Annual Review of Physical Chemistry - Volume 57, 2006

This is the story of a young person who grew up in Tel-Aviv during the period of the establishment of the State of Israel and was inspired to become a physical chemist by the cultural environment, by the excellent high-school education, and by having been trained by some outstanding scientists at the Hebrew University of Jerusalem and, subsequently, by the intellectual environment and high-quality scientific endeavor at the University of Chicago. Since serving as the first chairman of the Chemistry Department of the newly formed Tel-Aviv University he has been immersed in research, in the training of young scientists, and in intensive and extensive international scientific collaboration. Together with the members of his “scientific family” he has explored the phenomena of energy acquisition, storage and disposal and structure-dynamics-function relations in large molecules, condensed phase, clusters and biomolecules, and is looking forward to many future adventures in physical chemistry.

“What to leave out and what to put in? That's the problem.”

Hugh Lofting, Doctor Dolittle's Zoo, 1925

In this review, we highlight the progress made in the development of 4D ultrafast electron diffraction (UED), crystallography (UEC), and microscopy (UEM) with a focus on concepts, methodologies, and prototypical applications. The joint atomic-scale resolutions in space and time, and sensitivity reached, make it possible to determine complex transient structures and assemblies in different phases. These applications include studies of isolated chemical reactions (molecular beams), interfaces, surfaces and nanocrystals, self-assembly, and 2D crystalline fatty-acid bilayers. In 4D UEM, we are now able, using timed, single-electron packets, to image nano-to-micro scale structures of materials and biological cells. Future applications of these methods are foreseen across areas of physics, chemistry, and biology.

Atmospheric carbon particles originate from natural sources and from human activity. The processes that lead to their formation are varied and include fossil fuel combustion, biomass burning, and mechanical stress and wear of carbonaceous materials. In this review, we examine recent work on the structure and composition of carbon aerosol particles, and we describe how they react with the atmospherically abundant gases ozone, oxygen, sulfur dioxide, nitric acid, and nitrogen oxides. The study of carbon particles in the laboratory has shown that chemical reactivity depends strongly on the type of carbon used and on experimental conditions such as temperature and humidity. The variability in the results demonstrates the difficulty in extrapolating laboratory results to atmospheric conditions and in explaining the role of carbon particles in processes such as global warming and environmental chemical cycling.

Quantum-classical Liouville dynamics can be used to study the properties of open quantum systems that are coupled to bath or environmental degrees of freedom whose dynamics can be approximated by classical equations of motion. In contrast to many open quantum system approaches, quantum-classical dynamics provides a detailed description of the bath molecules. Such a description is especially appropriate for the study of quantum rate processes, such as proton and electron transport, where the detailed dynamics of the bath has a strong influence on the quantum rate. The quantum-classical Liouville equation can also serve as a starting point for the derivation of reduced descriptions where all or some of the bath degrees of freedom are projected out. Quantum-classical Liouville dynamics can be simulated in terms of an ensemble of surface-hopping trajectories whose character differs from that in other surface-hopping schemes. The results of studies of proton transfer in condensed phase and reactive dynamics in a dissipative environment are presented to illustrate applications of the formalism.

The motion of polar molecules can be controlled by time-varying inhomogeneous electric fields. In a Stark decelerator, this is exploited to accelerate, transport, or decelerate a fraction of a molecular beam. When combined with a trap, the decelerator provides a means to store the molecules for times up to seconds. Here, we review our efforts to produce cold molecules via this technique. In particular, we present a new generation Stark decelerator and electrostatic trap that selects a significant part of a molecular beam pulse that can be loaded into the trap. Deceleration and trapping experiments using a beam of OH radicals are discussed.

