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Spectroscopic characterization of the a$^3Π$ 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$^1Π$ $\leftarrow$ X$^1Σ^+$ transition around 227~nm are rotationally closed and thereby suitable for the main cooling cycle. The same holds for the narrow, spin-forbidden a$^3Π$ $\leftarrow$ X$^1Σ^+$ transition around 367 nm which has a recoil limit in the micro Kelvin range. We here report on the spectroscopic characterization of the lowest rotational levels in the a$^3Π$ state of AlF for $v=0-8$ using a jet-cooled, pulsed molecular beam. An accidental AC Stark shift is observed on the a$^3Π_0, v=4$ $\leftarrow$ X$^1Σ^+, v=4$ band. By using time-delayed ionization for state-selective detection of the molecules in the metastable a$^3Π$ state at different points along the molecular beam, the radiative lifetime of the a$^3Π_1, v=0, J=1$ level is experimentally determined as $τ=1.89 \pm 0.15$~ms. A laser/radio-frequency multiple resonance ionization scheme is employed to determine the hyperfine splittings in the a$^3Π_1, v=5$ 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$^3Π, v=0$ state. These measurements benchmark the electronic potential of the a$^3Π$ state and yield accurate values for the photon scattering rate and for the elements of the Franck-Condon matrix of the a$^3Π$ $-$ X$^1Σ^+$ system.