bluuebunny/crossref_metadata_2025 huggingface dataset:
Rate constants for the reaction of OH radicals with CHF2Cl, CF2Cl2, CFCl3, and H2 over the temperature range 297–434 °K
Authors: Atkinson R. , Hansen D. A. , Pitts J. N.
Published: February 2003

Rate constants for the reaction of OH radicals with CHF2Cl, CF2Cl2, CFCl3, and H2 have been determined over the temperature range 297–434 °K using a flash photolysis–resonance fluorescence technique. The following Arrhenius expressions were obtained: k1(CHF2Cl) =1.21×10−12e−(3250±300)/RT cm3 molecule−1 sec−1, k1(CF2Cl2) <1×10−15 cm3 molecule−1 sec−1 (T=297–424 °K), k1(CFCl3) <1×10−15 cm3 molecule−1 sec−1 (T=297–424 °K), k1(H2) =5.9×10−12 e−(3990±300)/RT cm3 molecule−1 sec−1. The rate constants for the reaction of OH radicals with H2, which were used to check the experimental system, are in good agreement with literature values. The atmospheric significance of the reaction of OH radicals with CHF2Cl is briefly discussed.
A kinetics study of the reaction of OH radicals with two C2 hydrocarbons: C2H4 and C2H2
Authors: Davis D. D. , Fischer S. , Schiff R. , Watson R. T. , Bollinger W.
Published: February 2003

The flash photolysis-resonance fluorescence technique has been utilized to study the kinetics of hydroxyl radical reactions with ethylene and acetylene at 300 K over a wide range of experimental conditions. (1) OH+C2H4 →k1 Products (e.g., C2H5O), (2) OH+C2H2 →k1 Products (e.g.,C2H2O+H). The bimolecular rate constant for Reaction (1) was observed to increase from (2.24–5.33) × 10−12 cm3 molecule−1⋅ sec−1 as the total pressure varied from (3–300) torr of helium. The rate of Reaction (2) was invarient with total pressure, and the value obtained for k2 was (1.65±0.15) × 10−13 cm2 molecule⋅sec−1. The observed pressure dependency of Reaction (1) brings the existing literature values for k1 into close agreement. Reaction (2) has been shown to be particularly sensitive to secondary processes and this is discussed in some detail.
The electronic structure of 1,2-di(p-tolyl)ethane and of pure and substituted [2.2]paracyclophane
Authors: Duke C. B. , Lipari N. O. , Salaneck W. R. , Schein L. B.
Published: February 2003

Two new ’’spectroscopic’’ CNDO model parametrizations, constructed to describe the electronic excitation spectra of conjugated hydrocarbons, are utilized to analyze these spectra for the double-benzene-ring molecules 1,2-di(p-tolyl)ethane, [2.2]paracyclophane, pseudopara-dibromo[2.2]paracyclophane, pseudopara-dicyano[2.2]paracyclophane, and 1,1,2,2,9,9,10,10 octafluoro[2.2]paracyclophane. Our CNDO/S3 model predicts orbital eigenvalue spectra which are in quantitative (ΔE∼0.1 eV) correspondence with the ultraviolet photoemission spectra of those molecules for which they have been measured. Its predictions of the ultraviolet absorption spectra of 1,2-di(p-tolyl)ethane are equally quantitative, although those for the absorption spectra of the [2.2]paracyclophanes are only qualitative (ΔE≲0.3 eV). A major success of the model is its correct prediction of the observed symmetry of the radical anion ESR spectra of 1,2-di(p-tolyl)ethane and [2.2]-paracyclophane: the latter result having eluded prior molecular-orbital models. Moreover, a substantial alteration in the symmetry of the ESR radical anion spectra is predicted for the case of 1,1,2,2,9,9,10,10 octafluoro-[2,2]-paracyclophane relative to [2,2]paracyclophane.
Fluids of hard spheres with nonadditive diameters
Authors: Adams D. J. , McDonald I. R.
Published: February 2003

Monte Carlo results are reported for the equation of state and radial distribution functions of mixtures of hard spheres with nonadditive diameters. Comparison is made with the predictions of perturbation theories of various types and methods based on a one-component reference fluid are found to give the most satisfactory results.
Zero field optically detected magnetic resonance of multiple localized states in 1,4-dibromonaphthalene crystals
Authors: Nishimura A. M. , Zewail A. H. , Harris C. B.
Published: February 2003

A systematic study of the localized states of 1,4-dibromonaphthalene was made, both as a dilute guest in a host crystal and as a trap site in the neat crystal, utilizing zero field magnetic resonance techniques. The kinetic parameters associated with the various ’’localized’’ triplet states have been determined. These include the relative intersystem crossing rates, the relative radiative rates from the spin states to the electronic and vibronic origins, and the total depletion rates from the individual spin sublevels. The results of these studies indicate that the correlation of the magnetic structure of trap states and the energy difference between traps and a host band is not straightforward, even for one-dimensional systems. Moreover, these results imply that spin exchange and equilibrium between traps and band states can severely influence the dynamics of energy migration and must be incorporated into any temperature dependent line shape analysis of the exciton transitions, particularly for 1,4-dibromonaphthalene because of the similarity in trap and exciton EPR frequencies.
Reply to Comment by A. Laubereau on ’’Intermolecular energy transfer in liquid benzene: Raman spectra, linewidth measurements, picosecond spectroscopy, and vibrational relaxation times’’
Authors: Griffiths J. E. , Clerc M. , Rentzepis P. M.
Published: February 2003

