Nanocalorimetryand angle-resolved photoelectron spectroscopy on sodium, aluminum,and water clusters

In this thesis, the thermodynamic properties and the electronic structure of the valence electrons of various cluster species in gas phase have been investigated. These studies were enabled by two major modifications of an existing experimental apparatus, which was designed to record caloric curves of free clusters with a photofragmentation method. First, a radio-frequency multipole ion trap was designed and set up to allow for temperature control of the clusters between 6 and 350 K. Second, a photoelectron imaging spectrometer, that offers the possibility to analyze the angular momentum character of photodetached electrons, was added to the existing setup. Thermodynamic data of higher quality than previously available has been obtained with the improved setup. A comparison of the results with calculations have yielded a detailed understanding of the premelting and postmelting behaviour of sodium clusters containing 139 and 147 atoms. Furthermore the thermodynamic properties of free, partially oxidized sodium clusters have been studied. The motivation of this study was to investigate the thermodynamics of a finite multicomponent system. The experimental results show a systematic depression of both the melting point and the latent heat as compared to pure sodium clusters. This is in contrast to the corresponding bulk system where the melting point does not change upon partial oxidization due to a phase separation of sodium and its oxide. In a finite system, however, even separated phases are not independent due to the interfacial energy. Calculations show that the interaction between the pure and the oxidized part of the cluster is causing the observed effects. Since the liquid part of the cluster has more structural freedom to adapt to the more rigid oxidized part, the interfacial energy is lower in this case. This promotes the liquid phase of the metallic part of the cluster and is the origin of the cluster-specific melting-point depression. In addition to the investigations on sodium based clusters, a study of the melting behavior of free water clusters has been performed. This species is of great relevance due to the ubiquity of water and the fact that nanometer sized water particles play an important role in Earth's atmosphere and in interstellar space, where the particles are exposed to a broad range of temperatures. The first ever measured experimental caloric curves of free, size selected water cluster anions (namely the clusters comprising 48 and 118 water molecules) yield several findings. At low temperatures, the curves of the clusters are practically identical to the bulk curve, demonstrating that the vibrational density of states of the nanoparticles and the bulk are similar. At particle-size specific temperatures around 100 K, the caloric curves significantly deviate from the bulk since the heat capacities increase dramatically. The assigned transition temperatures mark the onset of a gradual melting transition which takes place at a much lower temperature than expected (at only one third of…