Molecular effects in K-shell ionization by charged particles

Measurements of atomic inner-shell ionization cross sections for low-Z elements are complicated by small fluorescence yields and because elements do not occur naturally in atomic form. These conditions favor the measurement of Auger-electron yields to determine vacancy formation and they require the use of molecular gas targets. Because molecular structure has little influence on the energy levels of inner shells it is expected to have little effect on K-shell ionization probabilities. On the other hand, Auger-electron transitions involve molecular orbitals which introduce molecular effects in the spectra and could potentially influence yields. Variations in measured atomic K-shell ionization cross sections for different molecular targets have generally been found to be small (10-15%) and explained by simple geometric effects. Some recent measurements have, however, exhibited molecular effects as large as a factor of three which are beyond simple explanations. This has revived our interest in trying to further quantify the extent of molecular effects in K-shell ionization. We have employed a new method using well tested theoretical benchmarks to derive atomic cross sections from molecular targets to study K-shell ionization of carbon and fluorine in a number of fluorocarbon molecules excited by MeV protons and lithium ions. The atomic cross sections derived from these measurements are not found to exhibit effects of molecular structure within the experimental uncertainties of approximately 10% for protons and 25% for lithium ions.