Index of content:
Volume 48, Issue 2, 15 January 1968
48(1968); http://dx.doi.org/10.1063/1.1668679View Description Hide Description
The electronic angular momentum is calculated for the ground‐state, π‐electronic wavefunctions of C3H3, C5H5, C7H7, C4H4 +, C4H4 −, C6H6 +, and C6H6 − on the basis of simple molecular‐orbital theory. If the effective nuclear charge of carbon is taken as 3.18 and the carbon‐carbon bond distance as 1.40 Å then , in units of , is 0.393, 0.755, 1.331, 0.644, 0.644, 0.809, and 1.084 for C3H3, C5H5, C7H7, C4H4 +, C4H4 −, C6H6 +, and C6H6 −, respectively. Values of are also given for other values of effective nuclear charge and bond distance.
48(1968); http://dx.doi.org/10.1063/1.1668680View Description Hide Description
The thermal diffusion mechanism for hydrogen in zirconium is studied in terms of a simple kinetics model appropriate for nondilute as well as dilute solutions. The equations resulting from this model are correlated with available experimental data for zirconium hydride at high hydrogen contents. The results demonstrate that the thermal diffusion flux term for a concentrated solution must contain a factor to account for the nonavailability of interstitial sites. This factor is essentially proportional to the probability of an individual atom finding a vacant site on an adjacent lattice in which to jump. For δ‐ or ε‐phase zirconium hydride, this factor is given with reasonable accuracy by (2–H/Zr)/3.25.
48(1968); http://dx.doi.org/10.1063/1.1668681View Description Hide Description
Measurements of the optical properties of solid polystyrene in the wavelength range 1100‐2300 Å are reported. The real and imaginary parts of the complex index of refraction were determined from transmission measurements on a series of thin films. The absorptionspectrum shows a strong peak at 1925 Å plus minor features around 1350 and 2170 Å as well as evidence for a second major peak below 1100 Å. The measured values of the optical constants were used to calculate the complex dielectric constant and the characteristic electron energy‐loss function. The latter exhibits a maximum at 6.92 eV which appears to arise primarily from collective excitationeffects.
48(1968); http://dx.doi.org/10.1063/1.1668682View Description Hide Description
Shock‐wave compression measurements from 20 to 300 kbar are reported for hexagonal boron nitride. A phase transition occurs at 122 kbar and 230°C in good agreement with static data. The volume decrease is approximately 19% and the estimated slope of the phase line for the solid–solid transition is −3.6 × 10−2kbar °C−1. Crystallites of the dense phases recovered after shock compression above the transformation pressure were identified by x‐ray diffraction as zinc blende cubic with traces of wurtzite.
48(1968); http://dx.doi.org/10.1063/1.1668683View Description Hide Description
Neutron‐powder‐diffraction study has shown that, in the range 55°‐42.5°K, TbAu2 (tetragonal, iso‐structural to MnAu2) exhibits a linear‐transverse‐wave spin alignment propagating along the axis (the face‐centered description) and polarized in the axis direction. The wavelength of this static magnetization wave is incommensurable with the atomic repetition and varies 1.200‐1.186 in spacing units in the range 55°‐42.5°K. The rms moment per Tb at 42.6°K is 5.1 Bohr magnetons. At just below 42.5°K, a first‐order magnetic transition takes place and the wavelength of the transverse wave abruptly becomes equal to the spacing. The resulting antiferromagneticstructure persists down to 2°K and the saturation moment is practically identical to the ordered moment of the free Tb3+ ion, 9.0 Bohr magnetons. The crystallographic data in the range 300°‐5°K are also presented.
48(1968); http://dx.doi.org/10.1063/1.1668684View Description Hide Description
Total collision cross sections have been measured for helium atoms with energies between 500 and 1900 eV scattered by room temperature H2 and D2. Within the experimental precision of ±2% the cross sections of the two systems are identical. The results have been analyzed to obtain the following average potentials: He–H2: ; He–D2: . The measurements are in good agreement with a calculated He–H2 interaction of Krauss and Mies, suitably averaged over molecular orientation. The effect upon scattering of the anisotropy of the potential is discussed in the light of a classical perturbation theory of small angle scattering developed by Cross.
48(1968); http://dx.doi.org/10.1063/1.1668685View Description Hide Description
Satellite bands near the atomic lines of the emission spectrum of cesium in the presence of heavy noble gases were observed and their positions measured. For the series which also appear in absorption, the measured positions of the band maxima are in general agreement with those previously reported. Several interesting regularities in the data are noted and a simple interpretation is suggested.
Studies on the Vibrational Spectrum of the SO4 = Ion in Crystalline M2′M″(SO4)2·6H2O (M′ = K or NH4 and M″ = Mg, Zn, Ni, or Co): Observations on the Symmetry of the Sulfate Ion in Crystals48(1968); http://dx.doi.org/10.1063/1.1668686View Description Hide Description
Polarization studies on the Raman spectra of suitably oriented single crystals of the isomorphous M2′M″ (SO4)2·6H2O (M′ = K or NH4 and M″ = Mg, Zn,Ni, and Co) have been carried out to assign the symmetry species of the sulfate group frequencies. The theoretical principles underlying these experiments are briefly explained. Infrared absorption spectra of these sulfates have been recorded by the pressed‐disk and mull techniques. Infrared reflection data from the literature are also collected and analyzed along with the other data. The results relating to the multiplicity of the SO4 = frequencies are discussed in terms of the symmetry of the sulfate ion in these crystals.
