Transient Grating Signal Research Articles

Vibrational Population Relaxation and Dephasing Dynamics of Fe(CN)64-in D2O with Third-Order Nonlinear Infrared Spectroscopy

Third-order nonlinear IR spectroscopy has been used to study the vibrational population relaxation and the vibrational dephasing dynamics of the CN stretching mode of Fe(CN)64- in D2O. Transient-grating signals in the magic-angle condition show a biexponential decay upon the excitation of the T1u mode of the CN stretching motion. A fast-decaying component (∼700 fs) of the transient-grating signal is attributed to the rapid equilibration between the T1u mode and the Raman-active modes. The time scale of the slow-decaying component is around 23 ps, which corresponds to the vibrational population relaxation from the v = 1 state of the CN stretching mode. Anisotropy of the transient-grating signal decays with a time constant of 2.6 ps, which is due to the time evolution of the superposition states of the triply degenerate T1u modes. Three-pulse photon-echo measurements showed that the correlation function of the frequency fluctuations decays biexponentially with time constants of 80 fs and 1.5 ps. The time scales of the decay of the correlation function are similar to those for SCN- in D2O even though the coupling strength of the solute−solvent interaction depends on the systems. On the basis of these results, we discuss the vibrational dynamics of the CN stretching mode of Fe(CN)64- in D2O.

A Novel Method for Study of Protein Folding Kinetics by Monitoring Diffusion Coefficient in Time Domain

Molecular diffusion process after the photo-induced electron injection to ferric cytochrome c (Fe(III) cyt c) in guanidine hydrochloride (GdnHCl) 3.5M buffer solution is studied by the time-resolved transient grating technique. Circular dichroism studies have revealed that Fe(III) cyt c is unfolded under this condition but the reduced form, Fe(II) cyt c, is folded. Hence, this pulsed laser-induced reduction should initiate the folding process of cyt c. The observed transient grating signal shows prominent features, which have never been observed before. Based on several characteristic points, we concluded that the apparent diffusion coefficient (D) of Fe(II) cyt c after the reduction is time dependent, which must be associated with the protein folding dynamics. This time-dependent apparent D should reflect either the continuous time development of the hydrodynamic radius or population change of the unfolded and folded states during the folding dynamics. This is the first observation of the time-dependent apparent D during any chemical reaction, and this time-dependent measurement of D should be a unique and powerful way to study the protein folding kinetics from a viewpoint of the protein’s shape or the protein-water intermolecular interaction.

Two-color Transient Grating Spectroscopy of a Two-level System

A theoretical description and experimental demonstration of homodyne-detected two-color transient grating (2-C TG) signal are presented. By treating the coupled bath degrees of freedom as a collection of harmonic oscillators and using a short-time expansion method, approximated nonlinear response functions were obtained. An analytic expression for the two-color transient grating signal was obtained by carrying out relevant Gaussian integrals. The initial rising and decaying parts of the 2-C TG signal is shown to be critically dependent on the ultrafast inertial component of the solvation correlation function. The experimental results confirm the predictions of the theoretical model.

Analysis of transient-grating signals for reacting-flow applications

Single-shot transient-grating measurements for thermometry in pressurized reacting flows are examined in the context of rapid digital signal processing. Simple approaches are discussed for temperature determination and rejection of unwanted signals in real-time measurement applications. Examples of temperature data in pressurized postflame gases are presented in the form of probability-density functions (PDFs). Three contributions to the PDF half-widths are discussed. Analysis of phase-matching requirements indicates that beam steering as a result of density fluctuations affects the signal amplitude but not the grating period. Therefore, such stochastic beam deviations have little effect on the derived temperatures. Mode noise on the cw probe beam as well as linear light scattering are found to be insignificant in the frequency range of the observed transient-grating acoustic signature. Use of a single-mode laser for the pump beams is shown to enhance the signal intensity.

Denaturation of a Protein Monitored by Diffusion Coefficients: Myoglobin

Change of global protein structure as a function of concentration of a denaturant, guanidine hydrochloride (Gdn−HCl), was studied by using the laser-induced transient grating (TG) method for carboxymyoglobin (MbCO). From the time profile of the TG signal after the photodissociation reaction of the ligand, diffusion coefficients (D) of the protein were determined at various concentrations of the denaturant. The denaturation curve of MbCO monitored by D was compared with that monitored by the circular dichroism (CD) method. The −m value and Δ determined from the transition curve monitored by D are smaller than those obtained from the CD signal intensity. This noncoincidence of the two transition curves indicates that the global structure of Mb is still changing after the complete secondary structure (α-helices) deformation process. The smaller diffusion coefficient of unfolded MbCO compared to folded MbCO can be interpreted in terms of changes of the protein−water interaction and the surface roughness of th...

