bluuebunny/crossref_metadata_2025 huggingface dataset:
Order–disorder phase transitions in NH4BrxCl1–x crystals
Authors: Garland C. W. , Leung R. C. , Zahradnik C.
Published: February 2003

Single crystals of NH4BrxCl1–x have been investigated ultrasonically over a wide range of pressure, temperature, and composition. Using the elastic constant c44 as a convenient probe, we have established the p–T–x phase diagram in the region where the orientations of NH4+ ions undergo long-range ordering. Two critical surfaces exist: one corresponding to ’’ferro’’ ordering and one corresponding to ’’antiferro’’ ordering. These surfaces intersect in a line of triple points closely related to the bicritical points in magnetic spin–flop systems. The adiabatic elastic constants c11 and C′ (as well as the acoustic absorption associated with these waves) have also been measured along several isobaric paths. The results are consistent with the existence of a master p–T phase diagram valid for all values of x.
Study of electronic properties of the SF6 molecule by (e,2e) spectroscopy
Authors: Giardini-Guidoni A. , Fantoni R. , Tiribelli R. , Vinciguerra D. , Camilloni R. , Stefani G.
Published: February 2003

(e,2e) spectroscopy has been used to obtain information on the electronic properties of the SF6 molecule, in particular on the valence states. The ordering of the orbitals given by ESCA spectroscopy has been confirmed. The measured electron momentum distributions are compared with the ones calculated from literature wave functions and show a poor agreement. Namely, it is shown that the minimum energy criterion that is successful in calculating the energy eigenvalues is not always sufficient to determine a good wave function basis set suitable to reproduce other chemical-physics properties. Bonding character of the three outermost orbitals is evidenced well by their momentum distributions; two configuration interaction peaks following the valence orbitals are characterized as well.
Rydberg states of H3
Authors: Jungen M.
Published: February 2003

The energies of the Rydberg states of H3 are calculated for the equilibrium geometry of the ion H3+ in order to interpret the observed transitions. The equilibrium geometry is assumed to be D3h with R=1.650 bohrs. (AIP)
A Mössbauer study of iron bonding to solid ammonia and its photofragments
Authors: Barrett Paul H. , Pasternak Moshe
Published: February 2003

Mössbauer studies of isolated 57Fe atoms in solid ammonia (0.1% Fe) at low temperatures suggest the formation of a FeNH3 molecule characterized by a quadrupole coupling constant ‖e2qQ/2‖=1.90  (5) mm ⋅s−1 and isomer shift δ=+0.60  (5) mn ⋅s−1 with respect to iron metal. Irradiating with a visible-UV Xe-source results in photolyzed species revealing two doublets, with hyperfine constants different from the parent molecule. To the more intense site we assign a FeNH2 formulation and to the low intensity doublet, a FeNH molecule. Both isomer shift and quadrupole splitting systematics are consistent with a single ammonia molecule bound to iron via the nitrogen atom. Absorbers prepared with higher concentrations of 57Fe (0.6%) show additional lines superimposed on the typical FeNH3 doublet.
Franck–Condon effects in resonance Raman spectra and excitation profiles
Authors: Siebrand Willem , Zgierski Marek Z.
Published: February 2003

Resonance Raman spectra and excitation profiles are calculated for totally symmetric vibrations not involved in vibronic coupling and thus deriving all their intensity from Franck–Condon effects. Theoretical results are derived for harmonic oscillators undergoing displacements and frequency shifts upon electronic excitation of the molecule. The model does not only consider the effect of interference between several totally symmetric modes but also includes a number of basic refinements absent from most previous treatments such as inhomogeneous line broadening, perturbation of a weak resonance transition by the preresonance spectrum of a strong neighboring transition, and normal coordinate rotation (Dushinsky effect). The model calculations are used to interpret observed Raman spectra and excitation profiles of totally symmetric modes in the chromate ion and the β-carotene molecule.
Chemi-ionization in collisions of metastable argon with metastable krypton
Authors: Neynaber R. H. , Tang S. Y.
Published: February 2003

Absolute and relative cross sections were obtained for the associative-ionization (AI) reaction Ar*+Kr*→ArKr++e and the Penning-ionization (PI) reactions Ar*+Kr*→Ar+Kr++e and Ar*+Kr*→Ar++Kr+e. The studies were made by a merging-beams technique for a relative kinetic energy W of the reactants from nominally 0.01 to 2 eV for AI and 0.01 to 10 eV for PI. The Ar* represents a composite of Ar(4s 3P2) and Ar(4s 3P0), and Kr* a composite of Kr(5s 3P2) and Kr(5s 3P0). The Ar*–Kr* system is moderately attractive with a well depth of a few tenths of an electron volt. The PI reactions are directed with most of the Penning ions scattered in the c.m. system in the direction of their parent reactants. The fraction of AI to total ionization (AI plus PI) is only a few percent or less. Cross sections for total ionization QT vary as W−0.40. At W=0.033 eV, QT=308×10−16 cm2. Such a relatively large QT has also been observed in this laboratory for collisions between two metastable He atoms and between metastable He and metastable Ne.
Structure and dynamics of associated molecular systems. II. Atom–atom potentials and the properties of ammonia
Authors: Klein Michael L. , McDonald Ian R. , Righini Roberto
Published: February 2003

