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Applied Nanophotonics || Energy transfer processes
摘要： In this chapter, we introduce the general phenomenon of excitation energy transfer, explain radiative and nonradiative types of energy transfer, and derive the basic processes of energy transfer. We look at the F?rster resonance energy transfer (FRET) in particular. We also describe Dexter energy transfer, charge transfer, exciton diffusion, and exciton dissociation. Finally, we summarize the modifications of FRET when using nanostructures with mixed dimensionalities and in different assemblies.
关键词： F?rster resonance energy transfer,nanostructures,FRET,charge transfer,exciton diffusion,energy transfer,Dexter energy transfer,exciton dissociation
更新于2025-09-10 09:29:36
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Kinetics of Transient Protein Complexes Determined via Diffusion-Independent Microfluidic Mixing and Fluorescence Stoichiometry
摘要： Low-a?nity protein complexes and their transient states are di?cult to measure in single-molecule experiments because of their low concentrations. A prominent solution to this problem is the use of micro?uidic mixing devices, which rely on di?usion-based mixing. This is not ideal for multiprotein complexes, as the single-molecule ?uorescence signal is dominated by the already dissociated species. Here, we designed a micro?uidic device with mixing structures for fast and homogeneous mixing of components with varying di?usion coe?cients and for ?uorescence measurements at a de?ned single-molecule concentration. This enables direct measurement of dissociation rates at a broad range of timescales from a few milliseconds to several minutes. This further allows us to measure structural properties and stoichiometries of protein complexes with large equilibrium dissociation constants (KD’s) of 5 μM and above. We used the platform to measure structural properties and dissociation rates of heat shock protein 90 (Hsp90) dimers and found at least two dissociation rates which depend on the nucleotide state. Finally, we demonstrate the capability for measuring also equilibrium dissociation constants, resulting in the determination of both the kinetics and thermodynamics of the system under investigation.
关键词： single-molecule ?uorescence,micro?uidic mixing,dissociation rates,Hsp90,protein complexes
更新于2025-09-10 09:29:36
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Isomeric Effect of Mercaptobenzoic Acids on the Synthesis, Stability, and Optical Properties of Au <sub/>25</sub> (MBA) <sub/>18</sub> Nanoclusters
摘要： We report a simple size focusing, two-step “bottom-up” protocol to prepare water-soluble Au25(MBA)18 nanoclusters, using the three isomers of mercaptobenzoic acids (p/m/o-MBA) as capping ligands and Me3NBH3 as a mild reducing agent. The relative stability of the gas-phase multiply deprotonated Au25(MBA)18 ions was investigated by collision-induced dissociation. This permitted us to evaluate the possible isomeric effect on the Au?S interfacial bond stress. We also investigated their optical properties. The absorption spectra of Au25(MBA)18 isomers were very similar and showed bands at 690, 470, and 430 nm. For all Au25(MBA)18 isomeric clusters, no measurable one-photon excited fluorescence under UV?vis light was found, in neither solid- nor solution-state. The two-photon excited emission spectra and first hyperpolarizabilities of the clusters were also determined. The results are discussed in terms of the possible isomeric effect on excitations within the metal core and the possibility of charge transfer excitations from the ligands to the metal nanocluster.
关键词： Au25(MBA)18 nanoclusters,first hyperpolarizabilities,collision-induced dissociation,isomeric effect,optical properties,two-photon excited emission,mercaptobenzoic acids
更新于2025-09-10 09:29:36
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Dissociation dynamics of the water dication following one-photon double ionization. I. Theory
摘要： The measurement of the triple differential cross section in the body frame for double photoionization of a molecule can be made in principle by detecting the ionic fragments and the two photoelectrons in coincidence—but only if the dynamics and geometry of dissociation of the doubly charged molecular ion are known. A classical trajectory study of the nine lowest states of the water dication is presented using high quality ab initio potential-energy surfaces. Sampling from a semiclassical initial distribution of positions and momenta is used to approximate ionization from the Frank-Condon region of the ground vibrational state of neutral H2O. Excellent agreement in comparison with preliminary experimental momentum imaging measurements of double photoionization of water show that eight dication states can be unambiguously identified in the experiment with the aid of theory. The theoretical trajectory results allow body frame measurements of double photoionization to yield all eight states even though the usual assumption of direct dissociation, the “axial recoil” approximation, breaks down for three of the dication electronic states seen in the experiment.
