bluuebunny/crossref_metadata_2025 huggingface dataset:
Complex and simple relaxations observed in thermal-activation phenomena
Authors: Kogure Y. , Hiki Y.
Published: July 2002

Among the relaxations of thermal-activation type τ=τ0 exp(E/kBT), “complex” relaxations showing the following abnormal characteristics were noted: (1) a strong correlation between the activation energy E and the preexponential factor τ0; (2) unreasonably small values of τ0(≪10−14 s) and unusually large values of E. As examples, (a) the viscosity relaxation in amorphous materials near the glass transition, and (b) the anelastic relaxation in heavily cold-worked alloys are shown. These are compared with examples of relaxation with normal characteristics, or “simple” relaxation. In complex relaxation, the “compensation effect,” a linear decrease of log τ0 with increasing E, can be seen. Cooperative jumps of relaxing elements were taken into consideration, and a simple formalism for interpreting the complex relaxation was derived. The above examples of complex relaxation were analyzed by considering (a) the cooperative movement of several atoms over potential barriers existing in the amorphous materials, and (b) the cooperative movement of dislocation segments over a potential barrier resulting from impurity atoms.
Interface controlled conduction in a blue-light emitting SrS:Cu, Cl electroluminescent phosphor
Authors: Lee Yun-Hi , Shin Kyeong-Sik , Park Jung-Ho , Ju Byeong-Kwon , Oh Myung-Hwan
Published: July 2002

The leakage current mechanism of the electron beam evaporated SrS:Cu, Cl films was investigated as functions of electric field and temperature. The activation energy and interfacial barrier height of electronic conduction for Al–SrS:Cu, Cl–In2O3:Sn-glass structures were determined by analyzing direct current–voltage characteristics of these structures. We observed in this study that postannealing of the SrS:Cu, Cl films in H2S atmosphere leads to a reduction of leakage current as well as an increase of interfacial barrier height for the leakage current flowing in these films, regardless of the polarity of applied voltage. On the basis of this observation, we suggest that surface modification induced by the postannealing in H2S atmosphere may improve the electrical properties of the SrS:Cu films.
Edge-emission electroluminescence study of as-grown vertical-cavity surface-emitting laser structures
Authors: Ghosh Sandip , Constant Stephanie , Hosea Thomas J. C. , Sale T. E.
Published: July 2002

We report polarized edge- and front-emission electroluminescence studies on red-emitting vertical-cavity surface-emitting laser (VCSEL) structures. The measurements were performed nondestructively on pieces of as-grown wafers using indium–tin–oxide-coated glass electrodes. The front-emission spectra helped determine the Fabry–Pérot cavity-mode wavelength, while the edge-emission spectra were used to identify the wavelength of ground-state emission from the quantum wells (QWs) in the active region. However, measurements on edge-emitting laser (EEL) structures with a similar QW active region reveal that the peaks of the edge-emission spectra are always slightly redshifted with respect to front emission. We show that this arises due to reabsorption effects and then appropriately correct for it in the VCSELs by studying such shifts in the equivalent EELs. Thereafter, by comparing the experimental results with theoretical calculations and simulations, we estimate the composition, strain, and material quality of the QWs in the VCSEL active regions. Finally, we comment on the usefulness of comparing the two orthogonally polarized edge-emission spectra.
Investigation of the differences in stability of the OC⋅⋅⋅HF and CO⋅⋅⋅HF complexes
Authors: Curtiss Larry A. , Pochatko David J. , Reed Alan E. , Weinhold Frank
Published: July 2002

The structure and energetics of the isomeric H-bonded complexes OC⋅⋅⋅HF and CO⋅⋅⋅HF have been investigated by ab initio molecular orbital theory and by natural bond orbital analysis. Only with the inclusion of electron correlation is a significant preference for the experimentally observed OC⋅⋅⋅HF isomer found. The large effect of correlation upon the relative stability of the two isomers is apparently entirely an electrostatic effect caused by the correlation-induced sign reversal of the dipole moment of CO. Nevertheless, a molecular multipole expansion is found inadequate to account for the principal features of these H-bonded complexes and their relative stability. Contrary to a recent study, we find that ‘‘charge transfer’’ effects are highly significant contributions to the binding in these complexes. The differences in stability of OC⋅⋅⋅HF and CO⋅⋅⋅HF are attributed primarily to differences in the interaction of carbon and oxygen lone pairs of CO donating into the unfilled antibond on HF, i.e., to differences in nC→σHF* and nO→σHF* matrix elements.
A molecular dynamics study of the reaction H2+OH→H2O+H
Authors: Rashed O. , Brown N. J.
Published: July 2002

Classical trajectory calculations have been performed to determine the influence of translational temperature, H2 vibrational energy, H2 rotational energy, OH vibrational energy, and OH rotational energy on the reaction H2+OH→H2O+H. The potential energy surface was a modification of the Schatz–Elgersma analytical fit to the Walsh–Dunning surface. Reactivity increases with translational temperature, and is most strongly influenced by it. Rotational excitation of either or both molecules suppresses reactivity. Vibrational excitation of H2 enhances reactivity, and vibrational excitation of OH has no effect. A thermal rate coefficient was computed for the reaction at 1200 and 2000 K. The computed value compares favorably with the experiment at 2000 K, while the agreement at 1200 K is less satisfactory. The agreement between theory and experiment at both temperatures indicates that the potential surface is a reasonable representation of the HHOH potential energy surface.
Evidence for inhomogeneous broadening in vibrational overtone transitions: Formation of 1, 3-cyclohexadiene from c
i
s-1, 3, 5-hexatriene
Authors: Chuang Mei-Chen , Zare Richard N.
Published: July 2002

