bluuebunny/crossref_metadata_2025 huggingface dataset:
Broad band absorption studies of substituted methane and mono-substituted benzene compounds
Authors: Hindle P. , Walker S. , Warren J.
Published: February 2003

The broad band absorption of polar liquids of the substituted methane and mono-substituted benzene type have been examined in the far infrared region. Their integrated absorption intensity (Aobs) has been determined, and the plot of this against the square of the dipole moment (μ) divided by the average moment of inertia yields a straight line for both the symmetric-top and phenyl-Z molecules examined although their slopes differ. For the eight symmetric-top molecules by far the major part of the absorption may be accounted for by a rotational contribution to the integrated absorption (Al). For both types of molecules the internal field effects appear not to be of prime importance in influencing the Al/Aobs ratio and give some support to the view of Rothschild that the rotational motion in liquids is not very sensitive to molecular (dipole) association, providing momer species are maintained. In general, the liquids obey the relationship αmax = 0.403 μ2/Iav + 2.5, where αmax is the maximum value of the absorption coefficient. The use of this relationship in indicating strong association—polymerized units— is discussed as also its application in estimating the average moment of inertia.
The phase transitions in cesium azide: Temperature and pressure dependent Raman studies
Authors: Iqbal Zafar , Christoe C. W.
Published: February 2003

The high temperature and the high pressure induced phase transitions in polycrystalline cesium azide (CsN3) have been studied by Raman spectroscopy. The remarkable broadening of the Eg rotatory mode with increasing temperature and the observation of quasielastic low frequency scattering in the high temperature phase (Phase I) support the order–disorder character of the temperature induced transition. In the high pressure phase (Phase III), the Raman spectrum shows a decrease in the rotatory mode linewidth and splittings of the lines due to internal modes, suggesting that the new structure is more compact and involves at least two crystallographically nonequivalent azide ion sites. Phase III could not be obtained by lowering the temperature to 20 °K at 10−6 bar.
Radiationless transitions in aromatic molecules with nonbonding electrons: Role of vibronically active out-of-plane modes
Authors: Kanamaru N. , Lim E. C.
Published: February 2003

By virtue of their large frequency change and equilibrium displacement, the vibronically active out-of-plane bending modes are argued to be important accepting modes for radiationless transitions of aromatic molecules with nonbonding electrons. The conclusion is shown to be consistent with the position dependence of the deuterium isotope effect in T1(ππ*) → S0 radiationless transitions of nitrogen-heterocyclic and aromatic carbonyl compounds.
The local electric field. I. The effect on isotropic and anisotropic Rayleigh scattering
Authors: Burnham A. K. , Alms G. R. , Flygare W. H.
Published: February 2003

Isotropic and anisotropic Rayleigh light scattering intensities are reported for seven liquids. Anisotropic intensities as a function of refractive index for four molecules are also reported. We have used spectral analysis to separate the single molecule scattering from the complicating effects of collisional scattering and static pair correlations. Several macroscopic continuum models for the local electric field at a molecule in a liquid phase are summarized and these theoretical models are compared with our experimental results for scattering from single molecules. We have established the equivalence of the local field correction calculated from a fourth-power local field with that calculated from the dielectric fluctuation method for all models. We further show that of these local field corrections, only that calculated by the ellipsoidal Onsager–Scholte model is consistent with all our experimental data. The size and shape parameters of these models are related to molecular parameters.
The kinetics of phase separation in a liquid binary mixture
Authors: Mou Chung Yuan , Lovett Ronald
Published: February 2003

A dynamical theory for the spontaneous formation and growth of spherical drops in a metastable liquid phase is constructed out of hydrodynamics, nonequilibrium thermodynamics, and thermodynamic fluctuation theory and used to predict the nucleation kinetics for phase separation in a liquid binary mixture. The nucleation rate is considerably lower than that predicted by classical nucleation theory and the inhomogeneities which result from forming nuclei in a finite time cause the actual kinetic path to differ considerably from the classical path. A qualitative agreement with experiment is demonstrated.
Mössbauer studies of Fe2+ in anhydrous hemoglobin and its isolated subunits
Authors: Papaefthymiou G. C. , Huynh B. H. , Yen C. S. , Groves J. L. , Wu C. S.
Published: February 2003

The Mössbauer spectra of dehydrated αSH and βSH deoxyhemoglobin subunits were obtained over a temperature range from 77 to 200°K and compared with anhydrous hemoglobin spectra. The Mössbauer data for both subunits show a superposition of a ferrous high spin (S = 2) and a ferrous low spin (S = 0) quadrupole doublet of approximately equal intensity. No differences either in the isomer shifts or the quadrupole splittings were detected between the three hemoproteins. Consequently, the model for AHb–A with the α and β chains in different spin states proposed by previous investigators is probably incorrect. The observed quadrupole splittings, ΔEQ(T), are analyzed in a ligand field approximation, which gives the electronic structure of the low lying eigenstates of the ferrous ion at the two spin sites. The principal components of the EFG for the low and high spin iron sites calculated from the ligand field model are normal to the heme plane and have the values at T = 80°K of −0.09e 〈r−3〉 and 0.21e 〈r−3〉, respectively. At T = 80°K the asymmetry parameter, η, is zero for the low spin iron site and 0.39 for the high spin site. A possible interpretation of the origin of the spin state mixture based on the disruption of the hydrophobic interaction upon dehydration is discussed.
Molecular reorganization upon ionization and excitation: NH2, NH2+, and NH2− systems
Authors: Heaton M. M. , Cowdry R.
Published: February 2003