关键词： potential-energy surfaces,double photoionization,dissociation dynamics,classical trajectory study,water dication
更新于2025-09-09 09:28:46
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Encyclopedia of Spectroscopy and Spectrometry || Field Ionization Kinetics in Mass Spectrometry
摘要： The mass spectra of molecules, ionized by electron ionization (EI), field ionization (FI) or photoionization (PI) in the gas phase, usually show many peaks with varying intensities. These peaks result from fragment ions generated by a series of competing and consecutive unimolecular dissociation reactions of the molecular ions provided that the latter have obtained during their formation an amount of energy in excess of the ionization energy of the corresponding molecules. The resulting mass spectrum therefore is determined by the relative rates of these unimolecular dissociation reactions, which in their turn are related to the relative abundances of the fragment ions observed. It will be clear that studying the kinetics of ion dissociation can provide an invaluable insight into the foundation of a mass spectrum.
关键词： Gas-Phase Ion Chemistry,Unimolecular Dissociation,Ion Lifetimes,Mass Spectrometry,Field Ionization Kinetics
更新于2025-09-04 15:30:14
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Encyclopedia of Spectroscopy and Spectrometry || Ion Collision, Theory ☆
摘要： Collisions between an ion and neutral species result in a number of possible outcomes depending upon the chemical and physical properties of the two reactants, their relative velocities, and the impact parameter of their trajectories. These include elastic and inelastic scattering of the colliding particles, charge transfer (including dissociative charge transfer), atom abstraction, complex formation and dissociation of the colliding ion. Each of these reactions may be characterized in terms of their energy-dependent rate coefficients, cross sections and reaction kinetics. This article outlines a theoretical framework for discussing these processes that emphasizes simple models and classical mechanics. The discussion of collision processes has been divided into two categories: low-energy and high-energy thermal or quasi-thermal collisions. Experiments under conditions – swarms, drift tubes, chemical ionization and ion cyclotron resonance – are strongly influenced by long-range forces and often involve ‘capture collisions’ in which atom exchange and extensive energy exchange are common characteristics. High-energy collisions are typically impulsive, involve short-range intermolecular forces and are direct, fast processes.
关键词： Low-energy Collisions,Elastic and Inelastic Scattering,Reaction Kinetics,Theory,Complex Formation,Rate Coefficients,Charge Transfer,Atom Abstraction,High-energy Collisions,Ion Collision,Dissociation,Cross Sections
更新于2025-09-04 15:30:14
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Encyclopedia of Spectroscopy and Spectrometry || Ion Dissociation Kinetics in Mass Spectrometry ☆
摘要： The ionization processes taking place in the source of a mass spectrometer and the many possible activation processes available in multi-dimensional mass spectrometry lead to ions with a broad range of internal energy which can extend well above the lowest fragmentation threshold, thus allowing the molecular ion to dissociate within the typical time frame of a mass spectrometry experiment. Dissociation is a dynamical process characterized by its associated lifetime. The following questions need to be addressed. (i) How fast does a selected fragmentation process take place under given experimental conditions (state selection, well-defined energy or temperature, for example) and how is the excess energy partitioned among the reaction products? (ii) Which information can be inferred from the experimental data about the reaction mechanism?
关键词： Ion Dissociation Kinetics,Canonical Rate Constant,Unimolecular Rate Constant,Mass Spectrometry,Microcanonical Rate Constant
更新于2025-09-04 15:30:14
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The importance of thermal dissociation in CO<sub>2</sub> microwave discharges investigated by power pulsing and rotational Raman scattering
摘要： The input power of a CO2 microwave plasma is modulated at kHz rate in scans of duty cycle at constant average power to investigate gas heating dynamics and its relation to dissociation e?ciency. Rotational temperature pro?les obtained from rotational Raman scattering reveal peak temperatures of up to 3000 K, while the edge temperature remains cold (500 K). During the plasma “OFF”-period, the gas cools down convectively, but remains overall too hot to allow for strong overpopulation of vibrational modes (2200 K in the core). Fast optical imaging monitors plasma volume variations and shows that power density scales with peak power. As dissociation scales with observed peak rotational temperature, it is concluded that thermal processes dominate. A simple 0D model is constructed which explains how higher power density favors dissociation over radial energy transport. Thermal decomposition is reviewed in relation to quenching oxygen radicals with vibrationally excited CO2, to re?ect on earlier reported record e?ciencies of 90 %.
