Optical quenching of metastable magnesium

Doppler cooling on narrow transitions has become a crucial technique for preparing ultracold samples of alkaline-earth-metal and alkaline-earth-metal-like atoms. For lighter species, such as calcium and magnesium, this technique relies on artificial broadening (quenching) of the upper level of the narrow line. We report on quenching experiments on a ${}^{24}\mathrm{Mg}$ atomic beam. The branching ratio of the ${\left(3s4s\right)}^1{S}_0$ state was determined to be $\beta =(1.33\pm 0.53)\times {10}^{-5}$ from the measured quenching efficiency. The branching ratio combined with the known linewidth of this state yields a transition rate for ${\left(3s3p\right)}^3{P}_1\to {\left(3s4s\right)}^1{S}_0$ of ${\Gamma }_{23}=283\pm 114{\mathrm{s}}^{-1}$, i.e., one order of magnitude smaller than estimated from semiempirical data. We have applied different numerical approaches, including ab initio relativistic many-body calculations, to compute the transition probabilities of the ${\left(3s3p\right)}^3{P}_1\to {\left(3s4s\right)}^1{S}_0$ and ${\left(3s3p\right)}^1{P}_1\to {\left(3s4s\right)}^1{S}_0$ transitions. The results are in good agreement with our experimental observation. With the measured branching ratio, we expect a transfer efficiency of Doppler-cooled atoms into a quench magneto-optical trap (QuenchMOT) of approximately 1% for our experimental parameters. According to our simulations, the transfer efficiency can be increased by one order of magnitude for lower ensemble temperatures as recently demonstrated by two-photon cooling in our uv MOT.

Figure 1

(Color online) Modified Ramsey-Bordé setup used for quenching experiments.

Figure 2

(Color online) Relevant transitions for quenching in ${}^{24}\mathrm{Mg}$.