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Volume 44, Issue 4, 15 February 1966
44(1966); http://dx.doi.org/10.1063/1.1726857View Description Hide Description
Undoped KI and thallium‐doped KI are irradiated with x rays at 78°K. Luminescence and energy storage, in the form of trapped electrons and holes, are produced during irradiation. After irradiationphosphorescence is observed at 425 mμ and stimulation with light gives an emission quite similar to that during irradiation with bands at 302, 370, and 425 mμ. It is suggested that the luminescence produced during irradiation is predominantly from recombination. In stimulation the 425‐mμ band characteristic of thallium grows and decays slower than the 302‐ and 370‐mμ bands characteristic of the KI host. It is suggested that the phosphorescence is the result of the direct recombination of I2 − and Tl°, and that the same process is occurring during stimulation and irradiation. In addition it is observed that 370‐mμ emission produced during irradiation decreases in intensity with increasing temperature and that the 425‐mμ emission intensity increases. It is suggested that 370‐mμ emission decreases because of a radiationless transition and that 425‐mμ emission increases because of the increase of recombinations at thallium sites.
44(1966); http://dx.doi.org/10.1063/1.1726858View Description Hide Description
A method for the detection of excited neutral long‐lived particles by ionization of properly chosen molecules with different ionization potential was developed and applied to the study of long‐lived excited states of H2, N2, CO, and O2. Results in H2 and O2 confirm previous findings. In N2 a new state was found which releases in the transition to the ground state an energy of ∼11.4–11.6 eV. Also determined was its separated excitation function. This state is formed partly through a resonance capture of an exciting electron and subsequent decomposition of the unstable negative excited molecular ion N2 −*. In CO a new long‐lived excited state at ∼10 eV was detected, and the existence of another, higher lying, state was suggested.
44(1966); http://dx.doi.org/10.1063/1.1726859View Description Hide Description
The F‐center absorption in NaF, x irradiated at CO2 temperature, has been studied as a function of temperature after a preanneal at 100°C. Three absorptions occur in such crystals: (a) an unnamed band at 5.70 eV (218 mμ), (b) the K band at 4.35 eV (285 mμ) whose peak position and half‐width (1.0 eV) are not temperature sensitive, and (c) the F band which has its peak at 2.71 eV (334 mμ) at 80°K after correction for K‐band overlap. The data for half‐width of the F band as a function of temperature when fitted to the well‐known hyperbolic‐cotangent relation, indicate a value of H 0=0.358±0.002 eV for the half‐width at 0°K and a value of ν g =(7.7±0.2)×1012 sec−1 for the effective frequency of the phonons which interact with the F‐center electron in its ground state. That frequency (ν g ) appears to be approximately one‐half the frequency (ν0) of the longitudinal optical modes, for NaF and for the five other alkali halides for which data are published.
44(1966); http://dx.doi.org/10.1063/1.1726860View Description Hide Description
The expansion wave generated by the simultaneous arrival of the ``contact'' surface and the reflected rarefaction wave was used for a direct kinetic study of a condensation process, the association of CN radicals generated by shocking C2N2/Ar mixtures to about 2000°K in the reflected‐shock region. During the quenching‐expansion phase the concentration of CN radicals decreases due to the concurrent chemical association and the lowering in total density. The CN concentration was followed spectrophotometrically at 3883 Å.
The results were reduced in three ways: (A) unidirectional association to C2N2 in the presence of a third body, (B) relaxation to the local equilibrium via a three‐body process, and (C) bimolecular reaction with C2N2 and C m N m polymers (formed during the heating process) to produce higher polymers. The data show that at the onset of quenching, the removal of CN was sluggish. This is attributed to failure to attain equilibrium in the preparation of the sample. After ≈200 μsec the data were best fitted by the third mechanism. This conclusion is in agreement with results obtained by another method and is supported by a reaction scheme which accounts for the formation and decay of CN in the presence of C m N m species. For a 2% initial concentration of C2N2, an average polymer number of ≈10 was indicated.
