bluuebunny/crossref_metadata_2025 huggingface dataset:
Potential Curves for the Valence-Excited States of Silicon Monoxide. A Theoretical Study
Authors: Heil Timothy G. , Schaefer Henry F.
Published: February 2004

Ab initio quantum mechanical calculations have been carried out at 10 internuclear separations for those 72 molecular states of silicon monoxide which dissociate to a Si atom in 3P, 1D, 1S, or 5S state plus an oxygen atom in 3P, 1D, or 1S state. Full configuration interaction calculations were made from a minimal basis set of Slater-type orbitals, with the restriction that the core orbitals (1s, 2s, 2p Si and 1s O) were fully occupied. The results were strikingly similar to those obtained for CO in an analogous theoretical study. Ten bound states were found to dissociate to ground state Si plus ground state O; five of these states have been observed experimentally in SiO, but eight have experimentally known counterparts in CO. The predicted ordering of states is X 1Σ+, a 3II, a′ 3Σ+, 3Δ, e 3Σ−, I 1Σ−, A 1II, 1Δ, 5Σ+, and 5II. Molecular orbital configurations are assigned to each of these 10 states. Among 10 higher predicted bound states, the 1II III state, with calculated De=1.32 eV, is perhaps the most likely to be observed experimentally. The tentative assignment of the I 1Σ− state by Verma and Mulliken is considered unlikely on the basis of the present calculations. The A 1II potential curve is very flat as it approaches the dissociation limit, but, unlike CO, there is no maximum in this curve for SiO.
Chemical Lasers from the Reactions of ClF and ClF3 with H2 and CH4: A Possible Chain-Branching Chemical Laser
Authors: Krogh Ole D. , Pimentel George C.
Published: February 2004

Both HF and HCl chemical laser emission have been observed from the flash photolysis of ClF–H2–Ar and ClF3–H2–Ar mixtures. In contrast, only HF laser emission is produced by flash photolysis of ClF–CH4–Ar or ClF3–CH4–Ar mixtures. For the ClF–H2 system, the presence of 3→2 HF transitions and the appearance of HCl transitions show that the H+ClF reaction divides to form very roughly equal amounts of HCl and HF. For HCl, equal–gain temperature measurements place k2/k1 in the range 1.2–3.8. The ClF3–H2 chemical laser is extremely intense, but it displays much lower vibrational excitation of HF than that of ClF–H2 or of ClF3–CH4. There is evidence that chain-branching reactions occur, which enhances the potentiality of the ClF3–H2 system as a possible high-power, continuously operating chemical laser.
Pseudopotential Theory of Atoms and Molecules. II. Hylleraas-Type Correlated Pair Functions for Atoms with Two Valence Electrons
Authors: Szasz Levente , McGinn George
Published: February 2004

The method for the calculation of correlated pair functions developed in the first part of this series is applied to the calculation of Hylleraas-type functions for the valence electron pair of the atoms: Li−, Be, B+, C2+, Na−, Mg, Al+, Si2+, K−, Ca, Rb−, Cu−, Zn, and Ag. − Six-parameter expansions are computed for all these atoms and 15- and 20-parameter expansions are computed for Li− and Be. Using the results the first ionization potentials are computed and are found to be in good agreement with previous ab initio calculations and with the empirical values.
Lattice Dynamics and Spectroscopic Properties by a Valence Force Potential of Diamondlike Crystals: C, Si, Ge, and Sn
Authors: Tubino Riccardo , Piseri Luigi , Zerbi Giuseppe
Published: February 2004

Lattice dynamics and the vibrational spectra of diamond and diamondlike crystals of the elements of Group IV (Si, Ge, and gray Sn) are treated on the basis of a valence force potential. Experimental phonon dispersion curves from the literature are satisfactorily fitted by a six-parameter valence force potential. The one- and two-phonon absorptions are compared with the calculated density of states g(v) and with the density of combined states J(v). The results are satisfactory. The success in reproducing the spectroscopic properties of all four elements with only these parameters is consistent with the highly directional character of the covalent bonds in the crystal and the lack of ionic character. The different information derived from the widely used shell model and the valence force potential are briefly discussed.
Harmonic Wavenumbers and Normal Coordinates for Pyramidal Molecules of the Type XYZ2 (Y is N, P, As, Sb; X, Z are H, D, T)
Authors: Whitmer John C.
Published: February 2004

Using a harmonic general valence force field previously determined for NX3, PX3, AsX3 and SbX3 (X is H, D, T), the harmonic wavenumbers and normal coordinates have been calculated for pyramidal molecules of the type XYZ2 (Y is N, P, As, Sb; X, Z are H, D, T). For the nitrogen molecules, harmonic wavenumbers also were obtained for the nitrogen-15 isotope and hypothetical molecules in which X and Z take on masses of 6, 20, and 1000 amu. The wavenumbers for the nitrogen molecules, XNH2, XND2, and XNT2 over the range X=1–1000 amu, and HNZ2, DNZ2, and TNZ2 over the range Z=1–1000 amu are plotted.
Uses of Self-Focusing and Dispersion Effects in Time-Resolved Picosecond Spectroscopy
Authors: Topp M. R. , Rentzepis P. M.
Published: February 2004

