Ortho-para conversion of ${\mathrm{H}}_2$ adsorbed at the step atoms of a Cu(510) surface proceeds with a short conversion time constant around 1 s as observed in electron-energy-loss measurements of rotational populations. We suggest that this rapid conversion is related to the special character of the adsorption state, which involves a short ${\mathrm{H}}_2\mathrm{\text{−}}\mathrm{Cu}$ bond length of 1.8 Å. On the flat Cu(100) surface, conversion is found to occur at active sites, most likely step atoms.

• Received 27 November 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.096105