J. Phys. Chem. B 1997, 101, 10887-10895
10887
Experimental Studies of Vapor-Deposited Water-Ice Films Using Grazing-Angle FTIR-Reflection Absorption Spectroscopy Mark A. Zondlo, Timothy B. Onasch, Matthew S. Warshawsky, and Margaret A. Tolbert CIRES and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0216
Govind Mallick, Pamela Arentz, and Marin S. Robinson* Department of Chemistry, Northern Arizona UniVersity, Flagstaff, Arizona 86011-5698 ReceiVed: March 19, 1997; In Final Form: October 2, 1997X
Grazing-angle Fourier transform infrared reflection absorption spectroscopy was used to monitor the free OH stretch (or “dangling bond”) in vapor-deposited H2O-ice films between 94 and 120 K. Ice film thicknesses and the sensitivity of our instrument to water-ice molecules were determined by optical interference using a helium-neon laser. These calibrations indicate that the dangling bond signals observed in the present study are indicative of the surfaces of micropores present within the amorphous ice bulk. The largest dangling bond signal (corresponding to the largest number of micropores) was observed at 94 K under conditions of fast ice growth while the smallest signal was observed at 120 K under conditions of slow growth. The temperature and pressure dependence of the dangling bond signal during film growth was used to estimate a barrier to diffusion (Edif) for H2O on amorphous ice. We measured an upper limit of Edif ) 4.2 ((0.5) kcal mol-1, consistent with a theoretically derived value of Edif ) 2.5-3 kcal mol-1. The decay of the dangling bond over time (corresponding largely to the collapse of the micropores) was monitored in ice films roughly 100 nm thick. With initial deposition rates of 2 nm s-1, the decay took 125 and 175 min at 118 and 112 K, respectively. Faster deposition rates and colder temperatures decreased the decay rate.
I. Introduction In recent years the physical and chemical properties of ice have been investigated by a wide range of scientists, including physical and atmospheric chemists,1-3 astrophysicists,4-8 and theoreticians.9-11 Researchers are particularly interested in understanding the chemistry that takes place on the surface of ice films. Interest in this area has been stimulated, in part, by the discovery that heterogeneous reactions on ice clouds play an important role in stratospheric ozone depletion.12 As a result, several groups13-17 have employed grazing-angle Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) to investigate ice films. This technique, proposed theoretically by Greenler,18 purportedly offers enhanced surface sensitivity of thin films. To this end, grazing-angle FTIR-RAS has been used to study the interactions between ice and HCl19 as well as the ionization and solvation of ice-bound N2O5 and ClONO2.20 In this paper grazing-angle FTIR-RAS was used to monitor the temperature- and pressure-dependent integrated absorbance of the non-hydrogen-bonded OH stretch (“dangling bond”) in microporous/amorphous ice. These data were used to infer structural changes in amorphous ice and to estimate the barrier to diffusion on the ice surface. Amorphous (noncrystalline) water-ice forms at substrate temperatures e 130 K.21,22 Above 130 K, ice forms either a cubic (133-153 K) or hexagonal (above 153 K) crystalline structure.22 Depending on temperature and deposition rate, amorphous ice also can be microporous.23 Microporous ice forms when condensing H2O molecules have insufficient time to rearrange before being buried by subsequent adlayers. As a result, they are buried in random orientations leaving gaps within the ice bulk. When these gaps become * To whom correspondence should be addressed. E-mail:
[email protected].. X Abstract published in AdVance ACS Abstracts, November 15, 1997.
