Correlations between angular momentum orientation and exit velocity in gas–surface scattering: A probe of the dependence of collision dynamics on the position of impact

Abstract

The scattering of rotationally cold N2 from Ag(111) results in angular momentum alignment and orientation of the scattered molecules; measurement of the angular momentum polarization as a function of exit angle, final J state, and exit translation energy provides direct information on the dynamics of the collisions. In this paper, the orientation of the angular momentum vector of the scattered N2 molecules, A{1}1−(J) has been measured for slow, medium, and fast groups of molecules in single rotational states at fixed exit angles. With normal incidence scattering (θi=0°) and off-normal detection, for a given final J state, the ‘‘slow’’ molecules have a higher probability of tumbling backwards (‘‘back spin’’) than the ‘‘fast’’ molecules. Conversely, for glancing incidence scattering (θi=30°) with quasi-specular detection, the opposite trend is observed: the slow molecules have a higher probability of tumbling forwards (‘‘top spin’’) than the fast molecules. These experiments were simulated and analyzed using molecular dynamics trajectory calculations. The calculations show that the amount of gas kinetic energy transferred to the surface is sensitive to the narrow dispersion of impact sites and molecular orientations that lead to scattering into a given final rotational state at a given exit angle. The calculations demonstrate that for both incident angles, collisions near the top of a surface atom lead to slower final velocities than collisions with the hollow sites in analogy with the simple case of two colliding spheres. Therefore, the experimentally observed dependence of the angular momentum orientation on the exit velocity results from the correlation between the initial molecular bond angle and the impact site for scattering into a given J state and at a fixed exit angle.