Index of content:
Volume 42, Issue 11, 01 June 1965
42(1965); http://dx.doi.org/10.1063/1.1695808View Description Hide Description
The efficiency of Eu3+fluorescence in a variety of rare‐earth oxygen‐dominated host materials was measured under excitation by 2537 Å uv at 25°C. It was found to be of the same order as that of the commercial phosphor,magnesium arsenate:Mn in all such hosts. To account for the high efficiency and particularly for the insensitiveness of efficiency to the nature of the host lattice, it is proposed that europium—oxygen states are excited directly and that the corresponding absorbtion coefficient is of a magnitude to make this the dominant absorption process.
Gas‐Phase Photolysis of Cyclohexane in the Far Ultraviolet: Modes of Decomposition of the Neutral Excited Cyclohexane Molecule and Reactions of the Parent Cyclohexane Ion42(1965); http://dx.doi.org/10.1063/1.1695809View Description Hide Description
The photolysis of cyclohexane—cyclohexane‐d 12 (1:1) mixtures and of cyclohexane‐1,1,2,2,3,3‐d 6 was investigated using xenon (8.4 eV) and krypton (10, 10.6 eV) resonance radiation. It is demonstrated that the formation of hydrogen by molecular detachment is the major primary process In this process, the hydrogen is mainly eliminated from a single carbon atom. The resulting C6H10 diradical rearranges mainly to cyclohexene, a fraction of which decomposes to form 1,3‐butadiene and ethylene. It is shown that, because of collisional deactivation, the probability of the latter mode of decomposition decreases with an increase in pressure or an increase in wavelength. Other primary processes such as and, to a lesser extent, as well as processes leading to the formation of H atoms and CH3 radicals are also shown to occur.
In the photolysis at 1236 and 1165 Å, C6H12 + ions are formed which undergo H2‐transfer reactions such as The efficiency of these ion—molecule reactions does not depend on the concentration of C3D6 or oxygen, but is reduced by NO because of the occurrence of a charge transfer from the cyclohexane ion to NO. The same processes have been reported to occur in the gas‐phase photolysis of similar hydrocarbon mixtures.
Enthalpies of Mixing in Silver Bromide—Alkali Bromide and Thallium Chloride—Alkali Chloride Liquid Mixtures42(1965); http://dx.doi.org/10.1063/1.1695810View Description Hide Description
The enthalpies of mixing in silver bromide—alkali bromide and thallium chloride—alkali chloride liquid mixtures have been determined by high‐temperature reactioncalorimetry.
The following semiempirical relations represent the molar enthalpies of mixing to a good approximation:In these expressions N 1 and N 2 are the mole fractions of the two components, while δ12 = (d 1 — d 2)/d 1 d 2, when d 1 and d 2 are the sums of the ionic radii in the two component salts. The positive term U 0 + + differs from system to system. It represents a theoretical estimate of the contribution to the mixing enthalpy which arises from the dispersion interaction between next‐nearest‐neighbor cations, calculated by an adaptation of the method of Mayer.
These expressions are similar to the ones previously determined for the silver chloride—alkali chloride and silver nitrate—alkali nitrate systems, except for the coefficient of the term linear in δ12. This term is absent in the corresponding expression for the binary alkali halides and nitrates. For the considered silver systems, it increases in the sequence NO3 —<Cl—<Br—. The term is attributed to the partly covalent bonding between cations and anions in the silver and thallium salts.
42(1965); http://dx.doi.org/10.1063/1.1695811View Description Hide Description
The microwave spectrum of 1,1‐difluorobutadiene has been studied. The small inertial defect indicates that the molecule is planar. From the allowed transition selection rules and the magnitudes of the moments of inertia, the molecule is shown to exist predominantly as the trans isomer. Attempts to find the spectrum of the cis form were unsuccessful.
Correlations between Photoinduced EPR and Photoconductivity in TCNE—THF Solution Charge‐Transfer Complex42(1965); http://dx.doi.org/10.1063/1.1695812View Description Hide Description
Reversible photoinduced electron paramagnetic resonance(EPR) signals and photoconductivity were observed when a solution of tetracyanoethylene (TCNE) in tetrahydrofuran (THF) was irradiated in the charge‐transfer band of the complex formed between these two compounds. The eleven‐line hyperfine structure of the EPRspectrum which was obtained demonstrated the presence of TCNE negative‐ion radical. The concentration of this radical was found to be directly proportional to the square root of the light intensity. Second‐order decay kinetics were followed when the light was shut off. Both the EPR signal and the photoconductivity rose initially as the square of the time. The latter portions of the growth curves could be fit to the latter portions of a hyperbolic tangential growth curve. From these data a reaction mechanism was proposed. The rate law , where n is the concentration of radicals, t is the time, k, α, and β are rate constants, and L is the light intensity, described both the photoinduced EPR and the photoconductivity within the limits of experimental accuracy.
