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 C${\mathrm{H}}_4$, C${\mathrm{H}}_3$F, C${\mathrm{H}}_2$ ${\mathrm{F}}_2$, CH${\mathrm{F}}_3$, C${\mathrm{F}}_4$, CCl${\mathrm{F}}_3$, and CBr${\mathrm{F}}_3$. For each molecule, experiments yield the second-order polarizability $\overline{\chi }(-2\mathrm{}\omega ; \omega , \omega )$ and the third-order polarizability $\overline{\chi }(-2\mathrm{}\omega ; 0, \omega , \omega )$. 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.