Inner Shell Ionization and Ejection of Relativistic Electrons from High Power Laser Pulses

High power lasers have opened new vistas for light-matter interaction. With intensities more than 10¹⁹ W/cm², lasers interacting with rarefied gases can remove electrons from inner shells and accelerate them to relativistic speeds. For non-relativistic intensities, the ponderomotive force causes the ejection of electrons normal to the laser axis. At relativistic intensities, these electrons gain significant momentum along the direction of laser propagation and are ejected over a range of angles, θ, relative to the axis. There is strong evidence that as the intensity is increased, the electrons gain more energy and θ decreases. However, the exact relationship between the electron energy and the laser intensity is still complicated and not well known. Further, theoretical studies predicted a uniform and symmetric electron distribution around the laser axis for an ideal Gaussian focus and past studies have only measured it for varying θ along fixed azimuthal angles. In this presentation, we review the relationship between θ, the electron energy and the laser intensity, and share our recent work that, to the best of our knowledge, produced the first two-dimensional image of the ring-like electron ejection distribution from rarefied gas. We show that this distribution is highly asymmetric, plausibly due to aberrations in the laser focus. Our results may possibly enable a direct way to assess these aberrations and measure the intensity in the focus at full power.