bluuebunny/crossref_metadata_2025 huggingface dataset:
Detection of the NaO radical by microwave spectroscopy
Authors: Yamada Chikashi , Fujitake Masaharu , Hirota Eizi
Published: July 2002

The rotational spectrum of the NaO radical in the four lowest vibrational states was observed in the millimeter- and submillimeter-wave region. The NaO radical was generated by the reaction of the sodium vapor with N2O directly in a high-temperature absorption cell. Analysis of the observed spectra yielded the rotational and centrifugal distortion constants, Λ-type doubling constants, spin–orbit interaction constants, and hyperfine interaction constants in the four vibrational states. The NaO radical is clearly shown to have the 2Πi ground electronic state with an anomalously large Λ-type doubling constant and very small hyperfine interaction constants. The molecular constants derived are discussed in terms of the ionic character of the bond.
The diffusion coefficient of a reptating polymer
Authors: Deutsch J. M. , Madden T. L.
Published: July 2002

The dynamics of a polymer in a network of entanglements is studied. The viscosity has been examined for chains up to 50 links and is found to scale with chain length L as L3.41±0.14 in agreement with previous theoretical work, that attributes this anomalous exponent as a finite size effect because of the finite tube length. Numerical results for chains up to 100 links give that the diffusion coefficient D, scales as L−2.50±0.04. This result differs from theoretical predictions based on tube fluctuations which claim finite length effects are unimportant for diffusion and therefore imply a −2 power law dependence for D. The reason for this discrepancy is examined. Previous work used a one-dimensional rather than a three-dimensional diffusion coefficient as the starting point of analysis. By deriving the correct procedure for calculating the three-dimensional D, one sees that it has corrections to its asymptotic behavior which are quite large, of order L−1/2, rather than L−1 as was thought previously. This suggests that fluctuations in the tube length cannot provide an adequate explanation for the experimental data.
The electronic and electrochemical properties of poly(phenylene vinylenes) and poly(thienylene vinylenes): An experimental and theoretical study
Authors: Eckhardt H. , Shacklette L. W. , Jen K. Y. , Elsenbaumer R. L.
Published: July 2002

The electronic and electrochemical properties of poly(p-phenylene vinylene), poly(thienylene vinylene), and their derivatives with electron donating moieties such as methyl, methoxy, and ethoxy are studied using the newly developed electrochemical potential spectroscopy (ECPS) and optical spectroscopy. It is shown that electrochemically derived band gaps agree well with band gap values obtained from optical measurements. Substitution with electron donating groups substantially lowers the ionization potentials and band gaps. A similar effect can be attributed to the incorporation of a vinylene linkage between rings of the polymer backbone. Our results imply that through a proper choice of substituents and backbone structure one can adjust the electrochemical potentials over a wide range as well as red shift the absorption edge of these polymers. In the case of the alkoxythienylene vinylenes the absorption edge is shifted through the visible range of the spectrum into the near infrared (NIR) yielding polymers which become transparent and substantially colorless upon doping with electron donors or acceptors. The structure and the substitution effects of these polymers were modeled using the semiempirical quantum chemical modified neglect of differential overlap (MNDO) method. The MNDO-determined structure served as basis for the valence effective Hamiltonian (VEH) technique which was employed to calculate band structures, ionization potentials, and band gaps, and to study theoretically the effect of substituents on the band structure. Good agreement between experimental and theoretical values of ionization potentials, band gaps and the change of these parameters with substitution is found with the exception of methoxy (or ethoxy) groups. This fact is attributed to a failure of VEH to correctly account for the role of the oxygen atoms in these groups.
Highly correlated systems. Excitation energies of first row transition metals Sc–Cu
Authors: Raghavachari Krishnan , Trucks Gary W.
Published: July 2002

The low-lying dns2→dn+1s1 excitation energies of the first row transition metal atoms Sc–Cu are calculated using fourth-order M≂ller–Plesset perturbation theory (MP4) as well as quadratic configuration interaction (QCI) techniques with large spd and spdf  basis sets. The MP4 method performs well for Sc–Mn but fails dramatically for Fe–Cu. In contrast, the QCI technique performs uniformly for all excitation energies with a mean deviation from experiment of only 0.14 eV after including relativistic corrections. f functions contribute 0.1–0.4 eV to the excitation energies for these systems. The highly correlated d10 state of the Ni atom is also considered in detail. The QCI technique obtains the d9s1→d10 splitting of the Ni atom with an error of only 0.13 eV. The results show that single-configuration Hartree–Fock based methods can be successful in calculating excitation energies of transition metal atoms.
Triplet excitation properties in large scale multiconfiguration linear response calculations
Authors: Olsen Jeppe , Yeager Danny L. , Jo/rgensen Poul
Published: July 2002

It is shown that large scale MCSCF linear response (MCLR) calculations can be carried out efficiently when the perturbation operator has triplet symmetry. Methods have been developed to efficiently construct two electron density matrices containing triplet orbital excitation operators and two electron density matrices which connect singlet and triplet states. Direct CI methods have also been developed for two electron operators with spin rank one. Numerical calculations are presented for CH+ and the results compared with full CI results.
Optimal performance of a generalized Carnot cycle for another linear heat transfer law
Authors: Yan Zijun , Chen Jincan
Published: July 2002

