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

Abstract

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.

Characterisation of the b3Σ+, v = 0 state and its interaction with the A1Π state in aluminium monofluoride

“…This indicates that there is a significant electron density at both nuclei, which is in stark contrast to the situation in the a 3 state. In the latter state, the Fermi contact term for the 19 F nucleus is about seven times smaller than for the 27 Al nucleus [3]. The Fermi contact parameter for the aluminium nucleus b F (Al) and the spin-spin interaction parameter λ presented here are consistent with the previously determined values, but their uncertainty is reduced significantly.…”

“…In the a 3 state, each J-level has an e and an f component due to -doubling. Figure 1(a) shows the energy level diagram of the electronic states relevant to this study, together with a rotationally resolved spectrum of the b 3 + , v = 0 ← a 3 1 , v = 0 band in 1(b), and a sketch of the experimental setup in 1(c), which is similar to the one reported previously [3]. The molecules are produced by laserablating an aluminium rod in a supersonic expansion of 2% SF 6 seeded in Ne.…”

“…We have recently reported on the detailed energy level structure of the X 1 + , A 1 and a 3 states in AlF [3]. That study resolves the complete hyperfine structure, including the contribution due to fluorine nuclear spin, in all three states.…”

“…To study the b−a transition, the molecules must first be prepared in the a 3 state. Previously, we described this state and the a−X transition in detail [3]. The hyperfine structure in X 1 + is small and for the purpose of this study, the ground state is comprised of single rotational levels with energies given by BJ(J + 1).…”

“…Polar molecules, cooled to low temperatures, have wideranging applications in physics and chemistry [1,2]. Recently, we identified the AlF molecule as an excellent candidate for direct laser cooling to low temperatures trapping and cooling on the strong A−X transition, the molecules may be cooled to a few μ K on any (narrow) Qline of the spin-forbidden a 3 1 , v = 0 ← X 1 + , v = 0 band. To laser-cool AlF successfully, it is essential to study the detailed energy level structure of the molecule, to measure the radiative lifetime of the states involved and to investigate and quantify the potential decay channels to states that are not coupled by the cooling laser, i.e.…”