Index of content:
Volume 71, Issue 7, 01 October 1979

Vibrational spectroscopy of cation‐site interactions in phosphate glasses
View Description Hide DescriptionThirty different binary metal oxide glasses having the metaphosphate composition have been prepared containing cations from the following groups: alkali metals, alkaline earths, transition metals, and lanthanide and actinidemetals. Far infrared absorption assigned to the cation vibration in its oxygen cage has been measured for these glasses. Empirical ionic models are proposed correlating the absorption maximum with the cation mass, charge, and ionic radius. Discrepancies between the observed and predicted vibrational frequency indicate a more covalent interaction between the metal cation and the glass network. Raman intensities and vibrational frequencies of the network metaphosphate vibrations have been obtained and provide additional evidence about the cation–site interaction.

Experimental and computer modeling study of ^{81}Br isotope enrichment by isotopically selective, two‐step laser photodissociation of HBr
View Description Hide DescriptionMolecular bromine enriched in ^{81}Br has been observed mass spectrometrically following isotopically selective, two‐step laser photodissociation of HBr in mixtures with NO. The dependence of isotopic enrichment and product yield on various experimental parameters was investigated. An extensive computermodel was used to simulate the two‐step enrichment process and gave fair agreement with experimental results. The computermodel indicated that isotopic enrichment was limited in these experiments (i.e., ^{81}Br_{2}/^{79}Br_{2}?1.6, compared to the natural abundance ratio of 0.96) by Br isotope exchange during chemical scavenging and not by the initial two‐step photodissociation process. Computer modeling studies of the effect of adding high pressures of rare gas to the HBr–NO mixtures suggest that much higher isotopic enrichments are possible. Enrichment of ^{81}Br in the C_{2}H_{5}Br product of isotopically selective, two‐step laser photodissociation of HBr in C_{2}H_{4} mixtures has also been observed experimentally.

High pressure fluorescence studies of radiative and nonradiative processes in diphenyl hexatriene, diphenyl octatetraene, and retinyl acetate
View Description Hide DescriptionThe effect of high pressure on the radiative and nonradiative rate parameters of three all‐t r a n s polyenes: 1,6‐diphenyl‐1,3,5‐hexatriene (DPH); 1,8‐diphenyl‐1,3,5,7‐octatetraene (DPO); and retinyl acetate (Rac) has been investigated to 10 kbar in methylcyclohexane (MCH) and toluene and to 40 kbar in polymethylmethacrylate (PMMA) and polystyrene (PS). The dependence of the radiative rate on the energy gap between the two lowest excited singlet states is compared to that predicted by a simple first order intensity borrowing expression. Comparison shows good agreement for DPH and DPO. The expression appears to fail for Rac.

