bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-photon spectrum of phenylacetylene
Authors: Chia Lucille , Goodman Lionel
Published: February 2003

The two-photon (TP) Lb (B2)←A (A1) vapor fluorescence excitation spectrum of phenylacetylene has been obtained and analyzed. The spectrum is strongly Franck–Condon (FC) allowed (the experimentally determined value of δtotal is ∼7×10−49 cm4 s molecule−1 photon−1), with the principal FC mode being ν11 (analog of the ring breathing mode ν1 in benzene). Appreciable vibronic coupling character appears in the form of ν29 (analog of the benzene mode ν14). The TP spectrum is in striking contrast to the weakly allowed one-photon (OP) one, which is mainly allowed by vibronic coupling and resembles that of toluene. The TP spectrum reveals such pronounced delocalization of the π electrons over the ring and acetylene moieties that strong coupling theory (akin to that appropriate for aniline) is necessary to explain the intensity arising from mixing of the charge–transfer states (acetylene→ring* and ring→acetylene*) into the final and intermediate states. Perturbation theory and INDO/S calculations suggest that the dominant CT state in the TP intensity mechanism is acetylene→ring*. The largely forbidden character of the one-photon spectrum appears to arise from cancellation of Bb terms due to the two types of CT states in the OP intensity mechanism.
Magnetization modes in weakly coupled spin systems without magnetic equivalence
Authors: Brondeau J. , Canet D.
Published: February 2003

The concept of magnetization modes in longitudinal nuclear magnetic relaxation of spins 1/2 is reconsidered with regard to the total spin inversion operator Λ. It is shown that simplifications in the transition probability matrix can only occur through this operator: a two block separation is achieved provided that (i) relaxation takes place solely by intramolecular dipolar and random field interactions and (ii) the investigated spin system is weakly coupled. In the case of such a system involving n nonequivalent spins, a method is given for constructing 2n−1 antisymmetric (with respect to Λ) independent magnetization modes and 2n−1−1 symmetric independent magnetization modes from properly selected sums and differences of standard intensities relative to lines occupying symmetrical positions within each multiplet (a standard line intensity is defined as the difference between populations of the levels connected by the considered transition). The disconnected evolution matrices relevant to these two type of modes are easily set up from the initial transition probability matrix by means of transformation matrices deduced from the magnetization mode construction procedure. The effects of the inverting and observation rf pulses (selective, semiselective, and nonselective) are thoroughly studied. In particular, it is demonstrated that (i) symmetric modes can only be created by a selective pulse and (ii) the magnetization modes, as defined above, are directly observed by semiselective pulses or nonselective pulses of small flip angle. Otherwise mixing occurs.
The methyl iodide multiphoton ionization spectrum with intermediate resonance in the A-band region
Authors: Gedanken A. , Robin M. B. , Yafet Y.
Published: February 2003

The 5pπ→6p Rydberg excitations of methyl iodide-h3 and -d3 are observed using multiphoton ionization spectroscopy as two-photon resonances when excited in the 28 000–32 000 cm−1 region. The two-photon methyl iodide resonances abruptly cease at 32 000 cm−1, and at higher frequencies are replaced by resonances originating at the 2P1/2 and 2P3/2 levels of the iodine atom. A tentative assignment is also made for a two-photon resonance in the methyl radical in this region. The replacement of methyl iodide resonances with those of iodine and methyl fragments beginning at 32 000 cm−1 correlates directly with the onset of a one-photon intermediate resonance in the A band of methyl iodide. When pumping with light of frequency less than 16 000 cm−1, the 5pπ→6p excitations of methyl iodide are observed as four-photon resonances. However, on entering the A-band region via two-photon resonance beyond 32 000 cm−1, no fragment transitions appear. Instead, we observe the three-photon resonance to the (5pπ, 6s) state of the parent molecule. The different behavior is thought to be due to excitation to a directly dissociative component state (A1 or E) in the case of one-photon A-band resonance, and to a bound A2 state in the case of two-photon A-band resonance.
Photoionization of the neon–rare gas dimers near the neon 3s resonance transitions: A study of associative ionization half-reactions
Authors: Pratt S. T. , Dehmer P. M.
Published: February 2003

The relative photoionization cross sections of NeAr, NeKr, and NeXe are reported in the energy region of the first excited states of atomic neon, which includes the dipole-allowed 3s[3/2]01 and 3s′[1/2]01 states and the dipole-forbidden 3s[3/2]02 and 3s[1/2]00 states. All three molecular spectra show both a narrow band at approximately the position of the 3s′[1/2]01 atomic transition and a broader band blue shifted from this level. A similar, but much weaker, broad band is observed blue shifted from the 3s[3/2]01 atomic transition in NeAr. No molecular bands are observed near the dipole-forbidden atomic neon states. The broad molecular band blue shifted from the 3s′[1/2]01 transition in all of the dimer spectra results from excitation to the repulsive wall of NeRg*, followed by rapid autoionization to form NeRg+, similar to the associative ionization reaction Ne*+Rg→NeRg++e. The relative absorption cross sections for transitions to the discrete molecular state were calculated for each dimer using NeRg* excited state potential energy curves derived from metastable Ne*–rare gas scattering experiments. The relative photoionization cross sections were then estimated using the known energy dependence of the associative ionization cross sections, and the results are in good agreement with present observations.
Ion–dipole scattering: The differential cross section for the K+–CsCl system
Authors: Budenholzer F. E. , Gislason E. A. , Polak-Dingels P.
Published: February 2003

