Abstract. Free-free transitions during the scattering of electrons by helium in the presence of a linearly polarized CO 2 laser field are investigated both experimentally and theoretically. Signals for laser-assisted scattering at 22 eV with absorption or emission of up to two photons are measured at scattering angles between 20 o and 70 o , and are compared to the values obtained from an 11-state R-matrix Floquet calculation as well as using the low-frequency approximation of Kroll and Watson. The two sets of theoretical results are found to be in very good agreement for the scattering geometries considered in the experiment. The order of magnitude of the experimental results is reproduced by calculations with intensities in the region of 10 7 W cm −2 . Agreement is improved by averaging the theoretical results over the spatial distributions of the three beams as well as the temporal intensity profile of the laser pulse, and by allowing for some misalignment of the three beams in the experiment.

We have studied photoassociation of metastable 2 3 S1 helium atoms near the 2 3 S1-2 3 P2asymptote by both ion detection in a magneto-optical trap and trap-loss measurements in a magnetic trap. A detailed comparison between the results of the two experiments gives insight into the mechanism of the Penning ionization process. We have identified four series of resonances corresponding to vibrational molecular levels belonging to different rotational states in two potentials. The corresponding spin states become quasi-purely quintet at small interatomic distance, and Penning ionization is inhibited by spin conservation rules. Only a weak rotational coupling is responsible for the contamination by singlet spin states leading to a detectable ion signal. However, for one of these series Bose statistics does not enable the rotational coupling and the series detected through trap-loss does not give rise to sufficient ionization for detection.

The absolute cross section for associative ionization in H(1s) + H(3s) and D(1s) + D(3s) collisions has been measured, in the energy range 0.006–3.6 eV, in a merged beam apparatus. The 3s state is populated by exciting metastable atoms, with CW laser radiation, in a static field. For both isotopes, the cross section exhibits an E−1 behaviour at low energy, and a faster decrease above the ionization threshold of the 3s state. In the intermediate energy region, isotope-dependent oscillations of the cross section are observed, which we attribute to interferences between several reaction pathways.

We report on a joined experimental and theoretical study of differential cross-sections resulting from inelastic scattering of a monoenergetic electron by helium atoms in the presence of an intense carbon dioxide laser. In particular, we measured the signals of the scattered electrons during the simultaneous electron–photon excitation of He 21P state for the first three microseconds of the laser pulse. The signals were measured for an incident electron energy of 45 eV and showed a structure that emerged at small scattering angles. The latter was found to be sensitive to the nature of the transferred photons, as well as the intensity of the laser field. The experimental findings were supported by quantum calculations based on the second-order Born approximation in which the correlated electron–electron interactions were taken into account.

We study the role of Penning ionization on the photoassociation spectra of He(2 3 S1)-He(2 3 S1). The experimental setup is discussed and experimental results for different intensities of the probe laser are shown. For modelling the experimental results we consider coupled-channel calculations of the crossing of the ground state with the excited state at the Condon point. The coupled-channel calculations are first applied to model systems, where we consider two coupled channels without ionization, two coupled channels with ionization, and three coupled channels, for which only one of the excited states is ionizing. Finally, coupled-channel calculations are applied to photoassociation of He(2 3 S1)-He(2 3 S1) and good agreement is obtained between the model and the experimental results.