We report the first detailed decay studies of trapped metastable $\mathrm{}\left(2S\right)\mathrm{}$ hydrogen. By two-photon excitation of ultracold H samples, we have produced clouds of at least $5\times {10}^7$ magnetically trapped $2S$ atoms at densities greater than $4\times {10}^{10}{\mathrm{cm}}^{-3}$ and temperatures below $100\mu \mathrm{K}.$ 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 $K_2{=1.8\mathrm{}}_{-0.7}^{+1.8\mathrm{}}\times {10}^{-9}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ at $87\mu \mathrm{K}$ and $K_2{=1.0\mathrm{}}_{-0.5}^{+0.9\mathrm{}}\times {10}^{-9}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ at $230\mu \mathrm{K}.$ 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 $\mathrm{}\left(1S\right)\mathrm{}$ hydrogen with peak densities reaching $7\times {10}^{13}{\mathrm{cm}}^{-3}.$ From the one-body component of the metastable decay, we derive experimental upper limits for $K_{12},$ the rate constant for loss due to inelastic $1S-2S$ collisions.

• Received 30 September 2002

DOI:https://doi.org/10.1103/PhysRevA.67.022718