Evidence for coupled rotational and phonon quantum excitation in the scattering of a nearly monoenergetic HD beam from the Ni(001) surface.

Angular distributions and time-of-flight spectra of a nearly monoenergetic (Δv/v[bar_over_tilde:_approx._equal_to]2% FWHM) He seeded HD beam (ki=6.80 Å-1, Ei=32 meV) have been measured after scattering from a Ni(001) single-crystal surface along the <100> azimuth. The most intense peaks in the angular distributions are attributed to diffraction with rotational excitation while the rotationally elastic diffraction peaks are an order of magnitude smaller. All the time-of-flight distributions reveal intense peaks at ΔE=-11.06 meV corresponding to the HD rotational transition ji=0→jf=1. The rotational energy loss observed at angles between the diffraction peaks is attributed to incoherent inelastic scattering at defects. The other inelastic peaks in the time-of-flight spectra can be assigned to phonons which are created or annihilated in the rotationally inelastic collisions. The measured phonon dispersion curves suggest a predominant interaction with the transverse bulk edge and not with the Rayleigh mode. These results provide the first direct evidence for a coupling of rotational transitions to the phonons of a metal surface. The pronounced rotational inelasticity at step edges does not support the widely held view that nonactivated chemisorption occurs at step edges. [ABSTRACT FROM AUTHOR]