Catching Ions in Superfluid Helium Droplets Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qb98mj64d

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  • In this dissertation, I describe the experimental investigation of catching ions in superfluid helium droplets. The ultimate goal of our project is to build a coherent electron diffraction apparatus for atomic structure determination from oriented single macromolecules. This involves generating protein ions from electrospray ionization (ESI) and doping them in helium droplets. The doped droplets are then oriented by an elliptically polarized laser and exposed to a coherent electron beam for diffraction. The specific goal of my project is to explore the methods for sample preparation. I have started by exploring the doping conditions for continuous cesium ions from a thermionic emission source and then moved on to pulsed ions from an ESI source. I have also characterized the size and size distribution of the doped droplets. The relative ion doping efficiency of the continuous cesium ions is determined by both the kinetic energy of the ions and the size of the droplets. For reserpine and substance P from the ESI source, we have succeeded with a simple experimental setup by stopping the ions in the droplet beam with a DC bias. The resulting doping efficiency is observed to scale with the residence time of the ion beam in the droplet beam. To further control the size of the ion doped droplets for diffraction, I also have performed an exploratory experiment of collisional size reduction of ion doped droplets. These exercises and the resulting theoretical understanding of the doping mechanism of cations in superfluid helium droplets will be fundamental for the next stage of the overall project.
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