15min:

YOSUKE SEMBA AND SHUNJI KASAHARA, Molecular Photoscience Research Center, Kobe University, Kobe 657-8501, Japan; MASAAKI BABA, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.

The fluorescence quantum yield at the vibrational energy over 3000 cm^-1 in the S₁ ¹B_2u state of benzene is dramatically decreased. This phenomenon is called ''channel three'' and has been studied extensively. We have observed the ultrahigh-resolution spectrum of the S₁ S₀ 1²₀14¹₀ band ( E _excess = 3412 cm^-1) and found the Zeeman splitting which explained to be originating from the magnetic moment of the S₁ state induced by mixing with S₂ state by J-L coupling. In this work, we established the apparatus for the Doppler-free two-photon excitation (DFTPE) spectroscopy with a molecular beam and measured the S₁ S₀ 1¹₀14¹₀ and 1²₀14¹₀ band of benzene. When we measured the DFTPE spectrum in a gas cell, many rotational lines were observed but the pressure broadening was remained. On the other hand, when we measured the DFTPE spectrum in a molecular beam, the influence of collision was decreased. The typical linewidth was 6 MHz. The high-resolution and collision-free condition is useful to investigate the perturbing region with high sensitivity and accuracy.