Doppler cooling of buffer-gas-cooled barium monofluoride molecules

We demonstrate one-dimensional Doppler cooling of a beam of buffer-gas-cooled barium monofluoride (BaF) molecules. This represents a key step towards further cooling and trapping of BaF molecules. The dependencies of the cooling efficiency on the detuning of laser, the intensity of the laser, and the bias field are systematically studied. Furthermore, the numerical simulations with a Monte Carlo method agree quite well with our data.

Figure 1

The energy levels involved in laser cooling of BaF molecules. (a) Shows the electronic states and the vibrational states. The branching ratios are also included in the plot. A 736-nm laser is used to repump molecules in the $v=1$ state back to the $v=0$ state. (b) Shows the rotational states and hyperfine states involved in the cooling scheme. Multifrequency lasers are used to address the hyperfine sublevels.

Figure 2

Experimental setup. The total working distance is about 70 cm. BaF molecules are created in a buffer-gas cell and then collimated by an aperture. After that, molecules are laser cooled in the $z$ direction by the multireflected cooling laser. After passing through the cooling region, molecules in the $v=1$ state are repumped back to the $v=0$ state by a cleanup laser. Finally, molecules are detected by a probe beam and the fluorescence image is recorded by a camera.

Figure 3

(a) A typical molecular image of the molecular beam. It has been averaged 1500 times. (b) The molecular signal along the $z$ direction. The smooth solid lines are fitting curves with function (1). (c) Shows the normalized results, the smooth solid lines are fitting curves with function (2). In both (b) and (c), the solid blue line represents cooling laser off and the dotted red line represents cooling laser on. The experimental parameters are $\delta =-5$ MHz, $B=0.82$ G, $\theta ={70}^{\circ },P_0=167$ mW.

Figure 4

The dependence of cooling efficiency versus laser detuning. The dots are experimental data with $B=0.82$ G, $\theta ={70}^{\circ },P_0=167$ mW, and the solid line is from numerical Monte Carlo simulation. Both cooling and heating show up depending on the detuning. Error bars are standard deviations.

Figure 5

The dependence of the cooling efficiency on the laser power. The dots are experimental data with $\delta =-5$ MHz, $B=0.82$ G, $\theta ={70}^{\circ }$, and the solid line is the numerical result from the Monte Carlo simulation. Error bars are standard deviations.

Figure 6

The dependence of the cooling efficiency on the bias field. The dots are experimental data with $P_0=167$ mW, $\delta =-5$ MHz, and $\theta ={70}^{\circ }$, and the solid line is the numerical Monte Carlo simulation result. The best cooling appears around $B\simeq 1$ G. Error bars are standard deviations.