Research

Photoelectron spectroscopy and photoelectron circular dichroism of cold chiral anions


Photoelectron circular dichroism (PECD) has been observed in the photoionization of chiral molecules by circularly polarized light. It becomes apparent by an intensity difference of electrons emitted in forward and backward direction with regard to the propagation direction of the light. Its sign changes when the polarization changes from left to right-handed or when changing the enantiomer. While by now intensively studied for the photoionization of chiral neutral molecules this effect has not yet been demonstrated in photodetachment of anions. 


The initial approach of producing chiral anions was to form complexes of atomic gold anions and chiral molecules (alaninol, fenchone, menthone, 3-hydroxy-tetrahydrofuran) in an existing laser evaporation cluster source. The classical anion photoelectron spectra revealed a weak interaction of the gold atom and the chiral ligand with only moderate shifts of the observed bands from the binding energies of free Au-. For these anionic complexes no PECD on a magnitude seen typically in ionization of neutrals (~10%) has been detected. Possible reasons under consideration were the formation of intrinsically hot complexes in the laser evaporation source, stability and sensitivity issues in the electron detection by the velocity map imaging setup or that anions behave differently compared to the neutrals because of changes in the relevant interactions between the emitted electron and the final state.


For improvements of sensitivity and stability a photo-elastic modulator has been implemented for turning the light polarization from left to right-handed on a shot-by-shot basis. Together with other measures to improve ion and laser beam alignment, mechanical stability, and magnetic shielding a sensitivity for detecting PECD of better than 1 % is now reached. Further, with a combination of discharge source and entrainment by a cold supersonic Ar expansion cold molecular anions can be formed e.g. via deprotonation of alcohols.



Spectroscopy of neutral boron clusters using tunable VUV generated by 4-wave mixing


Boron clusters have been found to show interesting binding behaviour with delocalized aromatic systems due to the only three valence electrons of the boron atom. According to previous experiments in combination with calculations, anionic boron cluster1,2are able to form versatile structures like planar, ring-like and cages. For neutral B40that is suggested to be formed in these experiments by electron detachment from a minor abundant, high energy isomer of the anion, a highly stable cage structure has been predicted2. Parallels to the carbon fullerenes might be drawn, which call for a more direct investigation of the neutral clusters’ structures. 


For this, the neutral clusters can be size-selectively characterized by vibrational spectroscopy using the IR-VUV two color ionization scheme. The resulting infrared spectra are structural fingerprints and, by comparison with calculated IR spectra, can allow for isomer assignment. In IR-VUV two color ionization, VUV photons ionize the clusters near the ionization threshold where the ionization efficiency for cold clusters is low.


However, if the clusters can be heated in a preceding resonant IR excitation step a strong increase in the ionization efficiency can be detected. By measuring the ion yield as a function of the IR frequency, cluster size-specific IR spectra can be determined. It should be noted that for a given VUV photon energy, there is access to only a limited number of cluster sizes as the ionization energy depends on cluster size. So far, with a commercial VUV laser (F2laser, 7.9 eV) very few sizes of boron clusters (B11, B16, B17) could be characterized3. To overcome the limitation in photon energy VUV photons are now generated by four-wave mixing in Xenon. In the present configuration which shall cover an energy range of 6.5 eV to 8.2 eV, two dye lasers interact in a static gas cell and the resulting 2+1 difference frequency is used for ionization. After creating 7.9 eV photons in that way the spectra reproduce the results obtained via F2laser ionization.



Chemistry and Physical properties of Platinum Clusters


The metals of the platinum group (Ru, Rh, Pd (4d); Os, Ir, Pt (5d)), are widely used in heterogeneous catalysis for a range of economically and environmentally important processes. Nevertheless, the understanding of their structural and electronic properties is still limited and in several cases there exist discrepancies between experimental findings and theoretical predictions.[1] In-depth experimental characterization of gas phase clusters can provide more insights and allows for testing the suitability of the theoretical methods. Anion photoelectron spectroscopy (PES) probes the electronic structures of the clusters and when using high resolution variants even low-frequency vibrational structure, characteristic for metal clusters, can be resolved.
Here we focus on the anionic platinum trimer. In earlier studies no conclusive assignment of the PES spectrum has been obtained.[2] High resolution anion photoelectron spectroscopy using velocity map imaging
(VMI) allows to resolve a multitude of not yet observed vibronic transitions giving, in combination with Franck-Condon simulations, insights into the structures of the anionic and neutral states of this cluster.

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