bluuebunny/crossref_metadata_2025 huggingface dataset:
Delayed Fluorescence of Anthracene at Low Temperatures. Failure of Square Dependence on Exciting Light
Authors: Goode D. H. , Lipsett F. R.
Published: February 2004

The intensity of blue delayed fluorescence of anthracene excited by red light in the triplet absorption region was investigated as a function of temperature over the range 2°−300°K. It was found that as the temperature was lowered the square relationship between fluorescence intensity and excitation intensity broke down at comparatively low excitation intensities. In order to explain the results the theory of Siebrand [J. Chem. Phys. 42, 3951 (1965)] was extended to include the saturation of triplet traps and the decay of triplets by bimolecular annihilation. An expression was derived for the square law failure and this gave reasonable agreement with experimental results. It indicated that both saturation and bimolecular decay were present. Square law failure can provide a way to measure “total” bimolecular annihilation rates and collision rates between free triplet excitons and trapped triplet excitons.
Dipole Moments of the Lowest Singlet π* ← π States in p-Fluorophenol and p-Fluoroaniline
Authors: Huang Kwo-Tsair , Lombardi John R.
Published: February 2004

The Stark effect within the rotational fine structure is used to measure the change in dipole moment on excitation of p-fluoroaniline and p-fluorophenol. The spectra are observed in the 0–0 bands of the lowestlying singlet π* ← π transitions. The results are | Δμ | = 0.44 ± 0.10 D for p-fluorophenol and | Δμ | = 0.82 ± 0.09 D for p-fluoroaniline. If the sign of the change is taken to be positive, the results correlate well with the observed structural changes on excitation. This correlation indicates the extent of quinoidlike resonance structure to the electronic nature of the excited state.
γ Radiolysis of Liquids at High Pressures. VII. Oxidation of Iodide Ion by Hydrogen Atoms in Aqueous Solutions
Authors: Hentz Robert R. , Johnson Clarence G.
Published: February 2004

The H-atom oxidation of iodide, H + H+ + I−– → H2 + I, was studied in γ radiolysis of deaerated acidic iodide solutions at 20° and applied pressures in the range 0–6.38 kbar. Without added iodine, slopes of dose plots for [H2] and total iodine, [I2]t≡[I2] + [I3−], begin to decrease appreciably at relatively low doses and approach limiting values at high doses. Such behavior is a consequence of the reaction H + I3−(or I2) → H+ + I− + I2−(or I), for which k2 = 3.0 × 1010M−1·sec−1 was measured in solutions containing glucose and iodine with [I3−] ≥ 0.83 [I2]t. In solutions without added iodine, results are consistent with the established model for water radiolysis and with the previously established pressure independence of primary yields (g): at all doses [H2]=[I2]t; from initial slopes at both 1 atm and 6.38 kbar, G(I2)t = 4.1 = g(H2) + g(H); from the high-dose limiting slope at 1 atm, G(I2)t = 0.45 ≃ g(H2). Competition between Reactions [1] and [2] was studied by measurement of initial H2 yields from solutions with added iodine. Reaction [2] was assumed diffusion controlled with ΔV2‡ = 1.6 ml mole−1. For [I−] up to at least 0.6M [H2SO4 up to at least 0.8N, rate of Reaction [2] at all pressures is given by k1[H][I−][H+]t, in which [H+]t≡[H+] + [HSO4−]; i.e., HSO4− is indistinguishable from H+. Reaction [1] occurs in termolecular encounters via H + I−→(HI−)‡ followed by proton transfer from the acid ion present. At 1 atm, k1 = 5.3 × 106M−2·sec−1 and ΔV1‡≃−11 ml mole−1; ΔV1‡ = − 6.3 ml mole−1 for P in the range 2–6.4 kbar.
Nuclear Magnetic Relaxation in the Ethanol System. Overhauser Effect and Chemical Exchange
Authors: Arata Yoji , Fukumi Toshio , Fujiwara Shizuo
Published: February 2004

Proton relaxation is investigated in liquid ethanol containing 10−2–10−6M 2, 6-lutidine hydrochloride. Evidence is given that scalar coupling between the methylene and hydroxyl protons plays an important role in determining relaxation for these protons. Longitudinal relaxation rates were used along with Overhauser enhancement data to estimate the exchange rate for the hydroxyl proton. A mechanism is suggested to explain the results involving chemical exchange.
Flowing Afterglow Studies of Ion–Molecule Association Reactions
Authors: Bohme D. K. , Dunkin D. B. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2004

Positive and negative ion–molecule association reactions have been investigated in a flowing afterglow system at 82° and 280°K with helium as a third body over a pressure range 0.1–3.0 torr. Specific rates have been measured for the association reactions Ar++Ar, O++N2, N++N2, O2++O2, N2++N2, and O−+CO2. At low pressures the kinetics of association were termolecular as expected for the ground-state ions studied. The termolecular rate coefficients were found to decrease significantly with increasing temperature. At 82°K the kinetics of association for both O2++O2 and N2++N2 become bimolecular above helium pressures of about 1.6 and 0.2 torr, respectively. This observation is qualitatively, but not quantitatively, consistent with the simple mechanism usually invoked for association reactions. A consideration of available laboratory data on ion–neutral termolecular associations in which a stable complex is formed indicates a surprisingly systematic variation of the termolecular rate coefficient with the number of atoms in the product-ion cluster. The cluster ions N6+, N7+, N8+, N9+, and Ar3+ were identified mass spectrometrically at 82°K, apparently for the first time.
Collision-Induced Absorption Spectrum of Gaseous Oxygen at Low Temperatures and Pressures. I. The Δg1 ← 3Σg− System
Authors: Blickensderfer Roger P. , Ewing George E.
Published: February 2004

