Emission of x rays in collisions of xenon ions with metal surfaces

X-ray spectra emitted in collisions of 1500 ~ 3500-keV projectile Xeq+ (q = 12-29) ions with medium-Z Fe, Ni, and Cu targets as well as higher-Z Ta targets are reported. Interestingly, the lines from the collisions with higherZ targets are mainly composed of characteristic x rays of target atoms, while those from the collisions with medium-Z targets consist mainly of characteristic x rays of projectile ions and broad molecular-orbital x rays. The origins of all these lines and their charge-state dependence are analyzed in detail within the framework of the transient quasi-molecular states which are formed in the collisions. By analyzing the fluorescence yields and vacancy lifetimes of the collision partners, the reasons for the differences in producing x rays between medium-Z and higher-Z targets are discussed in detail for the first time.