Computational evidence for the existence of two mechanisms for the vibrational relaxation of O+2 by collision with Kr

By performing classical trajectory calculations on an empirical potential energy surface it has been possible to show that relaxation of O₂⁺ (v = 1) in collisions with Kr occurs via two competing mechanisms whose roles are related to the angular dependence of the attractive part of the atom-ion interaction. Quasiclassical calculations reproduce the minimum in the energy dependence of the relaxation rate coefficient.     

Vibrational energy relaxation dynamics of Si---H stretching modes on stepped H/Si(111) 1×1 surfaces

The vibrational energy relaxation rates of excited Si---H stretching modes on the monohydride steps of miscut H/Si(111) 1×1 surfaces are calculated using Bloch-Redfield theory combined with classical molecular dynamics (MD) simulation. The structure and vibrational frequencies of the surface are first investigated using the Car-Parrinello ab initio MD method. The calculated Si---Si---H bending frequencies and relaxed structures are then used to refine the empirical potential for the classical MD simulations. The lifetime of the excited Si---H stretching mode at the step is found to be shorter than the modes on the terrace. Both the magnitude and the trend of the calculated results agree well with the experimental measurement on the 9° monohydride stepped surface. The vibrational relaxation rate of the Si---H stretching modes on the 15° monohydride stepped surface are also calculated and predicted to have a slightly shorter lifetime than for the 9° surface.     

Calculation of the nmr relaxation time of dilute 129 Xe gas

The spin-lattice relaxation time T₁ of ¹²⁹ Xe gas is calculated with the kinetic theory due to Chem and Snider. A Lennard-Jones (12,6) potential functions is employed as a model for the spherical potential while the transient spin-rotation interaction is assumed to be responsible for the relaxation of the nuclei. Cross sections for spin transitions on collisions are calculated either quantum mechanically or semiclassically depending on the relative energy. The temperature dependence of T₁ is determined in the range 200–450 K. The calculated value of T₁ at 298 K and 1 amagat is 2.8 x 0⁵ s while the value measured by Hund and Carr is (2.0 ± 0.2) x 10⁵s.     

A quasiclassical trajectory study of vibrational energy transfer in collisions involving intermolecular attraction of moderate strength

Monte Carlo selected, quasiclassical trajectories have been computed on six potential energy hypersurfaces possessing potential minima or “wells” up to 50 kJ mol^?1 deep. The aim of the investigation has been to examine how vibrational energy transfer in A + BC(υ = 1) collisions is promoted by intermolecular attraction of moderate strength. Here results are reported for the mass combination m_A = 20 u, m_B = 1 u, m_C = u. The results show that even quite slight intermolecular attraction can enhance energy transfer, as long as the attraction does not just depend on the separation of A from the center-of-mass of BC. The mean loss of vibrational energy does not depend only the well depth but also on its “location” (in particular, the difference in r_BC at the minimum and in isolated BC) and on the angular anisotropy of the potential. Large transfers of energy do not occur only in complex-forming collisions; indeed, a high fraction of trajectories on all surfaces are direct but show similar transfer of energy as in the more complex trajectories on the same surface. The results of the calculations are discussed in relation to the mechanisms and rates of vibrational relaxation in collisions between radicals and between species. such as HF + HF, capable of forming hydrogen bonds.     

Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and absorption spectroscopy

The electronic structures of quaternary pnictides ZrCuSiPn (Pn=P, As) were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the core-line XPS and the XANES spectra indicate that the Zr and Cu atoms are cationic, whereas the Si and Pn atoms are anionic, consistent with expectations from simple bonding models. The Cu 2p XPS and Cu L-edge XANES spectra support the presence of Cu¹⁺. The small magnitudes of the energy shifts in the XPS spectra suggest significant covalent character in the Zr-Si, Zr-Pn, and Cu-Pn bonds. On progressing from ZrCuSiP to ZrCuSiAs, the Si atoms remain largely unaffected, as indicated by the absence of shifts in the Si 2p_3/2 binding energy and the Si L-edge absorption energy, while the charge transfer from metal to Pn atoms becomes less pronounced, as indicated by shifts in the Cu K-edge and Zr K, L-edge absorption energies. The transition from two-dimensional character in LaNiAsO to three-dimensional character in ZrCuSiAs proceeds through the development of Si-Si bonds within the [ZrSi] layer and Zr-As bonds between the [ZrSi] and [CuAs] layers.     

