EXPERIMENTAL EVIDENCE OF THE ν3 MODE IN NO3 VIA SLOW PHOTOELECTRON VELOCITY-MAP IMAGING OF COLD NO3−

With five electronic states within 2 eV, the nitrate radical (NO$_3$) has a rich vibronic landscape for which photoelectron spectroscopy is an ideal probe. Here, we use slow photoelectron velocity map imaging of cryogenically cooled anions (cryo-SEVI), a high-resolution variant of anion photoelectron spectroscopy, to investigate the vibronic structure of the $\tilde{X}^2A'_2$ state of NO$_3$. Our cryo-SEVI spectra are in excellent agreement with Franck-Condon simulations produced using a three-state K\"{o}ppel-Domke-Cederbaum (KDC) Hamiltonian constructed for the NO$_3$ radical. Together, the experimental and simulated spectra provide clear evidence that the $\nu_3$ fundamental resides near 1060 cm$^{-1}$, resolving a long-standing controversy surrounding this vibrational fundamental. Further, the appearance of activity along the $\nu_4$ mode in this cryogenically-cold system verifies its activity through a Herzberg-Teller interaction, rather than as a hot band as previously suggested