bluuebunny/crossref_metadata_2025 huggingface dataset:
The a 3Σ+ state of NaK. High resolution spectroscopy using laser-induced fluorescence (LIF)
Authors: Breford E. J. , Engelke F.
Published: February 2003

We use laser-induced fluorescence (LIF) combined with a high resolution grating spectrometer to study the D 1Π→a 3Σ+ transitions of the heteronuclear alkali dimer NaK. Intense perturbations of the D 1Π state permit high resolution measurements and facilitate study of the corresponding intensities. From these spectra, we evaluate the molecular constants of the a 3Σ+ state of NaK and estimate the parameters involved in the perturbations of the D 1Π state. Various features of the perturbing levels are discussed. We compare the results of our analysis with the results of spin-exchange scattering experiments on the NaK system and with theoretical calculations. In addition, very accurate dissociation energies, De(NaK, 1Σ+) =5268.1±0.8 cm−1 and De(NaK, a 3Σ+) =203.1±0.5 cm−1, are obtained.
An explanation of the preferential formation of less stable isomers in three-body reactions: Cl+NO2+M; ClO+NO2+M
Authors: Chang J. S. , Baldwin A. C. , Golden D. M.
Published: February 2003

Low-pressure limit rate constants for the three-body reactions, (1) Cl+NO2+M→ products and (2) ClO+NO2+M→ products, have been calculated according to Troe’s method [J. Chem. Phys. 66, 4758 (1977)]. The result for the reaction of Cl+NO2+M is in excellent agreement with the experimental finding of Niki et al. [Chem. Phys. Lett. 59, 78 (1978)]. An explanation is proposed to account for apparent discrepancy between the measured rate constants for ClO+NO2+M in the forward and reverse directions. The stratospheric implications are also discussed.
CrH and CrH2 molecules: ESR and optical spectroscopy at 4 °K
Authors: Van Zee R. J. , DeVore T. C. , Weltner W.
Published: February 2003

CrH and CrD molecules have been trapped in solid argon at 4 °K. Infrared spectra, when the large anharmonicity is accounted for, yield predicted gas phase vibrational parameters in the ground state ωe =1615 cm−1 and ωexe=17 cm−1. ESR spectra clearly establish that the molecule has a 6Σ ground state with zero-field-splitting ‖D‖=0.34(1) cm−1 in solid argon and approximate hyperfine coupling constants ‖A (H) ‖=49(5) MHz, ‖A⊥(53Cr) ‖=53(5) MHz, and g⊥?g∥=ge. Several ’’extra lines’’ (off-principal-axis absorptions) in the ESR support the assignment. CrH2 (and CrD2, CrHD) was observed in the IR spectra and may also have been detected in the ESR, which tentatively suggest a S=2 molecule with ‖D‖?0.02 cm−1.
An experimental and theoretical determination of the electron affinity of the ethynyl radical, HC2⋅
Authors: Janousek Bruce K. , Brauman John I. , Simons Jack
Published: February 2003

The cross section for electron photodetachment has been measured for the acetylide anion (HC2−) using an ion cyclotron resonance spectrometer in conjunction with a xenon arc lamp. Calculation of the photodetachment behavior near threshold and an estimate of the Franck–Condon factors for the anion→neutral transition allow us to determine EA (HC2⋅) = 2.94±0.10 eV. A theoretical determination using eighth-order perturbation theory gives an adiabatic electron affinity of 3.18±0.25 eV, in good agreement with the experimental result. The use of a thermochemical cycle with the experimental electron affinity and gas-phase acidity data gives a C–H bond dissociation energy in acetylene of 132±5 kcal/mol.
Reactions of Kr2+, Xe2+, and C2+ with atoms and molecules at 300 K
Authors: Viggiano A. A. , Howorka F. , Futrell J. H. , Davidson J. A. , Dotan I. , Albritton D. L. , Fehsenfeld F. C.
Published: February 2003

The reaction rate constants are measured for the destruction of Kr2+, Xe2+, C2+ with Nl, Ar, Kr, Xe, H2, N2, O2 at 300 °K. The reactions were carried out in a ion flow tube. (AIP)
Generalization of the Faulkner–Richardson method for calculating polyatomic Franck–Condon factors
Authors: Kulander K. C.
Published: February 2003

Faulkner and Richardson’s approach for evaluating multi-dimensional Franck-condon and Herzberg-Teller integrals is used to calculate Franck-condon factors for polyatomic molecules and generalized to include transitions from excited states to excited states. (AIP)
Reply to ’’Generalization of the Faulkner–Richardson method for calculating polyatomic Franck–Condon factors’’
Authors: Faulkner Thomas R.
Published: February 2003

A numerical test of Kulander’s extension of the authors’ method for calculating Franck-condon integrals for polyatomic molecules is reported. The 1Agg→1B24 transition in benzene between excited vibrational states are discussed. (AIP)
State-to-state differential cross sections for rotational transitions in Na2+He collisions
Authors: Bergmann K. , Engelhardt R. , Hefter U. , Witt J.
Published: February 2003

Laser induced fluorescence was used to study the rotational energy transfer for elastic and inelastic Na2-He collisions. (AIP)
A study of Hartree–Fock instabilities in Cr2(O2CH)4 and Mo2(O2CH)4
Authors: Benard Marc
Published: February 2003

