Knowledge Bank

The fluorescence quantum yield at the vibrational energy over 3000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ in the $S11B2u$ 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 $S1\leftarrow S01021401$ band ($Eexcess$ = 3412 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) 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. } ,{\bf 79}, 75 (2006).} }, {\bf 80}, 456 (2007).} In this work, we established the apparatus for the Doppler-free two-photon excitation (DFTPE) spectroscopy with a molecular beam and measured the $S1\leftarrow S01011401$ and $1021401$ 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.