The optimal performance of an engine operating between a finite high-temperature source and an infinite low-temperature reservoir is studied. It is assumed that there exists only the irreversibility of heat conduction in the cycle and heat transfer obeys another linear law [i.e., the heat-flux q∝Δ(1/T)] instead of Newton’s law. It is shown that the optimal configuration of the cycle is composed of two adiabatic processes and two isoheat-flux processes in which the difference of the reciprocal temperature of the working fluid and reservoirs must be maintained constant. Moreover, the relation between the optimal power output and the efficiency of such a cycle is derived, and some new results are deduced from it. The results are compared with the optimal performance of a generalized Carnot cycle for Newton’s heat transfer law. Consequently, the common character and the main difference of a generalized Carnot cycle for the two different heat transfer laws are expounded.
Bound–free 1 3Π→1 3Σ+ emission from the NaK molecule: Determination of the 1 3Σ+ repulsive wall above the dissociation limit
Authors: Masters Mark , Huennekens John , Luh Wei-Tzou , Li Li , Lyyra A. Marjatta , Sando Ken , Zafiropulos Vassilios , Stwalley William C.
Published: July 2002

We report the observation of bound–free emission on the 1 3Π→1 3Σ+ band of the NaK molecule. The spectra, which consist of oscillating continua in the near-infrared, have been analyzed to determine parameters describing the repulsive wall of the 1  3Σ+ state above the dissociation limit. Spectra calculated using a potential of the form Ae−BR +C for the 1 3Σ+ state were compared to experimental spectra to yield the following values: A=5.94×105 cm−1, B=1.605 Å−1, C=−220.520 cm−1. This potential, which is referenced to the bottom of the RKR 1 3Σ+ well (De =209.1 cm−1), is valid over the range R=3.4–4.5 Å (R=6.4–8.5 a.u.). The relative transition dipole moment of the 1 3Π→1 3Σ+ band has also been determined over a limited range in R (7.5<R<8.9 a.u.) through the study of relative intensities of various maxima within each oscillating spectrum. In the simulated spectra, the dipole moment was represented by a functional form D(R)=m(R−R0)+D0 where D0 was used to normalize the results to a recent theoretical calculation (D0=1.07 a.u. R0=8.034 a.u.). The best fit for the parameter m was determined to be 0.121±0.029 in atomic units.
Fluorescence from photoexcitation of CF3X (X=H, Cl, and Br) at 50–106 nm
Authors: Lee L. C. , Han J. C. , Ye Chao , Suto Masako
Published: July 2002

The photoabsorption and fluorescence cross sections of CF3H, CF3Cl, and CF3Br are measured in the 50–106 nm region using synchrotron radiation. Fluorescence spectra from photoexcitation of these molecules are dispersed to identify the emitting species to be mainly the excited CF*2, CF*3, and CF3X+*, but not CF+*3. The fluorescence excitation function of CF3X (X=F, H, Cl, or Br) is generally divided into three excitation bands; each band produces a specific excited species. The fluorescence yields of the studied molecules are determined and correlated with dissociation thresholds and ionization potentials. The excited ion states that may emit are discussed.
Thermodynamics and isotope effects of the vanadium–hydrogen system using differential heat conduction calorimetry
Authors: Luo Weifang , Clewley J. D. , Flanagan Ted B.
Published: July 2002

The V–H (D) systems have been investigated using reaction calorimetry at 298 and 323 K over hydrogen contents from H (D)/V=0→≊1.8. Relative partial molar enthalpies have been determined by absorption of small increments of hydrogen in the single phase regions; for regions where two solid phases coexist, integral or plateau enthalpies are obtained by absorption (desorption) of either large or small amounts of hydrogen. The following calorimetric results were obtained for the two solid phase plateau regions: ‖ΔH‖=40.6 kJ/mol H, 40.8 kJ/mol D for (α + β) and ‖ΔH‖=19.1 kJ/mol H and 23 kJ/mol D for (β′+γ) and (α′+ε), respectively. Calorimetric data, relative partial molar enthalpies are also given for the single phase regions. Several phase boundaries are intersected as the hydrogen contents increase in this temperature range and the behavior of the enthalpies at the phase boundaries has been determined and is discussed in relation to the nature of the coexisting phases.
Dynamics of ion pair interconversion in a polar solvent
Authors: Ciccotti Giovanni , Ferrario Mauro , Hynes James T. , Kapral Raymond
Published: July 2002

The reaction rate and mechanism of the interconversion between a contact ion pair and solvent separated ion pair in model polar solvents is investigated by molecular dynamics (MD) simulation. The full rate constant for the model reaction is estimated from the product of the transition state theory (TST) rate constant, determined from the potential of mean force between the ions in an equilibrium solvent, and the transmission coefficient, which depends on the details of the dynamics. The collective character of the solvent motion in the reaction barrier crossing is examined in some detail, and the important role of solvent dynamics in causing the reaction rate to markedly deviate from the TST rate is discussed. The MD results are compared with the predictions of Kramers and Grote–Hynes theories.
Anisotropic conductivity in stretch-oriented polymers measured with coherent microwave transient spectroscopy
Authors: Arjavalingam G. , Theophilou N. , Pastol Y. , Kopcsay G. V. , Angelopoulos M.
Published: July 2002

