Zero-Quantum-Defect Method and the Fundamental Vibrational Interval of H2+

The fundamental vibrational interval of H₂⁺ has been determined to be ΔG1/2=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 -R_H₂/n², 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)].