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arXiv · 2507.08634

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

Manuel Alejandro Lefrán Torres·David Rodríguez Fernández·Jaime Javier Borges Márquez·Marcos Roberto Cardoso·Luis Gustavo Marcassa·Amrendra Pandey·Romain Vexiau·Olivier Dulieu·Nadia Bouloufa-Maafa
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Abstract

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 $X^1Σ_g^+ (v_X = 0)\rightarrow B^1Π_u (v_B = 2)$ 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 $18 V/cm$ and $180 V/cm$, 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$_2^+$.

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Manuel Alejandro Lefrán Torres, David Rodríguez Fernández, Jaime Javier Borges Márquez, Marcos Roberto Cardoso, Luis Gustavo Marcassa, Amrendra Pandey, Romain Vexiau, Olivier Dulieu, Nadia Bouloufa-Maafa. 2025-07-11. Ionization Energy of Rb$_2$ by electric field-ionization of molecular Rydberg states. https://arxiv.org/abs/2507.08634

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