  We evaluate the density matrix of an arbitrary quantum mechanical system in
terms of the quantities pertinent to the solution of the time-dependent density
functional theory (TDDFT) problem. Our theory utilizes the adiabatic connection
perturbation method of G\"{o}rling and Levy, from which the expansion of the
many-body density matrix in powers of the coupling constant $\lambda$ naturally
arises. We then find the reduced density matrix $\rho_\lambda({\bf r},{\bf
r}',t)$, which, by construction, has the $\lambda$-independent diagonal
elements $\rho_\lambda({\bf r},{\bf r},t)=n({\bf r},t)$, $n({\bf r},t)$ being
the particle density. The off-diagonal elements of $\rho_\lambda({\bf r},{\bf
r}',t)$ contribute importantly to the processes, which cannot be treated via
the density, directly or by the use of the known TDDFT functionals. Of those,
we consider the momentum-resolved photoemission, doing this to the first order
in $\lambda$, i.e., on the level of the exact exchange theory. In illustrative
calculations of photoemission from the quasi-2D electron gas and isolated
atoms, we find quantitatively strong and conceptually far-reaching differences
with the independent-particle Fermi's golden rule formula.
