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The triple-photoionization cross section of atomic Li was measured near its threshold. Our results can be described by a threshold power law up to Ϸ5 eV with an exponent of 2.05͑25͒ which is consistent with a previous experiment and theoretical predictions. At higher energies, we find that the cross section is well described by the threshold law for double ionization. The present analysis is based on the notion that for sufficient excess energy, triple photoionization of Li is reasonably well described by a double ionization of the inner electrons followed by a shakeoff of the outer electron.

The photoion Li 2ϩ /Li ϩ production cross section ratio of ground-state atomic lithium has been measured for photon energies ranging from 80 to 424 eV. The absolute cross sections for the Li 2ϩ and Li ϩ yield are also derived. In this energy region, the Li 2ϩ /Li ϩ ratio reaches a plateau of about 1.0% before reaching a maximum of about 4.5%, then decreases slowly. Good agreement is found between the measured total photoionization cross sections of lithium and theoretical calculations. The Li 2ϩ /Li ϩ ratio is also compared to the He 2ϩ /He ϩ ratio from excited He(1s2s) for photon energies up to 70 eV above threshold. The branching ratio of Li 2ϩ to total Li ion production is also compared to the single-ionization cross section of electron impact on Li ϩ ions.

The photoion spectrum of atomic potassium was measured over the 3s → np excitation region with the photoion time-of-flight method and monochromatized synchrotron radiation. An unusual spectrum with paired windows structure was found instead of a simple regular Rydberg series. Such subsidiary windows have not been observed in the 3s → np resonances of Ar, which has a closed outer shell. Based on Dirac-Fock calculations, the dual window structure at 36.7 eV and at 37.4 eV was assigned to the 3s −1 3p 6 4s4p resonance. The line shape can be fitted by Fano's formula and the Fano parameters were obtained.

We have determined the Fano parameters and oscillator strengths of the triply photoexcited Li 2s 2 2p 2 P o autoionizing resonance in the Li ϩ and Li 2ϩ partial photoion yields. Using monochromatized photons we observed an asymmetric resonance profile not only in the Li ϩ but also in the Li 2ϩ partial cross section. The interaction between the triply excited state and the double-photoionization continuum takes place via a onestep double autoionization, since a two-step autoionization of the discrete state is energetically not possible. Our theoretical calculations using an R-matrix method for both the Li ϩ and Li 2ϩ partial cross sections show resonance profiles in good agreement with experimental result. ͓S1050-2947͑99͒50707-5͔