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Chinese Physics B

Laser-assisted Stark deceleration of CaF in its rovibronic ground (high-field-seeking) state

Y. T. Gu, Kai Chen, Yunxia Huang, Xiaohua Yang

Published Apr 1, 2019• 6 citations
Publisher

Abstract

A near-resonant, red-detuning laser-assisted Stark deceleration scheme is proposed to slow CaF in its high-field-seeking rovibronic ground state. The assisting Gaussian laser beam can confine CaF molecules transversely owing to the optical Stark effect. Simulations suggest that the present scheme is superior to previous Stark decelerators. Under typical experimental conditions, when the assisting laser frequency is red-detuned to the molecular transition ( λ ∼606.3 nm) by 5.0 GHz and the laser power is about 5.6 W, the proposed decelerator can achieve a total number at the order of 10 4 CaF molecules with a number density at the order of 10 8 cm −3 . The equivalent temperature of the obtained cold CaF molecules is 2.3 mK. Additionally, the desired assisting laser power can be as low as about 1.2 W if keeping the red-detuning value to be 1.0 GHz, which further suggests its experimental feasibility.

DOI
10.1088/1674-1056/28/4/043702

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A near-resonant, red-detuning laser-assisted Stark deceleration scheme is proposed to slow CaF in its high-field-seeking rovibronic ground state. The assisting Gaussian laser beam can confine CaF molecules transversely owing to the optical Stark effect. Simulations suggest that the present scheme is superior to previous Stark decelerators. Under typical experimental conditions, when the assisting laser frequency is red-detuned to the molecular transition ( λ ∼606.3 nm) by 5.0 GHz and the laser power is about 5.6 W, the proposed decelerator can achieve a total number at the order of 10 4 CaF molecules with a number density at the order of 10 8 cm −3 . The equivalent temperature of the obtained cold CaF molecules is 2.3 mK. Additionally, the desired assisting laser power can be as low as about 1.2 W if keeping the red-detuning value to be 1.0 GHz, which further suggests its experimental feasibility.
Abstract The traditional Stark deceleration method is difficult to apply in chemically stable polar diatomic molecules in their ground (Χ 1 Σ) state because the Χ 1 Σ state normally experiences little Stark shift and the rovibronic ground level is mostly high-field-seeking. To solve this problem, we propose a laser-assisted Stark deceleration scheme to decelerate such molecules in the present paper. Our results show that, owing to the transverse bunching effect of the applied red-detuning laser beam, the molecules of the high-field-seeking level | J = 0, M = 0〉 in the Χ 1 Σ state can be effectively decelerated. Furthermore, the present scheme is more effective because the interaction between the molecules and the combined fields can produce the pseudo-first-order Stark effect, and thus increase the depth of the effective potential. Compared to those molecules in the low-field-seeking state | J = 1, M Ω = −1〉 in the usual electrostatic Stark deceleration, a higher molecular density and lower velocity can be achieved under an equivalent initial phase angle.
We propose a novel scheme to decelerate a cw precooled molecular beam by using an optical Stark decelerator, which is composed of a red-detuned cw semi-Gaussian beam. The dynamical process of the optical Stark deceleration for a cw methane molecular beam is studied by Monte Carlo simulations. It is realized that the proposed optical Stark decelerator can be used to continuously slow a cw methane molecular beam with a longitudinal temperature of 30 mK, and a maximum reduction in the most probable speed of 0.388 m/s (corresponding to a relative change of 9.93%) can be obtained by using a single semi-Gaussian beam with a power of 500 W and a maximum central intensity of about 7.96 × 10 7 W/cm 2 .
We propose a promising scheme to decelerate a cw filtering out molecular beam using a red-detuned quasi-cw semi-Gaussian laser beam. By using Monte-Carlo simulation method, we demonstrate this optical Stark deceleration scheme, and study its decelerated dynamic process for a cw ND3 molecular beam as well as the dependence of the deceleration effect on the border width of the semi-Gaussian beam. Our study shows that the proposed scheme can be used to efficiently slow a cw ND3 molecular beam, and obtain a relative average kinetic-energy loss of 9.33% by using a single semi-Gaussian beam with a well depth of 7.3 mK.
We propose a promising scheme to realize the deceleration of a pulsed subsonic molecular beam by using a multistage optical Stark decelerator (i.e., a 1D quasi-cw traveling optical lattice), which is composed of two nearly counter-propagating, time-varying, red-detuned light fields with an intensity of ~10⁷Wcm⁻² and a fixed frequency difference between them. We also study the influence of the velocity reduced amount of the traveling lattice, the lattice power, the synchronous phase angle, the deceleration-stage number and the temporal profile of laser pulses on the molecular slowing results by using 3D Monte-Carlo method. Our study shows that the proposed decelerator cannot only be used to slow a pulsed subsonic beam from 240m/s to standstill, but also to obtain a cold molecular packet with a temperature of a few µK, and the corresponding fraction of cold molecules is 10⁻⁶-10⁻⁷, which strongly depends on the synchronous phase angle. And we also find that a pair of appropriate rising and falling times of laser pulses will lead to a better slowing effect than that produced by the top-hat temporal ones.
Cold molecules have great scientific significance in high-resolution spectroscopy, precision measurement of physical constants, cold collision, and cold chemistry. Supersonic expansion is a conventional and versatile method to produce cold molecules with high kinetic energies. We theoretically show here that fast-moving molecules from supersonic expansion can be effectively decelerated to any desired velocity with a rotating laser beam. The orbiting focus spot of the red-detuned laser serves as a two-dimensional potential well for the molecules. We analyze the dynamics of the molecules inside the decelerating potential well and investigate the dependence of their phase acceptance by the potential well on the tilting angle of the laser beam. ND 3 molecules are used in the test of the scheme and their trajectories under the impact of the decelerating potential well are numerically simulated using the Monte Carlo method. For instance, with a laser beam of 20 kW in power focused into a pot of in waist radius, ND 3 molecules of 250 m/s can be brought to a standstill by the decelerating potential well within a time interval of about 0.73 ms. The total angle covered by the rotating laser beam is about 5.24° with the distance travelled by the potential well being about 9.13 cm. In fact, the molecules can be decelerated to any desired velocity depending on the parameters adopted. This scheme is simple in structure and easy to be realized in experiment. In addition, it is applicable to decelerating both molecules and atoms.