Impact of nonadiabatic coupling between the conically intersecting (X)over-tilde(2)A(1) and (A)over-tilde(2)B(2) states of NO2 on the negative ion photoelectron spectra of NO2-

Negative ion photoelectron spectra of NO2- at 266 nm (4.660 eV) and 351 nm (3.532 eV) are theoretically calculated and compared with earlier experimen tal findings. A diabatic model Hamiltonian with linear vibronic coupling be tween the conically intersecting (X)over-tilde (2)A(1) and (A)over-tilde B- 2(2) states of NO2 is employed for this purpose. While the linear vibronic coupling model successfully reproduces the low resolution experimental phot oelectron spectrum at 266 nm, second order coupling terms are necessary for the high resolution spectrum at 351 nm. The vibronic structure of the (X)o ver-tilde (2)A(1) state of NO2 in the photoelectron spectrum is found to be almost unaffected by the nonadiabatic coupling, whereas, the latter has a strong impact on the vibronic structure of the (A)over-tilde B-2(2) state. The magnitude of the vibronic coupling constant, which has otherwise been o verestimated by earlier theoretical studies, is calculated ab initio and it s limits of validity are confirmed by the results from different ab initio calculations. (C) 1999 American Institute of Physics. [S0021-9606(99)01712- 2].