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1D cold photoassociative ionization of Na inside a storage ring

Rafael R. de Paiva, Riaz Muhammad, R. F. Shiozaki, A.L. de Oliveira, K. M. F. Magalhães, Olivier Morizot, Vanderlei Salvador Bagnato

Published Jul 31, 2009• 0 citations
OpenAlex

Abstract

Motivated by recent interest in low dimensional arrays of atoms, we experimentally investigated the way cold collisional processes are affected by the geometry of the considered atomic sample. More specifically, we studied the case of photoassociative ionization (PAI) both in a storage ring where collision is more unidirectional in character and in a trap with clear undefinition of collision axis. First, creating a ring shaped trap (atomotron) we investigated two-color PAI dependence with intensity and polarization of a probing laser. The intensity dependence of the PAI rate was also measured in a magneto-optical trap presenting equivalent temperature and density conditions. Indeed, the results show that in the ring trap, the value of the PAI rate constant is much lower and does not show evidences of saturation, unlike in the case of the 3D-MOT. Cold atomic collisions in storage ring may represent new possibilities for study.

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Motivated by recent interest in low dimensional arrays of atoms, we experimentally investigated the way cold collisional processes are affected by the geometry of the considered atomic sample. More specifically, we studied the case of photoassociative ionization (PAI) both in a storage ring where collision is more unidirectional in character and in a trap with clear undefinition of collision axis. First, creating a ring shaped trap (atomotron) we investigated two-color PAI dependence with intensity and polarization of a probing laser. The intensity dependence of the PAI rate was also measured in a magneto-optical trap presenting equivalent temperature and density conditions. Indeed, the results show that in the ring trap, the value of the PAI rate constant is much lower and does not show evidences of saturation, unlike in the case of the 3D-MOT. Cold atomic collisions in storage ring may represent new possibilities for study.
In this paper we employ a circulating ring of trapped atoms, that we have named the atomotron, to study cold collisions. The atomotron is obtained from a conventional magneto-optical trap when the two pairs of normally retroreflecting Gaussian laser beams in the $x\text{\ensuremath{-}}y$ plane are slightly offset. Circulating stable atomic orbits then form a racetrack geometry in this plane. The circulating atom flux behaves similarly to an atomic beam with an average tangential velocity much greater than the transverse components, and is therefore suitable for one-dimensional atomic collision studies. Using the atomotron, we have investigated the polarization dependence of ultracold photoassociation collisions between Rb atoms circulating in the racetrack. The ability to investigate collisions in ultracold circulating atomic rings reveals alignment and orientation properties that are averaged away in ordinary three-dimensional magneto-optical trap collision processes.
Three-dimensional quantum scattering calculations predict that the degree of optical shielding of cold collisions saturates with increasing intensity very differently from two-state models and is sensitive to the polarization of the shielding light. We report measurements showing how suppression of Na photoassociative ionization (PAI) rates is sensitive to the intensity and polarization of the shielding laser. At higher intensities, circular polarization suppresses PAI rates an order of magnitude more effectively than linear polarization, in qualitative agreement with our model calculations.
We report measurements of cold photoassociative ionization (PAI) spectra obtained from collisions within a slow, bright Na atomic beam. A high-brightness atom flux, obtained by optical cooling and focusing of the atom beam, permits a high degree of alignment and orientation of binary collisions with respect to the laboratory atom-beam axis. The results reveal features of PAI spectra not accessible in conventional magneto-optical trap studies. We take advantage of this high degree of alignment to selectively excite autoionizing doubly excited states of specific symmetry.
We study photo-associative ionization (PAI) for sodium atoms held in a magneto-optical trap (T < 1 mK). The process is investigated using one and two laser frequencies. For a single color PAI, we measure the rate constant as a function of light intensity and the result is compared with calculation based on semiclassical Bloch equation theory. The measured rate constant appears to saturate sooner than theory. In the two-color PAI (TCPAI) we investigate individual steps of the process. Different combinations of laser frequencies prepare different populations in each of the hyperfine ground state and the contribution of each one in this collisional process is investigated. The comparison with theory allows a fairly clear understanding of PAI and TCPAI.
We report the observation of direct, two-color photoassociative ionization (PAI) in collisions between 85Rb^{85}\mathrm{Rb}85Rb atoms optically cooled and confined in a magneto-optical trap. We also measure the resulting R2+{\mathrm{R}}_{2}^{+}R2+​ ion spectrum and compare it to earlier trap-loss fluoresecence spectra. Unlike PAI in Na, which ionizes principally by collisional autoionization, the Rb2∗{\mathrm{Rb}}_{2}^{\mathrm{*}}Rb2∗​ intermediate photoionizes directly to Rb2+{\mathrm{Rb}}_{2}^{+}Rb2+​ in a fixed optical field. These observations confirm PAI as a sensitive and broadly applicable spectroscopic tool.
Summary form only given. Collisions with cooled and trapped atoms have been the subject of intense investigation during the past years. Unprecedented details of the intermediate steps in the collision have been revealed. A remarkable feature in the ultracold system is the ability to control entrance-channel scattering flux and thereby control the exit channel of the collisional encounter. Here we present the continuation of our recent work where photoassociative ionizations (PAI) of sodium atoms were suppressed by application of an optical field. The new experiments show that suppression of almost 100% is possible when high intensity is used and a remarkable dependence with the polarization of the suppressor laser field is observed and calculated. This result shows a deviation from the simple picture based on the Landau-Zener avoided crossing model when high intensity is used.