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 D-3h framework which split into 44 states in C-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 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. (C) 2001 American Institute of Physics.