Carbon Dioxide Interactions with Crystalline and Amorphous Ice Surfaces

Observation of Carbonic Acid Formation from Interaction between Carbon Dioxide and Ice by Using In Situ Modulation Excitation IR Spectroscopy

Carbonic acid, H₂ CO₃ , is of fundamental importance in nature both in living and non-living systems. Providing direct spectroscopic evidence for carbonic acid formation is however a challenge. Here we provide clear evidence by in situ attenuated total reflection IR spectroscopy combined with modulation excitation spectroscopy and phase-sensitive detection that CO₂ adsorption on ice surfaces is accompanied by carbonic acid formation. We demonstrate that carbonic acid can be formed from CO₂ on ice in the absence of high-energy irradiation and without protonation by strong acids. The formation of carbonic acid is favored at low temperature, whereas at high temperature it rapidly dissociates to form bicarbonate (HCO₃ ^- ) and carbonate (CO₃ ^2- ). The direct formation of carbonic acid from adsorption of CO₂ on ice could play a role in the upper troposphere in cirrus clouds, where all the necessary ingredients to form carbonic acid, that is, low temperature, CO₂ gas, and ice, are present.

Surface science investigations of the role of CO₂ in astrophysical ices

We have recorded reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) data for a range of CO₂-bearing model astrophysical ices adsorbed on a graphitic dust grain analogue surface. Data have been recorded for pure CO₂, for CO₂ adsorbed on top of amorphous solid water, for mixed CO₂:H₂O ices and for CO₂ adsorbed on top of a mixed CH₃OH:H₂O ice. For the TPD data, kinetic parameters for desorption have been determined, and the trapping behaviour of the CO₂ in the H₂O (CH₃OH) ice has been determined. Data of these types are important as they can be used to model desorption in a range of astrophysical environments. RAIR spectra have also shown the interaction of the CO₂ with H₂O and CH₃OH and can be used to compare with astronomical observations, allowing the accurate assignment of spectra.

Surface abundance change in vacuum ultraviolet photodissociation of CO2 and H2O mixture ices

Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O((3)P(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO(2). Two different adsorption states of CO(2), i.e., physisorbed CO(2) on the surface of amorphous solid water and trapped CO(2) in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H(2)O. Since the absorption cross section of CO(2) is smaller than that of H(2)O at 157 nm, the CO(2) surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.

Surface properties of water ice at 150–191K studied by elastic helium scattering

A highly surface sensitive technique based on elastic scattering of low-energy helium atoms has been used to probe the conditions in the topmost molecular layer on ice in the temperature range of 150-191 K. The elastically scattered intensity decreased slowly as the temperature was increased to about 180 K, followed by a rapid decrease at higher temperatures. An effective surface Debye temperature of 185+/-10 K was calculated from the data below 180 K. The changes in the ice surface above 180 K are interpreted as the onset of an anomalous enhancement of the mean square vibrational amplitude for the surface molecules and/or the onset of a limited amount of disorder in the ice surface. The interpretation is consistent with earlier experimental studies and molecular dynamics simulations. The observed changes above 180 K can be considered as the first sign of increased mobility of water molecules in the ice surface, which ultimately leads to the formation of a quasiliquid layer at higher temperatures. A small shift and broadening of the specular peak was also observed in the range of 150-180 K and the effect is explained by the inherent corrugation of the crystalline ice surface. The peak shift became more pronounced with increasing temperature, which indicates that surface corrugation increases as the temperature approaches 180 K. The results have implications for the properties and surface chemistry of atmospheric ice particles, and may contribute to the understanding of solvent effects on the internal molecular motion of hydrated proteins and other organic structures such as DNA.

Chlorine Interactions with Water Ice Studied by Molecular Beam Techniques

The kinetics of chlorine interactions with ice at temperatures between 103 and 165 K have been studied using molecular beam techniques. The Cl(2) trapping probability is found to be unity at thermal incident energies, and trapping is followed by rapid desorption. The residence time on the surface is less than 25 microg at temperatures above 135 K and approaches 1 s around 100 K. Rate constants for desorption are determined for temperatures below 135 K. The desorption kinetics follow the Arrhenius equation, and activation energies of 0.24 +/- 0.03 and 0.31 +/- 0.01 eV, with corresponding preexponential factors of 10(12.08+/-1.19) and 10(16.52+/-0.38) s(-1), are determined. At least two different Cl(2) binding sites are concluded to exist on the ice surface. The observed activation energies are likely to be the Cl(2)-ice binding energies for these states, and the Cl(2)-surface interactions are concluded to be stronger than earlier theoretical estimates. The surface coverage of Cl(2) on ice under stratospheric conditions is estimated to be negligible, in agreement with earlier work.

Amorphous Solid Water Films: Transport and Guest−Host Interactions with CO2 and N2O Dopants

Guest-host interactions have been examined experimentally for amorphous solid water (ASW) films doped with CO2 or N2O. The main diagnostics are Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). ASW films deposited at 90 K are exposed to a dopant, and the first molecules that attach to a film enter its bulk until it is saturated with them. Subsequent dopant adsorption results in crystal growth atop the ASW film. There are distinct spectral signatures for these two cases: LO and TO vibrational modes for the crystal overlayer, and an easily distinguished peak for dopant molecules that reside within the ASW film. Above 105 K, the dopant surface layer desorbs fully. Some dopants residing within the ASW film remain until 155 K, at which point the ASW-to-crystalline-ice transition occurs, expelling essentially all of the dopant. No substantial differences are observed for CO2 versus N2O. It is shown that annealing an ASW film to 130 K lowers the film's capacity to include dopants by a factor of approximately 3, despite the fact that the ASW spectral feature centered at approximately 3250 cm(-1) shows no discernible change. Sandwiches were prepared: ASW-dopant-ASW etc., with the dopant layer displaying crystallinity. Raising these samples past 105 K resulted in the expulsion of essentially all of the crystalline dopant. What remained displayed the same spectral signature as the molecules that entered the bulk following adsorption at the surface. It is concluded that the adsorption sites, though prepared differently, have a lot in common. Dangling OH bonds were observed. When they interacted with a dopant, they underwent a red shift of approximately 50 cm(-1). This is in qualitative agreement with studies that have been carried out with weakly bound binary complexes. As a result of this study, a fairly complete, albeit qualitative, picture is in place for the adsorption, binding, and transport of CO2 and N2O in ASW films.

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.