The collision-induced absorption of the (0–0) and (1–0) bands of the Δg1 ← 3Σg− transition of oxygen has been studied in the gas phase from 87°–300°K. Low pressures were used and only binary collisions are important in inducing transitions. At low temperatures the absorption is enhanced because the attractive intermolecular potential causes clustering in the gas. However, no direct evidence for bound-state (O2)2 molecules was found. The frequency maxima of the absorption of collision pairs were only slightly shifted from those calculated from monomer oxygen. This suggests that the shapes of the internuclear potentials and separation of the Δg1 and Σg3 electronic states are little affected by these binary collisions. A near-Boltzmann relation of the broad band shapes, moreover, suggests that the intermolecular potentials for Δg1···3Σg−and3Σg−···3Σg+ interactions are nearly the same. The mechanisms of the induced absorption collisions are discussed.
Electron Attachment to N2O
Authors: Chaney E. L. , Christophorou L. G.
Published: February 2004

Electron attachment to N2O has been studied with the swarm method in mixtures with argon and nitrogen, and the measured absolute attachment rates have been combined with beam data on the production of O− from N2O. The cross section for the production of O− from N2O at 373°K has a peak value of 8.3 × 10−18 cm2 at 2.3 eV. In the energy region between ∼ 0.2 and ∼ 1.5 eV the cross section for the production of O− from N2O increases with increasing temperature (323° ≤ T ≤ 473°K), suggesting dissociative attachment from higher vibrational levels of N2O. The results on mixtures with N2 evidence the formation of N2O− as a primary product of electron attachment. The apparent three-body attachment rate coefficient increases with increasing electron energy, suggesting an activation energy for the formation of N2O−. The various swarm and beam data on electron attachment to N2O are compared and discussed. This study demonstrates further the potential of the swarm–beam technique.
Vibrational Assignment of Fluorene from the Infrared and Raman Spectra
Authors: Bree A. , Zwarich R.
Published: February 2004

The polarized infrared and Raman spectra of a single crystal of fluorene are reported over as wide an energy range as possible. Where this work has overlapped with a recently reported study the agreement has been satisfactory, although, with the data from the c-polarized infrared spectrum and from the low-frequency (50–400 cm−1) region presented here for the first time, several changes in the assignment of the molecular fundamentals are necessary. Some lattice modes of the fluorene crystal were found both in the infrared and Raman spectra and a partial assignment is given.
Angular and Wavelength Dependence of the Light Scattered from a Cloud of Particles Formed by Homogeneous Nucleation
Authors: Stein Gilbert D.
Published: February 2004

Details of the features of light scattering from ice particles formed via condensation by homogeneous nucleation are presented. An argon-ion laser is used, and the results indicate that dipole or Rayleigh scattering is applicable to the situation under study. In addition, measurements of the number and the mean radius of the particles are made.
Mass-Spectrometric Study of the Bimolecular Formation of Diatomic Helium Ion
Authors: DeCorpo J. J. , Lampe F. W.
Published: February 2004

The homonuclear diatomic ion of He has been studied by a pulsed mass-spectrometric technique. These investigations of the bimolecular formation of He2+ show that at least four families of excited states take part in the process. Three of these states have excitation functions with peaks at 36, 58, and 68 eV, while the fourth peaks between 30–34 eV. Kinetic studies performed at these electron energies yielded the effective lifetimes, as well as specific reaction rates for the corresponding families of the excited states. The observed rate constants (1.8 − 3.6 × 10−9cm3molecule−1·sec−1) correspond to reaction diameters comparable to collision diameters expected on the basis of simple theoretical consideration of the radius of the excited helium atom.
Thermodynamics of Linear Molecules at High Temperature: C3O2 and C5
Authors: Pickett Herbert M. , Strauss Herbert L.
Published: February 2004

In this paper we derive equations for the classical partition function of a linear molecule, taking account of anharmonicity and rotation–vibration interaction. We calculate the thermodynamic properties of C3O2 and C5 using a potential suggested by Morino, Kuchitsu, and Tanimoto. This potential, as well as a number of others, gives good agreement with the measured value of S°230 for C3O2 of McDougall and Kilpatrick.
Shock-Tube Study of the Dissociation Rate of CN
Authors: Fairbairn A. R.
Published: February 2004

The rate of the reaction CN+Ar→C+N+Ar is estimated to be 2.5 ± 1 × 10−10exp[(− 71 000 ± 6000) / T] cm3sec−1 over the temperature range 7000°–15 000°K from a study of the decay of CN radiation excited by the passage of a shock wave through a mixture of BrCN and Ar. The formation of C2 and N2 introduces a major uncertainty in the interpretation of the results.
Oxygen Evolution from Products Condensed at Liquid-Air Temperature in the Dissociated Water-Vapor System
Authors: Jones R. A. , Chan Walter , Venugopalan M.
Published: February 2004

