Hypothetical Atom Research Articles

Superatomic crystal emerging in transition-metal oxides: Molybdenum hollandite K2Mo8O16

Density-functional-theory-based electronic structure calculations are carried out to elucidate the origins of the observed electronic properties of molybdenum hollandite K${}_{2}$Mo${}_{8}$O${}_{16}$. We find that the Mo${}_{4}$ cluster in the double Mo chains behaves as a ``superatom,'' a hypothetical big atom with a single composite molecular orbital, and that the system can be regarded as a solid of the superatoms condensed into a simple monoclinic structure with four superatoms per unit cell, thereby yielding four energy bands near the Fermi level at half filling. Based on an effective model proposed, we argue that K${}_{2}$Mo${}_{8}$O${}_{16}$ is a Mott insulator with one electron per superatom, which exhibits strongly frustrated antiferromagnetic spin correlations in the superatomic crystal.

Stability of the Pd-Co-Ni-Cu-P Metallic Glasses

AbstractStability of Pd-Co-Ni-Cu-P metallic glass was investigated in terms of free energy using first principle cluster calculations, thermal analysis, and photoemission spectroscopy measurements. We found that the internal energy of the Pd-based metallic glasses is dominated by the electronic structure near the Fermi level. The analyses on the electronic structure and local atomic arrangements indicate that the substitution of cobalt or a hypothetical atom Co0.5Cu0.5 for nickel in the Pd40Ni40P20 metallic glass decreases the free energy of the Pd-Ni-P metallic glass by increasing entropy without altering significantly internal energy. On the basis of the idea mentioned above, we prepared Pd28Co24Ni24P24, Pd25Co25Ni25P25 and Pd40Co40/3Ni40/3Cu40/3P20 metallic glasses. These metallic glasses certainly showed the nearly highest TX, which directly reflect the activation energy against crystallization, among the Pd-based metallic glasses ever reported.

Absolute and total electron-capture cross sections in slowArq+−C60collisions

We present experimental absolute and total electron-capture cross sections for ${\mathrm{Ar}}^{q+}\ensuremath{-}{\mathrm{C}}_{60}$ $(q=4,$ 6, and 8--18) collisions at $3.3q \mathrm{keV}.$ The absolute scale is based on the ${\mathrm{C}}_{60}$ vapor pressure by Abrefah et al. [Appl. Phys. Lett. 60, 1313 (1992)], which together with the results of Mathews et al. [J. Phys. Chem. 96, 3566, (1992)] are the only possible choices among the many, widely dispersed, vapor pressures reported in the literature. In order to support this statement, we calculate total electron-capture cross sections modeling ${\mathrm{C}}_{60}$ as a pointlike object and as an infinitely conducting sphere (ICS). These model results are shown to differ little, since polarization and finite-size effects are relatively unimportant for the distant collisions at which the outermost (first) electrons are transferred. We are thus able to use the semiempirical formula for absolute ion-atom cross sections by Selberg et al. [Phys. Rev. A 54, 4127 (1996)], treating ${\mathrm{C}}_{60}$ as a hypothetical atom with ionization potential ${I}_{1}=7.6 \mathrm{eV},$ and the ICS model to define lower and upper bounds for the absolute cross-section scale. We find significant oscillations in the total electron-capture cross sections as functions of q.

Structure-minty odour relationships: suggestion of an interaction pattern

Structure–odour relationships for the minty note were studied using a set of 150 compounds (68 minty and 82 not minty) with known olfactory descriptions and chemical structures. The study was based on the HBD (hydrogen bonding and dispersion) theory which considers that interactions involving molecules and receptor sites are mainly hydrogen bonds and dispersion forces. All the compounds of the set were examined and a combination of direct comparisons and chi-square tests allowed identification of relevant structural elements for the minty note. Twenty-four compounds (8 menthol isomers, 4 menthone isomers and 12 carveol and carvone derivatives) were chosen for their very precise olfactory description and because they constitute 12 enantiomeric pairs. Their low-energy conformations were computed using the Sybyl force field. Superimpositions on reference compounds of the likely conformations of each enantiomer were made using the Sybyl package, taking into account relevant structural elements previously identified. These comparisons showed that substituents in minty compounds must meet precise geometrical requirements and that, in spite of differences of location and nature for the functional oxygen group, they can receive a hydrogen bond from the same hypothetical atom of the receptor sites. Finally, from these structural and geometrical characteristics, an interaction pattern was proposed for the minty odour. © 1998 John Wiley & Sons, Ltd.

