Volume 70, Issue 2, 15 January 1979
Index of content:

Lyman‐α and Balmer‐series fluorescence from hydrogen photofragments of H_{2}O vapor
View Description Hide DescriptionAtomic hydrogen Lyman‐α and Balmer‐series (3–9→2) fluorescence have been observed from photodissociation of H_{2}O, and the production cross sections have been measured. A line emission source provided the primary photons at wavelengths from 400–900 Å. The maximum fluorescence production cross sections for Lyman‐α and Balmer‐α are 1.72±0.34 Mb at 703 Å and 0.58±0.17 Mb at 672 Å, respectively. The photodissociation is mainly a direct process following absorption of a photon into the continuum state(s). The repulsive potential energy curves corresponding to the respective pseudodiatomic OH–H(n) molecule are constructed according to the Franck–Condon principle. The efficiency for converting H(2S) to H(2P) by H_{2} collisions, following photodissociation of H_{2}, is determined to be 56±10%, while that by H_{2}O, following photodissociation of H_{2}O, is estimated to be nearly 100%. The present data are important for further understanding of cometary photochemistry.

Statistical mechanics of helical wormlike chains. VIII. Angular correlation functions
View Description Hide DescriptionThe adaptation of the helical wormlike chain to a given real chain, or its rotational isomeric state model on the monomer‐unit level, is studied with angular correlation functions between two localized coordinate systems affixed to the chain or between two monomer units as functions of their separation along the chain, as defined as the expansion coefficients of the Green’s functions. Since these correlation functions for the latter model depend on the orientation of a localized system affixed to the monomer unit with respect to that unit, they are first formulated in the Flory system and then transformed to those in a different system appropriate for a comparison with the helical wormlike chain. This transformation is determined to preserve certain symmetry properties of the helical wormlike chain in the correlation functions for the rotational isomeric state model. Numerical computations are carried out for polymethylene, polyoxymethylene, poly(dimethylsiloxane), tactic polystyrene, and tactic poly(methylmethacrylate) chains. The analysis shows that the decay of the torsional correlation in the helical wormlike chain is slow, while the higher order correlation functions for the rotational isomeric state model decay slowly. The imaginary parts of the correlation functions are related to the asymmetry of the chain, and their symmetry properties for the two models do not completely agree with each other in the case of the asymmetric chains. However, the imaginary parts are of minor importance since their magnitudes are small compared to the real parts; in particular, they vanish for the symmetric chains. As a whole, there is rather good agreement between the two models. The analysis provides also a general procedure of determining the orientation of the localized system of the helical wormlike chain with respect to the monomer unit, and therefore also to, for instance, the principal axes of the polarizability tensor of the latter.

Time‐dependent birefringence, linear dichroism, and optical rotation resulting from rigid‐body rotational diffusion
View Description Hide DescriptionExpressions are derived for the time‐dependent relaxation behavior of a monodisperse suspension of arbitrarily shaped rigid bodies, after initial alignment by an externally applied field. We show that five exponential terms are necessary for a complete description of birefringence, linear dichroism, and optical rotation decay phenomena in a force‐free rotational diffusion process. The explicit form for the multiplicative coefficients of the exponential relaxation terms are presented; they are expressed in terms of the optical anisotropytensor and a tensorcharacteristic of the initial alignment conditions. Symmetry constraints that involve special relationships between the optical anisotropytensor, the alignment tensor, and the diffusiontensor or that involve the initial orientational distribution conditions, are shown to lead to a reduction in the number of required exponential relaxation terms. We concern ourselves mainly with alignment by means of an electric field in a Kerr cell, but other alignment techniques are treated in a general formalism with application to hydrodynamic flow fields. We also present an alternate formulation of the birefringence decay which provides physical insight as regards the sign and the monotonic or nonmonotonic behavior of the time‐dependent relaxation process.

