  Photoinduced charge dynamics in dimerized systems is studied on the basis of
the exact diagonalization method and the time-dependent Schr\"odinger equation
for a one-dimensional spinless-fermion model at half filling and a
two-dimensional model for $\kappa$-(bis[ethylenedithio]tetrathiafulvalene)$_2$X
[$\kappa$-(BEDT-TTF)$_2$X] at three-quarter filling. After the application of a
one-cycle pulse of a specifically polarized electric field, the charge
densities at half of the sites of the system oscillate in the same phase and
those at the other half oscillate in the opposite phase. For weak fields, the
Fourier transform of the time profile of the charge density at any site after
photoexcitation has peaks for finite-sized systems that correspond to those of
the steady-state optical conductivity spectrum. For strong fields, these peaks
are suppressed and a new peak appears on the high-energy side, that is, the
charge densities mainly oscillate with a single frequency, although the
oscillation is eventually damped. In the two-dimensional case without intersite
repulsion and in the one-dimensional case, this frequency corresponds to
charge-transfer processes by which all the bonds connecting the two classes of
sites are exploited. Thus, this oscillation behaves as an electronic breathing
mode. The relevance of the new peak to a recently found reflectivity peak in
$\kappa$-(BEDT-TTF)$_2$X after photoexcitation is discussed.