The results of ab initio self-consistent field calculations on NH2+ (3B1 and 1A1), NH2 (2B1 and 2A1), and 1A1 NH2− reveal that the electron distribution in 2a1 and 1b2 bonding orbitals and in the in-plane 3a1 valence orbital is most affected by ionization or excitation. The formation of the cationic states is accompanied by a shift to the nitrogen atom of 0.35 electron for the 1A1 and 0.25 electron for the 3B1 state. In NH2− formation, electron density shifts to the hydrogens. These changes occur mainly in the bonding orbitals. Excitation of ground-state NH2 causes a shift of about 0.10 electron onto the nitrogen, whereas excitation of 3B1 NH2+ shifts charge density onto the hydrogens. These changes occur in the valence orbitals. The ionization potential calculated for 2B1 NH2 (11.6 eV) agrees well with experiment, but the photodetachment energy obtained for NH2− (0.30 eV) is less than half the experimental value. Polarization functions in the basis set made negligible contributions to the molecular orbitals with the exception of the 2a1 bonding orbital. Optimization of the nitrogen polarization functions for the 2B1 NH2 and the NH2− systems resulted in only a small improvement in the energy and a more marked change in the charge distribution in NH2−.
Laser-induced evaporation of solid surfaces
Authors: Wei P. S. P. , Nelson D. J. , Hall R. B.
Published: February 2003

Atomic and ionic species evaporated by a CO2 laser pulse at power density up to 107 W/cm2 from surfaces have been studied by time-resolved spectroscopy. The time-of-flight results for three emitting species from surfaces at different temperatures are consistent with a thermal model for evaporation.
Infrared chemiluminescence investigation of fluorine atom substitution reactions
Authors: Moehlmann Jon G. , McDonald J. D.
Published: February 2003

The arrested relaxation infrared chemiluminescence technique has been used to investigate the fluorine atom substitution reactions of various olefinic and aromatic compounds. Analysis of the infrared emission spectra of the reaction products allows the determination of the product vibrational energy distribution. The observed vibrational energy distributions are compared to the distributions which would be obtained if the products exhibited statistical partitioning of the reaction energy. It was found that for all aromatic substrates studied and for olefinic substrates in which F was substituted for Cl or Br, the products exhibited statistical vibrational energy distributions, while nonstatistical distributions were obtained for olefinic substrates in which F was substituted for H or CH3. These results are discussed within a framework of exit channel effects on product energy distributions, and the effects of postcollision intramolecular relaxation.
ESR studies of triplet radical pairs of CO2− in irradiated single crystals of CH3COOLi⋅2H2O
Authors: Nunome Keichi , Toriyama Kazumi , Iwasaki Machio
Published: February 2003

Single crystals of CH3COOLi⋅2H2O irradiated at 77 °K gave ESR signals arising from triplet radical pairs in addition to ĊH3 radicals. Spectral analyses lead to the conclusion that a pair of CO2− ions are formed with the average separation of 5.95 Å. The unpaired electrons extend over the four neighboring Li cations giving 13-line superhyperfine structures. Strong nuclear spin forbidden transitions arising from the anisotropic coupling with 7Li nuclei were observed in one of the fine structure components. From this behavior the sign of the isotropic coupling of 7Li was determined to be positive. The structure and the trapping site of the paired CO2− ions interacting with Li were elucidated from the g and D tensors as well as the 7Li and 13C hyperfine coupling tensors. The isolated CO2− ions in the crystals annealed at room temperature have been found to undergo hindered oscillation around the O⋅⋅⋅O axis at 77 °K. As a very minute component the radical pairs of ĊH2CO2− were also found in the crystal stored at 77 °K after irradiation.
O2(a 1Δg) absorption bands in the vacuum ultraviolet
Authors: Katayama D. H. , Huffman R. E. , Tanaka Y.
Published: February 2003

The absorption spectrum of metastable O2, excited by a microwave discharge, has been photographed in second order of the helium continuum with a 6.65 m vacuum spectrograph. The band progression in the 835–900 Å region has been investigated further. From the rotational analysis of the well resolved bands, this progression was determined to belong to the transition, 1Φu–a1Δg. The vibrational quantum numbers were assigned by an isotope shift analysis with respect to 18O2. The molecular constants obtained for the 1Φu state were ωe = 1071±4 cm−1, xωe = 8.3±0.8 cm−1, Be = 1.116±0.008 cm−1, and αe = 0.014±0.005 cm−1.
Order–disorder phenomena in adsorbed layers described by a lattice gas model
Authors: Doyen G. , Ertl G. , Plancher M.
Published: February 2003

Order–disorder transitions in adsorbed phases on single crystal surfaces manifest themselves by variations of the low energy electron diffraction (LEED) patterns. The present paper contains a theoretical treatment of the statistical properties of the simplest structure within this framework, namely a layer which may be described by a square lattice gas model with repulsive interactions between nearest neighbors and giving rise to a c2×2-LEED pattern on the (100) surface of a fcc or bcc crystal. At a coverage ϑ=1/2 the relative intensities of the half-order LEED spots are, within the kinematic approximation, shown to be identical to the expectation value of the spin-correlation function of the two-dimensional Ising model, averaged over an area corresponding to the coherence width of the electron beam. For ϑ<1/2 no analytic solutions are available, but the problem may be treated by means of the Monte Carlo technique, the results of which for ϑ=1/2 agree quite well with those from the analytic solution. The order–disorder transition temperature is predicted to decrease strongly with decreasing coverage. Below ϑ=0.25 the distinction between ordered and disordered phases becomes more or less irrelevant, a fact made evident by a crude determination of the configurational entropies. The configurational energy, the specific heat of the adsorbate layer and some parameters characterizing short-range order are evaluated as further quantities. The latter data may be of some importance for the kinetics of ad- and de-sorption. Quantitative comparison with experimental results is so far only possible with the LEED data for the system H/W (100) where the agreement is rather good. In a series of other cases, at least the qualitative features of the present treatment are applicable.
Nonlinear fluctuations in master equation systems. I. Velocity correlation function for the Rayleigh model
Authors: Hynes James T.
Published: February 2003

