Session L47: Chemical Physics of Condensed Phase Dynamics

Quantum thermodynamics for systems out of equilibrium strongly interacting with their surroundings.

The performance of molecular and nanoscopic systems as nanodevices capable of transforming some form of energy into work is greatly determined by their interaction with the surroundings. When the interaction is strong, one may redefine the part of the universe that can be regarded as the system in order to achieve a full dynamic as well as thermodynamic description. In this work we study a single level strongly interacting with a fermionic bath that undergoes slow driving. The dynamics of the system is estimated using Nonequilibrum Green's functions NEGF. We explore different alternatives for the system bath separation under driving, identifying the corresponding terms for heat and energy transferred as well as the dissipation term. We also formulate the problem in terms of the renormalized spectral density. Our investigations indicate that none of the alternatives can fully reproduce the laws of thermodynamics suggesting that the notion of heat, expressed as the expectation value of some part of the Hamiltonian, is responsible for the inconsistency.   

A Variational Statistical-Field Theory for Polar Liquid Mixtures

Using a variational field-theoretic approach, we derive a molecularly-based theory for polar liquid mixtures. The resulting theory consists of simple algebraic expressions for the free energy of mixing and the dielectric constant as functions of mixture composition. Using only the dielectric constants and the molar volumes of the pure liquid constituents, the theory evaluates the mixture dielectric constants in good agreement with the experimental values for a wide range of liquid mixtures, without using adjustable parameters. In addition, the theory predicts that liquids with similar dielectric constants and molar volumes dissolve well in each other, while sufficient disparity in these parameters result in phase separation. The calculated miscibility map on the dielectric constant-molar volume axes agrees well with known experimental observations for a large number of liquid pairs. Thus the theory provides a quantification for the well-known empirical “like-dissolves-like” rule.   

Role of solvent environments in single molecule conductance used insulator-modified mechanically controlled break junctions.

We present a method for studying the effects of polar solvents on charge transport through organic/biological single molecules by developing solvent-compatible mechanically controlled break junctions of gold coated with a thin layer of aluminium oxide using plasma enhanced atomic layer deposition (ALD). The optimal oxide thickness was experimentally determined to be 15 nm deposited at ALD operating temperature of 300\textdegree C which yielded atomically sharp electrodes and reproducible single-barrier tunnelling behaviour across a wide conductance range between 1 G$_{\mathrm{0\thinspace }}$and 10$^{\mathrm{-7\thinspace }}$G$_{\mathrm{0.}}^{\mathrm{\thinspace }}$The insulator protected MCBJ devices were found to be effective in various solvents such as deionized water, phosphate buffered saline, methanol, acetonitrile and dichlorobenzene. The yield of molecular junctions using such insulated electrodes was tested by developing a chemical protocol for synthesizing an amphipathic form of oligo-phenylene ethynylene (OPE3-PEO) with thioacetate anchoring groups. This work has further applications in studying effects of solvation, dipole orientation and other thermodynamic interactions on charge transport.   

Inferring mixture Gibbs free energies from static light scattering data

We describe a light scattering partial differential equation for the free energy of mixing that applies to connected, isotropic ternary and quaternary liquid composition domains, including restricted domains which may not touch all binary axes. For restricted domains, contrasting light scattering efficiency patterns obtained at different wavelengths can correspond to the same underlying free energy, and supplement the available information. We discuss well-posed problems for this fully nonlinear, degenerate elliptic partial differential equation. Using Monte Carlo simulations, we provide estimates of the overall system measurement time and sample spacing needed to determine the free energy to a desired degree of accuracy, and indicate how measurement time depends on instrument throughput. These methods provide a way to use static light scattering to measure, directly, mixing free energies of many systems that contain liquid domains.   

\textit{In situ/operando} soft x-ray spectroscopy characterization of ion solvation and catalysis.

Many important systems especially in energy-related regime are based on the complexity of material architecture, chemistry and interactions among constituents within. To understand and thus ultimately control the varying applications calls for in-situ/operando characterization tools. We will present the recent development of the in-situ/operando soft X-ray spectroscopic in the studies of catalytic and alkali ion solvation under bias condition, and reveal how to overcome the challenge that soft X-rays cannot easily peek into the high-pressure catalytic cells or liquid electrochemical cells. Also the different feasible detection approaches can provide surface and bulk sensitivity experimentally from those \textit{in-situ} cells. The unique design of \textit{in-situ/operando} soft X-ray spectroscopy instrumentation and fabrication principle with examples in Ca, Na, Mg based solutions at ambient pressure/temperature and high temperature (\textasciitilde 250\textdegree C) gas catalysis will be shown.   

