THEORY OF LASER-INDUCED ASSOCIATIVE IONIZATION OF ULTRACOLD NA

The rate coefficient for the laser-induced associative-ionization reaction of ultracold Na in an optical trap is calculated as a function of the trap laser detuning from the Na(2S1/2,F = 2)--> Na(2P3/2,F = 3) resonance. Aided by a knowledge of the excited-state potential-energy curves and of the nature of the free-atom optical-pumping process, we propose the following mechanism, which we call photoassociative ionization (PAI): 2Na(3SIGMA(u)+)-->(homegaBAR)Na2*(0g- and 1g)-->(homegaBAR)Na2**(1u)-->Na2+ + e-. Maxima in the calculated PAI rate coefficient occur at detunings that are simultaneously one-photon resonant with bound levels of the long-range 0g- state and two-photon resonant with the lower rotational levels of a vibrational level lying 9 GHz below the dissociation threshold of the autoionizing 1u state. The calculated PAI spectrum (PAI rate coefficient versus trap-laser detuning) displays a series of broad peaks between -0.5 and -4 GHz detuning and a cutoff at -5 GHz detuning, as does the experimentally measured spectrum of Lett et al. [Phys. Rev. Lett. 67, 2139 (1991)]. The broad widths of the peaks in the PAI spectrum is due in part to the orientation averaging of the collision vector with respect to the electric-field vector and to the optical pumping of the Na2-->Na2* rovibronic transition. The calculated PAI rate coefficient at -0.6 GHz detuning is a factor of 4 higher than the experimental value. Fine-structure-changing transitions play a role in the doubly excited states, because of the 1u avoided crossing, but not in the intermediate states, because they have gerade symmetry and the only states that have been shown to undergo fine-structure-changing transitions with large probability have ungerade symmetry. Several predictions based on the proposed model and suggestions for future experiments are discussed.