Mechanism for Two-Roton Raman Scattering from Liquid Helium

We study couplings between the phonon-roton excitations of liquid helium and an electromagnetic field. By including the electronic excitations of the helium atoms as intermediate states, we find a second-order two-roton Raman scattering amplitude which may be large enough to be observable. The roton scattering cross section as a function of energy transfer is calculated from the experimentally known roton spectrum and the pair-correlation function. The lowest-order infrared-absorption process involves (qa)² and the quadrupole-dipole interaction and is too small to be observed.