Kinetics of the oxygen evolution from products (≈ 0.02 g) condensed at liquid-air temperature from water vapor dissociated in a microwave discharge at low flow rates (1.875 mmoles H2O/h) and low pressures (0.12 mm Hg) were studied at − 80°C by the method of isothermal decomposition. Although one-third of the oxygen was evolved in about 1 h, the oxygen evolution was completed over a period of several days only, suggesting the occurrence of an initial fast reaction followed by a much slower reaction. The shape of the kinetic curves varied somewhat for products from different experiments under apparently the same conditions, and the data did not follow first- or second-order kinetics. These results are compared to data earlier obtained using both uniform heating and isothermal decomposition methods, and its is shown that H2O2 decomposition cannot adequately explain the oxygen evolution. On the other hand, the slower reaction can be explained by decomposition of the postulated hydrogen superoxides.
Radiation-Induced Electrical Conductance in the Liquids Argon, Neopentane, and n-Hexane: The Influence of Electron Localization Efficiency on the Free-Ion Yield
Authors: Tewari P. H. , Freeman G. R.
Published: February 2004

An extra electron in liquid argon does not form a localized state. The free-ion yield in liquid argon is G(e−)fi ≈ 5, which indicates that all the ion–electron pairs formed during the x radiolysis of argon become free ions. The difference between the values of Gfi in neopentane (0.9) and n-hexane (0.13) is due to the smaller efficiency of electron localization in the former than in the latter liquid. The efficiency of electron localization is a function of the geometry of the hydrocarbon molecule.
Quantum Deviations in Two-Dimensional Gases
Authors: MacRury T. B. , Sams J. R.
Published: February 2004

The second virial coefficient, correct to the fourth power in Planck's constant, is derived for a two-dimensional gas in an external potential field. The molecules of the gas are assumed to interact via either a renormalized (12,6) potential or the Sinanoǧlu–Pitzer (12,6,3) potential, where the inverse cube term accounts for the perturbation of the gas–gas interactions by the field. As the magnitude of the perturbation increases the quantum defects at a given temperature are enhanced. The models are applied to data for CH4, CD4, H2, and D2 on a uniform graphitic surface. The results are inconclusive, since uncertainties in the data for the hydrogens preclude a (12,6,3) analysis for these gases.
Study of “Non-Fickian” Diffusion Anomalies through Time Lags. V. Simple Distance-Dependent Anomalies in Laminated Media
Authors: Petropoulos J. H. , Roussis P. P.
Published: February 2004

The investigation of distance-dependent “non-Fickian” time-lag behavior is here extended to a laminated (multiple) membrane consisting of n laminations at right angles to the flow coordinate X, each lamination also exhibiting a continuous variation in diffusion properties along X. The results obtained in Paper II of the present series are valid here also. In addition, the “non-Fickian time-lag increments” characterizing such a multiple membrane can be analyzed into components arising from the variation of diffusion properties within each lamination and also from the differences between the over-all diffusion properties of the individual laminations. The relative importance of these contributions as a function of n is considered, with particular reference to the time-lag properties of microporous powder compacts formed by stepwise compression.
X-Ray and Thermal Analysis of Quenched Ammonium Fluoride II and III: Three New Phases
Authors: Nabar M. A. , Calvert L. D. , Whalley E.
Published: February 2004

Ammonium fluoride II and III have been recovered at 100°K and atmospheric pressure and examined by x-ray powder diffraction and by thermal analysis. The structure of phase III is closely related to that of cesium chloride. Phase II when heated from 100°K at atmospheric pressure transforms to a new phase, designated V, at about 120°K and V in turn transforms to I at about 230°K. Phase III under the same conditions transforms at about 120°K to a new phase, designated VI, and this in turn transforms to another new phase designated VII at about 200°K. On further heating to 230°K, phase VII transforms to I. Phases V and VII are probably stacking variants of the wurtzite-type structure of ammonium fluoride I. Phases II and VI have powder patterns too complicated to analyze unambiguously on the data available.
Vibrational Spectra of Single Crystals and Polycrystalline Films of CH2Cl2 and CH2Br2
Authors: Brown Chris W. , Obremski Robert J. , Allkins James R. , Lippincott Ellis R.
Published: February 2004

Polarized infrared spectra of single crystals formed in a miniaturized high-pressure cell and the infrared and Raman spectra of polycrystalline films of CH2Cl2 and CH2Br2 have been measured. From their vibrational spectra it is suggested that the CH2Cl2 molecules occupy C2 sites in a hexagonal D3h crystal system and that the CH2Br2 molecules are on Cs sites in a hexagonal C6υ or D6h crystal system. Comparison of the vibrational spectra of the two methylene halides indicates that the intermolecular forces present in the solid phases of the two compounds have different origins. In CH2Cl2 there appears to be a considerable dipole–dipole contribution to the intermolecular forces. Conversely, in CH2Br2 atom–atom interactions (hydrogen–hydrogen repulsions) seem to account for the intermolecular forces. The Raman spectrum of a mixed crystalline sample of CH2Cl2 in an CH2Br2 lattice has been recorded in order to separately assign the components of bands due to the chlorine isotopes from those due to the factor-group splittings.
Optically Active Crystal Vibrations of the Alkali-Metal Nitrates
Authors: Nakagawa I. , Walter J. L.
Published: February 2004

The infrared and Raman spectra for the three alkali-metal nitrates LiNo3, NaNO3, and KNO3 have been measured in the region from 1600 to 30 wavenumbers, and a normal-coordinate analysis has been made for these compounds with a special emphasis on the rotational and translational lattice modes in the crystal. All three compounds were studied as D3d6 models and a modified Urey–Bradley force field was used in the calculations. The intramolecular and interaction force constants for all three nitrates are given. The effect of the interaction potential constants on the NO3− ion is discussed as well as the effect of the metal ion–nitrate ionic interaction in the crystal from a comparison of the calculated force constants. Although there are large changes in the rotational and translational lattice frequencies when comparing one alkali-metal nitrate with another, the lack of force constant dependence is interesting.
Lattice Energies and Hydration Thermodynamics of Tetra-alkylammonium Halides
Authors: Boyd Richard H.
Published: February 2004

