Transient Grating Signal Research Articles

Stability of Dimer and Domain–Domain Interaction of Arabidopsis Phototropin 1 LOV2

Transient grating signals after photoexcitation of Arabidopsis phototropin 1 light–oxygen–voltage 2 (phot1LOV2) domain without the linker were found to be very sensitive to temperature. In particular, the diffusion signal drastically increased with rising temperature. The signal was consistently explained by the superposition of the photo-induced dissociation and association reactions. This observation indicated the presence of an equilibrium between the monomer and dimer forms of the phot1LOV2 domain in the dark. The equilibrium was confirmed by a gel chromatographic technique. The equilibrium constants at various temperatures were calculated from the fraction of the dimer, and the stabilization enthalpy and entropy were determined. Interestingly, the transient grating signal of phot1LOV2 with the linker (phot1LOV2-linker), which exists as the monomer form, was also temperature dependent; the diffusion signal intensity decreased with increasing temperature. Because the diffusion signal reflects a conformation change of the linker upon photoexcitation, this temperature dependence indicated that there were two forms of the phot1LOV2-linker. One form exhibited a conformational change upon photoexcitation whereas the other form showed no change. These two forms are not distinguishable spectroscopically. The fraction of these species depended on the temperature. Considering the monomer–dimer equilibrium of the phot1LOV2 domain, we suggest that the nonreactive form possesses the linker region that is dissociated from the LOV2 domain. Because the dissociation of the linker region from the LOV2 domain is a key step for the conformation change of the phot1LOV2-linker to induce biological activity, we proposed that the phototropins could have a role as a temperature sensor.

Protein Diffusion Probed by the Transient Grating Method with a New Type of Photochromic Molecule†

A new type of photochromic molecule that can be used for diffusion coefficient (D) measurements of various proteins in solution is described. The absorption spectrum of this molecule is changed upon photoexcitation by the trans-cis isomerization reaction. Target proteins were labeled by this photochromic molecule in the dark and the translational motion of the proteins was detected by the transient grating (TG) method. The TG signal was simple enough to determine D accurately and was stable even for long-time irradiation by the laser light. The TG method using this probe molecule improves many drawbacks of the other techniques.

Energy dissipation in the ground-state vibrational manifolds ofβ-carotene homologues: A sub-20-fs time-resolved transient grating spectroscopic study

Transient grating (TG) signals in 8-carotene homologues, by using sub-20-fs excitation pulses, were measured in order to investigate the dependence of the vibrational coherence dynamics on the pi-conjugation length of these carotenoids. The experimental TG traces can be well reproduced by computational simulations based on a Brownian oscillator model using the spectral density determined from their respective resonance Raman spectra and their previously reported excited-state population-relaxation times. The total dephasing times of the ground-state vibrational modes of the homologues were determined by applying a wavelet transformation of their coherent oscillations, which were observed in the experimental TG traces. The total dephasing time decreases as the number of conjugated C=C double bonds increases. The decoherence of the ground-state vibrational modes in the homologues is mainly caused by system-bath interactions. The dephasing time of the C-C stretching modes strongly depends on the conjugation length, whereas that of the C=C stretching does not. This trend clearly shows that shorter-chain carotenoids have a specific major channel of energy dissipation to the environment (the C=C stretching), whereas the longer-chain carotenoids do not

Study of the translational diffusion of the benzophenone ketyl radical in comparison with stable molecules in room temperature ionic liquids by transient grating spectroscopy

Transient grating (TG) spectroscopy has been applied to the photoinduced hydrogen-abstraction reaction of benzophenone (BP) in various kinds of room temperature ionic liquids (RTILs). After the photoexcitation of BP in RTILs, the formation of a benzophenone ketyl radical (BPK) was confirmed by the transient absorption method, and the TG signal was analyzed to determine the diffusion coefficients of BPK and BP. For comparison, diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) in various RTILs were determined by the TG method using the photodissociation reaction of DPCP. While the diffusion coefficients of the stable molecules BP, DPA, and DPCP were always larger than those predicted by the Stokes-Einstein (SE) relation in RTILs, that of BPK was much smaller than those of the stable molecules and relatively close to that predicted by the SE relation in all solvents. For the smallest molecule CO, the deviation from the SE relation was evident. The diffusion coefficients of stable molecules are better represented by a power law of the inverse of the viscosity when the exponent was less than unity. The ratios of the diffusion coefficient of BP to that of BPK were larger in RTILs (2.7-4.0) than those (1.4-2.3) in conventional organic solvents. The slow diffusion of BPK in RTILs was discussed in terms of the fluctuation of the local electric field produced by the surrounding solvent ions.