Roles of light interstitials in magnetism of Fe

We present ab-initio electronic structure calculations for layered (FeX) 1(Fe) 5 (X=B, C, and N) and Fe 4Z of perovskite structure, where Z denotes a hypothetical atom with a nuclear charge Z (5 ≤ Z ≤8). The calculations show, firstly, that the typical-element atoms enhance the magnetic moments of surrounding Fe atoms and, second, that chemical bonding between Z and Fe atoms drastically changes its character when Z increases continuously from 5 to 8. The latter, which causes a sudden increase in the magnetization near Z ⋍ 7, may be important to understand the magnetism of FeN systems in general.

Quo vadis electron-positron lines?

In strong electric fields the neutral electron-positron vacuum state becomes unstable and decays through pair creation. In a hypothetical atom with a charge exceeding the critical value Zcr ≃ 173 a charged vacuum will be formed, signalled by the spontaneous emission of positrons. In the laboratory sufficiently strong electric fields can be created transiently in heavy-ion collisions with total nuclear charge Z1 + Z2 > Zcr. Detailed coupled channel calculations for the time dependent two-centre Dirac equation have been performed to describe the dynamics of the electron shell in superheavy quasimolecules. K-hole production as well as δ-electron and positron emission are found to be sensitively dependent on the strong field in a collision. The emission of positrons is strongly enhanced in high-Z collisions. Clear experimental signals for supercritical positron creation could be expected in collisions above the nuclear Coulomb barrier if a "gaint" dinuclear system is formed with a lifetime larger than about 10−20 sec.In the spectra of positrons produced in heavy-ion collisions narrow structures with unexpected properties have been found in several experiments performed at GSI (Darmstadt). The subsequent observation of correlated e+ e− emission compatible with the two-body decay of a neutral particle state with mass around 1.8 MeV has led to widespread theoretical speculations on the existence and the nature of these objects. However, despite intense experimental and theoretical research no satisfactory explanation has been found. New elementary particles are clearly ruled out and also models of composite extended objects are beset with problems.

Matrix isolation spectroscopy of metal atoms generated by laser ablation. II. The Li/Ne, Li/D2, and Li/H2 systems

Results of experiments on lithium-doped cryogenic solids (Ne, D2, and H2) prepared by laser ablation of solid lithium are presented, including near UV-visible absorption spectra and thermal annealing studies. Li atoms are found in more than one type of trapping site in each of the systems studied. In Li/Ne matrices deposited at T=5 K, the main optical absorption feature is a so-called ‘‘triplet’’ absorption with peaks near 628, 641, and 650.5 nm. This observation extends this familiar result from heavier alkali/rare gas matrices to the lightest M/Rg matrix. In Li/D2 and Li/H2 matrices deposited at T=3 K, the main absorptions show an ‘‘asymmetrical doublet’’ pattern with peaks near 655 and 672 nm and 650 and 671 nm, respectively. The successful isolation of Li atoms in such light matrix host systems supports previous conjectures about the microscopic mechanism for trapping fast laser ablated atoms. Several highly symmetrical hypothetical Li atom trapping site structures are proposed and discussed in conjunction with the results of the classical Monte Carlo simulations reported in the following article of this journal. The observed absorption line shapes are best explained in the context of a Jahn–Teller effect caused by dynamic distortions of the trapping sites by lattice vibrations, with negligible matrix modification of the Li atom spin–orbit splitting.

Optically excited Zeeman coherences in atomic ground states: Nuclear-spin effects.

Optical experiments in alkali-metal atomic gases are usually interpreted in terms of the {ital J}=1/2 ground state, treating the electronic ground state as a degenerate two-level system. While this simplified level scheme has been quite successful in describing many experimental results, the nuclear spin can lead to significant modifications of the behavior. Apart from the obvious differences, such as the existence of hyperfine splitting, some more subtle effects are present that modify the dynamical as well as the equilibrium behavior of the system. An an example, the optical pumping process in the true atomic ground state is nonexponential and slower by at least an order of magnitude, compared to a hypothetical atom with nuclear spin zero. The limitations of the {ital J}=1/2 model are analyzed theoretically and experimentally for atomic sodium, and experimental methods are demonstrated that can help disentangle the contributions from different hyperfine components.