^{13}C NMR rotating frame relaxation in a solid with strongly coupled protons: Polyethylene
View Description Hide DescriptionThe validity of interpreting measured rotating frame relaxation times,T _{1ρ} ^{C*}, in terms of molecular motion is investigated for crystalline, oriented, linear polyethylene (PE) as a representative rigid organic solid with reasonably strong dipolar couplings. T _{1ρ} ^{C*} data are presented at three temperatures, −100, 28, and 100 °C and for ^{13}C rf fields, ν_{1C}, in the range 35<ν_{1C}<90 kHz, for the orientation where B _{0} is parallel to the PE chain axes. With the exception of the T _{1ρ} ^{C*} data taken at ν_{1C}≳80 kHz and T=100 °C, all T _{1ρ} ^{C*} data observed were determined not by molecular motion, but rather by spin–spin effects in which the spin‐locked carbon system seeks to equilibrate with the proton dipolar system with a characteristic time constant T _{CH} ^{ D }(ν_{1C}), while at the same time the proton dipolar system is equilibrating with the lattice with a time constant T _{1D }. The T _{CH} ^{ D } values deduced from T _{1ρ} ^{C*} support existing theory which predicts that T _{CH} ^{ D }(ν_{1C}) ∝ exp(2πν_{1C}τ_{ D }) where τ_{ D } is the correlation time for dipolar fluctuations. A calculation of τ_{ D } from PE crystal structure agrees very well with the experimentally determined τ_{ D } of 24 μs. Further experimental proof of the importance of the spin–spin contribution to T _{1ρ} ^{C*} is demonstrated via changes in the carbon rotating frame magnetization, M _{ S L }(τ), for differing states of proton dipolar order. The question of extracting information about molecular motion from T _{1ρ} ^{C*} data is examined under the assumption that the total reduced correlation function for the local proton dipolar field at a carbon nucleus is the product of a Lorentzian reduced correlation function describing dipolar fluctuations and an exponential reduced correlation function describing molecular motion. It is shown for molecular motion in the long correlation time regime and for sufficiently large ν_{1C} that contributions to T _{1ρ} ^{C*} from molecular motion and dipolar fluctuations are cleanly separated. Pertinent background material for the interpretation of T _{1ρ} ^{C*} data is also presented; this includes effects of sample spinning, transients in carbon and proton magnetization, and ambiguities which arise when molecular motion is fast enough to cause some averaging of the static dipolar line shape. Criteria are offered whereby one can decide on the validity of interpreting T _{1ρ} ^{C*} data in terms of molecular motion. As an example, the T _{1ρ} ^{C*} data point for ν_{1C}=87 kHz and T=100 °C is interpreted in terms of molecular motion and the deduced correlation time, assuming a flip–flop motion of the polymer chains, is shown to be qualitatively consistent with published protonT _{1ρ} ^{H} results. Finally, ^{13}C longitudinal relaxation time measurements are discussed as a possible alternative to T _{1ρ} ^{C*} for obtaining information about molecular motion.

Low‐energy electron diffraction structural analysis of the (2×2) oxygen overlayer on the iridium (111) surface
View Description Hide DescriptionExposure of a clean Ir(111) surface at room temperature to approximately 20 L of oxygen produces a sharp (2×2) LEEDpattern. This LEEDpattern can be caused by two fundamentally different adsorbate superstructures: a p (2×2) array of oxygen atoms (one‐quarter monolayer coverage) or three independent, equivalent domains of a (1×2) superstructure rotated 120° with respect to one another (half‐monolayer coverage). Dynamical calculations have been performed for the three most symmetric adsites on this surface: the on‐top site and the two different types of threefold sites for both the p (2×2) and the three domains of (1×2) superstructures. The results show that the oxygen adatoms, either forming a p (2×2) superstructure or the three domains of (1×2) superstructure rotated 120° with respect to one another, reside on the threefold site directly above a vacancy in the second substrate layer with an interlayer spacing of 1.30 Å corresponding to an Ir–O bond length of 2.04 Å.

Phase diagram of antimony pentachloride to 43 kbar
View Description Hide DescriptionThe phase diagram of antimony pentachloride has been measured in the range −60–160 °C and 0–43 kbar to test the suggestion that the molecular solid ought to transform to an ionic solid, such as SbCl_{4} ^{+}SbCl_{6} ^{−} or SbCl_{4} ^{+}Cl^{−}, under pressure. In the experimental pressure range, it does not. The equilibrium lines for the L–I, L–II, and I–II transitions and their volume and entropy changes were determined.

Vibrational relaxation of UF_{6}: Ultrasonic measurements in mixtures with Ar and N_{2}
View Description Hide DescriptionUltrasonic attenuation has been measured in neat UF_{6} and in mixtures of 5% and 10% UF_{6} in Ar and N_{2} at 273 and 323 K. In all cases a single relaxation process was observed and attributed to relaxation of the total UF_{6} vibrational energy. Isothermal relaxation times were in the range (4–17) ×10^{−9} atm s. The vibrational relaxation rate inferred for UF_{6}–N_{2} collisions shows a slight negative temperature dependence, while the rates inferred for UF_{6}–UF_{6} and UF_{6}–Ar collisions are virtually independent of temperature over the range of these experiments. In this range, Ar and N_{2} are slightly less efficient collision partners (by factors of ∼1.5–3.0) than UF_{6} itself.

