Volume 32, Issue 4, 01 April 1960
Index of content:
32(1960); http://dx.doi.org/10.1063/1.1730904View Description Hide Description
The class of materials B2O3–H2O has been investigated over the range B2O3 to B2O3·3H2O. This includes ``dry'' glassy B2O3, ``wet'' glassy boron oxide of approximate composition B2O3·½H2O, monoclinic and orthorhombic forms of metaboric acid, B2O3·H2O, and triclinic orthoboric acid, B2O3·3H2O. The nuclear quadrupoleinteraction of the B11isotope was measured and comparisons of the structures are made on the basis of this interaction. It is shown that all substances contain similar planar BO3 triangular units. The presence of O–H–O hydrogen bonds, as in B2O3·3H2O, has a minor effect on the boron nuclear quadrupoleinteraction. It is concluded that BO4 tetrahedral units exist in several of the wet glass samples as well as in monoclinic metaboric acid.
32(1960); http://dx.doi.org/10.1063/1.1730905View Description Hide Description
Hydrogen atoms have been stabilized in nonequivalent lattice sites in matrices of the rare gases at liquid helium temperature. Electron spin resonancespectra of H atoms in argon, krypton, and xenon show that at least two trapping sites are involved in each case. In a neon matrix, H atoms have been stabilized in only one site. Attainability of the various trapping sites apparently depends on the initial energy of the H atom, a simple doublet spectrum being obtained when the atoms are deposited from the gas phase, while multiple trappingspectra are obtained when the atoms are produced by photolysis in the solid.
The hyperfine coupling contants and the electronic g factors for H atoms trapped in the various matrix sites have been determined. The deviation of the hyperfine coupling constant from the free‐state value is positive in some cases and negative in others. The experimental results are in good agreement with theoretical predictions. A complex multicomponent H atom spectrum was obtained by photolysis of HI in xenon. The subsplitting in the spectrum is attributed to magnetic hyperfine interactions with matrix nuclei (Xe129 and Xe131). The nature of the trapping sites in rare gas matrices is discussed. Evidence for trapping in substitutional sites and octahedral sites is presented.
32(1960); http://dx.doi.org/10.1063/1.1730906View Description Hide Description
A study is made of the matrix effects on the electron spin resonance(ESR)spectra of hydrogen atoms stabilized in nonpolar matrices. It is assumed that the perturbing effect of the matrix consists of van der Waals interactions and the overlap or Pauli exclusion forces. These two effects are treated separately and the results added to get the net result. The van der Waals effect, which is treated by perturbation theory, leads to a reduction in the hfs splitting. The overlap effect, which is treated by requiring that the hydrogen atom wave function be orthogonal to the wave functions of the matrix particles, leads to an increase in the hfs splitting. In addition, the exclusion effect tends to introduce a small amount of the unpaired electron charge density onto the matrix particles. This can lead to a change in the electronic g factor, and also to hyperfine interactions with the nuclei of the matrix particles. The theory gives a good qualitative picture of the various matrix effects and their dependence on various properties of the matrix atoms and molecules.
32(1960); http://dx.doi.org/10.1063/1.1730907View Description Hide Description
For measuring the absorption spectra of a powdered solid in contact with a gas and adsorbed gas molecules, a new disk technique has been developed by which the powder can be formed into optical samples having sufficiently high transmissivity and a well‐defined thickness. By the use of this technique the spectrum of zinc oxide under varying heat treatments and gaseous conditions was studied at 20° and 300°C mainly in the region 2000∼1200 cm—1. With samples made from zinc nitrate and evacuated at 300°C a distinct symmetrical band was observed at 1550 cm—1 which was investigated in detail. From its intensity, behavior, and correlation with absorption by free electrons, the band was attributed to an electronic absorption associated with zincvacancies in the crystal lattice. Samples made from zinc carbonate showed strong absorption by free electrons on being evacuated, affording a useful basis for observing its variation.
32(1960); http://dx.doi.org/10.1063/1.1730908View Description Hide Description
The electronic energy and dipole moment of LiH have been calculated with valence‐bond wave functions into which some radial correlation has been introduced by assigning different orbitals to different electrons. The energy as calculated from the radially correlated functions is 0.76 ev lower than from the corresponding uncorrelated functions. The calculated dipole moment agrees well with that obtained in previous calculations. Moffitt's concept of ``atoms in molecules'' is applied to the correlated and uncorrelated functions.
