High-resolution laser spectroscopy of NO2 just above the X(2)A(1)-A(2)B(2)conical intersection: Transitions of K-=1 stacks

The complexity of the absorption spectrum of NO2 can be attributed to a con ical intersection of the potential energy surfaces of the two lowest electr onic 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 rep orted on the feasibility of using the hyperfine splittings, specifically th e Fermi-contact interaction, to determine the electronic ground state chara cter of the excited vibronic states in the region just above the conical in tersection; 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 measu red by exciting a supersonically cooled beam of NO2 molecules with a narrow -band Ti:Sapphire ring laser. The energy absorbed by the molecules was dete cted by the use of a bolometer. In the region of interest, rovibronic inter actions play no significant role, with the possible exception of the vibron ic 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 transition s of the K-=1 stack. In the present paper we extend the analysis to transit ions of the K-=1 stack, from which, in addition to hyperfine coupling const ants, values of the A rotational constants of the excited NO2 molecules can be determined. Those rotational constants also contain information about t he 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 conc lusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In additi on, 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. [S0021-9 606(00)01508-7].