The total differential cross section has been measured for K+ scattered by CsCl over the range 10?Eϑ?2000 eV deg. Classical perturbation scattering theory calculations for a realistic intermolecular potential have also been carried out and agree well with the experiments. The results show that the scattering is dominated at small angles by the ion–dipole potential and at large angles by the spherically symmetric part of the repulsive potential. No rainbow scattering is observed.
Quantum effects on conformational equilibria
Authors: Stratt Richard M. , Desjardins Steven G.
Published: February 2003

Some relatively preliminary investigations are made into the nature of quantum effects on conformational equilibria with the aid of simple gas phase models and a semiclassical method. The case of ethane is first used to test the semiclassical method and to define the concepts of an effective rotational barrier and an effective zero point energy. Internal rotation in n-butane and some alkyl radicals is then examined. At liquid nitrogen temperatures, butane is predicted to have its trans/gauche equilibrium constant shifted relative to its classical value by an amount roughly equal to (and in the opposite direction from) the shift induced by a hypothetical hard sphere solvent. In the case of certain alkyl radicals, it is pointed out how the occurrence of quantum effects may actually help resolve the current disagreement between solution ESR experiments and theoretical predictions.
Infinite order sudden approximation for reactive scattering within classical mechanics. I. Theory
Authors: Jellinek Julius , Baer Michael
Published: February 2003

An infinite order sudden approximation for treating reactive scattering within classical mechanics is presented. It is shown that requiring the conservation of total energy in the subspace formed by applying this approximation is enough to define, for any given set of initial conditions, a unique trajectory which moves from the reagents to the products arrangement channel. A detailed description of how to apply this method for any three-atom reactive system is given.
Observation of O(1D) produced from photodissociation of HO2 at 193 and 248 nm
Authors: Lee L. C.
Published: February 2003

The photodissociation processes of HO2 were investigated, using ArF and KrF laser photons. HO2 was produced by a three-body reaction: H+O2+He→HO2+He. The primary products from HO2 photodissociation are O(1D)+OH(X 2Π). O(1D) then transfers its energy to O2 to form O2(b 1Σ+g), giving emission at 7620 Å. This O2 emission band is identified both from its wavelength and its rates of quenching by various gases. The O(1D) production is confirmed by its quenching rates with N2 and Xe. The cross section for the production of O(1D) at 248 nm is larger than that at 193 nm by a factor of 3. The electronic states and photodissociation processes of HO2 that lead to the production of O(1D)+OH(X 2Π) are discussed.
Infrared multiple photon absorption in a static electric field
Authors: Noid D. W. , Stine J. R.
Published: February 2003

The infrared multiple photon process in the presence of a static electric field has been examined theoretically by employing a classical model. It is found that even a relatively small electric field (in comparison to the electric field of the laser) can significantly increase the fraction of molecules in an ensemble that absorbs energy from the laser. This effect is most prominent at low laser powers. Although these results are for hydrogen fluoride they are in qualitative agreement with recent experiments for CF2HC1.
UV short-pulse fragmentation of isotopically labeled acetylene: Studies of emission with subnanosecond resolution
Authors: Craig B. B. , Faust W. L. , Goldberg L. S. , Weiss R. G.
Published: February 2003

We have studied fragment emission in the spectral range 200 to 900 nm, following photolysis of acetylene (1–10 Torr) by short ultraviolet pulses (25 ps, 266 nm, 2.5–12 mJ). The dominant component of emission is the C2 d 3Πg→a 3Πu Swan system. Weak singlet C2 emission is observed. CH is also observed, in the A 2Δ→X 2Π and C 2Σ+→X 2Π systems. With isotopic labeling and with streak camera recording, we have demonstrated distinct unimolecular and intermolecular processes yielding C2 d 3Πg. The unimolecular channel proceeds with risetime τ1 = 215±15 ps, while the pressure-dependent intermolecular component develops over a period of nanoseconds. We discuss the underlying kinetics and the mechanistic implications of the 25 ps excitation.
Vibrational frequencies of the ground and lowest excited triplet state of thioformaldehyde h2, d2, and h
d
Authors: Goddard J. D. , Clouthier D. J.
Published: February 2003

Harmonic vibrational frequencies calculated by ab initio methods with double zeta plus polarization basis sets are reported for the ? and ? states of H2CS, D2CS, and HDCS. These frequencies are scaled to predict as yet unknown experimental values. For the triplet state, vibrational energy levels are given for a double minimum potential in the out-of-plane bending coordinate within a rigid bender approximation.
Application of quasidegenerate many-body perturbation theory to the calculation of molecular excited valence state negative ion Feshbach resonances
Authors: Sun Hosung , Freed Karl F.
Published: February 2003

Quasidegenerate many-body perturbation theory (QDMBPT) is shown to generate a good method for representing the abstract Feshbach projectors, required in the evaluation of the energies of negative ion resonances, provided the valence space is sufficiently large to produce a good zeroth order description of these valence state resonances. Our recent advances in extending third order QDMBPT to large valence spaces are utilized to calculate potential curves and spectroscopic constants for the lowest four excited states of CH− and the vertical electron affinities to excited states of NH− and OH−. The experimentally assigned a 1Δ excited state of CH− is confirmed by the calculations, and a few more bound resonant molecular states are predicted. A simple one electron molecular orbital model is extracted from the large scale calculations to describe the zeroth order electronic structure of the excited states of CH−, NH−, and OH−.
Le Châtelier reciprocal relations
Authors: Gilmore Robert
Published: February 2003

