Index of content:
Volume 45, Issue 7, 01 October 1966
45(1966); http://dx.doi.org/10.1063/1.1727943View Description Hide Description
The microwave spectra of C5H7 35Cl and C5H7 37Cl have been observed and analyzed. If the spiropentane ring is assumed to be in the D 2d configuration, with methylene‐group parameters similar to cyclopropyl chloride, the following parameters are calculated from the observed moments of inertia: r(CCl)=1.74 Å, r(CC)=1.51 Å, and ∠RCCl=118°. The quadrupole coupling constants in the inertial axis system have been obtained for both isotopic species. For the 35Cl species, the values are χ aa =−30.98 MHz, χ bb =6.22 MHz, and χ cc =24.76 MHz. Evidence is presented which indicates that the chemical bond linking the chlorine to the ring is not axially symmetric and does not lie along the C–Cl internuclear line.
45(1966); http://dx.doi.org/10.1063/1.1727944View Description Hide Description
The general scheme of the interaction of triplet excitons is formulated to remove the degeneracy of spin energy levels of the system and predict the spread of each spin absorption line in the zero‐field magnetic resonancespectra.
An approximate function of quasi‐Lorentz type is derived for the shape of spectrum which represents the feature for different numbers of excitonsinteracting effectively. The maximum slope width is computed and it is proportional to the square of exciton density. Discussion is made in comparison with experimental data.
On the Eu3+ Fluorescence in Mixed Metal Oxides. III. Energy Transfer in Eu3+‐Activated Tungstates and Molybdates of the Type Ln2WO6 and Ln2MoO645(1966); http://dx.doi.org/10.1063/1.1727945View Description Hide Description
The fluorescence of unactivated and rare‐earth‐activated materials of the type Ln2WO6, Ln2MoO6, and Y2W1−x Mo x O6 (0≤x≤0.1) has been studied. From the experiments on unactivated materials it follows that energy transfer from tungstate to tungstate or mobyldate group takes place, whereas there is no transfer from molybdate to molybdate or tungstate group. Arguments in favor of exchange‐regulated energy transfer from tungstate or molybdate group to rare‐earth ion are put forward. The efficiencies of the rare‐earth‐activated tungstates and molybdates can be qualitatively understood assuming a strong dependence of the energy transfer on the angle tungsten (molybdenum)—oxygen—rare earth. In Y2W1−x Mo x O6:Eu the molybdate group acts as a killer of the fluorescence under short‐wave uv excitation and as a sensitizer under long‐wave uv excitation.
On the Eu3+ Fluorescence of Mixed Metal Oxides. IV. The Photoluminescent Efficiency of Eu3+‐Activated Oxides45(1966); http://dx.doi.org/10.1063/1.1727946View Description Hide Description
Four rules are proposed to explain the photoluminescent efficiency of Eu3+‐activated oxides: (a) uv absorbing groups in the host lattice should be nearest neighbors in the crystal structure to have sufficient wavefunction overlap; (b) the emission spectrum of the absorbing group should overlap the absorptionspectrum of the group; (c) the configuration consisting of the center of the absorbing group, the intermediary O2− ion, and the Eu3+ ion should be as collinear as possible; (d) O2− ions surrounding the Eu3+ ion must feel a high potential field.
If all these rules are satisfied, highly efficient Eu3+phosphors can be expected; if one or more of the rules are violated, phosphors with low efficiency must be expected.
These ideas are applied to a large number of Eu3+‐activated oxides. There is a very good agreement between experimental results and predictions from the rules.
As far as possible the physical background of the rules is discussed.
45(1966); http://dx.doi.org/10.1063/1.1727947View Description Hide Description
The consequences of each of the three approximations made by Kirkwood in deriving the Lennard‐Jones—Devonshire cell theory are elucidated by a comparison with exact results in the one‐dimensional case of hard rods. This comparison gives an explicit calculation of the one‐dimensional communal entropy and shows that the single‐occupancy approximation is exact at close packing, although it does not lead to thermodynamic properties which are analytic functions of density. For two‐ and three‐dimensional systems, numerical results show that the cell‐theory entropy predictions are more accurate than in one dimension because the particles are more localized. The more‐than‐one‐particle‐per‐cell theories provide simple and rigorous lower bounds on the entropy, but the convergence to the thermodynamic limit by considering more and more particles is slow.
