bluuebunny/crossref_metadata_2025 huggingface dataset:
Order parameter measurements in polypeptide liquid crystals
Authors: Murthy N. Sanjeeva , Knox James R. , Samulski Edward T.
Published: February 2003

An analysis of x-ray scattering from magnetically oriented liquid crystalline solutions of poly-γ-benzyl-L-glutamate (PBLG) in dioxane yields a value for the order parameter S?0.75. At the isotropic–liquid crystal phase transition, the critical order parameter Sc?0.5 appears to agree with predictions of mean-field theory. Away from the transition, S is independent of PBLG concentration and PBLG axial ratio but does decrease when the helix denaturing agent trifluoroacetic acid is added to the liquid crystal. The magnitude of and changes in S are discussed in terms of the inherent flexibility of helical synthetic polypeptides.
Circular dichroism theory for unordered polypeptides: A Green operator technique
Authors: Rabenold David A.
Published: February 2003

A Green operator technique used within a suspectibility framework is employed to express the circular dichroism of a diamide which is taken as a model for an unordered polypeptide system. The formulation contains all common mechanisms that give rise to circular dichroism and has the advantage that band shape functions can be included for each absorption band of each isolated nonoverlapping chromophore. The rotational strengths, transition frequencies, and polymer band shapes are determined by solving a complex eigenvalue problem.
Critical slowing down of dielectric relaxation in ferroelectric NH4HSO4
Authors: Tamaki Akira , Fujimura Tadao , Kamiyoshi Kan-Ichi
Published: February 2003

Dielectric constant and dielectric loss of NH4HSO4 along the ferroelectric c axis have been measured at 100 kHz, 300, 660, 1145, and 1440 MHz as a function of temperature. It was found that the Curie–Weiss law held both in paraelectric and in ferroelectric phases of the upper Curie point which was T0=−2.3 °C. Dielectric relaxation was observed both in paraelectric and in ferroelectric regions. The former relaxation was of the Debye type with a single relaxation time, which was expressed by a relation: τ=1.70×10−10/(T−T0) sec in the temperature region down to T−T0?0.5 °C. It was found that after the same specimen was repeatedly cooled and heated through the upper Curie point, this relaxation gradually changed into that with distributed relaxation times. The dielectric relaxation in the ferroelectric region was complex; that is, the characteristics of the relaxation differed with every specimen.
Absolute rate of the reaction of atomic hydrogen with acetaldehyde
Authors: Whytock D. A. , Michael J. V. , Payne W. A. , Stief L. J.
Published: February 2003

Absolute rate constants for the reaction of hydrogen atoms with acetaldehyde were obtained over the temperature range 298–500 K using the flash photolysis–resonance fluorescence technique. The rate constants gave the Arrhenius expression k= (2.23±0.39) ×10−11 exp(−3300±120/1.987T) cm3 molecule−1 s−1. Independent experiments with a discharge flow system employing resonance fluorescence detection of H gave a room temperature rate constant in agreement with this equation. These results are discussed and comparisons are made with previous work and theoretical predictions.
The spectroscopic channel basis for radiative and radiationless processes: The effective system excited states and interference effects in absorption spectra
Authors: Ziv Alan R. , Rhodes William
Published: February 2003

By transforming the molecular Hamiltonian, we find a basis, called the spectroscopic channel basis (SCB), in which the individual molecular excited states are coupled in sequence, with the final one having a nonradiative linewidth, which is relatively energy independent. The SCB provides a simple, single-channel path for the radiationless processes and is a useful basis for analyzing the spectral properties of the molecule in terms of the Green’s operator. We show that the matrix element of the Green’s operator (Green’s function) associated with light absorption obeys certain recurrance relations in the SCB. The spectral density function of each state of the SCB is uniquely determined by the absorption spectrum of the molecule. We describe a rigorous and formally exact method of calculating the spectral density functions and all of the spectroscopic parameters of the SCB from a given absorption spectrum. The absorption spectrum can be expressed as a sum of generalized Lorentzians (Lorentzians modified by interference effects of the Fano type) which correspond to effective excited states and which can be determined from the parameters of the SCB. The interference effects are determined by a new parameter which we term the asymmetry index.
Electrical conductivity of binary mixtures near the critical point
Authors: Shaw Ching-Hao , Goldburg Walter I.
Published: February 2003

The electrical conductivity σ of several liquid mixtures was measured near their respective critical points. The systems studied were pure phenol–water, KCl-doped phenol–water, and isobutyric acid–water. The measurements spanned the reduced temperature range 10−5≲ε≲10−2, where ε≡ (T-Tc)/Tc. All systems clearly showed the existence of a singular contribution to the conductivity. A least-squares fit of the data to the equation, (σ-σc)/σc∝εϑ+ background terms, showed that the singularity could be characterized by a critical exponent ϑ=0.70+0.15−0.10. A possible explanation of this singularity is offered which uses percolation theory in the mean field limit. This percolation approach yields ϑ=2β, in accord with our own observations. The exponent β (?1/3) characterizes the shape of the coexistence curve.
Dynamic polarizabilities of two-electron atoms, with rigorous upper and lower bounds
Authors: Glover R. M. , Weinhold F.
Published: February 2003

