Rotational-energy transfer of hydrogen molecule on solid surface

固体表面における水素分子の回転エネルギー移動

植田 寛和   ; 福谷 克之

Ueta, Hirokazu; Fukutani, Katsuyuki

Energy transfer processes in molecular adsorption on solid surface are a key to understand gas/surface interaction. Recent experimental progress clarifies the vibrational-energy relaxation process of molecules on solid surfaces. However, its rotational-energy relaxation process still remains unclear. Molecular hydrogen has two nuclear spin isomers, ortho-H (o-H) and para-H (p-H). Due to symmetry restriction, o-H and p-H with parallel and antiparallel proton spins have odd and even rotational states, respectively. Interconversion between these two species, ortho-to-para (o-p) conversion is accompanied by the nuclear spin flip and the rotational-energy relaxation. The former has been discussed previously. In the present contribution, we deal with the rotational-energy transfer through the o-p conversion of molecularly chemisorbed H on Pd(210). A combination of a pulsed molecular beam, photo-stimulated desorption, and resonance-enhanced multiphoton ionization techniques is used for probing the time evolution of o-H and p-H populations. The surface temperature dependence of the o-p and its reverse process of para-to-ortho (p-o) conversions are studied. It is found that o-p conversion rate accelerates with increasing surface temperature. Furthermore, from a comparison between the o-p and p-o conversion rates, the o-p conversion rate is faster than that of p-o at the same surface temperature. Based on the results obtained and the analysis of the rotational-energy of adsorbed H, phonon- and electron-mediated paths are evaluated as a possible rotational-energy transfer process, and thereby it is likely that the former path is responsible.