Photoinduced Phenomena Research Articles

Photoinduced oxidation of carbon nanotubes

Photoinduced phenomena are of general interest for new materials. Recently,photoinduced molecular desorption of oxygen has been reported in carbonnanotubes. Here we present, using thermopower measurements, that carbonnanotubes when exposed simultaneously to UV light and oxygen exhibitphotoinduced oxidation of the nanotubes. At least two plausible mechanisms forthe experimentally observed photoinduced oxidation are proposed: (i) a lowerenergy barrier for the adsorption of photo-generated singlet oxygen, or (ii) due tothe presence of defects in carbon nanotubes that may facilitate the formation oflocally electron-deficient and electron-rich regions on the nanotubes whichfacilitate the adsorption of oxygen molecules on the nanotubes.

Molecular reorientation by photoinduced modulation of rotational mobility.

To explain the large photoinduced molecular reorientation phenomena observed in dye-doped liquids and liquid crystals, the hypothesis was formed that the rotational mobility of dye molecules is strongly altered during their electronic excitation. Here, we report the direct measurement of a 30%-50% mobility decrease of photoexcited anthraquinone dye molecules dissolved in a cyanophenyl liquid host. This mobility reduction is ascribed to an excited-state reinforcement of intermolecular hydrogen bonding. These results provide fully independent evidence for the validity of current models of the photoinduced reorientation and a working demonstration of the design concepts of "fluctuating-friction" molecular motors. We propose that a light-induced modulation of molecular mobility associated with electronic photoexcitation is of general relevance to the behavior of photosensitive organic materials, currently investigated for applications in optical data storage, liquid-crystal displays, and organic optoelectronic devices.

Photochemistry of caged molecules: CD3Cl@Ice

Hydrocarbons formation following UV photo-induced dissociation of CD3Cl trapped and caged inside thin amorphous solid water (ASW) layers on Ru(001) has been measured for the first time under well-defined UHV conditions. Stable products such as C2D6, CHD3, CD3CD2Cl, CD3OH were detected via post-irradiation temperature programmed desorption. Specific reactivity pathways for the various photo-products were identified based on excitation wavelengths, ASW layer thickness, and parent molecules initial coverage dependence. Cross sections of (1–6)×10−19 cm2 and (1–3)×10−20 cm2 at 193 nm and 248, respectively, were measured. These photo-induced phenomena of caged molecules are discussed as a possible mechanism for the formation of hydrocarbons in interstellar space.

Photoproduction of long-lived holes and electronic processes in intrinsicelectric fields seen through photoinduced absorption and dichroismin Ca3Ga2−xMnxGe3O12garnets

Long-lived photoinduced absorption and dichroism in theCa3Ga2−xMnxGe3O12garnets with x < 0.06were examined versus temperature and pumping intensity. Unusualfeatures of the kinetics of photoinduced phenomena are indicative of theunderlying electronic processes. The comparison with the case of Ca3Mn2Ge3O12,explored earlier by the authors, permits one to finally establish the main commonmechanisms of photoinduced absorption and dichroism caused by random electricfields of photoproduced charges (hole polarons). The rate of their diffusion andrelaxation through recombination is strongly influenced by the same fields, whoselarge statistical straggling is responsible for a broad continuous set of relaxationcomponents (observed in the relaxation time range from 1 to about 1000min). For Ca3Ga2−xMnxGe3O12,the time and temperature dependences of photoinduced absorption anddichroism bear a strong imprint of structure imperfection increasing withx.