Methods are described for calculating rigorous upper and lower bounds to dynamic dipole polarizabilities for frequencies up to and beyond the first excitation threshold, even when (as usual) the field-free problem cannot be solved exactly. These methods were employed to calculate rigorous bounds to the dynamic polarizabilities of the two-electron atoms H−, He, and Li+, using well correlated trial wavefunctions up to 135 terms in length. The majority of previous theoretical and experimental results for these atoms can thereby be ruled out, including the highly precise Starkschall–Gordon values, which had heretofore appeared to be the most accurate available. For He, the noteworthy discrepancy between the many-body perturbation result of Dutta et al. and conventional variation–perturbation treatments is resolved in favor of the latter. For all three atoms the present results provide new high-accuracy standards for comparison with future theoretical and experimental determinations.
Photoionization and ion cyclotron resonance studies of the ion chemistry of ethylene oxide
Authors: Corderman Reed R. , LeBreton P. R. , Buttrill S. E. , Williamson A. D. , Beauchamp J. L.
Published: February 2003

Time-resolved photoionozation mass spectrometry (PIMS), ion cyclotron resonance spectroscopy (ICR), and photoelectron spectroscopy have been employed to study the formation of the ethylene oxide molecular ion and its subsequent ion–molecule reactions which lead to the products C2H5O+ and C3H5O+. Earlier observations that a structurally and energetically modified species (C2H4O+) * is an intermediate in the production of C3H5O+ are confirmed. The PIMS data detail the effects of internal energy on reactivity, with the ratio of C3H5O+ to C2H5O+ increasing by an order of magnitude with a single quantum of vibrational energy. Evidence is presented for the formation of (C2H4O+) * in a collision-induced isomerization which yields a ring-opened structure by C–C bond cleavage. This species contains considerable internal excitation which is relaxed in collisions with ethylene oxide or bath gases such as SF6 prior to reaction. The relaxed ring-opened C2H4O+ ion reacts with neutral ethylene oxide by CH2+ transfer to yield an intermediate product ion C3H6O+ which gives C3H5O+ by loss of H. Isotopic product distributions observed in a mixture of ethylene oxide and ethylene oxide-d4 are consistent with this mechanism. The effects of ion kinetic energy on reactivity are explored using ICR techniques. Increased reactant ion kinetic energy leads to collision-induced dissociation of C2H4O+ rather than isomerization to the open form.
Quenching of NH(b 1Σ+) by O and N atoms; isotope effect in the quenching of NH and ND(b 1Σ+) by He, Ar, and N2
Authors: Gelernt B. , Filseth S. V. , Carrington Tucker
Published: February 2003

Electronically excited NH and ND(b 1Σ+, v′=0) have been produced in the vacuum ultraviolet photolysis of NH3 or ND3 at pressures in the millitorr range. Absolute quenching rate coefficients have been determined for NH(b 1Σ+) with oxygen and nitrogen atoms as 1.78±0.09×10−11 and 3.38±0.07×10−11 cm3 molecule−1 sec−1, respectively, at 293°K. Quenching by He, Ar, and N2 is much slower, and for each of these quenchers the rate coefficient for quenching ND(b 1Σ+) is at least a factor of 10 lower than that for quenching NH(b 1Σ+).
Quantum corrections to the momentum distribution of moderately dense Boltzmann gases. I. Smooth potentials
Authors: Rainwater J. C. , Snider R. F.
Published: February 2003

The momentum distribution of a quantum Boltzmann gas at equilibrium is shown to be the usual Maxwell–Boltzmann term plus a correction term of higher order in density and fully quantum -mechanical in origin. For smooth potentials, the correction may be described by a semiclassical expansion, and the leading quantum part depends on momentum in the simple manner postulated by Imam-Rahajoe and Curtiss, although higher-order terms have a more complicated momentum dependence. At lowest order in density, a Sonine polynomial expansion and expressions for the moments of the quantum distribution are derived. The pressure tensor and energy density of a quantum gas, as determined from kinetic theory, are shown to be consistent with thermodynamics in the equilibrium limit, but only when the quantum part of the momentum distribution is properly taken into account.
Positive ion appearance potentials measured in CeI3
Authors: Chantry P. J.
Published: February 2003

The positive ions produced by electron impact on CeI3 have been studied with a mass spectrometer having a heatable collision chamber with an attached, separately heatable sample reservoir. The individual ion appearance curves have been measured with a resolution of 0.15 eV, using the retarding potential difference technique. The ion appearance potentials have been determined from the appearance curves by a linear extrapolation technique which accounts for the curvature in the immediate vicinity of the threshold. The appearance potentials give estimates of the heats of formation of the various ions and of the ionization potentials of the corresponding fragments of CeI3. The resulting recommended values are ΔHf(CeI+3) =128 kcal/mole (5.53 eV), I.P.(CeI3) =9.05 eV; ΔHf(CeI+2) =113 kcal/mole (4.92 eV), I.P.(CeI2) =6.75 eV; ΔHf(CeI+) =171 kcal/mole (7.41 eV); I.P.(CeI) =5.9 eV; ΔHf(Ce+) =228 kcal/mole (9.90 eV), I.P.(Ce) =5.65 eV.
Photolysis of formaldehyde films at 77°K
Authors: Glicker Sol
Published: February 2003

Solid films of H2CO:D2CO mixtures have been photolyzed at 123.6 and 147 nm. The isotopic distribution of molecular hydrogen was determined by end-product analysis. Using the hydrogen distribution, an estimate was made of the overall molecular processes, i.e., 40% and 49% at 123.6 and 147 nm, respectively. Implications of this work relative to the mechanism and other related low-temperature studies are discussed.
Fluctuations, stability, and generalized state functions at nonequilibrium steady states
Authors: Keizer Joel
Published: February 2003