By pumping the 4–0 and 5–0 C–H overtones of cis-1, 3, 5-hexatriene, the rates of 1, 3-cyclohexadiene formation are measured using a Stern–Volmer analysis. While the rate for 5–0 C–H excitation is faster than 4–0 C–H excitation, the latter is about one order of magnitude larger than that calculated from an RRKM treatment. Moreover, nonmonotonic variation in rates are found as a function of excitation wavelength within each overtone region. These observations cannot be explained by assuming pure homogeneous broadening of each overtone transition, but can be rationalized by including inhomogeneous contributions, which may originate from hot bonds in the initial state and/or from a rotational dependence of this electrocyclic reaction rate near threshold.
Towards a full CCSDT model for electron correlation
Authors: Urban Miroslav , Noga Jozef , Cole Samuel J. , Bartlett Rodney J.
Published: July 2002

Coupled cluster models for electron correlation which include the effects of single, double, and triple excitation operators are analyzed. An alternate version of the approximate CCSDT-1 method is implemented. In this version, the full CCSDT cluster operator eT1+T2+T3 is preserved in the creation of single and double excitation coefficients, but in calculation of triple excitation coefficients only the T2 operator is used. We also present a theoretical analysis of the simplest improvement for the evaluation of the contribution of triples beyond that obtained with fourth-order MBPT. In this approximation, an MBPT(4)-like calculation of the triples energy is evaluated with converged CCSD T2 coefficients. This is found to offer a good approximation to the converged CCSDT-1 results.
Reactions of iron clusters with hydrogen. III. Laser-induced desorption of H2 by multiphoton absorption
Authors: Liu K. , Parks E. K. , Richtsmeier S. C. , Pobo L. G. , Riley S. J.
Published: July 2002

Multiphoton absorption and ionization of hydrogenated (or deuterated) iron clusters generally leads to desorption of a specific number of H2 (or D2) molecules for a given cluster size and a given number of photons absorbed. Ionization via single photon absorption occurs without desorption. Experimental results demonstrate that the fragmentation pattern resulting from multiphoton absorption is independent of whether desorption precedes or follows ionization. From the number of desorbed molecules and the number and energy of absorbed photons an estimate of 1.3 eV for the desorption energy can be made without the necessity of modeling the desorption process. A simple statistical model of the process provides similar estimates of the desorption energies, and indicates that the energy has some dependence on cluster coverage.
The microwave and radio frequency rotation–inversion spectrum of (SO2)2
Authors: Nelson D. D. , Fraser G. T. , Klemperer W.
Published: July 2002

The radio frequency and microwave spectrum of (SO2)2 has been measured using the molecular beam electric resonance technique. The spectrum is characteristic of an asymmetric top in which the two nonequivalent SO2 subunits interchange roles through a low frequency (70 kHz) tunneling motion. The spectroscopic constants obtained for SO2 dimer are: (B+C/2) (MHz)  926.160(2),    (B−C/2) (MHz)   22.3207(1),   A−(B+C/2) (MHz) 6032.3(6),      ΔJ(MHz)    0.00217(2),  ΔJK(MHz)    0.0995(1),   δinv(MHz)    0.070(1),    μa(D)    1.4052(13).  The average distance between the centers of mass of the two subunits, RCM, is 3.825(10) Å. The magnitude of the weak bond stretching force constant ks is 0.0264(4) mdyn/Å. The relative orientation of the subunits is not well determined, but is demonstrated to be unlike the orientation of the nearest neighbors in the sulfur dioxide crystal.
Charge transfer: An electronic bath approach
Authors: McDowell H. Keith
Published: July 2002

A charge transfer theory is developed for systems in which an atom, molecule, or ion interacts for a period of time with a substrate. The Hamiltonian of the system is taken to be one-electron and the nuclear motion of the substrate is ignored. The theory is structured so as to mimic generalized Langevin theory; namely, the equations of motion for annihilation operators of spin orbitals localized at the interaction site are developed such that a primary zone is defined with the remainder of the substrate being included through a memory kernel and a driving term. An electronic bath approximation is presented which leads to equations of motion for the occupation numbers of spin orbitals in the primary zone. These equations have the property that primary-zone occupation numbers damp to an equilibrium state at long time independent of their initial values. Specific application to a substrate modeled as a semi-infinite, Huckel chain is presented and numerical results are obtained. The electronic bath approximation is found to produce results in agreement with numerically exact data.
An improved closure for the Born–Green–Yvon equation for the electric double layer
Authors: Caccamo C. , Pizzimenti G. , Blum L.
Published: July 2002

