A model of fast heating of nitrogen–oxygen mixtures excited by a gas discharge in a broad range of reduced electric fields
E
/
N
is presented. It is shown that in air at
E
/
N
⩽ 200 Td the main ...contribution to gas heating occurs due to dissociation reactions by electron impact of O
2
molecules and due to processes of quenching of electronically excited N
2
(
B
3
Π
g
,
C
3
Π
u
,
) molecules by oxygen and excited O(
1
D) atoms by nitrogen. At
E
/
N
> 400 Td, dissociation reactions by electron impact of N
2
molecules are dominant as well as the processes involving charged particles. The fraction of discharge energy converted to fast gas heating does not exceed 40%. An analysis of the experimental data on fast air heating in discharges at high reduced electric fields
E
/
N
is given.
It was shown that, in a broad range of reduced electric fields, a fixed fraction of discharge power η
E
spent on the excitation of electronic degrees of freedom, ionization and dissociation of molecules is converted to fast heating of nitrogen–oxygen mixtures. In air, the value of η
E
is about 30 ± 3%. The value of η
E
diminishes with decreasing share of oxygen in a mixture. The significant role of heat release in the pooling reactions of
molecules for fast gas heating in pure nitrogen and in nitrogen with small admixtures of oxygen is demonstrated. The simulation results agree with experimental data at
E
/
N
< 200 Td within the range of oxygen content δ = 0–20%.
The results of a numerical study on kinetic processes initiated by a pulsed nanosecond discharge in air at high specific deposited energy, when the dissociation degree of oxygen molecules is high, ...are presented. The calculations of the temporal dynamics of the electron concentration, density of atomic oxygen, vibrational distribution function of nitrogen molecules, and gas temperature agree with the experimental data. It is shown that quenching of electronically excited states of nitrogen N2(B3Πg), N2( 3Πu), N2(a′1Σu−) by oxygen molecules leads to the dissociation of O2. This conclusion is based on the comparison of calculated dynamics of atomic oxygen in air, excited by a pulsed nanosecond discharge, with experimental data. In air plasma at a high dissociation degree of oxygen molecules (O/O2 > 10%), relaxation of the electronic energy of atoms and molecules in reactions with O atoms becomes extremely important. Active production of NO molecules and fast gas heating in the discharge plasma due to the quenching of electronically excited N2(B3Πg, C3Πu, a′1Σu−) molecules by oxygen atoms is notable. Owing to the high O atom density, electrons are effectively detached from negative ions in the discharge afterglow. As a result, the decay of plasma in the afterglow is determined by electron-ion recombination, and the electron density remains relatively high between the pulses. An increase in the vibrational temperature of nitrogen molecules at the periphery of the plasma channel at time delay t = 1-30 μs after the discharge is obtained. This is due to intense gas heating and, as a result, gas-dynamic expansion of a hot gas channel. Vibrationally excited N2(v) molecules produced near the discharge axis move from the axial region to the periphery. Consequently, at the periphery the vibrational temperature of nitrogen molecules is increased.
An original technique for describing excited states of electrons in crystal structure has been considered by an example of lithium. It is shown that the electron spectrum in lithium changes only ...slightly at large values of lattice parameter (up to 8.77 Bohr radii). The lifetimes of excited electrons of external
s
and
p
states differ significantly at lattice parameters
d
< 8.77 Bohr radii. A metastable crystalline state of bcc lithium is found, which barely depends on the excitation power at a lattice constant equal to 6.55 Bohr radii, corresponding to the bcc lattice constant of lithium in the ground state.
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This article outlines the magnetic properties of single molecule magnets based on Dy-encapsulating endohedral metallofullerenes. The factors that govern these properties, such as the influence of ...different non-metal species in clusterfullerenes, the cage size, and cage isomerism are discussed, as well as the recent successful isolation of dimetallofullerenes with unprecedented magnetic properties. Finally, recent advances towards the organization of endohedral metallofullerenes in 1D, 2D, and 3D ordered structures with potential for devices are reviewed.
