  We investigate the excitation spectrum and momentum distribution of the ionic
Bose-Hubbard model by the standard basis operator method. We derive Green's
functions in the random phase approximation in Mott insulator, superfluid,
charge density wave, and supersolid phases. The excitation spectrum has gapped
modes and gapless Goldstone modes in the superfluid and supersolid phases. We
show that the momentum distribution has a peak at the zone corner in the
supersolid phase and the charge density wave phase close to the phase boundary.
In addition, we demonstrate that the momentum distribution can be explained by
the excitation spectrum and spectral weights of hole excitation modes.
