This paper presents an experimental investigation of cold collisions between metastable Helium
atoms in an optical trap at 1mK. Penning (PI) and associative (AI) ionization reactions are distinguished
using a mass spectrometer and studied under influence of near resonant laser light. Sensitive behavior of the
ion rate is observed when the laser is tuned close to resonance. Experimental ?ndings are well described,
on an absolute scale, by a semi-classical model we have developed for optical collisions and by a modi?ed
Julienne-Vigu´e model
