Transient Grating Method Research Articles

Cooperative two-photon effects in chalcogenide photoresists

Photosensitive chalcogenide glasses are very promising materials for pulsed holography, pulsed photolithography and pulsed optical information recording due to their exceptional sensitivity to pulsed light excitation. We present a comprehensive study of the dynamics of the pulse photoresponse of 0.3–1.0 μm thick chalcogenide glassy As 50Se 50 thin films using a transient-grating method. Both the decrease of transparency and a change of the dissolution rate were recorded after single Nd : YAG and ArF laser pulse and about (1–5)×1000 times larger energy was necessary to obtain the same changes using CW radiation. We show that when excited by a short laser pulse, two different time scales behavior and different intensity dependence of short- and long-time scale signals are present: the short-time signal behaves linearly with the input power but the long-time signal behaves quadratically with the input power. Such behavior reflects a fast electronic process followed by a slow structural rearrangement (the after-pulse effect). The obtained data indicate that the strong increase of photosensitivity following a short intense pulsed light excitation is due to a two-photon effect that aids the process of structural rearrangement as when two photons weaken or break the neighboring bonds, the probability of structural transformation increases significantly.

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.

Enthalpy changes and reaction volumes of photoisomerization reactions in solution: azobenzene and p-coumaric acid

The photoisomerizations of azobenzene in ethanol and p-coumaric acid in water were studied by the transient grating and photoacoustic methods. The absorption spectrum of cis- p-coumaric acid and the quantum yield of the isomerization reaction in water were measured by a photostationary absorption method at two wavelengths (Φ iso=0.46±0.05). The diffusion coefficient of p-coumaric acid in water at room temperature is 4.4×10 −10 m 2 s −1. This relatively small value suggests a hydrogen bonding between p-coumaric acid and water. Concomitant with the photoisomerization, the enthalpy changes (Δ H) and reaction volumes (Δ V) of azobenzene and p-coumaric acid were measured. The obtained values (Δ H=43±8 kJ/mol and Δ V=−3±5 cm 3/mol for azobenzene, Δ H=49±7 kJ/mol and Δ V=−0.7±0.5 cm 3/mol for p-coumaric acid) are discussed in terms of structural change in trans → cis isomerization.

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...

Optical Heterodyne Detected Transient Grating for the Separations of Phase and Amplitude Gratings and of Different Chemical Species

The sensitivity and stability of an optical heterodyne detected transient grating (OHD-TG) method without an active phase control system was examined, and the system was used to separate the phase and amplitude gratings. From the absorptive component at 633 nm after the photoexcitation of a photochromic molecule (a spiropyran molecule), the diffusion constant of only the merocyanine form is selectively determined. The phase grating component consists of both the spiro and merocyanine contributions. However, since the contribution of the merocyanine form is still predominant over the spiro form in the phase grating signal, the diffusion process of the spiro form cannot be observed clearly. By adjusting the phase between the local oscillator and probe fields, the contribution of the merocyanine form can be selectively reduced and the diffusion constant of the spiro form is successfully measured. The result shows that the selective reduction of one species by the OHD-TG technique has a great advantage for st...

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.

Translational motion and isomerization reaction near a solid–liquid interface studied by the interface sensitive transient grating method

Translational diffusion and cis– trans isomerization reaction of a liquid crystal molecule (N-(4-methoxybenzylidene)-4- n-butylaniline (MBBA)) at a solid–liquid interface were studied by the interface sensitive transient grating (TG) technique. The interface sensitivity was achieved by the total internal reflection for the probe light. In all solvents we used, the fringe length dependence of the decay rate constants of the species grating signal reveals that the diffusion constant at the interface region (49 nm) is similar to that in the bulk liquid. In some solvents, faster decays at interface than those in the bulk phase were observed, and they are mainly attributed to the enhanced cis– trans isomerization of MBBA at the interface. The enhancement is not sensitive to the solid surface conditions and depends on the solvents. These facts suggest that the enhancement originates from different solvent structures near the interface from those in the bulk phase. The isomerization rate measured in a porous glass was also increased from that in the bulk phase, which may support this suggestion. The interface sensitive TG signal of another photochromic molecule (spiropyran) was also detected.

