Dynamics at conical intersections: Threshold photoelectron-photoion coincidence (T-PEPICO) spectroscopy of the nitrate cation NO3+

Abstract

The nitrate cation NO_3+ is a D_3h mol. that is predicted to have several possibly dissociative Jahn-Teller active excited states lying close to the ground state. It poses many of the same challenges to theorists and experimentalists as its fiendish neutral cousin, the nitrate radical. In this joint exptl./theoeretical study, we report the Threshold PhotoElectron-PhotoIon Coincidence (T-PEPICO) spectroscopy of the cation NO_3+ using the DELICIOUS3 coincidence spectrometer on the DESIRS beamline at the Soleil Synchrotron. The expts. reveal dissociative states that lead to both NO_2+ and NO+ fragment ions upon photoionization and resolve definitively a controversy concerning the excited states of NO_3+. We observe a strong absorption with partially resolved vibronic structure in the 13.0-14.5 eV range, where four excited states - including three conical intersections - are predicted to lie and an addnl. absorption at higher energies. Photoelectron spectra and angular distributions derived from Velocity Map Imaging provide insights into the nature of the complex electronic structure, while photofragment kinetic and energy distributions provide information on the dissocn. dynamics. We have performed high level calcns. using coupled cluster methods to assign the obsd. spectra and explain the obsd. dynamics.