Abstract

Momentum shift is an important sign of nonadiabatic tunnelling ionization process, to investigate the mechanism of momentum shift, we use the strong-field approximation theory to track the formation of ionization momentum spectra of hydrogen atom under the action of different laser pulses in time domain. By observing the ionization momentum spectra of different structures with time, we find that the momentum shift is formed by the continuous interference and evolution of ionization signals over time. Meanwhile, we further analyze how subcycle and intercycle interference influencing the formation of momentum shift. Before the duration is long enough that intercycle interference emerges, momentum shift grows smoothly. This finding reveals the different intrinsic mechanisms for the formation of momentum shift in many-cycle and few-cycle laser pulses. This work lays foundation for deeper understanding of nonadiabatic tunnelling process and makes the regulation of momentum shift possible.