Encapsulation of dysprosium ions in fullerenes results in efficient air stable single molecule magnets, which can be used in preparation of various 1D, 2D, and 3D assemblies.
A review of experimental and theoretical investigations of the effect of atomic particles, and electronically and vibrationally excited molecules on the induction delay time and on the shift in the ...ignition temperature threshold of combustible mixtures is presented. The addition of oxygen and hydrogen atoms to combustible mixtures may cause a significant reduction in the ignition delay time. However, at relatively low initial temperatures, the non-equilibrium effect of the addition of atomic particles in ground electronic states is not pronounced. At the same time, the effect of excited O(1D) atoms on the oxidation and reforming of combustible mixtures is quite significant due to the high rates of reactions of O(1D) atoms with hydrogen and hydrocarbon molecules. In fuel-air mixtures, collisions with O(1D) atoms determine, under certain conditions, the dissociation of hydrocarbon molecules. Singlet oxygen molecules, O2(a1Δg), participate both in chain initiation and chain branching reactions, but the effect of O2(a1Δg) on the ignition processes is generally less important compared to oxygen atoms. The reactions of vibrationally excited molecules and the processes of VT-relaxation in combustible mixtures are discussed. The production of vibrationally excited N2(v) molecules in fuel-air mixtures at relatively low electric field is very important. However, at the moment, the effect of the reactions of N2(v) molecules on the oxidation and ignition of combustible mixtures is not completely clear, and requires further investigation. Therefore, with present knowledge, to reduce the ignition delay time and decrease the temperature threshold of combustive mixtures, the use of gas discharge systems with relatively high E/N values is recommended. In this case the reactions of electronically excited N2(A3Σu+,B3Пg,C3Пu,a′1Σu−) molecules, and atomic particles in ground and electronically excited states, are extremely important. The energy stored in electronically excited states of atoms and molecules is spent on the additional dissociation of oxygen and fuel molecules, on fast gas heating, and finally on triggering chain branching reactions.
Recent results from a search for multi-quark exotic states at D0 experiment (FNAL, USA) are presented. This includes the new data for possible tetraquark state X(5568) decaying to Bs0π±, in the ...channels with semileptonic decays of Bs0 mesons. Also, result from the J/ψπ system analysis and an evidence for exotic charged charmoniumlike state Zc(3900) in semiinclusive weak decays of B-flavored hadrons are presented as well.
Metal–cage and intracluster bonding was studied in detail by quantum theory of atoms in molecules (QTAIM) for the four major classes of endohedral metallofullerenes (EMFs), including ...monometallofullerenes Ca@C72, La@C72, M@C82 (M=Ca, Sc, Y, La), dimetallofullerenes Sc2@C76, Y2@C82, Y2@C79N, La2@C78, La2@C80, metal nitride clusterfullerenes Sc3N@C2n (2n=68, 70, 78, 80), Y3N@C2n (2n=78, 80, 82, 84, 86, 88), La3N@C2n (2n=88, 92, 96), metal carbide clusterfullerenes Sc2C2@C68, Sc2C2@C82, Sc2C2@C84, Ti2C2@C78, Y2C2@C82, Sc3C2@C80, as well as Sc3CH@C80 and Sc4Ox@C80 (x=2, 3), that is, 42 EMF molecules and ions in total. Analysis of the delocalization indices and bond critical point (BCP) indicators such as Gbcp/ρbcp, Hbcp/ρbcp, and |Vbcp|/Gbcp, revealed that all types of bonding in EMFs exhibit a high degree of covalency, and the ionic model is reasonable only for the Ca‐based EMFs. Metal–metal bonds with negative values of the electron‐density Laplacian were found in Y2@C82, Y2@C79N, Sc4O2@C80, and anionic forms of La2@C80. A delocalized nature of the metal–cage bonding results in a topological instability of the electron density in EMFs with an unpredictable number of metal–cage bond paths and large elipticity values.
