Toward an Understanding of the Surface Chemical Properties of Ice

Chem. , 1996, 100 (33), pp 14151–14160. DOI: 10.1021/ .... Site Competition during Coadsorption of Acetone with Methanol and Water on TiO2(110). Min...
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J. Phys. Chem. 1996, 100, 14151-14160

14151

Toward an Understanding of the Surface Chemical Properties of Ice: Differences between the Amorphous and Crystalline Surfaces Jason E. Schaff and Jeffrey T. Roberts* Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455-0431 ReceiVed: May 17, 1996X

The interactions of several compounds with the surfaces of amorphous and crystalline ice-h2 and ice-d2 in ultrahigh vacuum were studied using temperature-programmed desorption (TPD) and Fourier transform infrared reflection absorption spectroscopy (FTIRAS). The following compounds were studied: acetone, acetonitrile, carbon tetrachloride, chloroform, diethyl ether, 1-hexene, and 2-propanol. Without exception, compounds possessing a functional group capable of accepting a hydrogen bond from the OH group of water show one or more thermal desorption states from amorphous ice that are absent from the crystalline surface. Conversely, for compounds with no such functional group, the desorption spectra from amorphous and crystalline ice are essentially identical. The unique states exhibit kinetic isotope effects for desorption from ice-d2, whereas states that are common to both amorphous and crystalline surfaces exhibit no isotope effect. FTIRAS measurements show that adsorption of good hydrogen bond donors on the amorphous ice surface is accompanied by the disappearance of the vibrational mode associated with the free surface OH group of ice. When poor hydrogen bond donors are adsorbed on ice, the free OH group persists. FTIRAS measurements indicate that the free OH coverage on the surface of crystalline ice is approximately one-sixth of that on amorphous ice. Four possible explanations are considered for the observed differences between amorphous and crystalline ice, based on differences in surface area, porosity, permeability, and surface chemistry. It is concluded that the surface chemical properties of amorphous ice are different from those of crystalline ice, probably because the hydrogen bond donor ability of the crystalline surface is less than that of the amorphous surface.

Introduction The study of chemical processes on the surfaces of ice and solid acid hydrates has grown explosively over the past decade, primarily due to interest in the origins of the stratospheric ozone hole. It is now generally accepted that chemical processes occurring on the surfaces of particulates contained in polar stratospheric clouds (PSC’s) convert photoinactive chlorine reservoir species, such as chlorine nitrate and hydrochloric acid, into photoactive species, such as dichlorine and hypochlorous acid.1,2 Reactive processes on PSC’s also lead to denitrification of the polar stratosphere via precipitation of nitric acid hydrate particulates. Although the overall chemistry occurring on PSC’s is reasonably well understood, the mechanisms involved in these transformations are still unclear. Many groups have undertaken investigations of ice and acid hydrate surface chemistry through methods that attempt to mimic polar stratospheric conditions (180 K < T < 210 K, relevant partial pressures).3-6 This approach is well suited to the determination of overall product distributions, kinetics, thermodynamics, and reaction coefficients. It does, however, have some limitations, in that it cannot easily be used to study the individual steps of multistep reactions or to distinguish between surface and near-surface phenomena. Another difficulty is that the structure of the surfaces being studied in is not clear, since, under stratospheric conditions, the evaporation and recondensation rates for ice are on the order of many tens of monolayers per second.7 The resulting surfaces are, therefore, highly dynamic and cannot easily be interrogated with most structural probes. This paper describes a different approach to the study of ice surfaces, one that exploits the more controllable environment * To whom correspondence should be addressed. Telephone (612) 6252363, FAX (612) 626-7541, E-mail [email protected]. X Abstract published in AdVance ACS Abstracts, August 1, 1996.