Structural dynamics of distal histidine replaced mutants of myoglobin accompanied with the photodissociation reaction of the ligand.

Protein dynamics observed by the transient grating (TG) method are studied for some site-directed mutants at the distal histidine of myoglobin (H64L, H64Q, H64V). The time profiles of the TG signals are very sensitive to the amino acid residue of the 64 position. It was found that the sensitivity is mostly caused by the different rates of the ligand escape from the protein to solvent and the magnitude of the molecular volume change. Several molecular origins of the volume difference between MbCO and Mb, such as the electrostatic interaction in the distal pocket, movement of helices, and distal water, are proposed. Interestingly, the volume difference between the CO-trapped Mb inside the protein interior and Mb is similar to that of the partial molar volume of CO in organic solvent. The effect of mutation on the nature of the CO trapped site is discussed.

Carrier Dynamics in InGaN/GaN SQW Structure Probed by the Transient Grating Method with Subpicosecond Pulsed Laser

Carrier dynamics in GaN and InGaN/GaN SQW structures were observed by using the transient grating (TG) method with sub-picosecond pulsed laser at room temperature. The diffusion coefficients (D) of photo-created carriers were estimated by the decay rate time of TG signals and the photoluminescence (PL) lifetime. It was found that D depends on the emission wavelength (In composition). The relationship between the emission efficiencies and carrier diffusion was considered in terms of the spatial inhomogeneity of In composition.

Ultrafast exciton dynamics of J-aggregates in room temperature solution studied by third-order nonlinear optical spectroscopy and numerical simulation based on exciton theory

We report a study of the exciton dynamics in 1,1′-diethyl-3,3′-bis(sulforpropyl)-5,5′,6,6′ -tetrachlorobenzimidacarbocyanine (BIC) J-aggregates in water solution at room temperature by third-order nonlinear optical spectroscopy and numerical simulations based on exciton theory. The temporal profiles of the transient grating signals depend strongly on the excitation intensity as a result of exciton–exciton annihilation. On the other hand, the peak shift measurement gives information on the fluctuations of the transition frequency of the system. The peak shift decays with time constants of 26 and 128 fs. There is no finite peak shift on a longer time scale. The electronic state of J-aggregates is described by a Frenkel exciton Hamiltonian, and the exciton population relaxation processes is described by Redfield equations. Based on the numerical simulations, the peak shift data can only be explained even qualitatively when both exchange narrowing and exciton relaxation process are included in the model. The 128-fs component is assigned to a “hopping” time between exciton units. We confirmed that while the static disorder within an exction state that is partially delocalized due to static disorder is exchange-narrowed, the exchange narrowing of the dynamical disorder is not complete but appears as lifetime broadening, which competes with the exchange narrowing of the fluctuations. The effect of the exciton relaxation on the absorption spectrum is discussed.

Isomerization Dynamics of 1,1’-Diethyl-4,4’-Cyanine (1144C) Studied by Different Third-Order Nonlinear Spectroscopic Measurements

The isomerization of 1,1’-diethyl-4,4’-cyanine (1144C) in two solvents, ethanol and hexanol, was studied by three different third-order four-wave mixing techniques: three-pulse photon echo peak shift (3PEPS), transient grating (TG), and pump−probe (PP) to provide complimentary information on different aspects of the reaction dynamics. A double-sided Feynman diagram analysis was used to assist in the formulation of the kinetic behavior in the TG and PP signals. The ground-state recovery process can be described as a single exponential process which is dominated by the bond-twisting motion on the excited-state surface. The rate is strongly viscosity and temperature dependent. A larger yield of the isomer is obtained in less viscous solvents or with greater excess excitation energy. Apparently, contradictory wavelength-dependent decay behavior was found in the TG and PP signals. This can be understood by considering the different real and imaginary contributions to the signals from hot ground-state absorpti...

