Collisional effects in the formation of cold guided beams of polar molecules (original) (raw)

Velocity-selected molecular pulses produced by an electric guide

Physical Review A, 2010

Electrostatic velocity filtering is a technique for the production of continuous guided beams of slow polar molecules from a thermal gas. We extended this technique to produce pulses of slow molecules with a narrow velocity distribution around a tunable velocity. The pulses are generated by sequentially switching the voltages on adjacent segments of an electric quadrupole guide synchronously with the molecules propagating at the desired velocity. This technique is demonstrated for deuterated ammonia (ND3), delivering pulses with a velocity in the range of 20 − 100 m/s and a relative velocity spread of (16 ± 2) % at FWHM. At velocities around 60 m/s, the pulses contain up to 10 6 molecules each. The data are well reproduced by Monte-Carlo simulations, which provide useful insight into the mechanisms of velocity selection.

A cryofuge for cold-collision experiments with slow polar molecules

Science (New York, N.Y.), 2017

Ultracold molecules represent a fascinating research frontier in physics and chemistry, but it has proven challenging to prepare dense samples at low velocities. Here, we present a solution to this goal by means of a nonconventional approach dubbed cryofuge. It uses centrifugal force to bring cryogenically cooled molecules to kinetic energies below 1 K × kB in the laboratory frame, where kB is the Boltzmann constant, with corresponding fluxes exceeding 1010 per second at velocities below 20 meters per second. By attaining densities higher than 109 per cubic centimeter and interaction times longer than 25 milliseconds in samples of fluoromethane as well as deuterated ammonia, we observed cold dipolar collisions between molecules and determined their collision cross sections.

Guiding neutral polar molecules by electromagnetic vortex field

Journal of Modern Optics, 2020

It is shown, within classical mechanics, that the field of an electromagnetic vortex is capable of capturing and guiding neutral molecules endowed with a permanent electric dipole moment (PEDM). Similarly as in the case of the magnetic field applied to elementary particles or atoms, this effect turns out to be very delicate because of the small values of PEDM observed in real molecules. They amount to 2 × 10 5 efm (electron charge × fermi) or less, which requires the use of very strong electric fields. It has also been observed that there exists a threshold in field strength above which the particles are ejected from the trap. Trajectories of guided particles are usually quite chaotic, which is a consequence of non-linearity of the equations of motion. With a very special and precise adjustment of parameters, a regular (i.e., circular, in the transverse plane) trajectory can be obtained. The presence of an additional constant electric field pointing along the direction of the wave propagation might help to achieve the necessary tuning and realize such trajectories.

Cold ion–polar-molecule reactions studied with a combined Stark-velocity-filter–ion-trap apparatus

Physical Review A, 2013

We have developed a combined Stark-velocity-filter-ion-trap apparatus for the purpose of reaction-rate measurements between cold trapped ions and slow polar molecules under ultrahigh vacuum conditions. The prerequisite steps such as the characterization of velocity-selected polar molecules (PM), namely ND 3 , H 2 CO, and CH 3 CN, were performed using time-of-flight (TOF) measurements. We confirmed the generation of slow ND 3 , H 2 CO, and CH 3 CN molecules having thermal energies of a few Kelvin. Additionally, the number densities of the slow velocity-filtered polar molecules were determined to be in the range of n = 10 4 to 10 6 cm −3 by calibrating the TOF signals. In a first experiment, the Stark velocity filter was connected to a cryogenic linear Paul trap and reaction-rate measurements between laser-cooled Ca + Coulomb crystals and velocity-selected polar molecules were carried out. The observed reaction rates are of the order of 10 −5 s −1 , which are much slower than typical reaction rates of molecular ion-polar-molecule reactions at low temperatures. The present results confirm that reaction-rate measurements between velocity-selected polar molecules and sympathetically cooled molecular ions cooled by a laser-cooled Ca + Coulomb crystal can be performed. Next we measured the reaction rates between sympathetically cooled nonfluorescent stored ion molecules namely N 2 H + ions and velocity-selected CH 3 CN molecules at the average reaction energy of about 3 K. The measured reaction rate of 2.0(2)×10 −3 s −1 is much faster than those of the Ca + + PM reactions. This is strong evidence that the velocity-selected polar molecules undergo reactive collisions. We also confirmed that the present reaction-rate constant of CH 3 CN + N 2 H + → CH 3 CNH + + N 2 is consistent with the estimated values from the room temperature results and the trajectory-scaling formula of Su et al. In the future, the present velocity-filter combined cryogenic trap apparatus will enable us to perform systematic measurements of cold ion-polar-molecule reactions, which are important problems from a fundamental viewpoint and also contribute to astrochemistry.