The Hartree–Fock instability of the SCF wave functions obtained for the quadruply bonded complexes Cr2(O2CH)4 and Mo2(O2CH)4 is studied at the ab initio level. Several cases of singlet, nonsinglet, and nonreal instabilities are found for each of these complexes at the experimental value of the metal–metal bond length. For Cr2(O2CH)4, these instabilities are still found at much shorter Cr–Cr distances. The ’’broken symmetry’’ wave functions corresponding to some of the singlet and nonsinglet instability roots of Cr2(O2CH)4 are computed and analyzed. Each of these broken-symmetry wave functions is characterized by an important decrease of the bonding character of the metal–metal interaction with respect to the symmetry-adapted wave function. Singlet instability can thus be considered as an attempt to reduce the strong bonding character imposed by the symmetry constraints to the whole class of D4h quadruply bonded complexes. The case of MoCr(O2CH)4 for which the symmetry of the system is broken from D4h to C4v, is analyzed. An examination of the singlet instability matrices As+Bs obtained for Cr2(O2CH)4 and Mo2(O2CH)4 suggests that the presence of a multiple metal–metal bond strongly favors instability. A single determinant Hartree–Fock wave function might lead in such a case to an erroneous description of metal–metal multiply bonded systems. For this class of molecules, singlet stability equations could provide a simple and accurate procedure to check the reliability of Hartree–Fock wave functions.
Depolarized Rayleigh scattering by nitrobenzene in binary and ternary combination with acetone and isopropanol
Authors: Phillies George D. J. , Kivelson Daniel
Published: February 2003

The depolarized Rayleigh linewidth Γ, integrated depolarized scattering intensity Io, and viscosity η of solutions of nitrobenzene in acetone and isopropanol were studied as a function of temperature at different nitrobenzene concentrations. In all systems studied Γ−1 is linear in (η/T). The extrapolated value of Γ−1 at η/T→0 is zero for the interactive solvents which we studied. Relative intensity measurements indicate that Io does not depend strongly on temperature. Comparison is made with the studies of Alms et al. [J. Chem. Phys. 59, 5310 (1973)] on Γ for nitrobenzene in nonpolar solvents. From these and associated studies it appears that in nonpolar solvents a ’’slip’’ hydrodynamic model is applicable, but in polar solvents the appropriate boundary conditions are closer to ’’stick’’ and vary from solvent to solvent.
ESR of sulfur-substituted purines and nucleosides: Ionic radicals in mercaptopurine crystals at 77 K
Authors: Alexander Chester , Sagstuen Einar
Published: February 2003

Single crystals of the sulfur-containing purine base derivative 6-methyl-mercaptopurine (6MeMP) have been irradiated by 4.0 MeV electrons and studied by electron spin resonance techniques at 77 K. A light-sensitive resonance has been analyzed, and the ESR data suggest that the resonance is due to a π radical with resolved hyperfine coupling from α protons on C2 and C8 and from one nitrogen atom. The spin densities are approximately 0.35 and 0.17 on C2 and C8 and 0.08 on the nitrogen atom. This interpretation is supported by a comparison of simulated and observed spectra, and, in combination with molecular orbital calculations, it is suggested that the species responsible for the light-sensitive resonance is the anion (electron adduct) radical. Radical formation in different sulfur-containing purines is discussed and compared to that in the unsubstituted purine bases.
Theory of chemically induced magnetic polarization. Effects of S–T±1 mixing in strong magnetic fields
Authors: Adrian F. J. , Monchick L.
Published: February 2003

Chemically induced magnetic polarization due to mixing of the singlet and MS=±1 triplet sublevels of a diffusing radical pair in a strong magnetic field is investigated by an approximate stochastic Liouville model which considers separately the two-level S–T−1 and S–T1 mixings. As previously found for S–T0 mixing, the stochastic Liouville equations for S–T−1 level mixing can be written as a Bloch-type equation with diffusion, and a similar equation for the S–T1 mixing can be obtained simply by changing the sign of the magnetic field. These equations give a simple vector model of chemically induced polarization by S–T±1 mixing, including the all-important diffusion through the S–T−1 level crossing. For simple Brownian diffusion and an exchange interaction that decays exponentially with radical separation, the stochastic Liouville equations for S–T−1 mixing can be converted to a single integral equation for the resulting polarization, and a similar equation can be derived for the S–T1 case. The polarizations are small and can be well approximated by a single term in the Neumann series solution of the integral equation. Usually the entire S–T±1 polarization comes from S–T−1 mixing in the region where these levels cross. Significant polarizations occur only in those radical pairs where either a very large hyperfine interaction provides rapid singlet–triplet mixing or slow diffusion enables the pair to remain a relatively long time in the level crossing region.
Effect of temperature on energy transfer in liquid toluene and cumene
Authors: Beck G. , Ding A. , Thomas J. K.
Published: February 2003

Picosecond pulse radiolysis has been used to study the rate of energy transfer from irradiated liquid arenes to solutes at low concentration. At room temperature the rates of energy transfer to solutes such as biphenyl and diphenylanthracene are rapid and rate constants are in the range from 5 to 7×1010 M−1 s−1, in agreement with values measured via low energy photochemical excitation. With solutes such as CHCl3 and CCl4 the yield of singlet excited toluene was reduced via reaction of these solutes with a precursor presumably the electron which neutralizes the solvent cation producing excited states. Decreasing the temperature of the irradiated toluene systems to −70°C leads to a marked increase in the rate of energy transfer from toluene to a solute. The nature of this effect and the effect of added e− scavengers suggest that lowering the temperature leads to a change in the mechanism of energy transfer from direct energy transfer to the formation of solute ions followed by ion neutralization leading to solute excited states. The onset of this latter process coincides with increased light scatter by the toluene which may be due to the formation of large aggregates in the liquid.
Evaluation of high-order JWKB phase integrals
Authors: Barwell Margaret G. , Le Roy Robert J. , Pajunen Petri , Child Mark S.
Published: February 2003

