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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Multiphoton free-free transitions are detected in the scattering of electrons on argon atoms and on hydrogen molecules in the presence of an intense pulsed CO, laser field. The authors present measurements for several values of the incoming electron energy, the electron scattering angle, and the angle between the laser polarization vector and the electron momentum transfer. All observations are in qualitative agreement with a recent theoretical model based on a low-frequency approximation.

An apparatus consisting of an electron energy selector built into a mass spectrometer is described. With it, initial ionization yield curves may be obtained. The interpretation of these curves is discussed, and the following new measurements reported:[Formula: see text]