Available online at www.sciencedirect.com
NIM B
Beam Interactions
with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 3057–3062
www.elsevier.com/locate/nimb
Radiation effects in ice: New results
R.A. Baragiola *, M. Famá, M.J. Loeffler, U. Raut, J. Shi
University of Virginia, Laboratory for Atomic and Surface Physics, 351 McCormick Road, Charlottesville, VA 22904, USA
Available online 25 March 2008
Abstract
Studies of radiation effects in ice are motivated by intrinsic interest and by applications in astronomy. Here we report on new and
recent results on radiation effects induced by energetic ions in ice: amorphization of crystalline ice, compaction of microporous amorphous ice, electrostatic charging and dielectric breakdown and correlated structural/chemical changes in the irradiation of water–ammonia ices.
Ó 2008 Elsevier B.V. All rights reserved.
PACS: 61.80.Jh; 79.20.m; 96.35.Er; 96.35.Hv
Keywords: Ice; Radiolysis; Compaction; Amorphization; Charging; Enceladus
1. Introduction
The effects of the interaction of radiation with solid
water (ice) are of great importance in icy astronomical
bodies such as icy satellites, planetary ring particles, comets, trans-Neptunian objects and ice-coated interstellar dust
grains. Since those icy bodies lack substantial atmospheres,
energetic charged particles and photons can easily penetrate and impact the surface [1]. The energetic particles
include cosmic rays, ions emitted by stars (stellar winds,
flares, coronal mass ejection) and ions and electrons present in planetary magnetospheres. While many radiation
effects in ices have been identified in the laboratory, astronomical observations reveal a few: the generation of faint
atmospheres around icy satellites by sputtering, the synthesis of hydrogen peroxide and possibly of O2 and ozone, and
the charging of icy particles in Saturn’s rings.
The topic of radiation effects in ice is related to the
vastly larger topic of radiolysis of liquid water, where
research is motivated by the need to understand radiation
effects in living organisms and in nuclear waste in contact
with ground water. Liquid water and amorphous ice have
*
Corresponding author. Tel.: +1 434 982 2907.
E-mail address: [email protected] (R.A. Baragiola).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.nimb.2008.03.186
very similar short-range tetrahedral structure but significantly different radiation effects. This is because of the
much greater ability of molecules in the liquid to relax in
the presence of radiation products, such as electrons, ions
and radical species. An example of such relaxation is the
decrease of the ionization potential going from the gas to
the condensed phase because the ion solvates into a lower
energy state by attracting nearby molecules.
Here we are concerned with radiation effects caused by
ions. They include radiolysis (molecular decomposition
and synthesis), defect creation, sputtering of neutrals and
ions, electron emission and structural changes. Although
much of the effects are due to the low energy secondary
electrons generated in the medium by direct ionizations,
there are significant differences in effects caused by ions
and incident low-energy electrons. These differences originate in electron capture and loss and internal excitations
in ions that do not exist for electrons, differences in cross
sections for primary collisions, in the density of deposited
energy and in the proximity to the surface (low-energy electrons penetrate only a few monolayers). Additional differences exist with photolysis, which is radiation chemistry
induced by sufficiently energetic UV photons. In the case
of fast heavy ions, the density of ionization is so high that
prompt reactions between radiation products can occur in
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the projectile track, which is not possible in excitations by
energetic electrons or photons [2]. Thus, the many studies
of surface processes induced by low-energy electrons, very
interesting but too many to list here, are insufficient to
reveal the complexity of ion-induced effects. Compared
with radiation effects in oxides or in alkali halides, ion irradiation-induced defects in ice is an unexplored subject.
This paper is not intended to be a review. Rather it presents a brief overview of radiolysis (ion-induced radiation
chemistry) and new experimental results of diverse radiation effects. The reader is referred to recent reviews on sputtering of ice [3], astronomical applications [1] and inelastic
ion-surface collisions [4]. In the last topic, it is important to
cite the additional recent research by Souda et al. [5].
2. Microscopic processes
2.1. Energy deposition
An important projectile parameter that determines the
effect of irradiation in a small volume of matter is the
amount of energy deposited per unit path length, called
the linear energy transfer or LET. It differs from the projectile energy loss per path length, or stopping power, due to
projectile excitation and energy escape, particularly near
surfaces. For fast ions, the energy is deposited mostly in
ionizations and excitations, which are comparable. At
lower energies, knock-on (elastic) collisions with target
nuclei become important. The relative importance of radiation effects in ice caused by elastic and inelastic collisions
has been recently determined for the case of sputtering [6].
