bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron paramagnetic susceptibility and conduction electron spin resonance in hexaamminecalcium(zero)
Authors: David Thérèse , Damay Pierre , Sienko M. J.
Published: February 2003

The magnetic susceptibility of Ca(NH3)6 has been measured by the Faraday method over the range 1.5−300°K. The observed molar susceptibility, after correction for diamagnetism, decreases linearly from 130×10−6 at room temperature to 85×10−6 at 100°K, then rises linearly to 140×10−6 at 20°K, and finally falls rapidly to 60×10−6 at 1.5°K. The behavior below 100°K is reminiscent of that previously reported for Li(NH3)4 and may be due to an incipient metal−nonmetal transition. Conduction electron spin resonance (CESR) at X−band frequency has been measured for Ca(NH3)6 over the temperature range 4.2−240°K. The absorption signal is Dysonian and its second derivative peak−to−peak linewidth decreases linearly with temperature to about 40°K, below which the decrease becomes more rapid. The spin−lattice relaxation time varies approximately as T−1, even at very low temperatures. The measured g shift (−5×10−4) is small and independent of temperature; the small value relative to Ca suggests that the wavefunction of the conduction electrons at the Fermi level possesses more s character in Ca(NH3)6 than it does in Ca.
Molecular orbital theory of the hydrogen bond. X. Monosubstituted carbonyls as proton acceptors
Authors: Del Bene Janet E.
Published: February 2003

A b initio SCF calculations with a minimal STO−3G basis set have been performed to determine equilibrium structures and energies for a series of mixed dimers formed from water and substituted carbonyl compounds. These dimers may be represented by the general formula HOH⋅⋅⋅OCHR, where R may be CH3, NH2, OH, F, CHO, and C2H3. For each dimer in this series except water−formic acid, two equilibrium structures have been found, in which the substituent R is ’’cis’’ or ’’trans’’ to the proton donor water molecule with respect to the carbonyl CO bond. Except for a cyclic equilibrium water−formamide dimer, all equilibrium dimers have open−chain trans structures consistent with the General Hybridization Model. There is only a small variation in the hydrogen bond energies of equilibrium dimers HOH⋅⋅⋅OCHR, except for the water−formamide dimers which are significantly more stable than all others in this series. The structures and energies of the equilibrium dimers HOH⋅⋅⋅OCHR have been analyzed in terms of the effect of the substituent R on the proton accepting ability of the substituted carbonyl compound RCHO.
Hydrodynamic analysis of non-hard-core interactions in the rotational relaxation of molecules; ESR data
Authors: Hoel Donna , Kivelson Daniel
Published: February 2003

Molecular reorientation for symmetric tops is analyzed in terms of a stickiness factor (s) which measures, in terms of a hydrodynamic model, the lag of the fluid motion relative to that of the particle surface. For s = 1 and s = 0, the hydrodynamic problem is that of ’’stick’’ and ’’slip’’ boundary conditions for symmetric tops. For truly hydrodynamic situations, 0 ? s ⩽ 1, but since molecular reorientation is nonhydrodynamic, negative values of s are possible. Reorientational data obtained from ESR spectra, for vanadyl acetylacetonate in a number of solvents, including a series of alcohols, are analyzed in terms of the s factor.
Theory of unsymmetric polymer–polymer interfaces
Authors: Helfand Eugene , Sapse Anne Marie
Published: February 2003

Solutions have been obtained to equations which described the interface between two immiscible polymers and are more general than the equations first introduced by Helfand and Tagami. Gaussian random−walk statistics are assumed for the macromolecules. As a consequence of the present work, limitations of the earlier theory are removed, particularly the assumption that the properties of the two polymers when pure are identical. Calculations are performed for a variety of polymers and comparison with experiment is made.
Vibration–vibration energy transfer in laser excited SO2: Further evidence for a slow V–V step
Authors: Siebert Donald , Flynn George
Published: February 2003

The rate of fluorescence rise and fall has been measured for the ν3 state of SO2 as a function of pressure in pure SO2 and SO2−rare gas mixtures pumped by a Q−switched CO2 laser. The results of these experiments are consistent with rapid equilibration of ν1 and ν3 stretching modes (∼135 collisions) followed by a very slow equilibration of the ν1, ν3 stretches with the ν2 bend (∼2600 collisions) in pure SO2. The presence of metastable, vibrationally hot stretching modes with vibrationally cold bending modes suggests the possibility of achieving a population inversion in SO2 by infrared laser pumping. The parameters and possibilities for obtaining vibrationally selective chemical reactions in SO2 are considered.
PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. I. Outline of the method for closed-shell states
Authors: Ahlrichs R. , Lischka H. , Staemmler V. , Kutzelnigg W.
Published: February 2003

The methods of configuration interaction with double substitutions to pair natural orbitals (PNO−CI) and of the coupled electron pair approximation (CEPA) proposed by W. Meyer are improved by combination with a new scheme of the calculation of the pair natural orbitals (PNO) and an efficient iterative scheme for the diagonalization of the CI matrix. The relevant matrix elements for the closed shell case are tabulated, the quantities that are pertinent for an analysis of the correlation energy are defined, and the organization of the computer programs is described.
Densitometric measurements of the rate of sulfur dioxide dissociation in shock waves
Authors: Kiefer John H.
Published: February 2003

