Laser-Induced Ionization of Na Vapor: A new Mechanism for Producing Na2+

The production of Na2+ ions by off-resonant laser excitation in the 5800-6200 Å region mainly results from two-photon absorption by the Na2 molecule to highly excited gerade states followed by (a) direct ionization by absorbing a third proton or (b) coupling to the molecular Na2 D1πu Rydberg state which is subsequently ionized by absorbing a third photon. This mechanism, i.e., a two-photon resonance three photon ionization process, explains a recent experimental observation of Roussel et al.[1] It is suggested that the very same mechanism is also responsible for a similar observation reported by Polak-Dingels et al.[2] in their work using two crossed Na beams. In the latter two studies the laser-induced associative ionization processes were reported to be responsible for producing the Na2+ ion. From the ratio of molecular to atomic concentration in the crossed beam experiment [2] we estimate that the cross section for producing Na2+ through laser-induced associative ionization is at least four orders of magnitude smaller than ionization through the two-photon resonance three photon ionization process in Na2 molecules.