Fourier transform infrared spectrometry studies of surface and bulk porosity of water ice

VIBRATIONAL SPECTBOSCOPY ELSEVIER

Vibrational

Spectroscopy

I2 (I 996) 1- I4

Fourier transform infrared spectrometry studies of surface and bulk porosity of water ice A. Givan a, A. Loewenschuss a, C.J. Nielsen b3* ’ Department h Department

of Inorganic of Chemist?,

Chemistry, The Hebrew Uniclersiry of Oslo.

Received

16 October

Unil’ersir) P.O. Box

1995; accepted

of Jerusalem, 1033

Blindern.

6 February

Jerusalem 91904, Israel N-0315 Oslo, Nowa?

1996

Abstract Water ices used as mimics in laboratory studies of heterogeneous chemicalreactionshave beeninvestigatedby FTIR. Water ice asvacuumdepositedonto a CsI substrateat 5 K wasfound to be essentiallynon-porous.Inert gas pre-deposition and co-deposition followed by temperature cycling induced surface and bulk porosity into the produced layers. Porosity was monitored by the shape and position of the 3300 cm-’ coupledwater band,by the danglingOH high frequencymode,and

by the modesof hydrogenbondedandtrappedCO. The highestamountof danglingOH bondswasrevealedwith Ar/H,O depositionmixtureswhereasthe highestdegreeof bulk porositywasachievedwith neonasco-deposited gas. Keywords:

Infrared

spectrometry;

Water

ices; Water

vapour

deposition:

1. Introduction Water vapour condensation at low temperatures results in formation of amorphousice, H,O,,,, . The polymorphism of this solid is well establishedfrom thermal measurements[ 1,2], X-ray diffraction [3,4], surface [4-81, and vapour pressure[9] studies. Depending upon the preparation, the BET surface area is reported to vary from a few m2 g-’ [6,7] to several hundred m’ g- ’ [4,5,8]. Amorphous ice, prepared by vacuum deposition methods at temperatures below 77 K, is claimed to be microporous in nature with a significant fraction of the water molecules at the surface of the micropores [8]. Esti-

* Corresponding 0924.2031/96/$15.00 PI1 SO924-203

author. Copyright 1(96)00013-6

0 1996 Elsevier

Science

Matrix

isolation

mates of the micropore volume vary from 0.1 to 0.2 cm3 g- ’ [S,lO] and X-ray results [4] indicate a coherencelength of krypton filled pores of about 12 A. Reports on the ice preparation parametersindicate an extreme dependenceof the product upon deposition conditions, especially the deposition rate [l l] and the exact physical status of the vapours before deposition. It has been argued [3] that a slow deposition of single water molecules on a cold tip causes the formation of a more homogeneousand lessporous solid, probably due to the high heat of condensation causing local annealing, and that condensation of a cluster containing supersonically cooled molecular beam tends to produce a more heterogeneouscluster like porous ice. In recent years the structure of amorphousice has

B.V. All tights reserved

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Vibrational

been studied by both theoretical methods [ 12-151 and by infrared spectroscopy employing different adsorbates to show the existence of dangling OH bonds and penetration into the surface micropores [14-191. Mass spectrometric desorption studies of amorphous ice and its dynamics [20-221 report the evolution of gases trapped in water icicles and show the evidence of gaseous molecules being trapped in H,O,,,, capillaries, indicating their bulk porosity. A high level ab initio calculation of H,O complexes with CO has recently appeared showing three stable structures, in decreasing order of stability: OCHOH, CO..H-OH and a T-shaped H,O..n complex [231. The present infrared spectroscopic investigation, which is part of our on-going research on the characterisation of surfaces relevant to heterogeneous reactions, indicates that vacuum deposition of pure water vapour onto a very cold (5 K) tip results in ice layers which are essentially non-porous. We report several attempts to destabilise this solid and increase its permeability to gaseous molecules using inert ‘impurities’ such as argon and neon and CO as probes. The noble gases were introduced into the solid by thermal diffusion or co-deposition and then removed by further warming and pumping. This destabilisation method was previously applied successfully to amorphous low temperature N,O, [24], whereby the initially ionic layer was forced to revert to the molecular form. We show that these matrix isolation spectroscopy related methods can be effective in producing water ices of bulk porosity. Similar phenomena, namely

Table 1 Spectral data (cm-‘)

of the 3300 cm-’

Sample

band

Maximum

a Band appearing

upon annealing.

3235 3257 3370 3390 3300

12 (19961 I-14

low temperature co-deposition of water vapours with other molecules (including CO), as well as desorption of the guest molecules through temperature cycling are also encountered and investigated by infrared spectroscopy in the interstellar medium [25,26].

