The two-photon decay of metastable atomic hydrogen

The two-photon decay of the 22S½ state of atomic hydrogen has been directly observed using a fast coincidence counting technique. A hydrogen metastable beam apparatus has been constructed using a duoplasmatron proton source and charge-exchange in cesium vapour, and its characteristics measured. Tests are described which firmly establish the existance of the two-photon decay of atomic hydrogen, and the observed lifetime due to this process is consistent with the predicted value. The angular distribution of the emitted photons does not disagree with the predicted (1 + cos2e) factor.