Fig. 2: Examples of vibronic spectra measured for ²²⁶RaF.

a–f, The counts on the particle detector were measured as a function of the laser wavenumber of the resonant step. A fixed wavelength (355 nm) was used for the ionization step. a, The observed peaks corresponding to the vibronic spectra of the Δv = 0 band system of v″ = 0, 1, 2, 3, 4, scanned by the grating Ti:sapphire laser. b, c, The pulsed dye laser was used to scan electronic transitions in different wavelength ranges: the Δv = +1 band system of the A²Π_1/2 ← X²Σ⁺ transition with v″ = 0, 1, 2, 3, 4 (b) and the (v′, v″) = (0, 1) and (1, 2) band. d–f, The corresponding transitions to other electronic states: A²Π_3/2 ← X²Σ⁺ (d), B²Δ_3/2 ← X²Σ⁺ (tentatively assigned; e) and C²Σ⁺ ← X²Σ⁺ (f). The shape of the spectra is due to population distribution of different rotational states. The solid lines show the fit with skewed Voigt profiles. g, Scheme of the molecular energy levels. The estimated upper limit of the ionization potential (IP) is indicated. Three essential properties for laser cooling of RaF molecules were identified: 1) the short lifetime of the excited states ²Π_1/2 (T_1/2 < 50 ns), which will allow for the application of strong optical forces; 2) dominant diagonal transitions, (Δv = 0)/(Δv = ±1, Δv = 0) > 0.97, indicating a large diagonal Franck–Condon factor; and 3) the expected low-lying electronic states B²Δ_3/2, A²Π_3/2 and C²Σ⁺ were found to be above the A²Π_1/2 states, which will enable efficient optical-cooling cycles. Wavenumbers in the spectra are given in the rest frame of the molecule. In a–f, the error bars show the statistical uncertainties (1 standard deviation) for the number of resonantly ionized molecules obtained within each laser frequency interval.