PONDEROMOTIVE EFFECTS IN ZERO KINETIC-ENERGY PHOTOELECTRON-SPECTROSCOPY WITH INTENSE FEMTOSECOND PULSES

Zero kinetic energy photoelectron (ZEKE) spectroscopy may be an intere sting technique for monitoring the time evolution of state selected at omic 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 tu nable femtosecond UV pulses. High intensity introduces a new broadenin g due to ponderomotive effects in the ZEKE spectrum. It is demonstrate d that a three-photon ZEKE spectrum can be recorded at intensities whe re the broadening due to these ponderomotive effects (i.e. the ac Star k shift for high-n Rydberg states) is small in comparison with the eff ective laser bandwidth.