bluuebunny/crossref_metadata_2025 huggingface dataset:
Close-coupling optical model analysis: A refined optical potential surface for He*(2 1S)+H2
Authors: Martin D. W. , Siska P. E.
Published: July 2002

A complex-arithmetic version of Johnson’s log–derivative integration method has enabled a straightforward extension of the close-coupling approximation to complex-potential surfaces. The complex-potential close-coupling (CPCC) method is applied to the determination from our earlier scattering measurements of an improved anisotropic optical potential for the He*(2 1S)+H2 Penning ionization system, with the aid of a new, nonpiecewise potential surface function based on the improved Tang–Toennies model. The potential is compared to earlier ab initio results, and the CPCC method is used to assess the shortcomings of the complex-infinite-order-sudden (CIOS) approximation. Variation of differential and ionization cross sections with the initial rotational state of H2 predicted by CPCC may be experimentally observable, and may also provide an explanation for the observed H2/D2 isotope effect in quenching rates.
Resonant two-photon ionization spectroscopy of naphthalene clusters
Authors: Syage Jack A. , Wessel John E.
Published: July 2002

Mass resolved 2−photon ionization spectra of naphthalene clusters are reported. Differences are observed between spectral structures associated with totally symmetric and nontotally symmetric monomer vibrational modes in the excited state. This provides unique information concerning resonance interactions and geometry of clusters.(AIP)
Electron affinity of Cl: A model potential-quantum Monte Carlo study
Authors: Yoshida Takashi , Mizushima Yasukazu , Iguchi Kaoru
Published: July 2002

The MP-QMC procedure, which is a combination of the model potential (MP) and quantum Monte Carlo (QMC) method, is employed to determine the electron affinity of Cl, together with the fixed-node approximation. Only valence electrons are treated explicitly, while core electrons are replaced by an effective potential constructed with Gaussian-type functions. The expectation value of electron affinity of Cl atom is obtained from subtracting the valence energy of Cl− from that of Cl. The result is in very good agreement with experiment.
A study of vapor phase self-initiated thermal polymerization of styrene with an expansion chamber
Authors: Schmitt John L.
Published: July 2002

Studies of the self-initiated polymerization of styrene have been performed in the uniform environment of an expansion cloud chamber, using nucleation of liquid monomer drops for detection. These studies are the first of this kind in an expansion chamber and, although preliminary and designed to explore feasibility, have yielded many interesting results. The occurrence of the self-initiated nonterminated gas phase processes, previously observed in a diffusion cloud chamber, has now been confirmed in an expansion chamber. At 15 °C the initiation rate is measured to be 2 to 6 radicals cm−3 s−1 and the propagation constant is approximately 6×10−21 cm3 molecule−1 s−1. Additional studies in the presence of oxygen indicate the (expected) formation of polystyrene peroxide as well as its cleavage into additional radicals at 25 °C.
Integral equations for continuum percolation
Authors: Maran Stefano , Reatto Luciano
Published: July 2002

Continuum percolation is studied by means of integral equations for the connectedness function which can be derived from similar equations for the correlation function of fluids with the aid of a diagrammatic expansion. In particular we examine a refinement (which we call PY-d(2)) of the Percus–evick approximation first proposed by Stell and a HNC-type approximation for the connectedness function. These equations are applied to the system of randomly centered spheres, where they are solved numerically. The value of the critical density and the way in which the mean clusters size diverges are given much more accurate by the PY-d(2) approximation than by the PY approximation, while the critical exponents remain unchanged. We find that the HNC approximation does not possess a critical point where the mean clusters size diverges and that the long range behavior of the connectedness bridge function has a more important role in the percolation problem than in the thermal critical point.
Sampling the exact electron distribution by diffusion quantum Monte Carlo
Authors: East Allan L. L. , Rothstein Stuart M. , Vrbik Jan
Published: July 2002

By the accummulation of branching factors in diffusion quantum Monte Carlo (DQMC) and their use as statistical weights, instead of the standard deletion and replication of configurations, we can estimate the averages of (nondifferential) operators taken over the exact electron distribution. This requires only a trivial modification of existing DQMC codes. We illustrate our algorithm by computing ground-state properties for H2 and LiH which are related to the interelectron distance. We also estimate the dipole moment of LiH.
Subnanosecond-time-resolved emission spectroscopy of 1-methylindole and 2,3-dimethylindole in n-butanol
Authors: Montoro Teresa , Chabbert Marie , Tyrzyk Janusz , Lami Hans
Published: July 2002

Subnanosecond-time-resolved fluorescence spectra of 1-methylindole and 2,3-dimethylindole in n-butanol were recorded at temperatures of 118–293 K in order to elucidate the unusually large Stokes shift of indoles in polar solvents. Above 200 K the presence of an isoemissive point in the emission spectra indicates interconversion between a partially relaxed excited state (B* state) and a completely relaxed one (R* state). The B*→R* process is characterized by a rate parameter identical with the inverse of the longest longitudinal dielectric relaxation time of the solvent and thus due to the collective reaction of butanol tetramers far removed from the solute. Analysis of the time evolution of the spectral shift below 200 K, reveals that the B* emission spectrum undergoes a Bakhshiev-type continuous red shift characterized by a relaxation time which compares well with the Debye relaxation time of the second dielectric dispersion region and may therefore be attributed to the rotation of a butanol monomer close to the solute molecule. As concerns the nature of the emitting states, neither the 1Lb–1La level inversion interpretation nor the 1La/CT (1La state with increased charge–transfer character) interpretation is wholly consistent with the results of this investigation.
High resolution infrared laser induced fluorescence study of state mixing in methyl formate
Authors: Minton T. K. , Kim H. L. , Reid S. A. , McDonald J. D.
Published: July 2002