An extensive series of calculations are made to compare an improved closure of the Born–Green–Yvon (BGY) equation for the electric double layer of primitive electrolytes to existing Monte Carlo simulation and other theories, such as the modified Poisson–Boltzmann (version 5) (MPB5) and the hypernetted chain/mean spherical approximation (HNC/MSA), and its recent improvements. In contrast to these theories the BGY equation satisfies the contact theorem always. Furthermore, the bulk pair correlation functions used are the most accurate available, namely, the HNC bulk results. The results show very good general agreement to the computer work for high densities and surface charges. In particular, we are able to go to higher densities than those hitherto published in the literature. For low density 2–2 and 2–1 electrolytes, the agreement is not as good as for the other theories.
Analytic evaluation and basis set dependence of intensities of infrared spectra
Authors: Yamaguchi Yukio , Frisch Michael , Gaw Jeffrey , Schaefer Henry F. , Binkley J. Stephen
Published: July 2002

Equations are presented for the analytic determination of dipole moment derivatives with respect to nuclear coordinates for closed-shell, open-shell unrestricted, and open-shell restricted Hartree–Fock wave functions. The efficient evaluation of these derivatives and the resulting infrared intensities simultaneously with determination of the vibrational frequencies is discussed. Intensities are presented for a selection of test molecules with a wide variety of basis sets. It is concluded that basis sets of double-zeta polarized or higher quality usually give correct qualitative information about the ordering of the intensities, while smaller basis sets may not even predict the most intense mode correctly. Quantitative accuracy using the larger basis sets seems to be limited primarily by the use of the double harmonic approximation.
Dissociative chemisorption dynamics of H2 on Ni and Cu surfaces: Morphology and surface temperature effects
Authors: Lee Chyuan-Yih , DePristo Andrew E.
Published: July 2002

A new representation of the potential energy surface (PES) for diatomic molecule–solid surface interactions is presented. It is based upon a delocalized effective medium form (i.e., homogeneous gas analogy) plus empirical two-body terms and is transferable from face to face of a crystal. We have determined the parameters for H2–Cu and H2–Ni from experimental measurements of H–Cu and H–Ni adsorption energy, frequency, and height and from limited molecular beam scattering data. The PES for H2–Ni(100), Ni(110), Ni(111), and H2–Cu(110), Cu(100) are presented. A dynamical simulation using the classical trajectory-generalized Langevin equation ‘‘ghost’’ atom formalism is implemented in order to calculate the dissociative sticking probability as a function of incident kinetic energy and surface temperature. The calculated probabilities are in general qualitative agreement with those measured experimentally, and are superior to those based upon the previous many-body expansion approach [J. Chem. Phys. 84, 485 (1986)] in the following features: (1) the angular distribution of reflected molecules is more specular, (2) Ni(100) has a barrier for dissociation so that the dissociation probability depends on the translational energy, and (3) the many body expansion approach requires very high order terms for H2–Cu systems. Quantitative agreement is not obtained especially for the surfaces which exhibit barriers to dissociation. The PES for H2–Ni(111) is different from the previous one and suggests an interesting dissociation mechanism. It was also found that the participation of d electrons is essential for the structure sensitivity of the H2 dissociative chemisorption on Ni surfaces. On Cu with a 3d10 4s1 configuration, no promotion of 4s to 3d electrons is possible and much less structure sensitivity is shown.
Vibrational overtone spectroscopy of bound and predissociative states of hydrogen peroxide cooled in a supersonic expansion
Authors: Butler L. J. , Ticich T. M. , Likar M. D. , Crim F. F.
Published: July 2002

The vibrational overtone excitation spectra of both bound and predissociative states of hydrogen peroxide molecules cooled in a supersonic expansion show features that are obscured otherwise. Spectra of predissociative states are measured by detecting the decomposition product following excitation of an overtone vibration. Spectra of bound states are obtained by a two-photon excitation technique in which a second photon excites the molecule from its bound vibrational overtone state to a dissociative state. The features in the bound state (4νOH) spectrum are 0.08 to 0.13 cm−1 wide, reflecting small inhomogeneous broadening, but those to the predissociative state (6νOH) are 1.5±0.3 cm−1 wide. This width, which corresponds to a lifetime of about 3.5 ps, reflects coupling into the dissociative continuum.
Molecular beam translational spectroscopy of physisorption bound states of molecules on metal surfaces. I. HD on Cu(111) and Au(111) single crystal surfaces
Authors: Harten U. , Toennies J. P. , Wöll Ch.
Published: July 2002

A new method for studying resonances in the scattering of molecules from metal surfaces is presented. In the experiment the velocity distribution of specularly scattered molecules with an initially broad distribution is analyzed with a time-of-flight technique and reveals sharp dips and peaks with a width of <0.25 meV attributed to Feshbach resonances. The present results for HD–Cu(111) and HD–Au(111) are analyzed by comparison with a close coupling calculation to provide potential well parameters. The method has been used to study the surface temperature dependence of one of the resonances down to 40 K and provides insight into a possible elementary step in physisorption.
Scattering of highly vibrationally excited Li2 from He and Kr
Authors: Fuchs M. , Toennies J. Peter
Published: July 2002

