bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculated paramagnetic resonance parameters of a gallium arsenide cluster: Ga2As3
Authors: Arratia-Pérez Ramiro , Hernández-Acevedo Lucı́a
Published: July 2002

Dirac molecular-orbital calculations are reported for the trigonal bipyramid gallium arsenide Ga2As3 cluster, and comparisons are made to electron spin resonance spectra. The calculations show good agreement with the observed Δg and 71Ga hyperfine tensors. This theoretical study predicts both parallel and perpendicular components of the 75As hyperfine tensors, while experimentally only the perpendicular component was assigned. Furthermore, the calculated isotropic and anisotropic spin distributions show some discrepancy with the empirically deduced spin distributions. The discrepancy between the calculated and empirically deduced values may be traced back to the methodology used in the empirical treatment. The calculations predict that the single unpaired electron spin spend 31.4% of its time on each axial 71Ga nuclei, and, 12.4% of its time on each equatorial 75As nuclei.
An ab initio study of the monoxides and dioxides of sodium
Authors: Elliott Simon D. , Ahlrichs Reinhart
Published: July 2002

A study of structure and bonding in neutral and cationic sodium monoxides NamO (m=2–6) and dioxides NanO2 (n=1–9) is presented. Structural isomers, adiabatic ionization potentials and sodium bond energies are calculated and the factors leading to “outer shell” sodium are discussed. Monoxide and superoxide properties are investigated using a range of ab initio methods. Density functional theory (DFT) results are found to compare favorably with those from experiment and from more expensive theoretical approaches. DFT is therefore used for all the dioxide calculations. The dioxides are rationalized in terms of the three oxidation states O2−, O22− and 2O2−, representing three stages in the redox reaction between sodium and oxygen. Superoxide NaO2 may be reduced by sodium to peroxides NanO2 (n=2–4), containing O22− stabilized by an equatorial Na cage. O–O cleavage occurs when Na attacks apically, releasing charge stored in this cage and effecting a two electron transfer to 2O2−. The resulting compounds NanO2 (n=5–9) may be understood as sodium-bridged bis-monoxides.
Experimental investigation of radiative lifetimes of vibrational levels at the electronic ground state of C2−
Authors: Pedersen H. B. , Brink C. , Andersen L. H. , Bjerre N. , Hvelplund P. , Kella D. , Shen H.
Published: July 2002

Radiative lifetimes of the vibrational levels v=2 to v=4 of the X 2Σg+ ground state of the C2− molecular ion have been experimentally investigated by photodetachment in a storage ring. The lifetime of the v=3 level was 0.32±0.02 s for 12C12C−, and 0.22±0.02 s for 12C13C−. For both ions, the lower limit for the v=2 lifetime was 5 s, and the upper limit for the v=4 level was ∼100 ms. The measured lifetime for v=3 of 12C12C− is a factor of 4 smaller than found in a calculation [P. Rosmus and H. Werner, J. Chem. Phys. 80, 5085 (1984)]. The results point to the existence of at least one bound vibrational level of the lowest 4Σu+ state.
An explicit-solvent dynamic-dielectric screening model of electron-hole interactions in conjugated polymers
Authors: Moore Eric E. , Yaron David
Published: July 2002