The total infrared fluorescence excitation spectrum in the region of the 101→212 rotational transition in the aldehyde C–H stretch fundamental band of methyl formate has been measured with a resolution of ∼0.007 cm−1. Five peaks were observed, indicating that the zeroth order optically active rovibrational state is mixed with nearly isoenergetic optically inactive bath states. This study marks the first use of a single mode optical parametric oscillator (OPO) for a spectroscopic experiment, and it demonstrates a technique with the potential to probe the vibrational makeup of individual molecular eigenstates.
Site-selective excitation of Eu3+ substituted sodium β″ alumina single crystals
Authors: Brown A. P. , Simkin D. J.
Published: July 2002

Broad band excitation spectra of Eu3+ substituted sodium β″ alumina reveals that the Eu3+ ions occupy at least two distinct sites. Examination of the fluorescence spectrum excited by an ultraviolet laser (308 nm) reveals the presence of an additional site. 5D0→7FJ emission spectra characteristic of each site were obtained using site-selective excitation. A crystal-field analysis of the emission spectra thus obtained permits the assignment of these sites. The two inhomogeneously broadened 5D0→7F0 emissions at 569.4 and 576.7 nm are assigned as mid oxygen sites perturbed by various arrangements of the residual sodium ions, while the sharp 5D0→7F0 emission at 578.7 nm is assigned as the Beevers–Ross site.
Raman band shapes of simple molecules dissolved in liquid perfluorocompounds: Solute–solvent interaction study
Authors: Nie Chong-Shi , Liang Kang-Nin , Zhu Zi-Ying
Published: July 2002

The vibrational Raman band shapes of N2, O2, and CO2 dissolved in perfluorotripropyl amine (FTPA), perfluorodecalin (FDC), and perfluorobenzene (FB) have been measured under elevated pressure (10–55 atm.). The full width at half-height (FWHH) of N2 and O2 vibrational spectra in the perfluoro-solvents increases with the order FTPA<FDC<FB. The vibrational bandwidths of the CO2 Fermi diad show explicit concentration dependence on these perfluoro-solvents. The FWHHs decrease with decreasing concentration. The anisotropic IVH band shape of CO2 ν1 mode in solution state, which tends to broadening when CO2 concentration decreases, is reported. The solvent and concentration dependences of the vibrational and rotational band shapes can be rationalized on the basis of cavity structure of the liquid perfluorocompounds and of no specific interaction between the solute and the solvent.
How are nitrogen molecules bound to NO+2 and NO+?
Authors: Hiraoka Kenzo , Yamabe Shinichi
Published: July 2002

Thermochemical data, ΔH0n−1,n and ΔS0n−1,n for clustering reactions, NO+2(N2)n−1+N2⇄NO+2(N2)n and NO+(N2)n−1+N2⇄NO+(N2)n, were measured with a pulsed electron-beam high-pressure mass spectrometer. Both clusters have small binding energies, 5 kcal/mol>−ΔH0n−1,n>1 kcal/mol for n=1–12. Although the decrease of −ΔH0n−1,n with n is generally slow, a noticeable gap between n=5 and n=6 for NO+2(N2)n is found. The ab initio molecular orbital calculation revealed that the observed falloff is due to the change of the cluster structures. Measured ΔH0n−1,n agree with binding energies calculated with the 6-31+G* basis set within 0.6 kcal/mol.
Fourier transform–electron spin resonance study of spin polarization generated by triplet- and radical-pair mechanisms in a photoinduced electron transfer reaction
Authors: Plüschau M. , Zahl A. , Dinse K. P. , van Willigen H.
Published: July 2002

Fourier transform–electron spin resonance (FT–ESR) was utilized to measure the photoinduced charge transfer in the system Zn tetraphenylporphyrin/duroquinone with a time resolution of 10 ns. The separate observation of the intensity/time profile of different hyperfine lines in the spectrum, which was made possible by application of FT–ESR, enabled the independent determination of transient acceptor radical polarization originating from the triplet- and radical-pair mechanism. The observed temperature dependence and the absolute value of the electron transfer rate lead to the conclusion that the process is diffusion limited with no indication of an activation barrier.
Solvent dielectric effects on isomerization dynamics: Investigation of the photoisomerization of 4,4′-dimethoxystilbene and t-stilbene in n-alkyl nitriles
Authors: Sivakumar N. , Hoburg E. A. , Waldeck D. H.
Published: July 2002

New data for the photoisomerization dynamics of t-stilbene and 4,4′-dimethoxystilbene in polar solvents are reported. These data demonstrate that in n-alkyl nitriles, nonassociated polar solvents, it is possible to extract a solvated barrier for the isomerization. This result is contrasted with recent studies in n-alkyl alcohols, associated polar solvents, where this separation is not possible and clarifies the origin of the changing barrier height in n-alkyl alcohols. Comparison of reduced isomerization rates with models of chemical reaction dynamics in solution are drawn. Good agreement of the data with a coupled oscillator model is found.
Analytical gradients for unrestricted Hartree–Fock and second order Mo/ller–Plesset perturbation theory with single spin annihilation
Authors: McDouall Joseph J. W. , Schlegel H. Bernhard
Published: July 2002

