High Resolution Photoelectron Spectroscopy of Cryogenically-Cooled NO3ˉ

High-resolution anion photoelectron spectra of cryogenically cooled NO₃ˉ anions obtained using slow photoelectron velocity-map imaging are presented and provide new insight into the vibronic structure of the corresponding neutral radical. A combination of improved spectral resolution, measurement of energy-dependent intensity effects, temperature control, and comparison to theory allows for full assignment of the vibronic features observed in this spectrum. We obtain a refined electron affinity of 3.9289(14) eV for NO₃. Further, the appearance of Franck-Condon forbidden transitions from vibrationally cold anions to neutral states with excitation along the NO₃ v₄ mode confirms that these features arise from vibronic coupling with the excited state of NO₃ and are not hot bands as has been suggested. Together, the suite of experimental and simulated results provides clear evidence that the v₃ fundamental of NO₃ resides near 1050 cm⁻¹, addressing a long-standing controversy surrounding this vibrational assignment.