Two simple and reliable quadrature procedures for evaluating JWKB phase integrals of the form ∮dr f (r)/[E−U (r)]k+1/2 for integer k≳0 and any smooth potential U (r) are derived, tested, and compared. Their development allows calculations based on the higher-order JWKB quantization conditions to be done with virtually the same ease now associated with use of the first-order approximation. Applications to date include calculation of two-term and three-term (i.e., third- and fifth-order) JWKB eigenvalues and the direct calculation of diatomic molecule centrifugal distortion constants for both analytic and numerical potential energy functions.
PES of higher temperature vapors: Lithium halide monomers and dimers
Authors: Berkowitz J. , Batson C. H. , Goodman G. L.
Published: February 2003

The He (I) photoelectron spectra of ’’pure’’ monomers and dimers of the lithium halides have been obtained by using a double-oven technique to vary the dimer:monomer ratio. The lithium halide monomer spectra conform nicely to a model based on spin–orbit and ligand field interactions previously introduced. The first ionization potentials of the dimers are in each case higher than in the corresponding monomers. These conclusions are also deduced in this paper by ionic model calculations and molecular calculations of the X-α type, using the discrete variational method. The X-α calculations enable us to assign tentatively the dimer valence bands. These calculations have been extended to trimer and tetramer structures and indicate an oscillation in the first ionization potential in the series encompassing monomer through tetramer. Some experimental verification exists up to trimer. The oscillatory behavior is also obtained in a crude ionic model calculation. An experimental method using photoionization mass spectrometry is suggested for the study of larger clusters and is tested for Na2I2.
Exciton lifetimes in electron beam excited condensed phases of argon and xenon
Authors: Keto J. W. , Gleason R. E. , Soley F. K.
Published: February 2003

We report measurements of lifetimes of trapped excitons in condensed argon and xenon. For argon, decays are qualitatively similar to those found in the gas phases. For xenon, we find by both spectroscopy and the time dependence of the fluorescence that the n=1 and n=2 Γ (3/2) excitons radiating at 175 and 164 nm, respectively, are present for all temperatures from 80–133 K. The measured decay times are found to be independent of temperature and density. We discuss the validity of decay time measurements when using electron beams for excitation.
Studies of molecular association in H2O and D2O vapors by measurement of thermal conductivity
Authors: Curtiss L. A. , Frurip D. J. , Blander M.
Published: February 2003

The thermal conductivities of H2O and D2O vapors were measured in a modified thick hot wire cell between 358 and 386 K at pressures ranging from 100 to 1000 Torr. Analysis of the data indicates that molecular association to form a dimeric species is the main source of enhancement of the thermal conductivity of both vapors. The enthalpy and entropy of association of the H2O dimer are −3.59 kcal mol−1 and −18.59 cal deg−1 mol−1, respectively. The enthalpy and entropy of association of the D2O dimer are −3.66 kcal mol−1 and −18.67 cal deg−1 mol−1, respectively. The measured enthalpy of association of the H2O dimer is in agreement with recently reported ab initio molecular orbital calculations on the H2O dimer. The entropies of association of the H2O and D2O dimers are calculated theoretically and are found to be in agreement with the measured values.
Double-quantum EPR transition in AgCl:Ni
Authors: Holmberg G. E. , Slifkin L. M. , Hempel J. C.
Published: February 2003

At 4.2 K EPR absorption by Ni2+ in a substitutional octahedral site in AgCl:Ni results in a broad line at g=2.276. For high microwave powers, a narrow line with resolved superhyperfine structure appears at the same g value. This narrow line is attributed to a double quantum transition by the Ni2+ ion. Superhyperfine coupling parameters to the nearest neighbor chloride ions are derived by fitting the narrow line. The covalent bonding parameters for NiCl4−6 are calculated to be fs=0.67% and fσ=6.79% by means of the independent bonding model.
Reactions of S+(4S°) ions with O2 and NO at relative kinetic energies from 0.04 to 2 eV
Authors: Dotan I. , Fehsenfeld F. C. , Albritton D. L.
Published: February 2003

The reactions of S+ with O, and NO are studied in a helium-buffered flow-drift tube, at relative kinetic energies from 0.04 − 2ev. The significance of these reactions in interpreting the 32 and 34 e/m ion signals observed at 70–90 km in the ionosphere is discussed. (AIP)
Photoemission from atomic Rb 4p and Ag 4d levels
Authors: Süzer Sefik
Published: February 2003

The photoelectron spectra for Rb and Ag are reported. The configuration mixing and open shell interactive for 4p Rb and 4d Ag are discussed. (AIP)
Orientational phases of classical octopoles on a triangular lattice and the adsorption of methane on graphite
Authors: O’Shea Seamus F. , Klein Michael L.
Published: February 2003

Monte Carlo calculations have been used to study the effect of temperature on the orientational ordering of classical octopoles on two-dimensional triangular net with toroidal boundary conditions. In the absence of crystal field effects three distinct phases have been observed: a high temperature disordered phase, an intermediate phase of undetermined structure, and a low temperature ordered phase. When the crystal field is sufficiently strong that the octopoles sit as stable tripods on the surface free to rotate only about the axis normal to the surface, only two phases are observed, a high temperature disordered phase and a low temperature ordered phase. The relevance of the models to the registered adsorbed phase of methane on graphite is discussed and the experimental evidence is reviewed.
Temperature dependence of the reaction HO2+NO→OH+NO2
Authors: Howard Carleton J.
Published: February 2003

