Influence of radiative interatomic collisions on dark-state cooling.

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We discuss the role of light absorption by pairs of atoms (radiative collisions) in the context of a model for velocity selective coherent population trapping with emphasis on its application to metastable triplet helium. Our main interest is the application of such dark-state cooling schemes to the regime where individual quantum states get multiply occupied, a regime which is sometimes referred to with the term atom laser. We show that, because of radiative collisions, for positive detuning of the driving light fields from an atomic resonance the conditions for proliferation of the occupation numbers of the lowest lying states can only be marginally met. Multiple occupation of these states only occurs if a very efficient sub-Doppler precooling mechanism is operative. In the case of negative frequency detuning the requirements on this sub-Doppler mechanism are less restricting, provided one avoids molecular resonances. {copyright} {ital 1996 The American Physical Society.}