RESUMO
The physics of the ice crystal surface and its interaction with adsorbates are not only of fundamental interest but also of considerable importance to terrestrial and planetary chemistry. Yet the atomic-level structure of even the pristine ice surface at low temperature is still far from well understood. This computational study focuses on the pattern of dangling H and dangling O (lone pairs) atoms at the basal ice surface. Dangling atoms serve as binding sites for adsorbates capable of hydrogen- and electrostatic bonding. Extension of the well known orientational disorder ("proton disorder") of bulk crystal ice to the surface would naturally suggest a disordered dangling atom pattern; however, extensive computer simulations employing two different empirical potentials indicate significant free energy preference for a striped phase with alternating rows of dangling H and dangling O atoms, as suggested long ago by Fletcher [Fletcher NH (1992) Philos Mag 66:109-115]. The presence of striped phase domains within the basal surface is consistent with the hitherto unexplained minor fractional peaks in the helium diffraction pattern observed 10 years ago. Compared with the disordered model, the striped model yields improved agreement between computations and experimental ppp-polarized sum frequency generation spectra.
RESUMO
A new computational scheme is presented for calculation of sum frequency generation (SFG) spectra, based on the exciton model for OH bonds. The scheme is applied to unified analysis of the SFG spectra in the OH-stretch region of the surfaces of ice, liquid water, and acid solution. A significant role of intermolecularly coupled collective modes is pointed out. SFG intensity amplification observed for acid solutions in the H-bonded OH-stretch region is reproduced qualitatively and accounted for by enhanced orientational preference "into the surface" of the H(2)O bisectors within the hydronium solvation shell.
