Angular distribution of electrons elastically scattered from Ar in the presence of a 1.17 eV laser field

Experiments on free-free electron scattering by an Ar target are reported. The angular distribution of 350 eV electrons, elastically scattered at angles between $4^{\circ }$ and ${80}^{\circ }$ in the presence of 1.17 eV photons from a Nd:YAG laser, shows a pronounced maximum close to ${47}^{\circ }$ and is symmetric about this angle, as predicted by a Kroll-Watson approximation calculation for the experimental kinematics. The experiments test a term in the approximation that is a strong function of the magnitude and direction of the momentum transfer of the elastically scattered electrons.

Figure 1

Angular distributions of 350 eV electrons for single photon processes ($\ell =\pm 1$) calculated with the Kroll-Watson approximation (KWA). The results are given as the ratio of the free-free cross section to the field-free cross section, and all calculations are normalized to unity at ${45}^{\circ }$. The laser polarization direction lies in the scattering plane at ${135}^{\circ }$ to the electron beam (see Fig. 2). Calculations for three laser intensities are shown; note that all intensities less than $10{\mathrm{GW}/\mathrm{cm}}^2$ result in the solid curve.

Figure 2

Schematic of the experimental apparatus showing the relative directions of the laser beam, laser polarization, electron beam, and the momentum transfer. As the scattered electron detector is varied from $0^{\circ }$ to ${90}^{\circ }$ the momentum transfer sweeps through ${45}^{\circ }$ as shown; for ${90}^{\circ }$ scattering, it is perpendicular to the polarization direction.

Figure 3

Free-free angular distribution of 350 eV electrons scattered from an Ar target in the presence of a 1.17 eV laser field. The circles (red) are for a target density of approximately 1 Torr; the squares (blue) are for a target density of approximately 0.1 Torr. The solid (green) line is a Kroll-Watson approximation calculation for single-photon processes and polarization ${133}^{\circ }$ to the electron beam. The magnitude of the calculation in the figure corresponds to the best fit to the data. See text for details.