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 $\overline{\ensuremath{\chi}}(\ensuremath{-}2\ensuremath{\omega}; \ensuremath{\omega}, \ensuremath{\omega})$ and the third-order polarizability $\overline{\ensuremath{\chi}}(\ensuremath{-}2\ensuremath{\omega}; 0, \ensuremath{\omega}, \ensuremath{\omega})$. Both independent nonzero spatial components of each are measured. The bond-additivity model is shown to work well (\ifmmode\pm\else\textpm\fi{}15%) for third-order polarizabilities but poorly (discrepancies of about a factor of two) for second-order polarizabilities.