Total integral scattering cross sections have been measured for Li2 molecules in selected rotational and vibrational states up to v=21. The vibrationally highly excited states with energies up to 80% of the dissociation energy were prepared by Franck–Condon laser pumping. With increasing vibrational quantum number the observed cross sections show first a 10% decrease followed by a 20%–25% rise. A model potential has been developed based on an ab initio potential for the vibrational ground state to account for the effect of vibrational stretching. The results provide for the first time direct information on the bond-distance dependence r of the complete potential hypersurface V(R,r,γ) (R distance between centers of mass and γ angle between r and R) of an atom–molecule system.
Vibrational frequency shifts in dense molecular fluids
Authors: LeSar R.
Published: July 2002

A theory is presented for calculating the bond length and vibrational frequency shifts in dense molecular fluids. We combine equation-of-state calculations based on the effective spherical potentials of Shaw, Johnson, and co-workers with a simple model for the interactions of a molecule with the surrounding fluid. The repulsive interactions are approximated with the hard-sphere model of Schweizer and Chandler. We extend their model by including an improved treatment of the long-range attractive interactions and the centrifugal forces. We apply this model to fluid nitrogen and compare with static and shock wave experimental frequency shifts, as well as computer simulation data, over a wide range of pressures and temperatures. We find that while the results using this simple model are reasonably close to experiment in the region appropriate to shock conditions, agreement with data taken under static high-pressure conditions (at lower temperatures) is not as good. Inadequacies of the model and possible improvements are discussed.
A density functional approach to freezing transitions in molecular fluids: Dipolar hard spheres
Authors: McMullen William E. , Oxtoby David W.
Published: July 2002

We extend a density functional theory of atomic systems to simple molecules using first-order perturbation theory. We model the hydrogen halides as dipolar hard spheres and consider freezing into two orientationally different fcc lattices. In one of these, the dipoles may align parallel to a single, space-fixed axis while in the other, the dipoles in adjacent faces of the fcc unit cell are constrained to lie perpendicular to one another. The second structure approximates the experimentally observed molecular crystals of HF and HCl, and the phase diagram calculated from this model agrees qualitatively with experiment. At high temperatures, or for weak enough dipole moments, a plastic (orientationally disordered) solid precedes the molecular crystal. The plastic phase is unaffected by the dipolar perturbation but eventually becomes a molecular crystal upon lowering the temperature or increasing the density. We also discuss second-order perturbation theory, the effects of higher multipoles, and some general consequences of the density functional theory of molecular systems.
Energy-adjusted a
b i
n
i
t
i
o pseudopotentials for the first row transition elements
Authors: Dolg M. , Wedig U. , Stoll H. , Preuss H.
Published: July 2002

Nonrelativistic and quasirelativistic ab-initio pseudopotentials representing the Ne-like X(Z−10)+ cores (X=Sc–Zn) of the first row transition metals and optimized (8s7p6d1f )/[6s5p3d1f ]-GTO valence basis sets for use in molecular calculations have been generated. Excitation and ionization energies of the low lying states of Sc through Zn from numerical HF- as well as SCF- and CI(SD)-pseudopotential calculations using the derived basis sets differ by less than 0.1 eV from corresponding all-electron results.
Picosecond solvation dynamics of coumarin 153: The importance of molecular aspects of solvation
Authors: Maroncelli Mark , Fleming Graham R.
Published: July 2002

Solvation dynamics in polar liquids have been examined using the probe molecule coumarin 153 (Cu153) and picosecond spectroscopic techniques. Steady-state absorption and fluorescence spectra of Cu153 as a function of solvent show that the frequency of the electronic spectrum of this probe provides a convenient measure of solvation energetics. Both nonspecific dipolar and to a smaller degree H-bonding solute–solvent interactions are involved. Time-correlated single photon counting was used to observe time-dependent shifts of the fluorescence spectrum of Cu153 in a variety of alcohols, propylene carbonate, and N-methylpropionamide solvents as a function of temperature. These time-dependent spectral shifts provide a direct measure of the time dependence of the solvation process. Theoretical models that treat the solvent as a dielectric continuum do not adequately account for the observed solvation dynamics. In the solvents studied, such theories predict a single exponential shift of the fluorescence spectrum with a time constant equal to the longitudinal relaxation time (τL) of the solvent. We find that solvation is nonexponential in time and that the average time constant observed is, in general, greater than τL. As the dielectric constant of the solvent increases the disagreement between the observed and predicted solvation times becomes more pronounced. For N-methylpropionamide (ε0∼300 at 245 K) solvation is observed to occur 15 times slower than predicted. These results are discussed in terms of the importance of general molecular aspects of solvation not included in a continuum description. The rotational dynamics of Cu153 in a number of polar solvents has also been examined using time-resolved fluorescence anisotropy measurements. In addition to the ‘‘normal’’ diffusive rotational dynamics we observe a much faster component of the anisotropy decays in polar solvents. The latter rotational component is correlated to the observed solvation time and appears to be due to rapid rotation of the transition dipole of Cu153 in response to changes in the solvent environment.
A theoretical basis for the correlation between bond length and local mode frequency
Authors: Swanton David J. , Henry Bryan R.
Published: July 2002