The dissociation of SO2 in mixtures with Kr was investigated in incident shock waves over the temperature range 2900−5200 K. The rate of the low−pressure unimolecular dissociation was determined with a laser−beam deflection technique. Vibrational relaxation of the SO2 was unresolved and initial rates were estimated by extrapolation to the shock front. Arrhenius parameters obtained for the rate coefficient in SO2−Kr collisions are logA = 16.23 (A in cm3/mole⋅s), and an activation energy Ea = 112±4 kcal/mole. The rate coefficient for SO2−SO2 collisions is roughly a factor of 3.4 larger. These results fit well with recombination measurements and agree adequately with some previous shock tube data. However, they are in serious disagreement with the shock tube measurements of Olschewski, Troe, and Wagner [Z. Phys. Chem. 44, 173 (1965)].
Rough hard sphere theory of the self-diffusion constant for molecular liquids
Authors: Chandler David
Published: February 2003

A simple theory based on a hard sphere model is used to calculate the self−diffusion constant and the shear viscosity of liquid carbon tetrachloride. The theory accounts for the coupling between translational and rotational motions of molecules. This feature distinguishes the hard sphere theory presented herein with that given recently by other workers. It is shown that the coupling has a large (factor of 2) effect on the diffusion constant, and thus the coupling should not be neglected even for molecules as spherical as carbon tetrachloride. The approximations that are needed to arrive at a hard sphere theory for transport coefficients are discussed in detail. This analysis reveals the strengths and limitations of such a theory.
The ESR spectrum of PH4
Authors: Colussi A. J. , Morton J. R. , Preston K. F.
Published: February 2003

The ESR spectrum of PH4 and PD4 radicals produced in a neopentane matrix by γ radiation is reported. The results indicate that structure of the radical is bipyramidal, rather than tetrahedral or square−planar. (AIP)
Application of Rayleigh–Schrödinger perturbation theory to the hydrogen atom. III
Authors: Gray B. F. , Gonda I.
Published: February 2003

It is shown that energies calculated for the hydrogen atom by enclosing it in a spherical box with radius R and treating the Coulomb attraction as a perturbation are convergent for small R and agree with previously obtained exact results. This method is useful for calculating the energies of systems at high pressure where these can be stimulated by confinement to a small box. (AIP)
Electrostatic polarization in the exchange scattering of electrons of high kinetic energy. 11S → 23S transition in helium. Singlet–triplet excitation in CO
Authors: Klump Kirby N. , Lassettre Edwin N.
Published: February 2003

Excitation in forward scattering by electron impact of the 23S state in helium is studied in the kinetic energy range 100−300 eV. This extends to lower energies the investigation of Skerbele, Harshbarger, and Lassettre [J. Chem. Phys. 58, 4285 (1973)]. A minimum in the cross section is found for the first time at ∼225 eV, in approximate agreement with the calculations of Huo [J. Chem. Phys. 60, 3544 (1974)]. Abnormally high cross sections in forward scattering (ϑ = 0°) are expected (from Huo’s theory) for those singlet−triplet transitions for which the orbital term symbols (like S for 11S → 23S) are the same in ground and excited states. A search for such transitions is made in CO at 200 and 300 eV incident energies and an example is found.
Spinodal decomposition in a binary liquid mixture near the critical point
Authors: Schwartz Arthur J. , Huang John S. , Goldburg Walter I.
Published: February 2003

Light scattering was used to study the nonlinear behavior of spinodal decomposition in a binary mixture of 2−6 lutidine−water near its lower critical point. We have photometrically analyzed the time evolution of the differential scattering cross section following a quench into the thermodynamically unstable spinodal region. Our results are in very good agreement with a mean field theory proposed by Langer. Using this theory we have determined the temperature dependence of the fourth concentration derivative of the free energy ∂4f/∂c4. The results are consistent with scaling if the nonclassical critical exponents are used.
Vapor-phase infrared spectrum of nitrosyl cyanide
Authors: Dorko Ernest A. , Buelow Larry
Published: February 2003

The infrared spectrum of nitrosyl cyanide has been recorded in the gas phase between 4000 and 250 cm−1. Four vibrational fundamentals have been located. Assignments of the five in−plane and one out−of−plane fundamentals have been made on the basis of observed and calculated vibration−rotation bond contours and on the basis of a normal coordinate analysis. Force constants have been calculated for all of the fundamentals and an estimate of the purity of the normal modes has been made. Thermodynamic properties of nitrosyl cyanide have been calculated for several temperatures.
Conduction electron spin resonance study of the cubic phase of tetraamminelithium(zero)
Authors: Glaunsinger W. S. , Sienko M. J.
Published: February 2003

