bluuebunny/crossref_metadata_2025 huggingface dataset:
Indices of refraction of HF and F2. II
Authors: Penning D. F. , Weimer D. , Rumpel W. F.
Published: February 2004

The refractive indices of HF and F2 were experimentally determined in the visible spectral region. The values of n0 − 1 obtained for HF are: 1.38, 1.34, and 1.33 × 10−4 at 4360, 5460, and 5790 Å, respectively. For F2 the results for n0 − 1 are 2.14 × 10−4 at 4360 Å and 2.07 × 10−4 at 5460 Å. The measurements for F2 were made at ambient temperatures, and those for HF at approximately 35 °C.
ESR and ENDOR studies of DL-serine irradiated at 4.2 °K
Authors: Lee Joon Y. , Box Harold C.
Published: February 2004

The primary products formed by ionizing radiation in single crystals of DL-serine were studied by ESR and ENDOR spectroscopy. The crystals were maintained at 4.2 °K during irradiation and measurement. The primary reduction product is formed by the addition of an electron to the carbonyl oxygen of the carboxyl group. The primary oxidation product is a radical resulting from decarboxylation. A third product is a radical formed from serine by removal of hydrogen from the hydroxy group.
Raman spectroscopy and molecular reorientation in liquids: CDCl3 and CH3Br
Authors: Laulicht I. , Meirman S.
Published: February 2004

Raman scattering experiments on symmetrical stretching lines of liquid CDCl3 and CH3Br have been carried out at room temperature. Angular autocorrelation functions were calculated by Fourier transforming the spectra, and by eliminating the vibrational correlation function. It is found that vibrational relaxation contribution to band shapes is important for these molecules. Correlation times were found to be τ2 = 1.59 ± 0.03 psec for CDCl3 and 1.33 ± 0.07 psec for CH3Br, in agreement with previous results obtained in different ways. The times between collisions were found to be much shorter (∼ 0.01–0.03 psec) than those obtained by infrared absorption experiments but in fair agreement with recent results obtained by Bartoli and Litovitz. Second moments were found to be smaller than those predicted for pure rotational Raman bands in the classical limit. The last result may partly be explained by the important contribution of the vibrational relaxation to the band shape.
Physical cluster theory of point defect interactions. I. General formalism
Authors: Allnatt A. R. , Loftus E.
Published: February 2004

The configurations of point defects in a solid can be uniquely classified in terms of interacting physical clusters. Two point defects are counted as in the same physical cluster if their separation is less than or equal to some characteristic distance. The methods of Mayer cluster theory are used to derive a general formal exact law of mass action which relates concentrations and activity coefficients of physical clusters to concentration-independent equilibrium constants. The activity coefficients and distribution functions for physical clusters are obtained as series expansions in powers of physical cluster concentrations. From these results further expansions, individual terms of which converge even for ionic crystals, are obtained by the methods of the Mayer ionic solution theory. The relationship of these results to the Frenkel-Band theory of association in imperfect gases and to the Lidiard-Teltow theory of defect interactions in ionic crystals is briefly indicated and possible applications of the formalism suggested. A detailed application is given in the accompanying paper.
Physical cluster theory of point defect interactions. II. Application to AgCl doped with CdCl2
Authors: Allnatt A. R. , Loftus E.
Published: February 2004

The Lidiard-Teltow approximation (L) describes a point defect system of cation vacancies and divalent impurity cations in AgCl doped with CdCl2 as a collection of nearest-neighbor impurity-vacancy pairs and unpaired impurities and vacancies. The concentrations of the latter and related by a law of mass action using Debye-Hückel activity coefficients for unpaired species. The formalism of the previous paper is applied to the same equilibrium. The leading terms of the exact cluster expansions for activity coefficients in the mass action law are reduced analytically and calculated numerically in two approximations, P and T. The approximations L, P, and T agree well for concentrations at which an extension of L indicates that larger defect clusters (triplets, etc.) can be neglected but they tend to diverge at higher concentrations than this, i.e., at impurity contents larger than 7 × 10−3 for temperatures greater than or equal to 250°C but at lower concentrations for lower temperatures. The agreement between P and T approximations confirms the adequate convergence of the new cluster expansions; the agreement of the L approximation with them indicates that it is an accurate approximation although it neglects the effects of paired defects and the discreteness of the lattice on the Debye-Hückel shielding as well as certain other Coulombic interactions retained in P and T.
Study of the correlation effects in a three-electron model system using the projected Hartree-Fock method and the natural spin orbital formalism
Authors: Laforgue A. , Čížek J. , Paldus J.
Published: February 2004

Various forms of the projected Hartree-Fock (PHF) scheme are used to study the correlation effects in a simple three-electron model system, represented by the PPP model of the allyl radical. The natural spin orbital formalism is used to calculate the correlation energy and spin densities. The unprojected independent particle trial function, which is used in the PHF scheme, always has lower symmetry than the Hamiltonian. The symmetries which are violated, either individually or simultaneously, are those of spin, space, and alternancy. The correlation energy and the spin density on the central atom obtained by the PHF method are compared with the corresponding values given by the exact solution of the model. Both quantities are examined for the whole range of the coupling constant. Due to the simplicity of the model, any two-parameter symmetry breaking scheme gives results identical with the full CI exact solution. Thus, the same exact wavefunction is obtained in three different ways and has a different structure depending on the two-parameter scheme used. Among the single symmetry breaking schemes, the spin symmetry violation is the most effective one. Its further improvement, however, is not straightforward and addition of the space symmetry violation leads to a complete reconstruction of the unprojected wavefunction. On the other hand, addition of alternancy symmetry violation to the space symmetry or spin symmetry violation results only in minor corrections to the unprojected wavefunction. This suggests that one should exercise extreme caution in selecting the trial wavefunction for the PHF scheme.
Determination of dissociation energies for some alkaline earth (hydro-) oxides in CO/N2O flames
Authors: Kalff P. J. , Alkemade C. Th. J.
Published: February 2004

