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The low frequency spectra of the instantaneous normal modes of CCl 4 at temperatures of 260, 300, and 340 K, calculated with the full potential. The positive part of the spectrum corresponds to real frequencies, i. e., true oscillatory modes. The negative part of the spectrum corresponds to imaginary frequencies which are related to the structural evolution of the liquid.
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understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump-probe experiments measuring the vibrational lifetime of the T1u mode from the melting points to the boiling points of the two solvents show a dramatic solvent dependence. In CCl4 , the vibrational lifetime decreases as the temperature is in...
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... configu- rations were stored every 2 ps, and ;R was evaluated. Figure 4 displays the instantaneous normal mode spec- trum for CCl 4 at three temperatures calculated with the full potential. The spectrum is shown with both positive and negative values of the frequency. ...
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... comparison with Fig. 4, which was calculated with the full potential, Fig. 6 shows the density of states of CCl 4 calculated using a LJ potential. It can be seen that the LJ potential, while giving qualitatively similar results, does not yield the same spectrum produced by the potential that in- cludes internal degrees of freedom of the molecule. The maximum ...
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... using a LJ potential. It can be seen that the LJ potential, while giving qualitatively similar results, does not yield the same spectrum produced by the potential that in- cludes internal degrees of freedom of the molecule. The maximum frequency of both the real and the imaginary parts of the spectrum are substantially lower than seen in Fig. 4. However, the change in the density of states with tempera- ture at the peak of the real part of the spectrum is still 10%. The LJ spectrum probably gives a good description of the true translational spectrum; thus the high-modes in both liquids are primarily rotational. This is why CHCl 3 has a higher maximum phonon frequency, with ...
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... to 340 K. The fact that the change is smaller at low temperature can per- haps be seen more clearly by looking at the imaginary part of the spectrum. The decrease in the real part results in an in- crease in the imaginary part as stable modes are replaced with unstable modes as the temperature is increased. Look- ing at the imaginary portions of Figs. 4 and 5, it is clear that there is a greater increase in the area at the higher tempera- tures. At the 160 cm 1 point in the real part of Fig. 5, there is very little change going from 260 to 340 K. Given the signal-to-noise ratio in the calculation, it is not clear that there is any change. It is expected that there will be no sig- ...
Citations
... Raman scattering transforms into RRS when ω L approaches ω ji , the denominator decreases, the numerator of σ fg increases, and RRS is generated. Different interactions exist in the solute-solvent system [20][21][22]. The solute-solvent system has different types of interactions: vibrations between solute-solvent, solute-solute, or solvent-solvent molecules [23]. ...
A method of energy-transfer resonance of lycopene used to enhance stimulated Raman scattering (SRS) of a weak vibration C–O mode in tetrahydrofuran (THF) was developed in this study. Only C–H SRS was observed in pure THF at high energies. When lycopene was added, the C–O SRS located at 915 cm⁻¹ of the weak vibration mode in THF was observed. The maximum SRS enhancement of the C–O mode was achieved when the concentration was 3.72 × 10−6 mol/L because of the resonance enhancement of the solute, which transferred the excess vibrational energy to the solvent. Moreover, the pulse width compression phenomenon of the C–H vibration in the presence of C–O SRS was obtained.
... Recently, however, Zaccone and Baggioli have successfully derived such an analytical expression that is considered to be the universal law for the vibrational density of states of liquids. 2 Based on the overdamped Langevin dynamics, this universal law has defined the liquid VDOS in terms of the imaginary INMs, which have been shown to dominate the dynamics of liquids, particularly in the low energy region. [3][4][5][6] According to Zaccone and Baggioli (who we will refer to herein as Z-B) the VDOS for a liquid is given by the expression: 2 ...
