New measurements of the cross sections of doubly excited resonances in calcium

Abstract

The authors report new cross section measurements, believed to be the first involving synchrotron radiation together with a 'hot-wire' diode detector, in which they have observed the doubly excited series of Ca I at a much improved signal to noise ratio using the radiation from the DORIS II electron accelerator and the HASYLAB facilities. The data provide new information on the 'vanishing width' effect in Ca I and are compared with several calculations of the cross section reported in recent literature. The best representation of the detailed structure appears to be achieved by the most recent combination of MQDT with the R-matrix method. All the calculations (R matrix and MBPT) underestimate the cross section in the 3d5p peak by 20-40%.