Electronic excited state energy transfer,trapping by dimers and fluorescence quenching in concentrated dye solutions: Picosecond transient grating exp

Picosecond transient grating experiments are used to examine electronic excited state dynamics in concentrated dye solutions. A model based on radiationless excited state transport and trapping by dimers describes the phenomena responsible for fluorescence quenching. The trapping rate constant is found to have a cubic concentration dependence. Rhodamine 6G dimer lifetimes in glycerol and ethanol are 830 ps and <50 ps respectively. The difference arises due to the viscosity dependence of the dimer radiationless relaxation rate.     

The magnetic field influence on bridge-assisted electron transfer

The influence of an external magnetic field on the rate of electron transfer is studied theoretically for the case of a donor-acceptor complex with paramagnetic ions as bridging sites. We demonstrate for a bridge with a single and with two paramagnetic ions that the applied magnetic field causes a blocking effect of the electron transfer. Each paramagnetic ion staying in its electronic ground state is assumed to reduce its spin from if the transferred electron is coupled to it. Such a spin reduction determines the magnetic field dependence of the transfer rate in a specific manner. The magnetic field dependence is derived at low temperatures in utilizing the Wigner 6j-symbols method. For the case of two paramagnetic ions per bridge the exchange interaction between them has been additionally included into the calculations. It is responsible for the step-like dependence of the rate constant on the magnetic field, as well as for the creation of a rather narrow field-strength region where the rate constant drops to zero.     

Temperature dependence of the electronic factor in the nonadiabatic electron transfer at metal and semiconductor electrodes

The temperature dependence of the electronic contribution to the nonadiabatic electron transfer rate constant (k_ET) at metal electrodes is discussed. It is found in these calculations that this contribution is proportional to the absolute temperature T. A simple interpretation is given. We also consider the nonadiabatic rate constant for electron transfer at a semiconductor electrode. Under conditions for the maximum rate constant, the electronic contribution is also estimated to be proportional to T, but for different reasons than in the case of metals (Boltzmann statistics and transfer at the conduction band edge for the semiconductor versus Fermi–Dirac statistics and transfer at the Fermi level, which is far from the band edge, of the metal).     

The free energy gap and temperature dependence of the electronic transmission coefficient in electron-transfer reactions of the solvated electron

We have investigated the dependence of the electronic factor in electron-transfer rate constants on the energy gap and temperature by minimizing the electronic energy with respect to the instantaneous non-equilibrium solvent polarization. The electronic charge billows out and shrinks in the “normal” and strongly exothermic free energy region, respectively, but the effect is only comparable to the variation of the nuclear factor for low temperatures and strongly exothermic processes.     

Shock wave study on the thermal unimolecular decomposition of allyl radicals

We present the first direct study on the thermal unimolecular decomposition of allyl radicals. Experiments have been performed behind shock waves, and the experimental conditions covered temperatures ranging from 1125 K up to 1570 K and pressures between 0.25 and 4.5 bar. Allyl radicals have been generated by thermal decomposition of allyl iodide, and H-atom resonance absorption spectroscopy has been used to monitor the reaction progress. A marked pressure dependence of the rate constant has been observed which is in agreement with the results from a master equation analysis. However, our experimental results as well as our Rice-Ramsperger-Kassel-Marcus calculations seem to contradict the results of Deyerl et al. (J. Chem. Phys. 1999, 110, 1450) who investigated the unimolecular decomposition of allyl radicals upon photoexcitation and tried to deduce specific rate constants for the unimolecular dissociation in the electronic ground state. At pressures around 1 bar we extracted the following rate equation: k(T) = 5.3 x 10(79)(T/K)(-19.29) exp[(-398.9 kJ/mol)/RT] s(-1). The uncertainty of the rate constant calculated from this equation is estimated to be 30%.     

On Theoretical Treatments of Electronic Excitation Energy Transfer

In this paper, we review the generalized Forster-Dexter theory to treat photoinduced electronic energy transfer for a system in dense media and for an isolated system (i.e., a system in the collision-free condition). Instead of expressing the rate of energy transfer in terms of spectral overlap, the expression of the energy-transfer rate constant is obtained by evaluating a Fourier integral involved in the energy transfer rate constant using the saddle-point method. In this way, the energy-gap dependence, and the effect of temperature and the isotope effect on the energy transfer can be easily studied. The effect of bridge groups connecting between donor and acceptor chromophores on the intramolecular energy transfer is also studied.     

A nonmonotonic dependence of standard rate constant on reorganization energy for heterogeneous electron transfer processes on electrode surface

In the present work a nonmonotonic dependence of standard rate constant (k(0)) on reorganization energy (lambda) was discovered qualitatively from electron transfer (Marcus-Hush-Levich) theory for heterogeneous electron transfer processes on electrode surface. It was found that the nonmonotonic dependence of k(0) on lambda is another result, besides the disappearance of the famous Marcus inverted region, coming from the continuum of electronic states in electrode: with the increase of lambda, the states for both Process I and Process II ET processes all vary from nonadiabatic to adiabatic state continuously, and the lambda dependence of k(0) for Process I is monotonic thoroughly, while for Process II on electrode surface the lambda dependence of k(0) could show a nonmonotonicity.     

