Echo Peak Shift Measurements Research Articles

Ab Initio Molecular Dynamics Study of Aqueous Solutions of Magnesium and Calcium Nitrates: Hydration Shell Structure, Dynamics and Vibrational Echo Spectroscopy.

Ab initio molecular dynamics simulations are performed to study the hydration shell structure, dynamics, and vibrational echo spectroscopy of aqueous Mg(NO3)2 and Ca(NO3)2 solutions. The hydration shell structure is probed through calculations of various ion-ion and ion-water radial and spatial distribution functions. On the dynamical side, calculations have been made for the hydrogen bond dynamics of hydration shells and also residence dynamics and lifetimes of water in different solvation environments. Subsequently, we looked at the dynamics of frequency fluctuations of OD modes of heavy water in different hydration environments. Specifically, the temporal decay of spectral observables of two-dimensional infrared (2DIR) spectroscopy, three pulse echo peak shift (3PEPS) measurements and also of time correlations of frequency fluctuations are calculated to investigate the dynamics of vibrational spectral diffusion of water in different hydration environments in these solutions. The OD stretch frequencies of water molecules in the vicinity of both divalent cations are found to be red-shifted and also fluctuating at a slower rate than other water molecules present in the solutions. The Mg2+ ions are found to be strongly hydrated which can be linked to their lower tendency to form contact ion-pairs and essentially no water exchange between the cationic hydration shells and bulk during the time scale of the current simulations. The stronger hydration of Mg2+ ions make their hydration shells structurally and dynamically more rigid and make the dynamics of hydrogen bonds and vibrational spectral diffusion, as revealed through spectral observables of 2DIR and 3PEPS slower than that for the Ca2+ ions. The structural and spectral dynamics of water molecules outside the cationic solvation shells in the Mg(NO3)2 solution are also found to be relatively slower than that of the Ca(NO3)2 solution and pure water which show the effects of stronger electric fields of Mg2+ ions extending beyond their first hydration shells. Also, water molecules in the hydration shells of the NO3- ions are found to relax at a slower rate in the Mg(NO3)2 solution which manifests the effect countercations have on anionic hydration shells for divalent metal nitrate solutions.

Ultrafast solvation response in room temperature ionic liquids: Possible origin and importance of the collective and the nearest neighbour solvent modes

Recent three-pulse photon echo peak shift (3PEPS) measurements [M. Muramatsu, Y. Nagasawa, and H. Miyasaka, J. Phys. Chem. A 115, 3886 (2011)] with several room temperature ionic liquids (RTILs) have revealed multi-exponential dynamics with ultrafast solvation timescale in the range, 20 < τ(1)∕fs < 250, for both imidazolium and phosphonium RTILs. This is striking for two reasons: (i) the timescale is much faster than those reported by the dynamic Stokes shift (DSS) experiments [S. Arzhantsev, H. Jin, G. A. Baker, and M. Maroncelli, J. Phys. Chem. B 111, 4978 (2007)] and (ii) sub-hundered femtosecond solvation response in phosphonium ionic liquids is reported for the first time. Here, we present a mode coupling theory based calculation where such ultrafast solvation in 3PEPS measurements has been visualized to originate from the nearest neighbour solute-solvent interaction. Consideration of Lennard-Jones interaction for the nearest neighbour solute-solvent non-dipolar interaction leads to biphasic dynamics with a predicted ultrafast time constant in the ∼100-250 fs range, followed by a slower one similar to that reported by the 3PEPS measurements. In addition, the calculated fast time constants and amplitudes are found to be in general agreement with those from computer simulations. Different microscopic mechanisms for ultrafast solvation response measured by the 3PEPS and DSS experiments have been proposed and relative contributions of the collective and nearest neighbour solvent modes investigated. Relation between the single particle rotation and ultrafast polar solvation in these RTILs has been explored. Our analyses suggest 3PEPS and DSS experiments are probably sensitive to different components of the total solvation energy relaxation of a laser-excited dye in a given ionic liquid. Several predictions have also been made, which may be re-examined via suitable experiments.

