X-ray induced fragmentation of nucleic acids building blocks and their halogenated analogues
The purpose of this short term scientific mission hosted by Dr Paola Bolognesi was to study the ionisation/fragmentation of nucleic acids building blocks. Particularly, using photoelectron/photoion coincidence technique (PEPICO) and tunable synchrotron light, we can obtain site/state selective mass spectrum giving insights into the fragmentation dynamics following the ionisation. The systems under study were halogenated analogues of the pyrimidine molecule, namely the 2-Cl-pyrimidine, the 2-Br-pyrimidine, and the 5-Br-pyrimidine (see the molecular structures in Figure).
We perform several PEPICO measurements where, at a fixed photon energy, the photoion mass spectrum is recorded in coincidence with an electron of a given kinetic energy. As an example, the direct photoionisation of 2-Cl-pyrimidine at 100 eV photon energy is given in Figure where the mass spectrum associated to electronic states of increasing binding energies are shown. A marked selectivity in the fragmentation of these different ionic states is clearly visible. In the case of the X-ray photoabsorption, the PEPICO technique is applied by setting the photon energy for the resonant excitation of the 1s ® p* transition of a specific atomic site in the molecule and recording the mass spectrum in coincidence with the resonant Auger electron emitted.
The analysis of the different sets of results is in progress. Their presentation in international conferences as well as the publication in peer-reviewed journal is foreseen. In the near future, we expect to further the collaboration between our two groups in order to apply coincidence techniques (PEPICO, ion-ion coincidence after ionisation by ion-impact  or X-ray) to study the fragmentation dynamics of complex molecular systems.