2.2. The physico-chemical stage
The inelastic energy deposited by the projectiles in the
form of ionizations and excitations can decay into nonradiative and radiative channels. The latter are negligible
for water. The non-radiative channels are those where the
inelastic energy transferred is coupled to motion of the
water molecule as a whole or to dissociation fragments.
Dissociation occurs by electronic excitation to states
H2O* where the reduced intramolecular screening leads to
repulsion between fragments. The main dissociation channels in the gas phase are [7]
X þ H2 O ! H2 O ! H þ OH
H2 þ O
ð 90%Þ
ð 10%Þ
where X denotes an ionizing particle.
These dissociations can occur both by the incident ions
and by energetic secondary electrons produced in ionization collisions. In addition, there are less probable channels
such as 2H + O and the bipolar dissociation H+ + OH. In
dissociation, the lighter particle takes the largest part of the
kinetic energy. The molecular fragments will collide with
the surrounding molecule (the cage), lose their energy
and reform the molecule. In liquid water, thermal fluctua-
tions soften the cage making it easier for the fragments to
escape recombination into H2O. Thus, destruction of H2O
molecules is more difficult in ice: 100 eV of deposited
energy destroy, on the average, 0.5 molecules in ice at
73 K and 4.5 in water at 20 °C [8]. In addition, the cage
effect in the solid can reduce the probability of the
H2 + O channel relative to the H + OH channel shown
above since H2 will find it harder to leave the dissociation
site than H.
Ionization collisions produce electrons, H2O+ and ionized fragments. The electrons slow down quickly; when
their energy is smaller than 6 eV, they cannot produce
further excitations or ionizations, becoming sub-excitation
electrons, but they may still break up a water molecule by
dissociative recombination and attachment.
After this prompt physical stage, the energy evolves
more slowly, governed by typical vibrational times, of the
order of 0.1–1 ps. The electrons cannot recombine directly
with the ions efficiently because their kinetic energy cannot
be easily absorbed by the medium. They then degrade in
energy slowly by excitations of intramolecular vibrations
and phonons. In times >1 ps they slow down sufficiently
(though not actually thermalized) that they can efficiently
recombine with ions. The distance to which the electrons
move from their original atom is extremely difficult to calculate since it depends on elastic scattering by atomic cores,
vibrational excitations and the dynamic electric fields produced by the ion track. The latter force makes the evolution of secondary electrons highly dependent on LET. In
particular, the track field destroys the tendency of isotropy
produced by elastic scattering. The use of external electric
fields can remove electrons from near the track and into
vacuum, thereby affecting radiation effects. This is particularly prominent in solid argon [9].
In the chemical stage, ions and neutral products react in
chemical reactions. Their description requires reaction
rates beyond those known in the gas phase since the presence of other molecules can enable reactions by absorbing
momentum or decreasing activation barriers. Common
examples of chemical reactions [8] are incomplete because
they neglect excited states, many ion-molecule reactions
and processes in dense tracks.
3. Final products: radicals and stable molecules
The observation of trapped radicals in radiolyzed ice has
proven to be very hard, particularly because of the low
concentration of those species, a few percent or lower. Dissociation of water molecules in ice often leads to immediate
reformation of the molecules by the cage effect, since the
dissociation fragments suffer collisions with surrounding
molecules and cannot escape. The consequence is a substantially smaller yield of radiation products in solids as
compared to the gas phase. Up till recently we only had
fragmentary information on ice radiolysis [8,10]. A breakthrough is the work of Laffon et al. [11] using X-ray
absorption near the O–1s absorption edge to investigate
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3057–3062
the radiolytic products (containing oxygen) produced by
wide-band X-ray irradiation at 20 K. This type of excitation is mediated by photoelectrons inside the material
and therefore leads to lower LET than for projectile ions.
Nevertheless, the simultaneous observation of various radiation products and their temperature dependence provides
much new information. Keeping in mind that the technique
does not detect H, H2, ions or trapped electrons, the two
most important products are H2O2 and OH. Their concentration decreases steadily with annealing temperature and
OH disappears above 90 K. The less abundant O2 and
HO2 species have a similar concentration that peaks at
40 K and then also decays with temperature up to the maximum attainable, 150 K. On first order, amorphous and
crystalline phases of ice behaved similarly. This contrasts
with studies of c-irradiation of ice at 77 K that found OH
is the main product in crystalline ice, while comparable
amounts of OH and HO2 are formed in amorphous ice [12].
