Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

The interest in following the evolution of the valence electronic structure of atoms and molecules during chemical reactions on a femtosecond time scale is discussed. By explicitly mapping the occupied part of the electronic structure with femtosecond pump-probe schemes one essentially follows the electrons making the bonds while the bonds change. This holds the key to unprecedented insight into chemical bonding in short-lived intermediates and reveals the coupled motion of electrons and nuclei. Examples from the recent literature on small molecules and anionic clusters in the gas phase and on atoms and molecules on surfaces using lab-based femtosecond laser methods are used to demonstrate the case. They highlight how the evolution of the valence electronic structure can be probed with time-resolved photoelectron spectroscopy with ultraviolet (UV) probe photon energies of up to 6 eV. It is shown how new insight can be gained by extending the probing wavelength into the vacuum-ultraviolet (VUV) region to photon energies of 20 eV and more by accessing the whole occupied valence electronic structure with time-resolved VUV photoelectron spectroscopy. Finally, the importance of soft X-ray free-electron lasers with probe photon energies of several hundred eV and femtosecond pulses and in particular the key role of femtosecond time-resolved soft X-ray emission spectroscopy or resonant inelastic X-ray scattering for mapping the electronic structure during chemical reactions is discussed.     

Domain structure of phytochrome from Avena sativa visualized by electron microscopy

Highly purified phytochrome from Avena sativa was visualized by electron microscopy after negative staining with uranyl acetate and after rotary shadowing with platinum. The particle shape was variable in both types of specimens, but tripartite structures resembling a 'Y' were consistently observed. The tripartite substructure is composed of three globular domains each having a diameter of 7 to 8 nm and equally spaced in an equilateral triangle. The dimensions of the tripartite particle measured 15 nm between the centers of any two of the three particles. When phytochrome was digested with trypsin in a manner which releases the amino-terminal globular domain from the polypeptide, the tripartite structure was lost and only small globular particles were seen. We propose that the outer particles of this tripartite structure are the amino-terminal domains of the phytochrome dimer, and the central particle comprises the carboxyl domains of the two subunits.     

High-resolution transmission electron microscopy—Investigation of vanadium-tungsten oxides prepared by chemical transport reactions

Crystalline monophasic samples of three mixed vanadium-tungsten oxides (V_0.8W_0.2)₃O₇ ? (V,W)₉O₂₁, (V_0.65W_0.35)₂O₅ ? (V,W)₁₆O₄₀, and V_0.64W_0.36O_2.60 ? V₁₆W₉O₆₅ were prepared by chemical transport reactions. The block structures of these compounds were investigated by high-resolution electron microscopy. They are built up by square blocks with [3 × 3 × ∞], [4 × 4 × ∞], or [5 × 5 × ∞] cornersharing MO-octahedra. Other block sizes were only observed as defects. The thermal behavior of the compounds was investigated.     

4D electron microscopy visualization of anisotropic atomic motions in carbon nanotubes

We report the anisotropic atomic expansion dynamics of multi-walled carbon nanotubes, using 4D electron microscopy. From time-resolved diffraction on the picosecond to millisecond scale, following ultrafast heating at the rate of 10(13) K/s, it is shown that nanotubes expand only in the radial (intertubule) direction, whereas no significant change is observed in the intratubular axial or equatorial dimensions. The non-equilibrium heating occurs on an ultrafast time scale, indicating that the anisotropy is the result of an efficient electron-lattice coupling and is maintained up to equilibration. The recovery time, which measures the heat dissipation rate for equilibration, was found to be on the order of ～100 μs. This recovery is reproduced theoretically by considering the composite specimen-substrate heat exchange.     

4D scanning transmission ultrafast electron microscopy: Single-particle imaging and spectroscopy

We report the development of 4D scanning transmission ultrafast electron microscopy (ST-UEM). The method was demonstrated in the imaging of silver nanowires and gold nanoparticles. For the wire, the mechanical motion and shape morphological dynamics were imaged, and from the images we obtained the resonance frequency and the dephasing time of the motion. Moreover, we demonstrate here the simultaneous acquisition of dark-field images and electron energy loss spectra from a single gold nanoparticle, which is not possible with conventional methods. The local probing capabilities of ST-UEM open new avenues for probing dynamic processes, from single isolated to embedded nanostructures, without being affected by the heterogeneous processes of ensemble-averaged dynamics. Such methodology promises to have wide-ranging applications in materials science and in single-particle biological imaging.     