Experiments with single picosecond pulses have shown certain interesting effects in the transmission of pulses through various Kerr-active liquids, which prove useful in the study both of the laser pulses and of the liquid media. A technique is described for the accurate measurement of single-pulse intensities using three-photon fluorescence and the self-focusing effect. Using this technique it is possible to derive values of the effective Kerr constant for picosecond pulses. It is observed that the duration of the induced birefringence is not governed solely by the Debye reorientation time but also by other faster (·10−13-sec) mechanisms such as electron polarization or molecular ``rocking.'' The experimental results show that nitrobenzene and other molecules with relatively long Debye relaxation times have, under certain conditions, birefringence recovery times comparable to that of CS2. The experimental observations show that complete self-focusing is experienced by only a small portion of a laser pulse.
Collisional Relaxation Processes for the n=3 States of Helium. I. Excitation Transfer by Normal Atoms and by Electrons
Authors: Wellenstein H. F. , Robertson W. W.
Published: February 2004

The positive column of a glow discharge through helium served as a reactor where each of the n=3 states of helium was selectively modulated by absorption of radiation. By monitoring the in-phase component of radiation from adjacent n=3 states over a range of discharge currents and pressures, rate constants were determined for transfer of excitation between n=3 states by collision with normal atoms and with electrons.
Collisional Relaxation Processes for the n=3 States of Helium. II. Associative Ionization
Authors: Wellenstein H. F. , Robertson W. W.
Published: February 2004

Rate constants for the associative ionization reaction He * + He → He 2 + + e have been measured, where He* represents the n=3 states of helium. By line absorption from the metastable 21S and 23S states, the n=3 states in the positive column of a dc discharge in helium were selectively and periodically modulated. Using phase-sensitive detection, the changes in excited-state populations were determined spectroscopically while the corresponding in-phase variations in the concentration of He 2 + were measured with a quadrupole mass filter. The cross sections for associative ionization in units of 10−16 cm2 were determined to be: σ (3 3 P) = 1.6 ± 0.1, σ (3 1 P) = 3.1 ± 1, σ (3 3D) = 4.5 ± 0.5, and σ (3 1D) = 20 ± 4. σ (3 3S) and σ(31S) were apparently zero.
Population and Predissociation of Vibronic States of Nitric Oxide
Authors: Bubert H.
Published: February 2004

In a crossed beam experiment nitric oxide molecules are excited by electrons. From the observed emission of the γ bands (A 2Σ + → X 2Π) the mean electronic transition moment R̄e and the population of the vibrational levels v′=0, 1 and 2 of the A 2Σ+ state are determined. The population numbers are compared with values calculated from kinetic equations. Relative rate constants are given corresponding to the transitions D  2Σ +, v′ = j → A  2Σ +, v″ = j with j=1 and 2. From this it can be concluded that a predissociation occurs in the D  2Σ +, v′ = 2 state.
van der Waals Forces between Two Spheres
Authors: Mitchell D. J. , Ninham B. W.
Published: February 2004

The van der Waals free energy of interaction between two spheres is evaluated by an extension of van Kampen's method. For close separation the zeroth approximation is the Lifshitz result for the interaction between two planes modified by a geometric factor. In next order of approximation the free energy exhibits distinctively new features due to nonadditivity of molecular forces in condensed media interactions. The formulas coincide with earlier expressions of Langbein at large distance, but disagree at close separation since his results here are valid only for dilute media interactions.
Ion—Molecule Reactions in the System CO2–CH4
Authors: Kasper S. F. , Franklin J. L.
Published: February 2004

Ion—molecule reactions in the system CO2–CH4 have been studied at pressures 0.1–0.4 torr in the ion source of a quadrupole mass filter. By employing mobilities to complete the residence time of ions in the source, resonable values of the rate constants for several reactions have been determined. The values are somewhat higher than similar reaction rate constants for two of the reactions determined at low pressures. In one instance, the reaction apparently goes to equilibrium, and from the equilibrium constant a reasonable value for the heat of reaction in close agreement with literature values was determined.
Deuteron NMR in Formic and Acetic Acid Single Crystals
Authors: Adriaenssens Guy J. , Bjorkstam John L.
Published: February 2004

The deuteron quadrupole coupling constants and asymmetry factors in single crystals of DCOOH and HCOOD are found to be 161± 2 kHz, η=0.040± 0.025 and 165.1± 2.7 kHz, η=0.125± 0.030, respectively. These are compared with free molecule and single crystal results of others on the formate group. In addition, attention is called to influence of the X-fragment electronegativity upon the deuteron coupling constant in C–DX compounds with sp2 hybridization. The temperature independent CD3COOH results, 58± 2 kHz, η ≤ 0.1, indicate rapid hindered rotation of the CD3 group, with correlation time τ ≲ 2(10−8) sec, above 63°K. This result is shown to be in good agreement with the only other single crystal result available for sp3 hybridized deuterocarbon compounds, i.e., malonic acid.
Binary Collision Broadening: Approximate Methods
Authors: Reck G. P. , Hood R. J.
Published: February 2004

The difficulties associated with the determination of potential parameter estimates for excited electronic states from the shift and width of isolated electronic spectral lines within the impact limit are discussed and analyzed. Using the LJ potential as a model, the measured shift and width of the He–Kr system are compared to exact theoretical calculations generated from quantum and JWKB phase shifts over an extended velocity region. A detailed, but easily understood, velocity dependence is found. The effect of thermal averaging is discussed, and two new approximations using variants of the Massey—Mohr and stationary-phase approximation are developed and compared to the exact results. The relationship of the exact calculations, the new approximations, thermal averages, and many of the previously used approximations is examined in detail.
Electron Spin Resonance Study of Metal Ion Separations in Dimeric Copper (II) and Vanadyl Porphyrins
Authors: Boyd P. D. W. , Smith T. D. , Price J. H. , Pilbrow J. R.
Published: February 2004

