Vibrational dephasing of spherical top anions in aqueous solutions

The vibrational dephasing times for spherical top anions in aqueous solutions calculated from in the shape of the isotropic Raman bands of the fully symmetric vibrations of anions, depending on the temperature and the nature of the anion, are analyzed. It is concluded that the dephasing rates for the anions and their temperature dependences can be reasonably explained within the framework of a model that describes the vibrational dephasing of a diatomic molecule interacting with a solvent molecule by using the Morse potential.     

Raman study of temperature and pressure effects on vibrational relaxation in liquid CHCl3 and CDCl3

The Raman line shapes of the ν₁(A ₁)C-H and C-D stretching fundamentals in liquid CHCl₃ and CDCl₃ have been measured as a function of pressure from 1 bar to 4·5 kbar within the temperature range 30°C to 90°C. Densities have also been determined under the same experimental conditions. The vibrational relaxation rates are obtained from the isotropic component of the Raman band and the experimental results can be summarized as follows: (i) as T increases at constant density the vibrational relaxation rate increases; (ii) at constant T, the increase in density produces an increase in the relaxation rate; (iii) an increase in temperature at constant pressure results in an increased relaxation rate. The above three cases hold for the CDCl₃ liquid, whereas only (ii) may be stated for the CHCl₃ liquid.

The experimental vibrational data are interpreted in terms of the Kubo stochastic line-shape theory and the collinear-isolated-binary-collision model proposed by Fischer and Laubereau. Application of the Kubo formalism shows that vibrational dephasing is the dominant relaxation mechanism and that the modulation is fast both in liquid CHCl₃ and CDCl₃.

Interpretation in terms of the binary collision dephasing model leads to the following results: (i) the pure dephasing mechanism seems to be the dominant broadening mechanism for the isotropic Raman line shapes studied; (ii) the calculated dephasing rates as a function of density and temperature show agreement with the experimental data. In these calculations the elastic collision times are obtained from the modified Enskog theory.     

Dynamics of SCN- ions in molten thiocyanates and aqueous solutions by Raman spectroscopy

The Raman spectra of the v ₁(CN) stretching mode of SCN^- ions have been measured in molten thiocyanates, KSCN and NaSCN, at temperatures 450-600K. The vibrational and the rotational correlation functions are calculated, and the dynamics of SCN^- ions in the molten state are compared with those in aqueous solutions of KSCN, NaSCN, and LiSCN at concentrations 1–10 mol dm^-3 and at temperatures 303–353 K. The observed vibrational correlation functions are analysed by the stochastic line shape theory of Kubo, in which homogeneous and inhomogeneous broadening are treated simultaneously. Both broadening contributions to the isotropic spectra are extracted. The homogeneous broadening is found to increase with increasing temperature in both melts and aqueous solutions; the inhomogeneous broadening remains constant in molten KSCN while it decreases in aqueous solutions. As the result, the isotropic Raman bandwidth is considered to increase with temperature in the molten state and to decrease in aqueous solutions. Rotational correlation functions of SCN^- ions in these molten salts show the behaviour of the short time inertial rotation (t ? 0·15 ps, jump angle 20°), which is a little slower than the free rotation of a single ion. The long time exponential decay of the rotational correlation functions reflects the ultimate diffusional behaviour of the ionic reorientation. The rotational relaxation rate increases with increasing temperature in both melts and aqueous solutions. The vibrational dephasing rate decreases and the rotational relaxation rate increases as the cation size increases in melts. In aqueous solutions, the vibrational dephasing rate follows the same cation dependence as that in melts, while the rotational relaxation rate decreases as the cation size increases. This seems to be a consequence of the specific local structures in aqueous electrolyte solutions.     

Coherent molecular vibrational dynamics of CH2 stretching modes in polyethylene studied by femtosecond‐CARS

We have studied the coherent molecular vibrational dynamics of CH₂ stretching modes in polyethylene by time‐resolved femtosecond coherent anti‐Stokes Raman spectroscopy. We observed that the coherent vibrational relaxation of symmetric CH₂ stretching modes in polyethylene at room temperature is much faster than that previously measured in polyvinyl alcohol. In addition, it was detected that, at low temperature, the coherent vibrational relaxation of the symmetric stretching modes evidently becomes slower compared with that at room temperature. These temperature‐dependent measurements enable us to discriminate the contribution of pure dephasing mechanism, due to phonons and two‐level systems in polymer, from the contribution of lifetime of the vibrational excited state to the coherent vibrational relaxation of CH₂ stretching modes. We conclude that the coherent vibrational relaxation of symmetric CH₂ stretching modes at room temperature consists of the contribution of lifetime and approximately 1.5 times larger contribution of pure dephasing. Copyright © 2015 John Wiley & Sons, Ltd.     

