Ponderomotive Shifts Induced By Intense Laser-Driven Coherent Extreme Ultraviolet Radiation

Signatures of strong-field effects induced by an intense attosecond pulse train, synthesized by the superposition of a Ti:sapphire laser high-order harmonics are observed in atomic two-photon ionization. Ponderomotive shifts in the photoelectrons' kinetic energies have been measured in the non-resonant two-photon ionization of Helium atoms induced by the interaction with extreme ultraviolet radiation at intensities on the order of 10(15) W cm(-2). These results inaugurate the topic of strong-field phenomena in experimental laser based attosecond science.