ECAMP 15

The absorption of soft X-ray photons by biological matter can lead to core-level ionization, producing excited cation radicals with the deposition of large amount of energy. The excited molecule relaxes by different competing relaxation channels, emitting a photon or release of a secondary electron called Auger electron where the core-level vacancy is filled by an outer-valence electron and the excess energy is used to emit another outer-valence electron from the same molecule. The doubly charged molecule mostly undergoes fragmentation. If the biomolecule is embedded in an environment, then apart from the local decay channels there can be different non-local decay processes involving both the biomolecule and its neighbours. One such non-local decay channel is the intermolecular Coulombic decay (ICD) [1] where the energy released from the relaxation of the initially core ionized molecule is transferred to a neighboring molecule that uses it to emit one of its electrons. Another decay channel is the electron-transfer mediated decay (ETMD) [2] where the core vacancy of the initially ionized molecule is filled by an electron from the neighboring molecule and another electron is emitted from yet another neighbor. Thus two vacancies are formed on two neighbors while the initially ionized molecule becomes neutral. These nonlocal decay mechanisms have mostly been considered to have minor contribution in case of inner-shell (core-level) vacancies, accounting for a few percent compared to the predominant Auger decay channel. However, a recent study showed experimentally that core-level ICD is an important channel for X-ray induced core-level ionization of microsolvated pyrimidine molecules [3] and theoretical calculations revealed a high branching ratio of non-local channels along with predicting a significant intensity for core-level ETMD channel. In the context of radiation damage to biological matter, such local and non-local competing decay channels can produce several low energy secondary electrons and water radicals which are the key players for causing single and/or double strand breaks in the DNA/RNA of the cells.
In the present work, we study the X-ray photoionization and fragmentation of pyrimidine embedded in water cluster to experimentally verify the core-level ETMD channel using the photoelectron-photoion-photoion coincidence (PEPIPICO) spectrometer connected at the gas phase endstation of the Finnish-Estonian beamline (FinEstBeAMS) [4] at the MAX IV synchrotron radiation facility. The PEPIPICO coincidence maps measured in coincidence with the C 1s photoelectron of pyrimidine shows signature of non-local processes especially the ETMD channel causing ionization of the neighboring water molecules to distribute the internal energy to the environment when the pyrimidine is ionized by the initial irradiation.
References
[1] T. Jahnke et. al., Chem. Rev. 120, 11295 (2020)
[2] M. Förstel et. al., Phys. Rev. Lett. 106, 033402 (2011)
[3] A. Hans et. al., J Phys. Chem. Lett. 12, 7146 (2021)
[4] K. Kooser et. al., J. Synchotron Rad., 27, 1080 (2020)