2-PHOTON IONIZATION OF AN INNER-SHELL ELECTRON OF THE CL ATOM

We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine for E = 1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a Is electron moves to an empty p-orbital, and those in which an electron from an occupied p-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of a p-electron into the continuum. The second class of intermediate states is followed by the transfer of a Is electron into an empty p-state. The largest contribution comes from 3p --> d-continuum followed by Is --> 3p transition. Our result is sigma((2))/I = 2.06 x 10(41) cm(4)/W where sigma((2)) is the two photon ionization cross-section and I is the light intensity.