Physical Review A
It has recently been established that the radiative lifetime of the metastable 2S3 state of helium and the heliumlike ions is determined by single-photon magnetic dipole (M1) transitions to the ground state, rather than the two-photon process proposed by Breit and Teller. The theory of nl-n′l M1 transitions with n ′ is developed in the Pauli approximation and extended to two-electron systems. Terms arising from relativistic energy corrections and finite-wavelength effects are included. The results for hydrogenic systems are shown to be identical to those obtained in the relativistic four-component Dirac formulation. The coefficients in the Z-1 perturbation expansion of the 1s2sS3-1s2S1 M1 transition integral are evaluated through ninth order and used to calculate the M1 emission probabilities from the 2S3 state of the two-electron ions up to Fe XXV. The emission probability for neutral helium is 1.27 × 10-4 sec-1. The results are compared with recent solar coronal observations by Gabriel and Jordan, and with a measurement of the 2S3 state lifetime in Ar XVII by Schmieder and Marrus. © 1971 The American Physical Society.
Drake, Gordon W. F.. (1971). Theory of relativistic magnetic dipole transitions: Lifetime of the metastable 2S3 state of the heliumlike ions. Physical Review A, 3 (3), 908-915.