22 August 1997
ELSEVIER
CHEMICAL
PHYSICS
LETTERS
Chemical Physics Letters 275 (1997) 98- 102
The stretching mode of deposited CO as a probe for the
morphology of 5 K ices
A. Givan a, A. Loewenschuss a, C.J. Nielsen b
a The Hebrew University of Jerusalem, Jerusalem 91904, Israel
b Department of Chemistry, University ofOslo, Blindern, N-0315 Oslo, Norway
Received 20 March 1997; in final form 19 June 1997
Abstract
The band structure and spectral parameters (peak frequency, bandwidth) of the stretching mode of CO deposited at 5 K
upon various ice layers are indicative of CO • • - H20 complexations and thereby of the nature of the ice surface. It is shown
that for the former, a strained surface is sufficient and the ice does not have to possess uncoupled OH bonds. The order of
crystaUinity found is: pure ice deposited on CsI window > pure water deposited on argon and neon layers > neon/H20
solid mixture after neon release > argon/H20 solid mixture after argon release. © 1997 Elsevier Science B.V.
1. Introduction
Recent studies [1,2] have proven CO to be a
sensitive probe in the study of both surface and bulk
porosity in ices. The penetration by diffusion of CO
molecules to intemal and external voids in ice samples (at temperatures higher than 28 K) was found to
be associated with spectroscopic changes in the Uco
infrared absorption, which, in turn, could be used to
study structural defects in the ice solid. Langel et al.
[3] demonstrated that the morphology of low temperature ice samples could be investigated by the X-ray
diffraction patterns of thin layers of solid krypton
deposited on them. Their method revealed that during the first stage of deposition,° krypton atoms fill
the surface micropores ( r < 20 A [4]), then form a
monomolecular layer over the larger voids and outer
surface of the ice and only in a third step create a
thick multimolecular solid krypton layer on the ice
solid. Their study discerned three ice polymorphs by
the X ray patterns of the deposited krypton: a glassy
non-porous one, demonstrating low surface area, and
two polycrystalline polymorphs, one smooth and non
porous, the other microporous with a high surface
area. A distinction between the vitreous and polycrystalline natures of bulk ice could also be made in
our recent study of the temperature dependence of
the spectroscopic parameters (band positions, bandwidths and integrated intensities) of vapor deposited
pure and mixed ice layers [5].
In this study the morphology of ice samples is
characterized by the infrared profile of the Uco band
of thin CO layers deposited upon them at 5 K, a
temperature at which the diffusion probability is
practically zero.
2. Experimental
Vapors of water, previously deionised and thoroughly degassed, or their mixtures with A G A supplied Ar (5.7) or Ne (4.5), were sprayed on a KeI-F
0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
PII S0009-2614(97)00723-9
A. Givan et al. / Chemical Physics Letters 275 (1997) 98-102
coated CsI window maintained at 5 K by an Air
Products HS-4 Heliplex cryostat employing two HC-4
MK1 compressor modules. Temperature control was
facilitated by a Lakeshore model 330 temperature
controller using Si-diode sensors. Typical deposition
rates were several m m o l / h and the ice layer thicknesses were less than 0.5 micron. The CO gas deposited on the ice samples was also an AGA product.
The deposition rates of the CO gas were similar to
those of the water vapors. The much shorter deposition times of the latter resulted in CO layers calculated to be only 0.05 micron (assuming a smooth
non-porous ice surface). As differently formed ice
samples may possess a diversity of surface qualities
[4,6-11], the CO covering layers may be even thinner. The deposition nozzle parameters were: 1 mm
inner diameter and a distance of 30 mm from the
cold window. The warming rates of the investigated
layers were < 1 K / m i n . Infrared spectra were
recorded on a Bruker IFS-88 interferometer equipped
with a DTGS detector, with 32-128 superimposed
scans and resolutions of 0.5-2 cm -j. Ice samples
kept for extended periods (even exceeding 48 h)
showed no observable changes in their spectral features.
3. Results and discussion
Seven different ice samples were covered with
CO layers and their infrared spectra are compared
with those of a 10:1 C O / H 2 0 solid and a 10:1
H20/CO
ice. The Uco bands are reproduced in Fig.
1A and the matching '31~' water bands in Fig. 1B
and their spectral parameters are summarized in Table
1. Blanketing the ice (samples 2 - 7 below) with a
CO layer had no effect on their '3p,' water bands.
