The NO2 vibronic levels near the X2A1-A2B2 conical intersection:: Jet cooled laser induced fluorescence between 11680 and 13900 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 11680 and 13900 cm(-1). These observations allow us to characterize the vibronic levels resulting from the conical intersection between the X(2)A(1) and A(2)B(2) electronic PESs. Globally, we observed by LIF 78 vibronic levels of B-2 vibronic symmetry, among which 61 had previously been observed by ICLAS. Conversely, one vibronic level observed at 13088 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 B-2 vibronic symmetry calculated in that range; 85 X(2)A(1) levels of b(2) vibrational symmetry and 10 A(2)B(2) levels of a(1) vibrational symmetry. The missing levels are expected to have a weak A(2)B(2) electronic character and then a very weak intensity. Consequently, these missing levels are expected not to be significantly shifted by X(2)A(1)-A(2)B(2) 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(2)A(1) and A(2)B(2) PESs. The integrated density of states of vibronic levels of B-2 symmetry is carefully discussed up to 19360 cm(-1) and the next neighbor spacing distribution (NNDS) is presented for a (almost?) complete set of 65 levels located between 12500 and 13860 cm(-1). These data contributes to the characterization of the transition from regularity, below 10000 cm(-1), to quantum chaos, which is fully developed above similar to 16500 cm(-1) within the NO2 vibronic degrees of freedom. (C) 1999 American Institute of Physics. [S0021-9606(99)01509-3].