Correlations between ionization cross sections and threshold energies in electron-, positron-, proton- and antiproton-atom collisions

Strong correlations between the direct single-ionization cross sections and the target ionization energies are found for electron, positron, proton and antiproton impact on the noble gases. These correlations, extending from near threshold to high projectile energies, are similar to those previously observed for positronium formation, namely σ = A exp (- BE th ), where σ is the cross section for each process, E th is the relevant threshold energy and A and B are functions of the excess energy of the projectile. The generality of this form for the four projectiles is suggestive of a common underlying fundamental aspect of the collisions.