Dispersion of the electronic contribution to the third-order nonlinear susceptibility of H_2

We report a new value for the electronic contribution (the electronic hyperpolarizability) to the third-order nonlinear susceptibiity of H2. Our result is derived from a measurement of the ac (optical)-Kerr constant referenced to the resonant Q-branch susceptibility of D2. We apply a long-known dispersion formula to compare our value of the electronic hyperpolarizability with values determined by other nonlinear optical techniques. This dispersion formula is shown to account systematically for the results of other nonlinear techniques.