Ultrafast Solvation Dynamics in Room Temperature Ionic Liquids Observed by Three-Pulse Photon Echo Peak Shift Measurements

Three-pulse photon echo peak shift (3PEPS) measurement was applied to the investigation of the primary part (<100 ps) of the solvation dynamics in a series of imidazolium ionic liquids (IL) with an organic dye, oxazine 4 (Ox4), utilized as a probe. The ultrafast solvent response in the range of ≤300 fs exhibited dependence on the square root of the anion mass, indicating its relation with the inertial motion of anion. The inertial response of ILs with chloride anion was the fastest among other ILs with heavier and larger anions. Because Ox4 is a cationic dye, it holds a stronger interaction with the anion of IL, thus the ultrafast part of the solvation is strongly affected by the inertial motion of anions. The second solvation component in the range of ≤3.5 ps had better correlation with the reduced mass and the size of both ions included, indicating the beginning of a more global solvation process.

Three-Pulse Photon Echo Peak Shift Measurements of Capped CdSe Quantum Dots

In this article, the results of femtosecond three-pulse photon echo peak shift (3-PEPS) measurements are reported for a series of cadmium selenide core−shell semiconductor nanocrystals. The peak shift data were found to be highly dependent on the shell compositions and particle morphologies. The static inhomogeneity parameter was found to vary from 10 cm−1 for highly isotropic particles to 170 cm−1 for particles with a broad size distribution. We also discuss the high-frequency (≈212 cm−1) longitudinal optical (LO) phonon and the low-frequency (≈13−19 cm−1) longitudinal acoustic (LA) phonon modes (ω00) and how the damping times and frequencies also depend on the choice of capping material.

Dephasing in semiconducting single-walled carbon nanotubes induced by exciton-exciton annihilation

Three-pulse photon echo peak-shift measurements were performed on semiconducting single-walled carbon nanotubes embedded in a polymer matrix at room temperature. We found a striking dependence of the peak shift on the excitation intensity. Numerical simulations based on an interacting-boson model demonstrate that the intensity dependence originates from a highly nonlinear optical response initiated by exciton-exciton annihilation.

Femtosecond Photon Echo Spectroscopy of Semiconducting Single-Walled Carbon Nanotubes

Three-pulse photon echo peak shift measurements were performed on semiconducting single-walled carbon nanotubes embedded in polymer matrix at room temperature. Simultaneous modeling of the peak shift data in the limit of zero-intensity and the linear absorption spectrum enable us to extract an intrinsic homogeneous line width of 178 cm(-1), an inhomogeneous width of 698 cm(-1), and a Huang-Rhys factor of 0.04 for the radial breathing mode vibration. The peak shift data when combined with two-pulse photon echo and pump-probe measurements allows us to determine a pure exciton dephasing time scale of 78 fs at room temperature.

Ultrafast N−H Vibrational Dynamics of Cyclic Doubly Hydrogen-Bonded Homo- and Heterodimers

Hydrogen-bonded interfaces are essential structural elements in biology. Furthermore, they can mediate electron transport by coupling the electron to proton transfer within the interface. The specific hydrogen-bonding configuration and strength have a large impact on the proton transfer, which exchanges the hydrogen-bonded donor and acceptor species (i.e., NH...O --> N...HO). Modulations of the hydrogen-bonding environment, such as the hydrogen-bond stretch and twist modes, affect the proton-transfer dynamics. Here, we present transient grating and echo peak shift measurements of the NH stretch vibrations of four doubly hydrogen-bonded cyclic dimers in their electronic ground state. The equilibrium vibrational dynamics exhibit strong coherent modulations that we attribute to coupling of the high-frequency NH vibration to the low-frequency interdimer stretch and twist modes and not to interference between multiple Fermi resonances that dominate the substructure of the linear spectra.

Correlated Exciton Relaxation in Poly(3-hexylthiophene)

Two-color 3 pulse photon echo peak shift (2C-3PEPS) measurements on poly(3-hexylthiophene) (3PHT) demonstrate that spectral regions in the photoluminescence remain correlated with the excitation, despite large differences in energy (>0.5 eV). The observations are explained in terms of exciton-phonon coupling that is dominated by only two motions: one high frequency bond stretch and a low frequency torsional motion. Numerical simulations of the 2C-3PEPS are shown to be consistent with the experimental observations. The results demonstrate that initial intramolecular exciton relaxation in P3HT is not primarily a stochastic process, but is driven by strong, selective exciton-phonon coupling to torsional motions.