Excited statemolecular dynamics in polarizable environments

Many experimental measurements of molecular systems are performed in solutions, where the solvent forms a polarizable environment around the solute. The effects of this on important molecular processes such as vibrational relaxation or chemical reaction are often significant. In this talk, recent developements in efficiently simulating solvation effects in quantum molecular dynamics for excited electronic states are presented. These methods fall into the category of multiscale quantum mechanics/continuum methods. To adequately describe polarization of the solvent by the electronically excited states of molecules, state-specific methods have been pursued which allow for polarization effects based on the excited state charge density. Variational formulations of solvation models in linear response time-dependent density functional theory are described which allow analytical gradients and efficient molecular dynamics propagation. Further, recently developed simulation methods for nonequilibrium solvation effects are demonstrated.   

Evolution of molecular crystal optical phonons near structural phase transitions

Molecular crystals are increasingly important photonic and electronic materials. For example organic semiconductors are lightweight compared to inorganic semiconductors and have inexpensive scale up processing with roll to roll printing[1]. However their implementation is limited by their environmental sensitivity, in part arising from the weak intermolecular interactions of the crystal. These weak interactions result in optical phonons in the terahertz frequency range. We examine the evolution of intermolecular interactions near structural phase transitions by measuring the optical phonons as a function of temperature and crystal orientation using terahertz time-domain spectroscopy. The measured orientation dependence of the resonances provides an additional constraint for comparison of the observed spectra with the density functional calculations [2], enabling us to follow specific phonon modes. We observe crystal reorganization near 350 K for oxalic acid as it transforms from dihydrate to anhydrous form. We also report the first THz spectra for the molecular crystal fructose through its melting point. \\ 1. Krebs, FC., et al. J.Materials Chem., 2009, 19(30): p. 5442-5451 \\ 2. Singh, R., et al. J. Phys. Chem. C, 2012. 116(42): p. 10359–10364.   

Transient ultrafast coherent spectroscopy of 2-propanol

We use transient coherent spontaneous Raman spectroscopy to measure the coherence lifetimes of vibrational states in liquid propanol. By creating single-photon-level collective excitations of the vibrational states in the system we observe coherence oscillations due to simultaneous excitation of the 2885 cm$^{\mathrm{-1}}$, 2938 cm$^{\mathrm{-1}}$, and 2976 cm$^{\mathrm{-1}}$ modes. These lifetimes and oscillation frequencies agree with frequency-domain lineshape measurements.   

Molecular Level-Crossing Dynamics in Condensed Phase from an Optical Hanle Effect Perspective

The molecular optical Hanle effect typically involves field-induced level-splitting to measure excited state lifetimes. Here I will discuss how curve-crossing dynamics probed with a single shaped pulse in the weak field regime can be understood from a perspective of the molecular optical Hanle effect. I will discuss how pulse shaping can be utilized to investigate the curve-crossing dynamics occurring in a large organic molecule in solution. Both Experimental and theoretical results will be presented.   

Femtosecond Heterodyne Transient Grating Studies of Nonradiative Decay of the S$_2$ (1$^1$B$_u$$^+$) State of Peridinin: Detection and Spectroscopic Assignment of an S$_x$ Intermediate State

Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative relaxation dynamics of peridinin from the S$_2$ state to the S$_1$ (2$^1$A$_g$$^-$) state in methanol. A global target analysis indicates that S$_2$ decays in 12 fs to populate an intermediate state, S$_x$. The absorption and dispersion components of the transient grating signal exhibit a response that is very similar to that of $\beta$-carotene in benzonitrile solution. Numerical simulation of the experimental data indicates that the excited state absorption transition from S$_x$ has a larger oscillator strength than that of S$_1$, which rules out an assignment of S$_x$ to a vibrationally excited S$_1$ state. The lifetime of S$_x$ is found to be strongly dependent on the polar solvation timescale. This result indicates that nonradiative decay from S$_x$ to S$_1$ involves large-amplitude torsional motions and a concomitant formation of intramolecular charge transfer character. The present work provides the first evidence that peridinin has an ultrashort S$_2$ lifetime owing to the onset of torsional motions and shows that the S$_x$ acts as an active state for excitation energy transfer to chlorophyll in light-harvesting proteins.   

