Moritz Theisen
Graz University of Technology
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Featured researches published by Moritz Theisen.
Journal of Chemical Physics | 2011
Moritz Theisen; Florian Lackner; Wolfgang E. Ernst
We report the non-desorption of cesium (Cs) atoms on the surface of helium nanodroplets (He(N)) in their 6(2)P(1/2) ((2)Π(1/2)) state upon photo-excitation as well as the immersion of Cs(+) into the He(N) upon photo-ionization via the 6(2)P(1/2) ((2)Π(1/2)) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 6(2)P states. We compare laser-induced fluorescence (LIF) spectra with a desorption-sensitive method (Langmuir-Taylor detection) for different excitation energies. Dispersed fluorescence spectra show a broadening of the emission spectrum only when Cs-He(N) is excited with photon energies close to the atomic D(1)-line, which implies an attractive character of the excited state system (Cs∗-He(N)) potential energy curve. The experimental data are compared with a calculation of the potential energy curves of the Cs atom as a function of its distance R from the center of the He(N) in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 6(2)P(1/2) ((2)Π(1/2)) to the 6(2)S(1/2) ((2)Σ(1/2)) state help to explain the experimental data. The stability of the Cs∗-He(N) system allows to form Cs(+) snowballs in the He(N), where we use the non-desorbing 6(2)P(1/2) ((2)Π(1/2)) state as a springboard for ionization in a two-step ionization scheme. Subsequent immersion of positively charged Cs ions is observed in time-of-flight mass spectra, where masses up to several thousand amu were monitored. Only ionization via the 6(2)P(1/2) ((2)Π(1/2)) state gives rise to a very high yield of immersed Cs(+) in contrast to an ionization scheme via the 6(2)P(3/2) ((2)Π(3/2)) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, Cs(2) (+)-He(N) aggregates are observed in time-of-flight mass spectra.
Journal of Physical Chemistry A | 2011
Moritz Theisen; Florian Lackner; Wolfgang E. Ernst
The study of small clusters is intended to fill the knowledge gap between single atoms and bulk material. He nanodroplets are an ideal matrix for preparing and investigating clusters in a superfluid environment. Alkali-metal atoms are only bound very weakly to their surface by van der Waals forces. Due to the formation process, high-spin states of alkali-metal clusters on He nanodroplets are favorably observed, which is in contrast to the abundance in other preparation processes. Until now, the prevailing opinion was that stable clusters of the heavy alkali-metal atoms, rubidium (Rb) and cesium (Cs) on He nanodroplets, are limited to 5 and 3 atoms, respectively (Schulz et al., Phys. Rev. Lett. 2004, 92, 13401). Here, we present stable complexes of Rb(n)⁺ and Cs(n)⁺ consisting of up to n = 30 atoms, with the detection of large alkali-metal clusters being strongly enhanced by one-photon ionization. Our results also suggest that we monitored both high-spin and low-spin state clusters created on nanodroplets. The van der Waals bound high-spin alkali-metal clusters should show strong magnetic behavior, while low-spin states are predicted to exhibit metallic characteristics. Alkali-metal clusters prepared in these two configurations appear to be ideal candidates for investigating nanosized particles with ferromagnetic or metallic properties.
Journal of Chemical Physics | 2009
Moritz Theisen; Martin Linke; Max Kerbs; Henk Fidder; Mohamed Madjet; Angelica Zacarias; Karsten Heyne
A method is presented that combines femtosecond polarization resolved UV/visible pump-IR probe spectroscopy and density functional theory calculations in determining the three-dimensional orientation of an electronic transition dipole moment (tdm) within the molecular structure. The method is demonstrated on the approximately planar molecule coumarin 314 (C314) dissolved in acetonitrile, which can exist in two ground state configurations: the E- and the Z-isomer. Based on an exhaustive search analysis on polarization resolved measurement data for four different vibrational modes, it is concluded that C314 in acetonitrile is the E-isomer. The electronic tdm vector for the electronic S(0)-->S(1) transition is determined and the analysis shows that performing the procedure for four vibrational modes instead of the minimally required three reduces the 1sigma probability area from 2.34% to 2.24% of the solution space. Moreover, the fastest rotational correlation time tau(c) for the C314 E-isomer is determined to be 26+/-2 ps.
ChemPhysChem | 2010
Martin Linke; Moritz Theisen; Theodore von Haimberger; Mohamed Madjet; Angelica Zacarias; Henk Fidder; Karsten Heyne
A new mixed experimental and theoretical approach for determining the exact three-dimensional orientation of electronic transition dipole moments (tdms) within the molecular frame is discussed. Results of applying this method on Chlorophyll a and the dye Coumarin 314 (C314) are presented. For C314 the possible influence of a mixture of E- and Z-isomers in the sample on the determined electronic tdm is investigated. Moreover, the robustness of the method is investigated with the C314 data.
Physical Chemistry Chemical Physics | 2010
Moritz Theisen; Florian Lackner; Wolfgang E. Ernst
Physical Chemistry Chemical Physics | 2011
Florian Lackner; Günter Krois; Moritz Theisen; Markus Koch; Wolfgang E. Ernst
European Physical Journal D | 2011
Moritz Theisen; Florian Lackner; Francesco Ancilotto; C. Callegari; Wolfgang E. Ernst
Journal of Physical Chemistry Letters | 2011
Moritz Theisen; Florian Lackner; Günter Krois; Wolfgang E. Ernst
XVIIIth Symposium on Atomic, Cluster and Surface Physics 2012 | 2012
Florian Lackner; Günter Krois; Moritz Theisen; Markus Koch; Wolfgang E. Ernst
International symposium on small particles and inorganic clusters XVI | 2012
Markus Koch; Florian Lackner; Martin Ratschek; Moritz Theisen; Alexander Volk; Evelin Fisslthaler; Werner Grogger; Wolfgang E. Ernst