Charge and Structural Dynamics in Photoinduced Phase Transition of (EDO-TTF)₂PF₆ Examined by Picosecond Time-Resolved Vibrational Spectroscopy

Abstract

Using time-resolved near-infrared reflectance spectroscopy and time-resolved mid-infrared vibrational spectroscopy, we studied photoinduced phase transition of the charge-ordered insulating phase in a charge-transfer complex (EDO-TTF) 2 PF 6 (EDO-TTF: ethylenedioxy-tetrathiafulvalene) in the hundred picosecond range after photoexcitation. The temporal profiles at 0.83−1.03 eV, which are a characteristic of the photoinduced charge-disproportionate phase immediately after photoexcitation, suggested the formation of a new metastable phase in the hundred picosecond range. Time-resolved vibrational spectra at 1300−1700 cm −1 , where charge-and structure-sensitive CC stretching vibrational modes are located, elucidated that the nature of the new phase is very close to that of the high-temperature metallic phase and it takes about 100 ps for the new phase to emerge accompanied with charge and structure fluctuation.

Local response to light excitation in the charge-ordered phase of(EDO−TTF)2SbF6

“…This is a clear indication that this photoinduced state is different from both the insulating and the high temperature metallic states. This is in agreement with recent femtosecond electron diffraction [9] and infrared [8] studies of (EDO-TTF) 2 …”

“…This is not the case in our study and this is an indication of the local nature of the structural trapping of the electronic excited state. This local trapping was theoretically introduced to explain the two-step dynamics [6][7][8][9], which we will discuss in the last part of this paper.…”

“…Additional experimental measurements [6] showed that the reflectivity spectrum of the photoinduced state differs on the ultrashort time scale from that of the metallic state at thermal equilibrium, when seen over a broad energy range from 69 meV to 2.1 eV. [8] and ultrafast electron diffraction measurements [9] unveiled that the photoinduced phase similar to the metallic one of the high temperature phase is only reached around 100 ps. During the relaxation from the photoexcited electronic state to the photoinduced (D 1 D 0 D 1 D 0 ) state, a coherent phonon with frequency around 2 THz was also observed [5].…”

“…It exhibits first-order insulator-to-metal (I-M) phase transition at thermal equilibrium and a gigantic ultrafast change of reflectivity under irradiation by a laser pulse in the I phase [5]. * It was shown by time-resolved optical [6,7] and vibrational spectroscopy [8] as well as ultrafast electron diffraction [9] that the photoinduced transformation proceeds stepwise. The initial electronic excited state is trapped at the molecular level within 40 fs following the electron-phonon coupled precursor state [7], while the crystal transformation is driven by slower propagation at the 100 ps time scale.…”

The Earliest Stage of Photoinduced Phase Transition in a Strongly Correlated Organic System Using a 10-fs Pulse

“…The photoinduced state at around 100 fs was assigned to the (1010) photoinduced original phase by measuring the transient reflectivity spectrum ranging from 18 m to 590 nm using a 120-fs pulse [6]. Afterward, the photoinduced phase was found to turn to the charge melting (0.5, 0.5, 0.5, 0.5) metallic phase with time constant of 94 ps by time-resolved infrared vibrational spectroscopy [7]. To observe the earliest stage of this PIPT process, we measured the reflectivity change (R/R) after photoexcitation of the CT band using the 10-fs pulse.…”