A reply is made to the preceeding Comment by Laubereau and the picosecond technique is clarified. (AIP)
A comment on the optical spectrum of C2H
Authors: Gilra Daya P.
Published: February 2003

It is pointed out that spectrum reported by Graham, Dismuke, and Weltner at 1 μ and attributed by them to C2H is actually part of the Phillips system of C2.
Weak exchange in the Heisenberg linear chain: Structure and EPR of [N(n-Bu)4]2[Cu(mnt)2]
Authors: Plumlee Karl W. , Hoffman Brian M. , Ibers James A. , Soos Zoltan G.
Published: February 2003

We report magnetic and x-ray diffraction studies on a single crystal of [N(n-Bu)4]2[Cu(mnt)2], where mnt = maleonitriledithiolato (C4N2S2)−2. The planar paramagnetic (S=1/2) [Cu(mnt)2]−2 anions crystallize in stacks with crystallographically equivalent sites, and the stacks are isolated by the cations. The compound crystallizes in space group C1i-P1̄ of the triclinic system, with unit cell dimensions a=11.149(5), b=13.201(8), c=9.403(6) Å, α=105.44(5) °, β=114.92(5) °, and γ=72.09(9) °, with Z=1. Static susceptibility measurements show exchange to be weak. Single crystal EPR studies yield a rich variety of multilined spectra, in spite of the magnetically concentrated nature of [N(n−Bu)4]2[Cu(mnt)2]. Analysis of the EPR spectrum in terms of a one-dimensional exchange, J0=0.0107 cm−1, at 4.2 °K between successive [Cu(mnt)2]−2 ions along the c axis yields a surprisingly detailed picture of one-dimensional exchange in a hitherto unobserved, weak coupling regime. The temperature dependence of J0 and the angular dependence of the Cu hyperfine splitting K (ϑ) permit spectra with both J0<K (ϑ) and J0≳K (ϑ). In the former case, the magnetic inequivalence of [Cu(mnt)2]−2 sites with different nuclear spin projection leads to a complicated EPR spectrum that can be quantitatively simulated to yield the value of J0. In the latter case, a typical single, exchange-narrowed EPR signal is observed. A single theoretical description based on one-dimentional exchange describes all the angular and temperature dependences of the [N(n−Bu)4]2[Cu(mnt)2] EPR spectra.
Theory of vibronic interactions: The importance of floating basis sets
Authors: Johnson W. Curtis , Weigang Oscar E.
Published: February 2003

The theory of vibronic interactions in polyatomic molecules is investigated in the nondegenerate case and the importance of using a floating basis set when expanding the electronic eigenfunction as a function of nuclear position is demonstrated. Normal Herzberg–Teller theory using a fixed basis set considers only interactions with the vibrating nuclei. The physical meaning of the extra terms in Liehr’s formalism, which uses a floating basis set, is found to be interactions with electron distributions moving with the nuclei. Furthermore, it is shown that Liehr’s formalism can be simplified if electron coordinates are related directly to nuclear position. Although a floating basis set is used to develop our equations, only unperturbed zero order electronic functions for the equilibrium nuclear configuration need be known to carry out most computations involving vibronic wavefunctions in our new formalism. This simplifies calculations considerably.
Cross sections for reorientation and rotational relaxation of oxygen
Authors: Gardiner W. C. , Pickett H. M. , Proffitt M. H.
Published: February 2003

Experimental measurements of the collisional EPR linewidths of molecular oxygen are reported for 17 X-band transitions at resonance magnetic fields up to 0.7 T. In addition to the expected gradual decrease of linewidth with rotational quantum number, a dependence upon orientation quantum number is also observed. The EPR linewidths are comparable to the higher values among the microwave results previously reported for the 60 GHz transitions and substantially larger than the corresponding Raman data.
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o SCF and CI studies on the ground state of the water molecule. I. Comparison of CGTO and STO basis sets near the Hartree–Fock limit
Authors: Rosenberg Bruce J. , Shavitt Isaiah
Published: February 2003

A b initio self-consistent field and configuration interaction calculations have been carried out for the ground state of the water molecule near its experimental equilibrium geometry. Several contracted Gaussian (CGTO) and Slater type (STO) basis sets have been used, and the effects of variations in the type and size of the basis set on the SCF and CI energies and on computed first-order one-electron properties have been examined. The CI calculations included all single and double excitation configurations relative to the SCF function. The largest basis set studied, a 39-STO (5s4p2d/3s1p) basis, provided an SCF energy of −76.064226 hartree, which is about 3 mhartree above the estimated Hartree–Fock limit, and a CI energy of −76.339802 hartree, which accounts for 74.5% of the estimated total correlation energy. The SCF and CI one-electron property expectation values for this basis are generally within 5%–10% of the experimental values. Ionization potentials, binding energy, and pair-correlation energies have also been computed with the 39-STO basis.
Integral Hellmann–Feynman analysis of nonisoelectronic processes and the determination of local ionization potentials
Authors: Simons Gary
Published: February 2003

The integral Hellmann–Feynmann theorem is extended to apply to nonisoelectronic processes. A local ionization potential formula is proposed, and test calculations on three different approximate helium wavefunctions are reported which suggest that it may be numerically superior to the standard difference of expectation values. Arguments for the physical utility of the new concept are presented, and an integral Hellmann–Feynman analysis of transition energies is begun.
X-ray diffraction study of liquid germanium tetrachloride and liquid tin tetrachloride
Authors: Rutledge Carl T. , Clayton Glen T.
Published: February 2003

