Statistical analysis of the computed X̃ 2A1/Ã 2B2 spectrum of NO2: Some insights into the causes of its irregularity

Abstract

We present a statistical analysis of the B2 symmetry lines of the nonadiabatic X̃ 2A1/à 2B2 spectrum of NO2 in the energy range 10 000–22 000 cm−1, obtained by ab initio diabatic potentials refined with respect to the experimental lines until 10 000 cm−1. The two most common statistical measures, the nearest neighbor spacing distribution and the Δ̄3 are taken into account. Both the statistics show a high degree of irregularity of the spectrum. The analysis is repeated in restricted ranges for which analogous investigations on experimental data are reported in the literature. The comparison is always satisfactory. Comparison is also discussed with respect to previous theoretical results obtained by other authors. We investigate the causes of the spectrum irregularity by removing systematically some selected couplings from the full nonadiabatic Hamiltonian. We find that the irregularity is due to two main causes; the sensible vibronic coupling between the two diabatic states X̃ 2A1 and à 2B2 and the stretch–bend couplings on the diabatic ground surface X̃ 2A1.