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).
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].
Alexander Schiffmann, Daniel Knez, Florian Lackner, Maximilian Lasserus, Roman Messner, Martin Schnedlitz, Gerald Kothleitner, Ferdinand Hofer and Wolfgang E. ErnstUltra-thin h-BN substrates for nanoscale plasmon spectroscopyPublikation in PURE anzeigen
Maximilian Lasserus, Martin Schnedlitz, Roman Messner, Florian Lackner, Wolfgang E. Ernst and Andreas W. HauserVanadium(V) oxide clusters synthesized by sublimation from bulk under fully inert conditionsPublikation in PURE anzeigen
Roman Messner, Alexander Schiffmann, Johann V. Pototschnig, Maximilian Lasserus, Martin Schnedlitz, Florian Lackner and Wolfgang E. ErnstSpectroscopy of gold atoms and gold oligomers in helium nanodropletsPublikation in PURE anzeigen
Maximilian Lasserus, Martin Schnedlitz, Daniel Knez, Roman Messner, Alexander Schiffmann, Florian Lackner, Andreas W. Hauser, Ferdinand Hofer and Wolfgang E. ErnstThermally induced alloying processes in a bimetallic system at the nanoscale: AgAu sub-5 nm core-shell particles studied at atomic resolutionPublikation in PURE anzeigen
Florian Lackner and Wolfgang E. ErnstPhoto-induced Molecule Formation of Spatially Separated Atoms on Helium NanodropletsPublikation in PURE anzeigen
Johann V. Pototschnig, Florian Lackner, Andreas W. Hauser and Wolfgang E. ErnstRydberg states of alkali atoms on superfluid helium nanodropletsPublikation in PURE anzeigen
Johann Valentin Pototschnig, Günter Krois, Florian Lackner and Wolfgang ErnstInvestigation of the RbCa Molecule: Experiment and TheoryPublikation in PURE anzeigen
Philipp Thaler, Alexander Volk, Daniel Knez, Florian Lackner, Georg Haberfehlner, Johannes Steurer, Martin Schnedlitz and Wolfgang ErnstSynthesis of Nanoparticles in Helium Droplets - a Characterization Comparing Mass-Spectra and Electron Microscopy DataPublikation in PURE anzeigen
Markus Koch, Andreas Kautsch, Florian Lackner and Wolfgang ErnstOne- and Two-Color Resonant Photoionization Spectroscopy of Chromium-Doped Helium NanodropletsPublikation in PURE anzeigen
Florian Lackner, Günter Krois, Thomas Buchsteiner, Johann Valentin Pototschnig and Wolfgang ErnstHelium Droplet Assisted Preparation of Cold RbSr MoleculesPublikation in PURE anzeigen
Johann Valentin Pototschnig, Günter Krois, Florian Lackner and Wolfgang ErnstAb initio study of the RbSr electronic structure: Potential energy curves, transition dipole moments and permanent electric dipole momentsPublikation in PURE anzeigen
Günter Krois, Florian Lackner, Johann Valentin Pototschnig, Thomas Buchsteiner and Wolfgang ErnstCharacterization of RbSr molecules: spectral analysis on helium dropletsPublikation in PURE anzeigen
Philipp Thaler, Alexander Volk, Florian Lackner, Johannes Steurer, Daniel Knez, Werner Grogger, Ferdinand Hofer and Wolfgang E. ErnstFormation of bimetallic core-shell nanowires along vortices in superfluid He nanodropletsPublikation in PURE anzeigen
Florian Lackner, Johannes Poms, Günter Krois, Johann Valentin Pototschnig and Wolfgang ErnstSpectroscopy of Lithium Atoms and Molecules on Helium NanodropletsPublikation in PURE anzeigen
Florian Lackner, Günter Krois and Wolfgang ErnstRydberg-Ritz analysis and quantum defects for Rb and Cs atoms on helium nanodropletsPublikation in PURE anzeigen
Günter Krois, Johann Valentin Pototschnig, Florian Lackner and Wolfgang ErnstSpectroscopy of Cold LiCa Molecules Formed on Helium NanodropletsPublikation in PURE anzeigen
Florian Lackner, Günter Krois, Markus Koch and Wolfgang ErnstRubidium on Helium Droplets: Analysis of an Exotic Rydberg Complex for n* Publikation in PURE anzeigen
Moritz Theisen, Florian Lackner and Wolfgang ErnstRb and Cs oligomers in different spin configurations on helium nanodropletsPublikation in PURE anzeigen
Moritz Theisen, Florian Lackner, Francesco Ancilotto, Carlo Callegari and Wolfgang ErnstTwo-step excitation of Rb atoms on He nanodropletsPublikation in PURE anzeigen
Florian Lackner, Günter Krois, Moritz Theisen, Markus Koch and Wolfgang ErnstSpectroscopy of nS, nP, and nD Rydberg series of Cs atoms on helium nanodropletsPublikation in PURE anzeigen
Moritz Theisen, Florian Lackner and Wolfgang ErnstCs atoms on helium nanodroplets and the immersion of Cs+ into the nanodropletPublikation in PURE anzeigen
Moritz Theisen, Florian Lackner, Günter Krois and Wolfgang ErnstIonization Thresholds of Alkali Metal Atoms on Helium DropletsPublikation in PURE anzeigen