Ab initio investigation of the vertical and adiabatic excitation spectrum of NO3

Abstract

The ground and excited electronic states up to approximately 100 000 ${\mathrm{cm}}^{\text{−1}}$ of the nitrate free radical $({\mathrm{NO}}_3)$ are investigated by complete active space self-consistent-field (CASSCF) and mulitreference–singles doubles configuration interaction calculations. Extended basis sets, containing polarization and diffuse functions, and an active space consisting of 13 orbitals and 17 electrons are used. A total of 28 electronic states is obtained within the $D_{\text{3h}}$ framework which split into 44 states in $C_{\text{2v}}.$ All calculated states are valence states and their character is discussed in detail. Oscillator strength and radiative lifetimes are determined from the CASSCF wave functions and give evidence for a strong transition that was not yet known experimentally. For the experimentally observed ${}^2{E}^{″}$ and ${}^2{E}^{\prime }$ states equilibrium geometries, harmonic frequencies, and adiabatic excitation energies are calculated in excellent agreement with experimental data. Important new insight is gained about the role of the low-lying electronic states in the photodissociation and the mechanism of this process is discussed.