An interplay of various damage channels in polyethylene exposed to ultra-short XUV/X-ray pulses

Polyethylene (PE) irradiated with femtosecond extreme ultraviolet or X-ray laser pulses in a single-shot damage regime is studied theoretically. The employed microscopic simulation tool XTANT-3 traces nonequilibrium electron kinetics, energy exchange between electrons and atoms, nonthermal modification of interatomic potential, and the induced atomic response. It is found that the nonthermal detachment of hydrogen atoms in bulk PE starts at the threshold deposited dose of similar to 0.05 eV per atom. With an increase in the dose, more hydrogen atoms detach from the carbon backbone. At a dose of similar to 0.3 eV per atom, hydrogen behaves like a liquid flowing around carbon chains. It is accompanied by the appearance of defect energy levels within the band gap. At a dose of similar to 0.5 eV per atom, carbon chains actively bend and cross-link. In the range of doses from similar to 0.5 eV per atom to similar to 0.9 eV per atom, the electronic excitation induces formation of new carbon structures embedded in the hydrogen liquid, such as benzene-like rings. The band gap collapses at such doses, merging the valence and the conduction bands. Finally, at doses above similar to 0.9 eV per atom, the carbon subsystem also melts into liquid. All of these damage mechanisms are mainly nonthermal, triggered by promotion of electrons from the valence into the conduction band of PE. At high doses, however, thermal electron-ion coupling is extremely fast causing equilibration of the electronic and the ionic temperatures within a hundred femtoseconds.

Automated summary

The theoretical study on polyethylene irradiated with femtosecond extreme ultraviolet or X-ray laser pulses reveals that nonthermal detachment of hydrogen atoms, formation of new carbon structures, collapse of the band gap, and finally melting of the carbon subsystem occur at specific doses of energy deposition per atom. The damage mechanisms are primarily nonthermal, initiated by electron promotion and leading to equilibration of electronic and ionic temperatures at high doses.