2. Experimental The Ar (5.7), Ne (4.5) and CO (4.7) gases used were supplied by AGA. Water vapour was taken from deionised water which was first submitted to several degassing cycles. The samples were deposited onto a Kel-F’ coated CsI window maintained at 5 K. Cooling was provided by an Air Products HS-4 Heliplex cryostat employing two HC-4 MKl compressor modules. Temperatures were controlled by a Lakeshore model 330 temperature controller using Si-diode sensors. Layers of less than 0.5 pm thickness were obtained at deposition rates below 15 pm01 rnp2 s-’ and deposition times of up to 45 min. The deposition nozzle had an inner diameter of 1 mm and was placed 30 mm from the cold window. The H,O/inert gas deposited layers were slowly warmed ( < 1 K min- ’ ) either to release the inert gas under external pumping or to facilitate CO diffusion. The infrared transmission spectra were recorded with a Bruker IFS 88 interferometer normally employing a DTGS detector and co-adding 32 scans at a nominal resolution of 1 cm-‘.

solid HZOcaSJ and CO bands obtained H,O

CO’S, CO in Ar ( 1:200) CO..H,O in AK (1:1:200) Pure ice Pure ice on Ne substrate Ar:H,O (4:l) Ne:H,O (4:l) CO:H20 (1:lO)

Spectroscopy

at 5 K CO bands

Width

330 330 380 330 300

2138.6 2138.6 2149.0 2138.5 2138.2 2139 2138.2 2138, 2152

2147. 2151 a 2152 a 2151 a

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3. Results and discussion We have prepared different water ices at 5 K from 6) pure H,O vapour, (ii) pure H,O vapour on top of pre-deposited layers of Ar or Ne, (iii) H,O/Ar mixtures, (iv) H,O/Ne mixtures, and (v) H,O/CO vapour mixtures. The spectral data for the 3300 cm-’ water ice band as well as the absorption bands of adsorbed/trapped CO are collected in Table 1. Reference spectra of solid CO, of H,O isolated as a trace species in solid CO, and of CO, H,O and CO/H,0 mixtures isolated in argon matrices were also recorded in order to facilitate the spectral interpretation. Some of these data are also included in Table 1 and agree with previous matrix isolation [27-321 and solid state data [33]. For the sake of clarity, the results and their discussion are given for each type of experiment, followed by summarizing remarks. 3.1. Pure water

ice

The 3300 cm-’ infrared band of amorphousice, as formed from vapour deposition onto the cold window at 5 K is shown in Fig. I (curve A). The

3525

3250 Wavenumber

2375

2700

cd

Fig. 1. The 3300 cm-’ infrared ice band obtained at 5 K from thin film deposits of: (A) pure water vapour; (B) a 4:l Ar:H,O mixture: (C) a 4: 1 Ne:H>O mixture.

_J,ie, 2160.0

21;o. 0

Fig. 2. Infrared spectra vapour deposition at 5 vapour deposition on annealing sample B to

2140.0 Wavenumber

2130.0

2120.0

cd

of a CO deposit on: (A) ice from a water K; (B) a temperature cycled ice from water a Ne substrate at 5 K, see text; (C) after 34 K.

band peaks at ca. 3235 cm-‘, it has a width of 330 cm-’ and shows clear shoulders around 3400 and 3 130 cm- ‘. In general, the band shape resembles previously presentedspectra[ 111.An examination of the non H-bonded OH stretching region above 3600 cm-’ did not reveal additional bands of significant intensity above noise level. No clear spectral changes were detected in the amorphousice band after several hundred monolayers of CO were slowly depositedon top of it. Fig. 2 shows the CO band (curve A), positioned at 2138.5 cm-‘, and with a band shaperesemblingthat of pure solid CO depositedat 5 K. Warming the double layer to 30 K did not changethe ice band, whereasthe CO band narrowed slightly due to the CO,,,, -+ a-CO transition around 23 K. Further slow heating to 40 K vaporized all the deposited CO without affecting the 3300 cm-’ ice band. In recent experimental infrared and theoretical simulation studies [ 12-191, the ices, prepared by different deposition methods at temperaturesbelow 90 K, are stated to have high surface areas with substantial amounts of micropores. Two low intensity infrared bands in the G> 3600 cm-’ region, at

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3720 and 3696 cm-‘, were attributed to dangling OH groups with coordinations of 2 and 3, respectively, most of them naturally found in the micropores [14]. Deposition of gaseous CO molecules on such a sample, followed by warming to temperatures above 30 K was reported to cause a penetration of the CO into the micropores, remaining there to be observed up to temperatures well above 40 K. This penetration was reflected in the infrared spectrum, both as a large shift of the 3696 cm-’ band and in the appearance of a new CO band at 2 152 cm- ’ [ 181. In the present study we found no notable evidence for the existence of dangling OH bonds, nor for CO penetration into surface defects or internal regions of the pure H,O,,,, samples as prepared by us. The experimental evidence indicates that pure amorphous ice, produced under our deposition conditions, is of non-porous nature and of low surface area. This may be related to the high heat of sublimation of water, AH,,, = 48.88 kJ mall’ [34], together with the fact that the cryostat used has its minimum cooling capacity when used close to the lowest temperature it can achieve. We also note that our peak position and bandwidth of the 3300 cm- ’ ice band do not fit in with the systematic patterns previously reported [ 111. This is a further indication of how strongly the experimental conditions influence the nature of the ice layer formed. The size of the spray-on nozzle was also reported to influence the amount of clusters in the deposited solid [l]. In this context it may be noted that slow deposition of H20 vapours onto a 77 K cold tip [2] resulted in a solid sample already containing significant amounts of cubic crystalline ice Ic, even though the transition HzOc,,’ + Ic does not occur below ca. 140 K. 3.2. Pure water neon layers

ice deposited on top of argon or

Water vapour sprayed onto a predeposited layer of argon at 5 K showed a featureless 3300 cm-’ band peaking around 3300 cm-’ and with a width of 330 cm-’ (Table 1). The analogous Ne experiment led to an ice spectrum which closely resembles that of pure water ice. The infrared band shapes remained essentially without change after a very slow temperature cycling (Ar: 5 K +30K-+5 K; Ne: 5 K-t 15 K -+ 5 K) during which the underlying and stirred up

