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The visible absorption spectrum of NO2 is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection 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 electronic 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 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 a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the (electronic) composition of the hybrid eigenstates. In the present paper we have restricted our analysis to transitions of K⎯=0 stacks. The fine- and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian 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 calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands.

The visible absorption spectrum of NO2 is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection 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 electronic 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 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 a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the (electronic) composition of the hybrid eigenstates. In the present paper we have restricted our analysis to transitions of K⎯=0 stacks. The fine- and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian 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 calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands.

It is demonstrated that enhanced mode suppression in stimulated photon echo experiments can be obtained by diagonal time gating of the echo. This technique is especially important when the optical dynamics of the system is non-Markovian. A two-mode Brownian oscillator model is used to analyze the effect of time gating on the stimulated photon echo. The method is demonstrated on a dye solution of DTTCI in ethylene glycol at room temperature. Experimentally, time gating of the echo is accomplished by means of femtosecond phase-locked heterodyne detected stimulated photon echo. The vibrational dynamics in this system are explored by conventional stimulated photon echo experiments, Especially stimulated photon echo-maximum shift measurements are found to be particularly useful. (C) 1996 American Institute of Physics.