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Investigations on the chemical modifications induced by swift heavy ions in PMMA thin films were carried out using beams of high d E /d x (2.2 GeV Bi, 14,090 eV/nm) and low d E /d x (2 MeV H, 19 eV/nm). The induced chemical modifications were monitored by XPS for films with initial thickness of 50 and 100 nm. For both beams, the irradiation decreased the amount of carbon atoms bound to oxygen (C O and C O C), with a larger decrease of the carboxyl moiety, as expected. However, the chemical changes induced by light and heavy ions were qualitatively different. For the same mean deposited energy density, proton irradiation induced a decrease of the relative intensity of the carbon–oxygen bonds up to ∼20% larger than the irradiation with Bi ions. This suggests a greater importance of particle ejection by unzipping of PMMA chains at high d E /d x , which tends to keep the O/C ratio closer to the pristine value.

In this paper, the surface effects of individual heavy ions impacting thin polymerlike films were investigated, using molecular dynamics simulations with the finite extensible nonlinear elastic (FENE) potential to describe the molecular chains. The perturbation introduced by the ions in the lattice was modeled assuming that the initial excitation energy in the ion track is converted into an effective temperature, as in a thermal spike. The track was heated only within the film thickness h, leaving a nonexcited substrate below. The effect of decreasing thickness on cratering and sputtering was evaluated. The results were compared to experimental data of thin polymer films bombarded by MeV-GeV ions and to simulations performed with the Lennard-Jones potential. While several qualitative results observed in the experiments were also seen in the simulations, irrespective of the potential used, there are important differences observed on FENE films. Crater dimensions, rim volume, and sputtering yields are substantially reduced, and a threshold thickness for molecular ejection appears in FENE simulations. This is attributed to the additional restrictions on mass transport out of the excited track region imposed by interchain interactions (entanglements) and by the low mobility of the molten phase induced by the spike. Fil: Lima, N. W.. Universidade Católica Do Rio Grande Do Sul; Brasil Fil: Gutierres, L. I.. Universidade Católica Do Rio Grande Do Sul; Brasil Fil: Gonzalez, R. I.. Universidad de Chile; Chile Fil: Müller, S.. Universidade Católica Do Rio Grande Do Sul; Brasil Fil: Thomaz, R. S.. Universidade Católica Do Rio Grande Do Sul; Brasil Fil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina Fil: Papaléo, R.M.. Universidade Católica Do Rio Grande Do Sul; Brasil

We show direct experimental evidence that radiation effects produced by single MeV heavy ions on a polymer surface are weakened when the length of the ion track in the material is confined into layers of a few tens of nanometers. Deviation from the bulk (thick film) behavior of ion-induced craters starts at a critical thickness as large as ∼40nm, due to suppression of long-range additive effects of excited atoms along the track. Good agreement was found between the experimental results, molecular dynamic simulations, and an analytical model. Fil: Papaléo, R. M.. Pontificia Universidade Católica do Rio Grande do Sul; Brasil Fil: Thomaz, R.. Pontificia Universidade Católica do Rio Grande do Sul; Brasil Fil: Gutierres, L.I.. Pontificia Universidade Católica do Rio Grande do Sul; Brasil Fil: De Menezes, V.M.. Universidade Federal Da Fronteira Sul; Brasil Fil: Severin, D.. GSI Helmholtz Centre; Alemania Fil: Trautmann, C.. GSI Helmholtz Centre; Alemania. Universitat Technische Darmstadt; Alemania Fil: Tramontina Videla, Diego Ramiro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina Fil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina Fil: Grande, P. L.. Universidade Federal do Rio Grande do Sul; Brasil