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Chinese Physics B

Optical Stark deceleration of neutral molecules from supersonic expansion with a rotating laser beam

Yongcheng Yang, Shunyong Hou, Lianzhong Deng

Published May 1, 2018• 4 citations
Publisher

Abstract

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.

DOI
10.1088/1674-1056/27/5/053701

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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.
We propose a versatile scheme to slow supersonically cooled molecules using a decelerating potential well, obtained by steering a focusing laser beam onto a pair of spinning reflective mirrors under a high-speed brake. The longitudinal motion of molecules in the moving red-detuned light field is analyzed and their corresponding phase-space stability is investigated. Trajectories of CH4\mathrm{C}{\mathrm{H}}_{4}CH4​ molecules under the influence of the potential well are simulated using the Monte Carlo method. For instance, with a laser beam of power 20 kW focused onto a spot of waist radius 40--100 \ensuremath{\mu}m, corresponding to a peak laser intensity on the order of $\ensuremath{\sim}{10}^{8}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$, a CH4\mathrm{C}{\mathrm{H}}_{4}CH4​ molecule of $\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$ can be decelerated to $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$ over a distance of a few centimeters on a time scale of hundreds of microseconds.
Cryogenic buffer-gas beams are a promising method for producing bright sources of cold molecular radicals for cold-collision and chemical-reaction experiments. In order to use these beams in studies of reactions with controlled collision energies or in trapping experiments, one needs a method of controlling the forward velocity of the beam. A Stark decelerator can be an effective tool for controlling the mean speed of molecules produced by supersonic jets, but efficient deceleration of buffer-gas beams presents new challenges due to longer pulse lengths. Traveling-wave decelerators are uniquely suited to meet these challenges because of their ability to confine molecules in three dimensions during deceleration and their versatility afforded by the analog control of the electrodes. We have created ground-state $\mathrm{CH}(X{\phantom{\rule{0.16em}{0ex}}}^{2}\ensuremath{\Pi})$ radicals in a cryogenic buffer-gas cell with the potential to produce a cold molecular beam of 1011{10}^{11}1011 molecules/pulse. We present a general protocol for Stark deceleration of beams with a large position and velocity spread for use with a traveling-wave decelerator. Our method involves confining molecules transversely with a hexapole for an optimized distance before deceleration. This rotates the phase-space distribution of the molecular packet so that the packet is matched to the time-varying phase-space acceptance of the decelerator. We demonstrate with simulations and an analytic one-dimensional model that this method can decelerate a significant fraction of the molecules in successive wells of a traveling-wave decelerator to produce energy-tuned beams for cold and controlled-molecule experiments.
Abstract We demonstrate the production of cold, slow NH3 molecules from a supersonic NH3 molecular beam using our electrostatic Stark decelerator consisting of 179 slowing stages. By using this long Stark decelerator, a supersonic NH3 molecular beam can be easily decelerated to trappable velocities. Here we present two modes for operating the Stark decelerator to slow the supersonic NH3 molecules. The first is the normal mode, where all 179 stages are used to decelerate molecules, and it allows decelerating the NH3 molecular beam from 333 m/s to 18 m/s, with a final temperature of 29.2 mK. The second is the deceleration-bunch mode, which allows us to decelerate the supersonic NH3 beam from 333 m/s to 24 m/s, with a final temperature of 2.9 mK. It is clear that the second mode promises to produce colder (high-energy-resolution) molecular samples than the normal mode. Three-dimensional Monte Carlo simulations are also performed for the experiments and they show a good agreement with the observed results. The deceleration-bunch operation mode presented here can find applications in the fields of cold collisions, high-resolution spectroscopy, and precision measurements.
We produce SO2 molecules with a centre of mass velocity near zero using a Stark decelerator. Since the initial kinetic energy of the supersonic SO2 molecular beam is high, and the removed kinetic energy per stage is small, 326 deceleration stages are necessary to bring SO2 to a complete standstill, significantly more than in other experiments. We show that in such a decelerator possible loss due to coupling between the motional degrees of freedom must be considered. Experimental results are compared with 3D Monte-Carlo simulations and the quantum state selectivity of the Stark decelerator is demonstrated.
The work depicted in this thesis demonstrate the successful operation of a 4 meter long traveling-wave Stark decelerator. This is a novel tool for deceleration of heavy diatomic molecules to arbitrarily slow beams with a very small velocity spread which can serve as ideal starting point for precision tests of fundamental physics. The first part of the thesis discusses the characterization of a new Supersonic beam source for creation of SrF molecules which serves as an ideal candidate for precision test of nuclear spin dependent parity violation. The later part of the thesis discusses the the results of deceleration of a SrF molecules in the N = 1 rotation state of the SrF(v = 0) electronic ground state from a supersonic beam with an initial velocity of 340 m/s to 265 m/s using a deceleration potential with 3.25 k V voltage amplitude using a the 4 m long traveling-wave decelerator built in our research lab. Increasing the voltage amplitude of the potential to 4.8 kV, we were able to decelerate molecules from 340 m/s to 250 m/s which corresponds to a deceleration strength of 6.6 km /s2. By cooling the valve by flushing cold nitrogen gas through the valve enclosure we were able to decrease the initial velocity of the supersonic beam. With the valve cooled to ~190 K, we demonstrated deceleration of SrF molecules from 290 m/s to 120 m/s which corresponds to the removal of almost 85% of the initial kinetic energy. The thesis described the operation of the decelerator and end with an outlook for precision tests using this novel tool.
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.