The liquid crystal response to instantaneous strong laser pulses is related to its electronic, orientational, as well as thermal nonlinear optical properties. In this work, the transients of the refractive index variation in a pentyl-cyanobiphenyl (5CB) liquid crystal doped with an anthraquinone dye are studied with nanosecond laser pulse excitation at 532 nm. The magnitude of the response and its sign are studied as functions of the dye concentration. and the ac electric field applied to a planarly or homeotropically aligned liquid crystal cell.