J. Phys. Chem. B 1999, 103, 8205-8215
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FEATURE ARTICLE Second Harmonic Generation Studies of Ozone Depletion Reactions on Ice Surfaces under Stratospheric Conditions Franz M. Geiger, Anthony C. Tridico, and Janice M. Hicks* Department of Chemistry, Georgetown UniVersity, Washington, DC 20057 ReceiVed: May 11, 1999; In Final Form: July 26, 1999
Hypochlorous acid, HOCl, an important species in the proposed heterogeneous mechanism for stratospheric ozone depletion, has been observed directly at submonolayer amounts on a single crystalline basal ice surface at 155-188 K, using the nonlinear optical method second harmonic generation. The ice is held in equilibrium with its vapor pressure. Second harmonic generation signals from 290 to 310 nm spectroscopically characterize the species and enable us to follow isothermal desorption kinetics in situ. HOCl desorbs as a single species with a ∆G/des ) 48 ( 4 kJ/mol, close to the cohesive energy of ice itself. The lifetime of HOCl on the clean ice surface at 185 K is estimated to be 4 s and the equilibrium surface coverage at 10-11 Torr HOCl to be around 4 × 1011 molecules/cm2, corresponding to about 0.1% of a monolayer. However, these same measurements performed on ice predosed with varying amounts of HNO3 show that the HOCl lifetime is lengthened by coadsorbed HNO3, depending on the HNO3 surface density.
1. Introduction The depletion of stratospheric ozone over the Antarctic continent has been the subject of intense field and laboratory work in the past decade.1-6 There is agreement that the phenomenon is due to chlorine reactions that are somehow catalyzed by the solid particles in the polar stratospheric clouds (PSCs).3,7,8 While an exact compositional and structural analysis of the particles is not yet possible, field and laboratory work suggests that the clouds are composed, at least in part, of ∼10 µm crystals of water ice (Type II PSCs) that form at temperatures below 188 K.9-14 Another category includes nitric acid hydrate (NAH) particles (Type I PSCs) that form at ∼195 K. As a result of their small size and because they grow slowly and under near equilibrium conditions, PSC particles are thought to be single crystals, with low index faces exposed.10,12 PSC particle lifetimes are on the order of minutes15 to hours.11,16 While the particles are not in complete equilibrium with the gas phase,17 probably local equilibrium is nearly achieved most of the time. From the vapor pressure of ice at 185 K (10-4 Torr),18 it can be estimated that the equilibrium evaporation and condensation rates of clean ice are approximately 100 monolayers of water per second.19 However, the presence of foreign molecules may alter these rates.15,20 It has been known since the mid 1980s that the reaction rate constants for the homogeneous gas-phase reactions involving ClONO2, HCl, and H2O are exceedingly slow21,22 such that they could be neglected in the computer modeling codes predicting rates of stratospheric ozone depletion.23 However, it is now known that “safe” forms of chlorine, namely chlorine nitrate * To whom correspondence should be addressed. E-mail: hicksj@ gusun.georgetown.edu. Fax: 202-687-6209.
