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 5x10^7 magnetically trapped 2S atoms at densities greater than 4x10^10 cm^-3 and temperatures below 100 \muK. We derive experimental values for the total 2S-2S two-body loss rate constant in this temperature regime. Our results are in the range of recent theoretical calculations. We also find experimental upper limits on the rate constant for loss due to inelastic 1S-2S collisions. 
David Landhuis, Lia Matos, Stephen C. Moss, Julia K. Steinberger,   Kendra Vant, Lorenz Willmann, Thomas J. Greytak, and Dan...