Inelastic collision rates of trapped metastable hydrogen

We report the first detailed decay studies of trapped metastable (2S) hydrogen. By two-photon excitation of ultracold H samples, we have produced clouds of at least 5×10⁷ magnetically trapped 2S atoms at densities greater than 4×10¹⁰cm^-3 and temperatures below 100μK. At these densities and temperatures, two-body inelastic collisions of metastable atoms are evident. Experimental values for the total two-body loss rate constant are K₂=1.8_-0.7^+1.8×10^-9cm³s^-1 at 87μK and K₂=1.0_-0.5^+0.9×10^-9cm³s^-1 at 230μK. These results are in the range of recent theoretical calculations for the total 2S-2S inelastic rate constant. The metastable clouds were excited in a gas of ground-state (1S) hydrogen with peak densities reaching 7×10¹³cm^-3. From the one-body component of the metastable decay, we derive experimental upper limits for K₁₂, the rate constant for loss due to inelastic 1S-2S collisions.