bluuebunny/crossref_metadata_2025 huggingface dataset:
Fully quantum study of vibrational energy transfer between H2 and D2
Authors: Alexander Millard H.
Published: February 2004

We present here the results of a detailed study of the collisional transfer of vibrational energy between H2 and D2. Following our work on D2–D2 collisions [J. Chem. Phys. 59, 6254 (1973)], we carry out a fully quantum treatment of the collision dynamics, assuming a collinear geometry. We have used one fully ab initio and four different model interaction potentials. For both the 10→01 and 11→02 processes, we find that the transition probabilities are extremely sensitive to the choice of potential surface. Another result is that nearest-atom model potentials yield inelastic probabilities which are too large. We further show that long-range forces play a far less important role in the case of these off-resonant processes than for the D2–D2 near-resonant collisions. Also of interest is the result that the calculated V–V transition probabilities are largely insensitive to the choice of a harmonic or Morse oscillator description of the colliding molecules.
Production of negative ions from CH3X molecules (CH3NO2, CH3CN, CH3I, CH3Br) by electron impact and by collisions with atoms in excited Rydberg states
Authors: Stockdale J. A. , Davis F. J. , Compton R. N. , Klots C. E.
Published: February 2004

Thermal electron attachment to nitromethane, methylcyanide, methyliodide, and methylbromide is compared with capture of electrons by these molecules from highly excited Rydberg states of atoms. Data on thermal electron attachment to CH3NO2 are consistent with a three-body attachment process, with the nature of the third body being important. The thermal energy electron attachment rate constant for CH3CN is ≤1.24×10−14 cm3 sec−1. Some results on dissociative electron attachment and ion pairing processes in CH3NO2, CH3CN, CH3I, and CH3Br are also presented.
Energy transfer during ``orbiting'' collisions
Authors: Dillon T. A. , Stephenson J. C.
Published: February 2004

Semiclassical collision theory is used to calculate the temperature dependence of cross sections for exchange of vibrational excitation from HF and DF to CO2. For collisions with kinetic energy less than four-fifths of the intermolecular potential well depth there exists a narrow range of impact parameter for which the scattering angle exceeds 2π (i.e., more than one revolution). That part of the trajectory in excess of a single revolution can be accurately represented by a simple model of a circular orbit with a constant angular velocity replacing the instantaneous classical turning point. Calculations are compared to experimental data and illustrate the significance of multiple revolution collisions.
Effect of an external electric field on the yield of free ions. II The initial distribution of ion pairs in liquid hydrocarbons
Authors: Mozumder A.
Published: February 2004

In this paper initial-distribution-averaged escape probability has been calculated for a variety of compounds using the procedure developed in the preceding paper. The present method allows computation at any external field strength and temperature although the possible effect of very high external fields on the initial distribution of the ion pairs has been ignored. It has been found that no unique form of distribution function is the ``best'' under all circumstances even though in many cases an exponential or a Gaussian form suffices. An extended initial distribution function tends to reduce the ``temperature inversion effect,'' i.e., the decrease of escape probability with temperature at high fields. Nevertheless at suitably high fields the escape probability averaged over several distribution functions is predicted to be relatively insensitive to temperature over a wide range; such an effect should be observable.
Gas-phase reaction of Ba with N2O. I. Measurement of production efficiency of excited states
Authors: Jones C. R. , Broida H. P.
Published: February 2004

Photon yields for the reaction of Ba with N2O are determined by observing chemiluminescence from flames in a rapid flow of buffer gas. The yield for BaO A−X photons is strongly dependent on inert-gas pressure and is measured to be 20% at pressures near 10 torr. Also at this pressure, the specific yield for production of BaO A-state molecules in a single vibrational level, v = 1, is found to have the surprisingly large value of 8%. The vibrational distribution of BaO A1Σ molecules in the flame is found to be a strong function of oxidizing reactant and buffer-gas pressure. A kinetic scheme accounting for gross features of the observations is presented.
Gas-phase reaction of Ba with N2O. II. Mechanism of reaction
Authors: Field R. W. , Jones C. R. , Broida H. P.
Published: February 2004

A reaction mechanism and a correlation diagram argument are presented to account for the high photon yield of the chemiluminescent reaction of Ba atoms with N2O. Most BaO molecules appear to be formed initially in the long-lived a3π state and to be transferred subsequently by collisions into the shorter-lived A1Σ+ state. This mechanism accounts for numerous kinetic and spectroscopic observations of BaO. The arguments suggest that reactions of alkaline earth atoms with N2O and with O3 may be suitable for electronic-transition chemical lasers.
Pulse radiolysis of liquids at high pressures. V. Absorption spectrum and yield of the solvated electron in liquid ammonia at 23 °C and pressures up to 6.7 kbar
Authors: Farhataziz  , Perkey Lewis M. , Hentz Robert R.
Published: February 2004