The effects of interchain interactions on the exciton-binding energy of conjugated polymers are explored theoretically, using rigid polyacetylene chains as a model system. An explicit quantum chemical description is used to describe the polarization that an electron and hole induce in the surrounding polymer chains. The motivation for explicitly including interchain interactions is to allow the standard parameters of semiempirical quantum chemistry to be used to make predictions for solid-state polymers. The model includes the time scales of both the electron-hole motion and the dielectric polarization. A free electron or hole forms an electronic polaron, in which the bare electron or hole delocalizes over about four unit cells before developing a polarization cloud. In the 1 1Bu exciton state, the time scale for electron-hole motion is comparable to that of the polarization. (If a fast dielectric response is assumed, the polarization energy is overestimated by about 60%.) For the Pariser-Parr-Pople Hamiltonian, polarization stabilizes a free electron-hole pair by about 1.5 eV and the exciton by about 0.2 eV, thereby lowering the exciton-binding energy by 1.3 eV. This reduction in exciton-binding energy occurs with relatively minor effects on the form of the exciton itself, indicating that the electron and hole must shed their polarization when they join to form an exciton. The electron-hole interaction in the exciton is then nearly identical to that on an isolated chain. This indicates, more generally, that the effective strength of the dielectric medium varies depending on the nature of charge fluctuations in a particular state. This observation may help resolve many issues concerning the relative importance of electron-electron interactions and electron correlation in these materials.
Universality in the fast orientational relaxation near isotropic–nematic transition
Authors: Ravichandran S. , Perera A. , Moreau M. , Bagchi B.
Published: July 2002

Detailed molecular dynamics simulations of Lennard-Jones ellipsoids have been carried out to investigate the emergence of criticality in the single-particle orientational relaxation near the isotropic–nematic (IN) phase transition. The simulations show a sudden appearance of a power-law behavior in the decay of the second-rank orientational relaxation as the IN transition is approached. The simulated value of the power-law exponent is 0.56, which is larger than the mean-field value (0.5) but less than the observed value (0.63) and may be due to the finite size of the simulated system. The decay of the first-rank orientational time correlation function, on the other hand, is nearly exponential but its decay becomes very slow near the isotropic–nematic transition. The zero-frequency rotational friction, calculated from the simulated angular velocity correlation function, shows a marked increase near the IN transition.
Molecular and ionic interactions in poly(acrylonitrile)- and poly(methylmetacrylate)-based gel electrolytes
Authors: Ostrovskii Denis , Brodin Alexander , Torell Lena M. , Appetecchi Giovanni Battista , Scrosati Bruno
Published: July 2002

Vibrational Raman and Fourier transform IR (FTIR) spectroscopy has been used to determine molecular and ionic interactions in poly(acrylonitrile)—(PAN) and poly(methylmetacrylate)—(PMMA) based gel electrolytes, containing LiClO4 salt and a mixture of ethylene carbonate (EC) and propylene carbonate (PC) as a gelating agent. A number of model systems, representing different stages of gel preparation, as well as the pure components, have also been studied. Vibrational spectra reveal interactions between EC and PC molecules in EC–PC mixture via electric dipoles located on C=O groups. In the salt-doped EC–PC system, additional bands close to the ring breathing and bending fundamentals of the carbonate ring of EC and PC, respectively, are attributed to solvent molecules interacting with the cations via Coulombic forces, the satellite band frequencies being well correlated to the cationic potentials. In the PMMA-based gels, no spectroscopically detectable interactions between the polymer matrix and the electrolyte were observed. In the PAN-based systems, in contrast, C≡N groups of PAN serve as attractive sites for both the solvent molecules and the cations, the preferable interaction being that between PAN and the solvent.
Femtosecond photodissociation dynamics of I2 studied by ion imaging
Authors: Larsen Jakob Juul , Mo/rkbak Nils Jakob , Olesen Johannes , Bjerre Nis , Machholm Mette , Keiding So/ren Rud , Stapelfeldt Henrik
Published: July 2002

Ion imaging is employed to analyze the fragments from timed Coulomb explosion studies of femtosecond (fs) molecular dynamics. The technique provides high detection efficiency and direct recording of the two-dimensional velocity of all ionized fragments. We illustrate the approach by studying photodissociation of iodine initiated by a weak femtosecond pump pulse and probed by an intense, delayed femtosecond pulse. The ion images, recorded as a function of the pump-probe delay, reveal the evolution of the internuclear separation of the dissociating I2 molecule. The experimental results are in good agreement with quantum mechanical wave packet simulations. We discuss the perspectives for extending the studies to photochemical reactions of small polyatomic molecules.
Spectroscopic detection and characterization of iodogermylene (HGel)
Authors: Harper Warren W. , Klusek Chad M. , Clouthier Dennis J.
Published: July 2002