44(1966); http://dx.doi.org/10.1063/1.1726861View Description Hide Description
Electron transferreactions between a diamagnetic molecule and its paramagnetic anion modify the NMRspectrum in two different ways. First, the NMR lines are broadened and secondly, the lines are shifted. The line broadening may provide information on rates of electron exchange; the line shift is related to the spin density at the proton.
The magnitude and the sign of the spin densities are quantities of great interest from a quantum‐chemical point of view. It is useful, therefore, to provide methods for measuring spin densities. Usually they are derived from hfs parameters in ESR psectra. In this paper it is shown how they may be detected by the NMR line shifts. A special advantage of the NMR method is that the sign of the spin density is also easily available, because it is directly related to the direction of the line shift.
Basic equations that describe both line broadening and line shifts are derived in this paper. The theory has been applied to p‐xylene and p‐diethylbenzene. Rates of electron transfer of p‐xylene have been obtained from the broadening of the NMR lines. A combination of ESR and NMR has been used to obtain the values of the spin densities at the sites of the methyl and methylene protons; contrary to predictions in the literature, the sign is negative. An explanation for this is given on the basis of MO theory.
44(1966); http://dx.doi.org/10.1063/1.1726862View Description Hide Description
A fundamental problem in the mass‐spectrometric study of the vapors in equilibrium with condensed phases at high temperatures is the determination of the absolute instrumental sensitivities to the several species in the vapor. An analysis is presented which shows that calibration for all the individual species can be accomplished without additional information if the absolute total rate of effusion is obtained as a function of temperature and is combined with mass‐spectrometric measurements of the relative intensities of the individual species. General equations are derived and two procedures for treating the case of two species are presented.
44(1966); http://dx.doi.org/10.1063/1.1726863View Description Hide Description
Variational calculations of the center of long‐range forces in HCl are described. The calculations are performed in two ways. In the first, the center of polarizability is calculated through a consideration of the polarization energy for the molecule placed in an inhomogeneous electric field, and the center is assumed identical with that effective in long‐range interactions. In the second, a direct calculation of the dispersion forces between HCl and the rare‐gases Ar and He is made, and from their angular dependence, the position of the effective center of attraction of HCl to the rare‐gas atom is deduced. The calculations, which take account of the anisotropy of the HCl orbitals, yield a value for the separation between the HCl center of mass and center of long‐range force equal to 0.11 Å, with a small variation from one rare gas to another. The long‐range isotropic force constant and the HCl polarizability components are also calculated through the present approach. Comparison of the calculated parameters with experiment is made whenever this is possible.
44(1966); http://dx.doi.org/10.1063/1.1726864View Description Hide Description
The microwave spectrum of vinylene carbonate has been studied in the frequency region between 12 000 and 26 500 Mc/sec. Rotational constants and moments of inertia have been derived for the vibrational ground state and for the first three excited states of the ring‐puckering vibrational mode. The molecule is planar, with no barrier at the planar configuration in the ring‐puckering potential function. Measured intensities of the vibrational satellites are fitted well by a purely quadratic potential function. The dipole moment, which lies exclusively along the a axis, is 4.57±0.05 D.
Energy Transfer between p‐Benzoylbenzoate and Europium Ethylenediaminetetraacetate in Water Solution44(1966); http://dx.doi.org/10.1063/1.1726865View Description Hide Description
Ultraviolet radiation absorbed by the p‐benzoylbenzoate (p‐BB−) ion in solution is transferred to an appreciable extent to the europium ethylenediaminetetraacetate (EuEDTA−) ion and is re‐emitted as the visible red fluorescence characteristic of Eu3+. Fluorescence intensity is greater in D2O than in water. The fluorescence is quenched by dissolved O2. Although evidence for mixed ligand complexes involving p‐BB− and EuEDTA− was obtained, fluorescence seems to result from ultraviolet absorption by free p‐BB− ions followed by intermolecular energy transfer to free EuEDTA− ions. The analogous water systems containing o‐BB− or m‐BB− are nonfluorescent, although all three of the solid hydrated salts Eu(o‐BB)3, Eu(m‐BB)3, and Eu(p‐BB)3 fluoresce brightly at room temperature.
