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

The ground and excited electronic states up to approximately 100 000 cm−1 of the nitrate free radical (NO3) 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 D3h framework which split into 44 states in C2v. 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 E″2 and E′2 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.