Abstract
We experimentally investigated the carrier-envelope phase (CEP) effect on above-threshold ionization (ATI) process of argon in frustrated tunneling ionization regime. For the first time, we found the asymmetrical fast oscillation of the ionization probability with a period around π/6 in ATI spectrum, to the best of our knowledge. Simulation results agreed well with the experimental data. Two kinds of electron trajectories were resolved to interpret the experimental observations. One was the tunneling ionization directly from the ground state, which was responsible for the asymmetry in spectrum, while the other one was tunneling ionization from the excited states, which contributed to the π/6 oscillation. Our results served as evidence that the bound states population was controlled by the CEP.