We construct a modified Morse potential which allows us to obtain equations which relate the equilibrium internuclear distance in a Morse oscillator re to the experimental quantities ωe and ωexe. The results provide a theoretical basis for the observed correlation of overtone spectral frequency shifts and changes in equilibrium bond lengths. We also use our results to discuss the basis of the many empirical and nonempirical relationships that have been proposed between re and spectral observables. We illustrate the applicability of our equations for re through calculations of CH bond lengths for a number of polyatomic molecules. We also use our approach to determine bond lengths for a number of diatomic molecules from measured spectroscopic constants. We compare these results to results obtained from other relationships.
Laser-assisted deposition of iron on Si(111)-(7×7): The mechanism and energetics of Fe(CO)5 decomposition
Authors: Swanson Jon R. , Friend C. M. , Chabal Y. J.
Published: July 2002

The adsorption and decomposition of Fe(CO)5 on the Si(111)-(7×7) surface has been investigated using multiple internal reflection Fourier transform-infrared, Auger electron, and temperature programmed desorption spectroscopies and low energy electron diffraction under ultrahigh vacuum conditions at 120 K. Resonant decomposition of adsorbed Fe(CO)5 via a multiphoton electronic excitation of the molecule was observed using laser photolysis. Iron deposition was induced efficiently by ultraviolet but not visible photons with high cross sections, in other words, only in the wavelength region where excitation of metal–ligand charge transfer bands and d–d transitions occur. In addition, Fe(CO)5 decomposition was induced using 1.6 keV electrons. No thermal reaction was apparent in temperature programmed desorption experiments. Only molecular Fe(CO)5 desorption was observed at temperatures of 150 and 170 K. Significant amounts of carbon were deposited from the electron induced decomposition while no residual oxygen or carbon were detected in the photodeposited Fe. No partially decarbonylated Fe(CO)x, x<5, fragments were detected subsequent to exposure to ultraviolet photons using infrared spectroscopy. In addition, no new features were observed in a temperature programmed desorption experiment after laser

photolysis. These data suggest that there are no surface stable Fe(CO)x, x<5, species in the photodeposition process. Instead, photolysis yields adsorbed Fe atoms without trapping of iron carbonyl fragments, even at low temperatures. Multiple photons were required to induce the Fe(CO)5 decomposition based on the fluence dependence of the photodecomposition yield. These data are consistent with gas phase photodecomposition energetics. The two mechanisms proposed are rapid sequential ejection of CO into the gas phase with loss of the first CO being rate limiting or simultaneous ejection of all CO. An unresolved low frequency shoulder did appear in the infrared spectrum after exposing the Fe(CO)5 covered Si(111)-(7×7) crystal to the electron beam, possibly due to formation of a distribution of surface stable Fe(CO)x, x<5, fragments. The relative Fe:Si Auger peak intensities after photolysis and annealing to 300 and 1000 K were different by a factor of 2 indicative of diffusion of Fe into the crystal and probable silicide formation. Molecular carbon monoxide could not be readsorbed on a surface where Fe(CO)5 had been photolytically decomposed and annealed to 300 or 1000 K, further evidence that isolated iron is not formed in the laser deposition process.
Efficient formulas for molecular integrals over the Hiller–Sucher–Feinberg identity using Cartesian Gaussian functions: Towards the improvement of spin density calculations
Authors: Momose Takamasa , Shida Tadamasa
Published: July 2002

Recurrence relations are derived for the molecular integrals arising in the use of the Hiller–Sucher–Feinberg identity. In comparison with the formulas obtained by other methods, the present result gives simpler formulas, and efficient computations are feasible. To perform the numerical integration the double exponential formula developed by Takahashi and Mori is found suitable for accurate and expeditious calculations.
Simplified treatment of organic substituents in SCF-CI calculations. The methyl group
Authors: Cimiraglia Renzo , Maynau Daniel , Persico Maurizio
Published: July 2002

The description of substituent groups, attached to an organic skeleton, by means of basis sets of lower quality with respect to the rest of the molecule, has been investigated and tested. Starting from a double-zeta Gaussian basis, two successive reductions of the basis set dimensions have been envisaged: the first is simply a contraction to single zeta, the second consists in constructing ‘‘supercontracted’’ basis functions as linear combinations of AOs centered on different atoms of the same substituent group. An optimization of contraction and ‘‘supercontraction’’ coefficients at the molecular level leads to much better results for the energy and other molecular properties, in comparison with the common practice of using the contraction based on atomic calculations. An unbalanced description, potentially implied in using basis sets of different quality in the same calculation, is thus avoided. The example of the methyl group shows that the contraction coefficients determined for a neutral molecular environment (the ethane molecule) can be used in SCF-CI calculations with different aims: for example, the study of the electronic excitation in acetone, of the protonation in amines, of radical and carbocation formation in alkanes.
Theoretical analysis of nonadiabatic effects on the predissociation of theA 2Σ+ state of OH
Authors: Lee Sungyul , Freed Karl F.
Published: July 2002

