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Physical Review Letters

Kinematically Complete Charge Exchange Experiment in the Cs++Rb Collision System Using a MOT Target

X. Fléchard, H. Nguyen, E. Wells, I. Ben-Itzhak, Brett D DePaola

Published Aug 31, 2001• 65 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.87.123203

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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.
Resolution plays a vital role in spectroscopic studies. In the usual recoil-ion momentum spectroscopy (RIMS), Q-value resolution is relied upon to distinguish between different collision channels: The better the Q-value resolution, the better one is able to resolve energetically similar channels. Although traditional COLTRIMS greatly improves Q-value resolution by cooling the target and thus greatly reducing the initial target momentum spread, the resolution of the technique is still limited by target temperature. However, with the recent development in RIMS, namely, magneto-optical trap recoil ion momentum spectroscopy (MOTRIMS) superior recoil ion momentum resolution as well as charge transfer measurements with laser excited targets have become possible. Through MOTRIMS, methods for the measurements of target excited state fraction and kinematically complete relative charge transfer cross sections have been developed, even for some systems having energetically degenerate or nearly degenerate channels. In the present work, the systems of interest having energy degeneracies or near degeneracies are Rb+{\mathrm{Rb}}^{+}Rb+, K+{\mathrm{K}}^{+}K+, and Li+{\mathrm{Li}}^{+}Li+ colliding with trapped Rb(5l)\mathrm{Rb}(5l)Rb(5l), where l=sl=sl=s and ppp.
A new MOTRIMS (magneto-optical trap-target recoil ion momentum spectroscopy) apparatus has been built at the LPC-CAEN, and recently tested with a beam of 2 keV Na + projectiles colliding on a trapped Rb target. In this work, we demonstrate the capability of the setup to provide, with a very high signal over background ratio, fully differential cross sections in scattering angle, initial state, and final state of the system. The fine control of the trapping lasers, the extraction of the recoil ions transverse to the ion beam axis, and the fast switching for the MOT magnetic field, are three characteristics promising for new studies including collisions with oriented targets or requiring electron detection.
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.