THE JOURNAL OF CHEMICAL PHYSICS 124, 104702 共2006兲
Decomposition of solid amorphous hydrogen peroxide by ion irradiation
Mark J. Loeffler, Ben D. Teolis, and Raul A. Baragiolaa兲
Laboratory for Atomic and Surface Physics, Thornton Hall, University of Virginia, Charlottesville,
Virginia 22904-4238
共Received 15 September 2005; accepted 11 January 2006; published online 10 March 2006兲
We present laboratory studies of the radiolysis of pure 共97%兲 solid H2O2 films by 50 keV H+ at
17 K. Using UV-visible and infrared reflectance spectroscopies, a quartz-crystal microbalance, and
a mass spectrometer, we measured the absolute concentrations of the H2O, O2, H2O2, and O3
products as a function of irradiation fluence. Ozone was identified by both UV and infrared
spectroscopies and O2 from its forbidden transition in the infrared at 1550 cm−1. From the
measurements we derive radiation yields, which we find to be particularly high for the
decomposition of hydrogen peroxide; this can be explained by the occurrence of a chemical chain
reaction. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2171967兴
INTRODUCTION
Solid hydrogen peroxide, like water ice, is a suitable
substance for fundamental studies of radiation chemistry because of the small number and limited complexity of the
radiation products. Extended radiolysis of both water and
H2O2 leads to solids containing similar radiation products,
but we expect significant differences because of the different
atomic composition; e.g., the extra O atom causes H2O2 to
be much more reactive. To date, the radiolytic behavior of
highly reactive molecules in the solid phase, such as condensed H2O2, has not been explored extensively.
Our interest in the radiolytic behavior of solid H2O2
stems from the discovery of this molecule on Europa,1 one of
the Galilean satellites around Jupiter, whose surface is composed mostly of water and is heavily irradiated with ions
from the inner magnetosphere. Most icy bodies in the outer
solar system and interstellar space lack atmospheres that can
shield impacts of energetic particles and photons, and for this
reason, we expect that hydrogen peroxide is ubiquitous. In
particular, it has been predicted to occur with abundances as
high as 5% in the icy mantles on grains in interstellar molecular clouds.2 In this environment, H2O2 can be produced
by radiolysis initiated by cosmic rays, which can penetrate
the clouds.
Previous experimental studies have shown that irradiation of pure water ice with energetic photons,3 low energy
electrons,4 and energetic ions5,6 can produce small but detectable concentrations of H2O2, which is consistent with the
observations on Europa and with the predictions for the interstellar medium. Of additional relevance in astrophysics is
that the destruction of H2O2 leads to the formation of O2, a
process that has been postulated to be a source of condensed
O2 observed on icy extraterrestrial surfaces such as Europa.7
Here we report experimental studies of the radiolysis of
solid amorphous H2O2 films in vacuum at 17 K by 50 keV
protons. A distinctive feature of our work is the combination
of several characterization techniques: infrared, visible, and
ultraviolet spectroscopies, microbalance gravimetry, and
mass spectrometry that allow us to measure quantitatively
the amount of H2O, O2, and O3 in the films, during irradiation and during subsequent warming. This combination of
techniques was possible by a specially designed port arrangement in our vacuum system and multitasking LABVIEWbased software.
EXPERIMENTAL SETUP
All experiments were performed in a stainless steel
vacuum chamber 共Fig. 1兲 on a radiation-shielded cryostat.
The base pressure of the chamber was ⬃10−10 Torr and one
to two orders of magnitude lower inside the shield. H2O2
films were grown by vapor deposition on an optically flat
gold mirror electrode of a 6 MHz quartz-crystal microbalance 共QCM兲. The areal mass Q 共mass/unit area兲 of the films
was determined by the change in the resonance frequency of
the crystal, which was measured with an Inficon IC/5 con-
a兲
Electronic mail: [email protected]
0021-9606/2006/124共10兲/104702/6/$23.00
FIG. 1. Experimental setup.
