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Volume 72, Issue 2, 15 January 1980

Electric deflection studies of halogen and interhalogen polymers
View Description Hide DescriptionElectric deflection studies of the polymeric halogen species, (Br_{2})_{2}, (ICl)_{2}, (Icl)_{3}, (ClF)_{2}, and (F_{2})_{2} are reported. The dimeric species listed above are found to be polar. The trimeric species (ICl)_{3} is also observed to be polar, a relatively unusual occurrence. A comparison is made between the gas phase dimer orientations and the local crystal structure of these halogens.

Linear response theory of molecular interaction: Correlated Π electron systems
View Description Hide DescriptionA many‐body theory of molecular interaction is presented for systems of nonoverlapping molecules containing delocalized Π electrons. The theory is based on a combination of LCAO MO theory and time‐dependent Hartree linear response theory. A multipole expansion of the Coulomb interaction is not employed rendering the theory applicable to large molecules. Some aspects of electron correlation are included. The infinite order result for the dispersion interaction is the change in the intermolecular correlation energy associated with ’’turning on’’ the Coulomb potential. A closed‐form infinite order expression for the intermolecular permanent moment interaction accounts for the modification of the bare Coulomb interaction by the polarizable system. Second order results that include all orders of intramolecular correlation for the dispersion and static interactions are also obtained. These are expanded in terms of the intramolecular correlation. A three‐body dispersion potential that includes all orders of intramolecular correlation is also presented. The theory is amenable to numerical analysis and provides a method to investigate the convergence of various perturbation series.

Extension of Glauber theory to account for intratarget diffraction in multicenter scattering
View Description Hide DescriptionA 1975 treatment of single‐double multiple scattering of electrons by randomly oriented vapor‐phase molecules is put on a firmer footing and extended to higher order. It is shown how to include effects of intramolecular Fresnel diffraction, as an electronscattered by one atom propagates to the next, without ever explicitly evaluating the scatteredwave function in the target. Theoretically derived estimates are made of the limitations of the approach at various levels of approximation. Numerical calculations are compared as a function of energy and scattering angle with newly available high quality calculations by Kohl and Arvedson (KA). In the expected range of applicability — roughly, 10 keV and higher − the present approach yields results that are satisfactory and (unlike those of KA) inexpensive enough for use in routine electron diffraction analyses.

Comparative studies of various methods in time domain spectroscopy
View Description Hide DescriptionVarious aspects of time domain and frequency domain analysis of the dielectric response function have been discussed in detail. Time domain results of several alcohols have been found to be in reasonably good agreement with the frequency domain results. The accuracy of the experimental results in the high frequency region is considerably improved by the application of successive synchronized sampling for drift reduction.

Time‐dependent multiconfigurational Hartree–Fock theory
View Description Hide DescriptionA time dependent version of multiconfigurational Hartree–Fock theory is formulated as an extension of the time‐dependent Hartree–Fock approximation. The time variation of the reference state is calculated using a form of Frenkel’s variation principle, which was suggested by Langhoff, Epstein, and Karplus. A linearized form of the equations provides linear response functions, which transform correctly under contact transformations generated by one particle operators. A consequence of this feature is that the various formally equivalent expressions for the oscillator strengths of electronic transitions will give the same result, apart from truncation errors when a finite orbital basis is used. Example calculations are carried out for some π electron systems within the Pariser–Parr–Pople model using a correlated reference state suggested by Clementi and Veillard.

The chemical shift tensor for a strongly hydrogen‐bonded proton: The carboxylic proton in KHCH_{2}(COO)_{2}
View Description Hide DescriptionThe chemical shifttensor for the carboxylic proton in potassium hydrogen malonate has been determined from a proton multiple pulse experiment carried out at 270 MHz. Both the values of the averaged chemical shift relative to tetramethylsilane (TMS), ?=−20.5 (5) and the chemical anisotropy, Δσ=27.6 (6), are consistent with the very strong hydrogen bond found for the carboxylic proton in KHCH_{2}(COO)_{2}. The directions of the eigenvectors are similar to those found for protonchemical shifttensors determined for other hydrogen‐bonded systems, i.e., the z‐principal axis is fairly close to the O⋅⋅⋅H⋅⋅⋅O vector off by 9 (2) °, and the x‐principal axis is close to the normal of the plane defined by the carboxylic groups [off by 8 (2) °].