The cubic phase of tetraamminelithium(zero), Li(NH3)4, has been stabilized below the normal solid−solid transition temperature by freezing lithium−ammonia solutions more dilute than 0.2 MPM. Conduction electron spin resonance (CESR) has been studied in the temperature range 11−160°K. CESR line shapes are in excellent agreement with Webb’s theoretical line−shape function. The relative resistivity, average particle size g shift, spin−lattice relaxation time, and relative paramagnetic susceptibility are extracted from the CESR spectra. The resistivity varies linearly with temperature down to 11°K; on the basis of the Block−Grüneisen theory, it is estimated that the Debye temperature is below 30°K. Average particle sizes are in the micron range. The g shift is temperature independent and equal to −8.3±0.3×10−4. For concentrations exceeding 0.01 MPM the spin−lattice relaxation rate varies linearly with temperature down to 11°K. Surface relaxation becomes important below 0.01 MPM. The impurity, surface, and bulk spin−lattice relaxation times are separated in a 0.001 MPM sample. Several spin−lattice relaxation mechanisms are evaluated, and it is concluded that the spin−orbit relaxation mechanism is dominant. The paramagnetic susceptibility of the cubic phase is temperature independent; it undergoes an 8% increase at the melting point, attributable to the decrease in density upon melting. In contrast to the hexagonal phase of Li(NH3)4, the cubic phase exhibits normal metallic behavior.
Microwave spectrum and structure of cyclopropene
Authors: Stigliani William M. , Laurie Victor W. , Li James C.
Published: February 2003

The microwave spectrum of cyclopropene has been investigated in the region of 18−40 GHz. The two monosubstituted 13C species have been assigned and a centrifugal distortion analysis carried out on the main isotopic species. Structural parameters obtained are r (C1C2) = 1.509±0.001 Å, r (C2C2′) = 1.2959±0.0004 Å, &C2C1C2′ = 50.84±0.05°, r (CH ethylenic) = 1.072±0.001 Å, r (CH methylenic) = 1.088±0.002 Å, &C2C2′H = 149.85±0.08°, &HC1H = 114.57±0.19°.
Time delays and diffusion controlled reactions
Authors: Monchick L.
Published: February 2003

The theory of diffusion controlled recombination of photolysis products is reconsidered and extended to reaction mechanisms with a time delay built in. Quantum yields for photodissociation turn out to be complete independent of the time delay, and, to a certain extent, of whether the random walk or Bronwnian motion theory of diffusion, i.e., whether the Smoluchowski or telegrapher’s equation, is used. Once the time dependence is considered, damped oscillatory phenomena appear that are a direct consequence of the time delay. Methods of introducing the structure of the solvent and hydrodynamic corrections to the diffusion tensor are taken up briefly in the Appendix.
Pyrolysis of ethylamine. I. Microwave spectrum and molecular constants of vinylamine
Authors: Lovas Frank J. , Clark Frank O. , Tiemann Eberhard
Published: February 2003

Microwave rotational spectra assignable to gas phase vinylamine (CH2=CHNH2) have been detected in the pyrolysis decomposition products of ethylamine. Stark effect and hyperfine structure measurements have aided the assignment of the spectrum and allow the determination of the 14N nuclear electric quadrupole coupling constants and the electric dipole moment for vinylamine. The spectrum of the first excited inversion vibrational state has been assigned. Relative intensity measurements indicate that the inversion state lies about 65±25 cm−1 above the ground state. The spectral evidence suggests that vinylamine has a nonplanar equilibrium structure with r0(C−N) = 1.40 Å and &CCN = 125°.
Raman spectra and ring puckering of silacyclobutane-d and -1,1-d2 and silacyclopent-3-ene
Authors: Lewis J. D. , Chao T. H. , Laane J.
Published: February 2003

Vapor−phase Raman spectra have been recorded for three cyclic organosilane molecules. Ten Δv=2 transitions were observed between 90 and 180 cm−1 for silacyclobutane; seven were recorded for silacyclobutane−1,1−d2. The undeuterated compound also shows a difference band series in the CH2 stretching region. For silacyclopent−3−ene a difference band series originating from an olefinic C−H stretching mode was recorded. All the spectra confirm previous assignments of far−infrared spectra.
S− formation by dissociative attachment in OCS and CS2
Authors: Ziesel J. P. , Schulz G. J. , Milhaud J.
Published: February 2003

The electron energy dependence and magnitude of the cross section for formation of S− from OCS and CS2 are studied using monoenergetic electron beams. The dissociative attachment cross sections exhibit a near−vertical onset followed by structure at higher energies. In the case of S−/OCS, this structure probably arises from the production of the CS fragment in a vibrationally excited state (v = 1), but this explanation may not apply to the case of S− from CS2. The peak cross section in OCS is 2.9×10−17 cm2 at 1.35 eV; in CS2 it is 3.7×10−19 cm2 at 3.35 eV.
Temperature coefficients of zero-field splittings of multiply oriented triplet states of aromatic molecules in single crystals
Authors: Chen Mon-Chao , Cullick A. S. , Gerkin Roger E.
Published: February 2003

Significant differences in the temperature coefficients of the component zero−field splittings from multiply oriented aromatic triplet states in single crystals have been observed in several systems and are reported in detail for 1,2;5,6−dibenzanthracene in p−terphenyl. The utility of these temperature coefficients in assigning triples of splittings associated with the distinct triplet species is demonstrated.
Molecular orbital theory of the hydrogen bond. XII. Amide hydrogen bonding in formamide–water and formamide-formaldehyde systems
Authors: Del Bene Janet E.
Published: February 2003