An analytical model describing the vibrational phonon density of states (VDOS) of liquids has long been elusive, mainly due to the difficulty in dealing with the imaginary modes dominant in the low-energy region, as described by the instantaneous normal mode (INM) approach. Nevertheless, Zaccone and Baggioli have recently developed such a model based on overdamped Langevin liquid dynamics. The model was proposed to be the universal law for the vibrational density of states of liquids. Distinct from the Debye law, g({\omega}) ~ {\omega}2, for solids, the universal law for liquids reveals a linear relationship, g({\omega}) ~ {\omega}, in the low-energy region. The universal law has been successfully verified with computer simulated VDOS for Lennard-Jones liquids. We further confirm this universal law with experimental VDOS measured by inelastic neutron scattering on real liquid systems including water, liquid metal, and polymer liquids. We have applied this model and extracted the effective relaxation rate for the short time dynamics for each liquid. The model has been further evaluated in the predication of the specific heat. The results have been compared with the existing experimental data as well as with values obtained by different approaches.
... Vibrational relaxation is controlled by a higher-order anharmonic process where vibrations are annihilated or created depending on the decay routes available [4,5]. When the active modes of a solvent system are excited, they may relax by intramolecular and intermolecular interactions with the other molecules present. ...
The Raman scattering parameters of linewidth and peak energy of liquids that are undergoing local nanoscale spatial confinement are investigated. This is done using several polar and nonpolar liquid molecules of varying size, weight, shape, and polarity that are placed inside different nanometer-sized silica glass pores. Nanoscale confinement is shown to produce changes in the Raman spectra that are related to different quantum effects. Raman spectra are collected from filled porous glass samples with average pore sizes of 2.5, 5, 10, and 20 nm. The resulting spectra are then compared to bulk liquid samples. The most salient spectral changes of individual vibrational modes, overtone modes, and combinations modes of methanol, deuterated methanol, acetone, benzene, carbon disulfide, carbon tetrachloride, and beta-carotene are graphed and tabularized. Results illustrate how different types of molecular vibrations respond to confinement, and how molecular geometrical restrictions affect Raman active modes based on the ratio of pore size to molecular size. Nano-confinement allows for the direct measurement of how intra-molecular and inter-molecular forces affect the expression of vibrational resonant peak energies as well as vibrational dephasing times. The outcome demonstrates a preference for polar molecules, specific vibrational types, and ratios of molecular size to cavity size.
... The following gives an empirical model that explains the gain to solutes from anharmonic effect [27,28]. We extend Kaiser's pioneering research [29,30] on energy transfer and decay of vibrations in mixed liquids for the generation of an excess of excited vibrations by RR process and energy transfer from C to M. In support of the proposed model, an order of magnitude calculation follows to explain the observed Raman small gain of M from an excess of M vibration with increasing C concentration, which in turn leads to more excited M from C molecules via the RR effect of C molecules. ...
... The salient observation of this study is that the carotene solute's RRS enhanced SRS signal from the vibrations of methanol. The solute-solvent system can have different types of interactions: vibrations between solute-solute molecules, solvent-solvent molecules, or solute-solvent molecules [27][28][29][30]. The spontaneous Raman signal at 2834 cm −1 (Fig. 1(a)) was a coupling effect. ...
A new nonlinear optical process, named enhanced stimulated Raman scattering (ESRS), is reported for the first time from resonance Raman in β-carotene-methanol solution. It is well known that absorption decreases the efficiency of the nonlinear optical and laser processes; however, we observed enhanced stimulated Raman peaks at the first and second Stokes from methanol solvent at 2834 cm⁻¹ with the addition of β-carotene solutes. This enhanced SRS effect in methanol is attributed to the resonance Raman (RR) process in β-carotene, which creates a significant number of vibrations from RR and the excess vibrations are transferred to methanol from anharmonic vibrational interactions between the β-carotene solutes and the methanol solvent, and consequently leads to the increased Raman gain.
