A Weingartshofer

Electron scattering in intense laser fields

The authors report quantitative measurements of multiphoton free-free (FF) transitions in a controlled three-beam experiment: electrons elastically scattered from argon in the presence of a CO2 laser pulse having an average flux density between 106 and 108 W cm-2. They present FF spectra for four incident electron energies, two scattering angles, and two directions of the laser polarisation vector. Electrons that have changed their kinetic energy by as many as eleven photons have been detected. From the spectra the absolute average flux density of the laser pulse was determined and the validity of the sum rule examined. Both results are indirect verifications of the theory.

Angular dependence of the elastic resonance scattering of slow electrons on argon and krypton

In an electron scattering experiment the angular dependence of the resonance doublets due to the 2P3/2 and 2P1/2 negative ion states has been studied for argon and krypton in the elastic channel. With an energy resolution of the spectrometer of slightly better than 30 meV (FWHM) it was possible to localize the energy positions of the 2P3/2 states to 11.07 eV for argon and 9.47 eV for krypton within 10 meV accuracy. The measured separation between the doublets was 172+or-5 meV for argon and 637+or-5 meV for krypton. A phase shift fit was performed on the angular dependences of the resonance effects and the potential scattering. Analysis of the results gave a natural width at half-height for the 2P3/2 resonances which lies between 3 and 4 meV in the argon case and is about 8 meV for krypton. For the best fit to the experimental data we found the following potential scattering phase shifts eta 0=4.99, eta 1=2.53, eta 2=1.17, and eta 3=0.10 for argon and eta 0=5.37, eta 1=2.54, eta 2=1.40, and eta 3=0.17 for krypton.

Simultaneous electron-photon excitation of He 23S: an experimental investigation of the effects of laser intensity and polarisation

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 CO2 laser delivering approximately 108 W cm-2 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 calculations.

Simultaneous electron-photon excitation of metastable states of rare-gas atoms

The authors report data demonstrating the laser-assisted electron excitation of the 21S state of helium and the 23P states of neon and argon in the field of a pulsed carbon dioxide laser. These are the first measurements of the simultaneous electron-photon excitation cross sections of atomic states other than the 23S state of helium. Comparison of the relative cross sections for these excitations with that for the He 23S state is made.

Multiphoton Free-Free Transitions

In an electron-atom (molecule) scattering process the electron changes its direction; it is accelerated and so it can emit electromagnetic radiation, so-called Bremsstrahlung. For electrons with energies in the order of 10eV the spontaneous emission of a photon with a measurable energy has such a low probability that it is totally negligible. However, in a sufficiently intense external laser field we can have induced emission and absorption; and if the intensity is moderately high, very distinct non-linear processes can be observed in which the scattered electrons change their energy by integer multiples of the photon energy. Because in such transitions the electron proceeds from one continuum state to another, these processes are also called free-free (F-F) transitions [1,2] and can be represented as

Direct observation of multiphoton processes in laser-induced free-free transitions

{e^ - }\left( {{E_i}} \right) + Atom + Laser \to {e^ - }\left( {{E_i} \pm N\hbar \omega } \right) + Atom + Laser.

Absorption and emission of radiation during electron excitation of the 2 sup 1 S and 2 sup 1 P states of helium

The cross-sections for free-free processes in electron-argon scattering in the presence of a pulsed CO2 laser have been measured as a function of laser flux. This represents the first attempt at an experimental investigation of the theoretical predictions contained in the Kroll-Watson equation. The dependence of the cross-sections on the various experimental parameters is discussed and we stress the advantages of using moderate laser powers (107W/cm 2) to carry out these tests of the fundamental interactions between free electrons and photons in the presence of an atomic potential.