The review of the photo-induced cooperative phenomena observed in a polydiacetylene (PDA) single crystal is reported. Recently extensive studies have been made on the photo-induced cooperative phenomena, especially photo-induced phase transition, because they are typical examples of the cooperative phenomena under non-equilibrium condition. Here, we report various characteristics of the photo-induced cooperative phenomena and its dynamics after the irradiation of the nanosecond pulsed laser light on the surface of PDA crystal as a typical example. The observed phase conversion efficiency has shown the clear threshold-like behavior. In addition, we also report the conversion dynamics with varying temperatures, excitation intensities and excitation photon energies. The obtained results clearly indicate that thecooperative electron-lattice interaction plays a key role in the dynamical process of the photo-induced phase conversion in PDA crystals.
Primary dynamics of photo-induced ionic (I)-neutral (N) phase transition in a charge transfer complex tetrathiafulvalene-p-chloranil (TTF-CA) was investigated by femtosecond reflection spectroscopy with varying excitation densities. In I-to-N transition, charge transfer excitation in the I phase produces N donor-acceptor (D0A0) strings within 200 fs immediately after the weak photo-excitation. These initial N states decay with a life time of 300 ps at 4 K, whereas those cause multiplication leading to macroscopic I-to-N conversion within 20 ps at 77 K just below the N-I transition temperature TNI. Such a multiplication process is not observed for the N-to-I transition at 90 K which is a reverse process of I-to-N transition. Ionic (D+A−) string with a lifetime of several picosecond was observed even for strong excitation in N-to-I transition. Vibrational coherence due to the collective motion of the D+A− dimerization as well as of N-I domain boundary in several tenth ps and sub-ps time regions is observed.
Perovskite-type doped manganites are ferromagnetic metals due to the so-called double-exchange mechanism. Recent experimental investigations, however, revealed that the double-exchange interaction strongly competes with the charge-ordering interaction. The competition causes the short-range charge-order in the paramagnetic phase and the phase separation observed in the vicinity of the phase boundary between the ferromagnetic metals and charge-ordered insulators. The competition is expected to govern also the pulse laser irradiation effects. In this article, we will introduce (1) temperature variation of the pulse laser irradiation effects on La0.7Ca0.3MnO3 film, (2) critical slowing down of the relaxation time of La0.7Ca0.3MnO3 film and (3) chemical substitution effects on the critical slowing down in La0.7Ca0.3MnO3 film. These experimental data are well understood in terms of the photoinduced formation of the short-range charge-order.
We have investigated the photoinduced phase transition of spin crossover complex [Fe(2-picolylamine)3]Cl2EtOH by Raman and infrared absorption spectroscopy and clarified that the symmetry breaking, which has not appeared in the thermal equilibrium state, takes place in the photoinduced phase. In order to clarifythe origin of the symmetry breaking, the local structure around iron ion has also been studied in the photoinduced phase by x-ray absorption spectroscopy. In addition, we proposed a qualitative modeltoexplain the appearance of a new-material phase, which has not been observed in the thermal equilibrium state but could be created by the photoinduced phase transition.
Photoinduced phase transitions via excited electronic states are discussed theoretically using a one-dimensional model composed of localized electrons and lattices under the adiabatic or diabatic approximation. We show that the global phase change by photoexcitation only at a site is possible, and we clarify conditions for the occurrence of such phenomena. In the adiabatic regime, depending on the intersite interaction, an initial local photoinduced change (i) remains locally, (ii) induces cooperatively a global phase change, or (iii) disappears and the system returns to the initial phase. Dynamical features of the case (ii) are characterized by the “deterministic domino” processes of the domain walls; the domain walls between the two phases move deterministically with a constant velocity without spontaneous emissions. In the diabatic regime, similar three types of photoinduced change are possible. The domain-wall dynamics is described as the “stochastic domino” process with a random velocity, which is accompanied by the successive radiative transitions. Related topics of the photoinduced domino theory are also introduced.
The chain polymerization in a monomolecular layer of unsaturated organic compounds on a surface is investigated using a scanning tunneling microscope (STM). We used self-ordered layers of 10,12-pentacosadiynoic acid or 10,12-nonacosadiynoic acid of diacetylene compounds on a graphite substrate. Ultraviolet light irradiation on the molecular layer induced the chain polymerization, and the created polymers can be clearly imaged by STM. The tunneling electrons from an STM tip can also excite a molecule, and the excited molecule initiates the extended chain reaction. The termination of chain reaction can also be controlled by making an artificial defect in advance using an STM tip. As a result, we can control the initiation and termination of chain polymerization with a spatial precision of the order of 1 nm.
Fragmentation of some organic compounds is inferred in terms of bond-dissociation energies of the atoms in those compounds. A big role of the dissociation energy for the fragmentation can be recognized, but fragmentation and ionization process are complex compared to the case of fragmentation of inorganic compounds. Besides the dissociation energy, proton transfer, formation of sub-stable ion, recombination of fragment radicals, cyclization etc. should be considered during the Ga+ primary ion irradiation.
We have studied the morphology and structure of splintered carbon nanotubes (CNTs) grown by plasma-assisted chemical vapor deposition with the addition of hydrogen sulfide gas. Typical multi-wall CNTs are usually grown using a mixture of methane and hydrogen. The addition of hydrogen sulfide gas caused the CNTs to splinter. We call these CNTs “frilled” or “cornhusk”. Transmission electron microscopic observation showed that both the size and the length of the frill increased in accordance with the fraction of H2S gas. S was not detected from any part of the frilled CNTs by electron energy loss spectroscopy (EELS). EELS spectra indicated that the σ-bond/π-bond ratio at the split part of the frill is larger than that at the apex. These results suggest that H2S gas acts as a catalyst to form the sp3-hybridized orbital in carbon materials. We propose a possibility of controlling the morphology and structure of CNTs by adding H2S.