ESR–ENDOR studies of x‐irradiated glucose‐1‐phosphate dipotassium salt
View Description Hide DescriptionSingle crystals of the dipotassium salt of gluocose‐1‐phosphate were grown out of water or deuterium oxide and x‐irradiated at 4.2 °K. The products of irradiation were identified by means of ESR and ENDORspectroscopy. In crystals grown out of water, three different alkoxy radicals were observed as oxidation products. In partially deuterated crystals only one of the aforementioned alkoxy radicals was produced. A hydroxyalkyl radical, RHĊOH was also produced by loss of hydrogen from the C6′ position. Electrons trapped at intermolecular sites were identified. The distances between the electron and the protons of the polar hydroxy groups forming the trap were deduced from ENDOR measurements and found to be relatively large, namely 1.732 and 1.738 Å.

Dissociation of metal carbonyls by metastable rare gases: Fe and Ni emission analysis
View Description Hide DescriptionChemiluminescence from Ni(CO)_{4} and Fe(CO)_{5} collisions with metastable He, Ne, and Ar atoms is described. The emission spectra are due to atomic Ni and Fe. An analysis of these spectra indicate a dissociativeenergy transfer process which is not spin–differentiated among the metal atom states. Because of this, certain low‐lying quintet states are seen here in emission for the first time. Steady state population analysis of all features permits a determination of the radiative lifetimes of these new states. For both carbonyls, a restricted statistical rate theory, in which the CO fragments are allowed to translate in one dimension only (the radial reaction coordinate) and are prohibited from rotating, gives good agreement with all the data. The nature of metastable electronic energy transfer is compared to photolytic energization, and a comparison of the likely excited electronic states involved in each indicates the source of the differences in product distributions observed by each method.

On electronic energy transfer in disordered systems
View Description Hide DescriptionA generalized master equation (GME) describing the incoherent motion of an excitation in a disordered system is developed. The connection of the GME to the semi‐Markovian theory of Scher and Lax, the generalized continuous random walk, and the self‐energy approaches to the temporal properties of the transport is discussed. The theory is used in a model calculation to compute the mean square displacement and the probability of the excitation to remain at the origin as one dimensional systems, in which only nearest neighbor interactions are included.

Trapping in dilute quasi‐one‐dimensional systems
View Description Hide DescriptionA model is developed for calculating the asymptotic trapping rate, by a sink, of an excitation migrating in a quasi‐one‐dimensional dilute system. The asymptotic behavior for the dilute limit differs considerably from that corresponding to a concentrated system. The migration is assumed to be incoherent and is described within the continuous time random walk approach. The competing roles of short range exchange coupling and of long range dipole–dipole coupling is also discussed using time scaling arguments.

Collisional relaxation of low lying electronically excited states of uranium: f ^{3} d s ^{2}(^{5} K ^{0} _{5}) and (^{5} L _{7} ^{0})
View Description Hide DescriptionLaser induced fluorescence techniques have been used to measure the rates of electronic relaxation of atomic uranium in a beam‐gas scattering apparatus. Cross sections for the collisional deactivation of the U f ^{3} d s ^{2}(^{5} K _{5} ^{0}) and (^{5} L _{7} ^{0}) states by H_{2}, D_{2}, HD, CH_{4}, N_{2}, and CO at room temperature are reported. Upper limits for quenching cross sections with He were also obtained. Relaxation of these uranium metastables by molecular hydrogen (H_{2}, D_{2}, and HD) and methane appears to proceed by resonant electronic‐to‐rotational or electronic‐to‐vibration/rotational energy transfer processes.