Ab initio study of mechanism of forming a spiro-heterocyclic ring compound involving Si from dichlorosilylenesilylene (Cl2Si=Si:)→Cl2Si=Si: and aldehyde

The mechanism of the cycloaddition reaction between singlet dichlorosilylenesilylene (Cl₂Si=Si:)→Cl₂Si=Si: and aldehyde has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has one dominant reaction pathway. The reaction rules presented is that the two reactants firstly form a four-membered ring silylene through the [2+2] cycloaddition reaction. Because of the 3p unoccupied orbital of Si: atom in the four-membered ring silylene and the π orbital of aldehyde forming a π → p donor–acceptor bond, the four-membered ring silylene further combines with aldehyde to form an intermediate. Because the Si: atom in the intermediate happens sp ³ hybridization after transition state, then the intermediate isomerizes to a spiro-heterocyclic ring compound involving Si via a transition state.     

Ab initio study of the formation of bis-heterocyclic compound involving Si and Ge from dichlorosilylene germylidene (Cl2Si=Ge:) and ethene

The mechanism of the cycloaddition reaction between singlet state dichlorosilylene germylidene (Cl₂Si=Ge:) and ethene has been investigated with CCSD(T)//MP2/6-31G* method, from the potential energy profile, we predict that the reaction has one dominant reaction pathway. The presented rule of the reaction is that the two reactants firstly form a Si-heterocyclic four-membered ring germylene through the [2+2] cycloaddition reaction. Due to the sp ³ hybridization of the Ge: atom in Si-heterocyclic four-membered ring germylene, the Si-heterocyclic four-membered ring germylene further combined with the ethene to form a bis-heterocyclic compound with Si and Ge.     

Electron energy structure of GaN when titanium or zinc atoms substitute for gallium

A calculation scheme based on density functional theory with full geometry optimization, modified for structures with translational symmetry is applied to study the electron energy spectrum and magnetic characteristics of hexagonal gallium nitride and structures such as Y _x Ga_1?x N (Y: donor (Ti) or acceptor (Zn) impurity). The dependence of relaxation shifts of interstitial atoms, the position of the chemical potential level, energy band boundaries, valence band widths, and energies corresponding to the intraband maxima of the density of states on the dopant concentration is discussed.     

Si-decorated graphene: a superior media for lithium-ions storage

The characteristics of lithium adsorption on Si-decorated graphene are investigated using first-principles density functional theory calculations. It is found that the Si atom is strongly adsorbed at the bridge site of the C–C bond with binding energy of about ?26.75 kcal/mol. We show that Si decorating turns Si:graphene complex into an electron-deficient system and significantly enhances the Li-storage capacity on the graphene. The obtained results indicate that up to eight Li-ions being adsorbed onto the Si-decorated graphene can form the stable complex. It is found, interestingly, that two Si atoms coated onto double-side of the graphene can strongly adsorb sixteen Li-ions. The analyses of electronic structures show a strong interaction between Li-ions and Si-decorated graphene leading to a high exothermicity. The stability of the sixteen Li-ions adsorbed on the Si:graphene system was evaluated with ab initio molecular dynamics simulation which have been carried out at room temperature. Our first-principles results are relevant to identify the potential applications of Si-decorated graphene as superior media for Li-ions storage.     

Low-lying electronic states of Ben clusters (n = 7, 10, 13) and of the Be7H system

Self-consistent-field (SCF) calculations have been performed by means of a pseudopotential (PP) technique on medium-size Be_n clusters (n = 7, 10, 13). Correlation effects have been taken into account through multireference double-excitation configuration-interaction (MRD CI) procedure. Particular attention has been paid to the existence of Be clusters with many nearly degenerated states of singlet and triplet spin multiplicity. The SCF ordering of these states is frequently reversed in CI. Planar and non-planar Be clusters show comparable high stabilities caused by a strong sp hybridization. Two sections of the potential energy surface for the interaction of a H atom with the Be₇(7,0) cluster have been determined. Two regions of low energy found in this energy surface: one inside and the second outside the cluster border, are separated by an energy barrier. The CI results, indicating the directly overhead position as the absolute minimum of the surface, are in qualitative agreement with previous SCF studies on similar systems. The modifications of the cluster geometry (shrinkage or relaxation) caused by the interaction with the H atom in the directly overhead position are found to be very small.     

