Time-Resolved Multiphoton Ionization Process of Xenon Investigated by Photoelectron Imaging Method

Femtosecond time-resolved multiphoton ionization dynamics of xenon was investigated using a homemade ion imaging detector. A comparison experiment comprising of the multiphoton ionization of Xe at 408 nm showed that the energy resolution of our homemade image detector was similar to that of a commercial detector. Under 272 nm femtosecond laser irradiation, photoelectrons with a kinetic energy of 1.57 and 0.26 eV, produced by three-photon ionization, corresponded to two different Xe+ spin states, respectively. For the ionization at 408 nm, an additional first-order above-threshold ionization of Xe was also observed. In the two-color femtosecond time-resolved experiments, the photoelectron kinetic energy spectra varied with the delay time between the pump and the probe. The photoelectron intensities produced by 3+1' and 4'+1 two-color multiphoton ionization schedules became stronger with an increase in the degree of overlap between the two laser beams. The kinetic energy of the photoelectrons produced by one-color multiphoton ionization showed obvious red shifts, which were modulated by the second laser beam. Depopulation of the excited states was also observed upon application of the second laser beam. The red shifts in the photoelectron kinetic energy spectra reflect the time-dependent dynamical modulation process of the laser induced ponderomotive effect in an atomic system