Molecular Beam Scattering from Supercooled Sulfuric Acid: Collisions of HCl, HBr, and HNO3 with 70 wt D2SO4

Surface Confinement of Finite-Size Water Droplets for SO3 Hydrolysis Reaction Revealed by Molecular Dynamics Simulations Based on a Machine Learning Force Field

As an important source for sulfuric acid in the atmosphere, hydrolysis of sulfur trioxide (SO₃) takes place with water clusters of sizes from several molecules to several nanometers, resulting in various final products, including neutral (H₂SO₄)-(H₂O) clusters and ionic (HSO₄)^--(H₃O)⁺ clusters. The diverse products may be due to the ability of proton transfer and the formation of hydrated ions for water cluster of finite sizes, especially the sub-micrometer ones. However, the detailed molecular-level mechanism is still unclear due to the lack of available characterization and simulations tools. Here, we developed a quantum chemistry-level machine learning (ML) model to simulate the hydrolysis of SO₃ with water clusters of sizes up to nanometers. The simulation results demonstrate diverse reaction paths taking place between SO₃ and water clusters of different sizes. Generally, neutral (H₂SO₄)-(H₂O) clusters are preferred by water clusters of ultra-small size, and a loop structure-mediated mechanism with SO₃(H₂O)_n≤4 structures and a non-loop structure-mediated mechanism with structure relaxation are observed. As the water cluster size increases to (H₂O)₈, a (HSO₄)^--(H₃O)⁺ ion-pair product emerges; and the Eigen-Zundel ion conversion-like proton transfer mechanism takes place and stabilizes the ion pairs. As the water cluster sizes further increase beyond several nanometers ((H₂O)_n≥32), the (SO₄)^2-[(H₃O)⁺]₂ ion-pair product appears. The reason could be that the surface of these water clusters is large enough to screen Coulomb repulsion between two tri-coordinated ion-pair complexes. These findings would provide new perspectives for understanding SO₃ hydrolysis in the real atmosphere and sulfuric acid chemistry in atmospheric aerosols.

Oxidation Enhances Aerosol Nucleation: Measurement of Kinetic Pickup Probability of Organic Molecules on Hydrated Acid Clusters

We investigate the uptake of the most prominent biogenic volatile organic compounds (VOCs)-isoprene, α-pinene, and their selected oxidation products-by hydrated acid clusters in a molecular beam experiment and by DFT calculations. Our experiments provide a unique and direct way of determination of the surface accommodation coefficient (α_S) on the proxies of ultrafine aerosol particles. Since we are able to determine unambiguously the fraction of the clusters to which the molecules stick upon collisions, our α_S is a purely kinetic parameter disentangling the molecule pickup from its evaporation. Oxidation increases the α_S of VOCs by more than an order of magnitude, because oxidized compounds form hydrogen bonds with the clusters, whereas the interactions of the parent VOCs are weaker and nonspecific. This work provides molecular-level insight into the condensation of single molecules into atmospheric particles, which has important implications for aerosol nucleation and growth.

Coexistence of Physisorbed and Solvated HCl at Warm Ice Surfaces

The interfacial ionization of strong acids is an essential factor of multiphase and heterogeneous chemistry in environmental science, cryospheric science, catalysis research and material science. Using near ambient pressure core level X-ray photoelectron spectroscopy, we directly detected a low surface coverage of adsorbed HCl at 253 K in both molecular and dissociated states. Depth profiles derived from XPS data indicate the results as physisorbed molecular HCl at the outermost ice surface and dissociation occurring upon solvation deeper in the interfacial region. Complementary X-ray absorption measurements confirm that the presence of Cl^- ions induces significant changes to the hydrogen bonding network in the interfacial region. This study gives clear evidence for nonuniformity across the air-ice interface and questions the use of acid-base concepts in interfacial processes.

