With the help of a theoretical analysis supported by numerical simulations, we show that one can observe unambiguous relativistic effects in atom-laser interactions in the regime of ultrahigh intensities. These effects, which can be observed in the high frequency stabilization regime, differ from ordinary high- $Z$ relativistic corrections and also from those resulting from standard relativistic dynamics of free electrons in a plasma. Their signature is the occurrence of notable modifications in above-threshold-ionization spectra resulting from a two-color scheme involving simultaneously a high frequency and a low frequency field.

• Received 4 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.2882