Rotational alignment of NO from Pt(111). Inelastic scattering and molecular desorption

Abstract

The rotational alignment distribution of NO has been measured subsequent to the molecule's interaction with a well characterized Pt(111) surface. Internal state distributions have been probed using 1 + 1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy in which lines of the NO A ²Σ⁺–X²Π(0, 0) band constitute the resonant transition. NO/Pt(111) scattering has been studied in two distinct regimes: inelastic scattering and trapping/desorption. In both cases, there is relatively no preferential alignment of rotation for J < 12.5. However, molecules with higher rotational angular momentum show a marked increase in alignment. Inelastically scattered molecules prefer to rotate in a plane normal to the surface (‘cartwheel’ motion), whereas desorbing molecules prefer to rotate in a plane parallel to the surface (‘helicopter’ motion). These measurements provide new insight into momentum transfer at surfaces and the nature of the transition state which leads to molecular desorption.