We have measured the rotational state distribution and the angular momentum alignment and orientation of N2scattered 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 thgr;exit. For a modestly glancing incident beam (thgr;i=30deg;) and incident translational energy,Ei=0.3 eV, the angular momentum alignment (tumbling vs helicoptering) at both quasispecular detection (thgr;exit=35deg;) and superspecular detection (thgr;exit=50deg;) 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 fromTs=90 K toTs=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 twohyphen;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 twohyphen;dimensional impact parameter and molecular orientation geometry results in a greater range of finalJstates, angular momentum polarizations, and velocities (exit angles) at high temperature than at low temperature. The resulting lsquo;lsquo;smearingrsquo;rsquo; 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 N2is to increase the exit angle averaging rather than to increase the inhyphen;plane forces.
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