4D scanning ultrafast electron microscopy: visualization of materials surface dynamics

The continuous electron beam of conventional scanning electron microscopes (SEM) limits the temporal resolution required for the study of ultrafast dynamics of materials surfaces. Here, we report the development of scanning ultrafast electron microscopy (S-UEM) as a time-resolved method with resolutions in both space and time. The approach is demonstrated in the investigation of the dynamics of semiconducting and metallic materials visualized using secondary-electron images and backscattering electron diffraction patterns. For probing, the electron packet was photogenerated from the sharp field-emitter tip of the microscope with a very low number of electrons in order to suppress space-charge repulsion between electrons and reach the ultrashort temporal resolution, an improvement of orders of magnitude when compared to the traditional beam-blanking method. Moreover, the spatial resolution of SEM is maintained, thus enabling spatiotemporal visualization of surface dynamics following the initiation of change by femtosecond heating or excitation. We discuss capabilities and potential applications of S-UEM in materials and biological science.     

Mean potential phase space theory of chemical reactions

A nonconventional application of phase space theory to the insertion reactions A+H(2), with A=C((1)D) and S((1)D), is presented. Instead of approximating the potential energies of interaction between separated fragments by their isotropic long-range contributions, as in the original theory, the latter are replaced by the accurate potential energies averaged with respect to Jacobi angles. The integral and differential cross sections obtained from this mean potential phase space theory (MPPST) turn out to be in very satisfying agreement with the benchmark predictions of the time-independent and time-dependent statistical quantum methods. The formal and numerical simplicity of MPPST with respect to any approach combining statistical assumptions and dynamical calculations makes it a promising tool for studying indirect polyatomic reactions.     

Electrochemically induced chemical reactions: Kinetics of competition with electron transfer

Acid—base or electrophile—nucleophile chemical reactions can be induced by electrochemical means in the case where the reaction becomes faster at the +1e (or ?1e) level than at the starting level. This is typically the case for SRN1 aromatic nucleophilic substitution. When bond cleavage occurs at the +1e (or ?1e) level, a competing route may be opened by the electroactivity of the cleaved species being higher than that of the substrate. Electron transfer at the electrode (ECE) or in the solution (DISP) thus appears as a possible side-reaction decreasing the efficiency of the electrochemical inducement. The kinetics of this competition is investigated in the context of cyclic voltammetry. The kinetic characteristics are shown to be dramatically different in the ECE and the DISP cases providing an example of the operational significance of the distinction between these two modes of electron transfer. Diagnostic criteria and procedures for rate constant determination are discussed. An experimental illustration of the role of the various operational parameters substrate and nucleophile concentrations, sweep rate, is given. It involves the electrochemically catalyzed aromatic nucleophilic substitution of 2-chloroquinolin by benzenethiolate in liquid ammonia.     

Applications of ultrashort shaped pulses in microscopy and for controlling chemical reactions

This article presents a new perspective on laser control based on insights into the effect of spectral phase on nonlinear optical processes. Gaining this understanding requires the systematic evaluation of the molecular response as a function of a series of pre-defined accurately shaped laser pulses. The effort required is rewarded with robust, highly reproducible, results. This approach is illustrated by results on selective two-photon excitation microscopy of biological samples, where higher signal and less photobleaching damage are achieved by accurate phase measurement and elimination of high-order phase distortions from the ultrashort laser pulses. A similar systematic approach applied to laser control of gas phase chemical reactions reveals surprising general trends. Molecular fragmentation pattern is found to be dependent on phase shaping. Differently shaped pulses with similar pulse duration have been found to produce similar fragmentation patterns. This implies that any single parameter that is proportional to the pulse duration, such as second harmonic generation intensity, allows us to predict the molecular fragmentation pattern within the experimental noise. This finding, is illustrated here for a series of isomers. Bond selectivity, coherent photochemistry and their applications are discussed in light of results from these systematic studies.     

