Target Dependence of Post-Collision Effects in Ionization by Proton Impact.*

We have measured singly charged recoil ions in coincidence with the momentum analyzed projectiles for ionization of Ne and Ar by 75 keV proton impact. From the data we extracted double differential cross sections (DDCS) for a broad range of fixed projectile energy losses as a function of the projectile scattering angle. Furthermore, the average scattering angle $\theta_{\mathrm{ave}}$ for a given energy loss were analyzed as a function of the ejected electron to projectile speed ratio v$_{\mathrm{e}}$/v$_{\mathrm{p}}$. Along with data obtained previously for H, H$_{\mathrm{2}}$, and He targets this made possible a systematic analysis of the dependence of post-collision effects on the target. For targets with a relatively small ionization potential $I$ pronounced minima in $\theta _{\mathrm{ave}}$ were found near v$_{\mathrm{e}}$/v$_{\mathrm{p}} \quad =$ 1, which were absent for targets with large $I$. This observation seems to suggest a decreasing importance of post-collision effects with increasing $I$. However, a possible alternative explanation is an increasing role of the repulsive nucleus-nucleus interaction with increasing $I$, which tends to lead to larger scattering angles. Furthermore, for v$_{\mathrm{e}}$/v$_{\mathrm{p}}$ \textless 1 $\theta_{\mathrm{ave}}$ decreases with increasing $I$, suggesting an increasing role of post-collision effects.