Resonance structures in free-free cross sections for electron-helium scattering

The authors report data showing the excitation of the He- 1s2s2 2S resonance obtained in electron scattering experiments in the field of an intense carbon dioxide laser. Resonance structure are observed as essentially pure Breit-Wigner profiles for electrons scattering through 13' under conditions where the electrons decrease their kinetic energy by the equivalent of one and two laser quanta. The experimental results are compared with calculations based on the low frequency approximation, the resonance being described by the Fano formalism. The agreement between experiment and theory is found to be quite good.