Photoinduced phase transition in tetrathiafulvalene-p-chloranil observed in femtosecond reflection spectroscopy

Abstract

Photoinduced transitions from ionic ͑I͒ to neutral ͑N͒ and neutral ͑N͒ to ionic ͑I͒ phases in an organic charge transfer (CT) complex, tetrathiafulvalene-p-chloranil (TTF-CA), were investigated by femtosecond pumpprobe reflection spectroscopy. Transient reflectivity changes of the intramolecular transition band of TTF sensitive to the degree of CT between a donor molecule of TTF and an acceptor molecule of CA are measured as a function of excitation energy, excitation density, and temperature. By adopting the multilayer model for the analysis of the obtained transient reflectivity spectra, we have derived the time characteristics of amounts and spatial distributions of photoinduced N ͑I͒ states in the I ͑N͒ phase. The results reveal that the I to N ͑IN͒ transition induced by the resonant excitation of the CT band at 4 K is composed of three processes; (1) formation of a confined one-dimensional (1D) N domain, that is, a sequence of D 0 A 0 pairs, just after the photoexcitation, (2) multiplication of the 1D N domains to the semimacroscopic N states up to 20 ps within the absorption depth of the excitation light, and (3) proceeding of the IN transition along the direction normal to the sample surface. At 77 K near the NI transition temperature ͑T c =81 K͒, the size of the 1D N domain initially produced is enlarged and its multiplication process is strongly enhanced. When the excitation energy is increased, the initial photoproduct is changed from the confined 1D N domain to the positively and negatively charged N states. The spatial size of the latter is considerably larger than that of the former, indicating that the introduction of charge carriers makes the neighboring I state strongly unstable. The dynamics of the photoinduced N to I ͑NI͒ transition has also been investigated. The 1D I domains are initially produced by lights, however, they decay within 20 ps even if the density of the I domains is increased. The results demonstrate that there is a clear difference of the dynamics between the photoinduced IN and NI transitions. In these photoinduced transitions, three kinds of coherent oscillations with the period of ϳ0.6, ϳ50, and ϳ85 ps have been detected on the photoinduced reflectivity changes, which are reasonably assigned to the dynamical dimeric displacements of molecules associated with the spin-Peierls instability, the shock wave driven by the sudden volume change due to the photoinduced transitions, and the oscillation of the NI domain boundary. On the basis of the results, dynamical aspects of the photoinduced IN and NI transitions have been discussed in detail.

State of the art and opportunities in probing photoinduced phase transitions in molecular materials by conventional and picosecond X-ray diffraction

“…Consequently the probed dynamics may be different: in particular nucleation processes may be strongly influenced by the vicinity of the surface and bulk dynamics by the propagation process. Moreover, the results of both optical and diffraction measurements unambiguously agree on the threshold behavior (Koshihara et al, 1999;Iwai et al, 2002;Tanimura 2004;Okamoto et al;2004;Collet et al, 2003). When the TTF-CA is excited in the region of the charge-transfer band (i.e.…”

“…Recent optical pump-probe reflectivity measurements have evidenced the photoinduced change of electronic state, both for the I-to-N and for the N-to-I transformations, on the 100 ps time scale (Koshihara et al, 1999, Iwai et al, 2002, Okamoto et al, 2004, Tanimura 2004. These laser-induced states relax to their thermodynamical stable phases in less than 1 ms.…”

“…Low laser penetration depth is a drawback in this X-ray experiment, as X-rays probe the whole volume of such 100 mm thick crystals. At 1.55 eV (800 nm), the laser penetration has been estimated to be on the order of few microns (Okamoto et al, 2004). Given that laser light is attenuated upon propagation in the bulk, the excitation density decreases and may drop below threshold after a given depth so that a typical crystal can be only partially transformed.…”

“…The high conversion efficiency, with many molecules transformed by a single photon, originates from cooperativity. From the X-ray and optical data, it is estimated that one photon transforms about few hundreds molecules (Koshihara et al, 1999, Iwai et al, 2002Okamoto et al, 2004;Collet et al, 2003). Low laser penetration depth is a drawback in this X-ray experiment, as X-rays probe the whole volume of such 100 mm thick crystals.…”

Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

“…Over the decades, a large number of ultrafast pump-probe experiments have been performed on TTF-CA in order to explore the photo-induced phase transition (PIPT) that occurs in this one-dimensional charge-transfer compound, most of them confined to the femto-and picoseconds time range [35,58,[69][70][71][72][73][74][75][76][77][78][79][80]. The studies were mainly restricted to low temperatures (typically 4 K) or very close to the phase transition (T ≤ T NI ).…”