EPR powder pattern analysis for Mn^{2+} in cubic crystalline fields
View Description Hide DescriptionThe EPRpowder pattern analysis for a d ^{5} spin system in a cubic crystalline field is discussed. From the study of the EPR absorption curves for the powder, it has been established that if divalent manganese occupies sites of cubic symmetry in single crystal MgO, CaO, and SrO, it also occupies cubic sites in the powder. The EPR spectra of Mn^{2+} in MgO, CaO, and SrO single crystals were first observed along different magnetic field orientations and compared to the subsequent powder spectra obtained after the same samples were ground to a fine powder. For each host, a one to one correspondence between each allowed EPR transition in the 〈100〉, 〈110〉, 〈111〉 single crystal spectra and the derivative peaks of the powder spectrum was found, leading to the conclusion that the powder spectrum is due solely to the contributions from these principal directions. Spin‐Hamiltonian parameters g, a, and A obtained from EPR measurements of the powder spectrum agree with those obtained from the single crystal, and both sets of parameters are in good agreement with those already reported for the single crystal samples. Forbidden hyperfine lines ‖ΔM _{ S }‖=1, Δm _{ I }=±1 were also observed in all the powder spectra. Contrary to some previous published works, it was not necessary to use any axial crystal‐field term to explain the forbidden lines.

The structural phase change in s‐triazine: Reconciliation of x‐ray diffraction and NQR measurements
View Description Hide DescriptionPrevious x‐ray diffraction studies of the low‐temperature phase of s‐triazine identified the order parameter with a shear strain of the unit cell and interpreted the data in terms of a weakly first‐order transition. Independent measurements of the nuclear quadrupole resonance spectrum showed that the ^{14}N lines split in the low temperature phase; this splitting was taken to be proportional to the order parameter and its variation with temperature was described in terms of that associated with a second‐order phase change, with a fluctuation contribution in the region of the transition. The present work shows that the first order nature of the transition is a necessary consequence of the symmetry of the system and the temperature dependence of the NQR frequencies can be explained if they are expressed as a sum of a term proportional to the order parameter together with a relatively large quadratic term whose magnitude can be qualitatively understood by a consideration of the environment of the nitrogen atoms.

Depolarized Rayleigh spectroscopy of polystyrene near the glass–rubber relaxation
View Description Hide DescriptionDepolarized Rayleigh scattering in polystyrene near the glass–rubber relaxation is studied using photoncorrelation spectroscopy. The observed correlation functions cannot be fit to a single exponential decay, but the empirical form C _{ H }(t)−1=δ+A exp[−2 (t/τ)^{β}] describes the results very well. Average relaxation times 〈τ〉= (τ/β) Γ (1/β) are determined from 100 to 130°C. The apparent activation energy for the change of 〈τ〉 with temperature is over 80 kcal/mole. Depolarized Rayleigh spectroscopy should prove to be a very useful tool in the study of relaxation behavior in polymers.

Synthesis, characterization, and pressure dependence of conductivity for a partially‐oxidized tetra‐aza macrocyclic palladium complex
View Description Hide DescriptionThe synthesis and characterization of the I_{2}oxidation product of a tetra‐aza macrocyclic complex of Pd (II) is described. The product is formulated as [Pd (TAAB)] [I_{3}]_{2.7} where TAAB is tetrabenzo [b, f, j, n] [1, 5, 9, 13] tetra‐azacyclohexadecine. The presence of I_{3} was confirmed by Raman and far‐infrared spectroscopy.Electrical conductivitymeasurements for pressed powder samples using a diamond‐anvil pressure cell showed an unusually large pressure dependence.

Electrical resistance and defect structure of stable and metastable phases of the system Nb_{12}O_{29}–Nb_{2}O_{5} between 800 and 1100 °C
View Description Hide DescriptionThe defectstructure of oxides of the system Nb_{12}O_{29}–Nb_{2}O_{5} have been studied by thermogravimetry and electrical resistancemeasurements. It has been clearly established that the stable oxides Nb_{2}O_{5−x }, Nb_{25}O_{62−x }, and Nb_{12}O_{29−x } are related to oxygen vacancies. On the other hand, the metastable phase of reduction of Nb_{2}O_{5−x } may be related to interstitial trivalent cations and it is suggested that such a defect of structure may be a step before the formation of crystallographic shear planes. In that case, the electric conductivity has been found to resemble a hopping process.

