bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional quantum dynamics of H2O and HOD photodissociation
Authors: Segev Eli , Shapiro Moshe
Published: February 2003

Three-dimensional quantum mechanical calculations of the VUV photodissociation of H2O and HOD on realistic potential surfaces are presented. The dynamical equations in the ground and excited states are solved by a coupled channels expansion using the artificial channel method. The photoabsorption spectrum in the 1360–1290 Å range is computed. A progression of ‘‘Feshbach-type’’ rotational resonances, whose positions coincide very nicely with the well-known diffuse bands of water is obtained. A bimodal rotational state distribution of the OH(2Σ) photofragment is shown to exist. It is a result of an interplay between the direct process (giving rise to an inverted ‘‘abnormal’’ distribution) and a compound process (resulting in a substantial contribution of a thermal-like component). The branching ratio for OH/OD production is shown to be a sensitive function of photon energy, as are the OH versus OD rotational state distributions. General implications concerning our ability to theoretically analyze and predict the dynamical behavior of the three-body system, as a result of this calculation are discussed.
Calculated long-range potential-energy curves for the 23 molecular states of I2
Authors: Saute M. , Aubert-Frécon M.
Published: February 2003

The interaction energy is calculated for the 23 molecular states of I2 as a sum of the quadrupolar electrostatic and dispersion energy terms in the multipole expansion for values of the internuclear separation R larger than 7 Å. For each state the potential energy curve and the multipolar expansion coefficients C5 and C6 are displayed. Results in fair agreement with experimental potential energy curves are obtained for the X O+g and the B O+u states. The relative importance of electrostatic and dispersion energy terms is quoted, as well as the attractive or repulsive feature of each state. The determination of the wave functions is also considered.
Laser photofragment spectroscopy of N+2: Evidence for predissociation of B 2Σ+u
Authors: Helm H. , Cosby P. C.
Published: February 2003

Band progressions of a Σ←Σ system showing predissociation in N+2 are observed using a coaxial fast-ion-beam-laser-photofragment spectrometer. The most consistent interpretation of these spectra is in terms of absorptions in the first negative system of N+2. Thus, the bands originate in five vibrational levels (v′′=32–36) of N+2 X 2Σ+g and populate quasibound states of at least three vibrational levels (v′′≳45) in the B 2Σ+u state which lie above the dissociation limit N+(3P0)+N(4S0). Measurements of the photofragment kinetic energies show that predissociation to the least exothermic fine-structure limit is favored.
The thermodynamic properties of diatomic molecules at elevated temperatures: Role of continuum and metastable states
Authors: Mies Frederick H. , Julienne Paul S.
Published: February 2003

A complete quantum formulation of diatomic partition functions uses the energy variation of the elastic scattering phase shift to represent the phase space associated with the molecular continuum states. The resonance structure in the phase shift, due to tunneling through rotational barriers, gives a rigorous interpretation of the metastable states which lie behind the barrier, and we can justify the need to include such states in the evaluation of thermodynamic properties. However, we also find that it is inconsistent merely to include the metastable phase space without considering the remaining contributions from the continuum. If either component is ignored in treating the dimer, then, of necessity, their presence will appear as the virial coefficients which cause nonideal behavior for the atomic fragments of the molecule. Both approaches are consistent and yield a proper equation of state for the vapor. We will show quantitative agreement between the exact quantal results and the approximate classical expressions for the partition function at high temperatures. Since the classical theory automatically includes all the effects of the molecular continuum, we suggest that this is both the simplest and most unambiguous procedure for extending thermodynamic tables to elevated temperatures. Explicit calculations are presented for Li2 and Na2.
The compression of solid CO2 at 296 K to 10 GPa
Authors: Olinger Bart
Published: February 2003

The volume of solid CO2 was measured at 296 K from 1 to 10 GPa using a high pressure, x-ray diffraction technique. The compression of NaF was used as the pressure gauge. The data are converted to the velocity plane and linearly fit. The volumes and bulk moduli are calculated from the fit as a function of pressure. Previous compression studies are discussed and a recent a priori calculation is shown to agree well with the present data.
Exciton–surface plasmon coupling: An experimental investigation
Authors: Pockrand I. , Brillante A. , Möbius D.
Published: February 2003

Langmuir–Blodgett monolayer assemblies, which contained dye molecules, have been deposited on silver films. Exciton–surface plasmon interactions have been studied with attenuated total reflection (ATR) spectroscopy. Reflectivity and dispersion curves for plasmon surface polaritons (PSP’s) at the metal interface are reported for both angular and wavelength scans. In agreement with theory dispersion curves from angle scans exhibit a double ‘‘back bending’’ at the transverse exciton frequency ωT (due to PSP interaction with the in plane component of the dye transition dipole moment) and at the longitudinal frequency ωL (due to PSP interaction with a perpendicular component). Correspondingly, dispersion curves from wavelength scans break into separate branches at these frequencies.
Adsorption of polymer chains at surfaces: Scaling and Monte Carlo analyses
Authors: Eisenriegler E. , Kremer K. , Binder K.
Published: February 2003

