bluuebunny/crossref_metadata_2025 huggingface dataset:
The electronic structure of NO2. II. The ? 2B2← ? 2A1 and ? 2B1←? 2A1 absorption systems
Authors: Gillispie Gregory D. , Khan Ahsan U.
Published: February 2003

The results of an accurate ab initio study of the electronic structure of NO2 have been applied to an analysis of the two important visible and near infrared absorption systems of this molecule. The long wavelength absorption (λ≳6000 Å) arises from an ? 2B2←? 2A1 transition. A theoretical absorption spectrum that is generated from the C2V ab initio potential surfaces of these two states qualitatively reproduces most of the features of an experimental low resolution absorption spectrum between 9000 and 6000 Å. The (0–0) band of the transition is predicted to be several times less intense than nearby hot bands even at temperatures as low as 300 K. The computed ? 2B2 spectroscopic parameters are Te=1.18 eV, Re=1.26 Å, ϑe=102°, ω1=1461 cm−1, ω2=739 cm−1, and μ=0.46 D. There is a marked difference between experimentally determined ? 2B2 rotational constants and those deduced from the ab initio equilibrium geometry; this datum adds to the rapidly increasing evidence for strong vibronic coupling of the ? 2B2 state with high vibrational levels of the ground electronic state. The theoretical spectroscopic parameters of the experimentally better understood ? 2B1 excited state are Te=1.66 eV, Re=1.20 Å, ϑe=180°, ω1=1192 cm−1, 2 ω2=960 cm−1, ω3=2040 cm−1, and μ=0.0 D. The adiabatic excitation energy, bond angle, and bending frequency are in good agreement with experiment. However, the theoretical equilibrium bond length is significantly shorter than the value deduced from the experimental spectroscopic data and possible reasons for this discrepancy are discussed.
Effective cross sections for excitation of atomic oxygen by electron impact
Authors: Haasz A. A.
Published: February 2003

Effective cross sections for excitation of atomic oxygen by 600–3000 eV electrons were obtained for the OII 4351.3 Å spectral line corresponding to the 3p′2D5/20 →3s′2D5/2 transition and the OII 4414.9 Å and OII 4417.0 Å lines of the 3p2D0→3s2P multiplet. Oxygen atoms were produced in the absence of O2 molecules by the N+NO→N2+O titration reaction. To minimize O atom recombination and elastic scattering of the electron beam, the N2–O gas mixture was expanded into a vacuum chamber resulting in a free jet flow. This provides a unique technique for excitation cross section measurements of free radicals and reactive gases.
Experimental and theoretical studies on M2X+(M=Li, Na; X=F, Cl)
Authors: Rechsteiner C. E. , Buck R. P. , Pedersen L.
Published: February 2003

Mass spectrometric work was combined with ab initio quantum chemical calculations at the STO-3G level to examine the stability and structure of the ions Li2Cl+, Li2F+, Na2F+, Na2Cl+. The stability (based on M2X+ dissociation) order is Li2F+≳ (Na2F+, Li2Cl+) ≳Na2Cl+ for both theory and experiment. The calculations give all ions in the linear configuration.
Properties of metastable krypton atoms in afterglows produced in krypton and krypton–nitrogen mixtures
Authors: Tracy C. J. , Oskam H. J.
Published: February 2003

Light absorption techniques are used to study the temporal dependence of the number densities of the 5s′00 and 5s12 krypton metastable atoms during the decay period (afterglow) of a pulsed discharge in krypton and krypton–nitrogen mixtures. The sensitivity of the temporal properties of the 5s′00 state of krypton to discharge pulse conditions was attributed to an electron destruction process. The 5s12 state was not affected by discharge excitation conditions, permitting the measurement of the diffusion coefficient (Dp0=29.3 cm2 sec−1 Torr) and of the two-body and three-body reaction rate constants in pure krypton (k1=2.44×10−15 cm3 sec−1 and k2=2.59×10−32 cm6 sec−1, respectively). In krypton–nitrogen mixtures the additional loss of the 5s12 state of krypton due to its reaction with nitrogen was determined to have a reaction rate constant of 3.33×10−12 cm3 sec−1. Light emission studies made using a flowing afterglow system show that this process populated a wide range of vibrational levels of the B 3Πg state of the nitrogen molecule.
Anomalous ultrasonic absorption in copper lanthanum nitrate
Authors: Hentz R. P. , Yun S. S.
Published: February 2003

Ultrasonic wave absorption and velocity data seem to indicate the existence of some type of phase transition in single crystal salts of copper lanthanum nitrate in the neighborhood of 240°K. The nature of the phase transition observed at 240°K could not be explained by this experiment. The absorption coefficient varies linearly with the frequency (10–200 MHz) and shows a temperature dependence of 1/Ta, where a ranged from 1.8 to 3.0 depending on the orientation except the anomaly at 240°K.
Sodium-23 nuclear magnetic resonance of resins and mixed solvent systems
Authors: Goldsmith Michael , Hor Doris , Damadian Raymond
Published: February 2003

