Fluorescence and fluorescence excitation spectra of jet-cooled carbazole

Abstract

The fluorescence excitation spectra of jet-cooled carbazole molecules at vibrational temperatures of 55 and 80 K and the fluorescence spectrum of these molecules excited by radiation at the frequency of a pure electronic transition are measured. As the vibrational temperature increases, the excitation spectra exhibit a series of lines of the same symmetry, which are caused by the interaction of the active vibration with a subensemble of optically inactive vibrations. The final symmetry of the totally and nontotally symmetric vibrations is determined from the shape of the rotational contours of the lines of vibronic transitions. The values of a decrease in the frequency of the nontotally symmetric vibrations in the first excited electronic state S 1 due to their interaction with the electronic state S 2 are calculated to be up to 100 cm−1. The frequencies of the pure electronic transitions in the absorption and fluorescence spectra coincide with each other and are equal to 30809 cm−1, the frequencies of vibrations in the ground state S 0 exceeding the frequencies of the corresponding vibrations in the excited state S 1. The degree of polarization of the integral fluorescence is determined for a series of vibronic transitions of the a 1 and b 2 final symmetry that are observed in the fluorescence excitation spectra, and the contribution of the intensity with the borrowed polarization θ⊥ to the integral fluorescence is calculated. It is found that the intensity θ⊥ is higher for the transitions of the b 2 symmetry and can reach ≈50%.