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.

Laser assisted electron excitation of the 23S state of helium is examined at electron energies near to the threshold for excitation in the field of a pulsed CO, laser delivering -10' W cm-' in the scattering region. The simultaneous electron-photon excitation cross section of helium is studied at various laser intensities and at different angles between the laser polarisation and the incident electron beam. These data are a first step towards elucidating the influence of various scattering parameters on this complex process and are compared with the results of recent calculations.

Energy and angular&dependences of the elastic and inelastic scattering of electrons from H2 have been measured in the energy range 10 to 16 eV and for scattering angles ranging from 10' to 120'. Three resonance series have been investigated starting at 11.30, ll. 50, and 13.63 eV, respectively. The last two series decay preferentially into one single electronic channel, while the first resonance (resonance I) decays into all energetically possible exit channels, i.e. , X'Z~, b Z'", B'Z'". Throughout, stress is laid on the importance of absolute cross sections and reported values, and excitation f unctions have been presented in absolute units. The cross sections for the elastic channel and some inelastic channels (X Z'", v = 1 and 2, and the dissociative continuum b Z'") are composed of resonant and nonresonant scattering and show pronounced and rapidly changing interference structures. Relative branching ratios for resonance series I in terms of absolute total cross sections have been listed for those channels where practically no direct scattering amplitude is present. Summarizing those results, one cgn state that the probabilities (branching ratios) for the decay of resonance series I into the three electronicstates X Z~, b Z'", and B Z'"of the H& molecule summed over all vibrational states of the exit channel are of the same order of magnitude.From the observed angular dependence of the scattered electrons, the configuration of the resonances has been determined and compared with existing theoretical predictions.