Theory of phase transitions in solid methanes. XII. Orientational order of molecules in phase III
View Description Hide DescriptionIn order to elucidate the structure of methanesolids in phase III, orientational orderings of molecules are studied on the basis of a model potential consisting of a crystalline field (one‐body potential), octopole–octopole type, octopole–hexadecapole type, and hexadecapole–hexadecapole type potentials, which are obtained through a multipole expansion of the sum of interatomic Lennard‐Jones type potentials. It is assumed that carbon atoms are fixed at sites of a fcc lattice, and that the orientational order has the period not larger than twice the lattice constant of the carbon fcc lattice. In the framework of classical molecular field approximation, we solve consistency equations for molecular fields, and examine stabilities of various solutions which branch off from phase I (disordered phase) or from phase II (partially ordered phase). To this end, Landau’s theorem on the phase transition of second order is extended such that it allows us to determine possible space groups for the lower temperature phase which are related to a given irreducible representation of the space group of the higher temperature phase. Two parameters are introduced to adjust the strengths of the multipolar interactions employed. Solutions with high branching temperatures are studied in detail within a range of parameters which yields the correct structure (F m3c) for phase II. Referring to the extinction rules found by neutron diffraction experiments on CD_{4}, we conclude that the structure of phase III is described by a solution with the tetragonal space group P4_{2}/m b c containing 16 molecules per unit cell (a subgroup of F m3c). Inclusion of the octopole–hexadecapole type potentials is found to be most essential for stabilizing this structure relative to phase II. Changes in site symmetries at the transition II → III are O→D _{2}, D _{2d } →S _{4}, and C _{ s }. Thus the O‐molecules in phase II also become ordered in phase III. However, in comparison with strong molecular fields at S _{4} and C _{ s } sites, molecules at D _{2} sites are subjected to a much weaker molecular field. Approximating this multisite structure by a two‐site model in tetrahedral fields, we analyze the observed heat capacity anomalies of CH_{3}D and CH_{2}D_{2} in phase III, and predict their tunneling level structures. Infrared and Raman spectra are also discussed. It is also found that a solution, C2/c containing four molecules per unit cell, becomes most stable at 0 K if the values of the hexadecapolar parameters are reduced and the crystalline field is strengthened.

Thermodynamics of the lanthanide trifluorides. II. The heat capacity of praseodymium trifluoride PrF_{3} from 5 to 350 °K
View Description Hide DescriptionThe heat capacity of PrF_{3} was measured by aneroid adiabatic calorimetry from 5 to 350 °K. The following results were obtained for PrF_{3} (c) at 298.15 °K: the heat capacityC _{ p }°= (92.65 ±0.09) J °K^{−1} mole^{−1}, the entropy S°= (121.22±0.12) J °K^{−1} mole^{−1}, the enthalpy (H _{ T }°−H _{0}°)= (17 946±18) J mole^{−1} and the Planck function −(G _{ T }° −H _{0}°)/T=(61.02±0.06) J °K^{−1} mole^{−1}. Recommended thermal functions from 400 to the melting temperature, 1672 °K, are presented in tabular form. The excess heat capacity due to the internal electronic energy levels of Pr^{3+} in PrF_{3} is calculated from spectroscopic data and compared with excess heat capacity derived from calorimetric results.

Rayleigh–Brillouin scattering and structural relaxation of a viscoelastic liquid
View Description Hide DescriptionThe linear response theory using a complete set of dynamic variables involving density, velocity, and energy fluctuations is used to analyze the Rayleigh–Brillouin spectrum of a viscoelastic liquid. The result is discussed in the fast and slow relaxation limits. In the former case, the Rayleigh–Brillouin spectrum is identical to that predicted by the classical hydrodynamic equations; whereas in the latter case a new structural central peak is found, in addition to the anomalous dispersion and relaxation effects present in the frequency and linewidth data associated with the Brillouin peak. The evolution of structural relaxation and its effect on the entire Rayleigh–Brillouin spectrum is described. The structural central peak is most pronounced when the frequency dispersion and the linewidth maximum are present. The theoretical result has been used to calculate the Rayleigh–Brillouin spectra of polypropylene glycol at various temperatures. The results of the frequency shift, the spectral linewidth, and the Landau–Placzek ratio are in good agreement with the experiment.

Low‐lying excited electronic levels of Cs_{2}UO_{2}Cl_{4}
View Description Hide DescriptionThe new electronic energy level which is at 311 cm^{−1} above the fluorescent doublet has been confirmed by fluorescence decay measurement. The fluorescent doublet is shown to belong to the same uranyl ion rather than two fluorescent singlets associated with two different uranyl ions. Two fluorescence decay rates are observed, and they are attributed to the intrinsic relaxation of the crystal and the relaxation of impurities or defects. The low‐lying energy levels of the impurities or defects are described. In addition, theoretical calculations on the temperature dependence of the fluorescence decay rates can be improved by including a temperature dependent contribution from multiphonon processes.

