Neutral calcium, strontium and barium: determination of f values of the principal series by the hook method

Abstract

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.