6.0 MeV u⁻¹carbon 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 C 6+ and C 4+ ions with water vapor. Theoretical calculations of the DDCSs were made for C 6+ 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 Z 2 -scaling law (i.e. first Born approximation) is not applicable for high-Z bare projectiles such as C 6+ ions. The SDCS of C 4+ ions was observed to be smaller than that of C 6+ ions by 50% in the low-energy region (<200 eV) due to the screening effect of its bound electrons in C 4+ ions, which could be explained quantitatively by taking account of an effective projectile charge.