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 X0.16em0ex¹Σ⁺ electronic ground state, in the A0.16em0ex¹Π state, and in the metastable a0.16em0ex³Π state. We determine the rotational, vibrational, and electronic branching ratios from the A0.16em0ex¹Π 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 A0.16em0ex¹Π - X0.16em0ex¹Σ⁺ transition and for trapping at high densities. The energy levels in the X0.16em0ex¹Σ⁺,v=0 state and within each Ω manifold in the a0.16em0ex³Π,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 X0.16em0ex¹Σ⁺,v=0 state, and between rotational and Λ-doublet levels in all three spin-orbit manifolds of the a0.16em0ex³Π,v=0 state. We measure the hyperfine splitting in the A0.16em0ex¹Π state using continuous wave (cw) laser-induced fluorescence spectroscopy of the A0.16em0ex¹Π,v=0←X0.16em0ex¹Σ⁺,v^''=0 band. The resolution is limited by the short radiative lifetime of the A0.16em0ex¹Π,v=0 state, which we experimentally determine to be 1.90±0.03 ns. The hyperfine mixing of the lowest rotational levels in the A0.16em0ex¹Π state causes a small loss from the main laser cooling transition of 10^-5. The off-diagonal vibrational branching from the A0.16em0ex¹Π,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 A0.16em0ex¹Π,v=0→a0.16em0ex³Π,v^'=0 transition is measured to be seven orders of magnitude lower than the strength of the A0.16em0ex¹Π,v=0→X0.16em0ex¹Σ⁺,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 X0.16em0ex¹Σ⁺, $v=0$, a0.16em0ex³Π,v=0 and A0.16em0ex¹Π,v=0, respectively, by recording cw laser excitation spectra in electric fields up to 150 kV/cm.