The nine Vco band profiles belong to the following
systems (recorded as deposited at 5 K):
(1) a 1:10 H 2 0 / C O solid deposited on the CsI
window (Fig. I A.I); (2) CO deposited on a pure ice
sample (Fig. 1A.2); (3) CO deposited on ice formed
by first vapor depositing H 2 0 onto a neon layer and
subsequently warming the sample to 15 K to remove
the underlying neon (Fig. l A.3); (4) CO deposited
on ice formed by first vapor depositing H 2 0 o n t o an
Ar layer and subsequently warming the sample to 50
K to remove the underlying argon (Fig. 1A.4); (5)
CO deposited on a 4:1 argon:H20 sample (Fig.
99
B.8
B.7
B.6
B~q
B.4
i
21~J
i
i
215&0
214U
Wtvelkr ¢ml
i
2130.0
i
I
2120.0 ~
I
,~75
I
i
335g
3025
Wamom~ cm1
~
2700
Fig. 1. The (Vco band (A.I-A.9) of CO layers deposited at 5 K
on various ice layers and the parallel '3tz' band of the ice
(B.1-B.9). (A and B).I A 1:10 H 2 0 / C O solid, deposited on the
Csl window. (A and B).2 CO deposited on a pure ice sample. (A
and B).3 CO deposited on ice formed by first vapor depositing
H 2 0 onto a neon layer and subsequently warming the sample to
15 K to remove the underlying neon. (A and B).4 CO deposited
on ice formed by first vapor depositing H 2 0 onto an Ar layer and
subsequently warming the sample to 50 K to remove the underlying argon. (A and B).5 CO deposited on a 4:1 argon:H20 sample.
(A and B).6 CO deposited on an ice formed by first vapor
depositing a 3:1 A r / H 2 0 mixture and subsequently warming it to
50 K, to remove the argon. (A and B).7 CO deposited on an ice
formed by first vapor depositing a 4:1 N e / H 2 0 mixture and
subsequently warming it to 29 K, to remove the neon. (A and B).8
CO deposited on an ice formed by first vapor depositing a 4:1
N e / H 2 0 mixture and subsequently warming it until crystallization at 165 K. (A and B).9 An 1:10 C O / H ~ O sample.
1A.5); (6) CO deposited on an ice formed by first
vapor depositing a 3:1 A r / H 2 0 mixture and subsequently warming it to 50 K, to remove the argon
(Fig. 1A.6); (7) CO deposited on an ice formed by
first vapor depositing a 4:1 N e / H 2 0 mixture and
subsequently warming it to 29 K, to remove the neon
(Fig. 1A.7); (8) CO deposited on an ice formed by
first vapor depositing a 4:1 N e / H 2 0 mixture and
subsequently warming it until crystallization at 165
K (Fig. 1A.8); (9) a 1:10 C O / H 2 0 sample (Fig.
1A.9).
Our results indicate that the surface qualities of
the underlying ice layers are well reflected in the
Vco peak frequencies, their bandwidths and the band
100
A. Givan et al. / Chemical Physics Letters 275 (1997) 98-102
substructures. The Vco bands (Fig. 1A, Table 1)
peak at 2138.4_ 0.2 cm -1, with three exceptions:
The solid CO containing 10% water, the 10% CO in
H 2 0 and the 3:1 argon:H20 sample from which the
argon was released by warming to 50 K (Fig. 1A.9,
1A.1 and 1A.4). The 2138.4 cm -1 band is assigned
to the crystalline form of CO solid, to which the
amorphous polymorph (CO)as transforms at about 23
K [12]. The presence of the more crystalline form of
CO solid already at 5 K is attributed in part to self
annealing occurring at condensation, due to the insulating effect of the underlying ice. The small red-shift
of Vco (0.5-1.7 cm -1) in these three samples is
accompanied by the emergence of an extra band in
the 2147-2150 cm -1 region, attributed in previous
studies to CO * H 2 0 species both in rare gas matrices
[13-20] and in mixed ice solids [1,2,13,21,22]. The
formation of C O - H 2 0 bonds in these samples is
also reflected in their Vco bandwidths, Av~/2; the
three largest are listed in Table 1. In part, this width
is due to unresolved Vco features of CO-rich
( C O ) n ( H 2 0 ) m ( n > m ) species [13]. Hence, the only
pure ice sample which shows evidence of surface
complexing CO molecules deposited on it at 5 K,
and for which the Vco (Fig. 1A.6) is similar in
shape, shift and width to those of mixed C O - H 2 0
ices, is the one formed by premixing the water vapor
with a large amount of argon, and subsequently
releasing most of the latter by warming it to about 50
K (beyond the first annealing stage of amorphous ice
at 30 K [5]). In the analogous neon experiment (ice
formed after neon thermal release, Fig. 1A.7) an ice
layer is produced which shows clear spectral evidence of bulk porosity [2], but the Vco band profile
of deposited CO shows only remnants of the 2148
cm -1 band of the C O * H 2 0 being formed on its
surface.