124, 104702-1
© 2006 American Institute of Physics
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104702-2
Loeffler, Teolis, and Baragiola
troller to a resolution of 0.1 Hz.8 The measured Q can be
converted to film column density ␩ 共molecules/ cm2兲 if the
film composition is known and converted to thickness if the
mass density is known.8 Since the frequency of the QCM
depends on temperature, we measured this dependence in a
blank experiment with no film deposited and used it to correct the data shown here.
Hydrogen peroxide, ⬎97% pure, was prepared by
vacuum distillation of a commercial grade 50 wt % H2O2
aqueous solution 共Fischer Scientific兲 in a glass ampoule and
vapor deposited onto the microbalance using an effusive
glass doser aimed at near-normal 共2.5°兲 incidence. The
choice of glass was dictated by the need to prevent the catalytic decomposition of H2O2 that occurs on metal surfaces.
Films were deposited at 110 K to a column density ␩
= 2.6⫻ 1018 H2O2 / cm2 共about 910 nm thick兲, chosen to be
larger than the maximum ion penetration depth 共2.0
⫻ 1018 H2O2 / cm2 or 700 nm for 50 keV H+ used in these
experiments兲9 to prevent alteration of the substrate. The relatively high deposition temperature was chosen so that impurities, formed when scattered H2O2 reacts with the chamber
walls, would not stick on the substrate, as verified by infrared spectroscopy and mass spectrometry.
After growth, the films were cooled to 17 K and irradiated at an incident angle of 9°. The proton beams were produced by an ion accelerator, mass analyzed, and scanned
uniformly over the film. A thin wire collector placed in the
ion beam path monitored the proton current and fluence. A
Dycor M200 quadrupole mass spectrometer 共MS兲 monitored
the species ejected 共sputtered兲 during irradiation or desorbed
during heating of the film.
The specular reflectance of the films on the gold mirror
was measured at an incident angle of 35° at infrared wavelengths and 22.5° at visible and UV wavelengths. The infrared spectra were recorded with a Thermo-Nicolet Nexus 670
Fourier transform infrared spectrometer at 2 cm−1 resolution,
and the UV-visible reflectance by an Ocean Optics S2000
charge-coupled device 共CCD兲 grating spectrometer in the
range ␭ = 0.2– 0.8 ␮m. The spectra were divided by the reflectance of the gold mirror substrate taken before film deposition. The ratios R共␭兲 were then converted to optical depth
units, −ln R共␭兲. Absorption band areas were derived after
subtraction of base lines that matched the continuum. In Fig.
2 we show infrared reflectance spectra taken before and after
irradiation with 1.8⫻ 1015 H+ / cm2, and in Table I we list the
frequencies of absorption band maxima. We note that irradiation leads to the appearance of water bands, the 1550 cm−1
O2 stretch band, several ozone bands, and a few unidentified
features.
RESULTS
Quantification methods
QCM and MS: Oxygen and water at high irradiation
fluences
Following irradiation with 4.7⫻ 1015 H+ / cm2, we heated
the film at a constant rate of 0.2 K / min, measuring the mass
loss due to desorption with the QCM 共Fig. 3兲 and the desorbed flux with the MS 关Fig. 4共b兲兴. The reading of the MS at
J. Chem. Phys. 124, 104702 共2006兲
FIG. 2. Infrared spectra of a solid H2O2 sample before 共1兲 and after 共2兲
irradiation to a fluence of 1.8⫻ 1015 H+ / cm2 at 50 keV. We abbreviate H2O2
as HP and H2O as W.
32 amu was due not only to O2 but also to O3 共Ref. 10兲 and
H2O2, since these molecules break up in collisions with the
walls and in the ionizer of the mass spectrometer. Based on
previous measurements with water films,11–13 we attribute
the small mass loss peak between 35 and 90 K to the desorption of O2 and O3 from the surface. Most of the oxygen
produced by radiolysis remained in the film until the temperature reached ⬃155 K, when it left abruptly, as shown in
the data of Figs. 3 and 4. We note that the desorption of O2
from oxygen-water mixtures also shows a sharp peak near
155 K 共Ref. 13兲. Integration of the peak in Fig. 4共a兲 after
subtracting a base line yields 35 ␮g / cm2. Adding this value
to that for the mass loss below 120 K, we obtain
39.3 ␮g / cm2 for the total areal mass of oxygen 共both O2 and
O3兲 produced by radiolysis of the 147 ␮g / cm2 hydrogen peroxide film.
