Sub-Doppler high-resolution excitation spectroscopy of the S1 ← S0 transition of dibenzofuran

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S1 ← S0 transition of dibenzofuran have been observed using the technique of crossing a collimated molecular beam and the single-mode UV laser beam. 3291 rotational lines of the 000 band and 3047 rotational lines of the 000+443cm−1 (5501:b2) band have been assigned. The 000 band has been found to be a b-type transition, in which the transition moment is along the twofold symmetry axis of this molecule, and only the ΔKa = ± 1 transitions were observed. The excited state is identified to be the S11A1(ππ∗) state. In contrast with this, the 000+443cm−1 (5501:b2) band has been found to be an a-type transition in which the transition moment is along the long axis in plane. It indicates that the intensity of this vibronic band arises from vibronic coupling with the S21B2(ππ∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.