The lattice energies of tetramethylammonium (TMA+) chloride, bromide, and iodide, tetraethylammonium (TEA+) iodide, and tetra-n-propylammonium (TPA+) bromide are calculated from their crystal structures and an adopted potential model for the nonionic interactions between H, C, N and halogen atoms. When combined with heats of solution in water, hydration enthalpies are arrived at and a division among cation and anion is made. The lattice energies are (in kilocalories per mole): TMA+Cl−, − 132.1; TMA+Br−, − 130.1; TMA+I−, − 123.8; TEA+I−, − 109.6; TPA+Br−, − 117.9. The hydration enthalpies are: TMA+, − 53; TEA+, − 42; TPA+, − 48 (based on values of − 80.3, − 72.2, and − 61.8 for Cl−, Br− and I−, respectively). Entropies of hydration of the TMA+ halides are determined from a calculation of gas-phase entropies and experimental entropies of the aqueous solutions, leading to values of s¯h° for TMA+Cl−, TMA+Br−, TMA+I− of − 47.5, − 42.5, and − 38.1 cal °K−1·mole−1, respectively. The hydration heats of the tetra-alkylammonium cations show a reversal in the usual trend of decreasing hydration heat with increasing size. This is due to the increasing importance of nonelectrolyte interactions relative to the charge effect as the size increases. The entropy loss on hydration of TMA+ is very high compared to that expected from extrapolation from smaller ions and is also apparently due to the approach to non-electrolyte character of large ions.
Vibration–Vibration Energy Transfer between Diatomic Molecules
Authors: Berend George C. , Benson Sidney W.
Published: February 2004

A classical method is presented to calculate resonant and near-resonant vibrational energy exchange between colliding diatomic molecules. The method entails a rigorous solution of the equations of motion of the four particles interacting in two dimensions. The interaction potential is described by six independent, atom–atom, Morse-type potential functions. Computer solutions provide the probability of the simultaneous energy-transfer process. Temperature-dependent transition probabilities, relaxation times, and exchange cross sections are calculated. The method is demonstrated on two reactions: N2(υ = 1) + CO(υ = 0)→N2(υ = 0) + CO(υ = 1) and N2(υ = 2) + N2(υ = 0)→2N2(υ = 1). Results show good agreement with experimental data and other theoretical calculations.
Structure and Conformation of Trimethylamine
Authors: Wollrab James E. , Laurie Victor W.
Published: February 2004

The ground-state rotational spectra of (CH2D) (CH3)2N, (CH3)315N, and (13CH3) (CH3)2N have been assigned yielding the effective moments of inertia. This information has been combined with that for (CH3)3N to give the following structural parameters: r(CN) = 1.451 ± 0.003 Å, r(CHs) = 1.109 ± 0.008 Å, r(CHa) = 1.088 ± 0.008 Å, ∢CNC = 110.9° ± 0.6°, ∢NCHs = 111.7° ± 0.4°, ∢NCHa = 110.1° ± 0.5°, ∢HaCHs = 108.1° ± 0.7°, ∢HaCHa = 108.6° ± 0.8°, where Hs lies in a symmetry plane. A C3υ equilibrium conformation has been established and the methyl groups appear to be tilted by 1.3° toward the unshared electron pair.
Effect of Solvent Perturbation on the S3 → S1 Internal Conversion Efficiency of Benzene, Toluene, and p-Xylene
Authors: Lawson Craig W. , Hirayama Fumio , Lipsky Sanford
Published: February 2004

The S3 → S1 internal conversion efficiency of benzene is found to depend critically on the effectiveness of environmental perturbation in mixing the S3 and S1 states. Evidence for this derives from the experimental correlation of the conversion efficiency with the intensities of the 0–0 absorptive and emissive transitions in solutions with the solvents benzene, methanol, isopropyl alcohol, tetrahydrofuran, ethyl ether, acetonitrile, hexane, cyclohexane, methylcyclohexane, decalin, isooctane, and perfluorinated hexane. Additionally, the importance of S3 − S1 mixing is shown to explain the marked increase in the efficiency of internal conversion observed upon methyl substitution of benzene.
Crossed-Beam Study of Ionization of Cs by Br2
Authors: Helbing R. K. B. , Rothe E. W.
Published: February 2004

The velocity dependence of the relative cross section for the formation of Cs+ from Cs and Br2 has been measured in the c.m. energy range 1.5–16.5 eV. A crossed-molecular-beam technique was used. The threshold energy is 1.66 ± 0.1 eV (c.m.). This leads to a nominal value of 2.23 ± 0.1 eV for the electron affinity of Br2. A maximum cross section is found near 6 eV.
2S1 / 2-State Centers in II–VI Compounds
Authors: Holton W. C. , Watts R. K.
Published: February 2004

The electron paramagnetic resonance spectra of the S1 / 22 ground states of Ge3+, Sn3+, and Pb3+ in ZnSe are reported. Ligand superhyperfine structure has been observed for Ga2+ in ZnS, and optically induced population inversion in the ground levels of Ga2+ and Tl2+ is discussed.
Multistate Kinetics in Nonsteady-State Nucleation: A Numerical Solution
Authors: Abraham Farid F.
Published: February 2004