Hydrogen atom in a spherical box. II. Effect on hyperfine energy of excited state admixture
View Description Hide DescriptionThe exact eigenfunctions of the hydrogen atom H^{0} trapped in a finite sphere are used to evaluate radial averages and allow, for example, evaluation of 〈r ^{−3}〉 to yield the 2phyperfineinteractionenergy. Inclusion of compressed 2p orbital in the ground state permits simultaneous fitting of the relatively large hyperfineanisotropy and the isotropic component, observed by EPR for H^{0} in α‐quartz, with an acceptable sphere radius and an admixture coefficient a _{2p } more reasonable (smaller) than is possible with the ordinary hydrogenic 2p orbital. The local uniform electric field required to generate the desired a _{2p } value is estimated.

The surface photovoltage of an increasing series of polyarenes: Anthracene, tetracene, and pentacene
View Description Hide DescriptionThe surface photovoltage signals and the associated relaxation times generated by a laser pulse at the surface depletion layers of anthracene (0.8 μV, 5.6 msec), tetracene (12. μV, 10.0 msec), and pentacene (17.5 μV, 20.0 msec) appear to vary with the increasing amount of electron delocalization. As expected, the photovoltage of these materials depends logarithmically on light intensity until a saturation value corresponding to the complete energy band flattening at the surface is achieved, and this energy band bending is larger for pentacene than it is for tetracene. The photovoltage signal is observed to decay exponentially following the laser pulse with a relaxation time that is independent both of the wavelength and intensity of the light. It is established that this is in agreement with theoretical predictions based on a simple model involving the recombination of the photoinjected charge. The photovoltage spectral dependence of all three polyarenes have maxima which correspond to maxima in the corresponding optical absorptionspectra due to the allowed singlet–singlet transitions. In addition, the photovoltage spectrum of anthracene has maxima that correspond to the ’’forbidden’’ singlet–triplet transitions, which are comparable in size to the photovoltage arising from the allowed singlet–singlet transitions. This observation implies that the dissociation of excitons to form free carriers is independent of the distance of the exciton from the anthracene surface. The corresponding singlet–triplet transitions for tetracene and pentacene are outside the spectral region examined and thus were not observed.

Photochemical studies on the ethyl radical
View Description Hide DescriptionPhotochemical studies are performed on dipropionyl peroxide and the ethyl radical in argon matrices. Matrix isolated dipropionyl peroxide photochemically produces the ethyl radical upon exposure to ultraviolet light in the λ≳2800 Å region. Irradiation of the ethyl radical by UV light with λ<2800 Å produces ethyl propionate, propionic acid, ethylene, and acetylene. The first three products are formed at a rate different from that for formation of acetylene. This different rate dependence is conveniently explained in terms of a site dependence for the photochemistry.

Understanding the infrared continuum spectrum of the N_{2}O dimer and other van der Waals complexes at low temperatures
View Description Hide DescriptionRecent diode lasermeasurements of the N_{2}O dimer 4.5 μm absorption band revealed a continuous band shape even though the experimental spectral resolution was 7×10^{−4} cm^{−1}. The theory of rotational relaxation and dimer formation in supersonic molecular beams is used to establish a lower bound to the (N_{2}O)_{2} rotational temperature of approximately T _{ R }?10 K for a source pressure of 17 atm. A band model approach is used to predict the temperature dependence of the band shape and line density for (N_{2}O)_{2}. The calculated band shape gives a good fit to the measured (N_{2}O)_{2}spectrum for rotational and vibrational temperatures of 30 K. The results obtained show that the line density is of the order 10^{4}–10^{5} lines/cm^{−1} for rotational and vibrational temperatures from 10 to 30 K. This range of line densities will yield a continuous band shape even if a spectral resolution, limited by beam divergence, of 2×10^{−5} cm^{−1} could be attained. It is shown that the smallest predissociative lifetime which could yield a discrete N_{2}O dimer spectrum is approximately 10^{−7} s. A simple expression for quickly estimating average line densities is presented and applied to a sampling of van der Waals complexes.