The properties of ammonia are briefly reviewed. Recently proposed models for the intermolecular forces between ammonia molecules are tested by calculating selected properties other than those used in their parameterization. Results are reported for the second virial coefficient of the gas, the zero wave vector lattice frequencies of the solid, and the atomic pair distribution functions for the liquid. It is concluded that none of the existing atom–atom models is entirely satisfactory.
Electron excitation of the singlet-g states of H2
Authors: Day R. L. , Anderson R. J. , Sharpton F. A.
Published: February 2003

The optical method and time-resolved spectroscopy have been used to study the electron impact excitation of the G 1Σ+g(v′=0,1,2,3) and I 1Πg(v′=0) Rydberg states and the K 1Σ+g(v′=0,1,2), M 1Σ+g(v′=0), and N 1Σ+g(v′=1,2) doubly excited states of the H2 molecule. Emission cross section measurements in the range 0.2 to 3.3×10−20 cm2 and radiative lifetime measurements in the range ∼21 to 70 ns are obtained at 50 eV incident electron energy and ∼30 mTorr H2 gas pressure. In addition, H2–H2* quenching rate data are obtained for several rovibronic levels of the singlet-g states over the pressure range ∼10 to 400 mTorr.
An effective nuclear charge model for the prediction of valence force constants in polyatomic molecules. I
Authors: Ohwada Ken
Published: February 2003

The effective nuclear charges have been defined using the quadratic force constants which are obtained by the application of perturbation theory to homonuclear diatomic molecules. It has been shown that these charges are applicable to the calculation of force constants in heteronuclear diatomic molecules. To make further application of the effective nuclear charges to polyatomic molecules, we have proposed effective intramolecular potential functions founded on the electrostatic theorem, and have attempted to predict the force constants of triatomic molecules. The results show that our method of approach using the effective nuclear charges is useful for the prediction of approximate valence force constants in polyatomic molecules.
The far-infrared vapor phase spectra of the methylanilines
Authors: Kydd R. A. , Krueger P. J.
Published: February 2003

The potential energy barriers opposing coplanarity of the amino group and aromatic ring in 2-, 3-, and 4-methylaniline are found to be 558, 528, and 588 cm−1, respectively. The results show a reasonable correlation with experimental 15N NMR results and also (combined with barriers for halo-substituted anilines) with Hammett σ substituent constants. The trend in barrier heights is consistent with the concept that the methyl groups influence the electron density of the amino nitrogen more through the π system than through an inductive effect involving the σ bonds, as calculations reported elsewhere indicate.
EPR and ligand-ENDOR measurements of Cu (i
i) in l-alanine single crystals
Authors: Calvo R. , Oseroff S. B. , Abache H. C.
Published: February 2003

EPR experiments have been performed in l-alanine single crystals doped with copper in order to measure the gyromagnetic tensor of Cu(ii) and the hyperfine coupling tensor with 63Cu. The hyperfine coupling and the nuclear quadrupole interaction tensors of a ligand nitrogen have been obtained by ENDOR at 4 K. The principal values of the hyperfine coupling of 14N are: AN(1)=27 190 KHz, AN(2)=27 560 KHz, and AN(3)=41 740 KHz and those for the nuclear quadrupole interaction are PN(1)=97 KHz, PN(2)=987 KHz and PN(3)=−1084 KHz. The principal directions of the four tensors evaluated in this work are noncoincident. ENDOR resonances of another nitrogen ligand and at least three protons have been detected. Our ENDOR measurements show a pseudonuclear spin–spin interaction between 63Cu and 14N nuclei. This effect is very large and arises from a high order mechanism involving the hyperfine couplings of the unpaired electrons of Cu(ii) with those nuclei. The data are explained with a second order perturbation calculation and it is shown that these effects should be considered in systems with low symmetry where the electrons are coupled to more than one magnetic nuclei.
The electronic structure of MnO42− as determined by ESR in K2SO4
Authors: Greenblatt M. , Pifer J. H.
Published: February 2003

The electron spin resonance of MnO42− has been studied at 4.2 K in single crystal K2SO4. The principal values of the g and A tensors are gx=1.9757, gy=1.9696, gz=1.9445, Ax=−21.0×10−4 cm−1, Ay=−25.5×10−4 cm−1, and Az=−147.6×10−4 cm−1. Az and gz make an angle of 7.8° and the only element of symmetry in the MnO42− tetrahedron is a xz mirror plane. The ground state is dxy similar to that of MnO2−4 in K2CrO4 but different from the dxy ground state of MnO2−4 in BaSO4 and BaSeO4. It is suggested that a dz22 ground state will be stabilized whenever the host XO4 tetrahedron is distorted so that the angle O(1)–X–O(2) is greatly different from the angle O(3)–X–O(3′) where O(1) and O(2) lie in the mirror plane.
Quantum statistical mechanical (QSM) approach to molecular relaxation phenomena: An irreversible thermodynamic theory for radiationless transitions
Authors: Wassam W. A.
Published: February 2003

The quantum statistical mechanical (QSM) approach to molecular relaxation phenomena is extended to allow for nonexponential decay and the incorporation of external disturbances. Microscopic models can be substituted into the final results to study the temporal evolution of multistate systems. The relationship of the present version of QSM theory to Onsager’s theory of linear irreversible processes is discussed. It is shown that kinetics and the principle of detailed balance emerge from QSM theory. The results of QSM theory are related to Fermi’s golden rule, the scattering theoretic formulation of radiationless transitions, and Kubo’s generating function technique.
Stochastic trajectory simulation of iodine recombination in liquids
Authors: Hynes James T. , Kapral Raymond , Torrie Glenn M.
Published: February 2003

