Theoretical investigation of Jahn–Teller and pseudo-Jahn–Teller interactions in the ammonia cation

Abstract

The spectroscopic and dynamic aspects of Jahn–Teller and pseudo-Jahn–Teller interactions in the ammonia cation are investigated within an ab initio based vibronic-coupling model approach. Multireference second-order perturbation theory (CASPT2) has been employed to obtain the potential energies of the ground state and the first excited state of NH3+ as a function of symmetry-coordinate displacements. Vibronic-coupling parameters determining the Franck–Condon, Jahn–Teller, and pseudo-Jahn–Teller activity of the normal modes have been obtained from the ab initio data. The vibronic structures of the X̃ 2A1 and à 2E photoelectron bands of ammonia have been calculated by numerical diagonalization of the vibronic Hamiltonian matrix. All six vibrational degrees of freedom are taken into account. The effects of Jahn–Teller and pseudo-Jahn–Teller interactions on the band shape of the à 2E photoelectron band are analyzed. The calculation of the time-dependent population probability of the à 2E state reveals a radiationless decay process on a time scale of 30 fs caused by a conical intersection of the X̃ and à potential-energy surfaces, which arises from the combined effect of the Jahn–Teller splitting of the à 2E state and the X̃–à pseudo-Jahn–Teller interaction. In the X̃ 2A1 band, the X̃–à pseudo-Jahn–Teller coupling results in the weak excitation of a single quantum of the degenerate bending mode. This theoretical result corroborates the earlier assignment of the vibronic structure of the X̃ 2A1 photoelectron band of NH3 by Edvardsson et al. [J. Phys. B 32, 2583 (1999)].