Low temperature ion-radical collisions

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The chemistry of various gaseous environments is dominated by reactions involving transient, highly-reactive atomic and molecular species; these environments include the interstellar medium (ISM, principally very low density gas clouds between the stars) the upper atmosphere, flames and combustion systems, electric discharges and plasmas. The key highly-reactive species present are either free-radicals - atoms or molecules which generally have an odd number of electrons so that at least one electron is 'unpaired' - or ionic species carrying a positive electrical charge (cations). Such species have a natural tendency to form new chemical bonds, and typically reactions of these species have very low activation energies, or even zero activation energy. This means that they typically have very fast reactions even at low temperatures - indeed many reactions of these species become faster as the temperature lowers. In order to model the chemistry of the environments above (which may contain hundreds of different chemical species), we need to know how fast the reactions of these species are, and how those reaction rates vary with temperature. For the interstellar medium the temperatures are very low (10 - 50 Kelvin) whereas in flames and plasmas the temperature may be very high. Thus knowing the reaction rates at room temperature does not generally provide sufficient information for modelling purposes.In this work we will measure the rates of reactions between free-radical species and ionic species over wide temperature ranges (from above 300 Kelvin to below 1 Kelvin). There is currently a vast knowledge gap in terms of measuring rates for reactions between two transient species - most work has been done with one transient species and one stable species. The reason for the current dearth of information from laboratory measurements is that both of the transient species involved in the reaction tend to be present in very low concentrations, and therefore current methods lack the sensitivity to detect the occurrence of reactions - the number of reaction product molecules formed per second is likely to be undetectably low.To make these measurements we will assemble a unique instrument which consists of a 'Zeeman decelerator' for producing the free-radical species with variable kinetic energies (and hence variable temperatures), and a laser-cooled ion trap for producing the cold target ionic species for reaction. The Zeeman decelerator uses the fact that free-radical species are typically magnetic (as they have unpaired electrons) and so their velocities can be controlled using magnetic fields - in this case the fields are created by a linear sequence of 12-100 solenoid coils through which the radicals pass. By decelerating the radical species (H, N and O atoms, or CH3 and CN molecules) we can control their kinetic energy and hence the temperature. For the ionic species we use a radiofrequency quadrupole to trap the ions (which are produced by laser ionization of neutral precursors), and by using laser cooling we can create a low density cloud of atomic ions (Ca+ in this case). The ions condense to form a 'Coulomb crystal' in which the ions take up positions in a regular array and the temperature can be as low as a few milli-Kelvin The Ca+ ions are constantly fluorescing and can be observed individually by imaging microscopy. Molecular ions (in this case CH+, C2H2+, CO2+, or C6H6+) can then be co-condensed ('sympathetically cooled') into the Coulomb crystal and trapped there for periods of hours.In the experiments, radicals from the Zeeman decelerator interact with trapped ions, and reactions occur producing a new ionic chemical species, which is also trapped. Thus the reaction rate is determined by monitoring product ion formation versus time. The unique ability of our proposed experiment derives from a combination of the very long trapping time of the ions and the capability to observe even single ions in the trap.