Rydberg constant ($R_{\infty }$)

The inverse-length constant in the infinite-nuclear-mass hydrogenic spectrum, $R_{\infty }=m_e{e}^4/[8{ϵ}_0^2{h}^{3}c]$. A nucleus of finite mass replaces it by $R_M=R_{\infty }\mu /m_e$, where $\mu$ is the electron-nucleus reduced mass. Some spectroscopic fits call $Z^2{R}_M$ an effective Rydberg constant; the charge-inclusive and reduced-mass-only conventions must be distinguished.