  Strong-field ionization of polar molecules contains rich dynamical processes
such as tunneling, excitation, and Stark shift. These processes occur on a
sub-cycle time scale and are difficult to distinguish in ultrafast
measurements. Here, with a developed strong-field model considering effects of
both Coulomb and permanent dipole, we show that photoelectron momentum
distributions (PMDs) in orthogonal two-color laser fields can be utilized to
resolve these processes with attosecond-scale resolution. A feature quantity
related to the asymmetry in PMDs is obtained, with which the complex electron
dynamics of polar molecules in each half laser cycle is characterized and the
subtle time difference when electrons escaping from different sides of the
polar molecule is identified.
