Studies of Molecular and Cluster Fragmentation Using Synchrotron Radiation: Measurements and Models

Abstract: A new spectrometer for studying photoinduced fragmentation has been designed and commissioned. The instrument produces a detailed view of the energy and angular emission of fragments, which in the case of small molecules can be related to their geometry at the moment of dissociation. The spectral profiles for single, double and triple ion coincidences are analyzed in terms of molecular alignment, and nuclear dynamics. A model based upon molecular symmetry and dipole excitation was developed for interpreting multi-coincidence spectra where all ionic fragments are measured. The basic principles of the model can be applied to higher-order fragmentation processes as well, and a picture of molecular dynamics has been obtained. In the case of molecular dissociation, anisotropy of the fragments and thus the geometry of the molecules at the moment of dissociation has been of special interest. The measurements performed on ammonia, sulfur dioxide, water, ozone and nitrogen show clear evidence of anisotropy arising from alignment upon core excitation. This provides information on the symmetry of the core excited state as well as geometry changes. The anisotropy indirectly provides information on localization/delocalization of the core excitation. Oxygen 1s to "sigma like" transitions show clear evidence for localized excitation in ozone but not in sulfur dioxide. Moreover the spectrometer is used to perform multi-coincidence studies of dynamic effects of argon cluster fragmentation. Both size and energy dependent measurements are reported. Evidence for nuclear rearrangements before fragmentation is found for small ( ~ 5) argon clusters by studying the fragmentation patterns around the argon 2p threshold. The time frame of dissociation for various cluster sizes is also presented. This in connection with a model describing the influence on the peaks shapes for dissociation on a picosecond to microsecond time scale in the spectrometer.