bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spin Relaxation of Optically Pumped Rubidium Atoms in Molecular Buffer Gases
Authors: McNeal Robert J. 
Published: January 2005

Short spin relaxation times for rubidium atoms, oriented by optical pumping, have been observed in the presence of benzene, ammonia, and dimethyl ether buffer gases. The disorientation cross sections of these gases in rubidium are obtained from relaxation times measured in mixtures with molecular nitrogen. The partial pressures of the gases were low compared to the nitrogen pressures in the mixtures, and the diffusion coefficient of rubidium in the mixtures was assumed to be the same as in pure nitrogen. The disorientation cross sections at 55°C are 7±1×10—19 cm2 for benzene, 8±2×10—18 cm2 for ammonia, and 3±1×10—18 cm2 for dimethyl ether. The cross section of carbon monoxide has also been obtained from the longer rubidium spin relaxation time in this gas. These measurements, complicated by a slow reaction of carbon monoxide with liquid rubidium, yield a cross section of 1±0.5×10—22 cm2 for carbon monoxide.
The cross sections of ammonia, dimethyl ether, and benzene are much larger than cross sections of other molecular buffer gases. A discussion of the relaxation processes in these gases is given. The large cross sections of polar gases with appreciable dipole moments are found to result from an interaction of the rubidium atoms with the electric field of the polar molecules during a collision. Virtual excitation to the p states of the atom by the electric field gives rise to an interaction of the spin of the valence electron with the orbital angular momentum of the p states. The direct contribution of the magnetic field of the moving polar molecule is also considered and found to be small compared to the electric interaction. The large cross section of benzene is assumed to result from a spin exchange interaction with a charge transfer complex of rubidium and benzene formed in very small amounts. If the assumed complex is responsible for the large benzene cross section, the equilibrium constant for its formation can be calculated from an estimate of the spin exchange cross section.
SCF Theory of Localized Electron Sets
Authors: Schmeising H. N. , Miller R. L. 
Published: January 2005

This paper describes a method within the LCAO—MO—SCF approximation for calculating pi electron localization energies for aromatic substitution reactions. Pilot calculations on butadiene, naphthalene, and azulene have been carried out using zero overlap (ZDO). A new result is that the relative reactivities of the unique positions in the azulene radical are found to be in agreement with experiment.
In the general theory we have divided a normally delocalized system into two or more sets of partially delocalized electrons. Each group of electrons is described by a unique set of molecular spin orbitals (MSO). Since, in general, it is impossible to guarantee that the wavefunctions of one set are orthogonal by symmetry to those of every other set, the resultant interset Lagrangian multipliers are taken into account by the introduction of interset coupling operators.
Integrated Intensity Measurements of the 1.9-μ Bands of CO2 in the Temperature Range 1400° to 2500°K
Authors: Breeze J. C. , Ferriso C. C. 
Published: January 2005

Measurements have been made of the total integrated band intensity of the 1.9-μ bands of CO2 in the temperature range 1400° to 2500°K. The gas was heated to the high temperature by a shock wave reflected from the rigid end plate of a shock tube. The experiment determines the total integrated band emission as a function of optical path length. The total emission is related to the integrated band intensity in a simple way.
The intensity in the 1.9-μ region of the CO2 spectrum arises from three combination bands, namely the (v3+4v2), (v3+2v2+v1), and (v3+2v1) bands. These band systems are in strong Fermi resonance. The bands have not been resolved; the total integrated intensity of the three bands was measured as a function of temperature. The temperature dependence of the absolute intensity is discussed in terms of a simple model using the harmonic oscillator approximation to the CO2 molecule. The results indicate that the intensity in the 1.9-μ resonant triplet of CO2 originates in the (v3+2v1) band. An extrapolation of the data using the derived temperature dependence gives an integrated band intensity of 2.07 (cm—2 atm—1) at STP for the total 1.9-μ CO2 band.
Statistical Thermodynamics of Spherical Droplets
Authors: Plesner Igor W. 
Published: January 2005

