  We present a numerical study of the charge dynamical structure factor
N(k,omega) of a one-dimensional (1D) ionic Hubbard model in the Mott insulator
phase. We show that the low-energy spectrum of N(k,omega) is expressed in terms
of the spin operators for the spin degrees of freedom. Numerical results of
N(k,omega) for the spin degrees of freedom, obtained by the Lanczos
diagonalization method, well reproduce the low-energy spectrum of N(k,omega) of
the 1D ionic Hubbard model. In addition, we show that these spectral peaks
probe the dispersion of the spin-singlet excitations of the system and are
observed in the wide parameter region of the MI phase.
