New determination of the third-order nonresonant susceptibility of H/sub 2/

A new value of the electronic contribution (the electronic hyperpolarizability) to the third-order nonlinear susceptibility of gaseous H/sub 2/ has been determined. A phase-modulated version of stimulated Raman (gain/loss) spectroscopy is employed for these measurements. A phase-modulated signal wave is generated at the probe frequency by use of two orthogonally polarized phase-modulated pump waves. With this technique a signal linearly proportional to either the real or imaginary part of the third-order susceptibility can be selected. High-resolution (10 MHz) cw laser sources are used in these measurements