Penning collisions of laser-cooled metastable helium atoms

We present experimental results on the two-body loss rates in a magneto-optical trap of metastable helium atoms. Absolute rates are measured in a systematic way for several laser detunings ranging from -5 to -30 MHz and at different intensities, by monitoring the decay of the trap fluorescence. The dependence of the two-body loss rate coefficient β on the excited state ($2^3{\rm P}_2$) and metastable state ($2^3{\rm S}_1$) populations is also investigated. From these results we infer a rather uniform rate constant $K_{\rm sp}=(1\pm0.4)\times10^{-7}$ cm3/s