Here we report on spectroscopic measurements of the aluminum monofluoride molecule (AlF) that are relevant to laser cooling and trapping experiments. We measure the detailed energy level structure of AlF in the X 1 Σ + electronic ground state, in the A 1 Π state, and in the metastable a 3 Π state. We determine the rotational, vibrational and electronic branching ratios from the A 1 Π state. We also study how the rotational levels split and shift in external electric and magnetic fields. We find that AlF is an excellent candidate for laser cooling on any Q-line of the A 1 Π -X 1 Σ + transition and for trapping at high densities.The energy levels in the X 1 Σ + , v = 0 state and within each Ω-manifold in the a 3 Π, v = 0 state are determined with a relative accuracy of a few kHz, using laser-radio-frequency multiple resonance and ionization detection schemes in a jet-cooled, pulsed molecular beam. To determine the hyperfine and Λ-doubling parameters we measure transitions throughout the 0.1 MHz -66 GHz range, between rotational levels in the X 1 Σ + , v = 0 state and between rotational and Λ-doublet levels in all three spin-orbit manifolds of the a 3 Π, v = 0 state. We measure the hyperfine splitting in the A 1 Π state using continuous wave (CW) laser-induced fluorescence spectroscopy of the A 1 Π, v = 0 ← X 1 Σ + , v = 0 band. The resolution is limited by the short radiative lifetime of the A 1 Π, v = 0 state, which we experimentally determine to be 1.90 ± 0.03 ns. The hyperfine mixing of the lowest rotational levels in the A 1 Π state causes a small loss from the the main laser cooling transition of 10 −5 . The off-diagonal vibrational branching from the A 1 Π, v = 0 state is measured to be (5.60 ± 0.02) × 10 −3 in good agreement with theoretical predictions. The strength of the spin-forbidden A 1 Π, v = 0 → a 3 Π, v = 0 transition is measured to be seven orders of magnitude lower than the strength of the A 1 Π, v = 0 → X 1 Σ + , v = 0 transition. We determine the electric dipole moments µ(X) = 1.515 ± 0.004 Debye, µ(a) = 1.780 ± 0.003 Debye and µ(A) = 1.45 ± 0.02 Debye in X 1 Σ + , v = 0, a 3 Π, v = 0 and A 1 Π, v = 0, respectively, by recording CW laser excitation spectra in electric fields up to 150 kV/cm.

Recently, we determined the detailed energy level structure of the X 1 + , A 1 and a 3 states of AlF that are relevant to laser cooling and trapping experiments [Truppe et al., Phys. Rev. A. 100 (5), 052513 (2019)]. Here, we investigate the b 3 + , v = 0 state of the AlF molecule. A rotationally resolved (1 + 2)-REMPI spectrum of the b 3 + , v = 0 ← a 3 , v = 0 band is presented and the lifetime of the b 3 + , v = 0 state is measured to be 190(2) ns. Hyperfine-resolved, laser-induced fluorescence spectra of the b 3 + , v = 0 ← X 1 + , v = 1 and the b 3 + , v = 0 ← a 3 , v = 0 bands are recorded to determine fine-and hyperfine structure parameters. The interaction between the b 3 + , v = 0 and the nearby A 1 state is studied and the magnitude of the spin-orbit coupling between the two electronic states is derived using three independent methods to give a consistent value of 10(1) cm −1. The triplet character of the A state causes an A → a loss from the main A−X laser cooling cycle below the 10 −6 level.

Spectroscopic studies of aluminum monofluoride (AlF) have revealed its highly favorable properties for direct laser cooling. All Q lines of the strong A1Π ← X1Σ+ transition around 227 nm are rotationally closed and thereby suitable for the main cooling cycle. The same holds for the narrow, spin-forbidden a3Π ← X1Σ+ transition around 367 nm, which has a recoil limit in the µK range. We here report on the spectroscopic characterization of the lowest rotational levels in the a3Π state of AlF for v = 0–8 using a jet-cooled, pulsed molecular beam. An accidental AC Stark shift is observed on the a3Π0, v = 4 ← X1Σ+, v = 4 band. By using time-delayed ionization for state-selective detection of the molecules in the metastable a3Π state at different points along the molecular beam, the radiative lifetime of the a3Π1, v = 0, J = 1 level is experimentally determined as τ = 1.89 ± 0.15 ms. A laser/radio frequency multiple resonance ionization scheme is employed to determine the hyperfine splittings in the a3Π1, v = 5 level. The experimentally derived hyperfine parameters are compared to the outcome of quantum chemistry calculations. A spectral line with a width of 1.27 kHz is recorded between hyperfine levels in the a3Π, v = 0 state. These measurements benchmark the electronic potential of the a3Π state and yield accurate values for the photon scattering rate and for the elements of the Franck–Condon matrix of the a3Π–X1Σ+ system.

Aluminum monofluoride (AlF) possesses highly favorable properties for laser cooling, both via the A$^1\Pi$ and a$^3\Pi$ states. Determining efficient pathways between the singlet and the triplet manifold of electronic states will be advantageous for future experiments at ultralow temperatures. The lowest rotational levels of the A$^1\Pi, v=6$ and b$^3\Sigma^+, v=5$ states of AlF are nearly iso-energetic and interact via spin-orbit coupling. These levels thus have a strongly mixed spin-character and provide a singlet-triplet doorway. We here present a hyperfine resolved spectroscopic study of the A$^1\Pi, v=6$ // b$^3\Sigma^+, v=5$ perturbed system in a jet-cooled, pulsed molecular beam. From a fit to the observed energies of the hyperfine levels, the fine and hyperfine structure parameters of the coupled states, their relative energies as well as the spin-orbit interaction parameter are determined. The standard deviation of the fit is about 15~MHz. We experimentally determine the radiative lifetimes of selected hyperfine levels by time-delayed ionization, Lamb dip spectroscopy and accurate measurements of the transition lineshapes. The measured lifetimes range between 2 ns and 200 ns, determined by the degree of singlet-triplet mixing for each level.

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