48(1968); http://dx.doi.org/10.1063/1.1668687View Description Hide Description
The photolysis of polystyrene and poly (α‐methylstyrene) was studied by ESR techniques using highly concentrated glassy solutions of the polymers near liquid‐nitrogen temperature. Every attempt was made to insure detection of the earliest free‐radical species. The use of poly (1‐vinylnaphthalene) and polystyrene‐2,3,4,5,6‐d 5 verified that the primary radical species is located on the polymer chain and not the ring. The ESR spectra obtained for both polystyrene and poly (α‐methylstyrene) are consistent with the structure . Interpretation of the observed spectra is made on the basis of the stereochemistry of the polymer chain.
48(1968); http://dx.doi.org/10.1063/1.1668688View Description Hide Description
The triplet fluorescence of CO, sensitized by xenon and excited by 1470‐Å radiation, has been investigated. As opposed to unsensitized fluorescence, where almost all the observed triplet emission is from the state, emission is observed from various vibrational levels of the states. It is shown that Xe metastable atoms are not involved in the collisional process that causes excitation, and therefore, a reaction between the resonant atom and CO must be occurring. Experiments carried out to compare the quenching of unsensitized and sensitized fluorescence by He, Ar, and N2 indicate that there is a precursor to the emitting CO* in the sensitized case, presumably a collision complex of and CO. The reaction Xe* + CO→XeCO appears to take place at every collision, and the complex has a lifetime of at least 2 × 10−6 sec before it decomposes to Xe and CO*. Rate constants were obtained for the quenching of CO* by various gases, including CO, and ratios of rate constants were found for XeCO quenching by He, Ar, N2, and CO.
48(1968); http://dx.doi.org/10.1063/1.1668689View Description Hide Description
A high‐resolution Zeeman effectmicrowave spectrometer has been used to determine the anisotropy in the magnetic susceptibility of OCS as . From this data, the individual components of the magnetic susceptibility and the quadrupole moment of OCS have been calculated as ; and , respectively.
48(1968); http://dx.doi.org/10.1063/1.1668690View Description Hide Description
A shock‐tube investigation of the dissociation of nitrogen diluted in argon is described which utilizes vacuum‐ultraviolet absorption at 1176 Å to monitor the disappearance of molecular nitrogen. Measurements of the dissociation rate coefficients for the process were obtained over the temperature range 8000° to 15 000°K where the collision partner, M, was Ar, N2, and N. The temperature dependence of the individual rate coefficients was found to be the same and was obtained with good precision. The magnitudes of the rate coefficients were generally lower than the currently accepted values obtained in previous shock‐tube investigations, especially the value for which, although it was found to be about 10 times greater than , was six times less than two previous measurements. The results of the investigation may be summarized by the following expressions for the dissociation rate coefficients: , , .
Encouragingly good agreement between the recombination rate coefficients, , inferred from the measurements of , with predictions of obtained from the theories of Benson and Fueno and Keck and Carrier, were obtained.
48(1968); http://dx.doi.org/10.1063/1.1668691View Description Hide Description
The EPR spectrum of Cu++: MgO at 1.2°K is described and shown to be characteristic of a Kramers quartet of states which have anisotropic transitions characteristic of a one‐ion‐per‐unit‐cell system. This is an octahedral symmetry property which may be realized either by the vanishing of the Jahn‐Teller effect at 1.2°K or by the presence of tunneling between equivalent tetragonally distorted states. The tunneling process generates a Kramers doublet and quartet separated by a small tunneling (inversion) splitting, as has been shown by Bersuker and O'Brien. A theoretical g‐value formula for the tunneling quartet is derived for the magnetic field in an arbitrary direction. The experimental g values are shown to be in good agreement with this formula, but they are not in agreement with a similar formula for the static octahedral symmetry quartet. The hyperfine structure is also shown to be better explained as a property of the tunneling quartet. The tunneling doublet has not been detected at 1.2°K and is presumed to be thermally depopulated. It is concluded that the paramagnetic resonance properties of Cu++:MgO at liquid‐helium temperaturesmeasured at 9 × 109 cps are determined by a dynamic tunneling effect. We have found no evidence for a “frozen‐out” tetragonal distortion.
48(1968); http://dx.doi.org/10.1063/1.1668692View Description Hide Description
In the high‐density limit, the Helmholtz free energy function for an assembly of rigid spheres at temperature has the asymptotic form , where is Boltzmann's constant, is the sphere diameter, the mean thermal de Broglie wavelength, and where is the close‐packed volume of the hard‐sphere crystal whose bulk volume is . The value of the constant for the tunnel model in a face‐centered cubic lattice is found to be , as compared with the single‐particle free‐volume cell value . In addition an upper bound for in a face‐centered lattice is calculated, with the result , a previous result being .