The Le Châtelier matrix Lαβ describes the response to perturbations of a system in thermodynamic equilibrium. The diagonal elements of this matrix embody Le Châtelier’s Principle, and the off-diagonal elements describe the response of one pair of conjugate thermodynamic variables to a perturbation involving another pair. The symmetry of this matrix Lαβ = Lβα gives rise to the Le Châtelier reciprocal relations.
Dipole sums on a surface with fractional coverage: Application to enhanced Raman spectra of halide ions on silver
Authors: Nichols Henry , Hexter Robert M.
Published: February 2003

A procedure is developed for calculating the dipole sum for a lattice of sites in which not all sites are occupied. The summation accounts for molecules far from the origin as well as for statistical fluctuations in site occupancy near the origin. Vibrational frequencies are then calculated at various coverage fractions for chloride ions adsorbed on silver (100) in which a dipolar interaction potential is used to couple individual Cl–Ag vibrational modes. One vibrational frequency is calculated for each particular distribution of Cl− ions. This process is repeated for a large number of distributions having the same coverage fraction and the relative occurrence of the frequencies are then displayed as a histogram. Calculated histograms at low coverage exhibit structural features characteristic of the number of nearest neighbors to the origin ion. These structural features agree qualitatively with recent experimental observations.
Statistical rate theory of interfacial transport. I. Theoretical development
Authors: Ward C. A. , Findlay R. D. , Rizk M.
Published: February 2003

After assuming that the transport of molecules between phases at thermal equilibrium results primarily from single molecular events, the expression for the rate of molecular transport between phases is developed by using a first order perturbation analysis of the Schrödinger equation and the Boltzmann definition of entropy. This leads to an Einstein-type relation with the constant of proportionality being the average rate of exchange between microscopic states of different molecular distributions. A hypothesis is introduced which leads to the conclusion that this exchange rate is unchanged as the system moves through the molecular distributions leading to equilibrium, and to it being equal to the molecular rate of exchange between phases in the final equilibrium state. This allows a complete expression for the rate of molecular transport between phases to be developed. The validity of the hypothesis can be examined by comparing the predictions that follow from the derived rate expressions with the available experimental data. This comparison is reported in subsequent parts of this work.
Molecular beam reaction Na3+SF6. A triple electron-jump mechanism
Authors: Rothe Erhard W. , Sinha D. , Ranjbar F.
Published: February 2003

In a beam-gas experiment, an unseeded supersonic sodium beams was made to collide with SF6 gas. The ionic products of single collisions of NA6 are examined. (AIP)
11B chemical shifts and quadrupole coupling constants in the alkaline–earth hexaborides
Authors: Sears R. E. J.
Published: February 2003

The 11B chemical shifts of CaB6, BaB6 and SrB6 are reported. Theses chemical shifts characterizes the above materials as semiconductors. (AIP)
Comment on ‘‘Analysis of the spin-Hamiltonian parameters for Mn2+ in orthorhombic and axial symmetry sites of the alkali chlorides’’
Authors: Edgar A. , Siegel E.
Published: February 2003

The contrast between the superposition model intrinsic parameter b2 of Mn2+ ions with Cl− ligands, as determined from Mn2+-doped alkali chlorides and from axial chlorides, is discussed. It is concluded that the discrepancy arises from the restricted nature of the lattice distortion model used in the analysis of the alkali chloride data. (AIP)
Collision effects in liquid CCl4: A molecular dynamics study
Authors: Samios J. , Dorfmüller Th.
Published: February 2003

The trajectories of a Lennard-Jones system with the 18-6 potential reproducing the thermodynamic properties of liquid CCl4 have been obtained by a molecular dynamics computer simulation at liquid densities. From these, the mean time between binary molecular collisions, the mean duration of a collision, and the force autocorrelation functions were calculated. The results display a marked discrepancy with respect to collision times estimated by cell model considerations. The result were shown to yield a consistent picture of the molecular dynamics of this system. The periodic component of the force autocorrelation function is shown to be connected to the maximum of the collision induced absorption specturm.
Molecular dynamics simulation of liquid anisotropy. II. Rise and fall transients on a picosecond time scale
Authors: Evans M. W.
Published: February 2003

The behavior of a liquid sample subjected to an external torque of picosecond duration is investigated by molecular dynamics simulation, using 108 C2v triatomics interacting with a 3×3 site–site Lennard-Jones potential. The resulting rise and fall transients are computed and their behavior is such as to fall on Langevin’s functions from the linear response region to saturation, i.e., when the energetic perturbation (E) is less than kT to the point at which it is about 10 times kT. The breakdown of the fluctuation–dissipation theorem is investigated at the point E/kT = 12, where the normalized fall transients no longer decay as the equilibrium autocorrelation functions but considerably faster. The simulation does not support the conventional view of rise and fall transients, which is based on Debye’s theory of rotational diffusion.
Migration of excess electrons in high density supercritical ammonia
Authors: Krebs P. , Heintze M.
Published: February 2003

