Vibrational branching ratios and shape resonant photoionization dynamics in N_2O

Abstract

Vibrational branching ratios and photoelectron asymmetry parameters for alternative vibrational modes in the photoionization of N_2O(7σ^(−1)) have been studied using accurate photoelectron continuum orbitals. Earlier dispersed ionic fluorescence measurements [E. D. Poliakoff, M. H. Ho, M. G. White, and G. E. Leroi, Chem. Phys. Lett. 130, 91 (1986)] revealed strong non‐Franck–Condon vibrational ion distributions for both the symmetric and antisymmetric stretching modes at low photoelectron energies. Our results establish that these features arise from a σ shape resonance which, based on its dependence on internuclear geometry, must be associated with the molecular framework as a whole and not with either of its fragments, N–N or N–O. This behavior accounts for the more pronounced deviations of the vibrational branching ratios from Franck–Condon values observed in the symmetric than in the antisymmetric mode. The σ continuum also supports a second shape resonance at higher energy which does not influence the vibrational branching ratios but is quite evident in the photoelectron asymmetry parameters around a photon energy of 40 eV. These vibrationally resolved studies of the photoelectron spectra of this polyatomic system provide an interesting example of the rich shape resonant behavior that can be expected to arise in polyatomic molecules with their alternative vibrational modes.