The iquad: chemical reactions at ultralow temperatures

We have succeeded in combining the quadrupole velocity selector with the Coulomb crystal ion trap to study a chemical reaction at temperatures around 1K.

A Coulomb crystal of Ca+ is prepared by laser cooling, and then exposed to a low flux of slow CH3F molecules.  We observe how the crystal changes over a period of an hour or so.

 

The figures above show an initially prepared Ca+ crystal, and its appearance after 35 minutes exposure to CH3F. The changes are caused by reaction, and the production of CaF+ ions, which do not fluoresce under these conditions. In fact, we can demonstrate that the product ions remain trapped, and are sympathetically cooled into the crystal.

Figures (a) and (b) above show another crystal that has undergone reaction to produce crystal (b). By applying a radiofrequency field we can measure the resonant frequencies of the crystals. The ions vibrate more vigorously at resonant frequencies determined by the mass, enabling a mass spectrum to be recorded. The two dips in figure (d) correspond to the CaF+ products and the Ca+ reactants observed for crystal (b) whereas the pre-reacting crystal (a) only shows one peak in figure (c). In (e) we show how the appearance of the crystal (b) changes as the radiofrequency is scanned.

By measuring the change in apparent volume  (equivalent to the number of reactant ions present) as a function of time, we can determine a rate constant for the bimolecular reaction.

The rate constant varies with the quadrupole voltage used for the velocity selector indicating a change in the rate with the effective temperature.


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