Stretch-oriented and doped polyacetylene and polyaniline are characterized using the coherent microwave transient spectroscopy technique. Conductivities parallel and perpendicular to the direction of elongation are determined. The measured orientation dependence of the sample transmissions are observed to follow the predictions of theory.
Hydrogen bonding between the nitrate anion (conventional and peroxy forms) and the water molecule
Authors: Shen Mingzuo , Xie Yaoming , Schaefer Henry F. , Deakyne Carol A.
Published: July 2002

A b initio quantum mechanical methods have been applied to the complexes of the nitrate anion (conventional D3h NO−3 and Cs ONOO− forms) with the water molecule, namely, NO−3⋅H2O and ONOO−⋅H2O. Equilibrium geometries and vibrational frequencies for a number of energy minima on the ground state potential energy surface have been determined using analytic energy first and second derivative techniques. The highest level of theory employed is the self-consistent field method with a double-zeta plus polarization basis set. The minimum energy structure (lying ∼15 kcal/mol below separated NO−3 +H2O) incorporates two hydrogen bonds in C2v symmetry. This theoretical dissociation energy agrees well with experiment. A second, more conventional single-hydrogen-bond structure is predicted to lie 2.6 kcal/mol higher in energy. At the most reliable level of theory, no distinct minimum was found for the hydrogen bonded structure between HNO3 (nitric acid) and OH−. Several minima (high lying energetically) between ONOO− and H2O are characterized theoretically.
Effective valence shell Hamiltonian and potential curves of the oxygen molecule from quasidegenerate many-body perturbation theory
Authors: Kanzler Alfred W. , Freed Karl F.
Published: July 2002

The effective valence shell Hamiltonian (Hv) is calculated for O2 using quasidegenerate many-body perturbation theory with an eight orbital valence space. A comparison is made of the accuracy of Hv results from a second vs third order truncation of the perturbation expansion. Potential curves for ten low lying valence states show that second order calculations produce dissociation energies and harmonic frequencies that are systematically too large. However, the third order Hv calculations correct the deviations present in second order. Our third order ground state spectroscopic constants compared well with those from a full configuration interaction calculation using the same basis set. Hv calculations are also performed using a second set of orbitals constrained such that the molecular valence space is the union of atomic valence spaces. The constrained orbital Hv calculations are designed for comparison with model valence shell Hamiltonians of semiempirical methods. Comparison of second and third order constrained calculations enables a determination of the reliable range of internuclear distances of the individual constrained Hv matrix elements. Third order constrained Hv matrix elements in the atomic orbital basis set are least squares fit to simple functions of inverse internuclear separation or orbital overlap for comparison with the forms used in semiempirical methods. Functional forms employed for second order Hv matrix elements are compared with previous fits to second order Hv matrix elements for S2 and CH in order to present systematic trends.
Time-resolved and static optical properties of vibrationally excited porphyrins
Authors: Rodriguez Juan , Kirmaier Christine , Holten Dewey
Published: July 2002

The effects of nuclear motion on the ground state and excited state optical spectra of porphyrins are examined in a number of experiments designed to generate excess vibrational energy within the macrocycle. These include time-resolved spectroscopic measurements following ultrafast radiationless transitions, and static measurements in the gas and in the condensed phase at various temperatures. The excess vibrational energy generated by highly exothermic radiationless transitions is found to induce significant red shifts in both the ground state absorption and excited state emission features. As the excess vibrational energy is dissipated on the time scale of about 10 ps, the optical features blue shift to their steady-state spectral positions. The red shifts found in the time-resolved spectra are also observed in the ground state absorption spectra of porphyrins in the gas phase at high temperature. We consider various mechanisms for the spectral shifts, including vibrationally induced reduction of the electronic energy gap by anharmonic expansion and/or vibronic coupling.
Interpretation of the Fermi hole curvature
Authors: Dobson John F.
Published: July 2002

Two different interpretations are given for the Fermi hole curvature parameter used recently by Stoll et al., by Colle and Salvetti and by Becke to estimate the size of the correlation hole, by Becke et al. to clarify aspects of chemical shell structure and bonding, and by Luken and Culbertson to discuss mobility of the Fermi hole. The first, more straightforward interpretation involves the number of ‘‘other’’ electrons to be found in a small neighborhood near a given electron. The notion of other electrons leads naturally to correlation functionals which correctly vanish when only one electron is present. The second interpretation, made explicit by use of the Wigner pair distribution, involves the density of relative kinetic energy of pairs of spin-parallel electrons at point r. Since, in a classical interpretation at least, the correlation hole in a nonuniform Coulomb system depends both on density and relative kinetic energy of colliding pairs, one expects that both the Fermi hole curvature and the density should be significant in constructing theories of the correlation energy of such systems.
Photodetachment of WO−3: The electron affinity of WO3
Authors: Walter C. W. , Hertzler C. F. , Devynck P. , Smith G. P. , Peterson J. R.
Published: July 2002