Calculation of isotope distributions in mass spectrometry. A trivial solution for a non-trivial problem

Isotope peak patterns in the mass spectra of chemical compounds result from the occurrence of polyisotopic elements. The various approaches and programs published for calculating such isotope distributions are complex or of limited applicability or have insufficient accuracy because of rounding errors, or need too much calculation time. The algorithm described here is simple, easy to understand, highly accurate and extremely fast. Instead of the known procedures involving stepwise development or expansion of polynomial expressions, the isotope distributions are calculated by combining precalculated patterns of hypothetical atom clusters. For large organic molecules, the calculation time is reduced by several orders of magnitude compared to the stepwise procedure, and accuracy is increased compared to the use of polynomial expressions with inappropriate neglect of smaller terms. Typical examples are discussed to illustrate the scope and limitations of this approach. Additional features of the program are the calculation of isotope peak patterns of mixtures and the calculation of high-resolution masses and intensities for any mass within a given isotope distribution.

The structural models of the Si/SiO 2 interface

In order to get a comprehensive understanding of a microscopic structure of the Si/SiO2 interfaces, various interface models constructed by using plastic balls and spokes have been characterized in terms of distortion energy and composition of the interface region in the models. The distortion energy has been represented by Keating-type potential. The composition has been simulated with our hypothetical atom probe measurements. The magnitude of the distortion energy at the interface depends on the orientation of crystalline Si and decreases in the order of (100), (110) and (111). The distortion energy at the (100)Si/SiO2 interface delineated by (111) facets is smaller than the one at the flat and abrupt interface. These results are in good agreement with TEM observations that the (111)Si/SiO2, interface is flat while the (100)Si/SiO2 interface is usually rough. A transition region which apparently seems to be stoichiometric SiO is identified by the hypothetical atom probe measurement between crystalline Si and SiO2 in the model of the (100)Si/SiO2 interface delineated by {111} facets.

Quantum chemical study on acidity of silica-alumina catalysts

Acidities of silica-alumina systems are studied by the CNDO/2-FA method. A hypothetical atom is suggested for saturating the covalence of the outermost oxygen atoms of the models. It turns out that the hydrogen charge Q H can serve as a rough measure of Brønsted acidity of silica-alumina systems, whereas the LUMO energy as a measure of Lewis acidity can only be used for comparing the generally accepted Lewis sites of the same kind of atom in the same valence state and with the same coordinate number. Using the Q H data, the strong Brønsted site is assigned to the structure AlOHSi, and the moderate one is assigned to the SiOH in connection with a four-coordinate aluminum. The Q H data do not support the view that Si-OH in connection with a three-coordinate aluminum or AlOH is the strong Brønsted site.

The concept of the photon

The idea of the photon has stirred the imaginations of physicists ever since 1905 when Einstein originally proposed the use of light quanta to explain the photoelectric effect. This concept is formalized in the quantum theory of radiation, which has had unfailing success in explaining the interaction of electromagnetic radiation with matter, seemingly limited only by the ability of physicists to perform the indicated calculations. Nevertheless, it has its conceptual problems—various infinities and frequent misinterpretations. Consequently an increasing number of workers are asking, “to what extent is the quantized field really necessary and useful?” In fact the experimental results of the photoelectric effect were explained by G. Wentzel in 1927 without the quantum theory of radiation. Similarly most electro-optic phenomena such as stimulated emission, reaction of the emitted field on the emitting atom, resonance fluorescence, and so on, do not require the quantization of the field for their explanation. As we will see, these processes can all be quantitatively explained and physically understood in terms of the semiclassical theory of the matter–field interaction in which the electric field is treated classically while the atoms obey the laws of quantum mechanics. The quantized field is fundamentally required for accurate descriptions of certain processes involving fluctuations in the electromagnetic field: for example, spontaneous emission, the Lamb shift, the anomalous magnetic moment of the electron, and certain aspects of blackbody radiation. (The Compton effect also fits here, but see later under references 8b and c.) Here we will outline how the photon concept originated and developed, where it is not required and is often misused, and finally where it plays an essential role in the understanding of physical phenomena. In our discussion we will attempt to give a logically consistent definition of the word “photon”—a statement far more necessary than one might think, for so many contradictory uses exist of this elusive beast. In particular consider the original coining of the word by G. N. Lewis: “[because it appears to spend] only a minute fraction of its existence as a carrier of radiant energy, while the rest of the time it remains an important structural element within the atom…, I therefore take the liberty of proposing for this hypothetical new atom which is not light but plays an essential part in every process of radiation, the name photon!” (our exclamation point). Clearly the present usage of the word is very different.

