Double photoionization of alkaline earth atoms and photoelectron spectroscopy of reactive intermediates

Double photoionization (DPI) of the outermost s2 electrons of Ca and Sr atoms in the vapour phase has been studied by measurements of the angular correlation between the directions of emission of the two electrons. This process has been studied using monochromatic radiation produced by the second generation Daresbury synchrotron radiation source (SRS). The extremely low cross sections of the DPI processes were enhanced by using incident radiation at wavelengths corresponding to autoionizing resonances of the neutral species. Measurements of DPI for Ca atoms have also been collected at a non resonant photon energy using the third generation Elettra SRS in Trieste. Discrepancies between the present experimental results and the limited existing theory are found for the non resonant process. Two-step double photoionization via intermediate excited ionic states has been studied in both Ca and Sr atoms. Measurements of angular distributions and angular correlations between the directions of emission of inner shell photoelectrons and the associated Auger electrons allowed detailed characterizations of the photoionization processes. Valence photoelectron spectroscopy of the short-lived reactive intermediates O2(a1Δg), OH and OD has been performed using the Daresbury SRS. Autoionizing resonances have been assigned to members of series of Rydberg states converging towards higher ionic limits. Measurements of angular distributions of photoelectrons allowed further insight into the photoionization processes to be obtained. Suggestions are presented for further experimental investigations. (author)