Electron spin resonance measurements have been made on a number of copper (II) and vanadyl porphyrins. The spectra recorded at a sample temperature of 77°K have low field components attributed to ΔM=2 transitions which provided evidence for the existence of dimer species in solution. In one case, namely copper hematoporphyrin (IX) dimethylester, a resonance centered at 950 G is thought to be due to a ΔM=3 transition arising from a trimer species. The theory of the ESR spectra of dipolar coupled S=12 transition metal ions with particular reference to systems involving copper (II) and vanadyl ions is outlined. The treatment has been developed to the point which makes a computer simulation of the ΔM=1 and ΔM=2 transitions possible in the systems studied. An important product of this is the evaluation of the distance between the metal ions in the dimeric species. The variation of this distance in the metalloporphyrins studied is related to subtle changes in structure brought about by the various substituents in the macrocylic system as well as to the nature of the transition metal ion.
Temperature Dependence of Nearly Resonant Vibration → Vibration Energy Transfer in CO2 Mixtures
Authors: Stephenson John C. , Moore C. Bradley
Published: February 2004

Laser-excited vibrational fluorescence measurements have given rates for nearly resonant vibrational energy exchange between the asymmetric stretch of CO2 and the stretching vibrations of 13CO2, N2O, CO, and 15N2 as a function of temperature and of 13CO and D2 at room temperature. For molecules with vibrational transition dipole moments the energy transfer cross sections, σ vv, decrease rapidly with increasing vibrational energy discrepancy, ΔE. Their magnitudes at resonance are accurately fit by first order calculations using the transition—dipole interaction, but that theory underestimates σ vv for Δ E ≥ 66 cm−1, especially at low temperatures. At resonance, σ vv is inversely proportional to temperature. As ΔE increases, σ vv decreases less rapidly with temperature (13CO2, Δ E = 66 cm−1), becomes temperature-insensitive (N2O, Δ E = 125 cm−1), and finally increases with temperature (12CO, Δ E = 206 cm−1). It appears that rotation—translation coupling terms in the intermolecular potential operate simultaneously with the transition dipole interaction to transfer more of ΔE into rotation than is allowed by the first-order selection rules. When the leading vibrational transition multipole term is dipole quadrupole (CO2–N2), σ vv is much less sensitive to ΔE. First-order calculations match CO2 – 14N2 (Δ E = 19 cm−1) well, but show some breakdown for CO2–15N2 (Δ E = 99 cm−1). The vibrational deactivation rates have been measured in mixtures of CO2 with N2O, 15N2, CO, and 18O2 as a function of temperature. The diatomics are similar in behavior to the rare gases. The rate for deactivation in N2O–CO2 collisions is much faster than the rates for pure CO2 or N2O and is nearly temperature independent. Nearly resonant processes are likely to be responsible. The rate of relaxation in pure N2O is also obtained.
Temperature Dependence of Phosphorescence Characteristics of Aromatic Hydrocarbons in Poly(methylmethacrylate)
Authors: Graves W. E. , Hofeldt R. H. , McGlynn S. P.
Published: February 2004

The thermal dependence of the phosphorescence parameters of a number of aromatic hydrocarbons has been studied in poly(methylmethacrylate) in the range 77–400°K. The data generated here, as well as that available elsewhere in the literature, has been analyzed in terms of two distinct temperature-dependent triplet-depletive processes: The process dominant in the low-temperature regime is probably intramolecular; the process which predominates at higher temperatures is intermolecular and represents a thermally assisted energy transfer from the lowest triplet state of the guest solute, T1G, to a state of the host plastic at 25100±500 cm−1. The ``plastic'' state is thought to be a triplet and is denoted T1P. Some independent evidence for the T1P designation is also provided.
Quantum Mechanical Investigation of the HeH3+ Energy Surface
Authors: Benson Merle J. , McLaughlin Donald R.
Published: February 2004

Results of an ab initio calculation of the potential surface of HeH3+ are reported, with emphasis on establishing the low energy reaction paths for HeH+ + H2 = He + H3+ within the single configuration molecular orbital (SCF) approximation. Using a basis set of (9s1p/4s1p) Gaussian functions centered on the (He/H) nuclei, the lowest potential energy paths are found to follow C2v symmetry, and these are examined in greater detail. A small number of configuration interaction (CI) calculations are reported involving all single and double excitations from the single configuration. From these calculations about 80% of the total correlation energy (or 90% of the p limit correlation energy) is obtained for the separated two electron fragments, and the CI surface is indicated to lie parallel to the SCF surface to within a few percent of the total energy change of the reaction. The C2v minimal energy reaction path is found to decrease monotonically with no reaction barriers or local minima, with the heat of reaction (at 0°K) calculated to be 2.63 eV exothermic from the SCF results and 2.62 eV exothermic from the CI results.
Reactions of Ethyl Radicals Produced in the Pyrolysis of Azoethane in Reflected Shock Waves
Authors: Izod T. P. J. , Kistiakowsky G. B. , Matsuda S.
Published: February 2004