Study of liquid crystal space groups using controlled tilting with cryogenic transmission electron microscopy

We developed a method that enables differentiation between liquid crystalline-phase particles corresponding to different space groups. It consists of controlled tilting of the specimen to observe different orientations of the same particle using cryogenic transmission electron microscopy. This leads to the visualization of lattice planes (or reflections) that are present for a given structure and absent for the other one(s) and that give information on liquid crystalline structures and their space groups. In particular, we show that we can unambiguously distinguish among particles having the inverted micellar cubic (space group Fd(3)m, 227), the inverted bicontinuous gyroid (space group Ia(3)d, 230), the inverted bicontinuous diamond (space group Pn(3)m, 224), and the inverted bicontinuous primitive cubic structure (space group Im(3)m, 229).     

Ultrafast electron diffraction: oriented molecular structures in space and time.

The technique of ultrafast electron diffraction allows direct measurement of changes which occur in the molecular structures of isolated molecules upon excitation by femtosecond laser pulses. The vectorial nature of the molecule-radiation interaction also ensures that the orientation of the transient populations created by the laser excitation is not isotropic. Here, we examine the influence on electron diffraction measurements--on the femtosecond and picosecond timescales--of this induced initial anisotropy and subsequent inertial (collision-free) molecular reorientation, accounting for the geometry and dynamics of a laser-induced reaction (dissociation). The orientations of both the residual ground-state population and the excited- or product-state populations evolve in time, with different characteristic rotational dephasing and recurrence times due to differing moments of inertia. This purely orientational evolution imposes a corresponding evolution on the electron scattering pattern, which we show may be similar to evolution due to intrinsic structural changes in the molecule, and thus potentially subject to misinterpretation. The contribution of each internuclear separation is shown to depend on its orientation in the molecular frame relative to the transition dipole for the photoexcitation; thus not only bond lengths, but also bond angles leave a characteristic imprint on the diffraction. Of particular note is the fact that the influence of anisotropy persists at all times, producing distinct differences between the asymptotic "static" diffraction image and the predictions of isotropic diffraction theory.     

An electron number distribution view of chemical bonds in real space

Several key concepts of chemical bonding theory, such as electron pair sharing, polarity, charge transfer, multiple bonding, etc., are shown to be recovered from the statistical properties of multivariate electron number distribution functions. The latter are constructed from the real-space atomic partition provided by the quantum theory of atoms in molecules. We present the basic formalism and several exemplifying calculations.     

Soft X‐ray emission electron microscopy: chemical state microscopy from interface and bulk

Scanning electron microscopy (SEM) has long been a workhorse of materials science and provides information on morphology, structure and elemental composition. However, information as to the chemical state of the elements is only available for deep lying core levels of the heavy elements and not the light elements. Whilst considerable advances have been made in high‐resolution wavelength dispersive spectroscopy (WDS) and energy dispersive spectroscopy (EDS), electron microscopy in the soft X‐ray region of ≈ 50–1500 eV is lacking. We present first results for a combined instrument of a soft X‐ray emission (SXE) spectrometer together with a spatially resolving (<100 nm) electron gun. Copyright © 2008 John Wiley & Sons, Ltd.     

Transmission electron microscopy characterization of colloidal copper nanoparticles and their chemical reactivity

