Deschepper, P., Lebrun, P., Palffy, L., & Pellegrin, P. (1981). New Approach To Lifetime Measurements of the Metastable 2s 1/2 Level in Hydrogen-like Ions - Application To the Phosphorus Ion. Physical review. A, Atomic, molecular, and optical physics, 24(3), 1633-1635. https://doi.org/10.1103/PhysRevA.24.1633 (Original work published 1981)