Temperature Dependence of OH Yield, Translational Energy, and

J. Phys. Chem. C 2007, 111, 14833-14842

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Temperature Dependence of OH Yield, Translational Energy, and Vibrational Branching in the Reaction of O(3P)(g) with Liquid Squalane Mhairi Allan, Paul A. J. Bagot, Matthew L. Costen, and Kenneth G. McKendrick* School of Engineering and Physical Sciences, Heriot-Watt UniVersity, Edinburgh, EH14 4AS, United Kingdom ReceiVed: May 29, 2007; In Final Form: July 8, 2007

The relative reactivity of the liquid surface of a long-chain, partially branched hydrocarbon (squalane, C30H62) with gas-phase O(3P) atoms has been measured as a function of liquid temperature. The O(3P) atoms were generated with a superthermal velocity distribution by 355 nm photolysis of NO2. Laser-induced fluorescence was used to detect the relative branching into specific OH product vibrational states. The yield of OH(V′)0) proves significantly less dependent on liquid surface temperature than the yield of OH(V′)1). Time-of-flight measurements of the escaping OH provide partially resolved product translational energy distributions. These profiles also differ between OH vibrational states. OH(V′)1) shows overall longer arrival times, but with a clear trend toward earlier times as the surface temperature is increased. OH(V′)0) shows little detectable variation of the distribution of arrival times over the range of temperatures investigated (263-333 K). We discuss the interpretation of these findings, taking account of earlier experimental work, which has indicated significant contributions from distinct “direct” and “trapping-desorption” reaction mechanisms, and new molecular dynamics simulations of the surface structure. There are a number of factors that may contribute, including both energetic and structural effects. It is not possible on the basis of the current evidence to discriminate conclusively between them. Nevertheless, we conclude, on balance, that structural effects may well be the more important. In particular, higher temperatures are predicted to promote more open structures. We speculate that this may enable more OH(V′)1) to escape before it is either vibrationally relaxed or, less probably, undergoes vibrationally enhanced reaction to produce H2O.

Introduction Processes that occur at gas-liquid interfaces are of direct practical interest in a diversity of fields. Viewed broadly, these include biological respiration, distillation, particle exchange at the surface of the sea, and mass accumulation on atmospheric aerosols. Despite this significance, and in contrast to homogeneous gas-phase reactions, direct studies of the dynamics of reactions at the gas-liquid interface were until recently unknown. This situation has begun to change, and a number of groups have now investigated inelastic scattering of gases from liquid surfaces.1-11 Based on observations of energy exchange with the liquid surface and residence time, these studies have shown that the inelastically scattered species very commonly have a bimodal translational energy distribution. The two components consist of a direct mechanism, where residence time is short and molecules leave the surface with a relatively high kinetic energy related to their initial energy, and a trappingdesorption mechanism, where molecules are trapped for a finite time at the surface before escaping with a Maxwell-Boltzmann distribution of velocities. Of particular interest in the present work is the surface oxidation of liquid hydrocarbons. In general, these processes impact on a number of applied fields such as combustion and lubricant oxidation. They are also involved in the processing of atmospheric aerosols, even those containing only trace levels of organic constituents. These particles form a micelle-like structure with a hydrophobic outer layer.12,13 On oxidation, predominantly by the hydroxyl radical but also in some cases * Corresponding author. E-mail: [email protected].

by other oxidants present in the atmosphere, hydrophilic sites are created on the outer layer. This has important consequences because it enables them, for example, to capture water and hence act as cloud-condensation nuclei. In addition, the oxidation of hydrocarbon surfaces plays a key role in the specialized, but important, degradation of polymers on the outer surfaces of spacecraft in low Earth orbit (LEO). In the altitude range 200-700 km, atomic oxygen is the most abundant species.14-16 Due to the spacecraft’s orbital motion, they collide with collision energies as high as 5 eV (∼480 kJ mol-1). This motivated an important series of experiments by Minton, initially in collaboration with Casavecchia and then with a number of other co-workers.17-21 These represent the first dynamic measurements of reactiVe collisions of gas-phase atoms at liquid surfaces. They were carried out using a pulsed beam containing hyperthermal oxygen atoms directed at a continuously refreshed liquid surface. The speciation, velocity, and angular distributions of the products were measured using a rotatable quadrupole mass filter. The main gas-phase products were inelastically scattered oxygen atoms, hydroxyl radicals, and water. The time-of-flight distributions for all three products again revealed thermal and nonthermal scattering channels. The “slower” component could be fitted to a Maxwell-Boltzmann distribution corresponding approximately to the temperature of the surface. This component was attributed to a trapping-desorption mechanism. The second component at higher translational energies was thought to come from a direct abstraction mechanism analogous to the corresponding homogeneous gas-phase reaction. Angular distributions of the products showed that the thermal component had a cosine

10.1021/jp074147p CCC: $37.00 © 2007 American Chemical Society Published on Web 09/14/2007

