Vanishing influence of the band gap on charge exchange of slow highly charged ions in freestanding single layer MoS₂

Charge exchange and kinetic energy loss of slow highly charged xenon ions transmitted through freestanding monolayer MoS₂ are studied. Two distinct exit charge state distributions, characterized by low and high charge exchange, are observed. They are accompanied by smaller and larger kinetic energy losses, respectively. High charge exchange is attributed to two-center neutralization processes, which take place in close impact collisions with the target atoms. Experimental findings are compared to graphene as a target material and simulations based on a time-dependent scattering potential model. Independently of the target material, experimentally observed charge exchange can be modeled by the same electron capture and de-excitation rates for MoS₂ and graphene. A common dependence of the kinetic energy loss on the charge exchange for MoS₂ as well as graphene is also observed, which additionally underlines the common nature of the two-center Auger neutralization process. Considering the similarities of the zero band gap material graphene and the 1.9 eV direct band gap material MoS₂, we suggest that electron transport on the femtosecond time scale is dominated by the strong influence of the ion’s Coulomb potential in contrast to the dispersion
defined by the material’s band structure.