Vibrational energy relaxation, nonpolar solvation dynamics and instantaneous normal modes: Role of binary interaction in the ultrafast response of a dense liquid (original) (raw)
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arXiv (Cornell University), 2023
Liquid dynamics play crucial roles in chemical and physical processes, ranging from biological systems to engineering applications. Yet, the vibrational properties of liquids are poorly understood when compared to the more familiar case of crystalline solids. Here, we report experimental neutron-scattering measurements of the vibrational density of states (VDOS) of water and liquid Fomblin in a wide range of temperatures. In the liquid phase, we observe a universal low-energy linear scaling of the experimental VDOS as a function of the frequency, which persists at all temperatures. Importantly, in both systems, we observe that the slope of this linear behavior grows with temperature. We confirm this experimental behavior using molecular dynamics simulations, and we explain it using instantaneous normal mode (INM) theory, whose predictions are in good agreement with the experimental and simulation data. Finally, we experimentally observe a sharp crossover at the melting point of water, below which the standard Debye's law is recovered. On the contrary, in Fomblin, we observe a gradual and continuous crossover indicating its glassy dynamics, and proving that the low-frequency power-law scaling of the VDOS is a good probe for the nature of the solid-to-liquid transition. Our results experimentally confirm the validity of INM theory and the success of a normal mode approach to liquid dynamics, which could pave the way towards a deeper understanding of liquids and their properties in general.
Solvation dynamics in liquid water
2015
Solvation dynamics in liquid water is addressed via nonequilibrium energy transfer pathways activated after a neutral atomic solute acquires a unit charge, either positive or negative. It is shown that the well-known nonequilibrium frequency shift relaxation function can be expressed in a novel fashion in terms of energy fluxes, providing a clearcut and quantitative account of the processes involved. Roughly half of the initial excess energy is transferred into hindered rotations of first hydration shell water molecules, i.e. librational motions, specifically those rotations around the lowest moment of inertia principal axis. After integration over all water solvent molecules, rotations account for roughly 80 % of the energy transferred, while translations have a secondary role; transfer to intramolecular water stretch and bend vibrations is negligible. This picture is similar to that for relaxation of a single vibrationally or rotationally excited water molecule in neat liquid wate...
Dielectric friction and solvation dynamics: Novel results on relaxation in dipolar liquids
In this article we present a new, general but simple, microscopic expression for time-dependent solvation energy of an ion. This expression is surprisingly similar to the expression for the time-dependent dielectric friction on a moving ion. We show that both the Chandra-Bagchi and the Fried-Mukamel formulations of solvation dynamics can be easily derived from this expression. This expression leads to an almost perfect agreement of the theory with all the available computer simulation results. Second, we show here for the first time that the mobility of a light solute ion can significantly accelerate its own solvation, specially in the underdamped limit. The latter result is also in excellent agreement with the computer simulations.
Solvation Dynamics in Monohydroxy Alcohols: Agreement between Theory and Different Experiments
The Journal of Physical Chemistry B, 1997
Recently three different experimental studies on ultrafast solvation dynamics in monohydroxy straight-chain alcohols (C 1 -C 4 ) have been carried out, with an aim to quantify the time constant (and the amplitude) of the ultrafast component. The results reported are, however, rather different from different experiments. In order to understand the reason for these differences, we have carried out a detailed theoretical study to investigate the time dependent progress of solvation of both an ionic and a dipolar solute probe in these alcohols. For methanol, the agreement between the theoretical predictions and the experimental results [Bingemann and Ernsting J. Chem. Phys. 1995, 102, 2691 and Horng et al. J. Phys. Chem. 1995 is excellent. For ethanol, propanol, and butanol, we find no ultrafast component of the time constant of 70 fs or so. For these three liquids, the theoretical results are in almost complete agreement with the experimental results of Horng et al. For ethanol and propanol, the theoretical prediction for ionic solvation is not significantly different from that of dipolar solvation. Thus, the theory suggests that the experiments of Bingemann and Ernsting and those of Horng et al. studied essentially the polar solvation dynamics. The theoretical studies also suggest that the experimental investigations of Joo et al. which report a much faster and larger ultrafast component in the same series of solvents (J. Chem. Phys. 1996, 104, 6089) might have been more sensitive to the nonpolar part of solvation dynamics than the polar part. In addition, a discussion on the validity of the present theoretical approach is presented. In this theory the ultrafast component arises from almost frictionless inertial motion of the individual solvent molecules in the force field of its neighbors. *
On the origin of the anomalous ultraslow solvation dynamics in heterogeneous environments
Journal of Chemical Sciences, 2007
Many recent experimental studies have reported a surprising ultraslow component (even >10 ns) in the solvation dynamics of a polar probe in an organized assembly, the origin of which is not understood at present. Here we propose two molecular mechanisms in explanation. The first one involves the motion of the 'buried water' molecules (both translation and rotation), accompanied by cooperative relaxation ('local melting') of several surfactant chains. An estimate of the time is obtained by using an effective Rouse chain model of chain dynamics, coupled with a mean first passage time calculation. The second explanation invokes self-diffusion of the (di)polar probe itself from a less polar to a more polar region. This may also involve cooperative motion of the surfactant chains in the hydrophobic core, if the probe has a sizeable distribution inside the core prior to excitation, or escape of the probe to the bulk from the surface of the self-assembly. The second mechanism should result in the narrowing of the full width of the emission spectrum with time, which has indeed been observed in recent experiments. It is argued that both the mechanisms may give rise to an ultraslow time constant and may be applicable to different experimental situations. The effectiveness of solvation as a dynamical probe in such complex systems has been discussed.
Physical Review E, 1999
An approximate expression describing the density dependence of vibrational energy relaxation rates in fluids in terms of thermodynamic and transport parameters of the fluid is developed on the basis of a classical statistical mechanical theory of vibrational energy relaxation of highly excited molecules in polyatomic solvents. The energy relaxation rate is expressed via the friction coefficient, which describes the interaction between solute oscillator and solvent molecules. The corresponding force-force time correlation function is expressed in terms of the dynamic structure factor of the solvent and the force of interaction between solute and solvent molecules. Approximating the dynamic structure factor appropriately leads to expressions for the density dependence of vibrational relaxation rates in terms of thermophysical solvent parameters. Using these expressions the density dependence of vibrational relaxation rates in supercritical ethane and propane both in the vicinity of the critical point and far from it are evaluated and compared with measured relaxation rates obtained under the same physical conditions. ͓S1063-651X͑99͒15710-6͔
Chemical Physics, 1994
The modeling of the vibrational relaxation of molecules in simple liquids was tested using a number of implementations of isolated binary collision theory. These investigations are valuable for both the knowledge that is gained and because the models are compared to the well studied experimental system of iodine vibrationally relaxing in liquid xenon. The model started with a one-dimensional trajectory calculation in order to predict the transition probability of vibrational relaxation as a function of vibrational energy in the iodine molecule. The density dependence of the vibrational relaxation from experiment and molecular dynamics over a large part of the Lennard-Jones phase diagram are compared to the prediction of a number of implementations. For the most part the simple models are successful, however, problems with the simple models and their implementation are also discussed.