Cold collision and high-resolution spectroscopy of buffer-gas-cooled BaF molecules

We reported a detailed experimental study of the cold collision of barium monofluoride (BaF) with buffer gas and the high-resolution spectroscopy relevant with direct laser cooling. BaF molecules are efficiently produced with laser ablation and buffer-gas cooled in a cryogenic apparatus. The laser cooling relevant transition $|X{}^2\mathrm{\Sigma },v=0,N=1\rangle$ to $|A{}^2\mathrm{\Pi },v^{\prime }=0,J^{\prime }=1/2\rangle$ is identified. The collision cross section with buffer gas is measured to be $1.4\left(7\right)\times {10}^{-14}{\text{cm}}^{-2}$, which is very suitable for buffer-gas cooling. Both rotational and vibrational temperatures are effectively cooled, and a large amount of molecules are quenched into the desired states. Our study provides an important benchmark for further laser cooling of the BaF molecule.

Figure 1

Schematic experimental setup. (a) Cryogenic apparatus. The whole system is an aluminum vacuum chamber; the 30- and 4-K shielding layers are attached to the first stage and the second stage coolers of a pulse tube refrigerator respectively. The buffer gas is precooled by the 30- and 4-K heat sinks before sending into the cell. Charcoal is used to help pump helium gas at 4 K. The temperature monitor is not presented here. (b) The cooling process from room temperate to 4 K. It takes roughly 8 h. (c) The warming process from 4 K to room temperature.

Figure 2

(a) Experimental setup for absorption spectroscopy measurement. The BaF molecules are produced by laser ablation in the buffer-gas cell. (b) Typical absorption signal of molecules with laser ablation. The pulse laser fires at $t=0\text{ms}$, the absorption signal shows up and then decays. (c) Normalized signal. The red line is the fitting function with formula (3). (d) Ablation laser power dependence of molecule production. The pulse laser is $532\text{nm}$, 2 Hz, $12\text{ns}, 1/e^2$ diameter is $0.5\text{mm}$. (e) The decay of laser ablation at one point. When the targets are hit by laser more and more at one point, the produced molecule number decays. The decay constant is about few hundred shots. All the data are measured with the He buffer-gas rate of 5 sccm.

Figure 3

Cold collision of BaF with buffer gas. (a) Absorption fraction decay curve with log scale. A long time after ablation, the higher-order diffusion mode of the molecule decays away. Then the curve can be fitted with a single exponential decay. The red line is the exponential decay fitting. (b) Decay time constant vs the flow rate. Higher flow rate means higher helium density, and thus lead to slower diffusion. The helium density linearly follows the flow rate at the diffusive limit when the flow rate is small. We fit data linearly at the beginning to get the slope to calculate the collision cross section.

Figure 4

Rotational number vs decay time constant. The helium flow rate is 5 sccm.

Figure 5

High-resolution spectroscopy. (a) Main cooling transition of BaF molecules. The hyperfine levels for $N=0$ and $N=1$ are shown. Because of the parity selection rule, the transitions from $|X,N=1\rangle$ to $|A,J^{\prime }=1/2,+\rangle$ and $|X,N=0\rangle$ to $|A,J^{\prime }=1/2,-\rangle$ are allowed. (b) Absorption spectroscopy of transition from $|X,N=1\rangle$ to $|A,J^{\prime }=1/2,+\rangle , f_1=11630.0843{\text{cm}}^{-1}$. (c) Absorption spectroscopy of transition from $|X,N=0\rangle$ to $|A,J^{\prime }=1/2,-\rangle$. $f_2=11630.2595{\text{cm}}^{-1}$. The fine structure of $N=1$ and the hyperfine structure of $N=0$ are resolved. The red lines in (b) and (c) are simply Gaussian fits to the measured data.

Figure 6

Rotational relaxation for buffer-gas cooling. (a) Theoretic calculation of the rotational distribution for different temperatures. Inset is the log scale plot. At room temperature, less than $1\%$ molecules are in $N=1$ state, while for 4 K, it is enhanced to more than $10\%$. (b) Experimental data for different rotational populations. All data are normalized with $N=0$ population. The lines are the theoretic curve for different temperature, and the best fitting shows the rotational temperature is $4.0\left(7\right)$ K.

Figure 7

Vibrational relaxation via buffer-gas cooling. (a) Theoretic calculation of the vibrational distribution. At room temperature, a small fraction of molecules will populate higher-lying vibrational states. At 4 K, almost all molecules populate the $v=0$ state. (b) Experimental absorption signal for $v=0$ and $v=1$ molecules from laser ablation. The population of $v=1$ is $10–100$ times smaller than $v=0$.