Effect of a magnetic field on the resonant multistep selective photoionization of gadolinium isotopes

A multistep photoionization with three resonant polarized photons has been used to separate the odd and even isotopes of gadolinium. Due to their hyperfine structure (I= 3 2 ), the 155,157Gd isotopes can be photoionized via a J=2→2→1→0 scheme with three п photons whereas the even isotopes cannot. If a time delay is introduced between the three laser pulses we show that the presence of a low intensity dc magnetic field affects the selectivity. The effect of this weak field on the 156Gd photoionization rate has been calculated independently of the optical pumping and the result is in good agreement with the experiment.