High-Resolution Photoelectron Spectroscopy of NO₃^- Vibrationally Excited Along Its ν₃ Mode

The nitrate (NO₃) radical has long been the subject of both experimental and theoretical studies due to its complex electronic structure resulting from vibronic interactions between its X̃²A₂^' and B̃²E^' states. In particular, the definite assignment of the fundamental of its degenerate stretching vibration (ν₃) is still under debate. Here, we report high-resolution photoelectron spectra of vibrationally pre-excited NO₃^- using the recently developed IR-cryo-SEVI technique. The anions are excited through infrared (IR) excitation near 1350 cm^-1, accessing the ν₃ and 2ν₃(e') vibrational levels with band centers at 1350.5 and ∼2700 cm^-1, respectively. The IR-cryo-SEVI spectrum for 2ν₃ pre-excitation shows clear evidence for an intense 3₂¹ transition. From the position of this feature (30031 cm^-1), the electron affinity of NO₃ also determined in this work (31680 cm^-1), and the IR excitation energy, we obtain a fundamental frequency of 1051 cm^-1 for the ν₃ fundamental of the NO₃ radical. This assignment and other features in the IR-cryo-SEVI spectra are supported by spectral simulations based on a vibronic Köppel-Domcke-Cederbaum Hamiltonian. The simulations also show that nearly all features in the IR-cryo-SEVI spectra arise because of pseudo-Jahn-Teller coupling between the X̃ and B̃ states of NO₃. The results and analysis presented here settle a long-standing controversy regarding the ν₃ frequency of NO₃.