Knowledge Bank

A Doppler-free absorption spectrum and the Zeeman effect of the $A1B2u\leftarrow X1A1g$ transition of benzene have been measured by means of two-photon absorption spectroscopy within an external cavity. Rotational lines were fully resolved and the accurate molecular constants have been determined for the $A1B2u(v14=1,v1=1)$ state. The Zeeman splittings for lines of a given J were observed to increase regularly with K 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 $A1B2u(u14=1,v1=1,J=43,K=43)$ level was determined to be $0.005\mu B$. The Zeeman splittings of the $K=J$ levels were observed to increase with increasing J. It is concluded that the $S1\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u$ state is mixed with the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E2u(\sigma ,\pi \ast or\sigma \ast ,\pi )$ state. The $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u(S1)\to \mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1u(T1)$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u(S1)\to \mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E1u(T2)$ intersystem crossings are suggested to occur through the vibronic interactions between $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E2u$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1u(T1)$ and between $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E2u$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E1u(T2)$ through the mixed $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E2u$ component, respectively, and the spin-orbit interaction between the the $S1\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}E2u(\sigma ,\pi \ast or\sigma \ast \pi )$ $states.a$