High-resolution laser spectroscopy of NO2 just above the X2A1-A2B2 conical intersection:: Transitions of K-=1 stacks

The complexity of the absorption spectrum of NO2 can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of (2)A(1) symmetry and the first electronically excited state of B-2(2) symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10 000 to 14 000 cm(-1) above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest, rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12 658 cm(-1), so that the fine- and hyperfine structure of each rotational transition could be analyzed by using an effective Hamiltonian. In the previous paper we restricted ourselves to an analysis of transitions of the K-=1 stack. In the present paper we extend the analysis to transitions of the K-=1 stack, from which, in addition to hyperfine coupling constants, values of the A rotational constants of the excited NO2 molecules can be determined. Those rotational constants also contain information about the electronic composition of the vibronic states, and, moreover, about the geometry of the NO2 molecule in the excited state of interest. The results of our analyses are compared with those obtained by other authors. The conclusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In addition, the A rotational constants correspond to geometries that are consistent with the electronic composition of the relevant excited states as expected from the Fermi-constants. (C) 2000 American Institute of Physics. [S0021-9606(00)01508-7].