Laser-induced electron tunneling is one of the most fundamental strong-field phenomena. In laser-induced electron tunneling, it is usually assumed that the highest tunneling rate appears at the peak of the laser electric field corresponding to the shortest tunneling barrier. Here we show that this fundamental assumption is broken in an orthogonally polarized two-color laser field. We find that a rapid change of the instantaneous initial momentum at the tunnel exit can lead to a distinct ionization time shift between the instant of the maximum ionization rate and the peak of the electric-field strength. This ionization time shift can be either positive or negative depending on the degree of nonadiabaticity of the electron-tunneling process, and it is mapped to a measurable momentum shift in the photoelectron momentum distribution. These findings have significant implications for resolving attosecond electron dynamics and are essential to quantify possible tunneling delay time during strong-field ionization.

• Received 20 May 2019

DOI:https://doi.org/10.1103/PhysRevA.100.023414