The influence of a hard wall on the configurations of long flexible polymer chains near the wall is studied, in the presence of a short-range attractive force between monomers and the wall. Particular attention is paid to the region around the temperature Ta below which the polymer becomes adsorbed to the wall, i.e., where the typical polymer linear dimensions perpendicular to the wall become independent of chain length. Both ideal noninteracting chains and chains with excluded volume interactions are treated. Polymer linear dimensions parallel and perpendicular to the wall and their probability distributions are studied, as well as the behavior of the monomer fraction at the surface and a distance z in the interior. The relation of polymer statistics to the problem of correlation functions in the n-vector model of magnetism in the limit n→0 is exploited to express both the exponents describing the various power laws and the crossover scaling functions near Ta in terms of results for the analogous problem of the critical behavior of magnets with a free surface. The predictions of this scaling theory are confirmed by Monte Carlo studies of self-avoiding walks on the tetrahedral lattice with a free surface, and estimates for the exponents involved are presented. A new sampling technique with a bias in favor of adsorbed polymer configurations is developed.
Rainbows in rotationally inelastic scattering: A comparative study of different model potential surfaces and dynamical approximations
Authors: Schinke Reinhard , Korsch H. Jürgen , Poppe Dirk
Published: February 2003

Rainbow structures in rotationally elastic and inelastic differential cross sections in atom–diatom collisions are investigated by comparison of three model potential energy surfaces labeled I, II, and III which are represented by V(R,γ)=V0(R)+V2(R)P2(cos γ). The cross sections are calculated within the quantal infinite-order-sudden (IOS) approximation. The anisotropic part V2 is the same for all potentials and purely repulsive. The isotropic part V0 for potential I is also repulsive and the differential cross sections show the well-studied rotational rainbow structures. Structural changes occur for collisions in potential II and III which have V0 terms being attractive at intermediate and large atom–molecule separations and having well depths of 10% and 25% of the collision energy, respectively. For example, the elastic cross section has no classical rainbow in the case of potential I but three in the case of potential III. The rainbow structures are analyzed within the classical and semiclassical versions of the IOS approximation and interpreted in terms of catastrophe theory. The quantitative comparison of the classical with the quantal IOS cross sections manifests possible quantum effects, i.e., tunneling into nonclassical regions and interference effects due to the superposition of several contributions (up to six in the present study). They can be very prominent and thus we conclude that much caution is needed if experimental data are compared with classical calculations. The accuracy of the IOS approximation is tested by comparison of classical IOS cross sections with cross sections obtained from exact classical trajectory calculations. The agreement is generally good with the exemption of the rainbow region and small angle, rotationally elastic scattering.
Gel deswelling under reverse osmosis. II
Authors: Geissler E. , Hecht A. M.
Published: February 2003

Previous observations of the rate of deswelling z0 of polyacrylamide gels under the influence of an externally applied pressure P showed that z02 is proportional to P. This behavior was interpreted as indicating the formation of a concentrated layer of gel at the deswelling surface, favored by an attractive potential between the gel and the membrane. In this article, optical measurements of the concentration gradient profile of a series of gels undergoing deswelling are reported. From these curves it is shown that for small degrees of deswelling, the collective diffusion constant D varies with concentration as c0.5, but increases more strongly with concentration at large degrees of deswelling. The zeroth and first moments of these curves are in good agreement with the calculated concentration increment induced by the external pressure, and the observed rate of deswelling of the gel, respectively. It is concluded that there is no evidence of a distinct controlling layer at the interface between the gel and the membrane, and the observed linearity between z02 and P is the consequence of the nonscaling behavior of D in the equation of motion.
Interference effects on relaxation in three-level systems: Breakdown of the rate equation description
Authors: Bagchi Biman , Oxtoby David W.
Published: February 2003

The stochastic Liouville equation and exact quantum mechanical simulations have been used to test the validity of rate law predictions for the vibrational energy relaxation in a three-level system. The model system consists of a low lying ground state which is radiatively coupled to only one of three closely spaced excited states. All three excited states are strongly coupled to baths of known statistical properties (either Gaussian or Poisson). We follow the energy exchange between the excited states after the light is turned off. Two different types of bath correlation have been considered: (A) The couplings between the different levels are the same and (B) the couplings are uncorrelated with each other. We find that relaxation differs for these two cases, in contradication to the prediction of the simple rate law description. We also present the first quantitative study of the interference between different pathways that can occur in such a three-level system. We find that the interference has a considerable effect on the decay of the middle level for the same bath (case A), even in the Markovian limit, while for independent baths (case B) interference becomes important only in the non-Markovian limit. These results are entirely different from the prediction of the conventional rate laws. We also discuss the effect of the excitation on the subsequent relaxation and the coherence transfer processes that lead to an oscillatory decay in the non-Markovian limit.
Quantum manifestations of classical stochasticity. I. Energetics of some nonlinear systems
Authors: Weissman Yitzhak , Jortner Joshua
Published: February 2003

