Relative level populations in beam-foil-excited C iv, N v, O vi, and F vii

We have studied the relative level populations in beam-foil-excited C iv, N v, O vi, and F vii by optical spectrometry in the range n=3-12. These Li-like ions have the configuration 1s²nl. The population distribution curves are quite different, ranging from a sharp, approximately (n*)^-3, decrease for C iv to flat maxima at n~7 and 8 for O vi and F vii, respectively. This gradual change may be explained as resulting from a near-resonant charge transfer from the valence band of the carbon foil to the projectile. For fixed n, states with large l are preferentially populated. The consequences of these results in cascade analyses of lifetime data from beam-foil measurements are briefly discussed.