Ionization cross sections and probabilities for removal of2sand2patomic electrons in the binary-encounter approximation

Calculations of ionization cross sections have been performed for removing electrons from $2s$ and $2p$ levels in hydrogenlike atoms by heavy-charged-particle impact using the binary-encounter approximation (BEA). The ratio of $\frac{{\sigma }_{L_{\mathrm{II}}}}{{\sigma }_{L_{\mathrm{I}}}}$ is predicted to peak when the projectile has a velocity equal to 0.3 times the velocity of the atomic electron. Experimental values of $\frac{{\sigma }_{L_{\mathrm{II}}}}{{\sigma }_{L_{\mathrm{I}}}}$, in agreement with Born predictions, peak at a velocity significantly lower than that predicted by BEA calculations. By transforming to coordinate space in an approximate way, predictions for the impact-parameter dependence are made. In the $2s$ case, the ionization probability has a pronounced local minimum for impact parameters near half the atomic radius, corresponding to a node in the $2s$ wave function. The ionization probabilities for arbitrary charged particles on arbitrary targets may be easily found by applying scaling laws to values tabulated over a wide range of projectile velocities.