  Optically induced ultrafast electronic excitations with sufficiently long
lifetimes may cause strong effects on phase transitions like structural and
nonmetal to metal ones. Examples are transitions diamond to graphite, graphite
to graphene, non-metal to metal, solid to liquid and vapor to liquid, solid. A
spectacular case is photo-induced water condensation. These non-equilibrium
transitions are an ultrafast response, on a few hundred fs-time scale, to the
fast electronic excitations. The energy of the photons is converted into
electronic one via electronic excitations changing the cohesive energy. This
changes the chemical potential controlling the phase transition. In view of the
advances in laser optics photon induced transitions are expected to become an
active area in non-equilibrium physics and phase transition dynamics.
Conservation laws like energy or angular momentum conservation control the time
during which the transitions occur. Since the photon induced effects result
largely from weakening or strenghtening of the bonding between the atoms or
molecules transitions like solid/liquid etc. can be shifted in both directions.
Photoinduced transitions will be discussed from an unified point of view.