(ClONO2) and HCl, can be “activated” by ice surfaces, producing hypochlorous acid (HOCl) or Cl2.7,8,24
ClONO2 (g) + H2O (s) f HOCl (ad) + HNO3 (ad) (1) HOCl (g) + HCl (ad) f Cl2 (g) + H2O (s)
(2)
ClONO2 (g) + HCl (ad) f Cl2 (g) + HNO3 (ad)
(3)
Both HOCl and Cl2 can be photolyzed with the advent of the polar spring to form Cl radicals which catalytically destroy ozone by well-known gas-phase reactions.1 Reactions 1-3 play a key role in the processes that ultimately lead to the annual formation of the ozone hole, but not much is known about the reaction mechanisms, their time scales, and the unique role of ice catalyzing the reactions. In this article, we focus on the behavior of HOCl adsorbed on the ice surface in an effort to begin to elucidate the mechanisms and unravel the kinetics of reactions 1-3. Reactions 1-3 occur at 25 km altitude where, at the end of the Antarctic winter, stratospheric temperatures typically fall to 185-200 K, with ambient pressures around 40 Torr. The chemical composition of the polar stratosphere determined during spaceborne and airborne missions is reported in terms of volume mixing ratios (VMRs) which correspond to partial molar volumes. Measured H2O mixing ratios are typically in the parts per million by volume (ppmv) range. HNO3 mixing ratios are found to be around 10 ppbv,25,26 and ClONO2 and HCl mixing ratios are around 1 ppbv.25,27,28 The partial pressures of the trace gases ClONO2, HCl, and HNO3 are thus between 10-8 to 10-7 Torr. HOCl has not yet been measured directly in the polar stratosphere during a PSC event, and its concentration has to be inferred from model calculations that are based on
10.1021/jp991559s CCC: $18.00 © 1999 American Chemical Society Published on Web 09/15/1999
8206 J. Phys. Chem. B, Vol. 103, No. 39, 1999
Figure 1. (a) Crystal structure of ice Ih (adapted from ref 38). Oxygen atoms are shown, hydrogen atoms are omitted for clarity. (b) Top view of the basal plane of ice Ih displaying the 3m symmetry of the surface. Open and closed circles represent oxygen atoms in the upper and lower bilayer, respectively. Hydrogen atoms are omitted for clarity.
measured chlorine and nitrogen concentrations.29 Typical partial pressures for HOCl determined in this way are around 10-11 to 10-10 Torr.29 We study the heterogeneous reactions on ice from a surface science point of view. Under stratospheric conditions, ice is hexagonal Ih and is uniaxial (see Figure 1a). Perpendicular to the c-axis lays the very flat basal face or (0001) surface of ice Ih. This surface has 3m symmetry (Figure 1b), and consists of a bilayer with water molecules spaced by 0.95 Å in the c-axis direction. Parallel to the c-axis lays the prism face or (1100) surface of ice Ih, a rougher surface. The perfectly terminated ice surface structures serve as models of ice surfaces; under various temperature and pressure conditions, the actual surface structures of ice, including possible reconstructions, are not known. Ice surfaces are difficult to study due to their high vapor pressures. Conventional ultrahigh vacuum (UHV) surface science methods have been utilized to study deposited water films19,30,31 and adsorption of foreign molecules on them.32-34 These studies are generally limited to total pressures and temperatures lower than those in the stratosphere, however, they have proven to be useful in elucidating structures and binding motifs on amorphous and polycrystalline films. For instance, Materer et al. measured six-fold LEED patterns from a thin ice film crystallized on a clean Pt (111) surface at 90 K,30 assigned to a (0001) surface. Supporting molecular dynamics simulations showed that large vibrational amplitudes exist down to tem-
Geiger et al. peratures of at least 90 K. From the simulation, the authors conclude that the ice surface has a full bilayer termination. Enhanced vibrational amplitudes at the surface of ice were confirmed experimentally by He scattering experiments.35 Buch, Devlin, and Rowland applied transmission infrared spectroscopy to study thin films of amorphous D2O ice and nanometer sized clusters of crystalline D2O ice at 10-120 K in high vacuum and combined the method with Monte Carlo computer calculations.36 Surface molecules that were 2-fold and 3-fold coordinated (i.e., dangling OH(D) groups) were identified by two narrow IR bands assigned to nonhydrogen bonded OH(D) groups in microporous amorphous ice. Only one band at 2725 cm-1 was found in the case of crystalline ice, in agreement with a relatively ordered surface structure. This one band was also observed by Schaff and Roberts, who used single-reflection FTIR spectroscopy applied to thin D2O ice films.32 Devlin et al. and Schaff and Roberts thus determined that crystalline samples have a low density of free or “dangling” OH groups, corresponding to a mostly nonpolar surface, whereas amorphous samples were found to have a polar surface resulting from highly abundant dangling OH groups. Related to surface order is the