The influence of nonlinear velocity fluctuations on the velocity correlation function Π (t) is studied for the Rayleigh model of a massive particle in an ideal gas as an example of a master equation system. It is shown that the Mori kernel K (t), which determines the decay of Π (t), has a slow mass-dependent decay on the time scale of the decay of Π (t) and has no well-behaved expansion in the mass ratio. Both features are contrary to standard assumption. The origins of the slow decay are traced to nonlinear fluctuations and the relationship to previous work on requisite conditions for exact exponential decay is discussed. The slow decay of Π (t) is shown to lead to divergent ’’Burnett’’ coefficients in macroscopic friction laws and the resolution of this difficulty is discussed. The relationship of the microscopic ’’bare’’ friction constant to the macroscopic friction constant is considered. Explicit expressions for Π (t) and K (t) for small mass ratio are obtained by mode–mode coupling analysis and perturbation methods. The influence of nonlinear fluctuation effects is found to be numerically negligible despite their long lifetime. The remaining deviation from standard Brownian motion results is examined numerically. The validity of some standard assumptions in mode–mode coupling theory is also examined.
Pitch–concentration relationships in multicomponent liquid crystal mixtures
Authors: Bak C. S. , Labes M. M.
Published: February 2003

The variation of helical pitch in multicomponent liquid crystal mixtures is derived using the concept of the long-range distortions induced by chiral molecules in a nematic matrix. The pitch in a mixture is a quadratic function of the number densities of component molecules of given molecular twisting powers. The pitch variation becomes a linear function of number densities only when the twisting powers satisfy certain special conditions. The nonlinear pitch relation formulated herein is shown to describe typical experimental data on nematic–cholesteric mixtures and allows determination of experimental values of the molecular twisting power.
Ion–molecule reactions in Ar at 296, 195, and 77 °K
Authors: Liu Wei-cheng F. , Conway D. C.
Published: February 2003

Ion–molecule reactions have been studied at 296 and 195 °K in Ar gas which has been purified by passage through a Ti furnace. The rate constant k1 for the reaction, Ar+(2P3/2) + 2Ar → Ar+2 + Ar, was measured in a drift tube at pressures reduced to 0 °C (P0) of 1.414–3.00 torr. Ions effused from the drift tube and were counted with a TOF mass spectrometer. k1 was corrected for side reactions with impurities and found to be (2.3±0.2) ×10−31 and (3.0±0.2) ×10−31 cm6/sec at 296 and 195 °K, respectively. It appears that Ar+ in the 2P1/2 state does not form Ar+2 at these temperatures. An upper limit to the rate constant for the reaction Ar+(2P1/2) + Ar → Ar+(2P3/2) + Ar + energy, was observed to be 2×10−16 cm3/sec at 296 °K. At 77 °K both states of Ar+ appear to form Ar2+ with a rate constant of (5.2±0.1) ×10−31 cm6/sec. The rate of formation of Ar+3 and the equilibrium between Ar+3 and Ar+4 were also studied at 77 °K.
Thermochemistry of the gaseous tungsten fluorides
Authors: Hildenbrand D. L.
Published: February 2003

Equilibrium and electron impact techniques, both employing mass spectrometry, were used to study the gaseous tungsten fluorides WF2, WF3, WF4, and WF5. The molecular species were generated by the reaction of both WF6 and SF6 with tungsten in an effusion cell at 1000 to 2200 K under demonstrable equilibrium conditions. Reaction enthalpies were derived from second-law analysis of various gaseous equilibria and from electron impact threshold measurements, yielding the standard heats of formation, ΔHf298, of WF2 (−20.6±3.2 kcal/mol); WF3 (−121.2±2.8 kcal/mol); WF4 (−222.0±2.5 kcal/mol); and WF5 (−309.1±2.0 kcal/mol). A lower limit of 92 kcal/mol was obtained for the standard heat of formation of WF. Information about molecular ionization potentials is available from the threshold appearance potentials. Thermochemical data were also obtained for gaseous WSF3, WSF2, and WS2F2. Various aspects of the chemical bonding are discussed.
Concentration dependent solute redistribution at the ice/water phase boundary. II. Experimental investigation
Authors: Gross Gerardo Wolfgang , Wu Chen-ho , Bryant Lynn , McKee Chester
Published: February 2003

Solute distribution curves from several hundred ice columns doped with small amounts of HCl, HF, NH4OH, and NH4F were investigated with the log linear approximate solution to the one-dimensional diffusion equation, discussed in an earlier paper. This led to the discovery of spontaneous convective effects which can be separated from a genuine concentration dependence of the distribution coefficient. The solutes studied show specific characteristics with respect to solubility in ice and concentration dependence of the distribution coefficient. These characteristic properties should eventually be correlated with specific structural relationships of these solutes (and the defects they create) to the ice lattice. The method of interpretation may be useful for unraveling the freezing history of natural ices of dilute solute content. The most direct and accurate method for measuring the distribution coefficient of (nonvolatile) solutes is by a combination of intense stirring and slow freezing rates.
ESR spectra of the carboxylated cyclohexadienyl radicals: 13C hyperfine constants and the effect of carboxylation on the g factors and proton hyperfine constants
Authors: Eiben Klaus , Schuler Robert H.
Published: February 2003

