bluuebunny/crossref_metadata_2025 huggingface dataset:
Contribution to the theory of dielectric relaxation in polar media
Authors: Fulton Robert L.
Published: February 2003

A discussion of the connection between the frequency dependent dielectric constant ε and the short range orientational correlation function d is given for a simple model of polar media. The introduction of separate spatial regions with the attendent difficulties in determining the connection between their (time dependent) correlation functions is avoided by noting that ε must satisfy two relations. The use of these relations allows the long range orientational correlations to be eliminated from the expression for ε, the resulting ε being given as a function of d. The result is a generalization of that given by Nee and Zwanzig [J. Chem. Phys. 52, 6353 (1970)]. A short discussion of the results is given, including a physical picture of the connection between the long and short range orientational correlations.
Energy and lifetime of O2− from analytic continuation of isoelectronic bound states
Authors: Herrick D. R. , Stillinger F. H.
Published: February 2003

We offer a method for determining the stability of dinegative ions for isoelectronic series exhibiting a bound singly charged anion. Nonlinear variational ground state energies obtained with bound, multiconfigurational wavefunctions are followed above the first ionization threshold as the nuclear charge (1/λ) is decreased. Resonance energies and widths for physical states are constructed by analytically continuing the energy around a singularity at a nonphysical λ=λ*. Results for the Ne isoelectronic series predict an O2− resonance state at 5.38 eV (width = 1.3 eV) above the O− + e− continuum theshold. The marked O2− instability suggests that estimates of the O− electron affinity, arising from thermochemical Born–Haber cycles, may require quantum corrections. We also discuss several chemical systems for which the method will be useful and which may lead to predictions of bound states existing in the continuum.
EPR of low spin d7 Fe+, Co2+, and Ni3+ cyanide complexes in NaCl and KCl
Authors: Jain S. C. , Reddy K. V. , Reddy T. Rs.
Published: February 2003

Low spin d7 complexes of type [M(CN)4⋅XY]n−, where M is Fe+, Co2+, or Ni3+, the axial ligands X, Y being Cl−, CN−, or NC−, have been produced and stabilized in NaCl and/or KCl crystals doped with diamagnetic Fe(CN)64− (d6), Co(CN)63− (d6) or Ni(CN)42− (d8), by a suitable combination of irradiation and heat treatment. These complexes were identified by EPR on the basis of the g factors which are characteristic of unpaired electrons in the dz2 orbital pointing towards the ligands X and Y, and the super hyperfine structure from the nuclei of these ligands observed in the EPR spectra. Systematic variations are noted in the EPR parameters, which are discussed in terms of changes in the nature of binding in these complexes in relation to the charge state of the metal ion, the host lattice spacings, and the nature of the axial ligands. In addition to showing that CN− and NC− are much stronger ligands than Cl−, the study pointed out the importance of back bonding in CN−- and NC−-type ligands.
Dissociation of H2+ ion by collision-induced vibrational excitation
Authors: Dastidar (Mullick) K Rai , Barua A. K.
Published: February 2003

Collision-induced dissociation of hydrogen molecular ion due to the vibrational excitation from ground vibrational state to the continuum of electronic ground state (1sσg) has been studied, using sudden approximation. In close encounters, the interaction between the nucleus of projectile and the two nuclei of molecular ion has been taken as the sum of two screened Coulomb-type interactions. Exact values for the ground vibrational state and quasiclassical wavefunctions for the continuum have been used. The angular distribution, energy distribution, and total vibrational dissociation cross section for fragments in a given excess energy range have been calculated with He, Ar, and Xe as projectiles. Cross sections for dissociation by Ar impact have been calculated for collision energies ranging from 3 to 50 keV, and for He and Xe at 10 keV. It is concluded that the electronic dissociation plays a dominant role even in the small excess energy region of dissociated fragments.
A π, τ, π/2 type pulse sequence method for the determination of T1 in rotational transitions
Authors: Mäder H. , Ekkers J. , Hoke W. , Flygare W. H.
Published: February 2003

We report a pulse sequence measurement of T1 in the J=0 → J=1 rotational transition in OCS in the pure gas, OCS–He mixtures, and OCS–CH3F mixtures at low pressures. This pulse sequence method is similar to the π, τ, π/2 pulse sequence used in nuclear magnetic resonance to measure T1. The pulse method of measuring T1 in rotational systems has several advantages over the analysis of transient absorption (transient nutation) results or the analysis of power broadened steady state absorption lines. We find T1=T2 for the J=0 → J=1 OCS transitions in pure OCS, OCS–He mixtures, and OCS–CH3F mixtures.
Resonances in the collinear inelastic scattering of He by H+2 below the reaction threshold
Authors: Chapman Frank M. , Hayes Edward F.
Published: February 2003

Collinear quantum mechanical calculations are reported for the inelastic scattering of He by H+2 below the reaction threshold. The inelastic transition probability curves show a severe oscillatory behavior similar to that recently observed in reaction probability curves for this same system. Perturbation calculations and considerations of the channel phase shifts indicate that the resonance structure is caused by the existence of quasibound (resonance) states of HeH+2. A simple picture is presented which accounts for these quasibound states.
Properties of intercalated 2H-NbSe2, 4Hb-TaS2, and 1T-TaS2
Authors: Meyer S. F. , Howard R. E. , Stewart G. R. , Acrivos J. V. , Geballe T. H.
Published: February 2003