The sodium-23 spin–lattice relaxation time (T1) was determined at 10 MHz on Dowex 50W-X2 ion exchanger resin as a function of the dielectric constant of the solvent. The relaxation time varied from 24.8 msec in distilled water (D=78.5) to values too short to determine in water–ethanol mixtures below a dielectric constant of 35. Similar studies were conducted on 250 mm sodium bisulfite (NaHSO3) in a series of aqueous–nonaqueous mixtures. The results indicate that T1 may be expressed as a function of the macroscopic dielectric constant for any binary solvent system. For systems with a dielectric constant greater than 65 the relationship is given by T1=a exp(bD), where a and b are constants for a particular solvent system.
Structural and orbital analysis of ethylene and acetylene on Ni(111) surfaces
Authors: Anderson Alfred B.
Published: February 2003

A molecular orbital study is made of the binding and structural distortions of ethylene and acetylene adsorbed on Ni (111). The following results emerge from the calculations. Disigma bridging positions are preferred. Two Ni atom clusters are sufficient for determining adsorbate structures which show distortions similar to those in coordination compounds. Acetylene, at monolayer coverage, occupies half the surface binding sites while ethylene occupies a quarter of the bridging sites. Acetylene readily dissociates into CH fragments occupying threefold surface indentations. The calculated energy levels for the various modes of adsorption are related to the photoemission spectra of Eastman and Demuth.
Storage and recombination of atomic H in solid H2
Authors: Rosen Gerald
Published: February 2003

A phenomenological rate process theory is developed for the storage and rapid recombination of atomic hydrogen free radicals in a crystalline molecular hydrogen solid at temperatures in the range 0.1 K≲T≲4 K. It is shown that such a theory can account quantitatively for the recently observed dependence of the storage time on the storage temperature, for the maximum concentration of trapped H atoms, and for the time duration of the energy release in the tritium decay experiments of Webeler. The theory predicts that maximum atomic hydrogen concentrations of the order 1020/cm3 are realizable for storage temperatures in the vicinity of 0.14 K.
Relativistic symmetry spinors for polyatomics
Authors: Oreg J. , Malli Gulzari
Published: February 2003

A systematic treatment of the simple rotation double groups in general (with any order n of the principal axis) is presented. The LCAS simple symmetry spinors of these groups are given in general for any values of the atomic quantum numbers ljm.
Relativistic symmetry spinors for polyatomics. II
Authors: Oreg J. , Malli Gulzari
Published: February 2003

A procedure for describing the effect of symmetry operations on the various atomic centers of molecules of the ’’simple rotation groups’’ is presented. The general form of the various projection operators and the four-component symmetry spinors for polyatomics of simple rotation double groups (SRDG) is given.
The microwave Q branch of the distortion moment spectrum of 13CH4
Authors: Ozier I. , Lees R. M. , Gerry M. C. L.
Published: February 2003

The distortion moment microwave spectrum of 13CH4 has been studied with a conventional Stark modulated instrument having a sensitivity of ∼1×10−11 cm−1. Six pure rotational Q branch transitions have been observed between 8 and 20 GHz. With the assumption that the three octic tensor distortion constants L4T, L6T, and L8T have the same values as in 12CH4, values have been obtained for the one quartic and two sextic tensor distortion constants. These are (in Hz) DT= (132 983.32±0.48); H4T=−(17.0026±0.0018); H6T= (11.1869±0.0011). The errors given here are the standard deviations obtained from the least squares analysis. The justification for the assumption is presented, and estimates are given for the absolute errors in DT, H4T, and H6T, as well as for the isotopic shifts in L4T, L6T, and L8T. The measured differences between the 13CH4 and 12CH4 values of DT, H4T, and H6T are discussed with reference to recent theoretical calculations. A method is given of using these differences to calculate the tensor energy splittings of 13CH4 from those previously published for 12CH4.
Information theoretic analysis of single collision energy transfer from highly vibrationally excited KBr
Authors: Crim F. F. , Fisk G. A.
Published: February 2003

The molecular beam deactivation of highly vibrationally excited KBr in single collisions with Ne, Ar, Co, N2, and CO2 is analyzed in terms of information theory. AIP
Sequential two photon excitation of the C2 Swan transitions and C2 relaxation dynamics in rare gas solids
Authors: Bondybey V. E.
Published: February 2003

The C2 d 3Πg–a 3Πu emission in solid Ar and Ne is excited both by sequential absorption of two identical photons and by two unlike photons using two separately tunable lasers. It is shown that excitation of guest molecules by sequential absorption should be a useful technique for populating levels unaccessible in a single step absorption process. The different phonon band shapes observed for the X 1Σg+–A 1Πu and d 3Πg–a 3Πu transitions are interpreted in terms of the electronic structure of the states involved. The vibrational relaxation in the d 3Πg state is fast and shows erratic variation with v. The lifetimes of the low a 3Πu levels vary from 65 μs for v=0 to less than 0.5 μs for v=3. The rate of the spin and symmetry forbidden radiationless crossing into the ground state shows a strong temperature dependence expected by theory. The efficient two photon excitation of the low d 3Πg levels leads to population inversion and stimulated emission on the d 3Πg→a 3Πu Swan bands.
Cross section measurements for photodetachment or photodissociation of ions produced in gaseous mixtures of O2, CO2, and H2O
Authors: Beyer R. A. , Vanderhoff J. A.
Published: February 2003

