We report experimental measurements of the differential cross sections for scattering of electrons in the range 8-20 eV from helium and argon through scattering angles of 10 degrees -140 degrees in the presence of a high intensity ( approximately 108 W cm-2) carbon dioxide laser. The majority of the results reported are scattering from argon under conditions for which the Kroll-Watson approximation, usually applied to these types of measurements, predicts zero cross sections. The cross sections reported range up to approximately 12% of the appropriate field-free elastic scattering cross sections in strong disagreement with the theoretical predictions.

The authors report experimental measurements of electron spectra resulting from the scattering of low energy (6.2-32 eV) electrons by helium atoms through an angle of 9 degrees in the presence of a high intensity ( approximately 108 W cm-2)CO2 laser. The intensities of the additional peaks caused by the presence of the laser which occur separated from the elastic scattering peak by multiples of the photon energy are much greater than expected on the basis of calculations, using the Kroll-Watson approximation (1973). The intensity of these free-free transitions also increases with decreasing electron energy, again in disagreement with the calculations. The authors propose that the disagreement between the experimental results and those calculated is due to the electrons scattering from a helium target polarized by the laser field.

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Multiphoton processes are detected in the scattering of electrons on argon atoms in the presence of a strong C0 2 -laser field. The observations are in accordance with a recently developed semiclassical model.We present here what we believe to be the first direct observation multiphoton absorptions and emissions by electrons in a strong laser field. Measurements were made in an electron-argonatom scattering experiment in the field of a focused, pulsed C0 2 laser with a peak power of 50 MW. The following multiphoton absorption [Eq.(1)] and emission [Eq.(2)] processes were studied:^"(.E^+Ar + laser -<* e~ {E { +nhv) + Ar +laser;(1) e~(£ t .)+Ar + laser-e~(E { -nhv) +Ar +laser;where E { is the incident electron energy, hv is the energy of a laser photon, and n can be any positive integer. The one-photon (n = l) processes have recently been reported by Andrick and Langhans 1 using a 50-W continuous-wave C0 2 laser as a light source, which after focusing resulted in a flux density of 6x 10 4 W/cm 2 . At this flux density, a first-order perturbation expansion with respect to the laser field is suitable and provides in the soft-photon limit the following simple relation 2 between the one-photon absorption (emission) cross section <2a ff {1) /da and the cross section without laser field do el /dQ: da p { da with T 2 given by 5 =4.86xl0-13 X 4 F£ j [ €# fc^] 2 ,where the laser wavelength X is expressed in units of microns, the flux density F in units of watts per square centimeter, the incoming electron en-ergy E { in eV, and the polarization e is normalized according to ?-?=l such that, for all incoming and outgoing electron momenta p t . and py, the quantity in brackets in always between 0 and 1. In the present experiment, however, flux densities in the order of at least F = 10 9 W/cm 2 have been achieved in the scattering center. At these F values the quantity T 2 in Eq. (4) is about 50, which means that a perturbation expansion with respect to the laser field no longer applies and multiphoton processes are expected to contribute significantly. In the case of a C0 2 laser, however, a semiclassical soft-photon approach 3 " 5 can be applied, which yields the following cross-section formula for a free-free transition with a net absorption (emission) of n laser photonsHere J n (T) is the Bessel function of the first kind and order n, and T is given by Eq. (4). Clearly, if |r|« 1 and w = ±l, Eq. (5) reduces to Eq.(3), which shows the connection between the nonperturbative and the perturbative treatments of the laser field. From J 0 2 (*) + 2SJ" 2 (*) = 1,we note the sum rule (n< 0 correspond to emissions; n> 0 correspond to absorptions of a net number of nhv)

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