The layered compounds 2H-NbSe2, 4Hb-TaS2, and 1T-TaS2 have been intercalated with organic molecules; and the resulting crystal structure, heat capacity, conductivity, and superconductivity have been studied. The coordination in the disulfide layers was found to be unchanged in the product phase. Resistance minima appear and the superconducting transition temperature is reduced in the NbSe2 complex. Conversely, superconductivity is induced in the 4Hb-TaS2 complex. Corresponding evidence of a large change of the density of states, negative for 2H-NbSe2 and positive for 4Hb-TaS2, was also observed upon intercalation. The transport properties of all the intercalation complexes show a pronounced dependence upon the coordination of the transition metal.
Correlation functions of classical fluids. IV. Perturbation theory on correlation functions and the scaling properties of the Lennard-Jones system
Authors: Lee Lloyd L.
Published: February 2003

A perturbation equation is derived from the formalism of inhomogeneous systems of Percus for the calculation of the radial distribution function (rdf). Two special aspects of this development are noted: (1) The derivation is via a mixture system; (2) the direct evaluation of the inhomogeneous direct correlation function C2(1,2/U) is circumvented by the application of a previous result (Ref. 19) on the Percus–Yevick (PY) approximation. The equation arrived at is quite simple and is amenable to numerical solution. As a test, we propose an ’’internal’’ perturbation, or ’’scaling’’ theory, on the Lennard–Jones (LJ) system. Tests on the temperature scaling and the density scaling were carried out with respect to the correlation functions and the thermodynamic properties. The results were compared with the molecular dynamics data. Remarkable improvement was found as compared with the corresponding PY results, especially at high densities and low temperatures. A ratio function w (r) is introduced, which is found to be essential in determining the performance of the perturbative equation. This w function is unity for the PY case and can be viewed as a correction to the regular PY theory. Its functional form is closely related to the direct correlation function and its behavior was determined for several cases.
Ornstein–Zernike equation for the direct correlation function with a Yukawa tail
Authors: Henderson Douglas , Stell George , Waisman Eduardo
Published: February 2003

The Ornstein–Zernike (OZ) equation with a core condition h (x) = −1 for x<1 and a direct correlation function of Yukawa form c (x) = K exp[−z (x−1)]/x for x≳1 was solved analytically by one of us recently [E. W., Mol. Phys. 25, 45 (1973)]. The equation is of interest (i) as the mean-spherical approximation for a potential that is the sum of hard-sphere and Yukawa terms; (ii) as a generalized mean-spherical approximation for a hard-sphere system; and (iii) as the key ingredient in the generalized mean-sphrerical approximations for ionic and polar fluids of Ho/ye, Lebowitz, and Stell, J. Chem. Phys. 61, 3253 (1974). Here we analyze the solution of the above equation to give a quantitatively useful picture of its character. A rapidly convergent expansion in K is obtained. In addition, a general cluster expansion for the solution of the OZ equation with arbitrary c (x) previously derived by one of us (G.S.) is applied to the equation to yield a complementary representation of its solution. Detailed numerical results are given.
Surface and bulk exciton transitions in the reflection spectrum of tetracene crystals
Authors: Turlet J.-M. , Philpott M. R.
Published: February 2003

The reflection spectrum of the 0–0 component of the first singlet transition of tetracene crystal has been measured for temperatures ranging from 300 to 1.7 °K. A discontinuous change in the Davydov splitting occurs near 186 °K signifying the occurrence of a phase transition. At the lowest temperatures two anomalous reflectivity minima are observed inside the b-polarized stopping band that are very similar to structure in the corresponding reflection band of anthracene crystal. The reflectivity minima are tentatively assigned to transitions to surface exciton states lying approximately 5 cm−1 and 160 cm−1 above the b-polarized exciton transition at 5349 Å. The other structure within the reflection band is interpreted as a coupling of the polariton to vibrational modes of intra- and intermolecular origins.
Quantum yields for the production of O(1S), N(2D), and N2(A 3Σ+u) from the vacuum uv photolysis of N2O
Authors: Black G. , Sharpless R. L. , Slanger T. G. , Lorents D. C.
Published: February 2003

Relative quantum yields have been measured for O(1S), N(2D), and N2(A 3Σ+u) production from N2O over the wavelength range 1100–1500 Å. The measurements of O(1S) were made by observing the 1S0 → 1D2 emission at 5577 Å. N(2D) was measured via the intensity of NO β bands generated by N(2D)+N2O → N2+NO(B 2Πr) followed by NO(B 2Πr) → NO(X 2Πr) % +hν (NO β bands). The N2(A 3Σ+u) was measured by the intensity of NO γ bands generated by N2(A 3Σ+u)+NO → N2+NO(A 2Σ+) followed by NO(A 2Σ+) → NO+hν (NO γ bands). The O(1S) quantum yield is close to unity over the 1280–1380 Å wavelength range. N(2D) exhibits a large yield for λ?1200 Å. The quantum yield of N2(A 3Σ+u) is ? 0.2 over the entire 1100–1500 Å region.
Quantum yields for the production of S(1S) from OCS (1100–1700 Å)
Authors: Black G. , Sharpless R. L. , Slanger T. G. , Lorents D. C.
Published: February 2003

The 4589 Å emission of S(1S0) has been used to measure the quantum yields of S(1S0) production from OCS over the wavelength interval 1100–1700 Å. A large yield is found from 1420 to 1600 Å with smaller yields being observed at both shorter and longer wavelengths.
A quasiclassical trajectory study of the H2+F2 reactions
Authors: Thompson Donald L. , McLaughlin Donald R.
Published: February 2003

