Abstract
The frequency dependence of the second hyperpolarizability (γ) of CH4 has been measured in the visible (477<λ<672 nm) by means of gas-phase electric-field-induced second-harmonic generation (ESHG). No resonances due to vibrational overtones are observed. Comparison of the ESHG results with the results of several other third-order nonlinear-optics experiments indicates that vibrational contributions to γCH4 are negligible.