A method used to obtain the first state-selected cross sections for the near-threshold photoionization of argon from an analysis of threshold electron spectra is described. The yield of photoelectrons in the wings of threshold peaks is normalized by division with the efficiency function of the penetrating field threshold electron analyser, convoluted with a Gaussian photon beam profile. Although very non-uniform within the first 150 meV, the analyser efficiency function was found to have a particularly simple analytic form suitable for further numerical treatment. We have discussed the measured ratios of the two components of the lowest Ar + 3p 5 2 P state in relation to the spectral repulsion theory of Fano and Cooper (1965, 1968). The potential of the technique is illustrated on the 3p 4 ( 3 P)3d 4 F 3/2 ion state cross section, showing enormous resonant peaks. The limitation in the general applicability of the method is also discussed.
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Cvejanović et al. (1995) studied this question.