A&A 397, 7-13 (2003)
DOI: 10.1051/0004-6361:20021473
G. Leto - G. A. Baratta
INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
Received 27 June 2002 / Accepted 7 October 2002
Abstract
Water ice is a ubiquitous material in space, on the surface
of planets and their moons, in comets and interstellar dust grains.
The structure of water ice strongly depends on
the deposition temperature. It can be amorphous if deposited at
low temperature, i.e. 16 K, or crystalline if the deposition
temperature is higher than 140 K.
In this paper we report the first experimental study of
the structural effects induced by Lyman-
UV photons in
crystalline water ice carried out by in situ infrared
spectroscopy. The effects induced by Lyman-
photolysis are
compared with those induced by ion irradiation carried out with the same
experimental apparatus.
We found that, as already observed in the case of fast ions, also UV
Lyman-
photons are able to fully amorphize the crystalline water ice
structure after a dose of
few eV per molecule.
This study has important astrophysical implications, in particular
because photons and fast ions are both present in space, they can induce
variations in the profile of the 3
m band of crystalline water ice.
Thus these effects have to be considered when one attempt to deduce information on the physical status
(e.g. temperature history) of the responsible dust from
the profile of the observed ice band.
Key words: astrochemistry - molecular processes - methods: laboratory - techniques: spectroscopic - ISM: molecules
Different forms of water ice exist depending on the deposition rate and
substrate temperature. If the deposition of water vapor is made at
temperatures between 140 and 150 K, crystalline cubic ice (Ic) is
produced. Crystalline cubic ice is stable under cooling, but if heated,
to about 200 K, it
is converted to crystalline hexagonal ice (Ih). When vapor is deposited at
temperatures below 100 K the resulting ice is amorphous (P/ASW,
Porous/Amorphous Solid Water). It is
possible to obtain several forms of amorphous water ice
(Boutron & Alben 1975; Hagen et al. 1983; Jenniskens et al. 1995); some of the sensible parameters are:
substrate temperature and nature, deposition rate,
deposition flow angle (Westley et al. 1998; Devlin 2001). P/ASW once formed is
unstable against temperature cycling: as T increases it is converted to
crystalline ice on a time scale which depends exponentially on T(Schmitt et al. 1989); at 150 K it is shorter than 10 s, at 20 K it is longer
than the age of the Universe (Schmitt et al. 1989).
Water ice has a well-known infrared spectrum that exhibits bands due to O-H stretching and
bending modes at 3 m and 6
m respectively, the libration band at 12
m, a
band at 4.5
m attributed to the combination of bending and libration,
and a lattice band at 45
m. Because of the asymmetry at the
surface or inside pores, small bands arise from molecules that do not saturate their possible bonds, so H and O dangling bonds appear in the 2.7-2.8
m range.
The shapes of the infrared absorption features of water ice strongly depend on the structure of the ice. As an example, Fig. 1, shows the 3-micron band (the most intense in the mid IR) for cubic ice obtained by depositing H2O vapor at 150 K and cooling to 16 K, and for an amorphous ice sample of the same thickness directly deposited at 16 K. Their profiles (peak position, shape, strength) are very different. Thus IR spectroscopy in the 3-micron region is a good technique to investigate structural changes of water ice.
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Figure 1: The 3-micron OH band in the case of crystalline and amorphous water ice. |
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If only equilibrium thermal processes are considered, the observation of
infrared features of P/ASW
ice in several astronomical objects would imply that the ice is formed directly at low T,
and it has never experienced temperature increases since the epoch
of its formation. However
experiments carried out on crystalline (cubic) ice (Baratta et al. 1991; Strazzulla et al. 1992; Moore et al. 1992)
showed that the transition
from crystalline to amorphous water ice can be induced by keV-MeV ion
irradiation at temperatures
between 10 and 100 K. It was found that at low temperatures (T < 55 K) the process is very efficient (few eV/molec are
sufficient to amorphize the ice) and at T < 20 K the profile of the
3-micron
band of ion-irradiated crystalline water ice is close to the band
of amorphous ice deposited at the same temperature. At higher
temperatures (T > 55 K)
the fraction of amorphous water ice
obtained after
irradiation is reduced and the crystalline water ice becomes more resistant
(greater doses are
needed to disorder the structure). Similar results (Lepault et al. 1983; Heide 1984; Dubochet & Lepault 1984) have
been found for high
energy electron irradiation of thin layers of ice crystals below 70 K.
