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Volume 41, Issue 11, 01 December 1964
41(1964); http://dx.doi.org/10.1063/1.1725725View Description Hide Description
The energy distribution of ions obtained by field ionization of He, Ne, Ar, and H2 near the surface of a tungsten emitter is measured with an improved retarding potential tube. Most ions are found to originate immediately beyond the critical distance determined by the condition that the electron tunnels above the Fermi level. The ion energy is determined by the ionizationenergy and an image force contribution, while there is no indication of a shift of the ground level of the gas atom due to the proximity of the metal surface. In the case of hydrogen the atomic ion is found to have an additional energy of ½ of the dissociation energy of the molecular ion. The half‐width of the distribution is between 0.5 and 0.7 eV for all the above gases, indicating an extremely narrow ionization zone with a depth of a few tenths of an angstrom. Calculation of the energy distribution by applying the WKB method to a square well gives the proper general features and the temperature effect but a half‐width too wide by a factor of 2, thereby exhibiting the limitation of a linear model.
41(1964); http://dx.doi.org/10.1063/1.1725726View Description Hide Description
The magnetic characteristics of 11 intermetallic compounds represented by the formula LnAg are reported for temperatures between 2° and 300°K. YAg is a Pauli paramagnet. CeAg and PrAg are ferromagnets with Curie temperatures 9° and 14°K, respectively. The compounds with Ln=Nd, Sm, Tb, Dy, Ho, Er, or Tm exhibit pronounced susceptibility peaks, from which it is inferred that they are antiferromagnetic at low temperatures. This may also be true for GdAg whose susceptibility behavior is anomalous below 200°K. At the upper temperatures studied Curie—Weiss behavior is observed, and effective moments in good agreement with the free tripositive ion moments are obtained for all except YAg, SmAg, GdAg, and ErAg. The lack of compliance for SmAg is ascribed to limitations in the experimental procedure used. GdAg obeys the Curie—Weiss Law above 200°K, but with an effective moment of 8.57 μB. The effective moment observed for ErAg varies from about 9 μB at 298° to 6 μB at 77°K.
41(1964); http://dx.doi.org/10.1063/1.1725727View Description Hide Description
The Verdet constants of a number of glasses have been measured with the object of studying the Faraday effect in glass.
The Verdet constant was measured for three types of glasses: (1) optical glasses, where the Verdet constant correlates well with the reciprocal dispersion; (2) diamagneticglasses, where glasses which contain high concentrations of ions that have sp—s 2transitions give rise to large Verdet constants; (3) rare‐earth glasses, i.e., paramagnetic ions, where Pr3+ and Dy3+ silicate and borate glasses have the largest Verdet constants. These latter rare‐earth data were compared to the theoretical expression of Van Vleck and Hebb and to the experimental data on metaphosphates of Berger et al.
41(1964); http://dx.doi.org/10.1063/1.1725728View Description Hide Description
In this paper we consider some of the consequences of the mixing of neutral exciton states (neutral excitation waves or Frenkel excitons) with charge‐transferexciton states (ion‐pair excitons) in the oriented gas model of aromatic molecular crystals.
The lowest‐energy charge‐transferexciton is described by suitable symmetry‐adapted functions built on the basis functions of a near‐neighbor positive and negative ion pair. It is then shown that the energy of this state is not greatly different from the p states of anthracene and naphthalene, and therefore that the ion‐pair exciton and Frenkel exciton states can mix. For the cases of anthracene and naphthalene, such configuration interaction leads to the following contributions to the Davydov splitting of the triplet manifold:where ΔE is the energy difference (in electron volts) between the triplet level and the near‐neighbor charge‐transfer level. In making the computations, it is necessary to consider carefully the effects of vibronic coupling and of configuration changes in the excited state. It is concluded that charge‐transferexcitons can make an important contribution to the Davydov splitting of a triplet manifold. This conclusion is supported by preliminary estimates of the interaction matrix elements for crystalline benzene.
41(1964); http://dx.doi.org/10.1063/1.1725729View Description Hide Description
The protonchemical shifts have been obtained from the NMR spectra of nine γ‐substituted pyridines. The substituent‐induced chemical shifts, which were calculated by subtracting from each observed value the corresponding proton shift of the unsubstituted pyridine, were found to be comparable to the shielding parameters of the analogous substituted benzenes. Moreover, these chemical shifts can be fitted to the additivity relationship of Martin and Dailey which was obtained from the proton shieldings in substituted benzenes. Therefore the shielding mechanisms in the pyridines are probably the same as those in the benzenes. The failure of the calculated chemical shifts of the α and β protons in the pyridines to agree with observed shifts, as was also the case with the corresponding substituted benzenes, is interpreted as further evidence that the shieldings at positions ortho and meta to the substituents are not determined by the pi‐electron density variations alone.
