Differential cross section ratios for p + He collisions

We have measured differential cross sections for single capture, double capture, and transfer excitation in p + He collisions. In the double to single capture cross section ratios R$_{DC}$ we observe peak structures around 0.5 to 1.0 mrad similar to those reported previously for the double to single ionization and transfer-ionization to single capture ratios R$_{DI}$ and R$_{TI}$, respectively. However, surprisingly in our data for R$_{DC}$ the relative importance of these peaks maximize at a relatively small projectile energy of about 50 to 75 keV while in R$_{DI}$ and R$_{TI}$ the structures become increasingly pronounced with increasing projectile energy and are not observed below approximately 200 keV. We also found a pronounced peak structure in the double ratio R =R$_{TE}$/R$_{DE}$, where R$_{TE}$ is the transfer excitation to single capture ratio and R$_{DE}$ the double to single excitation ratio$^{1}$. Our theoretical calculation qualitatively reproduces the peak structure in R if the elastic scattering between the projectile and the residual target ion is treated quantum-mechanically. Finally, we revisited doubly differential single ionization data reported earlier$^{2}$ and found peak structures in the ratios between cross sections for different electron energies. $^{1}$W. Htwe et al., PRL \underline {73}, 1348 (1994) $^{2}$ T. Vajnai et al., PRL \underline {74}, 3588 (1995)