Prevention of decoherence by two femtosecond chirped pulse trains

We study the vibrational energy relaxation (VER) and collisional dephasing as channels of coherence loss in a vibrational mode that is selectively excited using chirped pulse adiabatic passage in the Raman configuration. Based on the dressed state picture analysis we propose a method to reduce decoherence using femtosecond chirped pulse trains. When applied with a period close to the VER time, the pulse trains allow one to sustain high coherence in the selected vibrational mode.     

分子多振动模式的振动退相时间同时测量方法

利用作者自制的超连续谱激发时间分辨相干反斯托克斯拉曼散射(coherent anti-stokes Ranman scat-tering,CARS)谱仪同时测得反映分子组成的各种振动谱以及这些振动模式各自的退相时间.实验对苯甲腈样品分子振动谱中5个典型分子振动模式的振动弛豫过程进行了测量,一次获得了这种分子各振动模式对...     

Model for vibrational relaxation: Pure-dephasing and depopulation

The three dominant mechanisms giving major contributions to vibrational relaxation in molecular systems are (a) pure dephasing, (b) depopulation (or energy relaxation), and (c) resonant transfer. Here (c) is not considered but the effects due to thesimultaneous occurrence of (a) and (b) are treated within a stochastic model. In dealing with (a), the vibrational frequency is assumed to undergo random uncorrelated ‘jump’, due to fluctuations in the environment of the active molecule between a continuous set of values. The ensuing results are somewhat different from those of the commonly used Kubo model of vibrational dephasing, especially at long times and appear to be better suited in interpreting certain experimental data. The model is next extended to include the simultaneous occurrence of (b). The calculation leads to two important conclusions: (i) the lineshape is not just the convolution of those due to (a) and (b), and (ii) the lineshape is asymmetric, if the intermolecular interactions are not isotropic.     

Linear concentration dependence of vibrational width and vibrational dephasing paths in aqueous nitrate solutions

A theoretical model of vibrational dephasing of Raman active ions in aqueous electrolyte solutions is presented in which a probe ion is coupled to the bath by direct ion-solvent and ion-ion interactions. Expression for the vibrational width in terms of concentrations and efficiencies of the vibrational frequency modulation by ion-perturber interactions is given in the fast modulation scheme. The observed linear concentration dependence of the vibrational dephasing width of the v ₁(A'₁) mode of NO₃ ^- in aqueous solutions is reasonably well explained from this model, and efficiencies of the dephasing paths through NO₃ ^--water hydrogen bonding interaction and contact NO₃ ^--cation pair formation interaction are estimated. Anions in the solution give only a secondary effect to nitrate vibrational dephasing because of interionic repulsive forces.     

A simulation study of vibrational relaxation of I− 3 in liquids

The temperature dependence of the vibrational relaxation of a flexible model of triiodide in a Lennard-Jones solvent (xenon) has been studied using equilibrium molecular dynamics simulations. The internal dynamics of the ion is calculated from a previously published semi-empirical valence bond model with a limited number of basis states. Vibrational decorrelation rates of the symmetric and antisymmetric stretching modes were found from the time correlation functions of the normal coordinate velocities and the vibrational energy relaxation rates from the time correlation functions of the kinetic energy in each mode. The vibrational dephasing rates and the energy relaxation rates decrease slowly as the temperature is lowered and do not show a discontinuity when the fluid solidifies, although the reorientational diffusion rates change rapidly at low temperatures. In order to interpret the results, perturbation theory expressions for the relaxation rates were evaluated for simulations of a rigid model of the ion and found to agree well with the direct observations. These showed that, unusually, both the solvent force and its derivative, the solvent potential curvature, contribute to the dephasing of the symmetric mode. The relevant fluctuation correlation times are very short, which may explain the insensitivity of the vibrational relaxation to the state of the solvent.     