A colloidal synthesis method was developed to produce face centered cubic (fcc) Cu nanoparticles in the presence of surfactants in an organic solvent under an Ar environment. Various synthetic conditions were explored to control the size of the as-prepared nanoparticles by changing the precursor, varying the amount of surfactants, and tuning the reaction temperature. Transmission electron microscopy (TEM), selected-area electron diffraction, and high-resolution TEM were used as the main characterization tools. Upon exposure to air, these nanoparticles are oxidized at different levels depending on their sizes: (1) an inhomogeneous layer of fcc Cu₂O forms at the surface of Cu nanoparticles (about 30 nm); (2) Cu nanoparticles (about 5 nm) are immediately oxidized into fcc Cu₂O nanoparticles (about 6 nm). The occurrence of these different levels of oxidization demonstrates the reactive nature of Cu nanoparticles and the effect of size on their reactivity. Furthermore, utilization of their chemical reactivity and conversion of spherical Cu nanoparticles into CuS nanoplates through the nanoscale Kirkendall effect were demonstrated. The oxidization and sulfidation of Cu nanoparticles were compared. Different diffusion and growth behaviors were involved in these two chemical transformations, resulting in the formation of isotropic Cu₂O nanoparticles during oxidization and anisotropic CuS nanoplates during sulfidation.      

Real-time imaging of surface chemical reactions by electrochemical photothermal reflectance microscopy

Traditional electrochemical measurements based on either current or potential responses only present the average contribution of an entire electrode''s surface. Here, we present an electrochemical photothermal reflectance microscope (EPRM) in which a potential-dependent nonlinear photothermal signal is exploited to map an electrochemical process with sub-micron spatial resolution. By using EPRM, we are able to monitor the photothermal signal of a Pt electrode during the electrochemical reaction at an imaging speed of 0.3 s per frame. The potential-dependent photothermal signal, which is sensitive to the free electron density, clearly revealed the evolution of surface species on the Pt surface. Our results agreed well with the reported spectroelectrochemical techniques under similar conditions but with a much faster imaging speed. We further mapped the potential oscillation during the oxidation of formic acid on the Pt surface. The photothermal images from the Pt electrode well matched the potential change. This technique opens new prospects for real-time imaging of surface chemical reaction to reveal the heterogeneity of electrochemical reactivity, which enables broad applications to the study of catalysis, energy storage, and light harvest systems.

The potential-dependent photothermal signal, which is sensitive to the free electron density, map the evolution of surface species on the electrode in real time.     

Nanoscale chemical analysis of beam-sensitive polymeric materials by cryogenic electron microscopy

Nanoscale chemical analysis of functional polymer systems by electron microscopy, to gain access into degradation processes during the materials life cycle, is still a formidable challenge due to their beam sensitivity. Here a systematic study on the different stages of degradation in a P3HT-PCBM organic photovoltaic (OPV) model system is presented. To this end pristine samples, samples with (reversibly) physisorbed oxygen and water and samples with (irreversibly) chemisorbed oxygen and water are imaged utilizing the full capabilities of cryogenic STEM-EELS. It is found that oxygen and water are largely physisorbed in this system leading to significant effects on the band structure, especially for PCBM. Quantification proves that degradation concomitantly decreases the amount of CC bonds and increases the amount of C O C bonds in the sample. Finally, it is shown that with a pulsed electron beam utilizing a microwave cavity, beam damage can be significantly reduced which likely extends the possibilities for such studies in future.     

Dienophilicity of imidazole in inverse electron demand Diels-Alder reactions. 4. Intermolecular reactions with 1,2,4-triazines

Intermolecular inverse electron demand cycloadditions of 2-substituted imidazoles with various 1,2,4-triazines produced both imidazo[4,5-c]pyridines (3-deazapurines) and pyrido[3,2-d]pyrimid-4-ones (8-deazapteridines). The product distribution was controlled by reactant substituents and influenced by reaction temperature. A regioselective method for the preparation of 6-unsubstituted 1,2,4-triazines was also developed. By using this route to 8-deazapteridines, a new 8-deazafolate analogue was prepared.     

Fully adiabatic dissociative electrochemical electron transfer reactions induced by scanning tunneling microscopy

A theory of fully adiabatic dissociative electrochemical processes of the electron transfer that are induced by scanning tunneling microscopy is constructed. Adiabatic free energy surfaces are calculated and properties of their symmetry are examined under various conditions. Diagrams of kinetic regimes, which characterize possible kinetic processes, which may proceed in the system under consideration, are constructed in the space of model parameters. Dependence of activation free energy on the bias voltage, overvoltage, physical properties of a molecule, and intensity of interaction of a molecule with an electrode and the tip of the scanning tunneling microscope is explored.