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Journal of Physics B Atomic Molecular and Optical Physics

Low energy collisions of spin-polarized metastable argon atoms with ground state argon atoms

Thierry Taillandier-Loize, F Perales, J. Baudon, M. Hamamda, Valerij Bočvarski, M. Ducloy, Franck Correia, Nathalie Fabre, G. Dutier

Published Mar 8, 2018• 3 citations
Publisher

Abstract

Abstract The collision between a spin-polarized metastable argon atom in Ar* (3p 5 4s, 3 P 2 , M = +2) state slightly decelerated by the Zeeman slower-laser technique and a co-propagating thermal ground state argon atom Ar (3p 6 , 1 S 0 ), both merged from the same supersonic beam, but coming through adjacent slots of a rotating disk, is investigated at the center of mass energies ranging from 1 to 10 meV. The duration of the laser pulse synchronised with the disk allows the tuning of the relative velocity and thus the collision energy. At these sub-thermal energies, the ‘resonant metastability transfer’ signal is too small to be evidenced. The explored energy range requires using indiscernibility amplitudes for identical isotopes to have a correct interpretation of the experimental results. Nevertheless, excitation transfers are expected to increase significantly at much lower energies as suggested by previous theoretical predictions of potentials 2g( 3 P 2 ) and 2u( 3 P 2 ). Limits at ultra-low collisional energies of the order of 1 mK (0.086 μ eV) or less, where gigantic elastic cross sections are expected, will also be discussed. The experimental method is versatile and could be applied using different isotopes of Argon like 36 Ar combined with 40 Ar, as well as other rare gases among which Krypton should be of great interest thanks to the available numerous isotopes present in a natural gas mixture.

