Tightly Bound Electrons in a Uniform Electric Field

The energy spectrum of tightly bound electrons in a uniform electric field is studied for finite systems. It forms the Stark ladder for almost all states except those near the band edges (these can be understood as surface states) and in the case of extremely low fields such that the potential drop across the entire sample is of the order of the bandwidth. This fact is independent of the boundary conditions. A two-band tight-binding model is solved, taking into account the interband coupling, and it is found that the spectrum is that of two interspaced Stark ladders. The experimental situation is briefly reviewed and the limitations and feasibility of observing Stark ladders in solids is discussed.