The absorption spectrum and primary yield of the solvated electron in liquid ammonia (eam−) have been determined by nanosecond pulse radiolysis of liquid ammonia at 23°C and pressures up to 6.7 kbar. With increase in pressure from 0.009 kbar (vapor pressure at 23°C) to 6.7 kbar, the following changes occur in the absorption spectrum: The transition energy at the absorption maximum (Emax) increases from 0.67 to 0.91 eV; the observable portion W of the bandwidth at half-maximum, from Emax to the high-energy side of the spectrum, increases by 35%; and optical density at the absorption maximum (ODmax) decreases by 32%. The pressure and temperature dependences of Emax are attributed to changes in (1) size of the electron cavity and (2) the local dielectric constant associated with the solvation shell of the electron. At 23°C and 0 kbar, (∂lnEmax/∂P)T for eam− exceeds that for solvated electrons in water, methanol, and ethanol by a factor of ∼4. Such a result indicates that the cavity of eam−(V̄=98 ml mol−1) is more compressible than that of the electron in water (V̄ = 7 ml mol−1 for eaq−) and the alcohols. From the changes in W and ODmax with increase in pressure from 0.009 to 6.7 kbar, the extinction coefficient at the absorption maximum is estimated to decrease by 19% (similar to the result for eaq−) and the primary yield is estimated to decrease from 3.2 to 2.0 solvated electrons per 100 eV (in contrast with invariance of the eaq− yield). The decrease in primary yield is attributed to an increase in the probability of neutralization relative to separation of geminate NH4+−eam− pairs owing to the large negative activation volume of the neutralization reaction as a consequence of the large volume of eam−.
Mechanism of volume viscosity in the liquid metal system lead-bismuth
Authors: Flinn Jane M. , Gupta P. K. , Litovitz T. A.
Published: February 2004

Ultrasonic measurements in the Pb–Bi liquid metal system show that volume viscosity in these alloys differs from that observed in the pure metals Pb and Bi. The excess absorption in the alloys exceeds that in either of the pure metals at low temperatures and decreases with increasing temperature. We suggest that this absorption is due to two effects: (1) a structural volume viscosity similar to that observed in pure Pb and Bi; and (2) a volume viscosity due to concentration fluctuations.
Quantum yield for photoionization of F centers at room temperature
Authors: Gudat Albert E. , Scott Allen B. , Wagner Manfred
Published: February 2004

The transfer of charge following absorption of a light pulse by colored KCl at room temperature in an electric field was studied as a function of the field. At sufficiently high fields the charge should reach a saturation value from which the quantum yield for photoionization η can be obtained directly. Saturation is difficult to achieve experimentally in KCl; however, in 2 of 12 samples studied, saturation was clearly established and η was found to be 0.49±0.07 and 0.17±0.03. Taking η to lie between about 0.2 and 0.5, the trapping cross section for negative ion vacancies lies between 50% and 80% of the capture cross section at room temperature.
Molecular orbitals of PtF6 and E110 F6 calculated by the self-consistent multiple scattering Xα method
Authors: Waber J. T. , Averill F. W.
Published: February 2004

The ground state one-electron energy states of PtF6 and E110F6 were calculated by the self-consistent multiple scattering Xα method. The results for the platinum compound agree well with the experimentally observed ligand field transition in optical spectra. Relativistic effects on the hexafluoride of E110 are discussed, and a more complex spectra may be found.
Relaxation processes in water: Viscosity, self-diffusion, and spin-lattice relaxation. A kinetic model
Authors: Hindman J. C.
Published: February 2004

Experimental measurements of T1 for oxygen-17 water in n-heptane emissions were made in the supercooled region down to −31°C. The data were fit to the double exponential form of the rate equation with activation energies for the two processes of E1=11.85±0.57 and E2=3.54±0.08 kcal mole−1, respectively. Using transition state rate theory, activation entropies ΔS1*=35.3±2.3 and ΔS2*=4.63±0.25 cal deg−1 · mole−1 were calculated. It is proposed that both the low and high temperature reactions are kinetic processes. It is suggested that the large entropy term for the low temperature reaction indicates that the relaxation process involves the ``cooperative'' dissolution of a small cluster of hydrogen bonded molecules. An examination of the available data for viscosity and diffusion indicates that the high temperature relaxation process is the same as that for the rotational (T1) relaxation. It is suggested that this process involves the breaking of a single hydrogen bond. The question of whether the low temperature processes for viscous flow, diffusion, and rotational relaxation are related is discussed.
Effect of spin orbit interactions on the dipolar nature of the radiative microwave zero-field transitions in aromatic molecules
Authors: El-Sayed M. A.
Published: February 2004

In the absence of spin orbit interaction, each of the zero-field (zf) transitions of a triplet state of a polyatomic molecule is magnetic dipole allowed, with a magnetic dipole moment of two Bohr magnetons resulting from the change of the spin motion from one quantization direction in the molecule to the other. By the introduction of spin orbit perturbations, the radiative transition probability of a zf transition can be altered in the following manner. (1) Additional transition probability could be ``borrowed'' from (orbital type) magnetic dipole allowed triplet-triplet and singlet-singlet transitions. (2) Additional transition probability could be ``borrowed'' from the spin zf magnetic dipole allowed transitions of other triplet states. (3) Transition probability can be ``borrowed'' from (orbital type) electric-dipole allowed triplet-triplet and singlet-singlet transitions. (4) Contributions 1 and 2 above are shown not to alter the polarization of the zf transition, but contribution 3 could, for some Cs or C2v type molecules, cause the electric moment to be in a different direction from that for the magnetic moment resulting from contributions 1 and 2. (5) In contributions 1 and 3, triplet-triplet transitions between the state under examination and other triplet states are found to be more important than singlet-singlet transitions, whose contributions appear in second order in the spin orbit mixing coefficient. (6) Because of the symmetry of the spin orbit and the electric dipole operators, the zf transitions of molecules having a center of inversion are found to be totally magnetic allowed in nature (i.e., contribution 3 vanishes). A rough estimate of the spin orbit mixing coefficient required to make the transition probability for the electric dipole allowed part of the zf transition comparable to that for the magnetic dipole allowed part shows it to be within the value expected in some molecules with strong spin orbit perturbation, e.g., out of plane distorted haloaromatics, N-heterocyclic or carbonyl molecules. This analysis leads to the important conclusion that some of the zf transitions of some low symmetry molecules with large spin orbit interactions could possess a magnetic and an electric transition moment parallel to one another and should thus show natural circular dichroism in the microwave region.
Spin echo of spins diffusing in a bounded medium
Authors: Neuman C. H.
Published: February 2004

