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In molecular J-aggregates one often observes an increase of the fluorescence decay time when increasing the temperature from 0 K. This phenomenon is usually attributed to the thermal population of the dark Frenkel exciton states that lie above the superradiant bottom state of the exciton band. In this paper, we study this effect for a homogeneous one-dimensional aggregate in a host medium and we model the scattering between different exciton states as arising from their coupling to the host vibrations. A Pauli master equation is used to describe the redistribution of excitons over the band. The rates entering this equation are calculated within the framework of first-order perturbation theory, assuming a linear on-site interaction between excitons and acoustic phonons. Solving the master equation numerically for aggregates of up to 100 molecules, we calculate the temperature dependence of the fluorescence kinetics in general and the decay time scale in particular. The proper definition of the fluorescence decay time is discussed in detail. We demonstrate that, even at a quantum yield of unity, the possibility to directly interpret fluorescence experiments in terms of a simple radiative time scale depends crucially on the initial excitation conditions in combination with the competition between spontaneous emission and intraband phonon-assisted relaxation. [ABSTRACT FROM AUTHOR]

A mathematically strict analysis of the consequences of the requirement of wave function antisymmetry is presented. It is shown that the probability density for symmetric electron configurations is zero, i.e., these electron configurations in atoms are inaccessible owing to the action of fundamental forces termed by the author "the forces of symmetry". As a consequence, a fundamentally new, in computational aspect, quantum chemistry can be developed. A previously reported hypothesis that forbids two electrons with the same spins to have equal potentials is confirmed. [ABSTRACT FROM AUTHOR]

The correlation of electron counts from two detectors illuminated by a coherent electron beam is analyzed by associating the path of the electron beam through the lenses with the direct Coulomb interaction between two individual electrons. This is shown to lead to a full statistical description of the electron counts. The dominant contribution to the correlation is found to be due to the trajectory displacement caused by the repulsive Coulomb interaction between the first anode and the cathode tip, and the correlation of electron counts is found to depend on the amount of defocusing on the shift of the virtual source for two electrons within the correlation time. The Coulomb scatterings, which altered the direction of two neighbor electrons, occur during the acceleration, leading to a significant decrease in the electron density. The Coulomb potential with no screening will then cause large-angle scattering of nearest-neighbor electrons within the correlation time. These results are consistent with those obtained by a previous experiment. [ABSTRACT FROM AUTHOR]

Relativistic effects and electron correlation effects on the dipole moments of the coinage metal hydrides are investigated and compared employing one-component (scalar) relativistic approximations based on the mass–velocity and Darwin operator and, alternatively, the Douglas–Kroll-transformed spin-averaged no-pair Hamiltonian. The former of the two operators is found to perform quite accurately for CuH and AgH. For AuH the limits of the Pauli approximation seem to be reached, as can be inferred from a comparison with the values obtained within the spin-averaged Douglas–Kroll no-pair formalism. The coupled cluster calculations in the Douglas–Kroll no-pair approximation for relativistic effects establish the dipole moment values of the coinage metal hydrides as equal to 1.05 a.u. for CuH, 1.14 a.u. for AgH and 0.52 for AuH. The corresponding non-relativistic results are 1.14 a.u., 1.36 a.u., and 1.22 a.u., respectively. Some formal problems arising in applications of the Douglas–Kroll no-pair approximation are discussed. It is shown that the Hellmann–Feynman theorem leads to a rather complicated form of the first-order energy change due to external perturbation. The usual expectation value formula is, however, valid through terms proportional to 1/c4 and can be used in most applications. The invariance property with respect to a shift in the external potential is addressed for the Douglas–Kroll no-pair approximation in a finite basis set. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The lowest energies of molecular systems with fourfold symmetry can violate Hund’s multiplicity rule. We derive relationships between two electron integrals that in favorable cases allow one to predict when this will happen. These relationships result in a fairly simple inequality as a criterion. The systematics of the phenomenon are illustrated with a series of calculations on model systems consisting of hydrogen atoms. These are rings of three, four, five, six, seven, and eight members as well as octahedral and square planar arrangements. These show how the results and simple ideas of molecular orbital theory can be qualitatively modified when various levels of configuration interaction are allowed in the theoretical discription. [ABSTRACT FROM AUTHOR]