Ferromagnetic Spin-Ordering in Photo-active RbMn[Fe(CN)6

In RbMn[Fe(CN)6], 1,2) two kinds of magnetic elements, i.e., Fe and Mn, form a three-dimensional NaCl-type structure bridged by CN ions. Recently, Tokoro et al. has reported suppression of magnetization by irradiation of one-pulse laser shot (532 nm, 10mJ cm ) at 3 K. This observation suggests photo-induced ferromagnetic-to-antiferromagnetic transition, perhaps mediated by some photoinduced structural change. To clarify this novel photoinduced phenomenon, we first have to determine the spin structure of the ground state (before photo-irradiation). Crytallographically, RbMn[Fe(CN)6] belongs to the tetragonal space group (I4m2; Z 1⁄4 2) at low temperatures, in which Fe and Mn elements locate at 2a and 2b site, respectively. The low temperature valence state is considered to be high-spin Mn3þ (d; S 1⁄4 2) and low-spin Fe2þ (d; S 1⁄4 0) state. In this Note, we report the spin structure of RbMn[Fe(CN)6] determined by neutron powder diffraction by the Kinken powder diffractometer for high efficiency and high resolution measurements (HERMES). The observed magnetic Bragg reflection pattern is well reproduced by ferromagnetic alignment of the Mn3þ local spins along c. The Curie temperature is determined to be TC 1⁄4 13ð1ÞK. We will discuss a possible origin for the ferromagnetic coupling between the Mn3þ local spins. RbMn[Fe(CN)6] was prepared by reacting an aqueous solution (0.1mol dm ) of MnCl2 with a mixed aqueous solution of RbCl (1mol dm ) and K3[Fe(CN)6] (0.1mol dm ) to yield a light brown precipitate. With use of a SQUID magnetometer, we have measured temperature variation of magnetization M under a field of 10G. TheM{T curve increases steeply below 12K. The saturated magnetization is 3:3 B (at 7 T) at 3K. Neutron powder diffraction measurements were performed using the Kinken powder diffractometer for high efficiency and high-resolution measurements (HERMES) of Institute for Materials Research, Tohoku University, installed at the JRR-3M reactor at the Japan Atomic Energy Research Institute, Tokai, Japan. Neutrons with wavelength 1.8207 A were obtained by the (331) reflection of the Ge monochromator, and 120-B-Sample-180 collimation. The RbMn[Fe(CN)6] powders ( 1 g) sealed in a vanadium capsule (9mm ) with helium gas, and mounted at the cold head of a closed-cycle He-gas refrigerator. The diffraction data were integrated for 3 h. First, we investigated the lattice structure at 20K by Rietveld structural analysis (RIETAN2000) with the same model as ref. 4. Reflecting the Jahn–Teller instability of the Mn3þ (d) ions, the MnN6 octahedra are elongated along c: the Mn–N1 (out-of-plane) and Mn–N2 (in-plane) bond distances are 2.1997(37) A and 1.9581(29) A, respectively. On the other hand, distortion of FeC6 octahedra is much smaller: the Fe–C1 (out-of-plane) and Fe–C2 (in-plane) bond distances are 1.8868(42) A and 1.9167(58) A, respectively. Figure 1(a) shows neutron powder patterns at 5K ( TC; thick curve) and at 20K ( TC: thin curve) for RbMn[Fe(CN)6]. In the middle panel [Fig. 1(b)], we show magnetic components of the neutron powder pattern. Several magnetic Bragg reflections are observed at 18 , 21 , 29 and 35 . No trace of (002) magnetic Bragg reflection is observed, indicating that the easy axis is along c. This is consistent with the elongation of the MnN6 octahedra along c. In the Bottom panel [Fig. 1(c)], we plotted calculated intensities IM,cal of the magnetic Bragg reflections with ferromagnetic alignment of the Mn3þ local spins along c. IM,cal is expressed as IM.cal / mhkl Lð Þ jFMðQÞj; ð1Þ

Real‐time Fourier transform infrared study of free‐radical UV‐induced polymerization of hybrid sol–gel. I. Effect of silicate backbone on photopolymerization kinetics

AbstractFree‐radical photocurable hybrid sol–gel materials have gained special interest. They are becoming more and more widely used for applications in coatings, optics, sensors, catalysis, and so forth. The photochemical step is a fundamental step in the elaboration of this kind of hybrid sol–gel. However, little is known about the specifics of the photochemistry in this material. The relation between the organic and the inorganic part is investigated. Hydrolysis and condensation reactions were characterized by 29Si NMR. A precise description of the material before irradiation is of paramount importance to understand photoinduced phenomena. Real‐time Fourier transform infrared spectroscopy was used to examine the photopolymerization of hybrid sol–gel under UV irradiation. UV photopolymerization occurred efficiently in hybrid sol–gel although inhibition of free‐radical polymerization by molecular oxygen was pronounced. Important structural modifications during irradiation were also measured. They concern both inorganic and organic parts of the hybrid material. The condensation state of the silicate network was of crucial importance. The presence of the silicate backbone did not limit the final conversion ratio. On the contrary, photopolymerization occurred more efficiently for systems with a higher degree of condensation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 716–724, 2003

