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].
Wolfgang Ernst, Florian Lackner, Alexander Schiffmann, Martin Schnedlitz, Maximilian Ingo Lasserus and Roman MessnerUltra-thin h-BN substrates for nanoscale plasmon spectroscopyShow publication in PURE
Wolfgang Ernst, Florian Lackner, Andreas Hauser, Martin Schnedlitz, Maximilian Ingo Lasserus and Roman MessnerVanadium(V) oxide clusters synthesized by sublimation from bulk under fully inert conditionsShow publication in PURE
Wolfgang Ernst, Florian Lackner, Alexander Schiffmann, Martin Schnedlitz, Maximilian Ingo Lasserus, Harald Matthias Fitzek and Roman MessnerHelium nanodroplet assisted synthesis of bimetallic Ag@Au nanoparticles with tunable localized surface plasmon resonanceShow publication in PURE
Wolfgang Ernst and Florian LacknerPhoto-induced Molecule Formation of Spatially Separated Atoms on Helium NanodropletsShow publication in PURE
Wolfgang Ernst, Florian Lackner, Alexander Schiffmann, Martin Schnedlitz, Maximilian Ingo Lasserus and Roman MessnerSpectroscopy of gold atoms and gold oligomers in helium nanodropletsShow publication in PURE
Wolfgang Ernst, Florian Lackner, Andreas Hauser, Alexander Schiffmann, Martin Schnedlitz and Maximilian Ingo LasserusThermally induced alloying processes in a bimetallic system at the nanoscale: AgAu sub-5 nm core-shell particles studied at atomic resolutionShow publication in PURE
Wolfgang Ernst, Florian Lackner and Andreas HauserRydberg states of alkali atoms on superfluid helium nanodropletsShow publication in PURE