The g factors and proton hyperfine constants of 15 multiply carboxylated cyclohexadienyl radicals and all 19 of the carboxylated hydroxycyclohexadienyl radicals have been determined by the in situ radiolysis method. The g factors were measured with sufficient accuracy (±0.00002) to establish that they increase monotonically with total spin density on the positions substituted. The proton hyperfine data are interpreted to indicate that there is a 13% loss in spin density on the ring in the pentacarboxylate radical. Pronounced effects of asymmetric substitution are noted even when this substitution is at a position of low spin density. For the more highly charged hydroxycyclohexadienyl radicals the ESR spectra are sufficiently intense that the 13C containing radicals can be observed at the natural abundance level. Complete sets of 13C hyperfine data are reported for the 1,3,5-tricarboxylate, the three tetracarboxylate and the pentacarboxylate radicals. Definitive assignment of 13C hyperfine constants to the carboxyl groups attached to the conjugated system is possible. Values of 4.8, 1.3, and 6.8 G are attributed to positions 1, 2, and 3, respectively, in the pentacarboxylate radical. These hyperfine constants parallel almost exactly the proton values in the unsubstituted radical so that one can conclude that they can be used as a good indicator of the spin density on the adjacent ring carbon via a relation of the form aC(CO2−) = QC(CO2−) ρα. Correlation of the 13C and proton hyperfine data give a value of 15.8 G for QC(CO2−). Ring 13C hyperfine constants of ∼ 18, 12(2), 11(2), and 8.5 G are observed for each radical with the largest and smallest values being assigned, respectively, to the carbon atoms at positions 3 and 6. Comparisons among the different sets of values indicate variations from predictions based on the Karplus–Fraenkel treatment of the order of 10% so that specific assignment of the remaining four values is difficult. The hyperfine constants of the methylene protons in the substituted cyclohexadienyl radicals reflect very closely the loss in spin density to the carboxyl groups. Substitution in the hydroxycyclohexadienyl radicals, however, produces a far more pronounced and complicated effect that indicates a strong interaction between the OH group and the carboxyl groups substituted on the adjacent positions.
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o valence-bond calculations of the van der Waals interactions between π systems: The ethylene dimer
Authors: Wormer Paul E. S. , van der Avoird Ad
Published: February 2003

A multistructure valence-bond method for the calculation of van der Waals forces is presented which includes in one consistent formalism the electrostatic, induction, and dispersion forces and takes exchange correctly into account. The application of this method to the ethylene dimer leads to the following main conclusions: (1) The ’’first order’’ electrostatic forces are comparable in magnitude to the ’’second order’’ forces even though the molecules possess no permanent dipole moments. Dispersion forces are much larger than induction. Second order interactions are more isotropic than first order forces. (2) In the multipole expansions of the long range forces, the inclusion of the first term only is not sufficient for a good approximation to the interaction. (3) Exchange effects become nonnegligible at approximately 12 bohr, while the van der Waals minimum between two perpendicular molecules is at 9.4 bohr. At about 6 bohr, penetration effects make the multipole expansion meaningless. Possible simplifications for future calculations are suggested. A seemingly good van der Waals minimum which is calculated by the ab initio SCF method is shown to be caused in part by the (mathematical) effect of basis set enlargement. CNDO results disagree completely with the ab initio calculations.
Valence force potential calculations of the lattice dynamics of diamond-type crystals
Authors: Bell M. I.
Published: February 2003

Recent calculations of the lattice dynamics of diamond, silicon, germanium, and gray tin using a valence for potential are shown to share a serious defect with simpler calculations employing only first- and second-neighbor interactions: The zone-boundary phonon frequencies at the points X and L cannot be fitted properly in such a model. It is shown that this difficulty arises in any model, which, like the valence force potential introduced by McMurry, Solbrig, Boyter, and Noble, and since employed by a number of authors, includes interactions involving only first, second, and fifth neighbors. It is found that the addition of interactions between third (but not fourth or sixth) neighbors creates the possibility of correcting this failure, and an improved calculation for silicon by Solbrig gives evidence for the feasibility of this approach.
Binary nucleation. II. Time lags
Authors: Wilemski Gerald
Published: February 2003

The role of the time lag needed to attain steady state nucleation in binary vapors is discussed. Under appropriate conditions it is possible to obtain both a large predicted rate of binary nucleation and a large time lag. In this circumstance, homogeneous nucleation of the more concentrated component may be the predominant process. It is of obvious importance to the experimentalist to differentiate between these possibilities. Approximate formulas for the time lags are developed, and representative calculations for the ethanol–water system are reported.
Low frequency light scattering from liquid and supercooled ethyl benzoate
Authors: Bezot P. , Searby G. M. , Sixou P.
Published: February 2003

The low frequency depolarized and polarized light scattering spectra of ethyl benzoate have been studied over a temperature range of 150 °. At high temperatures, the hydrodynamic theories of Keyes–Kivelson and Andersen–Pecora are adequate to describe the fine structure of the depolarized spectra. At low temperatures, a weak depolarized doublet is seen, corresponding to propagating transverse excitations in the supercooled liquid. The above theories do not correctly describe the observed spectra at low temperatures.
Vibrationally excited OH from the reaction of H with NO2 observed by laser-induced fluorescence
Authors: Brophy John H. , Silver Joel A. , Kinsey James L.
Published: February 2003

