SURVIVAL MECHANISM FOR ROTATIONAL RAINBOWS IN HIGHLY ATTRACTIVE MOLECULE-SURFACE SYSTEMS - NO SCATTERING FROM PT(111)

Rotational rainbow scattering has been observed for NO from Pt(111) at translational energies considerably smaller than the NO-Pt well depth ( 1 eV). Rotational distributions at superspecular angles show a clea r rotational rainbow but scattering towards the surface normal is well described by a Boltzmann distribution. At incoming energies in the or der of the well depth, rotational distributions at all angles appear s tatistical, with an apparent temperature far exceeding the surface tem perature, reflecting the energy randomization expected for scattering via a deep well. Orientation-selective trapping caused by the highly d irectional interaction of NO with the Pt surface restricts the influen ce of the attractive well on direct scattering at low energies.