Laser trapping of metastable 3He atoms:: Isotopic difference in cold Penning collisions

By confining He-3 and He-4 in their respective magneto-optical traps. ionization rate coefficients for Penning collisions between two cold He(2s(3)S(1)) atoms have been measured. At 0.5 mK, the rate coefficients obtained are (3.8 +/- 1.1) x 10(-10) cm(3)/s for He-4-He-4 collisions and (1.1 +/- 0.4) x 10(-9) cm(3)/s for He-3-He-3, a difference of a factor of 3. Theoretical calculations have shown that such a large difference arises only when the lowest order partial wave alone contributes to the ionization, and that it is mainly due to the difference in the quantum statistical symmetry between He-3 and He-4. [S0031-9007(99)08899-7].