A drift tube mass spectrometer coupled with a continuous laser photon source has been used to study photodissociation of thermal energy positive ion clusters. Absolute photodissociation cross sections have been measured for O2 +(H2O)1,2, O2 +(O2), O2 +(CO2), and H+(H2O)n=1–4 at room temperature and for low field conditions (10 or 14.3×10−21 V m2). Photon wavelengths ranging from 676.4 to 454.5 nm were studied. The H+(H2O)n=1–4 were found to have small or zero cross sections; upper limits on their cross sections were obtained. The O2 +(H2O) and O2+(CO2) cross sections behave similarly over part of the photon energy range. Near 2.60 eV both ions have cross sections approaching 6×10−22 m2. The photodissociation of O2 +(CO2) produces a CO2 + photofragment. From the appearance of this fragment an upper bound on the dissociation energy of O2 +(CO2) was placed at 0.46 eV.
The structure of radical pairs in irradiated single crystal of n-eicosane as studied by electron spin resonance
Authors: Fujimura Takashi , Tamura Naoyuki
Published: February 2003

The ESR spectra of a single crystal of n-eicosane have been analyzed. The angular dependence of the fine splitting in ΔMs=1 spectra reveals that several different radical pairs are produced in the single crystal of n-eicosane irradiated at 77°K. Five different pairs are clearly identified; they are interchain type. Among these five radical pairs four species are produced in the adjacent chains aligned along the b axis; their interspin distances are 4.42, 4.54, 5.22, and 5.33 Å. One other is produced in the chains along the a axis and has the interspin distance of 4.25 Å. The presence of the intrachain radical pairs (biradical) is also suggested. The structure of radical pair found in n-eicosane was compared with that found in oriented polyethylene.
Preliminary interpretation of the infrared intensities of combination and difference bands of N2O and CO2.
Authors: Brown Kenneth G. , Person Willis B.
Published: February 2003

Expressions derived by Overend for the infrared absorption of combination and difference bands are tested against experimental data for N2O and CO2(ν3±ν1, ν3±ν2, and ν1±ν2, where data are available). From these comparisons, values of the electrical anharmonicities Pαss′(=∂2pα /∂qs∂qs′) are obtained. Overend’s prediction of an anomalously higher value for the transition moment of certain difference bands than for the corresponding combination bands is verified. For CO2, mechanical anharmonicity dominates the intensities perdicted for combination and difference bands; for N2O, electrical anharmonicity is comparable in its contribution to the intensities of combination and difference bands.
Statistical mechanics of helical wormlike chains. II. Operational method and moments
Authors: Yamakawa Hiromi , Fujii Motoharu , Shimada Jiro
Published: February 2003

General expressions for the characteristic functions and moments for the helical wormlike chain are derived by an operational method. It is then shown that the Wigner generalized spherical functions are useful as a basis set in the expansion of the Green’s function. In particular, the even moments 〈R2m〉 for the end-to-end distance with m=1–5 are evaluated numerically by generating necessary diagrams on a digital computer for several typical cases, including atactic and syndiotactic poly(methylmethacrylate) chains. The behavior of 〈R2m〉 with m=1–4 in the helix–coil transition is also shown. For comparison, a recurrence formula for the moments is also derived in the Appendix.
The triplet state of 1,1′-diethyl-2,2′-cyanine iodide in neat and mixed crystals
Authors: Marchetti A. P. , Scozzafava M.
Published: February 2003

The singlet–triplet absorption spectrum of crystalline 1,1′-diethyl-2,2′-cyanine iodide has been obtained. The absorption origin consists of two lines separated by ∼4.0 cm−1. Zeeman spectra were used to assign the higher energy more intense line to the Au factor group state and the lower energy line to the Bu factor group state. Emission spectra have been obtained from both dilute and concentrated solid solutions of the dye. The results from the concentrated samples indicate that the largest excitation–transfer interaction between translationally inequivalent molecules is less than 1 cm−1.
Application of molecular dynamics computations to the conformal ionic solution theory: Thermodynamics of binary molten salt mixtures
Authors: Saboungi Marie-Louise , Rahman Aneesur
Published: February 2003

An evaluation of the coefficient a (T,V) as defined in the equation for the excess Helmholtz free energy of mixing of binary solutions, ΔAmE=a (T,V)(λi−λj)2XiXj +..., derived from the conformal ionic solution theory is presented. This coefficient is shown to be negative and proportional to a fluctuation energy. The sign agrees with experimental calorimetric data of binary molten salt mixtures. A molecular dynamics computation of a (T,V) was made to compare theory and experiments. The calculated values are strongly dependent on the form of the repulsive contribution to the pair potentials (Tosi and Fumi model and Michielsen, Woerlee, Graaf, and Ketelaar model). Criteria for testing the repulsive part of proposed pair potentials are suggested as a result of this investigation.
Vibrational calculations on the biphenyl crystal: The mixing between low frequency internal and lattice modes
Authors: Burgos E. , Bonadeo H. , D’Alessio E.
Published: February 2003

The low frequency optically active crystal vibrations of biphenyl have been calculated using atom–atom interaction potentials. The agreement with experimental data is good. It has been found that torsional and translational modes are very strongly coupled. The conformational change of the biphenyl molecule from gaseous to crystal state and the corresponding changes in the internal force field are discussed. Some assignments are reexamined taking into account the calculated results. The heat of sublimation is well reproduced from internal energy calculations.
Electronic transitions in CS2: A systematic assignment based on solid phase spectra
Authors: Monahan K. M. , Russell R. L. , Walker W. C.
Published: February 2003

