S₁ → S<i>_n</i> and S₂ → S<i>_n</i> Absorption of Azulene:  Femtosecond Transient Spectra and Excited State Calculations

Abstract

The transient spectra of azulene in solution have been measured in the spectral region 600−350 nm at room temperature, pumping into the S 1 and S 2 states with femtosecond pulses and probing with a delayed femtosecond white light continumm. The spectra are a combination of ground-state bleaching, stimulated emission, and excited-state absorption. The latter component gives a direct information on excited states which may be not active in the ground-state absorption. S 1 → S n and S 2 → S n absorptions have been discussed with the help of ab initio calculations of the MCSCF/CAS type with the 6-31G* basis set and including perturbative corrections. The calculated S 0 → S n, S 1, eq → S n, and S 2, eq → S n vertical excitation energies and oscillator strengths are in satisfactory agreement with the experimental results. It has been found that electronic states, weakly active in the ground-state absorption, occur with high intensity in the femtosecond transient spectra, in particular in the energy range 36000−44000 cm -1 above the ground-state energy.