Nature of coordination bond in silatranes and its formation dynamics according to the ab initio calculations

Quantum-chemical calculations of 1-hydrosilatrane molecule with complete optimization of its geometry and at various fixed Si…N distances (2.0 to 3.7 Å) has been carried out at the MP2/6-31G(d) level. The silatranes coordination bond is formed of different atomic orbitals of Si and N atoms participating in a series of molecular orbitals. With the Si…N distance decreasing, contributions of the atomic orbitals in these molecular orbitals have been changed, number of the molecular orbitals has increased, and total energy of the molecule has decreased. At the coordination centers are getting closer, population of the nitrogen valence s and p _z orbitals have changed due to the corresponding bond angle change; the populations of Si and H orbitals are not significantly changed.     

Pulsed EPR characterization of encapsulated atomic hydrogen in octasilsesquioxane cages

Hydrogen atoms encapsulated in molecular cages are potential candidates for quantum computing applications. They provide the simplest two-spin system where the 1s electron spin, S = 1/2, is hyperfine-coupled to the proton nuclear spin, I = 1/2, with a large isotropic hyperfine coupling (A = 1420.40575 MHz for a free atom). While hydrogen atoms can be trapped in many matrices at cryogenic temperatures, it has been found that they are exceptionally stable in octasilsesquioxane cages even at room temperature [Sasamori et al., Science, 1994, 256, 1691]. Here we present a detailed spin-lattice and spin-spin relaxation study of atomic hydrogen encapsulated in Si(8)O(12)(OSiMe(2)H)(8) using X-band pulsed EPR spectroscopy. The spin-lattice relaxation times T(1) range between 1.2 s at 20 K and 41.8 μs at room temperature. The temperature dependence of the relaxation rate shows that for T < 60 K the spin-lattice relaxation is best described by a Raman process with a Debye temperature of θ(D) = 135 K, whereas for T > 100 K a thermally activated process with activation energy E(a) = 753 K (523 cm(-1)) prevails. The phase memory time T(M) = 13.9 μs remains practically constant between 200 and 300 K and is determined by nuclear spin diffusion. At lower temperatures T(M) decreases by an order of magnitude and exhibits two minima at T = 140 K and T = 60 K. The temperature dependence of T(M) between 20 and 200 K is attributed to dynamic processes that average inequivalent hyperfine couplings, e.g. rotation of the methyl groups of the cage organic substituents. The hyperfine couplings of the encapsulated proton and the cage (29)Si nuclei are obtained through numerical simulations of field-swept FID-detected EPR spectra and HYSCORE experiments, respectively. The results are discussed in terms of existing phenomenological models based on the spherical harmonic oscillator and compared to those of endohedral fullerenes.     

Structure,spectroscopy and electronic properties of neutral lattice-like (MgO) n clusters: a study based on a blending of DFT with stochastic algorithms inspired by natural processes

In this article, we explore the feasibility of using stochastic optimization techniques, which are inspired by natural processes, namely simulated annealing (SA) and genetic algorithm (GA) in association with DFT, to find out the global minimum structures of (MgO) _n clusters with n being in the range of 2–15. To check whether the structures are indeed the correct ones, we proceed to do several property calculations like IR-spectroscopic modes, vertical excitation energy, cluster-formation energy, vertical ionization potential, the HOMO–LUMO gap as well as polarizability and hyperpolarizability—both static and dynamic. We emphasize on the point that an initial determination of structure using SA/GA leads to very quick relaxation to structures which are very close to the structures predicted from quantum chemical calculations done from the outsets like DFT. The general pattern of these systems to form beautiful three-dimensional lattice networks is also evident from our study.     

