Damage mechanisms in polyalkenes irradiated with ultra-short XUV/x-ray laser pulses

Abstract

Although polymers are widely used in laser-irradiation research, their microscopic response to high-intensity ultrafast XUV or X-ray irradiation is still largely unknown. Here we comparatively study homologous series of alkenes. XTANT-3 hybrid simulation toolkit is used to determine their damage kinetics and irradiation threshold doses. The code simultaneously models the nonequilibrium electron kinetics, the energy transfer between electrons and atoms via nonadiabatic electron-ion (electron-phonon) coupling, nonthermal modification of the interatomic potential due to electronic excitation, and the ensuing atomic response and damage formation. It is shown that the lowest damage threshold is associated with local defect creation such as dehydrogenation, various group detachment from the backbone, or polymer strand cross-linking. At higher doses, the disintegration of the molecules leads to a transient metallic liquid state. We identify nonthermal effects as the leading mechanism of damage, whereas the thermal (electron-ion coupling) channel influences the kinetics only slightly in the case of femtosecond-pulse irradiation. Despite notably different properties of the studied alkene polymers, the ultrafast-X-ray damage threshold doses are found to be very close to ~0.05 eV/atom in all three materials: polyethylene, polypropylene, and polybutylene.

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