Spectroscopic characterization of the a(3)Pi 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 A(1)Pi <- X-1 Sigma(+) transition around 227 nm are rotationally closed and thereby suitable for the main cooling cycle. The same holds for the narrow, spin-forbidden a3 Pi <- X-1 Sigma(+) transition around 367 nm, which has a recoil limit in the mu K range. We here report on the spectroscopic characterization of the lowest rotational levels in the a(3)Pi state of AlF for v = 0-8 using a jet-cooled, pulsed molecular beam. An accidental AC Stark shift is observed on the a(3)Pi(0), v = 4 <- X-1 Sigma(+), v = 4 band. By using time-delayed ionization for state-selective detection of the molecules in the metastable a(3)Pi state at different points along the molecular beam, the radiative lifetime of the a(3)Pi(1), v = 0, J = 1 level is experimentally determined as tau = 1.89 +/- 0.15 ms. A laser/radio frequency multiple resonance ionization scheme is employed to determine the hyperfine splittings in the a(3)Pi(1), 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 a(3)Pi, v = 0 state. These measurements benchmark the electronic potential of the a(3)Pi state and yield accurate values for the photon scattering rate and for the elements of the Franck-Condon matrix of the a(3)Pi-X-1 Sigma(+) system. (C) 2022 Author(s).