Experimental investigations of resonant structure in doubly charged ion (A 2+ ) formation were carried out during Sr and Ba atom ionization by infrared laser radiation (ω = 8400-9100 cm −1 ). It has been shown that in this case the mechanism of ion formation is a two-electron process-the ions are produced immediately from the neutral atoms. It has been found for the first time that the resonant structure in A 2+ ion formation in such conditions is due to the excitation and ionization of neutral-atom mixed states. The atomic-state mixing in this case results from the Stark effect provided that the polarizability of these states is very high.

The results of measurements of the yield of singly and doubly charged ions against laser radiation frequency in non-linear Ca, Sr and Ba ionisation are presented in this paper. These data are analysed for the two-electron and cascade mechanisms of doubly charged ion formation. In the case of Ca ionisation, the experimental data imply the possibility of both cascade and two-electron mechanisms of Ca2+ ion formation.

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