Relative level excitation in ion-atom collisions as a function of the orbital-angular-momentum quantum number

We present experimental data on relative excitation cross sections from the collisions ${\mathrm{Li}}^{+}$-He, -Ne, -${\mathrm{C}}_6$ ${\mathrm{H}}_6$; ${\mathrm{Ne}}^{+}$-, ${\mathrm{Na}}^{+}$-, ${\mathrm{Mg}}^{+}$-, ${\mathrm{Mg}}^{2+}$-He; and ${\mathrm{Mg}}^{+}$-${\mathrm{C}}_6$ ${\mathrm{H}}_6$ at 10-150 keV. The ion-atom data show a preferential excitation of $d$ levels for fixed principal quantum number, whereas no preferential excitation exists in the ion-benzene collisions. The data are compared with various model predictions. The preferential $d$-level population seems to be a result of the symmetry properties of the transient quasimolecule formed during the collision for projectile velocities of the order of one atomic unit of velocity. It is concluded that neither the Born approximation, the Brinkman-Kramers approach, a statistical population viewpoint, or a velocity-matching viewpoint explains the data satisfyingly well in our energy regime. For $v\gg v_0$ (1 a.u.), $s>p>d>\mathrm{}\cdots$.