Chemiluminescence of S_{2} in solid argon
View Description Hide DescriptionUpon slow warming of previously photolyzed matrix samples of OCS/Ar, CS_{2}/Ar, OCS/Kr, and OCS/SF_{6} (M/R=100), the B (B ^{″}) →Xchemiluminescence of S_{2} is observed as reported earlier. In addition, two new progressions were recorded, a weak progression between 5580–6890 Å and a stronger one between 6680–8750 Å. With OC^{34}S, the 6680–8750 Å progression displayed isotopic shifts that allow deduction of ν_{ o o }=15 750±10 cm^{−1}, ω^{″} _{ e }=699±5 cm^{−1}, and ω_{ e } x _{ e } ^{″}=2.6±1 cm^{−1}. Similarly, the weaker 5580–6890 Å progression corresponds to ν_{oo}=20 870±30 cm^{−1}, ω^{″} _{ e }=724±6 cm^{−1}, and ω_{ e } x ^{″} _{ e }=2.9± 1 cm^{−1}, with a less favored, alternative vibrational assignment that gives ν_{ o o }=21 600±30 cm^{−1}, ω^{″} _{ e }=731±6 cm^{−1} and ω_{ e } x ^{″} _{ e }=2.9±1 cm^{−1}. These spectroscopic parameters point to assignments of the c ^{1}Σ^{−} _{ u }→a ^{1}Δ_{ g } and A′^{3}Δ_{ u } →X ^{3}Σ^{−} _{ g } transitions of S_{2} for the 6680–8750 Å and 5580–6890 Å emissions, respectively. These are the first observations of these transitions of diatomic sulfur, and one of them provides the first direct spectroscopic information illustrating that the c ^{1}Σ^{−} _{ u } state (T _{ o }=20 250±200 cm^{−1}) actually lies lower than the A ^{3}Σ^{+} _{ u } and A′^{3}Δ_{ u } states, as is the case for O_{2}.

Chemiluminescence in reactions of N^{+} ions with hydrogen and hydrocarbons. I. Rotational–vibrational product excitation
View Description Hide DescriptionChemiluminescent ion–molecule atom‐exchange reactions between N^{+} ions and H_{2}, CH_{4}, C_{2}H_{4}, C_{2}H_{6}, and C_{3}H_{8} were studied in a beam–gas arrangement. The collision energy ranged between 1.6 and 150 eV_{c.m.}. The observed product was in all cases NH (A ^{3}Π) radiating into the NH ground state. All reactions proceed nonadiabatically. The NH (A→X) emission spectra obtained were simulated on a computer. This yielded detailed rotational population distributions for both the v′=0 and v′=1 vibrational levels of NH(A), as well as the (v′=1)/(v′=0) vibrational population ratio. At high collision energy the distributions are affected by predissociation, but below 5 eV_{c.m.} in the case of CH_{4} the observed rotational distribution is that resulting directly from the reactive collision. In this case the dependence of the mean rotational energy on the collision energy strongly suggests a stripping mechanism for the reaction, while the reactions with the heavier hydrocarbons clearly do not proceed by stripping.

Chemiluminescence in reactions of N^{+} ions with hydrogen and hydrocarbons. II. Dependence of total reaction cross sections on the collision energy
View Description Hide DescriptionChemiluminescent reactions between N^{+} ions and H_{2} as well as some hydrocarbons were studied up to ∼450 eV_{ C M } collision energy. Emission from NH(A ^{3}Π) and, with hydrocarbons, from CH(A, B, and C) as well as from C and H atoms was observed. Except in the H_{2} case, the formation of NH(A) in a bimolecular exchange reaction occurs up to 150 eV_{ C M }. Below 20 eV_{ C M } this reaction appears to proceed by direct interaction between the N^{+} projectile and an H atom, while at high energies the primary interaction is between N^{+} and a C atom. The approximately exponential falloff of the cross section for NH(A) formation above 50 eV_{ C M } was explained in the case of CH_{4} using a soft‐sphere collision model. A hard‐sphere model was used in an attempt to explain the measured ratio of the N^{+}+C_{2}H_{4} and N^{+}+CH_{4} cross sections. Computer trajectory studies revealed the relative importance of various types of collision and led to semiquantitative agreement with experiment.

