NonvanishingJ=1↔0equal-frequency two-photon decay:E1M2decay of the He-like2S13state

Abstract

The selection rule which forbids $J=1\ensuremath{\leftrightarrow}0$ two-photon decay when the energies of the two photons are equal is shown to apply only when the multipolarities of the two photons are the same (i.e., $2E1$, $2M1$, etc.) and does not generally apply to mixed multipolarity amplitudes such as $E1M2$. A calculation of the two-photon decay rate for He-like ions in the $2\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$ state including both $2E1$ and $E1M2$ two-photon decay amplitudes is presented. It shows a significant contribution for the case of equal-energy photons. The ratio of the $E1M2$ contribution to the $2E1$ contribution is found to be surprisingly large. For example, it is 0.27 for two-photon decay of the $2\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$state in He-like uranium.