Experimental study of relative level populations in beam-foil-excited Ar VIII and Kr VIII

The relative level populations in beam-foil-excited sodiumlike argon (Ar VIII) and copperlike krypton (Kr VIII) have been studied at 2-MeV projectile energy. In Ar VIII, the $3p$ and the $3d$ terms are strongly excited. The level population decreases rapidly with increasing value of the principal quantum number n, reaches a minimum for levels with $n=5 or 6$, but increases then and reaches a strong maximum for levels with n11. As an example, the $11d$ level is populated a factor of 2 more than the $3d$ level. Also the Kr VIII data show a population maximum for levels with $n=11$. The strong $3p$ and $3d$ level excitations in Ar VIII are explained as selective inner-shell processes which can be understood in the molecular-orbital electron-promotion picture. The preferential population of highlying Rydberg states is explained as resulting from a near-resonance electron transfer from the valence band of the carbon foil to the projectile. This process takes place when the projectile leaves the back of the foil, and the distribution of population upon different n levels will vary with charge state. The level population for levels with fixed n increases rapidly with increasing value of the orbital-angular-momentum quantum number for levels in Kr VIII, Kr X, Kr XI, and Kr XIII. The results are discussed, and their influence on decay curves as well as their relations to multiply charged ion-atom collisions are pointed out.