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Trapping biomolecular ions in superfluid helium droplets


Bierau,  Frauke
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Bierau, F. (2011). Trapping biomolecular ions in superfluid helium droplets. PhD Thesis, Freie Universität, Berlin, Germany.

The He droplet technique is a versatile method for investigating molecules in a broad range of species at sub-Kelvin temperatures. In the framework of this thesis, an experiment has been setup which allows for studying intrinsic properties of large biomolecules such as peptides and proteins under well-defined conditions. The He droplet technique is combined with an electrospray ion source, a quadrupole mass filter, and a linear hexapod ion trap in which mass-to-charge selected gas-phase biomolecular ions are accumulated. The trap then serves as a pickup cell for He droplets that pass through the trap and pick up the species of interest, cool them down to their equilibrium temperature of approximately 0.37 K, transport them out of the trap, and such make them accessible for further investigation with the experimental method of choice, for example laser spectroscopy. Obtained spectra can reveal a high resolution comparable to those of isolated gas-phase molecules with little perturbances by the helium environment. Helium droplets are generated in a pulsed supersonic nozzle beam expansion. Two different He droplet sources have been installed and characterized. The sizes and size distributions of the droplets have been measured and it turns out that the most probable droplet sizes depend on the embedded particle, the respective source installed, and on the source conditions applied such as the expansion temperature and pressure which provides for manipulation of the droplet size as required.