Radiation damage in X-ray crystallography: a quantum-mechanical study of photoinduced defect formation in beeswax-analogue n-eicosane crystals

Abstract

We study the nuclear dynamics of n-eicosane ( $$\hbox {C}_{20}\hbox {H}_{42}$$ ) in the crystalline state after photoirradation at room temperature using adiabatic ab initio excited-state dynamics based on hybrid time-dependent density-functional theory. We consider the weak perturbation (absorption) limit, in which an excited electron and a hole are simultaneously created in the system, and the strong perturbation (photoemission) regime, in which one electron is removed. We examine the changes in the carbon chain conformation occurring over timescales of the order of ca. 5 ps relative to the unperturbed (ground state) crystal structure at room temperature, which we simulate using standard ab initio molecular dynamics based on hybrid density-functional theory. Whereas the system retains its ground-state structure in the photoemission limit, the formation of structural defects, in the form of local distortions of the chain geometry, is observed in the absorption limit. We attribute the formation of these defects to the nuclear screening of the electron–hole pair created by photoexcitation. We discuss these findings in the context of radiation damage in organic/biological macromolecules and X-ray diffraction techniques.