phenomenon of surface melting or the existence of a quasi-liquid layer, which has become a controversial issue in explaining the physical and chemical processes that are involved in the heterogeneous reactions leading to ozone depletion. A solid-liquid, liquid-gas system of interfaces is expected to behave differently with adsorbing species than a solid-gas interface. Fletcher suggested that the existence of a quasi-liquid layer on ice surfaces, first proposed by Faraday,37 could reduce the free energy of ice by allowing for disorder at the surface, with gradually increasing order within the surface layer.38 Important initial experiments by Fletcher and Adamson suggested the existence of a quasi-liquid layer on ice down to temperatures of about 240 K,39 well above stratospherically relevant temperatures of ∼190 K. Several other experimental40 and theoretical41-45 approaches suggest that a quasi-liquid layer does not exist at stratospherically relevant temperatures. Using glancing angle X-ray scattering techniques, Lied et al.46 showed that a quasi-liquid layer is nonexistent in both the basal and the prism faces of ice Ih at 190 K. The surface melting of ice has been shown to be extremely sensitive to the presence of salt impurities.47 Reactions 1-3 on vapor-deposited ice have been studied using two related techniques. In flow reactor measurements, reactants are delivered through a tube of ice by a carrier gas (the total pressure including the carrier gas is usually a few Torr) and reactions on ice are inferred by studying the effluent gases.7,48,49 These studies have the advantage of being able to screen many reactions and measure their efficiencies. Efficiencies are usually expressed in terms of a “reactive uptake coefficient”, or γ, which is the fraction of reactant molecules impinging on the surface that results in product molecules. The kinetic analysis can be complicated, however,50 and the ice is difficult to characterize.6,48,51,52 In Knudsen cell experiments, a chamber containing ice is exposed to a known amount of gas, then pumped out and analyzed for products.8,53 This method is also useful in screening for reactions and measuring reactive uptake coefficients. The pressures and temperatures can be stratospheric, and kinetic measurements can be enhanced by using a pulsed valve to deliver the reactants.53 As in the flow tube experiments, the ice is difficult to characterize. The interaction of HOCl with ice has been studied using both flow reactor and Knudsen cell techniques. Hanson and Ravishankara observed HOCl adsorption on ice at 191K and, using
Feature Article a partial HOCl pressure of around 10-7 Torr, found a fractional surface coverage of 0.01 or less.48 They measured an adsorption enthalpy ∆Hads for HOCl on ice held at 191 K of 58 ( 8 kJ/ mol, but it is important to note that the HOCl had been formed via the ClONO2 hydrolysis on vapor deposited ice. Hence, the adsorption energy includes the thermodynamics and kinetics of the ClONO2 adsorption and hydrolysis process, which are not known, and the other product of the reaction, HNO3, is also present on the ice. Pulsed valve experiments by Oppliger et al. found as well that HOCl interacts only weakly with ice at T > 173 K and pressures of 10-6 Torr; only up to 5% of a monolayer is found to form.53 Using HOCl partial pressures in the 10-510-7 Torr range, Abbatt and Molina showed that at 200 K, HOCl adsorption on ice and desorption from ice are reversible.54 Heats of adsorption ∆Hads were reported to be around 44 ( 8 kJ/mol. Banham et al. also studied the interaction of HOCl with ice.34 IR spectral features associated with HOCl/H2O mixtures at 140 K decrease with the addition of HCl (presumably by reaction 2) and disappear with heating to 165 K. Popov et al. performed a thermodynamic analysis of flow tube data by Abbatt et al.55 and Hanson and Ravishankara.48 Using an arbitrarily chosen desorption preexponential factor of υo ) 1013 s-1, they obtained a desorption barrier E/des of 50 kJ/mol.57 Theoretical calculations have been performed on HOCl (density functional theory)58,59 and on HOCl/water complexes (ab initio theory).60,61 The chlorine is thought to have a very slight positive (+0.13) partial charge,59 whereas the hydrogen’s partial charge is +0.32. As a weak acid in aqueous solution (pKa ) 7.54), HOCl is expected to interact as a hydrogen donor with the oxygen of water; this was observed in ab initio calculations of the HOCl‚H2O complex60 and HOCl interacting with four water molecules mimicking an adsorption site on ice.61 This latter work yielded a bonding energy of 37 kJ/mol. Molecular dynamics calculations performed on HOCl interacting with the basal surface of ice yielded a bonding energy of -60 kJ/mol; however, the optimized structure yielded HOCl with its hydrogen oriented away from the surface.43 Reactions 1-3 on a variety of surfaces (ice,7,8,53,54,56,62,63 nitric acid/water mixtures,48,54,64-69 and sulfuric acid/water mixtures65,66,70-73) have