S1089-5647(97)00987-5 CCC: $14.00
sufficiently wide, they prevent hydrogen bonding across pores, forming 3-coordinate, non-hydrogen-bonded OH groups at the pore surface known as “dangling bonds”.23 Techniques employing FTIR spectroscopy show that the dangling bond in water-ice appears at 3699 cm-1, shifted to higher frequencies than the H-bonded OH stretch (3378 cm-1).14,15,23 The dangling bond has been studied by other researchers using grazing-angle FTIR-RAS. Purportedly, this technique has detected dangling bonds at the true surface of amorphous ice (between 90 and 130 K), after the micropores have collapsed.13-15,19 In contrast, no dangling bonds have been observed at the surface of crystalline ice (170 K).15 These data have been used to suggest that the surface characteristics of amorphous ice are significantly different from those of crystalline ice.15 In contrast to the FTIR-RAS studies cited above, the dangling bonds monitored in this work reside predominantly on the surfaces of ice micropores located within the bulk ice structure. Because it is important to differentiate between dangling bonds on the surface of ice micropores versus dangling bonds on the true amorphous/crystalline surface, it was necessary to accurately characterize the sensitivity of the FTIR-RAS technique toward H2O-ice films. Two measures of surface sensitivity were employed. The first used optical interference techniques with a helium-neon laser to estimate the thickness of our thinnest detectable ice film. The second monitored the infrared spectra of CF4 adsorbed on ice at 95 K. Optical interference methods also allowed us to determine the thicknesses of ice films up to 1000 nm (1.0 µm). These results were checked by two independent measures of film thickness: (1) comparison of experimental FTIR-RAS spectra with calculated FTIR-RAS spectra and (2) determination of film thickness through knowledge of deposition rates and deposition times. © 1997 American Chemical Society
10888 J. Phys. Chem. B, Vol. 101, No. 50, 1997
Zondlo et al.
Figure 1. Schematic of FTIR-RAS apparatus from (A) front view and (B) top view.
an1 - 2bn2 ) mλ
II. Experimental Section A. Instrumental. All experiments were conducted in a newly constructed FTIR-RAS apparatus shown schematically in Figure 1. The high-vacuum instrument (base pressure ) 5 × 10-8 Torr) consists of two stainless steel chambers separated by a gate valve (Figure 1A). The top chamber supports a vertically mounted, optically flat aluminum (Al) substrate (d ) 9.14 cm) upon which ice was condensed, two KCl windows for the passage of the infrared beam, a Bayard-Albert ionization gauge, and a Baratron capacitance manometer. The ionization gauge pressure (PIG) was calibrated for water using the absolute pressure of the capacitance manometer (PCM), resulting in PCM ) 1.4PIG. Errors in pressure were estimated to be (15%. The bottom chamber houses a Balzer’s turbomolecular pump (240 L s-1) and a UTI quadrupole mass spectrometer. Water (HPLC grade, Aldrich) was introduced into the bottom chamber through a precision leak valve (Lesker) after undergoing at least three freeze/pump/thaw cycles on an attached vacuum glass line. Infrared spectra were collected using a Nicolet 550 Magna FTIR spectrometer with a silicon-carbide source and a liquid nitrogen cooled HgCdTe detector. Spectra represent the coaddition of 128 scans at 8 cm-1 resolution. The infrared beam was passed through a Molectron wire-grid polarizer before reflecting off the Al substrate at 83° ((2°) from the surface normal (Figure 1B). The substrate was cooled through thermal contact with a liquid nitrogen filled cryostat, and then resistively heated to the desired temperature (90-163 K) using a Kapton heater (Minco) controlled by a Eurotherm temperature programmer. The substrate temperature was measured by averaging the output of 2-4 radially spaced, T-type (copper-constantan) thermocouples permanently mounted to the aluminum surface with a thermally conductive stycast epoxy (Lakeshore Cryogenics). Radial gradients across the surface were (2 K, and absolute temperatures were accurate to (2 K. The aluminum surface was vacuum-sealed by a Teflon O-ring into a differentially pumped