Doubly differential cross section measurement in ion-atom collisions at low energy region.

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An apparatus, for studies of low energy ion-atom collisions, was developed and tested for doubly differential cross section measurements. An economical micro-computer controlled measuring-system was also constructed.As the first step of the project, we measured for one-electron capture processes in doubly-charged rare gas ion on rare gas system at about <E_(lab)> = 10 eV. From the measurements, we have noticed that the existance of "reaction window" at this collision energy region, that is, when a potential crossing point exists between 3-5 <ang> of internuclear distance then the transition probability around the point comes to be large. Differential cross sections for the process with larger exothermicity were found to have broader angular distributions. This is understood to be due to an effect of stronger Coulomb repulsion after the potential crossing, which locates at a little bit smaller internuclear distance. In <Ar^(++)> - He system, a de-excitation process of metastable state, <^1D> , was also observed. Comparing the results obtained in <Kr^(++)> - He, we conclude that the reaction path is as follows: <Ar^(++)> ( <^1D> ) + He <-!>> <Ar^+> ( <^2P> ) + <He^+> ( <^2S> ) <-!>> <Ar^(++)> ( <^3P> ) + He. This process is a spin-non-conservative collision process. A reaction path for the process was not found using selection rules predicted in radial coupling mechanism. Recently, Koike, Kitasato univ., developed a theory to find a reaction path considering the spin-flip of <Ar^+> ion in the intermediate quasi-molecular state. Detailed comparison with the experiment and theory is now in progress.