Effects of vibronic interaction and autoionization on the photoelectron spectrum of N2O

Abstract

High‐resolution and high‐sensitivity HeIphotoelectron spectra (PES) are reported for the first four valence levels of N2O. The vibrational structure, including many new peaks, is completely assigned for the X 2Π, A 2Σ+, and C 2Σ+ electronic states, and in all three cases excitation of a single quantum of the bending vibration is observed. The bending mode appears as a result of vibronic coupling within and between different electronic states. The Renner–Teller splitting is resolved in the (0,1,0) band of the transition to the X 2Π state. The relative intensities of the vibrational bands in the X 2Π and A 2Σ+ states are compared to the intensities determined from the threshold photoelectron spectra (TPES) of Frey e t a l. [Chem. Phys. Lett. 54, 411 (1978)]. In both states, autoionization of intermediate neutral states that are nearly degenerate with the molecular ion state to which they decay increases the relative intensities of the higher vibrational bands in the TPES. This resonant autoionization process can enhance bands selectively, and, in particular, the higher quanta of the symmetric stretch are systematically more intense in the TPES than in the HeI PES. New relative photoionization cross sections for N2O and its fragment ions are presented which show that even weak autoionization features can have a substantial effect on the intensities in the TPES.