Another process for the transformation from cubic to amorphous ice was
reported by Kouchi & Kuroda (1990). Photons from a deuterium lamp
(<11.3 eV) converted
cubic ice into amorphous ice at a temperature below 70 K as evidenced by
in situ reflection electron
diffraction. However that result has been questioned (Jenniskens et al. 1995) because the reported fluence (
photons/cm2) corresponds to a
dose of
eV/molec that is a
strikingly low value.
In this paper we describe an experimental study of the effects induced
by UV photolysis in crystalline water ice (Ic).
For the first time, a quantitative
description of the structural effects induced by UV Ly-
photons in
water-ice is given; the effects are compared with
those induced by ion irradiation in the same experimental setup.
In Sect. 2 we describe our experimental apparatus and procedures,
in Sect. 3 a description of the full set of experiments
and irradiation parameters is reported. The results are presented in
Sect. 4 and discussed in Sect. 5.
Summary and concluding remarks are reported in
Sect. 6.
The experimental work reported in this paper has been carried out in our
laboratory at Catania Astrophysical Observatory.
The experimental setup allows in situ IR spectroscopy
of frozen gases and refractory materials irradiated with fast ions and
Lyman-
photons.
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Figure 2: Schematic view of the experimental apparatus used for in situ IR spectroscopy of ion and UV photolyzed frozen gases. |
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The in situ IR spectroscopy is performed in a stainless steel vacuum
chamber (see Fig. 2) facing an FTIR spectrometer (Bruker Equinox 55).
Inside the vacuum chamber the pressure is kept below
10-7 mbar. An IR transparent substrate (crystalline silicon) is placed in
thermal contact with a cold finger whose temperature can be varied between 10 K and 300 K.
A needle valve is used to admit pre-prepared gases (or mixtures) into the
chamber where they freeze on the substrate. The thickness of the ice
film can
be controlled, during deposition, by looking at the interference pattern
(intensity versus time) given by a He-Ne laser beam reflected at an angle of
45
both by the vacuum-film and film-substrate interfaces
(see Baratta & Palumbo 1998, for further details on the technique used to measure
thickness).
The substrate holder forms a 45
angle with both the
IR beam and the ion beam and it is orthogonal to the UV beam. In this
way, spectra can be easily taken in situ, even during irradiation with ions
and UV photolysis, without tilting the sample. For this purpose the IR spectrometer
is positioned (by a movable optical bench) so that the IR beam is
transmitted by the substrate through a hole in the sample holder.
In the experiments here reported the structural evolution under irradiation
of deposited water ices is followed by monitoring the 3
m OH stretching
band. The spectra reported in the next sections have been acquired at oblique
incidence (45
)
with the electric vector perpendicular (s polarized)
to the
plane of incidence. The plane of incidence is the plane of the paper in
Fig. 2; this plane contains the electric vector of p polarized light while the plane of s polarization is perpendicular to the paper. As already shown (see Baratta et al. 2000) spectra taken at oblique incidence in s polarization are equivalent
to unpolarized spectra at normal incidence.
The ion implanter is a 30 kV accelerator from which ions with energy up
to 30 keV (60 keV for double ionizations) can be obtained. The ion beam
produces a cm2 spot on the target and current ranges between
100 nA/cm2 and tens of
A/cm2.
A hydrogen microwave discharge resonance lamp (Opthos Instruments),
capable of producing 10.2 eV photons, is interfaced to the vacuum
chamber through a MgF2 window. An
aluminum light collector is
placed at the end of the lamp in order to increase the number of UV photons that reach the sample.
The UV flux given by a microwave powered discharge lamp strongly depends on the operating
conditions and can vary over one order
of magnitude. We have developed a monitoring
system of the UV flux in order to have a reliable measure of the
total number of UV photons that reaches the sample during
photolysis. This was obtained by measuring the current
given by
photoelectric effect on a platinum wire placed at the end of
the aluminum light collector. The total charge measured
by the wire detector is proportional to the total UV photon number per
square centimeter that reached the sample (fluence). The calibration constant
of the wire was determined by measuring the O2
O3 conversion
rate in a UV photolyzed O2 ice film (see Gerakines et al. 1996; Gerakines et al. 2000) (see Baratta et al. 2002, for further details).
Photolysis of frozen O2 slab with Lyman-
photons produces
ozone (O3) with a yield of 1.92 [molecule/Ly-
photon]
(Groth 1937; Gerakines et al. 2000). In order to ensure that almost all the incoming Ly-
photons are
absorbed in the slab we deposit a 1
m thick O2 layer at 16 K.
