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Physical Review Letters

Penning-Trap Technique for Studying Electron-Atom Collisions at Low Energy

Michael D. McGuire, E. Norval Fortson

Published Sep 23, 1974• 11 citations
Publisher

Abstract

In a new technique for studying electron collisions, the electrons are confined by electromagnetic fields and exposed to a known low density of atoms or molecules. Collisions rates are measured by electron diffusion in the trap. The use of spin-oriented atoms allows triplet and singlet contributions to be separated. Results of such studies are reported for room-temperature collisions of electrons with potassium atoms and with nitrogen molecules.

DOI
10.1103/physrevlett.33.737

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Collisions between ultracold neutral atoms have for some time been the subject of investigation, initially with hydrogen and more recently with laser cooled alkali atoms. Advances in laser cooling and trapping of neutral atoms in a Magneto-Optic Trap (MOT) have made cold atoms available as the starting point for many laser cooled atomic physics investigations. The most spectacularly successful of these, the observation of Bose-Einstein Condensation (BEC) in a dilute ultra-cold spin polarized atomic vapor, has accelerated the study of cold collisions. Experimental and theoretical studies of BEC and the long range interaction between cold alkali atoms is at the boundary of atomic and low temperature physics. Such studies have been difficult and would not have been possible without the development and advancement of laser cooling and trapping of neutral atoms. By contrast, ion-atom interactions at low temperature, also very difficult to study prior to modern day laser cooling, have remained largely unexplored. But now, many laboratories worldwide have almost routine access to cold neutral atoms. The combined technologies of ion trapping, together with laser cooling of neutrals has made these studies experimentally feasible and several very important, novel applications might come out of such investigations . This paper is an investigation of ion-atom interactions in the cold and ultra-cold temperature regime. Some of the collisional ion-atom interactions present at room temperature are very much reduced in the low temperature regime. Reaction rates for charge transfer between unlike atoms, A + B(+) approaches A(+) + B, are expected to fall rapidly with temperature, approximately as T(sup 5/2). Thus, cold mixtures of atoms and ions are expected to coexist for very long times, unlike room temperature mixtures of the same ion-atom combination. Thus, it seems feasible to cool ions via collisions with laser cooled atoms. Many of the conventional collisional interactions, exploited as a useful tool at room temperature and higher, are greatly enhanced at low energy. For example, collisional spin transfer from one species of polarized atoms to another has long been a useful method for polarizing a sample of atoms where no other means was available. Because optical pumping cannot be used to polarize the nuclear spin of Xe-129 or He-3 (for use in nmr imaging of the lungs), the nuclear spins are polarized via collisions with an optically pumped Rb vapor in a cell containing both gases. In another case, a spin polarized thermal Cs beam was used to polarize the hyperfine states of trapped He(+)-3 ions in order to measure their hyperfine clock transition frequency. The absence of an x-ray light source to optically pump the ground state of the He(+)-3 ion necessitated this alternative state preparation. Similarly, Cd(+) and Sr(+) ions were spin-oriented via collisions in a cell with optically pumped Rb vapor. Resonant RF spin changing transitions in the ground state of the ions were detected by changes in the Rb resonance light absorption. Because cold collision spin exchange rates scale with temperature as T(sup -1/2) this technique is expected to be a far more powerful tool than the room temperature counterpart. This factor of 100 or more enhancement in spin exchange reaction rates at low temperatures is the basis for a novel trapped ion clock where laser cooled neutrals will cool, state select and monitor the ion clock transition. The advantage over conventional direct laser cooling of trapped ions is that the very expensive and cumbersome UV laser light sources, required to excite the ionic cooling transition, are effectively replaced by simple diode lasers.

