Scattering of N-2 from Ni(111)

A cold (T-rot <10 K) beam of N-2 with an initial translational energy of 0. 40 eV strikes an Ni(111) surface at surface temperatures from 300 to 873 K at several incident angles from 15 to 60 degrees. The rotational energy and angular distributions of the scattered molecules are probed using (2 + 1) resonance-enhanced multiphoton ionization. Molecules scattered in the specu lar direction have mean rotational energies that are independent of surface temperature, whereas those scattered at angles far from the specular show a dependence on surface temperature, caused likely by multiple collisions w ith the surface before escape. A rotational rainbow, seen in systems such a s CO-Ni(lll) and N-2-Ag(111), is not seen in this system. For molecules tha t scatter close to the specular direction, approximately 10% of the initial translational energy is converted into rotational energy of the scattered N-2. For surface temperatures above room temperature, the angular distribut ions indicate that molecules that scatter into low-J states also tend to ex it in a broad peak (10-20 degrees FWHM) near the specular, and this peak is broadened with increasing incident angle. The molecules that scatter into high-J states have a much broader distribution, indicating that they are tr apped rotationally during the initial collision and suffer multiple collisi ons before leaving the surface. (C) 2000 Elsevier Science B.V. All rights r eserved.