DIRECT MEASUREMENT OF ELECTRONIC DEPHASING USING ANISOTROPY

In a recent theoretical treatment of coherence effects in the anisotro py of optical experiments, it was predicted that the decay of the anis otropy that exists for molecules with degenerate states prior to the o nset of electronic dephasing could be utilized to measure electronic d ephasing directly. The first experimental observation of this coherent anisotropy and its ultrafast decay are presented. The data were obtai ned by means of femtosecond pump-probe experiments on room temperature solutions of magnesium tetraphenylporphyrin in tetrahydrofuran. In th is system, which has a doubly degenerate excited state due to the four fold molecular symmetry, the anisotropy decays with time constants 210 fs and 1.6 ps.