Annual review of physical chemistry最新文献

Actinides are inherently unstable and undergo nuclear decay processes with a concurrent release of energy. Consequently, they are used for nuclear power generation, nuclear weapons, and nuclear medicine. However, the radioactive decay processes also pose significant technological problems for the safe treatment and storage of spent nuclear materials. Cost-effective extraction of the actinides is the key first step in the remediation of nuclear waste, but the appropriate chemical means have yet to be determined. Our present understanding of the chemistry of actinides is limited, with the role of the 5f electrons posing a set of particularly challenging questions. The work reported here is focused on the use of electronic spectroscopy to probe the bonding of small molecules in the gas phase that contains thorium or uranium. Analyses of these data, carried out within the framework of ligand field theory, reveal clear evidence that the 5f electrons are spectators that retain their atomic metal ion character.

Dynamics of molecular interactions with solid surfaces, such as scattering, adsorption/desorption, diffusion, and reaction, are affected by energy dissipation at surfaces. Recent progress in experimental studies of surface dynamics has stimulated intense interest in theoretical investigation of microscopic mechanisms and pathways of energy transfer. This review summarizes recent developments in modeling such processes, emphasizing new understandings of electronically adiabatic and nonadiabatic energy dissipation mechanisms and dynamics in representative systems, using various theoretical methods. In particular, machine learning has been leveraged to represent high-dimensional adiabatic potential energy surfaces, electronic friction tensors, and effective multielectron diabatic Hamiltonians. When integrated with mixed quantum-classical dynamics methods, such as molecular dynamics with electronic friction and independent electron surface hopping, these first-principles-based simulations provided unprecedented insights into the roles played by adiabatic and nonadiabatic energy dissipation channels in surface dynamics and in-depth interpretation of experimental observations.

In my undergraduate studies at the Massachusetts Institute of Technology (MIT), the short-lived chemical physics major allowed me to evade a number of courses required for chemistry majors. Thus, it was possible to take many physics courses and most of the advanced PhD-level courses in physical chemistry. I also took the introductory electrical engineering course in computer programming. The latter allowed me to write lots of computer code as a part of my (passing, but largely unsuccessful) senior thesis directed graciously by Professor Walter Thorson. As recommended by MIT Professor John C. Slater, I moved to Stanford University with Professor Frank Harris as my PhD supervisor. Frank was the perfect advisor for me, providing very close direction during the first year, and then allowing me to develop more and more independently. Within a few days of my twenty-fifth birthday, I became an assistant professor of chemistry at the University of California, Berkeley. Eighteen years later, I moved to the University of Georgia as director of a new research institute.

Hydrated electrons are created in virtually every radiation environment and in many photochemical or electrochemical environments where liquid water is present, so their reaction products and reaction rate constants are naturally important in applications. Thanks to the strong optical absorbance of (e-)_aq, these rate constants are easy to measure, and a large database has been accumulated. It is not generally appreciated that no working theory of hydrated electron reaction rates presently exists. We discuss key experimental observations of hydrated electron reactions in the context of recent progress in theoretical and simulation developments toward understanding them, made possible by ever increasing computational power.

The oxidation of gas-phase SO₂ to sulfate aerosols has been a driver of urban air pollution since the Great Smog of London in 1952. Traditionally, this reaction has been perceived as a quintessential atmospheric aqueous reaction, occurring within condensed water such as cloud and fog droplets. This established view has been challenged by recent studies showing that, in urban air pollution, sulfate aerosols form predominantly through a heterogeneous SO₂ conversion at aerosol surfaces. This review summarizes recent advances in understanding this heterogeneous process, focusing on (a) why S(IV) oxidation is faster at the air-water interface, (b) how to experimentally determine the reaction location with the scaling relationships of apparent reaction kinetics, and (c) how to predict, or retrieve, the localized surface reaction kinetics with multiscale models. We conclude by discussing open questions and remaining challenges, with the central theme of how the interfacial heterogeneous process may redefine our understanding of atmospheric sulfur chemistry.

Friction is a phenomenon that manifests across all spatial and temporal scales, from the molecular to the macroscopic scale. It describes the dissipation of energy from the motion of particles or abstract reaction coordinates and arises in the transition from a detailed molecular-level description to a simplified, coarse-grained model. It has long been understood that time-dependent (non-Markovian) friction effects are critical for describing the dynamics of many systems, but that they are notoriously difficult to evaluate for complex physical, chemical, and biological systems. In recent years, the development of advanced numerical friction extraction techniques and methods to simulate the generalized Langevin equation has enabled exploration of the role of time-dependent friction across all scales. We discuss recent applications of these friction extraction techniques and the growing understanding of the role of friction in complex equilibrium and nonequilibrium dynamic many-body systems.

As a nonlinear optical process, sum frequency generation (SFG) requires noncentrosymmetry across multiple length scales, ranging from individual molecular functional groups to their arrangements in space. This principle makes SFG not only intrinsically sensitive to molecular species at surfaces but also useful for studying 3D structures of crystalline biopolymers in natural materials. Examples of such biopolymers are cellulose, starch, and chitin in the polysaccharide family and collagen, silk, and keratin in the fibrous protein family. These biopolymers are noncentrosymmetric at multiple length scales, with chirality at the molecular scale, unit cell structure at the nanoscale, and crystallite orientation and polarity at the mesoscale; thus, they are SFG active. In this review, we describe how SFG can be used to determine nano- to mesoscale polarity and orientational orders of crystalline biopolymers interspersed in natural materials containing the same or similar biopolymers in amorphous states, which cannot be obtained with other characterization methods.

The optical centrifuge was demonstrated in 2000 as a tool for preparing ensembles of molecules in extreme rotational states. Highly rotationally excited molecules, so-called superrotors, are observed as products of photodissociation and molecular collisions, in high-temperature environments in the atmospheres of Earth and exoplanets, and in the interstellar medium. Traditional optical excitation is limited to small changes in rotation, limiting experiments to relatively low rotational states. In this review, I discuss the use of a tunable optical centrifuge to prepare molecules in selected ranges of excited rotational states and investigations of their collisional relaxation using state-resolved polarization-sensitive transient IR probing. I examine the decay dynamics of population, alignment, and translational energy release, focusing on experimental results, and compare them with simulations that overestimate observed relaxation rates. A clear picture of near-resonant and nonresonant energy transfer pathways emerges and establishes the means to distinguish superrotor and bath collision products.

Recent studies on ozone photodissociation in the Hartley and Huggins bands have provided new insights into the dissociation dynamics and product state distributions. A Λ-doublet propensity in the photodissociation has been identified through experiment and theory as the origin of the oscillatory O₂(a1Δ_g) rotational distributions and provides a promising diagnostic for determining the relative contributions of 3A' and 3A″ states in Huggins band spin-forbidden processes. Recent experiments on spin-forbidden dissociation have provided detailed information about the vibrational and rotational distributions of the O₂ products and the branching ratios between the O₂ electronic states, serving as a motivation for high-level theory.

In this review, we survey the use of extreme ultraviolet absorption spectroscopy to measure electronic and vibrational dynamics in transition metal complexes. Photons in this 30-100 eV energy range probe 3p → 3d transitions for 3d metals and 4f, 5p → 5d transitions in 5d metals, and the resulting spectra are sensitive to the spin state, oxidation state, and ligand field of the metal. Furthermore, the energy of the core level depends on the metal, providing elemental specificity. Use of tabletop high-harmonic sources allows these spectra to be measured with femtosecond to attosecond time resolution in a standard laser laboratory, revealing short-lived states in chromophores and photocatalysts that were unresolved using other techniques.