Super-Maxwellian helium evaporation from pure and salty water

Helium atoms evaporate from pure water and salty solutions in super-Maxwellian speed distributions, as observed experimentally and modeled theoretically. The experiments are performed by monitoring the velocities of dissolved He atoms that evaporate from microjets of pure water at 252 K and 4-8.5 molal LiCl and LiBr at 232-252 K. The average He atom energies exceed the flux-weighted Maxwell-Boltzmann average of 2RT by 30% for pure water and 70% for 8.5m LiBr. Classical molecular dynamics simulations closely reproduce the observed speed distributions and provide microscopic insight into the forces that eject the He atoms from solution. Comparisons of the density profile and He kinetic energies across the water-vacuum interface indicate that the He atoms are accelerated by He-water collisions within the top 1-2 layers of the liquid. We also find that the average He atom kinetic energy scales with the free energy of solvation of this sparingly soluble gas. This free-energy difference reflects the steeply decreasing potential of mean force on the He atoms in the interfacial region, whose gradient is the repulsive force that tends to expel the atoms. The accompanying sharp decrease in water density suppresses the He-water collisions that would otherwise maintain a Maxwell-Boltzmann distribution, allowing the He atom to escape at high energies. Helium is especially affected by this reduction in collisions because its weak interactions make energy transfer inefficient.

Coupling a Knudsen reactor with the short lived radioactive tracer (13)N for atmospheric chemistry studies

A Knudsen cell flow reactor was coupled to an online gas phase source of the short-lived radioactive tracer (13)N to study the adsorption of nitrogen oxides on ice at temperatures relevant for the upper troposphere. This novel approach has several benefits over the conventional coupling of a Knudsen cell with a mass spectrometer. Experiments at lower partial pressures close to atmospheric conditions are possible. The uptake to the substrate is a direct observable of the experiment. Operation of the experiment in continuous or pulse mode allows to retrieve steady state uptake kinetics and more details of adsorption and desorption kinetics.

Reactions of HCl and D2O with molten alkali carbonates

The acidic oxide SO₂ and protic acid HCl are among the gases released in the combustion of coal and the incineration of municipal waste. They are typically removed by wet or dry scrubbing involving calcium carbonate or calcium hydroxide. The molten alkali carbonate eutectic provides a liquid-state alternative that readily absorbs SO₂ and HCl and does not become covered with a passivating layer. Gas-liquid scattering experiments utilizing the eutectic mixture (44 mol % Li₂CO₃, 31 mol % Na₂CO₃, 25 mol % K₂CO₃) reveal that the reaction probability for HCl(g) + CO₃²⁻ → CO₂(g) + OH⁻ + Cl⁻ is 0.31 ± 0.02 at 683 K and rises to 0.39 at 783 K. Gaseous CO₂ is formed within 10⁻⁴ s or less, implying that the reaction takes place in a liquid depth of less than 1000 Å. When the melt is exposed to D₂O, the analogous reaction D₂O(g) + CO₃²⁻ → CO₂(g) + 2OD⁻ occurs too slowly to measure and no water uptake is observed. Together with previous studies of SO₂(g) + CO₃²⁻ → CO₂(g) + SO₃²⁻, these results demonstrate that molten carbonates efficiently remove both gaseous HCl and SO₂ while reacting at most weakly with water vapor. The experiments further highlight the remarkable ability of hot CO₃²⁻ ions to behave as a base in reactions with protic and Lewis acids.

Reactive collisions of sulfur dioxide with molten carbonates

Molecular beam scattering experiments are used to investigate reactions of SO(2) at the surface of a molten alkali carbonate eutectic at 683 K. We find that two-thirds of the SO(2) molecules that thermalize at the surface of the melt are converted to gaseous CO(2) via the reaction SO(2)(g) + CO(3)(2-) --> CO(2)(g) + SO(3)(-2). The CO(2) product is formed from SO(2) in less than 10(-6) s, implying that the reaction takes place in a shallow liquid region less than 100 A deep. The reaction probability does not vary between 683 and 883 K, further implying a compensation between decreasing SO(2) residence time in the near-interfacial region and increasing reactivity at higher temperatures. These results demonstrate the remarkable efficiency of SO(2) --> CO(2) conversion by molten carbonates, which appear to be much more reactive than dry calcium carbonate or wet slurries commonly used for flue gas desulfurization in coal-burning power plants.