Multichannel scattering calculations are used to determine the role of nonadiabatic interactions on the predissociation dynamics of several vibrational levels of the A 2Σ+ state of OH and the resulting oxygen atom fine structure branching ratios. Nonadiabatic effects are found to be very strong for the lower predissociative vibrational A 2Σ+ levels and to remain nonnegligible even for the higher vibrational states of A 2Σ+ despite the considerable degree to which the asymptotic fragment kinetic energy exceeds the oxygen atom fine structure splittings. Nonadiabatic effects generally increase in importance with the final total angular momentum J of the A 2Σ+ state because of the increased Coriolis couplings, but the spin sublevel and parity are of minor importance except at low energies. The oxygen atom fine structure branching ratios are shown to provide a qualitative probe of the predissociation pathway. Certain conditions are presented under which a good approximation to the branching ratios can be obtained by simple analytical treatments without the necessity of performing multichannel scattering calculations.
A study of the vibration–rotation and fine structure energy levels of the NSe radical by laser magnetic resonance
Authors: Brown John M. , Uehara Hiromichi
Published: July 2002

Vibration–rotation and fine structure transitions of the NSe radical in its X 2Π state have been studied by CO2 laser magnetic resonance at 11 μm. Transitions involving the five most abundant naturally occurring isotopes of selenium have been detected. These data were all analyzed together by invoking the appropriate isotopic relationships between molecular parameters. The vibrational band origin for N80Se is determined to be 944.542 16(9) cm−1 and the spin–orbit coupling constant Ã is 891.891 32(11) cm−1. From a consideration of the likely sign of the lambda doubling parameter (p+2q), it is suggested that B electronic state may be 2∑+ rather than 2∑−.
The Morse oscillator in position space, momentum space, and phase space
Authors: Dahl Jens Peder , Springborg Michael
Published: July 2002

We present a unified description of the position-space wave functions, the momentum-space wave functions, and the phase-space Wigner functions for the bound states of a Morse oscillator. By comparing with the functions for the harmonic oscillator the effects of anharmonicity are visualized. Analytical expressions for the wave functions and the phase space functions are given, and it is demonstrated how a numerical problem arising from the summation of an alternating series in evaluating Laguerre functions can be circumvented. The method is applicable also for other problems where Laguerre functions are to be calculated. The wave and phase space functions are displayed in a series of curves and contour diagrams. An Appendix discusses the calculation of the modified Bessel functions of real, positive argument and complex order, which is required for calculating the phase space functions for the Morse oscillator.
Geometrical dependence of the electrical properties of H+3
Authors: Augspurger Joseph D. , Dykstra Clifford E.
Published: July 2002

A b initio calculations using large basis sets with self-consistent field and fully correlated wave functions have been carried out to examine the geometry dependence of multipole moments, polarizabilities, and hyperpolarizabilities of H+3. These surfaces of properties were continued to the H+–H2 dissociation limit in order to demonstrate the change in the properties of H2 upon protonation. There is generally a decrease in the polarizability upon protonation, though at long range the proton’s interaction with H2 produces a general increase. The implications of this on vibrational state properties are considered.
Second-order Mo/ller–Plesset perturbation theory as a configuration and orbital generator in multiconfiguration self-consistent field calculations
Authors: Jensen Hans Jo/rgen Aa. , Jo/rgensen Poul , Ågren Hans , Olsen Jeppe
Published: July 2002

It is shown that the magnitude of the natural orbital (NO) occupation numbers of second-order Mo/ller–Plesset (MP2) perturbation theory can be used to select physically reasonable configuration spaces for ground state MCSCF calculations. When the MP2 NO’s are used as an initial guess for the orbitals, a second-order Newton–Raphson MCSCF calculation is in the local region from the first iteration. Fast convergence is therefore ensured to a stationary point with orbitals of similar structure as the MP2 NO’s, thereby reducing significantly the risk of converging to undesired stationary points.
The aluminum–polyimide interface: An electron-induced vibrational spectroscopy approach
Authors: Pireaux J. J. , Vermeersch M. , Grégoire C. , Thiry P. A. , Caudano R. , Clarke T. C.
Published: July 2002

Clean polyimide films spin-coated onto gold substrate have been fully characterized by high resolution electron energy loss spectroscopy (HREELS): 19 electron-induced vibrational bands could be resolved and assigned by comparison with IR spectroscopy. The attribution of these peaks constitutes a firm and necessary basis for the detailed study of the incipient interface formation with aluminum at room temperature. The metal being evaporated at a low rate from a Knudsen effusion cell, its equivalent coverage on the polymer could be monitored very efficiently and allowed a clear distinction between three reaction steps: (i) at an Al coverage below 4×1014 Al atom/cm2, the aluminum atoms interact only with the C–O carbonyl in the polymer to form a C–O–metal complex; (ii) between 4 and 10×1014 atom/cm2, no further Al ‘‘reaction’’ occurs, but the appearance of new CH and OH stretching bands is attributed to bond breaking in the polymer and diffusion of the residues towards the surface; (iii) after further metal evaporation, all the peaks reduce in intensity and the observation of a ν(Al–O) [or ν(Al–C) band] testifies the formation of an aluminum oxide (or carbide) at the surface. Although no dedicated theory can be invoked up to now for the interpretation of HREELS data collected with different electron impact energies and under different scattering geometries, the spectra measured from polyimide before and after aluminum condensation are consistent with a modification of the geometry of the polymer monomeric unit after bonding with the metallic atoms. The metallized polymer surface appears with an equal statistical spatial distribution of all the chemical groups.
Phosphorescence excitation spectroscopy in supersonic jets. The lowest triplet state of pyrazine
Authors: Tomer J. L. , Holtzclaw K. W. , Pratt D. W. , Spangler L. H.
Published: July 2002

