Metastability of2sStates of Hydrogenic Atoms

It has been found by Breit and Teller that double photon emission is the principal cause of decay of interstellar hydrogen atoms from their metastable $2s$ state and the mean life corresponding to this decay was bracketed by them between upper and lower limits. In the present note an account is given of a more accurate evaluation of the transition probability $A_{\tau }=(8.226\mathrm{}\pm 0.001)Z^6$ ${\sec }^{-1\mathrm{}}$, where $Z$ is the atomic number. Relativistic effects on the atomic wave functions have been neglected in the calculations which are therefore accurate only for small $Z$.