Rotational alignment in the UV-laser induced desorption of CO from Cr2O3(0001)

The rotational alignment of the laser-induced non-thermal desorption of CO adsorbed on an epitaxially grown film of Cr2O3(0001) has been studied using 193 nm (6.4 eV) laser light and a (1 + 1')-REMPI technique. At low rotational quantum numbers J the molecules desorb like a 'helicopter' (J-vector perpendicular to the surface) while at high J-values a 'cartwheel' like motion is preferred (J-vector parallel to the surface). CO is adsorbed in a flat geometry as a single species. The resulting angular momentum alignment in the laser desorption is interpreted qualitatively as a sum of effects due to corrugation of the surface in the initial state and a change of the adsorption geometry in the intermediate state before desorption.