Resonance energy transfer from a dye molecule to graphene

We study the distance dependence of the rate of resonance energy transfer from the excited state of a dye to the pi system of graphene. Using the tight-binding model for the pi system and the Dirac cone approximation, we obtain the analytic expression for the rate of energy transfer from an electronically excited dye to graphene. While in traditional fluorescence resonance energy transfer, the rate has a (distance)(-6) dependence, we find that the distance dependence in this case is quite different. Our calculation of rate in the case of the two dyes, pyrene and nile blue, shows that the distance dependence is Yukawa type. We have also studied the effect of doping on energy transfer to graphene. Doping does not modify the rate for electronic excitation energy transfer significantly. However, in the case of vibrational transfer, the rate is found to be increased by an order of magnitude due to doping. This can be attributed to the nonzero density of states at the Fermi level that results from doping.     

On the excess-energy dependence of radiationless decay rate constants

The results of calculations of the dependence of the radiationless rate constant on the excess of excitation energy within the two-electronic states model under the weak coupling and statistical limits are presented. It is assumed that the exact molecular states for a given electronic configuration are global in character containing equal contributions from all degenerated vibrational levels at a given excitation energy due to intramolecular vibrational relaxation (IVR). The results of calculations indicate an important role of the low-frequency vibrational modes, the potential energy surfaces of which cross between the two electronic states involved into the radiationless process. The sharp increase of the rate constant is predicted for the excitation energy below the diabatic crossing point, followed by saturation at higher energies. The calculated rate constants for the T₁→S₀ intersystem crossing in pyrazine and benzene are in good agreement with experimental observations. Some comments concerning the “channel-three” phenomenon in benzene are presented.     

Nonadiabatic quantum dynamics of C(1D)+H2→CH+H: coupled-channel calculations including Renner-Teller and Coriolis terms

The Renner-Teller (RT) coupled-channel dynamics for the C((1)D)+H(2)(X(1)Σ(g) (+))→CH(X(2)Π)+H((2)S) reaction has been investigated for the first time, considering the first two singlet states ??(1)A' and b(1)A' of CH(2) dissociating into the products and RT couplings, evaluated through the ab initio matrix elements of the electronic angular momentum. We have obtained initial-state-resolved probabilities, cross sections and thermal rate constants via the real wavepacket method for both coupled electronic states. In contrast to the N((2)D)+H(2)(X(1)Σ(g)(+)) system, RT effects tend to reduce probabilities, cross sections, and rate constants in the low energy range compared to Born-Oppenheimer (BO) ones, due to the presence of a repulsive RT barrier in the effective potentials and to long-lived resonances. Furthermore, contrary to BO results, the rate constants have a positive temperature dependence in the 100-400 K range. The two-state RT rate constant at 300 K, lower than the BO one, remains inside the error bars of the experimental value.     

Cross sections and thermal rate constants for the isotope exchange reaction: D(2S)+OH(2Pi)-->OD(2Pi)+H(2S)

We report state-to-state and overall thermal rate constants for the isotope exchange reaction D((2)S)+OH((2)Pi)-->OD((2)Pi)+H((2)S) for 0 K相似文献   

Phonon assisted site-to-site electronic energy transfer between Eu3+ ions in an amorphous solid

We have investigated the dynamics of phonon assisted electronic energy transfer between Eu³⁺ ions, experiencing different crystal fields in an amorphous Ca(PO₃)₂ matrix, using the technique of time-resolved fluorescence line-narrowing (TRFLN). From the concentration and temporal dependence of the transfer rate, the mechanism of the ion—ion electronic coupling has been identified as electric dipole—dipole. Both the excitation wavelength independence and the power law temperature dependence of the transfer rate identify the mechanism of the phonon participation in the transfer process as one involving a two-phonon process, with one-phonon action at each of two coupled ion sites.     

Quantum-chemical study of the mechanism of the influence of cations of d metals on the rate of the oxidation of phosphine by Cu(II) halides

A hypothesis concerning the formation of associates between intermediate complexes and cations has been introduced in order to account for the experimental dependence of the rate constant of the oxidation of phosphine by Cu(II) halides on the nature of the cation in the salt MCl₂ (M = Mn, Co, Ni). The quantum-chemical characteristics of the systems formed have been calculated by the Mulliken—Wolfsberg—Helmholz-SCCC method. It has been shown that the correct correlation between the electronic properties of the intermediates and the rate constant of the reaction upon the variation of M can be obtained only for a specific mechanism for the addition of M²⁺ to the complex.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 718–722, November–December, 1985.     

Temperature dependence of photoinduced electron transfer in hydrogen-bonded donor–acceptor systems

We have studied the temperature dependence of photoinduced electron transfer (PET) reactions in three hydrogen-bonded donor–acceptor systems in the range 220–298 K. For the hydrogen-bonded system in the normal region, the PET rate constant was found to increase with increase in temperature. For the two systems in the inverted region, the rate constants were nearly independent of temperature. We have analyzed the results using electron transfer theories.     

