Formation of doubly charged ions in the non-linear ionisation of alkali-earth atoms in the spectral range 15000-18700 cm⁻¹

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. 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 Ca 2+ ion formation.