The 115–300 nm absorption spectrum of solid CS2 reveals four previously unidentified maxima at 4.53, 5.13, 8.08, and 9.88 eV. The first three can be systematically assigned as 3Πg,1Πg, and 3Πu excitations. The fourth correlates with the CS2+(A 2Πu) ionization potential of the free molecule.
Spectra and kinetics of the Rb2 molecule
Authors: Drummond D. L. , Schlie L. A.
Published: February 2003

In an effort to evaluate the potential of the Rb2 molecule as a high power laser system, spectra of the A 1Σu to X 1Σg band have been taken in absorption and fluorescence, using 1020 atom/cm3 of noble gas as a buffer. The potential curves for both molecular states were estimated from the temperature dependence of the normalized absorption, using the classical Franck–Condon principle. The rate constant for molecular formation of the A 1Σu state is estimated to be 9×10−32 cm6/sec using xenon as a buffer gas. However, this estimate was made on the basis of the normalized fluorescence which was emitted in that band. There appears to be a significant amount of nonradiative loss from that state, as evidenced by its low fluorescence efficiency compared to the RbXe molecule. Therefore, our estimate of the molecular formation rate could be low by as much as a factor of three. These nonradiative losses might be due to predissociation from the a 3π to the repulsive χ 3Σ state. This question must be more thoroughly investigated before any attempt is made to lase this molecule.
Translational and rotational relaxation
Authors: Koura Katsuhisa
Published: February 2003

The translational relaxation and the translational nonequilibrium effects on the rotational relaxation are studied by solving the semiclassical Boltzmann equation with Monte Carlo simulation in the HCl–H2 system, where HCl formed in the exothermic reaction H+Cl2→HCl+Cl with nonequilibrium rotational and translational distributions with an excess translational energy is dispersed in the heat bath of H2. A simple hard sphere model with the line-of-centers cross section of upward rotational transition, the exponential model of downward transition probability proposed by Polanyi and Woodall, and the selection rule ‖ΔJ‖=1 is used. The explicit time-dependent translational and rotational distributions from initial nonequilibrium to final equilibrium are obtained. Even when the translational distribution is far from equilibrium at the initial stage of rotational relaxation, the difference in the rotational distribution between translational nonequilibrium and equilibrium is less than 10% at the initial stage. Therefore, the estimation of the initial rotational distribution by Polanyi and Woodall with the master equation, where the translational equilibrium is assumed, is valid.
Isostructural phase transitions due to core collapse. I. A one-dimensional model
Authors: Kincaid J. M. , Stell G. , Hall C. K.
Published: February 2003

This is the first of a two part study of a statistical mechanical system that exhibits an isostructural phase transition at high density and pressure. Here, in Paper I, we consider a one-dimensional fluid; in Paper II we go on to treat the same system in three dimensions, using the approximation methods developed and tested here for one of the systems studied exactly by Stell and Hemmer [J. Chem. Phys. 56, 4274 (1972)]. Our pair potential φ (r) consists of a hard core of diameter d plus a shoulder of constant positive magnitude V0 for d<r<d (1+λ), to which a weak long-range attraction term is added. We study the results of first-order perturbation theory in V0, as well as in f0=exp(−βV0)−1, β= (kT)−1, and find that used jointly they yield remarkably accurate results that promise to be similarly accurate in three dimensions, where an exact theory is lacking. In particular, we note that first-order perturbation theory in V0 becomes rigorously exact in one-dimension for fixed β as the density ρ approaches close packing; we argue (nonrigorously) that this result can be expected in any dimension. The first-order theory in exp(−βV0)−1 is correspondingly good in the opposite limit of ρ→0.
Isostructural phase transitions due to core collapse. II. A three-dimensional model with a solid–solid critical point
Authors: Kincaid J. M. , Stell G. , Goldmark E.
Published: February 2003

We give a theoretical treatment of isostructural phase transitions that result from partial collapse of the repulsive core of the interparticle pair potential. We use an approximation procedure that is tailored to the high-density regime in which an isostructural solid–solid transition is found for a potential that is the sum of a hard-sphere term and a repulsive shoulder term. A study is made of the transition and its critical point as a function of the potential parameters. We also give an extension of our theory to mixtures, as well as to the shouldered hard-sphere potential to which a long-range attractive tail is added.
Molecular reorientation and shear viscosity in dense liquids
Authors: Fury M. , Jonas J.
Published: February 2003

The viscosity dependence of reorientation correlation times τϑ for a number of neat liquid monosubstituted benzenes and several symmetric top molecules has been analyzed in terms of Hu and Zwanzig’s hydrodynamic slip limit and also the parameter κ introduced by Kivelson. The slip limit predictions are improved by introducing a weighting factor which emphasizes molecular shape. The analysis of our high pressure data shows that the parameter κ is independent of pressure and temperature, in agreement with the results of other studies. The coupling between translational and rotational motions as characterized by κ is dominated by the molecular shape.
Influence of solutes on material constants of liquid crystals and on electro-optical properties of twisted nematic displays
Authors: Schadt M. , Müller F.
Published: February 2003

