Direct observation of a two-photon radiative ionisation collision in crossed Na-Ba atomic beams

The authors report the observation of a two-step process in which Na*(4d) is first populated by a radiative collision between Na(3s) and Ba*(6p) in the presence of an intense optical field i.e. Na(3s)+Ba*(6s6p ¹P)+h(cross) omega to Na*(4d)+Ba(6s^{2 1}S) followed by photoionisation of the excited sodium Na*(4d)+h(cross) omega to Na⁺+e. The peak ion intensity is observed at a laser frequency close to the energy difference between Na*(4d) and Ba*(6s6p ¹P), and the magnitude of the cross section for the first step agrees well with theory.