We have numerically calculated the multistate kinetics for the nonsteady-state nucleation of a few cases of supersaturated water vapor. The calculations include: (a) the complete time dependence of cluster concentrations for cluster sizes 11–110 molecules, and (b) the “currents” which describe the net rate of passage of clusters for a given nearest-neighbor transition. From these exact solutions, we obtain, for a particular case: (a) the delay period (or time) as a function of cluster size; (b) the time lag as a function of cluster size; (c) the steady-state concentrations for the various cluster sizes; (d) the steady-state current; and (e) in general, an over-all picture of how the system relaxes into steady state. Where applicable, we will compare our results with previous work.
Atom Formation Rates behind Shock Waves in Oxygen
Authors: Watt W. S. , Myerson A. L.
Published: February 2004

Atomic absorption spectrophotometry has been used to monitor the rate of oxygen atom formation behind shock waves in oxygen–argon mixtures. These observations were made by observing the time history of absorption at the oxygen atom triplet near 1300 Å. The atomic absorption coefficients were determined experimentally in the shock tube. These observations lead to an oxygen–argon dissociation rate given by kO2 − Ar = 1.85(± 14%) × 1011T0.5exp(− 95 700 / RT) cm3mole−1·sec−1 over the temperature range 2850°–5550°K. In addition this technique is shown to provide a powerful method for examining the vibration–dissociation coupling region. Measurements of the induction time for a steady-state atom formation rate to be established were obtained in the temperature range 2850°–5000°K. These measurements are compared with previously published experimental and theoretical induction times for dissociation in oxygen–argon mixtures.
Energy-Level Scheme for Nd3+ in PbMoO4
Authors: Sengupta D. , Sharma Kailash K. , Artman J. O.
Published: February 2004

Crystal-field parameters appropriate for S4 symmetry were fit to the I9/24 and I11/24 sublevel splitting of Nd3+ in PbMoO4. Intermediate coupling wavefunctions were used. J–J mixing between the I9/24 and I11/24 states was considered. The values found are (in cm−1): B02 = 662.5, B04 = − 690.0, B06 = − 84.4, B44 = − 984.2, B46 = − 600.0, B4′4 = − 6.0, and B4′6 = − 0.8. The rms deviation of this fit was 4.8. We calculate ground-state g∥ and g⊥ values of 1.283 and 2.585; experimentally these are 1.351 and 2.592. By changing the Bmn slightly we obtain 1.348 and 2.586.
Quantum Solution of Collinear Reactive Systems: H+Cl2→HCl+Cl
Authors: Rankin Charles C. , Light John C.
Published: February 2004

Reaction-path coordinates are used in the quantum-mechanical calculation of reactive and nonreactive scattering probabilities for a collinear collision of an atom and a diatomic molecule. The parameters of the potential-energy surface were varied to demonstrate the influence of various features on the probabilities of reaction and the distribution of vibrational energy in the reaction H+Cl2→HCl+Cl. In addition to the features known previously, the sharpness of the curvature of the reaction path was identified as having a major influence on the vibrational energy of the products. All six open vibrational channels of the products were considered. The advantages and disadvantages of this method of numerical solution of the problem are discussed.
Hydrodynamical Properties of Stiff Linear and Ring Polymers
Authors: Imai Satoshi
Published: February 2004

The intrinsic viscosity and sedimentation coefficient of stiff linear and ring polymers are calculated by using the method developed by Fixman. The contradictory prediction on the behavior of [η]r / [η]l by Bloomfield–Zimm and Fukatsu–Kurata is discussed in relation to the Kirkwood–Riseman approximation. Our results for the intrinsic viscosity and sedimentation coefficient predict[η] − const = KηMaη,S0 = KMa + B(const),which agrees with the experimental observation by Crothers–Zimm on DNA.
Theory of Triplet Exciton Annihilation in Polyacene Crystals
Authors: Swenberg C. E.
Published: February 2004

An improved theoretical formulation of the triplet exciton bimolecular decay constant in organic crystals is developed which avoids the ad hoc introduction of a “weight factor” w used in previous theoretical treatments, allows for explicit calculations of all reaction constants pertinent to mutual exciton annihilation of triplets, and properly accounts for the anisotropy of the interaction responsible for the bimolecular annihilation. A simplified model for the effective density of final electronic states ρ(ε) is presented and explicit calculations are performed assuming that only the totally symmetric intramolecular C–C stretching modes are active and by including intermolecular vibrations phenomenologically. Calculations of the bimolecular decay constant (γ) and diffusion tensor, both with and without the inclusion of charge-transfer interactions, are performed for solid naphthalene, anthracene, and tetracene. We obtain γ (naphthalene) ∼0.1γ (anthracene) and γ (tetracene ∼10−5γ (anthracene); only for anthracene are experimental values available and here agreement between theory and experiment is excellent. Specifically, γ (theoretical) = 4 × 10−11cm3sec−1 compared with γ (experimental)∼(2 to 5) × 10−11cm3sec−1. The dependence of the bimolecular rate constant on the rate of encounter of two localized triplet excitons and the transition rate to final state is quantitatively analyzed. The intermediate states involved in the annihilating process are identified theoretically as charge-transfer excitons. The inverse of triplet–triplet exciton annihilation is predicted to have nonnegligible probability in pentacene and tetracene crystals and the corresponding transition rates are calculated and correlated with the low fluorescent efficiency of these crystals.
Ring Puckering in Five-Membered Rings. III. The Microwave Spectrum, Dipole Moment, and Structure of Cyclopentanone
Authors: Kim Hyunyong , Gwinn William D.
Published: February 2004

