  Starting from a quantum kinetic equation including the mean field and a
conserving relaxation-time approximation we derive an analytic formula which
describes the time dependence of the dielectric function in a plasma created by
a short intense laser pulse. This formula reproduces universal features of the
formation of collective modes seen in recent experimental data of femtosecond
spectroscopy. The presented formula offers a tremendous simplification for the
description of the formation of quasiparticle features in interacting systems.
Numerical demanding treatments can now be focused on effects beyond these gross
features found here to be describable analytically.
