Spectroscopic characterization of aluminum monofluoride with relevance to laser cooling and trapping

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