  We study single-photon ionization of aligned H$_2^+$ in a high-frequency
low-intensity laser field. We focus on the case where the laser frequency is
not far larger than the ionization potential of the target. The calculated
photoelectron momentum distribution through numerical solution of
time-dependent Schr\"{o}dinger equation shows clear interference patterns. By
developing a theory model applicable for high-frequency laser field, we show
that the interference patterns can not be explained by the interference of the
electronic wave with the observed momentum between these two atomic centers of
the molecule. The Coulomb potential influences remarkably on the momentum of
the emitting electronic wave responsible for this interference. Our results
suggest a manner for probing the structure and the electron dynamics of the
molecule in single-photon ionization.