The effect of reflecting boundaries on the spin echo measurement of diffusion in a geometrically bounded medium is calculated by considering the accumulation of phase shifts during the spin echo process. The distribution of phase shifts is shown to be Gaussian in the limits when the time between pulses is either long or short compared to the time required for a spin to diffuse from one boundary to another. The distribution is assumed to be Gaussian in the intermediate case, and the problem is reduced to the calculation of the mean square phase change due to diffusion. Expressions for spin echo amplitudes are derived for the cases of planar, spherical, and cylindrical boundaries, and forms are derived that are valid when the time of the measurement is very long or very short compared with the time required for diffusion between boundaries.
Molecular beam scattering from clean and carbon monoxide covered platinum (111) crystal surfaces
Authors: Bernasek S. L. , Somorjai G. A.
Published: February 2004

Scattering distributions of monatomic and diatomic gases from clean and CO covered Pt single crystal surfaces indicate efficient translational vibrational (TB-VS) energy transfer when the surface is CO covered. This efficient energy transfer is attributed to low frequency bending modes of the CO molecules on the surface that absorb the incident particle energy. The increased surface residence time resulting from this efficient energy transfer could be responsible for higher sticking probabilities of incident gases and increased reaction rates for adsorption limited surface reactions.
Density and temperature dependence of the isotropic-nematic transition
Authors: Pynn R.
Published: February 2004

An equation that relates the density and temperature of a fluid at a transition between isotropic-liquid and nematic phases is proposed and found to be consistent with available data. Both steric repulsions and van der Waals interactions are included in the discussion.
Relaxation experiments in binary mixtures of Kneser liquids. II
Authors: Hunter J. L. , Dossa D. , Haus J. , Sette D.
Published: February 2004

In a paper published in this Journal in 1971, relaxation curves for many binary mixtures of CS2, CH2, Cl2, CH2Br2, C6H6, and CCl4 were presented, but no attempt was made to interpret these results quantitatively. It has always been our intention to interpret these results in a following paper. In the present paper, it is found that reaction rates for vibrational relaxation energy exchange varied considerably with concentration in many binary mixtures. For those mixtures where variations of reaction rates are strongly dependent on concentration, an empirical formula has been found. Although this variation in reaction rates was empirically determined at low frequency, theory shows that reaction rates, though concentration dependent, should be independent of driving frequency. We have therefore used the empirically determined reaction rates in dichloromethane-carbon tetrachloride to predict relaxation curves of this mixture over frequencies and concentrations where the strength of relaxation is very sizable.
Ionization in liquid hydrocarbons: γ radiolysis yields and electron range distribution functions
Authors: Dodelet J.-P. , Freeman G. R.
Published: February 2004

The use of an exponential distribution of initial separation distances in thermalized electron-ion pairs in γ irradiated liquid hydrocarbons leads to the calculation of total ionization yields of Gtot = 5–6. The distribution possesses a power function tail (y−3) at large initial separations (y). The concepts of ionization and excitation in a liquid are briefly dicussed.
Rate constant of OH + OH = H2O + O from 1500 to 2000 K
Authors: Rawlins W. T. , Gardiner W. C.
Published: February 2004

It was shown several years ago that concentration profiles of the OH radical in the shock-initiated combustion of lean ([H2]/[O2] = 0.1) hydrogen-oxygen-argon mixtures at low pressures (≃ 30 kPa) and high temperatures (1200–2000 K) exhibit transient maxima prior to attainment of partial equilibrium. At that time, the maxima could not be accounted for quantitatively in terms of the accepted mechanism of the H2–O2 reaction. The profiles have been reanalyzed utilizing more sophisticated computational techniques and increased knowledge of the reaction mechanism. The occurrence of maxima at temperatures above 1500 K was found to depend upon the ratio of the rate constants of the elementary reactions O+H2→OH+H and OH+OH→H2O+O. Using the rate constant expression 1.6×1014 exp(−56.6 kJ/R T) cm3 mol−1 s−1 for the former reaction, the rate constant expression for the latter was found to be 5.5×1013 exp(−29 kJ/R T) cm3 mol−1 s−1. This latter expression does not extrapolate linearly on an Arrhenius plot to the available room temperature data. The effect of boundary layer growth on the data analysis is discussed.
Precision neutron diffraction structure determination of protein and nucleic acid components. XV. Crystal and molecular structure of the amino acid L-valine hydrochloride
Authors: Koetzle Thomas F. , Golic Ljubo , Lehmann Mogens S. , Verbist Jacques J. , Hamilton Walter C.
Published: February 2004