Vibrational phase relaxation of O–H stretch in bulk water: Role of large amplitude angular jumps and negative cross-correlations among the forces on the O–H bond

The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time ( T 2) of the O–H stretch in bulk water calculated from the frequency fluctuation time correlation function ( C ω ( t)) is in the range of 70–80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of C ω ( t) is found to be responsible for the ultrashort T 2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of C ω ( t). (A) The large amplitude angular jumps of water molecules (within 30–40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen ( F O( t)) and hydrogen ( F H( t)) atom of the O–H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate.

Strongly Interacting Organic Conjugated Dendrimers with Enhanced Two-Photon Absorption

We report a strongly interacting new dendrimer system with an extended spectroscopic unit (coherent domain) beyond the trimer configuration. This report focuses on the mechanism of enhancement of the two-photon absorption (TPA) response of this material as a function of the dendrimer generation number. This enhancement is strongly correlated to the size and geometry of the spectroscopic unit in a strongly interacting macromolecular system. These systems are investigated by a wide variety of time-resolved spectroscopy methods including time-resolved fluorescence, transient absorption, three-pulse photon echo peak shift measurements as well as TPA cross section measurements. The extensive combination of modern spectroscopic methods strongly indicates that the spectroscopic unit (domain) in high generations covers the G1 (nine linear building blocks) and exceeds the trimer size.

Ultrafast Exciton Dynamics in a Branched Molecule Investigated by Time-Resolved Fluorescence, Transient Absorption, and Three-Pulse Photon Echo Peak Shift Measurements

The excited-state dynamics of a branched octupolar molecule (tris-4,4‘,4‘ ‘-(4-nitrophenilethynyl)triphenylamine (T-NPTPA)) with charge-transfer character is investigated by complementary measurements of time-resolved fluorescence, transient absorption, and three-pulse photon echo peak shift. The data are compared with those obtained for the linear molecule dimethylnitroaminotolane (DMNAT) representing a linear building block of the octupolar system. Time-resolved fluorescence as well as transient absorption experiments showed the presence of an intermediate C3 symmetry state for the trimer with a depolarization time faster than ∼50 fs. Three-pulse photon echo peak shift measurements for the trimer revealed a large initial fast component of the peak shift decay. The initial peak shift during the first 100 fs was found to be much higher for the trimer as compared to the linear building block. This is in accordance with the time-resolved anisotropy results suggesting a different character of the initial excitation of the trimer. The higher peak shift value for the trimer suggests a smaller total coupling of electronic transition frequency to the nuclear motions, which may be the result of the delocalized character of this initial excited state. A residual peak shift (∼2 fs) at long population times was also observed and was interpreted as an indication of the residual static inhomogeneity (inhomogeneous broadening) in the system.

Exciton–bath coupling and inhomogeneous broadening in the optical spectroscopy of semiconductor quantum dots

The homogeneous line shape and inhomogeneous broadening of colloidal quantum dots are measured at ambient temperatures and in solution. Three-pulse echo peak shift (3PEPS) measurements of nanocrystalline CdSe semiconductor quantum dots in toluene solvent at 298 K are reported. Modeling of the 3PEPS data using a detailed model designed to account for the optical properties of semiconductor quantum dots, suggests that Debye-type relaxations attributed to reorientation of the trioctylphosphine and trioctyl phosphine oxide capping ligands are significant. It is shown that 3PEPS spectroscopy can quantify precisely the inhomogeneous broadening of the optical spectrum, brought about mainly by the distribution of nanocrystal sizes.

Two-Dimensional Analysis of Integrated Three-Pulse Photon Echo Signals of Nile Blue Doped in PMMA

The integrated three-pulse photon echo signal of Nile blue doped in poly(methyl methacrylate) (PMMA) was measured two-dimensionally by varying the first coherence period and the subsequent population period. The electronic dephasing time was in the femtosecond time range even at low temperatures, whereas rephasing and echo formation were still possible even when the population period was extended to >100 ps. Oscillation was present in the echo signal even when population period was extended to longer values than the dephasing time of the vibrational coherence. Echo peak shift measurement indicated a presence of small spectral diffusion in the picosecond time range. A computer simulation was carried out from a model spectral density. The phonon mode of PMMA was represented by a Brownian oscillator centered at ∼116 cm-1, and it reproduced the temperature dependence of the echo signal surprisingly well.

