Ion Mobility and Photoelectron Spectroscopy

In atomic cluster research, generally both geometric and electronic properties of clusters have to be considered. Until now, however, experiments have yielded only limited information. Usually, either the geometry or the electronic structure, but not both, can be determined. The present work is the first to successfully combine an ion mobility spectrometer (IMS, yielding a cluster's shape) with a photoelectron spectrometer (PES, yielding a cluster's electronic levels). Thus one can measure isomer-separated photoelectron spectra in a systematic way. Both the number of atoms in a cluster and their arrangement are well defined. This does not depend on the conditions inside the cluster source. The proof of principle of the new IMS/PES method was given using carbon cluster anions.The new method would be helpful e. g. for the examination of silicon hydride mixed clusters (Si_m H_n). At silicon bulk surfaces, hydrogen adsorption strongly changes crystal structure and conductivity. It is still unknown what this means with respect to clusters. However, despite the relevance of semiconductor and nanotechnology, only few experimental research has been done considering either the geometric or the electronic structure of Si_m H_n clusters. As it has not yet been possible to apply the new IMS/PES method on these clusters, a comparison of conventional photoelectron spectra and theoretical simulated spectra is presented. Spectra habe been measured for Si_m H_n- anions, where m = 3 10 and n = 0 2. As a result, the observed clusters' structure may be concluded. Moreover it appears that for cluster anions, electron affinity plays an important role in the selection of isomers inside the source

Isomer-resolved ion spectroscopy

We demonstrate the isomer-resolved spectroscopy of gas-phase ions, with different geometries separated prior to spectroscopic probe using ion mobility techniques. Specifically, ring and chain isomers of carbon cluster anions with 10̄ 12 atoms have been separated by ion mobility/mass spectrometry and examined by photoelectron spectroscopy. This methodology should also apply to other ion spectroscopies, including IR photodissociation

Highest electron affinity as a predictor of cluster anion structures

Small clusters have a range of unique physical and chemical phenomena that are strongly size dependent. However, analysis of these phenomena often assumes that thermodynamic equilibrium conditions prevail. We compare experimentally measured and ab initio computed photoelectron spectra of bare and deuterated silicon cluster anions produced in a plasma environment. We find that the isomers detected experimentally are usually not the ground-state isomers, but metastable ones, which indicates that cluster relaxation is strongly limited kinetically by a dwell time that is much shorter than the relaxation time. We show that, under these conditions, the highest electron affinity replaces the traditional lowest total energy as the appropriate criterion for predicting isomer structures. These findings demonstrate that a stringent examination of non-equilibrium effects can be crucial for a correct analysis of cluster properties