Origin of initial rise process of ultrafast exponential response in liquids

Ultrafast optical Kerr effect (OKE) measurement was performed for various liquids. It was found that any response function obtained from OKE measurement shows a slow initial rise early after excitation even when the material response contains an exponential response at a later time which indicates that the Debye relaxation model breaks down in such a ultrafast time range. It is shown that the finite rise process leading to an exponential response is closely related with the crossover from the dynamical to statistical regime and comes from the anharmonic couplings among numerous microscopic oscillators.