The title reaction has been studied using laser magnetic resonance detection of HO2 radicals in a discharge flow system. For the range 403≳T≳232 K, k1= (7.9±1) (T/300)−0.83±0.18 ×10−12 cm3 molecule−1 s−1. Upper limits are placed on the rate constants for the reactions of (HO2+NO) that yield HNO and HOONO: k<0.0012 k1. Measurements have also been made that indicate that the reactions of HO2 with CO and N2O are very slow at 300 K. These rate constants are less than about 5×10−17 cm3 molecule−1 s−1.
Crystal-field analysis of triply ionized rare earth ions in lanthanum trifluoride
Authors: Morrison C. A. , Leavitt R. P.
Published: February 2003

Previously reported spectra of nine triply ionized lanthanides in LaF3 are analyzed by using a parametrized C2 crystal-field Hamiltonian. Initial crystal-field parameters Bnm in the fitting procedure were obtained from point charge lattice sums Anm using previously derived ρn, where Bnm=ρnAnm. The largest rms deviation of theory from experiment was 18.1 cm−1 (Pr3+), and the smallest was 2.1 cm−1 (Ho3+). The resulting Bnm were used to obtain a smoothed set of crystal field parameters for the entire lanthanide series whose largest rms deviation was 20.8 cm−1 (Pr3+) and whose smallest was 3.5 cm−1 (Ho3+).
An L2 calculation of the 1s and 2s photoionization cross sections of Ne
Authors: Martin Richard L. , Daasch W. Robert , Davidson Ernest R.
Published: February 2003

Discrete Gaussian orbitals are used to approximate the continuum for neon photoionization. Results within 2% of the exact numerical static exchange answer are obtained for both the 1s and 2s subshells by Stieltjes imaging. An alternative to the Tchebycheff method for obtaining the cross section from the moment distribution is given.
Ni2+ coordination in aqueous NiCl2 solutions: Study of the extended x-ray absorption fine structure
Authors: Sandstrom Donald R.
Published: February 2003

Extended x-ray absorption fine structure measurements are reported for aqueous NiCl2 solutions of molar concentration 2.78 and 3.74. Analysis of the Fourier filtered spectra by a multiparameter fitting procedure reveals a first coordination sphere consisting of six water molecules with an Ni–O bond distance of 2.07 Å, and no first sphere Ni–Cl coordination. Measurement of an outer coordination sphere consisting of both Cl− ions and water molecules is also reported. The relationship of these results to previously postulated ordered structures in these solutions is discussed.
Infrared absorption intensities of allene
Authors: Koga Yoshinori , Kondo Shigeo , Nakanaga Taisuke , Saëki Shinnosuke
Published: February 2003

The infrared absorption intensities of the fundamental bands of CH2=C=CH2 and CD2=C=CD2 were measured in the gas phase. The Coriolis interaction between ν7 and ν10 bands of CD2=C=CD2 was analyzed to obtain information about the signs of (∂p/∂Qi). The perturbation was found to be positive. Utilizing the obtained data, the correct sign combination was determined. The polar tensors and the atomic effective charges as well as the local intensity parameters have been obtained using the determined sign combination of (∂p/∂Qi). The results were discussed in connection with the empirical rules for the signs of CH bond stretching and bending motions. Predictions of this rule were found to be well satisfied in this molecule.
Extended basis first-order CI study of the 1A′, 3A″, 1A″, and B̃ 1A′ potential energy surfaces of the O(3P,1D)+H2(1Σg+) reaction
Authors: Howard Robert E. , McLean A. D. , Lester W. A.
Published: February 2003

Potential energy surfaces for the 1A′, 3A″, 1A″, and B̃1A′ states of the O(3P,1D)+H2(1Σg+) reaction are computed using a first order configuration interaction (FO–CI) wave function, an extended contracted Gaussian basis set, and iteration over natural orbitals (INO). As judged by the well depth, vertical excitation energies, and H2O(1A1) properties, the surfaces appear to be in good agreement with experiment. Several features important in collision dynamics, including a 14.0 kcal/mole abstraction barrier on the 3A″ surface, are reported. The positions of several surface crossings are described and discussed.
Monte Carlo studies on the hydrophobic hydration in dilute aqueous solutions of nonpolar molecules
Authors: Okazaki Susumu , Nakanishi Koichiro , Touhara Hidekazu , Adachi Yoshinori
Published: February 2003

Monte Carlo simulations have been carried out for 64 water molecules and two aqueous solutions each containing 63 water molecules and one nonpolar molecule in the canonical NVT ensemble at 298.15 K and 18.07 cm3 mol−1 using the Metropolis method. The ST-2 and Lennard-Jones potentials are used for water–water and water–solute interactions. Two nonpolar molecules roughly simulate methane and isobutane, respectively. The energy stabilization due to the introduction of nonpolar solute is clearly recognized. This energy stabilization is dependent on the size of solute and is pronounced in the case of methane. Internal energy, radial distribution function and related properties have been evaluated separately for vicinal and distant water molecules to the solute. Water molecules in the first hydration shell of methane are influenced by the presence of the solute and lead to a kind of structuration. Thermodynamic and structural properties of vicinal water to methane differ from those of pure water. This is also observed, though less pronounced, for isobutane solution. The Helmholtz free energy difference between pure water and aqueous methane solution has also been evaluated by the umbrella sampling technique.
The pressure dependence of the binary diffusion coefficients of the systems He–Ar, He–N2, He–O2, and He–CO2 at 300 and 323 K: Tests of Thorne’s equation
Authors: Arora P. S. , Dunlop Peter J.
Published: February 2003

A Loschmidt cell has been used to measure the pressure dependence at two temperatures of the binary diffusion coefficients of four systems containing helium. The results are used (a) to show that the Thorne–Enskog theory for moderately dense gases is not adequate to describe diffusion in these systems and (b) to indicate that a temperature dependence of the Thorne correction could not be detected.
Spectroscopic studies of the reactions Si+F2, SiH4+F2, and SiD4+F2
Authors: Armstrong R. A. , Davis S. J.
Published: February 2003