The formation of mono-oxides and mono-hydroxides of the alkaline earth metals calcium, strontium, and barium is investigated by a flame spectrometric method. By comparison of the atomic resonance line intensities of sodium and of the alkaline earth metals mentioned, the ratio of the total molecular compound concentration to the free atomic metal concentration is measured in two CO/N2O flames with varying water input. Dihydroxide molecules appear to be of minor importance in our flames based on published dissociation energies for these species, and the atom to total molecule ratio is corrected for their presence by calculation. From the resulting relative oxide plus monohydroxide concentrations, dissociation energies for these compounds are determined with a trial-and-error method by third-law calculations. The resulting energies are compared with literature values, and ``best values'' are given. These are Do(CaO) = 3.75 ± 0.20 eV, Do(SrO) = 4.06 ± 0.10 eV, Do(BaO) = 5.30 ± 0.10 eV, Do(CaOH) = 4.44 ± 0.10 eV, Do(SrOH) = 4.38 ± 0.10 eV, and Do(BaOH) = 4.88 ± 0.06 eV. The electronic partition functions for the oxide molecules are assumed to be 3, 3, and 6, respectively.
Orbital ground state reversal in the high-spin complex Fe(II) (N, N′-dicyclohexylthiourea)6(ClO4)2
Authors: Latorre R. , Abeledo C. R. , Frankel R. B. , Costamagna J. A. , Reiff W. M. , Frank E.
Published: February 2004

Mössbauer spectroscopy of iron (II)(N, N′-Dicyclohexylthiourea)6 (ClO4)2 shows the existence of two isomers. At low temperatures (T < 200°K) a form A with quadrupole splitting ΔE = 3.31 mm/sec is stable. At and above room temperature a form B with ΔE = 1.32 mm/sec is stable. For 200 °K < T < 300°K both forms are observed with the B/A ratio increasing with increasing temperature. The values of ΔE are typical of Fe2+ with an orbital singlet and an orbital doublet ground state for A and B, respectively. The isomer shift and magnetic susceptibility results indicate that both forms of the molecule contain high-spin ferrous ions. Infrared spectra show that in both forms in the iron is bound to the sulphur atoms of each of the six dicyclohexylthiourea ligands. Mössbauer spectra in external magnetic fields show that the sign of the principal component of the electric field gradient is negative for form A and positive for form B, and that the asymmetry parameter of the electric field gradient tensor is negligible in both forms. The only consistent interpretation of all the results outlined above is in terms of a trigonally distorted octahedral coordination of the Fe2+ for both isomers. In form A, the distortion is such that the prolately deformed (t2g0) orbital is lowest while in form B the oblately deformed degenerate pair (t2g+) and t2g− are lowest. The foregoing suggest a low symmetry ligand field component (Δ) of variable sign, going from a compression (Δ > 0) along the trigonal axis at low T to an elongation (Δ < 0) at higher temperatures. The transition is discussed in terms of perturbations of the coordination sphere possibly induced by temperature dependent changes in the hydrogen bonding to the perchlorate anions.
On the phase transition in a gas of rodlike particles
Authors: Brenner Stephen L. , McQuarrie Donald A. , Olivares Dorys
Published: February 2004

The effect of attractive forces on the order-disorder phase transition in a gas of long hard rods is examined. The model system is that proposed by Zwanzig and consists of a gas of rectangular parallelepipeds which can orient only in the direction of three perpendicular coordinate axes. We extend Zwanzig's model by including a rectangular parallelpiped of attractive potential which surrounds the hard core in a symmetrical manner. A study is made of the effects of well depth and well width on the parameters characterizing the first-order phase transition from an isotropic gas in which the long axes of the rods point in random directions to an anisotropic (ordered) gas. It is observed that even a shallow well (e.g., with a depth of 0.1 kT) can yield significant changes in the transition parameters. The results of this study may have implications in the study of nematic ordering in liquid crystalline systems.
Circular dichroism of (+)-2-butanol in the vacuum ultraviolet. A comparison of theoretical and experimental values
Authors: Snyder Patricia Ann , Johnson W. Curtis
Published: February 2004

The circular dichroism and absorption spectra of (+)-(S)-2 butanol have now been measured to 1350 Å. Theoretical rotational strengths calculated using an independent systems approach are compared to the measured values. The results support use of the extended Kirkwood theory for calculation of rotational strengths. Our method for applying the theory is described in detail. Using a model in which the first three electronic transitions in simple alcohols are assigned as n → σOH*, n → σCO*, n → 3sO, we obtain good agreement in sign and magnitude between calculated and experimental rotational strengths. The importance of including the generally ignored gij type term of Kirkwood is clearly illustrated. It is expected to dominate for transitions which are both magnetically and electrically allowed when the static fields are small.
Electron spin resonance spectra and covalent bonding in the square-planar CuCl42− and CuBr42− ions
Authors: Chow Chee , Chang Kun , Willett R. D.
Published: February 2004

