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 O 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.