At temperatures above that of the oxygen outburst, three
distinct changes in the sublimation rate were detected by the
QCM 共Fig. 3兲. We attribute these changes to desorption of
H2O bound to other H2O molecules, of H2O bound in the
H2O2 · 2H2O dihydrate compound, and of pure H2O2 共most
of which was beyond the penetration depth of the ions兲,
based on separate infrared measurements of water-H2O2
mixtures made in our laboratory14and in previous reports.15
Then, from the QCM data of Fig. 4, we derived the amount
of water produced during the irradiation by integrating the
rate of mass loss curve up to 176 K, after subtracting the
peak of oxygen at ⬃155 K. We find that the amount of water
produced is 59.4 ␮g / cm2 共1.99⫻ 1018 molecules/ cm2兲, after
a small correction for H2O2 desorbed between 150 and
165 K.
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104702-3
J. Chem. Phys. 124, 104702 共2006兲
Decomposition of amorphous HOOH
TABLE I. Peak positions for the absorption features present in the film before irradiation and after reaching
1.8⫻ 1015 H+ / cm2. The numbers in parentheses are the uncertainties in the least significant digits.
Sample irradiated by 1.8⫻ 1015 ions/ cm2
Virgin sample at 17 K
Position
共cm−1兲
Molecule
Position
共cm−1兲
Molecule
Position
共cm−1兲
710共12兲
886共1兲
1347共1兲
1649共6兲
H 2O 2
H 2O 2
H 2O 2
H 2O
803共10兲
888共3兲
1038共1兲
1109共2兲
H 2O
H 2O 2
O3
O3
2343共1兲
2841共2兲
3384共1兲
3706
2269共14兲
H 2O 2
1138共2兲
??
3676共3兲
2817共1兲
H 2O 2
1257共1兲
??
3630共3兲
3339共4兲
3920共15兲
4719共10兲
5994共30兲
6502共30兲
H 2O 2
H 2O 2
H 2O 2
H 2O 2
H 2O 2
1389共sh兲
1550共1兲
1655共1兲
2114共1兲
2000共4兲
H 2O 2
O2
H 2O
O3
H 2O
4010共10兲
4749共15兲
5123共9兲
6654共13兲
Some of this desorbed water originates from the decomposition of the dihydrate compound. This is the equilibrium
phase of hydrogen peroxide in the depth penetrated by the
protons, since the H2O2 concentration was much lower than
the eutectic value of 33%.16 Between ⬃168 and 176 K, after
the sublimation of pure water ends, we observe a broad
structure in the sublimation of water that we attribute to the
decomposition of the dihydrate 关Fig. 4共a兲兴. We then find that
⬃3.6⫻ 1017 H2O / cm2 共10.7 ␮g / cm2兲 are bound to H2O2 in
the dihydrate compound. Therefore, from the stoichiometry
of the compound, 1.8⫻ 1017 H2O2 / cm2 or 10.1 ␮g / cm2 exists in the irradiated ice. This value is an upper limit since
some of the unirradiated H2O2 below the proton penetration
depth will likely diffuse into the irradiated region during
warming above ⬃125 K, as seen by Loeffler and
Baragiola.14
UV-visible spectroscopy: Quantification of O3
The areal mass Q for oxygen species, 39.3 ␮g / cm2, is
the sum of the values for O2 and O3 since these molecules
FIG. 3. Mass loss on the microbalance due to sublimation of a H2O2 film
irradiated to 4.7⫻ 1015 H+ / cm2. 1 ␮g / cm2 = 1.88⫻ 1016 O2 / cm2 or 1.77
⫻ 1016 H2O2 / cm2.