Fluctuation–dissipation postulates, which describe the kinetic effects of molecular processes, are used to characterize nonequilibrium steady states. Attention is restricted to stable, noncritical states which develop in systems with inputs that are time independent. For these systems it is shown that the steady state distribution is Gaussian, which provides a generalization of the well-known Einstein formula for equilibrium states. For certain systems it is shown that the time dependence of the covariance matrix of the extensive variables gives a necessary and sufficient condition for the stability of a noncritical state. These considerations are illustrated for the steady states accompanying diffusion, heat transport, chemical reactions with linear coupling, and certain nonlinear chemical reactions. These examples show that the covariance matrix is not necessarily related to the local equilibrium entropy. When the covariance matrix is invertible, it can be used to construct generalized state functions which reduce to familiar thermodynamic functions at equilibrium. The generalization of the entropy, called the σ function, is related to stability, the probability density, and generalized ’’thermodynamic forces’’ in precisely the same way as the entropy is at equilibrium.
The gas phase photolysis and γ radiolysis of ethylenimine
Authors: Scala A. A. , Salomon D.
Published: February 2003

The vacuum ultraviolet photolysis and mass spectral cracking pattern have been used to elucidate the chemistry occurring during the gas phase γ radiolysis of ethylenimine. In the 147 nm photolysis of ethylenimine, the major primary reactions occurring are (1) C2H4NH→C2H4+NH, φ=0.38, and (2) C2H4NH→CH3+[H2CN], φ=0.47. The mass spectrum of ethylenimine indicates that in the γ radiolysis of ethylenimine, where ions as well as neutral excited states are produced, the mechanism includes the ionic analog of Reaction (2): C2H4NH+→CH3+H2CN+. The mass spectrum also suggests the inclusion of Reaction (10) in the γ-radiolysis mechanism: (10) C2H4NH+→C2H4N+ + H. Although it cannot be proven conclusively, because hydrogen atoms and ethylenimine radicals are produced in secondary reactions, spectroscopic evidence suggests that the neutral analog of Reaction (10) may occur in the 147 nm photolysis: (9) C2H4NH→C2H4N+H. Except for reaction (1), which has no ionic analog in the mass spectrometer and which probably occurs from a valence excited state, the similarity of the neutral and ionic reactions occurring in the 147 nm photolysis and γ radiolysis of ethylenimine suggests that the Rydberg excited state, (nN→4p), populated by 147 nm photons exhibits quasi-ionic behavior. It seems likely that useful correlations of vacuum ultraviolet photolyses (8–12 eV), γ radiolyses, and mass spectra should be possible for many polyatomic molecules which frequently exhibit Rydberg excitations at these energies.
Argon hydrochloride, Ar⋅HCl, bond energy by infrared spectroscopy
Authors: Miziolek Andrzej W. , Pimentel George C.
Published: February 2003

The infrared absorption of argon (200–760 torr) and hydrogen chloride (2–6 torr) mixtures has been re-examined in the spectral region 2860–3010 cm−1 (the missing Q branch region) over the temperature range 195–298°K. The temperature dependence of two absorption features of the Ar⋅HCl complex, at 2887 and 2879 cm−1, lead to a bond energy estimate that depends upon the assumptions made about the internal degrees of freedom of the complex. Agreement with experiment can be reached for well depths near 1.2 kcal/mole, a result that is relatively insensitive to the choices of the vibrational frequencies and anharmonicities but which does depend upon the extent to which the energy level manifolds are truncated to avoid molecular excitation in excess of the bond energy. This bond energy contrasts with the commonly accepted value of 0.4 kcal/mole; the possible origin of the disparity is considered.
A graphical representation of the memory function for spin relaxation
Authors: Friedman Harold L. , Blum Lesser , Yue Genevieve
Published: February 2003

The perturbation expansion of the memory function can be written as a series of graphs on white and black circles which are rather analogous to the white and black circle graphs for spatial correlation functions of equilibrium systems developed by Morita and Hiroike, De Dominices, and Stell. As in the equilibrium theory, the graphical representation of the memory function is especially convenient for generating resummed (or ’’renormalized’’) series based on summations of subclasses of infinite numbers of graphs. The resummed series are useful approximations to the memory function even when the perturbation that makes the spins relax is strong. Our most general results enable one to express the memory function as a generalized continued fraction or to calculate the time-correlation function as the solution to an integrodifferential matrix equation. The graphical representation of the latter result is surprisingly simple: the graphs are isomorphous with the graphs representing the perturbation expansion of the time derivative of the time-correlation function, but the bonds and faces which join the circles have different meanings.
Viscoelastic and inertia effects on the angular velocity autocorrelation function
Authors: Chow T. S.
Published: February 2003

Encouraged by the excellent agreement between the hydrodynamic model and the molecular dynamic calculation of the linear velocity autocorrelation function of a hard sphere translating in a simple fluid, a similar treatment is extended to the angular velocity autocorrelation function (AVCF) of rotating Brownian particles. Both the inertia and viscoelastic effects are simultaneously included in our hydrodynamic fluctuation model. An inertia dependent drag torque is used and a Maxwell viscoelastic relaxation time is introduced. The AVCF is solved analytically and is given in closed form. By choosing physically reasonable values for the viscoelastic parameters, good agreement is obtained with the AVCF determined by molecular dynamic calculation.
Spectroscopic studies of diatomic noble gas halides. II. Analysis of bound-free emission from XeBr, XeI, and KrF
Authors: Tellinghuisen Joel , Hays A. K. , Hoffman J. M. , Tisone G. C.
Published: February 2003