Photodetachment cross sections for WO−3 have been measured near the detachment threshold (3.0–3.7 eV). A fast beam of WO−3 is merged with a coaxial cw laser beam and the photodetached electrons are detected with a magnetic confinement slow electron collector. The photodetachment spectrum yields an electron affinity for WO3 of 3.33+0.08−0.15 eV, somewhat lower than values obtained in thermodynamic equilibrium measurements. In addition, a lower limit of 3.7 eV is obtained for the electron affinity of HWO4.
Theoretical foundations of dynamical Monte Carlo simulations
Authors: Fichthorn Kristen A. , Weinberg W. H.
Published: July 2002

Monte Carlo methods are utilized as computational tools in many areas of chemical physics. In this paper, we present the theoretical basis for a dynamical Monte Carlo method in terms of the theory of Poisson processes. We show that if: (1) a ‘‘dynamical hierarchy’’ of transition probabilities is created which also satisfy the detailed-balance criterion; (2) time increments upon successful events are calculated appropriately; and (3) the effective independence of various events comprising the system can be achieved, then Monte Carlo methods may be utilized to simulate the Poisson process and both static and dynamic properties of model Hamiltonian systems may be obtained and interpreted consistently.
Simulation of polyethylene above and below the melting point
Authors: de Pablo Juan J. , Laso Manuel , Suter Ulrich W.
Published: July 2002

Polyethylene at equilibrium is studied by computer simulation. Configuration space is sampled efficiently by a novel Monte Carlo simulation scheme developed for the study of long molecules at high densities. Simulations are carried out in an isobaric-isothermal statistical-mechanical ensemble which permits calculation of the density of the polymer matrix at specified conditions of pressure and temperature. A systematic study of the polymer at different temperatures indicates a phase transition; in agreement with experiment, at low temperatures, the polyethylene model studied here crystallizes spontaneously. At temperatures above the melting point, the simulated melt is described accurately by the model.
Nonperturbative method for core–valence correlation in pseudopotential calculations: Application to the Rb2 and Cs2 molecules
Authors: Foucrault M. , Millie Ph. , Daudey J. P.
Published: July 2002

The core–valence correlation is introduced into ab initio relativistic pseudopotential calculations by modifying the existing core polarization potential. The salient feature of the method presented here is the use of an l-dependent cutoff parameter (which is related to spherical harmonic functions) for solving the multicenter integrals over the 1/r4 - and r/r3 -type operators. The method is tested on the Rb2 and Cs2 molecules considered as two valence-electron problems. Reliable results for the molecular spectroscopic constants (Re, Te, De, and ωe ) are obtained for the ground state and the lowest excited states. Deviation from the experimental values ranges from 0.05 to 0.1 Å for Re, seldom exceeds 2 cm−1 for ωe, and is of the order of 100 cm−1 for De for most of the excited states.
Application of density functional theory to infrared absorption intensity calculations on main group molecules
Authors: Fan Liangyou , Ziegler Tom
Published: July 2002

Approximate density functional theory has been evaluated as a practical tool for calculations on infrared vibrational frequencies and absorption intensities. The density functional schemes included the local density approximation (LDA) by Gunnarson [Phys. Rev. B 10, 1319 (1974)] as well as a self-consistent nonlocal density functional method (LDA/NL) in which the gradient corrected exchange term by Becke [Phys. Rev. A 38, 3098 (1988)] and the gradient corrected correlation term by Perdew [Phys. Rev. B 33, 8822 (1986)] has been added to LDA. The LDA and LDA/NL schemes have been applied to calculations on the infrared vibrational frequencies and absorption intensities of H2O, NH3, H2CO, C2H4, CH3OH and oxirane. The calculations were carried out with two basis sets of respectively double-ζ plus polarization (DZP) and triple-ζ plus polarization (TZP) quality. The study has demonstrated that vibrational frequencies are relatively insensitive to the level of density functional theory as well as to the choice of basis set. The calculated frequencies are, in general, in better agreement with experiment than values obtained by ab initio Hartree–Fock calculations. Large basis sets of TZP quality are, on the other hand, required for intensities. It is further shown that a small but consistent improvement in the calculated ir intensities is obtained by including nonlocal corrections. The LDA/NL method with a TZP basis set affords ir intensities and frequencies of the same quality as post-Hartree–Fock methods with an average deviation in the intensities of 20%–40%.
Hydrogen bonding in the symmetry-equivalent C2h dimer of 2-pyridone in its S and S2 electronic states. Effect of deuterium substitution
Authors: Held Andrew , Pratt David W.
Published: July 2002

The properties of the two intermolecular N–H⋅⋅⋅O bonds that are responsible for the formation of the cis-peptide-like dimer of 2-pyridone (2PY) have been examined using deuterium substitution of the bridging hydrogen atoms as a probe. Studies of the fully resolved S2←S0 electronic spectrum of (2PY)2 in a molecular beam show that the protonated dimer has a symmetry-equivalent planar C2h structure in its S0(1Ag) and S2(1Bu) states. Analogous studies of (2PY)2–d1 and (2PY)2–d2 show that (2PY)2 and (2PY)2–d2 are energy delocalized dimers in their S2 states, with an exciton splitting of less than 20 cm−1. Effective structures of the symmetric dimers in both states are derived from the measured rotational constants. A comparison of these structures shows that there is a distortion of the hydrogen-bonding geometry when hydrogen is replaced by deuterium, along both in-plane and out-of-plane coordinates. ΔR(N–H⋅⋅⋅O)=0.008 Å, Δθ[C=O⋅⋅⋅(H)–N]=0°, and Δφ (the dihedral angle)=0.96° in S0 (2PY)2–d2 and ΔR=0.003 Å, Δθ=0°, and Δφ=0.86° in S2 (2PY)2–d2. The implications of these findings for currently accepted models of the hydrogen bond are discussed.
Structures of the aluminum oxides studied byab initiomethods with natural bond orbital analysis
Authors: Nemukhin A. V. , Weinhold F.
Published: July 2002

