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The influence of linearly and circularly polarized laser fields on the dynamics of fast electron-impact excitation in atomic helium is discussed. A detailed analysis is made in the excitation of 2 1 S, 3 1 S and 3 1 D dressed states of helium target. By using a semiperturbative treatment with the Sturmian basis expansion, we take into account the target atom distortion induced by a laser field. Important differences appear between the angular distributions depending on the different states of polarization, in particular the circular polarization presents an experimental interest. We give new features (intermediate resonances) for both polarizations, concerning the n = 2 states of helium for emission and the n = 3 for the absorption, in term of laser frequency. Qualitative differences from the case of laser-assisted elastic collisions have been evidenced.

The differential cross section for electron-hydrogen atom collisions in the presence of a CO2 laser field is analysed as a function of the incident electron energy. We show that the criticism of the second Born treatment of elastic scattering and excitation by fast electrons is unjustified for various geometrical configurations. Detailed calculations of the scattering amplitudes are performed by using the Sturmian basis expansion.

The second Born approximation has been used to calculate the triple differential cross sections for electron-impact ionization of atomic hydrogen in the presence of a linearly polarized laser field. The interaction of the laser field with the unbound electrons is treated in a nonperturbative way by the use of Volkov waves. We have used the wave function proposed by Joachain et al (1988 Phys. Rev. Lett. 61 165) for the ejected electron moving in the combined field of the residual + H photon and of the laser. We present a more precise treatment of the small-energy region of the incident electron by performing the calculation of the second Born term in this energy region. Detailed calculations of the scattering amplitudes are evaluated by using the Sturmian basis expansion. The second-order Born amplitude is capable of giving a reasonable correction to the first Born approximation model in studying laser-assisted (e, 2e) problems in the low energy range.

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