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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.