Observation of Rotational Transitions forH2,D2, and HD Adsorbed on Cu(100)

Energy losses observed when low-energy electrons were scattered from a Cu(100) surface exposed at $T\sim 15$ K to ${\mathrm{H}}_2$ (and ${\mathrm{D}}_2$,HD) lie close to rotational and rotational-vibrational transitions (${\mathrm{H}}_2$,${\mathrm{D}}_2$) of the free molecules. Both $o$- and $p$-${\mathrm{H}}_2$ and -${\mathrm{D}}_2$ physisorb on the surface, with negligible conversion on the time scale of the experiment. The relative populations were different from those characteristic of the adsorbing gas, suggesting a $j$-dependent sticking coefficient.