bluuebunny/crossref_metadata_2025 huggingface dataset:
Lifetime-separated spectroscopy: Observation and rotational analysis of the BaO A′ 1Π state
Authors: Pruett J. Gary , Zare Richard N.
Published: February 2003

The BaO A′ 1Π−X 1Σ+ band system has been investigated using laser−induced fluorescence. The BaO is formed in the gas−phase reaction Ba + CO2 under single−collision conditions, and the A′ 1Π and A 1Σ+ states are simultaneously excited by a pulsed tunable dye laser with a 0.2 Å bandwidth. The weak, long−lifetime A′−X fluorescence is separated from the strong, short−lifetime A−X fluorescence by delaying the observation of the A′−X emission until the A−X emission dies away. The lifetime of the BaO A′ 1Π state is found to be 9±1 μsec, more than an order of magnitude longer than that of the the BaO A 1Σ+ state. We have observed the (v′,0) band progression for 9 ? v′ ⩽ 18 as well as the (10,1) and (13,1) bands of the A′−X system, and they are found to be represented by the A′ state vibrational constants ν00 = 17 573±10 cm−1, ω0 = 442.38±1.1 cm−1 and ω0x0 = 1.693±0.025 cm−1. The (12,0), (17,0), and (18,0) bands have been rotationally analyzed, yielding the rotational constants B12 = 0.20849±0.00022 cm−1, B17 = 0.20290±0.00012 cm−1, and B18 = 0.20109±0.00014 cm−1. These results confirm that the shapes of the potential curves for the A′ 1Π and a 3Π states of BaO are remarkably similar for low vibrational levels. We also conclude that the A′ 1Π state is not the emitter responsible for the many−line emission spectrum seen in the Ba + N2O and Ba + O3 reactions at low pressures, nor is it the ’’dark’’ precursor responsible for the high photon yields observed in these chemiluminescent reactions at high pressures.
Calculation of heavy-atom kinetic isotope effects: Application of the high temperature approximation to models with three-element reaction coordinates
Authors: Wei G.-J. , Yankwich Peter E.
Published: February 2003

Calculations of the temperature dependent factor (TDF) in the isotopic ratio of specific rate constants kC12/kC13 for the decomposition of formic acid were carried out exactly and in the first order and second order high temperature approximations for a variety of reaction coordinate eigenvectors comprising displacements in three internal coordinates. The principal objective was to determine the consistency, if any, with which the features and general value levels of surfaces obtained by plotting exact values of ln(TDF) as functions of RCEV composition could be predicted from the corresponding surfaces in the first and second order approximations. The first order approximation is not usually a reliable guide to ln(TDF) for either value or gradients of value as functions of reaction coordinate composition at temperatures ? 1000°K. The second order approximation is often a reliable guide to gradients in the value of ln(TDF) in the range 1000−300°K, but the indications of value may be crude.
Determination of the sign of the spin–spin interaction in He2(a3Σ+u)
Authors: Vierima T. L.
Published: February 2003

In a measurement of the radio-frequency transition spectrum of the N=1 and N=3 rotational levels of He2(a3Σ+u) (probably v=0) in a molecular beam magnetic resonance experiment, we determined the finestructure spacings to within an inversion of the levels, and extracted the magnitude of the spin–spin interaction constant λ. This paper reports an independent determination of the level order, to obtain the sign of this interaction.
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o ground state energy surfaces for CO2 and CO2−
Authors: Pacansky J. , Wahlgren U. , Bagus P. S.
Published: February 2003

SCF ab-initio computations are performed for the ground states of CO2 and CO2−. The CO2 and CO2− potential surfaces have been obtained over a large region of space; in particular, the intersection of these two surfaces. Our results predict that the stability of CO2− depends strongly on whether it is formed near the equilibrium bond angle (135°), the most stable situation, or at significantly different angles. The calculations show that the 6a1 molecular orbital of CO2− is diffuse in character and that the computed equilibrium geometry (bond angle, 135.3°, bond length, 2.35 bohr) and electron affinity (−0.36 eV) are consistent with experiment.
Löwdin orthogonalization as a minimum energy perturbation
Authors: Aiken John G. , Jonassen Hans B. , Aldrich Haven S.
Published: February 2003

Let f1,...,fn be the atomic orbitals of a molecule in a molecular orbital calculation. There are advantages to using an orthonormal set e1,...,en instead of f1,...,fn in the computation. Of all the possible choices for the ei, we show that the Löwdin orthogonalization is the one which moves the fi the least. More precisely the sum of the squares of the lengths of the difference vectors ei − fi attains a minimum when the ei are obtained from the fi by the Löwdin orthogonalization.
Calculation of molecular electric polarizabilities and dipole moments. I. General theory and application to the hydrogen molecule
Authors: Svendsen E. No/rby , Hameka Hendrik F.
Published: February 2003

We analyze a number of different procedures for calculating molecular electric polarizabilities. We find that one particular method should give reliable polarizablitiy values for nonpolar molecules, having no permanent electric dipole. It seems questionable that this method will yield accurate polarizability values for polar molecules, but in that case it may be used to calculate the electric dipole moment. We use the method to calculate the electric polarizabilities of the hydrogen molecule and the hydrogen molecular ion and we obtain satisfactory results.
Conformational statistics of short RNA chains
Authors: Rubin H. , Kallenbach N. R.
Published: February 2003