Measurements are reported on the changes in properties of nematic liquid crystals with strong positive dielectric anisotropy which are induced by doping the liquid crystals with nonmesomorphic solutes. The dopands decrease the dielectric anisotropy and the viscosity of liquid crystals, as well as influencing the elastic constants k11 and k33 and the ratio k33/k11. The consequent changes of the static and dynamic electro-optical properties of twisted nematic displays when using doped liquid crystals are due to changes of the material constants. The threshold voltage of TN displays decrease by as much as 40% when using a solute concentation of 8% whereas the on-transient times on TN displays decrease by a factor of 4.
Electron traps in irradiated crystalline ice
Authors: Kawabata Keisuke
Published: February 2003

Using the pulse radiolysis method, the effects of repeated pulses on the slowly-decaying components of et− in an ice crystal after a pulse, have been studied around 100°K and the effect of warming the sample after an intense irradiation. The trap of the stabilized electron is produced by the electron irradiation. The radiation-produced traps disappear on warming to 150°K. An ESR signal of et− in irradiated-crystalline ice is detected. Possible kinds of the trap are discussed. The experimental results are explained by assuming the trap is a radiation-produced vacancy of a water molecule at the lattice.
Very high resolution study of photoabsoprtion, photoionization, and predissociation in H2
Authors: Dehmer P. M. , Chupka W. A.
Published: February 2003

Relative photoabsorption and photoionization cross sections for H2 (para and ordinary) have been measured at 78°K from 715 to 805 Å for para-H2 and from 745 to 805 Å for ordinary H2, with a wavelength resolution of 0.016 Å. This resolution represents a factor of 3 improvement over the previous data reported from this laboratory, and in addition, the new data have significantly improved statistics. This enables observation and identification of the R (0) npσ and npπ Rydberg series in para-H2 to principal quantum numbers of approximately 40 for series converging to H2+(2Σg+, v=1–6). Linewidths and relative intensities were measured for a large number of these levels and the results are compared to calculations using a quantum defect theory (QDT) approach. Two-channel QDT is used to assign nearly all the prominent structure in the para-H2 spectrum. The strengths and limitations of the simple two-channel theory and the necessity for extension to multichannel calculation for a complete understanding of the spectrum are discussed. The ionization efficiency for these Rydberg states is always close to unity for states which autoionize with Δv=−1. Decay by predissociation and/or emission compete to varying degrees with autoionization for those states which cannot autoionize with Δv=−1. For states which decay to a detectable extent by fluorescence, the measured autoionization efficiencies yield estimates of rates for highly suppressed autoionization processes which are in good agreement with theoretical calculations. Similarly, comparison of autoionization and predissociation rates yields some estimates of the latter rates for slowly decaying states
Simulation of dilute single crystal free radical ESR spectra
Authors: Harriman John E.
Published: February 2003

The fact that ESR spectra are obtained as a function of magnetic field strength at a fixed frequency makes the simulation of complex spectra either difficult or computationally expensive. Conventional perturbation theory cannot be successfully applied unless nuclear Zeeman energies are negligible, and other methods are difficult to apply when hyperfine interactions with several nuclei are significant. A method based on the partitioning of the spin Hamiltonian has been developed and found to be suitable for typical, organic free radicals with delocalized spin distributions. Nuclear quadrupole interactions have not been included but could easily be added. Extension to S≳1/2 would be possible, but inclusion of zero-field splitting would be difficult.
Absolute rate of the reaction of O(3P) with hydrogen sulfide over the temperature range 263 to 495 K
Authors: Whytock D. A. , Timmons R. B. , Lee J. H. , Michael J. V. , Payne W. A. , Stief L. J.
Published: February 2003

The technique of flash photolysis coupled with time resolved detection of O via resonance fluorescence has been used to obtain rate constants for the reaction of O(3P) with H2S at temperatures from 263 to 495 K and at pressures in the range 10–400 torr. Under conditions where secondary reactions are avoided, the measured rate constants for the primary step obey the Arrhenius equation k= (7.24±1.07) ×10−12 exp(−3300±100/1.987 T) cm3 molecule−1 sec−1. The results are discussed and comparisons are made with previous work and theoretical predictions. Experiments with D2S show that the reaction exhibits a primary isotope effect, in support of a hydrogen abstraction mechanism.
Picosecond stimulated Raman effect in ethanol
Authors: Pochon Etienne , Bourene Maurice
Published: February 2003

Time delays in the stimulated Raman emission from ethanol, excited with a mode locked Nd3+ glass laser, have been studied using echelons associated with a Kerr cell. By taking into account group velocity variations in all the dispersive media traversed by the light beam, the true time resolution of the spectrum has been determined. Our results indicate that there is no time delay between the Stokes, Rayleigh (laser pulse), and anti-Stokes bands. With regard to the Stokes and Rayleigh bands, this agrees with a theoretical prediction for our experimental conditions.
Oscillating CO oxidation on a Pt surface
Authors: Dagonnier R. , Nuyts J.
Published: February 2003

A phenomenological model is proposed for the kinetics of oscillating oxidation of CO on SiO2-supported Pt pellets. The numerical solutions are in qualitative agreement with experiments. The surface temperature, as opposed to the ambient temperature, plays a crucial role.
Nucleation in a nonuniform vapor
Authors: Becker Carol , Reiss H.
Published: February 2003

