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 molecules implicitly assumes the adiabatic approximation for both initial and final states. Since the use of this approximation in the aforementioned scattering regime has recently been questioned, we propose a calculation scheme free from the adiabatic assumption in the final molecular state. This scheme generalizes the Gersch-Rodriguez-Smith approach, explicitly including the electronic degrees of freedom and the Coulomb interaction, and provides analytical formulae for the asymptotic response function and for the first final state effect correction. A practical calculation is performed on the simple H2 molecule. Results show an asymptotic term very close to the standard one, but a first final state effect term (that is proportional to the inverse of the momentum transfer) exhibiting large discrepancies with respect to its equivalent derived from a semi-empirical internuclear potential.