The reaction H+NO2→OH+NO, which is widely used as a source of OH radicals for gas kinetic studies, has been thought to yield Oh almost exclusively in the ground state. However, recent experiments indicate that as much as 30% to 50% of the total energy may appear in vibrational excitation. In this study the reaction is studied using a laser-induced fluorescence method. The results indicate that as little as 10% of the total energy is in vibrational excitation.
Binary nucleation. I. Theory applied to water–ethanol vapors
Authors: Wilemski Gerald
Published: February 2003

A quantitative study of nucleation in vapor mixtures of ethanol and water near 273 °C is presented. First, Reiss’ theory of binary nucleation is reexamined. The theory is modified slightly in order to yield the proper limit for homogeneous nucleation in a one-component system. Moreover, a corrected expression for the equilibrium concentration of mixed clusters is derived. Calculations of the critical vapor activities needed to produce a visible condensate are presented and compared with the results of Flood’s cloud chamber experiments. The agreement is only fair, but qualitative accord is found.
Vanishing rainbows near orbiting and the energy dependence of rainbow scattering: Relation to properties of the potential
Authors: Greene E. F. , Hall R. B. , Mason E. A.
Published: February 2003

The energy threshold behavior of elastic rainbow scattering near the transition to orbiting is derived. Analysis of the energy dependence of the rainbow angle shows that the full range from high energies down to orbiting can be fitted with two parameters. Thus measurements of the rainbow angle can give essentially only two pieces of information about the potential. For potentials of common shapes such measurements are sensitive to regions of the potential just beyond the minimum and give information about the shape of the potential in this range. However, neither a minimum nor a point of inflection in the potential is necessary for rainbow scattering.
Fluorescence quenching of ultraviolet excited aromatic solutions by chloroform and several related chlorinated methanes
Authors: Saperstein David , Levin Eugene
Published: February 2003

Relative fluorescence quenching rate constants for solvent excitation to S1, S2, and S3 are reported for a number of the solvent–quencher (S–Q) systems in which the solvent was either benzene, toluene, o-xylene, p-xylene, or anisole and the quencher was either CH2Cl2, CHCl3, CDCl3, CH3CCl3, CFCl3, or CCl4. The magnitude of the experimental rate constants are found to follow qualitatively, but not quantitatively, the hypothesis that the rate-determining step in the quenching process is the formation of a DSAQ (donor–acceptor) exciplex by an excited solvent molecule (Sn, n=1, 2, or 3) and a quencher molecule. Tentative explanations are offered for these findings.
Comment on the accuracy of Rabitz’ effective potential approximation for rotational excitation by collisions
Authors: Green Sheldon
Published: February 2003

Cross sections for rotational excitation of HCN by low energy collisions with He have been computed with the effective potential approximation of Rabitz and compared with accurate quantum close-coupling results. Elastic cross sections are found to agree to about 20%; inelastic cross sections agree in general magnitude but not in detailed values for specific quantum transitions.
Energy transfer mechanisms in Tb(OH)3 from line shapes of band-to-band exciton fluorescence
Authors: Cone R. L. , Meltzer R. S.
Published: February 2003

Mechanisms of energy transfer between Tb3+ ions in Tb(OH)3 have been isolated for several states of the Tb3+ ion by means of line shape measurements of the band-to-band fluorescence from the two lowest exciton bands of the 5D4 manifold to one exciton band in the 7F6 (ground) and 7F5 (first excited) manifolds. The interionic energy transfer matrix elements were determined by comparing the observed line shapes with calculations of the exciton band-to-band transitions in which the energy dispersion was varied. A consideration of the simple nature of the single-ion states, an examination of the selection rules on the various energy transfer mechanisms, and an analysis of the range dependence of these mechanisms has enabled us to conclude that (1) the exchange plays a major role in both the 7F5 and 7F6 bands, (2) the electric multipole interaction does not make the dominant contribution to the energy transfer matrix elements in either manifold, (3) the magnetic dipole–dipole interaction plays a small but significant role particularly in the 7F6 manifold, and (4) the virtual phonon exchange plays a role comparable to that of exchange in the 7F6 band, a result which is of special interest in view of the few cases in which its importance has been clearly demonstrated.
Infrared absorption spectra of gaseous HD. I. Collision-induced fundamental band of HD in the pure gas and HD–He mixtures at room temperature
Authors: Prasad R. D. G. , Reddy S. Paddi
Published: February 2003

The infrared absorption spectra of the fundamental band of HD in the pure gas for densities up to 50 amagat and in HD–He mixtures for different base densities of HD and a number of total gas densities up to 190 amagat have been studied at room temperature. The collision-induced features of the band in the pure gas and in HD–He mixtures are similar to those previously observed for the spectra of H2 and D2 in the pure gases and in their binary mixtures with helium, respectively. In addition, the absorption profiles of the pure gas show the allowed sharp R1(1) transition due to the individual HD molecules. The binary absorption coefficients of the band derived from the measured integrated intensities are 2.32×10−35 and 0.94×10−35 cm6 s−1 for pure HD and HD–He, respectively. An analysis of the profiles of the enhancement of absorption of the band in HD–He has been performed by assuming appropriate line shapes, and the three half-width parameters, δd and δc of the overlap transitions and δq of the quadrupolar transitions, are obtained. The half-width δc of the intercollisional interference dip of the Q branch increases with density ρ of helium and fits to the equation δc = aρ − bρ2(a≫b). This observation and the conclusions derived from it are similar to those obtained previously for H2–He.
Molecular beam measurements of the D–H2 potential and recalibration of the reactive cross section
Authors: Gengenbach R. , Hahn Ch. , Toennies J. P.
Published: February 2003