The ozone production was monitored
via the 1040 cm-1 band assuming that the band strength is
[cm/molecule] as measured in the gas phase 1985.
The number of impinging Ly-
photons is evaluated through the
amount of O3 produced during the initial linear phase.
The ratio between the photon fluence measured with the Ozone method and
the integrated
wire current, has been verified to be constant within the whole range of
flux provided by the lamp
(
photons cm-2 s-1).
One of the objectives of this work was to verify if Ly-
photons were
able to induce amorphization of Ic H2O at
low temperature. This was achieved by looking at the
3
m band profile evolution during Ly-
photolysis; the band profile evolution was compared with that observed
for fast ions (H+,
He+, Ar++) irradiation performed in the same experimental
setup.
In the case of ion irradiation the dose has been calculated from the
knowledge of the stopping power (eV cm-2 molecule-1, computed
by using the TRIM Montecarlo simulation program) and of the integrated ion
flux (ions cm-2), which is directly related to the integrated ion current.
In the case of Ly-
photons the energy released per molecule can be
evaluated once the total number of absorbed photons is known.
The number of
Ly-
photons absorbed by the irradiated sample depends on the
thickness of the sample and on its absorption coefficient.
The absorption law can be described by an
exponential of the type
e-ad, where a is the absorption coefficient and d is the thickness. We have evaluated the
absorption coefficient for Ly-
photons in water
ice making different experiments in which we used samples with different thickness. We have found an absorption coefficient for water
ice of 28
m -1 (the full description of this method can be
found in Baratta et al. 2002).
The imaginary part of the refractive index of water ice at
m
is
(see Warren 1984). By using the
relation
we obtain
m-1,
that is close to our measure.
In our experiments the ice film thickness was about 500 Å so that
the ice sample is fully processed by photons through the whole
thickness.
The dose has been calculated from the knowledge of the integrated flux
(photons cm-2) of impinging photons measured by the wire detector during the
experiment and of the percentage of impinging UV photons absorbed in the sample
(estimated by using the measured absorption coefficient).
The ratio between the energy absorbed by the ice in eV/cm2 (number of
absorbed UV photons/cm2 multiplied by 10.2 eV) and the number of
water molecules/cm2 gives the irradiation dose in eV/molecule.
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Figure 3: Evolution of the crystalline water ice band under UV photolysis. |
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Figure 3 shows the
evolution of the 3-micron band of a water ice Ic sample at 16 K
photolyzed by 10.2 eV Lyman-
photons. The band profile
at the end of photolysis is close to that of an amorphous sample having
the same thickness. Few eV/molecule
(
5) are enough to complete the process.
Figure 4 shows four spectra: as deposited amorphous
ice (16 K), Ic ice sample (16 K), and the spectra of the same samples but
after UV photolysis. The two last spectra are so similar to be
hardly distinguishable in the figure.
This result can be interpreted as an indication that the Ic sample was
amorphized by Lyman-
UV photolysis. From
Fig. 4 it is
also evident that irradiated ices are slightly different (because of
different chemical
and/or lattice structure) compared to the as deposited amorphous
water ice.
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Figure 4: Spectra of a crystalline and of an amorphous water ice sample at 16 K; the spectra of the same samples after UV photolysis are also shown. The photolyzed spectra are hardly distinguishable. |
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It has been already shown that Ic water ice is efficiently amorphized by fast ion irradiation (Baratta et al. 1991; Strazzulla et al. 1992; Moore et al. 1992) here we confirm the results previously reported with new experiments conducted using different ions/energy values. Figures 5 and 6 show the evolution of the 3-micron band of Ic ice in the case of irradiation with 30 keV H+and 60 KeV Ar++ions.
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Figure 5: Evolution of the crystalline water ice band under 30 keV H+ ions irradiation. |
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In both cases there is a clear profile evolution towards the amorphous shape with increasing irradiation dose. After a few eV/molecule the profile modification is almost complete. As already observed for UV photolysis, the 3-micron band profiles of ion irradiated Ic and amorphous ices are the very same.
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Figure 6: Crystalline H2O irradiated by 60 keV Ar++. |
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Figure 7:
Evolution of the 3-micron band peak position under Ion
irradiation and UV photolysis. Diamonds, triangles, and plus symbols are used in the case of 30 keV H+, 60 keV Ar++, and UV Ly-![]() |
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The evolution of the peak position of the 3 m band under
irradiation is shown in Fig. 7 for 30 keV H+, 60 keV Ar++and
UV Ly-
photons. A clear trend of the peak position to
move towards a region at about 3258 cm-1 is observed during both
ion irradiation and UV photolysis; this regardless of the fact that the
starting point is an amorphous or a crystalline ice. In fact, with the only exeption of 30 keV H+on ASW, all peaks ended within 5 cm-1 (one standard deviation) from the mean peak position value (3258 cm-1).
