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Physics World

Collisions feel the cold

Keith B. MacAdam

Published Dec 1, 2001• 0 citations
Publisher

Abstract

For many years atomic-collision experiments basically involved measuring the deflection of fast projectiles that had passed through gaseous targets, or measuring the amount of light given off by atoms and molecules as they were bombarded with different projectiles. In recent years, however, the atomic-physics community has aspired to much more: we want to prepare a target of non-interacting atoms or molecules in a particular quantum state, strike it with projectiles of perfectly known speed, direction and internal state, and then record the time, frequency, speed, energy, direction, spin and internal state of every fragment that emerges from the collisions.

DOI
10.1088/2058-7058/14/12/27

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For many years atomic-collision experiments basically involved measuring the deflection of fast projectiles that had passed through gaseous targets, or measuring the amount of light given off by atoms and molecules as they were bombarded with different projectiles. In recent years, however, the atomic-physics community has aspired to much more: we want to prepare a target of non-interacting atoms or molecules in a particular quantum state, strike it with projectiles of perfectly known speed, direction and internal state, and then record the time, frequency, speed, energy, direction, spin and internal state of every fragment that emerges from the collisions.
The density matrix parametrisation of collisionally excited atomic ensembles is generalised to account for projectile and target spin polarisations. The density matrix elements, containing spin-resolved alignment and orientation parameters, can be determined by measurements of the 'generalised Stokes parameters' introduced by Andersen and Bartschat (1994) to describe scattered-projectile-polarised-photon coincidence experiments. Focusing on electron impact excitation of light alkali-type targets and mercury, the present experimental status of such experiments is reviewed, in particular with regard to the 'perfect scattering experiment' whereby all independent scattering amplitudes are determined.
Over the recent years a new research field emerged that combines technologies for cooling and trapping of neutral atoms and charged ions in one experimental apparatus. These atom-ion hybrid traps enable the investigation of atom-ion interactions at unprecedentedly low collision energies targeting the ultracold s-wave regime. In this thesis I report on experiments performed in an hybrid atom-ion apparatus combining single Ba^+ or Rb^+ ions with ultracold neutral Rb atoms. The main focus of this work is studying the emerging three-body reaction dynamics in a cold charged gas. First, I will present measurements investigating cold reactions in the hetero-nuclear Ba^+ - Rb system. So far, most theoretical and experimental studies have been dealing with reactions between two particles. However, we find that three-body reactions already dominate over two-body ones at atomic densities down to 10^{−11} cm^{−3} once collision energies are in the low mK regime. We then investigate the energy scaling of inelastic ternary collisions between an ion and two atoms. A power-law scaling of the three-body rate coefficient is predicted by theory. We find agreement with the experiment if we include the ion’s kinetic energy distribution that we access numerically. The next study elaborates on the very first observation of ternary reactions in a hybrid cold atom-ion setup. For this we immerse a single Rb^+ ion into ultracold Rb atoms with sufficiently reduced excess micromotion. Occurring reactions are signaled by the creation of highly energetic Rb^+ ions. I will also showcase novel techniques to investigate the atom and the ion system by using the respective other one which were developed by us over the course of the last years. In order to reduce the total ion kinetic energy in a Paul trap one has to compensate the ion’s excess micromotion. This is routinely being done by analyzing the ion’s fluorescence. By observing collision energy dependent elastic atom-ion collisions in an ultracold atom cloud we directly infer the ion’s micromotion energy. This way we can perform atom-ion experiments with optically dark Rb^+ ions and good excess micromotion. Without the need of visible laser light for micromotion compensation we are able to investigate the evolution of quasi-static stray electric fields in a Paul trap over a period of several months. The last study I will present is targeted at neutral Rb only. There, we spectroscopically access the molecular state distribution of Rb_2 molecules formed in neutral three-body recombination by state-selective ionization of these molecules. Trapping and subsequent detection of these ions is being done with our Paul trap, which has the role of a very sensitive single ion detector in these measurements.
A surface-electrode decelerator and deflector for Rydberg atoms and molecules has been developed with the goal of performing collisional experiments. Translationally cold H2{\mathrm{H}}_{2}H2​ molecules in a supersonic beam were excited to Rydberg-Stark states of principal quantum number n=31n=31n=31, loaded into electric traps moving at a predetermined speed above the surface of a bent printed circuit board, decelerated, and deflected from the original direction of the supersonic beam by an angle of ${10}^{\ensuremath{\circ}}$. The phase-space characteristics of the deflected beam were characterized by measuring the time-of-flight distribution and images of the Rydberg molecules and comparing them to the results of numerical particle-trajectory simulations. More than 1000 H2{\mathrm{H}}_{2}H2​ molecules were deflected per experimental cycle at a repetition rate of 25 Hz. The phase-space characteristics of the deflector make it attractive to study ion-molecule reactions at low collision energies.