The model employed by Hill for investigating the nature of the transition region between two phases in a one-component system is extended to spherical interfaces. The surface tension and location of Gibbs' dividing surfaces in droplets with a radius of 3 to 25 molecular diameters (the molecules are hard spheres with an attractive potential) are calculated at a reduced temperature of 0.6, providing the numerical basis for investigating the dependence of surface tension on droplet size. It is found that the surface tension in all cases is about 35% of that of a plane interface, but is almost independent of droplet size.
The values of the surface tension relative to that of a plane interface are compared with those obtained from nucleation experiments in jets, using the Becker—Doering theory of nucleation; the agreement is poor, which is not surprising, considering the rather complicated substances used for the experiment.
From the values of the surface tension at the Gibbs surface of tension, the work of formation, W/kT, of a droplet of the size in question can be calculated. Since the method here amounts to finding the critical nucleus in a supersaturated system, this quantity determines a ``critical nucleus for condensation''; from this can be found the ``critical supersaturation'' to be compared with existing cloud-chamber measurements on low-molecular-weight vapors. Assuming that thermal fluctuations will suffice to initiate condensation if W/kT is less than 75, the critical nucleus for condensation is found to contain about 50 molecules, and the corresponding critical supersaturation is found to be S=2.6, in reasonable agreement with most experiments. Some questions concerning the applicability of thermodynamic concepts to microscopic droplets are briefly discussed.
Statistical Mechanics of Defect-Containing Solids. II. Ionic Crystals
Authors: Allnatt A. R. , Cohen Morrel H. 
Published: January 2005

The cluster expansions for the partition function and defect distribution functions derived previously are studied in detail for the case of ionic crystals with the object of calculating activity coefficients, defect concentrations, and defect distribution functions at low defect concentrations. A diagram classification procedure analogous to that in the Mayer theory of ionic solutions is used to obtain nondivergent expansions for defect activity coefficients and distribution functions. The discreteness of the lattice requires some modification of the diagram summation techniques employed in the solutions theory. The theory of association of defects of the sort considered by Lidiard and Teltow is formulated more precisely in terms of the defect distribution functions. The formal multicomponent expressions are studied in more detail for the case of Schottky defects and impurity ions in a sodium chloride lattice. The results parallel those of the Mayer ionic-solution theory, the principal difference being that the Debye—Hückel potential of average force appearing in the final Mayer expressions is everywhere replaced by Ae2 exp(—κRξ)/RD, where A and ξ are structure and concentration dependent and go to unity in the continuum limit.
As an example for the case of activity coefficients, calculations of the contributions from cycle diagrams and terms of next lowest order in concentration have been made for divalent impurity ions and cation vacancies in sodium chloride. The pair correlation function for oppositely charged defects and the degree of association have been calculated for the doped crystal. The theory reduces to that of Lidiard in the limit of zero concentration but differs at finite concentrations. However, calculation of the contribution of ``triangle diagrams'' to the activity coefficients indicates that below 500°C the expansions do not converge rapidly enough to be of value at concentrations of experimental interest because of the low dielectric constant. It was found that in the range of temperature and composition for which the theory converged, the parameters A and ξ differed little from unity.
Analysis of A2B2 NMR Spectra. II. Nonsymmetrical 1,2-Disubstituted Ethanes
Authors: Hirst Robert C. , Grant David M. 
Published: January 2005

The features of A2B2 proton magnetic resonance spectra of nonsymmetrical 1,2-disubstituted ethanes are discussed in detail. It s shown that under certain conditions the magnitudes and signs of all the coupling constants may be obtained for this type of A2B2 spectra. Typical calculated spectra along with several experimental spectra are given to illustrate these conditions. With one exception, experimental data are given only for molecules for which all four of the coupling constants are obtained.
It is found that the geminal coupling constants are of opposite sign from the rotationally averaged values for the vicinal couplings. The values of the geminal couplings are approximately those found by other workers in the corresponding monosubstituted methanes. Small vicinal substituent effects on the geminal coupling constants are reported for several of the substituent groups. Assignment of the chemical shift of the particular methylene group is based on the comparison of the A2B2 chemical shifts with the chemical shifts in the related ethyl compounds.
Description of the Distribution of Electrons in the Methane Molecule
Authors: Turner Almon G. , Saturno Antony F. , Hauk Peter , Parr Robert G. 
Published: January 2005

