We report photoelectron energy spectra from multiphoton ionization of xenon by 150-fs pulses wavelength tuned between 575 and 675 nm. The spectra show a number of structures that are interpreted as Stark-induced resonances. Measuring the energy of these structures for different wavelengths yields the intensity and wavelength dependencies of the corresponding Stark shifts. For intensities up to 3×${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{-}2}$, states between 7p and 6f were found to undergo giant shifts (up to 1.5 eV). These results are compared with the predictions of a second-order perturbation calculation.

• Received 3 May 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.2208