Reversible Gas Adsorption in Coated Wall Flow Tube Reactors. Model Simulations for Langmuir Kinetics

A kinetic model has been developed to simulate reversible gas adsorption in coated wall flow tube reactors (CWFTs). The motivation of this work is to provide the theoretical framework for modelling studies in support of the results obtained from CWFT studies on the adsorption and desorption behaviour of atmospheric trace gases on ice surfaces at temperatures relevant to the upper troposphere/lower stratosphere (190–230 K). The model consists of an axial sequence of individual flow tube sections of equal volumes in which the gas phase is homogeneously mixed and interactions with the coated wall occur by adsoption and desorption exclusively. The adsorption rate is assumed to be kinetically controlled and not to be transport limited. Moreover, chemical reactions are not considered. Simulations have been performed for the temporal behaviour of the gas phase concentration at the exit of the flow tube as a function of laboratory time for typical operation procedures of CWFTs with moveable injectors i.e. (i) instantaneous injection of the gas, (ii) instantaneous termination of the gas flow and (iii) movements of the injector with constant velocities. It is found that the temporal profiles show complex behaviours due to the overlap of adsorption and desorption upon successive exposure of the gas to different length of the ice surface. The validity of the model is demonstrated for the adsorption of acetone on an ice surface at 200 K as a case study.

Chemistry and Microphysics of Atmospheric Aerosol Surfaces: Laboratory Techniques and Applications

A number of current techniques are presented by which the chemistry of interaction of selected gas phase species with atmospheric surfaces as well as the microphysical behaviour of such surfaces can be investigated. The techniques discussed include (i) the coated wall flow tube reactor, (ii) the Knudsen-cell / DRIFT spectroscopy, (iii) the surface aerosol microscopy and (iv) the molecular beam scattering technique. In each of these methods specific and robust information is deduced on the kinetics and thermodynamics of gas adsorption and reaction on surfaces. Specific examples include the adsorption of acetone on ice surfaces, the adsorption and reaction of SO2 on iron oxides, the hygroscopic and phase behaviour of binary and ternary salt solution droplets (ammonium sulphate and ammonium sulphate / dicarboxylic acids solutions) as well as on the dynamics of inelastic collisions of noble gases on super-cooled sulphuric acid surfaces. In addition we also show how quantum chemistry can be utilized to assist in interpreting absorption energies on structurally different ice surfaces. Whilst each example represents different aspects of heterogenous atmospheric interactions, they jointly represent significant progress in laboratory investigations of multi-phase atmospheric chemistry with substantial potential for application to other systems and/or problems.

Surfactant control of gas transport and reactions at the surface of sulfuric acid

Aerosol particles in the atmosphere are tiny chemical reactors that catalyze numerous reactions, including the conversion of benign gases into ozone-destroying ones. In the lower stratosphere, these particles are often supercooled mixtures of water and sulfuric acid. The different species present at the surface of these droplets (H(2)O, H(3)O(+), HSO(4)(-), H(2)SO(4), and SO(4)(2-)) stand at the "gas-liquid frontier"; as the first to be struck by impinging molecules, these species provide the initial environment for solvation and reaction. Furthermore, aerosol particles may contain a wide range of organic molecules, some of which migrate to the surface and coat the droplet. How do ambient gases dissolve in the droplet if it is coated with an organic layer? At one extreme, monolayer films of insoluble, long-chain alcohols can dramatically reduce gas transport, packing so tightly at the surface of water that they impede water evaporation by factors of 10,000 or more. Shorter chain surfactants are expected to pack less tightly, but we wondered whether these incomplete monolayers also block gas transport and whether this system could serve as a model for understanding the surfaces of atmospheric aerosol particles. To address these questions, our research focuses on small, soluble surfactants such as butanol and hexanol dissolved in supercooled sulfuric acid. These amphiphilic molecules spontaneously segregate to the surface and coat the acid but only to a degree. Gas-liquid scattering experiments reveal that these porous films behave in surprisingly diverse ways: they can impose a barrier (to N(2)O(5) hydrolysis), be "invisible" (to water evaporation), or even enhance gas uptake (of HCl). The transition from obstacle to catalyst can be traced to specific interactions between the surfactant and each gas. For example, the hydrolysis of N(2)O(5) may be impeded because of its large size and because alcohol molecules that straddle the interface limit contact between N(2)O(5) and its H(3)O(+) and H(2)O reaction partners. However, these same alcohol molecules assist HCl dissociation because the alcohol OH groups provide extra interfacial protonation sites. Interestingly, butanol does not impede water evaporation, in part because the butyl chains pack much more loosely than insoluble, long-chain surfactants. Through these investigations, we hope to gain insight into the mechanisms by which surfactants on sulfuric acid and other aqueous solutions affect transport and reactivity at the gas-liquid interface.

