Volume 43, Issue 1, 01 July 1965
Index of content:
43(1965); http://dx.doi.org/10.1063/1.1696455View Description Hide Description
A systematic study of quantum efficiencies and lifetimes has been carried out for the visible fluorescences of Pr3+ in single‐crystal LaF3. An analysis of the results as a function of Pr3+ concentration is given in terms of a two‐site model in which a Pr3+ ion can either be in an isolated site or in a site where it is coupled to other impurity ions. This model leads to ion—ion interaction ranges varying from ∼5 Å (nearest neighbor) to ∼12 Å depending on the fluorescent level studied. Throughout this range of interaction lengths, Dexter's model of electrostatic ion—ion interaction shows that the dominant contribution comes from the quadrupole—quadrupole term. Temperature effects in the interaction are consistent with the lack of exact resonance between the electronic levels. It appears that the lattice can readily absorb excess energy up to ∼1000 cm−1.
A set of samples doped with a second rare earth (Ce3+, Nd3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+) has demonstrated the existence of selection rules for ion—ion interaction in addition to energy‐matching requirements. The system 0.5% Pr3+: x% Ce3+ has been studied in more detail.
43(1965); http://dx.doi.org/10.1063/1.1696436View Description Hide Description
Analyses of the diffusive flux of reactants to catalytic walls and end plate of a cylindrical tube are presented for a number of geometries of experimental interest. Specifically, the reactant concentration profiles are examined in a cylinder composed of sections of different catalytic activities. The results of the analysis are applied to some published experimental data for oxygen‐atom recombination on metaloxide surfaces. In addition, an analysis is presented of the influence of the surface area of the catalyst on the concentration flux of reactant. This effect is of importance in the evaluation of the catalytic properties of solids from measurements of mass‐diffusion gradients.
43(1965); http://dx.doi.org/10.1063/1.1696447View Description Hide Description
We have studied the electron paramagnetic resonance spectra of Mn(CN)5NO2− and Cr(CN)5NO3− as dilute solutions in single crystals of various diamagnetic hosts. From these measurements, we have obtained the spectroscopic splitting factor tensor and the hyperfinetensors to the central metal atom and nitrosyl nitrogen. In the case of Cr(CN)5NO3−, we have also measured the hyperfinetensor to carbon‐13 in the equatorial and axial cyanides. We have measured the quadrupole coupling constant to 55Mn in Mn(CN)5NO2−. We have attempted to construct a coherent picture of the electronic structure of the complexes from these data. This attempt has been only partly usccessful, primarily because the spectroscopic splitting factors are much nearer the free spin value than one expects on the basis of the hyperfine data.
43(1965); http://dx.doi.org/10.1063/1.1696463View Description Hide Description
Relaxation of ESR of Cu2+ ions is investigated experimentally in aqueous solution. Dependencies of line‐widths on Cu2+ concentration, temperature, anion species, and field strength are examined. It is shown that the mechanism via spin—orbit coupling is the main source of relaxation.
43(1965); http://dx.doi.org/10.1063/1.1696467View Description Hide Description
A simple description of the stimulated Raman emission of Stokes radiation, and of the induced absorption found by Stoicheff at the anti‐Stokes frequency, is based on the semiclassical theory of radiation. This theory gives the well‐known formulas for absorption and stimulated emission when carried to the first order in the time‐dependent perturbation due to the interaction of the light beam with the molecules. If there are two light beams of frequencies ω L and ω, the second‐order perturbation may be associated with stimulated emission at the Stokes frequency ω=ω−1=ω L —ω R and with induced absorption at the anti‐Stokes frequency ω=ω1=ω L +ω R , where ω L is the laser frequency and ω R a Raman‐active vibrational frequency; in both cases, there is a molecular transition to the excited vibrational state. The highly directional and very sharp anti‐Stokes emission at ω1 arises from the conversion by molecules in intense beams at ω L and ω−1 of two ω L photons into an ω−1, ω1 pair. Like spontaneous emission from excited states, the production of this anti‐Stokes radiation requires the full quantum theory of radiation for an adequate description. The emission of ω1 and the observed directional absorption of ω−1 due to the reverse conversion ω−1+ω1→2ω L are explained. The anti‐Stokes emission is proportional to the Stokes intensity and to the square of that of the laser, and to the square of the molecular number density; its sharpness is due to the absence of a molecular transition. The generation of photon pairs ω−n +ω n , where ω n =ω L +nω R , can occur for all values of n, but it is most efficient for n=1, due to resonance in the appropriate molecular hyperpolarizability and to the intensity of the Stokes beam at ω−1.
