Doppler-Free Two-Photon Excitation Spectroscopy and the Zeeman Effect in the ''Channel Three'' Region of C6H6

Doppler-free two-photon excitation spectra and the Zeeman effects for the 1 0 2 14 0 1 and 1 0 1 14 0 1 bands, whose vibrational excess energies are 3412 and 2492 cm -1 , respectively, of the S 1 1 B 2u ← S 0 1 A 1g transition in gaseous benzene-h 6 have been measured. Rotationally resolved lines up to a rotational excess energy of 945 cm -1 in the 1 0 1 14 0 1 band, whose excess energy is isoenergetic with the one of the 1 0 2 14 0 1 band at low J, have been observed. The density of perturbation is observed to increase around the K = 0 and K = J levels in the 1 0 1 14 0 1 band as the rotational energy increases, but line broadening is not observed. It was reported that only the K = 0 lines at low J and the K = J lines at high J were observed as a sharp line and the rest was washed out by broadening in the 1 0 2 14 0 1 band. The assignments are confirmed by the Zeeman spectra. The character and magnitude of Zeeman splittings in both bands could be understood as originating from the electronic orbital angular momentum arising from a mixing of the S 1 1 B 2u and S 2 1 B 1u states via J-L coupling. The levels of the S 1 1 B 2u state are found not to be mixed with a triplet state by the Zeeman effect.