32(1960); http://dx.doi.org/10.1063/1.1730909View Description Hide Description
The relative intensities in the nitrogen afterglow, from 5000 to 11 000 A, have been measured in pure nitrogen and after addition of several foreign gases. Most of the bands in the recently discovered Y — B 3Π g system are overlapped by B 3Π g — A 3Σ u + bands but intensity assignments have been made. Changes in relative intensities accompanying changes in external conditions show that the bands can be classified into five groups of different kinetic origins. The vibrational levels of the B 3Π g and Y states just below De , the energy of two separated ground state nitrogen atoms, are not populated. At low temperature the highest populated levels of the B 3Π g and Y states tend to a limit about 850 cm—1 below De . It is suggested that this is approximately equal to the dissociation energy of the 5Σ g + state of the nitrogen molecule. Some of the emission from the low vibrational levels of the B 3Π g state is associated with the radiation cascade Y→B 3Π g →A 3Σ u +, but the rest has a different kinetic origin. The emission from B 3Π g (v=7, 6, 5) is probably the second step in the cascade 3Δ u →B 3Π g →A 3Σ u +.
Foreign gases modify the vibrational population distribution of the upper levels of the B 3Π g state, each in its characteristic fashion. Some gases quench the emission from the lowest vibrational levels of B 3Π g more effectively than the rest of the spectrum. The quenching of the emission from the higher levels depends on the pressure of nitrogen. A mechanism accounting for most of the afterglow emission is discussed.
32(1960); http://dx.doi.org/10.1063/1.1730840View Description Hide Description
A treatment is developed by means of which it is possible to calculate an equilibrium constant for the formation of a collision complex from a pair of chemically interacting atoms. For iodine atoms, this leads to a value of the rate constant for I+I+M→I2+M, if the mechanism is 2I⇌I2 *(1Σ), I2 *+M→I2+M. The rate constant so calculated agrees with that obtained by a statistical procedure [J. Keck, J. Chem. Phys. 29, 410 (1958)].
The calculation also furnishes enough information about the mechanical details of collisions so that the contribution of molecular states other than 1Σ to recombination may be deduced. As a result a fairly conclusive discussion may be given of the plausibility of the various detailed mechanisms thus far suggested. A most likely mechanism and a possible calculation based on it are proposed.
32(1960); http://dx.doi.org/10.1063/1.1730841View Description Hide Description
32(1960); http://dx.doi.org/10.1063/1.1730842View Description Hide Description
Chemical shifts of Al27 for aluminum alkyl halides, halides, alkyls, alkoxides, AlH4 —, and Al(OH)4 — are given. The magnitude of the resonance line width of aluminum compounds as liquids or in solution is indicative of the cubic or noncubic molecular symmetry around the Al27 nucleus. The chemical shift of Al27 in AlH4 — is calculated by a variation procedure using both valence bond and molecular orbital type wave functions. Requirements on variational functions due to time reversal and inversion symmetry are given. Factors determining the shifts of other aluminum compounds are discussed.
Electrical Properties of Organic Solids. I. Kinetics and Mechanism of Conductivity of Metal‐Free Phthalocyanine32(1960); http://dx.doi.org/10.1063/1.1730843View Description Hide Description
Techniques involving the use of high‐intensity, short‐duration light pulses have been applied to the study of the kinetics of photoconductivity in films of metal‐free phthalocyanine. These experiments, in conjunction with measurements of steady‐state photoconductivity, are consistent with the following scheme. The principal route for the formation of charge carriers is via the first excited singlet state, although the lowest triplet state can, to some extent, contribute to charge carrier production. The mobility of the carriers is low and is concentration‐dependent, being lower at higher carrier concentration. The decay of the photocurrent is the result of a diffusion‐limited bimolecular recombination, with a capture radius of approximately one molecular diameter. The experiments indicate that carriers produced thermally in the dark do not interact with light‐produced carriers.