A theoretical investigation of the reaction dynamics of the reactions H2(v=0,2,4, or 6) + F2(v=0 or 6) → H+HF+F, 2HF, F2H+H, H2F+F, F2+2H, and H2+2F has been accomplished using Monte Carlo averaging over quasiclassical trajectories on a reasonable semiempirical potential-energy surface. The details of the dynamical mechanism of the reaction processes were investigated by examining geometrically restricted reaction paths and selected restricted initial states. Reaction cross sections have been computed and estimates of rate coefficients have been made. The computed thermal rates are many orders of magnitude lower than thermal collision frequencies. Reaction thresholds are lowered significantly for vibrationally excited hydrogen.
Reaction rates of O + CH3Br and O + CH3Cl
Authors: Westenberg A. A. , deHaas N.
Published: February 2003

Rate constants for the reaction O + CH3X → CH2X + OH (X=Br,Cl) have been measured over a wide temperature range using a discharge flow reactor, with pseudo-first-order decays of CH3X measured mass spectrometrically. The Arrhenius expresssion k (O+CH3Br) = 3.5×1013 exp(−4560/T) cm3 mole−1⋅sec−1 was obeyed over the range 500°–1000°K with a probable error of about ±20%. Direct measurements of the ratio k (O+CH3Br)/k (O+CH3Cl) showed the two rate constants to be equal within experimental error (±10%) over the same temperature range. Comparisons with literature data are made.
Emission spectra of heteronuclear diatomic rare gas positive ions
Authors: Tanaka Y. , Yoshino K. , Freeman D. E.
Published: February 2003

An electric discharge in a binary mixture of rare gases produces several discrete emission band groups confined to a narrow wavelength region characteristic of the pair of rare gases and closely related in energy to the ionization energy difference between the two atoms. Such spectra were observed for nine of the ten binary mixtures of He, Ne, Ar, Kr, and Xe. The energy differences between band groups of a given mixture agree closely with the doublet splittings of the lowest states of the atomic ions. The suggestion is offered that the emission spectrum is carried by the heteronuclear ion (XY)+ which undergoes a bound–bound charge–transfer transition X+Y →XY++hν, in which X is the lighter and Y the heavier element in the mixture.
Comparison of activation energies with barriers to rotation for ammonium and borohydride compounds
Authors: Smith David
Published: February 2003

The activation energy observed experimentally by the study of the temperature dependence of the spin–lattice relaxation time has been determined from the hindered rotational energy levels of NH4+, ND4+, and BH4− in a number of their compounds. There are usually two anharmonic levels above the top of the barrier to rotation. The activation energy corresponds to the energy difference between the ground harmonic level and the energy levels just above the highest anharmonic level. The activation energy determined from the hindered rotational energy levels are in good agreement with the observed values.
Theory of electric field effect on electronic spectra and electronic relaxation with applications to F centers
Authors: Lin S. H.
Published: February 2003

A main purpose of this paper has been to present a microscopic theory of electric-field induced absorption spectra; the related phenomenon, the Kerr effect, is studied by using the Kronig–Kramers relation. Our attention is focused on the temperature effect, band shapes, moment relations, and the differences in field induced absorption spectra between allowed transitions and symmetry-forbidden transitions. To illustrate the application of theoretical results, we have investigated the field induced spectra of F centers in alkali halides. It is shown that the F band is composite, consisting of three bands. We have shown that the technique of the electric-field induced optical absorption can be used to resolve the hidden bands. We have also investigated the electric field effect on radiative and nonradiative processes. It has been shown that from the measurement of the electric field dependence of lifetimes of excited electronic states, one can determine the variation of radiationless transitions with the electric field which in turn can be used to study the energy gap law and temperature effect in radiationless transitions. Theoretical results have been applied to F centers in alkali halides. The feasibility of observing the electric field effect on lifetimes of organic molecules has been discussed and the field strength required for observing the electric effect for polar and nonpolar molecules has been suggested.
High resolution study of photoionization processes in O2
Authors: Dehmer P. M. , Chupka W. A.
Published: February 2003

Relative photoionization efficiency curves were determined for the production of O+2, O+, and O− from molecular oxygen in the wavelength region of the helium continuum. The photoionization efficiency curve for O+2, determined at a wavelength resolution (FWHM) of 0.07 Å throughout most of the region from 1030–580 Å, shows rotationless autoionization structure which is broadened due to the processes of predissociation and/or autoionization. The photoionization effciency curves for fragment ions from ion pair formation and from dissociative ionization were determined at wavelength resolutions of 0.07 and 0.15 Å, respectively. The data show considerable autoionization structure and provide information about the mechanisms for these dissociation processes. The ion pair photoionization efficiency curve shows steplike structure at threshold resulting from direct ionization from individual rotational levels of the ground state of O2. A value of the electron affinity of atomic oxygen of 1.462±0.003 eV was determined from the wavelength corresponding to ionization from the O2(3Σ−g, v=0, J=1) ground state.
ESR linewidths in solution. VII. Nondiffusional anisotropic spin–rotational contributions in hydrogen bonding solvents
Authors: Hoel Donna , Kivelson Daniel
Published: February 2003

