Multi-MeV Electrons from Above-Threshold Ionization of the Neon K-shell

We present measurements of integrated electron energies produced by above-threshold ionization (ATI) of neon in a laser field with intensity exceeding 10^20 W/cm^2. We observe electrons with energy exceeding 10 MeV ejected in the laser forward direction above a threshold intensity of 2 × 10^20 W/cm^2. We compare to ATI models using both tunneling (ADK-PPT) and barrier suppression ionization and observe the onset of ATI at a higher threshold intensity than predicted by these models.