Unrestricted Hartree–Fock (UHF) wave functions and Mo/ller–Plesset perturbation theory (UMPn) based on a single spin-unrestricted reference determinant can contain significant contamination from unwanted spin states. This contamination may lead to large distortions of the potential energy surface, particularly at the UMPn level. A simple approximation to projected UMPn theory (PMPn) improves the shape of the potential energy surface significantly. Formulas for analytical gradients of the PUHF and approximate projected UMP2 energies with single annihilation have been derived and programmed. This code has been applied to the optimization of transition states for H+C2H4, H+CH2O and H+C2H2 at the PMP2/6-31G* level.
Theoretical study of the radiative lifetime for the spin-forbidden transition a 3Σ+u→X 1Σ+g in He2
Authors: Chabalowski Cary F. , Jensen James O. , Yarkony David R. , Lengsfield Byron H.
Published: July 2002

The radiative lifetime for the spin-forbidden, dipole-allowed transition a3Σ+u→X1Σ+g in neutral He2 was calculated. This transition is assumed to derive its intensity by spin–orbit (SO) induced couplings which are treated using first-order perturbation theory. The first-order corrections to the wave functions are calculated directly in the configuration state function (CSF) basis by solving a system of linear equations given by first-order perturbation theory for the perturbation to the zeroth-order X1Σ+g and a3Σ+u wave functions. This approach eliminates the need to solve explicitly for many eigenstates of the unperturbed Hamiltonian which would be required if the spectral representation for the perturbed wave function were used. The results show a rapidly changing electric transition dipole moment as a function of internuclear seperation, R(He–He), over the bound region of the a3Σ+u potential energy curve, i.e., R(He–He)=1.5 to 4.0 bohr. The transition dipole reaches a maximum near the small barrier to dissociation around R(He–He)=4.5 bohr. A vibrational analysis gives the lifetime of the v=0 level of a3Σ+u to be approximately 18 s, which is consistent with a recent experimental estimate of 10 s (v=?) as a lower bound.
Reactions of Ar+, Ne+, and He+ with SiF4 from thermal energy to 50 eV c.m.
Authors: Weber M. E. , Armentrout P. B.
Published: July 2002

Guided ion-beam techniques are used to measure the cross sections for reaction of SiF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Charge transfer followed by loss of F atoms are the sole processes observed. All SiF+x (x=0–4) products are observed, except for SiF+4 from reaction with Ne+ and He+, and Si+ from reaction with Ar+. At high energies, the dominant products are SiF+3 in the Ar system, and SiF+ in both the Ne and He systems. There is some evidence in the Ne system for an excited state of SiF+3 at 5.7 eV. In the Ar+ and Ne+ reactions, the observed energetics are consistent with literature thermochemistry, but with He+, reaction barriers are observed. A value of ΔH0f,298 (SiF+3)=−30.1±0.9 kcal/mol is derived, which is in agreement with previous values but is much more precise. The observed product distributions and energetics are explained by consideration of the potential energy surfaces and the difference in ionization potentials of the rare gases. Finally, the relationships of these reactions to plasma deposition and etching are discussed.
Translational dynamics in simple dense fluids. II. Self-diffusion
Authors: Kumar Bijoy , Evans Glenn T.
Published: July 2002

Our previous kinetic theory of dense fluids is applied to the calculation of the self-diffusion coefficient of a hard sphere in a hard sphere bath. Corrections beyond an Enskog theory are obtained by the expansion of the recollision operator in terms of a truncated set of four functions; the properties and utility of these four functions, two representing caging and the other two, momentum flows, are determined by the comparison of the computed friction coefficients with the molecular dynamics of Alder et al. Agreement of the present work with existing theories is good, whereas agreement with the molecular dynamics results is still only qualitative.
A theoretical investigation of microscopic dynamics and time scales of predesorption processes
Authors: Muckerman J. T. , Uzer T.
Published: July 2002

When an internal mode of an absorbate is excited with enough energy to break the absorbate–surface bond, the adsorbate may desorb in a manner similar to predissociation (or unimolecular reaction) in isolated molecules. In this article, we set up a simple, microscopic model for this ‘‘predesorption’’ process and investigate the influence of various aspects of the model on the time scale of predesorption. These comparisons include quantum mechanical vs classical treatment, and the inclusion vs neglect of surface vibrations. For this simple analog of a ‘‘unimolecular reaction,’’ we find that classical mechanics predicts shorter lifetimes; and that the effect of surface vibrations is far less pronounced in quantum mechanics.
Translational kinetic energy release in dissociative electron attachment to C2Cl4, CCl4 , CFCl3, and 1,1,1-C2Cl3F3
Authors: Walter C. W. , Smith K. A. , Dunning F. B.
Published: July 2002