From variations in the linewidths of the various hyperfine components in the ESR spectra of vanadyl acetylacetonate (VOAA) dissolved in alcohols, the reorientational correlation time τϑ⊥ about an axis perpendicular to the V–O axis can be determined. In order to obtain these times, one needs to know the molecular magnetic parameters; these parameters can be determined in the glassy state, and a detailed analysis is given which indicates that they are effectively independent of temperature and phase. The residual linewidth α″, that part of the linewidth which cannot be attributed to anisotropies in the molecular magnetic parameters modulated by molecular reorientations, has previously been attributed to spin–rotational interactions. If the reorientational motion is diffusive, one can calculate the theoretical isotropic (spherical) spin–rotational contribution α″sph provided τϑ⊥ is known. It is found that α″/α″sph is close to unity at low η/T, where η is the coefficient of shear viscosity, and may in fact be slightly less than 1; at high η/T the ratio α″/α″sph is often much larger than 1, in some cases as large as 30. We have associated this discrepancy between α″ and α″sph with anisotropic spin–rotational relaxation. At low η/T the rotational motion is diffusive and more or less isotropic; at large η/T the angular momentum about the V–O axis relaxes much more slowly than about an axis perpendicular to the V–O axis. Whereas the motion about an axis perpendicular to the V–O axis is diffusive and τϑ⊥ is proportional to η/T, the motion about the V–O axis is nondiffusional, at least at large η/T, and τϑ is not simply proportional to η/T. The nondiffusional motion at high η/T may arise from holes within the alcohol medium in which the VOAA molecule can rotate reasonably freely about the V–O axis; at higher temperatures this structure may become less evident and the rotation less free and more diffusive. It may be that tha analyses of the residual linewidths α″ in viscous media in terms of spin–rotational relaxation could provide a sensitive test of the anisotropy and nondiffusional nature of molecular rotations; however, it has not yet been proved that α″ is entirely due to spin–rotational interactions, as we have assumed.
Weak and strong interactions in chemical reactions
Authors: Baer Michael
Published: February 2003

The over-all reactive process is assumed to follow a two-step mechanism each characterized by a different kind of interaction: (i) the weak interaction that takes care of the product rotational distribution; and (ii) the strong interaction that governs the product vibrational distribution. If we make this assumption it is possible to couple three-dimensional classical rotational distributions with collinear quantum mechanical vibrational distributions. This procedure leads to a modified vibrational distribution which is found to be in agreement with experiment.
The nuclear mass dependence of the adiabatic correction to the Born–Oppenheimer approximation
Authors: Bardo Richard D. , Wolfsberg Max
Published: February 2003

A simple formula is derived for the nuclear mass dependence of the adiabatic correction to the Born–Oppenheimer approximation for molecules with zero total angular momentum in nondegenerate electronic states. The importance of this result to the calculation of isotopic exchange equilibrium constants is pointed out.
On j
Z-conserving coupled-states approximation for molecular collisions
Authors: Choi B. H. , Tang K. T.
Published: February 2003

Recently, McGuire and Kouri introduced the jZ-conserving coupled-states method for inelastic scattering between an atom and a diatomic molecule in order to reduce the dimension of coupled equations. In this Letter the conditions under which that method might be expected to be valid are investigated. It is found that in cases where high values of j are involved, particularly if neither j nor j′ is equal to zero, the results of the jZ-conserving method must be viewed with caution.
Solvated electron yields in pulse-irradiated liquid 1-propanol
Authors: Dixon R. S. , Lopata V. J.
Published: February 2003

The optical absorption spectrum of the solvated electron in 1-propanol and its decay at various temperatures following a pulse of electrons have been studied, and the relative yields of geminate and free solvated electrons have been obtained. From these, the absolute yields have been estimated over the temperature range 150–298 K.
On the specific rates for electron scavenging in nonpolar solvents
Authors: Funabashi Koichi , Magee John L.
Published: February 2003

An attempt is made to formulate a theory of the reaction rate constant for electron scavenging which is valid for high mobility nonpolar liquids. It is proposed that the scavenging mechanism is an electron capture process which is completely analogous to the gas phase process. Application to the liquid state of theory developed for the use in the gas phase requires careful consideration of the relative energies of the quasifree (i.e., V0) and captured states of the electron and it is shown that a reasonable conjecture (that the polarization energy of a scavenger negative ion is the same in all hydrocarbons) allows a semiquantitative correlation of the known experimental results. When the energy of the negative-ion state of the scavenger in the Franck–Condon region is above the V0 level (e.g., C2H5Br in TMS), the reaction rate is shown to increase with V0, while if the former is below the V0 level (e.g., CCl4 in neopentane), the reaction rate decreases with V0. The temperature dependence of the observed rates is qualitatively consistent with the proposed model.
Photoion and photoelectron spectroscopy of oxygen
Authors: Gardner J. L. , Samson James A. R.
Published: February 2003

Photoion and photoelectron kinetic energy distributions of O2 have been measured at a number of incident monochromatic wavelengths extending down to 304 Å. By correlating energies and intensities in the ion and electron spectra at each wavelength, dissociation limits are assigned for the 2Πu(III), c 4Σ−u, and 4Σg(σg2s) molecular states of O2+. Other processes are suggested to explain the remaining peaks appearing in the photoion spectra.
Excitation transfer, lifetimes, and satellite bands in Cs(7p)–Xe collisions
Authors: Kielkopf J. F.
Published: February 2003