Microwave spectra of cyclopentanone and 2-d-cyclopentanone are reported. Eight vibrational satellites have been assigned for cyclopentanone, and two conformers have been observed for 2-d-cyclopentanone. The rotational constants of the ground states are: 6621, 3351.54, and 2410.40 MHz for the normal isotopic species of cyclopentanone; 6309, 3322.55, and 2382.82 MHz; and 6258, 3344.54, and 2361.96 MHz for the two conformers of the 2-deutero isotopic species. It was found that the nature of the ring-puckering potential is such that the ring is permanently twisted and undergoing bending and twisting vibration quite independently. The lower limit to the height of barrier-hindering pseudorotation was set about 400 cm−1. The structural parameters, r;(C1–C2), r;(C2–C3), ∠(C2–C1–C5), and the twist angle τ, were calculated by the iterative procedure of the weighed least-squares analysis. They are 1.504 ± 0.01 Å, 1.557 ± 0.007 Å, 110.5 ± 0.7°, and 23.6 ± 0.4°. The components of dipole moments were determined by the Stark effect: μa = 3.25 ± 0.02 D and μc = 0.00 ± 0.15 D.
Pulse Radiolysis of Benzene Cyclohexane Mixtures
Authors: Thomas J. K. , Mani I.
Published: February 2004

The radiolysis of benzene and mixtures of benzene and cyclohexane have been studied by the nanosecond pulse-radiolysis technique. In pure benzene, two short-lived transitory species are observed with absorption maxima at 320 mμ and t1/2 = 112 nsec and 515 mμ and t1/2 = 18.5 nsec. The effect of temperature and dilution in C6H12 shows that the 515-mμ peak is associated with the excimer of benzene. The effect of added solutes and of benzene dilution in C6H12 suggest that the 320-mμ absorption is associated with a free radical which is formed by the reaction of a benzene triplet with benzene. The effect of benzene concentration on the excited singlet and triplet yields and on the reaction of the ionic species is also studied. It is shown that the geminateion recombination time is shorter in benzene than in C6H12, and that ion recombination is a major source of energy transfer in the radiolysis of benzene/C6H12 mixtures.
NMR Subspectral Analysis Applied to Polyfluorobenzenes C6HnF6−n. IV. 1,2,4-Trifluorobenzene
Authors: Lustig Ernest , Hansen Elizabeth A. , Diehl Peter , Kellerhals Hanspeter
Published: February 2004

The 1H and 19F bands of the nuclear magnetic resonance spectrum of 1,2,4-trifluorobenzene have been analyzed. The principles of subspectral analysis guided the application of iteration by computer and the use of the ABC parameters of this ABCMXZ system, obtained from a totally decoupled 1H spectrum, in the assignment of all 177 observed peaks.
Electric Field Effects on the Dielectric Properties and Molecular Arrangements of Cholesteric Liquid Crystals
Authors: Baessler H. , Labes M. M.
Published: February 2004

Measurements of dielectric constants of a cholesteryl chloride–cholesteryl myristate mixture having a strongly temperature-dependent helical pitch have been performed in the presence and absence of an applied dc electric field. An analysis of the molecular arrangement in samples oriented by wall effects is presented, as well as the perturbations in this arrangement which occur both thermally and by the application of an electric field. Elastic moduli of bending and torsional strain are estimated from the threshold electric fields observed to (a) induce helical perturbation and (b) destroy the helical structure in the transition to a nematic phase.
Satellites Induced by Foreign-Gas Perturbation of the Lennard-Jones Type
Authors: Kieffer Jean
Published: February 2004

The paper presents a calculation of the intensity of a spectral line emitted under perturbation by foreign atoms, its purpose being to provide a better understanding of the formation of satellites which often flank resonance lines. As a typical perturbation, a Lennard-Jones interaction with suitable parameters is assumed for both electronic states involved in the transition. A numerical evaluation of integrals is required in this case, and the work is carried out in the limit of the statistical theory. A satellite appears on the red side of the main line up to a perturber density depending on the parameters in the potential, in our case up to 1021 cm−3. The satellite is located in the vicinity of the frequency ωg = ΔEm / ℏ, where ΔEm is the differential energy shift at its minimum. Details of shape and shift depend on the perturber density. Temperature does not appear in this analysis because of its restriction to the statistical theory.
Band Shapes of the Electronic Spectra of Complex Molecules
Authors: Siano Donald B. , Metzler David E.
Published: February 2004

Structureless bands of ultraviolet absorption spectra of hydroxypyridine derivatives in solution have been examined. It is shown that the log–normal distribution provides a convenient four-parameter empirical description of the bands and gives a much better fit than the two half-width Gaussian. The log–normal yields a resolution of overlapping bands by a method of least squares that is very satisfactory. A simple configuration-coordinate model is presented that lends some physical meaning to the parameters of the log–normal. It yields normalized third and fourth moments that are near those of the log–normal.
Polarization Assignments of the Electronic Spectrum of Purine
Authors: Chen Holly Ho , Clark Leigh B.
Published: February 2004