We present a comprehensive theoretical analysis for the low-lying isomeric structures, energetics, and vibrational properties of dinuclear aluminum oxides Al2On (n=1–4) to aid interpretation of experimental spectroscopic data for these species. We also carried out natural population and natural bond orbital (NBO) analysis of the correlated and uncorrelated ab initio wave functions in order to elucidate the general bonding principles governing these species. We find that the equilibrium structures generally exhibit high ionic character (viz., effective Al3+ and O2− ionic units), but with significant modifications due to covalency, which is generally enhanced by electron correlation. Although certain previous experimental assignments are confirmed by our studies, in other cases the theoretical results strongly contradict suggested assignments. For a significant number of the reported experimental lines, we currently have no good theoretical candidate species.
Raman-vibronic double-resonance spectroscopy of benzene dimer isotopomers
Authors: Henson B. F. , Hartland G. V. , Venturo V. A. , Felker P. M.
Published: July 2002

The results of Raman-vibronic double resonance experiments on benzene dimer are reported. The results were obtained by mass-selective, ionization-detected stimulated Raman spectroscopies. The data pertain to the ν1, ν2, and ν6 fundamentals of numerous dimer isotopomers. The results are discussed in terms of the geometry of the dimer. They show that the species is characterized by two inequivalent benzene sites with one of the sites of low and the other of higher symmetry. These two sites give rise to different Raman resonance frequencies, different vibrational dynamics, and markedly different S1↔S0 vibronic spectra. It is argued that all of the experimental results are consistent with a T-shaped equilibrium geometry in which the benzene moiety at the top of the T is freely rotating about its C6 axis.
Calculation of barriers to proton transfer using variations of multiconfiguration self-consistent-field methods. II. Configuration interaction
Authors: Luth Karl , Scheiner Steve
Published: July 2002

Various means are tested of including additional electron correlation into multiconfiguration self-consistent-field (MCSCF) methods for computing proton transfer potentials in HF2−, H7N2+, H3O2−, and H5O2+. Configuration interaction allowing single excitations (CIS) and configuration interaction with single + double excitations (CISD) calculations are performed following MCSCF expansion of the wave function using various different MCSCF reference wave functions. The CISD results are excellent, being fairly independent of choice of reference space although it is important that the occupied orbitals be balanced between the donor and acceptor. Localizing the occupied molecular orbitals prior to the MCSCF part of the calculation results in a further improvement since it is possible to use a smaller number of occupied orbitals and thereby allow more virtuals to be included. These results are compared to configuration interaction computations using the canonical orbitals and which are not preceded by MCSCF preparation of the wave function.
Mobilities of positive ions in sulfur hexafluoride gas
Authors: Li-Aravena F. , Saporoschenko M.
Published: July 2002

The mobilities of the ions SF2+, SF3+, SF4+, SF5+, and S2F7+ in SF6 gas have been measured using an electrical shutter method with an electron ionization ion source. Measurements were made at gas temperatures of 22–36 °C over a range of E/N, the ratio of electric field strength to gas number density, from 55–550 Td (1 Townsend (Td)=10−17 V cm2) and over the pressure range 0.1–0.7 Torr. The zero-field reduced mobilities of SF2+, SF3+, SF4+, SF5+, and S2F7+ were determined to be 0.64, 0.61, 0.64, 0.65,and 0.48 cm2 V−1 s−1, respectively.
Modified neglect of diatomic overlap approach to a molecular dynamics study of C20–C60 carbon clusters
Authors: Piccitto G. , Pucci R.
Published: July 2002

We introduce an algorithm that couples the molecular dynamics (MD) method to the modified neglect of diatomic overlap (MNDO) approximation. This approach is applied to the study of the carbon clusters Cn with n ranging from 20 to 60. We calculate the relative stability of some well-known structures as polyacetylenic rings and fullerenes. The results obtained are in substantial agreement with calculations available in the literature. The main emphasis of the paper is on the possibility of finding, with our approach, new isomers starting from different initial configurations or after thermal excursions. For some values of n, new stable isomers are found. The results allow us also to give explanations of the observed low abundance in the mass spectra of the clusters with n ranging between 30 and 40 (the so-called ‘‘dead’’ region).
Sticking and reaction of O2 on a potassium covered graphite surface
Authors: Sjövall P. , Kasemo B.
Published: July 2002