The main difference that is expected for the case of ion
impact is a higher production of molecular products, facilitated by the high density of radicals in the ion tracks. H2
and O2 molecules have been observed during sputtering
with a wide range of ions. The reader is referred to a recent
paper by Teolis et al. that discusses detailed experiments on
the sputtering of oxygen from ice [13] and the enhancement
of O2 retention (which enables the synthesis of ozone)
resulting from simultaneous irradiation and water deposition [14].
Remote observation of long-lived radicals and stable
molecular products of radiation in ice can be studied by
infrared spectroscopy and compared with laboratory
results. The only clear example of radiation product in
ice is hydrogen peroxide, which was reported in the Jovian
satellite Europa by Carlson et al. [15]. The chemical state of
this molecule in Europa’s ice was determined through laboratory studies [16].
a volume along the path of the ion is transiently liquefied
and re-freezes extremely fast inhibiting the diffusion necessary to achieve the lower energy state of a crystalline phase.
In addition to amorphization of crystalline ice at low
temperatures, vapor-deposited amorphous ice can be crystallized at 110 K by high-energy ions [20] or electrons [21].
In this case, the volume transiently heated by the energy
deposited by the projectile must cool sufficiently slow to
allow relaxation of the structure to a crystalline state.
5. Compaction of microporous ice
Amorphous ice grown from the vapor phase at low temperatures (<100 K) is microporous, with a high internal
surface area (several hundred m2/g) that results in a high
gas adsorption capacity. The micropores, less than a nm
wide [22], have a characteristic infrared absorption signature, weak narrow bands near 2.7 lm due to O–H stretch
vibration of dangling bonds of incompletely coordinated
molecules on the pore surfaces. The stability of micropores
under radiation is important to understand the gas adsorption capacity of interstellar and cometary ices. Two groups
of studies of compaction of microporous amorphous ice
have appeared recently [23,24]. Our studies were done at
40 K and the porosity was characterized by ultraviolet-visible spectroscopy, infrared spectroscopy and methane
adsorption/desorption. These three techniques provide different and complementary views of the structural changes
in ice resulting from irradiation. Below we give a brief summary of our results; more details can be found in [24].
Fig. 1 shows the decrease of the dangling O–H absorption bands when irradiating a 1018 H2O/cm2 ice film with
100 keV Ar+ ions. The bands at 3720 and 3696 cm1 correspond to the O–H stretch absorption in doubly- and triplycoordinated water molecules on the walls of the pores. The
integrated absorbance of these bands is indicative of the
4. Phase changes
2.67
Optical Depth
Ice grown from the vapor phase can exist in different
phases [4]. At temperatures above about 190 K is in the
hexagonal crystallographic phase, which is the standard
phase on Earth at normal pressures and temperatures.
Condensation below 190 K but above 135 K, leads to
cubic crystalline ice, which transforms into hexagonal ice
if warmed above 160–200 K. Films condensed on substrates colder than 130 K are amorphous, i.e. lacking
long-range crystalline order while keeping local tetrahedral
ordering. The amorphous phase is metastable and converts
irreversibly to cubic ice at a rate that depends on temperature [4]. Several studies have demonstrated that energetic
ions readily amorphize crystalline ice at low temperatures
[17–19]. The fraction of crystalline ice decays nearly exponentially with fluence. For 100 keV Ar on ice at 70 K,
Fc0.5 2.5 1012 ions/cm2, a very small fluence, is the
value at which the crystalline fraction drops to 50%. The
basic idea to understand amorphization is to consider that
3059
2.68
2.69
2.70
2.71
2.72
2.73
0.01
3740
3720
3700
3680
Wavenumber (cm-1)
Fig. 1. Decrease in the strength of the dangling O–H bands in an ice film
(initially 440 nm thick) during irradiation with 100 keV Ar+ at 40 K at
fluences (from top to bottom): 0, 0.18, 0.62, 2.16 and 8.22 in units of
1013 ions/cm2. From Raut et al. [24].
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R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3057–3062
Normalized band area, porosity
1.0
0.8
0.6
Porosity
DB
0.4
0.2
0.0
0.01
0.1
1
10
Irradiation Fluence (1013 ions/cm2 )
Fig. 2. Fluence dependence of ice compaction at 40 K measured through
the reduction of the normalized porosity (s) and the normalized total area
of the dangling bond (DB) adsorptions ( ). From Raut et al. [24].
total pore surface area. As shown in Fig. 1, both dangling
bond bands disappear at the same rate during ion irradiation. Fig. 2 shows the change in porosity and total band
area of the dangling bond absorption due to irradiation.
