| The metastable 1s2s3 S1 state of helium is the longest-lived neutral atomic state with a lifetime of 7859 seconds. This property has important astrophysical applications in the determination of temperature and density conditions in low density sources such as planetary nebulae.;This value of the 1s2s3 S1 → 1s2 1 S0 transition rate evaluated in this work is 1.2724255998(6) x 10-4s-1, where the uncertainty in this result is given in parentheses.;This lifetime is determined by single photon relativistic magnetic dipole (M1) transition rates and is evaluated numerically using large basis size variational Hylleraas wave functions for all the heliumlike ions through the isoelectronic sequence up to Ar+16. The coefficients of a Z -1 expansion, based on the results from the variational calculation, are evaluated up to ninth order with the zeroth and first order coefficients being determined analytically. This Z-1 expansion is used to evaluate the lowest order M∣1 transition rates for heliumlike ions through the isoelectronic sequence from K+17 to Fm+98. The results for helium are compared with experimental measurements by Moos and Woodsworth(1975), recent experimental results by Hodgman et al.(2009) and high precision numerical calculations by Lach and Pachucki (2001). For heliumlike ions, results are compared with several electron beam ion trap measurements for heliumlike lithium, carbon, oxygen, neon, and sulfur. |
