Active vibrations of indene cation studied by mass-analyzed threshold ionization spectroscopy

We applied the resonant two-photon mass-analyzed threshold ionization (MATI) technique to cation spectra of indene by ionizing via eight different intermediate vibronic levels. The adiabatic ionization energy was determined to be 65 721 ± 5 cm−1. Most of the pronounced MATI bands result from the active vibrations involving in-plane ring deformation motions of the indene cation. Analysis of these MATI spectra suggests that the molecular geometry of the indene cation in the ground D0 state resembles that of neutral species in the electronically excited S1 state.