Raman and DFT study of hydrogen‐bonded 2‐ and 3‐chloropyridine with methanol

Precise polarized Raman measurements of 2‐chloropyridine (2Clpy) in the region 560–1060 cm⁻¹ and 3‐chloropyridine (3Clpy) in the region 680–1080 cm⁻¹ at different concentrations in mole fraction of methanol were made to calculate the isotropic part of the Raman spectra, which has contributions only from vibrational dephasing. A detailed analysis of the Raman spectra was carried out to see the variation of peak position and linewidth. The dephasing is mode specific. The trigonal bending mode of 3Clpy has two components when it is mixed with methanol. The relative intensities of these two bands are used to calculate the equilibrium constants. The ring‐breathing mode of 3Clpy, on the other hand, remains single in the mixture. The appearance of a new band corresponding to the trigonal bending mode, as well as the nonappearance of that of the ring‐breathing mode, is also shown by the density functional theory (DFT) study of gas phase and methanol‐solvated complexes. The vibrational dephasing time for the hydrogen‐bonded ring‐breathing mode is calculated from the linear Raman linewidth and peak position data. For other modes, it was not possible to calculate the dephasing time because of the nonavailability of a suitable theoretical model. Contrary to 3Clpy, in 2Clpy the ring‐breathing mode becomes a doublet but the trigonal bending mode remains single. It is seen that the hydrogen‐bonding capacity of chloropyridines is highly influenced by the position of the Cl atom. Single and double components of these modes are also explained by DFT calculations. We obtained excellent match of the experimental and theoretical spectra with the B3LYP/6‐31 + G (d,p) method. Copyright © 2008 John Wiley & Sons, Ltd.     

A role of solvent in vibrational energy relaxation of metalloporphyrins

The formation of a vibrationally excited photoproduct of metalloporphyrins upon (π, π*) excitation and its subsequent vibrational energy relaxation were monitored by picosecond time-resolved resonance Raman spectroscopy. Stokes Raman bands due to a photoproduct of nickel octaethylporphyrin (NiOEP) instantaneously appeared upon the photoexcitation. Their intensities decayed with a time constant of 300 ps, which indicates electronic relaxation from the (d, d) excited state (B_1g) to the ground state (A_1g), being consistent with the results of transient absorption measurements by Holten and coworkers. Anti-Stokes ν₄ and ν₇ bands for vibrationally excited (d, d) state of NiOEP decayed with time constants of 10 and 300 ps. The former is ascribed to vibrational relaxation, while the latter corresponds to the electronic relaxation from the (d, d) excited state to the electronic ground state. While the rise of anti-Stokes ν₄ intensity was instrument-limited, the rise of anti-Stokes ν₇ intensity was delayed by 2.6±0.5 ps, which indicates that intramolecular vibrational energy redistribution has not been completed in subpicosecond time regime. To study a mechanism of intermolecular energy transfer, solvent dependence of the time constants of anti-Stokes kinetics was investigated using various solvents. No significant solvent dependence of the rise and decay constants was observed for NiOEP. For an iron porphyrin, we observed two phases in intermolecular energy transfer. The fast phase was insensitive to solvent and the slow phase depended on solvents. A model of classical thermal diffusion qualitatively reproduced this behavior. For solute-solvent energy transfer process, low-frequency modes of proteins seem to be less important.     

The application of incoherent light for the study of femtosecond-picosecond relaxation in condensed phase

A convenient technique for the ultrashort-relaxation-time measurement using temporally incoherent light instead of short pulses can be applied to the studies of relaxation processes. Theoretical studies on measuring various types of relaxation times by this method are summarized. We have applied this technique to the studies of the electronic dephasing in a polydiacetylene film, the vibrational dephasing in dimethylsulfoxide and the relaxation of optical Kerr effect in CS₂ and nitrobenzene.     

溶液和熔化状态下分子间的能量传递

通过二维红外光谱研究了GdmSCN/KSCN=1/1,GdmSCN/KS¹³CN=1/1和GdmSCN/KS¹³C¹⁵N=1/1三种混合晶体在熔融和溶液状态下的共振和非共振的分子间振动能量传递的性质. 在这些样品中,给体/受体的能量差越大,能量传递越慢. 而能量传递的快慢与拉曼光谱无关. 非共振能量传递与给体/受体的能量差的关系不能用声子补偿的机理来描述. 相反,它们的关系却可以用退相位机理来定量描述. 在熔融状态下,共振和非共振能量速率与温度的依赖关系也与退相位机理的预测相符合. 这一系列的结果表明只要分子的运动(平动和转动)远远快于非共振能量传递速率,那么退相位机理不仅在溶液中占主导,而且在熔融状态下(纯液体,不含溶剂)也占主导.     

Coherent coupling between vibrational modes of C-H bonds at different positions studied by femtosecond time-resolved coherent anti-Stokes Raman scattering

We performed femtosecond time-resolved coherent anti-Stokes Raman scattering(fs-CARS) measurements on liquid toluene and PVK film.For both samples,we selectively excited the CH stretching vibrational modes and observed the expected quantum beat signals.The frequency of the well-defined beats is in good agreement with the energy difference between the two simultaneously excited modes,which demonstrates that a coherent coupling between the vibrational modes of the C-H chemical bonds exists at the different positions of the molecules.The dephasing times of the excited modes are obtained simultaneously.     