In this paper we present the results of a semiclassical investigation and a quantum mechanical study of the bound energy spectrum of the Henon–Heiles Hamiltonian (HHH) and of the Barbanis Hamiltonian (BH). We have derived a simple semiclassical formula for the energy levels E, and for their sensitivity dE/dε with respect to the strength ε of the nonlinear coupling for the HHH, and established general relations between E and its derivatives dnE/dεn (n⩾1). Numerical quantum mechanical computations of the energy levels were conducted for the HHH and for the BH. The nonlinear coupling constant was adjusted so that for the HHH there will be ∼150 states up to the classical critical energy Ec and ∼300 states up to the dissociation energy ED. The E values were obtained by direct diagonalization using a basis containing 760 states, while the values of dE/dε were computed utilizing the Hellmann–Feynman theorem. Good agreement between the semiclassical and the quantum mechanical spectra was observerd well above Ec. These results raise the distict possibility that the semiclassical approxmation for these nonlinear systems does not break down in the vicinity of Ec and that the bound level structure does not provide a manifestation of the classical transition from quasiperiodic to chaotic motion.
The motional collapse of the methyl C–H stretching vibration bands
Authors: MacPhail R. A. , Snyder R. G. , Strauss Herbert L.
Published: February 2003

The asymmetric CH stretching bands of the methyl groups of a number of alkane systems have been examined as a function of temperature. At low temperatures, two bands are seen for a CH3 group. These bands collapse into one as the temperature is raised. Examination of crystals of alkanes with both odd and even numbers of carbon atoms, alkanes in urea clathrates, and deuterium substituted alkanes allows the separation of intramolecular and intermolecular contributions to the band splittings and widths. Model Hamiltonians that express the stretching energy levels as a function of both the methyl torsional coordinates and the external coordinates of the alkanes are set up. These are used in the Redfield equations to derive expressions for the splittings and widths in terms of parameters which give the coupling of the various types of motion. The splittings are found to be proportional to the average of cos 5ϑ, where ϑ is the torsional angle, and a contribution to the width is found to be due to fluctuations in this quantity. Comparison of the equations with the observed widths shows that the relaxation times of the torsional motion, which are greater than 0.2 but less than about 20 ps, are fast enough to yield the motionally narrowed limit for the spectrum as required in the derivation of the Redfield equations.
Microstructure of two-phase random media. I. The n-point probability functions
Authors: Torquato S. , Stell G.
Published: February 2003

The microstructure of a two-phase random medium can be characterized by a set of general n-point probability functions, which give the probability of finding a certain subset of n-points in the matrix phase and the remainder in the particle phase. A new expression for these n-point functions is derived in terms of the n-point matrix probability functions which give the probability of finding all n points in the matrix phase. Certain bounds and limiting values of the Sn follow: the geometrical interpretation of the Sn and their relationship with n-point correlation functions associated with fluctuating bulk properties is also noted. For a bed or suspension of spheres in a uniform matrix we derive a new hierarchy of equations, giving the Sn in terms of the s-body distribution functions ρs associated with a statistically inhomogeneous distribution PN of spheres in the matrix, generalizing expressions of Weissberg and Prager for S2 and S3. It is noted that canonical ensemble of mutually impenetrable spheres and the associated set of ρs define, in the limit of an unbounded system, a statistically homogeneous and isotropic medium, as does (trivially) a canonical ensemble of mutually penetrable spheres.
Stereochemical effects in the quenching of Na*(3 2P) by CO: Crossed beam experiment and ab initio CI potential energy surfaces
Authors: Reiland Werner , Tittes Hans-Ulrich , Hertel Ingolf V. , Bonačić-Koutecký Vlasta , Persico Maurizio
Published: February 2003

Electronic to vibration-rotational-translational energy transfer in the quenching of Na(3 2P3/2) by CO has been studied with state of the art crossed atomic, molecular, and laser beam techniques at 0.16 eV initial kinetic energy, and by ab initio CI calculations for the potential energy surfaces involved in the process. Double differential quenching cross sections are measured as a function of scattering angle and energy transferred to the molecule. A pronounced structure in the energy transfer spectra as well as a partial backward scattering is attributed to two different mechanisms, a ’’direct’’ one and one which proceeds through ’’complex’’ formation. The observations are explained by the calculated potential energy surfaces (PES) for the first excited states Ã 2A′(Ã′ 2A″) and the ground state X̃ 2A′ which exhibit two crossing seams below the 2.1 eV excitation energy: (i) one for colinear approach of Na* on the carbon side of CO with its lowest energy 1.06 eV at Rc(Na–CO)= 5.5a0, rc(C–O) = 2.35a0 responsible for the direct process and (ii) one for colinear approach of Na* on the oxygen end of CO with 1.28 eV at Rc= 4.9a0 and rc= 2.47a0, allowing the quenching after ’’complex’’ formation. The angularly integrated cross sections are maximal (27 Å2/eV) at an energy transferred to the molecule equivalent to five vibrational quanta. Comparison with bulk data suggests strong rotational excitation (two vibrational quanta in the average) as can be rationalized from the anisotropy of the X̃ 2A′ PES near the crossing region. Total quenching cross sections and their temperature dependence can be explained by the absorbing sphere model using the calculated location and energy of the crossing seams.
A
b i
n
i
t
i
o studies on the stabilities of even- and odd-membered Hn+ clusters
Authors: Wright L. R. , Borkman R. F.
Published: February 2003