Conventional nucleation theory describes nucleus formation in a uniform vapor as the addition or loss of single molecules from clusters of a given size. However, under nonuniform conditions, clusters can also be removed by diffusing away from the region of interest. Although there are many practical situations in which nucleation does occur under nonuniform conditions, the conventional theory cannot account for the loss of clusters by diffusion. It is therefore the purpose of this paper (1) to formulate a new theory which does include loss by diffusion and (2) to make a quantitative study of the diffusion effect in order to delineate the range of conditions under which it becomes important. The new theory is derived in the context of a typical diffusion cloud chamber by considering the diffusion loss from the narrow slab where nucleation occurs. The supersaturations which give rise to a unit steady rate of nucleation are then compared with and without the diffusion term for model compounds under a variety of conditions. These quantitative studies on typical model compounds show that the diffusion effect first becomes important for vapor pressures as low as 10−8 torr. Thus, in the cloud chamber, nonuniformities do not cause serious errors. More importantly, in polluted atmospheres, even very low concentrations of vapors of involatile organic substances (e.g., sulfinic acids) can nucleate and give rise to the particulate matter commonly found in photochemical smog.
Exactly solvable models for vibronic coupling in molecular spectroscopy. I. Nontotally symmetric harmonic mode
Authors: Gregory A. R. , Henneker W. H. , Siebrand W. , Zgierski M. Z.
Published: February 2003

Molecular vibronic coupling is studied on a model consisting of two electronic states coupled by one vibrational mode. If adiabatic potentials corresponding to the two states are available (e.g., from experiment), the model can be solved exactly in terms of a coupling function defined implicitly and self-consistently by means of a chosen or calculated diabatic potential. Alternatively and equivalently, the model can be formulated and solved in terms of one adiabatic and two diabatic potentials. In the harmonic-oscillator approximation, the coupling function reduces to one parameter modulated by the electronic energy separation. Near the limit of weak vibronic coupling, the model reduces to the well-known linear Herzberg–Teller approximation. A formally equivalent but complementary linear perturbation approximation is developed for strong coupling. For intermediate coupling, where both of these perturbation schemes are shown to fail, the model is solved by two complementary numerical procedures of which at least one converges quickly for any given set of physically acceptable parameter values. Detailed results are presented for coupling through a nondegenerate, non-totally-symmetric normal mode. Energy level diagrams as well as absorption and emission spectra are calculated for the full range of coupling parameters and energy gaps compatible with the harmonic approximation. The results show anomalous level spacings, intensities, and isotope effects, especially under intermediate-coupling conditions. The anomalies can be qualitatively understood on the basis of avoided crossings in energy-level correlation diagrams. As an example, the model is used for a qualitative explanation of anomalous intensity variations of the 1Lb transition in aromatic hydrocarbons.
Librational motion and phase transition in sodium azide
Authors: Raich J. C. , Gillis N. S.
Published: February 2003

The librational motion of N3− ion in the α and β phases of sodium azide is described in terms of an effective field treatment for rotational excitations. A rigid lattice model incorporating both electrostatic and short-range interactions is employed. The order–disorder aspects of the phase transition in sodium azide are discussed in terms of the pocket state model. The estimated q=0 librational frequencies are compared with the results of Raman scattering measurements. The calculated tilt angle for the azide ion in the α phase is found to agree with experiment. The model provides additional evidence for a small amount of order–disorder character of the α–β phase transition in sodium azide superimposed on its structural aspects.
Physical adsorption on small spherical particles and spherical pores
Authors: Schmeits M. , Lucas A. A.
Published: February 2003

The formal expression for the attractive part of the van der Waals interaction between an atom and the surface of a solid sphere or a spherical cavity is determined. The atom is represented by its dynamic polarizability and the solid by a frequency-dependent dielectric function. Aside from the geometrical effect of the surface curvature, one takes account of a size dependence of the dielectric function. Quantitative evaluation for argon atoms physisorbed on silver spheres shows that the dispersion energy is reduced with respect to the flat surface, whereas it is enhanced in the case of a cavity.
Dynamics of the reaction of N+ with H2. III
Authors: Farrar J. M. , Hansen Steven G. , Mahan Bruce H.
Published: February 2003

The reaction of N+ with H2 was studied using a new ion–molecular beam apparatus designed to operate in very low ion energy regime.(AIP)
Photoexcited chemiluminescence spectroscopy: Detection of hydrogen atoms produced from single vibronic level photolysis of formaldehyde (Ã 1A2)
Authors: Lewis Roger S. , Tang Kenneth Y. , Lee Edward K. C.
Published: February 2003

A sensitive and quantitative spectroscopic method of detecting the hydrogen atoms formed in photolysis has been developed. The near infrared emmision from HNO (Ã A′′→Ã 1A′) produced by recombination of H and NO is monitored.
Chemi-ionization in thermal energy collisions of K2 and Cs2 with halogen molecules
Authors: Rothe Erhard W. , Mathur B. P. , Reck Gene P.
Published: February 2003

Chemi-ionization studies were conducted for K2 and Cs2 colliding with Cl2, Br2, I2 and ICl at thermal energies. (AIP)
Chemical reactions stimulated by superthermal beams
Authors: Helbing R. K. B. , Hsieh Y. F. , Pol V.
Published: February 2003

Superthermal neutral atomic beam reactions were studied using potassium beams and thermal neutral sodium beams impinging on a tungsten surface. (AIP)
The identification of UN in Ar matrices
Authors: Green David W. , Reedy Gerald T.
Published: February 2003