Collisional relaxation of highly excited uranium atoms in gases
View Description Hide DescriptionA laser induced fluorescence technique has been used to measure the electronic relaxation rates of highly excited uranium atoms. Cross sections for the collisional deactivation of atomic uranium in the 15 632 and 24 560 cm^{−1} states by He^{3}, He^{4}, H_{2}, D_{2}, Ne, and CH_{4} at room temperature are reported. Relaxation of these high lying states by foreign gases is very efficient and is likely governed by an inter‐state‐mixing process leaving the excited uranium atoms in very close lying neighboring levels.

Proton spin T _{1} relaxation dispersion in liquid H_{2}O by slow proton‐exchange
View Description Hide DescriptionWe have measured the longitudinal protonspin relaxation time T _{1} in pure water as a function of temperature (10, 37, 80°C) and Larmor frequency (50 Hz⩽ν_{0}⩽50 kHz) by means of field‐cycling techniques. T _{1} becomes frequency dependent below ν_{0}≈5 kHz due to the slow proton‐exchange between different oxygen environments (^{16}O, ^{17}O, ^{18}O), which modulates the magnetic ^{1}H–^{17}O interaction. The proton‐exchange time τ_{ e } was found to be Arrhenius‐like with activation energy 13.4 kJ/mol and pre‐exponential 1.5×10^{−6} s (37 °C:τ_{ e }=2.6×10^{−4} s). This is in fairly good agreement with results obtained previously in the literature by more indirect NMR methods. The new access to proton‐exchange reduces the experiemtnal error limits and allows measurements at extremely small frequencies, where the familiar T _{1ρ} technique no longer works.

The role of thermal energy in the laser induced dissociation of complex molecules
View Description Hide DescriptionThe role of thermal energy in the laser‐induced unimolecular decomposition of a complex molecule, UO_{2} (hfacac)_{2}⋅THF, has been investigated experimentally and theoretically. In the temperature range 70–150 °C, the laser‐induced decomposition yield was observed to vary exponentially with the initial temperature of the molecules at low yields, changing to a linear variation at high yields. The laser‐induced decomposition process has been represented by a model that simulates the pumping of an ensemble of molecules, initially spread over a broad Boltzmann distribution, through a series of dissociative levels. This model is able to reproduce the experimental observations assuming a ’’loose’’ transition state that leads to a value for the A factor in the range 10^{16}–10^{18} sec^{−1}.

Statics and dynamics of the freely jointed polymer chain with Lennard‐Jones interaction
View Description Hide DescriptionUsing a dynamic Monte Carlo method we investigated the thermodynamics of static and time‐dependent properties of the freely jointed polymer chain with Lennard‐Jones type intramolecular forces. The Θ temperture is calculated to k _{ B }Θ/−=3.70±0.01 (− is the Lennard‐Jones energy parameter). We give estimates for the equilibrium values of end‐to‐end distances, radii of gyration, and densities for chains consisting of up to N=63 segments and for various temperatures. The detailed structure of the extended coil state (T≳Θ) and the dense globular state (T<Θ) are demonstrated by their average internal distances and their structure functions. The results seem to confirm the predicted scaling laws. Especially we found that for T=Θ the mean size of the coil is ∼N ^{1/2} but the detailed structure is different from a random walk shape. Although the specific heat exhibits a maximum below the Θ temperature, the finite size effects are too serious to confirm the predicted logarithmic divergency. The dynamics of our Monte Carlo method simulate the Rouse model. The density correlation function and the associated relaxation time are calculated. The results are consistent with dynamic scaling predictions. The segment correlation function exhibits the initial ∼t ^{1/2} behavior and the diffusional behavior ∼ t at large times.

The construction of modified virtual orbitals (MVO’s) which are suited for configuration interaction calculations
View Description Hide DescriptionA simple method, requiring the work of a single SCF iteration, is presented for the construction of modified virtual orbitals (MVO’s). Test calculations on H_{2}O, Ar, and two states of CH_{2} show that for the truncation of orbitals at the CI level, the MVO’s are far more efficient than canonical SCF virtual orbitals or such related SCF virtuals as improved virtual orbitals (IVO’s) and for some cases approach the efficiency of natural orbitals. MVO’s are shown to be suitable for configuration selection based on energy contributions. The CI wave function is observed to be more compact with MVO’s, allowing better interpretation of the results.

