THE INTERACTION OF CO WITH NI(111) - RAINBOWS AND ROTATIONAL TRAPPING

Angularly resolved rotational state distributions of CO scattered and desorbed from a clean, single-crystal Ni (111) surface were measured using (2 + 1 ) resonance-enhanced multiphoton ionization. Molecules scattered from the surface displayed highly non-Boltzmann rotational distributions that varied with incident translational energy and detection angle, but not with surface temperature. A rotational rainbow was seen in the scattering distribution and interpreted as arising from the interaction of the weakly attractive 0 end of the CO molecule with the Ni (111) surface. Up to total rotational-to-translational energy conversion was seen at incident translational energies of 0.26-0.45 eV. This energetic cutoff was the result of rotational trapping and was caused by the strongly attractive interaction of the C end of the molecule with the surface. The rotational state distributions of molecules desorbed from the Ni (111) surface were well fit by Boltzmann distributions each with a temperature which is 0. 82 +/- 0.08 of the surface temperature.