Translational energy release in dissociative electron attachment to C2Cl4, CCl4 , CFCl3, and 1,1,1-C2Cl3F3 has been investigated at subthermal electron energies through studies of collisions involving K(nd) Rydberg atoms. For CCl4 and CFCl3 , the observed mean translational energy release is in good agreement with the predictions of quasiequilibrium theories of unimolecular decay, suggesting that the excess energy of reaction is well-distributed among the internal modes of the intermediate negative ion prior to dissociation. In contrast, the data for 1,1,1-C2Cl3F3 indicate rapid dissociation before effective redistribution of the excess energy can occur. Implications of this as regards measurement of free-electron attachment cross sections at very-low energies are discussed.
Vibronic structure of TiO+ from multiphoton ionization photoelectron spectroscopy
Authors: Sappey Andrew D. , Eiden Greg , Harrington Joel E. , Weisshaar James C.
Published: July 2002

We apply the techniques of resonance enhanced multiphoton ionization (REMPI) and time-of-flight photoelectron spectroscopy (TOF-PES) to TiO molecules cooled in a pulsed nozzle expansion to obtain vibronic spectra of gas phase TiO+. The adiabatic first ionization energy is refined to I1(TiO)=54 999±52 cm−1=6.819±0.006 eV, which yields D0(Ti+–0) =159.9±2.2 kcal/mol. For the X 2Δ state of TiO+, we resolve spin–orbit pairs of vibrational levels for v=0–14, yielding ωe=1045±7 cm−1 and ωexe =4±1 cm−1. The spin–orbit splitting ΔEso =210±6 cm−1 permits confirmation of the state symmetry by comparison with the known spin–orbit splittings of the X 3Δ state of TiO. We also observe a new excited B 2∑+ state at T0=11 227±17 cm−1 with ωe =1020±9 cm−1 and ωexe =6±2 cm−1. This state is distinct from the A 2∑+ state (average frequency 860±60 cm−1) previously observed by Dyke and co-workers. From components of certain PESs apparently due to one or more metastable states of TiO, we infer the existence of a previously unobserved state of neutral TiO at T0=2980 cm−1, possibly the 3∑− state. Finally, we discuss the electronic structure and vibrational frequencies of TiO, TiO+, and other third row metal oxides from both molecular orbital and ligand field points of view in order to understand the ordering of electronic states and certain trends in vibrational frequencies. The molecular orbital model readily explains why nominally isoelectronic neutral and cationic metal oxides, such as TiO+ and ScO, are electronically quite dissimilar.
Shape resonance and non-Franck–Condon effects in (2+1) resonant enhanced multiphoton ionization of O2 via the C 3Πg state
Authors: Braunstein M. , Stephens J. A. , McKoy V.
Published: July 2002

We report vibrationally resolved photoelectron angular distributions for photoionization of the C 3Πg Rydberg state of O2. Comparison is made with recent experimental measurements of angular distributions which employ (2+1) resonant enhanced multiphoton ionization of the C 3Πg state. The present theory treats the process as single-photon ionization from an unaligned Rydberg state, and qualitatively accounts for much of the observed trends. Non-Franck–Condon effects induced by the kσu shape resonance lead to a substantial dependence of the angular distributions on the vibrational state of the X 2Πg ion. Discrepancies between our theoretical results and experiment are qualitatively discussed and tentatively attributed to residual electron correlations.
Conjugation length dependent transport in conducting polymers from a resistor network model
Authors: Baughman R. H. , Shacklette L. W.
Published: July 2002

The effect of conjugation length (L) upon electronic conductivity components of conducting polymers is derived using a generalized resistor network model. Results are obtained for polymers which contain a statistical distribution of defects which limit conjugation, as well as for regular copolymers which have a fixed phase relationship between interruptions in conjugation on neighboring chains. The short-conjugation-length limits of the derived equations are identical with those previously obtained by evaluating molecular aspects of charge transfer. More specifically, when interchain transport fully limits both chain-direction conductivity (σ1) and an orthogonal conductivity (σ2), the calculated electrical anisotropy is σ1/σ2=L2/6d2F, where d is the interchain separation in a hopping direction, and the product d2F is the mean square average projection of the interchain vector on the electric field direction. The present analysis extends predictive capabilities over the entire range from short conjugation lengths to infinite conjugation lengths. For long conjugation lengths terminated by effectively infinite barrier defects, σ1/σ1(∞) and σ2/σ2(∞) are calculated from the parameters which define polymer structure and, for the former ratio, the ratio of infinite chain conductivities parallel [σ1(∞)] and orthogonal [σ2(∞)] to the chains. A general relationship, appropriate for a still wider range of conjugation lengths, is derived between σ1/σ1(∞) and [σ2/σ1(∞)]1/2(L/d)/F1/2, where the geometrical parameter F is of order unity in directions of high σ2. Using this relationship for a polymer of known structure, the chain-direction electrical conductivity in the infinite-chain limit can be derived from measurements of σ1, σ2, and average conjugation length. Good agreement is obtained between the calculated and observed dependence of conductivity upon conjugation length for available polymers, in which bulk conductivity is limited by interchain hopping.
The binding energies of one and two water molecules to the first transition-row metal positive ions
Authors: Rosi Marzio , Bauschlicher Charles W.
Published: July 2002