A neutron diffraction study of L-valine · HCl has been carried out: space group P21, a = 10.382(2), b = 7.066(1), c = 5.4407(9) Å, β = 91.40(2)°, Z = 2. The structure has been refined by full-matrix least-squares techniques with anisotropic temperature factors for all atoms and with a Type II anisotropic extinction correction, leading to a conventional R value of 0.031. All hydrogen atoms have been located with a precision of 0.005 Å. The structure is stabilized by a three-dimensional network of one O–H … Cl and three N–H … Cl hydrogen bonds, one for each hydrogen atom that is expected to participate in hydrogen bonding. The potential energy barrier for torsional motion of the ammonium group is estimated to be 6.4 kcal mole−1, or about 3 times larger than those found for the methyl groups. This difference reflects the effects of hydrogen bonding.
Scattering of alkali halides by electrons. I. CsF
Authors: Slater R. C. , Fickes M. G. , Becker W. G. , Stern R. C.
Published: February 2004

The scattering of CsF by 0.69–6.81 eV electrons has been measured with the molecular beam recoil technique. A novel kinematic analysis is used to fit center of mass differential cross section models to the data. Absolute total cross sections determined in this fashion are a factor of 2 smaller than first Born approximation predictions at low energies. Absolute momentum transfer cross sections are an order of magnitude smaller than theory.
Kinematics of small angle scattering
Authors: Fickes M. G. , Stern R. C.
Published: February 2004

In beam-recoil measurements of the scattering of molecules by electrons or photons, deflected molecules are observed at very small angles, comparable to the angular divergences of the observed and crossing beams. A kinematic analysis is presented that explicitly includes the effects of these divergences. Distinguishing features of the analysis include the transformation to apparatus fixed vs ``laboratory'' coordinates and the calculation of the integrals over a finite detector in center of mass coordinates. The latter feature permits a simple recipe for inversion of laboratory observations to obtain a center of mass differential cross section.
Adsorption of CO on a Ni(111) surface
Authors: Christmann K. , Schober O. , Ertl G.
Published: February 2004

Carbon monoxide adsorbed on Ni(111) forms around θ = 1/3 an ordered √3 × √3/R 30c structure whose unit cell is continuously compressed upon further increasing the coverage until at saturation (θ=0.53 corresponding to 1 × 1015 molecules/cm2) a densely packed layer is reached. The work function increases by 1.31 eV. At θ=1/3 the adsorption energy decreases from its value of 26.5 kcal/mole by 3 kcal/mole. CO adsorption causes the appearance of two peaks at 6 and 14 eV in the electron energy loss spectra, which are ascribed to excitations of electrons from chemisorption levels. The sticking coefficient remains nearly constant up to medium coverages which can be described by a ``precursor'' state model. The results show close similarities with those reported for other nickel surfaces.
Simple basis sets for molecular wavefunctions containing atoms from Z = 2 to Z = 54
Authors: Roetti Carla , Clementi Enrico
Published: February 2004

The double-zeta orbital exponents for Slater-type functions have been reoptimized for the ground state functions of the atoms with Z = 2 to the atoms with Z = 36, and accurately computed for the ground state wavefunctions of the atoms with Z = 37 to the atoms with Z = 54. Thus the entire series of basis sets for accurate double-zeta functions for the atoms from He(1S) to Xe(1S) are now available for molecular computations.
Annihilation of positrons in gaseous and liquid NH3 and in NH3–Ar and NH3–Ne mixtures
Authors: McNutt J. D. , Kinnison W. W. , Ray A. D.
Published: February 2004

The lifetime spectra of positrons annihilating in gaseous NH3, in NH3–Ar and NH3–Ne gas mixtures, and in liquid NH3 have been measured as a function of density and temperature. The annihilation rate of slow positrons in NH3 gas, λNH3, has a strong nonlinear dependence on gas density and temperature, indicating a complex positron-ammonia interaction. A Zeff/Z ratio of 1860 was observed for 22.5°C NH3 gas at 2.4×10−4 g/cm3. Above gas densities of about 0.0015 g/cm3 at 22.5°C, λNH3 is found to be equal to the value measured in liquid NH3 at 0.607 g/cm3. λNH3 is additionally found to be independent of the partial density of Ar or Ne in the gas mixtures. 1Zeff for orthopositronium annihilation in NH3 has a temperature independent value of 0.60±0.03 throughout the gaseous and liquid density range investigated.
VUV dissociative excitation cross sections of H2O, NH3, and CH4 by electron impact
Authors: Morgan H. D. , Mentall J. E.
Published: February 2004

Absolute excitation functions for excited fragments resulting from electron bombardment of H2O, NH3, and CH4 by low energy electrons (0–300 eV) have been measured in the VUV (1100–1950 Å). The predominant emission for each molecule was the H Lyman-α line while the OI, NI, CI, and CII emissions were at least an order of magnitude weaker. Absolute cross sections at 100 eV are given along with the appearance potential of the various processes and the possible dissociative-excitation channels through which such processes proceed.
Raman spectral shifts relevant to electron delocalization in polydiacetylenes
Authors: Baughman R. H. , Witt J. D. , Yee K. C.
Published: February 2004

