Nonsequential two-photon absorption from the K shell in solid zirconium

We report the observation of nonsequential two-photon absorption from the K shell of solid Zr (atomic number Z = 40) using intense x-ray pulses from the Spring-8 Angstrom Compact Free-Electron Laser (SACLA). We determine the generalized nonlinear two-photon absorption cross section at the two-photon threshold in the range of 3.9-57 x 10(-60) cm(4) s bounded by the estimated uncertainty in the absolute intensity. The lower limit is consistent with the prediction of 3.1 x 10(-60) cm(4) s from the nonresonant Z(-6) scaling for hydrogenic ions in the nonrelativistic, dipole limit.