14834 J. Phys. Chem. C, Vol. 111, No. 40, 2007 distribution about the surface normal while the nonthermal component was much more sharply peaked in a direction near the specular angle. More recently, Nesbitt and co-workers have performed a reactive scattering study of fluorine atoms at a squalane surface via a distinct spectroscopic approach.22 They used highresolution IR-laser direct absorption to probe the rovibrational quantum states of the nascent HF. They also examined the velocity distributions of HF parallel to the surface through Doppler-resolved absorption profiles. The HF reaction product was found to have a bimodal rotational distribution. Once again, one component was attributed to a trapping-desorption mechanism and the other was attributed to a hyperthermal abstraction mechanism. Until very recently,23 no theoretical investigations of the dynamics of the interfacial reaction between oxygen atoms and liquid hydrocarbons have been reported. Previously, there had, however, been some related work on the structure of the liquid surface. Harris performed molecular dynamics simulations of the liquid surfaces of n-decane (C10H22) and n-eicosane (C20H42) to examine molecular-level details of the interface structure, the surface tension, and their dependence on chain length and temperature.24 From this work he found that the outer edge of the surface was dominated by chain ends: the methyl groups protrude further into the vacuum, on average, compared to the methylene groups. Subsequently, Siepmann and co-workers published a number of papers on the modeling of the larger, branched-chain molecule squalane (also the subject of the current work) using Monte Carlo methods.25-28 As expected, they found that the squalanevapor interface does not appear to be molecularly sharp. Squalane molecules protrude through the interface, and the density profile declines to the vacuum value over a significant distance. They did not identify any selective enrichment of a specific group at the interface. Prior to the most recent theoretical work23 on reactive scattering with liquid squalane itself, the calculations by Troya and Schatz,29 and independently by Hase and co-workers,30,31 on O(3P) atoms scattering from model alkanethiolate selfassembled monolayer (SAM) surfaces represented the most important connection between current theory and experiments on liquids. Troya and Schatz29 studied the dynamics of hyperthermal O(3P) atoms with a hydrocarbon SAM using a hybrid quantum mechanics/molecular mechanics (QM/MM) interaction potential. Their results showed the existence of a variety of different scattering channels. These include inelastic scattering, H-atom abstraction, O-atom addition to the chain, C-C bond breakage, and double H-atom abstraction to form water. The probability of each reaction channel was found to depend on the incident angle of the incoming O(3P) atoms. Hase30,31 also employed QM/MM methods to study energy transfer in collisions of O(3P) atoms with a variant of the alkanethiolate SAM surface. Interestingly, they concluded that there are three different mechanisms that contribute: direct scattering from the top of the SAM, physisorption on the top of the SAM followed by desorption, and penetration into the SAM. They found the dominant trajectory type was dependent on the collision energy and the incident angle. The most recent calculations now reported by Kim and Schatz23 on O(3P) with squalane adopted a similar approach, but with a novel dynamic partitioning of atoms between the QM and MM regions. H-atom abstraction was found to be the dominant channel at the relatively high energies that they examined. A number of interesting correlations between the dynamics of different product channels and

Allan et al. incident angle were again identified. Some of these were distinct from those of the model SAMs, particularly the overall lower efficiency of energy transfer in liquid squalane. In our own previous work, we have employed laser-induced fluorescence (LIF) to detect nascent hydroxyl radicals produced in the reaction of O(3P) atoms with a liquid hydrocarbon surface.32-34 This use of spectroscopic detection for nascent gas-liquid interfacial reaction products, which was at that time unique, yielded new information on their internal energy distributions, complementing the velocity measurements of Minton and co-workers.17-21 We were consequently able to discover that a significant fraction of vibrationally excited OH is produced in the reaction. We measured the rotational distributions for OH(V′)0) and OH(V′)1) at two different liquid surface temperatures.34 A partial dependence of the product rotational temperature on liquid temperature was found. From this we concluded that there were at least two components to the OH rotational distributions, as noted above to have subsequently been found by Nesbitt and co-workers22 for HF from the analogous, but much more exothermic, reaction of F with squalane. Our LIF observations therefore provided independent support for Minton and co-workers’17-21 conclusion that the O(3P) reaction takes place via more than one mechanism, conveniently labeled as direct and trapping-desorption components. Importantly, our original rotational temperatures33 were measured at the peak of the OH radical appearance profiles, where we inferred there are roughly equal contributions from the two mechanisms. It was subsequently possible for us to measure rotational distributions of the higher velocity component in isolation by exploiting its shorter time of flight.34 These new distributions were indeed consistent with the faster OH being produced in a direct mechanism, strengthening the earlier interpretation. Further to this, we performed molecular dynamics simulations of liquid squalane with the aim of characterizing the distribution of potentially reactive sites at the surface.35 We found a modest, but discernible, preference for methyl groups to protrude into the vacuum, which diminished at higher temperatures. Monte Carlo simulations of the flight paths of projectiles incident on the surface were carried out, mimicking those of the O(3P) atoms in our experiments. These indicated that the weak preference for protrusion of the methyl groups had, in practice, very little effect on the ratio of primary, secondary, and tertiary hydrogen atoms first encountered by the incoming atoms. All three H-C bond types were attacked with probabilities that did not differ markedly from their statistical ratios in squalane. In the current paper, we extend the understanding of the O(3P) + liquid squalane system by reporting new results on the influence of squalane temperature on reactivity. In particular, we measure the temperature dependence of the yield of each of the ground and vibrationally excited states of OH. We combine this with detailed measurements of the time-of-flight appearance profiles of each of these product states as a function of temperature. We discuss the interpretation of the results in the context of the established mechanisms, and consider possible explanations for two distinct, previously unobserved effects. Experimental Technique and Simulation Method A detailed description of the experimental apparatus has been published previously,33 so only a brief outline is presented here. At the heart of the experiment is a stainless-steel wheel, of 50 mm diameter. It is mounted on an axle, which rotates it at 0.5 Hz through the hydrocarbon liquid of interest contained in a

Reaction of O(3P)(g) with Liquid Squalane temperature-controlled copper bath. This arrangement provides a continuously refreshed liquid hydrocarbon surface, which in these experiments was squalane (C30H62, 2,6,10,15,19,23hexamethyltetracosane), supplied by Sigma-Aldrich (99%) and used without further purification. Squalane has acquired a status as the benchmark molecule in this field. This is primarily because of its commercial availability and its advantage of having a very low vapor pressure (10-8 Torr) over a wide range of temperatures. In our current experiments, the temperature was varied from 263 to 333 K. Particularly careful attention was paid to keeping all other parameters constant (most critically, photolysis and probe laser pulse energies, gas pressure, and the distance of the common laser axis from the wheel). To ensure reliable, reproducible results, experiments were repeated on at least three different days. The bath temperature was adjusted in a nonsequential manner between measurements to eliminate systematic error. The first and last measurements of any set were made at a common temperature to verify consistency of the signal size. O(3P) atoms were generated at a mean, but precisely controlled, distance of 5 mm from the wheel surface by photolysis of a carefully controlled pressure (nominally 1 mTorr) of NO2 (BOC, 98.3%) using the 355 nm third harmonic of a Nd:YAG laser (Continuum Surelite II-10). The photolysis laser supplied pulses at 10 Hz, with a nominal length of 4-6 ns and energy of typically 70 mJ, measured at the entry to the vacuum chamber. The spatial distribution of the O(3P) produced in this manner is described by an anisotropy parameter of +0.7.36 The photolysis laser was horizontally polarized, so roughly half of the O(3P) was directed toward the liquid surface. The collision energy of the O(3P) atoms is broadly distributed around an average value of 15.8 kJ mol-1 in the laboratory frame, with an average speed of 1340 ms-1. On impact with the liquid, some O(3P) atoms abstract a hydrogen atom, generating OH radicals that escape from the surface. These are probed by LIF on the A2Σ+-X2Π (1,0) and (1,1) bands, in the current experiments on the Q1(1) line, using a Nd:YAG (Continuum Surelite II-10) pumped dye laser (Sirah Cobra Stretch). This supplied ca. 1 mJ, 4-6 ns pulses measured at the entrance to the vacuum chamber. The fluorescence emitted by the electronically excited OH radicals is collected by a liquid light guide (Ultrafine Technology Ltd.) mounted 1 cm directly above the common laser axis. The fluorescence passes through custom interference filters before being converted into a signal by a photomultiplier tube (PMT; Electron Tubes Ltd.). This signal is in turn digitized and passed to a PC, which collects data and controls the wavelength and timing of the lasers using custom-written LabVIEW programs. The methods used for the molecular dynamics simulations have also been described in detail elsewhere;35 therefore, only a brief description will be provided here. Simulations were performed using the TraPPE (transferable potential for phase equilibria) force field developed by Siepmann and coworkers.25-28 A united atom model was used to describe all the CH3, CH2, and CH units. All simulations were performed in the NVT ensemble using the program DL_POLY2 on an Intel Xeon 3.0 GHz dual processor workstation. The starting configuration consisted of 288 squalane molecules arranged parallel to the interface. Periodic boundary conditions were applied to a box of fixed dimensions (65 × 65 × 186 Å3) containing a slab of liquid and two vacuum interfaces. The total internal energy of the system was used to track progress toward equilibration. Once equilibrium had been established, the ratios of primary/secondary/tertiary sites were analyzed in the form

