A METHOD FOR EXTRACTING NEAR-THRESHOLD PARTIAL CROSS-SECTIONS FROM THRESHOLD PHOTOELECTRON-SPECTRA

A method used to obtain the first state-selected cross sections for th e near-threshold photoionization of argon from an analysis of threshol d electron spectra is described. The yield of photoelectrons in the wi ngs of threshold peaks is normalized by division with the efficiency f unction of the penetrating field threshold electron analyser, convolut ed with a Gaussian photon beam profile. Although very non-uniform with in the first 150 meV, the analyser efficiency function was found to ha ve a particularly simple analytic form suitable for further numerical treatment. We have discussed the measured ratios of the two components of the lowest Ar+ 3p5 2P state in relation to the spectral repulsion theory of Fano and Cooper. The potential of the technique is illustrat ed on the 3p4 (3P)3d4F3/2 ion state cross section, showing enormous re sonant peaks. The limitation in the general applicability of the metho d is also discussed.