been proven to occur using both flow reactor7,48,54,56,63,65-68,70-72,74,75 and Knudsen cell methods.8,53,62,69 Reactions (1-3) occur readily on water ice surfaces, while on solid nitric acid/water mixtures, their efficiencies depend on the humidity.64,76 Some authors have suggested that reaction 2, HOCl with HCl yielding Cl2 and H2O at the ice surface, occurs through an ionic mechanism.5,49,77 HOCl may dissociatively adsorb to H+ and OCl- or HO- and Cl+ at the surface. There is no direct experimental evidence to support this hypothesis. Chu et al. examined the reaction of HOCl with HCl on water vapor deposited ice films held at 188 K.49 HCl and HOCl partial pressures of 6 × 10-8 to 2 × 10-6 Torr were used, and a “reactive uptake coefficient” of HOCl on an HCl covered ice film was determined to be 0.13 ( 0.08. Some authors have suggested that reaction 3 actually occurs in two steps (reactions 1 and 2 sequentially), a mechanism in which HOCl is retained on the ice surface under stratospheric conditions.7,24,48 Other groups have observed that reaction 1 occurs with a noticeable delay,48,53,78whereas reaction 3 occurs rapidly.53 This implies that reaction 3 can proceed without HOCl as an intermediate. We have developed a method in which a single crystal of clean ice held in equilibrium with water vapor is interrogated directly using the nonlinear optical laser method surface second
J. Phys. Chem. B, Vol. 103, No. 39, 1999 8207 harmonic generation (SHG).79-81 In SHG, light at a fundamental laser frequency, ω, is frequency doubled to produce light at 2ω. Due to symmetry constraints, SHG is forbidden in centrosymmetric media within the electric dipole approximation. At the surface, however, symmetry is broken and the secondorder process is allowed,82 thus making the method surface selective without contribution from the bulk phases. Surface SHG has been used to study a variety of surfaces, for example, metals, semiconductors, liquids, and glasses.83-86 In the present application, a single crystal of ice mimicks PSC particles and allows us to bypass problems associated with partitioning of adsorbates to grain boundaries, which can affect the observed kinetics and reactivity. Another feature of the work is that the ice surface is maintained in equilibrium with water vapor. The laser method also allows us to probe the structure of the clean ice surface suggesting that it is indeed crystalline. SHG allows the spectroscopic identification of adsorbed species in situ, a key point in this study. Previous work focused on the adsorption of a nonreactive polar probe molecule, p-tolualdehyde (TA), on the ice surface.81 At low coverages of approximately one-tenth of a monolayer, TA was shown to adsorb as a tightly bound species, probably acting as a hydrogen bond acceptor of dangling OH groups on the ice surface. SHG from the adsorbed TA/ice surface is consistent with a 3m symmetry, epitaxial with the underlying crystal lattice. This species begins to desorb at 185 K and higher. A second species appears to be physisorbed isotropically and begins to desorb at 170 K and higher. The present study is intended to address some of the basic molecular level questions regarding the adsorption of small molecules on ice surfaces. In particular, the stratospherically relevant molecule HOCl is probed. Is the adsorbed species molecular? Are there one or more types of adsorption sites? What is the lifetime of HOCl at the surface? How does HOCl desorb? What is the effect of coadsorbed HNO3? We will also address the reaction of HOCl with HCl on ice surfaces. Our study of reaction 1 using SHG is reported elsewhere.87 2. The SHG Approach There are several distinct advantages to employing secondorder nonlinear optical techniques in the study of ice surfaces. First, the methods derive their surface sensitivities from symmetry and do not require UHV, allowing one to study the dynamic ice/water vapor interface. The methods can be used to follow processes ranging from ultrahigh vacuum to even higher than atmospheric pressure. Indeed, we have observed SHG from the ice/liquid water interface.88 Second, the methods can be used to probe the surface in a direct manner in real time and in situ. Third, the methods are nondestructive. By using various polarizations of the incident light and measuring specific polarizations of the SHG signal, it is possible to gain information regarding the symmetry of the surface.84 In general, the intensity of the reflected SHG is
I2ω )
32π3ω2 2 sec θ| b 2ω‚χ 6(2): bω b ω|2Iω2 3 c
(4)
where θ is the angle of incidence, b ω and b 2ω are the Fresnelcorrected polarization vectors for the fundamental and second harmonic light fields at the surface, respectively, 6 χ(2) is the macroscopic second-order nonlinear susceptibility of the medium, and Iω is the intensity of the incoming light.84,89 Surfacegenerated SHG signals are typically weak (10 to 100 photons per second in this study) but are collected above a zero background.
