Energy and momentum distributions and projections in the scattering of CO from Ag(111)

We have studied the cross correlations between the rotational state distributions, angular momentum alignment, and velocity distributions of monoenergetic, rotationally cold CO and N2 scattered at 0.75 eV from Ag(111). Measurements were made for both normal incidence and glancing incidence beams and for specular and off-specular detection. The comparison between N2 and CO is most dramatic. (1) For N2 the rotational state selected velocity distributions are very narrow while for CO the rotational state selected velocity distributions are wide. (2) For glancing incidence beams, N2 exhibits a higher degree of parallel momentum conservation than CO. (3) The velocity resolved rotational rainbows for N2 are more prominent for glancing incidence while the velocity resolved rotational rainbows for CO are more prominent for normal incidence. (4) There is 100% cartwheeling type alignment for N2 in medium and high exit rotational states while for CO the alignment is weak except at the very highest rotational states where it is still <100% cartwheeling. Our data can be interpreted as showing that the N2 molecules at these relatively high energies collide with a slightly corrugated surface and have nearly linear trajectories. Conversely, the CO scattering data are consistent with scattering from a more corrugated surface. The CO molecules have a permanent dipole moment, therefore the gradient for the CO–Ag(111) gas surface potential with respect to molecular orientation is larger. In addition, CO has a deeper physisorption well on Ag(111). Thus, the CO molecules probe deeper into the corrugated repulsive portion of the potential and have a more inelastic collision that results in greater rotational and phonon excitation but lower exit translational energy. The lower alignment for CO scattering into high J states is consistent with the CO molecules having curved exit trajectories and/or multiple collisions with the surface. For both N2 and CO, rotational excitation into high J states scales with the normal component of incident translational energy, but the phonons can be excited by both the parallel and normal components of the incident translational energy.