A b initio SCF calculations with a minimal STO−3G basis set have been performed to determine the equilibrium structures and energies of dimers having formamide as the proton donor molecule and either water or formaldehyde as proton acceptor molecules. The structures of dimers in which the N−H proton ’’s−trans’’ to the carbonyl group is hydrogen bonded (t dimers) are consistent with structures anticipated from the general hybridization model. In these dimers, there is essentially free rotation of the proton acceptor molecule about the intermolecular line. When hydrogen bond formation involves the ’’s−cis’’ proton (c dimers), a single equilibrium formamide−water and formamide−formadehyde dimer exists, the structure of which is strongly influenced by the secondary factors of dipole alignment and long−range interaction. These factors are also responsible for the increased stability of c dimers relative to t dimers. A set of 1:2 formamide:water trimers has been constructed from the equilibrium formamide−water dimers, in which the formamide molecule forms two hydrogen bonds. Only in three open−chain trimers are the hydrogen bonds stronger than those of the corresponding dimers. CI calculations have also been performed to determine the effect of hydrogen bond formation on n→π* transition energies in the dimers and trimers. The n→π* transition energy of the proton acceptor formaldehyde molecule increases in the formamide−formaldehyde dimers. In these dimers, the magnitude of the blue shift is determined by the dimer hydrogen bond energy. The formamide n→π* band is also blue shifted to some extent in the formamide−water and formamide−formaldehyde dimers, even though the n→π* transition originates in the proton donor molecule. The blue shift of the formamide n→π* band in the open−chain trimers having formamide as the central molecule is equal to the sum of the blue shifts in the corresponding dimers.
An investigation of electron thermalization in irradiated gases using CCl4 as an electron energy probe
Authors: Warman John M. , Sauer Myran C.
Published: February 2003

The attachment of electrons to CCl4 has been studied in pulse irradiated argon and its admixtures with several atomic and molecular compounds under conditions where the timescale of attachment is comparable with that of electron thermalization. Using the known energy dependence of the rate constant for attachment, the resulting complex electron decay kinetics have been quantitatively related to the energy relaxation process. Values of the energy exchange rate coefficient Ku defined by the relationship d?/dt = −Ku⋅[? − (3/2) kT]⋅NM have been determined for 18 compounds of varying complexity and dipole moment. The values of Ku found encompass the range from 1.3×10−13 cm3 sec−1, for argon, to 1.5×10−8 cm3 sec−1, for acetone. The results are compared with energy exchange parameters available from other sources. Thermalization times and ranges of electrons in irradiated gaseous and liquid systems are calculated and their relevance to radiation chemistry is discussed.
Longitudinal diffusion of K+ ions in He, Ne, Ar, H2, NO, O2, CO2, N2, and CO
Authors: James D. R. , Graham E. , Akridge G. R. , Gatland I. R. , McDaniel E. W.
Published: February 2003

Longitudinal diffusion coefficients, measured with a drift tube mass spectrometer, are reported for potassium ions in helium, neon, hydrogen, nitric oxide, oxygen, and carbon dioxide for E/N in the range 1−700 Td (E is the electric field strength, N the neutral gas number density, and 1 Td = 10−17 V⋅cm2). These results, together with those previously reported for potassium ions in argon, nitrogen, and carbon monoxide, are compared with the theories of Wannier and of Mason, Whealton, and Viehland, which relate mobilities and diffusion coefficients.
The dynamics of dissociation of diatomic molecules and mass effect
Authors: Wong W. H. , Burns George
Published: February 2003

The effect of the mass of the third body upon the dynamics of dissociation of Br2 at 6000°K was studied using 3D classical trajectory calculations. It was found that the dissociation cross section and the impact parameter, being sensitive functions of Br2 total energy, are also sensitive functions of the mass of the third body. However, the total equilibrium dissociation rate coefficient (one−way flux) varies but little with the mass of the third body. The latter finding is in qualitative agreement with the variational theory of reaction rates and the available experimental data.
A Mössbauer effect study of 57Fe in transition metal monoborides
Authors: DeYoung D. B. , Barnes R. G.
Published: February 2003

The phase characteristics and nuclear hyperfine properties of 45 transition metal monoborides M1−x Fex B were studied by Mössbauer effect spectroscopy of 57Fe. Transmission measurements with powders were completed in the temperature range 4.2−1000 K; additional scattering and 18 kOe applied field data were collected at 300 K. Measured 57 Fe nuclear hyperfine parameters are discussed in terms of atomic bonding, charge transfer, and spontaneous magnetism in the borides. Within the context of the rigid band model, the isomer shift data for all boride phases indicate an electron transfer from boron to the metal 3d band. Similarly, the effective magnetic hyperfine field in ferromagnetic monoborides is proportional to the magnetization and also reveals a possible charge transfer from boron to metal. Quadrupole coupling constant results show a strong ionic contribution to the electric field gradient at the 57Fe site in the monoborides. Measurements on FeB confirm the existence of two distinct structural modifications possessing different magnetic hyperfine fields but the same Curie temperatures. The defect (α) phase of FeB appears to be stabilized by the addition of manganese.
Complex dependence of the positron annihilation rate on methane gas density and temperature
Authors: McNutt J. D. , Summerour V. B. , Ray A. D. , Huang P. H. , Johnson D. A.
Published: February 2003

Annihilation rates have been measured for positrons in methane gas at temperatures of 22, 4.5, −28, and −50°C at densities in the range 6.6×10−4−1.1×10−1 g/cm3, in 22.5 and −28°C methane gas as a function of applied electric field strength and gas density, and in 22°C methane−argon gas mixtures as a function of argon concentration. A complex dependence of the slow positron annihilation rate on methane density and temperature was observed. The orthopositronium annihilation rate became temperature dependent at high densities. Both the positron−methane and the orthopositronium−methane annihilation rates were found to be independent of argon concentration in the gas mixtures. Electric fields produced a decrease in the slow positron annihilation rates, but had no measurable effect on the orthopositronium rates. The annihilation behavior of slow positrons is discussed in terms of several annihilation models, and empirical analyses of the data in terms of these models are presented. Only the single−stage formation of a positron−methane complex can account for all the observed phenomena.
Effect of surface plasmons on transitions in molecules
Authors: Philpott Michael R.
Published: February 2003