Electronic radial distribution functions for GeCl4 and SnCl4 in the liquid state at 23 °C have been calculated from x-ray diffraction intensity distributions obtained by use of theta–theta reflection diffractometry. Both liquids show intermolecular effects at distances equivalent to the Cl–Cl intermolecular distance. Results are compared with electron diffraction results for these molecules and with data from CCl4 and SiCl4 in the liquid state.
Quasiclassical selection of initial coordinates and momenta for a rotating Morse oscillator
Authors: Porter R. N. , Raff L. M. , Miller W. H.
Published: February 2003

The classical orbits of a rotating Morse oscillator are calculated by means of Hamilton–Jacoby theory after truncating the Hamiltonian to permit analytical solution. Except at very high J, the approximate analytic orbit for the radial coordinate is in good agreement with that obtained by numerical integration of the exact equations of motion. Bohr quantization gives an expression for the rotation–vibration energy correct through quadratic terms in (v+1/2) and J (J+1), where v and J are the vibrational and rotational quantum numbers, respectively. The principal result is an analytic prescription for obtaining values of the coordinates and momenta, given v, J, and a set of random numbers, that facilitates properly weighted quasiclassical selection of initial states of diatomic molecules in trajectory calculations.
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o and semiempirical SCF–MO computations for N+4; comparison of spin unrestricted and spin restricted methods
Authors: Conway D. C.
Published: February 2003

Various basis sets (all but one minimal) were used in spin unrestricted ab initio SCF–MO computations on N+4. Computed values of the N+4 dissociation energy, D (N2−N+2), ranged from 49–72 kcal/mole for linear N+4 at a fixed internuclear separation R of 2.00 Å. Smaller dissociation energies were obtained by a spin restricted ab initio method in which the α- and β-spin eigenvector matrices were forced to be the same. All dissociation energies are larger than the experimental bond energy, but the smallest value was obtained with the extended basis set by the spin restricted ab initio method. There De(N2−N+2) was found to be 32.7 kcal/mole at Re=1.90 Å. Minimal basis set semiempirical computations were found to give results similar to those obtained by ab initio computations.
Photophysics of C−2 (B 2Σ+u) in rare gas lattices: Vibrational relaxation through intermediate a 4Σ+u levels
Authors: Bondybey V. E. , Brus L. E.
Published: February 2003

Time and wavelength resolved fluorescence techniques are used to investigate C−2 (B 2Σ) radiationless transitions following pulsed, tunable laser excitation. The purely radiative lifetimes in each host are Ne, ?64 nsec; Ar, ?40 nsec; Kr, ≃38 nsec; and Xe, ≃34 nsec. The principal B 2Σ vibrational relaxation mechanism is a sequential process: crossing into a 4Σ, vibrational relaxation within 4Σ, and reverse crossing into B 2Σ. The observed intermediate lifetimes strongly suggest that v=0 4Σ lies between v=0 and 1 of B 2Σ; this fact allows assignment of gas phase B 2Σ↔a 4Σ rotational perturbations reported by Herzberg and Lagerqvist. The unperturbed a 4Σ constants are Te=19 448.4 cm−1, ωe=1074.04 cm−1, Be=1.1348 cm−1, ωexe=24.68 cm−1, and αe=0.0035 cm−1. Vibrational relaxation within a 4Σ, and intersystem crossing, show a strong multiphonon relaxation energy gap law. The variation in behavior among the 12–12, 12–13, and 13–13 isotopes indicates that the photophysics is dominated by accidental near degeneracies with A 2Π (and probably X 2Σ) levels in addition to 4Σ levels.
Approximate reaction rates and probabilities for a class of exothermic chemical reactions
Authors: Shipsey Edward J.
Published: February 2003

An approximate expression for the total rate constant for certain exothermic reactions is given. The approximation can be expressed simply in terms of integrals in a region about the saddle point. More complicated procedures for computing the total probability for reaction from a given reactant state are presented. This involves solution of a set of coupled differential equations in a restricted region.
Vibrational energy transfer rates for the CO–CH4, CO–CF4, and CO–SF6 systems
Authors: Richman Dennis C. , Millikan Roger C.
Published: February 2003

The vibrational fluorescence quenching method has been used to measure the temperature dependence of the CO(v = 1) collisional transfer rates with CH4, CF4, and SF6 between 100 and 350 K. The V–V energy transfer rates for the CO–CF4 system were found to be much faster than the CO–CH4 system even though the fundamental band center energy mismatch is greater for CO–CF4. Also, the CO–CF4 system exhibited an inverse temperature dependence, indicating near resonant processes possibly arising from a multiple quantum exchange.
The reaction H2O+(D2,D)H2DO+
Authors: Jones C. A. , Wendell K. L. , Koski W. S.
Published: February 2003

The reaction H2O+(D2,D)H2DO+ was studied over an energy range 0.75 to 69 eV using a scattering spectrometer. The results are consistent with previous measurements at higher temperature and it is concluded that the reaction proceeds via a direct mechanism down to energies below 0.14 eV. (AIP)
Comment on ’’Intermolecular energy transfer in liquid benzene: Raman spectra, linewidth measurements, picosecond spectroscopy, and vibrational relaxation times’’
Authors: Laubereau A.
Published: February 2003