Molecular Motion and Relaxation in Free‐Radical Solutions of Benzene, Toluene, and Some Ethers as Studied by Dynamic Nuclear Polarization43(1965); http://dx.doi.org/10.1063/1.1696483View Description Hide Description
Magnetic double‐resonance methods have been used to investigate the spectrum of motion, the proton and electron spin relaxation in organic solutions of free radicals. Studies of both frequency and temperature dependences of the dynamic nuclear polarization and the relaxation times yield information on the solvent‐nucleus—free‐radical interactions and on the molecular motion. Measurements in very weak magnetic fields (15 G) realize the conditions of ``extreme narrowing,'' data at 1070 and 3230 G fall into the main region of decay of the Overhauser effect. The results in radical solutions of benzene, toluene, dimethoxymethane, dimethoxyethane, and diethoxymethane can be explained by a pure dipole—dipole interaction governed by translational random motions of the spin‐carrying molecules. Correlation times,activation energies, and other parameters are evaluated. An influence of different free radicals (molecules and ions) could obviously not be observed. Toluene at low temperatures displays an exceptional behavior. Some consequences for the phenomena of Overhauser polarization,proton, and electron spin—lattice relaxation in general are discussed. This especially includes the high‐frequency limit of the dynamic nuclear polarization and the dependences of both leakage and saturation parameters important for all applications.
43(1965); http://dx.doi.org/10.1063/1.1696484View Description Hide Description
The experiments performed by Campbell and Hilderband have provided intensity curves for x rays scattered from liquid xenon. They have computed, from their data, the radial‐distribution functions g. The Lennard‐Jones 6–12 potential with constants suggested by Beattie, Barriault, and Brierly and the Slater potential with Kane's constants have been used to compute g's and scattering intensities for comparison with those of Campbell and Hilderband. The agreement between experiment and theory is poorer than that found for neon, argon, and krypton, and arguments are presented why the experimental intensities should be remeasured.
Sigma and Pi Changes in Valence States of Pi‐Electron Theory and One‐Center Coulomb Repulsion Parameters43(1965); http://dx.doi.org/10.1063/1.1696485View Description Hide Description
Individual contributions to the total energy change ΔE of the reaction 2C→C++C− are calculated with valence states appropriate to π‐electron systems. Methods for the calculation of the open‐shell Hartree—Fock (HF) and correlation energies of valence states are given. The HF orbitals entering the main part of ΔE, the one‐center repulsion integral γ pp , are those of the 1s 2η32pz 2 valence state of C−. They yield a γ pp =12.72 eV, already remarkably close to the semiempirical ``γ pp '' of Pariser—Parr theory needed to reproduce, for example, the electronic spectrum of benzene. Both the σ and π orbitals and core correlations of each carbon species change strongly with the state of ionization. The energy of σ HF orbital changes in the reaction is 3.16 eV, of π orbital changes−4.19 eV. Such effects may not cancel out between the different electronic states of an organic molecule, though their net effect on the ΔE of the carbonreaction is −1.0 eV. The rest of ΔE is the 2pz 2 correlation, also −1.0 eV. A method is given for the prediction of other ΔE's (i.e., the Pariser—Parr ``γ pp '') for heteroatoms. The values obtained are 12.91 eV for pyridine‐type nitrogen, 16.47 eV for pyrolle‐type nitrogen, 15.38 eV for carbonyl oxygen and 18.85 eV for furan‐type oxygen.
