Knowledge Bank

``Rotational line strengths of two-photon absorption (TPA) processes in a symmetric top molecule have been developed by the powerful angular momentum theory. Experimentally accessible polarization states and frequencies of the photons are considered case by case. For TPA processes of identical frequency, $\updownarrow \updownarrow$ and $\to \to$ polarization states form a complete study. On the other hand, polarization ratios inferred from TPA processes of non-Identical frequencies should give new insights into the excited state assignments. The application of the full treatment to benzene $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u\leftarrow 1A1g$ two-photon excitation spectra reveals new $\Delta K$ selection rules and polarization ratios, which can be critically tested by higher resolution (Doppler-free) TPA spectra.