44(1966); http://dx.doi.org/10.1063/1.1726866View Description Hide Description
The Macedo—Litovitz hybrid equation for the viscosity of liquids is derived using statistical‐mechanical arguments. The formulation is based upon the assumption that diffusion in liquids is governed by (1) the presence of an adjacent free volume of certain size into which a molecule can jump and (2) the acquisition of sufficient energy by the diffusing molecule to escape from the force field of its neighbors.
44(1966); http://dx.doi.org/10.1063/1.1726867View Description Hide Description
It is shown that hole traps are induced in anthracene crystals by irradiating the crystals with either gamma or x rays. These traps are detected by measuring the steady‐state current—voltage characteristic of the crystal in the space‐charge‐limited current region. A modified type of Kallmann—Pope cell employing a sodium sulfate solution as one electrode and sodium iodide—iodine solution as the hole injecting electrode was used. After irradiating the crystals with gamma rays or x rays, part of the current—voltage curve shows a square‐law dependence and the current saturation occurs at higher voltages than in the nonirradiated crystals. By applying space‐charge‐limited current theory and interpreting the voltage at which the departure from the square law occurs as the trap‐filled‐limit voltage for the induced deep traps, the density of these traps can be estimated. Using this calculated density it is found that one deep trap is created for every 45 keV of incident energy absorbed in the crystal.
Electric, Magnetic, and Magnetic Resonance Properties of the Nonstoichiometric Sodium—Vanadium Bronzes44(1966); http://dx.doi.org/10.1063/1.1726868View Description Hide Description
Electric and magnetic studies have been carried out on the system Na x V2O5 (0.24<x<0.33). Single‐crystal electrical‐conductivity measurements on Na0.33V2O5 show semiconducting behavior from 77° to 500°K. A room‐temperature conductivity of 230 Ω−1·cm−1 with an activation energy of about 0.02 eV is observed. Thermoelectric power amounts to −116 μV/K° and is constant from 273° to 500°K. The Hall voltage is too small to be observed, indicating >1019 carriers/cc. Gouy‐susceptibility measurements have been carried out on a series of bronzes of variable Na/V ratio in the range 1.5° to 300°K. All specimens are highly paramagnetic and follow a Curie—Weiss law between 63° and 300°K. Weiss constants vary from −139° to −157°K; Curie constants from 0.119 to 0.159. The effective magnetic moments range from 1.94 to 2.01 Bohr magnetons. Below 63°K, the reciprocal susceptibility decreases more rapidly than linearly with temperature. No evidence of an antiferromagnetic transition is observed. The results, together with other electrical, ESR, and NMR data, cannot be explained by a hopping model but are consistent with a 3d t 2g band model in which a majority of V4+ donor sites remain un‐ionized over the temperature range investigated.
High‐Temperature Vapor‐Pressure Studies of UO2 by the Effusion Method and Its Thermodynamic Interpretation44(1966); http://dx.doi.org/10.1063/1.1726869View Description Hide Description
A high‐temperature effusion investigation under vacuum of the vapor pressure of UO2 is presented over the temperature range from 2200° to 2800°K. A least‐square analysis of the experimental results yielded the equationThe latent heat of sublimation ΔH°298=148.6 kcal/mole was determined by applying the third‐law method of thermodynamics. Blackbody hole readings, corrected for both the transmissivity of interposed windows and the cavity temperature, were checked by measuring the spectral emissivity of the optically smooth polished tungstensurface at 0.65 μ, which decreased from 0.44 at 2300°K to 0.416 at 2850°K.
Compatibility tests at 2800°K revealed no signs of a noticeable reaction between UO2 and the tungsten effusion cell.
44(1966); http://dx.doi.org/10.1063/1.1726870View Description Hide Description
The rate of dissociation of nitrogen behind strong shock waves in nitrogen—argon mixtures was determined using time‐resolved interferometric measurements of gas density. Dissociation rates for impact by N2, N, and Ar were found by varying the mixture composition from 5% N2 to 100% N2. Temperatures from 6000° to 9000°K were covered in the experiments. The effectiveness of the N atom in causing N2dissociation was found to be more than that of the N2 molecule by a factor of 10 to 15, whereas that of the argon atom was found to be less than that of N2 by a factor of 2.5. A large discrepancy between these results and those reported recently by Cary is partially resolved by an alternative interpretation of his data. A difficulty in fitting all of the data by using a single set of rate constants is described and several possible explanations are suggested.
