Ultrafast dynamics in the DNA building blocks thymidine and thymine initiated by ionizing radiation.

Understanding how energetic charged particles damage DNA is crucial for improving radiotherapy techniques such as hadron therapy and for the development of new radiosensitizer drugs. In the present study, the damage caused by energetic particles was simulated by measuring the action of extreme ultraviolet (XUV) attosecond pulses on the DNA building blocks thymine and thymidine. This allowed the ultrafast processes triggered by direct ionization to be probed with an optical pulse with a time resolution of a few femtoseconds. By measuring the yields of fragment ions as a function of the delay between the XUV pulse and the probe pulse, a number of transient processes typically lasting 100 femtoseconds or less were observed. These were particularly strong in thymidine which consists of the thymine base attached to a deoxyribose sugar. This dynamics was interpreted as excited states of the cation, formed by the XUV pulse, rapidly decaying via non-adiabatic coupling between electronic states. This provides the first experimental insight into the mechanisms which immediately proceed from the action of ionizing radiation on DNA and provides a basis on which further theoretical and experimental studies can be conducted.

Development and characterization of a multiple-coincidence ion-momentum imaging spectrometer.

The design and performance of a high-resolution momentum-imaging spectrometer for ions which is optimized for experiments using synchrotron radiation is presented. High collection efficiency is achieved by a focusing electrostatic lens; a long drift tube improves mass resolution and a position-sensitive detector enables measurement of the transverse momentum of ions. The optimisation of the lens for particle momentum measurement at the highest resolution is described. We discuss the overall performance of the spectrometer and present examples demonstrating the momentum resolution for both kinetics and for angular measurements in molecular fragmentation for carbon monoxide and fullerenes. Examples are presented that confirm that complete space-time focussing is possible for a two-field three-dimensional imaging spectrometer.

Role of the Renner-Teller effect after core hole excitation in the dissociation dynamics of carbon dioxide dication.

The fragmentation of the doubly-charged carbon dioxide molecule is studied after photoexcitation to the C 1s(1)2π(u) and O 1s(1)2π(u) states using a multicoincidence ion-imaging technique. The bent component of the Renner-Teller split states populated in the 1s→ π* resonant excitation at both the carbon and oxygen 1s ionization edges opens pathways to potential surfaces in highly bent geometries in the dication. Evidence for a complete deformation of the molecule is found in the coincident detection of C(+) and O(2)(+) ions. The distinct alignment of this fragmentation channel indicates rapid deformation and subsequent fragmentation. Investigation of the complete atomization dynamics in the dication leading to asymmetric charge separation shows that the primary dissociation mechanisms, sequential, concerted, and asynchronous concerted, are correlated to specific fragment kinetic energies. The study shows that the bond angle in fragmentation can extend below 20°.