Measurements of vibrational frequencies of the matrix-isolated UN molecule are reported; leading to the identification of a linear N-U-N molecule. (AIP)
Laser photoluminescence of zirconium oxide at 4 K
Authors: Lauchlan L. J. , Brom J. M. , Broida H. P.
Published: February 2003

Laser induced photoluminsecence spectra of ZrO trapped in Ne at 4 K have been obtained. Continuous wave tunable dye and cw argon ion lasers were used to excite ZrO from the X 1Σ+ ground state to the e 1Π and B 3Π electronic states, respectively. Red emission from e 1Π (v=0) →X 1Σ+(v=0,1,2) was observed. In addition, e 1Π (v=0) and e 1Π (v=2) have been found to undergo matrix induced intersystem relaxation into the E 3Π and A 3Φ states, respectively. Infrared emissions of ZrO have been observed and tentatively identified as belonging to E 3Π→X′ 3Δ and e 1Π→c 1Δ. Bandheads of observed transitions are shifted not more than 2% from known gas-phase values. Line shapes of the transitions to the ground state from the e and B states are characterized by a sharp zero-phonon line accompanied by a multiphonon sideband. An additional system overlapping the e 1Π−X 1Σ+ system has been observed and is identified as the A 3Φ2→X′ 3Δ1 band system. The energy of the low-lying c1Δ and X′ 3Δ states are determined to be 5200 and 1650 cm−1 above the ground state.
Electronic states of Zn2. A
b i
n
i
t
i
o calculations of a prototype for Hg2
Authors: Hay P. Jeffrey , Dunning Thom H. , Raffenetti Richard C.
Published: February 2003

The electronic states of Zn2 are investigated by ab initio polarization configuration interaction (POL-CI) calculations. Molecular states dissociating to Zn(1S)+Zn(1S, 3P, and 1P) and Zn(3P)+Zn(3P) are treated. Important effects from states arising from Zn+(2S)+Zn−(2P) are found in the potential energy curves and electronic transition moments. A model calculation for Hg2 based on the Zn2 curves including spin–orbit coupling leads to a new interpretation of the emission bands in Hg vapor.
Atomic scattering from a sinusoidal hard wall: Comparison of approximate methods with exact quantum results
Authors: Masel Richard I. , Merrill Robert P. , Miller William H.
Published: February 2003

Several approximate procedures for the calculation of the intensities of atoms elastically scattered from solids are compared with nearly exact quantum calculations from a sinusoidal hard wall potential. The quasiclassical method reproduces the qualitative features of the ’’rainbow’’ envelope of the scattered intensities present in the exact results but it does not contain any of the quantum interference arising from multiple scattering within a single unit cell and thus does not reproduce the rich structure in the intensities of individual diffraction beams. CCGM theory predicts only specular scattering for the hard wall, independent of roughness. Semiclassical theory at normal incidence gives the same analytical results as the Kirchoff approximation. This result agrees to within 3% of the exact calculations for surface roughness from 0.02 to 0.1 of a lattice constant a at ka vectors greater than 22. At lower ka vectors deviations as large as 60% are observed for the 0.1 roughness while at 0.02 roughness deviations are still less than 2%. Renormalization of the results from the 0.1 surface reduces its deviations from the exact calculation to less than 10% even at low ka vectors. The Kirchoff result agrees with the semiclassical one only at normal incidence and can be used only for hard wall potenials.
Velocity dependence of sensitized fluorescence in collisions of metastable argon and helium atoms with nitrogen
Authors: Sanders R. A. , Schweid A. N. , Weiss M. , Muschlitz E. E.
Published: February 2003

The interactions of metastable argon and helium atoms with nitrogen have been studied by observation of the fluorescence of electronically excited N2 and N2+. An effusive, velocity filtered, metastable atom beam intersects a supersonic nozzle beam of nitrogen seeded with helium to extend the relative energy range. The nozzle beam is modulated and the photon signal is extracted from background using a dual-channel photon counter. Relative cross sections for the production of N2(C 3Πu) in metastable argon–nitrogen collisions are reported over the energy range 0.06 to 0.36 eV. Cross sections for the production of N2+(B 2Σu+) in metastable helium–nitrogen collisions are reported over the energy range 0.03 to 0.16 eV.
Flowing afterglow studies of hydronium ion clustering including diffusion effects
Authors: Bierbaum V. M. , Golde M. F. , Kaufman F.
Published: February 2003

The termolecular hydration sequence of H3O+ in helium buffer gas H3O+(H2O)n−1+H2O+He⇄knk−nH3O+(H2O)n+He is studied using the flowing afterglow technique. The rate constants k1, k2, k3, and k−3 and the equilibrium constants K3 and K4 are measured, discussed, and compared with other values. The effects of mass discrimination, ion–electron dissociative recombination, and differing diffusion rates of the ions are considered, and the diffusion coefficients of the ions H3O+(H2O)n, n=0 to 4, are estimated.
On bases for irreducible representations of O (3) suitable for systems with an arbitrary finite symmetry group
Authors: Patera J. , Winternitz P.
Published: February 2003