Molecule
CO2
H 2O 2
H2O, H2O2
Dangling
bond
Dangling
bond
Dangling
bond
H 2O 2
H 2O 2
H 2O
H 2O
are indistinguishable with the QCM and also with the MS,
due to the efficient cracking of ozone before detection mentioned above. In Q, we neglect atomic oxygen because of its
high reactivity.12 The contribution of O3 to Q is obtained by
using the strong Hartley absorption band of ozone that appears in the UV-visible reflectance at ⬃255 nm 共O2 has neg
ligible absorption in the 225– 700 nm region we observe兲.
Figure 5共a兲 depicts the UV-visible spectral reflectance of the
film before and after irradiation; the large oscillations result
from interference between reflections from the substrate and
the film surfaces. By matching this interference pattern with
a theoretical calculation based on the Fresnel equations,17 we
derive the density, 1.6 g cm−3, for unirradiated H2O2 and the
optical constants of the film 关Fig. 5共b兲兴. Using the peak cross
section of 1.1⫻ 10−17 cm2 for the Hartley band,18 we calculate a column density of 7.4⫻ 1016 O3 / cm2, which corresponds to 5.9 ␮g / cm2. Subtracting this value from the total
Q, we obtain the areal mass of O2 to be 33.4 ␮g / cm2, cor-
FIG. 4. Sublimation of a hydrogen peroxide film irradiated to 4.7
⫻ 1015 H+ ions/ cm2 共a兲 Mass loss rate during heating at 0.2 K/min. 共b兲 Mass
spectrometer reading at mass 32. The mass 32 signal below 140 K was
below the noise level in the MS. The large rise in 共b兲 beginning at 180 K is
O2 from H2O2 decomposition off the chamber walls.
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104702-4
J. Chem. Phys. 124, 104702 共2006兲
Loeffler, Teolis, and Baragiola
FIG. 6. The production of water, dioxygen, and ozone in a film of 2.6
⫻ 1018 H2O2 / cm2 irradiated with 50 keV H+ to a fluence of 4.7
⫻ 1015 H+ / cm2. The lines in the figure are the fits described in the text. For
H2O2, we only include the H2O2 that is in the path of the ion beam, and thus
we subtract the column density corresponding to the underlying unirradiated
hydrogen peroxide 共5 ± 1.8⫻ 1017 H2O2 / cm2兲.
FIG. 5. Top: The UV-vis reflectance before 共—兲 and after 共-兲 irradiation with
50 keV H+ of a film initially of 2.6⫻ 1018 H2O2 / cm2 to a fluence of 4.7
⫻ 1015 H+ / cm2. The oscillations are a result of the interference in the film.
Bottom: absorption coefficient ␣ before 共—兲 and after 共-兲 irradiation.
responding to 6.28⫻ 1017 O2 / cm2. The results of all the
measurements discussed above are summarized in Table II.
Infrared spectroscopy and the fluence dependence
of H2O2, H2O, O2, and O3
We now normalize the area of the infrared bands of H2O
共1655 cm−1兲, O2 共1550 cm−1兲, and O3 共1037 cm−1兲 measured
during irradiation to the column densities calculated above
for high fluences to obtain the fluence dependence of the
column densities of those species. To quantify the destruction
of H2O2, we do not use the strong ⬃2800 cm−1 共3.5 ␮m兲
band of H2O2, because the absorbance of this overtone depends strongly on the molecular environment.14 Rather, we
used two other infrared absorption bands: the sharp O–O
stretch feature at 886 cm−1 that shifts less than 2 cm−1 and
does not change shape during irradiation and the 1347 cm−1
bending mode absorption. The fluence dependence of H2O2
TABLE II. The second column gives radiation yields determined from the
initial slope of the fluence dependence in Fig. 6; the numbers in the parentheses are uncertainties in the least significant digits. ␩⬁ are the column
densities and C⬁ the concentrations for the irradiated layer at 4.7
⫻ 1015 H+ / cm2.