The strong ultraviolet emission bands (’’Spectrum I’’) of XeBr, XeI, and KrF have been photographed following electron-beam excitation of appropriate noble gas/halide mixtures at moderate to high pressures. These diffuse spectra are analyzed through trial-and-error theoretical simulations. The upper-state vibrational frequencies are estimated to be 120±10, 112±8, and 310±20 cm−1 for XeBr, XeI, and KrF, respectively. The analysis also yields approximate shapes for the lower-state potential curves in the Franck–Condon region. Lasing is observed in XeBr (2818 Å) and KrF (2484, 2491 Å) but not in XeI.
Time-of-flight studies of rotational quantum excitation of H2 in collision with protons at Ec.m.=3.7 eV
Authors: Rudolph Klaus , Toennies J. Peter
Published: February 2003

In a crossed beam apparatus, a monoenergetic pulsed proton ion beam is scattered from a nozzle beam of n-H2 or p-H2 in their rotational ground states. The energy loss of the scattered ions is analyzed using a time-of-flight technique. Rotational quantum transitions corresponding to Δ j up to 20 could be resolved in the angular region of 0<ϑ?55°, which extends up to the rainbow angle. Only a small amount of vibrational excitation is obtained at Ec.m.=3.7 eV. The measured time-of-flight spectra have been transformed into the center of mass system to provide distributions of relative transition probabilities. These agree qualitatively with the predictions of Giese and Gentry based on Monte Carlo averaged classical trajectories for 10 eV. The measured probability distribution has a maximum for elastic scattering (Δj=0) at all angles. It drops to between 5% and 35% for Δj=2 transitions, and for most angles decreases only gradually with increasing Δj. At center of mass angles of 15° and 23°, the rotational probability distribution shows a second distinct maximum at Δj=8. At angles greater than 27°, the transition probability distribution falls off nearly monotonically. The first moment of the energy loss distribution shows two maxima at about 23° and 30°. The largest maximum at 30°, which is roughly one-half the rainbow angle, was predicted by Giese and Gentry.
Generation of atomic H in a hydrogen matrix by tritium decay
Authors: Zeleznik Frank J.
Published: February 2003

The generation of atomic H in a hydrogen matrix by tritium decay was studied by a phenomenological rate theory. Theoretical expressions were developed for the dependence of storage time, recombination time, and maximum density of trapped H atoms as a function of temperature for a given tritium concentration. An application to Webeler’s recent experiments implies that Δts=0.145 T−1.886 exp(0.860/T) and nmax×10−16=0.158 T−1.886 exp(0.860/T), where Δts is the storage time in hours, nmax is the maximum trapped H-atom density in atom cm−3 and T is the temperature in kelvins.
Laser-excited spectra and reorientation of the S−2 ion in alkali halides
Authors: Sawicki C. A. , Fitchen D. B.
Published: February 2003

Resonance Raman and emission spectra are reported for the S−2 molecular ion in several alkali halide crystals. At 6 K, the emission consists of one or more vibronic series of sharp zero-phonon lines with associated isotope structure and phonon sidebands. The Raman spectra exhibit several overtones of the ground-state S−2 vibration. In NaI:S−2, nine overtones are observed, each with isotope structure and phonon side-bands. The Raman and emission spectra are analyzed and compared for NaI, KI, and RbI. These data support the model of a single S−2 site with Πg splitting. Time dependence of the polarized emission intensity is seen at 6 K in KCl, KBr, NaBr, KI, and RbI, but not in RbCl, RbBr, or NaI. This phenomenon is explained and analyzed in terms of the reorientation kinetics of the optically-excited ions. The multiple-order Raman scattering efficiencies are shown to agree reasonably well with the theory of Hizhnyakov and Tehver.
EPR spectroscopic investigations of metal halides doped with transition metal ions. III. Palladium (II)−doped AgCl
Authors: Eachus R. S. , Graves R. E.
Published: February 2003

Divalent palladium ions are substitutionally incorporated into silver chloride during the melt growth of single crystals. These ions trap holes and/or electrons during exposure. The predominant trapped-electron center, (PdCl6)5−, has an effective depth of 0.61±0.01 eV and a room temperature lifetime of 0.5 msec. A second palladium-based complex, [(PdCl6)4− (Ag+vac)], is believed to trap photoholes above 80°K, but it can be made to behave as a secondary electron trap by cooling below 80°K, the temperature at which self-trapping of the photohole predominates. The room temperature lifetime of [(PdCl6)3− (Ag+vac)] has been calculated to be 1.1 msec from an effective trap depth of 0.57±0.01 eV.
Theoretical and experimental studies of the N2O− and N2O ground state potential energy surfaces. Implications for the O−+N2→N2O+e and other processes
Authors: Hopper Darrel G. , Wahl Arnold C. , Wu Richard L. C. , Tiernan Thomas O.
Published: February 2003