Vibration—Rotation Spectra of Monomeric HCl, DCl, HBr, DBr, and HI in the Rare‐Gas Lattices and N2‐Doping Experiments in the Rare‐Gas Lattices44(1966); http://dx.doi.org/10.1063/1.1726871View Description Hide Description
Extensive experiments are performed on trapping of the HCl, DCl, HBr, DBr, and HI molecules in rare‐gas lattices. Concentration, deposition rate, and temperature‐dependent studies yield the monomerspectra of the molecules. The HCl and DCl monomers in Ar, Kr, and Xe show hindered rotational spectra that is interpreted on the basis of 0 h site symmetry. HCl in Ne also appears to rotate. HBr and DBr in Ar shows additional splitting which is attributed to either site splitting due to loss of one of the near‐neighbor atoms or site distortion toward D 3h symmetry. HBr and DBr in the larger Kr lattice give spectra similar to the HCl spectra. The spectra indicate the predominant local symmetry is 0 h and the barriers to rotation in the lattices are given from an interpretation of the spectra. The barriers to rotation are interpreted by extension of an electrostatic model.
Calculations of rotational energy levels have been performed for the linear molecule in a lattice site of D 3h symmetry. The perturbed rigid‐rotor levels are given as a function of lattice—molecule coupling constants. No experimental evidence was found in these studies for D 3h site symmetry.
N2‐doping experiments are performed with HCl in the argon lattice. N2doping (1%—4%) is found to give predominately two sites for the HCl molecule; one site similar to the pure HCl in Ar case and another site in which the rotation is frozen out giving a single transition for HCl. The frozen site is attributed to the nearest‐neighbor presence of N2.
Detailed discussions are given in interpreting the spectra in terms of a hindered rotor or a librator. Most of the spectra is interpreted on the basis of a hindered rotor but some spectra such as the N2‐doping experiments require the librator analogy similar to CO in Ar and CN− in KCl.
The vibrational shifts of the monomers in the rare‐gas lattices are given for all cases studied where the predominant shift is to lower energy. These shifts are also qualitatively explained by localized nonbonded dispersion and overlap repulsion effects.
44(1966); http://dx.doi.org/10.1063/1.1726872View Description Hide Description
The infrared spectrum of VOF3 has been studied and interpreted in terms of a C 3vmolecular symmetry. All six fundamentals have been observed: the a 1 frequencies are 1057.8, 721.5, and 257.8 cm−1 and the e frequencies are 806, 308, and 204.3 cm−1. The infrared spectrum of POF3 has also been studied and the lowest e fundamental, earlier observed in the Raman spectrum, has been confirmed.
Nuclear Magnetic Relaxation in Ionic Solution. I. Relaxation of 23Na in Aqueous Solutions of NaCl and NaClO444(1966); http://dx.doi.org/10.1063/1.1726873View Description Hide Description
The relaxation time of the nuclei of sodium ions has been measured for aqueous solutions of sodium chloride and sodium perchlorate at 25°. The longitudinal and transverse relaxation times are equal and depend on solution composition according to the rate lawwith T 1 in seconds and c X in moles of X per liter, for single and mixed electrolyte solutions at concentrations less than 1M. Deviations from the equation are small up to 3M total concentration, and also in D2O if the coefficients are first multiplied by the ratio of solventviscosities.
The limiting value of T 1 at low concentration agrees roughly with the calculation of the effect of fluctuating electric‐field gradients due to reorientational Brownian motion of water molecules coordinated to the sodium ion, but there are some very uncertain factors in the calculation. The coefficients of the rate law may be related to the distance of closest approach of the ions in various ways, with the result that this distance is much smaller for Na+ClO− 4 than for Na+Cl−.