S0022-3654(96)01447-5 CCC: $12.00

afforded by ultrahigh vacuum (P < 1 × 10-6 Pa) and low temperatures (95 K < T < 170 K). An advantage of this approach is that, at temperatures below 140 K, the evaporation rate of ice is essentially zero, which means that ice surfaces are essentially quiescent, at least with respect to evaporation. Another advantage is that very thin ice films, on the order of several tens of monolayers, can be deposited under low vapor pressure conditions. Such films have high surface area to volume ratios, which makes it comparatively easy to distinguish between surface, near-surface, and bulk phenomena. The most obvious drawback of a low-temperature and -pressure environment is that the conditions are removed from those present in the stratosphere, although some work suggests that this may not be as much of a concern as one might at first believe.8 The chemical behavior of ice surfaces is of interest for reasons unrelated to stratospheric chemistry. In particular, the surface chemistry of molecular solids is generally poorly understood and is a subject which has not been extensively studied. Ice should be a particularly good candidate for developing methods to study such systems, since one type of chemical interaction, namely hydrogen bonding, may dominate the surface chemical behavior. Hydrogen bonding is generally easier to investigate than other weak intermolecular interactions due to the ready availability of deuterated compounds and the strong isotope effects produced by deuterium substitution. This paper concerns the chemical behavior and structure of the ice surface. In particular, we concentrate on a detailed description of both spectroscopic and kinetic probes for studying the hydrogen bond donor ability of ice surfaces and how this behavior is related to the surface structure of the ice films. These studies, preliminary results for which have been published previously,9 provide concrete evidence for a link between structure and surface chemical behavior of the ice films. © 1996 American Chemical Society

14152 J. Phys. Chem., Vol. 100, No. 33, 1996 Experimental Section 1. TPD Experiments. Temperature-programmed desorption (TPD) experiments were conducted in a stainless steel ultrahighvacuum chamber of base pressure approximately 1 × 10-8 Pa. The chamber was pumped continuously with a 300 L‚s-1 ion pump and intermittently with a titanium sublimation pump. Background gases were primarily H2, CO, He, H2O, and CO2. The chamber was equipped with a mass spectrometer (ExtrelC50, 1-800 amu), a double-pass cylindrical mirror electron energy analyzer with a coaxial 5 keV electron gun (Phi 15255G) for Auger electron spectroscopy (AES), and a precision x-y-z-θ sample manipulator (Thermionics EPM-200-RNN). The mass spectrometer was equipped with a movable pinhole shield approximately 5 mm in front of the entrance to the ionizer. Gases were admitted to the chamber using variable leak valves and deposited on a single-crystal metal surface, either W(100) (8 mm diameter) or Pt(111) (10 mm diameter). Adsorbates were deposited by positioning the sample crystal within 5 mm of the outlet of a 15 mm o.d. diameter tube connected to the leak valve. Exposures were measured as the product of the dosing time and the increase in total chamber pressure for the dose. Throughout this paper, exposures have been corrected for the enhancement factor of the dosers, which is approximately 100. Assuming a sticking coefficient of unity, a crystal temperature of 100 K, and complete accessibility of all surface sites, a monolayer of a typical adsorbate corresponds to an exposure of approximately 10-4 Pa‚s. The metal substrates were in thermal contact with a liquid nitrogen reservoir and could be cooled to approximately 95 K. A tungsten wire filament was used to radiatively heat the samples. The W(100) sample could also be heated via electron bombardment. Sample temperatures were measured using either a W/5% Re-W/26% Re (tungsten) or a type K chromel-alumel (platinum) thermocouple, with electronic ice points substituting for reference junctions. All data collection equipment was interfaced to an IBM compatible 286 computer. The W(100) sample was cleaned with oxygen according to established procedures,10 and the Pt(111) sample could be cleaned either via argon ion sputtering, using a converted ion gauge as a sputter gun, or via high-temperature oxygen treatment.11 Cleanliness of the metal surfaces was established using AES. 2. FTIRAS Experiments. The IRAS experiments were conducted in a separate stainless steel ultrahigh vacuum chamber of base pressure approximately 3 × 10-8 Pa, pumped continuously by a 400 L‚s-1 ion pump, and intermittently by a titanium sublimation pump. The background gas composition was similar to that in the chamber used for TPD. The chamber was equipped with a mass spectrometer (UTI 100-C, 1-300 amu) with glass pinhole cap for TPD studies and a precision x-yz-θ sample manipulator (Thermionics EM-201-RNN). Sample gases were admitted using variable leak valves similar to those found in the TPD chamber and were deposited on a singlecrystal metal substrate, either W(100) or Pt(111). The metal crystals were in thermal contact with a liquid nitrogen reservoir and could be cooled to approximately 100 K. Samples could be heated and cleaned by methods identical to those used in the other UHV chamber, and temperatures were measured using the same types of thermocouples found in the TPD chamber. Infrared spectra were collected using a commercial FTIR spectrometer (Nicolet Magna 550) that was modified for a single reflection absorption experiment. The instrument was controlled using an IBM compatible 486 computer. The beam was admitted into the chamber through a calcium fluoride window and focused onto the sample at an angle of approximately 4° to the metal surface using a calcium fluoride lens. The reflected