The ground state potential energy surface of the nitrous oxide negative ion is characterized and related to that of the neutral molecule by a synergetic theoretical–experimental approach. Ab initio multiconfiguration self-consistent-field/configuration interaction (MCSCF/CI) and other calculations for N2O−(X 2A′) yield the minimum energy geometry (ReNN, ReNO, AeNNO) = (1.222±0.05 Å, 1.375±0.10 Å, 132.7±2°), the vibrational frequencies (ν1,ν2,ν3) = (912±100 cm−1, 555±100 cm−1, 1666±100 cm−1), the dipole moment μ =2.42±0.3 D, and other properties. The N2O− molecular ion in the X 2A′ state is found to have a compact electronic wavefunction—one with very little diffuse character. The MCSCF/CI bending potential energy curve from 70° to 180° for the X 1Σ+(1 1A′) state of N2O as well as the bending curve for the X 2A′ state of N2O− are also reported. The dissociation energy D (N2–O−) =0.43±0.1 eV and, thus, the adiabatic electron affinity E.A.(N2O) =0.22±0.1 eV and the dissociation energy D (N–NO−) =5.1±0.1 eV are determined from beam–collision chamber experiments. Corrections are made for both the dispersion in the ion beam and the translational motion of each target gas. The combined theoretical and experimental results yield a vertical electron affinity V.E.A.(N2O) of −2.23±0.2 eV and enable the construction of angular dependent Morse functions to represent the neutral and ionic surfaces. This construction leads to the determination of the minimum intersection locus as (V*, R*NN, R*NO, A*NNO) = (0.67±0.1 eV, 1.18±0.05 Å, 1.28±0.10 Å, 154±3°). The predicted activation energy of this critical point with respect to the asymptote O−, N2—0.21±0.1 eV—and the position of the critical point with R*NN well outside of the N2 (v=0) outer turning point imply that the reaction O−+N2→N2O+e will be strongly facilitated by reagent vibrational excitation.
Study of HF doped ice by pulsed NMR
Authors: Barnaal Dennis , Kopp Marcel , Lowe I. J.
Published: February 2003

The spin–lattice relaxation times of protons in HF doped ice I have been measured in the laboratory frame (T1) and in the rotating reference frame (T1ρ) as a function of temperature and HF concentration. An activation energy of 5.66±0.1 kcal/mole is found for both T1 and T1ρ for temperatures above −100°C, and 1.8±0.2 kcal/mole for T1 for temperatures between −100 and −200°C. In the range 10−6–10−4 HF/H2O molecular ratio, T−11 is approximately proportional to the 3/4 power of the concentration of HF. Analysis of the behavior of T1 vs temperature shows that the defects induced by HF are different from those that are T1 effective in pure ice and are probably the same as those effective in dielectric relaxation. At high HF concentrations, we suggest that the proton magnetization relaxes through dipolar interaction with diffusing fluorine nuclei. The concentration dependence of T1 may thus be a combination of three relaxation mechanisms: that intrinsic to pure ice, that which is dominant for moderate HF content, and that which dominates at high HF concentration.
Thermal expansion of isobutyric acid+water near the upper critical solution temperature
Authors: Morrison Graham , Knobler Charles M.
Published: February 2003

We report the precise measurement of volume changes as a function of temperature near the critical solution point of isobutyric acid–water. We find not only evidence of a critical anomaly in the thermal expansion but also a small range of temperature in which it is negative, a requirement for thermodynamic consistency. Although the uncertainty in α, the critical exponent determined from these measurements, is on the same order as α itself, we find that α is consistently in the range 0.08–0.14; that is comparable to the Ising value of 1/8.
Multiphoton ionization spectrum of t
r
a
n
s-hexatriene in the 6.2 eV region
Authors: Parker David H. , Sheng S. J. , El-Sayed M. A.
Published: February 2003

The multiphoton ionization spectrum of gaseous trans hexatriene in the region 6.0 to 6.6 eV is reported. (AIP).
On the scattering of high velocity helium atoms by hydrocarbons
Authors: Freeman Gordon R.
Published: February 2003

It is suggested that the scattering properties of C(CH3)4 for He atoms be considered as caused by four C–C bonds rather than by the four CH3 groups.(AIP)
Model for trapped electrons in nonpolar glassy solids
Authors: Nishida Masahiko
Published: February 2003

A model calculation for 3-methylpentane and methylcyclohexane is tried on the basis of the model including the short-range exchange interaction of a trapped electron with core electrons of the nearest-neighbor carbon atoms, using the quasifree electron energies derived from experimental data. As a result, reasonable optical transition energies are obtained. The exchange interaction is found to contribute about 25% of the calculated optical transition energy.
Theory of monolayer physical adsorption. I. Flat surface
Authors: Steele William A.
Published: February 2003

The general statistical mechanical expression for the adsorption isotherm of a classical gas interacting with an inert solid is simplified by imposing the two-dimensional approximation and by Fourier expanding the factors in the configurational integral that arise from the periodic gas–solid interaction. The leading term in the resulting expression describes a two-dimensional fluid; approximate pair correlation functions and thermodynamic properties of such a system of Lennard-Jones atoms are evaluated and compared with published computer simulations. The method of calculation consists in an adaptation of statistical mechanical perturbation theory which allows one to treat two-dimensional systems at the temperatures and densitites that are of interest experimentally. Reasonably good agreement with the simulations is obtained. The usefulness and significance of these results are discussed.
Electron impact excitation of the Rydberg states in O2 in the 7–10 eV energy-loss region
Authors: Trajmar S. , Cartwright D. C. , Hall R. I.
Published: February 2003

Electron impact energy-loss spectra of O2 in the 7–10 eV region has been investigated at low impact energies and high scattering angles. Under these conditions a number of new transitions have been found which do not appear in optical spectra. Bands at 8.595, 8.826, 9.045, and 9.27 eV have been assigned to the 3sσg 1Π (v′=0, 1, 2, and 3) excitations, respectively, and the idenfification of the corresponding 3Πg bands have been reconfirmed. New transitions appearing at 9.13, 9.32, 9.51, 9.58, and 9.78 eV cannot be unambiguously assigned on the basis of the presently available information.
Ultrasonic investigation of the critical point at the top of p-T-X miscibility dome in 2-methylpyridine+D2O
Authors: Garland Carl W. , Nishigaki Kazu
Published: February 2003

