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The fundamental vibrational interval of H₂^+ has been determined to be G₁/₂=2191. 126 614 (17) cm^-1 by continuous-wave laser spectroscopy of Stark manifolds of Rydberg states of H₂ with the H₂^+ ion core in the ground and first vibrationally excited states. Extrapolation of the Stark shifts to zero field yields the zero-quantum-defect positions -R₇₂/n^2, from which ionization energies can be determined. Our new result represents a 4-order-of-magnitude improvement compared to earlier measurements. It agrees, within the experimental uncertainty, with the value of 2191. 126 626 344 (17) (100) cm^-1 determined in nonrelativistic quantum electrodynamic calculations V. Korobov, L. Hilico and J. -Ph. Karr, Phys. Rev. Lett. 118, 233001 (2017).
Doran et al. (Tue,) studied this question.