The rate constant of polymer reversal inside a pore

Translocation of biopolymers through pores is implicated in many biological phenomena. Confinement within a pore often breaks ergodicity on experimental and/or biological time scales by creating large entropic barriers to conformational rearrangements of the chain. Here, we study one example of such hindered rearrangement, in which the chain reverses its direction inside a long pore. Our goal is twofold. First, we study the dependence of the time scale of polymer reversal on the pore size and on the polymer length. Second, we examine the ability of simple one-dimensional theories to quantitatively describe a transition in a system with a complex energy landscape by comparing them with the exact rate constant obtained using brute-force simulations and the forward flux sampling method. We find that one-dimensional transition state theory (TST) using the polymer extension along the pore axis as the reaction coordinate adequately accounts for the exponentially strong dependence of the reversal rate constant on the pore radius r and the polymer length N, while the transmission factor, i.e., the ratio of the exact rate and the TST approximation, has a much weaker power law r and N dependence. We have further attempted to estimate the transmission factor from Kramer's theory, which assumes the reaction coordinate dynamics to be governed by a Langevin equation. However, such an approximation was found to be inadequate. Finally, we examine the scaling behavior of the reversal rate constant with N and r and show that finite size effects are important even for chains with N up to several hundreds.     

Experimental and theoretical study of the reaction of the ethynyl radical with nitrous oxide, C2H + N2O

We investigated the rate constants and reaction mechanism of the gas phase reaction between the ethynyl radical and nitrous oxide (C(2)H + N(2)O) using both experimental methods and electronic structure calculations. A pulsed-laser photolysis/chemiluminescence technique was used to determine the absolute rate coefficient over the temperature range 570 K to 836 K. In this experimental temperature range, the measured temperature dependence of the overall rate constants can be expressed as: k(T) (C(2)H + N(2)O) = 2.93 × 10(-11) exp((-4000 ± 1100) K/T) cm(3) s(-1) (95% statistical confidence). Portions of the C(2)H + N(2)O potential energy surface (PES), containing low-energy pathways, were constructed using the composite G3B3 method. A multi-step reaction route leading to the products HCCO + N(2) is clearly preferred. The high selectivity between product channels favouring N(2) formation occurs very early. The pathway corresponds to the addition of the terminal C atom of C(2)H to the terminal N atom of N(2)O. Refined calculations using the coupled-cluster theory whose electronic energies were extrapolated to the complete basis set limit CCSD(T)/CBS led to an energy barrier of 6.0 kcal mol(-1) for the entrance channel. The overall rate constant was also determined by application of transition-state theory and Rice-Ramsperger-Kassel-Marcus (RRKM) statistical analyses to the PES. The computed rate constants have similar temperature dependence to the experimental values, though were somewhat lower.     

Temperature dependence of rate constants of thermal activated processes and vibrational relaxation in starvation kinetics

The purpose of this paper is to study the conditions under which the rate constant exhibits the Arrhenius type of temperature dependence and the damping effect on the rate constant. According to the starvation kinetics, the rate of reaction is determined by the rate of energy flow in the high temperature range. We shall show that the rate of energy flow (vibrational relaxation) reaches a finite limit in the high temperature condition.     

Complex Franck—Condon overlap integral in nonradiative decays

Considering the nuclear coordinate (Q) dependence of the electronic energy denominator appearing in the virbonic coupling matrix element, a complex Franck—Condon overlap integral which is needed in order to evaluate the nonradiative decay rate constant not only in the weak coupling but also in the strong coupling case is derived. The real part of the overlap integral plays an important role in the weak coupling case. The imaginary part is originated from the potential energy surface crossing regions and, consequently, contributes to the nonradiative decay rate constant in the strong coupling case. When the Q-dependence of the electronic energy denominator is neglected, the complex overlap integral leads to the results ontained by using the usual Herzberg—Teller expansion method. It is shown that the complex integral is expressed by the optical Franck—Condon overlap integral multiplied by a correction factor when the nonradiative decay from the vibrationless state is considered.     

Electronic mechanism of STM-induced diffusion of hydrogen on Si(100)

We have observed a scanning tunneling microscopy (STM) induced lateral transfer of a single hydrogen atom on the Si(100) surface. The transfer rate of the hydrogen atom is proportional to the electron dose, indicating an electron-assisted transfer mechanism. Measurements of the relations between the transfer rate and the sample bias and temperature give further support for an electronic mechanism. The bias dependence of the transfer rate shows a peak, and from a first principles electronic structure calculation we show that the position of the peak is related to the energy of a localized surface resonance. We propose that the hydrogen transfer is related to inelastic hole scattering with this surface resonance. We develop a microscopic model for the hydrogen transfer, and using the experimental data we extract information on the resonance lifetime and the transfer yield per resonant electron. The transfer takes place by tunneling through a small excited state transfer barrier. The transfer rate is increased if the hydrogen atom before the resonant excitation is vibrationally excited, and this gives rise to an increasing transfer rate with increasing sample temperature.