Report on a second round robin measurement of the thermal conductivity of CVD diamond

Abstract The results of a round robin series of measurements of the thermal conductivity and thermal diffusivity of chemical-vapor-deposited (CVD) diamond are reported. The present round robin (RR2) is an outgrowth of an earlier series (RR1) aimed at understanding which measurement techniques are best suited to measuring the high conductivity of CVD diamond. RR2 includes diamond specimens of higher homogeneity, non-diamond specimens of lower conductivity as controls, and measurements by 14 laboratories using five techniques, with all techniques but one used by more than one laboratory. The data are converted to thermal conductivity at 25 °C to facilitate comparisons among specimens and laboratories. The statistical analysis excludes outliers using Chauvenet's criterion, resulting in data from typically two or three laboratories being excluded for each specimen. The analysis arrives at mean values in the range 1300–2000 W m −1 K −1 for the diamond samples, as expected because of the conditions of preparation, with uncertainties in the range 1.5–4%. For the non-diamond materials, SiC and AlN, the results are 268 W m −1 K −1 ±2.2% and 178 W m −1 K −1 ±2.5%, respectively. Comparing techniques, the d.c.-heated bar is found to be the most accurate, typically ±5% or better. Under favorable conditions, Angstrom's thermal wave method can apparently yield relative uncertainties of ±5–10%, and the mirage effect ±5–15%. Too few laboratories used the transient thermal grating and laser flash methods to make general comments on their accuracy, but one expects a comparable accuracy if used on specimens that are fine-grained and thermally isotropic. Deviations, sometimes large, from these optimal accuracies are examined, and it is suggested that certain experimental details are important for achieving accuracy. It is also found that the estimates of experimental uncertainty provided by most laboratories seriously underestimate the actual deviations of their data from the mean conductivity.

Induced transient gratings as a probe of the early-stage kinetics of phase-separating liquid mixtures

Because of the large number of nucleated domains and the polydispersity of the droplet distribution, the early-stage kinetics of a first-order phase transition remains difficult to explore. We investigate this issue by using laser-induced concentration variations to drive a transition locally in a liquid mixture with two intersecting pump beams whose interference pattern traps the nucleated droplets on the optical fringes. The time-resolved reflectivity of a third probe wave on the induced droplet grating allows us to characterize the droplet growth at its early stage, which illustrates the efficiency of transient grating methods in quantifying the kinetics of out-of-equilibrium processes.

Diffusion of Photochemically Generated Intermediate Radicals in Water−Ethanol Mixed Solvents

Diffusion processes of intermediate radicals created by the photoinduced hydrogen abstraction reactions of benzoquinone (BQ) and acetophenone (AP) in ethanol−water mixed solvents are studied by using the transient grating (TG) method. The electrically neutral radicals and the anion radicals are created selectively by adding sodium hydroxide (NaOH) or sulfuric acid (H2SO4) to the solutions. The intermediate species and the chemical stability of the radicals were monitored by the transient absorption and time-resolved EPR methods. The diffusion constants (D) of the radicals and the parent molecules are determined simultaneously. D of the anion radicals are similar to those of the neutral radicals in any mixtures of the solvents. D of the neutral and anion radicals (DR) are much smaller than those of the parent molecule (DP) in ethanol (DP/DR ∼ 2.8 for BQ and DP/DR ∼ 2.1 for AP in ethanol), while DP become closer to DR with increasing water content in the solvent (DP/DR ∼ 1.3 for BQ and QP in 90% water solut...