Schaff and Roberts beam exited the chamber through a second calcium fluoride window and was focused into an MCT-A detector using two mirrors. Beam throughputs averaged approximately 18% for both crystals under optimal conditions. Outside of the UHV chamber, the IR beam was enclosed in a dry air purge. The cutoffs of the calcium fluoride optics allowed for collection of spectral data between 4000 and 1000 cm-1, although the last 300 cm-1 on each end of this spectral range tended to show high noise levels. All spectra, unless stated otherwise in the text, were collected using 512 scans at a resolution of 4 cm-1, with an average data point spacing of 1.95 cm-1. Background spectra were normally acquired after the sample of interest had been desorbed from the crystal substrate. 3. Chemicals. Deionized water was subjected to multiple osmotic filtrations using a Millipore purification system. Water samples were then degassed prior to use each day via multiple freeze-pump-thaw cycles. Deuterated water (CIL, 99.9% isotopic purity), acetone (Fischer, 99.7%), acetonitrile (EM Science, 99.8%), 2-propanol (Fischer, 99.9%), chloroform (Fischer, 99.9%), carbon tetrachloride (EM Science, 99.99%), and 1-hexene (Aldrich, 99+%) were degassed prior to use each day and otherwise used as received. Diethyl ether (tech grade) was dried over molecular sieves and degassed before use each day. Oxygen (Matheson, 99.6%) and argon (Matheson, 99.9995%) were used as received. Results 1. Characterization of Ice Thin Films. a. Desorption Studies. Ice-h2 films approximately 80 monolayers (ML) thick were prepared by condensing water vapor onto a cold (≈100 K) metal substrate at a rate of approximately 0.75 ML‚s-1. For these deposition conditions, the films are in the metastable amorphous state.12 Previous work has shown that, for thicknesses greater than 6 ML, the films are free of pinholes that extend from the ice-vacuum to the ice-metal interface.13 The sublimation energy of a thin ice film was determined to be 47 ( 5 kJ‚mol-1, based on the average of the values from five experiments.14 Thin films of ice-d2 were prepared in an identical fashion, and the sublimation energy, again the average of five experiments, was found to be 51 ( 4 kJ‚mol-1. For some experiments, amorphous ice thin films were annealed to induce crystallization. Ice-h2 films were crystallized by heating briefly to ≈174 K, followed by annealing at 150-155 K for 60 s. Based upon bulk thermodynamics, the resulting films are probably in the cubic crystalline phase,15 although some work suggests that the phase behavior of ice in this regime may be more complicated than was originally believed,16 particularly for ice films grown on a Pt(111) surface.17 Thin ice-d2 films were annealed in a similar fashion, except that the initial brief heating was done at ≈180 K. For both types of film, annealing leads to the desorption of several monolayers of the thin film. Heats of sublimation of the crystalline films, averaged across five experiments, were calculated assuming zeroth-order kinetics and found to be 52 ( 1 and 55 ( 1 kJ‚mol-1 for the normal and deuterated films, respectively. Both of these values compare favorably with the literature values of 50.1 and 51.7 kJ‚mol-1.18 b. Infrared Studies. Thin (10-15 ML) films of amorphous ice-d2 deposited on Pt(111) were studied using infrared reflection absorption spectroscopy (IRAS). Thinner films were used for these studies than for the TPD studies in order to enhance sensitivity for surface structural features. Ice-d2 was used for most of the infrared experiments. A typical spectrum of an ice-d2 thin film is shown in Figure 1a. The spectrum is broad and featureless between 2200 and 2700 cm-1, as has been observed by other researchers,19 with a small, sharp feature at