These results may be contrasted with those for the
4:1 argon:H20 ice sample from which argon was not
released prior to the CO deposition (Fig. 1A.5 and
lB.5). The '3ix' band region (Fig. lB.5), clearly
indicates the presence of a large proportion of 'free'
decoupled OH bonds from small H 2 0 clusters by the
intense peak at 3704 cm-1 [2,5]. However, the Vco
band profile [Fig. 1A.5] is narrower and shows only
remnants of the 2148 cm- l CO* H 2 0 band. It must
be concluded that the existence of 'free' OH bonds
is not the only condition for the formation of
CO* H 2 0 or similar species. We thus suggest that a
creation of CO * H 2 0 bonds independent of the pres-
Table 1
Main peak frequencies, bandwidths and secondary band frequencies of Vco (cm- i )
Sample
Vco
A v 1/2
Secondary bands
1:10 H 2 0 / C O solid, deposited on the CsI window (Fig.lA.1).
CO deposited on a pure ice sample (Fig. 1A.2).
2137.9
2138.3
6.7
2.6
CO deposited on ice formed by first vapor depositing H20 onto a neon layer and subsequently
wanning the sample to 15 K to remove the underlying neon (Fig. IA.3).
2138.2
5.7
2150
2142(w, sh)
2136 (s,sh)
2142 (m, sh)
CO deposited on ice formed by first vapor depositing H20 onto an Ar layer and subsequently
warming the sample to 50 K to remove the underlying argon (Fig. 1A.4).
2138.4
4.1
2136 (s, sh)
2142 (m, sh)
CO deposited on a 4:1 argon:H20 sample (Fig. 1A.5).
2138.2
5.5
CO deposited on an ice formed by first vapor depositing a 3:1 A r / H 2 0 mixture and subsequently
warming it to 50 K, to remove the argon (Fig. lA.6).
2136.7
6.5
2136(w, sh)
2142(w, sh)
2136(s, sh),
2147 (vw)
2142 (m, sh)
CO deposited on an ice formed by first vapor depositing a 4:1 N e / H z O mixture and subsequently
warming it to 29 K, to remove the neon (Fig. 1A.7).
2138.3
3.5
2148(w)
2142 (w, sh)
CO deposited on an ice formed by first vapor depositing a 4:1 N e / H 2 0 mixture and subsequently
warming it until crystallization at 165 K (Fig. 1A.8).
2138.6
3.9
2136 (w, sh)
2142 (vw, sh)
1:10 C O / H z O sample (Fig. 1A.9).
2137.5
9.2
2136 (vw, sh)
2147 (m)
A. Givan et a l . / Chemical Physics Letters 275 (1997) 98-102
I
I
2155.0
2145,0
Wavenumber cm ~
Fig. 2. Difference spectrum (baseline corrected) of CO deposited
on ices from A r / H 2 0 mixtures, after thermal release of the rare
gas and as deposited (curves 1A.5-1A.6, respectively), showing a
marked increase in the CO* H 2 0 band in the latter.
ence of free or dangling OH ice bonds which is the
working assumption of Buch, Devlin and co-workers
[23-29]. While the presence of argon in A r / H 2 0
mixed ice may physically interfere with the formation of CO* H 2 0 bonds, the point made here, becomes even more prominent by considering the clear
emergence of a band representing CO* H 2 0 complexation in the difference of curves 1A.5-1A.6
shown in Fig. 2. Thus in spite of the drastic reduction in the free OH bonds (curve lB.6 versus lB.5),
and the removal of the interference of solid argon, a
marked growth in this band is noted. Moreover, in
the ice formed after the thermal release of neon (Fig.
lB.7), there is no interference of frozen rare gas
clusters, and even though weak 'free OH' bands are
discernible in the spectrum, the band of deposited
CO (Fig. 1A.7) shows no evidence of CO attachment
to the ice surface. To explain the fact that CO forms
such bonds at low temperatures (5 K), it may be
argued that releasing the argon from the mixed ice
solid creates a strained metastable ice which enables
the formation of CO* H 2 0 species upon deposition,
without the need for warming and diffusion.