Electron paramagnetic resonance spectra of copper-doped potassium tetrachloropalladate and potassium tetrabromopalladate have been observed. Both hyperfine and superhyperfine structures are resolved at room temperature. The CuCl42− and CuBr42− units are in a square-planar configuration. The data can be fitted with an axial spin Hamiltonian. The parameters are as follows; g∥ = 2.232, g⊥=2.049, A∥Cu=163.6 × 10−4cm−1, A⊥Cu=34.5 × 10−4cm−1, A∥Cl=23.3 × 10−4cm−1, and A⊥Cl=5.3 × 10−4cm−1 for CuCl42− and g∥ = 2.143, g⊥=2.0438, A∥Cu=189.5 × 10−4cm−1, A⊥Cu=45.8 ×−4cm−1, A∥Br=123 × 10−4cm−1, and A⊥Br=27.9 × 10−4cm−1 for CuBr42−. The MO parameters are evaluated exactly. There are strong covalent in-plane σ and π bonds and out-of-plane π bonds. The covalency is larger for Br− than Cl− as a ligand. Data were also obtained on single crystal copper-doped tetraphenylarsonium trichloropalladate. The pure salt contains Pd2Cl62− dimers, and the EPR spectra are consistent with PdCuCl62− dimeric units in the doped species. The spin Hamiltonian parameters are essentially the same as in CuCl42−, with g∥ = 2.245, g⊥=2.045, A∥Cu=155 × 10−4cm−1, A⊥Cu=57 × 10−4cm−1, A∥Cl=23.8 × 10−4cm−1, and A⊥Cl=5.2 × 10−4cm−1. Dimerization does not change significantly the character of the Cu–Cl bonds.
Lifetime of the D 3Σu+ state of N2
Authors: Kurzweg L. , Egbert G. T. , Burns D. J.
Published: February 2004

Using the method of delayed coincidence with electron excitation of N2, we have measured the lifetime of the D 3Σu+ state of N2 to be 14.1 ± 1.0 nsec.
Weak Raman bands from water
Authors: Walrafen G. E. , Blatz L. A.
Published: February 2004

Weak, broad, Raman bands have been observed from water and heavy water at shifts above and below those corresponding to the intramolecular bending and stretching vibrations. Bands near Δv̄ = 1190 cm−1 for D2O and Δv̄ = 1625 cm−1 for H2O, in addition to relatively sharp bands at 1210 cm−1 for D2O and 1640 cm−1 for H2O are thought to arise from intramolecular bending vibrations of D2O and H2O molecules that are either partially, or fully hydrogen bonded. Other weak bands can be explained as combinations of known intra- and intermolecular vibrations, in particular, librations.
Interpretation of glory undulations in scattering cross sections: Amplitudes
Authors: Greene E. F. , Mason E. A.
Published: February 2004

The interpretation of the amplitude ΔQ of the glory undulations is given in terms of the coefficients Ri of a semiclassical expansion of the form ΔQ = v1/2 (R0 + R1 E−1 +···) where v is the relative velocity and the E the relative kinetic energy. The Ri give a measure of the force in the vicinity of the distance of closest approach. These coefficients can be used with the analogous coefficients for the spacing of the undulations to provide combinations that depend only on the shape of the potential and not on its scale. Examples are given for square-well and Lennard-Jones (n, 6) potentials.
Theory on transient effects in time resolved ESR spectroscopy
Authors: Pedersen J. Boiden
Published: February 2004

A general theory is presented which in a unified way is able to explain the reported intensity behavior in the new field of time resolved ESR spectroscopy. Radicals with a short lifetime have been observed to have highly abnormal spin polarizations which have earlier been explained by a spin selective reaction. The production of polarization as well as other kinetic features are treated in a theoretical satisfactory way and the experimentally observed enhancement is directly related to fundamental quantities, such as the spin lattice relaxation time, the lifetime of the radical, and the polarization production per collision (the quantity calculated by the microscopic theories). It is shown that the time dependence of the intensity in many cases directly reflects the kinetic of the system and that the two commonly appearing cases of initial polarization and recombination polarization may be distinguished by the time behavior of the intensity. The effect of field inhomogeneities and different relaxation times T1 and T2 which becomes important when the radical decay is of the order of or faster than the spin lattice relaxation time is discussed. The method is based on a set of coupled Bloch equations modified to include a proper description of the radical kinetics, the polarization production, and the quenching of polarization due to Heisenberg spin exchange. Solutions of these equations are presented for a number of commonly appearing systems.
Nitrogen-14 nuclear quadrupole resonance study of the guanidinium ion
Authors: Oja Tõnis
Published: February 2004

Numerous ionic compounds containing the guanidinium ion have been investigated by means of nitrogen-14 nuclear quadrupole resonance techniques. The nitrogen resonance spectrum of the amino groups of the quanidinium ions yielded, with a few exceptions, an average e 2qQ/h = 3553.6 kHz and an average asymmetry parameter η = 0.39493. The Townes-Dailey theory is employed in interpreting the quadrupole coupling data to yield the electronic populations of the bonds. In two cases (guanidinium acetate and guanidinium carbonate) large deviations from the average quadrupole coupling parameters were observed. These deviations are interpreted, by means of a modified Townes-Dailey theory, to indicate that hydrogen bonds are formed between the guanidinium ions and the anions.
Effects of column asymmetry on thermal diffusion separations
Authors: So/rensen Jan P. , Willis Max S. , Stewart Warren E.
Published: February 2004