A b initio SCF and SD–CI calculations have been performed for the hydrogen positive ion clusters Hn+ with n = 3–9. The optimum structures of the clusters were all determined to consist of a nearly equilateral H3+ core surrounded by neutral H and/or H2 ligands at the apices. The most interesting new finding is that the even-membered clusters with n = 4, 6, 8 have binding energies comparable to the odd-membered clusters with n = 5, 7, 9. This result is striking since the odd-membered clusters have been observed in a number of previous experiments; whereas no even-membered H+n clusters have ever been observed experimentally. The conclusion appears to be that kinetic (but not thermodynamic) factors preclude formation of H+4, and other even-membered hydrogen ion clusters, in experiments performed to date. Our best calculations indicate that H4+ is stable by at least 5 kcal/mol relative to H3++H. The other Hn+ species studied have probable binding energies in the range 3–7 kcal/mol.
The δ-approximation (reflection) method applied to a two-photon process
Authors: Baierl P. , Kiefer W.
Published: February 2003

The δ-approximation or reflection method, which is able to well explain continuous absorption spectra of diatomics has been applied for a two-photon process. It is shown to what extent this approximation can be employed for the description of the band profiles observed in a Raman scattering process in 79Br2, where resonance takes place with the continuum of a repulsive and a dissociative excited electronic state. By means of this approximation, simple diagrams can be drawn which allow a qualitative explanation of the change in band profiles of resonantly enhanced vibrational–rotational Raman transitions.
Calculations of induced moments in large molecules. II. Polarizabilities and second hyperpolarizabilities of some polyenes
Authors: Papadopoulos M. G. , Waite J. , Nicolaides C. A.
Published: February 2003

Coupled SCF perturbation theory has been used to calculate the polarizability α and second hyperpolarizability γ of a number of polyenes. An extended basis CNDO wave function has been employed. Our computational approach is based on optimizing the exponents of a small basis set with respect to the experimental values of α and γ of ethylene. We have chosen this scheme having considered in detail several other options. Our results are in reasonable agreement with the limited number of experimental values available for these molecules.
Eigenmodes of the quantized polarizable fluid
Authors: Ho/ye J. S. , Olaussen K.
Published: February 2003

The equilibrium properties of a polarizable fluid that consists of interacting quantized Drude oscillators have recently been evaluated in an extended mean spherical approximation (MSA). Here we show that the thermodynamic properties that come from this result may be interpreted in terms of a set of quantized harmonic oscillators whose frequency distribution may be determined.
The theory of double-beam three-photon absorption
Authors: Andrews David L.
Published: February 2003

This paper deals with the theory of double-beam three-photon absorption, in which the excitation process involves the concerted absorption of three photons from two laser beams which simultaneously irradiate the sample. Equations are derived for the rate of absorption under various polarization conditions, and the dependence on the angle between the two beams is examined in detail. It is shown how a set of experiments at a suitable angle with specified polarizations allows the determination of five molecular parameters which characterize the symmetry of the excited state. It is demonstrated how the results thereby provide for an unambiguous assignment of the excited state to one of six different symmetry classes. It is also shown that the double-beam experiment allows access to excited states which are forbidden under single-beam excitation.
Classical two-dimensional model for rotational-vibrational atom–diatom inelastic scattering
Authors: Serri J. A. , Bilotta R. M. , Pritchard D. E.
Published: February 2003

The classical hard ellipse model for atom–diatom rotationally inelastic (RI) collisions is generalized to include simultaneous vibrational excitation by assuming that this excitation is proportional to the square of the component of momentum transferred along the major axis of the ellipse. Calculations are presented which compare level-to-level RI angular distributions of vibrationally elastic (Δv = 0) and vibrationally inelastic (Δv = 1) scattering. These calculations reproduce the main features observed in recent measurements of level-to-level rovibrationally inelastic scattering for Na2 with Ar. With Δv = 0 and 1, the scattering distributions display rainbow structure whose angular position increases nearly linearly with Δj. In addition, the Δv = 1 scattering shows considerable suppression of small angle scattering (which also has small Δj). We emphasize that this suppression of forward scattering results from a kinematic exclusion of small angle scattering for impulsive inelastic collisions with a slowly rotating molecule irrespective of the origin of inelasticity.
A study of the unimolecular decomposition of the (C2H2)2+ complex
Authors: Ono Y. , Ng C. Y.
Published: February 2003

