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

Abstract

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S 1 ← S 0 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 0 0 0 band and 3047 rotational lines of the 0 0 0 + 443 cm − 1 ( 55 0 1 : b 2 ) band have been assigned. The 0 0 0 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 Δ K a = ± 1 transitions were observed. The excited state is identified to be the S 1 1 A 1(ππ ∗) state. In contrast with this, the 0 0 0 + 443 cm − 1 ( 55 0 1 : b 2 ) 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 S 2 1 B 2(ππ ∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.