Motivated principally by dynamic issues, we have observed and assigned the laser-induced phosphorescence excitation spectrum of pyrazine in the collision-free environment of a supersonic jet. The origin of the lowest triplet state (T1) lies at 26 820 cm−1 and exhibits a symmetric parallel-type rotational contour, confirming that this state is 3B3u (nπ*) with an equilibrium geometry that is similar to those of the S0 (1Ag) and S1 (1B3u , nπ*) states. Thirty vibrational bands were also observed in the ∼4000 cm−1 interval between the T1 and S1 origins. Of these, the 13 lower energy bands all exhibit parallel-type contours and may be assigned as T1←S0 transitions, principally involving totally symmetric modes. The 17 higher energy bands exhibit both parallel and perpendicular contours and may be assigned as S1←S0 hot band transitions, some involving nontotally symmetric modes. No evidence for a second, ππ* triplet state lying below the S1 origin is found, nor is there any evidence for rapid relaxation of any of the zero-order T1 levels at a resolution of ∼1 cm−1. We conclude from these and other available spectroscopic data on the isolated molecule that the intersystem crossing dynamics of S1 pyrazine is governed by the interaction of the two largely nested potential surfaces, S1 and T1, zero–order nπ* states that appear to differ primarily in the extent to which they interact vibronically with other zero-order states in manifolds of the corresponding multiplicity.
Vibrational excitation and quenching of N+2 in collision with He at relative energies below 1 eV
Authors: Kriegel M. , Richter R. , Lindinger W. , Barbier L. , Ferguson E. E.
Published: July 2002

A selected ion flow drift tube (SIFDT) has been used to measure the vibrational excitation of N+2 ions in collisions with He from the detected threshold to about 0.8 eV relative collision energy. The vibrational quenching of N+2 by He has been measured over the same energy range. The measured excitation and quenching rate constants are found to satisfy the principle of detailed balance, implying no significant role for rotational excitation in the excitation or quenching processes. The quenching rate constant fits a linear Landau–Teller plot, −ln KqαKE−1/3cm, in spite of the fact that the collisions at all energies studied are well within the low energy adiabatic regime in which the collision time is long compared to h/ΔEv.
Fast–slow variable analysis of the transition to mixed-mode oscillations and chaos in the peroxidase reaction
Authors: Larter Raima , Steinmetz Curtis G. , Aguda Baltazar D.
Published: July 2002

An examination of the complex oscillations and chaotic behavior observed in a model of the peroxidase-catalyzed oxidation of NADH is made via numerical simulation and a fast/slow variable analysis. The NADH is considered to be a slowly varying species, compared to oxygen and the two key free-radical intermediates in the mechanism. By considering NADH to be a parameter which modulates the dynamics of a reduced model composed of the remaining fast variables, the origin of the complex oscillations is elucidated. Possibilities for the origin of the observed chaotic behavior are also suggested by this approach. The observation of birhythmicity and metastable chaos in this system may also be related to the two inherent time scales in the kinetics.
Theoretical study of the scandium and yttrium halides
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Partridge Harry
Published: July 2002

Theoretical spectroscopic constants are reported for the X 1Σ+ and a 3Δ states of the fluorides, chlorides, and bromides of scandium and yttrium. The calculated D0 values (eV) are ScF(6.00), ScC1(4.55), ScBr(3.90), YF(6.72), YC1(5.36), and YBr(4.74). All systems are predicted to have 1Σ+ ground states once the effect of unlinked higher excitations are incorporated using the coupled-pair functional approach and relativistic effects are included using either perturbation theory or relativistic effective core potentials. The singlet–triplet splitting is larger for the yttrium halides and decreases for both metals from F to Br. A complete-active space self-consistent-field plus multireference configuration-interaction study of the spectroscopic constants and radiative lifetimes is presented for most of the singlet and triplet states of ScF and YCl below 28 000 cm−1. The theoretical Te value computed for the B 1Π state of ScF indicates that the longest wavelength peak (9405 Å) for the B 1Π←X 1Σ+ system observed in absorption in a neon matrix probably corresponds to the 2–0 band. The calculations provide an unambiguous assignment of two previously unassigned band systems in the laser excitation spectrum of the YCl molecule.
Angular momentum dependent barriers: Possible key to understanding angular distributions for the reactions of fluorine with H2, D2, and HD
Authors: Hayes Edward F. , Walker Robert B.
Published: July 2002