A quantum theory of the coupling of excited molecules to the surface plasmon modes of a metal is described. For radiative decay, formulas are given for the separate decay into s photon, p photon and surface plasmon channels valid for all molecule−metal separations. Calculations of the quantum yield of fluorescence are described. The total decay rate is shown to equal that given by classical theory. Other processes considered briefly are the Raman effect and resonance energy transfer between identical molecules.
Vibrational energy dependence of the cross section and product distribution of acetylene ions in acetylene
Authors: Buttrill S. E.
Published: February 2003

The reactions of acetylene ions with acetylene to produce C4H2+ and C4H3+ were studied using photoionization mass spectrometry from 1090 to 1030 Å. In this wavelength region, acetylene ions are formed with none, one, or two quanta of vibrational energy in ν2, the carbon−carbon stretching mode. The total reaction cross section of C2H2+ was independent of vibrational energy, but the ratio of C4H2+ to C4H3+ formed decreased from 0.48 for v2 = 0 to 0.35 for v2 = 1. These results are consistent with the qualitative trends predicted by a quasiequilibrium theory treatment of the reaction.
First principles derivation of the effective valence shell Hamiltonian for large molecules
Authors: Westhaus Paul , Bradford Edward G. , Hall David
Published: February 2003

We construct an effective Hamiltonian for the valence shell of a large electronic system. The procedure begins by classifying the complete one electron space according to core, valence, and excited orbitals. An N−electron subspace 𝒯N spanned by Slater determinants with a fixed set of Nc core orbitals and different sets of Nv = (N − Nc) valence orbitals is defined. A canonical transformation on the Coulomb Hamiltonian is used to eliminate the interaction between 𝒯N and its orthogonal complement, ?N, thereby defining an effective N−electron Hamiltonian. This effective Hamiltonian is expanded in a cluster development of linked one−, two−, three−,⋅⋅⋅body operators in terms of which the conditions of having vanishing matrix elements between 𝒯N and ?N can be explicitly formulated. Then starting with this effective N−electron Hamiltonian we construct an equivalent Hamiltonian to operate in the space of antisymmetrized products of valence orbitals. To within a constant (times the identity on the valence space) this equivalent Hamiltonian—dubbed the valence shell Hamiltonian—has the same matrix elements between antisymmetrized products of valence orbitals as does the effective N−electron Hamiltonian on 𝒯N. This formalism is then applied to generate an effective pi−electron Hamiltonian for planar conjugated molecules. We conclude by deriving explicit formulas for the transformed dipole transition operators.
Photoionization and ion cyclotron resonance studies of the reaction C2H4+ + C2H4 → C3H5+ + CH3
Authors: LeBreton P. R. , Williamson A. D. , Beauchamp J. L. , Huntress W. T.
Published: February 2003

High pressure photoionization mass spectrometry has been employed to study the ion molecule reaction C2H4+ + C2H4 → C3H5+ + CH3 at wavelengths between 700 and 1180 Å. Measurements have been made of the apparent ionization cross section of both C2H4+ and C3H5+ as a function of photon energy. The threshold energy for the reaction coincides with the ionization threshold of the parent ion at 10.51 eV. The reaction cross section decreases as the internal energy of the ionic reactant increases. Vibrational structure, observed in the C2H4+ photoionization efficiency curve immediately above threshold (10.51−11.7 eV) permits a rough estimate of the change in reaction cross section with vibrational excitation. For the first three observed levels the change in cross section is ? 10%. However, for the fourth and fifth levels the cross section has dropped by approximately 25%. In the energy region between 11.7 and 12.1 eV a slight increase in the C2H4+ photoionization efficiency curve appears which is not reflected in the C3H5+ curve. A large increase in the photoionization efficiency curve of C2H4+ occurs at 12.2 eV, the threshold of the 2B3 first excited electronic state of the ion. A dramatic decrease is observed in the reaction probability of C2H4+ formed at energies above 12.2 eV. The cross section for reaction in the excited 2B3 state is ? 13% of the cross section for reaction in the lowest vibrational level of the ground electronic state. The observed decrease in reactivity is independent of repeller voltage at ion exit energies below 3 eV. Pressure studies reveal that after one or two collisions, the reactivity of C2H4+ is re−established, suggesting that relaxation of internal excitation is an efficient process. The change in reactivity of C2H4+ with internal energy is compared to the effects of translational excitation as determined using the techniques of ion cyclotron resonance spectroscopy.
Electron reactions and electron transfer reactions catalyzed by micellar systems
Authors: Grätzel M. , Kozak John J. , Thomas J. K.
Published: February 2003

