Angle-dependent charge exchange and energy loss of slow highly charged ions in freestanding graphene

The scattering of ions in solids is accompanied with momentum transfer and electronic excitations resulting in the slowing down of the ion. The amount of energy transferred in a single scattering event depends on the particular trajectory which can be traced back through the scattering angle. Performing scattering angle dependent measurements of slow highly charged Xe ions transmitted through freestanding single-, bi-, and trilayer graphene allows us to determine the charge exchange and energy loss for different minimal interatomic distances. Interestingly, the charge exchange shows an increase with scattering angle by a factor of less than 2, while the energy loss increases by more than a factor of 10 for 3 degrees compared to forward direction. Our results can be compared to a time-dependent potential model and show that determination of the stopping cross section is not straightforward even with angle-dependent data at hand.