The Pauli exclusion principle (PEP) can be considered from two aspects. First, it asserts that particles that have half-integer spin (fermions) are described by antisymmetric wave functions, and particles that have integer spin (bosons) are described by symmetric wave functions. It is called spin-statistics connection (SSC). The physical reasons why SSC exists are still unknown. On the other hand, PEP is not reduced to SSC and can be consider from another aspect, according to it, the permutation symmetry of the total wave function can be only of two types: symmetric or antisymmetric. They both belong to one-dimensional representations of the permutation group, while other types of permutation symmetry are forbidden. However, the solution of the Schrödinger equation may have any permutation symmetry. We analyze this second aspect of PEP and demonstrate that proofs of PEP in some wide-spread textbooks on quantum mechanics, basing on the indistinguishability principle, are incorrect. The indistinguishability principle is insensitive to the permutation symmetry of wave function. So, it cannot be used as a criterion for the PEP verification. However, as follows from our analysis of possible scenarios, the permission of states with permutation symmetry more general than symmetric and antisymmetric leads to contradictions with the concepts of particle identity and their independence. Thus, the existence in our Nature particles only in symmetric and antisymmetric permutation states is not accidental, since all symmetry options for the total wave function, except the antisymmetric and symmetric, cannot be realized. From this an important conclusion follows, we may not expect that in future some unknown elementary particles that are not fermions or bosons can be discovered.

The VIP-2 collaboration runs an apparatus in the Gran Sasso underground laboratories of the Italian Institute for Nuclear Physics (INFN) designed to search for anomalous X-rays from electron-atom interactions due to violations of the fundamental antisymmetry of multi-electron wavefunctions. The experiment implements the scheme first proposed by Ramberg and Snow, where a current source injects electrons into a metal strip (the experiment’s target). In this paper we describe the structure of a Monte Carlo program to simulate a new upgrade of the experiment, where the anomalous X-ray emission is modulated by an arbitrary time-varying input current. A novel feature of the simulation algorithm is that the Monte Carlo program is based on a mixture of analytical and numerical methods. We report preliminary, exploratory results on the expected detection rate for different modulations of the injected current; these results are a starting point on the way to optimize the modulation scheme and indicate a large potential improvement of the detection sensitivity.

The modern state of the Pauli exclusion principle is shortly discussed. We describe the discovery by Pauli, his principle for electrons, and how it was generalized for all elementary particles in the framework of quantum mechanics. The motivations and theoretical conceptions that induced the experiments for verification of the Pauli exclusion principle are analyzed. The results and methodology of two different types of experiments are discussed: (1) the search of unusual atoms and nuclei in the stable non-Pauli states, and (2) the experiments in which the emitted radiation of non-Pauli transitions is measured. In conclusion, the comments on the discussed experiments that follow from the general quantum mechanical conceptions and group theory are formulated.

Friedel oscillations appear in density of Fermi gases due to Pauli exclusion principle and translational symmetry breaking nearby a defect or impurity. In confined Fermi gases, this symmetry breaking occurs also near to boundaries. Here, density oscillations of a degenerate and confined Fermi gas are considered and characterized. True nature of density oscillations are represented by analytical formulas for degenerate conditions. Analytical characterization is first done for completely degenerate case, then temperature effects are also incorporated with a finer approximation. Envelope functions defining the upper and lower bounds of these oscillations are determined. It is shown that the errors of obtained expressions are negligible as long as the system is degenerate. Numbers, amplitudes, averages and spatial coordinates of oscillations are also given by analytical expressions. The results may be helpful to efficiently predict and easily calculate the oscillations in density and density-dependent properties of confined electrons at nanoscale. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The halogen bonds YX⋯CO are not primarily electrostatic in nature. • The σ-hole magnitude of YX has influences on all the stabilizing components of the LMOEDA method. • The PAEM function is a good descriptor of the covalence of the X⋯C interactions. Abstract A theoretical study of linear YX⋯CO complexes (in which YX are diatomic interhalogens or hydrogen halide) has been performed to high-level quantum chemical calculations. Calculations show that at the equilibrium geometry, the stabilizing effect that arises from the Pauli exclusion principle is more important than the electrostatic interactions. In addition, the stabilizing terms of the LMOEDA scheme increase with the σ-hole magnitude, being the exchange term the most affected one, followed by electrostatics, polarization and dispersion components. The PAEM function measured on the X⋯C interatomic interaction line gives a measure of the covalence of these interactions. [ABSTRACT FROM AUTHOR]