The internal energy effects and the energetics of the ion–molecule reactions C2H2++C2H2 → C4H3++H, C4H2++H2, and C4H2++2H have been studied by photoionization of neutral van der Waals acetylene dimers. The appearance energies for C4H3+ and C4H2+ from (C2H2)2 were found to be identical and have the value of 10.90±0.05 eV (1137±5 Å). These measurements suggest activation energies of ∼0 and ∼12 kcal/mol for the C4H3++H and the C4H2++H2 channels, respectively. The ratio of the fragmentation probabilities for the formation of C4H2++H2 and C4H3++H from C2H2+(X̄ 2Πu,v) ⋅ C2H2 was found to decrease from approximately 0.70 to 0.45 as the ionization photon energy was increased from the ionization threshold of C2H2 (11.40) to ∼14.60 eV, an observation in qualitative agreement with the prediction by the quasiequilibrium theory. The energy dependence measurements for the relative fragmentation probabilities of C4H2+ and C4H3+ from (C2H2)2+ support the conclusions that the C4H3+ ions fragment further to form C4H2+ + H with little kinetic shift effects and that the C4H2+ + 2H channel is mainly responsible for the substantial increase in the intensity of C4H2+ observed previously in electron impact and charge transfer experiments at energies ⩾16 eV. This experiment also reveals evidence that the high vibrationally excited levels of the X̄ 2Πu state are as effective as the Ā 2Σg+ state in the formation of C4H2++2H provided the reaction is energetically allowed. The pressure dependence measurements for the relative intensities of C4H2+ and C4H3+ show that the solvation effects do not affect the fragmentation probabilities for the formations of C4H2++H2 and C4H3++H at energies below the thermochemical threshold (∼14.93 eV) for the C4H3++2H channel, whereas at energies higher than 14.93 eV the association of additional acetylene molecules to dimer ions will strongly favor the C4H3++H formation at the expense of the C4H2++2H channel.
Enhanced photofragmentation on a silver surface
Authors: Goncher G. M. , Harris C. B.
Published: February 2003

Photofragmentation has been observed for molecules near a roughened Ag(110) surface using low intensity UV excitation at 363.8 nm. The effect shows a nonlinear dependence on illumination intensity at intensities less than 100 W/cm2, and is dependent on molecule-surface separation. (AIP)
Excited state absorption spectroscopy and state ordering in polyenes. II. α,ω-diphenylpolyenes
Authors: Goldbeck Robert A. , Twarowski Allen J. , Russell Eric L. , Rice Jane K. , Birge Robert R. , Switkes Eugene , Kliger David S.
Published: February 2003

Calculated and observed excited singlet state absorption (Sn←S1) spectra of a series of diphenylpolyenes are presented. In diphenyloctatetraene and diphenylhexatriene, the S1 state is assigned as an 1Ag state, in agreement with results from two-photon spectroscopic studies. In diphenylbutadiene, we assign the S1 state as a 1Bu, although two-photon studies have indicated that 1Ag state lies slightly below the 1Bu. It appears that a 1Ag state is the lowest excited state of diphenylbutadiene in its ground state geometry, but when the excited states relax to their equilibrium configurations, the 1Bu becomes the S1. Good agreement between the PPP–CI calculations and experimental Sn←S1 spectra demonstrates the potential usefulness of this technique in assigning ππ* excited states of large molecules.
Quantum mechanics of local mode A
B
A triatomic molecules
Authors: Sibert Edwin L. , Hynes James T. , Reinhardt William P.
Published: February 2003

The quantum mechanics of the vibrational stretching dynamics in ’’local mode’’ triatomic molecules is examined. A model for H2O is taken as a prototype. The quantum analysis exploits the corresponding classical analysis of the companion paper, in which an approximate but accurate Hamiltonian is derived via the techniques of nonlinear mechanics. Quantization of this Hamiltonian gives H2O vibrational energies in excellent agreement with direct quantum calculations. The corresponding overtone-combination spectrum of the H2O model is analyzed in terms of local and normal mode behavior with the aid of the twofold hindered rotor perspective provided by the Hamiltonian. The splittings in the spectrum are related to the quantum dynamics of energy transfer. A semiclassical WKB analysis is also used to relate the splittings to classical energy transfer rates and quantum dynamical tunneling and reflection probabilities.
Excited states of polar negative ions
Authors: Garrett W. R.
Published: February 2003

The spectra of electronically excited states of strongly polar negative ions are discussed in terms of general features that may be predicted for such systems. The general properties are then studied through a systematic treatment of lithium halide and lithium hydride anions. Only one or two excited electronic levels exist for these systems, and the binding energies are so low that a limited number of bound rotational levels are associated with each of the excited states. For the states of lowest binding energies, abnormal rotational level spacings are demonstrated. Also, a discussion is given of the implictions for electron scattering and photodetachment studies, of the higher lying dipole states which cross over into the continuum.
Polydisperse systems: Statistical thermodynamics, with applications to several models including hard and permeable spheres
Authors: Salacuse J. J. , Stell G.
Published: February 2003

The statistical thermodynamics of polydisperse systems of particles is investigated. A Gibbs–Duhem relation is obtained and the equilibrium conditions relevant to a two-phase system are derived. Systems of hard spheres, and hard spheres with Kac tails, are treated as illustrative examples with analytic results given in the context of scaled-particle (Percus–Yevick) theory as well as the polydisperse generalization of the thermodynamic approximation of Mansoori et al. An exact treatment of the analogous one-dimensional systems is also given. Quantitative results using a Schultz distribution of diameters are presented. A model of interpenetrable particles introduced previously by one of us—the permeable-sphere model—is also considered. Its thermodynamics and pair distribution functions are shown to be exactly obtainable in the context of the Percus–Yevick approximation. For this model, polydispersivity in both particle size and particle impenetrability is considered analytically. The pair potential for this model is discontinuous at the interparticle diameter; generalization of the model for which the pair potential is continuous is also introduced as a model of an effective polymer–polymer potential.
Collisional deactivation of vibrationally highly excited polyatomic molecules. I. Theoretical analysis
Authors: Troe J.
Published: February 2003