Observation of the time-dependence of excitation transfer between Cs 7p 2P1/2 and 2P3/2 levels caused by collisions with xenon are reported. The 2P levels were selectively excited with an N2 laser-pumped dye laser, and the spectrum of the fluorescence radiation was observed with 5 nsec time resolution. The lifetimes of the levels were found to be (2P1/2) 154±2 nsec and (2P3/2) 135±2 nsec, and the 2P1/2 to 2P3/2 transfer cross section at 325±10 °K is 0.150±0.002 Å2. An analysis of the time-dependent decay versus xenon partial pressure shows that the cross section for quenching or collision induced radiation is less than 4×10−3 Å2. This demonstrates that collision induced transitions cannot be important in the formation of near red satellites on the Cs 6s–7p lines perturbed by Xe. In addition, the radiative decay constant for the satellite is identical to that for the line within experimental uncertainty. The correspondence of satellite and line decay curves provides a method of identifying satellites with their parent line.
Model studies of chemisorption. Interaction between atomic hydrogen and beryllium clusters
Authors: Bauschlicher Charles W. , Liskow Dean H. , Bender Charles F. , Schaefer Henry F.
Published: February 2003

The interaction between hydrogen atoms and Be metal clusters has been studied by ab initio electronic structure theory. Self-consistent-field (SCF) calculations have been carried out using both minimum and larger basis sets of contracted Gaussian functions. Both spatially restricted and unrestricted SCF methods were used, and different results were obtained in several cases. Reasons for the choice of this particular model system are discussed. Clusters as large as ten Be atoms have been considered, as have four different sites for the approach of the H atom. The electronic structure is discussed on the basis of predicted orbital energies and Mulliken atomic populations.
Diatomics-in-molecules potentials for N(4S) collisions with O2(3Σg−)
Authors: Wilson C. Woodrow
Published: February 2003

Collisions between ground state N atoms and O2 molecules are of three distinct types. These correspond to doublet, quartet, and sextet states of the collision complex. Potentials operative in the doublet and quartet collisions are computed by the diatomics-in-molecules method. Analysis of these potentials suggests reaction occurs only through the quartet collisions. The experimentally observed activation energy is found to arise from dynamic considerations on the barrierless surface.
Electronic excitation in collisions of fast protons and hydrogen atoms with NO, N2O, CO2, and OCS
Authors: Birely John H. , Johnson Peter A.
Published: February 2003

Analyses of radiation in the 115–850 nm range produced by collisions of 10–25 keV H+ and H with NO, N2O, CO2, and OCS under thin target conditions have been carried out. In agreement with our earlier observations for CO2, the most prominent features for the polyatomic targets are emitted by excited parent molecule ions. Except in the case of OCS, for which weak CO+ (B 2Σ+–X 2Σ+) bands were detected, the cross sections for dissociative excitation to yield diatomic fragments are extremely small. These observations in conjunction with results obtained elsewhere for other projectile ions provide further evidence that in the competition among allowed excitation channels, details of the electronic structure of the collision partners override energy considerations. For an NO target, the NO+ (A 1Π–X 1Σ+) bands are observed, and, for all of the molecules studied, atomic lines owing to dissociative excitation are prominent in the spectra.
Threshold photoelectron–photoion coincidence mass spectrometric study of ethylene and ethylene-d4
Authors: Stockbauer Roger , Inghram Mark G.
Published: February 2003

Experimental curves have been obtained for the fragmentation of ethylene and ethylene-d4 ions as a function of the internal energy of those ions using threshold photoelectron–photoion coincidence mass spectrometry. The results are compared with the previous results of photoionization mass spectrometry, Hei photoelectron–photoion coicidence, charge exchange experiments, and with quasiequilibrium theory (QET) calculations. The discrepancies between results of these previous experiments and QET calculations do not appear in the present data. It is suggested that ion–molecule reactions competing with charge exchange has led to erroneous conclusions in the interpretation of the charge exchange data. It is concluded that QET does describe the fragmentation of ethylene and ethylene-d4 within the limits of the data and calculations available. The secondary ion fragmentation C2H4+ → C2H3++H → C2H2++2H is discussed in detail with regard to the C2H3+ fragment ion internal energy distribution.
Crystal nucleation in normal alkane liquids
Authors: Uhlmann D. R. , Kritchevsky G. , Straff R. , Scherer G.
Published: February 2003

The homogeneous nucleation of crystals in normal alkane liquids has been studied using a droplet technique for a range of alkanes between C16H34 and C5H12. The critical undercooling for nucleation, ΔTN, relative to the melting point TE, increases significantly with decreasing length of the alkane chain n: ΔTN/TE increases from 0.05 for C16H34 to 0.20 for C5H12, with separate variations noted for the odd-n and even-n alkanes. Use of a spherical nucleus model gives molar surface energies σM relative to the molar heat of fusion, ΔHM, which also increase significantly with decreasing n but show little odd–even variation: σM/ΔHM increases from 0.07 for C16H34 to 0.28 for C5H12. Because of the pronounced increase in surface energy with decreasing chain length, the model of a spherical nucleus is suggested to be inappropriate for the alkane liquids (at least for the high-n alkanes). Using a preferred cylindrical nucleus model with separate side- and end-surface energies, a side-surface energy of about 6 erg cm−2 is estimated from the present data, at least for the high-n alkanes. Using this model, the end-surface energies are found to increase with decreasing chain length. This increase is taken to reflect the increasing importance of chain localization and chain end effects with decreasing chain length. For the small-n alkanes, the side surface energy is also suggested to increase with decreasing chain length, again associated with chain localization effects in the liquid adjacent to the crystal (nucleus) surface.
Dimer formation in supersonic water vapor molecular beams
Authors: Calo J. M.
Published: February 2003