Spectroscopic studies of the reactions, Si+F2→SiF+F, SiH4+5/2F2→SiF+4HF, and SiD4+5/2F2→SiF+4DF are reported. Chemiluminescence, atomic absorption, and laser induced fluorescence techniques have been employed to probe these reactions. In all the reactions, SiF(A 2Σ→X 2Π) and SiF(a 4Σ→X 2Π) chemiluminescence was observed. Overall system photon yields of the SiH4/F2 reaction for producing SiF(A→X) and SiF(a→x) emission were ∼5×10−2% and 3×10−5%, respectively. The yields for the analogous SiD4/F2 reaction were about a factor of 2 lower. An energy resonance between SiF and SiH molecules was identified and studied. The results of these studies indicate that these reactions cannot be utilized to chemically produce an SiF electronic transition laser.
Fine structure of 3s,3d: 3Σ, 3Π, 3Δ complex of H2 by Doppler-free, laser spectroscopy
Authors: Lichten W. , Wik T. , Miller Terry A.
Published: February 2003

We have used the method of Doppler-free laser spectroscopy to resolve, for the first time, the fine structure of the H2, 3s,3d complex: h  (3s)3Σg+, g  (3d)3Σg+, i  (3d)3Πg, and j  (3d)3Δg. A collimated beam of H2 molecules in the metastable c 3Πu state was crossed with a tunable, dye laser beam. The absorbed light quenched the metastable molecules and changed the detected molecular beam intensity. The 3s,3d fine structure intervals have been measured to an accuracy of a few MHz (∼0.0001 cm−1) for all of the rotational levels in the v=0, 1 levels, which are connected by allowed transitions to the N=2 rotational level of the c  (2p)3Πu state of parahydrogen. The fine structure constants of the N=1 rotational level of the i  (3d)3Πg− state, extrapolated to the equilibrium internuclear separation are Aπ+γ=−145±4MHz and Bπ0−√6Bπ2=958±13MHz and MHz. These results are discussed in terms of the transition from united to separated atom values. The remaining states are coupled to each other by configuration interaction and rotational interactions. By parametrizing such interactions, we find that an account of the fine structure of these states can be given by considering only interactions within the triplet 3s, 3d complex. With only one exception, interactions with states outside this complex do not have to be taken into account. This analysis gives values for 15 spin interaction constants.
Diffusion-controlled chemical reactions modeled by continuous-time random walks
Authors: Helman W. P. , Funabashi K.
Published: February 2003

In the kinetic theory of pseudo-first-order diffusion-controlled chemical reactions, the diffusion motion of one component can be suppressed as long as the relative diffusion coefficient is assigned to the other component. The continuous-time-random-walk (CTWR) treatment of the pseudo-first-order reactions reaffirmed this well-known notion in the case of an exponential jump time distribution. In the case of a long-tail jump time distribution of Scher–Montroll, the decay dynamics depends upon which component is assigned the role of random walker, especially at low survival fractions.
Structural relaxation time of liquid water in the two-state model
Authors: Endo Harumi
Published: February 2003

The structural relaxation time at constant temperature and pressure in the two-state model of liquid water is calculated over the temperature range 0–100°C assuming a finite enthalpy difference (ΔH) between the two states. The relaxation time obtained here proved to be nearly the same as the residence time of a molecule in a quasicrystalline site as obtained from the inelastic neutron scattering of liquid water, while the correlation time for the reorientation of a dipole in a molecule as obtained from NMR measurements is somewhat larger than the structural relaxation time. The effect of nonvanishing ΔH is to make the relaxation time about one-half as compared when ΔH is assumed to be zero.
Cluster dynamics: A classical trajectory study of A + An?A*n+1
Authors: Brady John W. , Doll Jimmie D. , Thompson Donald L.
Published: February 2003

The dynamics of the collision of an atom A with a small cluster of atoms, An, leading to the formation of a quasibound A*n+1 complex, which subsequently decays, has been studied using classical trajectories. Pairwise Lennard-Jones potentials (with parameters appropriate for argon) were used to describe the identical point masses (Ar). The results illustrate the feasibility of direct calculations of microscopic rates for nucleation processes. The dissociation of collisionally formed A*n+1 (n=3,4, and 5) occurs by first-order exponential decay. Furthermore the energy dependence of the dissociation rate constants appears to be well described by the RRK functional form.
Anisotropic motion and spectral line shapes
Authors: Baram Asher
Published: February 2003

The effect of highly anisotropic motion on magnetic resonance line shapes is discussed. The details of the motion and the effect of the orientation of the rotation axis were considered. Utilizing the asymptotic method, slow motion and fast motion expressions for the spectrum were obtained, based on the appropriate powder spectrum as a starting point.
On the use of memory functions in the study of the dynamical properties of ionic liquids
Authors: Lantelme Frédéric , Turq Pierre , Schofield Peter
Published: February 2003

The molecular dynamics simulation results for a model of liquid LiBr are used to study the ionic motion in fused salts. It is shown that the particles exhibit a Brownian-like motion which is described by the generalized Langevin equation. The memory function introduced in this treatment is taken as a sum of two exponential terms M (t) = ω12  exp (−‖t‖/τ1)+ω22  exp (−‖t‖/τ2). The parameters are calculated from the velocity autocorrelation functions. The contributions of the two terms to the friction coefficient of the cation ξ =ω21τ1+ω22τ2 are respectively, 65% and 35%. The relaxation times are τ1=0.167 and τ2=2.19, both in units of 10−13s. An attempt is made to interpret the short time behavior in terms of relaxation in the force field of neighbouring particles. The longer decay is attributed to a collective mode of motion. This analysis is applied to the experimental data for physical LiBr with varying 6Li/7Li ratio.
Low temperature vibrational spectroscopy. III. Structural aspects and detection of phase transitions in crystalline alkali metal and tetramethylammonium hexabromotellurates and platinates
Authors: Berg Rolf Willestofte
Published: February 2003