41(1964); http://dx.doi.org/10.1063/1.1725730View Description Hide Description
The effect of low‐energy (15–200‐eV) electrons on hydrogen, oxygen, carbon monoxide, and barium adsorbed on tungsten has been investigated by a field‐emission technique. Desorption cross sections σ were determined from work function and Fowler—Nordheim pre‐exponential changes and are significantly smaller than would be expected for comparable molecular processes. Marked variations in cross section with binding mode within a given system were found. Thus σH=3.5 10—20 cm2 and 5×10—21 cm2 for processes tentatively interpreted as the splitting of molecularly adsorbed H2 and desorption of H, respectively; σ0=4.5×10—19 cm2 for a loosely bound state and σ0≤2×10—21 cm2 for all other states; σBa<2×10—22 cm2 under all conditions. In the case of CO (reported in detail elsewhere), three binding modes observed previously could be confirmed and differentiated by their different cross sections: σvirgin=3×10—19 cm2; σβ=5.8×10—21 cm2, σα=3×10—18 cm2; conversion by electrons of virgin to β σ vβ≥10—19 cm2. These results are interpreted in terms of transitions from the adsorbed ground state to repulsive portions of excited states, followed by de‐exciting transitions which prevent desorption. Arguments are made to show that the excitation cross sections should be essentially ``normal,'' i.e., ∼10—16 to 10—17 cm2, and that the much smaller over‐all cross sections observed are due to high transition probabilities to the ground state, estimated as 1014 to 1015 sec—1. A detailed calculation for the case of exponentially varying transition probabilities and repulsive upper states is presented and discussed, and the variations in cross section with binding mode made plausible. It is shown that low‐energy electron impact constitutes a sensitive tool for studying chemisorption.
41(1964); http://dx.doi.org/10.1063/1.1725731View Description Hide Description
The low‐energy electron impact desorption of CO adsorbed on W has been investigated by means of a field emission technique. It was found that the previously postulated binding modes of CO could be distinguished and confirmed by their differing desorption cross sections which are σvirgin=3×10—19 cm2, σvirgin to β≥10—19 cm2, σβ∼3×10—21 cm2, σα=3×10—18 cm2 for 80‐V electrons. Evidence for thermal conversion of virgin states to both β and α states was obtained; electrons also seem to convert virgin to β states. In addition to desorption,dissociation with carbon formation occurred. Although quantitative estimates are difficult, the ratio of dissociation to desorption was estimated to be between 0.5 and 0.1, and seems to be least for α and greatest for β desorption. The results on carbon formation show that (at least) a large fraction of CO in the α and virgin modes is adsorbed with the C end of the molecule only. It is pointed out that electron desorption is a sensitive tool for probing chemisorption states and is able to reveal details like interconversion which are not readily detectable by other means.
41(1964); http://dx.doi.org/10.1063/1.1725732View Description Hide Description
When electrically excited nitrogen was used as an absorber, about one hundred absorption bands were observed in the 1080–1300‐Å region. They are not due to absorption by ordinary nitrogen, but are produced by transitions from the vibrationally excited (v=4–13) ground stateX 1Σ g + to the j 1Σ u +, b 1Π u , l 1Π u , m 1Π u , and b′ 1Σ u + states.
41(1964); http://dx.doi.org/10.1063/1.1725733View Description Hide Description
The theory of Aden and Kerker on the scattering of electromagnetic waves from two concentric spheres has been applied to the interaction of light with two‐layer metallic colloids. Starting with gold nuclei 59 Å in ``diameter,'' silver layers 5 to 220 Å in thickness have been grown epitaxially. The optical absorption spectra of these hydrosols containing two‐layer particles are compared with the spectra calculated from the theory of Aden and Kerker. The calculated and experimentally observed spectra are in relatively good agreement in the wavelength range 380–700 mμ. Large discrepancies occur between 300 and 380 mμ for hydrosols containing particles with silver layers less than 15 Å. This is attributed to a deviation of the silver refractive index from that of bulk silver for these layer thicknesses. The experimental absorption peaks are generally found to be broader than the calculated results. This is due in part to size and shape anisotropy. Other possible factors are lattice imperfections and trapped ions at the gold—silver interface.
41(1964); http://dx.doi.org/10.1063/1.1725734View Description Hide Description
The optical absorption and fluorescence spectra of trivalent thulium in single crystals of yttrium oxide and the absorptionspectrum of pure thulium oxide, are investigated. All the 4f 12 electronic energy levels except the 1 S 0 level are identified. The three Slater Fk parameters and the spin—orbit parameter ζ, giving the best over‐all agreement between the observed spectrum and the calculated spectrum are: F 2=446.70 cm—1, F 4=70.491 cm—1, F 6=7.534 cm—1, and ζ=2617 cm—1. A detailed crystalline‐field splitting perturbation calculation is carried out for trivalent thulium and trivalent erbium ions in the C 2 sites of the Y2O3 lattice. The predicted crystalline‐field splitting for these ions is compared with the thulium data reported in this paper and with the erbium data reported earlier. The average deviations between the predicted and the observed splittings for 71 crystalline Stark levels in the Tm3+spectrum and 65 levels in the Er3+spectrum, are 13 and 11 cm—1, respectively.
