Neutron-photon studies of condensed matter

A theory of simultaneous photon absorption and inelastic neutron scattering is developed by treating the photon and neutron-matter interactions perturbatively. The leading-order mixing between the interactions shows that the neutron scattering cross-section is proportional to the dynamic structure factor (or Van Hove function) evaluated at an energy that is enhanced by the photon energy. The photon induced modification of the scattering vector is negligible. Thus, the proposed technique affords the possibility of measuring the dynamic structure factor at large energies and modest wavevectors which is a domain that is usually difficult to access because of kinematic constraints. The theory is developed in detail for some models of nuclear and magnetic systems. The results show that, in most cases, the experiments are likely to demand the use of very high intensity light sources. A particularly promising application appears to be in the study of electron plasmas since, using readily available pulsed lasers, the neutron cross-section is comparable with that for pure magnetic scattering.