The hydroxyl radical, OH, is the most important cleansing agent in the Earth's atmosphere, removing the majority of trace gases by oxidation, including greenhouse gases and CFC replacements. It is intimately involved in the chemistry that generates photochemical smog, which includes many substances harmful to health, such as ozone and particulate matter. In this review, the technique of laser-induced fluorescence for the detection of OH in the atmosphere is described, using as an example the fluorescence assay by gas expansion (FAGE) instrument developed at the University of Leeds. The comparison of measured OH concentrations at a given field site with those calculated by an atmospheric model, which is a mathematical representation of the underlying chemistry, provides one of the best methods to test whether the key chemical and physical processes are understood. Examples are given for field measurements made in clean and polluted environments.

Recent experimental and theoretical investigations of excitons in conjugated oligomer nanoaggregates, thin films, and crystals are reviewed. The review focuses on the technologically important unsubstituted oligo-phenylene vinylenes (OPVn) and oligo-thiophenes (OTn), which exhibit side-by-side herringbone crystal packing. Many of the salient photophysical properties displayed by OPVn and OTn solid phases, including the large Davydov splitting, the rich variety of peaks due to vibronic coupling in both absorption and emission, and the unusual behavior of the emission origin, are accounted for in a model including excitonic coupling between molecules, linear exciton-phonon coupling, and disorder.

Aerosols play a critical role in a diverse range of scientific disciplines. To characterize and quantify their role, it is essential that the fundamental details of mass and heat transfer between the aerosol particle and surrounding medium, the properties of multiphase droplets, and the coagulation of aerosol droplets be more fully explored. Elastic and inelastic light scattering can provide information on particle size, composition, morphology, and temperature. In addition, spatial inhomogeneities in composition and temperature can be probed. We review how such techniques can be used to develop an understanding of the chemical and physical dynamics of a single aerosol droplet.

Modern ultrafast spectroscopic techniques provide new opportunities to study chemical reaction dynamics in liquids and hold the possibility of obtaining much of the same detailed information available in gases. Vibrational energy transfer studies are the most advanced of the investigations and demonstrate that it is possible to observe state-specific pathways of energy flow within a vibrationally excited molecule (intramolecular vibrational relaxation) and into the surrounding solvent molecules (intermolecular energy transfer). Energy transfer in liquids and gases share many common aspects, but the presence of the solvent also alters the relaxation in both obvious and subtle ways. Photodissociation is amenable to similarly detailed study in liquids, and there are informative new measurements. Bimolecular reactions have received the least attention in state-resolved measurements in liquids, but the means to carry them much further now exist. Studying photodissociation and bimolecular reaction of molecules prepared with initial vibrational excitation in liquids is a realistic, but challenging, goal.

In recent years, developments in near-field techniques exploiting far-field illumination of a pointed, apertureless probe have demonstrated a newfound excitement. This is due in part to the advantages afforded by apertureless techniques that allow for the practical implementation of spectroscopic contrast mechanisms at length scales below 100 nm. These mechanisms include Raman and infrared absorption for chemical contrast, as well as materials contrast based on dielectric dispersion. In this review, we briefly describe the evolution of the field from the “classical” aperture-based approach toward the development of near-field optical microscopy with pointed probes. We highlight advances in state-of-the-art theory that describe the field distribution under an illuminated probe, as well as advances in the experimental implementation of scattering and excitation probe techniques.

A qualitatively new understanding of the nature of ions at the liquid water surface is emerging. Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models. Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions. This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations. Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging.

This contribution concerns a brief outline of structural environment models where correlated electronic structure response methods are utilized for the determination of nonlinear optical properties of molecules. The presentation provides theory and applications of a heterogeneous dielectric media model and a quantum mechanical–classical mechanical model at the level of correlated electronic structure response methods. The correlated electronic structure response methods include the multiconfigurational self-consistent field method.

Excitation of metal nanoparticles with subpicosecond laser pulses causes a rapid increase in the lattice temperature, which can impulsively excite the phonon modes of the particle that correlate with the expansion coordinates. The vibrational periods depend on the size, shape, and elastic constants of the particles. Thus, time-resolved spectroscopy can be used to examine the material properties of nanometer-sized objects. This review provides a brief overview of the steady-state and time-resolved electronic spectroscopy of metal particles, which is important for understanding why vibrational motion appears in transient absorption traces. I also describe how the vibrational modes observed in the experiments are assigned, and what information can be obtained from the measurements. Our work has been mainly concerned with noble metal particles (gold and silver) in aqueous solution. The different shapes that have been examined to date include spheres, rods, and triangles, all with different sizes.