Heat of reaction and reaction volume for the formation of ethers from diazo compounds in methanol

The heat of reaction and reaction volume for formation of ethers from several diazo compounds with two phenyl groups in methanol were determined by the transient grating and photoacoustic methods. It was found that the reaction volume significantly contributes to the photothermal signal and that the reaction volume is sensitive to the molecular structure of the diphenyl groups. The reaction volume is as large as 51 ml/mol for the creation of diphenylmethyl methyl ether. This large volume change is interpreted in terms of the rearrangement of phenyl groups by the reaction. The partial molar volume of N 2 and diffusion constants of N 2 and diazo compounds are determined simultaneously.

Relaxation of nitrobenzene from the excited singlet state

The lifetime of the lowest excited singlet (S1) state of nitrobenzene in ethanol was measured using a picosecond time-resolved transient grating method. The lifetime was found to be 6 ps and the rate constant of the intersystem crossing relaxation process from the S1 state was determined to be 1.3–1.6×1011 s−1. The relaxation mechanism from the S1 state is also discussed.

Solvent Density Dependence of the Photolysis Quantum Yield in Supercritical Fluids.

The transient grating (TG) method has been applied to studies on the photolysis quantum yield (φd) of iodine and diphenyl disulfide in supercritical fluids. The solvent density dependence of φd for iodine obtained by the TG method is quite different from the result by the transient absorption method (B. Otto, et al., J. Phys Chem., 81, 202 (1984) ), if the TG signal is analyzed only by the thermal grating contribution. If we assume that the difference comes from the overestimate of φd from the TG method, the origin could be ascribed to the neglect of volume grating term. Underr this assumption, the estimated reaction volume for the dissociation of iodine should be a considerably large negative over the density regions studied.

Diffusion of electrically neutral radicals and anion radicals created by photochemical reactions

Diffusion processes of the intermediate radicals created by the photochemical reactions of ketones in alcoholic solvents are investigated by using the transient grating (TG) method. The electrically neutral radicals and the anion radicals of acetophenone, benzaldehyde, xanthone, benzophenone and benzil were created selectively by controlling the concentration of sodium hydroxide (NaOH) in alcoholic solvents. The translational diffusion constants (D) of the anion radicals, the neutral radicals, and the parent stable molecules can be successfully measured under the same conditions by this method. It is found that both the neutral and anion radicals diffuse slower than the parent molecules. Values of D of the anion radicals, the neutral radicals and the parent molecules are compared in detail in wide ranges of solvent viscosities, solute sizes and temperatures. Under any conditions, D values of the charged radicals are similar to those of the neutral radicals. A possible origin of such a similarity is discussed in term of the intermolecular charge polarizabilities of the radicals.

Electrohydrodynamics of Dye Molecules in an Isotropic Liquid Crystal Studied by the Transient Grating Method

The transient grating method using a nanosecond laser pulse as the pump and a CW laser beam as the probe is applied to a dye (methyl red)-doped isotropic liquid crystal (MBBA) under a dc electric field. The diffracted signal decays biexponentially due to the faster (in the μs time scale) heat diffusion and the slower (in the ms time scale) mass diffusion of the dye molecule at zero electric field. No significant changes of the heat diffusion by the electric field are observed. The decay by the mass diffusion is greatly affected by the electric field, but the electrohydrodynamic convective flow is not observed. The complementary grating effect is observed in the mass diffusion under the weak electric field, which is explained well with the hydrodynamic model. The viscosity of the medium and the solute−solvent interactions are affected by the electric field. The activation energy of diffusion, which corresponds to the activation energy of viscosity in the hydrodynamic model, decreases with the electric field. The intermolecular association of methyl red and MBBA increases under the electric field so that the diffusional friction grows. The viscosity decrease by the electric field gives greater effect on the diffusion than the diffusional friction due to intermolecular association. Therefore, the diffusion becomes faster under the electric field.

