Ultrafast solvent response upon a change of the solute size in non-polar supercritical fluids

Absorption and emission lineshapes and solvation dynamics of NO in supercritical Ar

We perform a theoretical study of electronic spectroscopy of dilute NO in supercritical Ar fluid. Absorption and emission lineshapes for the A(2)Sigma(+)<--X(2)Pi Rydberg transition of NO in argon have been previously measured and simulated, which yielded results for the NO/Ar ground- and excited-state pair potentials [Larregaray et al., Chem. Phys. 308, 13 (2005)]. Using these potentials, we have performed molecular dynamics simulations and theoretical statistical mechanical calculations of absorption and emission lineshapes and nonequilibrium solvation correlation functions for a wide range of solvent densities and temperatures. Theory was shown to be in good agreement with simulation. Linear response treatment of solvation dynamics was shown to break down at near-critical temperature due to dramatic change in the solute-solvent microstructure upon solute excitation to the Rydberg state and the concomitant increase of the solute size.

Electronic spectroscopy of NO–(Rg)x complexes (Rg=Ne,Ar) via the 4s and 3d Rydberg states

We have employed (2 + 1) resonance enhanced multiphoton ionization spectroscopy to investigate the 3d and 4s Rydberg states of the NO molecule when bound to the surface of Rg(x) clusters (Rg = rare gas). We observe that the spectra of the NO-Ar(x) species converge in appearance as x increases, and this is discussed in terms of two Rg atoms interacting with the NO+ core, with other Rg atoms being "outside" the Rydberg orbital. We show that the interaction of each of the Rg atoms with the NO is essentially independent for the NO-Rg2 complexes: both by comparing our spectra for Rydberg states of NO-Rg and NO-Rg2, and from the results of ab initio calculations on NO+ - Rg and NO+ - Rg2. In addition, we discuss the disappearance of some electronic bands upon complexation in terms of Franck-Condon factors that are very sensitive to the angular coordinate. We relate our results to those of the bulk by comparing to the previously reported electronic spectroscopy of NO in both Rg matrices and He nanodroplets.

The molecular origins of nonlinear response in solute energy relaxation: The example of high-energy rotational relaxation

A key step in solution-phase chemical reactions is often the removal of excess internal energy from the product. Yet, the way one typically studies this process is to follow the relaxation of a solute that has been excited into some distribution of excited states quite different from that produced by any reaction of interest. That the effects of these different excitations can frequently be ignored is a consequence of the near universality of linear-response behavior, the idea that relaxation dynamics is determined by the solvent fluctuations (which may not be all that different for different kinds of solute excitation). Nonetheless, there are some clear examples of linear-response breakdowns seen in solute relaxation, including a recent theoretical and experimental study of rapidly rotating diatomics in liquids. In this paper we use this rotational relaxation example to carry out a theoretical exploration of the conditions that lead to linear-response failure. Some features common to all of the linear-response breakdowns studied to date, including our example, are that the initial solute preparation is far from equilibrium, that the subsequent relaxation promotes a significant rearrangement of the liquid structure, and that the nonequilibrium response is nonstationary. However, we show that none of these phenomena is enough to guarantee a nonlinear response. One also needs a sufficient separation between the solute time scale and that of the solvent geometry evolution. We illustrate these points by demonstrating precisely how our relaxation rate is tied to our liquid-structural evolution, how we can quantitatively account for the initial nonstationarity of our effective rotational friction, and how one can tune our rotational relaxation into and out of linear response.