stainless steel sleeve maintained at temperatures above the ice frost point. In this way, the aluminum substrate was the only surface in the chamber cold enough to condense ice. B. Film Thickness. Ice film thicknesses were determined by optical interference using a helium-neon laser (λ ) 632.8 nm). The laser beam passed into the top chamber and was incident on a growing ice film at θ1 ) 22° ( 4° from the surface normal. The reflected light was detected by a photodiode and converted to a digital signal. Constructive interference occurs when the path length difference between the reflected (an1) and refracted (2bn2) beams (Figure 2) is an integral number of wavelengths (m):
(1)
By invoking Snell’s law (n1 sin θ1 ) n2 sin θ2) and various trigonometric identities, this equation can be expressed in terms of film thickness (x):
2n2x - 2n1 x sin θ1 tan θ2 ) mλ cos θ2
(2)
By using the refractive indices for vacuum (n1 ) 1) and ice (n2 ) 1.3084) at 632.8 nm, the solution to eq 2 yields x ) 252 nm at one constructive interference fringe. Because it was relatively easy to locate the first fringe in the interference data, we used 252 nm as a reference thickness. A typical optical interference fringe during the growth of an ice film at 165 K is shown in Figure 3A. In this figure the helium-neon signal from the photodiode is plotted as a function of time. The photodiode signal at any given time (t) was related to ice film thickness (x) by the following relation:
x(t) )
( (
))
2(s0 - st) f arccos 1 2π h
(3)
where f is the thickness at the first constructive interference (252 nm, determined by eq 2), s0 is the initial photodiode signal before ice nucleation, st is the photodiode signal at time t, and h is the signal difference between complete constructive and destructive interference thicknesses. Because the HeNe signal is fairly insensitive to changes in ice film thickness near the fringe turning points, film thicknesses in these regions (175-275 nm) were determined by a different method. With a constant H2O pressure over the film, film growth rates were calculated according to the equation24
( )
-Ed υ0 dx RP exp ) dt F(T)[2πmkT ]0.5 F(T) RTs g
(4)
where R is the condensation coefficient (R ) 1),25 P is the partial pressure of water, F(T) is the temperature-dependent density of the ice (F ) 0.82 g cm-3 at Ts ) 94 K; F ) 0.85 g cm-3 at Ts ) 105 K; and F ) 0.93 g cm-3 at Ts ) 120 K),25 υ0 is the zero-order desorption preexponential (2.8 × 1030 molec cm-2 s-1),25 m is the molecular weight of H2O (18.0 g mol-1), Tg is the impinging gas temperature (298 K), k is the Boltzmann constant (1.381 × 10-23 J K-1 molec-1), Ed is the desorption activation energy (48.12 kJ mol-1),24 and Ts is the substrate temperature.
Vapor-Deposited Water-Ice Films
Figure 2. A schematic of the 3-layered vacuum-ice-Al system used to determine (a) film thickness by optical interference methods at nearnormal incidence (θ1 ) 22° ( 4°) using unpolarized light from a helium-neon laser and (b) the calculated grazing-angle (θ1 ) 83° ( 2°) spectra of water-ice films using Ep- and Es-polarized light.
Figure 3. (A) Helium-neon optical interference pattern for ice grown at 165 K at a water partial pressure of 8.7 × 10-6 Torr. (B) Plot of the integrated absorbance of the main OH stretch (2850-3800 cm-1) for crystalline ice (Ep-polarized light) versus ice film thicknesses as determined from the helium-neon optical interference data at 165 K. A weighted linear least-squares fit yielded the equation y ) 2.08x 1.48 ((2σ slope ) 0.04 absorption units nm-1, (2σ y-int. ) 1.1 absorption units).
By knowing the elasped time before the first fringe and the corresponding deposition rate, film thicknesses near the fringe turning points could be more accurately calculated in this fashion than by optical interference alone. We note, however, that this method is only applicable when the pressures were constant in time and when calibrated to an accurate film thickness. For example, using P(H2O) ) 6.0 × 10-6 Torr and a substrate temperature of 94 K, the growth rate (dx/dt) calculated from eq 4 corresponded to 1.0 nm s-1. At this growth rate it should take 252 s to grow a film to the first constructive interference
