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

Abstract

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_s) examined (300 to 650 K), the scattered NH₃ rotational distributions are Boltzmann‐like. Complete rotational accommodation is observed for T_s≤ 380 K, while for T_s> 380 K, the NH₃ rotational temperature is less than T_s. Angular and velocity analysis of the scattered NH₃ for T_s= 300 K show cosine and Maxwellian (T=300 K) distributions, respectively. These findings indicate a trapping–desorption scattering mechanism with a surface residence time below 10 μs at T_s= 300 K. 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.