The master equation for collisional deactivation of vibrationally highly excited molecular states has been solved numerically for representative collision models. The results can conveniently be represented by generalized analytical solutions for the diffusion limit of the master equation. The time evolution of the moments 〈E(t)〉 and 〈[E−〈E(t)〉]2〉 is related to the energy transfer averages 〈ΔE〉 and 〈ΔE2〉.
Alkali oxide diatomics: Explanation of the change in ground state symmetry from LiO(2Π) CsO(2Σ+)
Authors: Allison Janet N. , Goddard William A.
Published: February 2003

The ground state of CsO is 2Σ+, whereas the ground state of LiO is 2Π. Using purely ionic wave functions, we explain the origin of this effect in terms of a balance between Pauli repulsion (favoring 2Σ) and quadrupole attractions (favoring 2Π) and provide spectroscopic parameters (Re, ωe, De) on LiO, NaO, KO, RbO, and CsO from ab initio correlated wave functions.
On hybridization and bonding in the alkaline earths: Be, Mg, and Ca
Authors: Bauschlicher Charles W. , Bagus Paul S. , Cox Brian N.
Published: February 2003

We have computed the dissociation energy (De) for tetrahedral Be4, Mg4, and Ca4 from configuration interaction wave functions: Be4 (59.2 kcal/mol), Mg4 (11.8 kcal/mol), and Ca4(18.3 kcal/mol). We are able to relate these De’s to the degree of s to p hybridization of the alkaline atom. Using a minimum basis we have computed SCF wave functions for Be13 and Mg13. Using the central atom as a model for a bulk atom, we are able to relate the degree of hybridization in the metal to the heats of sublimation. Combining these calculations and other experimental data on alkaline earths, we can explain many of the similarities and differences between these compounds on the basis of arguments related to degree of hybridization and bond strength.
Absolute rate constants for the hydroxyl radical reactions with ethane, furan, and thiophene at room temperature
Authors: Lee J. H. , Tang I. N.
Published: February 2003

Interests in the gas phase oxidation of heterocyclic organic compounds containing oxygen, nitrogen, or sulfur have become increasingly evident due to the important roles that these compounds may play in combustion processes, atmospheric chemistry, and photochemical air pollution. The rate constants for the reactions of OH with furan (C4H4O) and thiophene (C4H4S) have been determined for the first time under pseudo-first-order conditions, using the discharge flow-resonance fluorescence technique. The measured room temperature rate constants are (2.31±0.40)×10−13, (1.05±0.08)×10−10, and (4.77±0.63)×10−11 cm3/molecule−1 s−1 for ethane, furan, and thiophene, respectively. The results are discussed in terms of structural differences and probable reaction pathways.
Dielectric enhancement due to electrochemical double layer: Thin double layer approximation
Authors: Chew W. C. , Sen P. N.
Published: February 2003

The dielectric constant and conductivity of a dilute ensemble of immobile, spherical particles with fixed surface (zeta) potential Φo, immersed in an electrolytic solution, is obtained in the thin double layer approximation δ≪a, δ being the thickness of the double layer, and a the radius of the particles. Equations of motion for coions and counter-ions are solved by the method of matched asymptotics. The equations of motions, linearized in the applied electric field Eo and with coefficients that are functions of the unperturbed potential (zeroth order in Eo), are solved to second order in (δ/a). The term giving enhancement in the real part of the effective dielectric constant of the ensemble ε1e, is second order in δ/a; but the series converges if (δ/a)t2/(1–t2)≪1, where t=tanh(eΦo/kBT), e being the ionic charge, kB the Boltzmann constant, and T the absolute temperature. The static value of ε1e, to this order, is ε1e∼36fε1t2/(1–t2)2, where f is the volume fraction of particles, ε1 the real part of the dielectric constant of the solution. When Φo→∞, therefore, t→1, ε1e diverges as ε1e∼9/4fε′ exp(eΦo/kBT). The present treatment is free from the approximations of previous analytical results. When applicable, the theory agrees well with experiments over three decades in frequency, with one adjustable parameter Φo. Comparison with other theories are made.
Unimolecular reaction paths of electronically excited species. IV. The C̃ 2Σ+g state of CO+2
Authors: Praet M. Th. , Lorquet J. C. , Raşeev G.
Published: February 2003

The geometrical structure of the low-lying states of CO+2 has been calculated ab initio. The C̃ 2Σ+g/2A1 state is found to be slightly bent in its equilibrium geometry. A new assignment of the vibrational structure of the corresponding band in the photoelectron spectrum is suggested. State C̃ is predissociated by two competitive channels. One of them leads to O++CO, the other to CO++O. The mechanism of these predissociations involves a slow, rate-determining, intersystem crossing to a bent ã 4B1 state. The population of state ã has a choice between dissociating to O++CO fragments and undergoing a further, much faster, intersystem crossing to the ground state X̃ which dissociates to CO++O. Since radiationless transitions between X̃ and ã are relatively rapid, the state which is lower in energy (i.e., X̃) has a much larger population than the other (i.e., ã). Hence, the CO++O channel prevails as soon as it is energetically accessible. The rate-determining step of both processes is the intersystem crossing between states C̃ and ã. Its rate constant is estimated by a statistical method due to Zahr, Preston, and Miller, recasted in a simpler form. A value of about 4×107 s−1 is obtained.
Effect of annealing on the secondary relaxations in glasses
Authors: Johari G. P.
Published: February 2003

