Knowledge Bank

A tunable dye laser has been used to investigate the two-photon absorption of biphenyl and related compounds. The method used is the absorption of two identical photons, of either linear or circular polarization, with fluorescence detection. This provides sufficient information to distinguish totally symmetric electronic transitions $(Ag\leftarrow Ag)$ from other allowed types of two-photon transitions $(Bg\leftarrow Ag)$. We see two distinct bands. The lower one (33 100 to $38000cm-1$) is due to a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bg$ excited state, which is the lowest excited singlet of the molecule. The upper one (38 000 to $40000cm-1$ and beyond) seems to be due to a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ excited state. No published calculation on biphenyl predicts this result.