State-Resolved Molecular Photoionization Dynamics of Polyatomic Systems: Effects of Non-Linear Changes in Molecular Geometry.

Abstract

An important topic in the absorption of vacuum ultraviolet photons by molecules is the correlation between electronic and nuclear degrees of freedom during photoionization. However, no previous investigations have probed the correlation between bending excitation and photoejection dynamics over a wide spectral range. We present the first such studies by reporting the influence of bending excitation following CO2 3sigmau --1 and N2O 7sigma--1 photoionization over the photon energy range (15 eV < hvexc < 200 eV). Using dispersed fluorescence spectroscopy in conjunction with synchrotron radiation, we determine the vibrational branching ratio v+ = 0,1,0 0,0,0 for the CO2+ (B 2Sigmau+) and N2O+ (A 2Sigma+) electronic states. The relative rate of production of the upsilon2 = 1 upper vibrational state varies over a broad ionization energy range, and in ways that are largely unanticipated. These branching ratios exhibit a strong thermal dependence, and we are able to separate out effects due to hot-band excitation from those that are due to vibronic coupling. The data indicate that the continuum electron is responsible for the observed energy dependence in CO2 3sigma u--1 photoionization. This is a previously unobserved result. Additional studies examine the influence of simultaneous excitation in the bending and symmetric stretching modes in N2O+ [A 2Sigma+, v + = (1,1,0)] to determine the effect of changing the energy separation of vibronically coupled potential surfaces. Finally, the CF4 + [D 2A1, v+ = (1,0,0,0)/(0,0,0,0)] branching ratio is studied, which provides the first experimental observation of a predicted low-energy shape resonance in this photoionization pathway.