The theory of the Clusius-Dickel column in static operation is extended to include eccentric misalignment of the inner cylinder. Variable-property effects are included. An orthogonal expansion method is used, and a computer program is provided for general application of the method. The new theory is useful for determining alignment tolerances in commercial separation columns or in absolute measurements of αT. The theory is supported by experiments on separation of gaseous 20Ne from 22Ne.
On the equivalence of V6 to torsional flexing for molecules with threefold barriers to internal rotation
Authors: Lees R. M.
Published: February 2004

A comparison is made of the approaches of Lees and Baker and of Ewig and Harris to the Hamiltonian for internal rotation, both of which showed that the effects of a V6 term in a threefold torsional potential are identical to those of torsional flexing, i.e., to the distortion of molecular geometry due to torsion. The two approaches are proved to give identical results in the first order of perturbation. Earlier contrasting arguments that spectra could be explained by a V6 term but not by torsional flexing are examined and found to contain an incorrect averaging procedure in the perturbation treatment. Values of the torsional flexing term equivalent to published values of V6 are obtained to first order and are discussed. Although experimental information is not yet sufficiently precise or extensive to permit quantitative conclusions, it appears that torsional flexing could account for a significant part of the reported V6 in some cases.
Vibrational coupling effects and potential functions for cis - and trans -1,2-difluorodiimide and cis - and trans -1,2-difluoroethylene
Authors: Pearce Robert A. R. , Levin Ira W.
Published: February 2004

Separate six parameter internal valence force fields were determined for cis- and trans-1,2-difluorodiimide (F–N=N–F), using a least-squares procedure. For the cis- and trans-1,2-difluoroethylenes (F–CH=CH–F), a 17 parameter overlay internal valence force field was fitted to the in-plane vibrational frequencies, and separate four parameter general valence force fields were calculated for the out-of-plane vibrations. For both sets of molecules, potential energy distributions are reported and discussed, with particular reference to the unusual couplings between the F–X stretching and F–X=X bending coordinates. These couplings are investigated in detail, especially for the 1,2-difluorodiimides, and it is concluded that they are predominately due to kinetic effects.
Vibrational relaxation of shock-heated N2 by atomic oxygen using the ir tracer method
Authors: Eckstrom D. J.
Published: February 2004

We have studied the effects of oxygen atoms on the vibrational relaxation of shock-heated N2 over the temperature range from 1200 to 3000 °K, using the CO tracer technique. The measured relaxation times pτN2–O vary from 0.8 to 2 atm · μsec in this range with an uncertainty of ± 50%, in excellent agreement with existing high-temperature shock tube and low-temperature flow tube results. The weak temperature dependence of all these results is in marked contrast to theoretical predictions. Some limited measurements of the relaxation of CO by oxygen atoms yield values of pτCO–O of approximately 0.04 atm · μsec, in good agreement with other recent measurements. This rate is approximately 25 times as fast as the N2–O rate and, coincidentally, is approximately the same as that for the relaxation of O2 by O atoms. The greater rapidity of the CO–O and O2–O relaxation, compared with that of N2–O, may be the result of atom-exchange processes in the former cases.
Vibrational spectral study of molecular orientation in vitreous fibers
Authors: Miller Philip J. , Exarhos Gregory J. , Risen William M.
Published: February 2004

Preferential orientation of polymeric chains with respect to the principle axis in completely noncrystalline, vitreous fibers is demonstrated by laser Raman and infrared reflectance study of sodium metaphosphate glass fibers. The polarization dependence of Raman intensities in 24 scattering arrangements have been obtained by varying the incident and scattered radiation propagation and polarization directions and are compared to calculated Raman intensity ratios for uniaxial orientation of the chains. Infrared dichroic ratio data were obtained from a Kramers-Kronig transformation analysis of polarized reflection spectra on oriented fibers, and employed to compute the averages of the angles the metaphosphate chains make with the fiber axis.
Burnett theory of planar thermal force with surface accommodation
Authors: McCoy B. J. , Cha C. Y.
Published: February 2004

The force on a plane surface in a rarefied gas caused by a temperature gradient normal to the surface is studied. In the free-molecule regime the force depends on the normal-momentum accommodation coefficient. For the transition regime the second-order Burnett constitutive equations for stress and heat flux provide the necessary coupling between force and temperature gradient. Force versus Knudsen number has a maximum which is related to both normal-momentum and thermal accommodation coefficients. The results compare favorably with the dusty gas model predictions and with experimental data. Relationships to other rarefied gas transport phenomena, e.g., thermal transpiration, are demonstrated.
Perturbation theories for the calculation of molecular interaction energies. I. General formalism
Authors: Chipman Daniel M. , Bowman J. D. , Hirschfelder J. O.
Published: February 2004

In the calculation of the interaction energy of two atoms, it is particularly convenient to take the zero order Hamiltonian H(0) to be the sum of the atomic Hamiltonians. Then H(0) does not reflect the full permutational symmetry of the exact Hamiltonian H. Many different perturbation expansions with such a ``nonsymmetric'' H(0) have been proposed. In this paper, these expansions are reviewed and classified within a very general perturbation framework. Also, a new ``optimized'' method is proposed which, by virtue of the Rayleigh-Ritz variational principle, is guaranteed to give better results than any of the previously suggested methods. For simplicity the analysis is restricted to the case that the spin-free zero order function φ(0) and the spin-free exact wavefunction ψ are both nondegenerate. As an example, the construction of the first order wavefunction in each method is discussed in detail for the simple case of a single symmetry operation.
Molecular formulation of the internal viscosity in polymer dynamics, and stress symmetry
Authors: Bazúa Enrique R. , Williams Michael C.
Published: February 2004

