The role of the electronic degrees of freedom in neutron Compton scattering from molecular systems

We show that the standard treatment of deep inelastic neutron scattering from moleculesimplicitly assumes the adiabatic approximation for both initial and final states. Since the use ofthis approximation in the aforementioned scattering regime has recently been questioned, wepropose a calculation scheme free from the adiabatic assumption in the final molecular state.This scheme generalizes the Gersch-Rodriguez-Smith approach, explicitly including theelectronic degrees of freedom and the Coulomb interaction, and provides analytical formulaefor the asymptotic response function and for the first final state effect correction. A practicalcalculation is performed on the simple H2 molecule. Results show an asymptotic term veryclose to the standard one, but a first final state effect term (that is proportional to the inverse ofthe momentum transfer) exhibiting large discrepancies with respect to its equivalent derivedfrom a semi-empirical internuclear potential.