42(1965); http://dx.doi.org/10.1063/1.1695813View Description Hide Description
The configuration integral for an electrolyte in a continuous dielectric medium is calculated using the method of collective coordinates. The pair potential energy is taken to be Coulombic (with fixed dielectric constant) for all interionic distances outside some separation a and zero inside. This form permits handling the potential energy completely within the collective‐coordinate framework, but has the disadvantage of limiting the concentration to which the treatment is applicable.
For low concentrations, the configurational partition function can be approximated to yield an analytic expression for the activity coefficient. For higher concentrations a numerical integration is used to calculate the mean molal activity coefficients for univalent salts in water at 25°C. The theoretical values obtained from the numerical integrations are within experimental error of the observed values up to molalities of 0.30 to 1.0, depending on the salt considered. As the concentration of the electrolyte goes to zero, the activity coefficient correctly tends to the Debye—Hückel limiting law.
42(1965); http://dx.doi.org/10.1063/1.1695814View Description Hide Description
Ionization of atoms by collisional transfer of electronic excitation energy, A*+B→A+B++e, is considered theoretically. A formula for the cross section is derived for the case where the transition A→A* is optically allowed and the excitation energy of A* is larger than the ionization potential of B, and is applied to several examples where A* is He (21 P).
42(1965); http://dx.doi.org/10.1063/1.1695815View Description Hide Description
As surmised by Riedel and by Pitzer, many substances which do not conform to the principle of corresponding states have reduced properties whose deviations from this principle can be expressed in terms of a single parameter. It is shown that a simple and useful parameter for a given substance is the number of carbon atoms in the normal alkane which most closely resembles the given substance.
42(1965); http://dx.doi.org/10.1063/1.1695816View Description Hide Description
The magnetic susceptibility and specific heat of the metallic sodiumtungsten bronzes Na x WO3 are explained by a model in which the Fermi energy does not depend on the sodium concentration x. The disappearance of metallic conductivity at small x is associated with finite cluster sizes of sodium atoms. Expressions are derived for the dependence of the conductivity on x in the metallic region.
42(1965); http://dx.doi.org/10.1063/1.1695817View Description Hide Description
It is shown that corrections to inhomogeneously broadened electron spin resonancelinewidths arising from cross‐relaxation effects can be calculated exactly using the moment formalism. Corrections of about 10% to the total width arising from ``cross‐relaxation corrections'' are found for inhomogeneous broadening which is three times the ``spin‐packet'' width.
42(1965); http://dx.doi.org/10.1063/1.1695818View Description Hide Description
Photoionization‐efficiency curves for the molecule and fragment ions of CH4 and CD4 are obtained in the wavelength region 1000 to 600 Å. The electronic structure of the molecule ion is related qualitatively to the shape of the parent‐ionization‐efficiency curve and the implications of the complicated vibronic behavior of the CH4 + ion are discussed. A qualitative explanation is given for the observed isotope effect on the parent‐ionization‐efficiency curve and on the fragment‐ion threshold energies. New upper bounds for the ionization energies I(CH4) = 12.71 eV and I(CD4) = 12.87 eV are obtained. Other thermochemical values deduced from the study are: D(CH3–H) = 4.41 eV, D(CD3−D) = 4.55 eV, D(CH2–H) = 4.87 eV, and ΔHf (CH2) = 4.07 eV. Zero‐point differences for methyl and methylene ions and neutrals are also estimated.
Spectral Intensities of the Trivalent Lanthanides and Actinides in Solution. I. Pr3+, Nd3+, Er3+, Tm3+, and Yb3+42(1965); http://dx.doi.org/10.1063/1.1695840View Description Hide Description
We have correlated the experimentally measured intensities of the solution absorption spectra of Pr3+, Nd3+, Er3+, Tm3+, and Yb3+ in three different solvents with a theoretical expression derived by Judd. The expression,is derived from a theory in which the ligand‐field interactions with the central 3+ ion cause a mixing of higher configurations of opposite parity into the fN configuration and gives rise to induced electric‐dipole transitions. A prerequisite for the intensity calculations is the computation of complete intermediate coupling eigenvectors for the energy levels investigated. Other possible mechanisms such as magnetic‐dipole and electric‐quadrupole transitions are considered. The oscillator strengths of important magnetic‐dipole transitions are tabulated.
The results give the first experimental verification of the ability of the theory to account for large changes in certain bands of the same lanthanide observed in different solvents. Conclusions are drawn respecting the T λ's obtained in a given medium for all the lanthanides studied, and also for those obtained for a given lanthanide in different media. The J dependence of the T λ's is removed to facilitate their intercomparison, and the resulting parameters are tabulated. The T λ's for Yb3+ are obtained by an extrapolation of the values computed for Tm3+ and Er3+. The nature of the model required by the theory is discussed.
