15min:

J. WANG, A. DOI, S. KASAHARA AND HAJIME KATO, Molecular Photoscience Research Center, Kobe University, Kobe 657-8501, Japan; M. BABA, Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8501, Japan.

A Doppler-free excitation spectrum and the Zeeman effect of the A~¹B_2u(v₁₄=1) X~¹A_1g(v=0) transition of C₆D₆ have been measured by means of two-photon absorption spectroscopy with counter-propagating light beams of identical wavelength within an external cavity. Rotational lines were fully resolved, and 1357 Q_Q lines of J=0-64, K=0-64 have been assigned, and the molecular constants of the A ¹B_2u(v₁₄=1) state have been determined. Perturbations centerd at K=28-29 were observed for J \geq 30, and these are identified as originating from the perpendicular Colioris interaction. The Zeeman splittings for lines of a given J were observed to increase proportionally to K² and reach a maximum at K=J. This demonstrates that the magnetic moment lies along the c axis (perpendicular to the molecular plane). The magnetic moment of the A ¹B_2u(v₁₄=1,J=64, K=64) level was determined to be 0.011 µ_B. The Zeeman splittings of the K=J levels were observed to increase linearly with J. From the analysis, the magnetic moment is shown to be originating mostly from mixing of the S₁ ¹B_2u and S₂ ¹B_1u states by the J-L coupling (electronic Coriolis interaction). The Zeeman splitting which could be identified as originating from a singlet-triplet interaction was not observed.