Electron spectra from multiphoton ionization of xenon at 1064, 532, and 355 nm

We report an electron spectroscopy study of continuum-continuum transitions that follow multiphoton ionization. Xenon was ionized by 10-ns multimode pulses at wavelengths of 1064, 532, and 355 nm, at intensities up to ${10}^{13}$ W/${\mathrm{cm}}^2$. The photoelectrons are confined by a diverging magnetic field and energy analyzed by means of the time-of-flight technique. Our setup allows an acceptance angle of $2\pi$ sr at an energy resolution of 15 meV. At a wavelength of 1064 nm, 11-photon ionization is energetically allowed but we report spectra in which the 12- and up to 19-photon ionization processes show higher relative probabilities. The intensity dependence of the separate processes gives a result that deviates strongly from lowest-order perturbation theory. An interpretation of the width of the peaks in the electron spectra based on the ponderomotive force is proposed.