Monoiodogermylene has been detected for the first time using pulsed discharge and laserinduced fluorescence techniques. HGeI and DGeI were produced by an electric discharge through argon seeded with H3GeI or D3GeI. Although the vibronic structure in the spectra was very limited, all three excited state vibrational frequencies have been obtained for both isotopomers. Analysis of the partially resolved rotational structure of the 000 bands gave the following approximate r0 structures, with the bond angles constrained to previous ab initio values: r0″(Ge–I)=2.525(10) Å, r0″(H–Ge)=1.593(15) Å, θ0″(HGeI)=93.5°, r0′(Ge–I)=2.515(10) Å, r0′(H–Ge)=1.618(15) Å, and θ0′(HGeI)=116.2°. The fluorescence lifetime of monoiodogermylene in the lowest rovibronic levels is 1.515±0.004 μs and shows significant variations on deuteration and with rotational and vibrational level.
On the effects of absolute laser phase on the interaction of a pulsed laser with polar versus nonpolar molecules
Authors: Brown Alex , Meath William J.
Published: July 2002

The absolute laser phase dependence of the time-dependent populations of the molecular states, including the steady-state (long time) populations of the states, associated with the interaction of a molecule with a pulsed laser is investigated using illustrative two-level examples. One-photon transitions, including the effects of permanent dipoles, are discussed as a function of the pulse duration, intensity, and (absolute) laser phase, for selected laser frequencies. The effects of laser phase can be large, depending on the values of the pulse duration for a given frequency and intensity. The effects of permanent dipoles, relative to no permanent dipoles, are significant for large laser field strengths ε0. When the laser-molecule coupling parameter b=μ12ε0/E21⩾0.2, where μ12 and E21 are the transition dipole and energy difference between the ground and excited states, respectively, the dynamics of the pulse-molecule interaction are (strongly) phase dependent, independent of pulse duration, whereas the corresponding steady-state populations of the molecular states may or may not be phase-dependent depending on the pulse duration. Analytical rotating wave approximations for pulsed laser-molecule interactions are useful for interpreting the dynamics and the steady-state results as a function of field strength and pulse duration, including the effects of permanent dipole moments. The results reported in this paper are based on molecular parameters associated with an S0→S1 electronic transition in a dipolar molecule. However, they are presented in reduced form and therefore can be scaled to other regions of the electromagnetic spectrum. Short, intense pulses at or beyond the limits of current laser technology will often be required for the types of absolute laser phase effects of this paper to be appreciable for electronic excitations. The discussion, in the UV-VIS, also suffers from the use of a two-level model and from the requirement of field intensities that can be beyond the Keldysh limit. For other spectral regions, these absolute laser phase effects will be much more readily applicable.
Low-lying states of Tl2 calculated by the configuration interaction methods based upon relativistic effective core potentials and two-component spinors
Authors: Kim Myeong Cheol , Lee Hyo-Sug , Lee Yoon Sup , Lee Sang Yeon
Published: July 2002

We have modified a Dirac–Fock program package, MOLFDIR [L. Visscher et al., Comput. Phys. Commun. 81, 120 (1994)], to perform two-component molecular spinor calculations based upon the relativistic effective core potentials (REPs) incorporating effective spin–orbit operators. The modified MOLFDIR can be used to perform two-component REP calculations instead of four-component all-electron calculations at various levels of theory including the configuration interaction (CI) level. As a test case for the multireference CI methods, the low-lying states of Tl2 are calculated with the two-component CI method and the more conventional spin–orbit CI method. Results indicate that the two-component method has some advantages in describing the ground state of Tl2 while excited states are similarly described by both methods.
Molecular dynamics simulation of methanol clusters
Authors: Zakharov Viktor V. , Brodskaya Elena N. , Laaksonen Aatto
Published: July 2002

