Direct inelastic scattering of N2 from Ag(111). IV. Scattering from high temperature surface

Abstract

We have measured the rotational state distribution and the angular momentum alignment and orientation of N₂ scattered from Ag(111) at 540 K. Using resonance enhanced multiphoton ionization (REMPI), we are able to probe the scattered flux as a function of the exit angle θ_exit. For a modestly glancing incident beam (θ_i =30°) and incident translational energy, E_i =0.3 eV, the angular momentum alignment (tumbling vs helicoptering) at both quasispecular detection (θ_exit=35°) and superspecular detection (θ_exit=50°) is only weakly dependent upon the surface temperature. However, the angular momentum orientation (clockwise vs counterclockwise rotation) is strongly affected by the surface temperature. Raising the surface temperature from T_s =90 K to T_s =540 K causes the orientation to decrease substantially. Stochastic trajectory calculations were carried out in conjunction with the experiments. They reveal that at low temperature there is an averaging over two important initial conditions: the two‐dimensional impact parameter and the molecular orientation geometry. At high temperature there is also an averaging over the instantaneous positions and momenta of the surface atoms. Hence, a given two‐dimensional impact parameter and molecular orientation geometry results in a greater range of final J states, angular momentum polarizations, and velocities (exit angles) at high temperature than at low temperature. The resulting ‘‘smearing’’ accounts for the changes in rotational state distribution and polarization as a function of exit angle observed at high temperature. The major effect of averaging over the positions of the surface atoms (thermal roughening) upon the orientation of the scattered N₂ is to increase the exit angle averaging rather than to increase the in‐plane forces.