We have measured ultrafast reflectivity changes of (EDO-TTF) 2PF6 in the infrared region (0.51 eV - 1.03 eV) after excitation with a 1.58 eV photon in order to reveal the mechanism of ultrafast photo-induced insulator-to-metal phase transition. We found that the temporal profiles of reflectivity change have two components: a faster component within 3 ps associated with a 1-ps or 0.5-ps period oscillation and a slower component lasting over hundreds of a picosecond. In addition, we found that the sign of a reflectivity change is often different from that expected from the thermally induced phase transition. These results indicate that the PIPT takes place via a complicated process due to strong electron and phonon coupling.
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