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Extreme ultraviolet light can easily ionize the valence shell of molecular systems and produce molecular cations in their ground or excited electronic states, which usually undergo ultrafast electronic and nuclear rearrangements. With the advent of intense and ultrashort light sources in the XUV region of the spectrum, e.g. FLASH in Hamburg, the time-resolved exploration of the dynamics of valence-ionized molecules and clusters is gaining in momentum. In this talk, the coupled nuclear-electronic dynamics of protonated water clusters H^+(H_2O)_n upon photoioization is discussed from a
theoretical perspective. First, the photoionized Zundel cation, n=2, is considered. This is the smallest water cluster in which an excess proton is shared between two water molecules thus forming a strong hydrogen bond, and is an important player in acidic chemistry. We describe its nuclear dynamics quantum mechanically and find that the motion of the newly formed electron hole and the shared proton are strongly coupled, leading to irreversible electronic and proton rearrangements in a few fs after photoionization, followed by Coulomb explosion. Later, the dynamics of larger clusters is discussed based on a quantum treatment of the electrons and a classical treatment of the nuclei, and a time-resolved picture of their fragmentation and relaxation pathways is also provided.
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