Chalcogenide glasses as prospective materials for optical memories and optical data storage

This paper first reviews recent progress in the understanding of the nanometer-scale mechanism of reversible photostructural changes in chalcogenide glasses, and its relevance to various photo-induced phenomena. Then the principles of phase-change optical recording are described. Finally, a novel technique for overcoming the diffraction limit in optical recording is considered, namely the super-resolution near-field structure. This technique, used in combination with multicomponent Te-based chalcogenides as a recording medium, is believed to be a prototype for future optical-storage devices.

Possible Photoinduced Spin Transitions in Bis(phenylmethylenyl)[2.2]paracyclophanes. A Spin−Orbit Coupling Study

A possible mechanism for the spin transitions in various stacking conformations of bis(phenylmethylenyl)[2.2]paracyclophanes, which have close-lying lowest singlet, triplet, and quintet spin states, is theoretically investigated by using diphenylcarbene dimers as models. Spin−orbit coupling (SOC) matrix elements, which play an essential role in the spin transition phenomena, are calculated with the effective one-electron spin−orbit Hamiltonian. The SOC between the first excited singlet state and the first excited triplet state and that between the first excited triplet state and the lowest quintet state are strong. The SOC between the first excited quintet state and the first excited triplet state and that between the first excited triplet state and the lowest singlet state are also strong. These results demonstrate that the spin conversion between the low-spin singlet state and the high-spin quintet state can occur via the first excited intermediate-spin triplet state. We propose that possible photoinduced spin-crossover phenomena can be observed in these organic molecular systems.

Photoexcitation of Bipolarons in Conducting Polymers

We theoretically study the photoexcitation process in a conducting polymerwith a non-degenerate ground state, where the carrier without spin isa bipolaron. The dynamical simulation reveals a photoinduced phenomenon in which apositive bipolaron is split into two polarons by absorbing one photon. Therelaxation time of this photoinduced bipolaron split is about 40 fs. It ispredicted that this photoinduced phenomenon can be detected experimentallyby observing the change of magnetic susceptibility.

Valence-alternation pairs as a possible cause of the unexpectedly strong fast relaxations in an oxide glass

Unexpected structural and dynamical features of non-crystalline As 2O 3, revealed through various light scattering techniques, indicate that this glass is distinctively different form other common oxide glasses. In view of its structural similarity with other chalcogenide glasses, i.e. As 2S 3 and As 2Se 3, it is suggested that the peculiar spectral properties of As 2O 3 may originate from structural configurations like those usually employed to account for photoinduced phenomena in chalcogenide glasses. The lone-pair electrons of oxygen atoms and the structural flexibility characterizing As 2O 3 compared to other tetrahedrally coordinated oxide glasses seems to be the key factor for the observed unique properties of this glass.

Two-photon absorption spectroscopy of As2S3 glass

Spectral dependence of two-photon absorption in a chalcogenide glass, As2S3, has been studied using a tunable light source. The result shows that two-photon and two-step absorptions occur, and the two-photon absorption spectrum appears to be exponential. These results and roles of nonlinear processes in photoinduced phenomena are discussed.

Photo-induced Cooperative Phenomena in Organic and Inorganic Crystals. A Case of .PI.-Conjugated Polymer: Polydiacetylenes.

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.

Photo-Induced Degradation in Strained ZnSe

Photo-induced degradation phenomena in strained ZnSe were investigated. (111) ZnSe substrates were bi-axially strained due to film deposition, and their cleaved (110) planes were exposed to a focused He-Cd laser for several thousands of seconds at room temperature. Although photoluminescence intensities decreased with the irradiation for all samples, the decreasing rates depended on the strains. Compressed regions degraded faster than diluted regions. No-strained regions degraded slower than the others. Conventional ZnSe based lasers have an active layer with compressive strains. Our results show that the laser with a diluted or no-strained active layer possibly survives longer.