The bonding of water to the transition metal positive ions is electrostatic in origin. The electrostatic bonding is enhanced by a variety of mechanisms: mixing in 4p character, 4s–3d hybridization, and 4s promotion into the compact 3d orbital. The importance of these effects varies between the different metal ions due to changes in the separation of the metal ion atomic states. Furthermore the changes in the metal–water repulsion when a second water is added also changes the relative importance of the different metal asymptotes. The second water binding energy varies from being 11 kcal/mol smaller than the first for Mn+ to 3 kcal/mol larger for V+ and Fe+.
Time-dependent theory of Raman scattering for systems with several excited electronic states: Application to a H+3 model system
Authors: Heather Robert , Metiu Horia
Published: July 2002

The time-dependent formulation of Raman scattering theory is used to study how nonadiabatic interactions affect the Raman spectrum of a model H+3 system, which has two excited electronic states. We start with a formula derived by Heller which gives the Raman scattering cross section as the Fourier transform (over time) of a time-dependent overlap integral. The latter is calculated with a method proposed by Fleck, Morris, and Feit, and extended to curve crossing by Alvarellos and Metiu. In performing these calculations we are especially interested in displaying effects typical of systems having more than one upper state. If the incident laser populates two electronic states there are several ways (i.e., excite to state one and emit from state two, excite to state one, and emit from state one, etc.) by which the Raman process can reach a given final state, and this leads to quantum interference. This interference is manifested in the Raman cross section as approximate selection rules controlling which final states can be reached through the Raman process. These selection rules depend on the relative orientation of the transition dipoles that radiatively couple the ground electronic state with the excited electronic states. The magnitude of the nonadiabatic contribution to the Raman emission, e.g., the contribution from absorbing to state one and emitting from state two, can be determined from the polarization dependence of the Raman emission if the transition dipoles have neither parallel nor antiparallel relative orientation.
Quantum molecular dynamics study of electron transport in an external field
Authors: Kalia Rajiv K. , Vashishta P. , de Leeuw S. W.
Published: July 2002

A quantum molecular dynamics method is proposed to study the transport of a quantum particle interacting with a classical system at finite temp− eratures in the presence of an applied electric field. Using this method the mobility of an excess electron in dense helium gas is calculated at 77 k. (AIP)
Theoretical study of memory kernel and velocity correlation function for condensed phase isomerization. II. Velocity correlation function, barrier crossing rates, and generalized Smoluchowski equation
Authors: Zhu S.-B. , Lee J. , Robinson G. W. , Lin S. H.
Published: July 2002

The time dependent friction (TDF) introduced in paper I for barrier crossing problems is used to derive a functional form of the angular velocity autocorrelation function (AVACF) associated with the reactive coordinate. This theoretical expression is then compared with AVACFs derived directly from molecular dynamics angular velocity data. A connection between this AVACF model and Zwanzig’s waiting-time description can be made. The TDF functions from paper I can also be used to deduce barrier crossing rates using the Kramers–Grote–Hynes formalism. A special case leads to the ‘‘extended’’ Kramers equation, which was found in previous work to be congruous with experimental barrier crossing results. Using methods introduced by Okuyama and Oxtoby, a generalized Smoluchowski equation can also be derived. This Smoluchowski equation permits an examination of free diffusion and of the approach to equilibrium. Throughout papers I and II the theme is raised concerning the effects of nonlinear coupling on the methodology for analyzing barrier crossing problems. Seemingly, the appealing Langevin approach can be retained providing the parameters in this formalism are considered strictly empirical.
Understanding dephasing in mixed molecular crystals. II. Semiclassical dephasing and superradiance for hundreds of coupled absorbers
Authors: Spano Frank C. , Warren W. S.
Published: July 2002

We numerically investigate optical dephasing from as many as 500 interacting two-level absorbers by making the semiclassical approximation on the radiation–matter interaction which includes retarded dipole–dipole coupling and superradiant coupling. We calculate the superradiant decay rate following single pulse optical excitation, the photon echo decay and the fluorescence. By carrying out an expansion to the second order in time we demonstrate a fundamental difference between the homogeneous free-induction decay and photon echo decay which is due to radiation damping. We show that an echo sequence is capable of reversing superradiant damping in some cases. Specific computations for pentacene doped mixed molecular crystals, where ‘‘domains’’ of resonant impurities are likely to exist, are presented and compared with experimental results.
Fourier transform infrared and Raman studies on solid nitrogen dioxide: Temperature cycling of ordered, disordered, and multicomponent layers
Authors: Givan A. , Loewenschuss A.
Published: July 2002

Fourier transform infrared (FTIR) spectra of nitrogen dioxide layers deposited at various rates and substrate temperatures, are reported. Bands assignable to N2O4(D2h) (ordered and disordered layers), to O=N–O–NO2 ‘‘D’’ and ‘‘D′’’ isomers, to NO+NO−3 nitrosonium nitrate and to the NO2 monomer were observed. The relative amounts of species depends upon deposition conditions. Temperature cycling effects were followed up to 205 K. Solid N2O4(D2h) behaves like a normal, stable molecular solid, its disordered layer exhibiting small frequency shifts, but also being stable with respect to temperature cycling. The observation of NO+NO−3 bands is related to the existence of the D′ isomer and of monomeric NO2. Raman experiments overemphasize the NO+NO−3 form due to resonant enhancement.
Spin statistics: An error in Landau and Lifschitz’ Q
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Authors: Jonas David M.
Published: July 2002

