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Attosecond snapshots of valence electrons Chemical reactions are triggered by the dynamics of valence electrons in molecular orbitals. These motions typically unfold on a subfemtosecond scale and have eluded real-time access until now. Attosecond spectroscopy (an attosecond is 10 −18 seconds), first applied to tracking electronic transitions from one quantum state to another, has now been extended to follow the hyperfast (subfemtosecond) motion of electron wavepackets in the valence shell — the bond-forming electrons — of krypton ions. This first proof-of-principle demonstration uses a simple system, but the expectation is that attosecond transient absorption spectroscopy of this type will ultimately reveal the elementary electron motions in molecules and solid-state materials that determine physical, chemical and biological properties. Attosecond technology (1 as = 10 −18 S) promises the tools needed to directly probe electron motion in real time. These authors report attosecond pump–probe measurements that track the movement of valence electrons in krypton ions. This first proof-of-principle demonstration uses a simple system, but the expectation is that attosecond transient absorption spectroscopy will ultimately also reveal the elementary electron motions that underlie the properties of molecules and solid-state materials. The superposition of quantum states drives motion on the atomic and subatomic scales, with the energy spacing of the states dictating the speed of the motion. In the case of electrons residing in the outer (valence) shells of atoms and molecules which are separated by electronvolt energies, this means that valence electron motion occurs on a subfemtosecond to few-femtosecond timescale (1 fs = 10 −15  s). In the absence of complete measurements, the motion can be characterized in terms of a complex quantity, the density matrix. Here we report an attosecond pump–probe measurement of the density matrix of valence electrons in atomic krypton ions 1 . We generate the ions with a controlled few-cycle laser field 2 and then probe them through the spectrally resolved absorption of an attosecond extreme-ultraviolet pulse 3 , which allows us to observe in real time the subfemtosecond motion of valence electrons over a multifemtosecond time span. We are able to completely characterize the quantum mechanical electron motion and determine its degree of coherence in the specimen of the ensemble. Although the present study uses a simple, prototypical open system, attosecond transient absorption spectroscopy should be applicable to molecules and solid-state materials to reveal the elementary electron motions that control physical, chemical and biological properties and processes.

Planar tetracoordinate silicon, germanium, tin, and lead (ptSi/Ge/Sn/Pb) species are scarce and exotic. Here, we report a series of penta-atomic ptSi/Ge/Sn/Pb XB2Bi2 (X = Si, Ge, Sn, Pb) clusters with 20 valence electrons (VEs). Ternary XB2Bi2 (X = Si, Ge, Sn, Pb) clusters possess beautiful fan-shaped structures, with a Bi–B–B–Bi chain surrounding the central X core. The unbiased density functional theory (DFT) searches and high-level CCSD(T) calculations reveal that these ptSi/Ge/Sn/Pb species are the global minima on their potential energy surfaces. Born–Oppenheimer molecular dynamics (BOMD) simulations indicate that XB2Bi2 (X = Si, Ge, Sn, Pb) clusters are robust. Bonding analyses indicate that 20 VEs are perfect for the ptX XB2Bi2 (X = Si, Ge, Sn, Pb): two lone pairs of Bi atoms; one 5c–2e π, and three σ bonds (two Bi–X 2c–2e and one B–X–B 3c–2e bonds) between the ligands and X atom; three 2c–2e σ bonds and one delocalized 4c–2e π bond between the ligands. The ptSi/Ge/Sn/Pb XB2Bi2 (X = Si, Ge, Sn, Pb) clusters possess 2π/2σ double aromaticity, according to the (4n + 2) Hückel rule.

