Electron-capture-to-continuum cusp in U 88 + + N 2 collisions

For the collision system ${\text{U}}^{88+}+{\text{N}}_{2}$ at a collision energy of 90 MeV/u, the energy distribution of electrons being nonradiatively captured from the target into the projectile continuum has been measured under an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam axis. This measurement of the electron-capture-to-continuum cusp with the highest effective projectile charge ${Z}_{p}^{\mathrm{eff}}=88$ at a near-relativistic collision velocity of $\ensuremath{\beta}\ensuremath{\approx}0.41$ is shown to be characterized by a strong asymmetry in the cusp shape. By comparing the data to measurements of the radiative-electron-capture-to-continuum cusp for the same collision system, the opposite asymmetry of the cusp is traced back to the varying underlying mechanisms. The experimental results are compared with the two theoretical calculations available for this process, one of them in the semirelativistic impulse approximation and the other in the nonrelativistic continuum-distorted-wave approach. A corresponding fully relativistic treatment may be motivated by the presented experimental data.