Absolute signs of hyperpolarizabilities in the liquid state

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Abstract
We discuss several techniques for the determination of the absolute sign of molecular hyperpolarizabilities in the liquid state. One of these techniques is the comparison of the phase of the harmonic field produced by the liquid with that generated by the glass walls of the electroded cell. Another method is the phase comparison with a crystal quartz plate of known absolute orientation. Using these methods we determine that the hyperpolarizability of nitrobenzene is positive, i.e., the molecular nonlinearity and dipole moment are parallel. By mixing nitrobenzene with other liquids, we can readily determine their absolute signs, e.g., we have found that liquid CH3I is positive in agreement with measurements in the gas phase.