Experiments with an ion-neutral hybrid trap: cold charge-exchange collisions

• Published in: Applied physics. B, Lasers and optics Vol. 114; no. 1-2; pp. 75 - 80
• Main Authors: Smith, W. W., Goodman, D. S., Sivarajah, I., Wells, J. E., Banerjee, S., Côté, R., Michels, H. H., Mongtomery, J. A., Narducci, F. A.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 2014
• Subjects: Activation; Cold traps; Collisions; Constants; Engineering; Hybrid systems; Lasers; Nanostructure; Optical Devices; Optics; Photonics; Physical Chemistry; Physics; Physics and Astronomy; Quantum Optics; Rate constants; Charge Exchange; Paul Trap; Trapping Time; Entrance Channel; Effective Core Potential
• ISSN: 0946-2171; 1432-0649
• DOI: 10.1007/s00340-013-5672-2

Due to their large trap depths (∼1 eV or 10,000 K), versatility, and ease of construction, Paul traps have important uses in high-resolution spectroscopy, plasma physics, and precision measurements of fundamental constants. An ion-neutral hybrid trap consisting of two separate but spatially concentric traps [a magneto-optic trap (MOT) for the neutral species and a mass-selective linear Paul trap for the ionic species] is an ideal apparatus for sympathetic cooling. However, over the past few years, hybrid traps have proven most useful in measuring elastic and charge-exchange rate constants of ion-neutral collisions over a wide temperature range from kilo-Kelvin to nano-Kelvin. We report some initially surprising results from a hybrid trap system in our laboratory where we have loaded the Paul trap with Ca + ions in the presence of a Na MOT (localized dense gas of cold Na atoms). We find a strong loss of Ca + ions with MOT exposure, attributed to an exothermic, non-resonant ion-neutral charge-exchange process with an activation barrier, which leads to the formation of Na + ions. We propose a detailed mechanism for this process. We obtain an estimated measure of the rate constant for this charge exchange of ∼2 × 10 −11   cm 3 /s, much less than the Langevin rate, which suggests that the Langevin assumption of unit efficiency in the reaction region is not correct in this case.