Anisotropic potential energy surfaces for atmospheric gas: unsaturated hydrocarbon molecule interactions from differential scattering experiments

Thesis (Ph. D.)--University of Washington, 1997The total differential scattering cross sections of several atmospheric gas-unsaturated hydrocarbon molecule systems (He, Ne, Ar, H$\sb2$, N$\sb2$, CO, and NO with $\rm C\sb2H\sb2,\ C\sb2H\sb4,\ C\sb3H\sb4,$ and $\rm C\sb4H\sb2)$ were measured using bolometric detection in a crossed-molecular beam apparatus. The observed angular scattering distributions exhibit damped rainbow and diffraction oscillations, providing information on the intermolecular potential energy surface anisotropy of each system. Light reduced-mass systems containing He and H$\sb2$ show fast diffraction oscillations, while heavy reduced-mass systems with Ar, N$\sb2$, CO, and NO show predominately a single broad rainbow oscillation. The Ne-$\rm C\sb2H\sb2,\ C\sb2H\sb4,$ and $\rm C\sb3H\sb4$ systems at high angular resolution show both rainbow and diffraction features.Anisotropic Lennard-Jones N-6 and Barker model potential energy surfaces were determined for each system by a 'best-fit' comparison between the experimental cross section and a theoretical cross section, calculated using infinite-order-sudden (IOS), JKWB, and Born approximations. Adjustment of potential parameters $\epsilon$ and R$\sb{\rm m},$ expanded in Legendre polynomial functions of the proper molecular rotational symmetry, allowed adequate fits to the experimental data. Barker model potentials provide better descriptions of the collision dynamics due to better estimates of the long-range interactions. In comparison, the Lennard-Jones potentials are consistently more anisotropic in the well region and more steeply repulsive at short-range.The damped diffraction oscillations were observed to be most sensitive to the potential size anisotropy and rather insensitive to the potential shape anisotropy. The relative size anisotropies for the various systems increase in agreement with the number of C-C bonds along the hydrocarbon's molecular symmetry axis. The damped rainbow oscillations were observed to be sensitive to the potential shape anisotropy and less sensitive to potential size anisotropy. He, Ne, and H$\sb2$--hydrocarbon interactions are found to have isotropic well depths, while the Ar, N$\sb2$, NO, and CO--hydrocarbon interactions are anisotropic. The Ar hydrocarbon systems prefer T-shaped structures. Interestingly, the acetylene-diatom systems have more stable linear configurations, while the ethylene-diatom and allene-diatom systems prefer T-shaped geometries. The IOS results need to be compared with more accurate close-coupled quantum scattering calculations to determine the contributions of rotational inelastic transitions