Spectroscopy

12 (1996)

l-14

noble gases were released and pumped off. In particular, in either spectrum the evidence for dangling OH bonds was scarcely discernible above noise level in the appropriate high wave number region. The experiments with an argon underlayer sometimes resulted in the ice film peeling off. Hence, in the following we refer only to the better controlled neon experiments. Only when CO was deposited onto the temperature recycled ice film did two very weak absorptions emerge at 3690 and 3673 cm-‘. The former band disappeared upon very slight sample heating to above 10 K, whereas the 3673 cm-’ band remained essentially unchanged to 25 K. Above this temperature it first broadened considerably and centred around 3640 cm-‘, then it diminished to noise level at about 30 K. Fig. 2 shows the corresponding CO band just after deposition at 5 K of an amount roughly corresponding to 500 monolayers (curve B), see later. The spectrum revealed a peak at 2138.2 cm-’ containing an additional broad component as well as a weak satellite around 2 147 cm- ’ . Annealing experiments and band deconvolution indicate that the main CO band consists of three components centred at ca. 2136, 2138 and 2141 cm-‘. Upon annealing for a few minutes at 20 K the 2136 cm- ’ component vanished. At higher temperatures the band broadened; at 34 K most of the deposited CO had evaporated and was pumped off, and the spectrum showed only a weaker broad component of the main CO band whereas the satellite band shifted from 2147 to 2 15 1 cm ’ with almost the same intensity (curve C). The two bands found in the non H-bonded OH stretching region [ 14-191 must have gained significant intensity after the impinging of CO molecules onto the ice surface. The lower wave number component at 3673 cm-‘, which also showed a temperature dependence related to the 2147/2151 cm-’ CO satellite band, may be assigned to some form of an OH..CO complex. Only with increasing temperature (and the higher mobility of the adsorbed CO> did this band relax to the approximate shape and position around 3640 cm- ’ reported and discussed by Devlin [ 181 and by Sadlej et al. 1231. Following the present as well as earlier matrix isolation results [27-321, which too often are not referred to by other authors, we then attribute the 2147 cm-’ band to an ill-de-

A. Ghan

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Spectroscopy

fined H,O/CO multimer complex, which by heating and evaporation of CO is gradually transformed into the better defined hydrogen bonded CO complex (0-H..CO) associated with the 2151 cm-’ band. It may also be noted that ab initio calculations indicate a +13 cm-’ shift in the vco mode upon this hydrogen bonding [23]. The broadening of the 2138 cm-’ CO band is attributed to CO molecules non specifically physisorbed on the polar ice surface. We have no definite spectral evidence for the existence of the predicted CO..H-OH and T-shaped H*0..7~ complexes [23]. However, the 2138 cm-’ band remaining at 34 K in this experiment, as well as in some of the experiments to be discussed in the following sections, is definitely not due to solid CO,,,, or a-CO. The spectral changes by annealing and the almost complete evaporation of CO by warming to only 34 K indicates that our samples consisted basically of two distinct solid layers, with only a weak interaction between CO and the H,O,,,, surface. At 34 K the pressure of a-CO is above lo-* Pa [35], and as the external pressure in the cryostat is less than lop6 Pa we do not expect more than a monolayer of CO to remain on the ice surface. The total amount of CO actually present in the beam was estimated using the integrated band intensities (IBI) reported for H,O,,,, [11,36], solid CO [36,37] and solid CO:H,O mix-

3.3. Co-deposition

5

of H,O and Ar vapour at 5 K

We decided to deposit a 4: 1 mixture of Ar:H,O at 5 K using the same experimental conditions as described above, which led to the formation of a

r

I

I

I

I

3750

3700

3650

3600

cm?

1-14

tures [36,38]. As the integrated band area for curve C in Fig. 2 is ca. 0.45 cm-’ (base lo), the amount of CO is ca. 2 X 1016 molecules cm-*. Taking the average area occupied by an adsorbed CO molecule in the complete monolayer as 16 A2 (or 6 X 1014 molecules cm-*) and the sample thickness, estimated from the IBI of the 3300 cm- ’ band, as roughly 0.5 pm one arrives at a surface area of ca. 300 m* g-‘. Although the above estimate is very rough, this number appears too large compared with BET estimates [4-81. We therefore suggest that a significant part of the infrared absorption originates from CO molecules enclosed in micropores by ‘creeping water’ as previously suggested [22]. This interpretation is also supported by the experimental data presented in Section 3.3. The heat transfer of stirred up Ne or Ar atoms from underlying noble gas layer into the amorphous ice and the possible subsequent penetration of gas molecules facilitated by warming of the system, is thus effective to some extent in enlarging the ice surface area. An alternative method is described below.

3800

Wavenumber

12 (1996)

2160.0

I

I

I

2150.0 2140.0 Wavenumber

2130.0 cm-'

2120.0

Fig. 3. Infrared spectia of the ‘free’ OH and CO spectral regions of CO deposits on an ice originating from deposition at 5 K of a 4:l Ar:H,O mixture. (A) Ice as deposited at 5 K; (B) CO deposited on sample A; (C) sample B annealed to 33 K; (D) sample B after further annealing to 37 K; (El redeposition of CO on sample D followed by annealing to 37 K.

