Direct time-resolved UV-absorption study on the ultrafast internal conversion of cycloheptatriene in solution

Abstract

We present the first direct measurement of the internal conversion time constant of cycloheptatriene (CHT) in the liquid phase by transient absorption spectroscopy in the femtosecond time domain. The pump and probe wavelengths were 266 and 400 nm, respectively. The experiments were carried out in solutions of CHT in cyclohexane, methanol and glycol. In all cases the excited state lifetime was found to be τ=(110±10) fs. The internal conversion is therefore, in good approximation, independent of polarity and viscosity of the surrounding solvent. The influence of the different group velocities in the liquid samples on the time resolution was minimized by using highly absorbent samples with CHT concentrations 0.02 mol l-1. In these samples the pump pulse is absorbed completely within a very thin layer of the solution of an optical depth of less than 100 µm. Thus, our time resolution is controlled by the 65 fs widths (FWHM) of the pump and probe pulses only. After the internal conversion of CHT there is a residual absorption due to the vibrationally excited ground state molecules. The corresponding absorption cross-section fully agrees with the absorption cross-section of thermally excited CHT containing an average energy identical to the original electronic excitation energy.