X-ray emission from the metastable components of slow ar¹⁶⁺ beam on metallic surfaces

K-shell X-ray emission was observed in the neutralization process of Ar¹⁶⁺ interacting with beryllium and molybdenum at slow speed (less than Bohr velocity). The K-shell X-ray emission came from the metastable component of Ar¹⁶⁺ beam with the configuration (1s2s) produced from electron cyclotron resonance ion source. The intensity ratio of K _β X-ray emission to K_α X-ray emission (I _Kβ/I_Kα) in the neutralization process decreased when the kinetic energy of the projectile increased. This is closely related to the flight time the projectiles spend above the surface, which is dominated by both the work function of the metal and the normal velocity component of the incident ions to the surface according to the classical over-barrier model. As the relation between I_Kβ/I_Kα and the above-flight time shows, the longer time above the surface the larger ratio due to the above-surface contribution. Furthermore, the smaller work function leads to the smaller contribution from the above surface.