Electrical Properties of Organic Solids. II. Effects of Added Electron Acceptor on Metal‐Free Phthalocyanine32(1960); http://dx.doi.org/10.1063/1.1730844View Description Hide Description
The addition of ortho‐chloranil to the surface of films of metal‐free phthalocyanine has been found (a) to increase the dark conductivity of such films by as much as 107, (b) to increase the steady‐state photoconductivity by as much as 105, and (c) to result in the formation of unpaired electrons whose concentration decreases reversibly as a result of illumination. These systems exhibit a light‐induced polarization, the phthalocyanine layer becoming more positive with respect to the ortho‐chloranil layer. Kinetic studies demonstrate that, upon illumination, a single process (time constant=60 sec) results in the increase in conductivity, the decrease in unpaired spins and the increase in polarization. The results are consistent with the following scheme. An electron transfer from phthalocyanine to ortho‐chloranil occurs in the dark at room temperature producing holes in the phthalocyanine layer and ortho‐chloranil negative ion radicals (high conductivity,ESR signal). Illumination results in the transfer of an electron from an excited phthalocyanine molecule to the ortho‐chloranil negative ion producing further phthalocyanine holes and ortho‐chloranil double negative ion (increase in conductivity, increase in polarization, decrease in ESR signal). By equating spin concentration with charge carrier concentration (phthalocyanine holes) it is possible to calculate a mobility of 10‐4 cm2/v/sec for holes in the phthalocyanine layer. With this value, a quantum yield of unity is calculated for the production of charge carriers in doped phthalocyanine. The experiments indicate a quantum yield of less than 10‐1 for undoped phthalocyanine. The over‐all results of adding a strong electron acceptor to a film of phthalocyanine are thus to (a) produce charge carriers in the dark, (b) increase the quantum yield for production of charge carriers by light and (c) increase charge carrier lifetime.
32(1960); http://dx.doi.org/10.1063/1.1730845View Description Hide Description
The origin of irreversibility in the phenomenon of crystal diffusion is examined from a dynamical point of view. It is shown that straightforward considerations lead to the establishment of two time scales, one for jumps between lattice sites and one for vibrations about a given lattice point. The dissipation of energy is shown to occur almost entirely by relaxation of ``recaptured'' particles, little energy being lost in the transit across the barrier. The quasi‐equilibrium and nonequilibrium formulations are compared and shown to be quite similar in structure and functional dependence.
32(1960); http://dx.doi.org/10.1063/1.1730846View Description Hide Description
A ``variational'' theory, which gives a least upper bound to the rate of a chemical reaction, is presented. The reaction is represented by the motion of a point in phase space across a trial surface dividing the ``initial'' and ``final'' chemical states. The trial surface is well defined in regions of phase space where interactions causing reaction are negligible, but is subject to arbitrary variations otherwise. It is shown that a least upper bound to the reaction rate can be obtained by calculating the rate at which representative points cross the ``trial'' surface and then minimizing this rate with respect to allowed variations of the surface. Explicit calculations of the recombination rate of attracting atoms in the presence of repulsive third bodies are made for a simple trial surface having one adjustable parameter. At low temperatures, the experimental rate constants are quite close to the theoretical bounds; at high temperatures, the experimental data fall away from the bounds in a manner which can be understood in terms of various approximations contained in the theory. Promising methods of improving the agreement between theory and experiment are discussed.
32(1960); http://dx.doi.org/10.1063/1.1730847View Description Hide Description
A modified quantum‐mechanical Boltzmann equation has been derived for the general case in which the molecules have degenerate internal states. This is an equation of the Boltzmann type for a quantity which is simultaneously a Wigner distribution function in molecular phase space, and a density matrix in internal state space. In particular, the nondiagonal terms of this density matrix have been included in the formalism, resulting in the collision term being modified from the usual Boltzmann expression. Thus the collisions are described in terms of combinations of the Lippmann‐Schwinger scattering matrix rather than the collision cross section. For nondegenerate states the usual collision term is obtained again.
32(1960); http://dx.doi.org/10.1063/1.1730848View Description Hide Description
Four bands of the Y 3Σ u —←X 1Σ g + system of nitrogen have been observed in absorption in a path of 3.4 meter‐atmospheres at 1518 A (0–0), 1484 A (1–0), 1453 A (2–0) and 1423 A (3–0). These bands consist of an unresolved Q branch ( QQ, QP, QR), and SR and 0 P branches.
From a comparison of the experimental and calculated intensity distribution, it is shown that the upper state is 3Σ u — rather than the alternative assignment of 3Δ u .
It is further shown that the Y 3Σ u — state is identical with the upper state of certain infrared bands recently found in the active nitrogen afterglow. Those bands are thereby identified as Y 3Σ u —→B 3Π g . The upper state was designated ``Y'' by Kistiakowsky and Warneck.
