  We investigate the microscopic dynamics at the initial stage of photoinduced
phase transitions in tetrathiafulvalene-$p$-chloranil by exact diagonalization.
We first show that the one-dimensional extended Peierls-Hubbard model exhibits
a neutral phase with small ionicity and negligible dimerization and an ionic
phase with moderate ionicity and dimerization. Aside from these phases, we find
a doubly ionized phase with strong dimerization that we call the "dipole"
phase. These ground-state phases are characterized by various order parameters
and the Zak phase, which is relevant to electronic polarization. We further
explore the microscopic dynamics of the three phases triggered by short
monocycle optical pulses. The electronic order parameters and lattice
displacement suggest that the neutral-ionic, ionic-neutral, and dipole-ionic
transitions are induced. Furthermore, clear spectroscopic changes are observed
in the time-dependent spectral density and pump-probe conductivity. A detailed
analysis of the spectroscopy demonstrates the generation of coherent
charge-transfer strings via multiphoton absorption and the crucial roles of the
excited states and the metastable ground state at the new lattice position for
the ultrafast dynamics.
