J K Holmes

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.

Laser-assisted elastic electron-atom collisions: low electron energy and small scattering angle

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.

Differential cross sections for laser-assisted elastic electron scattering from argon

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.

Laser-induced free?free transitions in elastic electron scattering from CO2

This paper presents measurements of laser-induced free–free transitions of electrons scattered from CO2 molecules in the ground electronic state at incident electron energies of 3.8 and 5.8 eV under a pulsed CO2 laser field. The differential cross section of free–free transitions involving absorption and emission of up to two photons was measured at various scattering angles with the polarization of the laser either parallel with or perpendicular to the momentum change vector of the scattered electrons. The results of the parallel geometry are found to be in qualitative agreement with the predictions of the Kroll–Watson approximation within the experimental uncertainty whereas those of the perpendicular geometry show marked discrepancy with the Kroll–Watson predictions.

Low-energy electron - helium scattering in a laser field

We report measurements for the scattering of electrons by helium atoms through in the presence of a laser. The incident energy of the electrons was in the range 0.2 - 2.5 eV and the polarization of the laser was arranged to be parallel to the direction of the electron momentum transfer. The data were compared with the results of simple calculations assuming the Kroll - Watson approximation.

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.

Experimental and theoretical study of free–free electron–helium scattering in a CO2 laser field

Free–free transitions during the scattering of electrons by helium in the presence of a linearly polarized CO2 laser field are investigated both experimentally and theoretically. Signals for laser-assisted scattering at 22 eV with absorption or emission of up to two photons are measured at scattering angles between 20° and 70°, and are compared to the values obtained from an 11-state R-matrix Floquet calculation and from the low-frequency approximation of Kroll and Watson. The two sets of theoretical results are found to be in very good agreement for the scattering geometries considered in the experiment. The order of magnitude of the experimental results is reproduced by calculations with intensities in the region of 107 W cm−2. Agreement is improved by averaging the theoretical results over the spatial distributions of the three beams as well as the temporal intensity profile of the laser pulse, and by allowing for some misalignment of the three beams in the experiment.

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