An experimental and theoretical study of dimer formation in supersonic water vapor molecular beams is presented. Terminal dimer mole fractions, as a function of source pressure, temperature, and orifice diameter, were measured by sampling fully expanded jets with a quadrupole mass spectrometer employing a phase-sensitive pulse counting technique. The results are in good agreement with a termolecular formation and bimolecular destruction kinetic model for the dimer if a sudden cessation is assumed for the reverse reaction at low reduced temperatures. The model makes use of an expression developed for the equilibrium dimer mole fraction of polar molecules, and takes into account the increase in collision diameter for water molecules due to the reduced kinetic energy of relative motion at low temperatures. The source pressure at which the terminal dimer mole fraction attains a maximum is found to be proportional to D−0.630 and T3.50, where D0 is the orifice diameter and T0 is the source temperature.
Dynamics of chain molecules. I. Solutions to the hydrodynamic equation and intrinsic viscosity
Authors: Perico Angelo , Piaggio Paolo , Cuniberti Carla
Published: February 2003

An exact numerical solution to the equation of motion for a flexible macromolecule in solution is given for the bead and spring model incorporating the preaveraged Oseen hydrodynamic interaction. The limits to be imposed on the strength of the interaction are confirmed and discussed. The data obtained for the eigenvalues λK, the eigenvectors QjK, and the quantities μK show that the hydrodynamic interaction affects the relaxation times and the shapes, the mean square length of the modes remaining practically unchanged with respect to the free-draining results. By comparison with the intrinsic viscosity experimental data, the values of r/b (hydrodynamic interaction strength) and b (mean dimension of the subchain) may be obtained. The method has been applied to PS and PDMS. Defined and reasonable values of r/b and b have been obtained. The range of r/b of major interest (0.2–0.3) gives small draining effects over a large molecular weight range; this makes the intrinsic viscosity depend approximately on M0.5, although with a different proportionality constant.
Carbon-13 chemical shielding tensors in ammonium hydrogen malonate
Authors: Chang J. J. , Griffin R. G. , Pines A.
Published: February 2003

Chemical shielding tensors have been determined in single crystal ammonium hydrogen malonate by proton-enhanced 13C NMR. The average principal values of the carboxyl tensors are σ11 = −112.8±2, σ22 = −45.8±3, and σ33 = 25.6±2 ppm relative to benzene. Using the crystal structure determined in the preceding paper by Chapuis et al., the orientations of the tensor principal axes have been determined and assigned to specific carbon sites. The carboxyl tensor is oriented with the most shielded element perpendicular to the plane of the carboxyl group and the least shielded element σ11 in the plane of the carboxyl group making an angle of 25 ° relative to the bisector of the CO bonds, rotated towards the long CO bond, displaying the effects of hydrogen bonding. The principal values of the methylene tensors are σ11 = 65.6±2, σ22 = 76.3±2, σ33 = 110.2±2 ppm. The more or less unique, most shielded element, is in the C–C–C plane and is nearly along the C–C bond adjacent to the out-of-plane carboxyl group.
Predissociation effects in the B 3Π0+u state of iodine
Authors: Vigue J. , Broyer M. , Lehmann J.-C.
Published: February 2003

The existing data on the magnetic and natural predissociations of the B 3Π0+u state of molecular iodine are briefly discussed. In order to explain some recent experiments the possibility that these two predissociations are due to one single electronic state is studied theoretically. This hypothesis leads to the existence of an interference effect between the two predissociations. The experimental results are shown to be in good agreement with this theory, and the values of some predissociation parameters are deduced. Some experiments are suggested in order to explain a few discrepancies existing between our values and those of previous authors.
Penning ionization of D by He metastables
Authors: Neynaber R. H. , Magnuson G. D.
Published: February 2003

A merging-beams technique has been used to study the Penning ionization reaction He*+D → He+D++e over a range of interaction energy W from 0.1 to 10 eV. (AIP)
Electrical phenomena accompanying phase changes of dilute aqueous solutions of amines
Authors: Stephen Thomas , Weber A. H.
Published: February 2003

The freezing potential was investigated during the phase change of dilute aqueous solutions (10−5 to 8×10−4 N) of methylamine (CH3NH2), dimethylamine [(CH3)2NH], and trimethylamine [(CH3)3 N]. The potentials relative to the liquid were ice negative for all three amine solutions indicating preferential incorporation of the OH− ion into the ice structure over the positive methyl-, dimethyl-, or trimethyl-ammonium ion. The voltages pulses (’’kicks’’) observed at the onset of freezing (methylamine solution, 8.2 V; dimethylamine solution, 6.1 V; trimethylamine solution, 2.04 V—all average values) are explained in terms of the steric factor and electric dipole moment of these positive ions. The largest freezing potentials for aqueous methyl-, dimethyl-, and trimethylamine solutions (55, 43, and 47 V, respectively) were obtained approximately at 2.2×10−5N, 2.2×10−5N, and 6.4×10−5N, respectively, with freezing potentials (44 V) obtained over the widest range of normalities (2×10−5 to 8.2×10−5N) for aqueous solutions of trimethylamine. The freezing potential dropped sharply for methylamine solution for concentrations greater than 5×10−5N. The behavior of dimethylamine solution was intermediate between that of the other two amines.
Emission properties of vinylcarbazole polymers
Authors: Johnson G. E.
Published: February 2003

