title: Retardation effects and the Born-Oppenheimer approximation: theory of tunneling ionization of molecules revisited.
abstract: We show that retardation in adjusting an electronic state to an instantaneous internuclear configuration caused by the finiteness of the electron's velocity breaks the validity of the Born-Oppenheimer (BO) approximation at large electron-nuclei distances. This applies even to the ground state. As a result, the BO approximation in the theory of tunneling ionization of molecules breaks down at sufficiently weak fields. We also show that to account for nuclear motion the weak-field asymptotic expansion for the tunneling ionization rate must be restructured. The predictions for the rate using the BO approximation and the asymptotic expansion are compared with numerical results for a one-dimensional three-body system modeling a diatomic molecule, with both electronic and nuclear motions treated exactly.
authors.affiliation: National Research Center "Kurchatov Institute," Kurchatov Square 1, Moscow 123182, Russia and Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia.
authors.forename: Oleg I
authors.initials: OI
authors.lastname: Tolstikhin
authors.forename: Lars Bojer
authors.initials: LB
authors.lastname: Madsen