Several comments are made to a recent paper by Griffiths, Clerc and Tentzepis. It is questioned that the picosecond technique applied in this paper provides information on the lifetime of the upper vibrational state.
The O Kα and C Kα emission and O K absorption spectra from O2 and CO2. IV
Authors: LaVilla Robert E.
Published: February 2003

The oxygen and carbon Kα emission and oxygen K absorption spectra of the molecular gases O2 and CO2 are reported. The spectra were obtained on two different spectrometers, both with single flat crystal configurations. All the emission spectra were excited by direct electron bombardment and are in fair agreement with an interpretation based on allowed electric dipole transitions between single vacancy states and molecular orbital theory for the molecules. The oxygen K absorption of O2 and CO2 is dominated by a strong resonance peak before the K-shell threshold which is interpreted as being due to the transition of oxygen K electrons to the first unoccupied valence orbital of the respective molecules.
Auger analysis of thin oxide films on Pb–In alloys
Authors: Chou N. J. , Lahiri S. K. , Hammer R. , Komarek K. L.
Published: February 2003

The composition and stability of thin oxide films (?35 Å), thermally grown on Pb–In alloy substrates at room temperature and various oxygen pressures, were studied quantitatively by Auger electron spectroscopy. The results indicate that all oxide films are enriched in indium in varying amounts, depending on the alloy composition and oxygen pressure. For alloys contaning more than 10 at.% In, the surface layer of the oxide consists of pure In2O3 only; for alloys of lower In concentrations a mixture of PbO and In2O3 is formed. The mixed oxide appears to be unstable since the Auger data monitored during vacuum storage are indicative of the reduction of PbO with attendant formation of additional In2O3 occuring near the alloy–oxide interface.
Metric geometry of equilibrium thermodynamics. III. Elementary formal structure of a vector-algebraic representation of equilibrium thermodynamics
Authors: Weinhold F.
Published: February 2003

The thermodynamic geometry established in an earlier paper is taken as the basis for an abstract vector-algebraic representation of equilibrium thermodynamics. In this representation, thermodynamic field variables appear as abstract Euclidean vectors whose lengths and internal angles describe the equilibrium properties. A variety of thermodynamic indentities, stability conditions, and other relationships are derived and shown to have simple and natural geometric significance in the new framework. The geometric viewpoint is also found to suggest certain lines of development—such as the use of self-conjugate (’’normal’’) field variables—with no obvious counterpart in the traditional differential formalisms. The formal ideas are illustrated throughout with elementary applications to properties of a simple homogeneous fluid.
Boundary layer technique applied to the master equation which describes the dissociation and recombination of diatomic molecules in inert diluents
Authors: Hogarth W. L. , McElwain D. L. S.
Published: February 2003

The master equation developed by Brau in his formal stochastic theory to describe dissociation, recombination, and internal relaxation processes of a diatomic gas by inert third bodies is approximated, for lower orders, by the use of the boundary layer technique. This method shows that there are two characteristic time scales in the problem instead of the infinite number introduced by Brau. On the first-time scale, a fast phase, the solution explicitly shows the nature of the internal relaxation processes involving a redistribution of molecules and indicates only a small change in the number of atoms. The second-time scale, a much slower phase, characterizes dissociation and recombination and the solution explicitly shows the approach to equilibrium of the atom concentration and the small effect of dissociation or recombination on the internal relaxation processes.
Determination of solute properties by solvent vapor pressure measurements. II. Simple electrolytes, polyelectrolytes, and complex ions
Authors: Amdur S.
Published: February 2003

A recently developed empirical method for the determination of solute parameters by solvent vapor pressure measurements is further explained and demonstrated. The results of our laboratory and those of two other laboratories have shown that the osmotic coefficient of water is practically constant over the range 0.02–0.1m. This is consistent with the above-mentioned method. Examples are given for solutions of simple electrolytes, polyelectrolytes, and complex ions.
Spin–lattice relaxation in coupled three spin systems of the A
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Authors: Fagerness Paul E. , Grant David M. , Kuhlmann Karl F. , Mayne Charles L. , Parry R. Bryce
Published: February 2003

Relaxation in coupled three spin systems of the AIS type is described by an extension of the two spin case treated by Solomon. The proper linear combinations of spin level populations are presented as normal mode variables in a system of coupled differential equations so that maximum reduction in the number of differential equations is achieved. Decoupling experiments further reduce the AIS problem to a set of equations similar to those encountered in the two and one spin cases. The formalism includes in a natural way dipolar relaxation effects due to cross-correlated motion of internuclear vectors. A theoretical expression is derived to describe these effects. In the normal mode variables these terms group together and relate total magnetization variables with a multiplet relaxation mode. Experimental results on the three spin system 13CHFCl2 are presented in terms of the above formalism.
The even–odd effect in liquid crystals. A simple model
Authors: Pink D. A.
Published: February 2003

We have developed a model of molecules composed of rigid segments and hydrocarbon chains which interact via van der Waals forces, and it has been used to study the even–odd effect in the nematic–isotropic transition in liquid crystals. Because it is mathematically simpler than other models which have examined this effect, analytical results can be obtained in a molecular field approximation. It shows explicitly the reasons for certain peculiarities observed and identifies the causes of the even–odd effect in the transition temperature and rigid segment order parameter. Finally, the transition temperatures calculated agree more closely with experiment than those obtained by other theories.
The complete structure of isoxazole from naturally occurring isotopic forms by double resonance modulated microwave spectroscopy
Authors: Stiefvater Otto L.
Published: February 2003