A stochastic trajectory simulation of iodine recombination in dense liquid solvents is presented. The calculations utilize a mean force potential which contains direct I–I interactions as well as solvent structure effects. Dynamical solvent effects are accounted for by a random force and friction coefficient. The time dependent probability of reaction for two initially separated radicals is determined. The choice of initial separations and atomic velocity distributions is appropriate for secondary recombination. The results of this study show the importance of including the strong direct chemical forces between the I atoms; the validity of simple diffusion equation approaches can thus be assessed. Effects due to solvent structure are quantitatively examined and are interpreted in terms of ’’caging’’ in dense fluids. The computer simulation results are also compared with the solution of the Smoluchowski equation for this problem and effects due to friction coefficient variation are discussed.
Collision induced relaxation of an electronically excited molecule: Evidence for low energy resonance enhanced vibrational deactivation
Authors: Sulkes Mark , Tusa James , Rice Stuart A.
Published: February 2003

The paper reports a study of collision induced vibrational relaxation of electronically excited iodine. A novel technique is used to probe the energy dependence of the collision cross section, namely, variation in the position of illumination along the axis of a supersonic free jet. Experimental results are reported which indicate that the relaxation induced by collisions with extremely small relative kinetic energy is very efficient. The evidence available is consistent with a model in which the vibrational deactivation is enhanced by orbiting resonances or rotational resonances of the colliding pair.
The effect of surface roughness on surface enhanced Raman scattering
Authors: Gersten Joel I.
Published: February 2003

A molecule adsorbed on the surface of a conducting prolate ellipsoid will Raman scAtter with an enhanced cross section. The relevance of this fact to the phenomenon of surface enhanced Raman scattering (SERS) is discussed. (AIP)
Rayleigh, Mie, and Raman scattering by molecules adsorbed on rough surfaces
Authors: Gersten Joel I.
Published: February 2003

Cross sections for Rayleigh and Mie scattering on a roughned conducting surface are given. The Rayleigh scattering is strongly enhanced by roughness. Rayleigh and Mie scattering are found to interfere with each other. The experimental consequences for surface enhanced Raman Scattering (SERS) are discussed. (AIP)
Theoretical studies of energy transfer in disordered condensed media
Authors: Blumen A. , Klafter J. , Silbey R.
Published: February 2003

In this paper we consider energy transfer among impurity molecules in disordered systems. We use the result of Gochanour, Andersen, and Fayer [J. Chem. Phys. 70, 4254 (1979)] for the generalized diffusion coefficient D(t) in terms of the decay function Φ(t), in conjunction with separate calculations of Φ(t) in the pair approximation to compute D(t). Both multipolar and exchange interactions and both short and long time limits are considered. Comparisons to earlier work are made.
Time-correlation functions for restricted rotational diffusion
Authors: Wang C. C. , Pecora R.
Published: February 2003

The dynamical model of rotational diffusion of rod-shaped molecules in a conical volume is applied to the calculation of time autocorrelation functions of 1st and 2nd order spherical harmonic functions of the rod’s orientation angles. Numerical results are obtained for the various correlation functions as a function of the polar angle ϑ0 within which the molecule’s axis is confined. Applications to dielectric relaxation, dynamic light scattering, and fluorescence depolarization experiments are pointed out.
Comparison of reagent translation and vibration on the dynamics of the endothermic reaction Sr+HF
Authors: Gupta Arunava , Perry David S. , Zare Richard N.
Published: February 2003

The endothermic reaction Sr+HF(v=0)→SrF+H has been studied as a function of collision energy (4–14 kcal/mole) using a crossed beam geometry in which a seeded HF beam intersects a thermal Sr beam. At the same total energy this reaction is compared to the Sr+HF(v=1, J=1) reaction carried out under beam-gas conditions with a pulsed HF laser as the excitation source. In both cases the SrF products are detected by laser induced fluorescence. Using the Ba+HF→BaF+H reaction as an internal reference, the cross section of the Sr+HF(v=1) reaction is found to be 1–10 times greater than for the Sr+HF(v=0) reaction when the same total energy is supplied as reagent translation. Product internal energy distribution and total relative cross sections as a function of collision energy are also measured. Phase space calculations are able to reproduce most of these results. This agreement and other arguments suggest that the Sr+HF reaction often proceeds via multiple encounters which scramble the reagent energy modes during the course of a reactive collision.
Li2 and Na2 3Σg+–3Σu+ excimer emission
Authors: Konowalow Daniel D. , Julienne Paul S.
Published: February 2003

A b initio calculations show the 3Σg+–3Σu+ in Li2 and Na2 dimers to be primarily a near-infrared continuum with respective v′=0 lifetimes of 62 and 15 nsec. The peak stimulated emission cross section for v′=0 is 4.5×10−−6 at 1.3μ for Li2 and 1.8×10−15 cm2 at 0.83μ for Na2. These calculations suggest a tunable high gain, near-infrared laser excimer if the 3Σg+ state can be populated sufficiently rapidly.
The chemical dynamics of the reactions of O(3P) with saturated hydrocarbons. I. Experiment
Authors: Andresen P. , Luntz A. C.
Published: February 2003

