Vibrational scattering anisotropy in O2—dynamics beyond the Born–Oppenheimer approximation

Abstract

Born–Oppenheimer and Franck–Condon approximations are two major concepts in the interpretation of electronic excitations and modeling of spectroscopic data in the gas and condensed phases. We report large variations of the anisotropy parameter ( β ) for the fully resolved vibrational sub-states of the X ^2 Π _g electronic ground state of O ^+ _2 populated by participator resonant Auger decay following excitations of K-shell electrons into the σ ^☆ resonance by monochromatic x-rays. Decay spectra for light polarization directions parallel and perpendicular to the electron detection axis recorded at four different excitation energies in the vicinity of the O 1s → σ ^☆ transition are presented. Breakdown of the Born–Oppenheimer approximation is for the first time selectively observed for the lower vibrational sub-states, where two quantum paths—resonant and direct—leading to the same final cationic state exist. The higher vibrational sub-states can only be populated by resonant photoemission; hence no interference between these channels can occur.