Methyl group rotation and bonding in solid hexamethyldialuminum, Al_{2}(CH_{3})_{6}
View Description Hide DescriptionContinuous wave proton second moment and line shape studies of the solid state trimethylaluminum dimer, Al_{2}(CH_{3})_{6}, have shown that all the methyl groups in this compound rotate about their triad axes at 77° K. An activation energy of 840±40 cal/mole was found for this methyl group rotational process from the temperature dependent static and rotating frame spin–lattice relaxation times (T_{1} and T_{1ρ}, respectively) studies. These observations tend to support the two‐electron three‐center Al–C–Al bond model for the bridging methyl group carbons.

Lattice relaxation model of the n=1 exciton emission in solid Ne
View Description Hide DescriptionThe emission of pure solid Ne is mostly due to atomically trapped n=1 excitons. Using the a b i n i t i o repulsive potentials of the molecular excited states and the long‐range vander Waals interactions the isotropic cavity formed around an isolated Ne* atom is calculated. The lattice relaxation is extended out to the 16th shell. The static and dynamical shifts obtained in this localized description are found to be in good agreement with experimental data.

Induced intensity and band shape of the w ^{1}Δ_{ u }←X ^{1}Σ^{+} _{ g } system in pure solid α‐N_{2}
View Description Hide DescriptionThe crystal‐field induced intensity and the band shape of the w ^{1}Δ_{ u }←X ^{1}Σ^{+} _{ g } transition in pure solid N_{2} are treated theoretically. The static crystal field generated by the molecular quadrupoles mixes ^{1}Δ_{ u } and ^{1}Π_{ u } states and its magnitude suffices to explain the observed intensity of the transition in the solid. The band shapes can be interpreted as Stokes‐type multiphonon structures due to totally symmetric local breathing modes with a weaker librational contribution. The dependence of the Stokes parameter and of the widths of the zero‐ and one‐phonon bands on the intramolecular Franck–Condon factor illustrates that the Stokes‐type excitation of phonons is diminished by increased delocalization through intermolecular vibronic resonance.

Practical application of extended valence bond diatomic calculations to the method of diatomics‐in‐molecules for NeHe^{+} _{2}
View Description Hide DescriptionPotential energy curves for the lowest two ^{2}Σ states and the lowest ^{2}Π state of NeHe^{+} were calculated using the valence bond method with minimal and extended basis sets. It is shown how this a b i n i t i o information, together with information on the He_{2}, He^{+} _{2}, and NeHe molecules obtained from other sources, can be incorporated into a minimal basis diatomics‐in‐molecules (DIM) description of NeHe^{+} _{2}. Particular emphasis is given to the incorporation of the extended a b i n i t i o w a v e f u n c t i o n s into the DIM procedure. Two sets of semiempirical DIM potential energy surfaces for NeHe^{+} _{2} were computed using the minimal and extended valence bond diatomic wave functions and energies as input. These surfaces are compared and discussed with regard to other electronic structure calculations on NeHe^{+} _{2}. The lower potential surfaces for NeHe^{+} _{2} are particularly relevant to the collision‐induced dissociation of He^{+} _{2} by Ne to form Ne^{+} ions.

A b i n i t i o SCF MO calculations of the potential surfaces of thiocarbonyls. II. H_{2}CS, HFCS, CIFCS, and Cl_{2}CS
View Description Hide DescriptionPotential energy surfaces and equilibrium geometries for the ground and first excited triplet states of H_{2}CS, HFCS, ClFCS, and Cl_{2}CS have been calculated using a b i n i t i o SCF MO methods. The triplet state out‐of‐plane bending mode frequencies have been calculated using a perturbation theoretical treatment and a suitable analytic approximation to the potential along the out‐of‐plane bending coordinate. Where experimental results are available, good agreement exists between calculated and experimental data. The equilibrium out‐of‐plane angle in the lowest triplet state is correlated with the decrease in the p electron density on the carbon atom accompanying the change from the planar to the equilibrium out‐of‐plane conformation.