In Quantum Mechanics, Landau and Lifschitz derive a formula for the direct calculation of rovibronic statistical weights. If it were correct, this formula would dramatically reduce the effort necessary to calculate statistical weights in large molecules. Unfortunately, due to a common misapplication of the Pauli principle, the formula derived is incorrect, as are the subsequent examples and problem solutions. The correct fomula derived in this paper is used to calculate statistical weights for hydrogen, ethylene, and ammonia which agree with those of Herzberg, Townes and Schawlow, and Bunker.
Li+–OH− intra- and interionic interactions derived from decomposition of a
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o three body force
Authors: Okazaki Susumu , Okada Isao
Published: July 2002

Three body forces acting on lithium, oxygen, and hydrogen atoms have been calculated directly by ab initio molecular orbital methods for more than a thousand relative intra- and interionic Li+-OH− configurations in order to evaluate the accurate interactions. The forces have been decomposed in two ways. The first method expresses the forces on O and H atoms of OH− ion in terms of the translational, the rotational, and the vibrational components. The vibrational forces have been optimized to the differential form of the Morse function and some spectroscopic parameters for OH stretching mode have been evaluated. It is clarified that they are largely dependent on the coordinating position of Li+ ion. A Li+ ion on the oxygen side causes a blue shift of the spectrum while the ion in the hydrogen side takes a red shift. These are directly related to the inhomogeneous broadening in Raman scattering and infrared absorption spectra. The second method leads the forces to a set of pairwise additive interatomic interaction forces. It has been also shown that the interatomic forces obtained here can be applied to higher order many body systems with good precision. The evaluation of the many body force is remarkably improved by considering the three body forces for the pair of the ions.
The vibron–phonon sidebands in the Fourier-transform infrared spectra of the molecular crystal CO2
Authors: Löwen H. W. , Bier K. D. , Jodl H. J. , Löwenschuss A. , Givan A.
Published: July 2002

FTIR investigations on polycrystalline CO2 at various temperatures show Stokes and anti-Stokes sidebands coupled to the vibrational modes v3, v2, and v+/v− (Fermi dyad). The latter is infrared forbidden and may be eliminated from spectra of good quality polycrystalline films. The main peaks in the structure of the sidebands may be assigned to phonons from points of highest symmetry in the Brillouin zone. The vibron–phonon coupling process is mode specific and driven by electrical anharmonicity in case of v+ and v−. For the infrared allowed fundamentals, and especially at the Γ point, the coupling process is also driven by mechanical anharmonicity. The dispersion of these internal modes at k=0 influences the coupled phonons and thus the one phonon density of states is correctly mirrored by the v+ and v− sidebands only.
Optogalvanic observation of the CO W 1Π–B 1∑+ transition
Authors: Sekine Shigeyuki , Adachi Yukio , Hirose Chiaki
Published: July 2002

The rotational structure of the W 1Π–B 1∑+ (0–0) band was resolved and the values of spectroscopic parameters were derived as B′0 =1.5549±0.0068 cm−1, r0=0.1257±0.0003 nm, and ν00=15 891.6±0.6 cm−1. The observation of the W 1Π–B 1∑+ (0–0) band supports Ogawa and Ogawa’s assignment that the W 1Π state is the first member of the nsσ Rydberg series converging to the A2 Πi state of CO+ ion. The W 1Π–B 1∑+ (1–0) band, on the other hand, was not observed with expected intensity and the presence of dissociative perturbation has been suggested.
Rotational energy distribution of products in fast hydrogen atom–molecule collisions
Authors: Kim Y. H. , Micha D. A.
Published: July 2002

Starting from the formalism of collisional time-correlation functions, an expression is derived for the double differential cross section (with respect to scattering angles and final rotational energies) of molecules with a thermal distribution of initial rotational states, colliding with fast atoms. This expression is valid when the duration of the collision is short compared with the periods of internal motions of target molecules. The formulation leads to simple distributions in terms of error functions, which can be used to parametrize experimental results. The parameters in turn give the average rotational energy and its standard deviation for the final state. This procedure is followed to interpret recent experimental measurements of the final rotational distributions of CO and CO2 colliding with fast hydrogen atoms obtained from the photolysis of hydrides. For CO(v=1) and CO2(0001), in which the collisions are impulsive, the derived formula fits the experimental distribution very well. For CO(v=0), in which long-duration, complex-forming collisions play an important role, an additional statistical distribution can be introduced to satisfactorily explain the experimental results.
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o studies on the radiationless deexcitation mechanism of ketene and its rearrangement to oxiranylidene
Authors: Yoshimine M.
Published: July 2002

Multiconfiguration self-consistent-field (MCSCF) and multireference configuration interaction (MRCI) calculations have been performed to investigate a possible surface crossing between the ground state and first excited 1A″ state of ketene, and to determine a rearrangement path between ketene and its isomer oxiranylidene. The crossing of the excited 1A″ surface with the ground 1A′ surface is near the excited state equilibrium geometry, providing a firm basis for a fast internal conversion mechanism for electronically excited ketene. The transition state for the ketene to oxiranylidene rearrangement is found to lie 81 kcal/mol above the 1A1 state of ketene and 5 kcal/mol below the CH2(1A1)+CO(1∑+) dissociation limit. Mechanisms emerging from the present study for radiationless deexcitation, and for isotope scrambling in the photolysis of doped ketene, are presented and discussed.
Optical properties of molecules forming a uniform, uniaxial layer on a small, isotropic sphere
Authors: Mamiche-Afara Suzanne , Robichaud Louis , Dignam Michael J.
Published: July 2002