... de nos expériences et des données de la littérature synthétise les temps de déphasage et de relaxation des populations du mode T 1u dans différents solvants à 300 K. bration des molécules constituant le solvant sont différents. Une étude comparative de la relaxation de W(CO) 6 dans différents solvants peut être trouvée dans les références[22] et[86]. Pour qu'il y ait transfert d'énergie vers le solvant, il faut que le mode de vibration de W(CO) 6 soit résonnant avec un mode ou une combinaison de modes de vibration des mo- ...
We built an experimental set-up in order to generate infrared stimulated photon echoes at the femtosecond timescale. The purpose is to examine the short time vibrational dynamics of metal carbonyls (W(CO)₆ and Fe(CO)₅) trapped in cryogenic matrices (4-50 K). This environment, resulting from the condensation of a gas mixture containing the impurity and an inert gas (N₂, CH₄, Ar, etc.), is well suited to study systems in their ground state. An excited molecular vibration is always damped in the time domain. It corresponds in the frequency domain to a broadening of the absorption line. The study of the vibrational dynamics aims at examining the physical causes of this spectral broadening. Typically, there are three kinds of causes: intramolecular phenomena, interactions between trapped molecules and interactions between the impurity and the environment. Photon echoes allow distinguishing between the homogeneous and the inhomogeneous contributions of the spectral broadening and characterizing dephasing process, population relaxation and spectral diffusion. Among the obtained results, we highlighted the influence of phonons that are specific to molecular matrices (ex: N₂ libration and CH₄ rotation) on the vibrational dephasing. Moreover, we observed the influence of the phase transition of solid methane at 20 K on the vibrational dynamics. We also showed that the vibrational dynamics depends on the site in which the molecule is trapped. Finally, when exciting several vibrational modes, we are also able to study intramolecular couplings.
... The population relaxation of W(CO) 6 embedded in different solvents was extensively studied. 11,14,17,24,39 It appears that it is strongly solvent dependent because of the lack of intermediate vibrational levels in the W(CO) 6 molecule. The first fundamental mode below the T 1u mode has an energy around 580 cm −1 , 21 and the slow step of the intramolecular relaxation from v = 1 of the T 1u mode should be the first step. ...
... In particular, T 1 is as long as 700 ps 14 at room temperature with CCl 4 as solvent because CCl 4 has no vibrational level of intermediate energy, the relaxation should thus occur in a high order process, involving at least three vibrations and a phonon. 39 Similarly, in nitrogen and rare gas matrixes, there is no available vibrational mode of the host under 2000 cm −1 , except phonons with frequencies lower than 100 cm −1 , and T 1 is thus expected to be very long. Otherwise, our measurements of S(T) show a nonmonoexponential decay, indicating that the population relaxation involves more than one characteristic time. ...
... The first interpretation is to assign the longest time to a recovery time T g , assuming T 1 ≠ T g . T 1 = T g was usually assumed in the previous studies of W(CO) 6 in liquids at room temperature 11,14,17,39 because of a fast vibrational relaxation in the vibration−rotation manifold of levels, helped by an important population of thermal phonons. In matrixes at low temperature, the cascade can be highly slowed down, and T g > 40 The estimation of T 1 is 180 ± 30 ps in nitrogen, and 80 ± 20 ps in krypton, i.e., the same order of magnitude in both matrixes. ...
Vibrational dynamics of the T1u CO stretching mode of tungsten hexacarbonyl is explored when the molecule is embedded in a nitrogen matrix at low temperature. Experiments combined IR absorption spectroscopy and IR stimulated photon echoes at the femtosecond timescale. W(CO)6 is found to be trapped in two main families of sites differing by their symmetries (called hereafter Oh and D2h ). In Oh sites, the vibrational coherence is strongly temperature dependent, exhibiting a coupling with librational phonons of the nitrogen lattice. Perturbation in D2h sites results in the splitting of the T1u band in three components. Each component is inhomogeneously broadened, with dephasing times in the tens of picoseconds, weakly coupled to the lattice phonons. Experiments in solid krypton are performed to compare the effect of atomic and diatomic host lattices. Dephasing time in Kr does not depend on temperature and remains in the hundreds of picosecond, highlighting the molecular origin of dephasing process in N2 . Additionally, non linear signals show oscillations due to quantum beats and polarization interferences between different frequency components of the induced third order polarization, giving information, in particular, on the overtone vibrational transition.
