Presentation on theme: "PROPERTIES OF TRAPPED Ca + IONS James Oldham, University of Oxford 63 rd Molecular Spectroscopy Symposium 17 th June 2008."— Presentation transcript:
PROPERTIES OF TRAPPED Ca + IONS James Oldham, University of Oxford 63 rd Molecular Spectroscopy Symposium 17 th June 2008
Introduction Motivation and Experimental Apparatus Simulation Model Findings from Simulations – Motions of Trapped Ions – Ion Temperature Dependencies – Bi-component Crystals Conclusions 2
Motivation Provide theoretical data about Coulomb crystals in context of current experiments. Determine thermal properties of ions in a crystal which will affect collision energies: – Dependence on trap parameters. – Dependence on ion positions. Determine extent that time-dependent potential changes ion energies. Investigate positions of reactant and product ions in bi-component crystals seen during a reaction. 3
Experimental Apparatus 4 Photo-ionised Ca + trapped with linear RF (Paul) trap. Primarily cooled with 397nm diode laser. 866nm diode laser used to repump ion population from metastable 2 D 3/2.
Coulomb Crystals ‘Condensed’ state of a one-component plasma Arises from equilibrium between trapping and repulsive Coulomb forces. Molecular dynamics (MD) simulations provide details such as ion energies, positions of non-fluorescing ions. 5
Modelling Trapped Ca + Ions 6
Simulation of a Coulomb Crystal 7 Simulated image of 1080 Ca + Coulomb crystal
Types of Ion Motion Ion motion comprised of : – secular motion, random thermal motion of ions; – micromotion, high frequency ordered motion induced by RF field. In adiabatic regime, types of motion are independent of one another. 8
How hot are the ions? Two different kinetic energies can be attributed to ions: – Secular energy – Effective energy Secular distributions match Boltzmann distributions at equal T sec. No match between effective energy and Boltzmann distributions. 9 Secular velocity distribution of a 2685Ca + Coulomb crystal with mean secular temperature of 15mK, and corresponding Boltzmann distribution.
Ion Secular Temperatures Varying magnitude of heating force alters T sec in simulations. T sec of experimental crystals obtained by comparing experimental and simulated images. Micromotion has negligible impact on images. 10
Effective Energies of Ions 11 For a trapped ion, Average radial ion displacement increases for larger or more oblate crystals. Greater range of energies expected for large/oblate crystals. Experimental images of an ion string and prolate and oblate crystals.
Effective Energy Distributions 12 Distributions formed by taking ion effective energies over many timesteps in simulation. Distributions dependent on number of ions and shape of crystal Clear shell structures of crystals seen. Changing secular temperature of ions has little effect on distribution.
Reaction Progression Simulated images of bi-component crystals replicate experimental images. Imply product ions formed and trapped experimentally. CaF + reside at greater radial distances than Ca +. 13
Conclusions Experimental crystals accurately reproduced by use of time-dependent MD simulation, allowing greater knowledge of crystals, including kinetic energy profiles including and excluding micromotion. Trap parameters and shape/size of crystal strongly affect effective energies of ions, although secular temperatures not significantly dependent. Micromotion generally dominates kinetic energy of a radially displaced ion. Simulations of bi-component crystals indicate that product ions are formed when CH 3 F is present in chamber. 14
Acknowledgements Alex Gingell Martin Bell Dr. Stefan Willitsch Prof. Tim Softley EPSRC 15