Coherent coupling between vibrational modes of Cben H bonds at different positions studied by femtosecond time-resolved coherent anti-Stokes Raman scattering

We performed femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) measurements on liquid toluene and PVK film. For both samples, we selectively excited the CH stretching vibrational modes and observed the expected quantum beat signals. The frequency of the well-defined beats is in good agreement with the energy difference between the two simultaneously excited modes, which demonstrates that a coherent coupling between the vibrational modes of the C-H chemical bonds exists at the different positions of the molecules. The dephasing times of the excited modes are obtained simultaneously.     

液相硝基甲烷分子振动特性的相干反斯托克斯拉曼散射光谱

构建时间分辨相干反斯托克斯拉曼散射（CARS）光谱系统,从微观层次研究硝基甲烷的分子相干振动动力学特性.实验中采用超连续白光作为斯托克斯光,通过调整斯托克斯光的时间延迟,得到不同振动模式的CARS光谱.通过对振动弛豫曲线的拟合,获得硝基甲烷分子不同振动模式的振动失相时间.结果表明C–H键伸缩振动比C–N键伸缩振动更容易受热声子的影响.在热加载下,硝基甲烷分子的C–H键有望首先被激发并引起初始化学反应.     

Dynamics of Si-H vibrations in an amorphous environment

We present results of the first vibrational photon-echo, transient-grating, and temperature dependent transient-bleaching experiments on a-Si:H. Using these techniques, and the infrared light of a free electron laser, the vibrational population decay and phase relaxation of the Si-H stretching mode were investigated. Careful analysis of the data indicates that the vibrational energy relaxes directly into Si-H bending modes and Si phonons, with a distribution of rates determined by the amorphous host. Conversely, the pure dephasing appears to be single exponential, and can be modeled by dephasing via two-phonon interactions.     

Some aspects of SERS temporal fluctuations: analysis of the most intense spectra of hydrogenated amorphous carbon deposited on silver

Surface‐enhanced Raman scattering (SERS) spectra of hydrogenated amorphous carbon (a C:H) deposited on silver substrates have been recorded with a confocal Raman microscope. When scattered radiation is collected during a short time from an area of a few square micrometres, the subsequently measured SERS spectra often exhibit strong temporal changes (fluctuations). In this paper we present examples of spectra for which the intensity maxima of the fluctuating narrow Raman bands are significantly higher than that of the background (the background is usually dominated by two broad Raman bands centred at about 1350 and 1590 cm⁻¹). In a series of successively measured spectra, one can find spectra with noticeably different total integral intensity. This suggests that the results of averaging the spectra revealing strong and weak fluctuations may be different (at least in intensity). The influence of some electrolytes on the SERS spectral fluctuations is also analysed. Our experiments revealed that the efficiencies of quenching of the SERS spectral fluctuations by various electrolytes are significantly different. We suggest that only anions directly interacting with the metal surface quench strong SERS fluctuations, and that the large differences between chloride and perchloride solutions are caused by differences in the strength of interaction of Cl⁻ and ClO₄⁻ anions with the silver surface. Copyright © 2007 John Wiley & Sons, Ltd.     

Vibrational relaxation dynamics in transient grating spectroscopy studied by rate equations based on time-dependent correlation function

A modified model, a set of rate equations based on time-dependent correlation function, is used to study vibrational relaxation dynamics in transient grating spectroscopy. The dephasing, the population dynamics, and the vibrational coherence concerning two vibrational states are observed respectively in organic dye IR780 perchlorate molecules doped polyvinyl alcohol matrix. The result shows that in addition to the information concerning system-environment interac- tion and vibrational coherence, the vibrational energy transfer can be described by this modified model.     

Vibrational spectroscopy investigation using density functional theory on 7‐chloro‐3‐methyl‐2H‐1,2,4‐ benzothiadiazine 1,1‐dioxide

The solid phase Fourier transform infrared (FTIR) and Fourier transform (FT) Raman spectral analysis of 7‐chloro‐3‐methyl‐2H‐1,2,4‐benzothiadiazine 1,1‐dioxide (diazoxide), an antihypertensive agent was carried out along with density functional computations. The optimized geometry, wavenumber and intensity of the vibrational bands of diazoxide were obtained by DFT‐B3LYP level of theory with complete relaxation in the potential energy surface using 6‐31G(d,p) basis set. A complete vibrational assignment aided by the theoretical harmonic frequency analysis has been proposed. The harmonic vibrational wavenumbers calculated have been compared with experimental FTIR and FT Raman spectra. The observed and the calculated wavenumbers are found to be in good agreement. The experimental spectra coincide satisfactorily with those of calculated spectra. Copyright © 2009 John Wiley & Sons, Ltd.