45(1966); http://dx.doi.org/10.1063/1.1727948View Description Hide Description
Isenthalpic solidification of a unary (one‐component) supercooled liquid results in either two‐phase invariant equilibrium or in single‐phase univariant equilibrium, depending on the degree of supercooling prior to solidification.
When postsolidification univariant equilibrium obtains, a determination of the specific heat at constant pressure, Cp l, of the supercooled phase relative to the stable, solid phase can be made by measuring the adiabatic temperature rise during recalescence.
Values of Cp l for highly supercooled phosphorus over the temperature range −1° to +18°C have been determined with this method. No evidence for the changes in molecular association deduced from earlier viscosity data on supercooled phosphorus was found in the temperature variation of the specific‐heat function. The enthalpy and specific‐heat values determined for supercooled liquid phosphorus agree with published values for the stable liquid above the normal melting point, Te =44°C.
45(1966); http://dx.doi.org/10.1063/1.1727949View Description Hide Description
The pyrolysis of ethylene, highly diluted by neon, was studied in the temperature range 1710°—2170°K with reflected shock pressures of 225 to 1600 torr. The products are C2H2 and H2 with some diacetylene at higher temperatures. The rate law is over a threefold ethylene and sixfold neon pressure range and the rate constant is given by .
In experiments with equimolar C2H4+C2D4 mixtures the hydrogen formed initially contains as much HD as H2 although at the same temperatures the reaction H2+D2→2HD is found to be relatively slow. In experiments with C2H4+D2 mixtures as much HD as H2 is formed in pyrolysis. Some relatively slow isotopic exchange between ethylenes and between C2H4 and D2 occurs, forming mainly C2H3D in the latter case.
An unexpectedly high sensitivity of the mass spectrometer to hydrogen formed in the pyrolysis suggested that it may be formed in vibrationally excited states. A number of diagnostic experiments could neither confirm nor reject this possibility.
Both a simple unimolecular reaction C2H4+M→C2H4 *→C2H2+ H2 *+M (where H2 * is a vibrationally excited molecule) and several free‐radical chain mechanisms of the Rice—Herzfeld type can describe the observed kinetics, although in all cases the relatively low observed activation energy is very difficult to explain. Thus no clear‐cut identification of the most probable reaction mechanism is possible.
45(1966); http://dx.doi.org/10.1063/1.1727950View Description Hide Description
The unusual magnetic properties of the 1Σ+ BH molecule calculated earlier from perturbed (coupled) Hartree—Fock theory have been confirmed, and rendered more nearly gauge invariant. Predictions of the spin—rotation constant for hydrogen are C H=−14.9 kc/sec with B as origin, and C H=−18.2 kc/sec with H as origin, as compared with the earlier values of −8.5 and −25.3 kc/sec, respectively. Also, the shielding at H is σH=24.46 ppm with B as origin and σH=25.44 ppm with H as origin, as compared with our previous values of 22.55 and 27.62 ppm, respectively. In addition, we present an interpretation of the molecular paramagnetism, the antishielding at B, and the large spin—rotation constant of B, in terms of orbital current densities. It is found that these extreme properties calculated for BH result from a paramagnetic current due to excitations of the lone‐pair electrons on the boron atom to an unoccupied pπ orbital on this atom.
45(1966); http://dx.doi.org/10.1063/1.1727951View Description Hide Description
A (110) surface of W that is covered by ½ a monolayer of O atoms will adsorb about ¼ monolayer of strongly held CO molecules and about half as many weakly held molecules. These are half the amounts that can be adsorbed on a clean (110) face. The CO is desorbed in its original form in two states at about 450° and 1100°K. No CO2 is formed. All the O is removable at 1950°K, apparently as oxides of W. The composite O–CO covered surface is a disordered structure if formed at room temperature, but if this is heated to about 1000°K a complex ordered surface is produced. This structure, having C (11×5) symmetry bears some resemblance to the C(9×5) structure produced in similar fashion without any oxygen. Experiments carried out in reverse order show that oxygen is not adsorbed at room temperature on a surface first saturated with CO, but if a CO saturated surface is held at 1000°K during exposure to O2, the same C(11×5) structure is formed as before. Exposure of a bare (110) surface to CO2 at a pressure below 10−7 torr results in negligible adsorption.