Methanol clusters have been studied based on Molecular Dynamics simulations. The methanols are described using the three-site OPLS potential model by Jorgensen. Five separate clusters with sizes up to 512 molecules are treated at two different temperatures, 200 K and 300 K, respectively. Several properties, for example, the local density, electric potential and the normal pressure, are calculated. Also, the surface potential of methanol is computed, based on the radial profiles of the electric potential. It is shown, once again, that the quadrupolar contribution to the surface potential is important to the total value of the surface potential. Using the size dependence of the work of cluster formation, it is found that the surface tension of methanol decreases monotonously upon decreasing the radius of the dividing surface approaching a limiting value. The calculated limiting value for surface tension agrees well with that, observed experimentally for the plane surface of methanol.
Chemisorption site of methanethiol on Pt{111}
Authors: Kim S. S. , Kim Y. , Kim H. I. , Lee S. H. , Lee T. R. , Perry S. S. , Rabalais J. W.
Published: July 2002

The chemisorption site of the simplest prototypical model alkanethiol compound, methanethiol [CH3SH], on a Pt{111} surface in the temperature range 298–1073 K has been investigated by means of time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low-energy electron diffraction (LEED). TOF-SARS spectra of the scattered and recoiled ions plus fast neutrals were collected as a function of crystal azimuthal rotation angle δ and beam incident angle α using 4 keV Ar+ primary ions. At room temperature, the adsorption of methanethiol produces a partially disordered overlayer that gives rise to a diffuse (3×3)R30° LEED pattern and three-fold symmetry in the scattering profiles. Heating this surface layer results in the sequential dehydrogenation of the methanethiol and the formation of S–C species at elevated temperatures. By ∼373 K, hydrogen is absent from the TOF-SARS spectra and a sharp (3×3)R30° LEED pattern is observed. The model developed from the scattering data is consistent with the preservation of the adsorption site at elevated temperatures, but a change in the S–C bond angle with respect to the surface plane. For the fully dehydrogenated species, the S atoms reside ∼1.6±0.2 Å above the surface in face-centered-cubic (fcc) three-fold sites and the C atoms reside ∼1.5±0.4 Å in hexagonal-close-packed (hcp) three-fold sites. It is proposed that the remarkable stability of this SC adsorbate results from bonding of both the S and C atoms to the surrounding Pt atoms, i.e., a Pt-stabilized SC moiety.
Calculation of atomization energies by a multiconfigurational localized perturbation theory—Application for closed shell cases
Authors: Dunietz Barry D. , Murphy Robert B. , Friesner Richard A.
Published: July 2002

In a recent publication [L. A. Curtiss et al., J. Chem. Phys. 106, 1063 (1997)] the enthalpies of formation of 148 molecules are calculated via the Gaussian-2 (G2) scheme, and compared to reliable experimental values. The set of molecules assembled can be used as a benchmark to test reliability of different theoretical models to predict thermochemical properties. The performance is impressive, with an average absolute deviation of only 1.58 kcal/mol. We have developed an alternative model, the J2 model which is based on the generalized valence bond-localized Mo/ller–Plesset (GVB-LMP2) method. In this work the J2 model is tested by calculation of the enthalpies of formation of closed shell molecules containing only first row atoms from the G2 set of molecules. This is achieved by using the GVB-LMP2 method to calculate atomization energies and using experimental values to shift the energies to heats of formation. The J2 theory is shown to perform statistically better than the much more expensive G2 method. This is shown not only by a lower mean absolute deviation but even more importantly by a lower maximum deviation. The most dramatic improvement is obtained for the systems containing carbon–fluoride bonds. An explanation for this result is presented.
Validation of molecular simulation by comparison with experiment: Rotational reorientation of tryptophan in water
Authors: Daura Xavier , Suter Raymond , van Gunsteren Wilfred F.
Published: July 2002