The extent of π electron delocalization in solid-state polymerized diacetylenes is derived from observed Raman vibrational frequencies. Raman intense C=C and C≡C vibrations indicate that the polymer backbone is best represented by (=C–C≡C–C=)n. However, vC=C is linearly correlated with vC≡C over a wide frequency range and both frequencies increase with decreasing phase perfection. This suggests a significant fractional contribution δ from the mesomeric structure (–C=C=C=C–)n, which decreases the π bond order of C=C to 1-δ and C≡C to 2-δ. Both vC=C and vC≡C are calculated as a function of δ and compared with observed frequencies for variously substituted polydiacetylenes in different physical states. Resulting ΔvC=C/ΔvC≡C values obtained using two different bond-length to bond-order approximations are in good agreement with the observed value (1.19±0.12).
Molecular photodissociation
Authors: Mukamel Shaul , Jortner Joshua
Published: February 2004

In this paper we present a quantum mechanical model for direct photodissociation and for predissociation of polyatomic molecules in terms of a sequential decay scheme involving multiple coupled continua, where each continuum corresponds to a different internal vibrational state of the fragments. The coupling matrix elements between the ``initial'' state and the continuum states are in general determined by the appropriate vibrational overlap factors for the polyatomic radical, while intercontinua coupling for a triatomic molecule occurs only between adjacent vibrational continua. The time evolution of the system was handled by the Green's function method. Explicit theoretical expressions for the final vibrational distribution of the fragments in the photofragmentation of linear triatomic molecules were derived, which are determined by the initial coupling to the different continua and by a wave matrix which couples the various dissociative channels. The wave matrix was evaluated for some simple realistic models for the intercontinua coupling. The available experimental data for the vibrational distribution of the CN(B2Σ) radical resulting from photodissociation and predissociation of XCN molecules are well accounted for in terms of our theory.
Hall mobility measurements in HF doped ice
Authors: Ham Joe S. , Rose Douglas N.
Published: February 2004

Hall measurements on HF doped water ice crystals showed no signals comparable with those reported by Bullemer and Riehl on pure crystals. This may either be due to the considerably different techniques of their dc method compared to our double frequency method or it may be a result of the different nature of the samples. We find a small signal which can only be removed from the noise by extended runs that indicates a mobility of +4.2±2.8×10−3 cm2/V·sec at − 5°C. The sign is characteristic of positive charge carriers. This is of the same order of magnitude as the drift mobility reported in the literature.
Pressure induced spectra of axially symmetric and slightly asymmetric quadrupolar molecules
Authors: Zwicker Judith O. , Cole Robert H.
Published: February 2004

Expressions are derived for the transition moments and absorption resulting from the electric moment induced in one molecule of a pair by the field of the quadrupole moments of a neighbor with axial or near axial symmetry. Values of relevant matrix elements of squares and products of direction cosines are included. For axial polar molecules, the infrared absorption has a different frequency dependence than that by permanent dipoles, permitting separation of the effects. For asymmetric rotors with nearly axial inertial symmetry, the absorption transitions with ΔK=0 are associated with one of the two quadrupole moments and transitions with ΔK=±2 with the other, again permitting spectral resolution of the different contributions. These possibilities are analyzed in some detail for the polar molecules CH3F and CHF3 and for the asymmetric rotor C2H4.
Infrared chemiluminescence studies of the reaction of fluorine atoms with monosubstituted ethylene compounds
Authors: Moehlmann Jon G. , Gleaves John T. , Hudgens Jeffrey W. , McDonald J. D.
Published: February 2004

The technique of arrested relaxation infrared chemiluminescence has been utilized for the observation of emission from the polyatomic substitution products of fluorine atom reactions with ethylene, propene, vinyl chloride, and vinyl bromide. A new apparatus for performing such experiments is described. The entire apparatus, including the scanning Michelson interferometer used in obtaining the emission spectra, is cooled to liquid nitrogen temperatures or lower. The resulting improvement in signal to noise ratio allows emission spectra to be recorded down to frequencies as low as 720 cm−1. Analysis of the chemiluminescence data allows the determination of the partitioning of reaction energy over the product vibrational modes. The results indicate statistical partitioning occurs for the vinyl chloride and bromide reactions, while the ethylene and propene reactions exhibit nonstatistical partitioning. The difference in energy partitioning for these reactions is attributed to the existence of a potential energy barrier in the exit channel for the latter two reactions, while no such barrier exists for the former reactions.
Dielectric relaxation of some aliphatic esters in cyclohexane solution
Authors: Crossley J. , Koizumi N.
Published: February 2004

The dielectric constant and loss data, measured at seven microwave frequences, for seventeen aliphatic esters in cyclohexane solution at 25°C have been analyzed for mean relaxation times and Cole-Cole distribution parameters. The relaxation times for ethyl esters are slightly longer than those for corresponding methyl esters. In both cases, however, the relaxation time does not lengthen significantly as R in RCOOR′ is increased from C5 to C22. It would appear that the dielectric absorption of aliphatic esters is similiar to that for aliphatic ketones and is dominated by an intramolecular relaxation process.
Excitation and pre-excitation of glow curves in natural semiconducting diamonds
Authors: Chen R. , Winer S. A. A. , Kristianpoller N.
Published: February 2004