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Geiger et al.
SHG originates from the second-order nonlinear susceptibility of the surface 6 χ(2), which is a third rank tensor. Different tensor elements of 6 χ(2) contribute to the polarization-analyzed SHG output for an anisotropic surface with 3m symmetry compared to that for an achiral isotropic surface.84,91 For the 3m surface, χ4 ) χxxx ) -χxyy is nonzero and independent, while it is zero for an isotropic surface. (We follow the convention that the surface is the xy plane, z is the perpendicular axis, and the second-order polarization P(2) i ) ∑χijkEjEk). For example, for jk
input light polarized in the surface plane (s), the reflected s-polarized SHG generated from the 3m surface is given by 2 2 2 I2ω s ) A|sin φ[χ4 sin φ - 3χ4 cos φ]|
(5)
where φ is the rotation angle about z. Rotational anisotropies in SHG predicted by eq 5 have been observed, for example, for Si(111).90 For the isotropic surface
I2ω s )0
(6)
Additionally, for input light polarized perpendicular to the surface (p), s-polarized SHG signals are not allowed for isotropic achiral surfaces, yet can result from the component χxyy characteristic of 3m surfaces. These distinctions are important in work on the basal ice surface, which is 3m symmetric.30,35 The macroscopic surface response 6 χ(2) can be related to the (2) molecular level by the model 6 χ ) Ns〈R 6(2)〉, where the microscopic second-order nonlinear polarizability R 6(2) is averaged over all orientations of the molecules. Ns is the number density of molecules at the surface. The square root of the SHG signal is thus proportional to Ns if the orientation of the molecules is constant with surface coverage. Through the use of tunable lasers, second-order nonlinear optical methods can be utilized as a surface electronic spectroscopy.92 The perturbation expansion for the second-order nonlinear polarizability R 6(2) for SHG contains terms related to electronic transitions in the molecules and is usually described by electric dipole transition terms such as82
6(2) R ijk
)-
4π2e3 h2
〈a|µ bi|b〉 ‚ 〈b|µ bj|c〉 ‚ 〈c|µ bk|a〉
∑ b,c (ω - ω
ba
+ + iΓba)(2ω - ωca + iΓca) . . . (7)
where Γ represents damping coefficients, b µi is the electric dipole transition moment, a, b, and c represent the ground, intermediate, and final state respectively, and the summation is over all excited states. As ω and/or 2ω, the frequency of the input light or the second harmonic frequency, approach a natural resonance frequency of the molecule, R 6(2) and thus the SHG efficiency increases. Surface SHG electronic spectra of molecular monolayers as well as surface states in metals not found in the bulk have been reported.81,84,91 3. Experimental Section The laser system consists of a picosecond Nd3+-YAG laser (Spectra Physics 3800S) and a 4 MHz cavity-dumped tunable dye laser operating with Rhodamine-6G (tunable from 580 to 620 nm) at 100 mW average power. Dye laser pulses are 900 fs fwhm and have ∼20 nJ energy. The beam is focused on the ice sample to approximately 30 µm. SHG is collected in a reflection geometry (see Figure 2); the quartz window passing the SHG is not shown in this diagram. The SHG is then separated from the fundamental frequency by two Corning 7-54
Figure 2. Arrangement for probing the basal ice surface using SHG: LN2 ) liquid nitrogen Dewar; H ) heater; I ) ice; RGA ) residual gas analyzer.