Photoassociation, photoluminescence, and collisional dissociation of the Sr_{2} dimer
View Description Hide DescriptionWe report observations and analysis of the laser‐excited photoluminescence of Sr_{2}. The spectra contains some discrete molecular line spectra, but is dominated by extensive, deeply modulated continua characteristic of bound–free transitions of excimer systems. Dye laser and argon laser excitation of the Sr_{2} dimer is used. Some of the spectra is attributed to fluorescence from dimers created by laser induced photoassociation of free Sr atoms. Techniques are developed to obtain Morse potential parameters by computer modeling of the observed spectra. We find for the X ^{1}Σ^{+} _{ g } state, D _{ e }=1100±100 cm^{−1}, ω_{ e }=39.6±1 cm^{−1}; for the A ^{1}Σ^{+} _{ u } state, D _{ e }=5460±100 cm^{−1}, ω_{ e }=83±3 cm^{−1}; and R _{ e } ^{″}−R _{ e } ^{′}=0.49±0.02 Å. Strong collision‐induced predissociation is observed and attributed to a potential curve crossing. Cross sections for dissociation by argon are reported as a function of laser excitation wavelength.

Deformation mechanism of polymer spherulite by linear isothermal viscoelastic theory
View Description Hide DescriptionA mathematical representation for the deformation mechanism of polymer spherulite is proposed through a linear isothermal viscoelastictheory to investigate the relaxation mechanism of strains and stresses within the spherulite. This description may be represented by the extension of the method proposed by Chen and Wang concerning an elastic problem. Displacements within the spherulite are obtained as solutions of equations of motion omitting an inertia term by assuming two potential functions of complex quantities. Viscoelastic stiffnesses of lamellae in the equations are obtained by applying the correspondence principle to a composite model, in which the mechanical behavior of amorphous phase and grain boundary region is assumed to be dependent upon a three‐element viscoelasticmodel of Maxwell type. As to the results, both the relaxations of strains and stresses within the spherulite are found to be dependent upon the relaxation of viscoelastic constants in bulk.

Mechanical behavior of crystal lattice strains detected by x‐ray diffraction technique in a polymer spherulite to external excitation
View Description Hide DescriptionA estimation of crystal lattice strain detected by dynamic x‐ray diffraction technique is proposed by using a linear isothermal viscoelastictheory. In the actual calculation, the results with regard to the polar angle dependences of the apparent dynamic crystal lattice strain as well as the crystal lattice strain normalized by sample strain, are obtained and are tested by the experimental ones. In addition, from the theoretical calculation, the distribution of the in‐phase component of crystal lattice strain is affected by the magnitude with regard to the real part of mechanical quantity such as viscoelastic Young’s modulus, while that of the out‐of‐phase component is affected by the magnitude with regard to the imaginary part of the mechanical quantity.

A new method for observing the rotational spectra of weak molecular complexes: KrHCl
View Description Hide DescriptionA new method of observing the rotational transitions of weak molecular complexes by combining the techniques of microwaveFourier transform spectroscopy, a Fabry–Perot cavity, and a pulsed nozzle source of molecules is described. High resolution and good sensitivity are achieved. Twelve isotopic species of the KrHCl molecule involving all combinations of the ^{86}Kr, ^{84}Kr, ^{82}Kr, H, D, ^{35}Cl, and ^{37}Cl isotopes are assigned and the rotational constants, Cl nuclear quadrupole coupling constants, and centrifugal distortion constants are all given. The molecular structure indicates a linear equilibrium structure with the H(D) atom between the Kr and Cl atoms. However, the inverse moments of inertia measured here indicate large amplitude vibrations of the H(D) atom relative to the Kr–Cl internuclear line. Estimates of the Kr–Cl stretching frequency and force constants, which show considerable dependence on the isotopic species, are also given. Bending frequencies and force constants are also given.