Photoionization mass spectrometry of molecular clusters using synchrotron radiation

The photoionization and dissociative ionization of molecular aggregates using synchrotron radiation is reported. The main objective of the review is to consider the intracluster relaxation processes after ionization. For hydrogen-bonded systems proton transfer is dominant. For small clusters (n<4) appearance potentials, ionization potentials, absolute proton affinities, proton solvation energies and intermolecular bond energies in the ionic clusters are deduced. For van der Waals aggregates proton transfer can also be used to characterize the intermolecular bond in the ionic cluster. Aggregates of CH₄, SiH₄, CH₃F show proton transfer in contrast to simple aromatic compounds, which reveal no proton transfer. From the fragmentation pattern and appearance potentials relaxation by intracluster ion molecule reactions is discussed. In heterogeneous clusters intracluster Penning ionization is observed. The shift of the charge transfer resonances depends on the π-electron density in the aromatic system. The width and spectral position of these resonances are influenced by the cluster size.