LIGHT-ION-INDUCED K-SHELL IONIZATION IN THE ADIABATIC REGION

K-shell ionization induced by fast ions was simulated by the excitation of a harmonic oscillator perturbed by time-dependent coupling of a parallel spring. Permanent coupling of the spring corresponds to the atom where both nuclei are united. In this adiabatic limit the two systems have an increased energy and reduced length scale. The ratio of the united-atom cross sections to the experimental ones was compared to the ratio of the oscillator excitation probabilities calculated in the adiabatic limit and by the coupled-channel method. Strong similiarities between the behavior of both types of ratios were observed, and it was pointed out that the adiabatic approximation overestimates the transition probabilities even at moderate collision velocities. The effective nuclear charges were deduced from the model and used for the cross-section calculation with hydrogenic Dirac wave functions. For the proton data, calculated and experimental cross sections agree within 10%. Due to the strong coupling of states in the adiabatic region, the effective nuclear charges exceed the united-atom limit by 20%.