Study on Vibrational Relaxation Process in Solution by the Transient Grating Spectroscopy

Time-resolved transient grating spectroscopy has been applied to the study of the vibrational energy relaxation process in solution. The vibrationally hot azulene molecule in the ground state was produced by the photo-excitation of the S1 ← S0 electronic transition and the successive rapid internal conversion process. The vibrational energy dissipation process was detected by the propagation of the acoustic wave of the transient grating (TG) signal. By comparing the solvent temperature rise time determined by the TG method with the energy dissipation process of azulene determined by the transient absorption spectroscopy, the mechanism of the vibrational energy dissipation process was discussed. This spectroscopic method has been applied to the other molecules e.g., p-nitroaniline and N,N-dimethyl-p-nitroaniline, and solvated electron.

Transient Dimerization and Conformational Change of a BLUF Protein: YcgF

The photochemical reaction dynamics of YcgF, a BLUF protein, were investigated by the pulsed laser-induced transient grating (TG) technique. The TG signal showed three reaction time constants: 2.7 micros, 13 micros, and 2 ms. The fastest was tentatively attributed to relaxation of the excited triplet state of the chromophore, flavin adenine dinucleotide (FAD), and the others represented conformational changes of the protein. The TG signal provided clear evidence that the diffusion coefficient (D) of the photoproduct (3.8x10(-11) m2 s-1) was significantly less than that of the reactant (8.3x10(-11) m2 s-1), with a time constant of 2 ms at a protein concentration of 700 microM. Interestingly, the rate constant increased in proportion to the concentration of the protein, indicating that protein dimerization was one of the main reactions occurring after photoexcitation. The significant reduction in D indicates that a conformational change leading to an increase in interactions with water molecules occurs upon formation of the signaling state. The 13 mus dynamics was attributed to the conformational change that induced transient dimerization. This conformational change might be an essential process for the creation of the signaling state. A detailed scheme for the photochemical reaction of YcgF is proposed.

Mechanism and Origin of Exciton Spin Relaxation in CdSe Nanorods

The dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = +/-1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = +/-1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = +/-2 contributes as the size of the nanocrystal increases. This idea is examined by simulations of 3-TG signals with a kinetic model for exciton spin relaxation considering the states in the exciton fine structure. Also, it is revealed that the rate of exciton spin relaxation has a strong correlation with the diameter, d, of the nanorod, scaled by the power law of 1/d4, rather than other shape parameters such as length, volume, or aspect ratio.

Diffusion of Platinum Ions and Platinum Nanoparticles during Photoreduction Processes Using the Transient Grating Method

The photoreduction process of PtCl(6)2- to Pt nanoparticles in poly(N-vinyl-2-pyrrolidone) solutions upon UV light irradiation was investigated by monitoring the change in the diffusion coefficient (D). The D values of chemical species during UV irradiation was measured by the laser-induced transient grating (TG) method. The TG signal of the PtCl(6)2- solution before UV irradiation was composed of three kinds of contributions, the thermal grating, the species grating due to the creation of PtCl4(2-), and the species grating due to the depletions of PtCl6(2-). Upon UV irradiation of the solution, the species grating signal due to PtCl6(2-) diminished and then the TG signal of Pt nanoparticles gradually appeared. This result indicates that the gradual clustering of Pt0 atoms into Pt nanoparticles occurs after all PtCl(6)2- ions are photochemically reduced to PtCl(4)2- and subsequently transformed to Pt0 atoms with a short delay. With increasing time of the UV irradiation, the TG signal intensity increased, while D of the Pt nanoparticles did not change. This suggests that the number of Pt nanoparticles increases, but the size of the Pt nanoparticles with the polymer layer is unchanged, in the course of the UV irradiation.

Coherent Electronic and Nuclear Dynamics for Charge Transfer in 1-Ethyl-4-(carbomethoxy)pyridinium Iodide

Although polaronic interactions and states abound in charge transfer processes and reactions, quantitative and separable determination of electronic and nuclear relaxation is still challenging. The present paper employs the amplitudes, polarizations, and phases of four-wave mixing signals to obtain unique dynamical information on relaxation processes following photoinduced charge transfer between iodide and 1-ethyl-4-(carbomethoxy)pyridinium ions. Pump-probe signal amplitudes reveal the coherent coupling of an underdamped 115 cm(-1) nuclear mode to the charge transfer excitation. Assignments of this recurrence to intramolecular vibrational modes of the acceptor and to modulation of the intermolecular donor-acceptor distance are discussed on the basis of a high-level density functional theory normal-mode analysis and previously observed wave packet dynamics of solvated molecular iodine. Nuclear relaxation of the acceptor induces sub-picosecond decay of the pump-probe polarization anisotropy from an initial value of 0.4 to an asymptotic value of -0.05. Electronic structure calculations suggest that relaxation along the torsional coordinate of the ethyl group is the origin of the anisotropy decay. Electric-field-resolved transient grating (EFR-TG) signal fields are obtained by spectral interferometry with a diffractive optic based interferometer. These measurements show that the signal phase and amplitude possess similar dynamics. Model calculations are used to demonstrate how the EFR-TG signal phase yields unique information on transient material resonances located outside the laser pulse spectrum. This effect can be rationalized in that the real and imaginary parts of the nonlinear polarization are related by the Kramers-Kronig transformation, which allows the dispersive component of the polarization response to exhibit spectral sensitivity over a larger frequency range than that defined by the absorption bandwidth.

