO2 ,
共17兲
OH + HO2 → H2O + O2 ,
共18兲
as suggested by the increase with temperature in the H2O2
destruction cross section.42 Therefore, both trapped OH and
H2O2 are possible precursors to trapped O2 formation, but, in
both cases, O2 synthesis is controlled by the amount of OH
left unconsumed by reactions with hydrogen. The process is
FIG. 8. 共Color online兲 A comparison of the temperature dependence of O2
and H2O2 production by ion irradiation. The saturation O2 column densities
after 100 keV Ar+ irradiation calculated from the integration of the depth
profiles of Fig. 5 共filled circles兲 and from Eq. 共4兲 共filled triangles兲 are also
shown, together with the saturation O2 desorption yield 共stars兲. The saturation values of the H2O2 column densities produced by 100 keV H+ 共open
triangles兲, 100 keV Ar+ 共open circles兲, and 50 keV Ar+ 共inverted triangle兲
irradiation are reproduced from Ref. 42.
augmented at high temperatures 共i.e., ⬎ ⬃ 100 K兲, as enhanced hydrogen out-diffusion 共Fig. 6兲 leaves more excess
OH near the ice surface, resulting in more O2 formation.
Moreover, O2 destruction by additional ions X
X + O2 → O + O,
共19兲
O + H → OH,
共20兲
O + H 2 → H 2O
共21兲
is diminished by hydrogen loss as reactions with H and H2
become less frequent, thereby increasing the likelihood of O2
reformation from O via Eq. 共11兲. The three temperature regimes are seen in Fig. 8, where we compare the temperature
dependence of O2 synthesis to that previously measured for
H2O2. While oxygen and peroxide predominate at high and
low temperatures, respectively, neither species is generated
efficiently at intermediate temperatures 共Fig. 8兲 because OH
escape from the ion tracks is compensated by its destruction
in reactions 共12兲 and 共14兲 with hydrogen.
SUMMARY AND CONCLUSIONS
We have used mass spectrometry together with a quartzcrystal microbalance to measure the ejection of molecular
oxygen from ion-irradiated water ice and the depth profile of
trapped O2. This new approach to the question of oxygen
radiosynthesis in ice shows that the fluence dependence of
O2 sputtering is due predominantly to the buildup of trapped
O2 near the ice surface, thereby demonstrating that most
sputtered O2 is first trapped in the ice, with only a small
fraction above 130 K ejected immediately on creation from
another precursor species 共OH or H2O2兲. This finding has
important implications for icy satellites, i.e., that the presence of trapped oxygen in the surface ice is required for the
existence of an oxygen exosphere sustained by O2 sputtering
from the satellite surface. This suggests a close interaction
between gaseous and surface bound oxygen on these bodies,
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134704-8
with sputtering and perhaps readsorption of gaseous O2 acting together to exchange and distribute oxygen between the
exosphere and the surface.
Our results show the crucial role of hydrogen outdiffusion at the ice surface in facilitating trapped O2
formation—a process that could have profound consequences for other irradiated astrophysical ices possessing
high surface area to volume ratios such as regoliths or grains,
in which O2 synthesis can be augmented by enhanced hydrogen escape from the surfaces. Grains rich in radiolytic oxygen, for instance, could be later incorporated into larger objects such as comets, satellites, or planets during their
formation.
Finally, the results help to explain the opposite temperature dependencies of O2 and H2O2 synthesis during ion irradiation in terms of the competing phenomena of OH diffusion away from the ion track and hydrogen escape from the
ice. The process begins with the dissociation of water molecules by a projectile ion into H and OH radicals which, at
low temperature 共i.e., ⬍ ⬃ 80 K兲, is followed by the formation of H2O2 from OH in the ion track. With increasing temperatures, H2O2 synthesis is suppressed due to OH diffusion
away from the track, but while most of the OH is subsequently consumed in reactions with H or H2, reforming water, excess OH remains near the ice surface where hydrogen
can escape before reacting. The leftover OH is converted to
trapped O2 either through a reaction with H2O2 or by dissociation into H + O via another projectile ion followed by
O-atom recombination, thus making OH or H2O2 the precursor to trapped O2 formation. Hydrogen escapes more efficiently with increasing temperature, thereby enhancing the
formation of O2. Trapped O2 can then undergo diffusion
共thermal and radiation induced兲 deeper into the ice or to the
surface, where it escapes by sputtering or thermal desorption.
Together, these processes determine the shape of the depth
distribution of trapped O2. The syntheses of oxygen and hydrogen peroxide are favored above and below ⬃80 K, respectively 共Fig. 8兲, which implies preferred formation of O2
on the icy Jovian satellites at their reported surface temperatures and of H2O2 on the colder surfaces found in the Saturnian system and beyond.
ACKNOWLEDGMENTS
This research was funded by NSF Astronomy through
Grant No. AST0807830 and by NSF Planetary Atmospheres
through Grant No. NNG06GF30G.
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