... After introduction of adequate coordinate displacements, the INMs provide an instantaneous decoupled description of the vibrational motions of the molecule at the corresponding timedependent configuration. 35 Though the INMs were originally developed to study shorttime dynamics properties of liquids, 36−41 this concept has been widely extended to deal with vibrational dynamics of polyatomic molecules. 42−45 Consequently, we use the NA-ESMD framework in combination with ES-INM analyses performed on each individual electronic excited state involved in the process. ...
The nonadiabatic excited-state molecular dynamics (NA-ESMD) method and excited-state instantaneous normal modes (ES-INMs) analyses have been applied to describe the state-specific vibrations that participate in the unidirectional energy transfer between the coupled chromophores in a branched dendrimeric molecule. Our molecule is composed of two-, three-, and four-ring linear poly(phenyleneethynylene) (PPE) units linked through meta-substitutions. After an initial laser excitation, an ultrafast sequential S(3) → S(2) → S(1) electronic energy transfer from the shortest to longest segment takes place. During each S(n) → S(n-1) (n = 3, 2) transition, ES-INM(S(n)) and ES-INM(S(n-1)) analyses have been performed on S(n) and S(n-1) states, respectively. Our results reveal a unique vibrational mode localized on the S(n) state that significantly matches with the corresponding nonadiabatic coupling vector d(n,(n-1)). This mode also corresponds to the highest frequency ES-INM(S(n)) and it is seen mainly during the electronic transitions. Furthermore, its absence as a unique ES-INM(S(n-1)) reveals that state-specific vibrations play the main role in the efficiency of the unidirectional S(n) → S(n-1) electronic and vibrational energy funneling in light-harvesting dendrimers.
... Measurements of vibrational relaxation of the carbonyl chromaphore in tungsten hexacarbonyl in CCl 4 and CHCl 3 show very different behaviors between the two solvents. 45 In fact, the temperature dependence of the vibrational lifetimes of the carbonyl chromaphore is opposite for CCl 4 , where the relaxation time decreases with increasing temperature, compared to CHCl 3 , where the relaxation time increases with increasing temperature. Similarly, in another study, the vibrational lifetimes of the carbonyl chromaphore in ethyl trichloroacetate were shown to differ by a factor of 2 between the two solvents. ...
Deprotonation of the alkoxysulfonium intermediate has been shown to be rate-determining in the Swern oxidation of benzyl alcohol. Directly following this rate-determining step is the intramolecular syn-β-elimination of the ylide. In the present study, intramolecular (2)H kinetic isotope effects (KIEs) are used to gain insight into this syn-β-elimination step. As a result of the stereogenic sulfur center in the ylide intermediate, two diastereomeric transition states (endo-TS1 and exo-TS1) must be assumed to contribute to the intramolecular KIE. The intramolecular (2)H KIE determined at -78 °C is 2.82 ± 0.06. Attempts to reproduce this measurement computationally using transition state theory and a Bell tunneling correction yielded a value (1.58) far below that determined experimentally. Computational analysis is complicated by the existence of two distinct transition structures owing to the stereogenic center. Two extremes of Curtin-Hammett kinetics are explored using energies, vibrational frequencies, and moments of inertia from computed transition structures. Neither Curtin-Hammett scenario can reproduce the observed KIE to any acceptable degree of fidelity. Evidence based upon previous kinetics measurements and calculations upon a model system suggests that the stereogenic sulfur center is not likely to undergo inversion to a significant degree at the temperatures at which the Swern oxidation is performed here. Proceeding under the assumption of no stereoinversion at the sulfur center, calculations predict a nearly linear Arrhenius plot for the KIE--even with the inclusion of a one-dimensional tunneling correction. By contrast, the experimentally determined temperature dependence shows a significant concave upward curvature indicative of the influence of tunneling. Notably, KIEs measured in CCl(4), CHCl(3), CH(2)Cl(2), dichloroethane, and chlorobenzene at -23 °C showed little variance. This finding discounted the possible influence from dynamical effects due to incomplete vibrational relaxation. An ad hoc amplification of the imaginary frequencies corresponding to the first-order saddle points corresponding to endo-TS1 and exo-TS1 allowed us to reproduce the experimental temperature dependence of the KIE using only two adjustable parameters applied to a kinetic scenario that involves four isotopomeric transition states. The cumulative data and computational modeling strongly suggest that, even though the intramolecular (2)H KIE observed in these experiments is small, this reaction requires a multidimensional description of the tunneling phenomenon to accurately reproduce experimental trends.
