The NO2 vibronic levels near the X 2A1–A 2B2 conical intersection observed by laser induced dispersed fluorescence

Abstract

Following our previous laser induced dispersed fluorescence (LIDFS) study on NO2 [J. Chem. Phys. 95, 5686 (1991)], we observed the vibronic levels up to 13 900 cm−1 by LIDFS. These observations allow one to characterize the effect of the conical intersection between the X 2A1 and A 2B2 electronic potential energy surfaces (PESs). This effect has been investigated by ab initio methods in the same range by Leonardi et al. [J. Chem. Phys. 105, 9051 (1996)]. Globally we observed 420 vibronic levels of A1 or B2 symmetry up to 13 900 cm−1, while 259 were observed previously up to 12 000 cm−1. Most of these levels belong to the X 2A1 state and only 8 to the A 2B2 state. Below 12 000 cm−1, most of the levels belonging to the X 2A1 state have been vibrationally assigned and only a few are significantly mixed with those of the A 2B2 state. In contrast, each vibrational level of the A 2B2 state is mixed with few nearby high vibrational levels of the X 2A1 state via vibronic interactions. The set of the X 2A1 vibrational levels is assigned and completed up to 11 700 cm−1 for the a1 vibrational symmetry (171 levels) and up to 11 000 cm−1 for the b2 symmetry (104 levels). Above these energies the X 2A1–A 2B2 vibronic interactions preclude secure vibrational assignment of most of the levels. The dominant electronic and vibrational characters have been used for the assignments of some levels. The set of zero order vibrational levels of the A 2B2 state can be described by polyads because there is an approximate 2:1:2 ratio between the three vibrational frequencies. The four lowest polyads of B2 vibronic symmetry are analyzed. In addition to the previously observed vibrationless level (0,0,0) of the A 2B2 state (first polyad), the vibrational levels (0,1,0) (second polyad), (0,0,1) or (1,0,0), and (0,2,0) (third polyad) and (0,3,0) (belonging to the fourth polyad) have been observed. Numerous vibronic levels, previously observed by absorption (ICLAS) and/or by LIF, have also been observed by LIDFS. A semiquantitative analysis of these vibronic interactions is presented. The A and B̄ rotational constants of numerous vibrational levels have also been measured. Globally, these results allow a better understanding of the low energy range of X 2A1–A 2B2 vibronic interaction.