The low-frequency infrared and Raman spectra of A2[TeBr6] with A=K, Rb, Cs, NH4, (CH3)4N, (CD3)4N and of A2[PtBr6] with A=K, (CH3)4N, and (CD3)4N were recorded as a function of temperature down to ∼100 K. The spectra of the Rb and Cs compounds changed little. For the other salts new, and in some cases sharp, bands were observed at low temperatures. The spectra have been assigned, and in most cases a confirmation of previous results was obtained. The assignment ambiguity in the literature on the ν4 mode of [PtBr6]2− has been solved, placing it at ∼ 130 cm−1. The majority of the new low temperature bands were interpreted in accordance with the known occurrence of phase transitions breaking the ideal antifluorite K2[PtCl6]-type symmetry, but it appears that the method of vibrational spectroscopy does not invariably reflect the presence of phases of lower symmetry. In the spectra of the tetramethylammonium compounds, methyl torsional IR bands were observed with increasing sharpness at lower temperatures. This behavior can be correlated with a gradual ordering of methyl torsional disorder. The potential energy barrier against methyl group rotation was found to be of the order 4–5 kcal/mol, showing that the methyl groups are not pseudo freely rotating. The anharmonicity of the vibrations in tetramethylammonium hexabromotellurate seems to increase abnormally at lower temperatures, possibly due to enhanced methyl–bromine interaction.
On the temperature dependence of 23Na asymmetry parameter in beta-alumina: ’’Trapped’’ vs ’’free’’ sodium
Authors: Polak Micha , Vaughan R. W.
Published: February 2003

23Na nuclear magnetic resonance results of Cheung, et al and Bailey, et al are interpreted in the light of model given by Wolf. (AIP)
Reply to comment in ’’Theory of resonant Raman scattering. III. Toward the total synthesis of molecular spectra and Raman excitation profiles within the generalized vibronic theory and from an exciton-in-molecule viewpoint’’
Authors: Jacon Marcel
Published: February 2003

This paper is concerned with resonance Raman scattering by a diatomic molecule which is excited in a dissociative continuum and whose electronic transition moment is evaluated in the Condon approximation.
Photochemical studies on the ethyl radical
Authors: Pacansky J. , Coufal H.
Published: February 2003

Photochemical studies are performed on dipropionyl peroxide and the ethyl radical in argon matrices. Matrix isolated dipropionyl peroxide photochemically produces the ethyl radical upon exposure to ultraviolet light in the λ≳2800 Å region. Irradiation of the ethyl radical by UV light with λ<2800 Å produces ethyl propionate, propionic acid, ethylene, and acetylene. The first three products are formed at a rate different from that for formation of acetylene. This different rate dependence is conveniently explained in terms of a site dependence for the photochemistry.
Understanding the infrared continuum spectrum of the N2O dimer and other van der Waals complexes at low temperatures
Authors: Bernstein L. S. , Kolb C. E.
Published: February 2003

Recent diode laser measurements of the N2O dimer 4.5 μm absorption band revealed a continuous band shape even though the experimental spectral resolution was 7×10−4 cm−1. The theory of rotational relaxation and dimer formation in supersonic molecular beams is used to establish a lower bound to the (N2O)2 rotational temperature of approximately TR?10 K for a source pressure of 17 atm. A band model approach is used to predict the temperature dependence of the band shape and line density for (N2O)2. The calculated band shape gives a good fit to the measured (N2O)2 spectrum for rotational and vibrational temperatures of 30 K. The results obtained show that the line density is of the order 104–105 lines/cm−1 for rotational and vibrational temperatures from 10 to 30 K. This range of line densities will yield a continuous band shape even if a spectral resolution, limited by beam divergence, of 2×10−5 cm−1 could be attained. It is shown that the smallest predissociative lifetime which could yield a discrete N2O dimer spectrum is approximately 10−7 s. A simple expression for quickly estimating average line densities is presented and applied to a sampling of van der Waals complexes.
Excitation of Cd, Zn, and Sr by a beam of active nitrogen
Authors: Fahey D. W. , Parks W. F. , Schearer L. D.
Published: February 2003

Excitation of the electronic levels of Cd, Zn, and Sr is observed when these metal vapors collide with a thermal-energy, active nitrogen beam. The beam is extracted from a glow discharge in pure N2. The active beam component is inferred to be vibrationally excited N2 in the A 3Σ+u electronic state. The absolute relative intensity of the emission lines in each element was measured. The excitation rates of the Cd and Zn target levels were found to depend exponentially on their energies indicating an effective temperature of approximately 4000°K. We believe that this temperature is related to the vibrational temperature of the N2(A 3Σu+) states that excite Cd and Zn in energy transfer collisions. The excitation rates of the Sr levels did not show an exponential energy dependence, which is a result consistent with N2(A 3Σu+) as the active species. The potential of such an emission study as a sensitive beam diagnostic is noted.
Electron spin resonance of 127I atoms trapped in Xe matrices
Authors: Iwasaki Machio , Toriyama Kazumi , Muto Hachizo
Published: February 2003