New Spectroscopy for Photo-Induced Phenomena Using Combination of Synchrotron Radiation and Laser

The new experimental systems for investigating the photo-induced phenomena have been constructed. They consist of the photoelectron spectroscopy system based on the synchrotron radiation, the laser system based on the mode-locked pulse lasers, and the experimental system for time-resolved measurements. The systems have been applied to study the surface photo-voltage (SPV) effects on the surfaces of GaAs (100) and GaAs-GaAsP superlattice. The results showed that the temporal profiles of the surface photo-voltage effects have several components in the nano- and micro-second ranges, which depend on the surface conditions.

Feasibility of optically controlled integrated Mach–Zehnder device based on Azo dye-doped PMMA thin films

Azoaromatic (Azo) polymers have attracted much attention as materials for nonlinear and optoelectronic applications. In this work, nonlinear properties of the concerned material are first characterized by m-lines spectroscopic method. Then, photoinduced phenomena reported in the literature are summarized and experimentally found out. Therefore, these photoinduced properties are used to design and manufacture an optically controlled integrated device based on light wave intensity dependence of refractive index.

Molecular photodesorption from single-walled carbon nanotubes

Probing the photoelectrical properties of single-walled carbon nanotubes (SWNTs) led to the discovery of photoinduced molecular desorption phenomena in nanotube molecular wires. These phenomena were found to be generic to various molecule–nanotube systems. Photodesorption strongly depends on the wavelength of light, the details of which lead to a fundamental understanding of how light stimulates molecular desorption from nanotubes. The results have important implications to nanotube-based molecular electronics, miniature chemical sensors, and optoelectronic devices.

Photoinduced phenomenon in polymers

Numerical simulation has shown that a polymeric molecule can possess a photoinduced phenomenon---photoinduced polarization reversion---in which the electric dipole of the polymeric molecule is reversed by absorbing one photon. This paper provides an analytic theory by means of a response function to prove that a polymeric molecule with a bipolaron has a negative static polarizability and explains the physical origin of this photoinduced phenomenon in detail. This paper also presents a dynamical calculation for the photoinduced polarization reversion, from which the relaxation time for the dipole reversion can be determined.

Anisotropic thermal strain measurement at the thermochromic phase transition in a polydiacetylene crystal

The anisotropic thermal strains of a polydiacetylene crystal induced by a heat cycle were measured from macroscopic changes in the sample dimensions. Measurement accuracy was superior to that of the x-ray diffraction method. Linear thermal expansion in phase A and a strain induced by the phase transition between phases A and B were clearly observed in the a and c axes. The reflectivity change was also monitored at the exciton energy of phase A. Both results exhibited similar transition behavior with hysteresis. On the other hand, in the b axis expansion was much smaller than that of other axes, and no hysteresis was found at the phase transition. The results support the supposition that the phase transition is a structural change of side groups. The volume change between phases A and B was measured to be 1.9% at 410 K, which indicates the possibility of an optically driven actuator using photoinduced phase-transition phenomena.

Photoinduced phenomena and shallow levels in K1−xLixTaO3

Li admixture greatly enhances n-type photoconductivity of KTaO3 crystals. Thermally stimulated current experiments indicate that this phenomenon is related with the presence of hole-trapping levels with depths of 50–100 meV. This is in good agreement with recent energy structure calculations showing the existence of levels at these energies above the top of the VB, due to perturbed O2- p-states. The study of visible photoluminescence and thermo-luminescence shows that the hole-trapping centre also controls the emission process. We conclude in favour of radiative recombination involving O−Ta4+polaron pairs. Keywords: ferroelectric perovskites: photoconductivity

Photoinduced effect in glassy crystallineAs4Se3:Negative photoinduced anisotropy

In studying the optical-transmission properties of the glassy crystalline chalcogenide composition ${\mathrm{As}}_{4}{\mathrm{Se}}_{3}$ with polarized light, we have observed a photoinduced phenomenon, called here negative vectoral photoinduced anisotropy. The negative anisotropy is ascribed to ${\mathrm{As}}_{4}{\mathrm{Se}}_{3}$ molecular units embedded in an amorphous network. During the course of illumination, the negative anisotropy gradually decreases in amplitude and is replaced by the well-known, previously observed positive vectoral anisotropy. A microscopic mechanism for the occurrence of negative anisotropy is suggested.