We note that porosity was originally calculated as 1 q/
qc from measurements of the density q and the published
value of qc = 0.94 g/cm3 for compact ice [25]. Thus, in
terms of density, irradiation compacts the ice from
q = 0.69 to 0.92 g/cm3. We proposed that the transient
local heating around the path of the ions and the collision
cascade started by recoiling water molecules activates the
decrease of the internal surface energy of porous ice, as
seen in the case of thermally annealed ices. The fluence
required to drop the intensity of the dangling bonds to
50% is 3 1012 ions/cm2, close to the value for crystallization given above (Section 4). However, the decay of
porosity is much slower, requiring a fluence of
1.7 1013 ions/cm2 to decay by 50%. Thus irradiation
reduces the total surface area faster than the pore volume.
This might occur by coalescence of small pores into larger
ones, preferential destruction of smaller pores or smoothing of the roughness of the pore walls.
The results imply that cosmic rays can cause compaction
in the icy mantles of the interstellar grains, which can
explain the absence of dangling bond features in the infrared spectrum of molecular clouds.
6. Electrostatic charging and dielectric breakdown
Electrostatic charging of ice is important in planetary
rings where the particles charge up by being immersed in
a plasma [26]. These particles are so small that electrostatic
forces are comparable to gravitation; the interplay between
those forces creates structures ‘‘spokes” in the ring of Saturn [27]. Since we have not landed on any icy object in
space we do not have direct information on surface potentials. Of interest is to understand how electrostatic charging
occurs and how charges migrate in the ice depending on
environmental conditions.
The process of charge trapping starts with the creation
of electron-ion pairs in ionization collisions of the projectile and secondary electrons with water molecules. Electrons that do not immediately recombine are much more
mobile than ions. They can trap, or escape from the surface
resulting in secondary electron emission. In the laboratory
we use thin ice films on a substrate and thus another possibility is migration to the substrate. However, electron
emission and migration to the substrate leave excess positive charges in the film. The ions are relatively immobile
and can trap at defect levels (energy states located between
the valence and conduction bands) resulting in an effective
positive polarization of the insulator. Since water ice is an
insulator (band gap 11 eV and high resistivity), one
expects electrostatic charging effects when it is irradiated
by charged particles. The trapped charges, together with
their images in the gold substrate, produce an electric field
that can affect the energy and trajectory of ejected species
such as secondary electrons and ions. This electric field is
limited to the value at which dielectric breakdown occurs.
To study electrostatic charging of ice we measured the
energies of ejected ions during irradiation of ice films with
100 keV Ar+ for different film thicknesses. The total number of ions emitted is orders of magnitude smaller than that
of secondary electrons and hence they give a negligible contribution to charging. We measured the mass and energy
distribution of positive secondary ions during irradiation
with 100 keV Ar+ at 80 K using a Hiden EQS-300
energy-selected quadrupole mass spectrometer. Water dissociation products such as H+, H2+, O+ and OH+ are present in the spectrum as well as new species such as O2+ and
H2O2+ formed by radiolysis. The most intense peak,
together with H+, corresponds to H3O+. The relative yields
of (H2O)nH+ cluster ions to the protonated water molecule
(Yn/Y1) follow an exponential decay function in agreement
with previous reports [28]. Since the H3O+ ions are intense
and have lower energy than the protons, we chose them for
analysis.
Fig. 3 gives the dependence of the peak kinetic energy of
this ion on film thickness for irradiation with 100 keV Ar+
at 45° incidence on a sample that was initially 8000 Å. Successive data points in the figure were taken as the film was
eroded by the ions (down to 3000 Å), which took a fluence
of 2.4 1016 ions/cm2. The data points between 1500 and
3000 Å were taken with fresh ice samples. The inset shows
an example of the energy distribution measured with an
energy resolution of 0.05 eV. The figure also shows a
Monte Carlo simulation of the distribution of implanted
Ar under the same conditions. For thicknesses below the
maximum range of the projectiles, the kinetic energy of
the emitted ions is relatively constant. For small thicknesses, the ionization produced by the ions and recoils provide
an easy path for excess charges to migrate to the metal
substrate. For film thicknesses larger than the maximum
ion range, the secondary ion energy rises sharply up to
R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3057–3062
In addition to these experiments, we also performed new
experiments on 1:2 ammonia–water mixed ices at 20 K,
where we observe an additional effect: the formation of
dangling bonds in the sample, as evidenced by their infrared absorption features (Fig. 4) which, as mentioned above,
Counts Per Second
2.7
400
2.72
2.74
2.76
0.10
14
0.08
20
40
60
Energy (eV)
80
2
11 x 10 ions/cm
3
0.8
0
200
0
0
100
0.06
1000 2000 3000 4000 5000 6000 7000 8000 9000
0.04
Thickness (Å)
7. Radiation effects in ice mixtures
There is substantial literature on radiation chemistry of
mixtures of condensed gases with water, for application in
astrochemistry. We have recently studied the irradiation of
condensed mixtures of ammonia and water to simulate
processes that may occur in the outer solar system. In particular, we were interested in learning if ion irradiation of
ammonia–water ice could explain some extraordinary
observations of Saturn’s icy moon Enceladus by the Cassini spacecraft. The instruments on board observed first
unexpected amount of N+ near the orbit of this satellite
[29] and then unexpected emission of plumes of water
vapor, nitrogen and icy particles from the southern polar
region of Enceladus [30]. Our experiments using 100 keV
proton irradiation at 70 K [31] showed that warming radiolyzed ammonia–water ice caused the emission of nitrogen,
water and possibly icy particles, manifesting as pressure
spikes. The ice particles may result from the exfoliation
of blisters, which were observed in the optical microscope.