Photothermal Expansion of Droplets in Emulsions Detected by the Transient Grating Method

Abstract A nonlinear optical effect due to the thermal energy after the photoexcitation of a light-absorbing chromophore or a metal ion contained in micro droplets (oil in water (O/W) and water in oil (W/O)) was investigated by the time-resolved transient grating method. The temporal profiles of the diffusive components of the thermal grating signals are very different from that observed in a homogeneous solution, and also depend on the emulsion systems. The profiles were consistently analyzed in terms of the droplet heating, thermal expansion, and subsequent cooling process. The size of the droplets determined from this analysis agrees with that from the dynamic light scattering method.

Study on the Photolysis Quantum Yield of Diphenyl Disulfide by the Transient Grating Method

Abstract Transient grating (TG) spectroscopy, photoaccoustic spectroscopy, and transient adsorption spectroscopy have been applied to studying the nongeminate recombination of photodissociated diphenyl disulfide (DPDS) in liquids. The photolysis quantum yield can be easily determined by an analysis of the time profile of the thermal grating signal, if the contribution of the population grating can be neglected. The quantum yield of the photodissociation for DPDS in isooctane has been determined to be 0.43 ± 0.02, which coincides with the values evaluated from the acoustic strength in the TG signal and the acoustic intensity in an ordinary photoacoustic measurement. This result is compared with a pico-second transient absorption measurement (T. W. Scott and S. N. Liu, J. Phys. Chem., 93, 1393 (1989)).

Diffusion Process of the Benzyl Radical Created by Photodissociation Probed by the Transient Grating Method

Translational diffusion processes of the benzyl radical (BR) created by the photodissociation from dibenzyl ketone (DBK) were studied by using the transient grating (TG) method in organic solvents ...

Nonradiative Relaxation Processes and Electronically Excited States of Nitrobenzene Studied by Picosecond Time-Resolved Transient Grating Method

The exothermic nonradiative relaxation processes from photoexcited nitrobenzene (NB) are studied by the picosecond time-resolved transient grating method. The decay rate constants and energy of the excited NB are determined. The lifetime of the lowest excited singlet state is found to be very short ( e10 ps) and a surprisingly short lifetime of the lowest excited triplet state (480 ps) is detected. From quantitative measurements of the thermal energies released from the excited states of NB, the lowest excited triplet state is determined to be located at (22-27) 10 3 cm -1 . The triplet formation is very efficient and its quantum yield is found to be grater than 0.80. From the temperature and solvent dependence of the triplet lifetime, it is suggested that motions of the nitro group may cause the surprisingly rapid relaxation from the lowest excited triplet state to the ground state. The photophysical properties and excited states of monosubstituted nitrobenzenes are also discussed.

Picosecond Time-Resolved Transient Grating Method for Heat Detection: Excited-State Dynamics of FeCl3 and o-Hydroxybenzophenone in Aqueous Solution

Transient grating signals after photoexcitation to electronically excited states of FeCl3 and o-hydroxybenzophenone (o-HBP) in aqueous solutions were investigated at various temperatures with short-pulsed laser systems. The observed signals can be expressed by a superposition of the density grating (Dens.G) signal and the temperature grating (Temp.G) signal. Above 4 °C, an acoustic oscillation is predominant in the signal, while the acoustic oscillation disappears and a pure Temp.G signal is observed clearly at 4 °C. The rise profile of the pure Temp.G signal was analyzed in terms of the deactivation from the photoexcited state, and it is found that a very fast thermalization process takes place among the solvent molecules. The lifetime of the charge-transfer state of [Fe(OH)(H2O)5]2+ is determined to be 55 ps from the rise profile of the Temp.G signal. Using a subpicosecond laser system and the population grating signal, two time constants (400 fs and 7.5 ps) are obtained for the dynamics of o-HBP. The dynamics of the photoexcited states of FeCl3 and o-HBP are also discussed. Since the temporal response of the Temp.G component is 10−100 times shorter than that of the Dens.G signal, this signal can be used for studying ultrafast heat-releasing processes.