The NO2 vibronic levels near the X 2A1–A 2B2 conical intersection: Jet cooled laser induced fluorescence between 11 680 and 13 900 cm−1

Following our previous LIF [J. Chem. Phys. 95, 5701, (1991) and 103, 1732 (1995)] and ICLAS [Chem. Phys. 190, 207 (1995)] studies on NO2, we observed by LIF the NO2 vibronic levels between 11 680 and 13 900 cm−1. These observations allow us to characterize the vibronic levels resulting from the conical intersection between the X 2A1 and A 2B2 electronic PESs. Globally, we observed by LIF 78 vibronic levels of B2 vibronic symmetry, among which 61 had previously been observed by ICLAS. Conversely, one vibronic level observed at 13 088 cm−1 by ICLAS has not been observed by LIF. The 17 new levels have been observed thanks to a better sensitivity (about a factor 10) and resolution (0.3 GHz). The LIF intensities range over four orders of magnitude. The 79 (78+1) observed vibronic levels represent 83% of the 95 levels of B2 vibronic symmetry calculated in that range; 85 X 2A1 levels of b2 vibrational symmetry and 10 A 2B2 levels of a1 vibrational symmetry. The missing levels are expected to have a weak A 2B2 electronic character and then a very weak intensity. Consequently, these missing levels are expected not to be significantly shifted by X 2A1–A 2B2 vibronic interaction. In addition, 85 hot bands have been observed by LIF in the same range (among which only 15 have been observed by ICLAS). For most of the vibronic levels, the rotational constants (A,B) and spin splittings have been determined from their few lowest rotational levels. The zero order vibronic levels can be classified according to a polyad number, N=N(bend)+2N(stretch). The present results cover the polyads four to seven. This experimental data, combined with laser induced dispersed fluorescence spectra (LIDFS), [J. Chem. Phys. 108, 6638 (1998)] which give access to polyads one to five, contributes to a quantitative interpretation of the strong interaction between the X 2A1 and A 2B2 PESs. The integrated density of states of vibronic levels of B2 symmetry is carefully discussed up to 19 360 cm−1 and the next neighbor spacing distribution (NNDS) is presented for a (almost?) complete set of 65 levels located between 12 500 and 13 860 cm−1. These data contributes to the characterization of the transition from regularity, below 10 000 cm−1, to quantum chaos, which is fully developed above ∼16 500 cm−1 within the NO2 vibronic degrees of freedom.