Li ion diffusion in the anode material Li12Si7: ultrafast quasi-1D diffusion and two distinct fast 3D jump processes separately revealed by 7Li NMR relaxometry

The intermetallic compounds Li(x)Si(y) have attracted considerable interest because of their potential use as anode materials in Li ion batteries. In addition, the crystalline phases in the Li-Si phase diagram turn out to be outstanding model systems for the measurement of fast Li ion diffusion in solids with complex structures. In the present work, the Li self-diffusivity in crystalline Li(12)Si(7) was thoroughly probed by (7)Li NMR spin-lattice relaxation (SLR) measurements. Variable-temperature and -frequency NMR measurements performed in both the laboratory and rotating frames of reference revealed three distinct diffusion processes in Li(12)Si(7). The diffusion process characterized by the highest Li diffusivity seems to be confined to one dimension. It is one of the fastest motions of Li ions in a solid at low temperatures reported to date. The Li jump rates of this hopping process followed Arrhenius behavior; the jump rate was ~10(5) s(-1) at 150 K and reached 10(9) s(-1) at 425 K, indicating an activation energy as low as 0.18 eV.     

Evidence of defect-promoted reactivity for epoxidation of propylene in titanosilicate (TS-1) catalysts: a DFT study

Our density functional theory study of hydroperoxy (OOH) intermediates on various model titanosilicalite (TS-1) Ti centers explores how microstructural aspects of Ti sites effect propylene epoxidation reactivity and shows that Ti sites located adjacent to Si vacancies in the TS-1 lattice are more reactive than fully coordinated Ti sites, which we find do not react at all. We show that propylene epoxidation near a Si-vacancy occurs through a sequential pathway where H(2)O(2) first forms a hydroperoxy intermediate Ti-OOH (15.4 kcal/mol activation energy) and then reacts with propylene by proximal oxygen abstraction (9.3 kcal/mol activation energy). The abstraction step is greatly facilitated through a simultaneous hydride transfer involving neighboring terminal silanol groups arising from the Si vacancy. The transition state for this step exhibits 6-fold oxygen coordination on Ti, and we conclude that the less constrained environment of Ti adjacent to a vacancy accounts for greater transition state stability by allowing relaxation to a more octahedral geometry. These results also show that the reactive hydroperoxy intermediates are generally characterized by smaller electron populations on the proximal oxygen atom compared to nonreactive intermediates and greater O-O polarization--providing a potential means of computationally screening novel titanosilicate structures for epoxidation reactivity.     

Effect of the erbium dopant architecture on the femtosecond relaxation dynamics of silicon nanocrystals

Femtosecond pump-probe absorption spectroscopy is used to investigate the role of Er(3+) dopants in the early relaxation pathways of photoexcited Si nanocrystals. The fate of photoexcited electrons in three different Si nanostructures was studied and correlated with the effect of Er-doping and the nature of the dopant architecture. In Si nanocrystals without Er(3+) dopant, a trapping component was identified to be a major electron relaxation mechanism. Addition of Er(3+) ions into the core or surface shell of the nanocrystals was found to open up additional nonradiative relaxation pathways, which is attributed to Er-induced trap states in the Si host. Analysis of the photodynamics of the Si nanocrystal samples reveals an electron trapping mechanism involving trap-to-trap hopping in the doped nanocrystals, whereby the density of deep traps seem to increase with the presence of erbium. To gain additional insights on the relative depths of the trapping sites on the investigated nanostructures, benzoquinone was used as a surface adsorbed electron acceptor to facilitate photoinduced electron transfer across the nanocrystal surface and subsequently assist in back electron transfer. The established reduction potential (-0.45 V versus SCE) of the electron acceptor helped reveal that the erbium-doped nanocrystal samples have deeper trapping sites than the undoped Si. Furthermore, the measurements indicate that internally Er-doped Si have relatively deeper trapping sites than the erbium surface-enriched nanocrystals. The electron-shuttling experiment also reveals that the back electron transfer seems not to recover completely to the ground state in the doped Si nanocrystals, which is explained by a mechanism whereby the electrons are captured by deep trapping sites induced by erbium addition in the Si lattice.     

