Time-resolved Fourier-transform infrared reflection study on photoinduced phase transition of tetrathiafulvalene-p-chloranil crystal

We have measured temporal change of infrared reflection spectrum of ionic phase of tetrathiafulvalene- p -chloranil (TTF-CA) crystal induced by 10 ns pulsed laser excitation at 2.35 eV with using a step-scan Fourier-transform infrared spectrometer. The excitation reduced the magnitude of the a g -mode bands around 980 cm -1 and 1380 cm -1 and enhanced reflection intensity at 1630 cm -1 . The time evolution of both the a g -mode changes and the enhancement at 1630 cm -1 are described by the sum of two decay components: one decayed within 2 μ s and the other showed very gradual decay within the time range used in the present study. The fractional changes of reflectivity for the fast decay components were almost identical for all a g modes. The 1630 cm -1 enhancement can be ascribed to the carbonyl stretching vibration of CA. The position of this peak shifts slightly to low-wavenumber side from the CA peak of the neutral phase in thermal equilibrium. Consequently, the fast decay component is ascribed to the photoinduced phase. The slow decay component is conceivably due to the metastable center formed by the excitation, because we could not resolve the spectral change corresponding to the slow component in the range between 1595 and 1660 cm -1 , where the carbonyl stretching vibration of TTF-CA is observed. The degree of charge transfer of the photoinduced phase is evaluated to 0.52 ± 0.08 by comparing the peak position of the transient enhancement with those of the carbonyl stretching vibration modes in ionic and neutral phases in thermal equilibrium.