Possibility of experimental separation of resonances and cusps from background in electron scattering

It is shown, with the help of model calculations, that a laser field can be used to suppress the background in electron scattering cross sections and to pick out only those parts which are rapidly varying as functions of the incoming electron energy. Therefore it becomes possible to measure the pure Breit-Wigner peaks and threshold effects, independent of the fact that interference between the resonances and the background in radiationless electron scattering is strong. The results are interpreted within the low-frequency limit for free-free transitions. The ac Stark shift of the resonance is also observed