The kinetics of the reaction of hydrated electrons with pyrene, pyrene butyric acid, and pyrene sulfonic acid (PSA) have been investigated in aqueous solutions of cetyltrimethylammonium bromide (CTAB). With all three solubilizates the formation of the electron adduct (P−) occurs very rapidly with rate constants ≳1011M−1⋅sec−1. These abnormally high rate constants are shown to be due to fast trapping of eaq− in the positive potential field of the micelle and subsequent efficient penetration of electrons into the micellar interior. A similar enhancement was observed for electron transfer reactions between CO−2 and solutes solubilized in or on the micelle. For example CO−2 readily transfers an electron to pyrene sulfonic acid on the surface of the micelle. This reaction does not occur in homogeneous solution but is catalyzed by the positive electrostatic surface potential. Addition of electrolyte drastically reduces the rate of eaq− and CO2− with solubilizates. The Debye−Hückel theory of electrolytes was invoked to elucidate the role of the charged micellar interface in facilitating the penetration of the electron into the micelle and promoting the electron transfer reaction on the surface. Ion distributions were calculated via solution of a Poisson−Boltzmann equation, generalized to take into account the probable change of the microscopic dielectric constant in the vicinity of the micellar interface.
A molecular orbital investigation of chemisorption. II. Nitrogen on tungsten (100) surface
Authors: Anders Leon W. , Hansen Robert S. , Bartell L. S.
Published: February 2003

The relative bonding energies of nitrogen chemisorbed at three symmetric sites on a W(100) surface, represented by finite arrays of tungsten atoms [L. W. Anders. R. S. Hansen, and L. S. Bartell, J. Chem. Phys. 59, 5277 (1973)] were obtained by means of the extended Hückel molecular orbital theory (EHMO). The preferred site for nitrogen chemisorption was found to be the five coordination number (5 CN) site or the fourfold site with a tungsten atom below four tungsten atoms surrounding the nitrogen atom. The 5p orbital repulsive energy, in the case of hydrogen chemisorption, could be adequately approximated by the sum over pairs of empirical exponential repulsive terms; in the case of nitrogen chemisorption, this same method was approximately 10% in error at the equilibrium bond distance, and repulsive energies were therefore obtained from calculations including tungsten 5p orbitals but with smaller arrays.
Central-force model for liquid water
Authors: Lemberg Howard L. , Stillinger Frank H.
Published: February 2003

We propose a new class of Hamiltonian models of liquid water based on resolution of the monomeric unit into three effective point charges. Interacting through central forces only, the three charges automatically assume the molecular structure. Two important effects are built into this model which have been neglected in similar attempts: intramolecular modes of vibration and the capacity for self−dissociation in the liquid phase. In addition, a large number of microsopic properties of water can be expressed in very simple terms for this representation: The pressure and internal energy, second virial coefficient, high−frequency elastic moduli, and dielectric function are discussed in explicit terms. A convenient algorithm for computing low−order quantum corrections (proportional to h/2) to thermodynamic properties is given as well. To illustrate the general class of central−force models, we provide a concrete realization which has been determined by fitting phenomenological potentials to a nearly linear hydrogen bond of proper energy and dimer configuration. In order to elucidate the microscopic consequences of assuming central−force interactions in water, we have investigated the energy variation of small polymers (dimers and trimers) and the solvated proton near their minimum energy configurations. On a qualitative level, the results of these initial computations provide considerable encouragement for the view that water molecule interactions can be realistically approximated by linear combinations of central forces.
A test of the validity of the combined phase-space/trajectory method
Authors: Anderson James B.
Published: February 2003

The validity of the combined phase−space/trajectory method of analysis of chemical reactions in thermal systems is tested by comparison of three−dimensional trajectory calculations for a reaction A + BB ? AB + B similar to the reaction F + H2 ? HF + H. Two sets of reactive trajectories at fixed total energy were generated using the combined method. For one, the dividing surface was located near the saddle point, in the entrance valley (4300 trials, 1001 reactive). For the other, the dividing surface was located around the corner in the exit channel (41702 trials, 244 reactive). Comparison of the two sets of reactive trajectories shows they are identical within statistical error and suggests the results are independent of the dividing surface location, as required for the method to be rigorously valid.
Effect of curvature of the reaction path on dynamic effects in endothermic chemical reactions and product energies in exothermic reactions
Authors: Duff James W. , Truhlar Donald G.
Published: February 2003

Collinear quasiclassical trajectories are examined for two realistic potential energy surfaces for atom−diatomic molecule reactions for two reaction attributes: (1) vibrational energy of the products of a thermal−energy exothermic reaction; (2) threshold energy for endothermic reaction of ground−state reagents. Eight different mass combinations are studied. The potential energy surfaces differ primarily in the amount of potential energy released in an exothermic reaction before and in the region of large curvature of the minimum−energy path and in the curvature of the repulsive potential energy contours when all three atoms are close. For attribute (1), we find the results are qualitatively correlated by the theory of Hofacker and Levine although, contrary to previous work, one potential energy surface shows high mixed energy release (in the language of Polanyi and co−workers) but low excitation to product vibration for five different mass combinations. For reaction attribute (2), we find one surface has a high translational threshold (or no reaction at any energy) for six mass combinations, while the other surface shows this behavior in only three cases. Thus, this type of surface provides an exception to previous generalizations that extra vibrational energy is required for very endothermic reactions with late barriers. This demonstrates the importance of the location of the curvature of the reaction channel for such reaction attributes. Very accurate determinations of potential energy surfaces will be required to make reliable predictions of reaction attributes such as (1) and (2) for real systems. Analysis of the details of the trajectories shows that the high threshold can generally be attributed to reflection before the saddle point of the surface rather than to recrossing the saddle point region. The vibrational excitation of reagents in nonreactive collisions is also strongly effected by curvature of the minimum−energy path.
Studies of excited state charge-transfer interactions with picosecond laser pulses
Authors: Chuang T. J. , Eisenthal K. B.
Published: February 2003