Rotation spectra of isotopic forms of isoxazole (ONCHCHCH) have been studied in their natural abundance by double resonance modulated microwave spectroscopy. Earlier data have been refined and extended to include the three deuterium forms, also in natural abundance. From the eight isotopic species the complete structure of isoxazole has been determined. Bond distances (in Å) are O(1) –N(2) =1.399±0.001, N(2) –C(3) =1.309±0.002, C(3) –C(4) =1.425±0.002, C(4) –C(5) =1.356±0.001, C(5) –O(1) =1.344±0.001, C(3) –H(3) =1.077±0.001, C(4) –H(4) =1.074±0.001, and C(5) –H(5) =1.075±0.002. The ring angles, beginning at the oxgyen atom, are (uncertainties <0.2°) 108.8°, 105.3°, 112.3°, 103.0°, and 110.6°; the angles between the CH- and adjacent ring bonds are C(4)C(3)H(3) =129.1±0.3°, C(3)C(4)H(4) =128.5±0.4° and C(4)C(5)H(5) =133.4±0.5°. During the course of the structure study, satellite spectra due to each of the eleven bending vibrations of isoxazole were assigned and correlated with ir work through relative intensity measurements. The reassignment of the lowest in-plane bending mode as ν13=844 cm−1 is suggested. On the basis of the results on isoxazole and other molecules the likely impact of the DRM technique on structure determinations by microwave spectroscopy is considered.
The far infrared absorption spectrum of N2 in the gas and liquid phases
Authors: Buontempo U. , Cunsolo S. , Jacucci G. , Weis J. J.
Published: February 2003

The collision induced spectrum of N2 has been measured in the gas phase at T=300 °K and T=124 °K and in the liquid phase at T=66 °K. The measurements at room temperature have been extended down to 8 cm−1. It has thus been possible to observe the translational component. From the analysis of the band shape the single rotational profile has been derived. The integrated absorption coefficients in the gas phase and the moment analysis of the spectrum are in agreement with the quadrupolar induction mechanism. The presence of cancellation effects has been deduced from the intensity of the spectrum measured in the liquid phase. The absorption coefficient was also obtained from molecular dynamics calculations and compared with the experimental one. Because of the correspondence of their shape we have concluded that also in liquid phase the spectrum is essentially due to the quadrupolar induction mechanism.
Kinetic theory of the coupling of thermal radiation with a rarefied diatomic gas
Authors: Saraf J. R.
Published: February 2003

A kinetic model is proposed for the coupling of a radiation field with a diatomic gas with nondegenerate internal energy states. The contributions of molecular and radiative momentum transport to the model equation are included. The model obeys H theorem, conserves the number of molecules, and satisfies the conservation laws of momentum and energy for the isolated system of photon and molecular fluids.
Continuum spectra and potentials of Li–noble gas molecules
Authors: Scheps R. , Ottinger Ch. , York G. , Gallagher A.
Published: February 2003

The normalized emission spectrum of the Li(2p−2s) resonance transition has been measured in the presence of 10–1000 Torr of noble gas. Fluorescence at 100–3000 Å from the atomic line, due to the A–X and B–X bands of Li–noble gas molecules, has been measured as a continuum with 1.5 nm resolution. The lithium is optically excited in a 670 °K cell. The lithium is optically thin (∼10−6 Torr) so that the continuum emission per excited lithium atom is obtained from the normalized emission. The attractive A-state potentials for Li–Ar, Kr, and Xe have been constructed by analysis of the A–X band spectra in the limit of high and low noble-gas densities. At high densities each spectrum is due to an equilibrated vibrational distribution of A-state molecules, whereas at low densities only free collision states contribute. The ratio of the two spectra thus depends on the excited state potential and has been used to determine this potential where it is attractive. Absorption and stimulated emission coefficients for Li–noble gas vapors are also deduced from the data.
The residual entropy of rectilinear trimers on the square lattice at close packing
Authors: Van Craen J.
Published: February 2003

A technique equivalent to the matrix method of Kramers–Wannier is used to compute the residual entropy per particle on semi-infinite strips of width n, completely covered by non-overlapping rectilinear trimers. This thermodynamic quantity is directly related to the largest eigenvalue of a matrix M which has been constructed for two kinds of boundary conditions: periodic ones and simply edges. In the latter case we also have computed the number of distinct arrangements of trimers on close-packed n×n squares. All calculations have been carried out with equal relative activity for horizontal and vertical trimers. For the semi-infinite systems this results in a somewhat different mean density for the two types of molecules. The thermodynamic limit has been approached in three totally independent ways by extrapolating three series of data: the residual entropy per trimer on a ∞×∞ square has been determined as SR/k = 0.4754±0.0001.
The molecular Zeeman and hyperfine spectra of LiH and LiD by molecular beam high resolution electric resonance
Authors: Freeman Richard R. , Jacobson Abram R. , Johnson David W. , Ramsey Norman F.
Published: February 2003

