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The hook method (distortion of interferometric channel-spectra in the neighbourhood of absorption lines by anomalous dispersion) has been used to measure the gf values for Ca I, Sr I and Ba I. The gf values of lines in the principal series of Mg I, Ca I and Sr I have been plotted by the quantum-defect method to obtain a value of the photoionization cross section at the threshold. Extrapolations of the bound state gf values for Mg I, Ca I and Sr I yield cross sections at threshold of 2.36, 2.75 and 11.2 Mb respectively, all about two times larger than previous values based on vapour-pressure data.
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Parkinson et al. (1976) studied this question.
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