Spectrally resolved nonlinear optical response of weakly confined excitons under femtosecond laser pulse excitation in GaAs thin films

We have investigated the dynamical properties of the weakly confined excitons in GaAs thin films by a transient grating method (three-beam four-wave mixing) with the use of femtosecond-laser pulses. The time-domain signals under the exciton-excitation conditions show a nonlinear optical response with a rise time of 4 ps and a long decay time of 120 ps. Moreover, in the case of that the probe pulse is parallel polarized to the two excitation pulses, the ultrafast optical response comparable to the pulse width appears. These characteristics of the nonlinear optical response is different from the experimental results by the picosecond-laser-pulse excitation. The spectra of the transient grating signals at the various time delays show several peaks at the energies, which agree with the energies of excitons confined in the thin films. This fact demonstrates that the characteristics of the nonlinear optical response of the weakly confined excitons are drastically changed by the spectral width of the excitation laser-pulse.

Diffusion coefficients as a monitor of reaction kinetics of biological molecules

For revealing spectrally silent dynamics in chemical reactions, a new method, the time-dependent diffusion coefficient, is presented. Principles and typical examples of this method, in particular applications to biologically related reactions, are reviewed. The pulsed laser induced transient grating signal of the photo-decomposition reaction of caged ATP showed that the diffusion coefficient increases gradually with time reflecting the molecular size decrease by the dissociation. Hence, this rate should be a direct measurement of the photo-dissociation rate of ATP from the caged state. In an application to a protein folding reaction, the time-development of the diffusion coefficient was observed during the folding reaction. This time dependence was interpreted in terms of the intermolecular interaction change; i.e., conversion from the intermolecular hydrogen bonding to intramolecular one. It was found that the change of the hydrogen bonding network occurred by the two state manner in entire refolding process of cytochrome c. The unique feature of this time-dependent diffusion coefficient method is discussed.

Time-Resolved Thermodynamics: Heat Capacity Change of Transient Species during Photoreaction of PYP

Heat capacity changes of short-lived transient species in different time ranges were measured for the first time by using the thermal component of the transient grating and transient lens signals at various temperatures. This method was applied to the transient intermediates of Photoactive Yellow Protein (PYP). The temperature dependence of the enthalpy change shows that the heat capacity of the short-lived intermediate pR2 (also called I1 or PYP(L)) species is the same as that of the ground state (pG) species within our experimental accuracy, whereas that of the long-lived intermediate pB (I2 or PYP(M)) is much larger (2.7 +/- 0.4 kJ/mol K) than that of pG. The larger heat capacity is interpreted in terms of the conformational change of the pB species such as melted conformation and/or exposure of the nonpolar residues to the aqueous phase. This technique can be used for photochemical reactions in general to investigate the conformational change and the hydrophobic interaction in a time domain.

Efficient method for the calculation of time- and frequency-resolved four-wave mixing signals and its application to photon-echo spectroscopy

An efficient method has been developed for the calculation of third-order time- and frequency-resolved optical signals. To obtain the general four-wave mixing signal, seven auxiliary density matrices have to be propagated in time. For the special cases of two-pulse photon-echo and transient-grating signals, two or three density matrices, respectively, are required. The method is limited to weak laser fields (it is thus valid within the third-order perturbation theory) but allows for any pulse durations and automatically accounts for pulse-overlap effects. To illustrate the method, we present the explicit derivation of the three-pulse photon-echo signal. Any other third-order optical signal can be calculated in the same manner. As an example, two- and three-pulse photon-echo and transient-grating signals for a weakly damped displaced harmonic oscillator have been calculated.

Determination of Thermal Diffusivity of a Manganite Thin Film

Thermal diffusivity has been investigated in a manganite thin filmLa0.6Sr0.4MnO3 by means of transient grating (TG)technique at room temperature. A new method, which is generalized totwo-layered samples of the thin film deposited on a semi-infinitesubstrate, is established to fit the TG signals. The thermaldiffusivity of the La0.6Sr0.4MnO3 thin film with athickness of 200 nm on an MgO (100) substrate is determined to be0.92 mm2/s, which is slightly smaller than that of the single crystalsample (1 mm2/s).

