  Tunneling ionization is one of the fundamental electron dynamics, which has
wide applications in ultrafast physics. When frustrated tunneling ionization
(FTI) is considered, the tunneling rate is not equivalent to ionization rate.
However, it is hard to resolve the effects of FTI and direct tunneling
ionization (DTI) in ionization spectrum experimentally. Here we report the
first observation of the asymmetrical fast oscillation in above-threshold
ionization (ATI) spectrum of Argon as function of carrier-envelope phase (CEP),
to the best of our knowledge. Simulation results identify that in the
experimental ATI spectrum, the {\pi}/5 oscillation originates from the quantum
interference of electrons in FTI, while DTI is responsible for the asymmetry.
Our results provide clear evidence to resolve the effects of direct tunneling
and FTI in a new physical regime.