The uv excited glow peaks at ∼150 K and ∼250 K were investigated. The mainly blue thermoluminescence (TL) peak appearing at 150 K in semiconducting diamonds was found to have also red and infrared components all appearing at the same temperature. Another red peak appearing at 260 K included an infrared component at a somewhat higher temperature. The intensity of this red peak behaved superlinearly with the dose of the 360 nm excitation. It was possible to pre-excite the 250 K blue peak using infrared light when the exciting light was 225 nm; however, this caused a partial bleaching of the 150 K blue peak. The investigation also included pre-excitation at various temperatures, intensity vs dose measurements of pre-excitation, and intensity vs dose measurements at various temperatures. It was found that the red TL peak produced by 500 nm could be excited more efficiently at 180 K than at liquid nitrogen temperature. The energy level model is reviewed in accordance with the new results.
Comments on the significance of orientation variables in the transport properties of gases subject to external fields
Authors: Schruben Dale L. , Condiff Duane W.
Published: February 2004

Transport coefficients of linear dipolar molecules in an electric field are predicted to possess an E/P dependence due to correlations of orientation with linear and angular momentum. The effect is found to be of far greater significance than E2/P dependent contributions, which are shown to be due to field-induced orientation polarization. Limiting effects of strong orientation polarization are determined in calculations of the viscosity tensor of fully aligned spherocylinders.
NMR line shapes for exchanging systems at high rf field
Authors: Kaplan Jerome , Yang Ping Pin , Fraenkel Gideon
Published: February 2004

A method is described by which NMR line shapes at high rf may be calculated for chemically exchanging systems. First order exchange results in a set of manifestly linear density matrix equations that are solved with standard methods. Second order exchanging systems, a model example of which is considered here, give rise to second order density matrix equations. Inspection of sample values of density matrix elements reveals that products of off-diagonal elements are small enough to drop from the equations, while products of off-diagonal and diagonal elements, respectively, may be linearized as ρi jAρk,kB=ρi jA/n B. This selective neglect of bilinear terms (SNOB) leads to a set of first order density matrix equations. Inspection of the calculated line shapes reveals that NMR at high rf in exchanging systems allows more precise determination of rate constants for exchange (compared to low power) in the slow, and, in a limited sense, in the fast exchange limits.
Lattice dynamics of mixed crystals: HCl–DCl
Authors: Gussoni Mariangela , Zerbi Giuseppe
Published: February 2004

The method of ``negative eigenvalue theorem'' has been applied for the determination of the vibrational density of states of mixed crystals of HCl and DCl. The corresponding atomic motions have been derived, thus making it possible to understand the dynamics of these systems. The fitting with the infrared spectrum has been obtained by weighting the density of states with a transition moment based on a rigid ion assumption.
Mass-spectrometric identification of the paramagnetic component of a molecular beam. The focusing of radicals
Authors: Tal'rose V. L. , Kudrov B. V. , Leipunskii I. O. , Morozov I. I. , Larichev M. N.
Published: February 2004

The technique of identification of a paramagnetic component of the molecular beam in an inhomogeneous magnetic field is used for investigation of elementary chemical reactions. The focusing of a halogen atom beam as a function of the magnetic field strength is determined. A detailed mechanism of reaction F+CF3I→IF+CF3 is proposed. The dependence of the focusing of CF3 and CH3 radicals obtained in chemical reactions on the field strength is discussed.
Interpretation of the absorption spectrum of single crystal UCl4
Authors: Hecht Harry G. , Gruber John B.
Published: February 2004

Temperature-dependent polarized absorption spectra of single crystal UCl4 have been investigated in the range 4000–25 000 Å. The spectra have been assigned and analyzed using the U4+ ion in a field of D2d point group symmetry as a model. An rms deviation of 91 cm−1 in the fit to 34 assigned levels was achieved using the following set of parameters: F2=172.6±1.8; F4=38.79±1.13; F6=2.565±0.101; ζ=1666±11; A 20〈 r2〉=−291.3± 82.6; A 40〈 r4〉=−378.4± 30.8; A 44〈 r4〉=−5940± 195; A 60〈 r6〉=−141.4± 30.3; A 64〈 r6〉=557.6± 184.6 (all units in cm−1). These optical data are interpreted to show that the ground state belongs to the Γ4 irreducible representation, with the first excited state being a Γ5 level at approximately 140 cm−1. The parameters are discussed in terms of the bonding characteristic of the coordinated complex ion.
Calculation of the AX and AA X NMR spectral lineshapes in the presence of spin-lattice relaxation
Authors: Hughes D. G. , Smith M. R. , Switzer D. A. M.
Published: February 2004

Using the density matrix approach, a calculation is made of the NMR absorption lineshape for the AX spin system, where A and X are spin 1/2 nuclei of different species which are coupled by the scalar interaction ℏJI(A)·I(X) and are subjected to fluctuating magnetic fields which cause spin-lattice relaxation of both nuclei. An expression is given for χ″(ω) in terms of J and the spin-lattice relaxation rates WA and WX, for the case where the fluctuating fields are isotropic with no correlation between the various components. A similar calculation is made of the X spectrum for the AA′X nuclear spin system. An analytic expression is presented for χ″(ω) in terms of WA, WX, and the spin coupling constants JAA′, JAX, and JA′X. Equations connecting the density matrix elements involved in the AA′ spectrum are also derived for this case. The relevance of these lineshape functions to conventional high resolution NMR is briefly considered.
Anticrossings and microwave transitions between electronic states of H2
Authors: Freund Robert S. , Miller Terry A.
Published: February 2004