The dielectric loss in a rigid-molecular glass (toluene–pyridine) and in poly(propylene oxide) has been measured from 50 to 2×105 Hz during the course of isothermal annealing at several temperatures. The decrease in the loss on annealing is predominantly due to a decrease in the number of molecules participating in the secondary relaxation. Measurements of the loss at different temperatures, but a fixed frequency, indicate a similar effect of annealing. Explanations for the secondary or β relaxations in amorphous polymers in terms of the motion of the side group attached to the main chain seem inconsistent with the observed temperature dependence of the amplitude of secondary relaxation. Several implications for our concepts of small scale molecular motions in glasses arising from these results are considered. It is suggested that the discrepancy between the relaxation rates determined from the loss spectrum at a constant temperature and from the loss-temperature plots at a constant frequency arises from a rapid increase in the amplitude of secondary relaxation with temperature.
Ordered structure in dilute solutions of highly charged polymer lattices as studied by microscopy. I. Interparticle distance as a function of latex concentration
Authors: Ise N. , Okubo T. , Sugimura M. , Ito K. , Nolte H. J.
Published: February 2003

The coexistence of ordered and disordered regions has been observed in dilute solutions of highly charged monodisperse latex particles, a copolymer of styrene and styrenesulfonate, by using a metallurgical microscope. Unlike latex particles of low charge numbers, the interparticle distances (2Dexp) in the hexagonal ordered regions are smaller than the calculated average values for a face-centered-cubic distribution (2D0,h) at low polymer concentrations (0.2–2 vol %). However, at high concentrations, up to 12%, the 2Dexp value approaches 2D0,h. The results are consistent with a ‘‘two-state structure’’ containing ordered and disordered regions and indicate the existence of ‘‘attractive’’ interactions between the charged solutes. The findings are also consistent with those for other species of high charge densities such as synthetic macroions and simple ions.
Time dependent linear quantum systems
Authors: Landovitz L. F. , Levine A. M. , Ozizmir E. , Schreiber W. M.
Published: February 2003

The solution to the linear Heisenberg equations which result from the most general quadratic time dependent Hamiltonian is developed. The Green’s function is obtained, and the transition amplitudes between harmonic states are calculated.
Iodine monofluoride B 3Π(0+)→X 1Σ+ lasing from collisionally pumped states
Authors: Davis S. J. , Hanko L. , Shea R. F.
Published: February 2003

An optically pumped IF laser operating on the B 3Π(0+)→X 1Σ+ transition has been studied. Vibrational levels v′=1, 2, 3, 4, 5, and 6 of IF(B) were pumped by a dye laser pulse and lasing was observed both from the level pumped and from lower levels populated as a result of collisions with bath gas molecules. Time delays from 0.5 to 3.0 μs were observed between termination of the pump pulse on v′=5 and lasing out of v′=0. The longer delays result from slower vibrational relaxation at lower pressures. The stable nature of v′=0 reservoir indicates that the B–X electronic transition is very attractive as a potential chemical laser system.
The rheological properties and fracture of a molecular dynamic simulation of sodium silicate glass
Authors: Soules Thomas F. , Busbey Robert F.
Published: February 2003

We extend earlier molecular dynamic calculations to a study of the rheological properties and failure of sodium silicate glass. Under biaxial compressions glass flow and stress relaxation are observed. The simulated glass ‘‘breaks’’ by forming cavities when subjected to a sudden large biaxial expansion. A microscopic model of a fiber behaves elastically up to stresses of near one million psi and then breaks with significant plastic deformation. The strength of the fiber is related to the ultimate strength of the glass. Under severe uniaxial compression the microscopic fiber undergoes a change in its atomic structure to a layered structure.
Density functional theory of nonlinear optical response
Authors: Zangwill A.
Published: February 2003

This work is concerned with the formulation and testing of a self-consistent nonlinear response theory of optical processes based upon the density functional formalism. An expression is developed for the third order susceptibility of a spherically symmetric system using a generalization of a time-dependent local density approximation (TDLDA) previously developed for linear response. Explicit calculations of the coefficient of third harmonic generation for the rare gases are in good agreement with experiment. Applications to other nonlinear phenomena are pointed out.
D2O product angular and translational energy distributions from the oxidation of deuterium on Pt(111)
Authors: Ceyer S. T. , Guthrie W. L. , Lin T.-H. , Somorjai G. A.
Published: February 2003

