Rotationally inelastic gas--surface scattering: NH3 from NH3-saturated W(100)

Two-photon resonant three-photon laser ionization spectroscopy is employed to study rotationally inelastic NH3 molecular beam--surface scattering from NH3-saturated W(100). For all surface temperatures (T/sub s/) examined (300 to 650K), the scattered NH3 rotational distributions are Boltzmann-like. Complete rotational accommodation is observed for T/sub s/≤ 3800K, while for T/sub s/> 3800K, the NH3 rotational temperature is less than T/sub s/. Angular and velocity analysis of the scattered NH3 for T/sub s/ = 3000K show cosine and Maxwellian (T = 3000K) distributions, respectively. These findings indicate a trapping--desorption scattering mechanism with a surface residence time below 10 μs at T/sub s/ = 3000K. Scattering of a nuclear-spin-state (para) enriched NH3 beam prepared using electric deflection techniques indicates that nuclear spin equilibration (ortho--para) is not achieved during the gas--surface encounter