Saturation effect in the excitation of heliumlike Si projectiles in the intermediate velocity range

The saturation effect in the excitation cross sections has been investigate d for Si12+ ions colliding with gas target atoms (atomic number Z(t) betwee n 2 and 54) in the (8-12)-a.u. velocity range, both experimentally and theo retically. The experimental excitation cross sections have barn deduced fro m the measured Ly x rays following the decay of the excited atoms. Theoreti cal calculations have been performed using the first Born approximation, th e symmetric eikonal continuum distorted wave (SECDW) approximation, and the Schwinger variational principle (SVP). Similar to the earlier observations on the heavier ion species, the saturation has been found in both the expe rimental and the SVP cross sections in the entire velocity range. The SECDW approximation, however, has been found to underestimate the cross sections at large values of Z(t) in the presently investigated systems. In the ligh t of these findings, the role of higher-order affects within different theo retical models has been reviewed and the possible interaction mechanisms ha ve been discussed. [S1050-2947(98)00112-7].