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14835

Figure 1. (a) Measured appearance profiles of OH(V′)0) LIF signal after reaction of O(3P) atoms with liquid squalane surface. Profiles were recorded on the Q1(1) line of the OH A-X (1,0) band. Bath temperature ) 333 K; p(NO2) ∼ 1 mTorr; distance surface-probe laser ) 5 mm. The figure shows 22 scans measured over six different days. (b) Measured appearance profiles of OH(V′)0) LIF signal after reaction of O(3P) atoms with liquid squalane surface. Approximately 20 scans were averaged at each temperature. Error bars indicate the two sigma uncertainties in the mean. Profiles were recorded on the Q1(1) line of the OH A-X (1,0) band. Bath temperature ) 333 (blue), 323 (green), 299 (red), and 263 K (black); p(NO2) ∼ 1 mTorr; distance surfaceprobe laser ) 5 mm.

of “z-density” plots (average densities of different united atom types as a function of distance normal to the interface) and by visual inspection of the reconstructed surfaces. Results LIF Measurements. Appearance profiles for OH(V′)0) are shown in Figure 1. Figure 1a shows the reproducibility of the individual profiles as measured in separate experiments, conducted over several different days, at one particular temperature (333 K). In Figure 1b, all profiles at a given temperature, also obtained over several different days, have been averaged. Note that the temperatures used (333, 323, 299, and 263 K) were not equally spaced. In all cases, the relative OH LIF intensity on the Q1(1) (1,0) line is plotted against the time delay between the photolysis and probe lasers. The peak in the profile at 12 µs agrees with the approximate round-trip time expected on the basis of the velocities for O and OH species traveling to and from the liquid surface to the laser axis. We stress that no further normalization has been applied, so the relative signals at

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Figure 2. Corrected appearance profiles of OH(V′)0) after reaction of O(3P) atoms with liquid squalane surface. Profiles were recorded on the Q1(1) line of the (1,0) band and corrected for the measured rotational temperature at the peak, as described in the text. Bath temperature ) 333 (blue), 323 (green), 299 (red), and 263 K (black); p(NO2) ∼ 1 mTorr; distance surface-probe laser ) 5 mm.

different temperatures may be compared directly. The error bars indicate the statistical variations (2σ) at each delay time. It is clear that changing the surface temperature over this 70 K range has only a very modest effect, if any, on the amount of OH(V′)0) that is observed. It is conceivable, in principle, that the measurements on a single rotational line in Figure 1 could disguise some change in the total amount of OH(V′)0) being produced because of a change in the OH rotational temperature with surface temperature. We have measured previously33 the rotational distributions at two surface temperatures at the upper and lower ends of the current range. These distributions were found to be adequately described by single Boltzmann temperatures. At the peak of the appearance profile, we found OH(V′)0) rotational temperatures of 304 and 330 K for liquid temperatures of 273 and 343 K, respectively. Knowing the rotational temperatures, the measurements on a single line can be scaled to reflect total populations over all rotational levels, as shown in Figure 2. The corrections for the intermediate liquid temperatures were made by interpolation, assuming a linear relationship between liquid and rotational temperatures. Since the rotational temperature used is that at the peak, it should be noted that this correction is only valid at this point in the profile. It is known that there is less dependence at the rising edge, as expected and discussed at length previously.34 Nevertheless, it is clear from a comparison of Figures 1b and 2 that the correction has only a minor effect on the profiles, implying at most a statistically marginal increase in OH(V′)0) production with temperature. This indicates that there is no significant hidden temperature dependence in the uncorrected profiles, and the details of how any such correction is applied are not critical in practice. Figure 3 shows the corresponding results (averaged over multiple separate experiments, uncorrected for rotational temperature) for the first vibrationally excited state, OH(V′)1). In marked contrast to V′ ) 0 in Figure 1, the yield of V′ ) 1 shows a distinct dependence on temperature. We have confirmed that the corresponding correction for known33 modest changes in the OH(V′)1) rotational temperature at the peak has only a minor effect. If anything, it amplifies slightly the already clearly increased production of OH(V′)1) at higher temperatures because of the positive correlation between liquid and product rotational temperatures.

Allan et al.

Figure 3. Measured appearance profiles of OH(V′)1) LIF signal from reaction of O(3P) atoms with liquid squalane surface. Profiles were recorded on the Q1(1) line of the (1,1) band. Approximately 40 scans were averaged at each temperature. Bath temperature ) 333 (blue), 323 (green), 299 (red), and 263 K (black); p(NO2) ∼ 1 mTorr; distance surface-probe laser ) 5 mm.

Figure 4. Normalized appearance profiles of OH(V′)0) and OH(V′)1) LIF signals after reaction of O(3P) atoms with liquid squalane surface. Profiles were recorded on the Q1(1) line of the (1,0) and (1,1) bands, respectively. Bath temperature ) 333 (blue), 323 (green), 299 (red), and 263 K (black); p(NO2) ∼ 1 mTorr; distance surface-probe laser ) 5 mm. The inset shows Monte Carlo predictions of the peak region of the profiles for OH molecules thermally desorbing at each liquid temperature, as described in the text.

