First determination of the NO_2 A~2B_2 stretching frequencies by jet cooled intracavity laser absorption spectroscopy around 11 000 cm~(-1)

摘要

Using intracavity laser absorption spectroscopy combined with a superosic slit jet, we have observed and analyzed seven vibronic cold bands of NO_2 between 10 800 and 11 680 cm~(-1). The vibronic energies, relative intensities, A, B, and C rotational constants, and the spin-rotation constants have been determined. The rotational constants play a crucial role in the vibronic assignments. The seven observed states are vibronically mixed. However, three of them have a dominant A ~2B_2 electronic character while the four others have a dominant X ~2A_1 character. The vibrational assignments and energies of the three A ~2B_2 levels are: (1, 0, 0) at 10 999.42 cm~(-1), (0, 2, 0) at 11 210.50 cm~(-1), and (0, 0,2) at 11 283.15 cm~(-1). The frequencies of the symmetric stretch, (omega_1 approx = 1265 cm~(-1)), and bending, (omega_2 approx = 738 cm~(-1)) modes derived from the (1,0,0) and (0,2,0) levels are in agreement with ab initio calculations. In contrast, the frequency of the antisymmetric strenth, omega_3 approx = 775 cm~(-1), derived from the observed (0, 0, 2) level, agrees only with the ab initio value obtained by Kaldor while other ab initio values ranging from 390 to 1750 cm~(-1) were reported. The four levels with a dominant X~2A_1 character have been tentatively vibrationally assigned. The present experimental data allows for a first experimental determination of the two A ~2B_2 stretch vibrational frequencies which are necessary to model the X~2A_1 - A~2B_2 vibronic interactions, i.e., the conical intersection between X ~2A_1 and A ~2B_2 potential energy surfaces.