Excess electron mobility measurements in supercritical ammonia, which show a transition from the quasifree to the localized electron state in the density range from about 1×1020 to 3×1021 molecules/cm3 have been extended to densities comparable to that of liquid ammonia (n≲1.5×1022 molecules/cm3; 410⩽T⩽460 K). The drift mobility reaches a minimum of μ = 3.2×10−2 cm2 V−1 s−1 at about n≃4.7×1021 molecules/cm3. By further increasing the fluid density, the mobility increases again by a factor of ∼ 5 approaching a plateau (maximum) with μ≃1.65×10−1 cm2 V−1 s−1 at n≃1.5×1022 molecules/cm3 (T = 410 K). The temperature dependence of the electron mobility is strongest at the density where the minimum occurs. At higher densities, the temperature dependence is drastically reduced. From these facts and other available experimental results, it is concluded that a ’’trap-controlled hopping’’ transport of the electron dominates at ammonia vapor densities n<nc (nc = 8.23×1021 molecules/cm3 is the critical density of ammonia). At higher densities (n≳nc), the observed mobilities can be interpreted to be due to a weakly activated small-polaron hopping which is influenced by the molecular dynamics in supercritical ammonia. An ionlike motion of the localized (solvated) electron as often supposed can be excluded to be an essential transport channel in supercritical ammonia.
Discovery and Raman study of an alkane phase similar to the high pressure intermediate phase of polyethylene
Authors: Fiedler K. H. , Wunder S. L. , Priest R. G. , Schnur J. M.
Published: February 2003

The behavior of tetracontane (C40H82) was studied as a function of pressure by optical microscopy and by Raman spectroscopy. A phase similar to the high pressure intermediate phase of polyethylene was found to interpose between the solid and liquid phases. This new phase exists for pressure up to about 1.4 kbar. The temperature range of stability of the new phase is less than 0.5 K at atmospheric pressure and greater than 1 K at 1 kbar. The properties of the new phase indicate that the 40 carbon alkane may be a suitable model system for the high pressure behavior of polyethylene.
Nonresonant coherent motion of triplet excitons in molecular pairs
Authors: Reineker P. , Richardt D. , Schmid U.
Published: February 2003

ESR properties of coherent triplet excitons moving between the differently oriented molecules of a nonresonant AB pair in a crystal of the anthracene type are investigated. The model Hamiltonian contains the different electronic excitation energies of the molecular pair, the electronic transfer integral, the Zeeman energy of the triplet spin in an external magnetic field, and the fine structure interactions at the two differently oriented molecules of the pair. As a function of the strength and orientation of the external magnetic field energy eigenvalues, ESR transition energies and transition probabilities for microwave fields orthogonal and parallel to the static magnetic field are calculated analytically in the high field case. The relations of the theoretical results to experimentally observable quantities are discussed.
An ESR study of an x-ray irradiated Ph3AsF2 single crystal
Authors: Geoffroy M. , Hwang J. , Llinares A.
Published: February 2003

X-irradiation of Ph3AsF2 single crystals produces a radical exhibiting hyperfine coupling with one 75As nucleus and two 19F nuclei. The three hyperfine tensors are determined and show that the unpaired electron is delocalized in 4s (17%) and 4p (55%) arsenic orbitals and in a 2p (∼12%) orbital of each fluorine atom. The ESR parameters are consistent with either Ph2AsF2 or Ph3AsF−2 but theoretical estimates of the geometry of these two species together with the experimental angle of 135° between the principal axes of the two fluorine hyperfine coupling tensors are more in accord with the structure Ph3AsF−2.
Vacuum UV spectra of photolyzed C2H2 in solid Ar at 8 K
Authors: Chang Kok Wai , Graham W. R. M.
Published: February 2003

The results are reported from a study of absorption spectra in the range 1200 to 3000 Å of the products from the photolysis of acetylene trapped in Ar matrices at ∼8 K. The principal features of the observed spectra are two-band systems appearing at 1660 and 1950 Å. The band system at 1660 Å is identified as the 3R-X Rydberg transition of the C4H2 diacetylene molecule. The vibrational structure exhibits vibrations in the stretching mode ν′2∼2020 cm−1 and overtones of the C–H bending mode 2ν′6∼550 cm−1 in the (σ2u π4u π3g 3pσ) 1Πu Rydberg state. These frequencies are shifted to 1930 and 400 cm−1, respectively, on deuteration of the parent molecule. It is proposed that the most likely assignment for the band system at 1946 Å is the C2H ethynyl radical and that the principal progression in ∼2175 cm−1 and shorter progressions in ∼630 cm−1 correspond to the ν′3 C≡C stretching mode, and even multiples 2ν′2′ of the deformation mode. These vibrational assignments are supported by the effects of deuterating the C2H2 parent: ν′3 remains essentially unchanged at ∼2180 cm−1 while 2ν′2 shifts to ∼520 cm−1. On the basis of the extended progression in the C≡C stretching mode, the increase in the ν′3 mode from 1848 cm−1 in the (4σ2 1π4 5σ)X 2Σ+ ground state to 2175 cm−1 in the excited state, and the expected relative strength of the 2Σ+ ← X 2Σ+ transition, it is argued from simple molecular orbital considerations that the upper state of the 1946 Å transition is (4σ1π4 5σ2)2Σ+, with C2H retaining its linear geometry.
The Ewald–Oseen theorem in the x-ray frequency region: A microscopic analysis
Authors: Oxtoby David W. , Novak Frank , Rice Stuart A.
Published: February 2003