A further, distinct way of comparing the profiles in either Figure 1b or 3 is to normalize them to the peak signal sizes. This involves only a small adjustment for OH(V′)0), because the data are already very similar. Nevertheless, it highlights, as shown in Figure 4, the very similar shape of the V′ ) 0 appearance profiles at all four temperatures. As we have discussed previously,33 geometric and other experimental averaging effects mean that these profiles are only a partially resolved measure of the translational energy distributions of the OH radical. Nevertheless, they indicate clearly that changing the surface temperature does not affect the OH(V′)0) translational energies substantially. This is especially true in the region of the higher-velocity rising edge of the profile. Any modest change is restricted to later times. In contrast, the similarly normalized corresponding profiles for OH(V′)1), also shown in Figure 4, show clearly quite distinct behavior. Not only is

Reaction of O(3P)(g) with Liquid Squalane

Figure 5. z-Density plots for equilibrated simulations at (a) 263 and (b) 333 K. Dashed-dotted lines are total density from all atom types; solid lines are primary, dashed lines are secondary, and dotted lines are tertiary united carbon atoms.

the peak arrival time for V′ ) 1 generally later, but there is also a very apparent and systematic shift to earlier times, interpreted as a corresponding increase in translational energy, as the liquid surface temperature is raised. (The predicted profiles also shown in Figure 4 are described and discussed below.) Molecular Dynamics Modeling. To aid our understanding of the experimental results, we have carried out molecular dynamics (MD) simulations to investigate possible influences of the liquid surface structure. Our own previous35 MD simulations for squalane were performed at 298, 400, and 500 K. To improve the directness of the comparison with the data from the current measurements, we report here further simulations of squalane at 263 and 333 K, corresponding to the minimum and maximum experimental temperatures. The MD simulations were initiated at 400 K to establish rapid equilibration of the starting configuration, run for 0.4 ns at 263 K, and then run at 333 K for a further 0.4 ns. Figure 5 shows the corresponding z-density plots for the three united atom types of squalane at 263 and 333 K. As found previously, particularly at 298 K,35 there is a modest preference for the methyl groups to protrude into the vacuum, as revealed by the primary density crossing the initially more abundant secondary density at larger distances. This is present at both current temperatures, with the degree of protrusion decreasing only slightly at the higher temperature (solid lines in Figure 5). Nevertheless, as in our previous work, all three united atom types are exposed to incoming projectiles in a near-statistical ratio in the outer layers of the surface. We have analyzed some of the other attributes of the surface as in our previous work,35 and we find corresponding modest but quantifiable changes as the surface is heated. The interface region becomes slightly more extended and atomically rougher. The distance over which the density drops from 50% of its bulk value, which defines the dividing surface at 0 Å, to 5% increases from 4.1 to 4.8 Å on going from 263 to 333 K. As support of the physical plausibility of these structural changes, the density in the bulk region is predicted to decrease by 5.6% over the same temperature range, which compares very favorably with an experimentally observed variation of 5.3%.37 Discussion There are several key new findings in the experimental data that we seek to explain. The squalane reactivity is essentially

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14837 temperature independent for production of OH(V′)0) (Figure 1 or 2). In contrast, it is distinctly temperature dependent for OH(V′)1) (Figure 3). A second clear distinction between the vibrational levels is that higher liquid temperatures yield visibly faster OH molecules for V′ ) 1, but not for V′ ) 0 (Figure 4). To help clarify the mechanistic framework in which we attempt to interpret these results, we have indicated pictorially in Figure 6 the processes that we include in our analysis. In summary, these include the following: approach of the incident superthermal O(3P) atom (step 1), which either undergoes direct reaction to produce OH(V′)0) or OH(V′)1) (steps 2g and 2v, respectively) or is trapped at the surface (step 3); direct escape of the nascent OH in either V′ ) 0 or V′ ) 1 (steps 4g and 4v); trapping and thermal accommodation of each of these levels (steps 5g and 5v); subsequent escape of each (steps 6g and 6v); secondary reaction of each (steps 7g and 7v) to produce H2O, which may desorb from the surface (step 9) but would not be observed in our experiments; and vibrational relaxation (step 8v), obviously confined to OH(V′)1). The essence of this discussion is to consider whether the relevant steps might be expected to be temperature dependent and, if so, whether this dependence is compatible with the experimental observations. We address first the apparent temperature independence of squalane reactivity to produce OH(V′)0). Before discussing the contrast with V′ ) 1, it is interesting to note briefly that this seemingly “null” result is, in fact, an important observation in its own right. It indicates that there cannot be a significant contribution from a mechanism in which O(3P) atoms are initially trapped at the surface (step 3 in Figure 6) and then react via a thermally activated process. If that had been the case, the typical activation energies for abstraction of H from a saturated hydrocarbon by O(3P) of 20-40 kJ mol-1 38 would suggest a variation of a factor of ca. 10-50 in the magnitude of the rate constant over the 70 K experimental temperature range. This reinforces the view that reaction is promoted by the initial superthermal velocities of the incoming O(3P) atoms (step 1). If this energy is dissipated through inelastic processes at the surface (step 3), the O(3P) atom is apparently much less likely to go on to react. We now discuss the conceivable explanations for the contrasting behavior of OH(V′)1) and OH(V′)0). In summary, these fall into two broad categories, which are distinguished by the underlying physical cause of the variable V′ ) 1 yield. The first is some form of thermal activation, where the energy in surface modes affects the initial reactivity (step 2 in Figure 6). The second is a temperature-dependent structural effect. Within this category of structural effects, there is also a second distinction between a temperature-dependent enhancement of the amount of V′ ) 1 produced (in step 2v), or of the amount that surViVes and escapes from the surface (either directly, through step 4v, or indirectly, via steps 5v-6v). We begin with the proposition that the observed positive temperature dependence of the yield of OH(V′)1) might be evidence for some type of activated behavior in step 2. This requires an underlying mechanism, most plausibly that the increased thermal population in the C-H vibrations of squalane is responsible for the enhanced reactivity. We stress, however, that this must effectively produce OH(V′)1) almost exclusiVely in step 2v: otherwise, a similar variation in yield with temperature would be seen for V′ ) 0 in step 2g. Appealing to independent evidence, there is actually some support from recent molecular beam scattering experiments on the corresponding gas-phase reaction of O(3P) with the parent compound, CH4,39 for the proposal that vibrational excitation in the hydrocarbon