42(1965); http://dx.doi.org/10.1063/1.1695841View Description Hide Description
A nuclear magnetic resonance study of 85Rb, 87Rb, and 19F in paramagnetic RbMnF3 is reported. The measured shifts of the rubidiumnuclear resonances are the first reported for the case of an alkali nucleus in the paramagnetic XMF3perovskite compounds (X = Na, K, Rb, Cs; M = Mn, Ni, Co). The shift for rubidium is in the opposite direction from that expected from a 5s contact hyperfine interaction. The isotropic hyperfine interaction of the fluorine indicates the presence of 0.51% unpaired 2s spins in the F— orbitals from each Mn+ + neighbor; this result is in good agreement with previous work for the F— resonances in these paramagnetic materials. The anisotropic shift of the F— resonance was examined in a powder sample and a 2p hyperfine coupling coefficient obtained.
42(1965); http://dx.doi.org/10.1063/1.1695842View Description Hide Description
The heat of fusion of solid parahydrogen has been measured for the first time at pressures above that of the triple point. The measurements were carried out up to a pressure of 338 atm. The heat of fusion was found to be a linear function of the pressure, and its values are well reproduced by the equation ΔHf = 0.04415 P+28.04, in which ΔHf is the heat of fusion in calories per gram mole and P the pressure in atmospheres. The value given by this equation for the triple point of hydrogen is 28.04, which compares with literature values of 28.03, 28.08, and 28.3 cal/mole. A few independent measurements were also made of temperatures corresponding to various pressures along the melting line for parahydrogen; most of the measuredtemperatures agree within 0.1°K of those previously reported in the literature.
42(1965); http://dx.doi.org/10.1063/1.1695843View Description Hide Description
42(1965); http://dx.doi.org/10.1063/1.1695844View Description Hide Description
Band shapes, wavelengths, and intensities are presented and discussed for the K emission spectra of oxygen and fluorine from some simple oxides and fluorides. It is shown that band shapes and peak positions are dependent upon the target material from which they are emitted. Evidence is presented illustrating that this dependence is in part due to such solid‐state properties as crystal structure, valence, and coordination.
Results were obtained from a flat‐crystal vacuum spectrometer with a thin window flow proportional counter using electron excitation.
42(1965); http://dx.doi.org/10.1063/1.1695845View Description Hide Description
Values of the quadrupole coupling constants |eQq|/h obtained at the temperature of liquid nitrogen for powdered specimens of deuterated ammonium sulfate and ammonium chloride have been found to be 161.9±0.3 and 175.7±0.7 kc/sec, respectively. Absolute values of the asymmetry parameter were estimated to be less than 0.2 from the powder spectra of the two salts. To obtain the deuteron quadrupole split spectrum of ND4Cl at liquid‐nitrogen temperature, it was found necessary to use very low rf field levels and also to irradiate the sample with a 60Co source to reduce the spin—lattice relaxation time. Second‐moment measurements were made on the single‐line spectrum of ND4Cl between room temperature and −83°C, from which an activation energy for rotational motion of the ND4 + ion was calculated to be 5.0±0.5 kcal/mole. At the temperature of the ferroelectric phase transition in (ND4)2SO4, a significant alteration of the electric field gradient was revealed by the change in the separation of the peaks in the five‐line deuteron magnetic resonance spectrum.
42(1965); http://dx.doi.org/10.1063/1.1695846View Description Hide Description
In this paper we use a simple model—a substitutional diatomic molecule in a linear monatomic chain—to examine the problem of intermolecular vibrational relaxation in the solid phase. A binary collision theory of vibrational deactivation is proposed for the relaxation process, with the motion of the collision partners governed by the normal modes of the lattice. The collision frequency is calculated by using an analysis similar to that of the Slater theory of unimolecular reactions. The model calculations predict the relaxation time to be very long compared with the vibrational period of the diatomic molecule. We thus conclude that, in a molecular crystal composed of polyatomic molecules, the excitation energy will be removed by the surrounding medium, not by the transformation of internal energy into lattice modes, but rather by other processes such as the formation of vibration excitons etc.
42(1965); http://dx.doi.org/10.1063/1.1695848View Description Hide Description
The intrinsic viscosity [η] of polymer chains is recalculated by methods similar to those of Kirkwood and Zimm. The theory is based on the preceding paper by Fixman, with the adoption of a smoothed excluded‐volume potential and the following first‐order approximation to the operator L which determines the segment coordinate distribution function ψ; for L acting on the nonequilibrium part of ψ, only the diagonal part of L in the free‐draining basis representation is used. This corresponds qualitatively to the Zimm theory in Hearst's version. However, the avoidance of preaveraging the Oseen hydrodynamic interaction tensor in the present work leads to larger values for the relaxation times of various normal modes when the hydrodynamic shielding effect is not negligible. The effect is more pronounced for lower modes, amounting to about eight percent increase in the lowest mode excited by the velocity gradient in the nondraining limit. The value of [η] in the same limit is found to be [η]=2.68×1023 L 3/Mw (L: the root‐mean‐square end‐to‐end distance, Mw : the molecular weight of the polymer; cgs units) and is about 5% lower than that obtained by the Zimm treatment. A similar correction to the flow birefringenceextinction angle is also given.