We address the problem of the proper microscopic description of the propagation of x rays in condensed matter. In the optical region of the spectrum, Fresnel boundary conditions may be used; these boundary conditions can be regarded as a consequence of the Ewald–Oseen extinction theorem. This theorem is, however, generally derived in the dipole limit, where the wavelength of the light is large compared with the dimensions of the scatterers. Since x-ray wavelengths are comparable to atomic dimensions, it is necessary to re-examine the validity of the Ewald–Oseen theorem and the consequent boundary conditions for x-ray optics. In this paper, we demonstrate that the Ewald–Oseen extinction theorem may also be derived in the limit where the radiation frequency is high compared with atomic transition frequencies, a condition which holds for x-ray radiation. This result therefore justifies the use of macroscopic dielectric theory with Fresnel boundary conditions to describe the x-ray reflectivity of liquid surfaces.
Theoretical treatment of the spontaneous predissociation of Br2, B 3Π(0u+)
Authors: Clyne Michael A. A. , Heaven Michael C. , Tellinghuisen Joel
Published: February 2003

The spontaneous heterogeneous predissociation of the B 3Π(0u+) state of bromine is discussed. Theoretical treatment of the heterogeneous interaction is presented, with emphasis placed on the role of the Franck–Condon principle in predissociation. Numerically generated vibrational wave functions are used to calculate predissociation probabilities for the B state. From these calculations the parameters of the repulsive potential responsible for the predissociation are determined, and this potential curve is identified as the 1Π(1u) state. Good agreement with experimental results was obtained with a 1Πu potential of the form U(R) = 1.594×104/R9.384. This crosses the B state curve between v′ = 4 and 5, which is two levels higher than indicated in previous work. The new 1Πu curve is compatible with other theoretical and experimental results.
Wigner distribution trajectory method for collision induced dissociation
Authors: Gray Joni C. , Truhlar Donald G.
Published: February 2003

A new way to calculate probabilities for collision induced dissociation is proposed and tested. The method involves classical trajectories whose initial conditions are determined by a Wigner transform of the initial quantum state. The results are compared to accurate quantal calculations and are found to be more accurate than those calculated by the standard quasiclassical trajectory method, especially in the threshold region.
Laser fluorescence study of MgO formed from Mg(3s3p 3P)+O2 and N2O under single-collision conditions
Authors: Dagdigian Paul J.
Published: February 2003

The reactions of metastable Mg(3s3p 3P0) atoms with O2 and N2O have been investigated under single-collision conditions by the laser-fluorescence technique. The MgO reaction product has been detected in the ground X 1Σ+ and low-lying a 3Π and A′ 1Π electronic states. The relative populations in the first six vibrational levels of X 1Σ+ and a rotational temperature characterizing the rotational distribution in v = 0 have been determined from B–X excitation spectra. The relative rates of formation for the X 1Σ+, a 3Π, and A 1Π states have been obtained from comparison of integrated fluorescence intensities in the B–X, d–a, and D–A band systems. For both reactions, little of the reaction exoergicity is channeled into MgO electronic or vibration-rotation energy. The dynamics of these reactions are disussed and are contrasted with those of the homologous Ca and Sr reactions.
Numerical Hartree–Fock–Slater calculations on diatomic molecules
Authors: Becke A. D.
Published: February 2003

A completely numerical method is presented for the calculation of Hartree–Fock–Slater wave functions in diatomic systems. The method is numerical in the sense that no LCAO basis sets are employed. Muffin tin or other cellular approximations are also avoided. All molecular functions are defined on a two-dimensional discrete mesh in prolate spheroidal coordinate space. The method is mathematically very simple, and numerical accuracy is easily controlled by changing the number of mesh points. Calculations on the molecules B2, C2, N2, CO, O2, and F2 are reported, and we compare dissociation energies, bond lengths, vibrational frequencies, and charge moments with recent LCAO results and with experiment. These calculations indicate that the method works very nicely for the molecules considered, and also that the Hartree–Fock–Slater theory describes molecular systems remarkably well.
Theoretical characterization of negative ions. Calculation of the electron affinities of carbon, oxygen, and fluorine
Authors: Botch Beatrice H. , Dunning Thom. H.
Published: February 2003

The electron affinities of carbon, oxygen, and fluorine have been calculated using a compact MCSCF wave function with a [4s, 4p, 3d] Gaussian basis set. This wave function describes radial correlation of the 2p electrons which is found to have a large differential effect between atom and anion, and also includes the (2s2→2p2) near-degeneracy effect. Radial correlation of the 2p electrons increases the calculated electron affinity by as much as 1.2 eV over the Hartree–Fock value. An orbital model is discussed, which ascribes this effect to the diffuse nature of the orbital occupied by the (Z+1)st electron of the anion. Configuration interaction calculations based upon the MCSCF wave function result in electron affinities comparable in magnitude to the large basis set calculations of Yoshimine and Sasaki [Phys. Rev. A 9, 17 (1974)]. Use of the MCSCF wave function as the reference function for the CI calculations efficiently includes the effect of the triple and quadrupole excitations (relative to the HF wave function) found to be important by Yoshimine and Sasaki.
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o calculation of molecular spin-orbit coupling constants using a universal even-tempered basis set of exponential functions
Authors: Cooper David L. , Wilson Stephen
Published: February 2003