6

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et al./ Vibrational

non-porous ice when pure water vapour was deposited. As apparent from Fig. 1, the resulting 3300 cm-’ band (curve B) differs significantly from that of pure ice. Similar spectral features, although not so extreme, have been presented for H,O/N, codeposits [39]. In fact, the spectrum looks more like those of the small clusters (H20& [40] and (H,O),, [41]. The partly split band peaks at 3370 cm-‘, which is 135 cm-’ blue shifted as compared with pure ice. In addition, the free OH spectral region, shown in more detail in Fig. 3, displays a strong overlapping band pair at ca. 3705 and 3700 cm- * accompanied by a weaker feature on the high wave number side at ca. 3720 cm- ’ (curve A). For both, the intensity is considerably higher than reported by Devlin coworkers [ 16,171. CO deposition onto this conglomerate ice had no influence on the ice spectrum (curves B) and resulted in a CO band similar to the one reported in Section 3.2, namely a strong band complex around 2138 cm-’ with a clear shoulder at 2136 cm-’ and a lower intensity component around 2147 cm-‘. Careful warming of this multilayer sample to 33 K (at which temperature part of the CO may diffuse into the H,O/Ar mixture and part of it may evaporate together with Ar) followed by retooling it to 5 K resulted in a smearing out of the sharper features of the 3300 cm-’ band but otherwise only in minor changes in the position and width of the band. However, as Fig. 3 shows, it caused the almost complete disappearance of the 3720 cm- ’ band, while the 3700 cm-’ doublet lost ca. 50% intensity and at the same time shifted the low wave number component to 3694 cm-‘, leaving a more clear shoulder at 3705 cm- ’ (curve C). As for the CO band, the warming cycle narrowed and shifted the main component to 2139 cm-’ and the 2152 cm-’ feature emerged as a discrete band. The 2139 cm-’ band may in part be due to matrix isolated CO, but the band width indicates that this is not the sole origin. Integrated band intensities of the 2120 to 2160 cm-’ spectral region indicated the remaining CO to be about l/3 of the original deposit with about 50% of the intensity in each component. Further annealing of the sample to 37 K for 30 min while evacuating to less than lO-‘j Pa resulted in the complete disappearance of the 2 139 cm- ’ band leaving the 2152 cm- ’ band with ca. 10% of the total

Spectrmropy

12 119961 1-14

initial intensity (curve D). At the same time the free OH band diminished by a factor of 2. Additional deposition of approximately the same amount of CO as in the first deposit, followed by the same temperature cycling had no influence on the water ice spectrum and only added to the total intensity of both of the two CO features (curves E). It is obvious from this experiment that there exist at least two types of CO/H,0 complexes and that the stabilities are quite different; the 0-H..CO complex associated with the 2151 cm- ’ band is clearly the stronger as also indicated by ab initio calculations [23]. During the first annealing to 37 K of the conglomerate Ar/H,O ice, the surface, but not the micropores, was stripped down to a monolayer of argon [22]. In a recent X-ray study [4] it was also suggested that, due to capillary condensation, the voids in a similarly prepared sample fill before the surface is covered. When the next layer of CO was deposited, the remaining argon might or might not act as heat carrier, but as the spectrum in Fig. 3 (curve E) shows, the 2138 cm-’ band is clearly very different from that in the Ne sub-layer experiment, Fig. 2 (curve C). The 2138 cm-’ band is now much broader, 10 versus 8 cm-‘, and actually weaker than the 2151 cm- ] band. We tentatively suggest that when the ice is covered with a monolayer of Ar (and the pores are filled with Ar as well), then CO is not able to penetrate the pores and be enclosed by the ‘creeping’ of the ice. This implies that the broad CO band around 2138 cm-’ in this experiment is entirely due to CO adsorbed on the surface either by CO..H-0 or n bonding and not originating from CO molecules adsorbed in the micropores as recently suggested [18]. Thus, the present results differ from those previously reported [18] with respect to the warming above 35 K, in both the absolute and relative intensities of the remaining CO features, and may be taken as evidence for diversities in the nature of the ices produced by the respective experiments. The co-deposition of inert gas/H,0 and CO/H,0 samples onto cold substrates was discussed in previous studies [4,20-22,36,38], which concluded that part of the rare gas (or CO) formed a separate layer on the amorphous ice. Our experiments also indicate that in spite of premixing the gases, the resulting solid layer contains large clusters of the frozen components.

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The shape, the partly resolved structure and the blue shift of the main 3300 cm-’ water band, as well as the appearance of the 3720 cm-‘, 3701 cm-’ free OH bands with rather high intensity, indicate a significant opening of the ice structure by the argon atoms in the sense of the production of substantial amounts of non H-bonded OH groups. Although the free OH modes in this experiment have much higher intensities, their behaviour during temperature cycling as well as the red shift of the 3700 cm-’ band upon deposition of a CO layer are similar in characteristics to the two dangling OH bond modes (3720 cm-‘, 3696 cm- ‘> stipulated by the previous studies [12-191. They, again, indicate the more pronounced cluster-like and more porous stmcture of ice produced by the present method of premixing the water vapour with condensable rare gases or other relatively volatile substances. 3.4. Co-deposition

