Theoretical study of the (D)over-tilde→(C)over-tilde emission spectrum of NO2

The 3 (2)A'((D) over tilde)-> 1 (2)A ''((C) over tilde) emission spectrum of NO(2) has been calculated by means of exact dynamics calculations and an accurate potential energy surface for the (C) over tilde state. The potential energy surface has been obtained by electronic structure calculations employing the internally contracted multireference configuration interaction method plus Davidson correction and the augmented correlation consistent polarized quadruple zeta basis set. The calculated spectrum, based on energies as well as intensities, agrees well with the measured one. Despite the two asymmetric C(s) potential wells of the (C) over tilde potential energy surface, the spectrum is best described by a C(2v) assignment in terms of symmetric stretch, bending, and antisymmetric stretch quantum numbers. The barrier separating the two wells is merely of the order of 500 cm(-1) with the consequence that only the two lowest states, (0,0,0) and (0,0,1), show a tunneling splitting. Essential for the correct assignment of the spectrum is the pronounced negative anharmonicity of the antisymmetric stretch mode. Excitation of the symmetric stretch mode is not directly seen in the main part of the spectrum. (C) 2008 American Institute of Physics.