Internal viscosity, the property of polymer ``submolecule'' resistance to its rate of extension, is derived from the mechanics of atomic bond rotations within the submolecule. The result for these deformational forces and velocities is F̂d = c C · V̂d in supervector notation for the whole set of submolecules, where C is the Zimm matrix and c is related to rotational energy barriers E(φ) and other molecular parameters. The controversy about stress symmetry predictions of all such models is resolved.
Deactivation of electronically excited sulfur atoms, S(3 1S)
Authors: Dunn O. J. , Filseth S. V. , Young R. A.
Published: February 2004

Deactivation of S(3 1S0) produced in the gas phase photolysis of carbonyl sulfide at 147.0 nm was studied with various molecules. The time dependence of the concentration of excited sulfur atoms is followed by their emission at 772.5 nm (1S0 → 1D2) and 458.9 nm (1S0 → 3P1). Absolute rate coefficients for removal (reaction plus collisional relaxation) of S(3 1S0) were measured for O2, CO, CO2, OCS, H2, NO, NO2, N2O, H2S, CS2, SO2, CH4, C2H6, C2H4, C2H2, and Ar. For many of these gases, removal rate coefficients for S(3 1S0) and O(2 1S0) are similar, but exceptions are noted as well.
Energy transfer between transition-metal complexes in double complex salts
Authors: Fujita Ichiro , Kobayashi Hiroshi
Published: February 2004

In a double complex salt consisting of cationic complex tris (2, 2′-bipyridine) ruthenium (II) [Ru(bipy)3]2+ and anionic chromium (III) complex [CrL6]3−, an excitation energy transfer from the excited ruthenium (II) complex to the chromium (III) complex is observed. When the 4T2g state of [CrL6]3− is lower than the ``metal to ligand'' charge-transfer singlet excited state 1CT of [Ru(bipy)3]2+, the phosphorescence of [Ru(bipy)3]2+ is completely quenched and the counterpart complex [CrL6]3− emits an enhanced phosphorescence even if it is irradiated at a wavenumber which excites exclusively [Ru(bipy)3]2+ but not [CrL6]3−. An efficient path of the excitation energy transfer in the double complex salt is the one from the 1CT state of [Ru(bipy)3]2+ to the 4T2g state of [CrL6]3−. When the 4T2g state of [CrL6]3− is higher than the 1C T state of [Ru(bipy)3]2+, the excitation transfer is competitive with emission and both [Ru(bipy)3]2+ and [CrL6]3− in the double complex salt phosphoresce Even such a chromium (III) complex behaves as an efficient quencher to excited [Rh(bipy)3]3+ in the double complex salt and emits a strong phosphorescence upon excitation of the [Rh(bipy)3]3+. Since the 4T2g state of such a chromium (III) complex is lower than the lowest singlet excited state of [Rh(bipy)3]3+, an excitation transfer takes place from the excited [Rh(bipy)3]3+ to [CrL6]3−.
Laser-induced rate processes in gases: Phase coherence in an N-level model
Authors: Stone James , Thiele Everett , Goodman Myron F.
Published: February 2004

We consider a gas of molecules absorbing light from a monochromatic laser source and also undergoing both phase randomizing and thermalizing collisions with a heat bath of inert molecules. The generalized master equation formalism, used previously to study a system of this type with a two-level absorber, is extended to an absorber with N equally spaced levels. The possibility of unimolecular decay from any or all of the N levels is included in the formalism. The well-known relaxation model of Rubin and Shuler and the chemical reaction model of Montroll and Shuler are built into the formalism as special cases when the light source is turned off. Specific calculations for an eight-level model are presented and compared with results expected for an incoherent light source. A coherent effect which increases the upper level populations at steady state is found to exist. A corresponding increase in the macroscopic chemical rate constant is calculated for the coherent model when unimolecular decay from the top level is included. A particularly simple and general result is found for the power absorbed by a harmonic oscillator. It is also shown that, for the N-level model, one does not necessarily find equal populations in the limit of infinite light field intensity.
Generalization of Hamilton's principle to continuous dissipative systems
Authors: Lebon G. J. , Lambermont J. H.
Published: February 2004

Some variational principles describing the behavior of nonstationary dissipative processes are presented. These criteria appear as an extension of Hamilton's principle to dissipative systems. Firstly, processes with vanishing barycentric velocity, i.e., of purely dissipative nature, are considered. The principle proposed is a slight modification of a criterion derived previously by the authors: The central quantity in the expression of the principle is however no longer the Legendre transform of the entropy per unit mass but is here the Legendre transform of the internal energy per unit volume. As an illustrative example, thermodiffusion is considered. Afterwards, the principle is generalized to describe the flow of a compressible, Newtonian fluid in which temperature effects are not neglected. After having shown that the Euler-Lagrange equations are equivalent to the conservation laws, some particular expressions of the principle, corresponding to special situations like the stationary case and fixed boundary conditions, are given.
New equation for the influence of pressure on melting
Authors: Reynolds C. L. , Faughnan K. A. , Barker R. E.
Published: February 2004