A contradiction exists between the gravitational singularity at the center of a black hole predicted by general relativity and the Pauli exclusion principle. General relativity theory asserts that collapsing stars over a certain size mass have no stable orbits and will become a gravitational singularity forming a "black hole," having finite mass within a point sized space inside an event horizon. On the other hand, the Pauli-exclusion principle predicts that for large collapsing stars, a quark-gluon plasma will be created to form a quark star at the central core inside an event horizon. Accordingly, the exclusion principle will preclude the collapse of a star into a gravitational singularity at the center of a black hole. A number of arguments in support of the quark star as black hole will be presented in this paper. It has been shown that the gravitational energy of a collapsed star (>3M☉), near the core of a quark star, exceeds the energy to create a quark-gluon plasma and the deconfinement energy of quarks from the neutron. It has also been shown that, instead of a black hole, a degenerate nonstrange quark star with a maximum density of 1.1 x 1025 kg/m3 could exist at the center of an event horizon within the Schwarzschild radius. The quark star core would be an ultrarelativistic degenerate Fermi gas that is stable for masses from 3M☉ to 20.69 x106 M☉. Calculations have also shown for stellar and rotating black holes that the quark star radius exists at the center and well within the radius of the Schwarzschild event horizon. However, the exclusion principle would preclude the formation of a gravitational singularity at the center. The problem for empirical science is that the quark star with an event horizon will have no emission of radiation and appear to observers to be similar to a black hole. Notwithstanding, the merger of two black holes for GW150914, producing gravitational waves, offers empirical evidence in the remnant event horizon for the existence of quark stars. [ABSTRACT FROM AUTHOR]

We reconsider the lower bound on the mass of a fermionic dark matter (DM) candidate resulting from the existence of known small dwarf spheroidal galaxies, in the hypothesis that their DM halo is constituted by degenerate fermions, with phase-space density limited by the Pauli exclusion principle. By relaxing the common assumption that the DM halo scale radius is tied to that of the luminous stellar component and by marginalizing on the unknown stellar velocity dispersion anisotropy' we prove that observations lead to rather weak constraints on the DM mass, which could be as low as tens of eV. In this scenario, however, the DM haloes would be quite large and massive, so that a bound stems from the requirement that the time of orbital decay due to dynamical friction in the hosting Milky Way DM halo is longer than their lifetime. The smallest and nearest satellites Segue I and Willman I lead to a final lower bound of m > 100 eV, still weaker than previous estimates but robust and independent on the model of DM formation and decoupling. We thus show that phase-space constraints do not rule out the possibility of sub-keV fermionic DM. [ABSTRACT FROM AUTHOR]

As one of the most widely used chemical concepts whose origin can be traced back to Lewis theory of bonding a century ago, a covalent bond involves sharing one or more pairs of electrons between atoms. A strong covalent interaction (SCI) is such a covalent bond that two or more electron pairs are shared, yielding a double, triple, quadruple, or even higher bond order. Despite its ubiquity and usefulness, a robust and generally applicable approach to accurately identify strong covalent interactions and determine their bond orders is still lacking. In this work, an SCI index is proposed from density functional theory using the Pauli energy, which is the contribution of the Pauli exclusion principle to the kinetic energy. Illustrative examples from organic, inorganic, and organometallic systems were provided. Its close relationship with the electron localization function (ELF) was elucidated. Both ELF and SCI generate similar results. Two complexes with a quintuple metal-metal bond have been confirmed. A stronger than quintuple bond has been showcased. This work should provide a robust approach to determine bond orders for strong covalent interactions in complex systems, pinpoint the physiochemical origin of strong covalent interactions, and rationalize the usefulness of both SCI and ELF. These tools should be able to be applied to other systems in different fields to effectively appreciate strong covalent interactions. [ABSTRACT FROM AUTHOR]