Electronic Structure of Transition Metal Complexes with Nitrosyl Ligand. Mössbauer Effect Investigation41(1964); http://dx.doi.org/10.1063/1.1725735View Description Hide Description
The Mössbauer effect was investigated with spin‐paired diamagnetic pentacyanonitrosylferrate (II) (sodium nitroprusside), spin‐paired paramagnetic nitroso‐bis‐dimethyldithiocarbamates iron (I), and with spin‐free pentaquonitrosyl iron (I) complexes.
Information on the nature of bonding in the first two complexes was obtained through detailed interpretation of the isomer shift and quadrupole coupling, measured with polycrystalline and single crystal samples.
Different possible interpretations are discussed for the results with the pentaquo complex.
41(1964); http://dx.doi.org/10.1063/1.1725736View Description Hide Description
Approximate SCF LCAO—MO calculations have been carried out for the ground‐state properties of four hydrogen halides; six internuclear distances were considered. In the model only the bonding electrons are considered explicitly. The effect of the inner electrons were taken account of by semiempirical methods. The electron pair is described by a single configurational wavefunction and the MO's are built from 1s and np AO's. The matrix elements are obtained utilizing the Mayer—Sklar, Mulliken, Pariser, and Pople approximations. As a secondary study the effect of the Wolfsberg—Helmholtz parameter K, in the resonance terms is studied.
The calculated binding energies, equilibrium bond distances, dipole moments, vibrational force constants, and breaking forces obey the same trends as in experimental data. Moreover the magnitudes for these calculated quantities are in substantial agreement with experiment. This agreement can be improved for a given physical property of the series by introducing the arbitrary parameter K and optimizing it for one member of the series.
It is concluded that the approximations characterizing the model, all of which had been previously used with π‐bonded systems, are equally well applicable to σ‐bonded systems.
Studies of Hydrogen‐Bonded Systems. I. The Electronic Structure and the Double Well Potential of the N–H···N Hydrogen Bond of the Guanine—Cytosine Base Pair41(1964); http://dx.doi.org/10.1063/1.1725737View Description Hide Description
For hydrogen‐bonded complexes we outline an LCAO—MO—SCF method which considers explicitly the σ electrons of the hydrogen‐bond region and the π electrons of the surrounding ligands. The protonic potential function is obtained by solution of these SCF equations for a series of proton positions. The electronic structure and the potential function of the middle N–H···N hydrogen bond of the guanine—cytosine base pair have been calculated by this method. The integrals appearing in the problem have been approximated semiempirically and several alternative approximations have been tested. The calculated π electronic structure and orbital energies conform with the result of a previous SCF calculation treating the H bond parametrically. The calculated charge distribution in the H‐bond region is in agreement with the mainly covalent character of the N–H bond and mainly electrostatic character of the H···N bond for equilibrium configuration. The empirical and electrostatic models agree well with our model with respect to the qualitative features of the double well potential characterizing a H bond. With respect to the magnitude there is a substantial decrease in the potential barrier calculated by us in comparison to that calculated from classical electrostatic interactions. This decrease can be partly related to the delocalization effect which is treated in our model and partly to the self‐consistent charge distribution utilized for the calculation of electrostatic interactions, which in the simpler models are estimated with more arbitrariness.
41(1964); http://dx.doi.org/10.1063/1.1725738View Description Hide Description
In an aqueous solution of a paramagnetic ion which exchanges water rapidly between its first coordination sphere and the solvent, a single oxygen‐17 NMR signal is observed shifted from its normal position in pure H2O. The magnitude of the shift is a measure of the ratio of the paramagnetic ion concentration to total (labile) water in the system. If a second ion for which the first coordination sphere water is sufficiently nonlabile (τ>∼10—4 sec) is added to the system, the decrease in the water available to interact with paramagnetic ions due to water retained in the nonlabile environment will produce an increase in the shift in the labile H2 17O resonance by the paramagnetic ion. The application of this ``molal shift'' method to the determination of the hydration numbers of certain diamagnetic and paramagnetic cations in aqueous solutions is demonstrated and discussed.