Molecule
H 2O
O2
O3
H 2O 2
G
共molecules/ 100 eV兲
12.3 共30兲
−21 共5兲
␩⬁
共molecules cm−2兲
C⬁
1.99⫻ 1018
6.28⫻ 1017
7.4⫻ 1017
⬍1.8⫻ 1017
0.692
0.22
0.026共13兲
⬍0.062
given in Fig. 6 is an average of the similar results derived
using these two bands. The band areas are scaled to the column density of H2O2 measured with the QCM before irradiation.
In Table III, we give values for the effective band
strength A* as a function of the column density ␩:
A* = B/共k␩兲,
where B is the band area of the infrared absorption feature at
zero fluence for H2O2 and at 4.7⫻ 1015 H+ / cm2 for the other
species. Here the factor k is the ratio of the length of infrared
beam path to the film thickness: 2.14 for our geometry. We
use the term effective band strength, to distinguish it from
the usual band strength A, measured in transmission experiments, since B is not directly proportional to column density,
i.e., Beer’s Law is, in general, not followed. This is a consequence of strong interference effects that are present in infrared reflection absorption spectroscopy 共IRAS兲. To use our
values of A* in a different geometry or with another film
thickness, they must first be corrected for changes in the
interference effect.5,19
Finally, we note that there is an error in the column
densities from using the same value of A* for all fluences
since absorption band strengths are affected by the changes
in the optical properties and thickness of the films, which in
turn change with irradiation. In the case of H2O2, this error is
larger at high fluences and it is estimated to be less than 50%
from comparisons of the infrared data from the two H2O2
bands mentioned above and the QCM data.5,19
With this analysis we obtain the fluence dependence for
H2O, O2, and O3 production and H2O2 destruction shown in
Fig. 6. Table II gives saturation concentrations and radiolytic
G values, defined as the number of molecules produced 共or
destroyed for H2O2兲 per 100 eV of deposited energy, in the
limit of low fluence. The definition of G implies that the
initial production is linear with fluence. This is observed for
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104702-5
J. Chem. Phys. 124, 104702 共2006兲
Decomposition of amorphous HOOH
TABLE III. Effective band strengths for our film in the infrared obtained from irradiation at 4.7
⫻ 1015 H+ / cm2.
A* 共cm molecule−1兲
H 2O a
共1655 cm−1兲
H 2O 2
共1347 cm−1兲
H 2O 2
共886 cm−1兲
O2
共1551 cm−1兲
O3
共1037 cm−1兲
7.6⫻ 10−18
2.1⫻ 10−17
3.02⫻ 10−19
2.8⫻ 10−20
6.1⫻ 10−18
We note that this band overlapped with the 1389 cm−1 H2O2 band 共see Fig. 2兲, so this strength could be an
underestimate.
a
H2O but not for ozone where the dependence is quadratic or
for O2 for which the very weak absorption does not allow us
to obtain data at low fluences.
DISCUSSION
Radiolytic processes
Irradiation effects in solid H2O2 have not been studied
previously but there are a number of reports of radiolysis of
hydrogen peroxide in various concentrations of liquid water
irradiated by weakly exciting ␥ rays, x-ray photons, and
electrons20–23 and a study of UV photolysis on frozen
H2O2 : H2O mixtures.24 Different effects are expected for
ionic projectiles that deposit a large amount of energy per
unit path length in the material 关large linear energy transfer
共LET兲兴, such as the overlap of reaction chains in the ion track
due to a high density of electronic excitations. The initial
decomposition reaction is believed to be
X + H2O2 → OH + OH,
共1兲
where X symbolizes the projectile. In addition, one photochemical study22 suggested, based on the results from
H2 18O : H2O2 experiments, that the decomposition under
2537 Å light might result from
X + H2O2 → H2O + O,
共2兲
O + H2O2 → OH + HO2 ,
共3兲
and that, in addition, the products in 共2兲 could result from
reaction 共1兲 followed by a subsequent reaction in the water
cage:25
2OH → H2O + O.