The molecular Zeeman spectra of LiH and LiD in the 1Σ ground electronic state have been measured in the first rotational state (J=1) and zeroth vibrational state (v=0) by molecular beam electronic resonance. By fitting the observed resonance lines of several different magnetic fields to an appropriate Hamiltonian, values for the rotational g factor (gJ), the anisotropies of the nuclear shieldings (σTH,σTLi), and the anisotropy of the molecular magnetic susceptibility (ξT) were measured. The results are     In order to fit the observed magnetic field spectra, accurate values of the hyperfine constants were required. The hyperfine spectra of several (J, v) states of LiH and LiD were measured. The results are     Numbers in parentheses are the uncertainties in the final figures.
Photostimulated reactions of isolated and paired radicals in solid hydrocarbons
Authors: Sprague E. D. , Willard J. E.
Published: February 2003

Ultraviolet radiation absorbed by radicals trapped in γ-irradiated 3-methylpentane (3MP), 3-ethylpentane (3EP), and 3-methylheptane (3MHp) glasses and in polycrystalline n-hexane at 77 K causes a major improvement in resolution of the ESR spectra without change in the radical concentration, indicating that the energy of the absorbed photons changes the nature, the configuration, or the environment of the radicals. The new spectral shape slowly reverts to the old in the dark at a rate which is dependent on the matrix and the temperature. Ultraviolet irradiation also accelerates the decay of paired radicals and other radicals which are removed by intraspur processes, but not of those which react by random encounter after diffusion. This effect is attributed to localized heating of the matrix in the immediate vicinity of a radical which has absorbed a photon. Secondary 3-ethylpentyl radicals in glassy 3EP undergo spontaneous conversion to an isomer or change to a new rotamer configuration in the dark and the process is reversed by exposure to uv light. The ΔMs=2 ESR spectrum of radical pairs in γ-irradiated 3MHp has been observed.
Absolute elastic differential electron scattering cross sections in the intermediate energy region. I. H2
Authors: Srivastava Santosh K. , Chutjian A. , Trajmar S.
Published: February 2003

A new method for measuring absolute electron-impact differential cross sections (DCS) for elastic scattering in gases is described. The technique uses a well-defined crossed-beam scattering geometry in which the relative target densities of an unknown and a secondary standard gas are accurately determined by a calibrated flowmeter and a pressure gauge. The method is applied to the measurement of elastic scattering from H2 (which, in the present work, also includes rotational excitation) relative to elastic scattering from He, the secondary standard. The relative DCS for H2 are then placed on the absolute scale by using recently measured absolute elastic DCS for He. Measurements for H2 are reported at incident electron energies of 3–75 eV, and in the angular range 20°–135°. Differential and integral cross sections at each energy are compared with previous measurements and recent theoretical calculations.
High resolution analysis of collision-induced N+2 spectrum
Authors: d’Incan Jean , Topouzkhanian Ari
Published: February 2003

A new band system ascribed to the N+2 molecule has been observed in the 3820–3920 Å region from hydrogen ion bombardment of nitrogen molecules. Particle energies up to 30 keV were displayed, at typical target pressures of 10−3–10−2 torr. Beam currents as high as 25 μA have been measured. The emission proceeds from a 4Σ+u→X 2Σ+g transition. Rotational analysis of the (0,0) and (1,1) bands leads to the following molecular constants derived by least-squares technique: Te=25 467 cm−1, ωe=2398 cm−1, ωexe=14 cm−1, Be=2.071 cm−1, αe=0.014 cm−1, re=1.078 Å, ν00=25 563.26 cm−1, and 3ε=0.69 cm−1.
Dense liquids. I. The effect of density and temperature on self-diffusion of tetramethylsilane and benzene-d6
Authors: Parkhurst H. J. , Jonas J.
Published: February 2003

Using the NMR spin-echo technique the self-diffusion coefficients of liquid tetramethylsilane and benzene-d6 have been measured as a function of temperature and pressure over the density range 2.5⩽ρ/ρc⩽3.5 and the temperature range 0.5⩽T/Tc⩽0.8. Densities have also been determined under the same experimental conditions. The experimental self-diffusion coefficients are successfully interpreted in terms of the rough hard sphere model of liquids based on the modified Enskog theory. In order to obtain a good agreement between the experimental and predicted values of D, the temperature dependence of the effective hard core diameter is assumed. The deviation of the experimental diffusion constants from those predicted from the hard sphere model at high densities (ρσ3≳0.93) is of particular interest. This discrepancy suggests a possible density dependence of the effective hard-core diameter for highly compressed liquids; or alternatively, the rough hard sphere theory begins to break down at densities for which the hard sphere fluid is metastable.
A model potential for chemisorption: H2+W(001)
Authors: McCreery Jane Hylton , Wolken George
Published: February 2003

A potential function to describe the interaction of a diatomic molecule with a solid surface is formulated as a modified LEPS potential. The surface is approximated by a rigid background potential, periodic in the plane of the surface. It is assumed that the individual atom–atom and atom–surface potentials are known and can be reasonably approximated by Morse functions. H2+W (001) is presented as an example. One adjustable Sato parameter is introduced and by varying this parameter a wide range of potentials can be generated. The functional form is sufficiently simple so that classical trajectory studies of the reaction dynamics are feasible. Several illustrative trajectories are presented.
Ultrasonic studies of liquids using hole theory
Authors: Narasimham A. V. , Manikiam B.
Published: February 2003