of H,O

and Ne vapour

at 5 K

A 4:l Ne:H,O mixture was slowly depositedat 5 K. Fig. 1 showsthe OH stretching region asrecorded at 5 K (curve C). The ‘coupled’ water band, peaking at 3390 cm-‘, is blue shifted 150 cm-’ relative to normal amorphousice and has a width of 330 cm-’ similar to the 4:l Ar:H,O sample although it displays less structure, Table 1. Fig. 4 shows the c > 3600 cm-’ side of the main band where two additional lines are again found at 3720 cm-’ and 3704 cm-’ (curve A) of about equal intensity. Compared with the argon experiment the former is thus of higher, the latter of lower intensity relative to the main band. In order to remove the co-deposited neon, the samplewas warmed in 0.5 K stepsto 11.5 K while keeping the pressure below 10m5 Pa by external turbomolecular pumping. The samplewas then kept at 11.5 K under pumping until the cryostat pressure was below 10e7 Pa. Further heating, first in 0.5 K stepslater in stepsof 1 K, releasedmore Ne and this process continued until 28 K where a temperature increasedid not seemto result in further Ne release. During this processthe water ice spectrum changed dramatically as shown in Figs. 4 and 5. The non-H bonded OH bands, Fig. 4, lost significant intensity already at 10 K (curve B) and at 28 K (curve C) essentially only a broad band is left around 3670

Spectroscopy

12 (1996)

1-14

7

3

I

3800

I

I

3750

3700 Wavenumber

I

3650

1

3600

cm-'

Fig. 4. Infrared spectra of the ‘free’ OH spectral region of an ice originating from deposition of a 4:l Ne:H,O mixture at 5 K. (A) As deposited at 5 K; (B) spectrum after raking the temperature to 10 K; (C) spectrum at 28 K; (D) spectrum at 155 K; (E) spectrum at IO K.

4000

I

3625

32% davenumber

2875

2500

cm-'

Fig. 5. The 3300 cm ’ infrared ice band originating from deposition at 5 K of a 4:l Ne:H20 mixture. (A) As deposited at 5 K; (B) spectrum after raising the temperature to 10 K; (Cl spectrum at28 K; (D) spectrum at 160 K; (E) spectrum after cooling of sample D to IO K.

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et al. / Vibrational

cm-‘. As seenfrom Fig. 5, the main coupled band gradually red shifted to ca. 3160 cm-’ beyond the peak wave number of the pure ice band. Both the elimination of the free OH bonds and the red shift of the coupled 3300 cm-’ band indicate that new strong H-bonds are formed at the expense of weaker ones ruptured by the evaporating gas molecules. These strong H-bonds are then responsible for the high degree of induced bulk porosity in the senseof a formation of voids in the solid capable of enclosing clustersof moleculesas describedbelow. Fig. 5 also includes the astonishing ice spectra resulting from subsequentannealing of this ice film to 165 K, as will be shown later. No significant changesoccurred in the OH region when gaseousCO was deposited at 5 K on the 5 K + 28 K + 10 K temperature cycled Ne:H,O ice. Fig. 6 shows the corresponding CO region dominated by the complex band at 2138.2 cm-‘. Subsequent warming of the system to temperatureshigher than 21 K causedthe appearanceof a new band in the CO region at 2 151 cm- ’ and in the OH region at 3640 cm-‘. Further warming of the ice substrate affected the two CO bands differently: the 215 1 cm-’ CO band gradually diminished with increasing

51 K 49 K 47K 45 K 43 K 41 K 39K 37 K 35 K 33 K 31 K 29 K 27K 25 K 20K 5K 2160.0

2150.0

2140.0 Wavenumber

2130.0

2120.0

cm?

Fig. 6. Infrared spectra obtained at different temperatures of a CO deposit on a previously temperature cycled ice originating from deposition of a 4:l Ne:H,O mixture at 5 K, see text.

Spectroscopy 12 11996) 1-14 155 K 150K 140K 130K 120K 1lOK 1OQK 90K 80K 70K 6OK 51 K 41 K 30K 20K 10K 2160.0

2150.0

2140.0 Wavenumber

2130.0

2120.0

cm“

Fig. 7. Infrared spectra obtained at different temperatures CO deposit on the same ice as presented in Fig. 6.

of a new

temperature, but was observed all the way up to 90 K; the 2138 cm- ’ band showed a monotonic red shift with temperature, a splitting into two components above 90 K, and disappearedonly around 155 K. This behaviour is demonstratedin Figs. 6 and 7 showing the CO bandsfrom two consecutive experiments with the same ice substrate. Redeposition of CO at 5 K on the ice previously annealedto 160 K produced only the CO,,,, band at 2138.2 cm-’ which remainedwithout significant changesup to 31 K and which completely disappeared at 40 K. Graphical representationsof intensity vs. temperature for the 2151 and 2138 cm-’ bandsas well as the peak wave number position for the main CO band around 2138 cm-’ vs. temperature are given in Figs. 8 and 9, respectively. A comparison of the present results with the argon experiment described in the previous section showsthat the water bandsbefore and after temperature cycling assumealmost exactly the same wave numbersin both cases.However, neon mixing is less effective in producing the small clusters with free dangling bonds, especially the 3-coordinated [ 12- 191 water bands are relatively weaker. Further, the ice produced by premixing and removing neon is far

A. Givan

0

et al. / Vibrational

0

0,o 0

50

100

TIKI

150

Fig. 8. Integrated band intensities as a function of temperature for the CO bands of the experiment presented in Fig. 7. (. ) The 2138 cm -I band; (0) the 2151 cm-’ band.

more effective in trapping gaseous contaminants as evidenced by the high temperature necessary to eliminate the CO related features. Insight into the processes of penetration of gases into amorphous ice and their subsequent thermal release can be gained by inspecting the temperature induced changes shown in Figs. 8 and 9. The temperature dependencies of the two CO bands at 2 15 1 and 2138 cm-’ are quite different (Fig. 8) and clearly imply that the two bands originate in CO molecules interacting significantly differently with the water ice matrix. Taking the v(C0) integrated band intensities as estimates of the amount of CO, it is found that approximately 60% of the CO associated with the 2138 cm-’ band is lost upon warming to 40 K (66% at 50 K etc., Fig. 8). In the very large 55-130 K

2138

21321 0

,

,

50

ICQ

,

TWI

Fig. 9. Temperature dependence of the ‘2138 cm-” the experiment presented in Fig. 7.