The Structural Dynamics and Ligand Releasing Process after the Photodissociation of Sperm Whale Carboxymyoglobin

The structural and energy dynamics and molecular diffusion after the photodissociation of sperm whale carboxymyoglobin were investigated by the transient grating (TG) method. The quantitative measurement of the TG signal provides the volume change and released heat within 10 ns (−5 cm3 mol-1 and 151 kJ mol-1, respectively) as well as with a lifetime of 700 ns at 20 °C (12 cm3 mol-1 and 8 kJ mol-1) without any assumption. From the signal intensity with a wide range of grating vector (105 to 8 × 106 m-1), the molecular origin of the 700-ns dynamics is clearly attributed to the ligand escaping process from the protein interior to the solvent. Hence, a four-state model is appropriate to describe the ligand dissociation reaction even at room temperature. Participation of the salt bridge between Arg45 and the heme 6-propionate side chain as well as the enthalpy change and the structural change of this photodissociation reaction is discussed.

Influence of intramolecular vibrations in third-order, time-domain resonant spectroscopies. I. Experiments

This is the first in a two-paper series that investigates the influence of intramolecular vibrational modes on nonlinear, time-domain, electronically resonant signals. Both Transient Grating (TG) and Three Pulse Photon Echo Peak Shift (3PEPS) signals were collected from several probe molecules: Nile Blue, N,N-bis-dimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide, and Rhodamine 6G dissolved in different solvents: benzene, dimethylsulfoxide, and acetonitrile. The effects of excitation of different vibronic transitions on the electronically resonant signals were identified by comparing signals collected with laser pulses at different excitation wavelengths. In the 3PEPS profiles, we find that excitation on the blue edge of the absorption spectrum causes a decreased initial peak shift values and more rapid initial decays, whilst in the TG signals, the magnitude of the “coherent spike” is strongly wavelength dependent. Additional thermally activated vibronic effects were studied via temperature dependent 3PEPS profiles. Our results reveal the sensitivity of the nonlinear signals to the excitation wavelengths and to the distinct vibronic structure of the different chromophores studied. Pronounced modulations in both the 3PEPS and TG signals originating from coherently excited vibrational modes were directly observed. Additional oscillations were observed that are attributed to difference frequencies and higher harmonics of the fundamental modes. In paper II we demonstrate that detailed account of the vibronic nature of the chromophore is required to describe the wavelength dependent signals.

Influence of intramolecular vibrations in third-order, time-domain resonant spectroscopies. II. Numerical calculations

We model recent experimental wavelength dependent Three Pulse Photon Echo Peak Shift (WD-3PEPS) and Transient Grating (WD-TG) signals considering both solvation dynamics and vibrational contributions. We present numerical simulations of WD-3PEPS and WD-TG signals of two probe molecules: Nile Blue and N,N-bisdimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide to investigate the influence of intramolecular vibrations in the signals. By varying the excitation wavelength, we show that the different initial conditions for the vibrational wave packets significantly affect the signals, especially through the contributions associated with high frequency modes, often neglected in experimental analyses. We show that the temporal properties of both WD-TG and WD-3PEPS signals display sensitivities to both the excitation wavelength and the vibronic structure of the specific probe molecule used. Several mechanisms for generating vibronic modulations in the signals are discussed and their effects on the signals are described. Quantitative agreement between experiment and simulated signals requires accurate characterization of the laser pulses, specifically the magnitude and sign of chirp has a significant effect on the initial temporal properties of the signals. We provide a description of the experimental considerations required for accurate determination of molecular dynamics from 3PEPS and TG experiments and conclude with a brief discussion of the implications of our results for previous analyses of such experiments.

Dynamics of structure and energy of horse carboxymyoglobin after photodissociation of carbon monoxide.

