Ultrafast solvation dynamics in water: Isotope effects and comparison with experimental results

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 water, although solvation relaxation is more non-local. In addition, we find a remarkable independence of the main relaxation channels on the newly created charges sign. Although the methodology is applied here to the simplest solute case, the approach is rather general, and it should be at least equally useful in more realistic and complex scenarios.

Section: A Energy Gap Relaxation Functionsupporting

confidence: 91%

Solvation Dynamics in Liquid Water. 1. Ultrafast Energy Fluxes

2015

“…[17][18][19][20][21][22][23][24][25][26][27][28][29] Thanks to this system's simplicity, the frequency shift is identical to the excited state ion-water solvent Coulomb energy. Consequently, the resulting excess energy's time variation could be readily expressed in terms of a sum of contributions of work on the solvent (and solute) configurational degrees of freedom.…”

Section: Introductionmentioning

confidence: 99%

Solvation Dynamics in Water: 2. Energy Fluxes on Excited- and Ground-State Surfaces

2016

This series' first installment introduced an approach to solvation dynamics focused on expressing the emission frequency shift (following electronic excitation of, and resulting charge change or redistribution in, a solute) in terms of energy fluxes, a work and power perspective. This approach, which had been previously exploited for rotational and vibrational excitation-induced energy flow, has the novel advantage of providing a quantitative view and understanding of the molecular level mechanisms involved in the solvation dynamics, via tracing of the energy flow induced by the electronic excitation's charge change or redistribution in the solute. This new methodology, which was illustrated for the case in which only the excited electronic state surface contributes to the frequency shift (ionization of a monatomic solute in water), is here extended to the general case, in which both the excited and ground electronic states may contribute. Simple monatomic solute model variations allow discussion of the (sometimes surprising) issues involved in assessing each surface's contribution. The calculation of properly defined energy fluxes/work allows a more complete understanding of the solvation dynamics even when the real work for one of the surfaces does not directly contribute to the frequency shift, an aspect further emphasizing the utility of an energy flux approach.

“…Its first application to solvation dynamics in I was to the classic idealized model system of an initially neutral monatomic solute in water which instantaneously acquires a positive/negative unit charge. [15][16][17][18][19][20][21][22][23][24][25][26] In II, the formalism was extended to the general case, for which both excited and ground electronic states of the solute are characterized by finite charge distributions.…”

Section: Introductionmentioning

confidence: 99%

Solvation Dynamics in Liquid Water. III. Energy Fluxes and Structural Changes

2017

In previous installments it has been shown how a detailed analysis of energy fluxes induced by electronic excitation of a solute can provide a quantitative understanding of the dominant molecular energy flow channels characterizing solvation-and in particular, hydration-relaxation dynamics. Here this work and power approach is complemented with a detailed characterization of the changes induced by such energy fluxes. We first examine the water solvent's spatial and orientational distributions and the assorted energy fluxes in the various hydration shells of the solute to provide a molecular picture of the relaxation. The latter analysis is also used to address the issue of a possible "inverse snowball" effect, an ansatz concerning the time scales of the different hydration shells to reach equilibrium. We then establish a link between the instantaneous torque, exerted on the water solvent neighbors' principal rotational axes immediately after excitation and the final energy transferred into those librational motions, which are the dominant short-time energy receptor.