Site selective fragmentation with soft X-rays: From gaseous polyatomic molecules, free clusters, polymers, adsorbates to biological macromolecules

Site selective fragmentation following core level photoexcitation with soft X-rays is reviewed using experimental results chosen in a variety of situations: free molecules, free van der Waals clusters, polymers, molecules adsorbed on metal surfaces, biological macromolecules. Preferential fragmentation is found to occur because the electronic excitation such as the Auger cascade is found localized in a region around the initially excited site and the bond breaking occurs through a repulsive surface at short times, typically on a sub-picosecond time scale. Such effects are small because other competing phenomena destroy the localization of the excitation, such as energy conversion between electronic and vibrational energy, effects of secondary electrons on surrounding atoms, charge transfer between the surface and the adsorbate or screening. In addition the heating of core excited system induces massive multifragmentation (free molecules and clusters) which restrains the appreciation of the damage on a long time scale. For large biological molecules, core level ionization induces multiple bond breaking compared to valence ionization and this induces irreversible damage but true site selective fragmentation is not established yet. © by R. Oldenbourg Verlag, 1996.