Quantum state-resolved CO2 collisions at the gas-liquid interface: surface temperature-dependent scattering dynamics

Energy transfer dynamics at the gas-liquid interface are investigated as a function of surface temperature both by experimental studies of CO2 + perfluorinated polyether (PFPE) and by molecular dynamics simulations of CO2 + fluorinated self-assembled monolayers (F-SAMs). Using a normal incident molecular beam, the experimental studies probe scattered CO2 internal-state and translational distributions with high resolution infrared spectroscopy. At low incident energies [Einc = 1.6(1) kcal/mol], CO2 J-state populations and transverse Doppler velocity distributions are characteristic of the surface temperature (Trot approximately Ttrans approximately TS) over the range from 232 to 323 K. In contrast, the rotational and translational distributions at high incident energies [Einc = 10.6(8) kcal/mol] show evidence for both trapping-desorption (TD) and impulsive scattering (IS) events. Specifically, the populations are surprisingly well-characterized by a sum of Boltzmann distributions where the two components include one (TD) that equilibrates with the surface (TTD approximately TS) and a second (IS) that is much hotter than the surface temperature (TIS > TS). Support for the superthermal, yet Boltzmann, nature of the IS channel is provided by molecular dynamics (MD) simulations of CO2 + F-SAMs [Einc = 10.6 kcal/mol], which reveal two-temperature distributions, sticking probabilities, and angular distributions in near quantitative agreement with the experimental PFPE results. Finally, experiments as a function of surface temperature reveal an increase in both sticking probability and rotational/translational temperature of the IS component. Such a trend is consistent with increased surface roughness at higher surface temperature, which increases the overall probability of trapping, yet preferentially leads to impulsive scattering of more highly internally excited CO2 from the surface.

Quantum-State-Resolved CO2 Scattering Dynamics at the Gas−Liquid Interface: Dependence on Incident Angle

Energy transfer dynamics at the gas-liquid interface have been probed with a supersonic molecular beam of CO2 and a clean perfluorinated-liquid surface in vacuum. High-resolution infrared spectroscopy measures both the rovibrational state populations and the translational distributions for the scattered CO2 flux. The present study investigates collision dynamics as a function of incident angle (thetainc = 0 degrees, 30 degrees, 45 degrees, and 60 degrees), where column-integrated quantum state populations are detected along the specular-scattering direction (i.e., thetascat approximately thetainc). Internal state rovibrational and Doppler translational distributions in the scattered CO2 yield clear evidence for nonstatistical behavior, providing quantum-state-resolved support for microscopic branching of the gas-liquid collision dynamics into multiple channels. Specifically, the data are remarkably well described by a two-temperature model, which can be associated with both a trapping desorption (TD) component emerging at the surface temperature (Trot approximately TS) and an impulsive scattering (IS) component appearing at hyperthermal energies (Trot > TS). The branching ratio between the TD and IS channels is found to depend strongly on thetainc, with the IS component growing dramatically with increasingly steeper angle of incidence.

