Decay channels after the photoexcitation of 3d electrons in Rb and Sr studied by photoion yield spectra

The relative ion yields for Rb n+ (n=1-3) and Sr n+ (n=1-4) have been determined using a combination of a time of flight mass spectrometer and monochromatised synchrotron radiation crossed with an atomic beam. The incident photon energies are around the threshold of 3d ionisation: 112-126 eV for Rb and 136-146 eV for Sr. For both Rb and Sr, the ion yield curves for multiply charged ions show prominent peaks corresponding to the excitation of 3d electrons to bound np (n=5, 6) states, while those for singly charged ions have no noticeable structure. At the resonance energies, Rb 2+ dominates among Rb n+ (n=1-3) ions and Sr 3+ among Sr n+ (n=1-4). It was found that excitation of the 3d electron followed by two or three successive autoionisations is an important possible route for the production of multiply charged ions.