Cluster Lab I

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.

Figure 1. Part of the experimental apparatus. Upon generation of the helium nanodroplets (HeN, N ~ 10 000) in the source chamber, they are doped by passing through resistively heated pickup cells (Pickup). Laser induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF) are the main experimental techniques used in our experiments. The beam terminates in a quadrupole mass spectrometer (QMS).


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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].

Figure 2. In their electronic ground state alkali atoms and molecules are located at the surface of helium nanodroplets (left). Upon excitation of a high Rydberg state of the atom, the ion core is expected to immerse into the helium droplet while the electron is orbiting outside (right).


Similar to free atoms, Rydberg states on helium droplets follow a systematic behavior which allows an organization of these states into Rydberg series, which gives much more information on the nature of these systems than the investigation isolated transitions [Lackner2011,Lackner2012]. Quantum defects and ionization thresholds obtained within a Rydberg-Ritz approach reveal insight into the screening mechanism that shields the Rydberg electron from the alkali ion core and demonstrate the attractive interaction between ion core and the droplet [Lackner2013].


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Latest Publications

2019
Wolfgang Ernst, Florian Lackner, Alexander Schiffmann, Martin Schnedlitz, Maximilian Ingo Lasserus and Roman Messner Ultra-thin h-BN substrates for nanoscale plasmon spectroscopy Show publication in PURE
Wolfgang Ernst, Florian Lackner, Andreas Hauser, Martin Schnedlitz, Maximilian Ingo Lasserus and Roman Messner Vanadium(V) oxide clusters synthesized by sublimation from bulk under fully inert conditions Show publication in PURE
Wolfgang Ernst, Florian Lackner, Alexander Schiffmann, Martin Schnedlitz, Maximilian Ingo Lasserus, Harald Matthias Fitzek and Roman Messner Helium nanodroplet assisted synthesis of bimetallic Ag@Au nanoparticles with tunable localized surface plasmon resonance Show publication in PURE
Florian Lackner Evaporation of an anisotropic nanoplasma Show publication in PURE
2018
Wolfgang Ernst and Florian Lackner Photo-induced Molecule Formation of Spatially Separated Atoms on Helium Nanodroplets Show publication in PURE
Wolfgang Ernst, Florian Lackner, Alexander Schiffmann, Martin Schnedlitz, Maximilian Ingo Lasserus and Roman Messner Spectroscopy of gold atoms and gold oligomers in helium nanodroplets Show publication in PURE
Wolfgang Ernst, Florian Lackner, Andreas Hauser, Alexander Schiffmann, Martin Schnedlitz and Maximilian Ingo Lasserus Thermally induced alloying processes in a bimetallic system at the nanoscale: AgAu sub-5 nm core-shell particles studied at atomic resolution Show publication in PURE
2017
Wolfgang Ernst, Florian Lackner and Andreas Hauser Rydberg states of alkali atoms on superfluid helium nanodroplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Lithium atoms on helium nanodroplets Show publication in PURE
2016
Florian Lackner Tracking dissociation dynamics of strong-field ionized 1,2-dibromoethane with femtosecond XUV transient absorption spectroscopy Show publication in PURE
2015
Wolfgang Ernst, Florian Lackner and Martin Schnedlitz Synthesis of Nanoparticles in Helium Droplets - a Characterization Comparing Mass-Spectra and Electron Microscopy Data Show publication in PURE
Wolfgang Ernst and Florian Lackner Investigation of the RbCa Molecule: Experiment and Theory Show publication in PURE
2014
Wolfgang Ernst and Florian Lackner Helium Droplet Assisted Preparation of Cold RbSr Molecules Show publication in PURE
Wolfgang Ernst and Florian Lackner Characterization of RbSr molecules: spectral analysis on helium droplets Show publication in PURE
Wolfgang Ernst, Markus Koch and Florian Lackner One- and Two-Color Resonant Photoionization Spectroscopy of Chromium-Doped Helium Nanodroplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Formation of bimetallic core-shell nanowires along vortices in superfluid He nanodroplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Ab initio study of the RbSr electronic structure: Potential energy curves, transition dipole moments and permanent electric dipole moments Show publication in PURE
2013
Wolfgang Ernst and Florian Lackner Spectroscopy of Lithium Atoms and Molecules on Helium Nanodroplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Spectroscopy of Cold LiCa Molecules Formed on Helium Nanodroplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Rydberg-Ritz analysis and quantum defects for Rb and Cs atoms on helium nanodroplets Show publication in PURE
2012
Markus Koch, Wolfgang Ernst and Florian Lackner Rubidium on Helium Droplets: Analysis of an Exotic Rydberg Complex for n* Show publication in PURE
2011
Markus Koch, Wolfgang Ernst and Florian Lackner Spectroscopy of nS, nP, and nD Rydberg series of Cs atoms on helium nanodroplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Ionization Thresholds of Alkali Metal Atoms on Helium Droplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Cs atoms on helium nanodroplets and the immersion of Cs+ into the nanodroplet Show publication in PURE
Wolfgang Ernst and Florian Lackner Two-step excitation of Rb atoms on He nanodroplets Show publication in PURE
Wolfgang Ernst and Florian Lackner Rb and Cs oligomers in different spin configurations on helium nanodroplets Show publication in PURE
2010
Wolfgang Ernst and Florian Lackner Forming Rb+ snowballs in the center of He nanodroplets Show publication in PURE

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All images © TU Graz/Institute of Experimental Physics

Group members
image/svg+xml


Prof. Wolfgang E. Ernst
wolfgang.ernstnoSpam@tugraz.at
+43 (316) 873-8140


Ass.Prof. Markus Koch
markus.kochnoSpam@tugraz.at
+43 (316) 873-8161

 
Dr. Florian Lackner
florian.lackner@.tugraz.at