A Theoretical Study of the Interaction of HCl with Crystalline NAT

Proton transfer and autoionization in HNO3·HCl·(H2O)n particles

The structure and spectroscopic properties of clusters of HNO(3)·HCl·(H(2)O)(n), with n = 1 to 6, have been calculated at the MP2/aug-cc-pVDZ level of theory. Altogether 22 different clusters have been found as stable structures, with minima in their potential energy surfaces. The clusters can be grouped in families with the same number of water molecules, and with close aggregation energies within each family. The addition of each new water molecule increments the aggregation energy of the clusters by a nearly constant value of 76.2 ± 0.1 Hartree. The proton transfer parameter and the coordination number of HNO(3) and HCl in each cluster have been evaluated, and the wavenumber shifts for the X(-)-H(+) vibration from the corresponding mode in the isolated molecules have also been predicted. These values allow classification of the acidic species in the clusters into three types, characterized by the strength of the hydrogen bond and the degree of ionization. A correspondence is found between the coordination number of HNO(3) and the magnitude of the X(-)-H(+) vibrational shift.

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.

Stimulated desorption of cations from pristine and acidic low-temperature water ice surfaces

Electron-impact ionization of low-temperature water ice leads to H+, H+2, and H+(H2O)(n=1-8) desorption. The 22 eV H+ desorption threshold is correlated with localized 2-hole 1-electron and 2-hole final states which Coulomb explode, while the 22 eV H+2 threshold is due to H2O+ unimolecular dissociation. The 70 eV primary cluster ion threshold is consistent with holes in the 2a(1) level and secondary ionization channels. All cation yields are sensitive to local structural changes and probe surface acidity. The cluster size distribution indicates hole-hole screening distances of 1-2 nm.