The Inhibition of N2O5 Hydrolysis in Sulfuric Acid by 1-Butanol and 1-Hexanol Surfactant Coatings

Gas-liquid scattering experiments are used to measure the fraction of N2O5 molecules that are converted to HNO3 after colliding with 72 wt % H2SO4 containing 1-hexanol or 1-butanol at 216 K. These alcohols segregate to the surface of the acid, with saturation coverages estimated to be 60% of a close-packed monolayer for 1-hexanol and 44% of a close-packed monolayer for 1-butanol. We find that the alkyl films reduce the conversion of N2O5 to HNO3 from 0.15 on bare acid to 0.06 on the hexyl-coated acid and to 0.10 on the butyl-coated acid. The entry of HCl and HBr, however, is enhanced by the hexanol and butanol films. The hydrolysis of N2O5 may be inhibited because the alkyl chains restrict the transport of this large molecule and because the alcohol OH groups dilute the surface region, suppressing reaction between N2O5 and near-interfacial H3O+ or H2O. In contrast, the interfacial alcohol OH groups provide additional binding sites for HCl and HBr and help initiate ionization. These and previous scattering experiments indicate that short-chain alcohol surfactants impede or enhance sulfuric acid-mediated reactions in ways that depend on the chain length, liquid phase acidity, and nature of the gas molecule.

Collisions and reactions of gaseous propanol with molten NaOH∕KOH

Molecular beam scattering experiments are used to investigate collisions of a protic molecule, deuterated 1-propanol (PrOD), with an extremely basic solvent, the 5149 mol % NaOH/KOH eutectic mixture. This powerful deprotonating medium readily absorbs PrOD from the gas phase. Nearly all PrOD molecules that thermalize at the surface of the melt enter the liquid and dissolve for long times, most likely residing as PrO- after deprotonation by OH-. The PrO- solvation time is controlled by dissolved H2O, which reprotonates the anion and liberates D --> H exchanged PrOH. We find no evidence for decomposition of the alcohol; at the 463 K temperature of the experiments, the hydroxide solution appears to store propanol reversibly.

Quantum-state resolved reaction dynamics at the gas-liquid interface: Direct absorption detection of HF(v,J) product from F(P2)+Squalane

Exothermic reactive scattering of F atoms at the gas-liquid interface of a liquid hydrocarbon (squalane) surface has been studied under single collision conditions by shot noise limited high-resolution infrared absorption on the nascent HF(v,J) product. The nascent HF(v,J) vibrational distributions are inverted, indicating insufficient time for complete vibrational energy transfer into the surface liquid. The HF(v=2,J) rotational distributions are well fit with a two temperature Boltzmann analysis, with a near room temperature component (T(TD) approximately equal to 290 K) and a second much hotter scattering component (T(HDS) approximately equal to 1040 K). These data provide quantum state level support for microscopic branching in the atom abstraction dynamics corresponding to escape of nascent HF from the liquid surface on time scales both slow and fast with respect to rotational relaxation.

Acetone Adsorption on Ice Surfaces in the Temperature Range T = 190−220 K: Evidence for Aging Effects Due to Crystallographic Changes of the Adsorption Sites

The rate and thermodynamics of the adsorption of acetone on ice surfaces have been studied in the temperature range T = 190-220 K using a coated-wall flow tube reactor (CWFT) coupled with QMS detection. Ice films of 75 +/- 25 microm thickness were prepared by coating the reactor using a calibrated flow of water vapor. The rate coefficients for adsorption and desorption as well as adsorption isotherms have been derived from temporal profiles of the gas phase concentration at the exit of the flow reactor together with a kinetic model that has recently been developed in our group to simulate reversible adsorption in CWFTs (Behr, P.; Terziyski, A.; Zellner, R. Z. Phys. Chem. 2004, 218, 1307-1327). It is found that acetone adsorption is entirely reversible; the adsorption capacity, however, depends on temperature and decreases with the age of the ice film. The aging effect is most pronounced at low acetone gas-phase concentrations (< or = 2.0 x 10(11) molecules/cm(3)) and at low temperatures. Under these conditions, acetone is initially adsorbed with a high rate and high surface coverage that, upon aging, both become lower. This effect is explained by the existence of initially two adsorption sites (1) and (2), which differ in nature and number density and for which the relative fractions change with time. Using two-site dynamic modeling, the rate coefficients for adsorption (k(ads)) and desorption (k(des)) as well as the Langmuir constant (K(L)) and the maximum number of adsorption sites (c(s,max)), as obtained for the adsorption of acetone on sites of types (1) and (2) in the respective temperature range, are k(ads)(1) = 3.8 x 10(-14) T(0.5) cm(3) s(-1), k(des)(1) = 4.0 x 10(11) exp(-5773/T) s(-1), K(L) (1) = 6.3 x 10(-25) exp(5893/T) cm(3), c(s,max)(1) < or = 10(14) cm(-2) and k(ads)(2) = 2.9 x 10(-15) T(0.5) cm(3) s(-1), k(des)(2) = 1.5 x 10(7) exp(-3488/T) s(-1), K(L)(2) = 5.0 x 10(-22) exp(3849/T) cm(3), c(s,max)(2) = 6.0 x 10(14) cm(-2), respectively. On the basis of these results, the adsorption of acetone on aged ice occurs exclusively on sites of type (2). Among the possible explanations for the time-dependent two-site adsorption behavior, i.e., crystallographic differences, molecular or engraved microstructures, or a mixture of the two, we tentatively accept the former, i.e., that the two adsorption sites correspond to cubic (1, I(c)) and hexagonal (2, I(h)) sites. The temporal change of I(c) to I(h) and, hence, the time constants of aging are consistent with independent information in the literature on these phase changes.