Hole theory of the liquid state has been used for the study of the propagation of ultrasonic waves in some organic liquids. Expressions are obtained for the ultrasonic absorption and dispersion and their variation with temperature and pressure using the concept of holes in a liquid. Available experimental data on the temperature and pressure variation of ultrasonic absorption and dispersion in liquids has been used to verify the validity of the aforesaid expressions. It has been found that the expressions could explain the variation of velocity and absorption in associated, polyatomic, and viscous liquids fairly well. However, the applicability of these equations to nonassociated liquids, especially variation of their absorption with temperature, has been found to be quite poor.
Lifetimes for OH and OD electronic states with resonance transitions in the region between 1700 and 1950 Å
Authors: Smith Wm. Hayden , Stella Gregory
Published: February 2003

The upper states of the OH and OD bands in the 1700–1950 Å region have been studied with our phase-shift radiative lifetime apparatus. We have found lifetimes varying from 1.8±0.2 to 2.9±0.4 ns. Lifetimes for OH and OD are essentially the same, indicating that the upper states are likely to be depopulated via purely radiative processes under our experimental conditions. Lifetimes have also been obtained for the C 2Σ+–A 2Σ+ transition in OH and OD. Our results are consistent with the preliminary analysis of Pryce (private communication) that the 1700–1950 Å bands in OH are partly due to the C 2Σ+–X 2Πi transition.
A spectroscopic study of fluorescence from the second excited singlet state of thiophosgene vapor
Authors: Oka T. , Knight A. R. , Steer R. P.
Published: February 2003

The S2–S0 absorption, emission, and excitation spectra of thiophosgene vapor have been measured and the observed single vibronic transitions assigned. Comparison of the absorption and excitation spectra reveals that emission is excited only when transitions to the upper state terminate in 3m4n, m or n=0 or 1, and that the upper state is depopulated by rapid radiationless processes when higher vibrational levels are populated. The most prominent bands in the emission spectrum are those in which the transitions terminate in 1p3q4r with 1⩽p⩽10, 1⩽q⩽5, and 1⩽r⩽4. All three types of spectra are consistent with a pyramidal excited state in which the equilibrium internuclear distances are substantially greater than those of the ground state. The implications of this fluorescence ’’anomaly’’ with respect to the nature of radiationless processes in the thiophosgene are discussed.
Determination of kinetic parameters by the frequency dependence of the NMR coalescence temperature
Authors: Gutowsky H. S. , Cheng H. N.
Published: February 2003

A method is described in which one determines the rate parameters for chemical exchange between two equally populated uncoupled sites by measuring the dependence upon NMR resonance frequency of the temperature at which the corresponding two high-resolution lines coalesce. The method is relatively simple and, when applicable, gives good accuracy with modest effort. It was tested by observing the coalescence of the methyl group resonances due to internal rotation in neat dimethyl-formamide, -acetamide, and -propionamide, at 60, 100, and 220 MHz. The results include an entropy of activation ΔS‡=0±0.2 e.u. for the three compounds, and an enthalpy, ΔH‡ of 20.9, 18.0, and 17.1±0.1 kcal/mole, respectively, where the errors indicate the approximate accuracy of the results.
Ambiguities in the interpretation of proton magnetic relaxation data in water solutions of Gd3+ ions
Authors: Koenig Seymour H. , Epstein Michael
Published: February 2003

We report measurements of the solvent proton spin–lattice relaxation rate 1/T1 in a water solution of GdCl3, pH 5.5, at 5, 15, and 25 °C. The data were obtained as a function of magnetic field, from 5 Oe to 12 kOe, corresponding to proton Larmor frequencies in the range 20 kHz to 50 MHz. The data are compared with theory, and it is shown that a satisfactory fit may be obtained for a range of values of the parameters of the theory. We give the results of a least squares fit of the theory to the data for two extremes of the range, one limit corresponding to the condition that the residence lifetime τM of a proton in the first hydration shell of the Gd3+ aquoion be comparable to the spin–lattice relaxation time T1M of the proton on the ion, the other limit corresponding to τM≪T1M. It is shown that knowledge of the temperature dependence of 1/T1 cannot distinguish between the two limits, and indeed, that unless a value for τM is obtained from independent experiments, the ambiguous results cannot be resolved. The source of the ambiguities is discussed in some detail since it is particularly clear in the present system, but considerably less so in systems containing complexes of macromolecules with paramagnetic ions, to which the present experiment is pertinent. It is also pointed out that, while there are many references to values for τM in the published literature, no experiments have been published that report values of τM for protons on the Gd3+ ion.
Energy transfer and spectral line shapes of impurities in crystals
Authors: Abram I. I. , Silbey R.
Published: February 2003

The problem of electronic energy transport is solids in discussed. A model system of two molecules interacting with a lattice of phonons is considered. The spectral line shapes and energy transport are treated in various limiting cases, and the connection between these two phenomena is discussed.
127I Mössbauer studies of thyroid compounds
Authors: Oberley Larry W. , Ehrhardt James C.
Published: February 2003

Mössbauer effect absorption spectra have been observed with 127I in the thyroid compounds MIT, DIT, T3, and T4. DIT and T4 were found to have similar spectra. The spectrum from MIT was statistically different from DIT and T4. T3 had a spectrum intermediate between MIT and DIT, although not statistically different from either. The effective charge on the iodine nucleus was more positive in MIT than in the other three compounds. All of the compounds exhibited a small s hybridization.
A Monte Carlo method for obtaining the interionic potential of mean force in ionic solution
Authors: Patey G. N. , Valleau J. P.
Published: February 2003