This project was inspired by the widespread use of the properties of coated spheres in modeling the dielectric and spectroscopic behavior, including surface enhanced Raman spectroscopy, of a variety of dispersed systems. In this paper the self-polarizing contributions to the absorption and elastic scattering properties of a coated sphere are obtained in the large wavelength limit, or dipole approximation, in which the coating is treated as a polarizable, uniaxially anisotropic, shell with the local optic axis directed radially. Numerical calculations for CO on Ag are provided to illustrate some of the phenomena predicted by the equations. Of particular interest are the dependence of the absorption band for CO stretch upon particle size, surface coverage, and molecular orientation, and the dependence of the surface plasmon resonance absorption on particle size and coverage. Some limitations of the model are examined.
Atom–diatom reactive scattering. II. H+H2 and its isotopomers, J=0
Authors: Webster Frank , Light J. C.
Published: July 2002

The solution of the reactive scattering problem by R-matrix propagation requires a number of choices which may affect the numerical accuracy. Although the formal definitions and development were given in detail in the preceding paper, in this paper we give (a) the specific choices and numerical procedures followed in our calculation, (b) a discussion and evaluation of the stability of the calculation with respect to these choices, and (c) results for three dimensional J=0 reactive scattering of H3, H2D, and HD2 systems in the energy range from threshold to about 1.30 to 1.50 eV.
Analysis of optical emissions produced by dissociative electron impact on CCl2F2
Authors: Jabbour Z. J. , Becker K.
Published: July 2002

We analyzed the optical emissions in the wavelength region 2000–8000 Å produced by dissociative electron impact on CCl2F2. Absolute photoemission cross sections have been determined for a variety of neutral and ionic fluorine and chlorine lines as well as for the strong diatomic CCl and CCl+ bands at 2778 and 2368 Å, respectively. In many cases comparisons between experimentally determined appearance potentials and spectroscopic and thermochemical data enabled a unique identification of the underlying break-up mechanism of the parent molecule upon electron impact. Atomic fluorine emissions which are the result of the total fragmentation of the parent molecule dominate the spectrum from 6000 to 8000 Å with absolute emission cross sections in the range of 0.1 to 3.5×10−19 cm2 at 100 eV for individual 3p→3s fine structure lines. The prominent continuous emission between 2200 and 4000 Å was found to consist of two contributions, the D 2B2→X 2B2 emission of the CCl2F+2 parent ion with an appearance potential of 14.2±1.0 eV and a second emission feature with an appearance potential around 42 eV which has not been uniquely identified.
A determination of the crystal structure of ice XI
Authors: Howe Rachel , Whitworth R. W.
Published: July 2002

Below 72 K ice doped with KOH transforms to an ordered phase known as ice XI. A powdered sample of such doped ice annealed for several days below 72 K was studied by neutron diffraction and found to be a two-phase mixture of ice Ih and ice XI. The ice XI has an orthorhombic unit cell, and the diffraction profile is consistent with the predicted Cmc21 structure.
Semiempirical configuration interaction calculations of shake-up satellites in formaldehyde, benzene, and benzaldehyde
Authors: Lunell S. , Keane M. P. , Svensson S.
Published: July 2002

The core photoionization shake-up spectra of benzaldehyde, benzene, and formaldehyde are discussed using a new computational package based on the method of intermediate neglect of differential overlap (INDO) including configurational interaction (CI). The computations include all singly excited and a large selection of the doubly excited states in the CI. The inclusion of the doubly excited states leads to a reduction of the overall intensities and to an elimination of artificial intensity in high and low shake-up peaks. A reassignment of one of the structures in the benzene spectrum has been made. In the case of benzaldehyde, where the shake-up spectrum is assigned for the first time, the calculated spectrum shows very good agreement with experiment for C1s ionization, whereas the O1s spectrum shows larger deviations due to a stronger core–valence interaction. By utilizing information from analogous calculations on the smaller molecules benzene and formaldehyde, however, it is possible to assign also the O1s spectrum in this case.
Computer modeling of naphthalene
Authors: Sediawan Wahyudi B. , Gupta Sumnesh , McLaughlin Edward
Published: July 2002

Molecular dynamics simulations have been performed to model naphthalene using an anisotropic two-site pair potential with the interactions between these sites modeled using the modified Gaussian overlap potential. This potential works reasonably well for thermodynamic properties of saturated liquid naphthalene and is shown to be comparable to an isotropic ten-site potential for thermodynamics and microscopic liquid structure. Adding point quadrupoles to the interaction sites further improves the comparison for density of the saturated liquid. However, the effect of electrostatics on thermodynamics in this case is much less than that found earlier for liquid benzene. Feasibility of perturbation theory, for the nonpolar potential, is demonstrated through the reference fluid simulations. Predictive methods are discussed in site frame as well as center frame and in a preliminary testing the latter is shown to be qualitative for predicting the residual Helmholtz free energy. The idea of anisotropic site–site pair distribution function has also been discussed.
Calculation of the cooperative and self-friction coefficients in a dilute suspension of spheres. II. Three-body effects
Authors: Jones Dumont M. , Muthukumar M. , Cohen Scott M.
Published: July 2002