A universal basis set of even-tempered exponential functions, consisting of nine 1s, six 2p, and three 3d functions on each atom, is used to perform matrix Hartree–Fock calculations for a number of diatomic molecules and molecular ions. Total energies and spin-orbit coupling constants are reported for the lowest 2Π states of the OH, CH, HeNe+, BH+, LiO, and CO+ molecules and comparison is made with results obtained using basis sets specifically constructed for these systems.
Ab initio vibrational-rotational spectrum of potassium cyanide: KCN. II. Large amplitude motions and rovibrational coupling
Authors: Tennyson Jonathan , van der Avoird Ad
Published: February 2003

The ten lowest vibrational states of KCN have been obtained ab initio using the close-coupling method of Le Roy and Van Kranendonk. Fundamental vibrations are found to lie at 114.7 (bending) and 293.0 cm−1 (K–CN stretch). Comparison is made with previous results obtained using the Watson Hamiltonian for nonlinear molecules and separating the rotations and vibrations. It is found that this model breaks down for the higher librations near and above the barrier to inversion. For these states the average geometry of the KCN molecule shifts towards the linear isocyanide (KNC) structure. Properties of the vibrational states and the rotational spectrum are analyzed.
Higher excited vibrational states of polyatomic molecules
Authors: Abram Izo , de Martino Antonello , Frey Robert
Published: February 2003

The overtone bands in the vibrational specturm of methane can be accounted for on the same footing as the fundamentals by use of a simple vibrational Hamiltonian and the application of conventional symmetry considerations and radiative selection rules. This approach gives a rough map of all stretch vibrational states up to ∼24 000 cm−1, by fitting the states of the appropriate symmetry to the observed overtone spectrum. All vibrational states are described in terms of anharmonic symmetrized internal coordinate (SIC) states which are very close to the eigenstates of the Hamiltonian. The one-quantum SIC states are identical to the normal modes while some multiquantum SICs present features analogous to the ’’local modes’’ conventionally used in the interpretation of overtone spectra. The discrete structure of the vibrational state–space obtained from this treatment is in conflict with the symmetryless quasicontinuous structure of state–space postulated to explain IR multiphoton absorption in polyatomic molecules. At the same time, it suggests that this process might take place mainly through an equidistant ladder of states, as is found in molecules with several identical bonds.
Infrared spectra of the hydrogen-bonded pi complex C2H4–HF in solid argon
Authors: Andrews Lester , Johnson Gary L. , Kelsall Benuel J.
Published: February 2003

The hydrogen-bonded pi complex C2H4--HF has been prepared by codeposition of C2H4 and HF with excess argon at 12 K. Isotopic substitution in both submolecules identified the complex and provided assignments to νs at 3731 cm−1, νl at 424 and 396 cm−1, and ν7c at 973 cm−1. The observation of a substantial perturbation on the out-of-plane bending mode of the C2H4 submolecule in the complex without a perturbation on the in-plane bending mode indicates that H–F is bonded to the pi electron system perpendicular to the plane of the molecule. The positions of νs and νl indicate a hydrogen bond of intermediate strength and the splitting in νl is characteristic of the anisotropy in the hydrogen bonding interaction with a π system.
A spectroscopic study of CH3F isolated in rare gas matrices
Authors: Apkarian V. A. , Weitz Eric
Published: February 2003

The hindered rotational/librational motion of symmetric top molecules isolated at substitutional sites in rare gas matrices is considered. It is shown that the dominant perturbation of the free rotor states is due to the reorientational forces of the cage atoms. An electrostatic model for symmetric top molecules trapped in octahedral fields that hinder the molecular tumbling motion is developed. The model is successfully applied to the qualitative interpretation of experimentally observed line shapes, linewidths, and spectral shifts of the ν3 mode of CH3F isolated in rare gas matrices. Exact quantitative predictions are not made due to the many model parameters, however, the spectra are compatible with a minimum barrier to tumbling of approximately 80 cm−1 and are compatible with free spinning motion. A complete analysis of the spectra requires the consideration of dynamic perturbations as well as possible sources of dynamic line broadening. The role of rotation-translation coupling is also explored. Experimental studies have provided a direct measure of the ν3 intergrated absorption coefficient from which a radiative lifetime of τ = 82±10 ms is obtained. Direct observation of 2ν3 yields the first anharmonicity constant ωeχe = 8.05±0.05 cm−1. Studies of matrices ’’capped’’ with pure rare gas have provided an insight as to the dynamics of aggregation with the conclusion that aggregation occurs at surfaces and grain boundaries. An absorption at 1034 cm−1 is assigned to molecules trapped in these locations. Implications with respect to energy transfer processes in these systems are also discussed.
Determination of lifetimes for the ν = 9, 12–16 vibrational levels of the a′ 3Σ+ state of CO and their identification from optical emission functions
Authors: Shadfar S. , Lorentz S. R. , Paske W. C. , Golden D. E.
Published: February 2003

The lifetimes for the a′ 3Σ+(ν = 9, 12–16) states of CO have been measured using pulsed low energy electron impact excitation and delayed coincidence detection after identification of these states was established from their optical emission function thresholds. The lifetimes determined for the ν = 12–16 vibrational levels are consistent with a v3 extrapolation of previous lifetime measurements of the lower vibrational levels. The collisional quenching cross sections were calculated from the pressure dependence of the measured lifetimes.
Configurations and dynamics of real chains. III. The excluded volume effect on inner segments
Authors: Allegra Giuseppe , Ganazzoli Fabio
Published: February 2003

