Saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride molecules

doi:10.1007/s11467-022-1194-x

Front. Phys.

2022, Vol. 17

Issue (6) : 62502 https://doi.org/10.1007/s11467-022-1194-x

RESEARCH ARTICLE

Saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride molecules

Wenhao Bu, Yuhe Zhang, Qian Liang, Tao Chen, Bo Yan(

)

Interdisciplinary Center of Quantum Information, State Key Laboratory of Modern Optical Instrumentation, and Zhejiang Province Key Laboratory of Quantum Technology and Device of Physics Department, Zhejiang University, Hangzhou 310027, China

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Abstract

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 A²Π_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.

Keywords cold molecule saturated spectroscopy buffer gas cooling BaF

Corresponding Author(s): Bo Yan

About author:

Tongcan Cui and Yizhe Hou contributed equally to this work.

Issue Date: 06 September 2022

Cite this article:

Wenhao Bu,Yuhe Zhang,Qian Liang, et al. Saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride molecules[J]. Front. Phys. , 2022, 17(6): 62502.

URL:

https://academic.hep.com.cn/fop/EN/10.1007/s11467-022-1194-x
https://academic.hep.com.cn/fop/EN/Y2022/V17/I6/62502

Fig.1 A schematic illustration of the experimental setup. The three layers from outside to inside are vacuum chamber,

30 K

shield and

4 K

shield, respectively. The BaF molecules are produced by laser ablation with the Nd:YAG laser in a cell, which is attached to a

4 K

cryogenic head to perform effective buffer gas cooling. The relative position of the BaF

2

target and the three laser beams for the spectroscopy detections are also shown. The propagation directions of the probe, pump, and reference beams are indicated by the arrows.

Fig.2 The saturated absorption spectroscopy for the main laser cooling transition,

X (ν = 0, N = 1, ?) ? A (ν = 0, J =

1 / 2, +)

, of the BaF molecule. (a) shows the Doppler-broadened absorption spectra (red dots, from the reference beam) and saturated absorption spectra (blue dots, from the probe beam). The solid lines are Gaussian fittings to the two spectra, respectively. (b) The Doppler-free saturated absorption spectra. The gray solid line is the fitted curve. Each peak in our model has been plotted individually, the color is matched with that in the inset. The positions of each hyperfine level in the

X (ν = 0, N = 1, ?)

are marked by the vertical black dashed lines. The inset shows the corresponding transitions and their branching ratios. The transition

F = 1 (J = 3 / 2) →

F ′ = 1

is marked by gray, for its branching ratio is negligible.

			$F ′ = 0$		$F ′ = 1$
$J$	$F$	$m F$	$m F ′ = 0$	$m F ′ = ? 1$	$m F ′ = 0$	$m F ′ = 1$

1/2	0	0	0	2/9	2/9	2/9
		$? 1$	0.1282	0.2493	0.2493	0
1/2	1	0	0.1282	0.2493	0	0.2493
		1	0.1282	0	0.2493	0.2493
		?1	0.2051	0.0007	0.0007	0
3/2	1	0	0.2051	0.0007	0	0.0007
		1	0.2051	0	0.0007	0.0007
		?2	0	1/6	0	0
		?1	0	1/12	1/12	0
3/2	2	0	0	1/36	1/9	1/36
		1	0	0	1/12	1/12
		2	0	0	0	1/6

Tab.1 Calculated hyperfine branching ratios for decays from

| A, J ′ = 1 / 2, + ?

state to

| X, N = 1, ? ?

state, also see [51, 60].

Fig.3 The hyperfine splitting

Δ

of states in

A 2 Π 1 / 2 (v = 0)

measurement. (a) The schematic illustration for the

P 1

and

Q R 12

branch transitions, which we used to measure the splitting

Δ

. (b) The typical data measured by the

P 1

branch [

X (ν = 0, J = N + 1 / 2) ? A (ν = 0, J ′ = N ? 1 / 2)

], the black dashed lines marks the positions of the ground states. (c) The typical data measured by

Q R 12

branch [

X (ν = 0,

J = N ? 1 / 2) ? A (ν = 0, J ′ = N + 1 / 2)

], the positions of the ground states is also marked by the black dashed lines. (d) Distribution of the measured

Δ

in different

J ′

state, the red and blue dots are obtained from

P 1 (N)

and

Q R 12 (N)

branches, respectively. The solid lines show the fitting curves accordingly.

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