Two-photon resonant multiphoton ionisation spectrum of Na₂

The production of Na 2 + by laser excitation in the 580-620 nm region mainly results from two-photon absorption of the Na 2 molecules to highly excited gerade state(s) and are subsequently ionised by a third photon. This mechanism, i.e. a two-photon resonance three-photon ionisation process, may account for several recent experimental observations in laser-irradiated sodium vapours. The assignment of the multiphoton ionisation spectrum and the dependence of the yield of Na 2 + on the Na 2 vapour density and the laser power density are presented.