45(1966); http://dx.doi.org/10.1063/1.1727952View Description Hide Description
Gas‐phase reactions of carbon vapor near room temperature with simple gases and their discharge products have been spectrally observed for the first time. Experimental arrangements for obtaining carbon vapor and mixing it with discharge products or gases in a low‐pressure flow system are presented. Spectra of the carbon vapor flames with oxygen, atomic oxygen, and atomic nitrogen have been obtained over the range 2200 to 7000 Å. Molecular‐oxygen flames emit the C2 Swan and CO triplet and third positive bands. Flames with atomic oxygen are much more intense than flames with molecular oxygen and emit the C3 4050 Å group in addition to C2 and CO bands. Bands from ν=2 in the C2 Swan system are found to be enhanced in atomic‐ and molecular‐oxygen flames. On addition of molecular or atomic hydrogen to the atomic‐oxygen flame, CH 4315‐ and 3900‐ Å and OH 3064‐ Å bands appear. Atomic‐nitrogen flames emit the CN red and violet systems with considerable intensity in bands from high vibrational levels. Spectra of these flames have been studied as a function of pressure, flow rates, carrier gas, and methods of obtaining carbon vapor. Mechanisms for formation and excitation of the emitters of the various bands are suggested.
45(1966); http://dx.doi.org/10.1063/1.1727953View Description Hide Description
Concentration‐dependent probabilities for flnding one of two kinds of impurities in a single, pair, or triple cluster within a simple cubic, body‐centered‐cubic, face‐centered‐cubic, or hexagonal‐close‐packed lattice are given as polynomials for which tabulated values are readily available. Both nearest‐neighbor interactions and next‐nearest‐neighbor interactions between foreign impurities in random distribution within the lattice are each separately treated. Graphical comparisons of the results indicate that the effect of introducing an additional impurity is similar to that of including next‐nearest‐neighbor interactions. In both cases cluster probabilities are decreased.
45(1966); http://dx.doi.org/10.1063/1.1727954View Description Hide Description
The absolute molecular electronegativities for 14 aromatic hydrocarbons are reported. For the hydrocarbons considered, a linear relationship exists between the electron affinity and the energy of the 1 La transition. The molecular electronegativity for some eight aromatic hydrocarbons is approximately constant. On this basis, it is possible to predict some ionization potentials which are in excellent agreement with recent experimental values. It does not appear reasonable to relate the intercept resulting from extrapolation of the aforementioned plot to hν=0 to the work function of graphite. Based on the measured electron affinities, it is possible to deduce a relationship with methyl affinities and calculate ΔE sol between neutral hydrocarbons and their mononegative ions. Certain theoretical approaches can be successfully employed to estimate electron affinities of aromatic hydrocarbons.
Internal‐Energy‐Transfer Efficiencies in Eu3+ and Tb3+ Chelates Using Excitation to Selected Ion Levels45(1966); http://dx.doi.org/10.1063/1.1727955View Description Hide Description
Fluorescence yields of solutions of several chelates of europium and terbium have been measured: (a) upon excitation in the ligand absorption bands and (b) upon selective excitation to individual upper levels of the rare‐earth ion. Yields with upper‐ion‐level excitation are lower than when the emitting level is excited directly and yields with ligand excitation are lower still. Efficiencies of energy transfer to the emitting level from upper ion levels and from the ligands, for the solutions studied, are calculated from the data.
The fluorescence yield observed when the emitting level is excited directly, together with the fluorescence lifetime, allows both radiative and radiationless rate constants for deactivation of the emitting level to be calculated. The enhanced fluorescence yields of chelates compared to unchelated rare‐earth ions are found to be due primarily to the enhancement of the radiative transition by the chelate environment rather than to a protective influence against quenching provided by the ligands.
For terbium chelates the strong temperature dependence of the fluorescence intensity is explained in terms of quenching of the emitting level via thermal excitation to the lowest triplet state of the ligand. This is indicated by the agreement between the activation energy for quenching and the energy difference between the emitting level and the triplet state.
45(1966); http://dx.doi.org/10.1063/1.1727956View Description Hide Description
The fluorescence lifetime of Eu(NO3)3 in CH3OH, CH3OD, and CD3OD and of EuCl3 in D2O has been determined as a function of temperature. The temperature‐dependent quenching of fluorescence from the 5 D 0 level is attributed to radiationless deactivation through the 5 D 1 level. From these and other results it is concluded that the 5 D 0 level of Eu3+ does not possess a mechanism for radiationless conversion to the ground level that does not involve the upper levels or solvent vibrations.
45(1966); http://dx.doi.org/10.1063/1.1727957View Description Hide Description
Several types of microwave double‐resonance experiments are performed using a double‐bridge superheterodyne detection scheme. The first set of experiments clarify some features of the double quantum effect and also confirm earlier experiments in a Stark‐modulated spectrometer.
