Tunneling ionization time delay under relativistic kinematics

Abstract We investigate the process of strong field tunneling ionization of hydrogen and Yukawa atoms under relativistic kinematics and observe a non-vanishing time delay related to a finite speed of light. The study is based on a numerical solution of the time-dependent Schrödinger equation with a relativistic Hamiltonian including the kinematic correction to the leading order. We employ the standard attoclock geometry with a circularly polarized driving laser pulse. By analyzing the photoelectron momentum distributions in the polarization plane and the spatial motion of the ionized wave-packets, we conclude that the account of the relativistic kinematics, which is a direct consequence of the finiteness of the speed of light, leads to a non-vanishing tunneling time delay of the order of several attoseconds. This is in contrast with the zero tunneling delay predicted theoretically by Torlina et al ( 2015 Nat. Phys. 11 503) and observed experimentally by Sainadh et al (2019 Nature 568 75).