Energy loss of aligned nitrogen molecular ions in an electron gas

Abstract

Nitrogen molecular ions (1.2–2.3 a.u. velocity) which emerge from thin carbon foils with their internuclear axes aligned along the direction of motion exhibit a diminishment of the average energy loss per atom relative to that of atomic ions having the same velocity. Assuming negligible damping of an electron gas with energy gap, we have calculated average stopping powers per atomic ion for an extended Brandt-Kitagawa-model charge distribution representing molecular nitrogen ions moving in carbon. The contribution from negligible-damped plasmon excitations displays a dependence on internuclear separation which differs greatly from that below threshold and does not agree with experiment. Electron-hole excitation is, however, the dominant mechanism for ions with velocities below 2.3 a.u. Below threshold the experimental trends in thin targets are reproduced by the calculations, but discrepancies in magnitude are present. Neglecting plasmon excitation, the experimental findings at 2.3 a.u. velocity can be reproduced approximately.