Ponderomotive effects in zero kinetic energy photoelectron spectroscopy with intense femtosecond pulses

Zero kinetic energy photoelectron (ZEKE) spectroscopy may be an interesting technique for monitoring the time evolution of state selected atomic products in ultrafast pump-probe experiments. We investigate here the effects of laser intensity by studying three-photon non-resonant ZEKE spectra of atomic xenon at the first ionization threshold with tunable femtosecond UV pulses. High intensity introduces a new broadening due to ponderomotive effects in the ZEKE spectrum. It is demonstrated that a three-photon ZEKE spectrum can be recorded at intensities where the broadenign due to these ponderomotive effects (i.e. the ac Stark shift for high-n Rydberg states) is small in comparison with the effective laser bandwidth.