The influence of potassium on the oxidation of graphite by molecular oxygen was studied using temperature programmed desorption/reaction spectroscopy (TPD/TPRS). Adsorbed potassium increases the sticking coefficient for oxygen on graphite by at least three orders of magnitude. The initial sticking coefficient is constant and of the order of unity when potassium is adsorbed at coverages between 0.056 and 1.0 ML (1 ML=5×1014 atoms/cm2). Possible mechanisms for the increased sticking are discussed. The saturation coverage of oxygen increases linearly with the K coverage in two different K-coverage regimes, indicating the formation of stoichiometric K–O adsorption layers. The K:O ratios at saturation are (assuming unity sticking) 1:1 and 2:1 at low and high coverage, respectively. The adsorbed K is thermally stabilized by oxygen on the graphite surface, and desorbs simultaneously with CO2 in three desorption peaks between 500 and 800 K after oxygen adsorption. The oxygen atoms are randomly mixed on the graphite surface prior to the formation of CO2, as concluded from the oxygen isotope distribution in CO2 after exposure of the K-covered graphite surface to 16O2 and 18O2.
Accurate three-dimensional quantum scattering calculations for the F+H2 reaction on a new potential energy surface
Authors: Kress Joel D.
Published: July 2002

Accurate 3D quantum reaction probabilities for the F+H2→HF+H reaction are presented for the new 5SEC potential energy surface (PES) of Truhlar and co-workers. This PES was found to be more reactive, on the average, than the closely related 5A PES of Truhlar and co-workers. The resonance structure in the probabilities also differs between the two PESs.
The effect of spectral dispersion for probe pulse upon the vibronic quantum beat in pump–probe experiments
Authors: Kawashima Hitoshi , Shida Tadamasa
Published: July 2002

The vibronic quantum beat in pump–probe experiments is studied for two types of experimental setups for the probe pulse, i.e., dispersing the pulse before and after the transmission through the sample. The induced transmittance is described as a coupling of a probe field and the third-order polarization. The third-order polarization is derived as a perturbative solution in which intramolecular vibrations are fully taken into account. On the basis of the resultant formulas, the effect of the bandpass of the monochromator is discussed. Numerical analyses are presented to demonstrate some aspects of the vibronic quantum beat.
Rotational distributions of molecular ions following core excitation and decay
Authors: Mahalingam M. , Lee K. , Hanson D. M.
Published: July 2002

The rotational distribution in the N2+ (B) state produced by core excitation and decay is reported for three different core excitation energies. In contrast to the case of valence excitation, the rotational branching ratios do not exhibit a significant dependence on excitation energy. This similarity is attributed to the uniformly high kinetic energies of the emitted electron for the three cases and provides evidence for the production of singly charged molecular ions in the core ionization continuum through an interchannel coupling mechanism.
On the stability of the one-electron bond
Authors: Hogreve H.
Published: July 2002

Binding in a system ZAZBe− consisting of two nuclear centres with positive charges ZA, ZB (and infinite mass) and one electron is studied as a function of ZA and ZB. Critical charge parameters separating the regime of stable, metastable, and unstable binding are defined and determined for the electronic ground state of the system. The metamorphosis of the involved potential energy curves and the behavior of other bond properties for varying charges is investigated and interpreted (e.g., as bifurcation effect). This includes an analysis of the electronic charge distribution and the arising electrostatic forces. In addition, the stability problem for vibrational motion in a given potential energy curve (associated with the H2+ molecular ion), but varying nuclear mass is treated. Our results are based on ab initio calculations having an accuracy superior to all comparable previous data for the ZAZBe− system. The qualitative aspects of the exhibited stability scenario are not restricted to one-electron systems, but can be expected as also being of relevance for more general situations in molecular stability.
Interaction of oxygen with Al(111) studied by scanning tunneling microscopy
Authors: Brune H. , Wintterlin J. , Trost J. , Ertl G. , Wiechers J. , Behm R. J.
Published: July 2002

The interaction of oxygen with Al(111) was studied by scanning tunneling microscopy (STM). Chemisorbed oxygen and surface oxides can be distinguished in STM images, where for moderate tunnel currents and independent of the bias voltage the former are imaged as depressions, while the latter appear as protrusions. An absolute coverage scale was established by counting O adatoms. The initial sticking coefficient is determined to so=0.005. Upon chemisorption at 300 K the O adlayer is characterized by randomly distributed, immobile, individual O adatoms and, for higher coverages, by small (1×1) O islands which consist of few adatoms only. From the random distribution of the thermalized O adatoms at low coverages a mobile atomic precursor species is concluded to exist, which results from an internal energy transfer during dissociative adsorption. These ‘‘hot adatoms’’ ‘‘fly apart’’ by at least 80 Å, before their excess energy is dissipated. A model is derived which explains the unusual island nucleation scheme by trapping of the hot adatoms at already thermalized oxygen atoms. Oxidation starts long before saturation of the (1×1) O adlayer, at coverages around ΘO≂0.2. For a wide coverage range bare and Oad covered surfaces coexist with the surface oxide phase. Upon further oxygen uptake both chemisorbed and oxide phase grow in coverage. Oxide nucleation takes place at the interface of Oad islands and bare surface, with a slight preference for nucleation at upper terrace step edges.

