Direct Inelastic Scattering of N sub 2 from Ag(111): II. Orientation.

The orientation of the angular momentum of N sub 2 scattered from clean Ag(111) is determined by resonance-enhanced multiphoton ionization. The orientation is the net helicity or handedness of the sense of rotation, i.e., clockwise versus counterclockwise. The orientation of the scattered N sub 2 is measured along a direction perpendicular to the scattering plane. The degree and sign of the orientation is found to depend strongly on the final rotational quantum number J and on the final scattering angle. The results require that there are forces acting in the plane of the surface during the scattering. The observed behavior can be reproduced qualitatively by a conventional hard-cube, hard-ellipsed model to which a tangential friction has been introduced to account for the in-plane forces. This produces a splitting of the rotational rainbow peak which leads to changes of sign of the orientation as a function of rotational quantum number. Thus orientation measurements provide a unique probe of in-plane gas-surface forces.