We interpret the observed common trends as an evidence that Ic ice
amorphize under UV photolysis or ion irradiation.
Baragiola et al. (2002) have recently reported a decrease in the porosity of amorphous
water ice under ion irradiation at temperatures below 100 K. This
finding could be in agreement with the slight 3-micron band profile
modification observed for irradiated amorphous water ice.
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Figure 8:
Area variation of the 3 ![]() |
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In Fig. 8, the normalized 3-micron band area of Ic and amorphous water ice irradiated with H+is plotted versus the irradiation dose. The total absorbance of the band decreases, for Ic ice, as the amorphization proceeds. The final value is about 20% lower than the starting Ic value. On the other hand, the 3-micron band area of amorphous water ice increases by 10%. This effect has also been observed for He+, and Ar++. In order to verify if the observed band area variation were in agreement with the hypothesized amorphization effect, we made two different experiments: in the first experiment we acquired IR spectra after deposition at 16 K and after warming up to 150 K; in the second experiment we aquired spectra after deposition at 150 K, after cooling at 16 K and after warming back to 150 K. The spectra of Ic ice samples at 150 K obtained in the two experiments were used to scale the area data of the second experiment to the column density (water molecules/cm2) of the first. The combined results are summarized in Table 1.
Under our standard vacuum condition the background water vapor deposition rate is quite low (less than 0.25 Å/min); in the worst case (60 min) this would give a neglectable (15 Å) contribution to the 3-micron band area for the 800 Å ice films considered in Table 1. This circumstance has been verified in the second (the longest) experiment by noting that the starting Ic band area at 150 K is equal, within 2 percent, to that obtained at the end of the experiment after cooling to 16 K, followed by warming up to 150 K (120 min elapsed).
If an ASW ice layer gives a band area of 100 cm-1 at 16 K (in an optical depth scale); an Ic layer of the same column density at the same
temperature would have a band area of 136 cm-1 (see
Table 1). The band areas of both ASW and Ic vary under
irradiation (see Fig. 8). We can estimate that the ASW area
after irradiation would increase to
cm-1 (
),
whether the Ic area would decrease to
cm-1 (
), leading to an
almost equal band area for Ic and ASW irradiated ices. The fact that the band
area of the irradiated amorphous is almost the same of that of the
irradiated Ic ice (for the same column density) implies that both ices
have the same band strength. This strongly supports the hypothesis that Ic
and amorphous ices have very similar structures after irradiation.
We could not verify if this behavior is also observed for the UV case due to
the higher background deposition rate of water during photolysis with the UV
lamp and to the longer time needed to reach a given dose compared to the
ion irradiation case. Since in UV experiments the deposited samples are
thinner than the ion irradiation experiments, the total amount of the
additional water ice deposited becomes a significant part of the sample. In particular, for the UV case, the 3-micron band area
monotonically increases during photolysis except for the first dose value during the experiment on Ic, where a moderate decrease (5 percent) is observed. By comparing
Figs. 3 with 5 we estimate
that an additional layer (15% in thickness) of UV photolyzed amorphous water ice was deposited during photolysis.
The efficiency of the process under UV photolysis and ion irradiation can be directly compared by looking at Figs. 3, 5, and 6. In the H+case an irradiation dose slightly greater than 2 eV per water molecule is enough to almost complete the amorphization process, whereas in the Ar++case it is 3.5 eV per water molecule. For UV photolysis the corresponding value of dose can be estimated to be 5 eV per water molecule.
Water ice form | Temperature [K] | Absorbance band area |
ASW | ![]() |
100 |
Ic a | ![]() |
![]() |
Ic b | ![]() |
![]() |
aIc obtained after warming up to 150 K the ASW
film deposited at 16 K (first raw). bIc obtained by direct deposition at high temperature (150 K), cooled to 16 K. |
In order to give a rough estimate of the cross-section of the amorphization
process for the different cases
we have analyzed the spectra of each experiment by using a simple
bilinear fit.