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Figure 6. Schematic representation of possible mechanistic steps involved in the reaction of O(3P) with a liquid hydrocarbon surface ultimately leading to production of OH(V′)0) and OH(V′)1) in the gas phase. Secondary steps involving ground (V′ ) 0) and vibrationally excited (V′ ) 1) OH are distinguished by subscripts “g” and “v”, respectively.

reactant does produce OH(V′)1) preferentially. However, for this argument to be plausible for the liquid squalane system, it must also be consistent with the inferred relative reactivity of ground-state and vibrationally excited H-C bonds. The Boltzmann fraction of H-C oscillators that are vibrationally excited in the current temperature range is extremely small. For the C-H stretching vibration that might most obviously be expected to be coupled to the reaction coordinate, this factor is on the order of 5 × 10-7. Even for the lower lying bending mode, suggested as being responsible for the enhanced reactivity in CH4,39 it is still around 1 × 10-3. Our results in Figure 3 show that the increase from 263 to 333 K represents almost half the total OH(V′)1) observed at the higher temperature. Therefore, if this increase were indeed entirely due to vibrationally excited squalane, a substantial proportion of all the observed OH(V′)1) must come from this mechanism. Our own previous measurements on liquid squalane33 have determined an OH(V′)1)/OH(V′)0) vibrational branching ratio of 0.07 ( 0.02 (measured at the peak of the V′ ) 0 profile at a liquid temperature of 298 K). This would correspondingly require the absolute reactivity of bending vibrationally excited squalane to be at least some 2 orders of magnitude higher than that of the ground state. No such increase in absolute reactivity is observed in the recent experiments on CH4, which suggest39 only a very modest enhancement of at most a factor of 2. On the other hand, the two systems may not be directly comparable with respect to vibrational promotion. The collision energies are higher (around 55 kJ mol-1) in the crossed-beam experiments39 and more narrowly defined than in the current work. Even though the barriers are also higher for CH4 (around 40 kJ mol-1)38 than for other hydrocarbons (closer to 20 kJ mol-1 for tertiary C-H bonds), a lower proportion of collisions therefore take place at

translational energies above the barrier in the current work. It might therefore be the case that vibrational excitation would be more effective in promoting reaction at the squalane surface than in the crossed-beam experiments on CH4. It is therefore probably not possible to eliminate conclusively vibrational activation of step 2 as the source of the selective temperature-dependent OH(V′)1) yield, but it would require a rather specific combination of vibrational-product selectivity (i.e., step 2v over 2g) and strong vibrational enhancement. Furthermore, if the systematic variation of OH(V′)1) yield with temperature in Figure 3 (note again that the temperature intervals are not equal) is analyzed in the conventional Arrhenius form, it results in a best-fit slope corresponding to an activation energy of only 4.9 kJ mol-1, or around 400 cm-1. This is incompatible with the frequency of any local vibrational mode of squalane that might have been expected on dynamic grounds to be responsible for its enhanced reactivity. In addition, if V′ ) 1 production were primarily the result of vibrational excitation in the squalane, this would correspondingly increase the total energy available to the products and therefore offset the higher kinetic energy otherwise available to OH(V′)0). As we consider further below, the data in Figure 4 strongly suggest that the direct component of V′ ) 0 does have significantly higher kinetic energies than any of the V′ ) 1. We therefore conclude that the balance of the evidence is not in favor of thermal activation through vibrational promotion of the initial reactive step (step 2) being the cause of the exclusive positive temperature dependence of the yield of OH(V′)1). In that case, we now examine the alternative proposition that this may be the result of structural changes in the liquid surface. Information on the molecular-scale surface structure of squalane through direct experiments is very limited, so the best source

Reaction of O(3P)(g) with Liquid Squalane currently is the MD simulations. As noted above, we have identified previously35 and quantified here for the current experimental temperature range that the thickness of the interfacial layer increases with temperature. This information is contained in Figure 5. As already stated, the distance over which the density drops from 50% to 5% of its bulk value increases by about 15%, from 4.1 to 4.8 Å, on going from 263 to 333 K. This implies a somewhat looser, more open structure over a greater range of depths at higher temperatures, which is intuitively reasonable. There is therefore support from the MD simulations that there are perceptible temperature-dependent changes in the surface structure, but the question remains of the physical mechanism by which this might lead to selective enhancement of the yield of OH(V′)1). Perhaps the most obvious suggestion would be that the production of V′ ) 1 in the initial reaction (step 2v in Figure 6) is increased at higher liquid temperatures. A potential mechanism for this could be greater access to sites producing predominantly V′ ) 1 as the surface structure opens up with temperature. Clearly, the prime candidate would be the tertiary H-C bonds on the squalane backbone. As is very well-known, they have the lowest bond strengths and corresponding highest reaction exothermicity. According to the normal “Polanyi rules”, they would also be expected to have lower, earlier associated barriers.38 There is indeed consensus from the earlier work in the gas phase that these sites are not only the most reactive but also give the highest proportional branching to OH(V′)1).38 The independent measurements of this branching are inconsistent at a quantitative level, though, so it remains unclear whether this could be responsible for the almost exclusive enhanced production of V′ ) 1 over V′ ) 0 observed here. A counterargument to this suggestion is that it seems physically unlikely that the improved access with increasing temperatures should apply solely to tertiary sites; if secondary sites are also more accessible, then appreciable enhanced production of V′ ) 0 would also be expected.38 Although our MD simulations do indicate a weak temperature-dependent, nonstatistical preference for primary H-C bonds to be present in the outermost layer, as discussed previously35 and confirmed here at the specific upper and lower temperature limits of the experiments, this ordering in the surface structure is very modest. Our Monte Carlo analysis of the paths of incoming projectiles35 suggests that is not sufficient to substantially perturb the chances of an O(3P) atom colliding with a primary, secondary, or tertiary site from their statistical proportions in the squalane molecule. Therefore, although we again cannot definitively exclude it, we believe that there are significant difficulties with accepting enhanced, near-exclusive production of OH(V′)1) at tertiary sites in step 2v as a satisfactory explanation for the experimental observations. Consequently, we consider the alternative possibility that the structural changes with temperature are responsible for variations in the surViVal probability of OH(V′)1). The qualitative idea is that the initial chances of producing nascent V′ ) 1 molecules in step 2v remain roughly constant, but because of the more diffuse structure at higher temperatures, there is more chance of them escaping from the surface. The simplest assumption is that this happens directly (step 4v), due to a lower probability of making the first, or at least the first few, secondary collisions that result in trapping via step 5v and hence at least partial loss of OH(V′)1). However, it is also possible that the probability of V′ ) 1 surviving trapping (resulting in successful escape via step 6v) could itself vary, most obviously due to a temperature dependence of the residence time of trapped molecules at the