A new equation for the variation of melting points Tm of metals as a function of pressure p has been developed on the basis of simplified atomic considerations. The equation, [ΔTm/Tm(0)]/βp≈ 2(γG − 1/3)(1 + C2βp), is thought to be in a more convenient form for comparison with experimental data than previous relationships. In this equation, β is the compressibility, γG the Grüneisen parameter, and C2 is a constant related to the anharmonicity. In checking the equation it was found that a value of C2 = −2.9 appears to be more accurate for the ratio a2/a12 than the value −2.5 reported by Slater (a1 and a2 are the coefficients in the expression −ΔV/V0 = a1p + a2p2, for the compression of a material). Obviously, the proposed equation for ΔTm should not be applied when the compression is not well described by the relation involving a1 and a2.
Influence of interband transitions of reflections from cubic crystals
Authors: Fulton Robert L.
Published: February 2004

A form for the contribution of the interband transitions to the susceptibility of a crystal of arbitrary shape is suggested. This contribution differs from that of the ``normal'' transitions which lead to the usual phenomenological Maxwell equations. The explicit form of the susceptibility of a semi-infinite cubic crystal is found, including the contributions of both interband and normal transitions. The reflection coefficients for light incident on the crystal surface from the vacuum are extracted from the general expression for the vector potential. The influence of interband transitions on the reflection coefficients r⊥ (perpendicular polarization) and r∥ (parallel polarization) are ascertained and compared to the effects of normal transitions. r⊥ is found to be related to the complex index of refraction by the same expression as given by classical optics. The expression for r∥, however, is found to differ from that of classical optics. For weak interband transitions it is found that the difference in phase of r⊥ and r∥ has the shape of a dispersion curve while classical optics gives an absorption curve. The differences between the reflection coefficients of classical optics and those in the presence of interband transitions are depicted graphically. Quite generally, interband transitions are predicted to have a definite fingerprint.
Experimental study of the sublimation of graphite at high temperatures
Authors: Zavitsanos P. D. , Carlson G. A.
Published: February 2004

The equilibrium vaporization of pyrolytic graphite was studied using rf induction heating and a time-of-flight mass spectrometer. rf induction heating was demonstrated for the first time as a compatible and practical heating technique for high temperature mass spectrometry. Relative ion intensities were measured for C1(g), C2(g), C3(g), and C4(g) in the temperature range 2320 K-3000 K and partial vapor pressures were obtained. The vapor pressures of C2 and C3 in the temperature range 2400 K-3000 K can be represented by logP(C2) (atm) = 9.971 − 44 230/T(K) and logP (C3) (atm) = 9.944 − 40 670/T(K). Heats of vaporization obtained by the second and third law methods are as follows: SpeciesSecond law ΔH°298(kcal/mole)Third law ΔH°298(kcal/mole)C1170.3 ± 1.1C2206.0 ± 1.3202.7 ± 0.6C3194.4 ± 1.2202.1 ± 1.5C4242.7Thermodynamic functions calculated by Strauss and Thiele were used forC3. JANAF thermodynamic data were used for all other species. The entropy change in the vaporization of C3 determined from this work is 45.55 eu at 2740 K.
Trajectory studies of atomic recombination reactions. V. Recombination of bromine atoms
Authors: Wong W. H. , Burns George
Published: February 2004

Recombination of Br atoms in Ar was studied by 3D trajectory calculations between 300 and 1500°K. The recombination path via unstable Br–Ar quasidimers was included in the calculations, which yield recombination rate constants in agreement with experiment within a factor of 2, provided that the ArBr Lennard-Jones interaction is of the order of 0.5 kcal/mole.
On the low-energy event in Hiraoka-Hamill experiments
Authors: Cheng Ing-yih , Funabashi K.
Published: February 2004

A theoretical analysis is made on energy loss spectra of low-energy electrons in organic thin films by Merkel, Hiraoka, and Hamill. The asymmetry of the first peak in the first-derivative curve for transmitted current does correspond to some loss event, as suggested by Hiraoka and Hamill, but their deconvolution method in locating the inelastic event is not unique. An alternative method is suggested. Relationship between the observed peaks and loss events is discussed, and it is pointed out that one-to-one correspondence does not necessarily exist between them.
Dissociation of vibrationally-rotationally excited I2 (B 3Πou+)
Authors: Gelb A. , Kapral R. , Burns G.
Published: February 2004

The collisional dissociation of I2 (B 3Πou+), initially excited near its dissociation limit in the presence of an excess of He, Ar, or Xe, has been studied, by trajectory calculations, as a function of the initial internal energy of the molecule. It was found that the percent dissociation is a steep function of temperature and total internal energy of I2 (B 3Πou+). The rotational energy of the molecule is less effective than the vibrational energy in inducing collisional dissociation. The results are compared with several experiments which have recently been performed on these systems. The present calculations are in rough agreement with some experiments on the I2 (B 3Πou+)–Xe system, but predict photochemical quantum yields for the I2 (B 3Πou+)–He system which are smaller than those obtained experimentally. Recent fluorescence quenching experiments are also discussed in light of the present findings. Some reasons for the discrepancies and additional experimental tests are suggested.
Ab Initio calculation of the vibrational and electronic properties of carbon dioxide
Authors: Vučelić Miloš , Öhrn Yngve , Sabin John R.
Published: February 2004

An ab initio LCAO-MO-SCF calculation is carried out on CO2 in order to obtain the vibrational spectrum, intensities, and electronic properties of the molecule. The results are presented and discussed, and comparisons are made with experiment where possible.
Tetrahedral methane without 2s → 2p promotion and hybridization: Direct calculation of the effects of promotion and hybridization in CH4, NH3, H2O, and H2S
Authors: Jarvie J. , Willson W. , Doolittle J. , Edmiston C.
Published: February 2004

