The measurement of the energy distribution of keV electrons backscattered elastically frommolecules reveals one or more peaks. These peaks are at nonzero energy loss and have an intrinsicwidth. The usual interpretation of these measurements is attractively simple and assumesbilliard-ball-type collisions between the electron and a specific atom in the molecule, and thescattering atom is assumed to behave as a free particle. The peak position is then related to the massof the scattering atom, and its width is a Compton profile of the momentum distribution of this atomin the molecule. Here we explore the limits of the validity of this picture for the case of electronsscattering from methane. The biggest discrepancy is found for electrons scattering from carbon. Forelectrons scattering from hydrogen the effects are substantial at relatively low incoming energiesand appear to decrease with increasing momentum transfer. The discrepancy is analyzed in terms ofthe force the atom experiences near the equilibrium position.
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