Further oxide formation progresses by nucleation of additional oxide grains rather than by growth of existing ones, until the surface is filled up with a layer of small oxide particles of about 20 Å in diameter. At very large exposures up to 5×105 L they cover the entire surface as a relatively smooth, amorphous layer of aluminum oxide. The difference in Al atom density between Al metal and surface oxide is accommodated by short range processes, with no indication for any long range Al mass transport. Based on our data we discuss a simpler two step model for the interaction of oxygen with Al(111), without making use of an additional subsurface oxygen species. The complex spectroscopic data for the O/Al(111) system are rationalized by the wide coexistence range of bare and Oad covered surface with surface oxide and by differences in the electronic and vibronic properties of the surface atoms depending on the number of neighboring O adatoms in the small Oad islands.
Dissociative chemisorption of nitrogen on Ru(0001)
Authors: Shi H. , Jacobi K. , Ertl G.
Published: July 2002

The dissociative chemisorption of nitrogen on clean and cesiated Ru(0001) surfaces has been studied using high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). N2 (at 300 K) chemisorbs dissociatively with a sticking coefficient of 2×10−6, independent of substrate temperature which was varied between 420 and 700 K. The saturation coverage is found at 0.5 monolayer. The energy of the N–Ru stretching vibration is 71 meV at the bare surface and 69 meV at the cesiated Ru(0001) surface. The activation energy for desorption is about 190 kJ/mol for small coverages. The kinetic data suggest the existence of an activation barrier in the entrance channel of adsorption. Preadsorption of 0.08 monolayer of Cs increases the sticking coefficient only by a factor of 1.3, and the maximum amount of adsorbed N is reduced due to blocking of adsorption sites through Cs.
Temperature induced mobility and recombination of atomic oxygen in crystalline Kr and Xe. I. Experiment
Authors: Danilychev A. V. , Apkarian V. A.
Published: July 2002

Recombinant molecular spectra of O2 in crystalline Kr and Xe, obtained from sudden recombination induced by laser pumping of the predissociative B state, and by adiabatic recombination induced thermally, are reported. All electronic states that correlate with O(3P)+O(3P) are populated by both methods in Kr. The excited triplet states are strongly perturbed in Xe. Laser induced fluorescence (LIF) spectra of atomic O are also reported. Two emissions are observed in Kr; one at 563 nm with a radiative lifetime of 11 μs, and another at 599 nm with a radiative lifetime of 1.4 μs. These are assigned to emissions from neutral O(1S) trapped in substitutional and interstitial octahedral sites, respectively. O atoms are generated in situ by photodissociation of either O2 or N2O. The subsequent thermally induced recombination of atoms is followed using LIF from atomic centers and molecular thermoluminescence. The recombination follows first order kinetics, from which it is inferred that atomic mobilities are characterized by long-range migration, with migration lengths of ≥300 Å. Site specific Arrhenius rate constants are extracted from the analysis of glow curves. Only two sites, interstitial and substitutional Oh, contribute to the glow curves in crystalline Kr. A third site is present in crystalline Xe. A broad distribution of activation energies is observed in vapor deposited matrices.
He beam scattering studies of the interaction of CO with iron (111)
Authors: Jiang P. , Zappone M. , Bernasek S. L.
Published: July 2002

The interaction of CO with Fe(111) has been studied by thermal energy atom scattering (TEAS) at surface temperatures of 250 and 400 K. The behavior of He scattering from both the clean and CO covered Fe(111) surface is different from that on smooth, close-packed surfaces such as Pt(111). Although the specular He intensity from the CO covered surface is not negligible, as it is on smooth surfaces, CO is seen to be a perfectly diffuse scatterer. A model to describe the scattering intensity as a function of CO coverage was developed which takes into account the He intensity scattered from the clean surface and the intensity scattered from the CO covered surface, with no interference effects. Effective cross sections of 56 Å2 for He scattering from molecular CO and 93 Å2 per molecule for He scattering from dissociated CO were obtained. Comparison of the He/CO/Fe(111) system is made with He/CO/Pt(111) and He/H/Pt(111) systems and clear differences are seen. As the temperature of the CO covered surface is increased there are two increases in scattered He intensity at ∼360 and 750 K, corresponding to the desorption of molecular and recombined CO, respectively. This study suggests that, although the interpretation of the data is more complicated than for smooth surfaces, TEAS is a viable technique for application to studies of adsorption on more corrugated surfaces.
van der Waals energies of cylindrical and spherical single layer systems
Authors: Witte N. S.
Published: July 2002

The van der Waals energies for cylindrical and spherical single layer systems are obtained, in the nonretarded limit, from the heuristic procedure of summing the normal surface electromagnetic mode frequencies. Both the zero and finite temperature results are presented. The geometrical dependence of the energies for the cylindrical and spherical systems is shown to satisfy inequalities involving simple model dielectric configurations. The exact energies have a first-order correction to the planar case, for a large inner radius in comparison to the film thickness, which is proportional to the mean curvature with a new Hamaker coefficient. It is conjectured that this applies to arbitrarily shaped smooth films with small curvature. In the two-body summation approximation the energies factorize into a geometrical factor, for which exact analytical forms are found, and the standard Hamaker constant. The effect of geometry on the van der Waals energy is shown to be important, in that the effective Hamaker constants for curved films of inner radius r and layer thickness w rises rapidly at r∼2.6w for the cylindrical and at r∼1.8w for the spherical system .
Two-electron exchange in collisions of neutral molecules
Authors: Drakes James. A. , McGregor Wheeler K. , Mason Arthur A.
Published: July 2002