44(1966); http://dx.doi.org/10.1063/1.1726874View Description Hide Description
The unique intensity distribution as a function of the angles between the principal magnetic axes and the applied magnetic field of the Δm=±1 ESR transitions of a collection of randomly oriented photoexcited triplet‐state molecules has been used as a magnetoselection device, to observe only the molecules whose principal magnetic axes are parallel to the magnetic field. The latter is possible because the shape of the derivative of the Δm=±1 ESR absorption curve gives special prominence to those resonant fields corresponding to transitions of molecules possessing a canonical orientation. Changes in the relative intensities of the derivative ESR lines of the magnetophotoselected collection of molecules were observed as a function of the angle between the external field and the polarization direction of the exciting monochromatic light. The results obtained for glasses of naphthalene‐d 8 and quinoxaline were analyzed in terms of the known ESRspectra and polarization of the electronic absorption bands; the conclusions agree very well with theoretical predictions. The known directions of polarization of the electronic absorption bands of phenanthrene‐d 10 were then used to assign the Δm=±1 ESRspectrum of phenanthrene‐d 10 in EPA glass at 77°K. The assignment is found to agree with the first‐order spectrum calculated using the published D and E values determined from single crystal studies. A simple procedure based on the magnetoselection method, and making use of the symmetry properties of π,π* and n,π* states, is then outlined by which assignments can be made for randomly oriented aromatic hydrocarbon or N‐heterocyclic molecules, of the Δm=±1 ESR lines corresponding to those molecules which have their molecular plane either perpendicular or parallel to the magnetic field. The polarization of the electronic transitions used to excite these molecules to their triplet states need not be predetermined. A discussion is given of the advantages of using the method of magnetophotoselection instead of the normal photoselection method (where optical luminescence is used for analysis) to such problems as the assignment of optical electronic transitions and the mutual orientation of donor and acceptor in triplet—triplet transfer processes.
Radial Distribution Functions for Binary Fluid Mixtures of Lennard‐Jones Molecules Calculated from the Percus—Yevick Equation44(1966); http://dx.doi.org/10.1063/1.1726875View Description Hide Description
Using the Lennard‐Jones 6–12 potential and the Percus—Yevick equations, the radial distribution functions for a number of binary mixtures have been calculated as a function of the Lennard‐Jones parameters and the thermodynamic state. These are compared with one‐component distribution functions and with those for binary hard‐sphere fluids. The effect of a hard‐sphere core, a softer attractive portion, and deviations from the combining rules ε12= (ε1ε2)½ and σ12= (σ1+σ2)/2 are also discussed. We find that the distances from the central molecules to the peaks of the distribution functions are entirely similar to those in the corresponding hard‐sphere systems. The peak heights are considerably different, however. These are most sensitive to the nature of the repulsive potential and to the combining rule for σ12.
44(1966); http://dx.doi.org/10.1063/1.1726876View Description Hide Description
There are two groups of phenomena that can cause frequency dispersion of capacity and resistivity at a perfectly polarizable interface. The first comprises artifacts which lead to the observation of an apparent frequency dispersion: Of these, penetration of the electrolyte to shielded parts of the electrode and the resulting transmission line phenomena are discussed. The second group comprises processes which lead to an inherent frequency dispersion of the double‐layer impedance: among these are, in particular, the relaxation phenomena of polar double‐layer constituents.
Experimental results have been obtained for the interface: Hg (dropping) /6NHCl aq., in terms of parallel capacity, Cp d1, and resistivity,Rp d1, of the electric double layer as a function of frequency, v. These results are consistent with the occurrence of dielectric relaxation of the water dipoles adsorbed at the interface, and characterized by a certain distribution of relaxation times around τ0≃10−6 sec. The results are not consistent with the values ∂ lnRp d1/∂ lnv expected from the effect of a possible penetration of electrolyte into the capillary.
On the basis of a molecular model of the relaxation process, a rough estimate of the energy of activation for the process which results in a change of direction of the water dipole is made which is consistent with the mean relaxation time, τ0, estimated on the basis of the dielectric properties of double‐layer water. It is shown that the frequency variation of the double‐layer capacity is less than the detection limits of most ac bridges up to very high frequencies, as long as the high‐atom polarization of water remains unchanged, and only dipole polarization is gradually reduced to zero with increase of frequency.