Direct inelastic scattering of N2 from Ag(111). III. Normal incident N2

Abstract

We have probed the angular momentum orientation of N2 scattered from cold Ag(111) when the N2 approaches the surface along the surface normal. Using resonance enhanced multiphoton ionization (REMPI) and pulsed molecular beam techniques, we are able to probe the flux backscattered along the surface normal. In accordance with the restrictions on cylindrically symmetric systems, the molecules backscattered along the surface normal have no angular momentum orientation nor does the entire scattered flux integrated over all exit angles. However, for detection away from the surface normal, we observe substantial angular momentum orientation; the degree and direction of orientation depends upon the rotational state being probed. Molecular dynamics calculations reproduce the experimental results semiquantitatively. The calculations show that for N2 incident along the surface normal, the exit angle is largely determined by the two-dimensional impact parameter of the molecule within the crystal unit mesh. However, the final rotational state, orientation, and alignment are determined largely by the molecular orientation geometry of the N2 during the collision. In essence, we have found a dynamical process which can partially differentiate between the two hidden initial conditions in a gas–surface collision: the two-dimensaional impact parameter and the molecular orientation geometry.