Highlights of the Past and a Challenge for the Future

I am of course deeply gratified to have my name assigned to this laboratory, which is to be devoted to a field in which I have worked for so large a part of my life. Laboratory work has always fascinated me because of the unexpectedness of what may be found and of the application which may be made of the discoveries. No more important example of the unexpected can be found than Roentgen's discovery of the x-rays. This was nearly fifty-three years ago and was one of the great steps leading up to the present atomic age. This discovery enlarged tremendously the known spectrum of radiant energy, which had previously been carried only from what we now call radio waves to heat and light, including the ultraviolet. The nature of the new radiations, at first unknown, was found also to be electromagnetic, thus joining them to the short-wavelength end of the previously known spectrum, that is, to the ultraviolet. Prior to Roentgen's discovery we had talked of a hypothetical indivisible and indestructible atom, and had recognized in the various chemical elements 88 different kinds of atoms. We had long been aware of a certain periodicity in the properties of these atoms but could only wonder why this was so. X-ray research showed the reason—that the atoms of the various elements all consist of the same building blocks and differ only in the number and arrangement of these particles, which we now recognize as electrons, protons, and neutrons. Our hypothetical atom, that indivisible particle of matter, was found to consist of a whole swarm of smaller particles, more than 300 in the case of the heaviest elements. We have thought that there were only as many kinds of atoms as there are chemical elements, but we know today that each chemical element may occur in several different atom forms (isotopes) and we now list some seven hundred different kinds of atoms. In the year following Roentgen's discovery, radioactivity was first observed. It was in the case of uranium, whose atoms proved to be unstable and spontaneously to undergo a series of internal changes until they finally become lead, thus exploding the idea of inherent permanence of the atom. We had felt very sure of the conservation of mass. If you burned a weighed piece of wood, weighed the remaining ash, and took account of the gases which came off, you found no diminution in total weight. We know now that mass may disappear as such, reappearing as energy, and that the two are mutually interchangeable. Were our measurements sufficiently precise, we would find, when we burn a piece of wood, that there is a loss in weight and that it corresponds exactly to the heat energy liberated. The weight loss would, however, in this case be relatively too small to show on even our most delicate balances, as it would amount to only about a ten thousandth of a millionth of the weight of the wood. We are here dealing with a chemical reaction in which nuclear changes are not involved.

Franklin's Discovery of the Electron

First Page

Periodic Structure of Atoms and Elements

A. C. CREHORE, in his recent papers on “The Hydrogen Molecule” in the Philosophical Magazine (October 1921, May 1922, and June 1922), makes use of the specially constituted atoms of hydrogen and helium discussed by him, and of a hypothetical atom, the atomic weight of which is 21/3, to build up some of the other atoms. He uses H particles, hydrogen with charge + e; α-particles, helium with charge + 2e; and particles of his hypothetical element, with positive charge +e. I think it is clear that he recognises the need for a helium particle with positive charge +e in addition, if neutral atoms are to be built up in the way he indicates. In one of his models one of the particles marked 4 is also marked with a dot, and this evidently means that it has a charge + e only. Calling these particles, glucinum or beryllium becomes α, γ, H and its structure appears to be γ—— α—— H, where the hyphens indicate electrons. Its isotope, of atomic weight 12, is —γ—H —γ—H…, forming a simple ring of unit-charged particles. Nitrogen is 2α, γ, 2H, and may have the structure

The Form of the Atoms in Relation to their Spectra

SINCE Balmer's important discovery in 1885 that it is possible to calculate the wave-lengths of the first nine lines of the hydrogen spectrum by means of a simple formula, the existence of series of lines, obeying simple mathematical laws, has been established in the case of the spectra of several other elements, notably by the researches of Rydberg and of Kayser and Runge. Among the various attempts that have been made to account for these series of lines, and, in general, for the different spectra, the most promising seems to be that of Prof. F. von Lindemann, of Munich, who in some recent papers1 has more fully developed an idea that he was first led to enunciate in 1888, after the publication of Lord Kelvin's Baltimore lectures on molecular dynamics. Prof. von Lindemann's method consists, not in deriving an empirical relationship between the wavelengths or frequencies of the spectral lines, but in investigating mathematically the possible waves which a hypothetical atom can send out into the luminiferous ether.