The ultrafast structural response of solid parahydrogen: A complementary experimental/simulation investigation

We present a complete characterization, based on femtosecond pump-probe spectroscopy and molecular dynamics simulations, of the ultrafast dynamics of electronic bubble formation in solid parahydrogen upon impulsive excitation of impurity-doped sites, which correlate with the lowest Rydberg state of the NO impurity. The high temporal resolution of the experiment allows us to identify three time scales in the structural dynamics. A first ultrafast expansion (<150 fs), associated with the release of approximately 80% of the excess energy available to the system after excitation, is accompanied by a transient narrowing of the spatial distribution of the first shell of H2 molecules around the impurity. In a subsequent stage (up to approximately 800 fs), the cavity expansion slows down, and energy starts to flow irreversibly into the crystal. Finally, the lattice undergoes a slow structural reorganization at the impurity site (5-10 ps). A weak low-frequency recurrence, probably associated with an elastic response of the crystal, is observed at approximately 10 ps. The absence of polarization dependence indicates that the dynamics is largely dominated by translational (radial) motions of the molecules surrounding NO and not by the rotational motion of the impurity. Molecular dynamics simulations with temperature corrections, to mimic zero-point fluctuations, fully support the experimental results and show that the bubble model is suited to describe the dynamics of the system. It appears that the response of the medium around the impurity at short times is typical of a liquid solvent rather than that of a solid.

Aqueous Solvation Dynamics at Metal Oxide Surfaces

Broadband transient absorption (TA) spectroscopy, three-pulse photon echo peak shift (3PEPS), and anisotropy decay measurements were used to study the solvation dynamics in bulk water and interfacial water at ZrO(2) surfaces, using Eosin Y as a probe. The 3PEPS results show a multiexponential behavior with two subpicosecond components that are similar in bulk and interfacial water, while a third component of several picoseconds is significantly lengthened at the interface. The bandwidth correlation function from TA spectra exhibits the same behavior, and the TA spectra are well reproduced using the doorway-window picture with the time constants from PEPS. Our results suggest that interfacial water is restricted to a thickness of less than 5 A. Also the high-frequency collective dynamics of water does not seem to be affected by the interface. On the other hand, the increase of the third component may point to a slowing down of diffusional motion at the interface, although other effects, may play a role, which are discussed.

Polar Solvation and Solvation Dynamics in Supercritical CHF3: Results from Experiment and Simulation

Solvation dynamics of the probe trans-4-(dimethylamino)-4'-cyanostilbene (DCS) have been measured in supercritical fluoroform at 310 K (1.04 Tc) and solvent densities over the range 1.4-2.0 rho(c) using optical Kerr-gated emission spectroscopy. Steady-state measurements and computer simulations of this and the related system coumarin 153 (C153) in fluoroform are used to help interpret the observed dynamics. The solvent contribution to the Stokes shift of DCS is estimated to be 2300 +/- 400 cm(-1) and nearly density independent over the range (0.7-2.0)rho(c). Spectral response functions are bimodal and can be fit to biexponential functions having time constants of approximately 0.5 ps (85%) and 3-10 ps (15%) over the observable range ((1.4-2.0)rho(c)). Computer simulations based on a 2-site model of fluoroform and assuming an electrostatic solvation mechanism appear to properly account for the magnitude and weak density dependence of the Stokes shifts but predict much faster solvation than is observed. Possible reasons for the discrepancy are discussed.

The effects of solute-solvent electrostatic interactions on solvation dynamics in supercritical CO2

We present here the results of molecular-dynamics simulation of solvation dynamics in supercritical CO(2) at a temperature of about 1.05T(c), where T(c) is the critical temperature, and at a series of densities ranging from 0.4 to 2.0 of the critical density rho(c). We focus on electrostatic solvation dynamics, representing the electronic excitation of the chromophore as a change in its charge distribution from a quadrupolar-symmetry ground state to a dipolar excited state. Two perturbations are considered, corresponding to different magnitudes of solute excited-state dipoles, denoted as d5 and d8. The d8 solute is more attractive, leading to a larger enhancement in CO(2) clustering upon solute electronic excitation. This has a large impact on solvation dynamics, especially at densities below rho(c). At these densities, solvation dynamics is much slower for the d8 than for the d5 solute. For both solutes, solvation dynamics becomes faster at densities above rho(c) at which solvent clustering diminishes. We show that the slowest solvation time scale is associated with solvent clustering and we relate it to solute-solvent mutual translational diffusion and the extent of change in effective local density resulting from solute electronic excitation.