  Radiation is important for the propagation of streamers in dielectric
liquids. Photoionization is a possibility, but the effect is difficult to
differentiate from other contributions. In this work, we model radiation from
the streamer head, causing photoionization when absorbed in the liquid. We find
that photoionization is local in space ({\mu}m-scale). The radiation absorption
cross section is modeled considering that the ionization potential (IP) is
dependent on the electric field. The result is a steep increase in the
ionization rate when the electric field reduces the IP below the energy of the
first electronically excited state, which is interpreted as a possible
mechanism for changing from slow to fast streamers. By combining a simulation
model for slow streamers based on the avalanche mechanism with a change to fast
mode based on a photoionization threshold for the electric field, we
demonstrate how the conductivity of the streamer channel can be important for
switching between slow and fast streamer propagation modes.