Conformational Dynamics of Phototropin 2 LOV2 Domain with the Linker upon Photoexcitation

Conformational dynamics of LOV2 domain of phototropin, a plant blue light photoreceptor, is studied by the pulsed laser induced transient grating (TG) technique. The TG signal of LOV2 without the linker part to the kinase domain exhibits the thermal grating signal due to the heat releasing from the excited state and a weak population grating by the adduct formation. The diffusion coefficients of the adduct product after forming the chemical bond between the chromophore and Cys residue are found to be slightly smaller than that of the reactant, which implies that the core shrinks slightly on the adduct formation. After that change, no significant conformational change was observed. On the other hand, the signal of LOV2 with the linker part to the kinase domain clearly shows very different diffusion coefficients between the original and the adduct species. The large difference indicates significant global conformational change of the protein moiety upon the adduct formation. More interestingly, the diffusion coefficient is found to be time-dependent in the observation time range. The dynamics representing the global conformational change is a clear indication of a spectral silent intermediate between the excited triplet state and the signaling product. From the temporal profile analysis of the signal, the rate of the conformational change is determined to be 2 ms.

Thermal diffusivity of La0.6Sr0.4MnO3thin film investigated by transient thermal grating technique

We investigate thermal diffusivity a of La 0.6 Sr 0.4 MnO 3 thin film by means of transient thermal grating technique. The transient thermal grating signal is analysised by Green's functions, and the thermal diffusivity a of La 0.6 Sr 0.4 MnO 3 film is determined to be 0.92 mm 2 s -1 . Comparison with that (∼1 mm 2 s -1 ) of the ceramic material, the smaller α-value is ascribed to the mismatch of lattice for the film material.

Kinetics of Intermolecular Interaction during Protein Folding of Reduced Cytochrome c

Kinetics of intermolecular interaction between reduced cytochrome c (Cyt c) protein and solvent during the protein-refolding process is studied by monitoring the time dependence of apparent diffusion coefficient ( D) using the pulsed-laser-induced transient grating technique. The refolding was triggered by photoinduced reduction of unfolded Fe(III) Cyt c in 3.5 M guanidine hydrochloride (GdnHCl) solution and the change in the diffusion coefficient was monitored in time domain. The relationship between D and the protein conformations under equilibrium condition were investigated at various GdnHCl concentrations using a photolabeling reagent. The time dependence of the observed transient grating signal was analyzed using these data and two models: a continuous change model of the intermolecular interaction and a two-state model. It was found that the TG signals in various time ranges can be consistently reproduced well by the two-state model. The dynamics of D is expressed well by a single exponential function with a rate constant of 22 ± 7 s −1 in a whole time range. The folding process of Cyt c is discussed based on these observations.

Transient grating experiment on supercooled water

We measured the acoustic and thermal relaxation dynamics on supercooled water in the temperature range from 42 to −17°C by heterodyne-detected transient grating (TG) experiments. The TG signal shows some particular and interesting features that are produced by the anomalous behaviour of the thermodynamic parameters of water. In particular, the thermal diffusion effects disappear in the TG data for a temperature of about 0°C. We found that this effect has to be ascribed to the combination of two different physical phenomena: the anomalous temperature dependence of water density and the contribution to the TG signal from the dielectric variation induced directly by the temperature. This term is usually neglected in both light scattering and TG studies; our data show clearly a non-negligible thermal decay that has to be addressed. The data are analysed using a model based on a simple formulation of the linearized hydrodynamic equations. A Markovian approximation for the memory function is used, since the structural relaxation times are much faster than the other responses analysed.

Coherent interactions in femtosecond transient grating.

Transient grating of a dye in liquid has been measured as a function of the electronic coherence period. A diffractive beam splitter and a pair of wedge prisms are implemented to achieve precise spatial phase overlap and interferometrically accurate control of the time delay between the pump pulses. As the electronic coherence period is varied, coherent interactions lead to an enhancement or loss of the sharp feature in the transient grating signal near time zero, which is usually called coherent spike. Sensitivity of the transient grating signal to the solvation process also changes by the coherence time delay. All the features can be accounted for by invoking third-order nonlinear response functions. Numerical simulations have been performed to corroborate our description. This work identifies a major source of the coherent spike in the transient grating and transient absorption experiments. In addition, it allows us to propose a method that measures the solvation function more efficiently than conventional transient grating technique does.

Multi-wave-mixing of two-dimensional excitons in semiconductors

The signals due to six-wave mixing (SWM) processes are observed for two-dimensional excitons and biexcitons. The signal intensity of the SWM process strongly depends on the pulse area of the excitation light. The pulse area is estimated by comparing the intensities of four-wave mixing (FWM) signals in two different directions. The photon echo and the transient-grating signals due to the biexcitonic state of GaSe were observed by SWM. When we change the order of the excitation pulses in our SWM experiments, the excitonic phase relaxation process can be observed in the negative delay-time region.