Our group uses electronic spectroscopy to study atoms, molecules and clusters inside or on the surface of superfluid helium nanodroplets. Helium droplets provide an ultracold (0.4 K) and weakly interacting spectroscopic matrix and play the role of a nanosized personal cryostat for dopant atoms and molecules. We are interested in the investigation of alkali atom – helium droplet Rydberg supercomplexes; revealing information on the electronic structure and stability of tailored molecules and clusters residing inside or on the surface of helium nanodroplets; and to understand the interactions between atoms, molecules and clusters with the surrounding helium. These questions require the knowledge on the electronic structure of these atoms and molecules, which are investigated by our group with various laser spectroscopic methods. Our experimental approaches are based on the use of continuous wave (cw) and pulsed laser systems for spectroscopy. Up to three synchronized pulsed lasers (Ti:Sapphire and dye lasers) in combination with a time-of-flight (TOF) mass spectrometer with angular reflectron are the tools used for resonant multi-photon ionization (REMPI) spectroscopy. The pulsed laser systems cover the spectral range from the near infrared to the ultra violet regime which makes REMPI-TOF a very versatile and powerful technique. Cw Ti:Sapphire and dye ring lasers provide narrow band light sources which are applied in our lab for laser induced fluorescence (LIF) spectroscopy and to obtain dispersed emission spectra by using a spectrograph.

Alkali atoms on helium nanodroplets: Rydberg states and Rydberg series

One of the most fascinating systems that can be prepared with alkali atom doped helium droplets is known as Sekatskii atom. This complex consists of an alkali ion core immersed into a helium droplet, with a size of approximately 5 nm and an orbiting electron. It is anticipated that these giant atomic systems are formed upon the excitation of alkali – helium droplet Rydberg states [Lackner2011,Lackner2012,Lackner2013]. The structure and stability of these systems is topic of current discussions and experiments. Many of our experiments paved the road to the spectroscopic investigation of alkali atom Rydberg states on helium droplets [Theisen2010,Theisen2011/Eur.Phys.J D]. We could show that the lowest electronic transition (D1 line) in Rb and Cs on the droplet does not lead to a desorption process [Theisen2011/J.Phys.Chem.Lett]. The atoms stay bound to the surface and the intermediate state can be used as a springboard for the excitation of Rydberg states or the efficient preparation of nm sized ions that contain so called “snowballs”. The term snowball arises because the alkali ion inside the droplet causes density oscillations in the liquid helium environment where the density is expected to locally exceed the density of solid helium [Theisen2010].

All images © TU Graz/Institute of Experimental Physics

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