Calculations of H2O microwave line broadening in collisions with He atoms: sensitivity to potential energy surfaces

Pressure-broadening of water transitions near 7180 cm(-1) by helium isotopes

In this study, pressure-broadening parameters for several H2O transitions near 7180 cm(-1) are obtained which describe collisions with (3)He and (4)He. The sensitivity of those parameters to choice of theoretical line profile (Galatry vs. Voigt) is investigated. H2O is an important species in atmospheric chemistry and astronomy. Because of this, basic fundamental research, which explores the nature of the H2O spectrum in the presence of different gases of varying physical properties, can provide useful reference data which can be applied in the fields of atmospheric and planetary remote sensing. Measurements were made using an intensity-modulated laser photoacoustic spectrometer. Results from the present work show that Galatry line profiles, with a constrained narrowing parameter, more accurately describe experimental spectra than Voigt profiles over a wide range of experimental pressure conditions. Average pressure-broadening parameters were found to be 0.0216 cm(-1)/atm and 0.0209 cm(-1)/atm for H2O in (3)He and (4)He, respectively, and were compared to a literature model for the mass-dependence of line broadening. Specific values were obtained for each transition with nominal combined uncertainties of 2-6%.

Proton order in the ice crystal surface

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.

Close-coupling study of rotational energy transfer and differential scattering in H2O collisions with He atoms

Quantum close-coupling scattering calculations of rotational energy transfer (RET) of rotationally excited H(2)O due to collisions with He are presented for collision energies between 10(-6) and 1000 cm(-1) with para-H(2)O initially in levels 1(1,1), 2(0,2), 2(1,1), and 2(2,0) and ortho-H(2)O in levels 1(1,0), 2(1,2), and 2(2,1). Quenching cross sections and rate coefficients for state-to-state RET were computed. Both elastic and inelastic differential cross sections are also calculated and compared with relative experimental results giving generally good agreement in all cases, but less so for inelastic results. Significant differences in the computed collisional parameters, obtained on three different potential energy surfaces (PESs), were found particularly in the ultracold regime. In the thermal regime, the rate coefficients calculated on each of the surfaces are generally in better agreement and comparable, but typically larger, than those obtained in a previous calculation. Unfortunately, a lack of absolute differential or integral inelastic experimental data prevents firm determination of a preferred PES.