The expansion effect experienced by the inner segments within an infinitely long polyethylene chain is investigated. Following Flory [J. Chem. Phys. 17, 303 (1949)], the chain free energy is expressed as the sum of an elastic and an excluded volume component, within the Gaussian approximation. Introducing the Fourier coordinates l̃(q) = ∑kl(k)eiqk [l(k) = general bond vector] allows us to formulate the latter component as a sum of pairwise contributions where the appropriate expansion is considered for each atom pair, unlike the mean-field approaches. Minimization of the free energy over the expansion factor α̃2(q) of the squared amplitude 〈‖l̃(q)‖2〉 produces an integral equation that eventually gives α2(k), i.e., the expansion of the square distance between kth neighboring atoms. In addition to asymptotic expressions for very small and very large expansions, numerical results are given in a wide range of segment lengths for different values of the excluded volume parameter and of the smallest number of bonds between two atoms in contact. It turns out that, even at comparatively long range, the chain expansion is sensitive to the latter parameter, which effectively embodies the detailed pattern of short-range interactions. On the average, the inner expansion factor α2(k) is more than twice larger than for the terminal atoms of a (k+1)-atom chain, using the classical Flory equation α5−α3 = C√k and the value of C suggested by Fixman on the basis of exact calculations [J. Chem. Phys. 23, 1656 (1955)]. Asymptotic analysis shows that α2(k)∝k1/5⋅(logk)2/5… for large k, thus reobtaining Flory’s well-known power law except for ’’weak’’ logarithmic factors. In the free-draining case, the half-peak time t1/2 of the incoherent dynamic structure factor scales as Q−11/3 [Q = 4π sin(θ/2)/λ] in the long-time limit, as already predicted, although different exponents appear at intermediate times.
The angular and velocity distributions of NO scattered from the Pt(111) crystal surface
Authors: Guthrie W. L. , Lin T.-H. , Ceyer S. T. , Somorjai G. A.
Published: February 2003

The angular and velocity distributions of NO scattered from the Pt(111) surface have been measured for a crystal temperature range of 475–1200 K and for an incident beam energy range of 265–1390 K. Both angular and velocity distributions confirm the coexistence of inelastic and trapping-desorption scattering processes. For beams with a kinetic energy of 〈KE〉/2k equal to 265 and 615 K, the molecules scattered into angles near the surface normal were found to be equilibrated with the surface for Ts between 475 and 900 K.
Reactions of homonuclear diatomic ions with metal surfaces. I. Model for X+2 beam–surface reactions in the low kinetic energy-near threshold region
Authors: Darko Theophilus , Baldwin David A. , Shamir Noah , Rabalais J. Wayne , Hochmann Petr
Published: February 2003

A simple quantum mechanical model has been developed to describe chemical reactions of low energy Ek (≲ 30 eV) beams of diatomic homonuclear cations X+2 with metal surfaces M to produce binary compounds MαXβ. The overall reaction is simulated by four elementary steps: (1) neutralization of the incoming X+2 by resonance and/or Auger electron transfer from M; (2) impact dissociation of X2;(3) de-excitation and thermalization of X; and (4) chemical reaction between X and M. The neutralization process is treated by a simplified quantum mechanical procedure involving resonance and Auger transition probabilities and Franck–Condon and Hönl–London factors for X+2 transitions to the ground and excited electronic states and rovibronic levels of X2. The probability of dissociation from a given rovibronic level is modeled through use of a function developed for decomposition of gas phase polyatomic molecules. For the slow ions treated herein, the model considers that de-excitation of nascent N atoms at the surface is fast and that thermalization occurs in the outer surface layers. The chemical reaction proper is governed by the reaction cross section σr between X and M. Model calculations using free-electron and modified free-electron bands for the metal and various manifolds of electronic and vibronic states for X+2 and X2 are presented in order to illustrate the sensitivity of the model to individual steps in the mechanism.
Reactions of homonuclear diatomic ions with metal surfaces. II. Nitridation of Al, Cu, Mo, and Ni by N+2 beams in the low kinetic energy-near threshold region
Authors: Shamir Noah , Baldwin David A. , Darko Theophilus , Rabalais J. Wayne , Hochmann Petr
Published: February 2003

Nitridation of Al, Cu, Mo, and Ni to produce very thin nitride surface films is induced by 0–50 eV mass selected N+2 beams. The efficiency of nitridation, monitored by the KL2L2 nitrogen Auger electron signal, exhibits a large kinetic energy Ek dependence below ∼30 eV. The threshold impact energy for nitridation varies for the different metals; it is nominally zero for Al and Ni, ∼4 eV for Mo, probably ≳0 for Cu, and does not occur at any energy below 200 eV for Ag. That the metal electronic structure is a critical parameter is evidenced by the facts that different ion doses are required to produce similar nitride signal levels on various metals and that plots of nitride intensity versus Ek exhibit structure or at least different shapes in the low Ek region. A simple quantum mechanical model [J. Chem. Phys. 76, 6408 (1982)] is used to simulate the nitridation reaction by delineation into four elementary steps: (1) neutralization of incoming N+2, (2) decomposition of N2 into 2N, (3) de-excitation and thermalization of N, and (4) chemical reaction proper. Least-squares fitting of the theoretically predicted dissociation efficiency versus Ek to experimental data provides estimates of the fraction of N+2 neutralizations occuring by resonance or Auger neutralization processes.
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o correlated potential energy surfaces for monomeric sodium and potassium cyanides
Authors: Marsden C. J.
Published: February 2003

