  The orientation-dependent strong-field ionization of CO molecules is
investigated using the fully propagated three-dimensional time-dependent
Hartree-Fock theory. The full ionization results are in good agreement with
recent experiments. The comparisons between the full method and single active
orbital (SAO) method show that although the core electrons are generally more
tightly bounded and contribute little to the total ionization yields, their
dynamics cannot be ignored, which effectively modify the behaviors of electrons
in the highest occupied molecular orbital. By incorporating it into the SAO
method, we identify that the dynamic core polarization plays an important role
in the strong-field tunneling ionization of CO molecules, which is helpful for
future development of tunneling ionization theory of molecules beyond single
active electron approximation.
