Dependence of ArgonK-Shell Vacancy Production on the Electronic Structure of Swift Heavy Projectiles

Thin gas targets of argon were bombarded with C, N, O, and F projectiles of various incident charge states at energies from 1 to 2 MeV/amu. Argon $K$ x rays were observed with a Si(Li) detector, and x-ray-production cross sections were obtained. For a given-energy projectile, the cross sections decrease approximately exponentially as the number of electrons bound to the incident projectile increases from 0 to 3. For three or more electrons, the measured cross sections change very little. Hartree-Fock calculations of fluorescence yields as a function of target configuration were used to obtain argon $K$-shell vacancy-production cross sections. Target configurations were chosen by comparing the experimental argon $K\alpha$ and $K\beta$ x-ray energies and $\frac{K\beta }{K\alpha }$ relative intensities with the values obtained by Hartree-Fock calculations. For projectiles with more than one electron, the vacancy-production cross sections scale as the square of projectile nuclear charge as expected for Coulomb ionization, but the absolute magnitudes of the cross sections do not agree with Coulomb-ionization results. For projectiles with 0 to 1 electron, however, Coulomb ionization alone cannot explain the observed cross sections. Charge exchange from the argon $K$ shell to projectile bound states has been proposed as a significant vacancy-producing mechanism for collisions involving highly charged projectiles. Although calculations of the charge-exchange cross sections are in reasonable relative agreement with the data, additional vacancy-producing mechanisms cannot be ignored.