A b initio molecular orbital calculations using large Gaussian basis sets have been performed for NaCN and KCN with full geometry optimization. Potential energy surface obtained are sensitive to the quality of polarization functions used on C and N. A bent structure, as found experimentally for KCN, is the lowest energy geometry for both NaCN and KCN, with the isocyanide form only 5–6 kJ mol−1 higher at the SCF level. Inclusion of electron correlation using third-order perturbation theory leads to a substantial relative destabilization of the isocyanide form for the both NaCN and KCN, and improves agreement between observed and theoretical geometries for KCN.
Electronic excitations of magnesium tetraphenylporphyrin in supersonic expansions
Authors: Even Uzi , Magen Yaacov , Jortner Joshua , Levanon Haim
Published: February 2003

In this paper we report the results of our experimental study of the fluorescence excitation spectrum of Mg tetraphenylporphyrin (MgTPP) seeded in supersonic expansions of He. The spectra of the ’’isolated,’’ internally cold, bare MgTPP molecule reveal a well-resolved vibrational structure for the S0→S1 transition (the Q band), as well as for the S0→S2 transition (the Soret, B band). The vibrational structure in the S1 state reveals low-frequency, large amplitude motion, giving rise to two distinct even-parity vibrational progressions, with frequencies ω1 = 14.5 cm−1 and ω2 = 24.5 cm−1. Higher vibrational excitations in the S1 manifold of MgTPP are in good agreement with the fundamental frequencies of chlorophyll a. The spectroscopy of the Soret band of MgTPP provided information regarding the dynamics of interstate radiationless relaxation of low-lying vibrational excitations of the S2 state, which decay on the ps time scale.
Electrohydrodynamic instabilities observed in a nematic phase under oblique boundary conditions
Authors: Igner Dan , Freed Jack H.
Published: February 2003

Electrohydrodynamic instabilities in the nematic phase of Merck ’’Phase V’’ with oblique boundary conditions were optically observed with a polarizing microscope in 25–100 μm ’’sandwich’’ cells. Oblique anchoring of the nematic was achieved by oblique evaporation of SiO on the plates. Two types of cells were used having the respective in-plane projection of the direction of evaporation on the two plates either parallel (p-type cells), or antiparallel (a-type cells). The low voltage dc instability observed for the p-type cells forms in an almost regular hexagonal pattern. By gradually increasing the voltage, the dc instability observed for the a-type cells forms at first as flows which originate at order disturbances created at imperfections in the SiO coating. Voltage increase causes these flows to detach themselves from the places of the imperfections and move solitarily. The moving flows are associated with what appears to be moving tilt inversion deformations (of splay-bend type) extending from the central part of the flow to some distance from it. When the voltage is further increased, a repeated process of replication of the flows, occurring on the associated tilt inversion deformations, leads to the creation of a periodic grid of moving flows. Other observed types of static and dynamic patterns under ac and dc excitation are reported, in particular: different types of cross rolls (ac conduction regime); variations of a pattern of what appears to be walls associated with flows, exhibiting an approximate wave number dependence on the electric field k∼E; a striped pattern associated with what appears to be twist walls and the propagating interference patterns associated with their oscillations; a toroidal flow (sometimes associated with closed inversion walls) which creates and caries along closed nematic threads (dc regime); a polygonal grid of turbulent flows (dc regime); a flow pattern correlated with the movement of the moving chevron pattern; a cellular fast turn-off pattern related to the chevron pattern. This cellular pattern appears at first as moving snakelike regions in the chevron pattern which are bordered by disclination lines. Some features of dark, spotlike figures appearing on the chevron pattern are described. Preliminary interpretations of some of the observations are offered.
Dynamics of diffusion-controlled reactions among stationary sinks: Scaling expansion approach
Authors: Tokuyama Michio , Cukier Robert I.
Published: February 2003

Diffusion-controlled reactions in a nondilute sink system are rigorously studied with the aid of a scaling expansion method. A space-time coarse graining is carried out in a manner consistent with an expansion in sink concentration to obtain macroscopic transport equations from microscopic equations. It is shown that, beyond the lowest order in sink concentration, the macroscopic transport equation for the reaction–diffusion process cannot be written in a conventional local form in space and time since a nonlocal contribution in space becomes important. Properties of the fluctuations around the macroscopic motion are also explicitly explored, and they are shown to be small in comparison with the macroscopic motion for three-dimensional systems with an appropriate choice for the size of a sink radius and obey a Gaussian process. An absorption process whose characteristic length is much longer than that of the reaction–diffusion process is also investigated, and a local damping equation in space and time is derived.
Lattice dynamics and phase transition of acetylene
Authors: Gamba Z. , Bonadeo H.
Published: February 2003

The statical properties and lattice dynamics of the two known phases of crystalline acetylene have been studied, using a model which takes account of electrostatical intermolecular as well as atom–atom interactions. Good agreement with observed statical and dynamical properties of both phases has been achieved. The calculated phonon dispersion curves and group symmetry considerations led us to the identification of the phonons in both phases which are possibly associated with the structural phase transition.