Molecular dynamics simulations aimed at analysis of the rotational reorientation of tryptophan and 3-methylindole in water have been performed. The dependence of the rotational relaxation time of tryptophan on several simulation and model parameters has been evaluated. The considerable sensitivity found for particular parameters illustrates the necessity of a detailed analysis before jumping to conclusions regarding the validity of a molecular model and force field based on comparing simulation with experimental data. The best agreement with experimental data is obtained when using the extended simple-point-charge (SPC/E) model for water together with a reaction-field correction for the long-range electrostatic interactions.
Evidence for a nuclear hexadecapole interaction in the hyperfine spectrum of LiI
Authors: Cederberg J. , Olson D. , Nelson A. , Laine D. , Zimmer P. , Welge M. , Feig M. , Höft T. , London N.
Published: July 2002

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of LiI7 to determine the nuclear hexadecapole interaction of the iodine nucleus. The nuclear magnetic octupole interaction was also considered but found to be marginally significant. A total of 172 transitions in vibrational states 0-3 and rotational states 1-6 have been included in a fit to determine the iodine nuclear quadrupole, spin-rotation, and hexadecapole interactions, the lithium quadrupole and spin-rotation interactions, and the tensor and scalar parts of the spin-spin interaction. Vibration and rotation dependencies of these constants have been determined. The results include: eHh=−0.0151(30), eQIqI=−194351.212(17)−8279.521(46)(v+1/2)+100.616(34)(v+1/2)2−0.3949(73)(v+1/2)3−6.41977(50)J(J+1)+0.10593(33)(v+1/2)J(J+1),eQLiqLi=172.613(52)−3.26(14)(v+1/2)+0.00145(87)J(J+1),cI=6.80260(32)+0.00303(49)(v+1/2)−0.000118(13)J(J+1), cLi=0.75872(72)−0.0088(11)(v+1/2), c3=0.62834(68)−0.0050(11)(v+1/2), c4=0.06223(36)+0.00041(26)(v+1/2), and eΩIωI′=0.000112(73), all in kHz with one standard deviation uncertainties for the last 2 digits in ( ).
Vanadium clusters: Reactivity with CO, NO, O2, D2, and N2
Authors: Holmgren Lotta , Rosén Arne
Published: July 2002

We have investigated the size-dependent reactivity of vanadium clusters, V10–V60, with CO, NO, O2, D2, and N2 by using a laser-vaporization source, low-pressure reaction cell and photoionization time-of-flight mass spectrometer. The reaction probability in a collision between a cluster and a gas molecule was determined for each gas for the first and second molecule. The reactivity was measured at two different cluster-source temperatures: room temperature (RT) and liquid-nitrogen temperature (LNT). We find that vanadium clusters react readily with all molecules investigated. With CO, NO, and O2, the reaction probability is high and exhibits relatively weak size dependence. With D2 and N2, the overall reaction probability is lower, and significant size variations are observed over the whole size range investigated, with coinciding minima and maxima for the two molecules. Upon cooling of the cluster source to LNT, the reactivity increases, but the overall size dependence is very similar to that observed at RT. The reaction probability with the second molecule, S2, is lower than that with the first one, S1, for all gases investigated, and, in most cases, the size dependence in S1 and S2 is similar. By analyzing the total relative abundance of each cluster size as a function of reaction-cell pressure, we find that reaction of Vn with CO, NO, and O2 results in size-dependent cluster-product fragmentation of clusters in the size range n≈10–20, most likely by metal-atom loss.
Photodissociation dynamics of the allyl radical
Authors: Deyerl Hans-Jürgen , Fischer Ingo , Chen Peter
Published: July 2002

