Rotationally resolved ultrahigh-resolution laser spectroscopy of the S _2A1 _1S_0A1_1 transition of azulene

摘要

We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 000 (a -type) and 000 +467 cm-1 (b -type) bands of the S_2 A1 _1 S_0 A1 _1 transition of jet-cooled azulene. The observed linewidth is 0.0017 cm-1, which corresponds to the lifetime of 3.1 ns in the S_2 state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S0 and S2 states (C2v symmetry). Rotational constants of the S_2 state are almost identical to those of the S_0 state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S_2 A1 _1 state of azulene was identified to be IC to the S_1B1 2 state. However, this S_2 → S_1 IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S1 B1 2 state, which is also responsible for the extraordinarily fast S_1 → S_0 IC in the isolated azulene molecule.