Surface Chemical Properties of Ice

Figure 1. Infrared spectra of ≈10 ML thick amorphous (a) and crystalline (b) ice-d2 films. Collection conditions are given in the text. The inset (c) is a difference spectrum for the crystallization of an amorphous film. Expanded regions are 2700-2800 cm-1.

2727 cm-1. This feature is assigned, based on work by Devlin and Buch,20 to the stretching mode of free OD groups on the surface of the ice film. Norton and co-workers have shown the mode to be nearly constant in intensity over a wide range of ice film thicknesses.21 The infrared spectra of crystalline ice-d2 thin films, prepared using the annealing procedure described above, were also studied. A typical spectrum of a crystalline film is shown in Figure 1b. Two significant differences between the spectra of the crystalline and annealed films are immediately apparent. First, the broad, featureless region of the amorphous spectrum develops into three sharp, distinct features at 2485, 2432, and 2345 cm-1. This change is consistent with the conversion of solid amorphous water to a crystalline phase.22 Second, for films this thin, the intensity of the surface free OD feature at 2727 cm-1 is significantly reduced upon crystallization. This reduction in the free OD stretch is not the result of the peak being obscured by the shoulder of the highest-frequency ice-d2 peak, as is show by the spectrum in Figure 1c, which is the result of using a spectrum of the initially deposited amorphous film as the background for the spectrum of a crystalline film. It should be noted that the reduction in intensity of the free OD feature is less for thicker films, the reasons for which will be discussed in detail later. 2. Adsorption on Ice-h2. The adsorption of acetone, acetonitrile, 2-propanol, diethyl ether, chloroform, and 1-hexene on thin films of amorphous and crystalline ice-h2 was studied using TPD. Sample desorption spectra for these compounds on films of amorphous and crystalline ice, as well as representative ice sublimation spectra, are shown in Figure 2. Water was monitored as m/e 20 (H218O+), acetone as m/e 58 (C3H6O+), acetonitrile as m/e 40 (C2H2N+), diethyl ether as m/e 59 (C3H7O+), 2-propanol as m/e 46 (C13CH5O+), chloroform as m/e 47 (C35Cl+), and hexene as m/e 84 (C6H12+). For the data in Figure 2, the adsorbate exposures, which were calibrated against the dose required to saturate the clean W(100) substrate, were as follows: 1.5 × 10-4 Pa‚s (2 ML) for acetone, 4 × 10-5 Pa‚s (1 ML) for acetonitrile, 3.2 × 10-4 Pa‚s (4 ML) for 2-propanol, 8 × 10-5 Pa‚s (0.75 ML) for diethyl ether, 4 × 10-5 Pa‚s (1 ML) for chloroform, and 1.6 × 10-4 Pa‚s (2 ML)

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Figure 2. Thermal desorption spectra of several species from ≈80 ML films of amorphous and crystalline ice: (a) acetone, (b) acetonitrile, (c) 2-propanol, (d) diethyl ether, (e) chloroform, (f) 1-hexene, and (g) multilayers of ice. The monitored mass numbers and exposures are given in the text. All spectra were collected with heating rates between 3.5 and 5 K‚s-1.