41(1964); http://dx.doi.org/10.1063/1.1725739View Description Hide Description
Although linewidths for the oxygen‐17 resonance were recorded in supercooled H2O and D2O, it has not been possible to obtain a signal from these pure solids.Linewidth measurements were also made to —79°C in a LiI–H2O eutectic solution.Ice with ∼10% ammonium fluoride in solid solution did not display the 17O resonance below the eutectic temperature. The deuterium resonance in a 0.15M NaCl—D2O solution was used to demonstrate the presence of liquid eutectic solution in polycrystallineice.
41(1964); http://dx.doi.org/10.1063/1.1725740View Description Hide Description
Quadrupole splitting of the deuteron magnetic resonance was measured in single crystals of solid D2O at —10°C. For deuterons on the hexagonal crystal axis, eQq 1/h equaled 213.2±0.8 kc/sec, and | η1 | was equal to 0.100±0.002. For the other deuterons, eQq 2/h was found to be equal to 216.4±1.0 kc/sec. No change in the line separations was observed at a temperature of —70°C. No evidence was found for hindered rotation of the D2O molecules nor for intermolecular deuteron transfer at —10°C. Lower limits to the barriers hindering these motions were estimated; these limits permit estimates to be made of hydrogen‐bond energies. The coupling constants in the solid are about 30% less than those reported for the free molecule. Most of this decrease must be ascribed to electronic structural changes brought about as a result of hydrogen‐bond formation.
41(1964); http://dx.doi.org/10.1063/1.1725741View Description Hide Description
An arbitrary function in the eigenfunction space of some quantum‐mechanical Hamiltonian may be thought to represent the initial configuration ψ(q,0) of a nonstationary state. The system develops in time according toDefining F(t)≡〈ψ(q,0), ψ(q,t) 〉, and hk ≡〈ψ(q,0), k ψ(q,0) 〉, and taking the Fourier transformwe obtainIn terms of the formal expansion in the energy eingenfunctionsthe Fourier transform representswhich exhibits, in principle, the entire eigenvaluespectrum. In this paper, a direct method of calculating eigenvalue spectra, based on the foregoing principle, is proposed. Two modifications are required for computational practicability: (i) use of a finite representation for the delta function and truncation of the summation (a); (ii) replacement of the integrals hk by . The modified spectral function is taken to bewith χτ(ω)≡sinτω/πω. The sequence GN (τ,ω,q′) is shown to converge as N→∞ if in the Expansion (b) the coefficients cn and c(ω) decrease with ω as exp(—ω/λ) or faster. Assuming convergence, the spectral function represents a broadened eigenvaluespectrum.
41(1964); http://dx.doi.org/10.1063/1.1725742View Description Hide Description
Mass spectrometric Knudsen effusion experiments have provided evidence for the existence of the suboxide species Be2O(g) in the equilibrium vapor above crystalline beryllium oxide at temperatures around 2300°K. Be2O+ ions appear to be formed both by simple ionization of Be2O(g) and by fragmentation of a larger molecule. The relative importance of the two processes has been shown to be temperature dependent. An average value of —8±10 kcal/mole was obtained for ΔHf°298[Be2O(g)] from third‐law calculations for three equilibria.
Mass spectral data are given for the equilibrium vapor above beryllium oxide at 2380°K in the presence of tungsten.
Integrated Intensity Measurements of the 5.3‐μ Fundamental and 2.7‐μ Overtone Bands of NO between 1400° and 2400°K41(1964); http://dx.doi.org/10.1063/1.1725743View Description Hide Description
Measurements have been made of the total integrated band intensities of the 5.3‐μ fundamental and the 2.7‐μ first overtone bands of NO in the temperature range 1400° to 2400°K. The NO gas sample was studied at high temperatures using a reflected shock‐wave technique.
Within the temperature range used in the present experiments, the integrated band intensity of the NO fundamental does not vary. However, the integrated intensity of the first overtone band of NO shows a small temperature dependence. The temperature dependencies of the two bands are in agreement with theoretical temperature dependencies determined assuming a harmonic oscillator approximation to the NO molecule. Using the theoretical temperature dependencies the experimental data have been extrapolated to give values of the integrated intensities α° at STP. The values of α° obtained for the fundamental and first overtone bands of NO are 76±7 cm—2·atm—1 and 2.8±0.5 cm—2·atm—1, respectively.
41(1964); http://dx.doi.org/10.1063/1.1725744View Description Hide Description
Mixtures of HCl and D2 and of DCl and H2 were irradiated with an ultraviolet mercury resonance lamp and analyzed mass spectrometrically for the different isotopic species of hydrogen. It was found that at room temperature the reaction H+DCl→HD+Cl is slower than the exchange reaction H+DCl→D+HCl. This is to be expected from a consideration of activation energies, but it is in apparent contradiction to earlier results obtained by Leighton and Cross and by Steiner and Rideal. The reaction between hydrogen atoms and hydrogen molecules was found to be much faster at this temperature than is usually estimated.