Spectroscopic characterization of the aΠ state of aluminum monofluoride.

Spectroscopic studies of aluminum monofluoride (AlF) have revealed its highly favorable properties for direct laser cooling. All Q lines of the strong AΠ ← XΣ transition around 227 nm are rotationally closed and thereby suitable for the main cooling cycle. The same holds for the narrow, spin-forbidden aΠ ← XΣ 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 aΠ state of AlF for v = 0-8 using a jet-cooled, pulsed molecular beam. An accidental AC Stark shift is observed on the aΠ, v = 4 ← XΣ, v = 4 band. By using time-delayed ionization for state-selective detection of the molecules in the metastable aΠ state at different points along the molecular beam, the radiative lifetime of the aΠ, 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 aΠ, 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 aΠ, v = 0 state. These measurements benchmark the electronic potential of the aΠ state and yield accurate values for the photon scattering rate and for the elements of the Franck-Condon matrix of the aΠ-XΣ system.