Theoretical and experimental studies on the captured electron population probability of hydrogen-like O and N ions in collision with Al surface

Abstract

The study of the interaction between highly charged ions and solid surfaces not only has great significance for basic scientific research such as atomic physics, astrophysics, and high energy density physics but also has promising application prospects in biomedicine, nanotechnology, surface analysis, and microelectronics. In this paper, the intermediate Rydberg states formed during highly charged O<sup>7+</sup> and N<sup>6+</sup> ions incident on Al surface are studied theoretically by using the two-state vector model. Both the probability of electron capture into different Rydberg states (<i>n<sub>A</sub></i> = 2 − 7) and the most probable neutralization distances are given. The calculation shows that the larger principal quantum number <i>n<sub>A</sub></i> is relevant to smaller probability. Therefore, the X-rays emitted by O<sup>7+</sup> and N<sup>6+</sup> ions incident on the Al surface come mainly from the de-excitation of the smaller <i>n<sub>A</sub></i> to the ground state. In order to confirm the calculations, we measured the X-ray emission spectra of O<sup>7+</sup> and N<sup>6+</sup> ions in collisions with the Al surface in the energy range of 3-20 keV/q. The experiments were performed at an ECR ion source located in Institute of modern physics. We also calculated the transition energies (n<i>p</i>-1<i>s</i>) from different high Rydberg states to the ground state by using the FAC code. The center of the measured <i>K</i> X-ray peak is close to the calculated transition energy from the principal quantum number n=2 to n=1, it is consistent with our results obtained by the two-state vector model as well. In addition, we found the experimental <i>K</i> X-ray yield for O<sup>7+</sup> ions incidence at lower energy collisions is almost the same with N<sup>6+</sup> ions, but larger at higher energy collisions. When the ion incident kinetic energy is low, the X-ray emission is mainly owing to the decay of “above the surface”hollow atoms. Because of the small difference in the critical distances for the capture of electrons by O<sup>7+</sup> and N<sup>6+</sup> to form hollow atoms, the X-ray yields produced in both cases are almost the same at low energy collisions. In contrast, as increasing the incident energy, the ions have a long-range in the target, so the contribution from the decay of “above the surface” and “below the surface”hollow atoms need to be considered at the same time.