Inner-shell ionisation of Xeq+–Au and Pb collision systems: MO picture

The generation of collision-induced vacancies and their transfer within the electronic inner shells are the source of several unprecedented phenomena, which have been revealed in recent experimental studies conducted using state-of-the-art next-generation accelerators and detectors. Despite the multitude of studies conducted over the years, our understanding of the atomic vacancy transfer mechanisms remains exiguous. The detection and analysis of collision-induced X-rays is a widely-used powerful technique for analysing atomic-scale phenomena. However, both experimental and theoretical investigations on ion–atom collision-induced X-rays have remained mostly restricted to high-energy light-ion impact on heavy atoms. To date, only a few studies have been reported on the investigation of very-low-energy heavy-ion impact on heavy atoms using X-rays, especially M-shell X-rays of the heavy atom. This study was conducted to evaluate the inner-shell ionisation of Au (135 and 508 μg/cm2) and Pb (107, 157 and 390 μg/cm2) targets due to low-energy (2–5 MeV) 54Xeq+-ion impact. The collision-induced X-ray spectra of both the collision partners were examined, and the corresponding intensity ratios and cross-sections were compared with those calculated using two well-known ionisation theories, viz. PWBA and ECPSSR. The measured cross-sections were underestimated by these theories. Moreover, the cross-sections measured in this study were found to be several orders of magnitude higher than those obtained in other reported studies on low-energy lighter-ion-impact on Au and Pb. The significant discrepancies observed between the experimental and theoretical values have been explained qualitatively within the framework of quasi-molecular orbital formation using level correlations. The insights obtained from the results of this study can be applied to analyse the vacancy transfer mechanisms in very-heavy systems (combined atomic number > 100).