Absolute integral cross sections have been measured for D–H2 at primary particle velocities v=7730 m/s and 9580 m/s with a 77 °K target. The number density of target particles was determined with an absolute accuracy of 1% by the method of dynamic expansion. The over-all error in the absolute cross sections is estimated to be about 1.5%. All available measurements of integral cross sections as a function of velocity were used to test literature potentials in a χ2 procedure. A new Born–Mayer–Spline–Van der Waals (BMSV) potential is proposed that is in reasonable agreement with recent ab initio calculations. The potential is also consistent with the differential total cross section measurement of Geddes, Krause, and Fite. This justifies the use of the new potential together with the differential measurements to perform an absolute calibration of the cross section for the D+H2 → HD+H reaction. The calibrated reactive cross section is found to be in reasonable agreement with measured bulk rate constants.
Generalized one-dimensional Ising model for polymer thermodynamics
Authors: Azbel’ M. Ya.
Published: February 2003

Polymer thermodynamics is described by the one-dimensional two-component Ising model (in an external field) which includes a Coulomb attraction. No assumptions are made about the component sequence. An arbitrary thermodynamic quantity is related to three particular distribution functions over the sequence composition. The experimental investigation of the dependence of thermodynamic quantities on the temperature and other independent variables is shown to reflect the sequence distribution function. A strong enough effective interaction results in a phase transition. The phase transition singularity can be either of finite order or of essential singularity type and depends on the sequence structure.
Potential function for the ν7 vibration of phosphorus pentafluoride
Authors: Bernstein Lawrence S. , Kim Janice J. , Pitzer Kenneth S. , Abramowitz Stanley , Levin Ira W.
Published: February 2003

The gas phase Raman spectrum of the ν7 fundamental of PF5 has been observed under spectral resolutions approaching 1 cm−1. With the use of a two dimensional harmonic oscillator basis set and a potential function of the form V (ρ,φ) = (1/2) aρ2 − b cos(3φ) ρ3 + cρ4, an adequate fit to the ν7 region was obtained. The barrier to axial–equatorial fluorine atom interchange was estimated to be 1371 cm−1 (3.92 kcal/mole).
Reaction, cavity, and dipole fields in a discrete lattice
Authors: Fulton Robert L.
Published: February 2003

Reaction, cavity, and dipole fields are evaluated in a simple model crystal. Aside from possible permanent dipole moments the electrical properties of isotropic molecules located on the sites of a cubic lattice are mimicked by a susceptibility appropriate to a collection of harmonic oscillators. Although qualitatively similar as a function of the dielectric constant ε, the reaction field in a discrete cubic lattice is found to be smaller than in a continuum. The cavity field in a discrete lattice is found to be larger than in a continuum and to diverge linearly in ε for large ε. It is found that the present expression for the reaction field is not in disagreement with data gleaned from studies of the dependence of chemical shifts on ε. Following the classic calculation of Onsager, the reaction and cavity fields are used to determine the dielectric constant of a polar crystal. In body centered cubic and face centered cubic lattices it is found that the expression for ε is singular at a temperature of about 0.30 TD, indicating a phase transition below the Debye temperature TD but nevertheless nonzero. Because of the anomalous dispersion of its energy bands, the simple cubic lattice undergoes a different sort of phase transition at a temperature of about 0.84 TD.
Light scattering from a binary liquid mixture above its critical consolute point
Authors: Thiel D. , Chu B. , Stein A. , Allen G.
Published: February 2003

Light scattering measurements were performed on the binary liquid mixture carbon tetrachloride–perfluoromethylcyclohexane at its critical concentration and immediately above its critical consolute temperature. Measurements of time averaged intensities as a function of scattering angle and temperature distance from the critical consolute temperature, T−Tc, yielded the critical parameters γ = 1.220±0.018, ξ0 = 2.28±0.21 Å, and ν = 0.626±0.013. Here γ describes the temperature dependence of the susceptibility χ according to the relation χ∝ε−γ, where ε = (T−Tc)/Tc. ξ0 and ν characterize the correlation length with ξ = ξ0ε−ν, which is a measure of the spatial extent of local concentration fluctuations. These values of γ and ν agree well with the scaling and universality hypothesis. Time dependence of the intensity as a function of scattering angle and of T−Tc was determined by measuring the photocurrent autocorrelation function. These measurements yielded decay rates for concentration fluctuations. Combination of decay rates, correlation lengths, and viscosities allowed a direct comparison with the mode–mode coupling theory of Kawasaki. Excellent coincidence of theory and experiment was found except in the vicinity of Kξ = 1, where decay rates were high when compared to theory. K is the magnitude of the momentum transfer vector. Sources of error in the critical parameters obtained from light scattering intensity measurements were analyzed in detail. A useful equation is derived which relates the actual scattering angle inside a cylindrical cell to eight experimental parameters including five ’’alignment’’ parameters which are subject to error. Graphical criteria are presented for estimating uncertainties in critical parameters γ, ξ0, ν, and η.
Detection of the amino radical NH2 by laser magnetic resonance spectroscopy
Authors: Davies P. B. , Russell D. K. , Thrush B. A. , Wayne F. D.
Published: February 2003