共4兲
In the condensed phase, the OH formed can either react with
surrounding molecules or recombine as a result of the cage
effect. Experiments in aqueous solutions show that the rate
constant for H2O2 decomposition is very high and depends
on concentration.23 This behavior has been explained by the
chain reaction26
OH + H2O2 → H2O + HO2 ,
共5兲
HO2 + H2O2 → H2O + O2 + OH.
共6兲
Therefore, if a hydroxyl radical encounters a H2O2 molecule it will likely destroy it, allowing the chain to propagate. We can combine 共5兲 and 共6兲 to determine that the energy released in each cycle of the chain reaction is 2.5 eV:
OH + 2H2O2 → OH + 2H2O + O2 + 2.5 eV.
共7兲
This exothermic chain reaction 共7兲 explains the large
value measured for the destruction of H2O2, G共H2O2兲 = 21, in
comparison to H2O, G共−H2O兲 = 0.48 共at 73 K, Ref. 27兲. It is
important to note that since at most 1.5% of the mass of the
irradiated region was eroded by sputtering in our experiments, the majority of the products in the final reaction 共6兲
remain in the ice.
The radicals required to start or propagate the chain reaction can also be provided by
X + H2O → OH + H,
共8兲
H + H2O2 → H2O + OH,
共9兲
H + O2 → HO2 .
共10兲
The termination step of the chain reaction has been debated
in literature;21–23,28 it appears that it could either be
HO2 + HO2 → H2O2 + O2
共11兲
2HO2 + H2O2 → 2H2O2 + O2 .
共12兲
or
Note that reactions 共6兲, 共11兲, and 共12兲 are sources of O2,
which can then contribute to the production of O3.
We note the lack of detection of the intermediate species
HO2. This may be due to efficient destruction in the tracks of
high LET particles 共as in our experiments兲 by radical-radical
reactions such as reaction 共6兲, 共11兲, and 共12兲, or
OH + HO2 → H2O + O2 .
共13兲
Unidentified species
So far we have shown the detection of the radiation
products: H2O, O2, and O3. In addition, there are a few unidentified absorption features 共Table I兲, which disappear when
the irradiated film is warmed to temperatures in the range of
90– 110 K. These features may be due to radicals or other
volatile molecules in the film. Possible candidates might be
HO2, H2O3, and H2O – O. Of the three, HO2 would be the
most likely if one assumes that the small feature at
1138 cm−1 corresponds to the 1101 cm−1 absorption of HO2
in solid Ar. However, there are no features within 100 cm−1
of the strongest HO2 absorption at 1389 cm−1 or its O–H
stretch at 3419 cm−1 共Ref. 29兲. The largest unidentified absorption shown here is at 1257 cm−1; this absorption was
also produced by laser irradiation 共266 nm兲 of a H2O : O3
mixture at 17 K30 and identified previously as the ␯6 mode of
H2O2. However, we do not use this assignment since we do
not observe the 1257 cm−1 absorption in dilute mixtures of
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104702-6
Loeffler, Teolis, and Baragiola
water and H2O2. Furthermore, we also dismiss H2O3 or
H2O – O since the reported infrared spectra of these species
in rare gas matrices do not show absorptions near these two
unidentified features, after accounting for reasonable
共⬃30 cm−1兲 matrix-related frequency shifts.
CONCLUSIONS
We have shown that the combination of several experimental techniques allows a fairly complete quantification of
the radiation products of hydrogen peroxide. We find that
understanding the high initial radiation yields requires considering a chemical chain reaction. At high fluences, the radiation products that we can detect are water, molecular oxygen, and ozone; but we cannot exclude OH, which we cannot
detect because of interference of its infrared bands with those
of water and hydrogen peroxide.
ACKNOWLEDGMENTS
This research was supported by the NASA Cosmochemistry Program. We thank W. H. Shoup for constructing the
glass manifold used in this article. One of the authors
共M.J.L.兲 thanks the Virginia Space Grant Consortium for a
fellowship.
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