Ionization Energy of Rb${}_2$ by electric field-ionization of molecular Rydberg states

We report the measurement of the ionization energy of the ${}^{85}{\text{Rb}}_2$ molecule through resonantly enhanced 2-photon ionization in a supersonic beam. The first photon excites the $X^1{\mathrm{\Sigma }}_g^{+}(v_X=0)\to B^1{\mathrm{\Pi }}_u(v_B=2)$ transition, while the second photon wavenumber is scanned over the 16720 cm${}^{-1}$-16750 cm${}^{-1}$ range, thus yielding a structured spectrum of Rb${}_2^{+}$ ions extracted by an electric field and recorded by mass spectrometry. We modeled the onset of the ionization signal as a function of the electric field strength between $18V\mathrm{/}cm$ and $180V\mathrm{/}cm$, leading to the Rb${}_2$ ionization energy $E_i=31497.3\pm 0.6$cm${}^{-1}$, and to the dissociation energy of the Rb${}_2^{+}$ ground state $D_0=6158.2\pm 0.6$ cm${}^{-1}$. Our measured value $E_i$ is found to be $149.3$ cm${}^{-1}$ larger than the one reported in the experiment by Bellos et al. [Phys. Rev. A 87, 012508 (2013)]. Our value of $D_0$ agrees with our theoretical determination using a quantum chemistry approach. Using a simple theoretical model, we assign unevenly spaced structures of the ionization spectrum to molecular Rydberg levels belonging to several series that converge to the lowest vibrational levels of Rb${}_2^{+}$.