  We discuss the role of light absorption by pairs of atoms (radiative
collisions) in the context of a model for an atom laser. The model is applied
to the case of VSCPT cooling of metastable triplet helium. We show that,
because of radiative collisions, for positive detuning of the driving light
fields from an atomic resonance the operating conditions for the atom laser can
only be marginally met. It is shown that the system only behaves as an atom
laser 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.

Atomic Physics;Atomic Physics