The methods of picosecond laser photolysis have been employed to study the inter− and intramolecular charge−transfer interaction between excited anthracene and N,N−diethylaniline in polar as well as nonpolar solvents. The formulation of reaction kinetics coupled with molecular reorientational motion is developed. The experimental results, at low diethylaniline concentrations, are analyzed in terms of diffusion model and Noyes molecular pair model. The transient behavior due to instantaneous flux in early time regions is clearly evident. The values of critical intermolecular distance and primary rate constant for electron transfer are determined and are consistent for all solutions studies. In the high diethylaniline concentration region, analysis of orientational relaxation times and the rate of charge−transfer reactions clearly reveals the dynamic nature of the interacting processes.
Photoionization study of the reaction O+2 (a 4Πu) with O2†
Authors: Dehmer P. M. , Chupka W. A.
Published: February 2003

A high−resolution photoionization study of the production of O+3 from the reaction of O+2 (a 4Πu) with O2 determined that the reaction proceeds primarily through the v′ = 5 to 10 vibrational levels of the O+2 (a 4Πu) state. The relative reaction cross section for O+3 formation increases rapidly from v′ = 4 to v′ = 9 where it is a maximum. For v′ = 10, the lowest vibrational level for which the reaction O+2 (a 4Πu) + O2 → O+2 + O + O is exothermic, the reaction cross section for observable O3+ is approximately 10% smaller than the cross section for v′ = 9, and for v′ ? 11 the cross section is relatively small. A study of the cross section as a function of translational energy showed that internal vibrational energy is significantly more effective than translational energy or product formation for total collision energies of up to ∼1 eV above threshold. A plausible surface crossing mechanism for the reaction is suggested. The calculated thermodynamic threshold for O+3 production corresponds to a wavelength of 747.2±1.4 Å, which nearly coincides with O+2 (a 4Πu, v′ = 4) convergence limit at 747.25 Å. A very small amount of reaction is observed at this vibrational convergence limit, however this is due at least in part to the thermal rotational and translational energy of the reactants.
A study of the role of vibration–vibration exchange on the collisional deactivation of vibrationally excited singlet molecular oxygen
Authors: Parker J. G.
Published: February 2003

A study of the role of vibration−vibration (V−V) exchange on the collisional deactivation of laser excited O2(1Δg) (v′ = 1) has been carried out. It is apparent that such exchange can be important only if deactivation of O2(2Δg) (v′ = 0) occurs via intersystem crossing (E−V) to the O2(3Σ−g) (v = 5) state. A theoretical study of such a deactivation mechanism has been carried out (II), yielding results similar in form to those obtained previously (I), in which only vibration−translation (V−T) and electronic−translation deactivation (E−T) were considered. In either case two time constants appear, one characterizing the electronic E−T or E−V process and the other the V−T process. In Case I, the V−T deactivation occurs in the 1Δg state, while in Case II it occurs in the 3Σ−g ground electronic state. Theory suggests that since the vibrational spacing in the 1Δg state is slightly less than in the 3Σ−g state, V−T deactivation should occur at a more rapid rate in the former. Analysis of experimental data on the effect of He on the vibrational phase shift of laser excited O2(1Δg) (v′ = 1) yields a vibrational time constant significantly smaller than that obtained from earlier measurements under the same experimental conditions for O2(3Σ−g). From this it is concluded that O2(1Δg) does not readily communicate vibration to the ground electronic state and that vibrational deactivation in this state must occur by a V−T process.
Anticrossings and microwave transitions between the k (4p) 3Πu, v = 1, N = 3 and the p (4d) 3Σ+g, v = 1, N = 5 levels of H2
Authors: Miller Terry A. , Freund Robert S.
Published: February 2003

Electric field induced anticrossings and electric dipole microwave transitions have been observed between the k (4p) 3Πu, v = 1, N = 3 and the p (4d) 3Σ+g, v = 1, N = 5 levels of H2. These ΔN = 2 anticrossings are forbidden except for mixing with N = 4 levels by the magnetic field. Our observations show that at zero field the 3Πu N = 3 level lies 0.2785±0.0001 cm−1 above the 3Σ+g N = 5 level, a separation significantly smaller than the optical value of 0.38 cm−1. Fine structure parameters for the k (4p) 3Πu state are consistent with values determined by MOMRIE from the N = 1 level. The magnetic g value and fine structure parameters for the p (4d) 3Σ+g state show that it lies very close to Hund’s case d.
Comments on the classical theory of energy transfer
Authors: Chance R. R. , Prock A. , Silbey R.
Published: February 2003