Molecular beam-laser induced fluorescence experiments have probed the nascent internal state distributions and excitation functions of OH formed in the technologically important reactions O(3P)+RH→OH+R⋅. RH is a saturated hydrocarbon and R⋅ is an alkyl radical. A variety of RH have been investigated corresponding to abstraction of primary, secondary, and tertiary hydrogens. The OH rotational state distribution is nearly identical for all RH and decrease rapidly from its peak at the lowest rotational level. This demonstrates that reaction occurs when O(3P) is collinear to a C–H bond in the hydrocarbons. The vibrational state distribution of OH depends markedly on the type of hydrogen abstracted, with vibrational excitation increasing dramatically across the series primary to secondary to tertiary. This is interpreted as a shift from a repulsive towards a more attractive surface across the series. Partitioning into the OH spin doublets shows that these reactions are midway between the adiabatic and diabatic limits with respect to the spin–orbit surfaces. Excitation functions measure the dynamic thresholds, and are in good agreement with activation energies obtained from thermal rate constants. The excitation functions for v=1 OH exhibit a sharp decrease at energies considerably above threshold. This suggests that excitation of internal modes of R⋅ occurs only at high collisions energies. All of these results are interpreted qualitatively in terms of a simple, but general, triatomic model for the interaction of O(3P) with RH, i.e., where R⋅ can be considered a structureless particle.
The strength of three-electron bonds
Authors: Baird N. Colin , Taylor Kathleen F.
Published: February 2003

A b initio LCAO–MO calculations, including complete CI within the double zeta plus polarization basis set of STO-4G expansions, are reported for three-electron HeX systems where the nuclear charge of X varies in steps of 0.1 from 2 to 1. The bonding energies and distances calculated for He2+ and for HeH agree well with experiment. The stabilization energy for HeX falls sharply toward zero with increasing difference in nuclear charges. These results are discussed with reference to polar three-electron bonds in polyatomic molecules.
Thermodynamics of sulfur chemisorption on metals. I. Alumina-supported nickel
Authors: McCarty Jon G. , Wise Henry
Published: February 2003

Sulfur chemisorption isosteres have been measured for nickel in powdered form and for nickel supported on two different alumina powders. The experiments were conducted in a closed-loop gas recirculation system containing one atmosphere hydrogen. Isotherms were determined by stepwise injection of H2S aliquots into recirculating hydrogen gas and analyzing for the H2S concentration as equilibrium was approached. Isosteres were measured by varying the sample temperature and monitoring the H2S/H2 ratio in the gaseous environment. A gas chromatograph and a photoionization detector was used to measure the H2S concentration to levels below 1 ppb. As monolayer coverage is approached the H2S/H2 ratio attains the equilibrium values reported for the bulk sulfide, Ni3S2. Adsorbed sulfur is very strongly bound to the surface of nickel. The heat of formation of chemisorbed sulfur with respect to 1/2 S2(g) at 800 K is 247 kJ mol−1 more negative than the heat of formation of Ni3S2. The heat of segregation exceeds 190 kJ mol−1. The results demonstrate that the equilibrium sulfur coverage exceeds one half monolayer on Ni/Al2O3 unless the H2S/H2 ratio is less than 1 ppb even at temperatures as high as 940 K.
Deuteron spin alignment: A probe for studying ultraslow motions in solids and solid polymers
Authors: Spiess H. W.
Published: February 2003

Deuteron spin alignment offers a new possibility to investigate extremely slow rotational motions in solids and solid polymers. A convenient theoretical description of the creation and detection of spin alignment by application of the Jeener–Broekaert pulse sequence is given for both static and slowly time dependent quadrupole coupling, as well as for spin–lattice relaxation of spin alignment. It is shown that the NMR signal following spin alignment yields a correlation function of the time dependent quadrupole coupling. This correlation function is evaluated explicitly for a deuteron on the corner of a regular tetrahedron undergoing tetrahedral jumps. Various applications of deuteron spin alignment are demonstrated experimentally, e.g., its use to obtain undistorted deuteron line shapes, from which the orientational distribution of partially ordered solids or solid polymers may be determined. In solid polyethylene it is shown that not only the deuterons in the crystalline regions but those in the mobile amorphous regions as well give rise to deuteron spin alignment. Tetrahedral jumps are investigated through deuteron echoes of a single crystal of hexamethylenetetramine. It is shown that the type as well as the time scale of rotational motions with correlation times 1 ms≲τ≲100s are obtained from the quadrupole coupling correlation function provided by deuteron spin alignment.
304 Å photoelectron spectra of atomic Hg, Tl, and Pb 5d levels; interaction of open shells
Authors: Süzer Sefik
Published: February 2003

Photoelectron spectra of atomic Hg, Tl, and Pb 5d levels are reported using He ii 304 Å radiation. The spectrum of Hg is simple and shows only a spin–orbit doublet. The spectra of Tl and Pb are complex, both consisting of eight lines. Using the energies from the optical data and the existing intermediate–coupling–configuration–intercation calculations, the Tl spectrum could be assigned. The calculated intensity distribution correlates with the observed one in Tl very closely. In the Pb spectrum only the two strongest lines could be identified and tentatively assigned to the 5d9(2D5/2)6s26p21/2 and 5d9(2D3/2)6s26p21/2 states of Pb+. Both in Tl and Pb, ionization from the 5d5/2 and 5d3/2 subshells can be understood in terms of a special kind of two-electron configuration-interaction mechanism. The 5d9(2D3/2) 6s26pn1/2 (n=1 for Tl and 2 for Pb) hole resulting from the ionization of an 5d3/2 electron, interacts strongly with the 5d9(2D5/2)6s26pn−11/26p13/2 states in which one electron from the (closed) 5d5/2 subshell drops to the 5d3/2 hole and another electron from the 6p1/2 subshell moves up to 6p3/2 subshell. This one-electron-down one-electron-up mechanism (a virtual Auger process) is favored for the 5d3/2 and not for the 5d5/2 subshell for energetic reasons and is responsible for the occurrence of multiline structure for one and not the other.
Electric deflection studies of halogen and interhalogen polymers
Authors: Chance K. V. , Bowen K. H. , Winn J. S. , Klemperer W.
Published: February 2003