filters, and its polarization is analyzed using a quartz Rochon polarizer. The SHG is further separated from the fundamental frequency by a monochromator and detected by single photon counting. The signal is normalized to the laser intensity by a reference SHG signal generated in transmission through potassium dihydrogen phosphate (KDP) powder in an index matched fluid. An identical detection system is used to collect the reference signal, correcting for the wavelength response of the detection system. All experiments are carried out in a cylindrical Pyrex vacuum chamber with a base pressure of 1.0 × 10-8 Torr (Figure 2). The chamber is pumped by a 170 L/s turbo pump (Balzers TPU 170). The partial pressures of gases present during an experiment are measured with a residual gas analyzer (RGA, AMETEK, Dycor MA200M). Ice samples are prepared by growth from the melt using a modification of the Bridgeman apparatus.79,80 At 273 K, the basal face grows faster than the prism face.38 This anisotropy in the growth rate can be reversed by applying a temperature gradient during ice growth since the thermal conductivity of ice is about 5% greater along the c axis than along the a axis.93 Thereby, it is possible to grow ice single crystals along the c axis94 with the basal (0001) plane exposed. After cleaving the ice using a razor blade, the ice sample (1 cm diameter, 3 mm thick) is adhered to a cold polished aluminum support. At present, it is positioned at an arbitrary rotation angle φ with respect to the three mirror planes. The support is in thermal contact with a liquid nitrogen Dewar and a ceramic heater (Spectra-Mat E-292) maintaining the sample at temperatures ranging from 110 to 270 K. A chromel-alumel thermocouple is embedded in the ice for accurate temperature measurements. The sample is annealed for 30 min at 190 K. Using a variable control leak valve vacuum sealed to a water reservoir (Fisher, HPLC grade), the water partial pressure in the chamber is adjusted such that the ice sample is kept in equilibrium (i.e., the evaporation rate equals the condensation rate; the water vapor pressure values were obtained from Marti and Mauersberger18 and Bryson et al.)95 HOCl could not be leaked into the vacuum chamber using a stainless steel leak valve because it reacts with the exposed metal surfaces. Instead, a Pyrex capillary leak valve consisting of a 10 cm capillary with
Feature Article a 50 µm orifice, a Teflon stopcock, and a 10 mL Pyrex sample bulb was used. With a 30 µm focused laser spot, one can find optically flat areas of the crystal (Rq < λ/10), yielding a reflection that is round and symmetric with no speckle visible to the eye in a darkened room. The optical flatness and also the fact that the SHG signals are well polarized (typical extinction ratio of 9:1) suggests that the surface is not significantly rough. Further, the observed SHG intensities for 45 input polarization, s output polarization are the same as that observed from the liquid water surface. This suggests but does not confirm that surface roughness does not contribute to the SHG signal. Willen and Dash observed unusually large planar facets of ice with the basal face exposed, and used a molecular level argument that, because it is difficult to nucleate in 2D on a very flat surface, the basal ice surface grows preferentially and efficiently. If it were rough with steps and defects, addition of water molecules on the basal plane would enlarge the prism plane (and this is not observed.)45 Note that when a nanosecond pulsed laser was utilized in the experiment instead of the picosecond system, we observed SHG signals only at the onset of surface damage, as evidenced by an increase in water partial pressure in the chamber with laser irradiation. The nanosecond damage threshold occurred at fundamental input wavelength 1064 nm at about 100 mJ/ cm2. This damage probably resulted from heating due to the very small absorption coefficient of water due to an overtone near 600 nm.93 With the picosecond laser, the energy density is
