Trapping of molecular hydrogen at surfaces via translational-rotational energy conversion

Using electron-energy-loss spectroscopy (EELS) intensities of rotational transitions and work-function changes as monitors, we show that molecular hydrogen gases, thermal ortho-para ${\mathrm{H}}_2$ mixture, para-${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD, adsorbing on a cold Cu(100) surface have different sticking coefficients. The relative sticking coefficients and the relative populations of ortho- and para-${\mathrm{H}}_2$ on the surface, as estimated by EELS, can be understood on the basis of a trapping mechanism involving conversion of center-of-mass kinetic energy into rotational energy.