Excitation of the symmetry forbidden bending mode in molecular photoionization

We present results on the energy dependence of the vibrational branching ra tio for the bending mode in CO2 3 sigma (-1)(u) photoionization. Specifical ly, we determine the upsilon (+) = (0,1,0)/upsilon (+) = (0,0,0) intensity ratio by detecting dispersed fluorescence from the electronically excited p hotoions. The results exhibit large deviations over a very wide energy rang e, 18 < h nu (exc) < 190 eV. Production of the upsilon (+) = (0,1,0) level of the ion from the upsilon (0) = (0,0,0) ground state is forbidden by symm etry, and while observations of such features are well established in photo electron spectroscopy, their appearance is normally ascribed to vibronic co upling in the ionic hole state. In this case, we find that such explanation s fail to account for the energy dependence of the branching ratio. These d eviations indicate that the continuum photoelectron participates in transfe rring oscillator strength to the nominally forbidden vibrational transition . A theoretical framework is developed for interpreting the experimental da ta, and Schwinger variational calculations are performed. These calculation s demonstrate that the continuum electron is responsible for the observatio n of the excited bending mode as well as its energy dependence. This is an intrachannel effect that is best described as photoelectron-induced vibroni c symmetry breaking. This appears to be a general phenomenon, and it may be useful in illuminating connections between bond angle and photoionization spectroscopies. The magnitude of these deviations display the utility of vi brationally resolved studies, and the extent over which these changes occur underscores the necessity of broad range studies to elucidate slowly varyi ng characteristics in photoionization continua. (C) 2001 American Institute of Physics.