The internal conversion (IC) pathways and times between S-3 and S-1 states are investigated by simulating the fluorescence depletion spectrum (FDS) of chlorophyll-a (Chl-a) in ethyl ether solvent using the perturbative density matrix method and the transient linear susceptibility theory. The calculated internal conversion (IC) times from S-2 to S-1 states and from S-3 to S-1 states are 110 and 143 fs, respectively, and vibrational relaxation rate constant is 8.5 ps(-1) for the Chl-a in ethyl ether at room temperature. The theoretical calculation shows that sequential process, S-3 -> S-2 -> S-1, is a main pathway for the IC process. (C) 2007 Elsevier B.V. All rights reserved.