Electric deflection studies of the polymeric halogen species, (Br2)2, (ICl)2, (Icl)3, (ClF)2, and (F2)2 are reported. The dimeric species listed above are found to be polar. The trimeric species (ICl)3 is also observed to be polar, a relatively unusual occurrence. A comparison is made between the gas phase dimer orientations and the local crystal structure of these halogens.
Linear response theory of molecular interaction: Correlated Π electron systems
Authors: Rabenold David A.
Published: February 2003

A many-body theory of molecular interaction is presented for systems of nonoverlapping molecules containing delocalized Π electrons. The theory is based on a combination of LCAO MO theory and time-dependent Hartree linear response theory. A multipole expansion of the Coulomb interaction is not employed rendering the theory applicable to large molecules. Some aspects of electron correlation are included. The infinite order result for the dispersion interaction is the change in the intermolecular correlation energy associated with ’’turning on’’ the Coulomb potential. A closed-form infinite order expression for the intermolecular permanent moment interaction accounts for the modification of the bare Coulomb interaction by the polarizable system. Second order results that include all orders of intramolecular correlation for the dispersion and static interactions are also obtained. These are expanded in terms of the intramolecular correlation. A three-body dispersion potential that includes all orders of intramolecular correlation is also presented. The theory is amenable to numerical analysis and provides a method to investigate the convergence of various perturbation series.
Proton magnetic resonance characterization of the intermediate (S=1) spin state of ferrous porphyrins
Authors: Mispelter J. , Momenteau M. , Lhoste J. M.
Published: February 2003

The temperature dependence of the paramagnetic shifts in square planar ferrous complexes has been investigated for a number of porphyrin derivatives. Large anomalies in the Curie law have been observed in newly synthesized substituted tetraphenylporphins in which the environment of both faces of the tetrapyrrolic ring is strictly controlled. The pseudocontact and contact contributions to the measured hyperfine shifts as well as their temperature dependence have been interpreted in terms of a model with two electronic states. Following this model, the nature of the ground state of the ferrous ion depends critically upon the energy of the dz2 orbital relative to that of the degenerate dxz and dyz orbitals. For most of the porphyrin derivatives, the ground state is 3A2g, strongly perturbed by the closely lying 3Eg excited state. This strong mixing by spin–orbit coupling explains the large orbital contribution to the magnetic susceptibility of these complexes. A small axial perturbation can induce a reversal of the ground state with corresponding inversion of the magnetic anisotropy. Such a behavior, predicted by the model, is observed at low temperature by proton NMR spectroscopy of some complexes in solution.
Laser-initiated chemical chain reactions
Authors: Nesbitt David J. , Leone Stephen R.
Published: February 2003

A detailed kinetic and experimental analysis is presented for chemical chain reaction processes initiated by well-controlled, low power laser pulses. Realtime evolution of the chain reaction is followed by direct detection of infrared chemiluminescence from vibrationally excited HCl product molecules produced by one of the propagation reactions in the chain. By appropriate choice of conditions, the chain reactions may be analyzed separately for pseudofirst-order, radical–reagent processes as well as for second-order, radical–radical events. The pulsed laser initiation technique is applied to three sample chain systems which exhibit distinctly different chain lengths, rates, and termination behaviors. These systems are Cl2/H2S, Cl2/H2, and Cl2/CH3SH. In the case of Cl2/H2S, detailed rate constant data are obtained for the fundamental chain propagation steps, and appropriate chain termination steps are assigned from the observations. The results demonstrate a new, general technique for the quantitative study of chemical chain reactions and related combustion processes.
Alkali halide photofragment spectra. II. Alkali bromide bond energies and excited state symmetries at 266 nm
Authors: Su Tzu-Min R. , Riley Stephen J.
Published: February 2003

Photofragment spectroscopy studies of the five alkali bromides at 266 nm show exclusive production of ground state 2P3/2 Br atoms in the photodissociation of LiBr and mixtures of ground and excited 2P1/2Br* atoms in the other bromides. Br*/Br branching ratios are 3.0, 2.5, 5.7, and 0.8 for NaBr, KBr, RbBr, and CsBr, respectively. From measured alkali atom recoil time-of-flight distributions the following bond energies (D00, kcal mol−1) are determined (all uncertainties ±1 kcal mol−1): LiBr-99.2; NaBr-85.9; KBr-89.7; RbBr-90.1; CsBr-92.1. The recoil angular distributions indicate that the parallel and perpendicular transitions leading to Br and Br* atoms have approximately equal intensities for the four heavier bromides, while for LiBr 29% of the dipole strength to the single exit channel Li+Br is parallel polarized. These results are discussed in the context of simple one-electron charge-transfer model for the optical excitations, and are compared with similar studies on the alkali iodides.
Differential cross sections for the j=0→1 rotational excitation in HD–Ne collisions and their relevance to the anisotropic interaction
Authors: Buck U. , Huisken F. , Schleusener J. , Schäfer J.
Published: February 2003

