Subcycle nonadiabatic strong-field tunneling ionization

We present a subcycle nonadiabatic strong-field tunneling theory and derive the position of tunnel exit, the transverse and longitudinal momentum distributions at the tunnel exit, and the ionization rate in an instantaneous laser field. These tunneling coordinates are shown to nonadiabatically couple with each other in an instantaneous laser field when the electron tunnels through the barrier. We have further incorporated the nonadiabatic tunneling theory with the quantum-trajectory Monte Carlo approach to investigate the nonadiabatic effect on the photoelectron angular distributions. The simulated photoelectron angular distributions with the nonadiabatic corrections have been validated by comparison with the ab initio results through numerically solving the time-dependent Schr\"odinger equation. The nonadiabatic coordinates at the tunnel exit play important roles in both the direct ionization and rescattering process. The nonadiabatic tunneling theory provides an intuitive understanding on subcycle dynamics of tunneling ionization.