Rotationally resolved ultrahigh-resolution laser spectroscopy of the S-2 (1)A(1)<- S-0 (1)A(1) transition of azulene

Abstract

We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 0(0)(0) (a-type) and 0(0)(0)+467 cm(-1) (b-type) bands of the S-2 (1)A(1)<- S-0 (1)A(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 S-0 and S-2 states (C-2 upsilon 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 (1)A(1) state of azulene was identified to be IC to the S-1 B-1(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 S-1 B-1(2) state, which is also responsible for the extraordinarily fast S-1 -> S-0 IC in the isolated azulene molecule. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3168394]