Electric field induced anticrossings and electric dipole microwave transitions have been observed between the d(3p) 3Πu, v=3, N=1 level and the i(3d) 3Πg, v=3, N=2 level of para H2. These observed signals correspond to normally forbidden changes in the electron spin magnetic quantum number MS of ±1 or ±2. They are weakly allowed, however, at the magnetic fields used (7 to 13 kG), because the electron spin S is not completely decoupled from the rotational motion N. The zero field separation between the levels is 1.9244±0.0007 cm−1, significantly larger than the optically determined value of 1.82 cm−1. The fine structure and Zeeman effect of the d(3p) 3Πu state are accurately known from previous work. In the i(3d) 3Πg, v=3, N=2 level, gN is measured to be −0.2202±0.0012, a value largely determined by L uncoupling in the (3d)3(Σ,Π,Δ)g complex but also influenced by unidentified perturbations. The measured spin-orbit and spin-spin interaction values are marginally significant, but they are needed to fit the data and are not inconsistent with estimates of their values.
Intermolecular potentials by the inversion of molecular beam scattering data. IV. Differential cross sections and potential for LiHg
Authors: Buck U. , Hoppe H. O. , Huisken F. , Pauly H.
Published: February 2004

High resolution differential scattering cross sections of lithium atoms by mercury have been measured in the thermal energy range. Supernumerary rainbows and rapid oscillations were well resolved. The data have been inverted to the interaction potential by the procedure described in Paper I [J. Chem. Phys. 54, 1923 (1971)]. The resulting potential (potential well depth ε=16.9×10−14 erg and distance rm=3.00 Å at the minimum) differs significantly in both the shape and the size parameters from the other alkali-mercury systems.
Electron spin-echo envelope decays and molecular motion: Rotational and translational diffusion
Authors: Brown I. M.
Published: February 2004

A description is given of the two-pulse electron spin-echo envelope behavior for dilute solutions of free radicals over a temperature range where the radical and matrix molecules are undergoing increased molecular tumbling. As the temperature is increased from 77 K, the phase memory time for the anthracene-d10 radical anion in 2-methyltetrahydrofuran (MTHF) first decreases, goes through a minimum between 104 and 140 K, increases to a maximum value at 180 K, and then decreases again up to 300 K. The temperature dependence of the phase memory time for the neutral radical 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl in MTHF is essentially the same. For peroxylamine disulfonate (PADS) in the glycerol-water and water matrix, only a minimum value is observed, since on the high temperature side of this minimum, the free induction decay signal is much larger than the echo. The low temperature side of these minima defines the slow tumbling region, i.e., τ ≪τc, where τc is the correlation time for the motion and τ is the interpulse time, whereas on the high temperature side τ≫τc. In the former region, the molecular tumblings render anisotropic interactions time dependent and produce additional local field fluctuations which account for the decrease of the phase memory time with increasing temperature. In the latter region the local field fluctuations are so fast that they average out the local fields and effect an increase in phase memory time with increasing temperature. Hence, the occurence of the minimum is analogous to a motional narrowing phenomenon. Expressions for the phase memory time and the shape of the echo envelope decay are derived assuming a Gauss-Markoff model for the rotational and translational diffusion effects. The predicted general behavior of the phase memory time is in agreement with observations in that a minimum value is predicted. Estimates of the minimum value of the phase memory time for the PADS radical in a glycerol-water matrix (85:15 wt%) indicate that at temperatures close to the minimum the phase memory time is probably determined by the local field fluctuations arising from a modulation of the nitrogen anisotropic hyperfine interaction by the radical rotational motions, whereas matrix hyperfine interactions and rotational and translational diffusion of the matrix molecules play only a minor role. The temperature dependence of the amplitude modulation in the echo envelope for the anthracene-d10 radical anion in MTHF is tentatively interpreted to mean that either the modulation pattern is less sensitive to radical rotations than is the phase memory time or, alternatively, the correlation time for matrix rotations is less than that for radical rotations. The decrease of the phase memory time at temperatures above the maximum value is due to Heisenberg spin exchange which is a result of radical-radical encounters brought about by increased radical translational diffusion.
Effect of photodecomposition on the infrared absorption of lead azide. II. Near infrared
Authors: Varma S. P. , Williams Ferd
Published: February 2004

The effects of photodecomposition on the near infrared intraazide ion absorption of lead azide films have been measured. The structure in the near ir is explained on the basis of the four types of azide lattice sites and the characteristic intraazide interatomic distances for each. The structure disappears during the early stage of the decomposition, indicating homogeneous destruction of the distinct types of sites in the thinner films. Changes in the near ir spectra are correlated with those in the far ir lattice modes and with those in the ultraviolet electronic spectra. The decomposition product has the optical properties of an amorphous material. The kinetics of the photodecomposition are analyzed and applied to the time dependence of the rate of nitrogen evolution.
Semiclassical theory of collisionally induced fine-structure transitions in fluorine atoms
Authors: Preston Richard K. , Sloane Chris , Miller William H.
Published: February 2004

A calculation of fine-structure transitions in F atoms impinging on both Xe and H+ has been carried out using a novel semiclassical theory that was proposed recently by Miller and George. The theory has the advantage of being conceptually simple and applicable to a wide class of situations. For Xe+F the cross section for the 2P3/2 → 2P1/2 excitation of F rises from its threshold (0.05 eV) to a value of ∼0.1 Å2 at a collision energy of 0.5 eV. The cross section for H++F is much larger, reaching a value of ∼1 Å2 at a collision energy of 0.25 eV, in reasonable agreement with recent quantum mechanical calculations.
Effects of polymorphism on positron lifetimes in tripalmitin
Authors: Walker W. W. , Kline D. C.
Published: February 2004