The conformational statistics of short ribonucleic acid (RNA) chains have been investigated by a Monte Carlo approach that includes effects due to base–base stacking interations. Using oligoadenylate chains up to ten residues in length as a model, we have determined the dependence of the radial distribution functions and its second moments 〈RN2〉 and 〈SN2〉, the mean square end-to-end distance and radius of gyration respectively, on temperature, due to these stacking interactions. Assuming a helical geometry for stacked residues and different possible coil geometries corresponding to unstacked residues, we find that when stacking is present at low temperatures, the chains are severely non-Gaussian for N ? 10, while at high temperatures where stacking interactions diminish, chains are substantially Gaussian for N ? 5 residues. Distribution functions for chains with termini constrained to fixed distances have been estimated, in order to permit evaluation of the probability for loop closure of different kinds in short chains. The results are in reasonable agreement with experimental values for short hairpin RNA structures, and predict enthalpic contributions arising from stacking interactions in the process of forming loops from small chains.
Reflection spectrum of the first singlet transition of crystalline anthracene at low temperatures
Authors: Turlet J.-M. , Philpott M. R.
Published: February 2003

The b-polarized reflection spectra off the (001) face of anthracene crystals grown from the melt, solution, and by vapor sublimation display fine structure in the region 25 000 to 25 500 cm−1 at temperatures below 70°K. Earlier observations of fine structure by Brodin et al. with sublimation crystals are confirmed and extended. The new results include (i) the temperature dependence of the fine structure between 120° and 2°K, (ii) the reflection spectrum of the higher vibronic transitions, and (iii) studies of the effects of surfaces disrupted by solvent evaporation and photo-oxidation.
Photodissociation of acetylene and bromoacetylene in the vacuum ultraviolet: Production of electronically excited C2H and C2
Authors: Okabe H.
Published: February 2003

The photodissociation of C2H2 and BrC2H in the vacuum ultraviolet produces nearly identical quasicontinuous emissions in the region 4000 to above 5500 Å. It is concluded that the continua are most likely associated with the electronically excited C2H (C2H*) in both cases. Based on this conclusion the following spectroscopic and thermochemical data are derived from the thresholds of incident wavelength to produce the continua (1305±3 Å and 1540±3 Å, respectively, for C2H2 and BrC2H): The electronic energy of C2H, E0(C2H) ? 4.11±0.05 eV, D0(Br–C2H) = 91±1 kcal mol−1 (381±4 kJ mol−1), and ΔHmDfo(BrC2H) = 64.2±1.5 kcal mol−1 (269±6 kJ mol−1). The photolysis of BrC2H yields the C2 Swan bands in addition to the C2H* by the spin forbidden process. The fluorescence yield has been measured as a function of incident wavelength for C2H2 and BrC2H. The fluorescence yield curves follow the absorption spectra of C2H2 and BrC2H, indicating that the C2H* is predissociated from the initially formed electronic states. The C2H* yield from C2H2 increases rapidly below the incident wavelength, 1200 Å. Bond dissociation energies and the heats of formation of haloacetylenes are estimated in comparison with the corresponding cyanogen halides. Primary photochemical processes in C2H2 are discussed.
An approximate molecular orbital method
Authors: Zerner Michael
Published: February 2003

An approximate molecular orbital method is developed for hydrogen and first row elements at the minimum basis set level. Although the basis set consists of only valence orbitals, the explicitly ignored inner shell is accommodated through pseudopotentials. The one center one electron terms are evaluated from atomic spectra. The two and three center one electron terms are exactly calculated. The two electron Coulomb matrix is evaluated rapidly in a mixed representation using a simple projection overlap. The two electron exchange matrix is estimated in a symmetrically orthogonalized set, with simple empirical corrections. The resulting method executes approximately as rapidly as does the popular INDO method. The model is applied to molecules up to the size of adenine, C5N5H5. Where comparison can be made with minimum basis set STO calculations, the method reproduces occupied molecular orbital eigenvalues, electron densities, and calculated bond distances to within a rms error of 0.03 a.u. The virtual molecular orbital eigenvalues are generally also within ±0.03 a.u., with the notable exceptions of homonuclear diatomic molecules. A simple empirical scheme allows calculation of experimental bonding energies with a rms error of ∼20 kcal in a study of 22 polyatomic molecules, and the results appear consistent enough to develop a bond energy additivity scheme such as devised by Pauling. Analysis is made of the several approximations used in the model.
Ion cyclotron resonance studies on the reaction of H2+ and D2+ ions with various simple molecules and hydrocarbons
Authors: Kim J. K. , Huntress W. T.
Published: February 2003

The rate constants and product distributions for the reactions of H2+ ions with N2, O2, and CO, and for the reactions of both H2+ and D2+ ions with CH4, CD4, NH3, H2O, H2S, C2H2, C2H4, and C2H6 have been measured by ion cyclotron resonance methods. Proton transfer dominates in the reaction with diatomic molecules, and charge transfer dominates in the reaction with polyatomic molecules. However, a significant amount of charge transfer was observed in the reaction of H2+ ions with O2 and CO, and a significant amount of proton transfer was observed in the reaction of H2+ ions with H2O. The reaction with methane to form methyl cations appears to involve a superposition of both dissociative charge transfer and proton transfer followed by H2 elimination. The extent of fragmentation resulting from H2+ charge exchange to CH4 is on the order of that observed in photoionization and Penning ionization of CH4 at 16 eV total available energy, and much larger than is observed in electron impact of CH4 at 16 eV.
Vibrational relaxation of CO2(001) by atoms
Authors: Buchwald M. I. , Wolga G. J.
Published: February 2003

