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M Tachiya

Publications and source records attributed to M Tachiya.

15 recordsLinked to original sources
PubMed2006

Selective ionization of oriented nonpolar molecules with asymmetric structure by phase-controlled two-color laser fields.

We report on the selective ionization of oriented nonpolar molecules with asymmetric structure by using phase-controlled two-color omega + 2omega laser pulses with an intensity of 1.0 x 10(13) W/cm(2) (tunneling ionization regime) and a pulse duration of 130 fs. The orientation of 1-bromo-2-chloroethane was monitored by the directional asymmetries of the forward-backward emission in dissociative ionization. The observed direction of orientation clearly confirms that molecular orientation is induced not by dynamic orientation but by selective ionization of oriented molecules, which reflects the structure of the highest occupied molecular orbital. This method can be applied for the vast majority of molecules.

Journal Article↗
PubMed2006

Stochastic model of photodynamics in multichromophoric conjugated polymers.

A stochastic model of triplet exciton dynamics in multichromophoric conjugated polymers is presented and analyzed in detail, with a focus on the single molecule spectroscopy observables. The model deals with the evolution of a discrete statistical distribution of triplets in isolated polymer molecules. This approach should provide more accurate quantitative information on the dynamic processes involved, as compared to the previously used two-state model which assumes that a conjugated polymer cannot contain more than one triplet. In particular, it allows for determination of the triplet-triplet annihilation rate.

Journal Article↗
PubMed2006

Dispersive-diffusion-controlled distance-dependent recombination in amorphous semiconductors.

The photoluminescence in amorphous semiconductors decays according to the power law t(-delta) at long times. The photoluminescence is controlled by dispersive transport of electrons. The latter is usually characterized by the power alpha of the transient current observed in the time-of-flight experiments. Geminate recombination occurs by radiative tunneling which has a distance dependence. In this paper, we formulate ways to calculate reaction rates and survival probabilities in the case carriers execute dispersive diffusion with long-range reactivity. The method is applied to obtain tunneling recombination rates under dispersive diffusion. The theoretical condition of observing the relation delta=alpha/2+1 is obtained and theoretical recombination rates are compared to the kinetics of observed photoluminescence decay in the whole time range measured.

Journal Article↗
PubMed2006

Direct observation of reactive trapped holes in TiO2 undergoing photocatalytic oxidation of adsorbed alcohols: evaluation of the reaction rates and yields.

To investigate the primary process of photocatalytic oxidation of TiO2, interfacial charge-transfer reaction of trapped holes formed in nanocrystalline TiO2 films by UV irradiation was directly measured by highly sensitive femtosecond and nanosecond transient absorption spectroscopy under low intensity excitation condition to avoid fast electron-hole recombination. Accordingly, the rates and yields of photocatalytic oxidation of several alcohols adsorbed on TiO2 were evaluated successfully.

Journal Article↗
PubMed2005

The reorganization energy of electron transfer in nonpolar solvents: molecular level treatment of the solvent.

The intermolecular electron transfer in a solute pair consisting of pyrene and dimethylaniline is investigated in a nonpolar solvent, n-hexane. The earlier elaborated approach [M. Tachiya, J. Phys Chem. 97, 5911 (1993)] is used; this method provides a physically relevant background for separating inertial and inertialess polarization responses for both nonpolarizable and polarizable molecular level simulations. The molecular-dynamics technique was implemented for obtaining the equilibrium ensemble of solvent configurations. The nonpolar solvent, n-hexane, was treated in terms of OPLS-AA parametrization. Solute Lennard-Jones parameters were taken from the same parametrization. Solute charge distributions of the initial and final states were determined using ab initio level [HF/6-31G(d,p)] quantum-chemical calculations. Configuration analysis was performed explicitly taking into account the anisotropic polarizability of n-hexane. It is shown that the Gaussian law well describes calculated distribution functions of the solvent coordinate, therefore, the rate constant of the ET reaction can be characterized by the reorganization energy. Evaluated values of the reorganization energies are in a range of 0.03-0.11 eV and significant contribution (more then 40% of magnitude) comes from anisotropic polarizability. Investigation of the reorganization energy lambda dependence on the solute pair separation distance d revealed unexpected behavior. The dependence has a very sharp peak at the distance d=7 A where solvent molecules are able to penetrate into the intermediate space between the solute pair. The reason for such behavior is clarified. This new effect has a purely molecular origin and cannot be described within conventional continuum solvent models.