The second set of experiments employs the double‐bridge technique and the wide bandwidth capability of the 30‐Mc/sec amplifiers to measure directly the restoration to equilibrium of a nonequilibrium system. Nonequilibrium conditions are produced in the OCS J=0, 1, and 2 rotational energy levels by applying high microwave power resonant with the J=0→1 transition and simultaneously observing the J=1→2 transition. The intensity of the J=1→2 transition was found to relax to equilibrium with a first‐order exponential dependence by direct observation of the amplitude of the 30‐Mc/sec carrier as a function of time after the signal irradiating the J=0→1 transition was cut off. The direct method of measuring rotational relaxation was carried out as a function of pressure. The linewidths were also measured in a steady‐state experiment in the same apparatus and the relaxation results are compared to the above direct method of observation. The direct method leads to relaxation times half as long as the relaxation times obtained from the corrected linewidth data (τeff=8.43 μsec·μ, τ=17.51 μsec·μ). This factor of 2 is explained by consideration of the basic difference between the two types of measurements.
The relaxation times of OCS in 1:1 ratios with Ar, He, and O2 were also measured by the direct method. In these cases the relaxation times lead to collision diameters of d OCS—OCS=10.2 Å, d OCS—Ar=8.3 Å, d OCS—He=5.5 Å, and d OCS—O2 =6.3 Å. The phenomena of the nonmixing of the J=1, m=0, and m=±1 states upon irradiation of the J=0→1, m=0→0 transition is investigated in the pure OCS and mixed OCS—O2 cases. No mixing was observed in either case. A plausible explanation is offered based on the second‐order Stark effect and the first‐order Zeeman effect, both of which are present during a collision.
Several methods of improving on the signal‐to‐noise ratio obtained in the direct method of observing rotational relaxation times are pursued with the interest being the extension to the measurement of vibrational relaxation. We have shown the feasibility of periodically repeating the above direct‐measurement experiment (at a time slower than the relaxation time) and observing the relaxation time as a phase shift due to the delayed response of the population in the molecular energy levels. In another experiment we have shown the feasibility of extracting the X‐band (J=0→1) modulation frequency from the 30‐Mc/sec carrier to obtain an amplitude depending only on the modulation efficiency or population modulation of the J=1→2 transition. By studying the amplitude of the modulation frequency as a function of modulation frequency, one can again extract relaxation information. Both of these latter methods could be applied in a Stark‐modulated spectrometer.
The future of relaxation experiments using microwave spectroscopy appears to be in the areas of observing transient species, in gaseous chemical kinetics, and studies of lifetimes of excited vibrational states in molecules.
45(1966); http://dx.doi.org/10.1063/1.1727958View Description Hide Description
It is suggested that the existing discrepancy between the experimentally determined H35Cl energy‐level constant Y 02 and its value, as predicted through Dunham's theory, represents a deviation from Born—Oppenheimer behavior. Relationships between 1Σ‐state molecular energy‐level constants, corrected to take account of deviations from Born—Oppenheimer behavior, are then used in a recalculation of the Dunham theoretical value for Y 02. The discrepancy is not removed through the present calculation, however. The fractional difference between the Dunham values with and without corrections for deviations from Born—Oppenheimer behavior is −0.29×10−3, as opposed to the fractional difference (0.649±0.065)×10−3 between the experimental and (uncorrected) Dunham theoretical values. The importance of the present calculation to an accurate spectroscopic determination of the velocity of light is indicated.
19F Spin—Rotation and Spin—Spin Interactions, 19F Magnetic Shielding, and Molecular Magnetic Moments for OCF245(1966); http://dx.doi.org/10.1063/1.1727959View Description Hide Description
The microwave spectrum of OCF2 is examined under high resolution using an L‐band waveguide and 1 and 5‐kc/sec modulation, where half‐widths at half‐height of 5–7 kc/sec are obtained. The 19F nuclear‐spin‐nuclear‐spin and the 19F spin—rotation interactions are observed. The experimental 19F spin—rotation constants along the principal inertial axes are Maa = | 19±3 | kc/sec, Mbb = | 13±3 | kc/sec, and Mcc = | 5±3 | kc/sec. The calculated magnitude of the nuclear contribution to the 19F spin—rotation constants show that the signs of the above values are all negative, which then leads to the calculated 19F paramagnetic shielding in OCF2 of σ p = −324×10−6.
