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

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 Delta K-a = +/- 1 transitions were observed. The excited state is identified to be the S-1 (1)A(1)(pi pi*) 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 B-1(2)(pi pi*) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states. (c) 2006 Elsevier Inc. All rights reserved.