Absolute cross sections for the transitions of the Kr atom into the 4s land 4p4nl states of the Kr + ion were measured in the 4s-electron threshold region by photon-induced fluorescence spectroscopy (PIFS). The cross sections for the transitions of the Kr atom into the 4sland 4p4nl states were also calculated, as weIl as the 4p4nln'I' doubly excited states, in the frame of LS-coupling many-body technique. The cross sections of the doubly-excited atomic states were used to illustrate the pronounced contributions of the latter to the photoionization process, evident from the measurements. The comparison of theory and experiment led to conclusions about the origin of the main features observed in the experiment.

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A method and apparatus have been developed for precise measurements of the ratios of total cross sections for electron-impact ionization and photoionization in rare gases based on the comparison of the total ion yields of the two ionization processes. Low uncertainties for the cross-section ratios are achieved above all by using a cryogenic electrical substitution radiometer as a primary detector standard in the soft-x-ray and vacuum ultraviolet spectral ranges in order to determine the impinging photon flux. On the basis of our measured cross-section ratios and well-known total photoionization cross sections we deduce absolute total electronimpact ionization cross sections of rare gases. As a result we present ratios of total electron-impact ionization cross sections to total photoionization cross sections and deduce total electron-impact ionization cross sections of neon in the energy range of electrons from 140 to 4000 eV and of photons from 100 to 1500 eV. Relative uncertainties as low as 1.3% for the cross-section ratios and 2.8% for the total electron-impact ionization cross sections have been achieved. A comparison of our cross-section data with published experimental and theoretical data is presented.

Resonancelike structures have been observed by means of photon-induced fluorescence spectroscopy in the Ar-35 photoionization cross section between the threshold and the Cooper minimum. They coincide with resonances classified in published absorption spectra of Ar by 3s^3p^nln'r excitations. Moreover, structures were found to correlate with resonances in the satellite production also studied by photoninduced fluorescence spectroscopy.PACS numbers: 32.80.FbResonance phenomena are prominent features of the interaction of photons with atoms. They are present in strong ionization channels,^ as well as in weak satellite processes,^ and stem from Rydberg series of the neutral atom. Interchannel interactions cause these discrete states to autoionize into continuum states which are degenerate in energy with states reached by direct photoionization from the ground state. The investigation of resonances can yield detailed information about the manifold of electron correlations.Resonancelike phenomena have not been observed so far in the photoionization of the AT-3S electron ^"^ though absorption spectra of Ar show pronounced window and absorption resonances above the AT'3S threshold.^ Also, until very recently,^ the published calculated AT'3S photoionization cross sections^"^^ showed a smooth passage from threshold on through the Cooper minimum. Thus they reproduced the main experimental features known so far. The calculations by Wijesundera and Kelly ^ include for the first time two-electron excitations which strongly influence the Ar-35 photoionization cross section in the form of narrow resonances. Though the selected doubly excited Ar configurations taken into account in these calculations are clearly not suflftcient -as will be shown in the following-the calculations certainly underline again the importance of electron correlations for an understanding of the photon atom interaction. It is demonstrated hereby that even a well studied process like the AT-3S photoionization is not understood completely. The implication of the presented results will hold also for photoionization of the outer s shell of the heavier rare gases. Moreover, new experiments with Ar, Kr, and Xe will be stimulated.We report in the following the first experimental evidence of the presence of resonances in the energy dependence of the Ar-B^ photoionization cross section. They have been observed for photon energies between the threshold and the Cooper minimum. Photon-induced fluorescence spectroscopy (PIFS) was used in the experi-