Fig. 2: Exciton capture process.

a Schematic depiction of the exciton capture process by a charged impurity. The collision parameter b is given by the distance between the initial trajectory of the exciton and a parallel trajectory passing through the center of the charged impurity. The collision parameter \({b}_{\max }\) divides trajectories that result in capture of the exciton and trajectories that result only in deflection of the exciton. b Shape of the total potential composed of the attractive second-order Stark shift and the repulsive centrifugal barrier term for two different collision parameters b. The collision energy of 24 μeV corresponds to the kinetic energy of the exciton which is determined by the recoil momentum of the photons. An exciton passing the impurity with large b results in a large angular momentum l which in turn increases the centrifugal barrier. The maximum value of the potential is larger than the collision energy, so the exciton becomes deflected. For small b, l is lower as well and the centrifugal barrier is reduced significantly. Here, the maximum value of the potential is below the collision energy an the exciton gets captured.