Rovibronic spectrum of the N3 radical in the X 2Πg state

Abstract

Based on ab initio electronic structure calculations, the three-dimensional potential energy functions for the electronic ground state X 2Πg of N3 have been generated and used in beyond Born–Oppenheimer calculations of the rovibronic energy levels by a variational approach accounting for anharmonicity, rotation–vibration, electronic angular momenta, and electron spin coupling effects. The vibronic levels (J=P) for energies up to 4300 cm−1 are given. The few experimentally known vibronic energy differences have been reproduced with an accuracy of better than 10 cm−1. For several vibronic levels also the rovibronic levels are reported. The electron–nuclear motion and anharmonic coupling effects have been analyzed. The strength of the Born–Oppenheimer breakdown depends strongly on the rovibronic symmetry. Like in the isoelectronic CO+2 ion, the anharmonic coupling in the vibronic states occurs mainly within the blocks of Fermi polyads for (2ν1+ν2) =constant. In low lying states the anharmonic coupling effects are weaker in N3 than in CO+2. The presence of the Fermi polyad blocks of interacting states strongly alleviates the characterization of the excited rovibronic states in the N3 radical.