In the long-wavelength limit, above-threshold ionization (ATI) is primarily the result of the interaction of a newly freed electron with the laser field. Classical physics requires that linearly and circularly polarized light produce very different ATI spectra. Measurements performed using both linearly and circularly polarized, picosecond, 10-μm pulses confirm these conclusions.

• Received 28 September 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.1259