Laser magnetic resonance spectra of the NH2 radical in the lowest vibrational level of its 2B1 ground electronic state have been detected, using far infrared H2O and D2O lasers. The spectra have been assigned to individual Zeeman components of the rotational transitions 220 − 331 (78 μm H2O laser), 111 − 220 (118.6 μm H2O laser) and 735 − 744 (108 μm D2O laser).
Vibrational deactivation of N2O(001) by N2O, CO, and Ar from 144–405 °K
Authors: Starr D. F. , Hancock J. K.
Published: February 2003

Vibrational deactivation of N2O(001) by N2O, CO, and Ar has been studied from 144 to 405 °K using the laser fluorescence method. The probability of vibration to vibration energy transfer, N2O(001) + CO(v=0) ? N2O(000) + CO(v=1) + ΔE = 81 cm−1, is 0.025 and is independent of temperature from 144 to 405 °K. We have performed theoretical calculations on this rate using the modified Sharma–Brau theory developed by Tam. The present results, together with previous experimental work at higher temperatures, suggest a good theoretical fit above room temperature, but not below. The deactivation of N2O(001) in collisions with N2O, CO, and Ar, N2O(001) + M → N2O(mnl0) + M + ΔE, has been determined to be 102 to 104 times slower than observed for the V→V exchange process. The N2O(001) intramolecular rates become smaller with decreasing temperature with the exception of the N2O self-relaxation rate. Below 250 °K this rate increases rapidly with decreasing temperature.
Homonuclear diatomic scattering from solid surfaces: A hard cube model
Authors: Nichols William L. , Weare John H.
Published: February 2003

A classical mechanical hard cube theory describing the scattering patterns of homonuclear diatomic gas beams scattered from well-defined solid surfaces is presented. The diatomic gas molecules are approximated by ellipses having rotational as well as translational energy. Scattering patterns are numerically computed with the assumption that only single collision processes are important. Analytical expressions for scattering patterns are derived by assuming that the ellipticity of a gas molecule is close to 1 and that the incident rotational energy is zero. The analytical scattering patterns exhibit the same qualitative features as the numerically computed patterns, though they do not agree quantitatively with them. The theory is briefly compared with previously presented hard cube models.
EPR investigation of Eu2+ in orthorhombic sites of RbBr
Authors: Muñoz P. E. , Rubio O. J. , Murrieta S. H. , Aguilar S. G. , Boldú O. J.
Published: February 2003

Accurate spin-Hamiltonian parameters describing the electron paramagnetic resonance spectra of Eu2+ in orthorhombic sites of RbBr single crystals have been determined at 77 and 300 K for the first time. Absolute signs of the spin-Hamiltonian parameters were determined for sample temperatures less than 4.2 K. The sign of the spin-Hamiltonian parameter b20 was found to be positive and therefore the same as that previously found for the same impurity in the isomorphic hosts of NaBr and KBr. We present a plot of the b20 term vs lattice parameter for Eu2+ ions in the alkali bromide series with results which should be noted in attempts to calculate theoretical 8S7/2 ground state splittings.
EPR discharge-flow kinetics: The H + ClF3 reaction
Authors: Pak S. J. , Krech R. H. , McFadden D. L. , MacLean D. I.
Published: February 2003

The reaction of hydrogen atoms and chlorine trifluoride is studied directly by the discharge-flow EPR technique over the temperature range of 243–313 K and total pressure range of 4–8 torr. Hydrogen atoms are produced by dissociation of molecular hydrogen in a microwave discharge. Absolute rate constant data from 43 experiments at four temperatures yield the Arrhenius equation, log(kex/cm3 mol−1⋅sec−1) = (11.9±0.1) − (1470±560)/ϑ, where ϑ = 2.303 RT/J mol−1. The experimental rate constant, kex, is interpreted as a lower bound for the reaction H+ClF3→HF+ClF2.
Computation of atomic many-electron integrals using a Markov chain generated Monte Carlo method
Authors: Gibbs R. L. , Dunn Tucson
Published: February 2003

A Monte Carlo method for evaluating complicated many-electron integrals is described. The process utilizes a Markov chain which properly chooses configurations of electrons in accordance with the form of Ψ chosen. The method inherently reduces the variance compared to the classical Monte Carlo method. Some explicit examples are shown for a four electron system.
Coherence in multilevel systems. I. Coherence in excited states and its application to optically detected magnetic resonance in phosphorescent triplet states
Authors: Breiland W. G. , Brenner H. C. , Harris C. B.
Published: February 2003

A method for monitoring the time development of the coherent components in a multilevel system using an observable which can measure only the relative populations in the levels is presented. Using standard density matrix formulation, expressions are obtained for the conventional and this new ’’probe’’ pulse method. For several of the more common coherence experiments, optically detected magnetic resonance of molecular excited triplet states is used to illustrate the principles. The effect of coherent averaging of relaxation terms in the spin Hamiltonian is discussed and specifically applied to excited triplet states in zero field. Finally, experimental results of coherence measurements are presented along with a detailed description of the apparatus and methods used.
The reduction of nucleotides by ionizing radiation: Uridine 5′ phosphate and cytidine 3′ phosphate
Authors: Box Harold C. , Potter William R. , Budzinski Edwin E.
Published: February 2003