for 1-hexene. The adsorbates were deposited at rates of between 0.01 and 0.05 ML‚s-1. The dashed lines in Figure 2 correspond to the approximate temperature at which 1 ML of an ≈80 ML thick ice film has sublimed into the gas phase. Thus, in most cases, desorption of the adsorbed molecules is complete before significant sublimation of the ice film has occurred. For acetone, two states, designated R- and β-acetone, are observed at 133 and 157 K, respectively, from the amorphous films. Only the R state is observed in significant yield from crystalline ice. An additional state is sometimes observed coincident with the water desorption peak, but this state is not consistently present and appears to be the result of either coadsorption of acetone from the chamber background gases during growth of the ice films or displacement of acetone from the chamber walls induced by the ice desorption. Similar spurious states were observed for most of the other adsorbates studied. The coverage dependencies of the acetone desorption spectra are discussed in detail elsewhere.23 Briefly, the R and β states grow simultaneously as a function of exposure on amorphous ice until 1.6 × 10-4 Pa‚s, at which exposure the β state saturates. The R state gives way to the multilayer sublimation feature at ≈2.7 × 10-4 Pa‚s on both types of films. Because it saturates with increasing exposure, and for other reasons detailed below, β-acetone can be unambiguously associated with an adsorbed state. Desorption spectra were obtained for acetone on amorphous and crystalline ice films grown on both types of metal substrate and in both vacuum chambers. In no case was there any significant variance in the observed results. Desorption spectra were also obtained for acetone adsorbed on ice films which had been grown at different temperatures, varying between 100 and 140 K. For films grown under ≈125 K, the desorption spectra were identical to the results described above for amorphous ice films. Once the growth temperature was increased to ≈130 K, however, the desorption yield of β-acetone began to decrease significantly. By ≈140 K, at which temperature the ice should be deposited in the crystalline form,15 the β state had become almost undetectable in the TPD spectra. Acetonitrile shows desorption states at 146 and 160 K, designated R- and β-acetonitrile, respectively, from both amorphous and crystalline films, but the β state is shifted up in

14154 J. Phys. Chem., Vol. 100, No. 33, 1996 temperature to 164 K on crystalline ice. On both types of ice, the β state grows in first, followed by the R state at 2 × 10-5 Pa‚s. The R state converges with the multilayer sublimation feature at an exposure of ≈1.3 × 10-4 Pa‚s. On the basis of experiments conducted on films of ice-d2 (see below), it appears that β-acetonitrile actually obscures a second, nearly coincident state, designated γ-acetonitrile on amorphous ice. 2-Propanol shows desorption states at 166 and 175 K, designated R- and β-2-propanol, on amorphous ice films, while only the R state is observed on crystalline ice films. On amorphous films, only the β state is present at very low 2-propanol exposures. Neither of these states can be unambiguously attributed to adsorbed 2-propanol, since neither saturates, even for 2-propanol exposures as large as 20 ML. The β state, does, however, grow much more slowly than the R state at high exposures. There is no significant net conversion of 2-propanol between the R and β states on amorphous ice films at ≈100 K, even if the film is allowed to rest for 45 min between adsorption and the acquisition of the TPD spectrum. Both states, at sufficiently high exposures, show desorption behavior which is either zeroth-order or pseudo-zeroth-order, with a sublimation energy of 56 ( 1 kJ‚mol-1 for the R state. The desorption energy for the β state could not be extracted, since its leading edge was obscured by the tail of the R state. This value is considerably higher than the literature value of 47.4 kJ‚mol-1 for the heat of sublimation of 2-propanol.18 On amorphous ice, diethyl ether shows three desorption states: R-ether in a broad peak at 128 K, β-ether at 166 K, and γ-ether at 176 K. On crystalline films, the β and γ states are replaced by a single state at 174 K, designated δ-ether. On amorphous films, the ether desorption states appear sequentially with increasing exposure in the order γ-ether, β-ether (at ≈2.7 × 10-5 Pa‚s), R-ether (at ≈5.3 × 10-5 Pa‚s), while on crystalline films the order of appearance is δ-ether followed by R-ether at 5.3 × 10-5 Pa‚s. On both types of films, the R state coalesces with the multilayer desorption feature at an exposure of ≈3.3 × 10-4 Pa‚s. The β and γ states can be unambiguously associated with adsorbed molecules, for reasons described below. Chloroform shows a single desorption state at 144 K, designated R-chloroform on both amorphous and crystalline ice surfaces. This state coalesces with the multilayer sublimation peak at exposures greater than ≈2 × 10-4 Pa‚s. The desorption behavior of chloroform from the two types of ice is indistinguishable. The desorption spectra of hexene from amorphous and crystalline ice are also essentially identical. Two desorption states, a multilayer feature at 135 K and R-hexene at 145 K, are observed. The R state grows in first as a function of pressure followed by the multilayer at ≈8 × 10-5 Pa‚s. The R state saturates at an exposure of ≈9.3 × 10-5 Pa‚s and is attributed to an adsorbed state. The effects of ice film thickness on desorption yields for the β state of acetone and for the β and γ states of diethyl ether were investigated. In all cases, approximately 2.5 ML of the compound of interest was adsorbed onto amorphous ice, where 1.0 ML represents the amount of material required to saturate the underlying metal substrate. Films with thicknesses of between 2 and 80 ML were investigated, and desorption yields of the adsorbed states were evaluated by integrating the corresponding TPD peaks with respect to time. For β-acetone, the desorption yield was essentially constant for films thicker than ≈8 ML, as is shown in Figure 3. For β-ether, the desorption yield became constant once the ice thickness became greater than ≈6 ML, while the yield of γ-ether saturated for ice films thicker than ≈10 ML. These results strongly indicate

