CHARGE-STATE DISTRIBUTIONS OF NOBLE-GAS ATOMS AFTER ELECTRON-CAPTURE BY PROTONS

By means of a combined coincidence and time-of-flight technique, charge-state distributions of the noble-gas atoms He, Ne, Ar, Kr and Xe have been measured after electron capture by 40-2000 keV/amu hydrogen ions. At low energies, where capture from the outermost shell of the target atom dominates, the singly charged state is the most abundant one, but large fractions of doubly and even triply charged ions are also seen. In helium and neon, the measured charge-state distributions at low energies resemble those found in photoionisation of the outermost shell, whereas in argon there seems to be a discrepancy. At high energies, where capture from inner shells of the target atom dominates, the charge-state distributions are governed by vacancy cascades which are capable of producing multiple ionisation. Initial vacancy distributions are estimated in the high-energy region by analysis of the measured charge-state distributions and discussed in relation to theory and previous data.