The laser fluorescence technique coupled with ESR detection of atoms has been used to measure the vibrational relaxation of CO2(001) by 0(3P), Cl(2P), and F(2P). The rates are kCO2M–O = 6.69×103 (sec-Torr)−1, kCO2–Cl = 4.91105 (sec-Torr)−1 and kCO2–F = 1.04×105 (sec-Torr)−1. These rates are much faster than those of rare gas atoms of similar atomic weights. Measurements were also made establishing limits for the relaxation of CO2 by H(2S), D(2S), and N(4S).
Thermal force on a disk
Authors: Davis Lorne A. , Adair T. W.
Published: February 2003

Measurements were made of the thermal force on thin disks with their planar surfaces perpendicular to a heat flux between parallel plates in monatomic and diatomic gases. The plate spacing and the diameters of the disks were of the same order of magnitude. The study of the thermal force was conducted as a function of the gas composition, mean free path, applied thermal gradient, disk size, and disk thermal conductivity. The data are described by a simple universal curve and for conditions approximating the Waldmann limit are in agreement with recent theory. Comparison is also made with other theoretical and experimental work.
Host excitation of luminescence from KI:Tl+ and RbI:Tl+ by slow electron impact
Authors: Huang Timothy , Hamill William H.
Published: February 2003

The luminescence of KI:Tl+ and RbI:Tl+ evaporated thin films in the range 30–300 °K is excited by electron impact from ∼4 to ?8 eV. Emission at 410 nm from KI:Tl+ at 300 °K was excited throughout the range 4.1–5.7 eV (the first optical exciton) and above. For 6.6 eV excitation at 70 °K, the emission spectrum may contain unresolved contributions from the host at 376 and 410 nm as well as from the activator at 336 and 430 nm, the last being much the strongest. Optical absorption is known to excite intrinsic luminescence bands at 410, 376, and 300 nm. In RbI:Tl+, the low-energy emission at 430 nm and 300 °K was excited by electron impact throughout the range 4.0–5.7 eV (the first optical exciton) and above. The temperature dependence of the lifetime for the 430 nm bands of KI:Tl+ agrees with results from the literature, while that of the intensity dependence agrees only in part. Characteristic electron energy losses of the host lattice are unmodified by Tl+ and the luminescence intensity is high, indicating host sensitization.
Energy transfer in CO A1Π, v′=9. I. Quenching and isotope effects
Authors: Melton L. A. , Yiin K.-C.
Published: February 2003

The atomic oxygen line at 1306 Å excites 12C16O A1Π, v′=9, J′=22 and 12C18O A1Π, v′=9, J′=6. For 12C16O A1Π, v′=9, the following total quenching rates have been measured (units, cm3 sec−1): 12C16O, 8.7×10−10; 12C18O, 8.6×10−10; He, 6.9×10−12; Ne, 2.0×10−12; Ar, 2.4×10−11; Kr, 1.2×10−10; Xe, 4.8×10−10; H2, 2.0×10−10; D2, 1.6×10−10; SF6, 1.4×10−10. For 12C18O, A1Π, v′=9, the following quenching rates have been measured: 12C16O, 6.7×10−10; 12C18O, 1.3×10−9; He, 3.7×10−12; Ne, 7.9×10−13; Ar, 2.2×10−11, Kr, 2.2×10−10; Xe, 5.4×10−10; H2, 2.6×10−10; D2, 1.6×10−10; SF6, 1.4×10−10. It is probable that He, Ne, Ar, and Kr quench CO A1Π, v′=9 by mixing the A1Π state with nearby electronic states and that CO, Xe, H2, D2, and SF6 quench both by mixing the A1Π state and by reaction and/or electronic energy transfer.
Molecular charge transfer. III. The role of incident-ion vibrational states in H+2–H2 and D+2–D2 collisions
Authors: Moran T. F. , Flannery M. R. , Albritton D. L.
Published: February 2003

Charge transfer cross sections for the H+2–H2 and D+2–D2 systems have been computed and compared with experimental measurements in the 1 eV to 5000 eV kinetic energy range. Cross sections have been calculated using a multistate impact parameter treatment of charge transfer in which the set of first order coupled differential equations resulting from the time dependent Schrödinger equation are solved numerically. At low kinetic energies the inclusion of a comparatively small number of product states results in converged cross sections. However, as the ion kinetic energy is increased, it becomes necessary to include as many as ∼100 final vibrational states in the wavefunction expansion of the system. Both the energy defects and vibrational overlaps of the different product channels have a strong influence on reactions at low kinetic energies but as the ion energy is increased the vibrational overlaps exert a relatively more important role in controlling the product state distribution. Vibrational overlaps have been computed using accurate wavefunctions which include the effects of vibration–rotation interaction. Variation of the theoretical cross sections with both ion kinetic energy and reactant ion vibrational state are in accord with experimental measurements.
Hindering and memory effects on the distribution of relaxation times in dielectric theory. I. Secondary dispersion regions in n-propanol
Authors: McMahon D. R. A.
Published: February 2003

An integrodifferential equation of the mth order, a generalization of the first order Markoffian equation, is derived for the single particle orientation conditional probability function where m is the average number of consecutive reorientation events over which significant memory of the initial conditions persist. The dipole autocorrelation function is derived and used in the linearized approximation to the Cole–Onsager equation. It is shown that the theory can account for the multiple dispersion features in the alcohols and model fitting to the available data for n-propanol at −145 and −150°C gives m of the order of 175 and 187, respectively. These high order memory effects are attributed to strong hindering of dipole reorientations by hydrogen bonding.
Radiative lifetimes in molecules studied by means of fast, molecular ion beams
Authors: Jo/rgensen S. W. , So/rensen G.
Published: February 2003