Schaff and Roberts

Figure 3. Desorption yield of several surface species from amorphous ice films as a function of ice film thickness. The solid lines that are drawn through the data points are intended to guide the eye, and do not represent a fit to the data.

Figure 4. Comparison of the desorption spectra of several small molecules from amorphous films of ice-h2 and ice-d2: (a) acetone, (b) acetonitrile, (c) 2-propanol, (d) diethyl ether, (e) chloroform, and (f) 1-hexene. For each pair of spectra, the upper trace is desorption from ice-d2, and the lower trace is desorption from ice-h2. The monitored mass numbers and exposures are given in the text. Heating rates for all experiments were in the range 3.5-5.0 K‚s-1. Some pairs of spectra have been offset slightly on the temperature axis in order to correct for differences in heating rates. Sensitivity factors are not necessarily identical.

that all of these states arise from material adsorbed on the surface or in the very near surface regions of the ice films. These results also correlate well with previous observations that, for films thicker than about 6 ML, the underlying metal substrate is completely isolated from the gas phase.13 It is worth noting that the total desorption yield of adsorbed states for acetone and diethyl ether is never more than half the amount of material required to saturate the clean W(100) surface. 3. Adsorption on Ice-d2. The adsorption of acetone, acetonitrile, diethyl ether, 2-propanol, chloroform, and 1-hexene on thin films of amorphous and crystalline ice-d2 was studied using TPD. Sample desorption spectra for each of these compounds on films of amorphous ice-h2 and ice-d2 are shown in Figure 4. Deuterated water was monitored as m/e 22 (D218O+), acetone as m/e 58 (C3H6O+), acetonitrile as m/e 40 (C2H2N+), diethyl ether as m/e 59 (C3H7O+), 2-propanol as m/e

