Ionization energy measurement of optically pumped rubidium molecules in a beam*

In this work, we have measured the ionization energy of Rb₂ molecules optically pumped in a supersonic beam. First, we use optically shaped broadband multimode diode lasers to rovibrationally cool the molecules trough transitions between the X¹Σ_g⁺ and the B¹Π_u  potentials. By optically shaping the multimode diode lasers, the X¹Σ_g⁺(v_X=0,J_X<10) states are dark, and the molecules are pumped into them. Then a pulsed dye laser excites the X¹Σ_g⁺(v_X=0,J_X<10) →B¹Π_u (v_B=4) transition, which is used because it allows better isotope discrimination (⁸⁵Rb₂). A second dye laser, synchronized with the first, excites the B¹Π_u (v_B=4) → X²Σ_g⁺(v_X=0) transition, photoionizing the molecule. The ionization happens at zero electric field, to avoid the excitation Rydberg molecular states. About 70 ns after the ionization, we apply a fast high voltage pulse to detect the molecular ions using a channeltron and a counter. The second laser is scanned in the 591 and 609 nm range. The results suggests that ionization energy is higher (∽60 cm^-1) than previous results in the literature, and they will be discussed during the presentation.