Energy transfer from an emitting molecule to an absorbing half−space is considered from the viewpoint of electromagnetic theory. The lifetime of a dipole emitter in the presence of a mirror is determined through a calculation of the complex Poynting vector in the dielectric surrounding the dipole. This method has the advantage over previous approaches to this problem in that the radiative and nonradiative components of the lifetime expression may be rigorously separated. The influence on emitter lifetime of a mirror of finite thickness is also described. A simple expression is derived describing the energy transfer rate in these layered systems. It is shown that nonradiative energy transfer results from coupling of the near field of the dipole to the surface plasmon modes in the metallic absorber. The Förster energy transfer rate law is discussed in the context of the present theory.
Multiple scattering treatment of the solvated electron in water
Authors: Moskowitz Jules M. , Boring Michael , Wood J. H.
Published: February 2003

The multiple scattering method of molecular orbital computations has been used to investigate the structure of an (excess) electron in water. The complex considered is a molecular ion consisting of four water molecules, situated on the vertices of a tetrahedron, plus the extra electron; the complex is placed in a dielectric cavity. The calculated excitation energy of 1.77 eV is in good agreement with the experimental value (1.72 eV). The computed equilibrium radius of the complex is in reasonable agreement with previous calculations. Our determination of a second ’’2p’’ state (5e) lying near the b2 state indicates another mechanism for the asymmetric line broadening of the absorption spectrum. Our computed self−consistent field potentials and charge densities are presented as they can be compared with previous model calculations.
Perturbation theory of fluids. An effective potential for liquid krypton
Authors: Mandel Frederic
Published: February 2003

The Weeks−Chandler−Andersen (WCA) perturbation theory is used to calculate an effective two body potential for krypton. A fit of the equation of state, isotope separation factor, and internal energy at three points in the ρ,T plane acts as a filter for screening out unacceptable potentials. It is found that the LJ 12−6 potential with ε = 119.8 and σ = 3.405 is indeed a highly accurate effective potential for argon, while for krypton a more repulsive potential is required to fit the experimental results.
Thermal force on a sphere
Authors: Davis Lorne A. , Adair T. W.
Published: February 2003

The thermal force on a sphere of insulating material centered between parallel plates has been measured in the three monatomic gases He, Ne, and Ar and the two diatomic gases N2 and HD over a broad range of mean free paths. These measurements are on a finite system, an apparatus where the sphere radius and the plate spacing are of the same order of magnitude. Behavior near the observed maximum of the thermal force is stressed. The translational heat flux characterizes the dependence of the thermal force on the gas. Comparison with existing theories is made for the three basic Knudsen regimes, and the data are compared to previous measurements of the thermal force on aerosols.
Cross-correlation functions for angular momentum and orientation
Authors: St. Pierre Anthony G. , Steele William A.
Published: February 2003

A general expression is derived that relates the generalized second moment of a rotational spectral band to an angular momentum−orientation time−correlation function for the ensemble of molecules. It is demonstrated that previous equations relating these spectral transforms to angular momentum correlation functions are recovered in the limit of short times. The extended diffusion models proposed by Gordon are used to calculate cross−correlation functions as a function of time, and the results are compared with several approximations, including the decoupling assumption widely used in spin−rotation relaxation. The cross correlations that appear in the spin−rotation problem are considered in detail. In contrast to the behavior of the spectral transform cross−correlation functions, it is explicitly demonstrated that the spin−rotation correlations for the extended diffusion models vanish after one collision.
Matrix reactions of alkaline earth metal atoms with oxygen molecules: Infrared spectra of the metal superoxide and metal dioxide species
Authors: Ault Bruce S. , Andrews Lester
Published: February 2003

Infrared spectra of the reaction products of oxygen molecules and alkaline earth metal atoms in argon and nitrogen matrices have been recorded. The ν2 interionic modes of barium, strontium, and calcium superoxides, MO2, have been located at 541, 474, and 498 cm−1, respectively, in nitrogen matrices. The metal oxide, MO, was isolated for each metal, and several bands of the metal oxide dimer were measured. Frequencies observed for the antisymmetric modes of (BaO)2 at 487 and 393 cm−1 indicate a D2h structure with an O−Ba−O angle of approximately 102°. Also, a band attributable to ν3 of a bent O−M−O metal dioxide species has been identified near 752 cm−1 for barium, 730 cm−1 for strontium, and 743 cm−1 for calcium, leading to a bond angle near 140° for all three species. A mechanism is proposed for the formation of the metal oxide from molecular oxygen, which includes MO2M and (MO)2 intermediates. It was also determined that nitrogen is much more effective as a matrix material in stabilizing ionic compounds than argon.
Nitrogen matrix reactions of alkaline earth metal atoms with ozone: Infrared spectra of the alkaline earth metal oxide molecules
Authors: Ault Bruce S. , Andrews Lester
Published: February 2003

The infrared spectra of the alkaline earth metal−ozone−nitrogen reaction mixtures were obtained, and the frequencies of the alkaline earth metal oxide molecules were measured in nitrogen matrices. These were found to be 613, 620, and 707 cm−1 for BaO, SrO, and CaO, respectively; these assignments were confirmed by oxygen−18 substitution. From the known gas phase fundamentals and 1Σ ground electronic states of SrO and BaO, and the anticipated matrix shifts, the ground state of CaO was determined to be 1Σ owing to near agreement between the known 1Σ CaO gas phase fundamental and the CaO nitrogen matrix frequency. Ozone was found to be an excellent precursor for oxygen atom abstraction reactions in nitrogen matrices, while nitrous oxide was considerably less productive. The pattern of reactivity of the alkaline earth metals was determined to be decreasing from barium to magnesium, in agreement with earlier researchers and the work presented in the preceding paper in this series.