The electron emission yield induced by mega-electron-volt H+, He2+, Li
3+, B5+,, and C6+ impact on aluminum, copper, and gold targets was mea
sured. We found a significant deviation of the results from a simple p
roportionality to the stopping power, especially for heavier ions and
low projectile velocities. Using a slightly modified model by J. E. Bo
rovsky and D. M. Suszcynsky [Phys. Rev. A 43, 1433 (1991)] our experim
ents could be well represented. In this model the collective electric
held generated along the projectile's path was taken into account. Con
sequently, the positive ion channel appears to be the dominant mechani
sm that leads beyond a projectile-independent yield-to-stopping power
ratio.