A shock tube coupled to a time-of-flight mass spectrometer has been used to study the reactions of ethyl radicals at temperatures much higher than before. The radicals were formed by heating mixtures containing azoethane highly diluted in neon to between 1000 and 1540°K in reflected shock waves at a total density of 9×1017 particle/cc. Detailed measurements of the products including ethylene, acetylene, ethane, methane, propane, n-butane, and methyl radicals have been made over a reaction time of 250 μsec. The effects of adding ethylene and azomethane to the reaction mixture have also been investigated. Experiments have been made on the thermal decomposition of n-butane over the range 1160–1550°K, and the ``apparent first-order'' decay rates for n-butane yielded an activation energy of close to 32.5 kcal/mole. Possible mechanisms for product formation are qualitatively discussed and a set of 12 elementary reactions is adopted to describe the experimental observations.
Motional Disorder in Hexasubstituted Chloromethylbenzenes
Authors: Chezeau J.-M. , Strange J. H. , Brot C.
Published: February 2004

The nuclear magnetic relaxation times T1 and T1ρ have been measured for protons in three solid hexasubstituted chloromethylbenzenes, 1,2,3-trichlorotrimethylbenzene, dichlorodurene, and tetrachloroparaxylene, from 77°K to their melting points. The results are interpreted in terms of in-plane molecular reorientation in the upper temperature range and methyl group reorientation in the lower temperature range. Activation enthalpies for these motions are obtained, and the effect of substituting chlorine atoms for methyl groups in hexamethylbenzene is discussed. It has been found necessary to invoke a distribution of correlation times to account for the measurement of in-plane reorientation in all samples studied. The results for 1,2,3-trichlorotrimethylbenzene are compared with those obtained by dielectric relaxation studies.
Products of the Vacuum-Ultraviolet Photolysis of Germane Isolated in an Argon Matrix
Authors: Smith George R. , Guillory William A.
Published: February 2004

The vacuum-ultraviolet photolysis of germane and the four deuterogermanes isolated in an argon matrix at temperatures between 4 and 25°K produces significant concentrations of several reactive species. Absorptions at 1839, 1813, 928, and 850 cm−1 have been assigned to a pyramidal GeH3 molecule and those at 1887, 1864, and 920 cm−1 have been assigned to a nonlinear GeH2 molecule. Digermane and the products of its photolysis are also produced and contribute considerably to the infrared spectra in the region below 1000 cm−1.
Mass Spectrometric Determination of the Dissociation Energy of EuC2(g)
Authors: Balducci G. , De Maria G. , Guido M.
Published: February 2004

The vapor phase in equilibrium with condensed europium dicarbide and graphite has been investigated by the Knudsen effusion-mass spectrometric technique over the temperature range 1800–2320°K. In addition to europium atoms, the dicarbide molecule EuC2(g) was identified and its bond and atomization energies were determined: D0 ° (Eu−C2)=129 ± 5 kcal/mole and Δ H0, atom ° (EuC2)=271 ± 4 kcal/mole.
Thermal Behavior of Solid CH3D in the Region T <10°K
Authors: Morrison J. A. , Norton P. R.
Published: February 2004

An attempt was made to determine the heat capacity of isotopically pure CH3D in the temperature regime where a Schottky-type anomaly was known to occur. Before measurements were performed, the calorimeter system was usually kept at about T=20°K for 24 h or so. The results did not agree with extensive ones obtained earlier by Colwell for CH3D of normal isotopic purity nor were they reproducible from day to day. It had been suggested by Colwell that conversion occurs between the nuclear spin species of this methane at low temperatures and that his results corresponded to the equilibrium mixture at each temperature. Some experiments were therefore done with isotopically and chemically pure CH3D to which had been added 0.17% of O2, the object being to try to catalyze the conversion. While the addition of the O2 had a small effect, the heat capacities were still not reproducible. A subsequent detailed study of a specimen of CH3D of normal isotopic purity disclosed a slow evolution of energy (characteristic time ≥30 min) when the specimen was first cooled below T=10°K. If the calorimeter vessel was held at temperatures of 4°K or less for several hours, then the heat capacities became reproducible and similar to but not identical with those obtained by Colwell. The cause of the slow process cannot be established with certainty but the magnitude of the energy released and the fact that the process can be reversed suggest that conversion between the spin species of CH3D is involved. Several experimental tests showed that the process could not be associated with the λ-type anomalies that occur in solid CH3D at T=16.1 and 23.1°K nor with pathological behavior of the calorimeter system.
Proton Magnetic Resonance Studies of Trimethylacetonitrile
Authors: El Saffar Z. M. , Schultz P. , Meyer E. F.
Published: February 2004

Proton magnetic resonance studies of the second moment S of the absorption spectrum and the spin—lattice relaxation time T1 have been made over a wide temperature range in solid trimethylacetonitrile, (CH3)3CCN. The results indicate that the dominant molecular motion occurring in this compound is that of the t-butyl group about the C–CN axis coupled with rotation of the methyl groups about their C3 axes. The ``one-step'' nature of the change in S and the temperature dependence of T1 which is characterized by a single minimum indicate that the two types of motion involved must occur in such rapid succession as to produce effectively simultaneous influence on S and T1. The motion persists above 80°K and is associated with an activation energy E=3.9±0.2 kcal/mole.
Thermal Diffusion of Polyatomic Molecules
Authors: Matzen M. Keith , Hoffman David K. , Dahler John S.
Published: February 2004

