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 the 𝑋 ̃ 2𝐴′2 and 𝐵 ̃ 2𝐸′ states. In particular, the definite assignment of the fundamental of its degenerate stretching vibration (𝜈3) is still under debate. Here, we extend the available spectroscopic information by reporting 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 cm−1 and ~2700 cm−1, respectively. The IR-cryo-SEVI spectrum for 2𝜈3 pre-excitation shows clear evidence for an intense 3_2^1 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 𝑋 ̃ and 𝐵 ̃ state of NO3. The results and analysis presented here settle a long-standing controversy regarding the 𝜈3 frequency of NO3.