Polarized reflection spectra of the (100) face of purine crystals have been measured to 185 mμ. The corresponding absorption spectra which are computed by a Kramers–Kronig transformation of the reflection spectra are analyzed so as to assign the transition moment directions of the electronic absorption bands. The analysis is made assuming the oriented-gas model and again for a case which treats intermolecular interactions. The polarization of the first absorption band (λmax = 294mμ) is out-of-plane as expected and is consistent with an n → π* assignment. The next band at 263 mμ is polarized in-plane (π → π*) at + 48° from the C4 − C5 axis (toward C6). The weak band at 250 mμ in the crystal is another perpendicular n → π* transition. The strong absorption in the 200-mμ region and below can be plausibly assigned as two transitions. One (at 200 mμ) is polarized in nearly the same direction as the 263-mμ band. The other band, at 190 mμ, can have reasonable intensity relative to the solution spectrum only if it is polarized out-of-plane. Some evidence for another weak, in-plane, long-axis band at about 230 mμ is found.
Perturbation Theory for Fluids
Authors: Praestgaard Eigil , Toxvaerd S/ren
Published: February 2004

Recently, Barker and Henderson have introduced a semimacroscopic approximation to the second order in the expansion of the configuration integral by considering the pair potential as the sum of a strong (repulsive) part and a weaker (long-range) part. We analyze this approximation and show that the essential part of it is to reduce the higher-order distribution functions to a second-order nonuniform distribution function, the nonuniformity coming from fixing a particle at the origin. The approximation can be done to all orders, and the series can be summed to give an expression for the free energy and pressure. The expression involves the grand partition function for the nonuniform system taken at the chemical potential for the reference system minus the perturbing potential. The functions of the nonuniform system are approximated by taking the functional form they have at uniformity and by using the product of the density and the radial distribution function of the reference system for the nonuniform density. The summed series can then be computed. For the three-dimensional square-well fluid it is confirmed that the convergence is very rapid down to reduced temperatures about 0.5. Comparison with the one-dimensional square-well fluid shows that below this temperature the convergence is slow and it is necessary to use the whole series.
Correlations in a Self-Avoiding Walk
Authors: Domb C. , Hioe F. T.
Published: February 2004

A fundamental feature of a self-avoiding walk on a crystal lattice is the correlation between any pair of steps. In this paper, exact enumerations for a number of two- and three-dimensional lattices are used to estimate the moments of this correlation. It is suggested that for a walk of n steps in the limit of large n the correlation can be characterized by a single function which is independent of lattice structure in a given dimension. An algebraic expression is put forward to represent this function. Agreement with Monte Carlo estimates is satisfactory.
Raman Spectrum of the Bromine Crystal
Authors: Suzuki Masako , Yokoyama Toru , Ito Mitsuo
Published: February 2004

The Raman spectrum of the bromine crystal was observed at 77°K, and it was compared with the corresponding spectrum of the chlorine crystal. The observed lattice vibrational frequencies and the crystal-field splitting of the intramolecular stretching vibration indicate stronger intermolecular interaction in the bromine crystal than that in the chlorine crystal. This conclusion is also supported from the normal coordinate calculation.
Effect of Stress on the Permeation of Helium through Borosilicate Glass
Authors: Laska H. M. , Doremus R. H.
Published: February 2004

McAfee's finding of an anomalous increase in the permeation rate of helium through a thin borosilicate glass bulb at high tensile stress and room temperature was confirmed. It was established that this increase was related to the breaking stress of the bulb and not to the absolute stress in the glass. A comparison of the steady-state permeation rate and the rate of uptake of gas before the steady state was attained showed that the anomalous increase resulted from a decrease in the effective thickness of the bulb, and not from a change in the diffusion coefficient or solubility of the helium in the glass. Some possible mechanisms for this thickness decrease are considered. No anomalous increase in permeation rate was found in the borosilicate glass at 200°C or in fused silica at room temperature.
Excimer Fluorescence of Benzene and Its Alkyl Derivatives—Concentration and Temperature Dependence
Authors: Hirayama Fumio , Lipsky Sanford
Published: February 2004

Fluorescence spectra and quantum yields have been obtained for methylcyclohexane solutions of benzene, toluene, ethylbenzene, cumene, p-, m-, o-xylene, and 1,3,5-, 1,2,3-, 1,2,4-trimethylbenzene as a function of aromatic concentration over the temperature range from 25 to −100°C. At low temperatures distinct excimer emissions were observed for all compounds studied. The intrinsic emission quantum yields of monomer φm and of excimer φe have been determined by a simple technique which requires no assumptions regarding the details of the monomer–excimer kinetics. With decreasing temperature, φe is observed to decrease contrary to the behavior of φm, suggesting that the rate constant for the excimer radiative transition decreases strongly as the temperature is lowered. Such temperature dependence is explained as arising from the existence of a substantial vibronic component in the transition moment that is induced by thermal excitation of upper-state vibrational motions (e.g., torsional, tilting, etc.) of one monomer with respect to the other. From analysis of the temperature dependence of the fluorescence, lower bounds on the excimer binding energies Eb have been determined. The difference between this lower bound and Eb is approximately equal to the activation energy for radiative decay of the excimer. An estimate of this activation energy indicates that, for the case of benzene, Eb > 0.36 eV. The probability for association of excited monomer to form excimer and the probability for dissociation of excimer to an excited and unexcited monomer have been determined for benzene at 25 and −78°C from an appropriate analysis of the fluorescence quenching effect of CCl4. Additionally it has been demonstrated that the observed increase in CCl4 quenching efficiency at high benzene concentrations is predominantly due to an energy migration process. The probability per encounter for formation of excimer has been determined for benzene to be ≈1.0 and to decrease with alkyl substitution in a manner consistent with the steric requirements of sandwich-type excimer configurations.
Electron-Impact Excitation Functions for Metastable States of N2
Authors: Freund Robert S.
Published: February 2004

