Comparative study of the photoassociation reaction of He + X+→ HeX+ (X = H,D): Including multiphoton transitions and dissociations

Using the time-dependent quantum wave packet method, the photoassociation (PA) processes of He + H ⁺→ HeH⁺ and He + D ⁺→ HeD⁺, driven by the sin²-shaped femtosecond laser pulse in the electronic ground state, including multiphoton transitions and dissociations, are investigated for a wide range of initial collision momenta spanning from 1 to 4 a.u. (or for the collision energy roughly in the ranges of 0.009∼0.148 eV and 0.006∼0.089 eV for HeH⁺ and HeD⁺ systems, respectively). It is found that, at some collision momenta, multiphoton transitions to deeply bound states are inevitable to occur and can greatly decrease the PA probability of the target state that selected is the vibrational state v = 6. For the dissociation process, the higher-order (two- and three-photon) dissociations, measured from the target state, tend to be significant at relative high collision energies, which implies that above-threshold dissociations may also be an important loss mechanism in the PA process. In addition, it is also shown that the higher-order dissociation is much stronger for HeH⁺ systems than that for HeD⁺ systems at a given collision momentum, and could be enhanced by the strong transitions among deeply bound states.     

Pressure‐Induced Intersite BiM (M=Ru, Ir) Valence Transitions in Hexagonal Perovskites

Pressure‐induced charge transfer from Bi to Ir/Ru is observed in the hexagonal perovskites Ba_3+nBiM_2+nO_9+3n (n=0,1; M=Ir,Ru). These compounds show first‐order, circa 1 % volume contractions at room temperature above 5 GPa, which are due to the large reduction in the effective ionic radius of Bi when the 6s shell is emptied on oxidation, compared to the relatively negligible effect of reduction on the radii of Ir or Ru. They are the first such transitions involving 4d and 5d compounds, and they double the total number of cases known. Ab initio calculations suggest that magnetic interactions through very short (ca. 2.6 Å) M? M bonds contribute to the finely balanced nature of their electronic states.     

Dynamical Bonding Driving Mixed Valency in a Metal Boride

Samarium hexaboride is an anomaly, having many exotic and seemingly mutually incompatible properties. It was proposed to be a mixed‐valent semiconductor, and later a topological Kondo insulator, and yet has a Fermi surface despite being an insulator. We propose a new and unified understanding of SmB₆ centered on the hitherto unrecognized dynamical bonding effect: the coexistence of two Sm?B bonding modes within SmB₆, corresponding to different oxidation states of the Sm. The mixed valency arises in SmB₆ from thermal population of these distinct minima enabled by motion of B. Our model simultaneously explains the thermal valence fluctuations, appearance of magnetic Fermi surface, excess entropy at low temperatures, pressure‐induced phase transitions, and related features in Raman spectra and their unexpected dependence on temperature and boron isotope.     

Template‐Free Synthesis and Mechanistic Study of Porous Three‐Dimensional Hierarchical Uranium‐Containing and Uranium Oxide Microspheres

A novel type of uranium‐containing microspheres with an urchin‐like hierarchical nano/microstructure has been successfully synthesized by a facile template‐free hydrothermal method with uranyl nitrate hexahydrate, urea, and glycerol as the uranium source, precipitating agent, and shape‐controlling agent, respectively. The as‐synthesized microspheres were usually a few micrometers in size and porous inside, and their shells were composed of nanoscale rod‐shaped crystals. The growth mechanism of the hydrothermal reaction was studied, revealing that temperature, ratios of reactants, solution pH, and reaction time were all critical for the growth. The mechanism study also revealed that an intermediate compound of 3 UO₃?NH₃?5 H₂O was first formed and then gradually converted into the final hydrothermal product. These uranium‐containing microspheres were excellent precursors to synthesize porous uranium oxide microspheres. With a suitable calcination temperature, very uniform microspheres of uranium oxides (UO_2+x, U₃O₈, and UO₃) were successfully synthesized.     

A Time‐Dependent DFT Study of the Absorption and Fluorescence Properties of Graphene Quantum Dots

Absorption and fluorescence spectra of graphene quantum dots (GQDs) have been computed by using time‐dependent density functional theory (TDDFT). Different functionals, including PBE, TPSSh, B3LYP, PBE0, CAM‐B3LYP, and LC‐ωPBE, have been tested and B3LYP/6‐31G(d) has been proven to be the most accurate method for our work. The bulk solvent effects of toluene and dichloromethane have been assessed by using the polarizable continuum model (PCM). The absorption wavelength of GQDs in solvents is red‐shifted compared with that in the gas phase. Edge functionalization effects analysis shows that a small number of substituted groups on GQDs induce a small redshift whereas a large redshift occurs when the edges of GQDs are all decorated. Little difference in the fluorescent emission was observed in solvents and in the gas phase. Molecular orbital transition and transition density matrix analysis have been performed. The electronic transition mainly occurs in the middle part of the structure of C132. The strong absorption of C132 corresponds to a S₀→S₃ transition and the fluorescence emission is ascribed to a S₁→S₀ transition, which indicates that Kasha’s rule is obeyed.     