150

CO band of

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12 (1996)

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9

temperature range, the amount of CO associated with this band remains essentially unchanged, desorbing only in the 130-155 K range where two ice transitions, H 20casj -+ Ic at 137 K and Ic + Ih at 158 K [3], occur. These infrared spectra indicate that most of the deposited CO evaporated again at a temperature where the vapour pressure reaches 0.1 Pa and that part of the CO was trapped in deep ice pores by creeping water from where it was only released when the surrounding solid underwent drastic structural changes. The CO was probably trapped as chain or sheet like clusters with a vibrational wave number close to that of CO dimers [30], which only clearly emerges after the removal of the external solid CO. On the other hand, the amount of H-bonded CO, estimated from the intensity of the 2151 cm-’ band [18,36,38] (Fig. 8) increases up to 35 K indicating thermal diffusion on the surface and into the solid at low temperatures, cf. the infrared spectra in Fig. 6. In the 35-95 K temperature range, the amount of H-bonded CO gradually decreases until desorption is complete at 95 K. The clear slope of the intensity curve for the 2151 cm-’ band may be contrasted with the flat region of the analogous curve for the 2138 cm-’ band, and is further support for the non-bonded character of the latter. The low intensity free OH bands in the high frequency region behave only in partial accordance with this picture as they lose intensity and cannot be discerned above the 35-50 K temperature range. The temperature dependence of the main CO band around 2138 cm-‘, Fig. 9, shows a gradual change from the initial 2138.7 to the final 2133.4 cm-‘. This suggests that CO probes the temperature induced changes in the trapping ice solid. The stages of such annealing were previously investigated by the mass spectrometry of the evolving gases upon warming of ice layers containing trapped gaseous substances [20-221 and accompanied by electron diffraction studies [35]. A comparison of the steps suggested by our results with those of Ref. [20,21,35] is as follows: 31 K: External CO,,,, and a-CO starts to evaporate. Ref. [21] gives the same temperature, but according to Ref. [35] this evaporation starts already at 23 K. The difference in temperatures is due to the fact that we can only discern intensity changes by infrared

IO

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spectroscopy when the evaporation rate becomes significant. 37 K: CO adsorbed on the ice surface starts to evaporate. At this temperature the 3670 cm- ’ free OH band also disappears. Ref. [35] gives 34 K as the starting temperature for this process. 93 K: The stage starting at this temperature spans over 50 K. It is at this temperature that the 2151 cm-’ CO band disappears (Fig. 8). Also, for all of this temperature range the intensity of the main CO band remains essentially constant (Fig. 8). The suggested slow annealing and amorphous to glassy transition [20] is not reflected in our spectra. 145 K: Phase transition. Ref. [35] gives the value of 143 K as the onset temperature for the + Ic transition and 157 K for the w,as, Ic + Ih transition. The high temperature to which CO persists in the solid ice and the similarity in the behaviour of the gas release to the mass spectrometric studies [2022,351 clearly indicate that the method presented in this section is efficient in producing a water ice with high bulk porosity. The porous nature of the water could be retained over repeated warming cycles and CO depositions to about 50 K, with similar spectral behaviour. In the experiments of Devlin et al. [18], the CO was sprayed onto a previously deposited water layer and the 2151 and 2138 cm-’ bands were attributed to CO molecules penetrating into .su@zce micropores, which collapse when the sample is warmed to 70-120 K. Annealing to 160 K, beyond the ice phase transitions, prevents any repenetration or readsorption of CO and CO sprayed onto a 5 K ice yielded only the solid CO(,,, band, which remained essentially unchanged until its disappearance at 40 K. As mentioned earlier in this section, annealing of the Ne/H,O ice conglomerate to 160 K resulted in an astonishing ice spectrum which is compared with the ‘normal’ ice Ic spectrum in Fig. 10. As can be seen, the intensities and position of the substructure bands are very different from those in the ‘normal’ ice (Ic or Ih). In fact, the spectrum with its three clear band maxima around 3355, 3205 and 3063 cm-’ is in much better agreement with the Raman spectrum of ice Ih, presented by Whalley 1421, than

Spectroscopy

12 (19961 l-14

4000 3625

3250 Uavenumber

2875

2500

cm-'

Fig. IO. Infrared spectra obtained at IO K of the OH stretching band in ‘normal’ ice annealed to 150 K (curve A) and the Ne/H20 ice annealed to 160 K, see text (curve B).

