K-shell ionization cross sections in asymmetric heavy-ion-atom collisions

Experimental x-ray production cross sections for Ne projectiles impinging on thick Al foils and Ar projectiles on Si foils reveal that in asymmetric atom-atom collisions, $K$-shell vacancies are created in both collision partners. Only in the almost adiabatic collision regime at very low velocities, corresponding to projectile energies $E_1<20\mathrm{}$ keV/amu, are all of the vacancies created in the collision partner with the lower atomic number, as predicted by the electron-promotion model of Pauli excitation. At higher energies, $E_1>25\mathrm{}$ keV/amu, the ratio of the $K$-shell x-ray production cross section of the heavier collision partner to that of the lighter collision partner increases approximately as the square of the incident ion energy, and can exceed unity. It provides experimental evidence for Pauli excitation of both $K$ shells under nonadiabatic conditions.