The transport properties of a multicomponent mixture of polyatomic gases in field-free space and in a constant, static magnetic or electric field are discussed. The kinetic equations are developed in detail for binary mixtures of monatomic—diatomic and diatomic—diatomic gases, and these equations are solved using a rigid ellipsoid collision model. Calculated thermal-diffusion factors are compared with experiment for several different isotopic mixtures of carbon monoxide.
Thermal Study of Entropies and Crystal Field Splittings in Heavy Rare Earth Trichloride Hexahydrates. Heat Capacities from 5–300°K
Authors: Spedding F. H. , Rulf D. C. , Gerstein B. C.
Published: February 2004

Heat capacity measurements were made from 5 to 300°K on salts of the type RCl3·6H2O, with R being Gd, Tb, Ho, and Lu. The lattice contributions to the heat capacities of the magnetic salts were obtained by using an interpolation between the heat capacities of the Gd and Lu salts. The crystal field splittings of the lowest J levels of the Tb3+ and Ho3+ ions were studied by comparing the calorimetrically determined magnetic entropies with those calculated on the basis of extant optical spectral data. This comparison indicated that the spectral levels were identified correctly in the case of the Ho salt but incorrectly in the case of the Tb salt. The total entropies of the Gd, Tb, Ho, and Lu salts at 298.15°K are 400.80, 403.84, 403.51, and 374.54 J mole−1·deg−1, respectively. The standard state partial molal entropies of the solutes for the above salts in aqueous solution were determined using the present measurements and previous data from solution calorimetry. The heat capacity of the Lu salt contains an anomalous peak between 279 and 285°K. The enthalpy change associated with the peak is 13.8±1 J. The entropy change is less than 0.3 J mole−1·deg−1. Possible causes of the anomaly are discussed.
Comparative Study of the Random-Phase Approximation, Hartree-Fock and Single Excited Configuration-Interaction Methods of Computing Excitation Properties of Some Molecules
Authors: Ho J. C. , Segal G. A. , Taylor H. S.
Published: February 2004

Selected excitation energies, potential surfaces of excited states, and oscillator strengths are computed using three methods: (1) the random-phase approximation (RPA), (2) the singly excited configurations-interaction (SECI) method based upon the ground-state restricted Hartree-Fock (RHF) determinant, (3) a variational or quasivariational method which is essentially an excited-state Hartree-Fock (HF) method and which is applied to each state of the system. The simplification of the intermediate neglect of differential overlap (INDO) method is used throughout and it seems not to affect the validity of the comparison of the different methods. The advantages and the disadvantages of respective methods are discussed. The molecules chosen for study are H2O, HCHO, CH2N2, and HCOOH. Conclusions are drawn on the basis of what are the most extensive set of semiempirical RPA calculations to date. The RPA method, within the approximations used and in the given examples, is found to have no advantages and to have several disadvantages relative to the conventional SECI method when static as opposed to dynamic properties are computed. This seems to be generally true for low-lying non-Rydberg molecular and atomic states.
Mechanism of Rotational Relaxation
Authors: Polanyi J. C. , Woodall K. B.
Published: February 2004

It has been known for some time from infrared chemiluminescence experiments that a nonthermal rotational distribution of hydrogen halide peaked initially at high rotational quantum number, J, relaxes to a thermal distribution without generating a peak at intermediate J [Discussions Faraday Soc. 44, 183 (1967)]. It is shown in the present study that this characteristic pattern of relaxation is well described by a model according to which ΔJ is unrestricted, except for the relation PJ−ΔJJ=N exp (— CΔE), where PJ−ΔJJ is the probability of a collision−induced transfer from J to J — ΔJ, ΔE is the energy difference between these two rotational states, and N and C are constants. This expression for PJ−ΔJJ ascribes a very much lower probability of rotational deactivation to the higher J levels. Three other, contrasting, models were tested; they were rejected since they failed to describe the observed pattern of relaxation adequately. Upper limits were ascribed to PJ−ΔJJ for ΔJ=1–5 in HCl–H2 collisions. This study provides a further method for correcting infrared chemiluminescence data for modest rotational relaxation and at the same time shows that the simple truncation correction procedure used until now is remarkably good.
Effect of Pressure on Pyrene Excimer Fluorescence in Toluene
Authors: Johnson P. C. , Offen H. W.
Published: February 2004

The pressure dependence of pyrene fluorescence in toluene has been measured to 10 kbar. The monomer and excimer fluorescence decay kinetics are analyzed to give the rate parameters for excimer formation, dissociation, and unimolecular decay as a function of pressure at 296, 320, and 340°K. Activation parameters are evaluated both as a function of temperature and as a function of pressure. The pressure dependence of the association is in one-to-one correspondence with the solvent viscosity. The rate of dissociation is strongly inhibited by high pressures. The volume change on excimer formation is −11 cm3/mole at 296°K. The enthalpy and free energy of excimer formation are −9.7 and −4.2 kcal/mole at 1 atm, and −3.6 and −6.7 kcal/mole at 10 kbar, respectively.
Vibrational Spectra and Molecular Symmetry of Diboron Tetrabromide in the Crystalline and Fluid States
Authors: Odom J. D. , Saunders J. E. , Durig J. R.
Published: February 2004

The infrared spectra of B2Br4 have been studied from 4000 to 33 cm−1 in the gaseous and crystalline states. The Raman spectra of the liquid at low temperatures and of the crystalline solid have been studied to 30 and 40 cm−1, respectively. Eleven of the twelve vibrational modes have been assigned whereas the low-lying torsional mode is calculated to be between 15 and 20 cm−1. Based on the vibrational analysis, it is concluded that the staggered D2d conformation is maintained in both the fluid and crystalline states. This behavior is contrary to what has been observed for both the B2F4 and B2Cl4 molecules. It is concluded that the increased steric interactions which would occur in the planar form preclude this conformation for the bromide.
Vibration—Rotation Interactions and the Choice of Rotating Axes for Polyatomic Molecules
Authors: Pickett Herbert M.
Published: February 2004

