  The Vlasov-Poisson-Boltzmann System governs the time evolution of the
distribution function for the dilute charged particles in the presence of a
self-consistent electric potential force through the Poisson equation. In this
paper, we are concerned with the rate of convergence of solutions to
equilibrium for this system over ${\mathbb{R}}^3$. It is shown that the
electric field which is indeed responsible for the lowest-order part in the
energy space reduces the speed of convergence and hence the dispersion of this
system over the full space is slower than that of the Boltzmann equation
without forces, where the exact difference between both power indices in the
algebraic rates of convergence is 1/4. For the proof, in the linearized case
with a given non-homogeneous source, Fourier analysis is employed to obtain
time-decay properties of the solution operator. In the nonlinear case, the
combination of the linearized results and the nonlinear energy estimates with
the help of the proper Lyapunov-type inequalities leads to the optimal
time-decay rate of perturbed solutions under some conditions on initial data.