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14839 surface. In this case, at least a component of the effect might be better considered as kinetic in origin, due simply to an increased rate of desorption of trapped molecules at higher temperatures. Nevertheless, there may well also be more genuinely structural effects resulting from varying rates of transport through a surface layer with a temperature-dependent density. We do not draw such a fine distinction at this stage and consider factors affecting the survival of V′ ) 1 collectively. In any event, a mechanism based on survival probabilities clearly requires there to be a selective loss process for V′ ) 1 that does not apply to V′ ) 0. The two obvious possibilities are vibrational relaxation to V′ ) 0 (step 8v) and vibrationally enhanced secondary reaction to produce H2O (step 7v), respectively. Our own experiments do not probe H2O production, but it is known to have been found as a nascent product in the molecular beam experiments of Minton and co-workers.17-21 Although it is probably not necessary to distinguish between these loss channels for the current purposes of trying to explain our observed temperature-dependent yield of V′ ) 1, it is nevertheless interesting to examine their likely contributions. It might actually have been possible to tell on empirical grounds, because the cascading population from vibrational relaxation (step 8v) should obviously appear in V′ ) 0, which we assume would be thermalized in the process (step 6g). However, because V′ ) 0 dominates the measured branching ratio,33 its proportional increase in population due to the downward transfer would be small. The resulting small decrease expected in the yield of the thermal component of V′ ) 0 as the temperature was increased would be within the statistical uncertainties in Figure 1b or 2, quite apart from the possibility of it being masked by other minor temperature-dependent factors affecting the V′ ) 0 yield. We are left to appeal to other indirect evidence to attempt to distinguish relaxation (step 8v) from reaction (step 7v). There is no obvious dynamic reason why excitation of the O-H “spectator” bond would be expected to strongly enhance the secondary abstraction reaction. This has indeed been confirmed in theoretical work40 on the related reaction of OH with ethane. There had been a previous contrary experimental report41 of vibrational enhancement of the reactivity of OH with CH4, but more recent Fourier transform infrared measurements42 of the secondary relaxation of nascent OH reaction products have cast doubt on it, suggesting that V′ ) 1 loss is dominated by vibrational relaxation. Even if the disputed, high level of vibrational enhancement of reaction of Yamasaki et al.41 were accepted, it only amounts to about 40% of the total loss of V′ ) 1 in gas-phase collisions with CH4. Ideally, we would have liked to be able to invoke similar results for larger molecules more similar to squalane. There is an extensive literature on the kinetics of reactions of thermal (i.e., V′ ) 0) OH with larger hydrocarbons.43 They are subject to relatively low activation barriers and have rate constants corresponding to reaction in 1 in ∼102 gas-kinetic collisions. We are not aware, though, of any prior state-selective information on collisions of OH(V′)1) with squalane nor, perhaps more surprisingly, with any other longer chain hydrocarbons. It is therefore only possible to speculate on which might be the dominant loss channel for V′ ) 1 in our experiments, but we believe, in conclusion, that the balance of the evidence is in favor of relaxation (step 8v) over reaction (step 7v). We note in passing that the presence of a reactive loss channel for both vibrational levels limits the maximum residence time at the surface to the order of nanoseconds or less. This is the basis of our assumption that the observed appearance times are entirely a measure of roundtrip times to and from the surface.

14840 J. Phys. Chem. C, Vol. 111, No. 40, 2007 Returning to our main line of argument, a more challenging question is whether the higher absolute rates of loss of OH(V′)1), through either relaxation or reaction, would be sufficient to result in the observed distinctive behavior in the yields of V′ ) 0 and V′ ) 1 with temperature. In a straightforward picture in which any structural change would affect equally the chances of both vibrational levels experiencing secondary collisions leading to trapping (steps 5g and 5v), then the key parameters would be the absolute fraction of nascent molecules avoiding such collisions, through steps 4g and 4v, and the rate constants for secondary reaction of OH(V′)0) (step 7g and combined reaction and vibrational relaxation of V′ ) 1 (steps 7v + 8v), respectively. These would each be in competition with the respective rates of desorption of trapped molecules (steps 6g and 6v). An answer to the question just posed could therefore be reached in principle if these parameters were known quantitatively. However, this neglects the possibility that the two vibrational levels might not have equal initial trapping probabilities through steps 5g and 5v. To explore this aspect, we turn now to the other new feature of our results, the distinct time dependencies of the appearance profiles for the two levels. This is displayed perhaps most clearly in the normalized form of the profiles in Figure 4. These data confirm the previous observation at a single temperature of 298 K34 that V′ ) 1 leaves the surface with a colder distribution of velocities than V′ ) 0. To help illuminate the possible contributions to each profile, we have added as an inset to Figure 4 further Monte Carlo simulations of the same type we have described previously.33 These show the thermaldesorption component predicted for each of the liquid temperatures in the current experiments. This should obviously apply equally to either V′ ) 0 or V′ ) 1 molecules that desorb after having been trapped (steps 6g and 6v), assuming the limit of complete thermal accommodation has been reached. As expected, the peak of the predicted profile shifts to earlier times, essentially in proportion to the T1/2 dependence of the average velocity, on top of an invariant average time for the O(3P) atoms to fly to the surface. The predicted times are obviously directly proportional to the assumed distances from the photolysis and probe beams to the surface. The distance that best matched the observed absolute peak arrival times for the V′ ) 1 profiles was 4.6 mm, in reasonable, if not perfect, agreement with the measured distance of 5.0 ( 0.5 mm. Regardless of such matters of detail, it is obvious that the predicted profiles reproduce much more closely the experimental V′ ) 1 profiles than those for V′ ) 0, both in terms of the absolute peak position and the variations with temperature. This actually makes it easier to be definitive about the interpretation of the V′ ) 0 profiles. The rising edge for V′ ) 0 is almost certainly dominated by the faster, direct component (step 4g in Figure 6). This is consistent with our previous conclusions based on the Monte Carlo modeling of flight times and the contrasting dependences of the rotational temperatures on liquid temperature at different points in the OH(V′)0) profiles.33,34 Only the slower V′ ) 0 molecules, if any, appear to be affected by the liquid temperature. This is all consistent with a mechanism in which the ratio of direct (step 4g) to trapped (step 6g) components varies only modestly with temperature for V′ ) 0. Even if it does vary to some extent, the observed nearly invariant yield would still be obtained if the survival probability of trapped V′ ) 0 (via steps 5g and 6g) is reasonably high, with relatively minor competitive loss through secondary reaction (step 7g). Any changes in the shape of the profile would also be relatively

