Rotational dynamics and electronic energy partitioning in the electron-stimulated desorption of NO from Pt(111)

Abstract

Using laser resonance ionization, we have examined the rotational dynamics of neutral NO molecules desorbed by electron impact from a cold (80 K) Pt(111) surface. Previously [A. R. Burns, E. B. Stechel, and D. R. Jennison, Phys. Rev. Lett. 58, 250 (1987)], we observed ‘‘hot’’ rotational temperatures in the range 481–642 K for the ν=0, 1, 2, and 3 vibrational levels. We now present more detailed data which do not reveal shifts or broadening in the rotationally selected NO velocity distributions as a function of rotational level J in the range J=2.5–29.5. The rotationally independent velocity distributions, peaked at 0.05 eV, as well as the rotational temperature can be understood within the framework of ‘‘electronically stimulated adsorbate rotation’’ (Burns et al., above). The model also predicts complete rotational alignment in a plane normal to the surface. In the laboratory, no alignment is observed, but this is most likely due to the presence of stray magnetic fields. We will also discuss our measurements which show an equal partitioning between the two lowest NO electronic states (2Π1/2 and 2Π3/2), and a preference for the π level which is perpendicular (antisymmetric) to the plane of rotation.