previously presented ice Ic or Ih spectra. This better resemblance may originate in a higher degree of unicrystallinity in our annealed ice, c.f. the sloping tail towards lower wave numbers, arising because the amorphous starting sample was extremely porous with so many free OH groups that the annealing initiated H-bond formation released enough energy locally to zip or actually melt together the individual ice clusters. The nature of the ice may be further elucidated by inspection of spectral region from 2000 to 500 cm-’ containing the vZ bending mode as well as the vL and the 2v, bands. We have collected some relevant spectra from this study in Fig. 11 and scaled them according to the 3300 cm- ’ band. From bottom to top, the spectra are of: (A) pure amorphous ice, (B) ice deposited on Ar, (Cl ice deposited on Ne, (D) 1O:l H,O:CO, (E) 4:l Ne:H,O, (F) Ne:H20 ice after temperature cycling 5 K + 28 K + 5 K, (G) Ne:H,O ice after temperature cycling 5 K + 160 K + 5 K. As can be seen, all our ice deposits, except the H,O on a Ne substrate (curve D), have broader librational bands than the pure amorphous ice. After annealing the 4:l Ne:H,O ice to 28 K, when the Ne bath had evaporated and been pumped off, the librational band shifted to higher

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II

ing to 160 K, the librational band narrowed and moved to higher wave numbers (curve G) and was now composed of two components at 840 cm-‘, as in ice Ic [43], and 925 cm- ’ , respectively. The total bandwidth was 175 cm-’ compared with ca. 175 in ice Ic [43], but the shape is different. Further, the vz mode and the 2v, mode in this annealed solid are completely mixed. From these observations we draw the conclusion that H-bonding and O-O distances are actually more regular in this annealed ice than in the ordinary or normal ices, whose spectra have been labeled by ice Ic or Ih. 3.5. Co-deposition I 2300

3175

I 1650 Wavewmber

I 1000

1325 cd

Fig. 1 I. Infrared spectra at 5-10 K of the 2300-1000 cm-’ region showing the v2 bending mode around 1650 cm-’ and part of the combination band and librational overtone around 2250 cm-‘. (A) Pure H20caSJ ice, (B) ice deposited on an Ar substrate, (C) ice deposited cm a Ne substrate, (D) 1O:l H,O:CO ice, (E) 4: I Ne:H>O ice, (F) 5 K + 28 K + 5 K temperature cycled Ne:H20 ice, (G) 5 K + 160 K + 5 K temperature cycled Ne:HZO ice.

wave numbers, the uz and 2v, levels coupled more strongly and the spectrum resembles that of ice deposited on a Ne substrate (curve Cl. After anneal-

of H,O and CO vapour at 5 K

A 1O:l H,O:CO sample was studied, mainly to gain information concerning trapping sites, band frequencies and annealing effects in comparison with the argon and neon co-deposition experiments (Sections 3.3 and 3.4, respectively), but also because of its importance to interstellar chemistry [36,38]. Spectra of the relevant OH and CO regions are reproduced in Fig. 12, and peak wave numbers are listed in Table 1. The H,O/CO ice displayed an almost symmetrical, Gaussian band peaking at 3300 cm-’ (300 cm- ’ bandwidth) and accompanied by a much weaker band at 3623 cm-‘. For CO the doublet bands at 215 I and 2138 cm- ’ were observed, the

1OOK QJ P d B a

I

I

I

3800

3525

3250 hvenumber

65K llSK 1OOK 90K 6SK SK 47K

90K

4SK 40K 35K 27K 17K 10K

40K

-

2975 cm-'

2700

6SK %K 47K UK

35K 27K 17K 10K

70.0

2157.5 2145.3 Wavenumber

2132.5 cm-'

2120.0

Fig. 12. Infrared spectra obtained at different temperatures of the 3300 cm - ’ ice band and the CO bands of a 5 K deposit of a 1O:l H mixture. (A) Cooled from I15 K, (B) further cooled to 5 K.

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A. Giuan et al. /Vibrational

latter with a low wave number shoulder. Temperature cycling while keeping the pressure below lop5 Pa induced changes in both the 3300 cm-’ water band and in the CO bands. This is also illustrated in Fig. 12 and is discussed for each band separately below. The 3300 cm-’ band red shifted about 30 cm-’ upon annealing to 90 K, which is larger than demonstrated by pure water ice. As can be seen in Fig. 12, the most pronounced change in band shape and peak position took place between 40 and 45 K. This range also marks the end of surface CO evaporation, see later. After the annealing cycle (10 K + 115 K + 5 K) the band shape was almost indistinguishable from that of pure water ice, but the peak position was ca. 35 cm-’ higher. The 3623 cm-’ band gradually lost intensity with increasing temperature but was still clearly visible at 115 K. A corresponding behaviour was also observed for the CO band at 2151 cm-‘, see later. The main CO band at 2138 cm-’ showed a temperature variation which was significantly different from that of the Ne/H,O prepared ice sample (Section 3.4). The peak intensity of the 2138 cm-’ band (and of the 2151 cm- ’ band as well) decreased as the sample temperature increased. However, the 2138 cm-’ band fully recovered its initial peak intensity upon cooling back to 5 K, whereas the low wave number shoulder and most of the 2150 cm-’ band vanished. The CO band at 215 1 cm- ’ clearly dropped in intensity with increasing temperature, similar to the observations in the neon experiment (Section 3.4.). However, it was still present at 115 K (Fig. 12). considerably higher than recently reported [36]. It is also quite clear from Fig. 12 that the 3623 cm-’ OH band and the 2150 cm-’ CO band showed a parallel temperature behaviour, indicating a common origin. The intensity variation of the 2150 cm- ’ band was similar to that encountered in the Ne/H,O experiment (Fig. 8). First there was a slight increase in the 5-30 K range, which we attribute to diffusion of external CO into the amorphous ice, then followed a monotonic decrease from 30 K and upwards, although somewhat slower than in the Ne/H,O experiment. Fig. 13 shows how the integrated band intensity of the 2120-2160 cm-’ region varied with tempera-

