Ab initio study of NO2. VII. Ã 2B2←X̃ 2A1 nonadiabatic Franck–Condon absorption spectrum Original Research Article

We have calculated the nonadiabatic Franck–Condon cold absorption spectrum of the X̃ 2A1/Ã 2B2 conical intersection of NO2, up to 22000 cm−1. The role of the Ã 2B2 Fermi polyads (v1, v2, 0) has been pointed out, both within and beyond the Born–Oppenheimer approximation. The diabatic and nonadiabatic results have been compared, both between themselves and with experimental results. We have shown that the nonadiabatic interactions give rise to a weaker and more chaotic intensity distribution, by mixing bright and dark states, by splitting the polyads into their multiplets, and by overlapping different polyads. The intensity maxima therefore correspond to the Ã 2B2 (1, v2, 0) character of the nonadiabatic states up to 15000 cm−1 (v2=5), and to more chaotic Ã 2B2 symmetric stretch-bending progressions of the nonadiabatic bands at higher energies. The survival probability of the initial Franck–Condon state has been calculated for three different sets of nonadiabatic states. The results in the energy and time domains nicely supplement and confirm each other, and highlight the beginning of the quantum chaos at about 16500 cm−1 and its full development above 17500 cm−1.