The ESR spectrum arising from the trapped 127I atoms has been observed in photolysis of HI in Xe matrices at 4.2 K. This may be the first clear detection of trapped halogen atoms in the solid matrices. The g and hyperfine coupling tensors are axially symmetric and the principal values are as follows: g∥=1.400, g⊥=2.532, A∥=889 MHz, and A⊥=1605 MHz. The quadrupole coupling tensor has been determined to be ‖ Q∥ ‖=74 MHz assuming axial symmetry. The approximate feature of the observed g and hyperfine tensors can be interpreted in terms of a simple crystal field model. However, a better understanding of the observed values requires a consideration of the effect of matrix wave functions as well as the dynamic Jahan–Teller effect. The radial parameters 〈r−3〉 estimated from the A and Q tensors are approximately consistent with the atomic beam data for free 127I atoms, giving conclusive evidence that the species is trapped 127I atoms.
Semiclassical calculation of bound states in multidimensional systems with Fermi resonance
Authors: Noid D. W. , Koszykowski M. L. , Marcus R. A.
Published: February 2003

A method is devised to calculate eigenvalues semiclassically for an anharmonic system whose two unperturbed modes are 2:1 degenerate. For some special states the periodic energy exchange between unperturbed modes is found to be very large. The quantum mechanical wave functions are examined and a correlation with the classical trajectories is described, both for quasiperiodic and the stochastic cases. A method used in the literature for calculating the stochastic limit is tested and found to break down when the present anharmonic system is separable.
Experimental and computer modeling study of 81Br isotope enrichment by isotopically selective, two-step laser photodissociation of HBr
Authors: Zittel P. F. , Little D. D.
Published: February 2003

Molecular bromine enriched in 81Br has been observed mass spectrometrically following isotopically selective, two-step laser photodissociation of HBr in mixtures with NO. The dependence of isotopic enrichment and product yield on various experimental parameters was investigated. An extensive computer model was used to simulate the two-step enrichment process and gave fair agreement with experimental results. The computer model indicated that isotopic enrichment was limited in these experiments (i.e., 81Br2/79Br2?1.6, compared to the natural abundance ratio of 0.96) by Br isotope exchange during chemical scavenging and not by the initial two-step photodissociation process. Computer modeling studies of the effect of adding high pressures of rare gas to the HBr–NO mixtures suggest that much higher isotopic enrichments are possible. Enrichment of 81Br in the C2H5Br product of isotopically selective, two-step laser photodissociation of HBr in C2H4 mixtures has also been observed experimentally.
High pressure fluorescence studies of radiative and nonradiative processes in diphenyl hexatriene, diphenyl octatetraene, and retinyl acetate
Authors: Brey Larry A. , Schuster Gary B. , Drickamer Harry G.
Published: February 2003

The effect of high pressure on the radiative and nonradiative rate parameters of three all-trans polyenes: 1,6-diphenyl-1,3,5-hexatriene (DPH); 1,8-diphenyl-1,3,5,7-octatetraene (DPO); and retinyl acetate (Rac) has been investigated to 10 kbar in methylcyclohexane (MCH) and toluene and to 40 kbar in polymethylmethacrylate (PMMA) and polystyrene (PS). The dependence of the radiative rate on the energy gap between the two lowest excited singlet states is compared to that predicted by a simple first order intensity borrowing expression. Comparison shows good agreement for DPH and DPO. The expression appears to fail for Rac.
Low-energy electron diffraction structural analysis of the (2×2) oxygen overlayer on the iridium (111) surface
Authors: Chan C.-M. , Weinberg W. H.
Published: February 2003

Exposure of a clean Ir(111) surface at room temperature to approximately 20 L of oxygen produces a sharp (2×2) LEED pattern. This LEED pattern can be caused by two fundamentally different adsorbate superstructures: a p (2×2) array of oxygen atoms (one-quarter monolayer coverage) or three independent, equivalent domains of a (1×2) superstructure rotated 120° with respect to one another (half-monolayer coverage). Dynamical calculations have been performed for the three most symmetric adsites on this surface: the on-top site and the two different types of threefold sites for both the p (2×2) and the three domains of (1×2) superstructures. The results show that the oxygen adatoms, either forming a p (2×2) superstructure or the three domains of (1×2) superstructure rotated 120° with respect to one another, reside on the threefold site directly above a vacancy in the second substrate layer with an interlayer spacing of 1.30 Å corresponding to an Ir–O bond length of 2.04 Å.
Phase diagram of antimony pentachloride to 43 kbar
Authors: Wu A. Y. , Whalley E.
Published: February 2003

The phase diagram of antimony pentachloride has been measured in the range −60–160 °C and 0–43 kbar to test the suggestion that the molecular solid ought to transform to an ionic solid, such as SbCl4+SbCl6− or SbCl4+Cl−, under pressure. In the experimental pressure range, it does not. The equilibrium lines for the L–I, L–II, and I–II transitions and their volume and entropy changes were determined.
Vibrational relaxation of UF6: Ultrasonic measurements in mixtures with Ar and N2
Authors: Cravens Dennis , Shields F. D. , Bass H. E. , Breshears W. D.
Published: February 2003

Ultrasonic attenuation has been measured in neat UF6 and in mixtures of 5% and 10% UF6 in Ar and N2 at 273 and 323 K. In all cases a single relaxation process was observed and attributed to relaxation of the total UF6 vibrational energy. Isothermal relaxation times were in the range (4–17) ×10−9 atm s. The vibrational relaxation rate inferred for UF6–N2 collisions shows a slight negative temperature dependence, while the rates inferred for UF6–UF6 and UF6–Ar collisions are virtually independent of temperature over the range of these experiments. In this range, Ar and N2 are slightly less efficient collision partners (by factors of ∼1.5–3.0) than UF6 itself.
The surface photovoltage of an increasing series of polyarenes: Anthracene, tetracene, and pentacene
Authors: Dahlberg S. C. , Musser M. E.
Published: February 2003

