Molecular second- and third-order polarizabilities from measurements of second-harmonic generation in gases

The temperature dependence of dc electric-field-induced optical second-harmonic generation has been measured for CH₄, CH₃F, CH₂{F}₂$, CH${F}₃$, C${F}₄$, CCl${F}₃$, and CBr${F}₃$. For each molecule, experiments yield the second-order polarizability ${{χ}}({-}2{ω}; {ω}, {ω})$ and the third-order polarizability ${{χ}}({-}2{ω}; 0, {ω}, {ω})$. Both independent nonzero spatial components of each are measured. The bond-additivity model is shown to work well (±{}15%) for third-order polarizabilities but poorly (discrepancies of about a factor of two) for second-order polarizabilities.