The energetics and structural volume changes after photodissociation of carboxymyoglobin are quantitatively investigated by laser-induced transient grating (TG) and photoacoustic calorimetric techniques. Various origins of the TG signal are distinguished: the phase grating signals due to temperature change, due to absorption spectrum change, and due to volume change. We found a new kinetics of approximately 700 ns (at room temperature), which was not observed by the flash photolysis technique. This kinetics should be attributed to the intermediate between the geminate pair and the fully dissociated state. The enthalpy of an intermediate species is determined to be 61 +/- 10 kJ/mol, which is smaller than the expected Fe-CO bond energy. The volume of MbCO slightly contracts (5 +/- 3 cm(3)/mol) during this process. CO is fully released from the protein by an exponential kinetics from 25 to -2 degrees C. During this escaping process, the volume expands by 14.7 +/- 2 cm(3)/mol at room temperature and 14 +/- 10 kJ/mol is released, which should represent the protein relaxation and the solvation of the CO (the enthalpy of this final state is 47 +/- 10 kJ/mol). A potential barrier between the intermediate and the fully dissociated state is DeltaH(*) = 41.3 kJ/mol and DeltaS(*) = 13.6 J mol(-1) K(-1). The TG experiment under a high wavenumber reveals that the volume expansion depends on the temperature from 25 to -2 degrees C. The volume changes and the energies of the intermediate species are discussed.

Heterodyne detected transient grating spectroscopy in resonant and non-resonant systems using a simplified diffractive optics method

We report a simplified optical heterodyne detected transient grating setup consisting of a single diffractive optical element based on modifications of the arrangement used by Miller and co-workers [J. Phys. Chem. A, 103 (1999) 10619]. Our arrangement features ease of alignment, suppression of scattering and is free of pump–probe contamination in the detected transient grating signals. The capability of our arrangement is demonstrated by measurements on a non-resonant system, CS 2, and a resonant system, malachite green in ethanol.

Energy Transfer in the B800 Rings of the Peripheral Bacterial Light-Harvesting Complexes of Rhodopseudomonas Acidophila and Rhodospirillum Molischianum Studied with Photon Echo Techniques

Eenergy transfer in the B800 ring of the LH2 antenna of the purple bacteria Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rs.) molischianum was studied at room temperature using three-pulse echo peak shift (3PEPS) and transient grating (TG) techniques. From the transient grating experiments, we found the B800 → B850 energy transfer rates to be 600−700 fs for both species. The anisotropy of the TG signal decays in about 1 ps for both species, which is ascribed to B800 → B800 energy transfer. The occurrence of B800 → B800 energy transfer was further substantiated by 3PEPS experiments. When measured over the whole B800 band, the initial peak shift of about 30 fs exhibited a fast <100 fs decay to about 10 fs due to the coupling to protein phonons, followed by a slow phase of about 1 ps, during which the peak shift decayed to 1−3 fs. Polarized 3PEPS experiments systematically resulted in smaller peak shift values for the third pulse polarized perpendicular to the first two than for the third pulse par...

Temperature-Dependent Volume Change of the Initial Step of the Photoreaction of Photoactive Yellow Protein (PYP) Studied by Transient Grating

The energetics and protein dynamics of photoactive yellow protein (PYP) were studied by transient grating (TG) and photoacoustic (PA) spectroscopies. The enthalpy difference (ΔH) between the ground state (pG) and the first intermediate (pR) at 20 °C was determined by the TG method as 160 kJ/mol, which is much larger than ΔH of the photoisomerized chromophore (p-coumaric acid) in water (50 kJ/mol). By simultaneous measurements of the TG and PA signals, the volume change (ΔV) for the pG → pR process is determined to be −7 cm3/mol (contraction) at 20 °C. Interestingly, the volume contraction increases with decreasing temperature. At 0 °C the volume change becomes ca. −15 cm3/mol. This temperature-dependent volume change may indicate the structural fluctuation of PYP protein in the solution phase.

Femtosecond Heterodyne-Detected Four-Wave-Mixing Studies of Deterministic Protein Motions. 1. Theory and Experimental Technique of Diffractive Optics-Based Spectroscopy

Analytic relations are developed to describe the coupling between experimentally observed optically heterodyne-detected (OHD) transient grating (TG) signals and the underlying molecular dynamics of heme proteins. The heterodyne detection was implemented using a diffractive optical element to generate the phase-matched beam pattern along with a reference beam for OHD. The phase stability between the reference and signal fields using this approach is shown to be excellent over several hours of signal collection. Under small signal conditions this leads to an enhancement in signal-to-noise of several orders of magnitude. The grating excitation provides an inherent acoustic reference that can be used to determine the absolute signal phase. This enables separation of the real and imaginary components to the nonlinear susceptibility as well as determination of both the absorption anisotropy and its real counterpart, the phase anisotropy. The relationship between the phase anisotropy and the observed OHD TG sign...

