  We present the first experimental observation of above-threshold ionization
(ATI) electrons produced by ionization of the neon K-shell in a laser field
where intensity exceeds 10$^{20}$ W/cm$^{2}$. An array of plastic scintillating
calorimeter detectors was used to measure the high-energy electrons at four
angles in the laser forward direction. Coarse energy resolution was obtained
using aluminum filters of several thicknesses to block lower-energy electrons.
A threshold intensity around $2 \times 10^{20}$ W/cm$^{2}$ is observed for
production of energetic ATI electrons in the laser forward direction, with
maximum electron energy exceeding 10 MeV. L-shell electrons with energies < 1.4
MeV are scattered further forward along the laser direction than expected. We
present comparisons of the measured total electron energies to the predictions
of a Monte Carlo models employing the ADK-PPT ionization model and the Augst
barrier suppression ionization model.

Atomic Physics