In a crossed molecular beam experiment, time-of-flight distributions of HD-molecules scattered from Ne at an energy of 31.5 meV have been measured using the pseudorandom chopper method. Each time-of-flight spectrum shows a clearly resolved inelastic peak due to rotational excitation of HD from j=0 to j=1. With the aid of these spectra, together with additional measurements of the total differential cross section, the angular dependence of the differential cross section for the excitation j=0→1 is derived over a large angular range (20° to 120°). The sensitivity of the data to the interaction potential is carefully studied and the measured inelastic differential cross sections are compared with calculations based on interaction potentials recently proposed for this system.
Determination of Au–O bond energy by glow discharge mass spectrometry
Authors: Hecq A. , Vandy M. , Hecq M.
Published: February 2003

The neutral species sputtered from several metallic targets are studied by glow discharge mass spectrometry. The dependence of ν=MO+/(M++MO+) on the discharge gas pressure allows the M–O bond energy to be determined. The experimental results suggest that the bond energy of AuO should be about 3 eV.
Ferroelectric phase transition in the superionic conductor Ag26I18W4O16
Authors: Scott J. F. , Habbal F. , Zvirgzds J. A.
Published: February 2003

An analysis of the dielectric measurements for the ferroelectric phase transition in the superionic conductor Ag26I18W4O16 is reported. The transition is interpreted as being slightly first order, with a hysteresis region less than 2.5 °K in width. The system obeys Curie–Weiss dependences in both phases, with the dielectric constant satisfying the equation in the paraelectric phase and in the ferroelectric phase. The ferroelectric transition is of the order–disorder-type and involves mobile silver ions. The frequency dependence of ionic conductivity has been measured in the high temperature phase. From 0.5 to 10.0 kHz it satisfies σ (ω) proportional to ω in contrast to the ω2 dependence predicted by Lines.
The electronic structure of HgCl2 and HgBr2 and its relationship to photodissociation
Authors: Wadt Willard R.
Published: February 2003

The low-lying excited states of HgCl2 and HgBr2 are investigated using effective core potentials in conjunction with a double-zeta plus polarization quality basis set and a moderate-size POL(1) CI wave function. A relativistic effective core potential is used for Hg, while nonrelativistic potentials are used for Cl and Br. The results are interpreted in terms of simple valence bond and molecular orbital pictures to give qualitative insight into the electronic structure of these species. The three lowest absorption features in HgCl2 and HgBr2 are assigned as 1 1Πu, 1 1Σ+u, and 2 1Σ+u, respectively. The initial two are assigned for the first time. Correlation and energy-level diagrams are used to explain the previous photodissociation experiments. The results are discussed briefly in relation to the development of mercury halide lasers. Finally, the recent experimental work of Husain, Wiesenfeld, and Zare on HgBr2, reported in the following paper, confirms in detail our calculations on the 1 1Σ+u state which gives rise to the second absorption band.
Vibrational energy relaxation in liquids
Authors: Velsko Stephan , Oxtoby David W.
Published: February 2003

A theory of vibrational relaxation of polyatomic molecules in liquids is presented which expresses the rate of decay of a vibrational state in terms of atomic force autocorrelation functions. These force correlation functions can be estimated from hydrodynamics or from an independent binary collision model. The results of calculations on small molecules dissolved in inert, cryogenic solvents are discussed, including the dependence of the rates on the details of the intramolecular potential surface.
Diffusive and percolative lattice migration: Excitons
Authors: Kopelman Raoul , Argyrakis Panos
Published: February 2003

A microscopic transport theory is developed for stochastic and correlated hopping on ordered and random lattices that contain a small fraction of supertraps and a small number of ’’hoppers’’ (i.e., excitons). It includes short-time (’’transient’’) behavior, which is of interest for both time-resolved and steady-state experiments. The relations with diffusion, percolation, random walk, and rate equations are exhibited and applications to energy transport in disordered molecular aggregates illustrate the approach, which is a combination of a rigorous analytical method and simple computer simulations of general validity. Simple analytical results, derived for special (limiting) cases, are compared with other methods, thus emphasizing the roles of time, dimensionality, anisotropy, clusterization, correlation of hops, and the order parameter of the lattice as well as the suitability of various approaches for dealing with these factors.
The water–water pair potential near the hydrogen bonded equilibrium configuration
Authors: Sceats Mark G. , Rice Stuart A.
Published: February 2003

An analysis is presented of the contributions to the potential energy of interaction between water molecules near the hydrogen bonded equilibrium configuration. From this analysis we synthesize an effective potential that includes the influences of long-ranged and nonadditive interactions. The influence of the hydrogen bond bending on the anharmonic properties of ice I and on the spectral properties of amorphous solid water, the denser ice polymorphs, and liquid water are used to test the consistency of the effective potential. We also use that potential to estimate the quasistatic distribution of hydrogen bond angles in liquid water, which distribution is an important characteristic of the random network model of water.
Sedimentation equilibrium in concentrated, multicomponent particle dispersions. Hard spheres in the Percus–Yevick approximation.
Authors: Vrij A.
Published: February 2003