The ultra-high-resolution structure of the high-potential iron–sulfur protein at 0.48 Å, the highest-resolution X-ray crystal structure of a protein reported so far. A metalloprotein structure at 0.48 Å resolution High-resolution X-ray crystal structures of proteins feature prominently in the scientific literature, but the resolutions of these structures are rarely better than 3.0–1.5 Å. This study reports the structure of the high-potential iron–sulfur protein (HiPIP) electron carrier protein from the thermophilic purple photosynthetic bacterium Thermochromatium tepidum bacterium at 0.48 Å. This is one of the highest resolution X-ray crystal structures of a protein reported to date. The ultra-high resolution structure of this metalloprotein has enabled the authors to perform a charge-density analysis of the iron–sulfur cluster at its centre, and to visualize the distributions of valence electrons around the iron and sulphur atoms in the Fe 4 S 4 cluster. The result contributes to the understanding of the relationship between structure and function of metalloproteins at the subatomic level. The fine structures of proteins, such as the positions of hydrogen atoms, distributions of valence electrons and orientations of bound waters, are critical factors for determining the dynamic and chemical properties of proteins. Such information cannot be obtained by conventional protein X-ray analyses at 3.0–1.5 Å resolution, in which amino acids are fitted into atomically unresolved electron-density maps and refinement calculations are performed under strong restraints 1 , 2 . Therefore, we usually supplement the information on hydrogen atoms and valence electrons in proteins with pre-existing common knowledge obtained by chemistry in small molecules. However, even now, computational calculation of such information with quantum chemistry also tends to be difficult, especially for polynuclear metalloproteins 3 . Here we report a charge-density analysis of the high-potential iron–sulfur protein from the thermophilic purple bacterium Thermochromatium tepidum using X-ray data at an ultra-high resolution of 0.48 Å. Residual electron densities in the conventional refinement are assigned as valence electrons in the multipolar refinement. Iron 3 d and sulfur 3 p electron densities of the Fe 4 S 4 cluster are visualized around the atoms. Such information provides the most detailed view of the valence electrons of the metal complex in the protein. The asymmetry of the iron–sulfur cluster and the protein environment suggests the structural basis of charge storing on electron transfer. Our charge-density analysis reveals many fine features around the metal complex for the first time, and will enable further theoretical and experimental studies of metalloproteins.

Auger decay of all levels of the double core-hole states 4d[sup.−2] of Xe[sup.2+] , including collective Auger decay (CAD) pathways, is investigated using the relativistic distorted-wave approximation. Large-scale configuration interaction calculations were performed to obtain level-to-level Auger decay rates. In addition to the typical Auger decay final levels associated with the configurations of 4d[sup.−1] 5s[sup.2] 5p[sup.4] , 4d[sup.−1] 5s[sup.1] 5p[sup.5] , and 4d[sup.−1] 5s[sup.0] 5p[sup.6] , evident contributions are identified from excited channels, leading to configurations such as 4d[sup.9] 4f[sup.1] 5s[sup.2] 5p[sup.3] , 4d[sup.9] 5s[sup.2] 5p[sup.3] 5d[sup.1] , 4d[sup.9] 5s[sup.2] 5p[sup.3] 6s[sup.1] , and 4d[sup.9] 5s[sup.2] 5p[sup.3] 6p[sup.1] . These contributions arise from strong electron correlation between the valence electronic orbitals and the 4d inner-shell orbital. The CAD rates and branching ratios (BRs) are determined for each double core-hole level with a minimum CAD BR of 1.28% and a maximum of 4.08% among all CAD channels. The configuration-averaged CAD BR is predicted to be 1.93%, which helps explain recent unexplained experimental findings. The inclusion of CAD processes enriches Auger electron spectroscopy, thereby extending potential applications of this important experimental tool in both fundamental and applied research.

Dependence of mechanical properties of binary platinum-rhodium alloys on valence electron ratio (VER), number of valence electrons (ev) and average atomic number of the alloys (Z) are investigated. The alloys have a high number of valence electrons (9 ≤ ev ≤ 10) and a wide range of average atomic numbers (Z = 45-78). Clear correlations between VER of the alloys and their mechanical properties are found. By increasing the VER of the alloy from 0.13 to 0.20 following the increase of rhodium content in the composition, the hardness, elastic modulus and ultimate tensile strength (UTS) of the alloy increases. Creep rates of the selected alloys clearly decrease with increasing VER at high temperatures (1500-1700°C), while stress rupture time at different temperatures consistently increases because of higher rhodium content in the alloy solid solution chemistry. Dependence of mechanical properties on valence electron parameters is discussed with reference to the atomic bonding.

Double photoionization (DPI) allows for a sensitive and direct probe of electron correlation, which governs the structure of all matter. For atoms, much of the work in theory and experiment that informs our fullest understanding of this process has been conducted on helium, and efforts continue to explore many-electron targets with the same level of detail to understand the angular distributions of the ejected electrons in full dimensionality. Expanding on previous results, we consider here the double photoionization of two 2p valence electrons of atomic carbon and neon and explore the possible continuum states that are connected by dipole selection rules to the coupling of the outgoing electrons in 3P, 1D, and 1S initial states of the target atoms. Carbon and neon share these possible symmetries for the coupling of their valence electrons. Results are presented for the energy-sharing single differential cross section (SDCS) and triple differential cross section (TDCS), further elucidating the impact of the initial state symmetry in determining the angular distributions that are impacted by the correlation that drives the DPI process.

