  Diatomic molecules (e.g., O$_2$) in intense laser field exhibit a peculiar
suppressed ionization behavior compared to their companion atoms. Several
physical models have been proposed to account for this suppression while no
consensus has been achieved. In this letter, we aim to clarify the underlying
mechanisms behind this molecular ionization suppression. Experimental data
recorded at midinfrared laser wavelength and its comparison with that at
near-infrared wavelength revealed a peculiar wavelength and intensity
dependence of the suppressed ionization of O$_2$ with respect to its companion
atom of Xe, while N$_2$ behaves like a structureless atom. It is found that the
S-matrix theory calculation can reproduce well the experimental observations
and unambiguously identifies the significant role of two-center interference
effect in the ionization suppression of O$_2$.