... This scenario is in contrast to other ultrafast temperature-dependent relaxation experiments where the solvent and solute are in thermal equilibrium; thus solvent viscosity changes and changes in the material density can significantly influence the measurements and should be explicitly included in the data analysis. 60,61 To further elucidate the effects of orientational relaxation on the 2DIR waiting-time signal decay, we analyze the results obtained from molecular dynamics simulations. The simulated photoproduct orientational decay of 13.4 ps is in excellent agreement with the experimental values at long time delays of 11.7 ps. ...
Transient two-dimensional infrared (2DIR) spectroscopy is applied to the photodissociation of Mn2(CO)10 to 2 Mn(CO)5 in cyclohexane solution. By varying both the time delay between the 400 nm phototrigger and the 2DIR probe as well as the waiting time in the 2DIR pulse sequence, we directly determine the orientational relaxation of the vibrationally hot photoproduct. The orientational relaxation slows as the photoproduct cools, providing a measure of the transient temperature decay time of 70 +/- 16 ps. We compare the experimental results with molecular dynamics simulations and find near quantitative agreement for equilibrium orientational diffusion time constants but only qualitative agreement for nonequilibrium thermal relaxation. The simulation also shows that the experiment probes an unusual regime of thermal excitation, where the solute is heated while the solvent remains essentially at room temperature.
... The experiments in this investigation were performed on liquid samples of 500 lm length of CS 2 , C 2 Cl 4 , CCl 4 and CDCl 3 at room temperature. These liquids were chosen for two reasons mainly: (i) all substances are common solvents for studies of molecular dynamics, and (ii) conventional Raman results and theoretical simulations for comparison are available in recently published work [9,15,20]. Fig. 1 presents, as a function of relative frequency Dm ¼ m À m Pu , the relative intensity changes registered in the mentioned liquids in the form of lnðI=I 0 Þ, which is directly proportional to the Raman gain factor gðDmÞ. Common to all results shown in Fig. 1 is an intensity increase (gain) at negative frequencies Dm < 0 and intensity decrease (loss) at Dm > 0. In all cases the Raman gain is zero at Dm ¼ 0, shows extrema in the region of jDmj ¼ ð10-30Þ cm À1 , and then decreases until it reaches zero again in the region between jDmj ¼ 100 and 200 cm À1 . ...
The low-frequency spectra of simple liquids have been studied with a stimulated Raman gain technique using tunable picosecond pulses in the mid-infrared region. The obtained results on CS2, CCl4, C2Cl4 and CDCl3 are rescaled to reduced Raman spectra, which are a representation of the low-frequency density of states of these liquids, weighted by the corresponding intermolecular polarizability. This thermal ‘bath’ is believed to accept or provide small portions of energy during vibrational relaxation processes. The technique demonstrated here has a large potential for time-resolved studies of vibrational relaxation and related phenomena.