The angular and translational energy distributions of D2O produced from the oxidation of deuterium on the (111) crystal face of platinum have been measured over the surface temperature range of 440–913 K. Although the angular distributions are described by a cosine function, the translational energy distributions deviate from the corresponding Maxwell–Boltzmann distributions. At the normal angle, the D2O mean translational energy 〈E〉/2k varies from 220 to 400 K over the temperature range investigated. Two mechanisms for the production of translationally cold product molecules are discussed.
Triplet positronium annihilation parameters in ethanol–pentene mixtures containing platinum
Authors: Ciottone Judith L.
Published: February 2003

Positron lifetimes and intensities were determined for simulated hydrogenation reaction mixtures consisting of 1/2, 1/1, and 2/1 ethanol–pentene molar ratios to which 0.25 mg platinum powder were added. Annihilation parameters were determined upon addition of each successive component, yielding long lifetimes (τ3) of 4.1±0.1 and 4.2±0.1 ns for 1-pentene and trans-2-pentene (99% trans), respectively. Large I2 increases were observed for cis-2-pentene and for 1/2 ethanol–cis-2-pentene, both containing platinum.
Theoretical and experimental determination of the lowest excited states of the Kr*2 excimer
Authors: Gadea F. X. , Spiegelmann F. , Castex M. C. , Morlais M.
Published: February 2003

The potential energy curves of the Kr*2 excimer dissociating into Kr (4s24p6 1S0)+Kr*(4s24p55s) are determined (i) theoretically from ab initio CI calculation and semiempirical SO coupling and (ii) experimentally from the temperature dependence of absorption profiles. The results are carefully examined with the help of theoretical simulation of line profiles using semiclassical or quantal models.
Reactions of atomic hydrogen with the Si(111) (7×7) surface by high resolution electron energy loss spectroscopy
Authors: Kobayashi H. , Edamoto K. , Onchi M. , Nishijima M.
Published: February 2003

High resolution electron energy loss spectroscopy (vibrational and electronic excitations) and monochromatic low-energy electron diffraction (LEED) have been applied to the study of atomic-hydrogen adsorption on the Si(111) (7×7) surface at ∼300 K. Adsorbed states of hydrogen are differentiated by taking the vibrational loss spectra of the Si(111) surface exposed to various amounts of hydrogen and of the same surface subsequently heated to high temperatures. The existence of four adsorbed states is proposed. Hydrogen adsorption is considered to proceed as follows. In the very early stage (fractional hydrogen coverage θ≲0.3), the SiH species are produced by the covalent bond formation of hydrogen atoms with the dangling bonds of the Si(111) surface atoms. Then, the Si–Si bond breaking and the desorption of SiH4 and, possibly, SiH3 occur, and the corrosion-induced SiH (with the bond axes parallel to and away from the surface-normal direction), SiH2, and SiH3 species are also produced. The hydrogen coverage is saturated at θ∼1.5, where the amount of the SiH2 and SiH3 species formed is estimated to be <1/4 of that of the total SiH species. The electronic transition associated with the SiH species is observed at 9.1 eV for low hydrogen exposure, which is shifted to lower energy by the increase in the exposure. Corrosion-induced surface roughening is reflected in the high background and in the degradation of the 1/7 order beam intensities of LEED angular profiles. Patch formation of the SiH, SiH2, and SiH3 species is evident from the slow decay of the 1/7 order beam intensities.
The interaction of CO and Pt(100). I. Mechanism of adsorption and Pt phase transition
Authors: Behm R. J. , Thiel P. A. , Norton P. R. , Ertl G.
Published: February 2003

It is known that clean Pt(100) normally exists in a reconstructed (‘‘hex’’) phase, that a metastable clean unreconstructed phase [the (1×1)] can be prepared, and that during adsorption of CO the hex→(1×1) transition occurs. In this and the following paper, we describe an investigation of the CO/Pt(100) system which clarifies the mechanism of this adsorbate-induced Pt phase transition. The experimental techniques included fast video-LEED techniques, thermal desorption spectroscopy, electron energy loss spectroscopy, and work function measurements. Adsorption of CO at low temperature (T≲400 K) is discussed in this paper. CO on (1×1)-Pt forms a c(2×2) overlayer near and at the ideal coverage of θ=0.5, in which the CO molecules occupy on-top adsorption sites. Repulsive CO–CO interactions cause this structure to form. CO adsorption on hex-Pt proceeds via formation of small areas with the same local structure, a c(2×2) layer of CO on (1×1)-Pt, even when the total coverage of CO is low (0.05<θ<0.5). This comparison between adsorption on the two phases indicates that the mechanism of the hex→(1×1) transition involves, effectively, ‘‘island’’ formation of adsorbed CO in spite of the repulsive character of the CO–CO interaction in the c(2×2). This apparent contradiction is resolved by considering the energy contribution of the Pt phase transition which accompanies the island formation, the whole process being well described as a ‘‘nucleation and trapping’’ mechanism. The nucleation process strongly limits the long-range order of the CO: At θ=0.5, the c(2×2) domains which result from CO adsorption on (1×1) are about five times larger than for hex-Pt, even though in the latter case, the substrate has been completely converted to (1×1)-Pt at θ=0.5. Differences in the long-range order of CO at θ≥0.5 are also observed in the surface work functions and in the distributions of occupied adsorption sites.