J. Phys. Chem. B, Vol. 101, No. 50, 1997 10889 fringe from the onset of ice nucleation. Experimentally, we found that it took as little as 180 s to grow such a film. Because the growth rate was uncontrollably fast immediately after nucleation, the incident water flux had to be adjusted to reestablish the desired pressure. Within this initial adjustment period it was difficult to accurately measure pressure for the thinnest films. In addition, during this phase of growth, it is unclear if the measured pressure was representative of the true H2O pressure over the ice surface. Therefore, this method of calculating film thicknesses can only be applied over the time scale of constant H2O pressures. Indeed, the experimentally determined deposition rates between the first (m ) 1) and second (m ) 2) interference fringes (when H2O pressures were steady) agreed to within 15% of the calculated deposition rates from eq 4. As a final check on film thickness, experimental FTIR-RAS spectra were compared with calculated FTIR-RAS spectra of water-ice films at 163 K. Calculated FTIR-RAS spectra of ice at 163 K were obtained using the equations formulated by Greenler18 which describe the classical optics of a 3-layered isotropic system (Figure 2). Mathematica software was used to solve for the ratio of parallel (Ep) or perpendicularly (Es) polarized outgoing (E1-) and incoming (E1+) electric fields on an aluminum substrate at a specified film thickness (x) and angle of incidence (θ1). Calculated spectral data were plotted as absorbance (A) versus wavenumber (cm-1), where A ) log R0/ R, and R and R0 are the reflectance (E1-/E1+) with and without an ice film, respectively. These values were compared to experimentally measured absorbance, A ) log I0/I, where I0 and I represent the reflected light intensity before and after passing through the ice film. The real refractive indices (n2) and the absorption indices (k2) for ice at 163 K as a function of wavelength (2700-3800 cm-1) were taken from Toon et al.26 The corresponding wavelength-dependent optical indices for metallic Al (n3, k3) were derived from room-temperature values reported in Palik.27 A curve-fitting program was used to fit an equation to the smooth curve obtained by plotting n3 (k3) against wavenumber (700-7000 cm-1). The equation was solved for n3 (k3) at the same wavenumbers as n2 (k2) for ice. No attempt was made to extrapolate n3 and k3 to 163 K. Sensitivity. The sensitivity of the infrared spectra to changes in the crystalline H2O-ice film thickness was determined from a calibration of infrared absorbance versus film thickness obtained using optical interference. These results were checked qualitatively by examining the adsorption of CF4 on ice at 95 K. Rowland et al.28 and Buch et al.29 have shown that CF4 at partial pressures of 0.001 to 0.150 Torr adsorbs on ice at 83 K to around monolayer coverages. For experiments examining CF4 coverage on ice, a vapor-deposited ice film at 140 K was annealed to 165 K and allowed to convert from an amorphous to crystalline ice state. The ice film was then cooled to 95 K and exposed to incrementally higher CF4 partial pressures ranging from 10-4 to 10-1 Torr. Infrared spectra were taken once the CF4 partial pressures were stable to within 1%. Gasphase spectra of CF4 were taken at 295 K in the absence of an ice film over the same range of CF4 partial pressures. An estimate of the sensitivity of FTIR-RAS toward CF4 is then made by comparing the spectral features of adsorbed CF4 to the calculated CF4 surface coverage on ice. III. Results A. Film Thickness. In Figure 4 we compare experimentally observed FTIR-RAS spectra of 163 K water-ice films on an Al substrate with calculated FTIR-RAS spectra based on the classical optics of an isotropic multilayered film as formulated
10890 J. Phys. Chem. B, Vol. 101, No. 50, 1997
Zondlo et al.
Figure 4. Comparison of experimental (s) and calculated (- - -) grazing-angle FTIR-RAS spectra (θ1 ) 83°) for crystalline water-ice films 0.1, 0.5, and 1.0 µm thick on Al using Es- and Ep-polarized light. Experimental ice films were vapor-deposited at 165 K. Calculated data were based on the optical constants of water-ice at 163 K and Al at 298 K.
