Observation of the Transition from Thomson to Compton Scattering in Optical Multiphoton Interactions with Electrons.

The energy and angular distribution of electrons ionized from neon and subsequently ejected from a high -intensity optical-frequency laser focus has been measured. The effects of the ionized electrons' interaction with high -intensity optical-frequency radiation are the focus of these studies. An electron in a relatively low-intensity optical -frequency (hbaromega~1 eV) plane wave oscillates at the field frequency about a constant average position. The acceleration of the electron during the oscillations causes the electron to radiate at the field frequency in a dipole radiation pattern. This scattering process, known as Thomson scattering in low -frequency fields (hbaromega<< rm mc^2, where mc^2 is the electron's rest energy of 511 keV), becomes Compton scattering as electron recoil becomes important at higher frequencies (hbaromega~ rm mc^2 or greater). If spatial variations are present in the incident field, the average position of the electron migrates from high- to low-intensity regions. The motion of the electron's average position is the same as the motion of a particle in an effective "ponderomotive" potential. Following ionization in a Gaussian laser focus, the force resulting from the ponderomotive potential causes electron ejection at 90 ^circ with respect to the laser vec k if the intensity is relatively low. At high field intensities, the standard description of Thomson scattering in an optical-frequency field breaks down. As an electron's motion becomes relativistic, Compton -like effects cause the electron to drift in the vec k-direction or recoil. This "high -intensity Compton scattering" results in electron ejection at an angle less than 90^circ from vec k. The experiments described in this thesis show a transition from the Thomson regime to the high-intensity Compton scattering regime at high laser intensities. Electrons interacting with relatively low laser intensities are observed at approximately 90^circ from vec k, as expected. Electrons interacting with high laser intensities are ejected with a significant component of momentum in the vec k-direction (25% of total momentum or 75^circ from vec k). The observed ejection angles are in excellent agreement with the predictions of high-intensity Compton scattering.