Experimental and theoretical study of free–free electron–helium scattering in a CO₂laser field

Abstract

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.

Simultaneous electron–photon excitation of helium in a CO2laser field

“…In the work reported here, we employ a set consisting of the eleven lowest states of helium, obtained by optimizing energies and oscillator strengths using the atomic structure package CIV3 [31]. The corresponding wave functions have been used in a recent study of free-free electron-helium scattering in a CO 2 laser field [4]. The wave functions are constructed from a set of twelve orbitals: 1s, 2s, 2p, 3s, 3p, 3d,4s,4p,4d,4f,5s,5p, where the bar denotes a pseudo-orbital.…”

“…The target state energies are presented in table 1, where they are compared with the most accurate values available [32]. We note in passing that the energies presented in columns 3-5 of table 2 in reference [4] are incorrect, and should be replaced by the values given here. The excitation thresholds are also compared to the values recommended by the National Institute of Standards and Technology (NIST) [33].…”

“…Most studies of low-energy laser-assisted electron-atom scattering have concentrated on free-free transitions, in which the target remains in its initial state after the collision (see for example [1][2][3][4][5] and references therein, as well as the reviews [6,7]). The target simply plays the role of a third body whose presence allows the electron in the field to absorb or emit photons.…”

“…In contrast, R-matrix Floquet theory [24][25][26] provides a rigorous approach to laser-assisted electron scattering by a general atomic target. In addition to free-free scattering [3,4,27,28], the method has been applied in a preliminary study of the SEPE process for electron-helium scattering in a Nd:YAG laser field, where the dominant influence of the He − (1s2s 2 2 S) resonance was demonstrated [29], as well as to excitation in a CO 2 laser field in connection with the feasibility of observing selection rules when the laser polarization axis is perpendicular to the scattering plane [30]. In this paper, we apply the theory to the study of the SEPE process for helium in the presence of a linearly polarized CO 2 laser field at scattering energies close to the lowest field-free excitation thresholds, for laser intensities between 10 7 and 10 8 W cm −2 .…”

Control of atomic dynamics in laser-assisted electron-atom scattering through the driving-laser ellipticity

“…Ref. [34], and references therein.) Besides having the appropriate ratio u p /(hω) 1 and a higher duty cycle (up to D ∼ 10 −8 ), the use of such powerful pulses holds the possibility of increasing the interaction volume V by orders of magnitude.…”