Metal–cage and intracluster bonding in endohedral metallofullerenes (EMFs), including mono‐ and dimetallofullerenes; metal carbide, nitride, and oxide clusterfullerenes; and Sc3CH§C80, was studied by quantum theory of atoms in molecules. The picture shows for La2§C78 a molecular graph in the vicinity of the La atoms (bond critical point (CP) red, ring CP yellow, cage CP green).
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The effect of the X-mode parametric decay into two short wavelength upper hybrid (UH) plasmons propagating in opposite directions is analyzed. Due to the huge convective power loss of both the UH ...plasmons along the inhomogeneity direction, the power threshold of the convective parametric decay instability (PDI), which can be excited in the presence of a monotonous density profile is derived to exceed the gyrotron power range currently available. In the presence of the magnetic island possessing the local density maximum at its O-point the daughter UH plasmons can be trapped in the radial direction that suppresses their energy loss from the decay layer in full and makes the power threshold of the convective two-plasmon PDI drastically (three orders of magnitude) lower than in the previous case. The possibility of the absolute PDI being due to the finite size of the pump beam spot is demonstrated as well. The power threshold of the absolute instability is shown to be more than two orders of magnitude lower than the threshold of the convective instability at the monotonous density profile.
Conspectus A characteristic phenomenon of lanthanide–fullerene interactions is the transfer of metal valence electrons to the carbon cage. With early lanthanides such as La, a complete transfer of ...six valence electrons takes place for the metal dimers encapsulated in the fullerene cage. However, the low energy of the σ-type Ln–Ln bonding orbital in the second half of the lanthanide row limits the Ln2 → fullerene transfer to only five electrons. One electron remains in the Ln–Ln bonding orbital, whereas the fullerene cage with a formal charge of −5 is left electron-deficient. Such Ln2@C80 molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln2@C79N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp2 carbon into an sp3 carbon and yielding the monoadduct Ln2@C80(CH2Ph). Because of the presence of the Ln–Ln bonding molecular orbital with one electron, the Ln2@C79N and Ln2@C80(R) molecules feature a unique single-electron Ln–Ln bond and an unconventional +2.5 oxidation state of the lanthanides. In this Account, which brings together metallofullerenes, molecular magnets, and lanthanides in unconventional valence states, we review the progress in the studies of dimetallofullerenes with single-electron Ln–Ln bonds and highlight the consequences of the unpaired electron residing in the Ln–Ln bonding orbital for the magnetic interactions between Ln ions. Usually, Ln···Ln exchange coupling in polynuclear lanthanide compounds is weak because of the core nature of 4f electrons. However, when interactions between Ln centers are mediated by a radical bridge, stronger coupling may be achieved because of the diffuse nature of radical-based orbitals. Ultimately, when the role of a radical bridge is played by a single unpaired electron in the Ln–Ln bonding orbital, the strength of the exchange coupling is increased dramatically. Giant exchange coupling in endohedral Ln2 dimers is combined with a rather strong axial ligand field exerted on the lanthanide ions by the fullerene cage and the excess electron density localized between two Ln ions. As a result, Ln2@C79N and Ln2@C80(CH2Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the Ln3+–e–Ln3+ fragment behaves as a single giant spin. Furthermore, the Ln–Ln bonding orbital in dimetallofullerenes is redox-active, which allows its population to be changed by electrochemical reactions, thus changing the magnetic properties because the change in the number of electrons residing in the Ln–Ln orbital affects the magnetic structure of the molecule.
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The trapping of a lower hybrid wave in the tokamak edge transport barrier is predicted, reducing by 3 orders of magnitude the excitation threshold for the absolute parametric decay instability that ...leads to side scattering of the ordinary microwave pump in electron cyclotron resonance heating (ECRH) experiments. This process is similar to the stimulated Raman scattering instability in laser physics and can result in substantial anomalous scattering of the pump wave, like in laser fusion experiments. The corresponding broadening of the ECRH power deposition profile can reduce the ability of this method to control the neoclassical tearing modes both in present day machines, as ASDEX-Upgrade, where the theory can be checked, and in fusion reactors such as ITER and DEMO.
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