Univ.-Prof. Dr. Paul Scheier
NanoBio Physics
Univ.-Prof. Dr. Paul Scheier
+43 (512) 507 52660
Dr. Elisabeth Gruber
+43 (512) 507 52670
Pickup of atoms and molecules in highly charged helium droplets enables the effective generation of size-selected clusters and nanoparticles. In addition, ions can be generated with a few helium atoms attached, which offer ideal conditions for spectroscopy.
Multiple ionization of pristine HNDs distributes charge centers in minimum energy configurations close to the surface of the droplets where they form a 2D-Wigner crystal. Dopants are polarized and attracted by these charge centers which leads to a homogeneous growth of singly-charged clusters and nanoparticles with an unprecedentedly narrow size distribution. These dopant cluster ions can be removed from the large host droplets still solvated with up to a few 100 He atoms via splashing of the HND upon surface collisions or controlled shrinking in a helium buffer gas.
Splashing of doped helium droplets leads predominantly to backscattering of embedded dopant clusters. However, by taking advantage of this effect, it is possible to cover surfaces within seconds with a film of monodisperse clusters or nanoparticles.
He tagged C60+ in front of an image of the sky. Laser spectroscopy of such weakly bound complexes enables the determination of absorption lines of cold cations and anion from the UV to the IR upon single photon absorption. These spectra are then compared with observational data obtained by ground and space based telescopes.
Currently, more than 200 molecules have been identified in the interstellar medium. In addition, astronomers have recorded some 600+ absorption bands in diffuse interstellar clouds where UV light is expected to ionize heavy elements and most molecules. Utilizing He tags as messengers for photoabsorption, it is possible to determine electronic transitions of cold molecular ions and clusters and compare them with absorption lines recorded by modern astronomical telescopes, both ground and space based.
Photo absorption of cold ionic complexes can be measured utilizing He messenger spectroscopy. The image shows a red laser hitting a sample of phthalocyanine – the structure of the molecule is also shown. This technique also enables to probe the effect of solvation including hydration of biomolecules as a function of the number of solvent molecules.
Laser spectroscopy of cold ions in combination with quantum chemical calculations provides insight into the interaction of molecules with mass selected metal cluster ions. Activation of otherwise inert molecules, such as CH4 or CO2 can be measured as a function of the cluster size which can be considered as a first step for a catalytic reaction. The low binding energy of He taggants is beneficial as absorption of a single IR photon will lead to its loss and can be recorded by mass spectrometry.