by Greenler.18 The two optical effects described by Horn et al.30 for water-ice on a gold (Au) substrate were evident in our calculations on Al. First, absorbance was negligible for Es-polarized light in the thinnest film (100 nm) since it excites a vibrational transition dipole moment in the film oriented parallel to the surface and is effectively canceled by the metal. The effect decreased as the distance from the surface was increased. Correspondingly, absorbance was enhanced at 100 nm for Ep-polarized light which excites a vibrational transition dipole moment oriented perpendicular to the metal substrate. This enhancement was observed for films up to ∼200 nm thick. Second, the absorbance peak under Ep-polarized light split with increasing film thickness. This effect was most dramatic at 1000 nm. Horn et al.30 attribute this response to the coupling of the absorbance index (k) with the real refractive index (n). As film thickness increases, the reflection maximum appears near the peak of absorption. As shown in Figure 4, reasonable agreement exists between calculation and experiment for both Es- and Ep-polarized light for films up to 1000 nm thick. This agreement confirms that the film thicknesses obtained by optical interference methods are of the right order of magnitude. Overall, however, the calculated absorbances were smaller than the experimental
FTIR-RAS data. This discrepancy may be due to the lack of sophistication in the model used. Greenler’s treatment,18 as compared to more recent models,31 assumes an isotropic film with none of the irregularities, grain boundaries, defects, and randomly oriented surfaces found in polycrystalline ice. Moreover, absorbance intensity can be influenced by the angle of incidence. The calculations used a fixed angle of 83° whereas our experimental angle was 83° ( 2°. A difference between our calculated spectra and Horn et al.30 also deserves note. We both saw essentially the same changes in peak shape for Es- and Ep-polarized light, but in their spectra the changes occurred between 3000 and 5000 nm, while in our data they occurred between 100 and 500 nm (Figure 4). Presumably the peak shape observed in our film at 1000 nm would match theirs for a film >5000 nm, but 5000 nm was the thickest film they reported. We cannot explain the cause of these differences. We have confidence in the values reported in Figure 4 since film thicknesses were measured independently by optical interference methods and were checked by using known film deposition rates and deposition times. B. Sensitivity. During growth of an ice film simultaneous measurements of film thickness and the integrated absorbance of the crystalline ice OH stretch (2850-3800 cm-1) were
Vapor-Deposited Water-Ice Films
J. Phys. Chem. B, Vol. 101, No. 50, 1997 10891
Figure 6. Grazing-angle FTIR-RAS spectra of amorphous water-ice films on Al at 94, 105, and 120 K using Ep-polarized light. All films are ∼100 nm thick. An enlargement of the dangling bond feature (3699 cm-1) is included in the inset.
Figure 5. (A) Infrared spectrum of an amorphous water-ice film using Ep-polarized light at 95 K when exposed to a CF4 partial pressure of 0.030 Torr. The peaks at 1242, 1264, and 1307 cm-1 are attributed to moleculary adsorbed CF4 on the ice surface. Gas-phase CF4 absorbs at 1282 cm-1 (antisymmetric stretch, absorbance ) 0.17) along with much weaker contributions in the 1240-1250 cm-1 region, including the 13CF antisymmetric stretch. (B) Plot of CF surface coverage on ice 4 4 versus CF4 partial pressure at 95 K. A linear least-squares fit yielded y ) 6.1 × 1014x - 2.7 × 1013 ((2σ slope ) 7 × 1013 molec cm-2 mTorr-1, (2σ y-int. ) 3.7 × 1014 molec cm-2).
obtained by optical interference and infrared spectroscopy, respectively. Figure 3B shows a plot of the integrated absorbance of the crystalline OH stretch versus film thickness (x < 60 nm) for a typical experiment at 165 K. A linear regression of this data, in combination with five other data sets, yielded an average slope of 2.1 ( 0.2 absorption unit nm-1 ((2σ). Although there are generally large errors for film thicknesses less than 25 nm, the slope of this line shows that changes in ice film thicknesses of 4 Å can be observed, given that the smallest discernible change for the OH integrated absorbance of crystalline ice is 0.7 absorption units. This sensitivity to the crystalline OH peak is approximately equal to the thickness of one ice bilayer where one ice bilayer has 1.15 × 1015 sites cm-2.22 The error bars associated with film thickness in Figure 3B are derived from a standard propagation of error in eqs 2 and 3 for θ1 ((4°), f ((4 nm), s0 ((0.001), h ((0.001), and A ((0.002). An additional test of surface sensitivity was obtained by exposing a thin H2O-ice film at 95 K to CF4 partial pressures ranging from 10-4 to 10-1 Torr. Figure 5A shows an infrared spectra of ice at 95 K when exposed to a CF4 partial pressure of 0.030 Torr. The largest peak (off-scale) at 1282 cm-1 is assigned to the gas-phase CF4 antisymmetric stretch (ν3) while the peaks at 1242, 1264, and 1307 cm-1 have been attributed to molecularly adsorbed CF4 on the ice surface.28,29 Gas-phase
