Direct determination of the adiabatic ionization potential of NO2 by multiresonant optical absorption

Abstract

The adiabatic ionization threshold of NO2 is observed by direct optical excitation in a three-color, triply resonant, three-photon transition from vibrationless levels of the bent ground state through the vibrational ground state of the linear Rydberg state to the ground state of the ion. The overall energy for this process for rotationless molecules is estimated to be 77320 ± 20 cm−1. The bent-to-linear two-photon transition to the Rydberg state is facilitated by a first-photon resonance with levels of NO2's mixed low-lying excited states. At higher energies, following intermediate selection of vibrationally excited 3pσ Rydberg states, transitions to series of high Rydberg states are observed uniformly converging to respective vertical ionization potentials. These transitions are marked by sharp bands below and in some cases above the adiabatic ionization threshold. Intensities suggest efficient autoionization.