Towards a higher-order description of Jahn-Teller coupling effects in molecular spectroscopy: The A∼2E″ state of NO3

Static and dynamic Jahn-Teller coupling effects in the 2E″ excited electronic state of the nitrate radical (NO3) are analyzed theoretically. An earlier vibronic coupling model proposed by us [S. Mahapatra, W. Eisfeld, H. Köppel, Chem. Phys. Lett. 441 (2007) 7] is extended here and the importance of higher-order coupling terms established. More specifically, the earlier analysis rests on a quadratic-coupling approach where terms up to second order in the nuclear displacements are retained. In the present analysis we find coupling terms of 3rd and 4th order to be important. This is related to low-energy dissociation thresholds in the system which render the (diabatic) potential energy surfaces anharmonic already for low excitation energies. The results are compared with the experimental photodetachment spectrum of Neumark and coworkers [A. Weaver, D.W. Arnold, S.E. Bradforth, D.M. Neumark, J. Chem. Phys. 94 (1991) 1740], and the agreement is much improved when the additional coupling terms are taken into account.