Journal Article↗
PubMed2005

The dynamics of solvation of an electron in the image potential state by a layer of polar adsorbates.

Recently, ultrafast two-photon photoemission has been used to study electron solvation at a two-dimensional metalpolar adsorbate interfaces [A. Miller et al., Science 297, 1163 (2002)]. The electron is bound to the surface by the image interaction. Earlier we have suggested a theoretical description of the states of the electron interacting with a two-dimensional layer of the polar adsorbate [K. L. Sebastian et al., J. Chem. Phys. 119, 10350 (2003)]. In this paper we have analyzed the dynamics of electron solvation, assuming a trial wave function for the electron and the solvent polarization and then using the Dirac-Frenkel variational method to determine it. The electron is initially photoexcited to a delocalized state, which has a finite but large size, and causes the polar molecules to reorient. This reorientation acts back on the electron and causes its wave function to shrink, which will cause further reorientation of the polar molecules, and the process continues until the electron gets self-trapped. For reasonable values for the parameters, we are able to obtain fair agreement with the experimental observations.

Journal Article↗
PubMed2005

Lithium ion effect on electron injection from a photoexcited coumarin derivative into a TiO2 nanocrystalline film investigated by visible-to-IR ultrafast spectroscopy.

The dynamics of ultrafast electron injection from a coumarin derivative (NKX-2311), which is an efficient photosensitizer for dye-sensitized solar cells, into the conduction band of TiO(2) nanocrystalline films have been investigated by means of femtosecond transient absorption spectroscopy in a wide wavelength range from 600 nm to 10 mum. In the absence of Li(+) ions, electron injection into the TiO(2) conduction band occurred in about 300 fs. In the presence of Li(+) ions, however, electron injection occurred within approximately 100 fs, and the oxidized dye generated was found to interact with nearby Li(+) ions. Possible positions of Li(+) ion attachment to the dye molecule were examined by means of semiempirical molecular orbital calculations. The electron injection efficiency was found to increase by a factor of 1.37 in the presence of Li(+) ions. The effects of Li(+) ions on the energy of the TiO(2) conduction band and the electronic interaction between the dye molecule and Li(+) ions are discussed, and the major cause for the acceleration of electron injection was suggested to be a conduction-band shift of TiO(2).

Journal Article↗
PubMed2005

A theoretical method to analyze diffusion of probe molecules in nanostructured fluids by fluorescence correlation spectroscopy.

In this paper, we study the fluorescence fluctuation correlation function in structured fluids where the diffusion coefficients of probe molecules have different values depending on the distance from initial position, and we derive two simple expressions. Both of them reproduce the exact numerical results rather accurately. One of the expressions contains a time-dependent diffusion coefficient and has a clear physical meaning. We show a procedure to analyze experimental data using the time-dependent diffusion coefficient which results from crossover from free diffusion inside a mesh to hindered diffusion through mesh structures.

Journal Article↗
PubMed2004

Effect of anisotropic diffusion and external electric field on the rate of diffusion-controlled reactions.

In this paper we investigate theoretically the effect of an external electric field on the rate constant of steady-state bulk diffusion-controlled reactions. We generalize previously derived results for isotropic diffusion in the absence of interparticle interaction [J. Chem. Phys. 87, 4622 (1987)] to the case where translational diffusion is anisotropic. A frequently occurring situation of transverse isotropy where D(x)=D(y) not equal to D(z) is considered in detail. We derive the first-order expansion for the reaction rate constant in terms of the electric field strength E, k(E)=k(0) (1+1/2epsilongamma), where gamma=k(0)/4piRD( perpendicular ), epsilon=qER/k(B)T, q is the charge, R is the contact distance, and D( perpendicular ) is the transverse diffusion coefficient. Numerical calculations show that this first-order expansion works well in the whole range of applicability of the Nernst-Einstein relation, i.e., for epsilon<1.