Anions formed by the addition of an electron to the uracil base were observed in single crystals of the barium salt of uridine 5′ phosphate x-irradiated at 4.2 °K. The hyperfine coupling tensor for the C6–H proton was deduced from ENDOR measurements; the principal values are −59.12, −32.92, and −16.24 MHz. Similar measurements were made on single crystals of cytidine 3′ phosphate. The principal values for the C6–H proton hyperfine coupling in the anion formed on the cytosine base are −59.26, −33.98, and −14.68 MHz.
The statistical treatment of the dielectric constant of a nonpolar gas
Authors: Bose T. K. , Navon A. , Springett B. E.
Published: February 2003

It is shown that the statistical treatment of Isihara for the dielectric second virial coefficient B of nonpolar gases is equivalent to that of Buckingham and Pople. The negative value of B for helium, determined theoretically by Isihara, is due to an error in his calculation.
Potential energy surfaces for H + Li2 → LiH + Li ground state surface from large scale configuration interaction
Authors: Siegbahn Per , Schaefer Henry F.
Published: February 2003

A b initio electronic structure calculations have been performed to determine the HLi2 potential energy surface. A contracted Gaussian basis set was employed: H(5s 1p/3s 1p), Li(8s 3p/4s 3p). In addition to selfconsistent-field (SCF) wavefunctions, full configuration interaction (CI) was carried out for the three valence electrons. For general geometry (point group Cs), the CI included 5175 configurations. For the diatomic molecules Li2 and LiH, these methods yield dissociation energies within 5 kcal/mole of experiment, and accurate spectroscopic constants are also predicted. The minimum on the HLi2 CI potential surface occurs for an isosceles triangle structure with r (H–Li) = 1.72 Å and on LiHLi bond angle of 95°. This minimum lies 22.4 kcal/mole below the separated products LiH + Li. The linear HLiLi minimum is much shallower, lying only 4.2 kcal/mole below the products. The much simpler single configuration SCF calculations yield qualitatively similar results. Furthermore, these features of the surface are quite analogous to those predicted for F + Li2 by Pearson and coworkers. The angular dependence of the surface between the C2v and C∞v extremes is discussed. The ’’electron jump’’ from covalent HLi2 to ionic H−Li2+ is seen to be much more gradual than was the case for FLi2. The electronic structure is described using a natural orbital analysis of the most important configurations in the wavefunction.
MCSCF calculations for six states of NaH
Authors: Sachs Edward S. , Hinze Juergen , Sabelli Nora H.
Published: February 2003

A b initio multiconfiguration self-consisting-field calculations are reported for the energies, electronic wavefunctions, and one-electron properties of the X1Σ+, A1Σ+, B1Π, a3Σ+, b3Π, and c3Σ+ states of NaH over a wide range of internuclear distances. In these calculations, only the two valence electrons are correlated. Three states (X1Σ+, A1Σ+, and b3Π) were found to be bound, with the following dissociation energies and internuclear separations (with known experimental values in parentheses): De (X1Σ+) = 1.878 (2.12±0.20) eV, Rmin (X1Σ+) = 3.609 (3.566) bohr; De (A1Σ+) = 1.203 (1.41±0.20) eV, Rmin (A1Σ+) = 6.186 (6.062) bohr; and De (b3Π) = 0.109 eV, Rmin (b3Π) = 4.458 bohr.
Rotation–vibrational analysis for three states of NaH and NaD
Authors: Sachs Edward S. , Hinze Juergen , Sabelli Nora H.
Published: February 2003

We have carried out a rotation–vibrational analysis for the X1Σ+, A1Σ+, and b3Π states of NaH and NaD using accurate ab initio calculated potential curves. The calculated values of Be, αe, Re, ωe, and ωexe (with known experimental values in parentheses) are NaH X1Σ+ : Be = 4.748 (4.886) cm−1, αe = 0.126 (0.129) cm−1, Re = 3.625 (3.562) bohr, ωe = 1183.17 (1172.2) cm−1, and ωexe = 21.23 (19.72) cm−1; NaD X1Σ+ : Be = 2.475 (2.5575) cm−1, αe = 0.0474 (0.0520) cm−1, Re = 3.624 (3.565) bohr, ωe = 826.60 (826.10) cm−1 and ωexe = 9.44 cm−1; NaH b3Π : Be = 3.533 cm−1, αe = 0.853 cm−1, Re = 4.202 bohr, ωe = 419.39 cm−1 and ωexe = 50.25 cm−1; NaD b3Π: Be = 1.763 cm−1, αe = 0.265 cm−1, Re = 4.294 bohr, ωe = 311.95 cm−1 and ωexe = 28.68 cm−1. The anomalous behavior of the Bv′s and ΔGv+1/2′s of the A1Σ+ state is satisfactorily reproduced by these calculations: for NaH, Bv (max ) = 1.9717 (1.941) cm−1 at v = 6 (6) and ΔGv+1/2 (max) = 381.37 (360.3) cm−1 at v = 9 (8); for NaD, Bv (max) = 1.0274 (1.012) cm−1 at v = 8 (8) and ΔGv+1/2 (max) = 275.34 (260.22) cm−1 at v = 13 (12). The wavefunctions obtained were also used to calculate average electric dipole moments resulting in the following values (for the lowest rotation–vibrational states): for NaH, μe (X1Σ+) = 2.674 a.u., μe (A1Σ+) = 0.189 a.u., and μe (b3Π) = 0.158 a.u., for NaD, μe (X1Σ+) = 2.668 a.u., μe (A1Σ+) = 0.178 a.u., and μe (b3Π) = 0.163 a.u.