A formal scheme is presented for describing the electron distribution in the methane molecule. Purely theoretical electron density contour maps for the ground state of the molecule are displayed, and various aspects of the electronic distribution are discussed. The electronic wavefunction employed is an analytical form composed of atomic orbitals of s through g type centered on the carbon nucleus.
The electronic charge density ρ is expressed as a series of normalized (to 4π) tetrahedral harmonics Tl (θ,φ), which are certain symmetrical linear combinations of spherical harmonics of the given l value. Namely, 4πρ(r,θ,φ)=ΣlAl(r)Tl(θ,φ),where the Al(r) depend only on r, the radial distance from the carbon nucleus. The functions Al(r) are tabulated, and contour maps of ρ are developed. The molecular octupole moment is predicted to be about 1.7×10—24e·cm3.
The distribution of electrons in a single carbon—hydrogen bond is considered from several points of view, with the purpose, among others, of elucidating the question of the sign and magnitude of the ``CH bond moment.''
Photochemistry of 1,3-Butadiene : Details of the Primary Processes and Mechanism of Photopolymerization
Authors: Haller I. , Srinivasan R. 
Published: January 2005

The photolysis of 1,3-butadiene in the vapor phase was studied in the presence of inert gases such as krypton, diethyl ether, and cyclohexane. In every instance, an increase in the pressure of the added gas led to a decrease in all of the volatile products. The behavior of 1,2-butadiene and 2-butyne were exceptional in that they showed a slight increase in yield when a small quantity of an inert gas was added. But at higher pressures, even in these cases, the yields decreased. A consideration of the radiative lifetime of the first-singlet excited state of 1,3-butadiene suggests that the observed pressure dependence cannot be attributed to a quenching reaction from that state. It seems likely that all of the volatile products in the vapor phase photolysis arise from a vibrationally excited ground state molecule of 1,3-butadiene that is formed by the internal conversion of the electronically excited singlet state. Photolysis of 1,3-butadiene in solution, which is equivalent to photolysis at very high pressures, yielded none of the volatile products that were observed in the vapor phase but only cyclobutene, bicyclo [1.1.0] butane, dimers, and a polymer. The cyclobutene and bicyclobutane are believed to be products of isomerization from the electronically excited state of 1,3-butadiene.
The formation of a polymer in the gas-phase photolysis was found to have an induction period and to depend on the square of the number of photons that were absorbed. In all probability, the polymer is formed in the photolysis of a volatile product(s) of the primary photodecomposition.
Forbidden Electronic Transitions in XeF2 and XeF4
Authors: Pysh Eugene S. , Jortner Joshua , Rice Stuart A. 
Published: January 2005

Transition strengths have been measured for the weak 2330 Å band in XeF2 (f=0.002) and for the two weak bands in XeF4 at 2280 Å (f=0.009) and 2580 Å (f=0.003). To investigate the origins of these weak transitions, the possibilities of vibronic and singlet—triplet transitions in XeF2 and XeF4 were examined. Using the Herzberg—Teller theory of vibronic transitions and a molecular orbital treatment of excited electronic states, estimated strengths of the relevant vibronic transitions have been calculated to be f=0.001 for both XeF2 and XeF4. The vibronic band in XeF2 borrows intensity from the symmetry allowed 1A1g→1A2u transition at 1580 Å (f=0.45), while in XeF4 the major contribution to the vibronic band is from the symmetry allowed 1A1g→1Eu transition at 1325 Å (f=0.8). A temperature dependence of the intensity of the 2330 Å band in XeF2 has been observed and found to be less than that predicted by the Herzberg—Teller theory.
The estimated strength of the singlet—triplet transition in XeF2 corresponding to the singlet—singlet transition at 1580 Å is shown to be small (f≦10−4) in spite of a heavy atom effect; the small transition strength persists because of the lack of nearby excited states of the required symmetry. In XeF4 the triplet excited state 3Eu corresponding to the singlet—singlet transition 1A1g→1Eu at 1840 Å (f=0.22) is permitted by group theoretical selection rules to mix with its own singlet state. Using an intermediate coupling scheme the estimated intensity of this singlet—singlet transition is calculated to be f=0.007. The theoretical estimates of the symmetry and spin forbidden transition strengths are used for the assignment of the weak electronic transitions in the xenon fluorides.
Coriolis Perturbations in the Infrared Spectrum of Ethylene
Authors: Smith W. L. , Mills I. M. 
Published: January 2005

