Cold Reactive Collisions between Laser-Cooled Ions and Velocity-Selected Neutral Molecules

Abstract

We report a new experimental method to study reactive ion-molecule collisions at very low temperatures. A source of laser-cooled ions in a linear Paul trap has been combined with a quadrupole-guide velocity selector to investigate the reaction of Ca+ with CH3F at collision energies E[over](coll)/k(B)> or =1 K with single-particle sensitivity. The technique represents a general approach to study reactive collisions between ions and polar molecules over a wide temperature range down to the cold regime.

Molecules and Ions at Very Low Temperatures

“…The efficiency of this 'sympathetic cooling' is limited by the magnitude of the momentum transfer between the laser-cooled atomic and the molecular ions which depends on the mass mismatch between the two species. Sympathetic cooling represents an extremely versatile technique to prepare a variety of translationally cold molecular ions ranging from diatomics [10,24] and small to medium-sized polyatomics [25] to multiply charged proteins. [26] Fig.…”

“…The trapping and sympathetic cooling of the product ions is confirmed by mass spectrometry and the theoretical modelling of the observed fluorescence images using molecular dynamics (MD) simulations. [10,27] Fig. 4 (a) shows the decrease of the number of Ca + ions from a small Coulomb crystal as a function of the time of reaction with velocity-selected CH 3 F molecules.…”

“…Our new setup combines a facility for the laser and sympathetic cooling of ions in a linear Paul trap with a quadrupole-guide velocity selector for the generation of continuous beams of translationally cold polar molecules. [10,11,27] The velocity selector serves to separate the slowest molecules from a thermal distribution in order to create samples of translationally cold molecules. The device used in our laboratory is an adaption of the design originally developed by Rangwala et al [28] consisting of four bent cylindrical electrodes arranged in a quadrupolar configuration as shown on the right-hand side of Fig.…”

“…With this simple method continuous beams of cold polar molecules with translational temperatures T trans ≥ 1 K and fluxes of 10 8 -10 9 molecules s -1 can be generated. [10,27] The beam of translationally cold molecules is guided towards the centre of the ion trap containing an ion Coulomb crystal. Fig.…”

“…The variety of recently developed cold-molecule sources reflects the interest in the many exciting potential applications such as ultra-high resolution spectroscopy, [8] quantum-information processing [9] and the study of chemical reactions in the cold regime. [6,10,11] Theoretical calculations predict that at ultralow collision energies many chemical reactions are dominated by quantum effects such as tunneling and the occurrence of reactive resonances which are sensitively dependent on the details of the potential energy surface of the relevant system. The study of such effects is not only of considerable interest in the context of low-energy collision physics, but also provides data for stringent tests of reactive-scattering models and high-level ab initio calculations.…”