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Physical Review Letters

Kinematic Slowing of Molecules Formed by Reactive Collisions

Ningning Liu, H. J. Loesch

Published Mar 8, 2007• 26 citations
Publisher

Abstract

We report the generation of a beam of slow (translationally cold) KBr molecules formed by exoergic reactive collisions in counterpropagating beams of K atoms and HBr molecules. The intensity of the slow beam is estimated to $4\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$. The velocity distribution (density) peaks at $42.5\text{ }\text{ }\mathrm{m}/\mathrm{s}$ corresponding to a formal temperature of 13 K. About 7% of the molecules move at velocities between 14.2 and $20\text{ }\text{ }\mathrm{m}/\mathrm{s}$ and an estimated fraction of 7% is slower than $14.2\text{ }\text{ }\mathrm{m}/\mathrm{s}$.

DOI
10.1103/physrevlett.98.103002

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We have generated a beam of translationally cold KBr molecules formed by exoergic reactive collisions in counterpropagating beams of K atoms and HBr molecules. The method relies on the extreme mass ratio of the products and the proper choice of the beam velocities (Liu, N.-N.; Loesch, H. J. Phys. Rev. Lett. 2007, 98, 10300). Here, we report the results of experiments on guiding the slow molecules from the site of their creation to the detector by a linear electrostatic quadrupole field. The device enhances the total intensity by a factor of 2.3 and the intensity at 14.2 m/s (1.4K) by an order of magnitude. The density velocity distributions, deduced directly from the observed time-of-flight profiles, peak at 20 m/s (2.9K). A numerical simulation of the guiding efficiency indicates that the polarization of the nascent molecules is first altered by a sudden change of the quantization axis from parallel to the initial relative velocity to parallel to the fringing field and thereafter follows adiabatically the local field as quantization axis. Drastic differences between the velocity and rotational state distributions of the molecules entering and leaving the energized quadrupole field are predicted. The counterpropagating beams can be used to continuously load an electrostatic trap. The equilibrium density of confined molecules is estimated to 1 x 10(7) cm(-3).