The constants of Y 3Σ u — as obtained here are: ω e =1517.69 cm—1; ω exe =12.22 cm—1; Be =1.472 cm—1; α e =0.0161 cm—1; T 00=65850.6 cm—1; re =1.2788 A.
The transition Y 3Σ u —←X 1Σ g + receives intensity from perturbations of Y 3Σ u — by 1Σ u + and 1Π u states lying at higher energies.
32(1960); http://dx.doi.org/10.1063/1.1730849View Description Hide Description
The relation between activation volume and activation energy of diffusive processes which was previously proposed is integrated to show how the activation free energy depends on the volume of the solid. The result is shown to be in agreement with the data of Nachtrieb et al. on the pressure dependence of the diffusion constant of sodium.
32(1960); http://dx.doi.org/10.1063/1.1730850View Description Hide Description
Molecular systems are observed in emission in solid products from a gas discharge trapped at liquid helium temperature. Previous tentative molecular assignments have been checked with the help of isotopic substitutions of oxygen and nitrogen. The Herzberg system (A bands) of oxygen (A 3Σ u + — X 3Σ g —) is analyzed and the molecular constants are derived for a molecule trapped in a nitrogen matrix. Another system (M bands) is attributed to the NO molecule (4II — X 2II).
32(1960); http://dx.doi.org/10.1063/1.1730851View Description Hide Description
The electric quadrupole splitting of the Na23nuclear magnetic resonance has been measured in single crystals as a function of pressure and temperature for NaNO3, and as a function of temperature for NaBrO3. It is found that the electric field gradient at the sodium nuclei varies as V —3.8 in NaNO3 and as V —2.0 in NaBrO3. These results, together with the previously reported V —1.9 dependence for Na23 in NaClO3, are compared with theoretical values computed from point charge models for the crystal lattices. It is concluded that the large volume dependence in NaNO3 arises from the anisotropic thermal expansion and compressibility of the hexagonal crystal plus the retention of size and configuration of the NO3 — group. The experimental results for NaClO3 can be explained in the same manner, assuming a rigid ClO3 — group; however, this model does not account for the V —2 dependence of the Na23quadrupole coupling observed in NaBrO3. The assumed rigidity of the ClO3 — group is supported by the small volume dependence, V —0.10, found for the Cl35quadrupole coupling in NaClO3, and also by calculations of the interionic contribution to the field gradient at the chlorine nuclei in NaClO3 and KClO3.
32(1960); http://dx.doi.org/10.1063/1.1730852View Description Hide Description
The diatomic and linear triatomic molecules such as the oxides and halides of the ``iron‐group'' elements are of importance in high temperature systems. Although the various high symmetry compounds of these elements, e.g., the solid oxides, hydrated complexes, etc., are sufficiently understood in terms of the present ligand‐field theory, no semiquantitative schemes have been utilized to understand and predict the spectroscopic and thermal properties of their simple, gaseous molecules. In this article, the observed dissociation energy vs atomic number curves for the linear gaseous oxides and the halides of the first transition series are compared with the corresponding energy curves of the solid oxides and the halides, and the similarities are pointed out. It is shown that the double maximum stabilization curves observed in the gaseous molecules as well can be understood in terms of the splitting of the (3d) shell of the transition atoms in a linear (C ∞v or D ∞h symmetry) ligand field with two independent splitting parameters, one of which is small. A method for predicting the low‐lying molecular spectral states is developed. The energies of the states depend on the splitting parameters and the electron interactions. The parameters are estimated from the first portion of the thermal data and used only for these elements. The electron interaction energy is obtained from the spectral terms of free atoms (ions). Unlike the case in the high symmetry applications, this procedure here takes into account ``electronic correlation''(configuration interaction) effects. The generality of the treatment is revealed by comparing the results of the simple ionic and covalent models with those of the molecular orbital method. Calculations of spectral states are made for ScO, TiO, VO, and CrO, and experimental data are discussed.
32(1960); http://dx.doi.org/10.1063/1.1730853View Description Hide Description
The infrared spectra of five isotopic derivatives of monomethyldiborane (CH3B2 10H5, CH3B2H5, CH3B2D5, CD3B2H5, and CD3B2D5) are analyzed and frequency assignments are made through correlation with trimethylborane and terminally substituted B2H5D and B2D5H.