Abstract
Recent studies of multiphoton ionization1 indicate the presence of an anomalously strong coupling for high Z materials. The principal findings, obtained at intensities up to ~1016 W/cm2, are (1) an unexpectedly strong multiphoton coupling strength resulting in multiple ionization of target atoms, (2) a strong Z-dependence of the coupling, and (3) ion charge state distributions which in some cases resemble those characteristic of Auger cascades. The results of these experiments suggest, for the heavy materials, a multiphoton analog of the collective atomic inner-shell response observed in both single-photon photoionization2 and bremsstrahlung.3 The relatively slow decline observed in the abundance of high ion states as Z increases qualitatively corroborates such an interpretation.