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.ice surface ͉ Monte Carlo simulations ͉ surface order ͉ dangling atoms ͉ ice-adsorbate interaction W ater ice is an intellectually challenging and fundamentally important solid. Ice and ice particles play a basic role in terrestrial, atmospheric, planetary, and interstellar phenomena (1-3). The remarkable properties of ice are determined by the unique ability of H 2 O to form four relatively strong hydrogen bonds to four neighboring water molecules, in an approximately tetrahedral arrangement. This gives rise to a rich bulk phase diagram comprising a variety of crystalline phases, in addition to muchdebated amorphous phases (1).Under ambient conditions, hexagonal ice is the dominant solid form of H 2 O. Here we focus on the surface structure of hexagonal ice, which hosts a variety of chemical reactions in natural environments; perhaps the most famous example being the sequence of reactions leading to ozone-hole formation, which is initiated by HCl and ClO adsorption on frozen stratospheric cloud particles (4). The interaction of ice with molecules, ions, and electrons is affected by the solvating properties of its surface. It has long been recognized that these properties are directly related to the structure and dynamics of the surface H-bond network, which has consequently received considerable attention in past research (see, for example, refs. 1, 3, and 5-16).Here, we address a feature of the ice surface that has not received extensive attention but that is expected to significantly influence its properties, including its interaction with adsorbates. Specifically, we argue that the pattern of dangling H (d-H) and dangling O (d-O) atoms on the ice surface is composed of ordered striped domains, with a...

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.