43(1965); http://dx.doi.org/10.1063/1.1696486View Description Hide Description
Data on dielectric constants of electrolyte solutions are used to evaluate directly the r −4 term in the asymptotic expression for the free energy of interaction of two ions in solution for large separation distance r. Use is made of the fact that for large separations each ion is in a uniform field due to the other, and that information about ions in uniform fields is obtainable from measurements of dielectric constants. For a Z:Zelectrolyte, for example, the r −4 term is found to be Z 2 e 2δ/8πε0 r 4, assuming the effect of overlapping solvent structures to be of shorter range; ε0 is the dielectric constant of the solvent and δ is the measured decrement in dielectric constant per unit concentration of added electrolyte. A similar result obtained when one of the particles is uncharged, δ now referring to the decrement observed when the neutral is added to solution. Typical values of the term are given for various substances using the data on δ's. This determination of the r −4 term permits some evaluation of ion—image force theories.
43(1965); http://dx.doi.org/10.1063/1.1696487View Description Hide Description
A series of self‐consistent‐field calculations are reported for the molecules C2, ethylene, and acetylene in a Gaussian basis. An upper limit for the correlation energy of ethylene and acetylene of 0.645 and 0.597 a.u. has been computed. The correlation energy is estimated to be too high by 0.045 to 0.084 a.u. based on the results of the C2 calculation.
43(1965); http://dx.doi.org/10.1063/1.1696488View Description Hide Description
Photolysis of diazomethane dimers in solid nitrogen produces methyleneimine, H2CNH, and HCN, as shown by infrared spectral study. The HCN frequencies show that it forms a strong hydrogen bond to the imine. Deuterium and 15N isotopic substitutions aid in the vibrational assignment of H2CNH. Normal coordinate analysis indicates that this prototype C=N stretching force constant is (9.3±0.5) × 105 dyn/cm despite difficulties in obtaining a satisfactory fit to all of the observed frequencies.
Vibrational Structuring in Optical Activity. I. Vibronic Coupling in the Circular Dichroism of Dimers43(1965); http://dx.doi.org/10.1063/1.1696489View Description Hide Description
The characteristics of circular dichroism arising from near‐degenerate electronic states under the influence of vibration are considered and compared to the ordinary absorption. A ``pseudo‐Jahn—Teller effect'' operates between limits of strong and weak coupling, the influence of which on ordinary absorption has been considered earlier. An exciton or coupled‐oscillator model is used which seems especially appropriate to optically active biaryls. For intermediate coupling, overlapping band systems of equal net rotatory strength but opposite sign will give a resultant dichroism altogether different in appearance from the ordinary absorption.
43(1965); http://dx.doi.org/10.1063/1.1696490View Description Hide Description
The excitation of electronic states of sodium by vibrationally excited nitrogen N2 ‡ has been demonstrated experimentally. The N2 ‡ was produced in the afterglow of a microwavedischarge in flowing N2. Identification of N2 ‡ as the source of the sodium excitation was accomplished by the selective removal of N2 ‡ from the afterglow by deactivation with N2O and by observation of the concurrent disappearance of sodium line emission. It is concluded that the observed excitation is representative of a general phenomenon whereby the electronic states of atoms can be excited by the transfer of vibrational energy from diatomic molecules. The results verify the generally assumed mechanism for the quenching of sodium resonance radiation by nitrogen.
43(1965); http://dx.doi.org/10.1063/1.1696491View Description Hide Description
Periodically pulsed single‐ and double‐resonance experiments exhibit a variety of features which should prove useful for detailed relaxation analyses of complex spin systems. Various aspects of the pulsed single‐resonance experiments are described using the Bloch equations, including the results of pulsing the receiver in synchronization with the rf field. Periodically pulsed double resonance experiments with a pulsing frequency of the order of the widths of the irradiated lines offer a technique for observing Overhauser intensity effects without double‐resonance complications. The method is illustrated for CFHCl2, CF2HCl, and CF3H, and is used to demonstrate explicitly that the Overhauser intensity changes vanish for large irradiation intensities.
Influence of the Dielectric Constant on the Yield of Free Ions Produced during Radiolysis of a Liquid43(1965); http://dx.doi.org/10.1063/1.1696492View Description Hide Description
The γ‐radiation‐induced conductance of liquid diethyl ether is consistent with the previously developed theory.