48(1968); http://dx.doi.org/10.1063/1.1668693View Description Hide Description
The catalytic decomposition of ammonia on tungsten has been investigated by field electron‐emission microscopy. Physisorbed ammonia causes a large lowering of work function (∼ − 2.8 eV). Ammonia chemisorbs at low temperatures by the formation of a coordinate bond and the heat of adsorption and activation energy for decomposition appear to be highest on planes of highest clean work function. Decomposition to nitrogen and hydrogen occurs in the interval 200°–400°K. Interaction of ammonia with a clean tungstensurface at higher temperatures causes the successive appearance of three species, presently interpreted as N, NNH2, and NNH3 +. The interaction of ammonia with nitrogen adatoms to produce the additional species will proceed at lower temperatures and therefore requires little activation energy. The rate‐limiting step in the decomposition of ammonia on tungsten is the decomposition on the surface of NNH2. The proposed scheme for decomposition satisfies the existing kinetic data for tungsten. The implication of these results on the structure of nitrogen adatoms is discussed.
48(1968); http://dx.doi.org/10.1063/1.1668694View Description Hide Description
A summary of interpretive methods regarding high‐field electronic Zeeman measurements is presented. The equations calculated for two active spin substates (zero field) of the lowest triplet state, are applied to 1,4‐dibromonaphthalene crystal spectra.
The Zeeman spectra of the electronic and vibronic origins of the lowest triplet state have different polarization, but they have very similar Zeeman effects indicating similar spin‐substate activity. The complete analysis enables us to show that the lowest triplet state is of species which correlates with in . The vibronic origins are shown to arise by spin‐orbit coupling with singlet states that are vibronically mixed with other singlet states.
Certain crystal‐field effects are apparent in the spectra and those are discussed in relation to two suggested types of mixing; namely, intermolecular spin‐orbit coupling, and intermolecular exchange coupling.
48(1968); http://dx.doi.org/10.1063/1.1668695View Description Hide Description
A study of the electronic spectrum of the transition of benzophenone crystals is presented. The transition originates at 24 194 cm−1, and it displays carbonyl stretching and ring torsion modes of the molecule. The observed polarization ratio of 2.5 ± 0.2 is indicative of the fact that at least two of the spin substates of the triplet can combine with the ground state in electric dipole radiation. The two transitions are (parallel to C–O) and (perpendicular to C–O in the R2CO plane) polarized with the former dominant by a factor of 3.
A polarized high‐field Zeeman study of the transition shows that the and spin designated states are the active ones; this result shows that the state is of (in ) type and that the state becomes active through vibronic coupling amongst higher states. The appearance of torsional modes in the spectrum is discussed in relation to this vibronic coupling.
48(1968); http://dx.doi.org/10.1063/1.1668696View Description Hide Description
The chemiluminescence from the reaction of oxygen atoms and carbon suboxide in a flow system has been reinvestigated. In addition to the “triplet” bands reported previously, two other band systems of CO were observed: the Herman bands, also in the red, and the Fourth Positive bands in the vacuum ultraviolet. The three electronic states show a common excitation limit of almost 9 eV above the ground state. The emission intensity in the red and in the vacuum ultraviolet vary together as the flame conditions are changed, showing that the three excited states are formed in a single reaction. Traces of hydrogen are not necessary for the chemiluminescence. The reaction responsible for the emission is probably . Quenching of the CO emission depends on the ratios (NO) / (O) and (O2) / (O). NO is a more efficient quencher than O2, in agreement with the behavior found for formed by the photolysis of C3O2. The vacuum‐ultraviolet emission from a O + C2H2flame was found to be identical to that of the O + C3O2flame. As required by the single‐reaction hypothesis, the “triplet” bands and probably the Herman bands were found in the spectrum of the O + C2H2flame. The vacuum‐ultraviolet photon yield from the O + C2H2flame was 10−4.3 ± 0.5/C2H2. Possible reactions which could form C2O in these flames are discussed.
48(1968); http://dx.doi.org/10.1063/1.1668698View Description Hide Description
The behavior of a spin system disturbed by a train of closely spaced pulses can, under the proper limiting circumstances, be described in terms of a time‐independent effective Hamiltonian. The necessary conditions on pulse spacing are discussed and a recipe is given for constructing the effective Hamiltonian. The method is applied to the previously employed trains of 180° or 90° pulses, obtaining the well‐known limiting behavior for small pulse spacing. An analysis is given of a recently reported experiment using a train of phase‐alternated 90° pulses. Two new experiments involving complex pulse sequences are proposed, one of which permits the arbitrary scaling of the effective chemical shifts in a high‐resolution NMR spectrum, and the other of which annihilates dipole‐dipole and quadrupole effects in solids, leaving the chemical shifts and scalar couplings.
48(1968); http://dx.doi.org/10.1063/1.1668699View Description Hide Description
Ellipsometry is applied to study the oxidation of platinum in sulfuric acid solution. Steady‐state and transient measurements show that oxide‐film formation commences when the potential is anodic to 0.95 V. The film is light absorbing. The film thickness increases linearly with potential. A mechanism of film growth based on place exchange is developed.