No abstract

Partial photodetachment and photoionization cross sections corresponding to highly excited residual atoms or ions are shown analytically to mirror one another in the neighborhood of a resonance. More precisely, any two groupings of partial cross sections are shown here to have components whose variations with energy near a resonance are equal in magnitude and opposite in direction. This work extends an analysis of Starace ͓Phys. Rev. A 16, 231 ͑1977͔͒ for the behavior of partial cross sections near a resonance to the case when the 2 parameter of Fano and Cooper ͓Phys. Rev. 137, A1364 ͑1965͔͒ tends to zero. ͓S1050-2947͑99͒50403-4͔

Eigenchannel R-matrix calculation results are presented for the photodetachment of K Ϫ in the energy region between the K(5s) and K(7p) thresholds. Present results are compared with prior experimental studies, including the recent relative K(5s) partial cross section measurements of Kiyan et al. ͓Phys. Rev. A 84, 5979 ͑2000͔͒, and resonances observed in electron-potassium scattering. Detailed analyses and identifications of 1 P o resonance structures are presented. Comparisons with H Ϫ and other alkali-metal negative ions provide further information on the structure and dynamics of negative ions.

New features are revealed in the low-energy photoionization spectrum of Ar by critically combining high photon resolution and differential photoelectron spectroscopic techniques. Two LS-forbidden doubly excited resonances are seen in the 3p(-1)(3/2, 1/2) partial cross sections which exhibit mirroring profiles, resulting in complete cancellation in the total photoionization cross section, as was predicted by Liu and Starace [Phys. Rev. A 59, R1731 (1999)]. These results demonstrate that a new class of weakly spin-orbit induced, mirroring resonances should be observable in partial, but not in total, collisional cross sections involving atoms, molecules, and solids in general.

The seeded Free-Electron Laser (FEL) FERMI is the first source of short-wavelength light possessing the full coherence of optical lasers, together with the extreme power available from FELs. FERMI provides longitudinally coherent radiation in the Extreme Ultraviolet and soft x-ray spectral regions, and therefore opens up wide new fields of investigation in physics. We first propose experiments exploiting this property to provide coherent control of the photoionization of neon and helium, carry out numerical calculations to find optimum experimental parameters, and then describe how these experiments may be realized. The approach uses bichromatic illumination of a target and measurement of the products of the interaction, analogous to previous Brumer-Shapiro-type experiments in the optical spectral range. We describe operational schemes for the FERMI FEL, and simulate the conditions necessary to produce light at the fundamental and second or third harmonic frequencies, and to control the phase with respect to the fundamental. We conclude that a quantitative description of the phenomena is extremely challenging for present state-of-the-art theoretical and computational methods, and further development is necessary. Furthermore, the intensity available may already be excessive for the experiments proposed on helium. Perspectives for further development are discussed.