A Hamiltonian for vibration and rotation in polyatomic molecules is described which is general for large and small amplitude motion and most choices of rotating axes. An analysis of the effect of a change in the definition of the rotating axes is presented and the convergence properties of perturbation expansions of the Hamiltonian are discussed. Some rotation—vibration terms converge slowly in an arbitrary set of axes, and the effect of these terms is shown to be equivalent to rotation from the original set of axes to a new set of axes in which the coriolis terms are minimized. Ways to find this new set of axes are discussed, and application of the results of this work to the microwave spectra of several molecules is presented.
Ground State of He2 by the Spin-Optimized Method
Authors: Kunik Dan , Kaldor Uzi
Published: February 2004

The Σg +1 ground state of He2 is investigated using the spin-optimized self-consistent-field (SOSCF) method. Molecular orbitals need not have g or u character. Over 40% of the correlation energy is included in the SOSCF function without giving up the concept of one orbital per electron, and a simple spin-coupling scheme, very close to maximal pairing, is obtained. Imposing symmetry restrictions on the orbitals leads to significantly higher energy and a more complicated coupling scheme. The dependence of orbitals on the internuclear distance is described.
Kerr Effect in Atoms and Molecules
Authors: Chang Tai Yup
Published: February 2004

A time-dependent double perturbation—variation formulation for the hyperpolarizabilities involved in the Kerr effect is presented which is suitable for quantitative study of the Kerr effect. It is applied to the hydrogen atom to generate accurate Kerr dispersion curves for a wide range of wavelengths, including the first two lines of the Lyman series. In the vicinity of resonance frequencies, the dominant term [(νi 2 − ν2)−3] approximation is shown to be good for quite a wide range of wavelengths.
Cluster Formation in Expanding Supersonic Jets: Effect of Pressure, Temperature, Nozzle Size, and Test Gas
Authors: Hagena O. F. , Obert W.
Published: February 2004

To study homogeneous condensation in an expanding nozzle flow, cluster beams were sampled from the core of the flow field and transferred into a vacuum chamber for further analysis. Sonic and hypersonic nozzles with throat diameters 0.015 cm ≤ d ≤ 0.15 cm were used. Source temperature was varied between 120 ≤ T0 ≤ 450° K, source pressure between 100 ≤ p0 ≤ 12 000 torr. Test gases were the rare gases (except He), N2, and CO2. The size of the clusters (=microdroplets or -crystals) and the intensity of the cluster beam was measured with a through-flow ionization detector with retarding potential system to get the mass-to-charge distribution of the cluster ions. The mean cluster size N̄ varied between 102 and 104 atoms/cluster. The mean cluster size remained almost constant with increasing T0 if p0 was increased simultaneously according to the isentropic relation p0T0γ /(1−γ)=const. Considering the various types of cluster-growth reactions one expects to get cluster beams with the same size, if p0 and T0 fall within the narrow range between the isentrope p0T0γ /(1−γ)=const and the line for equal bimolecular processes, p0T0(1.5γ −1)/(1−γ)=const. The experiments confirm this result. The same model predicts that a decrease of nozzle throat diameter d can be compensated by an increase of source pressure p0 such that p0dq=const with 0<q<1. The experimental scaling law for constant cluster size gives q=0.8 for argon and q=0.6 for CO2. Comparing different gases, the same cluster size N̄ was obtained for the rare gases if they were in corresponding states prior to expansion and if the reduced nozzle scale was the same. This confirms the model of ``corresponding jets'' which extends the thermodynamic principle of corresponding states to real gas effects in a time-dependent system like a nozzle flow, and which applies equally to condensation in slow and fast expansions, including the transition to molecular flow.
Paramagnetic Resonance of a Photoinduced Defect Associated with an Impurity in Cesium Azide
Authors: Owens F. J.
Published: February 2004

Single crystals of cesium azide (CsN3) when irradiated with uv light at 77°K yield an ESR spectrum arising from a spin-12 particle and having an isotropic g value of 2.041± 0.003. The spectrum displays hyperfine structure indicative of an equal coupling to a large number of equivalent nuclei. The hyperfine splitting is isotropic and is 7.0±0.05 G. The intensity distribution of the hyperfine lines is consistent with a coupling to eight azide ions which would constitute the nearest-neighbor environment of a cation site. The spectrum is attributed to an unpaired electron in the 1S1/2 state of a singly ionized substitutional cation impurity which must have no nuclear magnetic moment. Analysis of the material shows the presence of Ca impurities making an unpaired electron in the 1S1/2 state of Ca+ a likely prospect.
Thermodynamics of Lithium Isotope Exchange Reactions. II. Electrochemical Investigations in Diglyme and Propylene Carbonate
Authors: Singh Gulzar , Hall John C. , Rock Peter A.
Published: February 2004

