Electronic, vibrational, and rotational analysis of 1,2-benzanthracene by high-resolution spectroscopy referenced to an optical frequency comb

The electronic and vibrational structures of 1,2-benzanthracene-$h_{12}$ ($a$BA-$h_{12}$) and 1,2-benzanthracene-$d_{12}$ ($a$BA-$d_{12}$) were elucidated by analyzing fluorescence excitation spectra and dispersed fluorescence spectra in a supersonic jet on the basis of DFT calculation. We also observed the high-resolution and high-precision fluorescence excitation spectrum of the $S_1$ $\leftarrow$ $S_0$ 0$_0^0$ band, and determined the accurate rotational constants in the zero-vibrational levels of the $S_0$ and $S_1$ states. In this high-resolution measurement, we used a single-mode UV laser whose frequencies were controlled with reference to an optical frequency comb. The inertial defect is negligibly small and the molecule is considered to be planar, and the obtained rotational constants were well reproduced by EOM-CCSD calculation. Both $a$-type and $b$-type transitions are found to be included in the rotationally resolved spectrum, and the $a$-type contribution is dominant, that is, the transition moment is nearly parallel to the long axis of the $a$BA molecule. We concluded that the $S_1$ state is mainly composed of the $\mit\Phi$(B) configuration. The observed fluorescence lifetime (106 ns) is considerably longer than that of the $\mit\Phi$(A) system such as anthracene (18 ns). The transition moment for the lower state of mixed states becomes small reflecting a near cancelation of the contributions from the parts of the wavefunction corresponding to the two electronic configurations. The bandwidth of the $S_2$ $\leftarrow$ $S_0$ transirion is large and the structure is complicated. It is attributed to vibronic coupling with high-vibrational levels of the $S_1$ state.