Allan et al. slight, and restricted to later times, provided trapping-desorption (step 5g) does not dominate over direct escape (step 4g). The situation is clearly different for V′ ) 1. The source of the difference is the matter of interest, and in particular whether it is consistent with the structural explanation for the selective variation of the V′ ) 1 yield that we are currently examining. The much better match with the thermal desorption profiles could indeed be taken to imply that a higher proportion of the V′ ) 1 than V′ ) 0 molecules are trapped (via step 5v over 5g) and thermalized before desorbing (step 6v), as already suggested above. There is, in fact, an elementary physical reason why this might be expected, to which we have alluded briefly in the discussion of vibrational activation. In the initial reaction (step 2) the energy available to translation for V′ ) 1 (from step 2v) is lower than for V′ ) 0 (from step 2g) by simple energy conservation; the extra vibrational energy is a substantial fraction of the available reaction exothermicity. (Note that this fraction depends, in detail, on the H-C bond type.) This would be consistent with secondary encounters being more effective in leading to trapping, and subsequent thermalization, of V′ ) 1 (step 5v) because less momentum has to be dissipated than for V′ ) 0 (step 5g). However, caution must be exercised in concluding that the observed V′ ) 1 is entirely thermalized and produced via step 6v. Such a conclusion might reasonably have been reached from comparison of the Monte Carlo simulated profiles with the normalized V′ ) 1 profiles in Figure 4, taken in isolation. If that were the case, it would obviously not support the proposal that the increase in yield in the unnormalized profiles of V′ ) 1 in Figure 3 was due to an increased escape probability for the direct component (step 4v), which is one of the main arguments we are attempting to test. This increase would then have to be due to a temperature-dependent increased rate of escape of trapped molecules (step 6v) itself for either structural reasons or simply the kinetics of thermally activated desorption, as we have already noted above. This analysis neglects, though, the important point that the lower recoil velocity is doubleedged from a diagnostic point of view. It would also cause the direct component (step 4v) to be much less well resolved temporally from the thermal component (step 6v) for V′ ) 1 than the corresponding components clearly are for V′ ) 0. Such an overlap of contributions is consistent with our previous observation33,34 that measurements of V′ ) 1 rotational temperatures gave similar results at the rising edges and peaks of the profiles. This makes it very difficult to say to what extent the variations for V′ ) 1 in Figure 4, taken alone, are due to the effect of temperature on the thermally desorbed component (step 6v), or instead to a reduction in the trapping probability of a moderately faster direct component (step 4v). However, independent evidence suggests that there is a direct component to V′ ) 1 production (step 4v). Our previous measurements of rotational temperatures as a function of liquid temperature showed34 that the V′ ) 1 temperatures are somewhat below the liquid temperature at 273 K and, probably more significantly, do not fully track the increase to the higher liquid temperature of 343 K. This is consistent with an increasing component of rotationally colder, direct products at higher liquid temperatures. Trapping-desorption (step 6v) could indeed quite plausibly be the dominant contribution for V′ ) 1 at the lower end of the liquid temperature range (273 K), because the observed V′ ) 1 rotational temperatures are relatively close to the surface temperature.34 The reduction in trapping probability (step 5v) as the temperature is raised would not need to be very large to cause a significant increase in V′ ) 1 yield, through the direct

Reaction of O(3P)(g) with Liquid Squalane process (step 4v), if it were accompanied by a relatively low overall survival probability for the trapped V′ ) 1: as we have disused above, this would most plausibly be through vibrational relaxation (step 8v). Profiles resulting from the addition of an increasing component of moderately fast, directly escaping molecules (step 4v) to some underlying contribution from trapped molecules (step 6v) would be very difficult to distinguish from those of the trapped molecules alone. Hence, we are led finally to the conclusion on the suggested mechanism of enhanced selective survival of OH(V′)1) due to temperature-dependent structural changes that it is at least consistent with the variations in both the yields and the shapes of both the V′ ) 0 and V′ ) 1 profiles. In essence, V′ ) 0 is plausibly more likely to escape directly because of its higher recoil velocity, and less likely to be lost if it is trapped because of the absence of a vibrational relaxation channel. In contrast, V′ ) 1 may well be more likely to be trapped and, if so, more likely to be lost through vibrational relaxation. Within the assumptions of this mechanism, our results do not currently distinguish clearly between more escape of the direct V′ ) 1 product (step 4v in Figure 6) or of the trapped V′ ) 1 molecules (step 6v) as the temperature is increased. However, even if it remains plausible, this enhanced escape mechanism should certainly not be regarded as firmly established, because much of the analysis remains admittedly very speculative. Nevertheless, it is worth considering briefly the extent to which components of the mechanism might be supported by independent investigations of collisions at other liquid surfaces. There is certainly precedent for related temperature-dependent structural variations resulting in measurable effects in inelastic scattering. For example, the scattering of noble gas atoms from inert perfluorinated liquid surfaces appears to indicate that secondary trapping depends on recoil velocity and surface temperature.6 Of the noble gases examined, Xe was found to thermalize most efficiently at the surface. At low impact collision energies, thermalization appears to be essentially complete, with higher liquid temperatures pushing the appearance profile to earlier arrival times. In contrast, at higher collision energies a direct scattering component is clearly resolved from the thermal one. The thermal component shifts to earlier times at higher temperatures, but the direct component is unaffected. These effects mirror in several respects our interpretation of the appearance profiles of the OH reaction products. It is clear that interpretation of our observed OH yields and appearance profiles is complicated, even at the level of an attempted separation into competing direct and fully thermalized components as laid out in Figure 6. The real situation may, however, be more complex still. A simple binary division into two discrete mechanisms may itself be too simple. There is growing evidence from realistic dynamic simulations of other systems, including the very closely connected inelastic scattering of O(3P) from SAMs,31 that there may be effectively a continuum of mechanisms. These span direct scattering, through partial accommodation involving only a small number of secondary encounters at the outer surface of the liquid, to true trapping-desorption in which particles may penetrate quite deeply into the bulk. There are several interesting and subtly opposing physical effects in operation in these systems. A more open structure may decrease the probability of a secondary collision following reaction at a given depth in the sample, but conversely the incoming atoms may tend to penetrate to greater depths before reacting. In the inelastic scattering of Ar from PFPE,7 it has been inferred that the trapping probability of the