关键词： power pulsing,rotational Raman scattering,gas heating dynamics,thermal dissociation,CO2 microwave discharges
更新于2025-09-04 15:30:14
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Dissociative Photoionization of Chromium Hexacarbonyl: A Round-Trip Ticket to Non-Statisticality and a Detective Story in Thermochemistry
摘要： The fragmentation processes of internal energy selected chromium hexacarbonyl cations, Cr(CO)6+, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy at the VUV beamline of the Swiss Light Source. In the 9.3 to 21.5 eV photon energy range, Cr(CO)6+ dissociates by six sequential carbonyl ligand losses. The fragment ion fractional abundances, plotted in the breakdown diagram, along with the time-of-flight mass spectra for the first three metastable CO-loss channels were modeled using a statistical approach. Between 12 and 16 eV, the statistical model overestimates the degree of fragmentation, which is explained by enhanced kinetic energy release in impulsive CO loss on repulsive electronic states of the parent ion Cr(CO)6+, as confirmed by TD-DFT calculations. This is the first reported example for an embedded non-statistical unimolecular dissociation regime, bracketed by statistical regimes at low and at high energies. The statistical model was employed to derive 0 K appearance energies for Cr(CO)n+ (n = 0–5). The Cr(CO)5+ appearance energy and the literature CO–Cr(CO)5+ bond dissociation energy yield an adiabatic Cr(CO)6+ ionization energy of 8.195 ± 0.120 eV, independent of photoelectron spectroscopy measurements. The 0 K appearance energy of Cr+ is 61 kJ mol–1 higher than predicted based on literature enthalpies of formation, which we suggest is most likely due to an error in the enthalpy of formation of Cr(CO)6, and propose a revised ?fHo[Cr(CO)6, g] = –972.1 ± 4.1 and –968.9 ± 4.1 kJ mol–1 at 0 and 298 K, respectively, and ?fH298Ko[Cr(CO)6, c] = –1040.6 ± 4.2 kJ mol–1 for the crystalline state, based on the known enthalpy of sublimation. The measured Cr–CO bond dissociation energies in [(CO)nCr–CO]+ (n = 0–5), and the enthalpies of formation of the chromium carbonyl ion series are also reported.
关键词： unimolecular dissociation,non-statisticality,thermochemistry,Photoionization,PEPICO
更新于2025-09-04 15:30:14
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= 1,...,10)
摘要： Using fixed-node diffusion quantum Monte Carlo (DMC) simulation we investigate the structural properties and energetics of the linear and cyclic carbon clusters Cn for n (cid:2) 10. We calculate the binding energy, the electron correlation energy, the dissociation energy, and the second difference in energy. We also present an analysis of the structural properties of the clusters. It is found that the bond lengths, binding energies, and dissociation energies obtained from the DMC calculations are in excellent agreement with the available experimental results. The electron-correlation contribution to the binding energy indicates that in the case of the linear isomers, the clusters of odd-number size are relatively more favored than their neighbors of even-number size, whereas for the cyclic isomers, we do not observe the oscillation pattern. In the range of cluster size under investigation, we find that the electron-correlation impact in the binding energy of the cyclic clusters is larger than that of the corresponding linear ones, varying from 30% to 40% of the binding energy values. The electron correlation is also essential to the stability of the clusters enhancing by up to 52% their dissociation energy. A comparative analysis of the dissociation energy and second difference in energy indicates that the linear isomers C3 and C5 are the most stable ones.
关键词： dissociation energy,electron correlation,quantum Monte Carlo,carbon clusters,binding energy
更新于2025-09-04 15:30:14