Evaporation of Water and Uptake of HCl and HBr through Hexanol Films at the Surface of Supercooled Sulfuric Acid

Vacuum evaporation and molecular beam scattering experiments have been used to monitor the loss of water and dissolution of HCl and HBr in deuterated sulfuric acid at 213 K containing 0 to 100 mM hexanol. The addition of 1-hexanol to the acid creates a surface film of hexyl species. This film becomes more compact with decreasing acidity, ranging from approximately 62% to approximately 68% of maximum packing on 68 to 56 wt % D(2)SO(4), respectively. D(2)O evaporation from 68 wt % acid remains unaltered by the hexyl film, where it is most porous, but is impeded by approximately 20% from 56 and 60 wt % acid. H --> D exchange experiments further indicate that the hexyl film on 68 wt % acid enhances conversion of HCl and HBr into DCl and DBr, which is interpreted as an increase in HCl and HBr entry into the bulk acid. For this permeable hexyl film, the hydroxyl groups of surface hexanol molecules may assist uptake by providing extra sites for HCl and HBr hydrogen bonding and dissociation. In contrast, HCl --> DCl exchange in 60 wt % D(2)SO(4) at first rises with hexyl surface coverage but then drops back to the bare acid value as the hexyl species pack more tightly. HCl entry is actually diminished by the hexyl film on 56 wt % acid, where the film is most compact. These experiments reveal a transition from a porous hexanol film on 68 wt % sulfuric acid that enhances HCl and HBr uptake to one on 56 wt % acid that slightly impedes HCl and D(2)O transport.

Collisions of DCl with Pure and Salty Glycerol: Enhancement of Interfacial D → H Exchange by Dissolved NaI

The fate of DCl molecules striking pure glycerol and a 2.6 M NaI-glycerol solution is investigated using scattering, uptake, and residence time measurements. We find that dissolved Na+ and I- ions alter every gas-liquid pathway from the moment of contact of DCl with the surface to its eventual emergence as HCl. In particular, the salt enhances both trapping-desorption of DCl and interfacial DCl --> HCl exchange at the expense of DCl entry into the bulk solution. The reduced entry and enhanced desorption of thermalized DCl molecules are interpreted by assuming that Na+ and I- ions bind to interfacial OH groups and tie up surface sites that would otherwise capture incoming DCl molecules. These ion-glycerol interactions may also be responsible for enhancing interfacial D --> H exchange by disrupting the interfacial hydrogen bond network that carries the newly formed H+ ion away from its Cl- pair. This disruption may increase the fraction of interfacial Cl- and H+ that recombine and desorb immediately as HCl before the ions separate and diffuse deeply into the bulk.