An equation is derived for the sedimentation equilibrium of (colloidal or macromolecular) particles with hard-sphere interparticle potential in a low-molecular weight solvent, in the Percus–Yevick and Carnahan–Starling approximation. The concentration–distance profile has a sigmoidal shape. It is shown that the turbidity of the mixture shows a maximum at the inflection point of this profile, a phenomenon that is sometimes observed in practice. The treatment is subsequently extended to a p-component mixture of hard spheres (HS) with different HS diameters. First, the explicit role of the low-molecular weight solvent is removed and the sedimentation differential equation is expressed in terms of partial derivatives (∂ρi/∂μj)μ; i,j=1,...,p, where ρi is the particle number density of i and μj the chemical potential of j. This partial derivative measures the susceptibility of δρi to variations in a weak external (centrifugal) field working on species j and is expressed as an integral over the total correlation function hij(r) via a relation of Yvon. Second, the sedimentation equation is expressed in terms of matrices containing (∂μi/∂ρi)ρ, each connected with an integral over the direct correlation function cij(r). Third, Baxter’s factorization of the direct correlation matrix is used to derive a new sedimentation equation, also in factorized form, containing matrices with elements Qij, which have a simpler mathematical structure than the derivatives ∂μi/∂ρj in some theories. It turns out that for hard spheres treated in the Percus–Yevick approximation these Q matrices reduce to simple, closed expressions even for a p-component HS mixture. This leads to p differential equations for the concentration profiles that must be solved numerically. Finally a closed expression is given for (∂ρi/∂μk)μ in the Percus–Yevick approximation (compressibility version).
High pressure studies of luminescence efficiency and lifetime in La2O2S:Eu and Y2O2S:Eu
Authors: Webster G. , Drickamer H. G.
Published: February 2003

The effect of pressure to 120 kbars has been measured on the luminescence efficiency of La2O2S:Eu and Y2O2S:Eu in concentrations from 0.1% to 3% europium. Lifetimes were also measured for the lanthanum compounds. For La2O2S:Eu the emissions from 5D0 and 5D1 decreased in intensity with increasing pressure; the very weak 5D2 emission increased by a large factor, leveled in the region ∼40–80 kbars and decreased at the highest pressure; the emission fro 5D3 appeared at about 50–60 kbars and increased rapidly in intensity. These results could be explained essentially quantitatively in terms of the analyses of Struck and Fonger and the observed shift to higher energy of the charge transfer absorption. Increased concentration of dopant dampened the effect of pressure. This is also consistent with the analysis. In addition, the behavior of the lifetime with pressure confirmed the above results and showed that no significant new radiationless paths appear at high pressure. There were also large pressure effects in Y2O2S:Eu. While these differed quantitatively from the effects in La2O2S:Eu they could be explained in terms of the analysis and the difference in location of the charge transfer peak and its shift with pressure.
Photodestruction of ions containing sulfur dioxide
Authors: Hodges Ronald V. , Vanderhoff John A.
Published: February 2003

Photodestruction cross sections have been measured for six ions containing SO2 over the photon energy range 1.55–3.5 eV using a drift tube mass spectrometer and rare gas ion and dye lasers. The cross section for SO2− increases smoothly with photon energy from (0.9 to 2.4) ×10−18 cm2. The photodestruction spectra of SO2  (SO2)− and O2  (SO2)+ consist of broad, structureless bands. These bands are attributed to direct dissociation through repulsive excited states. Cross section for O2  (SO2)−, NO2  (SO2)−, and NO3  (SO2)− are measurably different from zero only at the upper end of the photon energy range.
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o effective valence shell Hamiltonian for the neutral and ionic valence states of N, O, F, Si, P, and S
Authors: Sun Hosung , Freed Karl F. , Herman Michael F. , Yeager Danny L.
Published: February 2003

The effective valence shell Hamiltonian, ℋv, which acts within a finite valence space and exactly describes all the valence state energies, is applied to several atomic systems. The n=2 (L shell) ℋv of the first row atoms, N, O, and F and n=3 (M shell) s and p orbital ℋv of the second row atoms, Si, P, and S, are evaluated through second order using STO 5s4p2d and 6s5p3d basis sets, respectively. The calculations are equivalent to a (perturbative) Bk approximation which incorporates all excited configurations and which chooses the primary (valence) space as all the valence K2(2s)m(2p)n and K2L6(3s)m(3p)n configurations, respectively. Using the calculated matrix elements of ℋv, the energies of all the valence states of the neutrals and ions are simultaneously determined from a single ab initio calculation on only one charge state of each of these atomic systems. To understand the dependence of ℋv on the choice of core and valence orbitals, several sets of orbitals, obtained within the same primitive orbital basis, are tested. The nature of the orbital dependence is discussed. The best choices yield an average deviation of calculated excitation energies, ionization potentials and electron affinities from all the available corresponding experimental values of 0.3–0.4 eV for all the atoms. It is to be emphasized that these errors pertain to all charge states of the atom even though the Bk calculation is performed explicitly for only one of these charge states. Some features of the calculated ℋv are discussed in relation to semiempirical theories. In particular, three-electron parts of ℋv, which have no counterparts in semiempirical theories, are negative and contribute substantially to most excitation energies, ionization potentials and electron affinities. This implies that the parameters of semiempirical theories of valence must incorporate the effect of three-electron interactions in an average fashion similar to the way that the one-electron Fock operator describes the average influence of the two-electron interactions. A detailed description of this relationship between semiempirical parameters and ℋv matrix elements is given elsewhere.
Lithium [Li*(2 2P1/2,3/2)] fine structure transitions induced by collisions with noble gas atoms
Authors: Elward-Berry Julianne , Berry Michael J.
Published: February 2003