Phase Transitions,Prominent Dielectric Anomalies,and Negative Thermal Expansion in Three High Thermally Stable Ammonium Magnesium–Formate Frameworks

We present three Mg–formate frameworks, incorporating three different ammoniums: [NH₄][Mg(HCOO)₃] ( 1 ), [CH₃CH₂NH₃][Mg(HCOO)₃] ( 2 ) and [NH₃(CH₂)₄NH₃][Mg₂(HCOO)₆] ( 3 ). They display structural phase transitions accompanied by prominent dielectric anomalies and anisotropic and negative thermal expansion. The temperature‐dependent structures, covering the whole temperature region in which the phase transitions occur, reveal detailed structural changes, and structure–property relationships are established. Compound 1 is a chiral Mg–formate framework with the NH₄⁺ cations located in the channels. Above 255 K, the NH₄⁺ cation vibrates quickly between two positions of shallow energy minima. Below 255 K, the cations undergo two steps of freezing of their vibrations, caused by the different inner profiles of the channels, producing non‐compensated antipolarization. These lead to significant negative thermal expansion and a relaxor‐like dielectric response. In perovskite 2 , the orthorhombic phase below 374 K possesses ordered CH₃CH₂NH₃⁺ cations in the cubic cavities of the Mg–formate framework. Above 374 K, the structure becomes trigonal, with trigonally disordered cations, and above 426 K, another phase transition occurs and the cation changes to a two‐fold disordered state. The two transitions are accompanied by prominent dielectric anomalies and negative and positive thermal expansion, contributing to the large regulation of the framework coupled the order–disorder transition of CH₃CH₂NH₃⁺. For niccolite 3 , the gradually enhanced flipping movement of the middle ethylene of [NH₃(CH₂)₄NH₃]²⁺ in the elongated framework cavity finally leads to the phase transition with a critical temperature of 412 K, and the trigonally disordered cations and relevant framework change, providing the basis for the very strong dielectric dispersion, high dielectric constant (comparable to inorganic oxides), and large negative thermal expansion. The spontaneous polarizations for the low‐temperature polar phases are 1.15, 3.43 and 1.51 μC cm^?2 for 1 , 2 and 3 , respectively, as estimated by the shifts of the cations related to the anionic frameworks. Thermal and variable‐temperature powder X‐ray diffraction studies confirm the phase transitions, and the materials are all found to be thermally stable up to 470 K.     

Theoretical investigation of the coexistence of α and β-nitric acid trihydrates (NAT) molecular conformations

In this letter we document the possibility of the existence of a second molecular configuration for nitric acid trihydrates. Density functional theory (DFT) methods have been used for studying the nitric acid trihydrates α and β-NAT conformations, their spectroscopic and thermodynamics properties and dipole moments have been calculated. This study describes the gas–solid phase transition of the NAT and provides two possible pathways for the molecular structure transformation between α and β-NAT.     

SANS from Pluronic P85 in d-water

Small-angle neutron scattering (SANS) has been used to investigate Pluronic P85 (EO₂₆PO₄₀EO₂₆) copolymer in deuterated water. A range of P85 fractions were measured for a wide sample temperature window. A rich phase behavior is reported. Unimers were observed below the critical micelle formation condition. At fixed P85 fraction, a number of micellar phases were observed upon increasing temperature; first spherical micelles, then cylindrical micelles, then lamellar micelles. At the highest temperature, a demixed lamellae phase was observed. Analysis of the SANS data consisted in fits to an empirical Guinier-Porod model that was appropriate for data fitting in the various phases at low P85 fractions. When the P85 fraction increased, an inter-particle structure factor was included to analyze SANS data from concentrated spherical micelles. At high P85 fractions, paracrystalline structures were observed as evidenced by an enhanced inter-particle interaction peak. A phase diagram for P85/d-water was obtained showing the various phases. Focusing on the spherical micelles phase for one sample composition, a core-shell model was used to fit SANS data and obtain sizes and scattering length densities. Using material balance equations, information such as the aggregation number (i.e., number of Pluronic macromolecules per micelle) and the number of hydration water molecules in the shell region are determined.