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

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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 (Ts) examined (300 to 650 K), the scattered NH3 rotational distributions are Boltzmann-like. Complete rotational accommodation is observed for Ts≤ 380 K, while for Ts> 380 K, the NH3 rotational temperature is less than Ts. Angular and velocity analysis of the scattered NH3 for Ts= 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 Ts= 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.