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We report the generation of a beam of slow (translationally cold) KBr molecules formed by exoergic reactive collisions in counterpropagating beams of K atoms and HBr molecules. The intensity of the slow beam is estimated to 4\ifmmode×\else\texttimes\fi1011  s\ensuremath−1 sr\ensuremath−14\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}4\ifmmode×\else\texttimes\fi1011  s\ensuremath−1 sr\ensuremath−1. The velocity distribution (density) peaks at 42.5  m/s42.5\text{ }\text{ }\mathrm{m}/\mathrm{s}42.5  m/s corresponding to a formal temperature of 13 K. About 7% of the molecules move at velocities between 14.2 and 20  m/s20\text{ }\text{ }\mathrm{m}/\mathrm{s}20  m/s and an estimated fraction of 7% is slower than 14.2  m/s14.2\text{ }\text{ }\mathrm{m}/\mathrm{s}14.2  m/s.
We have generated a beam of translationally cold KBr molecules formed by exoergic reactive collisions in counterpropagating beams of K atoms and HBr molecules. The method relies on the extreme mass ratio of the products and the proper choice of the beam velocities (Liu, N.-N.; Loesch, H. J. Phys. Rev. Lett. 2007, 98, 10300). Here, we report the results of experiments on guiding the slow molecules from the site of their creation to the detector by a linear electrostatic quadrupole field. The device enhances the total intensity by a factor of 2.3 and the intensity at 14.2 m/s (1.4K) by an order of magnitude. The density velocity distributions, deduced directly from the observed time-of-flight profiles, peak at 20 m/s (2.9K). A numerical simulation of the guiding efficiency indicates that the polarization of the nascent molecules is first altered by a sudden change of the quantization axis from parallel to the initial relative velocity to parallel to the fringing field and thereafter follows adiabatically the local field as quantization axis. Drastic differences between the velocity and rotational state distributions of the molecules entering and leaving the energized quadrupole field are predicted. The counterpropagating beams can be used to continuously load an electrostatic trap. The equilibrium density of confined molecules is estimated to 1 x 10(7) cm(-3).
We report the preparation and observation of translationally cold atoms using kinematic cooling. In these experiments, krypton atoms are cooled to subkelvin temperatures by elastic collisions in crossed atomic beams. Two independent velocity measurements indicate an upper-bound mean velocity of 13 m/s (Etrans/k=850mK{E}_{\mathrm{trans}}/k=850 \mathrm{mK}Etrans​/k=850mK) and are consistent with a much lower mean velocity of 4 m/s (Etrans/k=80mK{E}_{\mathrm{trans}}/k=80 \mathrm{mK}Etrans​/k=80mK) (kkk is Boltzmann's constant). The density of the cold atoms is measured to be 109{10}^{9}109 atoms/cm3{}^{3}3. Scattering calculations and diffusion models support these velocity and density measurements. The results demonstrate that cold, dense samples of ground-state atoms and molecules can be prepared by elastic collisions between identical collision partners.
Recently we developed a technique for producing cold molecules from a supersonic molecular beam via single collisions with a supersonic atomic beam [M. S. Elioff, J. J. Valentini, and D. W. Chandler, Science 302, 1940 (2003)]. This cooling technique necessarily produces the cold molecules at the relatively high density crossing of the atomic and molecular beams. In previous reports, secondary glancing collisions with the remnant atomic and molecular beams lead to rapid depletion of the cold molecules limiting the observation time to less than $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ with an estimated temperature of 440mK440\phantom{\rule{0.3em}{0ex}}\mathrm{mK}440mK. Here we present experimental conditions for the kinematic cooling technique which overcome the limitations of the previous experiments. We demonstrate the success of this experiment for the production of cold nitric oxide (NO) in the ground vibrational, j=7.5j=7.5j=7.5 rotational, state in order to compare with our previously reported data. With the present setup, we are able to extract the cold molecules from the pulsed molecular and atomic beams and we observe these cold NO(X)j=7.5\mathrm{NO}{(X)}_{j=7.5}NO(X)j=7.5​ persisting for over $150\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$. This long observation time demonstrates our ability to temporally and spatially separate the cold molecules from the parent atomic and molecular beams, simultaneously allowing for a better measurement of the final average velocity for the kinematically cooled molecules. From the data we find a final average velocity of the NO(X)j=7.5\mathrm{NO}{(X)}_{j=7.5}NO(X)j=7.5​ of approximately 4.5m∕s4.5\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}4.5m∕s, consistent with simulations. The final observed average velocity is equated to a temperature of approximately 35mK35\phantom{\rule{0.3em}{0ex}}\mathrm{mK}35mK, over an order of magnitude colder than our previous measurements.
The production of a translationally cold (T < 1 K) sample of bromine atoms with estimated densities of up to 10(8) cm(-3) using photodissociation is presented. A molecular beam of Br(2) seeded in Kr is photodissociated into Br + Br* fragments, and the velocity distribution of the atomic fragments is determined using (2 + 1) REMPI and velocity map ion imaging. By recording images with varying delay times between the dissociation and probe lasers, we investigate the length of time after dissociation for which atoms remain in the laser focus, and determine the velocity spread of those atoms. By careful selection of the photolysis energy, it is found that a fraction of the atoms can be detected for delay times in excess of 100 μs. These are atoms for which the fragment recoil velocity vector is directly opposed and equal in magnitude to the parent beam velocity leading to a resultant lab frame velocity of approximately zero. The FWHM velocity spreads of detected atoms along the beam axis after 100 μs are less than 5 ms(-1), corresponding to temperatures in the milliKelvin range, opening the possibility that this technique could be utilized as a slow Br atom source.
This introductory article contrasts molecular collisions, particularly reactive collisions, in the familiar "warm" domain with the ultracold regime where the relative deBroglie wavelengths become long compared with the range of interaction of the collision partners. Ultracold collisions have much greater sensitivity to entrance channel interactions, so offer the prospect of tuning by external fields to control onset of reaction. However, for ultracold collisions, kinematic constraints impose severe limitations on the observable dynamical properties. In the exit channel for appreciably exoergic reactions, the deBroglie wavelengths become short, so the exit dynamics are much like those for warm collisions. Reactions of alkali dimers, halides, and monoxide molecules are discussed that seem especially congenial for cold collision studies.
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. Published by the American Physical Society 2024