Anharmonic Coherent Raman Scattering inH2

The direct observation of anharmonic effect in coherently driven molecular vibrations is reported for low-pressure ${\mathrm{H}}_2$. Under intense excitation of the gas by light waves of frequencies $\omega$ and ${\omega }^{\prime }$, both electrical and mechanical anharmonicities contribute to a coherent polarizability modulation at $2(\omega -{\omega }^{\prime })$. The corresponding anharmonic scattering dominates, at low pressure, the usually observed harmonic double scattering. We discuss application of the effect to the determination of the second-order polarizability derivative.