Radiative lifetimes of excited levels in molecules have been studied by a spectroscopic investigation of the collision processes occurring between fast, molecular ion beams and helium atoms. This technique has been used to measure the lifetimes of the A 2Δ state in CH, the B 2Σ+ state in CO+, and the B 2Σ+ states in CO2+ with an accuracy of 10%–20%. A comparison with previously published results and a discussion on the perspectives of the method have been carried out.
Chemical accelerator studies of reaction dynamics: Ar+ + CH4 → ArH+ + CH3
Authors: Wyatt J. R. , Strattan L. W. , Snyder S. C. , Hierl P. M.
Published: February 2003

Chemical accelerator studies on isotopic variants of the reaction Ar+ + CH4 → ArH+ + CH3 are reported. Velocity and angular distributions of the ionic product as a function of initial translational energy have been measured over the energy range 0.39–25 eV center-of-mass (c.m.). The asymmetry of the product distribution with respect to the center of mass indicates that the reaction is predominantly direct over the energy range studied. The dynamics of the reaction are approximated by the spectator stripping model: The reaction exothermicity appears as product internal energy and product excitation increases with collision energy at the rate predicted by this model. The internal degrees of freedom of the neutral product have little effect on reaction dynamics, and product excitation appears to reside principally in the ionic product. Deviations from the spectator stripping model suggest the existence of a basin in the potential energy hypersurface for this reaction; the ArCH4+ complex which may be formed at low collision energies, however, preferentially decomposes via reaction channels other than that resulting in ArH+ formation.
A study of the magnetic state of O2+ in fluoride salts
Authors: DiSalvo F. J. , Falconer W. E. , Hutton R. S. , Rodriguez A. , Waszczak J. V.
Published: February 2003

Static magnetic susceptibility measurements for 4°K < T < 300°K are reported for the following dioxygenyl salts: O2AsF6, O2SbF6, O2Sb2F11, and O2PtF6. In contrast to previous studies, we find both spin–orbit coupling and crystal field effects to be important. While the As and Sb compounds are paramagnetic at 4.2°K and have small O2+–O2+ exchange, O2PtF6 is ordered at 4.2°K, probably due to O2+–Pt(V) exchange. ESR studies confirm the susceptibility results, except in O2PtF6 where a resonance was not detected, and show that the O2+ magnetic state is described by an axial spin Hamiltonian with g⊥ ∼ 1.98 and g∥ ∼ 1.74. The ESR linewidth of O2AsF6 below 70°K is ∼ 80 G. Above 100°K the linewidth rapidly increases so that above 180°K the line is unobservable. This increase may be due to O2+ motion. NMR shows that the AsF6 groups in O2AsF6 are also rotating above 200°K, probably in some hindered form.
Direct measurement of the radiative lifetimes of the A 2Σ+ (V′ = 0) states of OH and OD
Authors: German K. R.
Published: February 2003

The fluorescent decay of the A 2Σ+ (V′ = 0) state of OH and OD has been measured for N′ ? 8 for OH and N′ ? 9 for OD using pulsed dye laser excitation and at total gas pressures ? 1 mtorr. For the rotationless molecule, a radiative lifetime of 0.688 ± 0.021 μsec for OH and 0.692 ±0.021 μsec for OD was obtained, where the error bars are three standard deviations. In addition, OH exhibited a 8 ± 2% lengthening of the radiative lifetime between N′ = 1 and N′ = 8.
Molecular Rydberg transitions. I. Low-energy Rydberg transitions in methyl halides
Authors: Hochmann P. , Templet P. H. , Wang H.-t. , McGlynn S. P.
Published: February 2003

The energies of the rare gas transitions p6; 1S → (p5; 2PJ) nl; Γ, where p is the highest occupied p AO of the ground configuration and nl is the nth virtual l AO, exhibit a significant linear correlation, for fixed n and l, with the ionization energies p6; 1S → p5; 2PJ. Similar correlations exist for Rydberg transitions in CH3X molecules, X = Cl, Br, I. The assignment capability of the combined interatomic and intermolecular correlations is equivalent, at least in the case of the methyl halides, to that of the Rydberg equation for n ? 5.
Equilibrium constants of hydrogen–deuterium–tritium self-exchange reactions in water vapor as studied with a pulsed molecular-beam quadrupole mass filter
Authors: Pyper J. W. , Christensen L. D.
Published: February 2003

We measured the equilibrium constants KHDO = P2HDO/PH2O⋅PD2O, KHTO = P2HTO/PH2O⋅PT2O, and KDTO = P2DTO/PD2O⋅PT2O at 295.6°K (22.4°C), assuming the mass spectrometric sensitivities of the three species cancel in each expression of the equilibrium constant. The results obtained were KHDO = 3.82±0.05, KHTO = 3.66±0.06, and KDTO = 3.99±0.07. These values agreed with the current theoretical values of KHDO = 3.84, KHTO = 3.69, and KDTO = 3.97 at the same temperature. We discuss the discrepancy of the present value of KHDO as compared with previous experimental determinations, and we conclude that a direct measurement of the mass spectrometric sensitivity ratio for KHDO is still necessary to resolve the disagreement. We prepared the water mixtures that contained tritium by reacting the appropriate hydrogenic gases with oxygen over a platinum sponge catalyst.
Low energy scattering and the ’’free-fall’’ atomic model
Authors: Gryziński Michal/
Published: February 2003