The binary liquid system 2-methylpyridine+D2O has a closed miscibility loop at 1 atm. With increasing pressure the size of the two-phase region shrinks until the upper and lower consolute points merge at ∼101°C and ∼200 atm (at which point the critical mole fraction of D2O is ∼0.91). Such a critical point at the top of the miscibility dome is of a special type that has been predicted to have a weakly divergent adiabatic compressibility. Ultrasonic measurements at 1 MHz do not confirm this prediction, and possible reasons for the discrepancy between theory and experiment are discussed.
Monte Carlo trajectory study of Ar+H2 collisions. I. Potential energy surface and cross sections for dissociation, recombination, and inelastic scattering
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2003

Modified statistical electron–gas calculations using the methods of Gordon, Kim, Rae, Cohen, and Pack are carried out to obtain the interaction energy of Ar with H2 as a function of geometry. The results are combined with the accurate pairwise interactions, the long-range nonpairwise interaction, and the potential LeRoy and van Kranendonk fit to spectral data on the van der Waals’ complex to obtain a potential energy surface which is as accurate as possible at all geometries. This surface and the pairwise additive surface are then used in a Monte Carlo quasiclassical trajectory study of the cross sections (under shock-tube high-energy collision conditions) for complete dissociation, for production of quasibound states of H2, and for V–T, R–T, and V–R–T energy transfer. Except for R–T energy transfer, the accurate surface yields smaller cross sections than the pairwise additive surface does. The cross sections for dissociation are much smaller than predicted by the available-energy hard-sphere model but are larger than the inelastic cross sections for excitation to the highest bound vibrational energy levels. Initial vibrational excitation energy is more effective than rotational energy or relative translational energy in causing dissociation. Using the full potential surface the recombination cross section of the v=13, j=8 quasibound state of H2 is calculated at Erel=0.026 eV and is in good agreement with the result previously calculated by Whitlock, Muckerman, and Roberts using a less accurate, pairwise additive potential surface.
Dependence of the concentration and radiation intensity on reaction time and depth into the sample during the photochemical reaction of powdered sample
Authors: Simmons E. L.
Published: February 2003

Equations are derived that describe the dependence of the reactant concentration and radiation intensity on the reaction time and the depth into the sample during the photochemical reaction of a powdered sample. Expressions are also obtained for the rate of absorption of radiation by the reactant and the rate of appearance of photoproduct per unit area. The model used is based on a simple particle model, and assumptions are made that make the equations applicable only for short reaction times. The equations obtained are compared with those previously obtained.
Chemiluminescence spectra and photon yields for several Sn–oxidizer reactions
Authors: Capelle Gene A. , Linton C.
Published: February 2003

Tin vapor has been reacted with N2O, NO2, O2, and F2 to produce SnO and SnF in a flame. Reactions of Sn with O2 and NO2 produced very weak chemiluminescence, whereas Sn+Br2 and Cl2 produced no detectable light emission. The highest photon yield measured was 6.7% for SnO emission that resulted from the reaction of Sn+N2O in Ar at a Sn concentration of 3×1013/cm3. The maximum SnF yield measured was 0.3% for Sn+F2. The spectral character of the SnO chemiluminescence generated in the Sn+N2O reaction was found to be a strong function of pressure (0.5–25 Torr) and carrier gas identity (Ar, He, N2) in the flow-tube reaction system. Extension of the SnO a–X system yielded the following constants (all in cm−1): a3Σ+(1): Te=20622.6±2.5, ωe=554.0±1.7, ωexe=2.45±0.36 and X1Σ: ωe=823.40±0.99, ωexe=3.77±0.10.
One-dimensional models for superionic conductors
Authors: Wang J. C. , Pickett D. F.
Published: February 2003

Three one-dimensional models, interstitialcylike, vacancylike, and interstitiallike, are constructed to study properties of superionic conductors. Potential energy curves along the diffusion path due to all ions other than the conducting species are simulated by (V0/2) [1−cos(2πx/a)], where a is the lattice constant. By considering only the Coulombic and repulsive potentials among conducting ions, the activation energy Ea is calculated as a function of V0. It is shown that due to local correlative motions of conducting ions, the values of Ea are much smaller than V0. The correspondences between the models and actual superionic conductors such as β-alumina, β″-alumina, and α-AgI are discussed.
A calculation of the photodetachment energy of NH2−
Authors: Andersen Earl , Simons Jack
Published: February 2003

The equation-of-motion (EOM) method was used to calculate the photodetachment energy of NH2 −. An SCF level potential energy surface was calculated for NH2 − along the symmetric A1 vibrational coordinates. Several points were also calculated on the asymmetric-stretch surface to provide a comparison with the symmetric-stretch surface. From these data, corresponding surfaces for NH2 were obtained by adding the computed vertical detachment energy at each geometry. The calculated vertical photodetachment energy at the equilibrium geometry of NH2 − was 0.43 eV. By using experimental values for the vibrational energy difference between NH2 and NH2 −, the thermodynamic electron affinity of NH2 was calculated to be 0.42 eV. From the computed potential energy surfaces, spectroscopic constants were calculated and compared to experimental results. The use of d functions on the nitrogen center was investigated and found to give little change in calculated ionization energies.
Perturbation solution of the one particle generalized Smoluchowski equation
Authors: Harris S.
Published: February 2003