•Aug 1, 1997•0 citations
In a new technique for studying electron collisions, the electrons are confined by electromagnetic fields and exposed to a known low density of atoms or molecules. Collisions rates are measured by electron diffusion in the trap. The use of spin-oriented atoms allows triplet and singlet contributions to be separated. Results of such studies are reported for room-temperature collisions of electrons with potassium atoms and with nitrogen molecules.
Lately there has been rapid progress in techniques for trapping samples of neutral atoms at elevated densities and extremely low temperatures. Now a group at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, has used a new type of magnetic trap to confine rubidium atoms at the lowest kinetic temperature yet recorded for an atomic gas. Compelling motivations for this effort include the use of such ultra-cold gases in high-precision spectroscopy, low-energy tests of the "standard model" of particle physics, and studies of atomic collisions at very low relative velocities. Particularly exciting is the prospect of studying quantum collective phenomena such as Bose-Einstein condensation (p37).
Highly-ionized atoms with special properties have been proposed for interesting applications, including potential candidates for a new generation of optical atomic clocks at the one part in 1019 level of precision, quantum information processing and tests of fundamental theory. The proposed atomic systems are largely unexplored. Recent developments at NIST are described, including the isolation of highly-ionized atoms at low energy in unitary Penning traps and the use of these traps for the precise measurement of radiative decay lifetimes (demonstrated with a forbidden transition in Kr17+), as well as for studying electron capture processes.
Ultracold collisions between Cs atoms and Cs2{\mathrm{Cs}}_{2}Cs2​ dimers in the electronic ground state are observed in an optically trapped gas of atoms and molecules. The Cs2{\mathrm{Cs}}_{2}Cs2​ molecules are formed in the triplet ground state by cw photoassociation through the outer well of the ${0}_{g}^{\ensuremath{-}}({P}_{3/2})$ excited electronic state. Inelastic atom-molecule collisions converting internal excitation into kinetic energy lead to a loss of Cs2{\mathrm{Cs}}_{2}Cs2​ molecules from the dipole trap. Rate coefficients are determined for collisions involving Cs atoms in either the F=3F=3F=3 or F=4F=4F=4 hyperfine ground state, and Cs2{\mathrm{Cs}}_{2}Cs2​ molecules in either highly vibrationally excited states (${v}^{\ensuremath{'}}=32--47$) or in low vibrational states (${v}^{\ensuremath{'}}=4--6$) of the $a\text{ }^{3}\ensuremath{\Sigma}_{u}^{+}$ triplet ground state. The rate coefficients $\ensuremath{\beta}\ensuremath{\sim}{10}^{\ensuremath{-}10}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}$ are found to be largely independent of the vibrational and rotational excitation indicating unitary limited cross sections.
Using transverse laser excitation of a single collimated atomic sodium beam in combination with an efficient electron spectrometer, we have studied collisional (= Penning) and photoionization processes involving the laser-excited Na(3 p 3/2 ) and Na(3 d 5/2 ) atoms. As a result of our high electron energy resolution (6 meV) we have for the first time been able to measure the polarization-dependent lineshapes of the Penning electron spectra for collisions between two laser-excited atoms and to reliably separate overlapping contributions due to Penning and photoionization.
Collisions between ultracold neutral atoms have for some time been the subject of investigation, initially with hydrogen and more recently with laser cooled alkali atoms. Advances in laser cooling and trapping of neutral atoms in a Magneto-Optic Trap (MOT) have made cold atoms available as the starting point for many laser cooled atomic physics investigations. The most spectacularly successful of these, the observation of Bose-Einstein Condensation (BEC) in a dilute ultra-cold spin polarized atomic vapor, has accelerated the study of cold collisions. Experimental and theoretical studies of BEC and the long range interaction between cold alkali atoms is at the boundary of atomic and low temperature physics. Such studies have been difficult and would not have been possible without the development and advancement of laser cooling and trapping of neutral atoms. By contrast, ion-atom interactions at low temperature, also very difficult to study prior to modern day laser cooling, have remained largely unexplored. But now, many laboratories worldwide have almost routine access to cold neutral atoms. The combined technologies of ion trapping, together with laser cooling of neutrals has made these studies experimentally feasible and several very important, novel applications might come out of such investigations . This paper is an investigation of ion-atom interactions in the cold and ultra-cold temperature regime. Some of the collisional ion-atom interactions present at room temperature are very much reduced in the low temperature regime. Reaction rates for charge transfer between unlike atoms, A + B(+) approaches A(+) + B, are expected to fall rapidly with temperature, approximately as T(sup 5/2). Thus, cold mixtures of atoms and ions are expected to coexist for very long times, unlike room temperature mixtures of the same ion-atom combination. Thus, it seems feasible to cool ions via collisions with laser cooled atoms. Many of the conventional collisional interactions, exploited as a useful tool at room temperature and higher, are greatly enhanced at low energy. For example, collisional spin transfer from one species of polarized atoms to another has long been a useful method for polarizing a sample of atoms where no other means was available. Because optical pumping cannot be used to polarize the nuclear spin of Xe-129 or He-3 (for use in nmr imaging of the lungs), the nuclear spins are polarized via collisions with an optically pumped Rb vapor in a cell containing both gases. In another case, a spin polarized thermal Cs beam was used to polarize the hyperfine states of trapped He(+)-3 ions in order to measure their hyperfine clock transition frequency. The absence of an x-ray light source to optically pump the ground state of the He(+)-3 ion necessitated this alternative state preparation. Similarly, Cd(+) and Sr(+) ions were spin-oriented via collisions in a cell with optically pumped Rb vapor. Resonant RF spin changing transitions in the ground state of the ions were detected by changes in the Rb resonance light absorption. Because cold collision spin exchange rates scale with temperature as T(sup -1/2) this technique is expected to be a far more powerful tool than the room temperature counterpart. This factor of 100 or more enhancement in spin exchange reaction rates at low temperatures is the basis for a novel trapped ion clock where laser cooled neutrals will cool, state select and monitor the ion clock transition. The advantage over conventional direct laser cooling of trapped ions is that the very expensive and cumbersome UV laser light sources, required to excite the ionic cooling transition, are effectively replaced by simple diode lasers.