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 (X) over tilde (2)A(1) and (A) over tilde E-2 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 (A) over tilde E-2 photoelectron band are analyzed. The calculation of the time-dependent population probability of the (A) over tilde E-2 state reveals a radiationless decay process on a time scale of 30 fs caused by a conical intersection of the (X) over tilde and (A) over tilde potential-energy surfaces, which arises from the combined effect of the Jahn-Teller splitting of the (A) over tilde E-2 state and the (X) over tilde-(A) over tilde pseudo-Jahn-Teller interaction. In the (X) over tilde (2)A(1) band, the (X) over tilde-(A) over tilde 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 (X) over tilde (2)A(1) photoelectron band of NH3 by Edvardsson [J. Phys. B 32, 2583 (1999)]. (C) 2003 American Institute of Physics.