The photochemistry and photodissociation dynamics of the allyl radical upon ultraviolet (UV) excitation is investigated in a molecular beam by using time- and frequency-resolved photoionization of hydrogen atoms with Lyman-α-radiation. The UV states of allyl decay by internal conversion to the ground state, forming vibrationally hot radicals that lose hydrogen atoms on a nanosecond time scale. Two channels are identified, formation of allene directly from allyl, and isomerization from allyl to 2-propenyl, with a subsequent hydrogen loss, resulting in both allene and propyne formation. The branching ratio is between 2:1 and 3:1, with direct formation of allene being the dominant reaction channel. This channel is associated with site-selective loss of hydrogen from the central carbon atom, as observed in experiments on isotopically labeled radicals. Ab initio calculations of the reaction pathways and Rice–Ramsperger–Kassel–Marcus (RRKM) calculations of the rates are in agreement with the mechanism and branching ratios. From the measured Doppler profiles a translational energy release of 14±1 kcal/mol is calculated. The calculated value of 66 kcal/mol for the barrier to the 1,2 hydrogen shift from allyl radical to 2-propenyl is confirmed by the experimental data.
Conformational properties of isolated polyelectrolytes in poor solvents
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

The conformational behavior of isolated polyelectrolytes in poor solvents is investigated using theory and computer simulation. DiMarzio’s approach for the collapse transition in neutral polymers is extended to uniformly charged polymers [E. A. diMarzio, Macromolecules 17, 969 (1984)]. The theory predicts a discontinuous collapse transition with decreasing solvent quality in highly charged polymers. A discontinuous collapse transition is also observed when the charge fraction is decreased in very poor solvents. Monte Carlo simulations are reported for the collapse transition in a lattice model of polyelectrolytes. Under some conditions the distribution of polymer size is bimodal, consistent with the theory.
Determination of a methane intermolecular potential model for use in molecular simulations from ab initio calculations
Authors: Rowley Richard L. , Pakkanen Tapani
Published: July 2002

The possibility of obtaining an accurate site-site potential model suitable for use in molecular dynamics (MD) simulations of methane from ab initio calculations has been explored. Counterpoise-corrected (CPC), supermolecule, ab initio energies at the MP2/6-311+G(2df,2pd) level were computed for eleven relative orientations of two methane molecules as a function of C-C separation distance. C-C, C-H, and H-H interaction parameters in a pairwise-additive, site-site potential model for rigid methane molecules were regressed from the ab initio energies, and the resultant model accurately reproduced the ab initio energies. The model suggests that C-H attractions are dominant in weakly binding the methane dimer. CPC energies for methane trimers, tetramers, and a pentamer were also calculated at the same level. The results indicate that the n-mer energy per pair of interactions monotonically converges with increasing n, but that the assumption of pairwise additivity commonly used in MD simulations is reasonably valid. A limited number of higher-level calculations using MP4/6-311+G(2df,2pd) and MP4/aug-cc-pVTZ were also performed to investigate the possibility of obtaining the intermolecular potential model from higher accuracy calculations without a substantial increase in computer resources. Results suggest that a Ne-methane probe method is not useful in this regard, but that limited, high-level computations, coupled with more extensive lower-level values, may be used to improve the model at minimal cost.
Local structure of Ni2+ ions in fluorochloro- and fluorobromo-zirconate glasses
Authors: Buñuel M. A. , Garcı́a J. , Proietti M. G. , Solera J. A. , Cases R.
Published: July 2002

Nickel doped fluorochloro- and fluorobromo-zirconate glasses have been prepared following and modifying standard methods. Electron probe microanalysis has been carried out to obtain the actual chlorine and bromine content of the samples. X-ray and optical absorption measurements have been performed in order to study the Ni2+ neighborhood. Extended x-ray absorption fine structure and x-ray absorption near edge structure spectra at the Ni K edge show the progressive substitution of the fluorine ions in the first coordination shell of Ni2+ by chlorine or bromine ions when their concentration increases in the glasses. From the analysis of the absorption features due to the 3A2g → 1Eg(D) spin forbidden transition, up to four Ni2+ environments have been identified and assigned to: 6F−, 5F− and 1Cl− or 1Br−, 1F− and 5Cl− or 5Br− and 6Cl− or 6Br−. It is deduced that less than 10% of fluorine ions substitution is enough to have all the Ni2+ ions hexacoordinated to chlorine or bromine ions. NiCl2 and NiBr2 crystals have been found in the samples with an intermediate content of chlorine and bromine ions, respectively; Ni2+ ions tetrahedrally coordinated with Cl− or Br− ions are present in the most concentrated samples in chlorine or bromine ions, respectively. Values for the crystal field and Racah parameters, have been estimated from the absorption data for the hexa and tetracoordinated vitreous environments of Ni2+.
Rotational spectroscopy and molecular structure of 15N2–14N2O
Authors: Leung Helen O.
Published: July 2002

