Multiple photoionisation of Cs and Ba atoms due to creation of 4d hole states

Multiple photoionisation processes of Cs and Ba atoms due to creation of 4d hole states have been studied by measurement of photoion-yield spectra (energy dependence of photoion yield). Monochromatised synchrotron radiation was used to excite the 4d-shell electron, and measurements were made in the energy regions from the neighbourhood of the 4d-nl resonance excitation to the higher-energy tail of the big 4d- in f shape resonance. The main products in these regions are doubly-charged (Cs 2+ , Ba 2+ ) and triply-charged (Cs 3+ , Ba 3+ ) ions, and an appreciable number of Cs 4+ and Ba 4+ ions starts being produced around the lowest photoelectron shake-off (4d -1 nl -1 ) and shake-up (4d -1 nl -1 n ' l ' ) states, respectively. Although the relaxation of the 4d hole states proceeds mainly through stepwise processes consisting of the normal Auger process and autoionisation, Auger shake-off processes are found to have considerable probabilities (typically 0.19-0.38 in the case of Cs) in the first stage of the relaxation. The shake-off probability associated with the 4d photoionisation in Cs estimated from the present data amounts to 0.33+or-0.05 in the region of the higher-energy tail of the shape resonance.