Predissociations in the B ^{3}Π_{0} ^{+} state of I^{35}Cl and I^{37}Cl
View Description Hide DescriptionProfiles of lines in the 3–1 and 2–0 bands of the B ^{3}Π_{0} ^{+}←X ^{1}Σ^{+} system of I^{35}Cl and I^{37}Cl vapor, as well as in several fragmentary P and R branches associated with an adiabatic B′ state, have been recorded using a Fabry–Perot interferometer spectrometer. The potential curves of the B state, and of the adiabatic B′ state formed by an avoided crossing near 3.6 Å and 18 140 cm^{−1} between the B state and a repulsive Y O ^{+} state, have been determined using the intermediate coupling model of Child. Linewidths in v=2 of the B state are observed to be 0.034 cm^{−1}, independent of J and of isotopic composition; however, linewidths in v=3 are broader, corresponding to predissociative lifetimes in the subnanosecond range, and are found to o s c i l l a t e as a function of J. The major features of the observed linewidth pattern can be explained in terms of analytical approximations developed by Child and co‐workers, and provide evidence for a second curve crossing, near 2.94 Å and 17 975 cm^{−1}, of the B state and an Ω=1 state.

Excitation of Cd, Zn, and Sr by a beam of active nitrogen
View Description Hide DescriptionExcitation of the electronic levels of Cd, Zn, and Sr is observed when these metal vapors collide with a thermal‐energy, active nitrogen beam. The beam is extracted from a glow discharge in pure N_{2}. The active beam component is inferred to be vibrationally excited N_{2} in the A ^{3}Σ^{+} _{ u } electronic state. The absolute relative intensity of the emission lines in each element was measured. The excitation rates of the Cd and Zn target levels were found to depend exponentially on their energies indicating an effective temperature of approximately 4000°K. We believe that this temperature is related to the vibrational temperature of the N _{2}(A ^{3}Σ_{ u } ^{+}) states that excite Cd and Zn in energy transfer collisions. The excitation rates of the Sr levels did not show an exponential energy dependence, which is a result consistent with N _{2}(A ^{3}Σ_{ u } ^{+}) as the active species. The potential of such an emission study as a sensitive beam diagnostic is noted.

Kinetics of order–disorder transformations in alloys. III
View Description Hide DescriptionCalculations of specific heat vs temperature curves corresponding to measurements with finite heating rates have been carried out for alloys under various initial conditions utilizing the pair approximation of the Path Probability method of time dependent cooperative phenomena and are compared with experiments on Mg_{3}Cd, FeCo, and Cu_{3}Au. New experiments on Mg_{3}Cd suitable for comparison are also presented here. The main emphases of the comparisons are (1) the appearance of a specific heat subpeak below the main specific heat peak at T _{ c } for well annealed alloys, (2) the shift of the subpeak by a change of the heating rate, (3) the appearance of a specific heat dip for alloys quenched from higher temperatures, and (4) the appearance of two specific heat dips for highly disordered alloys quenched from above the critical point of order–disorder. Good qualitative agreement of theory and experiment is obtained in each of these four categories.

Electron spin resonance of ^{127}I atoms trapped in Xe matrices
View Description Hide DescriptionThe ESRspectrum arising from the trapped ^{127}I atoms has been observed in photolysis of HI in Xe matrices at 4.2 K. This may be the first clear detection of trapped halogen atoms in the solid matrices. The g and hyperfine coupling tensors are axially symmetric and the principal values are as follows: g _{∥}=1.400, g _{⊥}=2.532, A _{∥}=889 MHz, and A _{⊥}=1605 MHz. The quadrupole coupling tensor has been determined to be ‖ Q _{∥} ‖=74 MHz assuming axial symmetry. The approximate feature of the observed g and hyperfine tensors can be interpreted in terms of a simple crystal field model. However, a better understanding of the observed values requires a consideration of the effect of matrix wave functions as well as the dynamic Jahan–Teller effect. The radial parameters 〈r ^{−3}〉 estimated from the A and Qtensors are approximately consistent with the atomic beam data for free ^{127}I atoms, giving conclusive evidence that the species is trapped ^{127}I atoms.

