Laser‐induced ionization of Na vapor

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Abstract
The production of Na2+ ions by off‐resonant laser excitation in the 5800‐6200Å 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 photon 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. It is suggested that the very same mechanism is also responsible for a similar observation reported by Polak‐Dingels et al 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 of Polak‐Dingels et al. 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.

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