Abstract
The impact parameter dependence of Ar L-shell excitation in slow Ar-Ar collisions has been investigated both experimentally and theoretically. Experimental data have been obtained at 300, 500 and 700 keV collision energy by observing Ar L Auger electrons in coincidence with projectiles scattered through laboratory angles up to 40 degrees . The final charge states of the outgoing Ar ions have been analysed for large scattering angles. The experimental results are interpreted in terms of the molecular-orbital model of atomic collisions. L-shell excitation via rotational coupling close to the united-atom limit is treated theoretically in some detail. The probability for rotationally induced excitation in the vicinity of an arbitrary united-atom (n, l) shell is derived in the sudden approximation. Moreover, the 3d delta -3d pi -3d sigma rotational-coupling problem has been solved in the framework of numerical coupled state calculations. The good agreement between experiment and theory suggests that the enhancement of the Ar L-shell excitation over the 4f sigma background contribution, which is observed for impact parameters smaller than about 0.28 au, is effected by the 3d delta -4d pi -3d sigma rotational-coupling mechanism.