The geometry and inversion barrier of the sulphonium ion SH_{3} ^{+}. Dependence on basis set
View Description Hide DescriptionThe inversion barrier of the sulphonium ion, SH^{+} _{3}, has been calculated using a variety of basis sets. The dependence of the calculated barrier and geometry on basis set size and inclusion of electron correlation is studied. The best estimate of the inversion barrier, calculated with a polarized double zeta basis set and extensive configuration interaction, is 32.2 kcal/mole. Correlation effects on the barrier are small, but polarization functions are found to be crucial for an accurate calculation of the barrier.

Semiclassical calculation of bound states in multidimensional systems with Fermi resonance
View Description Hide DescriptionA method is devised to calculate eigenvalues semiclassically for an anharmonic system whose two unperturbed modes are 2:1 degenerate. For some special states the periodic energy exchange between unperturbed modes is found to be very large. The quantum mechanical wave functions are examined and a correlation with the classical trajectories is described, both for quasiperiodic and the stochastic cases. A method used in the literature for calculating the stochastic limit is tested and found to break down when the present anharmonic system is separable.

On a fundamental difference between energy functionals based on first‐ and on second‐order density matrices
View Description Hide DescriptionThe Euler equations and kernel F[γ] of an energy functional of the first‐order density matrix are compared to the corresponding quantities which result from Löwdin’s treatment of the extended Hartree–Fock equations (the latter are based on an energy functional ▪_{ v } dependent on the second‐order density matrix). Comparison of the functionals E _{ v } and ▪_{ v }, facilitated by transformation of Löwdin’s kernel to the Hermitian kernel ▪[γ] which is central to the extended Koopmans’ theorem, leads to a clarification of the fundamental difference between ionization and chemical potentials. A definition of chemical potential (electronegativity) appropriate to Hartree–Fock theory is proposed. Denoting the Fock operator by the symbol F ^{ N }[γ;x′,x], this definition is μ=−χ=FFd x d x′ F ^{ N }[γ;x′,x] [∂γ (x,x′)/∂N]. This reduces, in the special case of a system with a single valence electron, to a measure of the Hartree–Fock electronegativity proposed originally by Mulliken and by Moffitt; namely χ=−ε−J/2, where ε is an eigenvalue of the Fock operator, and J is a Coulomb integral evaluated for the canonical valence orbital χ_{ N }.

Semiclassical Monte Carlo methods
View Description Hide DescriptionThe problem of constructing quantum–mechanical corrections to classical equilibrium statistical–mechanical results is considered. These corrections (excluding interchange symmetry effects) can be computed within a classical framework if a temperature‐dependent effective potential is utilized. Two approaches to the construction of this potential are investigated. The first of these is a modification of an earlier procedure by Feynman. This method is similar in spirit to the empirical Pitzer–Gwinn approximation, and in fact is utilized to derive this earlier result. The second approach involves a general Monte Carlo prescription both for the construction of the effective potential and for obtaining the ratio of the quantum and classical–mechanical partition functions.

Non‐Markovian theory of collisional dephasing in near resonance light scattering
View Description Hide DescriptionWe present a general microscopic theory for near resonance light scattering (RLS) from collisionally perturbed atoms and molecules. The theory is based on the tetradic scattering formalism of Fano and Ben‐Reuven. We perform a systematic density expansion and show that the four‐time, many‐body, dipole correlation functions necessary for the evaluation of the RLS cross section may be rigorously expressed (to lowest order in pressure) in terms of three single‐particle, two‐time, correlation functions, two of which are associated with the absorption and emission line broadening and the third is a cross correlation function. Our most general expressions are valid for an arbitrary value of the bath correlation time (relative to the broadening), and interpolate smoothly all the way from the static limit of inhomogenous broadening up to the reverse, the Markovian (homogenous) limit. The stochastic Gaussian model of Takagahara, Hanamura, and Kubo is obtained also as a special case of the present formulation.