The free-fall atomic model has been applied for quantitative analysis of atomic and molecular collision processes. The model yields results which are in agreement with the measurements. It describes properly the low energy scattering of electrons (with respect to the energy dependence, as well as to the absolute cross section values) and van der Waals interaction and gives some insight into the nature of chemical bonds. The dynamic dipole was found to be a characteristic feature of π bonds.
ESR studies of [RhCl6]4− in various matrices
Authors: Shock John R. , Rogers Max T.
Published: February 2003

An ESR study of the [RhCl6]4− center in single crystals of AgCl has been made. The g values and chlorine hyperfine interactions observed for the axial chlorines are consistent with a d7 configuration and localization of the odd electron largely in the 4dz2 orbital. The same species has been studied in γ-irradiated single crystals of NaCl doped with [RhCl6]3− and the ESR parameters found were quite similar. Polycrystalline Na3RhCl6⋅12H2O, after γ irradiation at room temperature, appears to give the same species, although only g⊥ can be obtained from the spectrum. Molecular orbital theory has been used to interpret the chlorine hyperfine interactions in terms of bonding parameters for the complex. The delocalization found is smaller than that for [RhBr6]4−.
Thermomagnetic force at low pressures
Authors: Davis Lorne A.
Published: February 2003

The longitudinal component of the thermomagnetic force on a disk with surface perpendicular to the heat flux between parallel plates has been measured in N2 and HD at pressures near the observed maximum in the thermal and thermomagnetic forces. A comprehensive study of the thermomagnetic force in N2 was conducted as a function of pressure, magnetic field strength, disk size, disk thermal conductivity, and applied thermal gradient. The data on N2 and HD are in agreement with a recent theory derived for the Waldmann limit. In addition, a preliminary measurement of the thermomagnetic force on a sphere is reported.
Detection of dimers by 19F NMR in CdF2 doped with ErF3 and YbF3
Authors: Mustafa M. R. , Jones W. E. , McGarvey B. R. , Greenblatt M. , Banks E.
Published: February 2003

The 19F NMR of single crystals of CdF2 containing large concentrations of ErF3 or YbF3 has been studied at 30 and 46.5 MHz from 190° to 373°K. In addition to the main resonance, two types of weaker resonances were detected: one type displaying symmetry about the [111] axis and the other displaying symmetry about the [100] axis. The 111 lines are attributed to the lattice fluorides adjacent to one rare earth ion while the 100 lines are attributed to the interstitial fluoride. It is shown that the anisotropy of the interstitial fluoride resonance can only be explained by the presence of two rare earth ions at 90° angles and therefore the bulk of rare earth ions and interstitial fluorides must be present as (RE3+ − Fi−)2 dimers. The anisotropic portion of the NMR shifts is shown to be entirely due to the dipolar contribution of spin residing on the rare earth ion. Further it is shown that the isotropic shift is of opposite sign to that predicted by the equation currently used to explain contact shift of lanthanide ions.
Observation and analysis of the optical absorption and fluorescence spectra of green-colored Np:CaF2
Authors: Natarajan A. , Wang S. D. , Artman J. O.
Published: February 2003

The optical absorption spectra seen at room and cryogenic temperatures in green-colored Np:CaF2 crystals have been attributed to Np3+ substitution at Ca2+ [000] sites with three distinct modes of charge compensation and correspondingly lowered local symmetries. These are: A, C3v symmetry associated with the presence of an F− compensator at empty [(1/2)(1/2)(1/2)]-type sites; B, C3v symmetry related to the replacement of a nn F− at a [(1/4)(1/4)(1/4)]-type position by O2−; C, C4v symmetry attributed to the presence of an F− compensator at empty [00(1/2)]-type sites. A two-parameter fit to the five lowest excited multiplet centers yielded the following F2 and ζ values (in cm−1): A, 214.5 and 2032; B, 212.7 and 2001; C, 219.1 and 2083. The multiplet splittings of each of these three spectra have been fit with crystal potential (’’crystal-field’’) parameters. Fluorescence, consisting of several broad bands near 1.4 μm and four lines near 2.5 μm, was associated with the Np3+ type-C sites. At liquid He temperature, the 2.5 μm spectrum consisted of just one sharp line at 3929 cm−1. Vibrational effects were largely absent, both in absorption and fluorescence.
Multiple reflection time domain spectroscopy. II. A lumped element approach leading to an analytical solution for the complex permittivity
Authors: van Gemert Martin J. C. , Suggett Alan
Published: February 2003

A theoretical and experimental analysis of a simple lumped element model representing multiple reflection time domain spectroscopy (TDS) is presented. In the frequency domain, an analytical expression is derived relating the experimentally determined scattering coefficient S11(iω) and the complex permittivity. Permittivities thus determined for a number of cases in the frequency range 100 kHz to 2 GHz are shown to be a close approximation to the true permittivities in most circumstances. The conditions are discussed under which the analytical expression breaks down. The step response behavior for a Debye-type dielectric is also calculated, leading to an improved accuracy in measuring dielectric relaxation parameters directly from time domain waveforms.
Photon yields of several reactions producing diatomic strontium oxide and halides, and SrO (A′ 1Π–X 1Σ): A new band system
Authors: Capelle Gene A. , Broida H. P. , Field Robert W.
Published: February 2003