The concept of superatoms has long been used as a guide to understand the stability of clusters. However, little work has been done to compare the structural stability of ligand-protected Ag nanoclusters with the same kernel using the superatom concept. In this study, we construct six clusters based on the same Ag 13 kernel, including 5 e Ag 17 (SR) 12 , 6 e Ag 20 (SR) 14 , 6 e Ag 18 (SR) 12 , 7 e Ag 19 (SR) 12 (DMPA) 3 , 7 e Ag 25 (SR) 18 , and 8 e [Ag 25 (SR) 18 ] – (where R = -CH 3 , Methyl; DMPA = C 6 H 16 P 2 , 1,2-Bis(dimethylphosphino)ethane). The regulation of the valence electron count is achieved by varying the outer motifs—Ag(SR) 2 , Ag 2 (SR) 3 , Ag(SR) 3 , and Ag 2 (SR) 4 (DMPA)—which withdraw 1, 1, 2, and 2 valence electrons from the inner kernel, respectively. The results show that the 8 e [Ag 25 (SR) 18 ] – cluster, with its closed 1S 2 |1P 6 electron configuration, exhibits the least structural variation, a more balanced atomic charge distribution in the kernel Ag atoms, and a relatively large HOMO-LUMO gap. This work contributes to a better understanding of the superatom concept in the context of the structure of ligand-protected Ag nanoclusters.

The physical properties of molecular crystals are governed by the frontier orbitals of molecules. A molecular orbital, which is formed by superposing the atomic orbitals of constituent elements, has complicated degrees of freedom in the crystal because of the influence of electron correlation and crystal field. Therefore, in general, it is difficult to experimentally observe the whole picture of a frontier orbital. Here, we introduce a new method called “core differential Fourier synthesis” (CDFS) using synchrotron X-ray diffraction to observe the valence electron density in materials. By observing the valence electrons occupied in molecular orbitals, the orbital state can be directly determined in a real space. In this study, we applied the CDFS method to molecular materials such as diamond, C60 fullerene, (MV)I2, and (TMTTF)2X. Our results not only demonstrate the typical orbital states in some materials, but also provide a new method for studying intramolecular degrees of freedom.

At present, the question of the relationship between the characteristic martensitic transformation temperatures (MTT) and the electronic parameters of a system has not been fully studied. In the present work, an attempt to establish a similar relationship using the example of the concentration of charge carriers, n, was made. The field dependences of Hall resistivity ρH and magnetization M of the magnetocaloric Ni47−xMn41+xIn12 (x = 0, 1, 2) alloys were measured at T = 4.2 K and in magnetic fields of up to 80 kOe. The MTT were obtained from the temperature dependences of electrical resistivity and magnetization. It was observed that the MTT correlate strongly with both the valence electron concentration e/a and the electronic transport characteristics, which are the coefficient of the normal (NHE) R0 and anomalous (AHE) RS Hall effect and the concentration of charge carriers n.

Binary-shape memory alloys that are based on copper, mainly copper–aluminium, copper–zinc and copper–tin alloys, either with or without ternary elemental additions, are of special interest to the industry and academia because of their good shape recovery, ease of processing, larger recovery strain and lower cost. However, unlike Ni–Ti shape memory alloys, their uses are moderately limited due to shortcomings, such as stabilization of martensite due to ageing, brittleness and low mechanical strength. Therefore, efforts have been made over the years to overcome these limitations using appropriate ternary and quaternary elemental additions. This work takes into account the data obtained from the experimental work carried out by the authors of this paper as well as the data obtained from the experimental and theoretical works carried out by earlier researchers in this area that have been published in the literature over the years. It is observed in quaternary shape memory alloys based on copper that with an increase in the atomic radius of the quaternary element, the hysteresis width is found to increase. With the addition of ternary elements to binary Cu-based alloys (Cu–Al and Cu–Zn), and quaternary elements to ternary Cu-based alloys (Cu–Al–Fe, Cu–Al–Ni, Cu–Al–Mn, Cu–Zn–Al, Cu–Zn–Ni and Cu–Zn–Si), the M s temperature either increases or decreases. This influence is directly correlated with the e v / a ratio and c v values. It is also observed that as the concentration of electrons decreases, the M s temperature decreases too. In addition, in this paper, we have tried to obtain relationships between the M s temperature and the mass or atomic% of different elements through multiple regressions to generalize the interpretations.