Spectroscopy

12 (1996)

0

20

l-14

40

60

80

T [Kl

100

120

Fig. 13. Integrated band intensities (2160-2120 cm-‘) as a function of temperature for the CO bands of the experiment presented in Fig. 12.

ture. It is obvious that part of the CO was deposited as an external layer, which evaporated and was pumped off between 30 and 45 K. Judging by the IBIS alone, probably as much as 55% of the total CO amount was ‘external’. However, there was much more ‘internal’ CO in the present than in the recently reported experiment [36]. Electron diffraction patterns showed no change around 45 K [35], but according to a visual inspection [22] the cracks in porous amorphous ice disappeared within a few minutes around 45 K. We tentatively suggest that as the ‘external’ CO evaporates from the surface pores and grain boundaries, the ice restructures around these defects at ca. 45 K, and that this restructuring is the origin to the changed band shape of the 3300 cm-’ band. There has been some debate as to the origin of the two CO bands around 2151 and 2138 cm-’ in CO containing ices. It is obvious from Fig. 13 that the 215 1 cm- ’ band is a composite of several bands. The narrow component of external CO,,,,/cx-CO at 2138.6 cm-’ 1s clearly seen at temperatures below 35 K. However, it is also evident as a shoulder in the spectrum of the temperature cycled sample at 5 K. This suggests that the CO remaining in the sample above 45 K is already trapped during deposition as small clusters in cavities from where it cannot escape even at 115 K, but where there is a considerable degree of librational freedom leading to an apparent reversible loss of intensity at elevated temperatures. Further, the clusters have to be of considerable size for the solid state a-CO wave number to show up.

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Vibrational

4. Summary The experiments discussed above demonstrate that ice prepared by vapour deposition onto a 5 K substrate is essentially non-porous and with few non H-bonded hydroxyl groups at the surface. On the other hand, it is possible to produce amorphous water ices with surface and bulk porosity by means related to matrix isolation spectroscopy, i.e. inert gas predeposition and co-deposition followed by temperature cycling. Such ices can trap and contain significant amounts of volatile substances to much above their normal evaporation temperature. Of the experiments carried out, the most productive ones involve the deposition of premixed rare gas/water mixtures, with Ne mixtures yielding the most effective results in terms of porosity. For this latter sample, the 3300 cm-’ water band showed by far the largest shift upon temperature cycling (and rare gas release) to a wave number even below that of pure ice. The so prepared ice can keep CO trapped up to about 150 K. Further annealing of this amorphous and extremely porous ice to 160 K resulted in a very different ice spectrum which we suggest is the first infrared spectrum obtained of ice Ih. The ices were probed via the spectral features of CO, introduced by deposition or premixing. Two main CO signals observed are around 2138 and 215 1 cm-‘. The 2138 cm-’ band is shown to consist of at least three components, of which a narrow component at 2138.2 cm-’ originates from solid CO. Although speculative, the two other components of the band, around 2136 and 2141 cm-‘, could be due to n-bonded or CO..H-OH complexes. When CO is deposited on porous ice surfaces, the 2138 cm-’ band is accompanied by a weak satellite at 2147 cm-’ which we attribute to a (H,O),(CO), multimer complex. The 2138 cm-’ band is also associated with bulk trapping of CO, as evidenced by its persistence to temperatures close to 150 K. The band position, which remains close to that of solid CO, does not indicate a strong interaction with neighbouring H,O molecules and is therefore attributed to weakly interacting CO molecules, either via O-bonded [ 181 or accumulated in small clusters of CO trapped within larger ice cavities. The constancy of the intensity and wave number values over large temperature ranges,

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13

12 11996) I-14

followed by the abrupt disappearance, are in support of the latter suggestion. So also is the fact that in the porous ice no evidence was found for rotational motion, as seen in frozen CO/H,0 mixtures. The model of interstitial versus substitutional positions for CO of this wave number [36,38] is not well supported by the experimental observations, as the existence of H-bonding should be evident in both cases. Even though the CO water interaction is weak for this band, the gradual change in wave number with temperature is taken to reflect annealing changes in the surrounding water solid, most of which have parallels in mass spectrometric probing of gases evolving upon heating similarly produced samples 120-221. The 2151 cm-’ band, which to a certain extent finds its analog in our spectra as a weak band in the free OH region, is associated with H-bonded CO on the ice surface, OC..H-OH. Bands in the 2144 cm-’ to 2149 cm-’ range were assigned to the CO/H,0 dimeric complex in several matrix isolation studies [27-321. The present assignment is further supported by the temperature induced changes that this band reveals - an increase upon warming to 31 K, indicating solid penetration, followed by a gradual decrease in intensity until disappearance at 95 K. The temperature dependence of the intensity indicates a low interaction energy and may be related to the size of the cavities trapping the CO. In a previous study [44], we found that the interaction between weakly bound molecules (acetonitrile) significantly decreases with increasing size of the trapping site (Kr vs. Ar matrices).

Acknowledgements A.L. gratefully acknowledges the support by the European Environmental Research Organization (EERO) for a Short Term Fellowship at the University of Oslo.

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