  We theoretically study pump-photon-energy-dependent pathways of a
photoinduced dimer-Mott-insulator-to-metal transition, on the basis of
numerical solutions to the time-dependent Schr\"odinger equation for the exact
many-body wave function of a two-dimensional three-quarter-filled extended
Peierls-Hubbard model. When molecular degrees of freedom inside a dimer are
utilized, photoexcitation can weaken the effective interaction or increase the
density of photocarriers. In the organic dimer Mott insulator, $ \kappa
$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, the intradimer and the interdimer
charge-transfer excitations have broad bands that overlap with each other. Even
in this disadvantageous situation, the photoinduced conductivity change depends
largely on the pump photon energy, confirming the two pathways recently
observed experimentally. The characteristic of each pathway is clarified by
calculating the modulation of the effective interaction and the number of
carriers involved in low-energy optical excitations. The
pump-photon-energy-dependent pathways are confirmed to be realized from the
finding that, although the effective interaction is always and slowly weakened,
the introduction of carriers is sensitive to the pump-photon energy and
proceeds much faster.
