Crossing and anticrossing of resonances: the Wannier-Stark ladders

In the framework of regular perturbation theory we discuss the weak field crossing behavior of the resonances in double ladder (and double well) Stark problems. We get a precise condition for the anticrossing in terms of the Agmon length of the Zener barriers. This condition has a simple physical meaning: as a general rule we have anticrossing and beating effect if the lifetime of the system is larger than the beating period. Of course, we have full delocalization in the anticrossing case only.