The emission properties of poly(N-vinylcarbazole) (PVK), poly(N-ethyl-2-vinylcarbazole), and poly(N-ethyl-3-vinylcarbazole) have been investigated. Dilute fluid solutions of all three vinylcarbazole polymers are observed to exhibit a structureless emission band at energies approximately 5400 cm−1 to the red of the (0,0) band of the normal molecular fluorescence of isolated monomer compounds. On the basis of earlier work appropriate model compounds, this emission is assigned as that from an intrachain excimer state in which two interacting pendant groups, one of which is electronically excited, achieve an overlapping, eclipsed, sandwichlike configuration. The emission spectrum of PVK is characterized by the appearance of a second structureless emission band which appears at higher energy than the intrachain excimer fluorescence common to all three vinylcarbazole polymers. This emission band is unique to PVK and under certain conditions to its model compound, 1,3-bis(N-carbazolyl)propane. This emission is assigned as that from a second intrachain excimer. Contrary to the lower energy intrachain excimer, the polymer conformation appropriate for formation of this state exists prior to the initial excitation step.
Kinetic theory of ion cyclotron resonance collision broadening
Authors: Viehland Larry A. , Mason E. A. , Whealton J. H.
Published: February 2003

A rigorous kinetic-theory treatment of the collision broadening of ion cyclotron resonance lines, valid at all field strengths for arbitrary ion–neutral potentials and mass ratios, is presented. The theory justifies the use, in first approximation, of previously derived equations that are strictly valid only for the Maxwell model of constant mean free time between collisions; now the collision frequency, however, has a detailed molecular interpretation and is not in general constant. Computation of a second approximation shows that convergence is good, and that deviations from Lorentzian line shapes occur at all electric field strengths. The dependence of collision frequency on temperature, gas density, frequency, electric field strength, and ion–neutral potential is compressed into a dependence on a single effective temperature. It is shown how the experimental measurement of such dependence can be used to obtain information on nonreactive ion–neutral interactions.
Isotopic and vibronic coupling effects in the valence electron spectra of H2 16O, H2 18O, and D2 16O
Authors: Karlsson L. , Mattsson L. , Jadrny R. , Albridge R. G. , Pinchas S. , Bergmark T. , Siegbahn K.
Published: February 2003

Isotopic shift data for Hei induced high resolution valence electron spectra of H2 16O, H2 18O, and D2 16O are presented. From the differences in the vibrational subspectra we were able to deduce adiabatic ionization energies and geometries of the ions and to assign the excited vibrations to normal modes. The ν2 progression in the 2A1 band was found to be affected by strong vibronic coupling.
A
b i
n
i
t
i
o study of the π-electron states of t
r
a
n
s-butadiene
Authors: Hosteny R. P. , Dunning T. H. , Gilman R. R. , Pipano A. , Shavitt I.
Published: February 2003

Extensive ab initio configuration interaction calculations were carried out on the π-electron states of trans-1,3-butadiene. A double-ζ contracted Gaussian basis set, augmented with two diffuse 2p π functions on each carbon atom, was used in the calculations, which were based on a frozen σ core obtained from a ground-state SCF calculation. All excitations fro and to π orbitals were included in the CI treatment. Natural orbitals were obtained for many of the wavefunctions,and their spatial extenty was determined. Only five of the calculated excited states were found to have a valencelike charge distribution (computed vertical excitation energies in eV are given in parentheses): 1 3Bu (3.45), 13Ag(5.04), 21Ag(6.77), 33Bu (8.08), and 15Ag (9.61). These states all correlate with the valence N and T states of ethylene and can be readily described in terms of the ’’two-vinyl model’’ as either N T (the first two) or TT (doubly excited, the last three). Except for the doubly excited 21Ag, all low-lying singlet excited states were found to involve diffuse excited orbitals (as in the case of similar calculations on ethylene). Their identification as V states and/or Rydberg states is not always clear-cut. The calculated vertical excitation energies for the NT states are in good agreement with electron impact data, but owing to limitations of the frozen σ core, computed excitation energies for the diffuse states (particularly those related to the ethylene V state) are too higy by as much as 1 eV. The absorptions in the butadiene spectrum at 5.9 and 7.1 eV are believed to be due to the 11Bu and 21Bu states, computed at 7.05 and 8.06 eV, respectively.
’’Long time tails’’ in finite systems
Authors: Keyes T. , Ladanyi Branka
Published: February 2003

A theory is developed for the now well-known ’’long time tails’’ on the velocity correlation function for finite systems. The theory is used to calculate the system size dependence of the self-diffusion coefficient in two dimensions and the form of the ’’tail’’ in small two dimensional systems. The relationship between a three dimensional system with one small edge and a true two dimensional system is investigated.
Electron–ion recombination rate constants for some compounds of moderate complexity
Authors: Maier H. N. , Fessenden Richard W.
Published: February 2003

Electron–ion recombination has been studied for the systems N2, CH4, C3H8, neopentane, NH3, and CO2 by use of a microwave conductivity method coupled with pulse radiolysis. Rate constants were determined by analysis of the directly observed electron disappearance. All data pertain to 297 °K. Corrections were made for competing electron losses by extrapolation to zero of a plot of apparent second-order rate constant versus reciprocal of initial electron concentration. No attempt was made to try to identify the positive ions present. The rate constants for the first four compounds showed little change with pressure over the range 1–100 torr with the value found at 100 torr about 20%–50% higher than that at 1 torr. Values for pressures of 10–50 torr are 2.5×10−6 for N2, 3.9×10−6 for CH4, 5.8×10−6 for C3H8, and 4.2×10−6 for neopentane (all cm3 molecule−1 ⋅sec−1). The values found for NH3 varied from 4.6×10−6 at 1 torr to 11.2×10−6 at 10 torr. Data for CO2 were less reliable but showed a change of from 4×10−6 at 1.5 torr to 11×10−6 at 100 torr.
Millimeter- and submillimeter-wave spectrum and molecular constants of cuprous iodide
Authors: Manson E. Lowry , De Lucia Frank C. , Gordy Walter
Published: February 2003

