Cold state-selected radicals for the study of low temperature chemistry

Abstract

Generating a controllable and pure source of molecular free-radicals or open-shell atoms has been one of the primary barriers hindering the detailed study of radical processes in the laboratory. In this thesis, a novel source of state-selected radicals -- composed of a Zeeman decelerator interfaced with a newly-designed magnetic guide -- is introduced. This tuneable source generates a pure beam of velocity-selected hydrogen atoms that will enable the study of radical interactions with exceptional control over the properties of the radical species. In Zeeman deceleration, time-varying spatially inhomogeneous magnetic fields are used to create packets of translationally cold, quantum-state-selected paramagnetic particles with a tuneable forward velocity, which are ideal for cold reaction dynamics studies. In a proof-of-principle experiment, the Zeeman deceleration of nitrogen atoms in the metastable 2D5/2 state from 460 to 410 m/s is demonstrated for the first time. The covariance matrix adaptation evolutionary strategy (CMA-ES) is adopted in order to optimise deceleration switching sequences for the operation of a 12-stage Zeeman decelerator. Using the optimised sequences, a 40% increase in the number of decelerated H(2S1/2) atoms is observed compared to standard sequences for the same final velocity, imposing the same experimental boundary conditions. Furthermore, up to 98% of the initial kinetic energy of particles in the incoming beam is removed by the optimised sequences, compared to the removal of a maximum of 83% of kinetic energy with standard sequences. Three-dimensional particle-trajectory simulations show that the optimal solution uncovered by the evolutionary algorithm is not merely a local optimisation of the experimental parameters -- it is a new mode of operation that goes beyond the standard periodic phase stability approach typically adopted. A novel magnetic guide is designed and constructed to purify the post-deceleration beam. Only radicals with a selected velocity are transmitted through the guide; all other components of the incoming beam (radical species travelling at other velocities, precursor molecules and seed gases) are removed. The guide is composed of four Halbach arrays -- hexapolar focusing elements -- and two skimming blades. The relative positions of these components can be adjusted to tune the properties of the resulting beam and to optimise transmission for a given velocity. Experimental measurements of Zeeman-decelerated H atoms transmitted through the guide, combined with extensive simulations, show that the magnetic guide successfully removes 99% of H atoms travelling outside the narrow target velocity range.</p