Solute Dependence of Three Pulse Photon Echo Peak Shift Measurements in Methanol Solution

Three pulse photon echo peak shift (3PEPS) measurement has recently become a popular method to study solvation and protein dynamics. In this paper, dependence of the 3PEPS signal on probe solute molecules was studied using several xanthene dyes and styryl dye molecules in methanol and compared with pump−probe measurements. For xanthene dyes, the decay was faster when the blue side of the absorption spectrum was excited. This effect is considered to be the result of a limited observation window and vibrational relaxation of the excess energy through higher density of states in the excited state. Nonlinear coupling may be also need to be considered. It seemed that the 3PEPS signal was more sensitive to excited state dynamics rather than to the ground state ones. For styryl dyes, a gradual increase of the peak shift was observed in the picosecond region, which may be the result of photoinduced isomerization.

Energy Transfer in Light-Harvesting Complexes LHCII and CP29 of Spinach Studied with Three Pulse Echo Peak Shift and Transient Grating

Three pulse echo peak shift and transient grating (TG) measurements on the plant light-harvesting complexes LHCII and CP29 are reported. The LHCII complex is by far the most abundant light-harvesting complex in higher plants and fulfills several important physiological functions such as light-harvesting and photoprotection. Our study is focused on the light-harvesting function of LHCII and the very similar CP29 complex and reveals hitherto unresolved excitation energy transfer processes. All measurements were performed at room temperature using detergent isolated complexes from spinach leaves. Both complexes were excited in their Chl b band at 650 nm and in the blue shoulder of the Chl a band at 670 nm. Exponential fits to the TG and three pulse echo peak shift decay curves were used to estimate the timescales of the observed energy transfer processes. At 650 nm, the TG decay can be described with time constants of 130 fs and 2.2 ps for CP29, and 300 fs and 2.8 ps for LHCII. At 670 nm, the TG shows decay components of 230 fs and 6 ps for LHCII, and 300 fs and 5 ps for CP29. These time constants correspond to well-known energy transfer processes, from Chl b to Chl a for the 650 nm TG and from blue (670 nm) Chl a to red (680 nm) Chl a for the 670 nm TG. The peak shift decay times are entirely different. At 650 nm we find times of 150 fs and 0.5–1 ps for LHCII, and 360 fs and 3 ps for CP29, which we can associate mainly with Chl b ↔ Chl b energy transfer. At 670 nm we find times of 140 fs and 3 ps for LHCII, and 3 ps for CP29, which we can associate with fast (only in LHCII) and slow transfer between relatively blue Chls a or Chl a states. From the occurrence of both fast Chl b ↔ Chl b and fast Chl b → Chl a transfer in CP29, we conclude that at least two mixed binding sites are present in this complex. A detailed comparison of our observed rates with exciton calculations on both CP29 and LHCII provides us with more insight in the location of these mixed sites. Most importantly, for CP29, we find that a Chl b pair must be present in some, but not all, complexes, on sites A 3 and B 3. For LHCII, the observed rates can best be understood if the same pair, A 3 and B 3, is involved in both fast Chl b ↔ Chl b and fast Chl a ↔ Chl a transfer. Hence, it is likely that mixed sites also occur in the native LHCII complex. Such flexibility in chlorophyll binding would agree with the general flexibility in aggregation form and xanthophyll binding of the LHCII complex and could be of use for optimizing the role of LHCII under specific circumstances, for example under high-light conditions. Our study is the first to provide spectroscopic evidence for mixed binding sites, as well as the first to show their existence in native complexes.

Vibrational Dynamics of the OH Stretching Mode of Water in Reverse Micelles Studied by Infrared Nonlinear Spectroscopy

ABSTRACTVibrational dynamics of the OH stretching mode of water in the water pool of reverse micelles (H2O/Aerosol OT (AOT)/isooctane) are studied by nonlinear infrared spectroscopy such as transient grating method and three-pulse photon echo peak shift measurements. The W0 value (W0 =[H2O]/[AOT]) is changed from 2 to 40, which corresponds to a water pool diameter of a few nm to about 20 nm. Polarization dependent transient grating experiments show rapid anisotropy decay of the OH stretching mode, which could be due to resonant intra and intermolecular energy transfer. From the three-pulse photon echo peak shift experiments, the spectral diffusion is found to be dependent on W0.

