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Abstract

Here we report on spectroscopic measurements of the aluminum monofluoride molecule (AlF; boson) that are relevant to laser cooling and trapping experiments. We measure the detailed energy-level structure of AlF in the X¹Σ⁺ electronic ground state, in the A¹Π state, and in the metastable a³Π state. We determine the rotational, vibrational, and electronic branching ratios from the A¹Π 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 A¹Π - X¹Σ⁺ transition and for trapping at high densities. The energy levels in the X¹Σ⁺,v=0 state and within each Ω manifold in the a³Π,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 X¹Σ⁺,v=0 state, and between rotational and Λ-doublet levels in all three spin-orbit manifolds of the a³Π,v=0 state. We measure the hyperfine splitting in the A¹Π state using continuous wave (cw) laser-induced fluorescence spectroscopy of the A¹Π,v=0←X¹Σ⁺,v′′=0 band. The resolution is limited by the short radiative lifetime of the A¹Π,v=0 state, which we experimentally determine to be 1.90±0.03 ns. The hyperfine mixing of the lowest rotational levels in the A¹Π state causes a small loss from the main laser cooling transition of 10^-5. The off-diagonal vibrational branching from the A¹Π,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 A¹Π,v=0→a³Π,v′=0 transition is measured to be seven orders of magnitude lower than the strength of the A¹Π,v=0→X¹Σ⁺,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 X¹Σ⁺, v=0, a³Π,v=0 and A¹Π,v=0, respectively, by recording cw laser excitation spectra in electric fields up to 150 kV/cm.