Isomerization of one molecule observed through tip enhanced Raman spectroscopy

While exploring photoisomerization of azobenzyl thiols (ABT) adsorbed on Au(111), through joint scanning tunneling microscopy (STM) and tip-enhanced Raman scattering (TERS) studies, the reversible photoisomerization of one molecule is captured in TERS trajectories. The apparently heterogeneously photo-catalyzed reaction is assigned to cis-trans isomerization of an outlier, which is chemisorbed on the silver tip of the STM. In order to clarify the role of the silver tip of the STM, we perform systematic density functional theory (DFT) calculations. The results show that compared with the case on the flat Ag(111) surface, the energy difference between trans and cis states of ABT decrease as we add one silver atom or a tetrahedron silver cluster on Ag(111) surface which mimic the geometry of a silver tip. In particular, the trans stretches away from the surface on the tetrahedral silver cluster, and the energy difference between trans and cis decreases to 0.27 eV, from \textasciitilde 1 eV for ABT on the flat Ag(111) surface. This significantly increases the possibility of cis-trans isomerization, as observed in our experiments.   

Femtosecond Carrier Dynamics in Gold-MoS$_{\mathrm{2}}$ Hybrid Nanostructures

Small plasmonic nanoparticles are known to efficiently generate energetic hot carriers [1] that can be harnessed by injecting them across a Schottky barrier. To understand the role of plasmon-induced hot carrier generation across Schottky junctions in photocatalytic processes, we synthesized quasi-2D MoS$_{\mathrm{2}}$ monolayer flakes decorated with Au nanoparticles in ethanol. Our goal is to study ultrafast plasmon induced electron injection from Au nanospheres into MoS$_{\mathrm{2}}$ monolayer flakes. We will present femtosecond transient absorption measurements on MoS$_{\mathrm{2}}$/Au hybrid nanoparticles in ethanol solvent, and compare them with neat MoS$_{\mathrm{2}}$ flakes in ethanol. \newline ~[1] Nano Letters, 2015, 15 (9), p 6155   

Is DNA a metal, semiconductor or insulator? A theoretical approach

Over the last years, scientific interest for designing and making low dimensional electronic devices with traditional or novel materials has been increased. These experimental and theoretical researches in electronic properties at molecular scale are looking for developing efficient devices able to carry out tasks which are currently done by silicon transistors and devices. Among the new materials DNA strands are highlighted, but the experimental results have been contradictories pointing to behaviors as conductor, semiconductor or insulator. To contribute to the understanding of the origin of the disparity of the measurements, we perform a numerical calculation of the electrical conductance of DNA segments, modeled as 1D disordered finite chains. The system is described into a Tight binding model with nearest neighbor interactions and a s orbital per site. Hydration effects are included as random variations of self-energies. The electronic current as a function of applied bias is calculated using Launder formalism, where the transmission probability is determined into the transfer matrix formalism. We find a conductor-to-semiconductor-to-insulator transition as a function of the three effects taken into account: chain size, intrinsic disorder, and hydration   

Femtosecond Heterodyne Transient Grating Detection of Conformational Dynamics in the S$_0$ (1$^1$A$_g$$^-$) State of Carotenoids After Nonradiative Decay of the S$_ 2$ (1$^1$B$_u$$^+$) State

Transient grating spectroscopy was used to study the dynamics of nonradiative decay of the S$_1$ (2$^1$A$_g$$^-$) state in ß-carotene and peridinin after optical preparation of the S$_2$) state. The kinetics of the recovery of the absorption and dispersion components of the third-order signal exhibit significantly different time constants. For $\beta$-carotene in benzonitrile, the absorption and dispersion recovery time constants are 11.6 and 10.2 ps. For peridinin in methanol, the time constants are 9.9 and 7.4 ps. These results indicate that the initial product of the decay of the S$_1$ state is a conformationally displaced structure. The decay rate for the S$_1$ state and the conformational relaxation rate are both slowed in peridinin as the polarity of the solvent decreases; in ethyl acetate, the conformational relaxation time constant is 45 ps, which rules out a dominant contribution from vibrational cooling. These results indicate that the S$_1$ state develops intramolecular charge transfer character owing to distortions along torsional and out-of-plane coordinates, with a pyramidal structure favored as the most stable conformation. Recovery of the photoselected ground state conformation involves a reverse charge-transfer event followed by relaxation to a planar structure.