The two-photon emission from the triplet metastable state of He I

The theoretical expression for the probability of two-photon decay of helium in the 2 3 S 1 metastable level is derived. It is found that this expression is quite different from the corresponding expression for the 2 1 S 0 -> 1 1 S 0 transition. It is shown that the transition probability computed by Mathis in 1957 is overestimated by a very large amount and that A (2 3 S 1 -> 1 S 0 ) less, similar 10 -10 s -1 .