FT-IR study of photo-induced phase transition in organic charge-transfer crystal of TTF-CA

摘要

We have studied Fourier-transform IR-reflection spectra of tetrathiafulvalene-p-chloranil (TTF-CA) crystal under continuous excitation by 532-nm laser beam at 6 K. The excitation by the laser beam with the power (P) of 6.1 Wcm~(-2) induces an enhancement of the IR-reflection at around 1640 cm~(-1), which is ascribed to the carbonyl stretching vibration of CA molecules in the neutral phase. Meanwhile, the moderate excitation with P <3.5 Wcm~(-2) gives an enhancement in the intensity of the charge-transfer (CT) reflection and a reduction of the a_g-mode band but no change in the carbonyl stretching vibration band. The results show that there is a threshold intensity for inducing the phase transition. Both of the changes in the CT absorption and the a_g-mode band induced below the threshold intensity decays single exponentially with a lifetime of 23 min. The result suggests that the laser excitation with intensities less than the threshold intensity lead to formation of the metastable center accompanied with dissociation of donor-acceptor pair.