CF4 also absorbs weakly between 1240 and 1250 cm-1, including the gas-phase 13CF4 antisymmetric stretch, and may contribute to some of the absorbance observed in this region. As the CF4 partial pressure increased, the peak at 1307 cm-1 was observed to shift to higher wavenumbers due to splitting of the ν3 longitudinal and transverse modes of molecularly adsorbed CF4 asymmetric stretch, consistent with previous results.28,29 The adsorbed CF4 surface coverage was obtained in an analogous method to that described in Rowland et al.28 Briefly, the area of the condensed-phase CF4 peak was obtained by subtracting the integrated area of the gas-phase CF4 peak at a given partial pressure from the 95 K spectrum at the same partial pressure. This area was quantified to a surface coverage by assuming identical CF4 oscillator strengths for gaseous and adsorbed CF4. The condensed-phase coverage was calibrated using the gas-phase column which was calculated from the known CF4 partial pressures and the infrared path length (17.3 cm). A plot of the CF4 surface coverage versus CF4 partial pressure is shown in Figure 5B. Coverages as low as 4 × 1014 molec CF4 cm-2 can be observed in the infrared, suggesting a sensitivity to CF4 of around one monolayer. We note, however, that it is not possible to directly compare adsorbed CF4 sensitivity with that of crystalline or amorphous ice due to differences in peak shapes and oscillator strengths for the two molecules. Nonetheless, these experiments do show that FTIRRAS is a sensitive technique that can be used to detect near monolayer coverages of molecules, in good agreement with the optical interference data from the helium-neon laser. C. Temperature and Pressure Dependence of the Dangling Bond (90-120 K). The FTIR-RAS spectra of ice deposited at 94, 105, and 120 K are shown in Figure 6. The main OH stretch is located at ∼3430 cm-1 while the dangling OH bond is located at 3699 cm-1 (inset). All spectra were taken with Ep-polarized light, and each film was grown at a deposition rate ∼1.2 nm s-1 to a film thickness of roughly 100 nm. Note that the absorbance of the dangling bond decreased as temperature increased. The dangling bond signal was largest at 94 K and smallest at 120 K. Under conditions of rapid growth (>1 nm s-1), we could detect the dangling bond at temperatures up to 163 K, but the spectral feature was small and short-lived at this high temperature. The deposition rate and temperature dependence of the dangling bond signal are further illustrated in Figure 7A. In
10892 J. Phys. Chem. B, Vol. 101, No. 50, 1997
Zondlo et al.
Figure 8. Time decay of the integrated absorbance of the dangling bond in microporous/amorphous ice under equilibrium water vapor pressures for temperatures of 112 and 118 K. Deposition rates were at 2 nm s-1.
e6.4 × 10-7 Torr). Micropore collapse continued to be observed during film growth at 105 K after 150 min (corresponding to ice 450 nm thick, deposited at 0.05 nm s-1 at 3.2 × 10-7 Torr). IV. Discussion Figure 7. (A) Pressure and temperature dependence of the integrated absorbance of the dangling bond in amorphous ice (3680-3710 cm-1). (B) Pressure and temperature dependence of the corresponding Hbonded OH stretch in amorphous ice (3000-3850 cm-1).
this figure the integrated absorbances of the dangling bond versus film thickness are plotted for films deposited at 94, 105, and 120 K for H2O partial pressures between 3.2 × 10-7 to 3.2 × 10-5 Torr. It is easy to see that the largest dangling bond signal occurred at the coldest temperature. For example at 94 K, the integrated absorbance of a 300 nm film deposited at a pressure of 6.4 × 10-6 Torr reached 0.04, but at 105 and 120 K, the same pressure and film thickness gave dangling bond integrated absorbances of 0.02 and 0.005, respectively. Correspondingly, at a fixed temperature, the largest dangling bond signal occurred at the highest water pressure. This trend was seen most clearly at 94 K where the integrated absorbance was roughly 0.045 at 3.2 × 10-5 Torr, but dropped to 0.027 when the pressure was 6.4 × 10-7 Torr. We include in Figure 7B the corresponding integrated absorbance of the H-bonded OH stretch (3000-3850 cm-1) in which no analogous trends with temperature and pressure were observed. In Figure 8 we show the decay of the dangling bond over time in microporous/amorphous ice under static conditions. Ice films were deposited at a rate of 2 nm s-1 at 112 and 118 K until a film thickness of roughly 100 nm was reached. At this point, the water was turned off to halt further growth. (Residual water vapor pressures typically were
