On the vibronic level structure in the NO3 radical.: I.: The ground electronic statel

The model Hamiltonian approach of Koppel [Adv. Chem. Phys. 57, 59 (1984)] is used to analyze the electronic spectroscopy of the nitrate radical (NO3). Simulations of negative ion photodetachment of NO3-, the X (2)A(2)(')<- B E-2(') dispersed fluorescence spectrum of NO3, and the B E-2(')<- X (2)A(2)(') absorption spectrum are all in qualitative agreement with experiment, indicating that the model Hamiltonian contains most or all of the essential physics that govern the strongly coupled X (2)A(2)(') and B E-2(') electronic states of the radical. All 14 bands seen in the dispersed fluorescence spectrum below 2600 cm(-1) are assigned based on the simulations, filling in a few gaps left by previous work, and 7 additional bands below 4000 cm(-1) are tentatively assigned. The assignment is predicated on the assumption that the nu(3) level of NO3 is near 1000 cm(-1) rather than 1492 cm(-1) as is presently believed. Support for this reassignment (which associates the 1492 cm(-1) band with the nu(1)+nu(4) level) comes from both the model Hamiltonian spectrum and a Fourier-transform infrared feature at 2585 cm(-1) that is consistent with the large and positive cross anharmonicity between nu(1) and nu(4) needed for the alternative 1492 cm(-1) assignment. An apparent systematic deficiency exists in the treatment of the model Hamiltonian for levels involving nu(4). A discussion of the correlation between energy levels in the rigid D-3h and C-2v limits is illustrative, and provides insight into just how hard it is to treat the degenerate bending coordinate (q(4)) of NO3 accurately. (c) 2007 American Institute of Physics.