This paper reports our final results for the direct experimental determination of the equilibrium constants for the lithium isotope exchange reactions Li7(s)+6LiBr(digl)=6Li(s)+7LiBr(digl) Li7(s)+6Li+(PC)=6Li(s)+7Li+(PC)involving the solvents diglyme (digl, ε=7.2) and propylene carbonate (PC, ε=65.5). These reactions were studied in electrochemical double cells without liquid junction. The experimentally determined values of the equilibrium constants at 296.6°K are K1=1.035± 0.007 and K2=1.030± 0.005. The experimental values of K1 and K2 are compared with values calculated from statistical thermodynamic theory for various model reactions. A Born—von Kármán lattice calculation has been carried out for the isotopic lithium metals.
VH-Depolarized Light Scattering Spectra of Liquids at High k2η/ρΓ
Authors: Keyes Thomas , Kivelson Daniel
Published: February 2004

If the reorientational motion of liquids composed of nonspherical top molecules couples to the shear modes of the liquid, a k-dependent VH-depolarized light scattering spectrum results. At low values of (k2η/ρΓ) (where η is the coefficient of shear viscosity and ρ is the mass density), the spectrum exhibits a sharp Rayleigh wing line with half-width Γ and a k-dependent doublet near its peak. A simple theory is explored which shows that the doublet feature vanishes when (k2η/ρΓ) exceeds a critical value dependent upon both the angle of scatter and a dimensionless parameter (R) which measures the coupling between molecular reorientation and translational shear modes of the liquid. (k2η/ρΓ) increases with decreasing temperature and increasing molecular size; therefore, the theory indicates that the doublet should vanish at low temperature as observed.
Crystal Structure of (CH3CH2CH2NH3)2MnCl4
Authors: Peterson Ellis R. , Willett Roger D.
Published: February 2004

The crystal structure of (CH3CH2CH2NH3)2MnCl4 has been determined by Weissenberg single crystal diffraction techniques. The crystals are orthorhombic, space group Cmca, with a=7.29  Å, b=25.94 Å, and c=7.51 Å. Least squares refinement with anisotropic temperature factors yielded a final R value of 0.116. The outstanding feature of the structure is the existence of a two-dimensional metal—halogen framework formed by MnCl6 octahedra sharing corners. Each metal—halogen sheet is ``sandwiched'' between layers of propylammonium ions, thus effectively isolating it from adjacent sheets. This provides an exciting new class of compounds for studying two-dimensional magnetic interactions. The coordination sphere around the Mn(II) ion has approximately D4h symmetry with two Mn–Cl bond lengths of 2.475 Å and four bond lengths of 2.629 Å. Existence of N–H···Cl hydrogen bonds cause the four-fold axis of the distorted octahedra to be tipped 8.0° away from the crystallographic b axis. This produces a ``washboard'' effect when each metal—halogen sheet is viewed from the 〈 100 〉 direction and leads to nonlinear Mn–Cl–Mn bridges. A comparison with the structure of the corresponding copper(II) salts is made.
Establishment of an Ordered Array of Physisorbed Ethanol Molecules on Lithium Fluoride as Evidenced by the Diffraction of Helium Atoms
Authors: Mason B. F. , Williams Brian R.
Published: February 2004

A (001) surface of lithium fluoride was deliberately contaminated by several organic molecules at temperatures around 150°K. The molecules studied were; CCl4, C6H6, CH3OH, C2H5OH, and H2O. The adsorption mechanism involved completely reversible physisorption and in the case of C2H5OH molecules, reorientation occurred on the surface to form and ordered array, for a given range of surface temperatures. For temperatures below this range, an ordered array was not formed. The former case gave well-defined diffraction peaks in the scattering pattern of helium atoms, whilst in the latter case no evidence of diffraction was observed.
On the Determination of the Phases of Electromagnetic Scattering Amplitudes from Experimental Data
Authors: Gerber R. B. , Karplus M.
Published: February 2004

The scattering of electromagnetic waves by systems of unknown charge distribution is considered with the aim of establishing that the phase of the scattered radiation (relative to the incident one) can be computed theoretically from experimentally obtainable data, such as the intensity and the polarization of the scattered wave. By means of the unitarity relation of classical electromagnetic scattering theory, it is demonstrated that, subject to certain restrictions on the magnitude of the scattering data: (i) For an unpolarized incident beam and a spherically symmetric target, knowledge of the differential cross section and the polarization of the scattered radiation is sufficient to determine uniquely the phases of the scattering amplitude tensor. (ii) For a parity-invariant scatterer, and fully circularly or linearly polarized incident beams, knowledge of the differential cross section and the ellipticity of the outgoing radiation is sufficient to completely determine the phases of the scattering amplitude tensor. The scattering data must be available for both the left and the right circularly polarized incoming wave (or for two linearly polarized beams pertaining to the two orthogonal polarizations). (iii) In the above cases it is shown that the phases can be calculated systematically by converging iterative procedures. Conditions for convergence are established, and bounds are provided on the errors associated with the approximants obtained after an arbitrary (but finite) number of iterations. On the basis of result (ii) with the same data assumed to be known, a formal solution to the phase determination problem is given for a general scatterer. However, for this case rigorous results on convergence conditions and error bounds have not been obtained. An outline of the corresponding treatment valid within the framework of the quantum-electrodynamical theory of light scattering is given. To illustrate the proposed methods for phase determination, we apply them to the scattering of light by very simple systems. These include the free electron and an oscillating charge. We conclude by discussing the possibility of applying the above results to obtain solutions of the phase problem in systems of practical interest.
Theory of Zeeman Anisotropy Fluorescence Spectroscopy. Numerical Fit of Low-J Transitions in KCl:Sm2+
Authors: Ford Robert L. , Fong Francis K.
Published: February 2004