Energy transfer at a gas-liquid interface: kinematics in a prototypical system

A detailed characterization of collisional energy transfer at a liquid surface not only provides a framework for the interpretation of experimental studies but also affords insight into energy feedback mechanisms that may be important in multiphase combustion processes. We address this problem by performing simulations of a prototypical Lennard-Jones system, investigating the dependence of the energy transfer and incident-atom trapping probability on the liquid temperature, on the mass and angle of incidence of the impinging atom, and on the strength of the gas-liquid interaction. In general, in agreement with the results of experiments, these calculations point to the dominance of kinematic effects in determining the gross energy transfer, but they also attest to the important role played by surface roughening in the enhancement of energy transfer that accompanies an increase in the liquid temperature.

Theoretical Study of the First Acid Dissociation of H2SO4 at a Model Aqueous Surface

Electronic structure calculations on the H2SO4.(H2O)(4,6) model system embedded at the surface of an aqueous layer have been performed to examine the feasibility of the first acid dissociation of H2SO4 to an H2SO4-.H3O+ contact ion pair over a wide temperature range, with a special focus on the 190-250 K range relevant for atmospheric sulfate aerosols. The results indicate that the acid dissociation can be either thermodynamically favored or disfavored depending on the degree of solvation of the acid and the produced ions, as well as on the temperature.

Scattering, accommodation, and trapping of HCl in collisions with a hydroxylated self-assembled monolayer

Time-of-flight molecular beam scattering techniques are used to explore the energy exchange, thermal accommodation, and residence time of HCl in collisions with an OH-terminated self-assembled monolayer. The monolayer, consisting of 16-mercapto-1-hexadecanol (HS(CH(2))(16)OH) self-assembled on gold, provides a well-characterized surface containing hydroxyl groups located at the gas-solid interface. Upon colliding with the hydroxylated surface, the gas-phase HCl is found to follow one of three pathways: direct impulsive scattering, thermal accommodation followed by prompt desorption, and temporary trapping through HO--- HCl hydrogen bond formation. For an incident energy of 85 kJ/mol, the HCl transfers the majority, >80%, of its translational energy to the surface. The extensive energy exchange facilitates thermalization, leading to very large accommodation probabilities on the surface. Under the experimental conditions used in this work, over 75% of the HCl approaches thermal equilibrium with the surface before desorption and, for a 6 kJ/mol HCl beam, nearly 100% of the molecules that recoil from the surface can be described by a thermal distribution at the temperature of the surface. For the molecules that reach thermal equilibrium with the surface prior to desorption, a significant fraction appear to form hydrogen bonds with surface hydroxyl groups. The adsorption energy, determined by measuring the HCl residence time as a function of surface temperature, is 24 +/- 2 kJ/mol.

Quantum State-Resolved Energy Transfer Dynamics at Gas−Liquid Interfaces: IR Laser Studies of CO2 Scattering from Perfluorinated Liquids

An apparatus for detailed study of quantum state-resolved inelastic energy transfer dynamics at the gas-liquid interface is described. The approach relies on supersonic jet-cooled molecular beams impinging on a continuously renewable liquid surface in a vacuum and exploits sub-Doppler high-resolution laser absorption methods to probe rotational, vibrational, and translational distributions in the scattered flux. First results are presented for skimmed beams of jet-cooled CO(2) (T(beam) approximately 15 K) colliding at normal incidence with a liquid perfluoropolyether (PFPE) surface at E(inc) = 10.6(8) kcal/mol. The experiment uses a tunable Pb-salt diode laser for direct absorption on the CO(2) nu(3) asymmetric stretch. Measured rotational distributions in both 00(0)0 and 01(1)0 vibrational manifolds indicate CO(2) inelastically scatters from the liquid surface into a clearly non-Boltzmann distribution, revealing nonequilibrium dynamics with average rotational energies in excess of the liquid (T(s) = 300 K). Furthermore, high-resolution analysis of the absorption profiles reveals that Doppler widths correspond to temperatures significantly warmer than T(s) and increase systematically with the J rotational state. These rotational and translational distributions are consistent with two distinct gas-liquid collision pathways: (i) a T approximately 300 K component due to trapping-desorption (TD) and (ii) a much hotter distribution (T approximately 750 K) due to "prompt" impulsive scattering (IS) from the gas-liquid interface. By way of contrast, vibrational populations in the CO(2) bending mode are inefficiently excited by scattering from the liquid, presumably reflecting much slower T-V collisional energy transfer rates.

