Here we report on spectroscopic measurements of the aluminum monofluoride molecule (AlF; boson) that are relevant to laser cooling and trapping experiments. We measure the detailed energy-level structure of AlF in the $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ electronic ground state, in the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state, and in the metastable $a{\phantom{\rule{0.16em}{0ex}}}^{3}\mathrm{\ensuremath{\Pi}}$ state. We determine the rotational, vibrational, and electronic branching ratios from the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ 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 $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ - $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ transition and for trapping at high densities. The energy levels in the $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+},v=0$ state and within each $\mathrm{\ensuremath{\Omega}}$ manifold in the $a{\phantom{\rule{0.16em}{0ex}}}^{3}\mathrm{\ensuremath{\Pi}},v=0$ 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 $\mathrm{\ensuremath{\Lambda}}$-doubling parameters we measure transitions throughout the 0.1-MHz--66-GHz range, between rotational levels in the $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+},v=0$ state, and between rotational and $\mathrm{\ensuremath{\Lambda}}$-doublet levels in all three spin-orbit manifolds of the $a{\phantom{\rule{0.16em}{0ex}}}^{3}\mathrm{\ensuremath{\Pi}},v=0$ state. We measure the hyperfine splitting in the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state using continuous wave (cw) laser-induced fluorescence spectroscopy of the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}},v=0\ensuremath{\leftarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+},{v}^{\ensuremath{''}}=0$ band. The resolution is limited by the short radiative lifetime of the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}},v=0$ state, which we experimentally determine to be $1.90\ifmmode\pm\else\textpm\fi{}0.03$ ns. The hyperfine mixing of the lowest rotational levels in the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state causes a small loss from the main laser cooling transition of ${10}^{\ensuremath{-}5}$. The off-diagonal vibrational branching from the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}},v=0$ state is measured to be $(5.60\ifmmode\pm\else\textpm\fi{}0.02)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ in good agreement with theoretical predictions. The strength of the spin-forbidden $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}},v=0\ensuremath{\rightarrow}a{\phantom{\rule{0.16em}{0ex}}}^{3}\mathrm{\ensuremath{\Pi}},{v}^{\ensuremath{'}}=0$ transition is measured to be seven orders of magnitude lower than the strength of the $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}},v=0\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+},{v}^{\ensuremath{''}}=0$ transition. We determine the electric dipole moments $\ensuremath{\mu}(\mathrm{X})=1.515\ifmmode\pm\else\textpm\fi{}0.004$ Debye, $\ensuremath{\mu}(a)=1.780\ifmmode\pm\else\textpm\fi{}0.003$ Debye and $\ensuremath{\mu}(\mathrm{A})=1.45\ifmmode\pm\else\textpm\fi{}0.02$ Debye in $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$,
v=0, $a{\phantom{\rule{0.16em}{0ex}}}^{3}\mathrm{\ensuremath{\Pi}},v=0$ and $A{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}},v=0$, respectively, by recording cw laser excitation spectra in electric fields up to 150 kV/cm.
The simple structure of the BH molecule makes it an excellent candidate for direct laser cooling. We measure the branching ratios for the decay of the A^{1}Pi (v'=0) state to vibrational levels of the ground state, X^{1}Sigma^{+}, and find that they are exceedingly favourable for laser cooling. We verify that the branching ratio for the spin-forbidden transition to the intermediate a^{3}Pi state is inconsequentially small. We measure the frequency of the lowest rotational transition of the X state, and the hyperfine structure in the relevant levels of both the X and A states, and determine the nuclear electric quadrupole and magnetic dipole coupling constants. Our results show that, with a relatively simple laser cooling scheme, a Zeeman slower and magneto-optical trap can be used to cool, slow and trap BH molecules.
We report an experimental investigation on the Doppler-free saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride (BaF) molecules in a 4 K cryogenic cell. The obtained spectra with a resolution of 19 MHz, much smaller than previously observed in absorption spectroscopy, clearly resolve the hyperfine transitions. Moreover, we use these high-resolution spectra to fit the hyperfine splittings of excited A ( v = 0) state and find the hyperfine splitting of the laser-cooling-relevant A 2 Π 1/2 ( v = 0, J = 1/2,+) state is about 18 MHz, much higher than the previous theoretically predicted value. This provides important missing information for laser cooling of BaF molecules.