The surface photovoltage signals and the associated relaxation times generated by a laser pulse at the surface depletion layers of anthracene (0.8 μV, 5.6 msec), tetracene (12. μV, 10.0 msec), and pentacene (17.5 μV, 20.0 msec) appear to vary with the increasing amount of electron delocalization. As expected, the photovoltage of these materials depends logarithmically on light intensity until a saturation value corresponding to the complete energy band flattening at the surface is achieved, and this energy band bending is larger for pentacene than it is for tetracene. The photovoltage signal is observed to decay exponentially following the laser pulse with a relaxation time that is independent both of the wavelength and intensity of the light. It is established that this is in agreement with theoretical predictions based on a simple model involving the recombination of the photoinjected charge. The photovoltage spectral dependence of all three polyarenes have maxima which correspond to maxima in the corresponding optical absorption spectra due to the allowed singlet–singlet transitions. In addition, the photovoltage spectrum of anthracene has maxima that correspond to the ’’forbidden’’ singlet–triplet transitions, which are comparable in size to the photovoltage arising from the allowed singlet–singlet transitions. This observation implies that the dissociation of excitons to form free carriers is independent of the distance of the exciton from the anthracene surface. The corresponding singlet–triplet transitions for tetracene and pentacene are outside the spectral region examined and thus were not observed.
Non-Markovian theory of collisional dephasing in near resonance light scattering
Authors: Mukamel Shaul
Published: February 2003

We present a general microscopic theory for near resonance light scattering (RLS) from collisionally perturbed atoms and molecules. The theory is based on the tetradic scattering formalism of Fano and Ben-Reuven. We perform a systematic density expansion and show that the four-time, many-body, dipole correlation functions necessary for the evaluation of the RLS cross section may be rigorously expressed (to lowest order in pressure) in terms of three single-particle, two-time, correlation functions, two of which are associated with the absorption and emission line broadening and the third is a cross correlation function. Our most general expressions are valid for an arbitrary value of the bath correlation time (relative to the broadening), and interpolate smoothly all the way from the static limit of inhomogenous broadening up to the reverse, the Markovian (homogenous) limit. The stochastic Gaussian model of Takagahara, Hanamura, and Kubo is obtained also as a special case of the present formulation.
Investigation of the thermodynamic stability of the molecule LaIr(g) by high temperature mass spectrometry
Authors: Haque R. , Pelino M. , Gingerich K. A.
Published: February 2003

The high temperature Knudsen effusion mass spectrometric technique has been used to identify the previously unkown molecule LaIr in the gas phase over the La–Ir–Au–C(s) system. Evaluations of the enthalpy for the reaction LaIr(g) =La(g)+Ir(g) using the third and second law analysis yielded the following selected dissociation energy and standard heat of formation for the LaIr molecule. D°0(LaIr) =573±12 kJ mol−1 or 136±3 kcal mol−1, D°298(LaIr) =576±12 kJ mol−1 or 137±3 kcal mol−1, and ΔH°f,298(LaIr) =524±20 kJ mol−1 or 125±5 kcal mol−1. The dissociation energy of the gaseous molecule W2, which is isoelectronic with LaIr, is estimated to be 535±40 kJ mol−1 or 128±10 kcal mol−1.
Study of a magnetically oriented lyotropic mesophase
Authors: Amaral L. Q. , Pimentel C. A. , Tavares M. R. , Vanin J. A.
Published: February 2003

A study of a magnetically oriented lyomesophase formed by a quaternary system (Na decyl sulfate/water/decanol/Na sulfate) is reported. Small angle x-ray diffraction measurements have been performed on unoriented samples and samples previously subjected to the action of magnetic fields (H). Diffraction patterns show a diffuse inner halo at 80–140 Å and a sharp outer ring at 38 Å. They reduce to spots in the equator for oriented samples with the x-ray beam parallel to H; no diffraction figure is produced for the x-ray beam perpendicular to H. NMR spectra indicate that the director of the phase orients perpendicular to H. These results show that this lyomesophase has a structure not previously encountered. A model of finite planar micelles surrounded by water is proposed. In this model the micelles consist of a bilayer in the form of platelets, probably disk shaped. These platelets align in presence of magnetic fields, with their plane parallel to H, and further orientational restrictions are imposed by the container; they tend to be parallel and some periodicity appears in the director of the direction of the phase, perpendicular to the plane of polar heads. The mesophase satisfies the definition for nematics taking into account periodicities induced in nematic phases by magnetic fields.
Spin–orbit coupling and inelastic transitions in collisions of O(1D) with Ar, Kr, and Xe
Authors: Cohen James S. , Wadt Willard R. , Hay P. Jeffrey
Published: February 2003

Inelastic collisions of O(1D) with Ar, Kr, and Xe have been treated in the multistate Landau–Zener and the close-coupling approximations. The coupling mechanism is spin–orbit mixing. The dependence of the spin–orbit matrix elements on internuclear distance R is calculated using accurate configuration–interaction wave functions and an effective operator composed of one-electron, one-center terms. The R dependence is found to be very significant. Cross sections for transitions to the individual triplet fine-structure levels, as well as the total inelastic (quenching) cross section, are presented as a function of collision energy. The transitions occur primarily at curve crossings and the quenching rate constants were found to be significantly reduced by centrifugal barriers outside the crossing points. The calculated quenching rate constants at 300 K are (5.4±3.5) ×10−13, (6.0±0.7) ×10−12, and (3.0±0.2) ×10−11 cm3 molecule−1 s−1 for Ar, Kr, and Xe, respectively.