Rotational structure has been resolved and analyzed in two of the infrared-active perpendicular bands of C2H4 vapor: the Type b fundamental band, ν10, at 826 cm—1, and the Type c fundamental band, ν7, at 949 cm—1. Many of the individual PP and RR branch lines have been observed. The analysis has been confined to values of the quantum number K≥3, for which energy levels ethylene shows no detectable deviations from a symmetric-top rotational structure. The analysis reveals a Coriolis interaction between ν7 and ν10, and between ν4 and ν10, and values of the Coriolis constants ζ7,10z and ζ4,10y are obtained; these are related to normal coordinate calculations for the appropriate symmetry species, and force constants are derived to fit the observed zeta constants. The band center of ν10 has been revised from the original figure of 810 cm—1 to the new value, 826 cm—1, and the inactive frequency ν4 is estimated to lie at 1023±3 cm—1, in good agreement with the previous estimate of 1027 cm—1.
The change in the value of ν10 leads to a suggested change in the value of the Raman-active fundamental ν6 from 1236 to 1222 cm—1. New combination bands have been observed at 2174 cm—1, assigned as ν3+ν10; and at 2252 cm—1, assigned as ν4+ν6; also rotational structure has been resolved and analyzed in the ν6+ν10 band at 2048 cm—1.
The new data obtained for the C2H4 molecule are summarized in Table XII, with all of the other data presently available on the vibrational and rotational constants.
One-Center Basis Set SCF MO's. II. NH3, NH4+, PH3, PH4+
Authors: Moccia Roberto 
Published: January 2005

One-center-expanded SCF MO wavefunctions are reported for the ground states of NH3 and PH3 in their planar and in their calculated equilibrium configurations and of NH4+ and PH4+ in their calculated equilibrium configurations. The calculated molecular energies and the geometrical parameters of the determined equilibrium configurations agree well with the experimental values.
Good agreement is also obtained for the electric dipole moment and the proton affinity of NH3 while the calculated value of the height of the potential barrier for inversion appears to be too small. For PH3 a good value for the height of the potential barrier is obtained while the calculated electric dipole moment is too large. The calculated proton affinity of PH3 seems to be reasonable. A critical examination of the wavefunctions gives an account of the quality of the several results obtained.
One-Center Basis Set SCF MO's. III. H2O, H2S, and HCl
Authors: Moccia Roberto 
Published: January 2005

One-center expanded SCF MO wavefunctions are reported for H2O, H2S, and HCl in their ground states. The wavefunctions refer to the calculated equilibrium configurations which agree well with the experimental data. The corresponding computed molecular energies are —75.922436, —397.58906, and —458.83776 a.u., respectively, which are reasonably close to the experimental values. The energy obtained for H2O is found to be near the estimated Hartree—Fock value.
The computed electric dipole moment of H2O agrees well with the experimental value, while for H2S and HCl larger discrepancies are found.
Initiation of Deflagration Waves at Surfaces of Ammonium Perchlorate—Copper Chromite—Carbon Pellets
Authors: Evans Marjorie W. , Beyer Rodney B. , McCulley Leonard 
Published: January 2005

