In this work we describe a model for the electronic energy loss of bar
e ions at high velocities. Starting from first-order perturbation theo
ry we propose a simple formula to calculate the impact-parameter depen
dence of the electronic energy loss for all impact parameters. The phy
sical inputs are the electron density and oscillators strengths of the
atoms. A very good agreement is obtained with full first-order calcul
ations. [S1050-2947(98)01111-1].