Cold Reactive Collisions between Laser-Cooled Ions and Velocity-Selected Neutral Molecules

Physical Review Letters, 2008

We report a new experimental method to study reactive ion-molecule collisions at very low temperatures. A source of laser-cooled ions in a linear Paul trap has been combined with a quadrupole-guide velocity selector to investigate the reaction of Ca with CH 3 F at collision energies E coll =k B 1 K with single-particle sensitivity. The technique represents a general approach to study reactive collisions between ions and polar molecules over a wide temperature range down to the cold regime.

Efficient Stark deceleration of cold polar molecules

The European Physical Journal D, 2004

Stark deceleration has been utilized for slowing and trapping several species of neutral, groundstate polar molecules generated in a supersonic beam expansion. Due to the finite physical dimension of the electrode array and practical limitations of the applicable electric fields, only molecules within a specific range of velocities and positions can be efficiently slowed and trapped. These constraints result in a restricted phase space acceptance of the decelerator in directions both transverse and parallel to the molecular beam axis; hence, careful modeling is required for understanding and achieving efficient Stark decelerator operation. We present work on slowing of the hydroxyl radical (OH) elucidating the physics controlling the evolution of the molecular phase space packets both with experimental results and model calculations. From these results we deduce experimental conditions necessary for efficient operation of a Stark decelerator. PACS. 32.60.+i Zeeman and Stark effects-39.10+j Atomic and molecular beam sources and techniques

Slow molecular beams from a cryogenic buffer gas source

2024

We study the properties of a cryogenic buffer gas source that uses a low-temperature two-stage buffer gas cell to produce very slow beams of ytterbium monofluoride molecules. The molecules are produced by laser ablation inside the cell and extracted into a beam by a flow of cold helium. We measure the flux and velocity distribution of the beam as a function of ablation energy, helium flow rate, cell temperature, and the size of the gap between the first and second stages of the cell. We also compare the velocity distributions from one-stage and two-stage cells. The one-stage cell emits a beam with a speed of about 82 m s −1 and a translational temperature of 0.63 K. The slowest beams are obtained using the two-stage cell at the lowest achievable cell temperature of 1.8 K. This beam has a peak velocity of 56 m s −1 and a flux of 9 × 10 9 ground-state molecules per steradian per pulse, with a substantial fraction at speeds below 40 m s −1. These slow molecules can be decelerated further by radiation pressure slowing and then captured in a magneto-optical trap.

An experimental toolbox for the generation of cold and ultracold polar molecules

Journal of Physics: Conference Series, 2017

Progress toward ultracold chemistry: ultracold atomic and photonic collisions Jesús Pérez-Ríos, Maxence Lepers, Romain Vexiau et al. Thermodynamic of mixtures of ultracold Rydberg gases with different levels of excitation B V Zelener, S Y Bronin and A B Klyarfeld Nondestructive detection of polar molecules via Rydberg atoms M. Zeppenfeld Production and Detection of Ultracold Ground State 85Rb133Cs Molecules in the Lowest Vibrational Level by Short-Range Photoassociation

A quasi-analytic model of a linear Stark accelerator/decelerator for polar molecules

This article describes a quasi-analytic model of a linear Stark accelerator/decelerator for polar molecules in both their low-and high-field seeking states, and examines the dynamics of the acceleration/deceleration process and its phase stability. The requisite time-dependent inhomogeneous Stark fields, used in current experiments, are Fourier-analyzed and found to consist of a superposition of partial waves with well-defined phase velocities. The kinematics of the interaction of molecules with the partial waves is discussed and the notion of a phase of a molecule in a travelling field is introduced. Next, the net potential and the net force that act on the molecules are derived. A special case, the first-harmonic accelerator/decelerator, is introduced. This represents a model system many of whose properties can be obtained analytically. The first-harmonic accelerator/decelerator dynamics is presented and discussed along with that of the isomorphic biased-pendulum problem. Finally, the general properties of the velocity of the molecules in a phase-stable accelerator/decelerator are examined.