Knowledge Bank

A Doppler-free excitation spectrum and the Zeeman effect of the $A1B2u(v14=1)\leftarrow X\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1g(v=0)$ 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 $1357QQ$ lines of $J=0-64,K=0-64$ have been assigned, and the molecular constants of the $A\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u(v14=1)$ state have been determined. Perturbations centerd at $K=28-29$ were observed for $J\ge 30$, 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 $A\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u(v14=1,J=64)$ level was determined to be $0.011\mu B$. 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 $S1\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u$ and $S2\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B1u$ 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.