Double multiphoton ionisation via above-threshold ionisation in strontium atoms

Simple and double ionisation of a strontium atomic beam have been studied with a picosecond dye laser around 565 nm with intensities ranging from 10 to 1000 GW cm -2 . Resonances observed on the different ion signals as well as time-of-flight electron energy spectra clearly show that double ionisation is a two-step process which can involve above-threshold ionisation followed by resonant ionisation of the ion.