Rotational-Energy Transfer in H₂ Ortho–Para Conversion on a Metal Surface: Interplay between Electron and Phonon Systems

Clarifying energy transfer processes in molecular adsorption on solid surfaces is essential to understand the gas–surface interaction. Unlike the vibrational-energy transfer processes, which are thought to be well understood in detail, the rotational-energy transfer process still remains unclear. Considering the interconversion between ortho and para states of H 2 is accompanied by the nuclear spin flip and the rotational-energy transfer, the surface-temperature dependence of the ortho-to-para conversion of molecularly chemisorbed H 2 on Pd(210) is studied. The conversion rate is accelerated with an increase in surface temperature. Based on the conversion model proposed for metal surfaces, we analyze the temperature dependence of the conversion rate, taking into account both electron and phonon systems of the substrate. The rotational-energy transfer is most likely mediated by surface electrons with the assistance of the substrate phonons.