Electron-loss-to-continuum cusp in collisions of U89+ with N2 and Xe

We study the electron-loss-to-continuum (ELC) cusp experimentally and theoretically by comparing the ionization of ${\mathrm{U}}^{89+}$ projectiles in collisions with ${\mathrm{N}}_2$ and Xe targets, at a beam energy of 75.91 MeV/u. The coincidence measurement between the singly ionized projectile and the energy of the emitted electron is used to compare the shape of the ELC cusp at weak and strong perturbations. A significant energy shift for the centroid of the electron cusp is observed for the heavy target of Xe as compared to the light target of ${\mathrm{N}}_2$. Our results provide a stringent test for fully relativistic calculations of double-differential cross sections performed in the first-order approximation and in the continuum-distorted-wave approach.