Gage-equivalent forms of the Schroedinger equation for a hydrogenlike   atom in a time-dependent electric field
  Several gage-equivalent forms (including some novel ones) of the Schroedinger equation for a hydrogenlike atom in a time-dependent electric field of a laser pulse are presented. These forms allow to develop a perturbation theory for both small and rather large intensities of the electromagnetic field. 
Yuri V. Popov and Konstantin A. Kouzakov