Two-Dimensional Electronic Correlation and Relaxation Spectra: Theory and Model Calculations

Hybl et al. demonstrated a technique for recording two-dimensional Fourier transform electronic correlation and relaxation spectra based on detecting phase modulation of the signal electric field in a noncollinear femtosecond four-wave mixing experiment. A theoretical analysis of 2D correlation and relaxation experiments is presented for a system consisting of two electronic states each having two or more sublevels. The separation between absorption and dispersion mode 2D spectra in these experiments is investigated in detail for nonzero pulse duration and compared to related 2D NMR experiments based on a nonlinear optical definition of coherence order. Phase-twisted peaks, which mix absorption and dispersion line shapes, can occur under some circumstances. A 1D projection of the complex 2D spectrum is shown to equal the transient grating signal field, and the real part of this projection is related to the spectrally resolved pump−probe signal. Calculated 2D spectra for a two-level Bloch model, an underdamped Brownian oscillator, and a few models of polar solvent dynamics based on the correlation function approach to the nonlinear response developed by Mukamel and co-workers are presented. The real parts of the 2D spectra are primarily positive (indicative of ground state bleaching and excited state emission) but contain negative regions arising from excitation of coherent superposition states (e.g. vibrational wavepackets) in both the ground and excited electronic states. Assignment of the 2D spectra displaying wavepacket motion at the vibronic level is discussed, and the manifestations of wavepacket motion and vibrational relaxation in the 2D spectra are explored. As suggested by Hybl et al., an increase in pulse duration is found to affect a 2D spectrum primarily as a spectral filter that limits the range of the spectrum. The Gaussian correlation function characteristic of inertial solvent motion is found to be faithfully reflected in a homogeneous 2D cross width which is nearly independent of pulse duration. An alternative experimental method for obtaining only the real part of 2D spectra is proposed.

Photodissociation Quantum Yield of Iodine in the Low-, Medium-, and High-Density Fluids Studied by the Transient Grating Method

The transient grating (TG) method has been applied to the determination of the photodissociation quantum yield (φd) of iodine in various fluids at low-, medium-, and high densities. After a nanosecond laser excitation at 532 nm, the thermal grating created by fast processes such as excess energy relaxation, primary geminate recombination, and secondary geminate recombination appears as a fast rise of the TG signal, and the thermal grating created by the nongeminate recombination process appears as a slow rise of the TG signal. By comparison of the relative intensities of these signals, φd has been determined. We have demonstrated that the species grating and the volume grating are negligible in the TG signal in fluids such as argon at 323 K, krypton at 293 K, and liquid solvents at room temperature. Our results of φd in these solvents show density dependence similar to those previously measured by the transient absorption method (Dutoit, J.-C.; et al. J. Chem. Phys. 1983, 78, 1825. Luther, K.; et al. J. P...

Thermalization process after the relaxation of electronically excited states: Intramolecular proton transfer systems studied by the transient grating method

Photophysical and thermalization processes after photoexcitation of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (HMPB) in cyclohexane and 2-hydroxybenzophenone (HBP) in various solvents were investigated by the transient grating (TG) method. From the time profiles of the population grating (PG) signals, two distinct kinetics were observed for HMPB and HBP. For HMPB, the faster (600 fs) kinetics is attributed to the back proton transfer reaction in the ground state, and the faster process of HBP (400 fs) is attributed to the vibrational cooling in the S1 state. The slower one (∼30 ps) of both compounds is assigned to the vibrational cooling in the S0 state. In ethanol (hydrogen bonding solvent), the PG signal originated from the T1 state of HBP is apparent. However, the thermal energy from the T1 state is negligibly small and the triplet quantum yield was found to be less than 0.05. The photoexcited HBP relaxes to the ground state by the internal conversion dominantly even in ethanol. The thermalization rates of these molecules were measured from a point of view of the translational energy of solvents by the acoustic peak delay method of the TG signal. The results show that in the early step of the thermalization, there is a very fast cooling process (less than a few ps) which is due to the energy transfer from the photoexcited solute to (several) effectively coupled solvent molecule(s), and then the heated solvent molecule becomes cool by the thermal diffusion to the bulk solvents. The thermalization processes depend on both of the solute and solvent properties. The time development of the temperature calculated based on this thermalization model explains the experimental observations.