Ortho-para conversion of ${\mathrm{H}}_2$ and para-ortho conversion of ${\mathrm{D}}_2$ on an Ag surface were investigated by resonance-enhanced multiphoton ionization combined with photostimulated desorption. The conversion processes were found to be accelerated by laser irradiation at $193\mathrm{nm}$, while photo-irradiation at $532\mathrm{nm}$ had little effect on the conversion time. The natural conversion time of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ was estimated to be 610 and $1030\mathrm{s}$, respectively. The roles of electron transfer and rotational-energy dissipation in the conversion are discussed.

• Received 10 April 2008

DOI:https://doi.org/10.1103/PhysRevB.77.201404