Wavelength dependence of multiphoton ionization of xenon

We have studied the multiphoton ionization of xenon atoms by 1600.3em0exfs pulses at intensities of 5×10¹² and 1.3×10¹³0.3em0exW∕cm² and present photoelectron kinetic energy and angular distribution spectra measured with a photoelectron imaging spectrometer. A noncollinear optical parametric amplifier allows us to tune the wavelength of the laser pulse over a range between 500 and 7000.3em0exnm. Resonant and nonresonant processes as well as channel switching effects have been observed in this intensity and wavelength regime. Mainly resonant $(5+1)$-photon ionization via $ns$, $nd$, and nd^' Rydberg states was studied in the region of 505--6020.3em0exnm. Resonance structures were found related to the two fine structure ionization channels Xe⁺0.2em0ex²P3∕2 and Xe⁺0.2em0ex²P1∕2. In addition the $(6+1)$-photon resonant ionization and nonresonant 5- and 6-photon processes could be studied.