The bending corrected rotating linear model is used to study the angular distributions for the reactions F+H2, F+HD, and F+D2. The calculations are performed using two new surfaces (No. T5 and T5A) that were recently reported by Steckler et al. (Ref. 3). Analysis of the factors determining the shapes of these angular distributions indicates that the angular momentum dependence of the vibrationally adiabatic barriers in the entrance and exit channels plays a central role in determining the characteristics of the vibrational state-selected, reactive product angular distributions. While the distributions predicted for the reactions F+H2(v=0)→H+HF(v′=3), F+D2(v=0)→H=DF(v′=4), and F+DH(v=0)→H+DF(v′=4) are in good agreement with the high resolution molecular beam studies of these same reactions, the angular distributions predicted for the reaction F+HD(v=0)→D+HF(v′=3) do now show the forward scattering of products derived from high resolution molecular beam studies. This discrepancy appears to be due to a shortcoming in the T5 and T5a surfaces (i.e., the angular momentum dependent, vibrationally adiabatic barriers in the exit channel are too high in energy to permit forward ‘‘resonant’’ scattering at the low scattering energies corresponding to the experimental studies).
Energy-adjusted a
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i
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o pseudopotentials for the rare earth elements
Authors: Dolg M. , Stoll H. , Preuss H.
Published: July 2002

Nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials substituting the 1s–3d core orbitals with corresponding spin–orbit operators for the rare earth elements Ce through Yb have been generated. Excitation and ionization energies from numerical pseudopotential calculations differ by less than 0.1 eV from corresponding numerical all–electron results. The pseudopotentials for Ce have been tested in molecular calculations for the 3Φ ground state of CeO. The derived spectroscopic constants from quasirelativistic pseudopotential CI(SD) calculations including Davidson’s correction (Re=1.827 Å, De=6.95 eV, ωe=834 cm−1) are in good agreement with experimental values (Re=1.820 Å, De=8.19 eV, ωe=862 cm−1).
Intrachain loops in polymers: Effects of excluded volume
Authors: Chan Hue Sun , Dill Ken A.
Published: July 2002

We develop a theory for the formation of loops and intrachain contacts in polymer molecules which are subject to excluded volume. We use two methods: (i) exhaustive simulations of chain conformations on two-dimensional square lattices, and (ii) the Edwards path integral approach. The predictions are compared to those of the Jacobson–Stockmayer theory, which neglects excluded volume. Our results show that the cyclization probability in two dimensions depends on loop length to a power between −1.6 to −2.4, in contrast to the prediction of Jacobson–Stockmayer of a power of −1. In addition, the cyclization probability depends on the position in the chain, and end effects are significant. A principal result of the present work is the development of ‘‘topological’’ correlation functions among multiple loops in a chain. If two loops are far apart along the chain, they act independently, but as they approach each other, or if they are interlinked, then one can strongly hinder or enhance the likelihood of another. For these situations, the path integral theory is an improvement over Jacobson–Stockmayer, but misses some important features of short-ranged packing effects. A particularly striking conclusion is that in the presence of the most probable first loop, the formation of a second contact is strongly preferred to be in either of only two possible conformations: a helix or an antiparallel sheet. This suggests that the basis for secondary structure formation in globular proteins may be packing and conformational freedom, rather than hydrogen bonding or other specific interactions.
Simple accurate potentials for Ne–Kr and Ne–Xe
Authors: Barrow D. A. , Slaman M. J. , Aziz R. A.
Published: July 2002

Recent literature potentials for Ne–Kr and Ne–Xe systems are analyzed and critically assessed as to their abilities to predict dilute gas bulk and microscopic properties. Two new potentials are proposed for the Ne–Kr and Ne–Xe interactions which accurately predict a large number of properties and appear to be the best characterizations of these interactions.
Sticking and scattering in the molecular chemisorption regime: CO on Pt(111)
Authors: Harris J. , Luntz A. C.
Published: July 2002

The nondissociative sticking of CO on Pt(111) has been measured as a function of incident energy and angle. Both these sticking measurements and prior molecular beam scattering experiments are incompatible with simple theoretical models that describe successfully weak physisorption interactions. For strong chemical interactions, such as for CO–Pt(111), lateral and rotational corrugations in the interaction potential are large and play an important role in the scattering and sticking. Both translational to rotational and normal to parallel energy conversions cause substantial energy scrambling in the interaction. Classical trajectory calculations that incorporate these effects realistically predict a rather gentle fall off in sticking with incident energy, a scaling of sticking with total rather than normal energy and directly scattered distributions which consist of broad lobular components about the specular superimposed on an even broader diffuse component. All of these findings are in accord with experiment.
Atom- and radical-surface sticking coefficients measured using resonance-enhanced multiphoton ionization
Authors: Robertson Robert M. , Rossi Michel J.
Published: July 2002

Sticking coefficients γ of neutral transient species at ambient temperature were measured using in situ resonance enhanced multiphoton ionization (REMPI) of the transients in a low pressure reactor at mTorr pressure. The value of γ for I on a stainless steel surface was 0.16, whereas γ for CF3 free radical on the same surface was <0.01. The REMPI spectrum of SiH2 was observed for the first time, and by the use of different REMPI transitions a value of 0.10 was found for γ(SiH2 ) on a growing carbon-containing hydrogenated silicon surface at ambient temperature. This value increased to 0.15 for interaction of SiH2 with a growing surface containing exclusively Si and H. A lower limit for γ of >0.5 was found for highly vibrationally excited CF3 containing 5900 cm−1 of internal energy and for SiH2 containing 7000 cm−1 of internal energy. The surface was stainless steel in the former case and a carbon-containing Si and H surface in the latter case.