Strontium atoms were reacted with N2O, O3, O2, NO, Br2, Cl2, and F2 in a flow system to produce the diatomic oxide or halide. Photon yields were measured from these reactions in a bath of argon at pressures of 1.1–9.5 Torr. Photon yields in excess of 6×10−2 for the SrO A 1Σ–X 1Σ band system were observed with both N2O and O3. The total photon yield with F2 producing SrF is also high, exceeding 1×10−2, most of which is due to A 2Π→X 2Σ photons; the B 2Σ, C 2Π, D 2Σ, and E 2Π→X 2Σ systems are also observed, but they give progressively smaller contributions to the total yield. The previously unobserved SrO (A′ 1Π–X 1Σ) band system was seen in chemiluminescence from Sr+O3 and Sr+N2O reactions. Bandhead measurements allowed determination of molecular constants. The constants that correspond to the most probable SrO A′ 1Π vibrational numbering are ν00 = 9310(8), ωe = 472.8(14), ωexe = 2.06(6), Be = 0.2565(20), and αe = 0.0017(3) cm−1.
An application of the semiclassical approximation of the generalized phase shift treatment of rotational excitation: Ar–N2
Authors: Pattengill M. D.
Published: February 2003

The semiclassical approximation of the generalized phase shift (GPS) treatment of rotational excitation is applied to a previously considered collision model intended to represent thermal Ar–N2 scattering. In addition, calculations are performed in the sudden approximation limit. Results obtained are compared at the level of inelastic cross sections with those from previous classical trajectory and classical limit GPS calculations.
Close coupling test of classical and semiclassical cross sections for rotationally inelastic Ar–N2 collisions
Authors: Pack Russell T
Published: February 2003

Accurate quantum mechanical close coupling (CC) integral cross sections are reported for rotationally inelastic Ar–N2 collisions at room temperature using an empirical intermolecular potential. These cross sections are used to test the infinite order sudden (IOS) approximation and the results of several methods [classical trajectories (CT), full sudden (SA), generalized phase shift (GPS), and classical limit generalized phase shift (CGPS)] as reported in the preceding paper by Pattengill. The IOS approximation works very well for cross sections involving low rotational states. The CT method works well for all the cross sections examined, justifying the popular belief that the rotational motion of N2 is essentially classical at room temperature.
On the variation of the spin–orbit coupling in O+2
Authors: Raftery J. , Richards W. G.
Published: February 2003

Analysis of the A 2Πu–X 2Πg band system of O+2 shows that the spin–orbit coupling constants for both states vary markedly with vibrational quantum number; in addition, the sign of that for certain levels of the A 2Πu state is uncertain. This is adequately explained for the X 2Πg state on the basis of a variation of the electronic expectation value of the spin–orbit coupling with internuclear distance, the major variation coming not from the variation of the contributions of the four configurations in the wavefunction, but from variation with internuclear distance of the form of the 1πg orbital. For the A 2Πu state, it was not possible to obtain quantitative agreement, since the coupling constant in this case is a small number, the difference between two large quantities. However, it seems clear that in this case the variation was mainly due to variation in the contributions of the various configurations.
CNDO MO calculations for hydrogen atom adsorption on nickel atom clusters
Authors: Blyholder G.
Published: February 2003

CNDO molecular orbital calculations have been done for H atom interactions with various locations on (100), (110), and (111) surfaces of nickel atom clusters containing up to 10 nickel atoms. The CNDO procedure used was basically a modification of that introduced by Pople in which all molecular orbitals are doubly occupied except for the highest occupied MO which contains one electron. Equilibrium positions have been determined both with respect to distance from the surface and position on the surface. Hydrogen atoms are more stable in multicenter bonds over holes than when directly over a surface atom. The (111) surface produced the greatest binding energy. For the Ni clusters the s and p orbitals of the Ni atoms were primarily responsible for the nickel–hydrogen bonding with the d orbitals playing a relatively minor role. The adsorbed hydrogen atom carries a negative charge.
A study of a lower bounding formula for the overlap between the exact and an approximate wavefunction
Authors: Merkel Robert R.
Published: February 2003

Weinhold’s lower bounding formula for the overlap between the exact wavefunction and an approximate wavefunction is applied to the hydrogen atom. The lower bound is examined as a function of both the number of basis set terms used in the bounding inequality and the nonlinear variational parameter in this basis set. A method is then proposed and tested for approximating the integrals over H2 and H3 required by Weinhold’s technique as products of integrals involving only H. In the case studied, the resulting approximate overlap bounds converged to the true bound.
Two dimensional diffusion theory: Cylindrical diffusion model applied to fluorescence quenching
Authors: Owen Charles S.
Published: February 2003

A value for the diffusion constant of fluorescent quenchers can be inferred by fitting measured and theoretical fluorescence decay curves. In this paper, a formalism is presented describing diffusion-dependent fluorescence quenching in systems which are effectively two dimensional in nature. This model predicts somewhat different values for the diffusion constant from those one obtains by fitting the same experimental curve to the usual Smoluchowski model for diffusion in an isotropic (’’spherical’’) system.
The polar tensors, effective charges, and infrared intensities of X2CY molecules
Authors: Bassi A. B. M. S. , Bruns R. E.
Published: February 2003

The experimental polar tensors and effective charges for the X2CY molecules where X=F,Cl and Y=O,S have been calculated. The polar tensors, with the exception of ∂px/∂xy, and the effective charges of any one of these molecules can be calculated from these values for the other three molecules. The effective charges of carbon, oxygen, and sulfur are not transferable although those for fluorine and chlorine are transferable within the calculated experimental errors. The effective charge values appear to show trends which reflect the electronegativities of the atoms in the molecules. The sums of the gas phase infrared fundamental intensities of these molecules are shown to be interrelated.
Many-body perturbation theory applied to electron pair correlation energies. I. Closed-shell first-row diatomic hydrides
Authors: Bartlett Rodney J. , Silver David M.
Published: February 2003