Fairly accurate linear-combination-of-atomic-orbitals molecular-orbital self-consistent-field (LCAO MO SCF) wavefunctions have been calculated for CH4, both with and without the constraint that the first two a1 MO's be the 1s and 2s SCF AO's of the C atom. The calculations were carried out for tetrahedral and distorted-tetrahedral nuclear configurations, and it was found that the tetrahedral configuration is the most stable both with and without the constraint. To examine the effects of nonpromotional (zeroth-order) hybridization in CH4, calculations were also carried out in which the s-type basis functions (and four associated electrons) of the C atom were completely deleted and replaced with an appropriate effective nuclear charge. Again, a tetrahedral methane was obtained. Therefore we conclude that 2s → 2p promotion and/or hybridization does not cause the tetrahedral structure of CH4. Analogous calculations on H2O and NH3 show a decrease in bond angle due to the constraint from 106.7° to 98.1° for H2O and from [inverted lazy s] 107° to [inverted lazy s] 95° for NH3. Similar calculations for H2S, in which the six lowest-energy MO's are constrained to be the 1s, 2s, three 2p's, and 3s SCF AO's of the S atom, show a decrease of the calculated bond angle from [inverted lazy s] 93° to [inverted lazy s] 89°. As for CH4, the shapes of the angular-distortion potential energy curves for H2O, NH3, and H2S are nearly unaffected by the constraint. For CH4, NH3, and H2O, the molecular binding energies are decreased, respectively, 4.0, 2.2, and 1.3 eV, by the constraint.
Infrared study of the lambda transition and molecular configuration in crystalline ferrocene
Authors: Hyams I. J. , Ron Arza
Published: February 2004

The phase transition in crystalline ferrocene was investigated by following the intensity change with temperature of two infrared absorption bands associated with inactive molecular modes. It is suggested that at low temperatures there is an ordered lattice with noncentrosymmetric sites. There is no abrupt change in intensity at the phase transition indicating the persistence of the low-temperature short-range order into the high-temperature phase. The present study supports the suggested structure for the high-temperature phase in which there is a centrosymmetric lattice containing a predominant number of staggered molecules.
Dynamic nuclear polarization studies of intermolecular coupling of deuteron and unpaired electron spins
Authors: Bates Richard D. , Poindexter Edward H. , Wagner Burkhard E.
Published: February 2004

Observed 2H NMR enhancements with free radicals in solution are far below the dipolar limit. Leakage factors for the deuteron systems are much poorer than for the protonated analogs. This is the result of two factors: (1) unfavorable bulk nuclear relaxation times due to quadrupolar relaxation mechanisms, and (2) weak intermolecular coupling of nuclear and unpaired electron spins. Comparison of radical-induced nuclear relaxation rates for deuterobenzene with the free radicals GALV and BDPA and the analogous benzene solutions demonstrates the (γN)2 dependence of the dipolar coupling strength. The consequences of these results on future studies for nuclei with I ≠ (1/2) and small γN are discussed.
Microwave spectrum of allene-1, 1-d2
Authors: Hirota Eizi , Matsumura Chi
Published: February 2004

The J = 3 ← 2, K−1 = 2 and J = 4 ← 3, K−1 = 2 and 3 transitions were observed for the allene-1,1-d2 molecule. The B̄ = (B + C)/2 and (A−B̄)bp2 constants have been determined to be 7846.158 ± 0.01 MHz and 0.133 ± 0.005 MHz, respectively. The observed values of the two centrifugal distortion constants, DJ = 0.0024 ± 0.0005 MHz and DJK = 0.130 ± 0.005 MHz, were compared with the calculated to confirm the assignment of the weak spectra. The Stark effect of the J = 4 ← 3, K−1 = 3 transition was analyzed by comparing with computer-simulated spectrum, leading to the dipole moment of 0.0031 ± 0.0003 D.
Vibrational spectra and normal coordinate analysis for trans-2-butene-d and trans-2-butene-d8
Authors: Levin Ira W. , Pearce Robert A. R. , Harris W. C.
Published: February 2004

The vibrational spectra of trans-2-butene-d0 and trans-2-butene-d8 were recorded in the region 4000–100 cm–1. In conjunction with a normal coordinate analysis, vibrational assignments are proposed on the basis of Raman depolarization ratios for the liquid samples, isotopic shift ratios, and group coordinate considerations. Both assignments and vibrational coupling effects are discussed in terms of the calculated potential energy distributions. A 21-parameter modified internal valence force field was determined by least squares procedures from the observed polycrystalline frequencies of the two isotopic species.
Atomic coordinates of helical homopolypeptide chains
Authors: Tamburini B. , Tristo G. , Del Pra A.
Published: February 2004

A new mathematical method for the determination of the atomic coordinates of a helical polypeptide chain has been derived employing a function of the internal coordinates, i.e., valence angles, bond lengths, and rotation angles of the monomeric unit.
Temperature-independent density-of-states function for the S1 (0, 0) band of crystalline benzene
Authors: Colson S. D. , Netzel T. L.
Published: February 2004

The density-of-states function for the S1 (0, 0) band of crystalline benzene has been determined. The function is found to be independent of temperature between 12 and 20°K. This result clearly establishes the validity of the band-to-band transition method for determining density-of-states functions of bands that can be described by the restricted-Frenkel exciton formalism. The density-of-states function is determined more accurately than in the previous work and is used to derive a refined set of excitation exchange integrals. The mean of this density-of-states function agrees with the value predicted by a vibronic analysis of pure and isotopic mixed crystal spectra.
Microwave-induced triplet-triplet absorption in organic molecules
Authors: Clarke Richard H. , Hayes John M.
Published: February 2004

