Measurement of ionization threshold by electric field-dependent Rydberg state spectroscopy of Rb₂ molecule

Charge exchange processes between ultracold charged atom-dimer complex require the precise knowledge of ion molecule potential. This study presents the measurement of the ionization threshold of the ⁸⁵Rb₂ molecule in a supersonic beam, utilizing Resonance-enhanced two-photon ionization (RE2PI) with two distinct wavelengths. The first photon induces the X¹Σ_g⁺(ν_X=0) → B¹Π_u(ν_B=2) transition, while the second photon wavelength is varied across the range 16720-16750 cm^-1 to excite the B¹Π_u(ν_B=2) → X²Σ_g⁺(ν⁺) transition. Electric field-dependent Rydberg state spectroscopy allows us to discern the X²Σ_g⁺(ν⁺=0) state. The determined ionization threshold energy for the ⁸⁵Rb₂ molecule is E_i = 31497.8(3) cm^-1, with a dissociation energy of D₀= 6157.8(3) cm^-1, which is 149.8~cm^-1 higher than the value reported by Bellos et al. [Phys. Rev. A 87, 012508 (2013)]. The findings align well with theoretical models that consider Rydberg molecular states, confirming their accuracy in recent theoretical frameworks.