Diagrammatic many-body perturbation theory is formulated through third order and applied to LiH, BH, and HF with various sizes of two-center Slater orbital basis sets. The most extensive calculations use 46 orbitals to recover 94, 95, and 97% of the experimental correlation energy for the three molecules, respectively, when the perturbation expansion is carried through third order with pair restrictions and including selections of higher-order diagrams via denominator shifts. A detailed analysis of the ’’pair’’ correlation energies relative to SCF occupied orbitals is given, including both inter- and intrapair contributions for the different spin cases.
Generalized stochastic model for molecular rotational motion in dense media
Authors: Lindenberg Katja , Cukier Robert I.
Published: February 2003

We introduce a general stochastic model that encompasses all other commonly used stochastic models for the orientational motion of molecules in dense media. In this model the rotational motion is envisaged to proceed by an alternating sequence of collision and between collision events. These events are characterized by transition probabilities whose form can be chosen in accordance with the physical situation. The durations of the events are governed by probability density functions. These probability densities enable us to investigate the effects of finite durations of collisions and of time correlations between successive collisions. The effect of correlations between successive collisions is to produce nonexponential behavior in the time correlation functions of orientational variables. Indeed these time correlation functions can become negative and can have damped oscillatory behavior even in the complete absence of explicit inertial effects (free rotations). The finite duration of collisions insures that the short time expansion of the time correlation function does not diverge to any order in the time. In addition, we show that in dense gases and liquids with reasonable assumptions our model results agree well with exact molecular dynamics calculations to at least order t5. We also discuss the relation between the stochastic approach and the memory function approach.
25Mg and 14N nuclear quadrupole resonances in chlorophyll-a and magnesium phthalocyanine
Authors: Lumpkin O.
Published: February 2003

25Mg and 14N zero field nuclear quadrupole resonance are observed in chlorophyll-a and magnesium phthalocyanine by proton nuclear magnetic double resonance. In phthalocyanine, the observed resonances correspond to a quadrupole coupling constant of 3.79 MHz for 25Mg and 1.81 MHz for the 14N ligands. In chlorophyll, the 25Mg coupling constant is 3.73 MHz.
Rate constants for the reaction of the OH radical with H2 and NO (M=Ar and N2)
Authors: Atkinson R. , Hansen D. A. , Pitts J. N.
Published: February 2003

Absolute rate constants for the reaction of OH radicals with H2 and NO (M=Ar, N2) have been determined at 298±1 °K using a flash photolysis–resonance fluorescence technique. The rate constant for the reaction OH+H2 was determined to be (6.97±0.70) ×10−15 cm3  molecule−1 sec−1 in good agreement with recent literature values. Rate constants for the reaction OH + NO + M (M = Ar) were obtained over the pressure region 25–655 torr, while a rate constant for M = N2 was obtained at 25 torr total pressure. The low pressure third order rate constants for this reaction were determined to be (4.25±0.43) ×10−31 cm6 molecule−2 sec−1 (M=Ar) and (6.1±0.7) ×10−31 cm6  molecule−2 sec−1 (M = N2), in good agreement with the available literature values. The bimolecular rate constants at 760 torr total pressure were determined to be (5.2±0.8) ×10−12 cm3  molecule−1 sec−1 for M = Ar and (6.1±1.0) ×10−12 cm3 molecule−1 sec−1 for M = N2. A value of 8×10−12 cm3 molecule−1 sec−1 was obtained as a probable lower limit for the limiting high pressure second order rate constant from an extrapolation of the data.
Evaluation of the integrated energy distribution of CH3CF218F resulting from hot atom chemical activation
Authors: Smith W. S. , Rodgers A. S. , Tang Y.-N.
Published: February 2003

A general numerical method has been developed in order to evaluate the integrated distribution of excited molecules prior to unimolecular reaction. The method has been tested on a series of artificially designed distributions and kinetic models in order to determine its effectiveness and limits of applicability. It was then applied to the kinetic data of excited CH3CF218F. The distribution deduced from the pressure dependence of the decomposition and stabilization products indicates that the activating 18F-for-F primary process results in a very energetic distribution with at least 35% of the activiated molecules with energies above 165 kcal mole−1. A detailed analysis of the possible reaction channels indicates that both β-HF elemination and C–C bond scission contribute to the over-all mechanism but that the latter is nomimally pressure independent even at 30 000 torr.
Calculation of structural properties and vibrational frequencies of α- and γ-N2 crystals
Authors: Zunger A. , Huler E.
Published: February 2003

The effects of the zero-point energy, residual forces and torques on the molecules, and molar density, on the calculated lattice frequencies and structural properties of solid α- and γ-N2, are investigated. An intramolecular potential and a parametrized 12-6 atom–atom intermolecular potential are used to calculate lattice modes, intramolecular modes, sublimation energy, equilibrium unit cell parameters, Grüneizen coefficients, and P–V data. The second virial coefficient is also reasonably reproduced. The α-to-γ phase transition is not revealed by the employed intermolecular potential.
Kinetic theory of warm atoms: Non-Maxwellian velocity distributions and resulting Doppler-broadened emission-line profiles
Authors: Whipple E. C. , VanZandt T. E. , Love C. H.
Published: February 2003

The Boltzmann equation is formulated and solved for the following spatially homogeneous and steady-state system: Production of energetic [O(1S)] atoms by dissociative recombination of O2+; removal of the [O(1S)] by emission; and thermalization by elastic and excitation exchange collisions with the abmient gas, asumed to be entirely O(3P). This system is an approximation to the physics of [O(1S)] in the nightime F2 region of the upper atmosphre, but the method can be applied to similar problems. The collision integrals are formulated in terms of scattering kernels. Emission line profiles are calculated from the velocity distriubtion for various values of the elastic and exchange collision frequencies. The accuracy of kinetic temperatures inferred from such line profiles is discussed.
Angular distributions in the electron impact excitation of Xe at 20 eV
Authors: Williams W. , Trajmar S. , Kuppermann A.
Published: February 2003

