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Molecular Physics

State purified deceleration of SD radicals by a Stark decelerator

Omid Nourbakhsh, J. M. Michan, Tony Mittertreiner, David Carty, Eckart Wrede, Pavle Djuricanin, Takamasa Momose

Published Nov 20, 2015• 3 citations
Publisher

Abstract

Deceleration of supersonic beams of SD radicals by a linear Stark decelerator was demonstrated for the first time. SD radicals produced by an electric discharge of supersonic beams mainly occupied the lowest two rotational states of and in the 2Π3/2 state. Laser-induced fluorescence of the decelerated beam showed that, due to the proximity of the Stark shift between these two rotational states, the final radical packet still contained both the and rotational states when the decelerator was operated at a constant phase below 50°. We show that by controlling the timing sequence of the Stark decelerator, it is possible to create a decelerated molecular packet containing only a single rotational state of either or of the SD radicals with a tunable velocity. The creation of a cold radical beam containing a single rotation–vibration state will be a useful tool for the study of cold collisions and reactions in the quantum regime.

DOI
10.1080/00268976.2015.1109151

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Deceleration of supersonic beams of SD radicals by a linear Stark decelerator was demonstrated for the first time. SD radicals produced by an electric discharge of supersonic beams mainly occupied the lowest two rotational states of and in the 2Π3/2 state. Laser-induced fluorescence of the decelerated beam showed that, due to the proximity of the Stark shift between these two rotational states, the final radical packet still contained both the and rotational states when the decelerator was operated at a constant phase below 50°. We show that by controlling the timing sequence of the Stark decelerator, it is possible to create a decelerated molecular packet containing only a single rotational state of either or of the SD radicals with a tunable velocity. The creation of a cold radical beam containing a single rotation–vibration state will be a useful tool for the study of cold collisions and reactions in the quantum regime.
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.
For scientifically interesting diatomic free radical molecules, which typically possess large electric dipole moments, Stark deceleration of weak-field seeking states is an ideal method for cold sample production. We utilize a seeded, pulsed supersonic beam in combination with an electric discharge to realize an efficient free radical source that is well-suited for use with Stark deceleration. The source apparatus creates an intense OH molecular beam whose subsequent longitudinal phase-space manipulation by precisely sequenced, pulsed inhomogeneous electric fields is directly observed by in-situ laser-induced fluorescence (LIF) detection.
The Stark deceleration technique can produce molecular beams with very low velocities. In order to maximize the density of decelerated molecules, experimental parameters such as the velocity, the velocity spread and the spatial spread of the initial molecular beam as well as the operation characteristics of the decelerator have to be chosen appropriately. In this tutorial review, we describe procedures for the optimization of the density of Stark decelerated radicals for low-velocity applications which are of interest in, e.g., molecule trapping and cold-collision studies.
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.
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 experimentally test our homemade electrostatic Stark decelerator with 99 slowing stages with a supersonic ND 3 molecular beam. Relationships between the performance of the decelerator and various parameters, including the kinetic energy loss per stage versus the synchronous phase angle ϕ 0 , the final central velocity of a slowed molecular packet versus both the phase angle ϕ 0 and the decelerating stage number and the relative slowing efficiency versus the phase angle ϕ 0 , are studied both theoretically and experimentally. With our homemade Stark decelerator, a supersonic ND 3 beam is successfully decelerated from 330 to 24 m s −1 , producing a molecular packet with a translational temperature of ∼36 mK. Experimental results are in good agreement with their simulated and theoretically calculated counterparts. In addition, we propose and study a new mode to operate a Stark decelerator that allows one to prepare a slow, number-density-enhanced, high-energy-resolution molecular beam, which is essentially a complementary mode to the scheme proposed by Parazzoli et al (2009 New J. Phys. 11 055031) in a low velocity regime and promises important applications in the research of cold collisions and cold chemistry.