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14841 incoming projectile increases with temperature, due to a higher chance of multiple-bounce trajectories associated with increased surface roughness. This is the opposite of the decreased trapping probability we have suggested for the outgoing vibrationally excited reaction product. These two effects need not, however, be irreconcilable. Although we do not believe, for the reasons for discussed above, that the O(3P) atoms remain very reactive after the majority of their incoming momentum has been dissipated, it is possible that on a rougher surface more reaction takes place on the second bounce. These O atoms will on average have had a significant component of their momentum reversed in the direction away from the surface, which could promote a higher escape probability than for reaction on the first, incoming encounter. Speculation of this type simply serves as an example of the subtlety of the possible mechanisms that may remain to be uncovered. We have considered previously35 attempts to model the fate of the products by extension of our Monte Carlo tracking model. We do not expect, though, that such a simplified approach is likely to be very conclusive. We were able to estimate that a relatively high proportion of products (50-90%, depending on the assumed values of the model parameters) were indeed predicted to suffer a secondary collision. This is presumably a necessary, but not sufficient, condition to initiate the sequence leading to trapping. However, it is even more difficult to test with any rigor how the trapping probability might vary with temperature. Among the more fundamental difficulties, the angular scattering distributions in the local center-of-mass frame are unknown, and are potentially both H-C bond type and product vibrational-level dependent. Furthermore, a static model does not properly account for momentum conservation and hence recoil in the frame of the remaining squalane sample. This once again suggests the potential high value of realistic dynamic simulations, of the type previously reported for model SAM surfaces29-31 and now most recently for squalane itself,23 in disentangling the mechanisms of reactive collisions at liquid surfaces. Although very valuable in beginning this process, the calculations on squalane thus far23 are not directly comparable in detail to our experiments because of the much higher collision energies adopted. There is also a recognized difficulty that the semiempirical MSINDO Hamiltonian overpredicts the reaction exothermicity. This is itself likely to be a source of the prediction of much higher OH internal, especially rotational, energies than we observe. There has also been no attempt as yet to examine the influence of liquid temperature on product level dependent yields, which is the main thrust of the current work. Nevertheless, we eagerly anticipate further complementary theoretical modeling and refined experiments that might address these interesting questions. Conclusions We have uncovered an interesting and previously unobserved empirical fact that the yield of OH(V′)0) from reaction of O(3P) at a liquid squalane surface is essentially independent of liquid temperature in the range 263-333 K, whereas that of OH(V′)1) is not. We have considered possible explanations for the selective enhancement of a factor of around 2 in the OH(V′)1) signal. This could be due to a thermally activated increase in reactivity resulting from vibrational excitation of squalane molecules. We cannot eliminate this conclusively, but we do not consider it to be the most likely explanation because of the required scale of the increase in reactivity and the need for it to be highly vibrational product state selective. We have also examined the possibility that it is a structural effect, because

14842 J. Phys. Chem. C, Vol. 111, No. 40, 2007 our realistic MD simulations do predict a modest opening up of the surface structure with temperature. If the effect is structural, we believe it is unlikely to be due to enhanced exclusive access to more reactive tertiary H-C bonds. We have therefore considered a potential alternative structural explanation based on an enhanced surViVal probability, rather than greater production, of OH(V′)1) at higher temperatures. This would necessarily require a vibrational-state-selective loss mechanism for V′ ) 1. The obvious candidates are vibrational relaxation, which we consider to be the more likely, and vibrationally enhanced secondary reaction to produce H2O. It is also plausible that V′ ) 1 is more likely to be trapped in the first place than V′ ) 0 because of its lower recoil velocity. The enhanced survival mechanism does appear to be consistent with the currently available evidence, including the observed temperature-dependent normalized appearance profile for V′ ) 1 and the corresponding nearly invariant profile for V′ ) 0. Within this assumption, we are not currently able to distinguish, for experimental reasons, between enhanced survival of direct and trapping-desorbed components of V′ ) 1. However, we caution that this interpretation remains highly speculative, and there may well be other potential mechanistic explanations. Understanding of these subtle, product vibrational level dependent effects would clearly be aided by further realistic dynamic scattering calculations targeted at these aspects of this benchmark gas-liquid interfacial reaction. Acknowledgment. The authors thank the EPSRC for research grant funding, for a RCUK Academic Fellowship for M.L.C., and studentship support for M.A. References and Notes (1) Saecker, M. E.; Govoni, S. T.; Kowalski, D. V.; King, M. E.; Nathanson, G. M. Science 1991, 252, 1421-1424. (2) King, M. E.; Nathanson, G. M.; Hanning-Lee, M. A.; Minton, T. K. Phys. ReV. Lett. 1993, 70, 1026-1029. (3) Saecker, M. E.; Nathanson, G. M. J. Chem. Phys. 1993, 99, 70567075. (4) Saecker, M. E.; Nathanson, G. M. J. Chem. Phys. 1994, 100, 39994005. (5) Lipkin, N.; Gerber, R. B.; Moiseyev, N.; Nathanson, G. M. J. Chem. Phys. 1994, 100, 8408-8417. (6) King, M. E.; Saecker, M. E.; Nathanson, G. M. J. Chem. Phys. 1994, 101, 2539-2547. (7) King, M. E.; Fiehrer, K. M.; Nathanson, G. M.; Minton, T. K. J. Phys. Chem. A 1997, 101, 6556-6561. (8) Kenyon, A. J.; McCaffery, A. J.; Quintella, C. M.; Zidan, M. D. Chem. Phys. Lett. 1992, 190, 55-58. (9) Kenyon, A. J.; McCaffery, A. J.; Quintella, C. M.; Zidan, M. D. Faraday Discuss. 1993, 96, 245-254. (10) Perkins, B. G., Jr.; Haber, T.; Nesbitt, D. J. J. Phys. Chem. B 2005, 109, 16396-16405.

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