DOI
10.1088/1361-6455/aab52a

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Abstract The collision between a spin-polarized metastable argon atom in Ar* (3p 5 4s, 3 P 2 , M = +2) state slightly decelerated by the Zeeman slower-laser technique and a co-propagating thermal ground state argon atom Ar (3p 6 , 1 S 0 ), both merged from the same supersonic beam, but coming through adjacent slots of a rotating disk, is investigated at the center of mass energies ranging from 1 to 10 meV. The duration of the laser pulse synchronised with the disk allows the tuning of the relative velocity and thus the collision energy. At these sub-thermal energies, the ‘resonant metastability transfer’ signal is too small to be evidenced. The explored energy range requires using indiscernibility amplitudes for identical isotopes to have a correct interpretation of the experimental results. Nevertheless, excitation transfers are expected to increase significantly at much lower energies as suggested by previous theoretical predictions of potentials 2g( 3 P 2 ) and 2u( 3 P 2 ). Limits at ultra-low collisional energies of the order of 1 mK (0.086 μ eV) or less, where gigantic elastic cross sections are expected, will also be discussed. The experimental method is versatile and could be applied using different isotopes of Argon like 36 Ar combined with 40 Ar, as well as other rare gases among which Krypton should be of great interest thanks to the available numerous isotopes present in a natural gas mixture.
The resonant metastability exchange process in low-energy collinear collisions between metastable argon atoms (Ar* 3 P 2 ) polarized in spin ( M = +2) and ground-state Ar atoms from a nozzle beam is studied by means of a time-of-flight technique. A wide range of metastable atom velocities in the laboratory frame (275 m s −1 down to 50 m s −1 ) is obtained by use of a Zeeman slower, the counter-propagating laser beam of which is locked in frequency onto the 3 P 2 – 3 D 3 closed transition (λ = 811.5 nm). The accessible centre-of-mass energy range (8–27 meV) has not been explored so far, to our knowledge. Calculations based upon existing interatomic potentials of 2 g and 2 u symmetries are in reasonable agreement with experiment.
Metastable argon atoms Ar*(3P2), produced by electron bombardment of a nozzle beam of ground state Ar atoms, are slowed down from their initial thermal velocity of 560 m/s down to a few tens of m/s, using a standard Zeeman slower. In this decelerator, a repulsive force is induced by a counter-propagating σ+- polarized laser beam, locked in frequency on the3P2-3D3closed transition (λ= 811.5 nm) and then detuned by 340 MHz. A special profile of longitudinal magnetic field is adjusted in order to maintain atoms in resonance with light all over the device, by compensating the variable Doppler shift by the convenient Zeeman shift. Low velocities, e.g. 55 m/s, are accessible but due to spontaneous emission randomly distributed recoil momentums enlarge both angular and velocity distributions of the beam. Nevertheless, by placing off axis collimating slits and grating, it is in principle possible to observe transmission and/or diffraction phenomena specific of low velocities.
We investigate heteronuclear collisions in isotope mixtures of laser-cooled metastable (3P2{}^{3}\phantom{\rule{-0.16em}{0ex}}{P}_{2}3P2​) neon. Experiments are performed with spin-polarized atoms in a magnetic trap for all two-isotope combinations of the stable neon isotopes 20{}^{20}20Ne, 21{}^{21}21Ne, and 22{}^{22}22Ne. We determine the rate coefficients for heteronuclear ionizing collisions to ${\ensuremath{\beta}}_{21,20}=(3.9\ifmmode\pm\else\textpm\fi{}2.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm3{}^{3}3/s, ${\ensuremath{\beta}}_{22,20}=(2.6\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm3{}^{3}3/s, and ${\ensuremath{\beta}}_{21,22}=(3.9\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm3{}^{3}3/s. We also study heteronuclear elastic collision processes and give upper bounds for heteronuclear thermal relaxation cross sections. This work significantly extends the limited available experimental data on heteronuclear ionizing collisions for laser-cooled atoms involving one or more rare gas atoms in a metastable state.
This thesis presents investigations into the collision dynamics of cold metastable neon atoms in a magneto-optical trap (MOT). At very cold temperatures (approx. 1 mK) the atomic interaction occurs at long range and cannot be treated classically as the interaction between two nuclear point particles. In addition, the collision dynamics can be considerably altered in the presence of an optical eld. In this thesis results are presented for collisions that are altered by the presence of an optical eld and also collisions where the optical eld can be neglected. Controlled optical collisions of cold, metastable Ne atoms are performed by using a control laser tuned close to the (3s)3P2 to (3p)3D3 cooling transition. The control laser modies the rate of ionizing collisions by promoting a pair of colliding atoms onto an excited state potential. In order to induce collisions between pairs of trapped metastables, a control laser is tuned to the red of the transition where we measure a factor of 4 increase in the rate for ionizing collisions. We also investigate the eect of optical shielding by tuning the laser to the blue of the transition and observe a factor of 5 suppression in the ionization rate. However, the measured ionization spectrum does not contain resonances due to the formation of photoassociated molecules as predicted by Doery et al [1]. Instead, we observe a broad unresolvable ionization spectrum that is well described by established theory [2]. Collision cross sections are measured for neon in the (3s)3P2 metastable state with ground state, thermal atoms and molecules. The technique is based on the measurement of the loss rate due to collisions with a background gas. Generally, these type of collisions are considered undesirable, however, it is possible to infer information about the collision cross section if the rate of collisions is accurately known. This novel technique is described and collision cross section measurements are presented for metastable neon in the (3s)3P2 state with He, Ne, Ar, H2, O2, and N2. The average thermal energy of the collision ranges from 11 meV to 27 meV and the measurements have relatively small errors, of the order of 10%.
Absolute velocity-resolved total integral cross sections have been measured for collisions of helium singlet (2 1 S 0 ) and triplet (2 3 S 1 2) metastable (He*) atoms with ground-state helium, neon, and argon atoms in the thermal velocity range of 1.0-3.5*10 5 cm s -1 . Additional measurements on the He*-Ne system with a large input aperture at the detector to suppress the sensitivity of the apparatus to elastic scattering failed to show the previously predicted (Arathoon) sharply rising velocity structure in the inelastic excitation transfer cross sections.
Relaxation experiments have been performed between 0–200 G on optically polarized Rb atoms in argon, krypton or xenon, at pressures ranging from 0.1 to 20 torr. They clearly show the existence of Rb–Ar, Rb–Kr and Rb–Xe molecules of low binding energy (smaller than kT). Molecular parameters such as lifetime, spin-orbit coupling constant, formation rate, constant of mass action, etc., are measured, or evaluated when they cannot be directly measured. Experimental values of the diffusion coefficient of Rb in the gas (except xenon) and of the disorientation cross sections by usual binary collisions are also obtained.