Cold and Slow Molecular Beam

Quinten Alan Esajas, Artem Zapara, Sreekanth Chirayath Mathavan +3 more

Jianping Liu, Hou Shun-Yong, Bin Wei +1 more

In this paper, we investigate theoretically the Stark deceleration and cooling of subsonic NH3 molecular beams based on our second-generation electrostatic Stark decelerator with 180 stages. Firstly, we calculate the Stark shifts of NH3 molecules in the |J=1, K=1 ightangle states and show the stable area of longitudinal phase space for different synchronous phase angles. Secondly, we study the slowing performance of NH3 molecular beams in the traditional mode, and discuss the relationships between various parameters (such as the kinetic energy loss per stage, final velocity and the slowing efficiency) and the synchronous phase angle 0, as well as the dependence of final velocity on the applied voltages. It is found that a subsonic NH3 molecular beam can be decelerated from 280 to 6.7 m/s at 0=26.08 when the high voltages applied on the electrodes are 13 kV, corresponding to a removal of 99.9% kinetic energy. The translational temperature of the molecular packets in the moving frame is significantly reduced from 1.34 K to 80 mK. Finally, we study the slowing performance of NH3 molecules and the dependence of final velocity on the synchronous phase angle in an alternate operation mode. In this mode, a synchronous phase angle 0=0 is chosen to bunch the molecules by using the first 15 stages. The remaining 165 stages are then used to slow a subsonic molecular beam at a certain synchronous phase angle. Our result shows that a molecular beam with a mean velocity of 280 m/s can be decelerated to 20.7 m/s at 0=65.4 when the voltages applied are 6.5 kV, indicating a 99.4% kinetic energy removal, and the translational temperature of the molecular packets can be reduced from 1.34 K to 1.6 mK. By comparing the results obtained from the two operational modes, the temperature of the slowed molecular packet in the alternate mode is 50 times lower than that in the traditional mode. It is shown that our second-generation 180-stage Stark decelerator can effectively produce slow and cold molecules with relatively small electric dipole moment like NH3. These monochromatic NH3 molecular beams offer a promising starting point for high resolution spectroscopy, precision measurement, cold collisions and cold chemistry. This theoretical work provides a reliable basis in our further experimental research.

Sarayut Deachapunya, P. J. FAGAN, Andras Major +5 more

Jochen Küpper

A.M. Rowland

The use of photodissociation to produce cold, slow molecules from a molecular beam of cold, fast molecules is dubbed “photostop”. The essence of the scheme is thus: a pulsed molecular beam of a precursor molecule AB seeded in a noble carrier gas is crossed by a laser beam. The laser light dissociates AB, producing fragments A and B. These recoil from the dissociation site. The speed of the molecular beam and the wavelength of the laser beam are tuned so that the recoil velocity of those A molecules recoiling opposite to the molecular beam direction cancels out their initial velocity. This leaves a certain amount of A close to stationary in the laboratory frame. The photostop technique was first demonstrated with the production of cold, slow NO radicals by the photodissociation of nitrogen dioxide, both at Durham and in South Korea. The work described in this thesis began as a development of the NO photostop experiment, with an attempt made to magnetically trap NO. Latterly, the photostop project became part of the Millikelvin Molecules in a Quantum Array (MMQA) collaboration, the aim of which is the trapping of ten million cold, polar molecules. To this end, the photostop of SH radicals by the photodissociation of hydrogen disulphide was demonstrated.

Bin Wei, Shunyong Hou, Hengjiao Guo +3 more

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.