Dye laser excitation and interferometric fluorescence analysis techniques were used to determine collision induced fine structure transition rate coefficients in the Li*(2 2PJ)+ He, Ne, Ar, Kr, and Xe systems at a temperature of 555.7 K. All the noble gases induce transitions at ’’gas kinetic’’ rates, but differences in the individual magnitudes of the rate coefficients are probably sensitive to the long range portions of the Li*(2 2PJ)–noble gas interaction potentials. There is no appreciable isotope effect for 6Li*/7Li*+He, Ne, Ar systems. Simple models based upon localized transition concepts do not agree adequately with experimental results, suggesting that full dynamical computations using quantal and/or refined semiclassical formalisms are probably required for proper treatment of the Li*(2 2PJ)–noble gas systems and for stringent tests of various choices for the interaction potentials.
Solid state phase transitions and molecular reorientation in o
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a-carborane: An isomer effect
Authors: Beckmann Peter , Leffler Amos J.
Published: February 2003

The proton nuclear spin–lattice relaxation times at 30 MHz have been measured for ortho- and para-carborane between 170 and 310 K and between 210 and 320 K, respectively. Para-carborane undergoes solid–solid transitions at about 300 and 238 K while the ortho isomer shows only one transition at about 265 K. The details of the phase transitions are discussed and molecular reorientation in the isomers is compared.
On the direct energy transfer to moving acceptors
Authors: Allinger K. , Blumen A.
Published: February 2003

This paper studies the decay law of the excitation of a donor molecule, due to its microscopic long range interactions with moving acceptor molecules. Particular attention is given to the case of acceptor molecules randomly dispersed in a host liquid. From a general approach we derive an exact ensemble averaged formula for the decay of the donor excitation as a function of the molecular motion. The expression obtained is valid for all types of microscopic molecular interactions and for arbitrary ratios of acceptors to inert molecules; from this formula we determine the energy decay for motions which are slow or rapid relatively to the microscopic energy transfer times. If the motion is frozen, we retrieve the decay law valid for acceptors imbedded randomly in a solid matrix [A. Blumen and J. Manz, J. Chem. Phys. (to be published)]. For slow molecular displacements, which obey the Langevin equations of Brownian motion, the diffusion dependent decay law is presented; in the case of low acceptor concentration and multipolar interactions the expression reduces to the Yokota and Tanimoto result [J. Phys. Soc. Jpn 22, 779 (1967)], generalized to spaces of arbitrary dimensions. In the case of rapid motion, the decay becomes motion independent, and is not diffusion controlled.
Rayleigh and Raman scattering near the critical point of carbon dioxide
Authors: Garrabos Yves , Tufeu Roland , Le Neindre Bernard , Zalczer Gilbert , Beysens Daniel
Published: February 2003

An experimental study of the behavior of depolarized Rayleigh and Raman spectra of carbon dioxide when approaching the critical temperature along the critical isochore is presented. The study of the depolarized Raman scattering was carried out on the Fermi diad ν1, 2ν2 and was supplemented by a study of the polarized Raman scattering. Contributions to the different physical mechanisms which can give rise to the depolarized spectra are carefully analyzed. We have shown that the orientational dynamics of CO2 molecules are unchanged by critical slowing down of the diffuse hydrodynamical mode associated to fluctuations of the order parameter in the range 0.01 °C<T–Tc<10 °C. Moreover we found that the frequency variation of the polarized Fermi diad can be well described using only density terms. The variation of the spectral intensities of the different polarized and depolarized bands is well explained in terms of only turbidity and multiple scattering phenomena owing to the intensive quasielastic polarized Rayleigh scattering.
Potential energy surface for the Li+HF→LiF+H reaction
Authors: Chen Maynard M. L. , Schaefer Henry F.
Published: February 2003

The three dimensional potential energy hypersurface for Li+HF→LiF+H has been studied at the self-consistent field (SCF) and configuration interaction (CI) levels of electronic structure theory with a medium-sized basis set that included polarization functions. The ’’corner’’ of the reaction channel was first mapped by calculation of a lattice of points, then further calculations were carried out to characterize selected points along the minimum energy pathway more precisely. The classical reaction endothermicity was 2.9 kcal/mole, but with zero-point corrections, the reaction was found to be exothermic by 1.7 kcal/mole. As the Li atom approaches the diatomic, it first forms a bent complex with 4.5 kcal/mole of stabilization energy before reaching the transition state. The latter, also bent with an angle of 74° was located in the exit channel and is predicted to be 10 kcal/mole above the reactants. Force constants, vibrational frequencies, and zero-point energies of the complex and the transition state were calculated. After applying zero-point corrections to the transition state, the threshold energy for reaction was reduced to 6.4 kcal/mole, which will probably be further reduced to ∼4 kcal by higher order correlation effects. Our results were compared with previous theoretical efforts and with qualitative theories concerned with the transition state angle and its exit bias. A qualitative discussion of the dynamics over the surface emphasizing interrelationships between the translational, vibrational energy and the LiFH angle ϑ is also presented.