Annual Review of Physical Chemistry - Volume 63, 2012

We review the first ten years of attosecond science with a selection of recent highlights and trends and give an outlook on future directions. After introducing the main spectroscopic tools, we give recent examples of representative experiments employing them. Some of the most fundamental processes in nature have been studied with some results initiating controversial discussions. Experiments on the dynamics of single-photon ionization illustrate the importance of subtle effects on such extreme timescales and lead us to question some of the well-established assumptions in this field. Attosecond transient absorption, as the first all-optical approach to resolve attosecond dynamics, has been used to study electron wave packet interferences in helium. The attoclock, a recent method providing attosecond time resolution without the explicit need for attosecond pulses, has been used to investigate electron tunneling dynamics and geometry. Pushing the frontiers in attosecond quantum mechanics with increasing temporal and spatial resolution and often limited theoretical models results in unexpected observations. At the same time, attosecond science continues to expand into more complex solid-state and molecular systems, where it starts to have impact beyond its traditional grounds.

For more than two decades a cadre of physical chemists has focused on understanding the formation processes, chemical composition, and chemical kinetics of atmospheric aerosol particles and droplets with diameters ranging from a few nanometers to ∼10,000 nm. They have adapted or invented a range of fundamental experimental and theoretical tools to investigate the thermochemistry, mass transport, and chemical kinetics of processes occurring at nanoscale gas-liquid and gas-solid interfaces for a wide range of nonideal, real-world substances. State-of-the-art laboratory methods devised to study molecular spectroscopy, chemical kinetics, and molecular dynamics also have been incorporated into field measurement instruments that are deployed routinely on research aircraft, ships, and mobile laboratories as well as at field sites from megacities to the most remote jungle, desert, and polar locations. These instruments can now provide real-time, size-resolved aerosol particle physical property and chemical composition data anywhere in Earth's troposphere and lower stratosphere.

Recent advances in the growing field of nanoemulsions are opening up new applications in many areas such as pharmaceuticals, foods, and cosmetics. Moreover, highly controlled nanoemulsions can also serve as excellent model systems for investigating basic scientific questions about soft matter. Here, we highlight some of the most recent developments in nanoemulsions, focusing on methods of formation, surface modification, material properties, and characterization. These developments provide insight into the substantial advantages that nanoemulsions can offer over their microscale emulsion counterparts.

Super-resolution imaging methods now can provide spatial resolution that is well below the diffraction limit approaching virtually molecular resolution. They can be applied to biological samples and provide new and exciting views on the structural organization of cells and the dynamics of biomolecular assemblies on wide timescales. These revolutionary developments come with novel requirements for fluorescent probes, labeling techniques, and data interpretation strategies. Synthetic fluorophores have a small size, are available in many colors spanning the whole spectrum, and can easily be chemically modified and used for stoichiometric labeling of proteins in live cells. Because of their brightness, their photostability, and their ability to be operated as photoswitchable fluorophores even in living cells under physiological conditions, synthetic fluorophores have the potential to substantially accelerate the broad application of live-cell super-resolution imaging methods.

The recent literature on photochemical and photoelectrochemical reductions of CO2 is reviewed. The different methods of achieving light absorption, electron-hole separation, and electrochemical reduction of CO2 are considered. Energy gap matching for reduction of CO2 to different products, including CO, formic acid, and methanol, is used to identify the most promising systems. Different approaches to lowering overpotentials and achieving high chemical selectivities by employing catalysts are described and compared.

Neurotrophins are a family of target-derived growth factors that support survival, development, and maintenance of innervating neurons. Owing to the unique architecture of neurons, neurotrophins that act locally on the axonal terminals must convey their signals across the entire axon for subsequent regulation of gene transcription in the cell nucleus. This long-distance retrograde signaling, a motor-driven process that can take hours or days, has been a subject of intense interest. In the last decade, live-cell imaging with high sensitivity has significantly increased our capability to track the transport of neurotrophins, their receptors, and subsequent signals in real time. This review summarizes recent research progress in understanding neurotrophin-receptor interactions at the axonal terminal and their transport dynamics along the axon. We emphasize high-resolution studies at the single-molecule level and also discuss recent technical advances in the field.

Single-molecule fluorescence spectroscopy and super-resolution microscopy are important elements of the ongoing technical revolution to reveal biochemical and cellular processes in unprecedented clarity and precision. Demands placed on the photophysical properties of the fluorophores are stringent and drive the choice of appropriate probes. Such fluorophores are not simple light bulbs of a certain color and brightness but instead have their own “personalities” regarding spectroscopic parameters, redox properties, size, water solubility, photostability, and several other factors. Here, we review the photophysics of fluorescent probes, both organic fluorophores and fluorescent proteins, used in applications such as particle tracking, single-molecule FRET, stoichiometry determination, and super-resolution imaging. Of particular interest is the thiol-induced blinking of Cy5, a curse for single-molecule biophysical studies that was later overcome using Trolox through a reducing/oxidizing system but a boon for super-resolution imaging owing to the controllable photoswitching. Understanding photophysics is critical in the design and interpretation of single-molecule experiments.

Well-ordered, thin oxide films have drawn some attention in recent years as suitable oxide supports for modeling highly dispersed metal catalysts at the atomic scale. It turned out, however, that ultrathin oxide films may exhibit interesting catalytic properties in their own right. In this review, we discuss phenomena specifically connected to ultrathin oxide films to explain and understand the physicochemical basis of their reactivity in oxidation reactions. Two sets of systems are discussed, i.e., transition metal oxide films grown on metal substrates and native oxide films formed upon oxidation of metal surfaces.

Free-electron lasers are fourth-generation light sources that deliver extremely intense (>10¹² photons per pulse), ultrashort (∼10⁻¹⁴ s = 10 fs) light pulses at up to kilohertz repetition rates with unprecedented coherence properties and span a broad wavelength regime from soft (∼10 eV) to hard X-ray energies (∼15 keV). They thus enable a whole suite of novel experiments in molecular physics and chemistry: Inspecting radiation-induced reactions in cold molecular ions provides unprecedented insight into the photochemistry of interstellar clouds and upper planetary atmospheres; double core-hole photoelectron spectroscopy offers enhanced sensitivity for chemical analysis; the dynamics of highly excited molecular states, pumped by vacuum ultraviolet pulses, can be inspected; and vacuum ultraviolet or X-ray probe pulses generally hold the promise to trace chemical reactions along an entire reaction coordinate with atomic spatial and temporal resolution. This review intends to provide a first overview on upcoming possibilities, emerging technologies, pioneering results, and future perspectives in this exciting field.

Solid-state nuclear magnetic resonance (SSNMR) magic angle spinning (MAS) can be used to record high-resolution data dominated by site-specific information. Although MAS introduces high resolution by attenuating the anisotropic broadening, it also suppresses the nuclear dipole-dipole distance information that is the source of most structural data in the spectra. Such information can be reintroduced coherently and thus selectively by the application of a carefully chosen sequence of radiofrequency pulses, an approach that was introduced 20 years ago and is referred to as dipolar recoupling. This review presents the establishment of recoupling techniques in SSNMR and recalls the major steps achieved by the community throughout the last two decades. This review also presents emerging techniques and their corresponding new concepts. Finally, we present some recent developments based on second-order recoupling mechanisms and discuss their implications regarding dipolar truncation and the possibility to extract structural constraints in uniformly labeled systems.