DIRECT MEASUREMENT OF ELECTRONIC DEPHASING USING ANISOTROPY

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