The generalized Smoluchowski equation (GSE) can be used as the basis for a theory of the light scattering spectrum for a system of macromolecules in solution. We consider the solution of the one particle reduced GSE equation for the case of spherical macromolecules at low concentration through the use of a closure-effecting ansatz that relates the one and two particle distribution functions and an expansion in the macromolecule concentration. The equations which result are well defined in the mathematical sense, and solutions are obtained for the first two terms in the expansion in a form suitable for application the light scattering problem. Although we restrict ourselves here to the usual model of the GSE with the hydrodynamic interactions given by the Oseen expression, more complicated forms for the hydrodynamic interaction could also be accomodated in the theory at the expense of considerably more lengthy calculations. A brief, formal application of the results obtained to the light scattering problem is also included.
On the critical region of a simple fluid. III. Maxima and minima of thermodynamic functions
Authors: Stephenson John
Published: February 2003

The sealing law equation of state is used in the neighborhood of the critical point to obtain the asymptotic form of the loci of extrema of a variety of thermodynamic functions, such as the isothermal compressibility, the specific heats at constant volume and at constant pressure, and the isobaric coefficient of expansion. Various ’’nesting’’ relations between the loci are discussed and comparison is made with the predictions of van der Waals’ equation, and with calculations based on data for fluid argon. A detailed analysis is made of the shape is isochores of (∂P/∂T)v versus temperature in the vicinity of the critical point.
Information content of detailed differential reaction cross sections (molecular beam scattering velocity-angle maps) vs resolved product state distributions
Authors: Bernstein R. B. , Faist M. B.
Published: February 2003

Information theory provides a quantitative measure of the information content, or entropy deficiency, of different classes of experiments (and computer simulations), e.g., molecular beam scattering and infrared chemiluminescence. Using histogrammic representations of beam- or computationally derived polar (velocity-angle) flux contour maps, the entropy deficiency associated with the product translational recoil and angular distribution is readily calculated. This is compared with that from resolved distributions of product vibrational and rotational states (e.g., from ir chemiluminescence experiments). It appears that the gain in information associated with the knowledge of products’ angular distribution (even with the additional ’’mutual information’’ from the angle–recoil velocity correlation) is often outweighed by the substantial loss of information on the separate vibrational and rotational distributions. This so-called ’’equivocation’’ or VR→T loss can be an appreciable fraction of the total information content of the product state distribution. Product state angular distributions as obtained recently by the laser-induced fluorescence technique can provide substantially more information content.
Homogeneous nucleation in associated vapors. I. Acetic acid
Authors: Heist Richard H. , Colling Kevin M. , DuPuis Calvin S.
Published: February 2003

Homogeneous nucleation measurements on acetic acid vapor are reported. The presence of the relatively stable association clusters tends to stabilize the vapor with regard to homogeneous nucleation. The variation of the critical supersaturation with temperature for acetic acid vapor was found to agree well with the predictions of the Katz–Saltsburg–Reiss theory for nucleation in associated vapors.
Three-dimensional quantum mechanical studies of the H+H2 reactive scattering
Authors: Choi B. H. , Tang K. T.
Published: February 2003

A three-dimensional quantum mechanical study is made of H+H2 reactive scattering. The differential and total cross sections as well as the S-matrix elements are obtained from the adiabatic distorted wave model and the Porter–Karplus semiempirical potential surface. With the initial molecule in the ground rotational state, the energy dependence, rotational state dependence, and other properties of the reaction probabilities and of the cross sections for transitions to all possible states of the product molecule are determined. The reactive scattering from the threshold to 0.5 eV is predominantly backward. The 0→0 rotational transition contributes only a small fraction to the total reactive cross section; however, most product molecules prefer rotational states that are considerably lower than the highest one allowed by energy conservation. The reaction probability is found to be a smoothly decreasing function of the total angular momentum of the system. These results are very similar to our previous results for the D+H2 reaction except the threshold energy for the H+H2 is higher and the magnitude of the cross section for the reaction of the D+H2 is larger. A systematic study of the effects of the Pauli exclusion principle on cross sections of the scattering of three identical particles shows that in order to obtain the ’’observable’’ cross sections, the unsymmetrized results of the H+H2 and D+D2 reactions have to be multiplied by various ’’strange’’ numerical factors. These observable cross sections are compared with three different sets of close coupling results on the same potential surface. Our results are in close agreement with those of Kuppermann and Schatz and are in substantial disagreement with those of Wolken and Karplus. Our results near the threshold and at 0.5 eV are similar to those of Elkowitz and Wyatt but there are significant differences in energy dependence in the intermediate region. The present results are also compared with the three-dimensional classical trajectory results. The most significant quantum effect is caused by the spin statistics. Dynamically, there are excellent qualitative and quantitative agreements in many respects between the quantum and classical results, although the present quantum cross sections rise above the classical values at incident energies greater than 0.4 eV.
Studies of reactions of importance in the stratosphere. I. Reaction of nitric oxide with ozone
Authors: Birks John W. , Shoemaker Brian , Leck Thomas J. , Hinton Deborah M.
Published: February 2003

