Abstract
About 104 triplet metastable helium atoms have been trapped in a magneto-optical trap, based on the 23S1–23P2 transition at 1.083 μm (volume 0.1 mm3, temperature 1 mK). The losses of the trap are analysed by monitoring the ion current due to Penning collisions either with background gases, or between trapped atoms. The rate coefficient for these collisions between trapped metastable helium atoms has been found to be unusually large, and the crucial role of the quasi-resonant laser light has been experimentally demonstrated. The rate coefficient of the dominant process (He*(23S1) + He*(23P2) Penning collisions) has been measured by two different methods, and found to be Ksp ≃ 10-7 cm3 s-1 within an estimated factor of 4. The corresponding cross-section is 6 · 106 Å2.
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