Cold collisions of spin-polarized metastable hydrogen atoms

Cross sections and rate coefficients for collisions of two spin-polarized metastable hydrogen atoms are calculated at low energies and temperatures using complex scaling and Feshbach projection methods for the determination of the molecular potential-energy curves. The inclusion of the Lamb shift and fine-structure splitting of the excited n=2 atoms leads to an attractive van der Waals potential for two 2s(1/2) atoms with an exceptionally large coefficient. The quenching of the metastable atoms is dominated at temperatures below 20 mK by ionization. The calculated values for the total loss rate coefficient at 87 muK and 230 muK are within a factor of 2 or 3 of the error bars of recent measurements of metastable loss rate coefficients.