The 19F chemical shift in OCF2 is measured relative to Cl3CF giving 23×10−6. Relating the 19F shielding back to F2 as a reliable standard gives the total shielding at 19F in OCF2 as 236×10−6. Combining this with the paramagnetic shielding obtained from the 19F spin—rotation constants gives the diamagnetic shielding as σ d = 560×10−6, which is larger than the corresponding 19F diamagnetic shielding in F2 of 530×10−6.
The first‐order molecular Zeeman effect was also observed in OCF2 using a zigzag cell in a magnetic field up to 11 kG giving the diagonal elements of the g‐value tensor along the principal inertial axes of gaa = | 0.074±0.002 |, gbb = | 0.038±0.003 |, and gcc = | 0.028±0.003 |. Arguments are given to suggest that the signs of the g values are all negative, which yields the paramagnetic susceptibility of χ p = 127×10−6. The g values are combined with an estimate of the molar susceptibility to yield the average diamagnetic susceptibility in OCF2 giving a most probable value of .
A theory is presented which allows an estimation of either the molecular g values or the 19F spin—rotation interaction if one of the two quantities is known. The 19F parameters obtained in this study may be transferable to other systems.
Several low‐J transitions in OF2 are examined in a 4‐ft liquid‐N2 absorption cell, and the magnetic properties of 19F in this molecule are re‐examined.
45(1966); http://dx.doi.org/10.1063/1.1727960View Description Hide Description
Three‐body and wall recombination coefficients of atomic nitrogen were measured using an electron spin resonancespectrometer to determine the atomic nitrogen concentrations. Studies were made of nitrogen gas at pressures from 3 to 35 torr flowing in a cylindrical tube and nonflowing nitrogen gas at pressures from 0.3 to 7 torr. The three‐body recombination coefficient k 1 with molecular nitrogen as the third body was found to be (2.25±0.2)×10−32 cm6 sec−1. The wall recombination probability, λ, of quartz was found to range from 7×10−6 to 5×10−4, decreasing with increasing oxygen impurity; the variation for Teflon was measured to be from 2×10−7 to 2.5×10−5, increasing with increasing oxygen content. The sum of the two‐body radiative recombination and two‐body wall recombination was a significant part of the total atom recombination; the sum of these coefficients ranged from 0.35×10−15 cm3 sec−1 to 2.8×10−15 cm3 sec−1.
For intervals up to 2 sec following atomic‐nitrogen formation, the data indicate a slower decrease of atom concentration than can be accounted for by the measured mechanisms of recombination. The possible existence of an atom‐forming mechanism in the afterglow is discussed.
45(1966); http://dx.doi.org/10.1063/1.1727961View Description Hide Description
The two‐state approach to the structure of water is applied to D2O. Over the temperature range 4° to 100°C, the molar volume of both the close and open‐packed structures is estimated to be approximately the same in D2O as in H2O. The enthalpy difference between the two structures is found to be −2915 cal/mole in D2O compared to −2547 cal/mole in H2O, indicating an increase in the hydrogen‐bond strength of D2O compared to that of H2O. Estimates of the relaxational thermodynamic parameters of D2O are obtained. Only those which depend on the enthalpy difference exhibit an appreciable change from the corresponding parameters in H2O.
45(1966); http://dx.doi.org/10.1063/1.1727962View Description Hide Description
Previous studies of the Mössbauer effect for iron (IV) have been limited to the perovskitelike compounds SrFeO3 and BaFeO3. The structure of Sr3Fe2O7 is related, consisting of pairs of perovskite layers separated by a layer of strontium and oxygen ions. As in the previous studies of SrFeO3 and BaFeO3, samples of Sr3Fe2O6.9, Sr3Fe2O6.6, and Sr3Fe2O6.0 were prepared by equilibrating prereacted materials at various temperatures and oxygen pressures. Mössbauer spectra, measured at 298° and 4°K, indicated that the specimens of Sr3Fe2O6–7 are magnetically ordered at 4°K. The isomer shift of iron (IV) in Sr3Fe2O6.9 is −0.21 mm/sec at 298°K. The shift was more negative for the samples having a lower oxygen or iron (IV) content. Similarly at 4°K the magnetic hyperfine splitting went from 279 kOe for Sr3Fe2O6.9 to 232 kOe for Sr3Fe2O6.2. Quadrupole splitting was clearly evident in the trivalent component of these spectra and is attributed to the associated oxygen vacancy. The behavior is compared to that previously observed for systems containing iron (IV).