By assuming that, in first approximation, at each stage of irradiation the
irradiated ice can be described by two coexisting phases (crystalline and
irradiated amorphous) with varying volume fraction; we can write the relation:
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Figure 9:
Evolution of the amorphous fraction under ion/photon
irradiation at 16 K,
![]() |
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From Fig. 9 it is evident that in addition to the already noted
higher efficiency of ions compared to that of UV photons, slight differences exist also between different ions in the amorphization
process. In particular although 30 keV H+ and 30 keV He+ have
similar k values (1.66, 1.38 respectively), 60 keV Ar++ ions exhibit a lower cross-section (0.92).
The lower efficiency of Ar ions compared to H and He ions could be related to the different amount of anelastic (electronic
excitations) and elastic (nuclear collisions) energy components absorbed by the target. In particular the ratio between
electronic and nuclear stopping power is more than 10 for 30 keV H+ and 30 keV He+ but is less than 0.5 for 60 keV
Ar++ ions. This implies that in the case of Ar ions, a significant fraction of energy is released through elastic collisions
with the target nuclei, and a fraction of that energy is spent (loss) to excite phonons in the lattice instead of inducing
displacements. This could result in a significantly higher efficiency of H and He ions compared to Ar ions in damaging the Ic
lattice. In order to verify this hypothesis we got a rough estimate of the energy spent to excite phonons by using the TRIM
Montecarlo simulation program. We found that the fraction of
energy spent in phonons is 1%, 6% and 38% for 30 keV H+, 30 keV
He+ and 60 keV Ar++, respectively.
Hence, by considering only the energy released that either induces electronic
excitations or displacements we can correct the cross-section as:
1.66/(1-0.01)=1.68 molec/eV for H+,
1.38/(1-0.06)=1.47 molec/eV for He+ and
0.92/(1-0.38)=1.48 molec/eV for Ar++. The spread among the new
cross-sections is significantly lower than before and this
supports our hypothesis. In any case we want to stress that both fast ions and UV photons have comparable efficiency (within a
factor of 2.5) in amorphizing crystalline water ice.
Since the estimated value of k is 0.68 molec/eV for
photons, a single Lyman-
(that carries 10.2 eV) may induce the
depletion of about 7
water ice molecules from the cubic lattice. This is not surprising since every molecule is coordinated
with the neighbors in the lattice. Each oxygen atom has four
nearest-neighbor oxygen atoms arranged in a tetrahedral pattern, with a
single H atom between each near-neighbor oxygen pair. Each oxygen is
chemically bonded to two H atoms and is hydrogen bonded to two others.
The photodissociation of a single water molecule, breaking and
reformation of bonds, formation of radicals or other species such as
H2O2, affects the unit cell of the crystalline structure, hence a larger number of molecules are affected.
The results here discussed are in agreement, from a qualitative point of
view, with the only previous work on this topic by Kouchi & Kuroda (1990); UV photons do amorphize a pristine cubic water ice sample.
However, as already outlined (see Jenniskens et al. 1995) the Kouchi & Kuroda (1990) result suffered
from some quantitative mistakes. In fact, here we have shown that the cross section for Lyman-
induced amorphization is five orders of
magnitude lower (i.e. greater dose needed) than that reported by Kouchi & Kuroda (1990)
and is of the same order of magnitude measured for fast ions.
In this paper we report experiments on the effect induced by UV
photolysis of water ice samples. To probe the evolution of the
samples we have used IR spectroscopy of the 3 m band. We also
quantitatively compare the results with those of similar experiments
conducted in the same experimental setup irradiating with 30 keV
H+, 30 keV He+, and 60 keV Ar++ ions.
We have demonstrated that UV Lyman-
photons are able to amorphize
crystalline water ice.
The comparison of the efficiency of UV Lyman-
photons and fast ions to amorphize Ic water ice reveals that they are close within a factor of two.
In all the cases studied, the amorphization of the Ic ice sample is
reached for doses greater than a few eV/molec.
We estimate that the efficiency of UV photons to amorphize the ice is
slightly lower than the one of the 30 keV H+ ions.
In fact, a smaller dose (eV/molecule) is necessary to reach the
same amorphization stage in the case of the ion irradiation compared
to the UV case (cf. Figs. 5 and 3).
This study has important astrophysical implications. Photons and fast ions are both present in space: they can induce variations in the profile of the 3-micron band of crystalline water ice; therefore these effects have to be considered when one attempts to deduce, from the profile of observed ice bands, information on the temperature history of the responsible dust.
Acknowledgements
We are grateful to F. Spinella for his valuable support in the laboratory. This research has been financially supported by the Italian Space Agency (ASI). We also would like to thank the anonymous referee for careful reading and suggestions which have been very helpful in making this paper more clear.