Surface Chemical Properties of Ice 43 (C2H3O+), chloroform as m/e 47 (C35Cl+), and hexene as m/e 84 (C6H12+). For each of the first four compounds, a kinetic isotope effect is observed for at least one of the desorption states from amorphous ice. Specifically, those states which are unique to amorphous ice exhibit an isotope effect, while, except for R-2-propanol, those that are common to both types of ice exhibit no effect. For chloroform and hexene no isotope effects are observed in the desorption spectra from amorphous ice. Except for 2-propanol, the desorption spectra of all compounds from crystalline ice-h2 and ice-d2 were essentially identical. Also, again except for 2-propanol, none of these substances undergoes H-D exchange with either amorphous or crystalline ice-d2. The isotope effects during TPD from amorphous ice are summarized as follows. Acetonitrile has an additional desorption state at 175 K, designated γ-acetonitrile, from amorphous ice-d2, when compared to amorphous ice-h2. We attribute this additional state to surface hydrogen bound acetonitrile, shifted away from the normally nearly coincident surface physisorbed state through an isotope effect. For acetone, the β state undergoes an inverse isotope shift of approximately 3 K. For diethyl ether, both the β and γ states show positive isotope effects, of 2 and 4 K, on the amorphous films. Both the R and β states of 2-propanol showed positive isotopic shifts, of approximately 5 and 9 K, respectively, on deuterated films. Both desorption states also show extensive deuteration of the desorbing 2-propanol as determined by measuring the relative abundance of m/e 45 (C2H5O+) and m/e 46 (C2H4DO+) in the TPD spectra. Deuteration was approximately 35% for the R state and ranged from about 40%, at low coverage, to about 55%, at high coverage, for the β state. The lone R state of 2-propanol is shifted to higher temperature on crystalline films of ice-d2 by approximately 4 K and shows a level of deuteration comparable to that observed on amorphous ice-d2 films. 4. FTIRAS of Adsorbates on Thin Ice Films. The adsorption of acetone, acetonitrile, 2-propanol, diethyl ether, chloroform, carbon tetrachloride, and 1-hexene on thin films of amorphous and crystalline ice-d2 was studied using single reflection Fourier transform infrared spectroscopy. Sample infrared spectra for approximately 1 ML of each of these compounds on amorphous films are shown in Figure 5. The doses used in these experiments were not well enough calibrated to allow determination of more accurate adsorbate coverages. For amorphous ice-d2, the free surface OD stretching vibration disappears completely upon adsorption of submonolayer quantities of any of the first four compounds listed. That the free surface OD feature is titrated away by these adsorbates is unequivocally shown by the difference spectra for adsorption of small quantities of acetone, presented in Figure 6. These spectra were obtained by using the spectrum of a clean amorphous ice film as the background for spectra of acetone back-filled onto that same film. Note that the total decrease in intensity of the free OD stretch is comparable to that observed upon crystallization of a thin amorphous film as shown in Figure 1c. For thick ice films, it was found that adsorption of acetone on crystalline ice also “titrated” away intensity at 2727 cm-1. The origin of this effect will be discussed below. Upon adsorption of chloroform or carbon tetrachloride, the free surface OD peak is red shifted by approximately 30 cm-1 and considerably broadened. This effect is similar to one reported by Chesters and co-workers for the adsorption of chlorofluorocarbons on ice24 and is attributed to a physisorptive interaction between the adsorbates and the free surface OD groups. Adsorption of 1-hexene does not noticeably shift the position of the free OD band but does broaden the feature. This

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Figure 5. Infrared spectra of several small molecules on films of amorphous ice-d2. The exposures were typically sufficient to completely saturate the ice surface. Collection conditions are given in the test. The 1-hexene and carbon tetrachloride coverages are already sufficient to broaden the free OD peak below base line noise levels, a phenomenon that is described in the text.

Figure 6. Infrared difference spectra of various exposures of acetone on amorphous ice-d2: (a) 4, (b) 12, (c) 28, (d) 60, (e) 92, and (f) 130 × 10-6 Pa‚s exposures. Collection conditions are given in the text. Note the presence of two carbonyl states at higher coverages.

broadening increases with increasing 1-hexene exposure, until the free OD band is no longer discernible above spectral noise. With the exceptions of acetone and acetonitrile, none of the compounds studied show any significant changes in vibrational frequencies upon adsorption on ice films when compared with comparable exposures on the passivated Pt(111) surface. For very small doses of acetone (