The quenching behavior of neutral metastable species during collisions with a neutral partner is investigated. Usually, radiationless energy transfer can be described by invoking the radiation field as an intermediary between interacting molecules. However, for metastables, the dominant radiation transition moments are suppressed, most often by spin selection rules between the excited and ground energy states. The fast reaction rates for the quenching of the metastables suggest a different intermediary for such cases. Motivated by this phenomena, a two-electron exchange process is proposed as the quenching mechanism. During a collision, it is envisioned that the excited outer shell electron of the metastable effectively undergoes a change of molecular center and is replaced by an electron from the collision partner. The theoretical framework for this process is developed using the formalism of molecular quantum electrodynamics. The electron exchange term is obtained explicitly, using the minimal-coupling method.
On the existence of very large nonadditivities for clusters of distorted water molecules
Authors: Pastor Nina , Ortega-Blake Iván
Published: July 2002

We present a detailed analysis, both at the self-consistent field and electronic correlation levels, of the many-body expansion of water clusters occurring with intramolecular relaxation. The results show extremely large nonadditivities, which lead to the proposal of an alternative many-body expansion that shows convergence. From the results, it is clear that there is substantial intra–intermolecular coupling, an important contribution of the correlation energy to the stability of these clusters, and a crucial role of cooperativity in the stability of the condensed phases of water. It is also clear that these refinements should be included in water models attempting to simulate this substance.
Theory of dynamic magic angle spinning nuclear magnetic resonance and its application to carbon-13 in solid bullvalene
Authors: Luz Z. , Poupko R. , Alexander S.
Published: July 2002

Carbon-13 magic angle spinning (MAS) spectra of bullvalene, at a spinning frequency of about 4 kHz, are presented for the temperature range −30 °C to +85 °C. At low temperatures (<−10 °C), separate center peaks are observed for the aliphatic and olefinic carbons, as well as spinning sidebands for the latter. Upon heating to room temperature and above, the peaks broaden and eventually coalesce to a single line at the weighted average chemical shift frequency. These results are interpreted in terms of two independent processes—a concerted Cope rearrangement reorientation and symmetric threefold jumps about the molecular C3 axes. In both processes, the high order of the bullvalene crystals is preserved. A quantitative analysis of the spectra yields the following kinetic equations for, respectively, the Cope rearrangement reorientation and the threefold jump processes: kC(s−1)=1.38×1014 exp(−14.5/RT); kJ(s−1)=2.53×1019 exp(−21.0/RT), where R is in kcal/mol degree. The analysis of the spectra was performed by comparing the experimental results with simulated line shapes, computed using the formalism of Schmidt and Vega, which is based on Shirley’s theory of solving the Schrödinger equation with a time-periodic Hamiltonian. In a theoretical section, we rederive the line shape equation without, however, making use of Floquet’s theorem as formulated by Shirley. In an appendix, the equivalence of both approaches, as well as a derivation of Floquet’s theorem, are presented.
Adiabatic electron transfer in polar solvents. Crossover from activation transfer to tunneling
Authors: Dakhnovskii Yuri
Published: July 2002

An electron transfer reaction in a polar solvent has been considered in the region of the crossover from activation transfer to tunneling. An exact expression for the rate constant for an adiabatic potential surface has been found and has also been expressed in terms of a linear response theory for an arbitrary dielectric loss function of the solvent. The specific cases of electron transfer in water and inner-sphere electron transfer have been studied. The range for which quantum effects are important, in terms of the splitting between the adiabatic potential surfaces, has been found. The theory provides a simple numerical method for calculating the rate constant in this region using a few experimentally obtainable results. This is in contrast to previous methods where complex potential energy surfaces were required to numerically determine the rate.
Small angle neutron scattering studies of dilute supercritical solutions. I. Pure supercritical neon
Authors: Londono J. D. , Shah V. M. , Wignall G. D. , Cochran H. D. , Bienkowski P. R.
Published: July 2002

Small angle neutron scattering (SANS) results are presented for states of neon close to the critical isochore and supercritical temperatures between 47 and 60 K. The net intensities were converted to absolute units of differential cross section per unit sample volume by comparison with precalibrated secondary standards. The values of the differential cross section at zero scattering vector are in excellent agreement with values calculated from an equation of state. Evidence of three-body forces in these data are compared to that present in Krypton data reported earlier. It is concluded that small angle experiments of the sort reported here should be repeated for the heavier noble gases, for which effects from three-body forces are anticipated to be larger.
An electron paramagnetic resonance study of Er3+ in Bi4Ge3O12 single crystals
Authors: Bravo D. , López F. J.
Published: July 2002

The X-band electron paramagnetic resonance study of Er-doped Bi4Ge3O12 single crystals at 5 K is reported. The spectra have been attributed to Er3+ ions in the Bi3+ site, confirming the assumed trigonal site symmetry of Er3+ in previous crystal-field analysis. A detailed study of the measured g-factor values indicates that corrections to the existing crystal-field parameters Bnm should be performed. It is suggested that corrections to the sixth-rank parameters are particularly important, in agreement with spin-correlated crystal-field (SCCF) effects for Er3+.