Cooper-minimum-type structure in proton-induced L1 - and L3 -subshell x-ray line intensities of Pd measured with high-resolution x-ray spectroscopy

We report on high-resolution measurements of the L₁ - and L₃ -subshell x-ray lines of Pd induced by 0.5-3.0 MeV proton bombardment. The experimental results for the relative intensities of the Lβ₃ (L₁-M₃) , Lβ₄ (L₁-M₂) , and Lβ₆ (L₃-N₁) lines are compared to the theoretically predicted values employing L -subshell ionization cross sections calculated within the semiclassical approximation and the energy-loss Coulomb-repulsion perturbed-stationary-state relativistic theory. A structure of the L₁ -subshell cross section at low proton energies generated by the nodal structure of the 2s wave function, analogous to the well-known Cooper minimum in the photoabsorption cross section, has been observed also for the case of charged particle ionization. In addition, the influence of the multiple ionization on the intensity ratio of the two most intense L₁ -subshell x-ray transitions is studied by comparing the proton-induced intensities with the corresponding photoinduced value measured in an experiment employing monochromatic synchrotron radiation with an energy tuned just above the L₁ edge.