The metastable excitation function for N2 is mainly due to the A 3Σu+, E 3Σg+, and a 1Πg states. This interpretation, which is based on (1) the optical-excitation functions for these states, (2) their radiative lifetimes, and (3) new measurements of the metastable excitation function at a range of times after excitation, is consistent with other workers' data. The roles of several other metastable states of N2 are also discussed.
Random-Field Problem: Perturbations at a Lattice Site Due to Randomly Distributed Impurities
Authors: Duckett Steven W.
Published: February 2004

The problem of computing the field or other perturbation at a lattice site due to a random collection of impurity centers is considered. The Fourier transform of the field probability density is shown to be of the form exp[−n̄Ψ(t)], where n̄ is the average concentration of impurities and Ψ(t) is an integral that is a function of the individual center–impurity interactions. The Fourier transform is used to calculate numerous quantities that are functions of the random field. Emphasis is placed on computing luminescent decay forms and the energy-transfer probability from one center to surrounding acceptors, but the application of these calculations to the evaluation of other quantities (e.g., partition functions) is also considered. Graphs of some of the computed functions are presented.
On Intensity Alterations in C2 “Swan” Emission Spectra
Authors: Nieuwpoort W. C. , Bleekrode R.
Published: February 2004

The presence of intensity alternations in C2 Swan emission spectra of flames is analyzed. It is argued that the nonequilibrium rotational energy distribution is caused by collision-induced electronic transitions between the radiating A 3Πg state and the vibrationally excited A′3Σg− or A″3Σu+ states of C2. The selection rules are derived on the basis of the parity properties of the relevant states and the various terms of the multipole expanded interaction potential. The selection rules lead to an unambiguous choice between the collision-induced transitions A″3Σu+→A 3Πg or A 3Πg→A′3Σg−. On the basis of available experimental information of the pressure dependence of the effect, additional evidence is given that a formation process is responsible for the occurrence of the intensity alternations. Examples of intensity alternations in Si2 and N2 are briefly discussed.
Study of Tunneling Rotation of Methyl Groups by Electron Spin Resonance and Electron Nuclear Double Resonance
Authors: Clough S. , Poldy F.
Published: February 2004

Free radicals formed by γ irradiation of single crystals of 4-methyl-2,6-di-tert-iarybutylphenol are studied by resonance methods at low temperatures. The important part of the radical is the fragment Ċ–CH3, where the free-electron spin occupies a 2p orbital centered on the carbon atom attached to the methyl group and experiences a hyperfine interaction with each of the three protons dependent on the proton position, giving rise to structure in the electron spin resonance spectrum which is sensitive to motion of the protons. The hindered methyl group performs torsional oscillations and also undergoes tunneling rotation between its three equivalent orientations, the nuclear spin states being correlated with the motion through the exclusion principle, at least at temperatures sufficiently low for the fragment to be regarded as isolated. An almost symmetrical seven line ESR spectrum is then expected and is observed at 4.2°K. In first order the spectrum is independent of the tunneling frequency when this is large compared with the splitting due to the hyperfine interaction. The tunneling frequency is obtained from small relative shifts of the hyperfine lines measured by electron nuclear double resonance. From the hyperfine interaction tensors, the orientations of the methyl group symmetry axes are found. Above helium temperatures the ESR spectrum undergoes a change which is interpreted in terms of the onset of methyl-group rotations which are uncorrelated with the proton spin states and occur randomly. From this model, spectra are calculated as a function of the correlation time for the random motion. They show a progressive change from seven lines to the 1:3:3:1 quartet observed at high temperatures. The details of the observed transition are in good agreement.
Far-Infrared Absorption Spectrum and Pseudorotation of the Thiacyclopentane Molecule
Authors: Wertz D. W.
Published: February 2004

The far-infrared spectrum of thiacyclopentane vapor has been examined at a resolution of 0.5 cm−1 from 20 to 400 cm−1. The series of pronounced Q branches observed in the range 60–120 cm−1 has been assigned on the basis that the molecule is undergoing a pseudorotational vibration which is hindered by the potential function V(φ) in cm−1 = −12(772.7) (1−cos2φ) + 12(169.1) (1−cos4φ). The entropy of this model has been calculated and found to give excellent agreement with the calorimetric entropy.
Thermal Conductivities of Methyl Chloride–Sulfur Dioxide Mixtures
Authors: Manna A. , Srivastava B. N.
Published: February 2004

The thermal conductivities of binary mixtures of polar gases, methyl chloride, and sulfur dioxide have been measured over the temperature range 0°–200°C for various compositions by the hot-wire method. The results have been utilized to test the recent theories of heat conductivity of polyatomic gas mixtures. It has been found that the experimental data are in good agreement with those calculated from the Hirschfelder–Eucken formula, provided the pure component thermal conductivities are assumed to be given by the corresponding experimental values.
Fluctuations and the Gamma Expansion
Authors: Throop G. J.
Published: February 2004

The contribution of density fluctuations to the van der Waals equation is calculated. It is found that in fact these contributions are identical to the first-order term in the gamma expansion.
Intermolecular Potentials and Width of Pressure-Broadened Spectral Lines. I. Theoretical Formulation
Authors: Robert D. , Giraud M. , Galatry L.
Published: February 2004

A systematic approach utilizing the Anderson theory for the calculation of the width of pressure-broadened lines is developed for the most complete long-range intermolecular potential available. Particular attention is given to the problem of including in a consistent manner the various contributions of the different physical components of the potential. In addition to the usual results, terms arising from nonadditive contributions to the differential collision cross section, omitted in the past, are obtained and discussed. New terms are also found which result from using a more complete potential than has been considered heretofore.