Positron lifetime parameters have been measured in three polymorphic forms of tripalmitin. Differential scanning calorimetry and x-ray powder diffractometry were used to identify the polymorphic forms. Each form exhibited a characteristic long lifetime τ2. Values of τ2 obtained at 22°C were 2.30, 1.44, and 2.23 nsec for the α, β, and β′ forms, respectively. Lifetime measurements as a function of temperature were also carried out for the α and β forms.
Dependence of polyatomic ion mobilities on ionic size
Authors: Lin Shen Nan , Griffin G. W. , Horning E. C. , Wentworth W. E.
Published: February 2004

The ion mobilities of 11 negative ions in nitrogen have been measured using a plasma chromatograph. The ions range from small negative ions such as NO2− to large organic ions such as the parent negative ion of 1-naphthaldehyde, (C11H8O)−. The reduced ion mobilities for these negative ions, in addition to previously published ion mobilities of protonated hydrocarbons, were compared with the Langevin theory of ion mobility. The hard sphere average collision cross section between the ion and the neutral carrier gas N2 was estimated using two models: (1) static model where the ion is nonrotating and (2) rotational model where the ion is assumed to be rotating rapidly. Comparison of the calculated reduced ion mobilities with the experimental values reveals that the experimental collision cross sections are intermediate between those calculated using these two models. The experimental and calculated ion mobilities gave the best agreement using the mean collision cross section for the static and rotational models. The average deviation between the experimental and calculated ion mobilities was 4% for a total of 21 positive ions and 14 negative ions. These results suggest (1) the ion size is the dominant parameter affecting the ion mobility of large molecular ions and (2) the rotation of nonspherical ions must be considered.
Spectroscopic studies of the SO2 discharge system. I. The microwave spectrum and structure of S2O
Authors: Tiemann E. , Hoeft J. , Lovas F. J. , Johnson D. R.
Published: February 2004

The microwave spectrum of S2O has been reexamined in order to obtain reliable predictions of all rotational transitions up to J=50 in the frequency region 10–130 GHz and to determine the structure of the molecule. A centrifugal distortion analysis has been carried out on the ground state and lowest bending vibrational state of 32S32S16O. Centrifugal distortion corrected rotational constants have also been obtained for 34S32S16O and 32S34S16O. These observations allowed the determination of the structure: rs(S–S)=1.88248(10) Å, r(S–O)=1.4637(5) Å, ∠SSO=118.26(7)o.
Spectroscopic studies of the SO2 discharge system. II. Microwave spectrum of the SO dimer
Authors: Lovas F. J. , Tiemann E. , Johnson D. R.
Published: February 2004

Microwave spectra assignable to a dimer of SO have been detected in an electric discharge of SO2. A centrifugal distortion analysis has been carried out on the spectra from the ground state and the lowest excited vibrational state of 16O32S32S16O and the ground vibrational state of 16O32S34S16O. The molecule was found to have a planar cis configuration with the following geometry: r(SO)=1.458(2) Å, rs(SS)=2.0245(6) Å, and ∠SSO=112.7(5)o. The observed dipole moment of OSSO is μ=3.17(10) D.
Coil-stretch transition of dilute flexible polymers under ultrahigh velocity gradients
Authors: De Gennes P. G.
Published: February 2004

Because the hydrodynamic interactions are reduced by stretching, a solute polymer coil should unwind abruptly when a certain critical value of the velocity gradient is reached. Depending on the details of the velocity field (``longitudinal'', or ``transverse'', or more complex gradients) this coil ↔ stretch transition may be continuous (``second order'') or discontinuous (``first order''). In the latter case hysteresis should often be observed; a qualitative discussion of the associated relaxation times is given. Simple shear flow is an exceptional case, with no sharp transition. Some expected effects of the C↔S transition on the mechanical behavior, on optical properties, and on chemical degradation in flow are briefly analyzed.
New theory of active nitrogen
Authors: Young R. A.
Published: February 2004

The details of the collisional quenching of N2 (B 3Π)ν′1 are crucial to theories of active nitrogen. It is concluded that quenching of N2(B 3Π)ν′1 occurs by transfer to lower energy ν′2 levels of the A 3Σu+ state but with large rotational energy and that collisional transfer of N2(A) to N2(B) also produces high rotational energy in N2(B). The observed quenching of N2(B 3Π)ν′1 corresponds to rapid rotational relaxation and electronic state interchange that appear as fast vibrational relaxation down a ladder formed of both A and B vibrational levels. Formation of N2(A 3Σ)ν′2 by atom association and relaxation down the A−B ladder contributes a pressure independent term (above 1 torr) to the intensity of the N2 1st positive bands (1+) as suggested by Thrush and Golde. An additional process that involves the 5Σ and W 3Δ state, largely populated from N2(A) via N2(B), contributes a pressure dependence to the 1+ bands under suitable circumstances and permits population of low ν′ levels of B 3Π even in the absence of collisional energy degradation.
Vibrational Raman spectrum of orthorhombic BaBr2
Authors: Monberg Eric M. , Nicol Malcolm
Published: February 2004

Polarized Raman spectra are reported for a single crystal of the orthorhombic form of BaBr2. All 18 components of the first-order Raman spectra have been observed, and their symmetry classifications are assigned.