Electron impact energy-loss spectra of Xe have been studied at 20 eV incident electron energy from 5° to 135° scattering angle. Differential and integral cross sections for elastic scattering and for the excitation of electronic states up to 10.60 eV energy loss have been determined. The validity of the optical selection rules and the previously established angular behavior associated with direct and exchange processes were examined for this intermediate coupling case.
Excitation of the (5,2) band of the first positive band system of N2 by proton impact on nitrogen
Authors: Suchannek Rudolf G. , Sheridan J. Roger
Published: February 2003

Excitation of the (5,2) band of the first positive band system of N2 by proton impact on nitrogen was observed in a beam experiment. The intensity of the (5,2) band varied linearly with target gas pressure, and, therefore, the excitation of this band can be attributed to the impact of primary particles on nitrogen molecules. Excitation cross sections were measured for proton energies ranging from 3.35–20.1 keV. The excitation cross section attains its maximum value of 1.2×10−18 cm2 at approximately 5.0 keV and decreases smoothly towards higher proton energies. The measured cross sections seem to fit a Bethe plot. The quadratic pressure dependence of the intensity of the first positive band system, observed during earlier measurements, may have been due to a large contribution of neutral hydrogen atoms to the excitation of the first positive band system, which has masked out the excitation by protons.
Relativistic molecular calculations in the Dirac–Slater model
Authors: Rosen A. , Ellis D. E.
Published: February 2003

A new method is presented to calculate binding energies and eigenfunctions for molecules, using the Dirac–Slater Hamiltonian. A numerical basis set of four component wavefunctions is obtained from atom-like Dirac–Slater wavefunctions. A discrete variational method (DVM) has been applied to generate the binding energies and eigenfunctions for the molecule. Results are given for a series of molecules, including dihydrides H2X (X=O, S, Se, Te), diatomic indium halides InX (X=F, Cl, Br, I), and metal chlorides XCl (X=B, Al, Ga, In, Tl). Comparison is made with results from nonrelativistic calculations using the DVM with numerical Hartree–Fock–Slater-type wavefunctions and with other types of nonrelativistic calculations. In particular, relativistic level shifts and spin–orbit splitting have been analyzed. The theoretical ionization energies are compared with experimental results. Generally a very good agreement is obtained between the experimental and theoretical binding energies for the valence levels, calculated by a transition state procedure. Most of the calculations have been made with a minimal basis set consisting of the occupied atomic orbitals. For H2S calculations are also reported, using an extended basis set. The change in the binding energies for the valence levels is a few tenths of an eV compared with the results using a minimal basis.
Vibrational relaxation of HCl(v=1) by Cl atoms
Authors: Macdonald R. G. , Moore C. Bradley , Smith I. W. M. , Wodarczyk Francis J.
Published: February 2003

The vibrational relaxation of HCl(v=1) by Cl atoms at 294°K has been carefully measured by two different techniques. A discharge-flow-tube, laser-fluorescence method yielded a deactivation rate constant kCl = (8.8±2.6) ×10−12 cm3 molecule−1⋅sec−1. A refinement of the uv laser photodissociation method gives a value of kCl = (8±3) ×10−12. The wide ranges of the argon third body pressure and of the HCl pressure show that the process is clearly bimolecular and not termolecular.
Microwave spectrum of cyclopropane(1,1)dicarbonitrile
Authors: Pearson R. , Choplin A. , Laurie V. , Schwartz J.
Published: February 2003

The microwave spectrum of cyclopropane(1,1)dicarbonitrile has been studied between 18 and 40 GHz by modulated microwave double resonance spectroscopy. The molecule is a b-type asymmetric rotor with κ ? −0.20. Lines with J values up to 27 have been fit to a quartic Hamiltonian. Rotational constants obtained from this fit include A = 3779.65 MHz, B = 2549.21 MHz, C = 1726.75 MHz. The CC bond opposite the substituents has been estimated from the planar moment to be 1.485±0.01 Å.
Gas phase EPR linewidths and intermolecular potentials. III. Experimental results for the F–Ar, Cl–He, and Cl–Ar systems
Authors: Zegarski Bernard R. , Cook Thomas J. , Miller Terry A.
Published: February 2003

Previous linewidth measurements on the gas phase EPR spectral lines of the F atom perturbed by He have been extended to include the collision pairs, F–Ar, Cl–He, and Cl–Ar. The new results share with the F–He data the feature that different Zeeman transitions in the 2P3/2 state have different broadening cross sections. Also in all cases the 2P1/2 cross section is two or more orders of magnitude smaller than the 2P3/2 ones. Comparison of the 2P3/2 cross sections for the collision partners indicates that the angular symmetry of the intermolecular potential is the same for all. However, the lighter partners He and F have larger cross sections than are predicted by a simple dispersive force model. Evidence is presented which indicates that the 2P1/2 cross sections are dominated by nonadiabatic collisions, which connect the 2P1/2 and 2P3/2 levels.
Mode expansion method in polyelectrolytes
Authors: Iwasa Kunihiko
Published: February 2003

With the use of Andersen and Chandler’s collective variable method, the free energy of polyelectrolytes is estimated assuming that the polymer charge distribution is regarded as linearly distributed point charges. The result agrees well with the cluster expansion by Manning and Zimm.