Surfactant Control of Gas Uptake: Effect of Butanol Films on HCl and HBr Entry into Supercooled Sulfuric Acid

The entry of HCl into 60-68 wt % D(2)SO(4) and HBr into 68 wt % acid containing 0-0.18 M 1-butanol was monitored by measuring the fractions of impinging HCl and HBr molecules that desorb as DCl and DBr after undergoing H --> D exchange within the deuterated acid. The addition of 0.18 M butanol to the acid creates butyl films that reach approximately 80% surface coverage at 213 K. Surprisingly, this butyl film does not impede exchange but instead enhances it: the HCl --> DCl exchange fractions increase from 0.52 to 0.74 for 60 wt % D(2)SO(4) and from 0.14 to 0.27 for 68 wt % D(2)SO(4). HBr --> DBr exchange increases even more sharply, rising from 0.22 to 0.65 for 68 wt % D(2)SO(4). We demonstrate that this enhanced exchange corresponds to enhanced uptake into the butyl-coated acid for HBr and infer this equivalence for HCl. In contrast, the entry probability of the basic molecule CF(3)CH(2)OH exceeds 0.85 at all acid concentrations and is only slightly diminished by the butyl film. The OD groups of surface butanol molecules may assist entry by providing extra interfacial protonation sites for HCl and HBr dissociation. The experiments suggest that short-chain surfactants in sulfuric acid aerosols do not hinder heterogeneous reactions of HCl or HBr with other solute species.

Evaporation of Water through Butanol Films at the Surface of Supercooled Sulfuric Acid

The evaporation of water was monitored from 60, 64, and 68 wt % D(2)SO(4) at 213 K containing 0-0.18 M 1-butanol. Measurements were performed in vacuum using a mass spectrometer to record the velocities and relative fluxes of the desorbing D(2)O. In addition, the surface activity of butanol in the acid was characterized by hyperthermal argon atom scattering in conjunction with surface tension and butanol evaporation measurements. The segregated butyl species reach surface concentrations of approximately 4 x 10(14) cm(-2) (approximately 80% surface coverage) at 0.18 M bulk concentration. We find that the butyl films do not impede the evaporation of D(2)O from the acid to within the 5% uncertainty of the measurements. This result implies that small, soluble surfactants such as butanol form porous films that will not alter the growth or shrinkage of supercooled sulfuric acid droplets in the atmosphere.

MOLECULAR BEAM STUDIES OF GAS-LIQUID INTERFACES

Molecular beam scattering experiments provide a way to disentangle the elementary steps involved in energy transfer and chemical reactions between gases and liquids. After surveying the history and recent progress in this field, we review studies of the kinematics of gas-liquid collisions and proton exchange of HCl, DCl, and HBr with supercooled sulfuric acid and liquid glycerol. These experiments help to clarify the role of the surface region in controlling trapping and interfacial- and bulk-phase reactions.

Aqueous Solution/Air Interfaces Probed with Sum Frequency Generation Spectroscopy

An important issue for developing a molecular-level mechanism of heterogeneous interactions at the aqueous interface is determining changes in the interface with changes in the bulk composition. Development of the nonlinear spectroscopy, sum frequency generation (SFG) provides a technique to probe these changes. Several molecular and ionic solutes have been used to investigate changes in the structure of the aqueous interface. Molecular solutes include glycerol and ammonia. Ionic and associated ion complexes include sulfuric acid as well as alkali sulfate and bisulfate salts. Molecular solutes and associated ion complexes penetrate to the top monolayer of the aqueous-air interface displacing water from the interface. Specifically, the conjectured ammonia-water complex is observed with ammonia tilted, on average, 25-38° from the normal. Ionic solutes generate a double layer in the interfacial region due to the differential distribution of anions and cations near the interface. The strength of the double layer is dependent on ion size and charge. Due to the extreme size of the proton, the strongest field is generated by acidic solutes. As the ionic solute concentration increases, associated ion pairs form and these penetrate to the top monolayer. These results have wide implications because the aqueous interface is ubiquitous in atmospheric and biological systems.