  The spontaneous production of electron-positron pairs from the vacuum--in a
field configuration composed of a high-frequency electric mode of weak
intensity and a strong constant electric field--is investigated. Asymptotic
expressions for the single-particle distribution function ruling this
nonperturbative process are established by considering the low-density
approximation in the Boltzmann-Vlasov equation. An analytical formula for the
density rate of yielded particles is established which is shown to manifest a
nonperturbative dependence on both the strong and weak electric fields and to
interpolate between the tunneling and multi-photon regimes. It is shown
that--under appropriate circumstances--the produced plasma of electrons and
positrons might reach densities for which their recombinations into high-energy
photons occurs copiously. On the basis of this feature, an experimental setup
for observing the dynamically-assisted Schwinger effect is put forward.