Excited State Dynamics and Heterogeneity of Folded and Unfolded States of Cytochromec

Cytochrome c is an electron transfer protein whose redox and folding properties have received a great deal of attention. In this study, transient absorption, transient grating, and three pulse photon echo peak shift (3PEPS) measurements have been used to characterize the photophysics of the heme chromophore in the folded protein and in two different unfolded proteins. The data are interpreted in terms of a reactive three level system. A Soret excited state lifetime of 40 to 50 fs is observed, which is not sensitive to the folded state of cytochrome c. An intermediate time scale of 200 to 400 fs is also observed, which may be associated with the Q state lifetime. The longest time scale, assigned to ground state recovery, is approximately 2 to 4 ps. The dynamics are discussed in terms of the folded and redox states of the protein. The 3PEPS data show that the folded protein has little structural heterogeneity; however, the unfolded proteins show a larger degree of structural heterogeneity, the extent of which depends on the unfolding conditions.

Ultrafast Energy Transfer in LHC-II Revealed by Three-Pulse Photon Echo Peak Shift Measurements

We report the results of three-pulse photon echo peak shift (3PEPS) measurements on the light-harvesting complex II (LHC-II) of the green algae Chlamydomonas reinhardtii. Experiments were performed at two different excitation wavelengths, 670 and 650 nm, corresponding to Chl-a and Chl-b excitation, respectively. The 3PEPS data are analyzed using a new theory that incorporates the effect of energy transfer on third-order response functions. Our theoretical model separates energy transfer dynamics from the solvation dynamics arising from coupling of the electronic transitions to the protein environment. We suggest that the protein fluctuations can be described by an ultrafast solvation on a sub-100 fs time scale and a long time correlation (static disorder). Decay of the 670 nm peak shift reveals spectral equilibration time scales for Chl-a molecules that range from 300 fs to 6 ps and agree well with other experiments. 3PEPS data at 650 nm (Chl-b excitation) reveal rapid Chl-b to Chl-b energy transfer (<1 p...

Three pulse photon echo studies of nondipolar solvation: Comparison with a viscoelastic model

Three pulse stimulated photon echo peak shift (3PEPS) measurements were used to probe the solvation of a quadrupolar solute in three room temperature nondipolar solvents; benzene, CCl4, and CS2, and the results were compared with those for two polar solvents, methanol and acetonitrile, and one weakly polar solvent, toluene. Our data reveal three distinct solvent dynamical time scales; a sub-100 fs ultrafast component attributed to inertial motions, a slow (∼2–3 ps) component attributed to structural relaxation, and an intermediate time scale (∼600 fs) of uncertain origin. The six solvents were chosen to reflect a range of possible interactions, but exhibit similar dynamics, suggesting that similar mechanisms may be at work or that different mechanisms may exist, but occur on similar time scales. A viscoelastic continuum solvation model proposed to describe nonpolar solvation [J. Phys. Chem. A 102, 17 (1998)] was used for a preliminary analysis of our data.

Probing Solvation and Reaction Coordinates of Ultrafast Photoinduced Electron-Transfer Reactions Using Nonlinear Spectroscopies: Rhodamine 6G in Electron-Donating Solvents

The reaction kinetics as well as the solvation dynamics of the photoinduced electron-transfer (ET) reaction from the electron-donating solvents dimethylaniline (DMA) and diethylaniline (DEA) to rhodamine 6G (R6G) are elucidated using complementary information from transient grating (TG) and three-pulse photon echo peak shift (3PEPS) measurements. The data are contrasted with those obtained from TG and 3PEPS studies in the “unreactive” solvents ethanol and dimethyl sulfoxide. New methods are employed to model these data using nonlinear response functions expressed in terms of both solvation dynamics and reaction kinetics. A three-level model, including a component in the response function to account for excited-state absorption, is used to model the 3PEPS and TG data. It is also demonstrated that 3PEPS retrieves information concerning the reaction coordinate as well as solvation information. We conclude that for R6G/DMA, rapid photoinduced ET occurs on a time scale of τa ∼ 85 fs and for the R6G/DEA system τa ∼ 160 fs. An excited-state absorption contribution to the signals that we associate with back-electron transfer was observed with time constants τb = 4.0 ps for R6G/DMA (15% contribution) and τb = 6.9 ps for R6G/DEA (20% contribution). Subsequently, the cooling and relaxation (i.e. ground-state recovery) occurs on a time scale of τc = 19 ps (R6G/DMA) and τc = 50 ps (R6G/DEA). We attribute the τc to solvent-limited reequilibration on the ground-state free energy curve.