Monomeric cuprous iodide (CuI) in the vapor state has been produced as an intermediate product of the reaction of copper with iodine vapor at 1100 °K in a quasi-free-space microwave absorption cell. Rotational transitions of 63Cu127I and 65Cu127I were measured for J as high as 87→88 and for v as high as 11. Hyperfine splittings due to 127I were observed, and eqQ (127I) =−935(15) MHz was derived. Analysis of the rotational frequencies yields the following Dunham and equilibrium coefficients for 63Cu127I: Y01=2197.10172(98) MHz, Y11=−8.51120(66) MHz, Y21=0.00934(14) MHz, Y31=−0.0375(88) kHz, Y02=−0.67269(10) kHz, Y12=0.031(34) Hz, Y03=−0.00006858(10) Hz, Be=2197.102(2) MHz, ωe=264.897(18) cm−1, ωexe=0.715(2) cm−1, re=2.33831686(104) Å; for 65Cu127I: Y01=2151.89989(94) MHz, Y11=−8.24990(66) MHz, Y21=0.00896(14) MHz, Y31=−0.0356(88) kHz, Y02=−0.64530(10) kHz, Y12=0.030(34) Hz, Y03=−0.00006443(10) Hz, Be=2151.900(2) MHz, ωe=262.157(18) cm−1, ωexe=0.700(2) cm−1, re=2.33831664(102) Å. The Dunham potential constants for CuI are a0=239 366(34) cm−1, a1=−3.33365(36), a2=7.391(16), and a3=−13.20(18).
The diffusion of lithium in molybdenum
Authors: Wu C. H.
Published: February 2003

The diffusion coefficient of 7Li through molybdenum at temperatures around 950 °K has been measured. The diffusion coefficient in dependence on temperature can be expressed by D=D0e−Q/RT, where D0=0.65 cm2 sec−1 and Q=1.46±0.10 eV.
Phenomenological treatments of cross sections for proton and hydrogen impact below 1 keV on molecular nitrogen
Authors: Porter H. S. , Szydlik P. P. , Green A. E. S.
Published: February 2003

An analytic independent-particle model is used to construct static potentials to describe the interaction of hydrogenlike ions with atoms and molecules. Parameters of the ion–atom potential are determined from ab initio total energy minimization procedure. The elastic scattering of He+ from Ne and Ar is investigated as a test case and comparison is made with experiment. The model is then used in conjunction with low energy H+–N2 experimental data to construct differential and total cross sections for the scattering of protons and hydrogen in the energy range 10 eV to 1 keV from molecular nitrogen. Analytic forms are used to parametrize these cross sections to facilitate their use in the calculation of energy deposition by protons and hydrogen atoms in atmospheric gases.
Reactions of diatomic rare-gas ions with rare-gas atoms for primary-ion energies below 50 eV
Authors: Maier William B.
Published: February 2003

Reactions of He+2, Ne+2, and Ar+2 with rare-gas atoms have been studied for center-of-mass energies, E0, below 40 eV. Electron transfer, collision-induced dissociation of the primary ion, and ion transfer have all been observed. At E0=1 eV, electron transfer sometimes occurs with very large (∼50 Å2) cross sections; however, sometimes the cross section for electron transfer is small. The observed cross sections for electron transfer increase as E0 is decreased. Collision-induced dissociation of the primary ion occurs with maximum cross sections of 1.5–25 Å2; these cross sections fall off as E0 is lowered below some value. The cross sections for the collision-induced dissociation of He+2 are observed to be smaller than the cross sections for the collision-induced dissociation of Ne+2 and Ar+2. Ion transfer is observed in only two cases: Ar+2+Ne → ArNe++Ar and Ne+2+He → NeHe++Ne; the cross sections for these reactions increase as E0 is lowered.
Mixed adsorption of carbon monoxide and oxygen on tungsten (100), (110), and (111) single crystal faces
Authors: Anders Leon W. , Hansen Robert S.
Published: February 2003

Flash desorption spectra of C16O and C18O were measured for C16O dosed on tungsten (100), (110), and (111) at 300 K with varying 18O2 predoses and postdoses. Heavy doses of C16O resulted in flash desorption spectra characterized by a single α peak and two β peaks on each of W(100) and W(110) crystal faces, and single sharp α and β peaks on the W(111) crystal face. Predoses or postdoses of 18O2 resulted in extensive isotope exchange in all β peaks, and slight exchange in all α peaks. Postdosing of a C16O monolayer by 18O2 resulted in no change in total CO on the surface, although in cases where two β peaks occurred, that state desorbing at lower temperature was enhanced at the expense of that desorbing at higher temperature. With increasing predose of 18O2 the total β-CO resulting from a fixed C16O dose steadily decreased, and the amount of α-CO at first increased, then decreased. The results are broadly in accord with the Goymour–King model, according to which β states arise from dissociatively chemisorbed CO, α states from CO associatively adsorbed on isolated single sites. Isotopic exchange at low C16O doses followed by heavy 18O2 doses appears best interpreted in terms of random selection of oxygen from immediate neighbors in an immobile monolayer, rather than in terms of random mixing of oxygen isotopes over the entire monolayer.