The rate constant for the reaction NO+O3→NO2+O2 has been measured by the fast flow method over the temperature range 203–361 K. Loss of ozone in a large excess of nitric oxide was detected by molecular beam sampling into the electron impact ion source of a quadrupole mass spectrometer. The resulting Arrhenius expression is k=2.34±0.23×10−12 exp(−1450±50/T) cm3 molecule−1 s−1 which predicts k298=1.80×10−14 cm3 molecule−1 s in excellent agreement with previous determinations. Our results suggest a slight curvature in the Arrhenius plot, the activation energy increasing with increasing temperature.
Vibrational relaxation of DCl (v=1) by Cl and Br atoms and of HBr (v=1) by Br atoms
Authors: Macdonald R. Glen , Moore C. Bradley
Published: February 2003

The rates of vibrational relaxation of DCl (v=1) by Cl and Br are measured as (5.5±1.6) ×10−12 and (2.3±0.7) ×10−13 cm3 molecule−1⋅ sec−1, respectively. DCl is relaxed somewhat less rapidly than HCl in both cases. The rate for HBr (v=1) relaxed by Br atoms is found to be (2.6±1.0) ×10−12 cm3 molecule−1⋅ sec−1.
ESR studies of nearest neighbor Cr3+ pairs in MgO
Authors: Marguglio J.
Published: February 2003

Nearest neighbor Cr3+ pairs in MgO have been studied by ESR techniques. The spectrum associated with the S=2 spin state was observed. The appropriate spin Hamiltonian for the system is H=βH⋅g⋅S + Ds{S2z−1/3 S (S+1) }, where the z axis is along 〈110〉. The axially symmetric D tensor was measured to be 0.182±0.009 cm−1, 0.206±0.009 cm−1, and 0.231±0.009 cm−1 for the S=2 spin state. In addition, the temperature dependence of Ds was obtained and was attributed to thermal expansion of the crystal lattice.
Dynamics of the reaction of N+ with H2. IV. Reactive scattering at relative energies above 6 eV
Authors: Mahan Bruce H. , Ruska W. E. W.
Published: February 2003

Product velocity vector distributions for the reaction N+(H2,H)NH+ and its isotopic variants have been determined in the range of relative energies above 6 eV. The reaction is direct, and there is a critical relative energy above which the spectator stripping peak disappears and is replaced by a double intensity lobe structure. Some evidence of the reaction of electronically excited metastable ions, probably N+(1D), is found. The results are compared with the predictions of the sequential impulse model of reactive scattering. The agreement is generally good, except that more very large angle scattering is observed, particularly for the N+(HD,H)ND+ case, than is predicted by the model.
Laser-induced rate processes in gases: Dynamics of polyatomic systems
Authors: Goodman Myron F. , Stone James , Dows David A.
Published: February 2003

A model is proposed to describe the collision-free dynamics of a polyatomic molecule driven by an intense laser field. The evolution in time of stable and quasistable (chemically reactive) energy level populations driven on and off resonance are given, and analytic estimates are derived for the effects of dynamic power broadening (a–c Stark effect), mode and level degeneracies, and rapid unimolecular decay on the efficiency of laser pumping.
A model for laser isotope separation in SF6
Authors: Stone James , Goodman Myron F. , Dows David A.
Published: February 2003

We propose a model for the effect of CO2 laser radiation in decomposing and separating isotopes of SF6. Analytic expressions for oscillator strengths are derived for discrete-to-discrete, discrete-to-continuum, and continuum-to-continuum energy level transitions. Loss in oscillator strength as the system is driven up the manifold is compensated for by a cooperative power broadening that contains an explicit dependence on level degeneracies. Isotope selectivity and yield at low pressures are computed as a function of laser intensity, and a threshold in yield is found at approximately 14 MW/cm2.
Electron–phonon interaction in the spectra of p-benzoquinone in chemically mixed crystals
Authors: Loo B. H. , Francis A. H.
Published: February 2003

Phonon sidebands of unusual sharpness and number are observed in the phosphorescence spectrum of p-benzoquinone in chemically mixed crystals with p-dichlorobenzene and p-dibromobenzene. Phosphorescence–Microwave Double Resonance (PMDR) and thermal modulation spectroscopy are used to examine the role of localized phonon modes in the phosphoresence spectrum. Localized phonons are shown to be active in both distributing as well as inducing radiative intensity in the 3Blg→1Ag transition observed in phosphorescence. Some comments on the excited state geometry of p-benzoquinone are offered in view of our experimental results and recent results from other laboratories. JCP
Kinetic energies and angular distributions of oxygen atom photofragments produced by photodissociation of O2 and N2O in the vacuum ultraviolet
Authors: Stone E. J. , Lawrence G. M. , Fairchild C. E.
Published: February 2003

A gas phase chemi-ionization process is used to detect O atoms produced by photodissociation of O2 and N2O. For the photon energies used here the accessible O atom states are within the ground 3P levels and the metastable 1D and 1S levels. Kinetic energies of the O atom photofragments are determined using an atomic beam time-of-flight technique, and the O atom angular distributions are determined by varying the angle between the direction of the photon beam and the atomic beam flight path. Dissociative transitions within the Schumman–Runge continuum of O2 and the principal vacuum ultraviolet absorption continuum of N2O yield results in agreement with previous predictions. For the dissociation of O2 at wavelengths of 120 and 124 nm, the principal dissociation products are O(3P)+O(1D); and, at these wavelengths, the O atom angular distribution is consistent with previous conclusions that predissociation is important. Using existing theories of photofragment angular distributions, an asymmetry parameter having a value of (−0.61±0.05) is obtained for predissociation in O2 at the 10.0 and 10.3 eV absorption features.