Ionization Potential of the Helium Atom

The ground-state ionization potential of the ${\mathrm{He}}^4$ atom is found to be 5 945 204 223 (42) MHz. Along with lower-order contributions, this result includes all effects of relative order ${\alpha }^4$, ${\alpha }^3{m}_e{/m}_{\alpha }$, and ${\alpha }^5{ln}^2\alpha$. Effective operators derived in dimensionally regularized nonrelativistic quantum electrodynamics are employed. The average values of these operators are evaluated using a high-accuracy variational wave function constructed in an exponential basis.