Journal Article↗
PubMed2004

Coherent control of photofragment separation in the dissociative ionization of IBr.

We have investigated coherent control of the dissociative ionization of IBr using phase-controlled two-color omega+2omega laser pulses with an intensity of 1.0 x 10(12) W/cm and a pulse duration of 130 fs. The directional asymmetries of the photofragment angular distributions showed oscillation behavior dependent on the relative phase difference between the omega and 2omega pulses. The phase dependencies of the directional asymmetries observed for iodine ions and bromine ions were out of phase with each other. This result shows that a phase-controlled omega+2omega optical field can produce molecular orientation in which the optical field discriminates between parallel and antiparallel configuration of molecules that have a permanent dipole.

Journal Article↗
PubMed2001

Kinetics of diffusion-assisted reactions in microheterogeneous systems.

This review is focused on the basic theory of diffusion-assisted reactions in microheterogeneous systems, from porous solids to self-organized colloids and biomolecules. Rich kinetic behaviors observed experimentally are explained in a unified fashion using simple concepts of competing distance and time scales of the reaction and the embedding structure. We mainly consider pseudo-first-order reactions, such as luminescence quenching, described by the Smoluchowski type of equation for the reactant pair distribution function with a sink term defined by the reaction mechanism. Microheterogeneity can affect the microscopic rate constant. It also enters the evolution equation through various spatial constraints leading to complicated boundary conditions and, possibly, to the reduction of dimensionality of the diffusion space. The reaction coordinate and diffusive motion along this coordinate are understood in a general way, depending on the problem at hand. Thus, the evolution operator can describe translational and rotational diffusion of molecules in a usual sense, it can be a discrete random walk operator when dealing with hopping of adsorbates in solids, or it can correspond to conformational fluctuations in proteins. Mathematical formulation is universal but physical consequences can be different. Understanding the principal features of reaction kinetics in microheterogeneous systems enables one to extract important structural and dynamical information about the host environments by analyzing suitably designed experiments, it helps building effective strategies for computer simulations, and ultimately opens possibilities for designing systems with controllable reactivity properties.

Animals↗
PubMed1998

A model for diffusive transport through a spherical interface probed by pulsed-field gradient NMR.

In biological systems, because of higher intracellular viscosity and/or the restriction of the diffusion space inside cells, the (apparent) diffusion coefficient of an intracellular species (e.g., water) is generally smaller than when it is in the extracellular medium. This difference affects the spin-echo signal attenuation in the pulsed field gradient NMR experiment and thus affords a means of separating the intracellular from the extracellular species, thereby providing a basis for studying transmembrane transport. Such experiments have commonly been analyzed using the macroscopic model of Kärger (see Adv. Magn. Reson. 21:1-89 (1988)). In our previous study, we considered a microscopic model of diffusive transport through a spherical interface using the short gradient pulse approximation (J. Magn. Reson. A114:39-46 (1995)). The spins in the external medium were modeled with the "partially absorbing wall" condition or as having a small but finite lifetime. In the present paper, we extend our treatment to the case in which there is no limitation upon the lifetime in either medium. We also consider a simple modification of Kärger's model that more properly accounts for the restricted intracellular diffusion. Importantly, it was found that the exact solution within the short gradient pulse approximation developed here and the modified Kärger model are in close agreement in the (experimentally relevant) long-time limit. The results of this study show that when there is no limitation upon the lifetime of the transported species in either phase, the spin-echo attenuation curve is very sensitive to transport.

Diffusion↗
PubMed1997

Interpretation of passive permeability measurements on lipid-bilayer vesicles. Effect of fluctuations.

A stochastic model for migration dynamics of solute molecules from the bulk aqueous phase to the intravesicular water pool is developed with a goal to interpret recent passive permeability measurements of bilayer membranes. Previously neglected fluctuations of the number of solubilized species in the inner water pool of a vesicle are naturally incorporated into the model. For a homogeneous one-phase bilayer, the model predicts exponential long-time asymptotics of the migration dynamics with a rate constant given by a sum of the frequencies of exit and entry of a solute molecule from/into the intravesicular water pool into/from the bulk aqueous phase. The long-time constant is directly related to the membrane permeability.

Cell Membrane Permeability↗