The problem of constructing bases for representations of the rotation group O (3) adapted to an arbitrary discrete subgroup symmetry is considered. For each discrete subgroup Γ, we construct all Γ invariant operator valued functions of the generators of O (3). These invariant operators can be used to resolve the missing label problem in the O (3) ⊆Γ reduction and also to construct the corresponding bases functions.
A theory of self-consistent electron pairs. Computational methods and preliminary applications
Authors: Dykstra Clifford E. , Schaefer III Henry F. , Meyer Wilfried
Published: February 2003

The recently developed theory of self-consistent electron pairs (SCEP) is an iterative method of obtaining correlated wavefunctions. In its variational form, it is equivalent to a configuration interaction (CI) treatment including all single and double substitutions from a reference determinant. The computational application of the theory has been fully implemented and tested for a variety of chemical systems. Some theoretical refinements which resulted from these tests are presented. The chemical systems selected for this first SCEP study of molecular electronic structure test most of the anticipated difficulties in using the theory and include H2, LiH, BeH+, BH, Be2, CH2, H2O, H2CO, and HCCH. Some of the potential advantages of SCEP relative to conventional CI appear to be computational efficiency, variationally additive pair correlation energies, and the capability to treat systems nearly as large as can be studied with one-configuration self-consistent-field (SCF) theory. The method’s efficiency results largely from the avoidance of an explicit integrals transformation or construction and diagonalization of a large CI matrix. Because SCEP theory is formulated using Hartree–Fock-like operators, with the same dimensionality as the Fock operator, large basis sets are handled nearly as easily as with SCF calculations. One of the largest calculations reported here involved 42 contracted Gaussian functions and accounts for ∼88% of the valence shell correlation energy of singlet methylene. The equivalent CI wavefunction would include 2926 symmetry-adapted configurations. For the water molecule, an even more extensive SCEP treatment (equivalent to 4631 1A1 configurations) is reported.
Computational study of channel coupling arrays: Low energy e−–H scattering parameters in the 1S approximation
Authors: Kouri D. J. , Levin F. S. , Craigie Michael , Secrest Don
Published: February 2003

Low energy electron–hydrogen scattering parameters have been calculated using the new set of coupled equations for the transition operators for different arrangement channels derived recently. A key feature of this approach to multichannel scattering is the use of the channel coupling array W, whose elements link the various channels together and thus determine which arrangements are explicitly coupled in the equations. In this work, the dependence of the low energy scattering parameters on the elements Wij is studied numerically within an approximation retaining only the n=1, hydrogen ground state in the expansion of the channel Green’s operator. Because of the channel coupling, this is not the same approximation as the familiar static or static exchange approximation of more conventional calculations. The best results for singlet and triplet parameters taken together occur for the channel permuting array values W11=W22=0, W21=W12=1; these are the values that guarantee a connected integral equation for the matrix of transition operators. In particular, we find singlet and triplet scattering length values (in a.u.) of 6.230 and 2.077, which compare well with the variational results of Schwartz, 5.96 and 1.77.
The hard sphere ’’glass transition’’
Authors: Gordon J. M. , Gibbs J. H. , Fleming P. D.
Published: February 2003

We investigate the existence of a glass transition for the systems of hard spheres studied in computer simulations of molecular dynamics. An empirical best-fit equation of state is established for the metastable (supercooled or supercompressed) hard sphere liquid. Configurational thermodynamic properties are calculated from this, and their significance is discussed. Evidence is presented to support the proposition that a system of hard spheres exhibits a phenomenon similar to a glass transition in which ’’glass formation’’ is purely a kinetic phenomenon. For hard spheres there is no underlying second order transition temperature T2. The effective ’’cooling rate’’ associated with these computer simulations of molecular dynamics is investigated and found to be enormous (∼1011 deg/sec). The implications of these results with respect to the configurational entropy theory of the glass transition are discussed.
The configurational statistics of a polymeric chain confined by impenetrable barriers
Authors: Gaylord R. J. , Lohse D. J.
Published: February 2003

General expressions, applicable to any chain distribution function, are developed for the configurational statistics of a polymeric chain which is confined by any number of impenetrable barriers. The reflection principle, or method of images, is employed for this purpose. The procedure for obtaining approximate, analytical equations from these general expressions is then illustrated using the statistics of a real polymeric chain, to the order N−1, for the cases of: a loop (a chain with both ends attached to the same wall), a cilia (a chain with one end attached to a wall and the other end free), and a bridge (a chain with its ends attached to opposite walls), all between two walls.
Rayleigh–Brillouin scattering from methane
Authors: Hammond C. M. , Wiggins T. A.
Published: February 2003

The Brillouin displacements and the spectra of light scattered from CH4 in the density range from 2 to 20 amagat, y=2.1–21, have been measured and compared with hydrodynamic and kinetic theories. Using hydrodynamic theory it was found necessary to include both Burnett corrections in the equations describing translational motion and the effects of diffusion on the rotational energy relaxation time. An extended hydrodynamic theory is proposed which includes not only these corrections but, in addition, makes provision for a second internal energy relaxation. Comparisons of experimental results with this extended theory over the entire density range, with a kinetic description at the lower densities, and with translational hydrodynamic theory at higher density are presented. Owing to the success of the extended theory, it is suggested that the use of frequency dependent thermodynamic and transport coefficients is not required. Further, re-examinations of the kinetic and translational hydrodynamic formulations appear to be in order to extend their predicting ability.