By employing the computational methods of Jones and Muthukumar, and the hydrodynamic scattering formalism of Muthukumar and Freed, we have computed the translational friction and diffusion coefficients for a dilute suspension of spheres to second order in concentration. These results are compared with previous calculations and experiment.
Theroretical study of B2H+3, B2H+2, and B2H+
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

An ab initio molecular orbital study is presented of B2H+3, B2H+2, and B2H+ at the G1 level of theory, including correlation energy beyond fourth-order perturbation theory and large basis sets. The structures of these ions are found to contain no hydrogen bridges in contrast to the previously studied ions (B2H+6, B2H+5, and B2H+4) in this series. Good agreement is found with a recent photoionization measurement of the appearance potentials of the B2H+3 and B2H+2 ions from B2H6.
Sticking probability of light atoms on the graphite (0001) surface
Authors: Buch V.
Published: July 2002

Sticking of light gas atoms (He,D,H) on the (0001) surface of graphite is studied theoretically in the framework of the quantum mechanical perturbation theory. The formulas include a detailed microscopic description of the solid structure and vibrations. The calculations are employed in conjunction with the published experimental data to investigate the properties of the inelastic gas–surface coupling in the He/graphite system. The inelastic coupling is suggested to have a significant attractive component, and to be confined to highly localized spots on the surface. The low energy sticking probability is shown to be very sensitive to the weak coupling between the graphite layers. The high energy sticking via excitation of the optical out-of-plane phonons is found to be very inefficient. The isotopic effect is investigated in the H,D/graphite system. While the heavy isotope sticks about two times more efficiently in the low energy limit, the decay of the sticking probability with the energy is slower for the lighter isotope. Therefore the difference in the sticking probability of the two isotopes decreases strongly towards higher energies.
Excitation of chemical waves in a surface reaction by laser-induced thermal desorption: CO oxidation on Pt(100)
Authors: Fink T. , Imbihl R. , Ertl G.
Published: July 2002

If a Pt(100) sample is kept at steady-state conditions of O2 and CO partial pressures and temperature which are similar to those for the occurrence of autonomous temporal oscillations in the rate of CO2 formation, then the surface will be largely covered by adsorbed CO which inhibits oxygen adsorption and keeps the catalytic rate low. Irradiation of a small spot with a high power laser pulse causes momentarily local thermal desorption of CO and creation of a reaction front which propagates as a chemical wave across the surface area, as was monitored by the excursion of the integral reaction rate and locally by means of a work function (=oxygen coverage) probe placed at several mm distance from the spot of irradiation. The velocity for wave propagation rises from about 2 mm/min at 480 K to 4 mm/min at 507 K and is not noticeably dependent on the partial pressures. The mechanism is closely related to that for self-sustained kinetic oscillations of this system and exhibits the typical features of trigger waves: Coupling between autocatalytic reaction and diffusion, as well as the occurrence of a refractory period during which the system is ‘‘dead,’’ and of a threshold for the intensity of the excitation.
Exact analytical solution of the rotational–translational diffusion equation with mixed boundary conditions. An application to diffusion-controlled enzyme reactions
Authors: Baldo Marcello , Grassi Antonio , Raudino Antonio
Published: July 2002

A theoretical model was proposed for the orientational effects in diffusion-controlled enzyme reactions. To this purpose, we attained the exact solution of the rotational–translational diffusion equation (RTDE) with mixed boundary conditions (MBC). In steady-state conditions, the assumed MBC were: (i) the reactant molecules are chemically modified at the enzyme active site, provided that their relative orientation lies within a given angular range; (ii) the reactant molecules outside the active site or for uncorrect orientations are reflected; (iii) the concentration of reactants is constant at infinite distance from the active site. The developed exact analytical procedure led to a system of linear algebraic equations, which was numerically solved for the simple case of only one angular variable (plane–rotor approximation). The procedure was found to be well suited to calculate the kinetic constant of the enzymatic as a function of different physical parameters, such as the translational and rotational diffusion coefficients, the range of allowed orientations of the reactants, and the size of the active site of the enzyme.
Statistical dynamical theory of isomerization
Authors: Dumont Randall S.
Published: July 2002

An ergodic theoretic basis for the statistical theory of isomerization is provided. A strong mixing assumption is used to derive the absorbing boundary method of computing isomerization dynamics. In addition, the absorbing boundary method is shown to fail in systems exhibiting certain long-time correlations. In order to account for these correlations, we construct a new statistical theory termed the ‘‘flux renewal model.’’ The new model is based on the consistent application of strong mixing, with the incorporation of nonzero relaxation time. It utilizes statistical calculations to eliminate the explicit computation of long-time trajectories exhibiting characteristics of chaos. The flux renewal model is tested and compared with the absorbing boundary method via numerical computations of the isomerization dynamics of the chaotic siamese stadium billiard. The flux renewal model is shown to give the best approximation to the isomerization flux–flux correlation. It does this by simultaneously handling nonstatistical correlations and long-time scale exponential decay. In addition, with less computational effort, the flux renewal model generally provides more accurate rate constant estimates than the absorbing boundary method.