The COHe system revisited: V—T collisional relaxation from AB initio potentials

A new potential energy surface (PES), recently computed within an ab initio scheme for the CO + He system, is here used to calculate vibrationally inelastic cross sections via a coupled quantum treatment of the dynamics. Results are obtained with different decoupling approximations (BS and IOS) and compared with those yielded by a previously available PES [3] that showed too strong an attractive behaviour. The effect of anisotropic components in the potential is also analysed and discussed for both PES and appears to play a noticeable role only at low collision energies. Relaxation rates are also computed and used to obtain relaxation times within the Landau—Teller (LT) scheme. Comparison with experiments is very satisfactory at high temperatures and medium temperatures (3000 ? T ? 200 K) while low T behaviour is overestimated by the present calculations.     

Vibrational relaxation of compressed nD2 fluid

Experimental observations of the vibrational population relaxation time of ⁿD₂ fluid under pressures of up to 500 atm in the 25–85 K range are presented and described in terms of a semi-classical model for energy transfer in liquids. For comparison with the parameters of this model, a classical equivalent potential for quantum systems is derived from the “real” intermolecular potential.     

Structures and relative stability of medium- and large-sized silicon clusters. VI. Fullerene cage motifs for low-lying clusters Si(39), Si(40), Si(50), Si(60), Si(70), and Si(80)

We performed a constrained search, combined with density-functional theory optimization, of low-energy geometric structures of silicon clusters Si(39), Si(40), Si(50), Si(60), Si(70), and Si(80). We used fullerene cages as structural motifs to construct initial configurations of endohedral fullerene structures. For Si(39), we examined six endohedral fullerene structures using all six homolog C(34) fullerene isomers as cage motifs. We found that the Si(39) constructed based on the C(34)(C(s):2) cage motif results in a new leading candidate for the lowest-energy structure whose energy is appreciably lower than that of the previously reported leading candidate obtained based on unbiased searches (combined with tight-binding optimization). The C(34)(C(s):2) cage motif also leads to a new candidate for the lowest-energy structure of Si(40) whose energy is notably lower than that of the previously reported leading candidate with outer cage homolog to the C(34)(C(1):1). Low-lying structures of larger silicon clusters Si(50) and Si(60) are also obtained on the basis of preconstructed endohedral fullerene structures. For Si(50), Si(60), and Si(80), the obtained low-energy structures are all notably lower in energy than the lowest-energy silicon structures obtained based on an unbiased search with the empirical Stillinger-Weber potential of silicon. Additionally, we found that the binding energy per atom (or cohesive energy) increases typically >10 meV with addition of every ten Si atoms. This result may be used as an empirical criterion (or the minimal requirement) to identify low-lying silicon clusters with size larger than Si(50).     

X-ray photoelectron spectroscopic analysis of Si nanoclusters in SiO2 matrix

We investigated silicon nanoclusters Si(nc) in a SiO2 matrix prepared by the plasma-enhanced chemical vapor deposition technique, using X-ray photoelectron spectroscopy (XPS) with external voltage stimuli in both static and pulsed modes. This method enables us to induce an additional charging shift of 0.8 eV between the Si2p peaks of the oxide and the underlying silicon, both in static and time-resolved modes, for a silicon sample containing a 6 nm oxide layer. In the case of the sample containing silicon nanoclusters, both Si2p peaks of Si(nc) and host SiO2 undergo a charging shift that is 1 order of magnitude larger (>15 eV), with no measurable difference between them (i.e., no differential charging between the silicon nanoclusters and the oxide matrix could be detected). By use of a measured Auger parameter, we estimate the relaxation energy of the Si(nc) in the SiO2 matrix as -0.4 eV, which yields a -0.6 eV shift in the binding energy of the Si(nc) with respect to that of bulk Si in the opposite direction of the expected quantum size effect. This must be related to the residual differential charging between the silicon nanoclusters and the oxide host. Therefore, differential charging is still the biggest obstacle for extracting size-dependent binding energy shifts with XPS when one uses the oxide peak as the reference.