Isocharged Sequence Effect Of Direct-ionization On Helium

Direct single and double ionization of Helium induced by C^q+,O^q+(q=1～4) ions are studied both in experiment and in theory.The double- to single-ionization ratios R are measured by coincidence between time of flight(TOF) and position sensitive signals.The energy of these ions ranges from 20keV/amu-500keV/amu.This work was performed on the 2×1.7MV tandem accelerator in Lanzhou University.The experimental results indicated that the value of R₂₁ increase rapidly above 20keV/amu and when the collision energy is about 200keV/amu or 300keV/amu,R₂₁ reaches the maximum.Meanwhile,the value of R by partially stripped projectile is much larger than that by stripped ions.In theory,we explain the differences between partially stripped and stripped projectile by introducing into effect charge states of the first and the second ionization process.These two effective charges give better understanding of the dynamic screen and dis-screen abilities of the target and projectile outshell electrons in collision.The comparison between the different experimental values and the theoretical estimates shows a qualitative agreement.