A Monte Carlo method for obtaining the solvent-averaged interionic potential of mean force is described. The potential of mean force is obtained for two charged hard spheres immersed in a dipolar hard sphere solvent. The ions and solvent particles have the same hard sphere diameters and the ions bear single charges of opposite sign. The solvent particles are characterized by a reduced dipole moment μ*=1.0 which corresponds to a dielectric constant ε?7.8. The Monte Carlo result is compared with the ’’primitive model’’ and other approximations which have been suggested for the potential of mean force. These approximations are all found to be inadequate for small ionic separations. Some implications with respect to ’’ion pairing’’ are mentioned.
Matrix reactions of alkali metal chloride salts and HCl and DCl: Infrared spectra of the M+HCl2− and M+DCl2− ion pairs
Authors: Ault Bruce S. , Andrews Lester
Published: February 2003

The matrix reactions of alkali metal chloride salts and HCl and DCl have been investigated in argon matrices, as well as the reaction of lithium metal with HCl and DCl. From each reaction product a single intense band was observed which has been assigned to the antisymmetric stretch ν3 of the HCl2− and DCl2− anions in the M+HCl2− and M+DCl2− ion pairs. Comparisons to literature spectra, in conjunction with quartic deuterium shifts, indicate that the anion has a linear, centrosymmetric geometry in the isolated ion pair. A rough estimate for the bond strength of the hydrogen bond in this anion is 18±2 kcal/mole. Further, comparison to the spectra of earlier workers indicates that bands previously asigned to the HCl2 radical are best reassigned to the isolated HCl2− anion.
Hyperfine structure in the red emission system of NbN
Authors: Femenias J.-L. , Athenour C. , Dunn T. M.
Published: February 2003

A detailed study of hyperfine structure observed in the 0,0 band of the 3Φ (aβ) →3Δ (aβ) system of NbN has shown that the secondary hyperfine effects observed in this structure are not due to a perturbation in the 3Δ state but rather to the presence of a nonnegligible hyperfine effect in the 3Φ excited state. Calculations of the intensity factors corresponding to the observed transitions confirm this hypothesis and give results in excellent agreement with experiment.
The dynamical structure factor S (Q,ω) of solid α-N2
Authors: Weis J. J. , Klein M. L.
Published: February 2003

We report the first results of a classical computer simulation (molecular dynamics) study of the dynamical structure factor S (Q,ω) for a simple model of a homonuclear diatomic solid in the α-N2 structure. For a particular wave vector Q the peaks in S (Q,ω) can be identified with phonons and their associated widths as inverse lifetimes. The model consists of 250 molecules arranged with periodic boundary conditions in the cubic Pa3 structure and interacting via a Mie–Lennard-Jones (12–6) potential between their ends. Two runs were carried out with parameters such that for solid α-N2 they corresponded to the conditions (T=17 K, a=5.65 Å) and (T=35 K, a=5.684 Å). In neither case do the phonon peaks agree well with the predictions of quasiharmonic lattice dynamics. Moreover, at the higher temperature many of the S (Q,ω) peaks show a large quasielastic peak which we interpret as being because of ’’rotational diffusion’’ of the molecules.
Configuration and interstitial relaxation processes
Authors: Freed Karl F. , Fong Francis K.
Published: February 2003

A theory of orientational and interstitial relaxation processes is presented in which the phenomena are characterized as transitions between nonstationary zeroth order configurations. A number of different mechanisms are separated according to the relative timescales of the libron (interstitial) motion and relaxation and the phonon motions. These mechanisms lead to differing temperature dependences that should be amenable to experimental verification. The full (many) phonon participation is included and gives a polaronlike character to the relaxing libron (interstitial). The present theory of activated rate processes departs from the conventional transition state theory. The concept of the activation energy barrier to relaxation is seen to originate from an ensemble average of nuclear tunneling rates from the individual librational levels.
Vibration–vibration energy transfer in HF dimers
Authors: Shin Hyung Kyu
Published: February 2003

A theory of vibration–vibration energy transfer is developed for hydrogen fluoride molecules based on the model of energy transfer through the interaction of H of one HF molecule with F of the other in a nonrigid hydrogen-bond dimer. The theory is applied to HF(n)+HF(0) →HF(n−1)+HF(1)+ΔE for values of the vibrational quantum number up to 5. Calculations at 300 °K show the rapid energy exchange for these processes. Particularly fast rates are found for the collisions involving high vibrational states with small values of the energy mismatch. Reasonable agreement between the present theory and experiment is found for the deactivation processes with n=2, 3, 4, and 5.
Scattering of high-velocity neutral particles. XVIII. He– (CH3F, CH2F2, CHF3)
Authors: Amdur I. , Longmire M. S. , Jordan J. E. , Mason E. A.
Published: February 2003

Fast He beams have been scattered by room-temperature CH3F, CH2F2, and CHF3 to obtain the following average potentials (energy in eV and separation in Å): He–CH3F, V (r) =3.86×102/r8.34, 2.00<r<2.47; He–CH2F2, V (r) =3.88×104/r13.33, 2.22<r<2.58; He–CHF3, V (r) =4.06×105/r15.43, 2.33<r<2.63. Where they overlap, these results agree with independent measurements by Fink, King, and Freeman. The peripheral-force approximation, which assumes that the centers of force in a molecule are located at the nuclei of each atom, has been used to calculate potentials for the above systems from previous measurements on He–CH4 and He–CF4. The calculated potentials are in only fair agreement, being systematically high. Sources of the discrepancies are discussed.