Spectroscopic characterization of the a${}^3\mathrm{\Pi }$ 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${}^1\mathrm{\Pi }$ $\leftarrow$ X${}^1{\mathrm{\Sigma }}^{+}$ transition around 227~nm are rotationally closed and thereby suitable for the main cooling cycle. The same holds for the narrow, spin-forbidden a${}^3\mathrm{\Pi }$ $\leftarrow$ X${}^1{\mathrm{\Sigma }}^{+}$ 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${}^3\mathrm{\Pi }$ state of AlF for $v=0-8$ using a jet-cooled, pulsed molecular beam. An accidental AC Stark shift is observed on the a${}^3{\mathrm{\Pi }}_0,v=4$ $\leftarrow$ X${}^1{\mathrm{\Sigma }}^{+},v=4$ band. By using time-delayed ionization for state-selective detection of the molecules in the metastable a${}^3\mathrm{\Pi }$ state at different points along the molecular beam, the radiative lifetime of the a${}^3{\mathrm{\Pi }}_1,v=0,J=1$ level is experimentally determined as $\tau=1.89 \pm 0.15$~ms. A laser/radio-frequency multiple resonance ionization scheme is employed to determine the hyperfine splittings in the a${}^3{\mathrm{\Pi }}_1,v=5$ 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${}^3\mathrm{\Pi },v=0$ state. These measurements benchmark the electronic potential of the a${}^3\mathrm{\Pi }$ state and yield accurate values for the photon scattering rate and for the elements of the Franck-Condon matrix of the a${}^3\mathrm{\Pi }$ $-$ X${}^1{\mathrm{\Sigma }}^{+}$ system.