The theory of Zeeman anisotropy fluorescence spectroscopy of rare earth ions in single crystals has been formulated in terms of tensor operators. Numerical calculations are presented for the fit of the experimental ZAF observations for the D05 →7F0, 7F1, and 7F2 fluorescent transitions in KCl:Sm2+. Excellent agreement between theory and experiment has been obtained for the 7F0 and 7F1 levels using the nearest-neighbor C2v (1, 1, 0) Sm2+—K+ vacancy pair model. The poor fit for the 7F2 level in terms of the C2v model supports the finding of Heist, Chilver, and Fong that the 7282.8–Å line, one of the four prominent lines observed in the D05 →7F2 wavelength region, does not originate from the C2v (1, 1, 0) site.
Gas-Phase Reaction of NO2 and Ethylene at 25°C
Authors: Chao S. C. , Jaffe S.
Published: February 2004

The thermal reaction of NO2 and C2H4 was studied in the gas phase at 25°C. The products of the reaction were CH3NO2, CH3ONO2, NO, N2O, CO2, C2H4O, and H2O. A mechanism is presented to explain the formation of these products, and some rate constants are derived from the mechanism, the rate of consumption of NO2, and the rate of formation of the products.
On the Theory of the Thermomagnetic Torque
Authors: Park Won-Hoon , Dahler John S.
Published: February 2004

This paper presents a theory of the thermomagnetic torque which is applicable at pressures so high that Knudsen effects can be neglected. Kinetic theory techniques are used to derive momentum and energy boundary conditions appropriate to transport in the presence of a magnetic field. It is found that thermal creep generated by the field-induced annular flow of heat contributes substantially to the torque. Calculations based upon a combination of the kinetic theory of Levi, McCourt, and Beenakker and Senftleben—Beenakker experimental data are found to be in reasonable agreement with measurements of torque.
Equation-of-State Data of Solids from Shock Vaporization
Authors: Hornung K. , Michel K. W.
Published: February 2004

A spectroscopic technique is introduced to measure the amount of vapor species appearing from shocked metals upon unloading into vacuum. The degree of vaporization is correlated with the shock wave entropy, from which follows the possibility to determine the equation of state at pressures above 200 kbar.
Hydrogen Diiodide Radical: Infrared Detection through the Matrix-Isolation Technique
Authors: Noble Paul N.
Published: February 2004

When mixtures of iodine, hydrogen, and argon are passed through a glow discharge and the products are condensed at 16°K, infrared absorptions appear at 802.8 and 682.1 cm−1. Upon deuteration, these absorptions shift to 594 and 470.0 cm−1, respectively. The deuterium isotopic shifts and numerous close analogies with the spectra of HCl2 and HBr2 show that these absorptions can be assigned to the IHI (IDI) free radical in a linear, symmetric (D∞h) structure with ν3=682.1 (470.0) cm−1 and ν1=120.7 (124) cm−1. The vibrational potential function contains a large quartic term for v3, and the magnitude of the force constant for v1 suggests a hydrogen-bond strength in HI2 comparable to that in HI2−.
Near Hartree—Fock Calculations of the Force Constants and Dipole Moment Derivatives in Methane
Authors: Meyer W. , Pulay P.
Published: February 2004

The complete force field of methane is calculated from SCF—MO wavefunctions by applying the force method. Many different Gaussian basis sets are used, and the influence of different types of atomic functions on the force constants is discussed. The deviation of the calculated force constants from the observed values is similar for all treated basis sets. Therefore it is concluded that the deviation is mainly due to the neglect of correlation effects, and that there is little point in using very extended basis sets in Hartree-Fock calculations. The energy of our best calculation is estimated to be at most 0.01 a.u. above the HF limit. The results of this calculation are as follows (experimental values in brackets):

(1) Force constants in millidynes per angstrom: F11=5.578(5.842), F22=0.537(0.486), F33=5.382(5.383), F34=0.202(0.206), F44=0.512(0.458);

(2) Dipole moment derivatives in debyes per angstrom: μ3=−0.977 (± 0.833), μ4=−0.329 (± 0.373).

The interpretation of the force field is discussed on the basis of a population analysis.
Radiational Deuterium Effect in the Triplet—Singlet Transition of Benzene
Authors: Johnson Philip M. , Ziegler Lawrence
Published: February 2004

Detailed phosphorescence spectra of C6H6 and C6D6 in an argon matrix (20°K) were analyzed and found to contain emissions from several crystalline sites. Benzene's triplet decay time is found to be 11.7 and 17.5 sec in the two major sites. These are increased in the deuterated molecule to 14.5 and 29.5 sec. It is shown that the lifetime difference caused by deuteration, which had been ascribed entirely to different rates of radiationless T1—S0 decay for the two molecules, actually is due substantially to changes in the vibronic coupling of the radiational decay pathway as indicated by the lower emission intensity in certain C6D6 vibrational progressions relative to those in C6H6. The ratio of the quantum yields of phosphorescence of C6D6 to C6H6 is found to be 1.40 ± 0.03 while the ratio of radiative triplet decay rates is 0.82 ± 0.04..
Superposition Assumption. I. Low Density Fluid Argon
Authors: Krumhansl J. A. , Wang Shein-shion
Published: February 2004

The accuracy of the superposition assumption (SA) is checked by calculating the pair potential and its derivative from the Born—Green equation using the SA triplet function. These values are then compared with those for which the Monte Carlo triplet function was used. It is found that the SA results are poor.