A comparison of molecular hyperpolarizabilities from gas and liquid phase measurements

The first and second hyperpolarizabilities (β and γ) of eight molecules at a fundamental wavelength of 1064 were measured by gas-phase electric-field-induced second-harmonic generation (EFISH), gas-phase hyper-Rayleigh scattering (HRS), and liquid-phase HRS experiments. The EFISH measurements give accurate values of β and γ for these molecules in the gas phase, and the HRS measurements show that the effective β of these molecules in the liquid is enhanced over the gas-phase value by a factor which varies from ≈0.4 to 2.0, over and above the Lorentz local field factors. Combining all of the measurements provides an accurate, absolute determination of the effective β for HRS in the liquid phase. The results for CCl4, suitable as reference standards, are 〈βVV2〉1/2=18.6±0.7 au in the liquid phase and βxyz=20.7±1.6 au in the gas phase. Comparison of measurements between hydrogenated and deuterated molecules indicates that vibrational contributions to β are small.