Photostimulated desorption and change of the ortho-para ratio of ${\mathrm{H}}_2$ physisorbed on Ag at 7 K are reported. After pump laser excitation at 193 nm neutral ${\mathrm{H}}_2$ desorbing in a nonthermal process is state selectively detected by resonance-enhanced multiphoton ionization. At weak pump laser fluence the natural ortho-para conversion of ${\mathrm{H}}_2$ on Ag was monitored to proceed with a conversion time of $\sim 780\mathrm{s}$, which is in good agreement with predictions based on an electron-exchange-hyperfine-contact model. With increasing pump laser fluence, the ortho-para ratio decreases faster, suggesting photoassisted ortho-para ${\mathrm{H}}_2$ conversion.

• Received 3 September 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.096103