Pondermotive effects on angular distributions of photoelectrons

We report the observation of intensity-dependent angular distributions of photoelectrons in multiphoton ionization of xenon by 1064-nm light in the intensity range of (0.5–5)×${10}^{13}$ W/${\mathrm{cm}}^2$. The intensity dependence is shown to arise from the scattering of the photoelectrons from the ponderomotive potential created by the intense optical fields. This phenomenon is a general, final-state effect which can severely restrict the usefulness of angular distribution measurements in high-intensity multiphoton ionization.