Ion pair dissociation processes may be studied using coherent vacuum ultraviolet laser sources in a manner entirely analogous to photoelectron spectroscopy, albeit with the anion playing the role of a heavy electron. If the excitation energy is above the dissociation energy and the kinetic energy of the fragment is measured using ion imaging, this approach is termed ion pair imaging spectroscopy (IPIS) and is related to conventional photoelectron spectroscopy. If the excitation energy is just below the dissociation energy and pulsed-field dissociation is employed, this approach is analogous to mass analyzed threshold ionization (MATI) spectroscopy and is termed threshold ion pair production spectroscopy (TIPPS). These approaches provide a novel means of investigating ion thermochemistry and spectroscopy and superexcited state decay dynamics at high resolution.

With the rapidly changing materials needs of modern microelectronics, germanium provides an opportunity for future-generation devices. Controlling germanium interfaces will be essential for this purpose. We review germanium surface reactivity, beginning with a description of the most commonly used surfaces, Ge(100) and Ge(111). An analysis of oxide formation shows why the poor oxide properties have hindered practical use of germanium to date. This is followed by an examination of alternate means of surface passivation, with particular attention given to sulfide, chloride, and hydride termination. Specific tailoring of the interface properties is possible through organic functionalization. The few solution functionalization methods that have been studied are reviewed. Vacuum functionalization has been studied to a much greater extent, with dative bonding and cycloaddition reactions emerging as principle reaction mechanisms. These are reviewed through molecular reaction studies that demonstrate the versatility of the germanium surface.

Organic molecules adsorbed on solid surfaces display a fascinating variety of new physical and chemical phenomena ranging from self-assembly and molecular recognition to nonlinear optical properties and current rectification. Both the fundamental interest in these systems and the promise of technological applications have motivated a strong research effort in understanding and controlling these properties. Scanning tunneling microscopy (STM) and, in particular, its ability to manipulate individual adsorbed molecules, has become a powerful tool for studying the adsorption geometry and the conformation and dynamics of single molecules and molecular aggregates. Here we review selected case studies demonstrating the enormous capabilities of STM manipulations to explore basic physiochemical properties of adsorbed molecules. In particular, we emphasize the role of STM manipulations in studying the coupling between the multiple degrees of freedom of adsorbed molecules, the phenomenon of molecular molding, and the possibility of creating and breaking individual chemical bonds in a controlled manner, i.e., the concept of single-molecule chemistry.

Recent studies using a Raman microscope have shown that single protein crystals provide an ideal platform to undertake Raman difference spectroscopic analyses under nonresonance conditions. This approach, termed Raman crystallography, provides a means of characterizing chemical events within the crystal such as ligand binding and enzyme reactions. In many cases Raman crystallography goes hand in hand with X-ray crystallographic studies because the Raman results can inform the X-ray crystallographer about the status of chemical events in the crystal prior to flash freezing and X-ray analysis. In turn, the combined data from the Raman and X-ray analyses are highly synergistic and offer novel perspectives on structure and dynamics in enzyme active sites. In a related area, protein misfolding, Raman microscopy can provide detailed insights into the chemistry of the amyloid plaques associated with Alzheimer's disease and into the intermediates on the α-synuclein protein misfolding pathway implicated in Parkinson's disease.

Femtosecond time-resolved photoelectron imaging (TRPEI) is a variant of time-resolved photoelectron spectroscopy used in the study of gas-phase photoinduced dynamics. A new observable, time-dependent photoionization-differential cross section provides useful information on wave-packet motions, electronic dephasing, and photoionization dynamics. This review describes fundamental issues and the most recent works involving TRPEI.