The effect of a saturating microwave field in resonance with the zero-field levels of an organic triplet state on the triplet-triplet absorption is examined. The change in triplet-triplet absorption intensity is shown to be due to microwave alteration of the over-all steady state population in the lowest triplet state. The microwave-induced intensity changes for triplet-triplet absorption can be calculated in terms of the dynamical rate constants for the individual spin levels of the triplet state. Results of these calculations are compared with experimentally observed triplet-triplet absorption intensity changes for several nitrogen heterocyclic compounds.
Classical calculations of rotational and vibrational excitation in energetic atom-diatomic hydride collisions
Authors: Tait K. S. , Kolb C. E. , Baum H. R.
Published: February 2004

Classical rotational excitation probabilities for collisions of HF, HCl, and OH with three atomic species have been calculated in three dimensions using a Monte Carlo procedure. Results are presented for the rotational excitation of both rigid diatomic rotors and those coupled via a classical harmonic oscillator. Hard-sphere intermolecular potentials were employed to obtain results applicable to high-energy (≥ 1 eV) collisions.
Hyperfine interactions in the intramolecular antiferromagnet (Fe salen C1)2
Authors: Lechan R. , Nicolini C. , Abeledo C. R. , Frankel R. B.
Published: February 2004

Magnetic hyperfine interactions in (Fe salen C1)2 have been studied using the Mössbauer effect in external magnetic fields up to 80 kOe and for 4.2 ≤ T ≤ 80 °K. The low-temperature results are adequately described by a simple intramolecular exchange model with J = 7.0 cm−1 and a hyperfine interaction constant of −180 kOe per unit spin. At higher temperature the effective field approximation breaks down. Data for (Fe salen)2O are also briefly reported.
Cross sections for the alkali atom-Br2 reactions
Authors: Maya J. , Davidovits P.
Published: February 2004

The reaction cross sections of alkali metal atoms with bromine molecules have been obtained by direct measurements of alkali atom decay rates. The alkali atoms were produced in the presence of a known amount of bromine molecules by photodissociating the bromide of the particular alkali atom with a short pulse of uv light. As the alkali atoms reacted with bromine molecules their decay rate was monitored by observing the transmission of alkali atom resonance light through the vapor. The cross sections were computed from the decay rates. They are Na, 116 Å2; K, 151 Å2, Rb, 197 Å2, and Cs, 204 Å2. These results are accurate to about 15%. The results are compared with theoretical calculations.
Franck-Condon factors from selectively excited resonance fluorescence in the B-X system of S2
Authors: Meyer Keith A. , Crosley David R.
Published: February 2004

Franck-Condon factors, for transitions connecting the ν′ = 3 and ν′ = 4 vibrational levels of the B 3Σu− state of S2 to the ground X 3Σg− state, have been measured using selective excitation of resonance fluorescence. Observation of all ν″ terms for each ν′ allows a determination of the Franck-Condon factors without arbitrary normalization. Comparison of our results with theoretical predictions shows disagreement; this is not unexpected due to the extensive perturbations in the B state.
Hydrodynamic behavior of triplet excitons
Authors: Fleming Paul D.
Published: February 2004

In this work we consider the many triplet exciton states of molecular crystals. A model Hamiltonian, which is isomorphic to an anisotropic Heisenberg spin Hamiltonian axially symmetric about a uniform magnetic field, is introduced. This Hamiltonian can be viewed as a quasiparticle Hamiltonian in which the excitations are neither Fermion nor Boson but are so called Paulion operators. This Hamiltonian possesses conservation laws of exciton number and energy, and when Umklapp processes can be neglected it conserves exciton momentum. As a consequence of these conservation laws the long-wavelength collective behavior of triplets can be described by a picture very similar to ordinary particle hydrodynamics. However, because of kinematical differences it is necessary to introduce two additional transport coefficients.
Tests of approximate methods for the calculation of the static potential for electron scattering by CO
Authors: Truhlar Donald G. , Van-Catledge F. A.
Published: February 2004

The interaction potential between an electron and an unperturbed CO molecule is calculated as a function of the electron's position r (measured with the origin at the center of mass of CO) by the INDO (intermediate-neglect-of-differential-overlap) molecular orbital theory and by an approximate application (following Itikawa and Takayanagi) of Massey's method. The results are compared quantitatively to the ab initio extended-basis-set molecular orbital calculation of Gianturco and Tait. The potential is expanded as V(r) = ΣLVL(r)PL(r̂·R̂), where R̂ is a unit vector along the internuclear axis. The INDO VL(r) is fairly accurate for L = 0, 2, and 4 but not for L = 1. The approximate Massey's method calculation is fairly accurate only for r < 2.3 bohr for L = 0, 2, and 4.
Liquid crystalline phases in a doubly homologous series of benzylideneanilines-textures and scanning calorimetry
Authors: Smith George W. , Gardlund Zack G.
Published: February 2004

We have investigated the phase behavior of 35 N -(p -alkoxybenzylidene)-p -n -alkylaniline compounds, CnH2n + 1O–φ–CHN–φ–CmH2m + 1′ with n values ranging from 1 to 7 and m from 4 to 8 by means of thermal microscopy and differential scanning calorimetry. We report transition energies and entropies in addition to transition temperatures. From textures we determined the existence of nematic as well as smectic A, C, and B phases in many of the compounds. The microscopy and calorimetry evidence are consistent with a possible second order nature for the smectic C to smectic A transition. An interesting and useful smectic textural transition phenomenon is reported.