Abstract
A state-selective thermal desorption method was developed for the study of the rotational state dependence of adsorption-desorption kinetics of hydrogen molecules. The apparatus consisted of an adsorption-cell cooled by a compressortype cryohead and an ion detection system adopting a (2 +1) resonance-enhanced multiphoton ionization (REMPI) scheme. Granular activated alumina contained in the inner side of the adsorption-cell was used as an adsorbent for the hydrogen molecules. The sorvent was degassed at 523 K for 10 hours in ultrahigh vacuum. After cooling the cell to 14 K, normal hydrogen was admitted at a pressure of 6-8 × 10-3 Pa for 70 s. Assuming first order kinetics of desorption, desorption energies for hydrogen molecules in the rotational states J=0 and 1 were evaluated as 95 and 110 meV, respectively. These values are consistent with a model that relates the helicopter motion (Jz =± 1) of ortho hydrogen molecules to the higher desorption energy.
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