  Using a modified Cahn-Hilliard-Cook theory for spinodal decomposition in a
binary mixture that exhibits both diffusion and interconversion dynamics, we
derive the time-dependent structure factor for concentration fluctuations. We
compare the theory and obtain a qualitative agreement with simulations of the
temporal evolution of the order parameter and structure factor in a
nonequilibrium Ising/lattice-gas hybrid model in the presence of an external
source of forceful interconversion. In particular, the characteristic size of
the steady-state phase domain is predicted from the lower cut-off wavenumber of
the amplification factor in the generalized spinodal-decomposition theory.
