  We present measurements of the transient photoconductivity in pentacene
single crystals using optical-pump THz-probe spectroscopy. We have measured the
temperature and fluence dependence of the mobility of the photoexcited charge
carriers with picosecond resolution. The pentacene crystals were excited at 3.0
eV which is above the bandgap of ~2.2 eV and the induced change in the
far-infrared transmission was measured. At 30 K, the carrier mobility is mu ~
0.4 cm^2/Vs and decreases to mu ~ 0.2 cm^2/Vs at room temperature. The
transient terahertz signal reveals the presence of free carriers that are
trapped on the timescale of a few ps or less, possibly through the formation of
excitons, small polarons, or trapping by impurities.
