High-resolution laser spectroscopy of NO2 just above the (X)over-tilde(2)A(1)-(A)over-tilde(2)B(2) conical intersection: Transitions of K-=0 stacks

The visible absorption spectrum of NO2 is very dense and irregular, and sho ws signs of a chaotic frequency and intensity distribution in the higher en ergy region. The complexity of the spectrum is related to a conical interse ction of the potential energy surfaces of the two lowest electronic states. Above the conical intersection strong vibronic interactions lead to hybrid eigenstates, which can be viewed as mixtures of low vibrational levels of the electronically excited state and high vibrational levels of the electro nic ground state. As a contribution to the elucidation of the nature of the vibronic bands of NO2 we have measured high-resolution spectra of a number of vibronic bands in the region between 10 000 and 14 000 cm(-1) by exciti ng a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapp hire ring laser. The energy absorbed by the molecules was detected by a bol ometer, and in some cases, laser-induced fluorescence was detected. The hyp erfine structure is dominated by the Fermi-contact interaction and the magn itude of this interaction is a direct measure of the (electronic) compositi on of the hybrid eigenstates. In the present paper we have restricted our a nalysis to transitions of K = 0 stacks. The fine- and hyperfine structure o f each rotational transition can be analyzed by using an effective Hamilton ian approach. The very good agreement that is found between the calculated transition strengths and the measured line intensities is evidence that in the spectral region studied, rovibronic interactions play a minor role. The composition of the hybrid eigenstates is compared with ab initio calculati ons reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic b ands. (C) 1998 American Institute of Physics. [S0021-9606(98)01446-9].