Single and double ionization in 300-keVH++He collisions at small impact parameters

We have studied ionization for the collision system ${\mathrm{H}}^{+}$+He at 300 keV in a coincidence measurement, using a newly developed recoil spectrometer, which allows us to measure the charge state of the recoil ion without affecting the emitted electrons. Doubly differential probabilities [per energy interval and solid angle of the emitted electron(s) per scattered projectile at a fixed impact parameter] are determined as a function of the charge state of the He recoil ion. Comparison is made with results of different classical and quantum-mechanical collision theories. Strong deviations are found between the measured data and the predictions of the independent-particle model.