6.0 MeV u−1 carbon ion (C6+ and C4+)-induced secondary electron emission from water vapor

Abstract

We have precisely measured absolute double differential cross sections (DDCSs) for secondary electron emission produced in collisions of 6.0 MeV u−1 C6+ and C4+ ions with water vapor. Theoretical calculations of the DDCSs were made for C6+ ions using the continuum distorted wave-eikonal initial state model (CDW-EIS) in its straight-line version of the impact parameter approximation, showing general good agreement with experimental data, except in the intermediate- and high-energy region (> 50 eV), particularly at the backward angles (> 110°). On the other hand, the single differential cross section (SDCS), which was obtained by integrating the measured DDCSs over the solid angle, showed fairly good agreement with the CDW-EIS in the low-energy region (< 200 eV), while a significant discrepancy between the observed SDCS and the Rudd-model scaling (× 36) can be seen, suggesting that a simple Z2-scaling law (i.e. first Born approximation) is not applicable for high-Z bare projectiles such as C6+ ions. The SDCS of C4+ ions was observed to be smaller than that of C6+ ions by ∼ 50% in the low-energy region (< 200 eV) due to the screening effect of its bound electrons in C4+ ions, which could be explained quantitatively by taking account of an effective projectile charge.