Multiply-charged helium droplets

Helium droplets are very versatile cryogenic flying nano-reactors with unique potential for the formation of clusters and nanometer sized structures. Any kind of gas phase dopants can be picked up by them, including atoms, molecules and even the pickup of multiply charged proteins stored in ion traps has been demonstrated. Sequential pickup of two kinds of dopants leads to core-shell structures and pickup of dopants into large helium droplets sometimes leads to the formation of nanowires that grow along quantized vortices in the superfluid droplets. The absence of solvents other than He leads to ultra-clean conditions for the particle growth, providing that the vacuum is sufficiently free of contaminations from residual gas. The objective of the present proposal is the investigation of recently discovered multiply - charged helium droplets and their applications. Coulomb repulsion of charge centers in a liquid and a possibly superfluid matrix separates them into minimum energy configurations, which represent experimental solutions of the famous Thomson problem, i.e. the arrangement of charges on a conductive sphere. The charges will attract dopants and act as seeds for cluster growth and the uniform distribution of the charge centers will lead to a homogeneous cluster growth and a narrow size distribution of dopant clusters. This is in stark contrast to existing methods of cluster formation that lead to broad size distributions. Furthermore, depending on the size and charge state of the initially undoped helium droplet, several 1000 charged dopant clusters can be formed in one droplet. Experiments are proposed to explore the potential of multiply-charged helium droplets for matrix isolation spectroscopy of embedded (cluster) ions and as efficient breeders of mass- selected dopant clusters and nanoparticles. These dopant clusters will be studied inside the host droplets and after extraction both in the gas phase and deposited onto surfaces. Forefront techniques, such as high-resolution mass spectrometry, microscopy, and laser spectroscopy will be utilized to investigate the properties of the host droplets as well as the dopant clusters. Three PhD students will perform the experiments utilizing experimental setups that have to be adapted from existing ones and are constructed during the first year of this project. The PhD students will be supported and guided by the applicant and experienced postdoc toral researchers working in the group in Innsbruck. Furthermore, international collaborations with research groups in the USA will contribute to the success of this project.