This is the first high resolution measurement of the electron energy spectrum from high intensity above-threshold ionization (pondermotive energy ≫ ionization potential ≫hν) at intensities up to 7×${10}^{15}$ W/${\mathrm{cm}}^2$. In contrast to previous work, we have used a short pulse and a large focal spot along with low gas densities to minimize the effect of ponderomotive forces and collisions. Existing models do not agree well with electron energy spectra for either linearly or circularly polarized light. The electron temperature is too hot for the implementation of 820 nm multiphoton ionized recombination type XUV laser schemes.

• Received 16 November 1992

DOI:https://doi.org/10.1103/PhysRevLett.71.509