The blisters are thought to result from the accumulation of
bubbles of hydrogen and nitrogen molecules produced by
the radiolytic decomposition of ammonia.
DB
Optical Depth
0.00
3720
3700
3680
4.28
3660
4.29
3640
4.30
3620
3600
4.31
0.205
0.200
N2
0.195
2340
2335
2330
2325
2320
2315
Wavenumber (cm-1)
Fig. 4. Infrared spectra of a 1:2 ammonia–water mixed ice during
irradiation at 20 K with 100 keV H+ ions with fluence as a parameter.
(Top) region of the dangling bond O–H absorptions; (bottom) region of
the N2 absorption. The shift of the dangling bond with respect to that of
Fig. 1 for pure ice is due to gas attached to the dangling bond.
1.2
N2
0.3
1.0
-1
hundreds of eVs. We interpret the change in the kinetic
energy of the secondary ions as a direct evidence of the
positive electrostatic potential at the surface of the ice, created by trapped charges. Above some thickness the secondary ion energy shows an erratic behavior, indicative of
dielectric breakdown of the charged ice, occurring at
1.8 MV/cm in Fig. 3. Dielectric breakdown depends on
ion flux, ion energy (which affects the penetration depth),
ion type, substrate conditions and temperature of ice
(which affects the mobility of the unbalanced charges inside
ice). We are currently using the secondary ion method to
study the temperature dependence of charge trapping.
0.02
N2 Band Area (cm )
Fig. 3. Peak H3O+ energy versus ice film thickness at 80 K during
irradiation with 100 keV Ar+ at 45°. The solid and dashed lines are for
different films. The shaded area represents the ion range of 100 KeV Ar+
ions under the same conditions, also in a log vertical scale. The inset is a
H3O+ ion energy distribution for a 4200 Å fresh film grown at 80 K.
-1
0
Wavelength (m)
600
DB Band Area (cm )
Peak Energy (eV)
100
10
3061
DB
0.8
0.2
0.6
0.4
0.1
0.2
0.0
0.0
0
2
4
6
8
15
10
12
14
2
Fluence (10 ions/cm )
Fig. 5. Fluence dependence of the formation of N2 and dangling bond
(DB) absorptions as indicated by infrared spectroscopy in a 1:2 ammonia–
water mixed ice during irradiation with 100 keV H+ ions at 20 K.
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R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3057–3062
indicate the presence of pores or cavities with large internal
surface areas. The shift of the dangling bond with respect
to that of pure ice (Fig. 1) implies that the cavities contain
gas (likely N2) attached to the internal surface. Furthermore, we can follow the formation of N2 by measuring
the band area of the infrared absorption at 2327 cm1,
which is forbidden for the free molecule, but can be active
for N2 in a perturbing environment [32]. Fig. 5 shows that
the formation of N2 correlates to the formation of dangling
bonds, which also suggests that at these low temperatures
the trapped gas serves to stabilize the irradiation-induced
cavities. The effect of trapped N2 would explain the discrepancy between experiments with pure water, where we
have seen that the dangling bonds are not produced but
rather easily destroyed by ion irradiation.
8. Summary
We have given several examples of the effects of irradiation, which include structural changes (amorphization, creation and destruction of pores and cavities), electrostatic
charging and dielectric breakdown, and chemical alterations in mixed ammonia–water films. The underlying physics behind all these processes is the coupling of electronic
excitation energy into atomic and molecular motion
through repulsive potentials, for which the theoretical
understanding is still rather limited. Efforts in this area will
likely prove fruitful in many applications in astrochemistry
and radiation biology.
Acknowledgement
This material is based upon work supported by the National Science Foundation under Grant No. 0506565.
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