A study was made of the initiation of a deflagration wave at the surface of ammonium perchlorate—copper chromite—carbon pellets in nitrogen at 25 atm. The relationship between energy flux density I and exposure time τI, describing the boundary conditions necessary for initiation, was measured. Integration of the governing partial differential equation using experimentally determined chemical and physical parameters and the boundary conditions gave the temperature T(0, τI) of the initiating surface at the moment τI of cutoff of the energy pulse, and a lower bound for the minimum thermal ignition time τim.
T(0, τI) for ammonium perchlorate with 5% copper chromite was 380±31°C. It was drastically lowered when carbon was added, the amount of lowering being proportional to carbon concentration. It ranged from 300° to 215°C for carbon percentages ranging from 0.5 to 4. For given composition T(0, τI) was independent of flux density over the range 9 to 63 cal/cm2·sec. Initiation occurred substantially before the calculated minimum thermal ignition time τim. The results provide support for the view that thermal ignition of the solid material is not a prerequisite for initiation of a deflagration wave.
Semiempirical SCF—LCAO—MO Calculation of the Electronic Structure of the Guanine—Cytosine Base Pair: Possible Interpretation of the Mutagenic Effect of Radiation
Authors: Rein Robert , Ladik Janos 
Published: January 2005

The π-electronic structure of the guanine—cytosine nucleotide base pair has been investigated in the semiempirical SCF—LCAO—MO approximation.
An examination of the energy levels with respect to the single bases showed that the transition between the highest filled and lowest unfilled π orbitals (n*→n*+1) takes place between the levels originating from two different single bases. This statement follows from considering the n* and n*+1 levels of the base pair as perturbed ones of the guanine, cytosine single bases, respectively. This view is substantiated by the structure of the eigenvectors of the respective states. The transition between these states is associated with a transfer of charge, which in the investigated case involves the transfer of a unit charge from guanine to cytosine.
Since in the charge-transfer state the bases are essentially in an ionic form, a considerable increase in the probability of a single-proton tunnelling, proposed by Löwdin, may be expected. The interpretation of radiation-induced mutations in terms of the charge-transfer state and single-proton tunneling is further discussed.
Ultrasonic Absorption Measurements in Single Crystals of Naphthalene and p-Dichlorobenzene
Authors: Yun Seung S. , Beyer Robert T. 
Published: January 2005

The attenuation of ultrasonic waves has been measured in single crystals of naphthalene and p-dichlorobenzene at frequencies from 5 to 95 MHz for the former crystal, and 5 to 65 MHz for the latter. The absorption coefficients of the crystals were proportional to the square of the acoustic frequency. The change in absorption coefficient for the temperature range from 30°C down to —70°C was very small to about —40°C; a slight decrease was observed below this point. There was no appreciable change in the absorption coefficient within the limit of experimental error, up to a (uniaxial) pressure of 4.25×106 dyn/cm2 applied in a direction perpendicular to the direction of propagation of the acoustic wave. From these results, it is concluded that any contributions of dislocation damping and thermal conduction in the crystals to the total acoustic absorption were negligibly small.
The sound velocity was determined for these same crystals at room temperature. This velocity was to be found to be a linear function of the temperature, with negative slope, and shows approximately the same temperature coefficient in both cases.
Description of Crystallite Orientation in Polycrystalline Materials Having Fiber Texture
Authors: Roe R.-J. , Krigbaum W. R. 
Published: January 2005

The distribution of plane-normal orientations in a polycrystalline sample can be investigated by x-ray diffraction measurements. Our problem is to deduce from such a set of data the maximum amount of information concerning the distribution of crystallite orientations within the sample. We restrict our attention to samples having fiber texture. The plane-normal distribution function is expanded in a series of Legendre polynomials, the coefficients of which are to be evaluated from the experimental diffraction data. The crystallite distribution function is expanded in a series of spherical harmonics, the coefficients of which may be obtained by the solution of a set of simultaneous linear equations. Certain simplifications arising from symmetry within the unit cell, or in the crystallite distribution, are examined. The effects of series termination errors are investigated, and relations are presented which permit one to estimate the number of terms which will be required to achieve a specified degree of accuracy.
If diffraction from a particular plane is too weak to measure, the present treatment permits the construction of its plane-normal distribution from diffraction data for other planes. Also, the effect of overlapping reflections can be accounted for. Finally, one can obtain information concerning the distribution of orientations for a subset of crystallites in the sample, for example, those having a specified c-axis orientation.