Single ionization of He by 1 keV electron impact in the presence of an intense ($I=4\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$) laser field ($\lambda =1064\mathrm{nm}$) has been explored in a kinematically complete experiment using a reaction microscope. Distinct differences in the singly to fully differential cross sections compared to the field-free situation are observed which cannot be explained by a first-order quantum calculation. Major features, such as the number of photons exchanged and the modification of the energy spectrum of emitted electrons, can be understood qualitatively within a simple classical model.

• Received 30 July 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.153201