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Physical Review Letters

State Selective Charge Transfer Cross Sections for Na+ with Excited Rubidium: A Unique Diagnostic of the Population Dynamics of a Magneto-Optical Trap

X. Fléchard, Hai Truong Nguyen, R. Brédy, S. R. Lundeen, M. Stauffer, H. A. Camp, C. W. Fehrenbach, Brett D DePaola

Published Dec 11, 2003• 18 citations
Publisher

Abstract

It is shown how the newly developed technique of magneto-optical-trap recoil-ion momentum spectroscopy can be used to measure the temporal evolution of excited state fraction in such a trap. In this case, the fraction of atoms in a 5p state is measured. The technique can be generalized to allow the measurement of more complicated systems, e.g., a Rb sample having a mixture of 5s, 5p, 4d, and Rydberg states.

DOI
10.1103/physrevlett.91.243005

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Journal of Physics Conference Series•Nov 5, 2012•0 citations
It is shown how the newly developed technique of magneto-optical-trap recoil-ion momentum spectroscopy can be used to measure the temporal evolution of excited state fraction in such a trap. In this case, the fraction of atoms in a 5p state is measured. The technique can be generalized to allow the measurement of more complicated systems, e.g., a Rb sample having a mixture of 5s, 5p, 4d, and Rydberg states.
A novel apparatus has been developed in which atoms in a magneto-optical trap are used as targets in ion-atom collision experiments. The apparatus is an extension of earlier methodology in which the momentum of the recoiling target is measured and used to deduce the collision Q value and projectile scattering angle. In the present work, the low temperature of the target atoms yields increased momentum resolution, which in turn leads to improved Q value and scattering angle resolution. In addition, because the trapping process leaves some fraction of the atoms in an excited state, the new methodology is ideal for the study of collisions with excited targets. The prototypical system presented is low energy charge transfer between singly charged alkali ions and trapped rubidium atoms in the ground and first excited state.
A new magneto-optical trap-target recoil ion momentum spectroscopy apparatus has been built and tested at the LPC-CAEN. Dedicated to ion-atom collisions studies and excited fraction measurements, the setup combines a projectile ion beam line, a target of cold rubidium atoms provided by a magneto-optical trap (MOT), and a recoil ion momentum spectrometer. In a test experiment using a beam of Na(+) projectiles, we demonstrate its capability to measure, with a very high signal over background ratio, fully differential cross sections in scattering angle, initial state, and final state of the system. We detail, in this work, features that had not been described previously in the literature: an extraction of the recoil ions transverse to the ion beam axis, and a fast switch for the MOT magnetic field. Advantages of transverse versus longitudinal extraction are discussed, and future possibilities for the setup are presented.
The trapping lasers of a magneto-optical trap have been used to bring Rb atoms into well defined oriented states. Coupled to recoil-ion-momentum spectroscopy, this provided a unique MOTRIMS setup which was able to probe scattering dynamics, including the coherence features, with unprecedented resolution. The technique was applied to the low-energy charge exchange reactions Na+ + Rb(5p±1)→Na(3p,4s)+Rb+. The measurements revealed detailed features of the collisional interaction which were employed to improve the theoretical description. As such, it was possible to ascertain the validity of the intuitive models used to predict the most likely capture transitions.
Charge exchange is examined with unprecedented precision using the newly developed magneto-optical trap-target recoil ion momentum spectroscopy (MOTRIMS) technique. Initial and final state selective, charge exchange cross sections are obtained for 6 keV Cs+ colliding with rubidium in 5s and 5p states. For each charge transfer channel, cross sections differential in scattering angle are measured. These data are used to overturn previous long-standing conjecture as to the origin of oscillations seen in total charge exchange cross section measurements, and compare well with an enhanced Demkov model calculation.