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arXiv:1901.02482 · The Journal of Chemical Physics

Penning collisions between supersonically expanded metastable He atoms and laser-cooled Li atoms

Jonas Grzesiak, Takamasa Momose, Frank Stienkemeier, Marcel Carto Constantin Mudrich, Katrin Dulitz

Published Jan 17, 2019• 9 citations
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Abstract

We describe an experimental setup comprised of a discharge source for supersonic beams of metastable He atoms and a magneto-optical trap (MOT) for ultracold Li atoms that makes it possible to study Penning ionization and associative ionization processes at high ion count rates. The cationic reaction products are analyzed using a novel ion detection scheme which allows for mass selection, a high ion extraction efficiency, and a good collision-energy resolution. The influence of elastic He-Li collisions on the steady-state Li atom number in the MOT is described, and the collision data are used to estimate the excitation efficiency of the discharge source. We also show that Penning collisions can be directly used to probe the temperature of the Li cloud without the need for an additional time-resolved absorption or fluorescence detection system.

DOI
10.1063/1.5063709

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We describe an experimental setup comprised of a discharge source for supersonic beams of metastable He atoms and a magneto-optical trap (MOT) for ultracold Li atoms that makes it possible to study Penning ionization and associative ionization processes at high ion count rates. The cationic reaction products are analyzed using a novel ion detection scheme which allows for mass selection, a high ion extraction efficiency, and a good collision-energy resolution. The influence of elastic He-Li collisions on the steady-state Li atom number in the MOT is described, and the collision data are used to estimate the excitation efficiency of the discharge source. We also show that Penning collisions can be directly used to probe the temperature of the Li cloud without the need for an additional time-resolved absorption or fluorescence detection system.
Control of the combined Penning and associative ionization cross section is demonstrated with cold (~1mK) metastable Ne (3s 3 P 2 ) atoms in a magneto-optical trap (MOT). By illuminating the trapped atoms with a near resonant probe laser beam, increased ionization rates are observed at several detunings. The probe beam is swept through a region from +500MHz to −500MHz. The increase in the Penning and associative ionization cross section is observed in both the red and blue regions of the spectrum.
We have studied a cloud of cold metastable helium (He*) atoms interacting with near-resonant light at 1083 nm and 389 nm. The 1083 nm light allows for efficient loading of a large magneto-optical trap (MOT) and the 389 nm light is subsequently used to increase the density and reduce the temperature of the He* cloud during a brief compression stage. Cold collisions in the cloud yield ions and fast metastables which are monitored separately using calibrated microchannel plate (MCP) detectors. We thus measure absolute production rates of ions and fast metastables escaping from the MOT during the various stages of the experiment. We observe that 389 nm optical collisions, apart from Penning ionization, produce a relatively large flux of fast metastables, which we relate to the short-range behaviour of the molecular potentials involved. Furthermore, by rapidly switching between 389 nm and 1083 nm the ratio between the respective two-body loss rate constants is determined. Using these values, together with the observed time dependence of the cloud size, the temporal behaviour of the absolute ion production rate during the compression stage is well reproduced.
We have studied the optical collisions of cold, metastable helium atoms in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the associative ionization rate is increased with more than a factor 20 due to the presence of the light field. We present a simple, semiclassical model, which accounts for all the observed features, and which agrees, also on an absolute scale, with our experimental results.
We report on the efficient production of Li 2 + ions in a magneto-optical trap (MOT) of Li atoms prepared in an ensemble of 2 S , 2 P , and 3 S states. A linear Paul trap surrounding the MOT is used to trap and investigate the time evolution of the ion populations. Associative ionization of Li( 2 P 3 / 2 ) and Li( 3 S 1 / 2 ) atoms, with a giant cross section of ∼ 2.2 × 10 6 a 0 2 , is the dominant Li 2 + production channel. Li 2 + decays to Li + via photodissociation and also in the absence of light, which are investigated experimentally and theoretically. A fraction of the Li 2 + ions are long-lived in the presence of the MOT light, implying that they are deeply bound and hence immune to dissociation, which opens up the possibility for studying Li 2 + molecular ion interactions with alkali atoms.
Summary form only given. The understanding of the interaction of ultracold atoms will be substantially extended by the experimental study of ultracold non-alkaline systems. Due to the special internal structure and the high internal energy, metastable noble gases will play a crucial role. The objective of our work is to attain a detailed study of the atomic structure and the interactions in a gas of ultracold metastable neon atoms. Ensembles of high phase space density and, if possible, Bose condensates shall be achieved. The elementary pair interaction as well as the collective interactions are to be characterized. Of special interest is Penning ionization as it is the main loss process in dense metastable atom traps. The degree of its suppression in a spin polarized gas will strongly affect the feasibility of a quantum-degenerate system of metastable atoms. In addition, metastable noble gas atoms will allow the implementation of novel diagnostics for the investigation of Bose condensates due to their high detectability. We want to make use of this advantage in order to investigate higher order correlation functions and study the laser-like behavior of Bose condensates and atom-laser outputs. We have achieved loading of 4/spl times/10/sup 8/ atoms in a magneto-optical trap (MOT) and after efficient spin polarization of 2.5 /spl times/ 10/sup 8/ atoms in a Ioffe-Pritchard-type magnetic trap. We present a precision measurement of the lifetime of the 3s[3/2]/sub 2/-metastable state of neon which we could determine to 14.70(13)s. We report on the status of the determination of the elastic scattering rate, the rates of Penning ionization for spin-polarized and non-spin-polarized atom samples, and the implementation of evaporative cooling.
About 10 4 triplet metastable helium atoms have been trapped in a magneto-optical trap, based on the 2 3 S 1 –2 3 P 2 transition at 1.083 μm (volume 0.1 mm 3 , temperature 1 mK). The losses of the trap are analysed by monitoring the ion current due to Penning collisions either with background gases, or between trapped atoms. The rate coefficient for these collisions between trapped metastable helium atoms has been found to be unusually large, and the crucial role of the quasi-resonant laser light has been experimentally demonstrated. The rate coefficient of the dominant process (He*(2 3 S 1 ) + He*(2 3 P 2 ) Penning collisions) has been measured by two different methods, and found to be K sp ≃ 10 -7 cm 3 s -1 within an estimated factor of 4. The corresponding cross-section is 6 · 10 6 Å 2 .