Luminescence induced by swift atomic ions in samples with nanoscale dimensions

Abstract

Thin self-supporting foils with nanoscale dimensions (thickness: 40–1000 nm) consisting of a mixture of the polymer Formvar and the luminescent dye POPOP were irradiated with 12 C and 32 S atomic ions at energies from 2 to 36 MeV and 10 to 55 MeV, respectively. The relative luminescence yield originating from the impact of these ions was measured by applying the technique of time-correlated single photon counting. The 32 S induced yield increases nearly linearly with increasing ion energy in the whole energy range studied, whereas a constant value after an increase up to approximately 25 MeV is reached for the 12 C induced yield. In the energy range above the Bragg-maximum of the ion energy loss, the observed yield can be reproduced by the application of a model based on the energy density resulting from the secondary electrons which are set free in binary collisions with the projectiles. The luminescence yield measured at constant projectile energy increases linearly with foil thickness if the latter exceeds sufficiently the range of secondary electrons which are able to leave the sample. For thinner foils, a reduced luminescence yield was observed due to the larger fraction of secondary electrons which leaves the sample. The result is a smaller energy density along the ion track. The effect of escaping electrons is also shown to arise in microcrystalline samples, which have typical crystal dimensions in the nm-range, and leads to a new interpretation of earlier measurements.