The following values of G(free ion) were determined by the conductance method: n‐hexane, 0.10; cyclohexane, 0.10; p‐dioxane, 0.05; n‐butyl ether, 0.11; ethyl ether, 0.19; n‐butyl bromide, 0.27; n‐butyl chloride, 0.39.
Values of G(free ion) in other systems were obtained from spectroscopic and chemical studies reported in the literature.
G(free ion) is roughly proportional to the static dielectric constant of the liquid, over the range studied. A theory is presented which explains the results. It appears that the slowed‐down secondary electrons are ``trapped'' in the liquid and solvated before undergoing geminate recombination, even in alkanes.
Solvated electrons in alkanes would probably have a charge‐transfer type of absorptionspectrum, with a broad absorption maximum in the infrared.
The major portion of ion pairs that undergo geminate recombination in these liquids does so in a period of 10−10±1 sec.
A general mechanism is formulated for the initial stages of the ionic part of a liquid‐phase radiolysis reaction.
43(1965); http://dx.doi.org/10.1063/1.1696493View Description Hide Description
43(1965); http://dx.doi.org/10.1063/1.1696494View Description Hide Description
The adsorption of molecular and atomic hydrogen on multilayers of potassium, deposited on a tungstenfield emitter, and the coadsorption of hydrogen and potassium on tungsten have been investigated by a field‐emission technique. Molecular hydrogen does not appear to be chemisorbed on potassium at low temperatures. Atomic hydrogen seems to interact, most probably by the formation of a hydride structure with K atoms outermost from the surface. The coadsorption of H and K on tungsten leads to a decrease in work function over that caused by potassium alone, and to increases in the temperature required for potassiumsurfacediffusion on tungsten. These facts are explained in terms of a surface complex WHK in which H is negatively charged relative to K.
43(1965); http://dx.doi.org/10.1063/1.1696434View Description Hide Description
The infrared spectrum of ReF7 vapor and the Raman spectrum of liquid ReF7 suggest that this molecule, like IF7, belongs to Symmetry Group D 5h . The five expected Raman‐active fundamentals have been observed at 351, 485, 590, 650, and 737 cm−1. Of the five expected infrared‐active fundamentals, three have been observed at 299, 353, and 703 cm−1.
43(1965); http://dx.doi.org/10.1063/1.1696435View Description Hide Description
Measurements of the thermal conductivity, using a steady‐state technique, and of the specific heat, using a transient technique, have been performed between 4.5° and 1°K on commercial samples of polytetrafluoroethylene (Teflon), polychlorotrifluoroethylene (Kel‐F), nylon, and three samples of polyethylene of varying densities. In all cases the specific heat is proportional to T 3. For the polyethylene samples, the constant of proportionality was found to depend strongly on the density of the sample, indicating that amorphous regions have a higher specific heat in this temperature range than do crystalline regions. Extrapolation to completely crystalline material gives the heat capacity as C = 100 T 3 erg/°K·cm3, while extrapolation to completely amorphous material gives the heat capacity as C = 297 T 3 erg/°K·cm3. The thermal conductivity varies more rapidly than linearly with temperature except in the case of polychlorotrifluoroethylene. Analysis of these thermal‐conductivity measurements and some similar results which have been published previously on the basis of a combination of structurescattering and internal boundary scattering yield sizes of the internal boundaries which show good correlation with the average size of crystalline regions determined optically for the same samples.
43(1965); http://dx.doi.org/10.1063/1.1696437View Description Hide Description
A Monte Carlo procedure was used to simulate non self‐intersecting random walks on a high‐speed digital computer. The specific model considered hard cores of 3½ units and different rotational angles. The mean‐square end‐to‐end distance and the mean‐square radius of gyration were fitted to an equation of the form 〈J N 2〉 = aNb . It was found that the results obtained by considering nonlattice systems are not consistent with walks constrained to a lattice. This discrepancy is discussed in terms of intrinsic excluded volume and excess excluded volume. It is suggestive that the ratio 〈S N 2〉/〈R N 2〉 may be a more general parameter in discussing these systems.