For all the other samples, the Vco bands are
narrower, peak at the same wavenumber 2138.4 + 0.2
cm ~, and have the same substructure of two shoulders at 2142 _+ 1 and 2136_+ 1 cm -~. CO deposited
on vapor deposited pure ice (Fig. 1A.2) displays the
narrowest line (2.6 cm -~) with the weakest shoulders - two facts which seem interrelated. A bandwidth decrease is usually associated with a more
ordered phase [30] and so the more crystalline nature
of the pure ice sample, previously [5] deduced from
101
its own infrared spectroscopic features, is also reflected in the solid CO growth on top of it. The
bandwidth criterion as well as the influence of substrate crystallinity on the deposited solid crystallinity
are again analogous to the arguments of Langel et al.
[3] for the X-ray patterns of solid krypton deposited
on ice samples. Even if weak features of 'free OH'
bonds were obscured by the residual atmospheric
bands of spectrum B.2, no parallel CO* H 2 0 band is
to be discerned in spectrum A.2.
In a previous study [13], we assigned the 2142 + 1
c m - I band to the (CO) 2 H 2 0 trimeric species in an
argon matrix. We suggest that in the present experiments this frequency, observed in almost every sample studied, originates in vibrational modes of the
weak complexes between the first adsorbed
monomolecular CO layer and the ice surface and
may indicate that each water molecule on it 'feels'
the presence of two adjacent CO molecules. Support
for this assumption is that the band of this frequency
has its highest relative intensity in the most amorphous sample, i.e., ice from a A r / H 2 0 mixture after
argon release (Fig. 1A.6), whereas the weakest 2142
+ 1 cm 1 bands are found in the more 'crystalline'
samples of pure vapor deposited ice (Fig. IA.2) and
of ice from a N e / H 2 0 mixture annealed to 165 K
(Fig. 1A.8). In the amorphous sample, the larger
surface area and the cohesive energy of CO (0.088
eV [21]) may facilitate the suggested molecular arrangements, whereas the effect will be minimized in
the tightly bound (0.52 eV [21]) crystalline ice.
The second shoulder, at 2136_+ 1 cm l, was
attributed by us to (CO) n polymers in argon matrices
[13] as well as to CO clusters in bulk porous ice [2];
the latter is our preferred interpretation for the infrared bands of the present samples. This assumption
is again supported by the fact that as for the 2142 _+ 1
c m - l band, CO deposited on the two more crystalline ice substrates (Fig. IA.2, 1A.8), shows the
lowest relative intensity of the 2136 _+ 1 cm I band
and has the highest relative intensity in the clusterlike sample formed by argon release (Fig. I A.6), to
the extent that the latter does not demonstrate the
2138.4 cm -~ Uco maximum at all, but peaks at
2136.7 cm 1
Temperature induced shifts of Uco are in accord
with the arguments above. The frequency values of
the CO band deposited on the pure ice samples is
102
A. Givan et al. / Chemical Physics Letters 275 (1997) 98-102
constant in the 5 - 3 0 K temperature range. The mixed
solid samples, 10:1 C O / H 2 0 and 10:1 H 2 0 / C O
and the ices produced by rare gas thermal release
demonstrate an irreversible blue shift of their Uco to
2138.4 cm-1. These results are also in good accord
with those from the temperature dependence of the
ice band profiles reported in a previous study [5].
The present study leads to two main conclusions.
1. For the formation of CO * H 2 0 bonds upon CO
deposition onto an ice layer, the latter does not need
to have free, dangling or uncoupled OH bonds, a
strained surface being sufficient to facilitate complexation of CO to water molecules on an ice surface.
2. Uco band profiles of deposited CO on ice
samples are influenced by the substrate, and reflect
the ice order and crystallinity and even of its capability to absorb CO by diffusion at higher temperatures
(T > 30 K). The present results suggest an order of
crystallinity of: pure ice deposited on CsI window >
pure water deposited on argon and neon layers >
n e o n / H 2 0 solid mixture after neon release>
a r g o n / H 2 0 solid mixture after argon release.
Acknowledgements
AL gratefully acknowledges a Research Fellowship at the University of Oslo by NFR, the Norwegian Research Council.
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