OPTICAL COLLISIONS OF COLD, METASTABLE HELIUM-ATOMS

We have studied the optical collisions of cold, metastable helium atom s in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the asso ciative ionization rate is increased with more than a factor 20 due to the presence of the light field. We present a simple, semiclassical m odel, which accounts for all the observed features, and which agrees, also on an absolute scale, with our experimental results.