The rotational spectrum of 15N2–14N2O has been recorded in the 7–19 GHz region with a pulsedmolecular beam,Fourier transformmicrowavespectrometer. An internal motion of the 15N2 subunit has been observed and the nuclear quadrupole hyperfine structurein each internal motion state has been analyzed using the WatsonS-reduced Hamiltonian with the inclusion of nuclear quadrupole couplinginteractions. The spectroscopic constants of the ground internal motion state are not welldetermined since only 4 transitions have been observed, but they are similar to those ofthe excited internal motion state that are determined from the analysis of 14a- and b-type transitions. The rotational andcentrifugal distortion constants (in MHz) for the excited internal motion state areA=12 791.307 0(2),B=2 014.982 4(1),C=1 728.950 45(7),DJ=1.052 5(2)×10−2,DJK=3.933 7(3)×10−1,d1=−1.822(3)×10−3,d2=−1.118(6)×10−3. The nuclear quadrupole coupling constants (in MHz) in the excitedinternal motion state for the terminal 14N nucleus in N2O are χaa=0.3465(4),χbb=−0.7445(5), and χcc=0.3980(5), while those for the central 14N nucleus are χaa=0.1023(9),χbb=−0.2528(8), and χcc=0.1505(8). These spectroscopic constants are consistent with aT-shaped structure, with 15N2 forming the leg of the T. The intermolecular distance is3.691 Å. The 15N2 axis and the 14N2O axis make an angle of 13° and 81° with the intermolecularaxis, respectively. The nuclear quadrupole coupling constants show electric field gradient perturbationin the N2O subunit, with the field gradient at the central nitrogenaffected to a greater extent than that at the terminal nitrogen. This perturbation islikely due to electron charge redistribution in N2O upon complexation with N2.
Birefringence measurements in low-Tg photorefractive materials by means of electro-optic relaxation spectroscopy
Authors: Hoechstetter K. , Schloter S. , Hofmann U. , Haarer D.
Published: July 2002

The maximum achievable diffraction efficiency of organic photorefractive materials is strongly influenced by the properties of the nonlinear optical chromophores. Here we report on a novel evaluation method of frequency dependent ellipsometric measurements. We fit our theory to the phase shift between the alternating electric field and the detected signal as a function of the applied frequency. The presented method is based on an improved theoretical description of electro-optic relaxation spectroscopy when compared to earlier publications. It allows for the determination of the birefringence contribution CBR to the refractive index change in materials with low glass transition temperature Tg. We apply our method to data obtained on highly efficient photorefractive guest–host systems based on polysiloxane. The nonlinear contributions were also determined and the results are briefly discussed. An error estimate for the value of CBR yields an accuracy of about ±10%. A comparison of electro-optic relaxation spectroscopy with photorefractive methods for determining the ratio CBR/CEO is presented. We conclude that this method allows for an estimate of the orientational enhancement with a relatively simple setup which is independent of the photorefractive experiment.
The basis set convergence of the Hartree–Fock energy for H2
Authors: Jensen Frank
Published: July 2002

By using completely optimized basis functions, it is shown that the Hartree–Fock energy for the H2 molecule converges exponentially, both with respect to the number of basis functions of a given type and with respect to the highest angular momentum functions included. With a basis set including up to h-functions, a subnanohartree accuracy can be obtained. These results clearly suggest that estimation of the complete basis-set limit should employ different extrapolation schemes for the Hartree–Fock and correlation energies.
Cluster birth–death processes in a vapor at equilibrium
Authors: Soto Rodrigo , Cordero Patricio
Published: July 2002

A method is presented to analyze and observe, in molecular dynamic simulations, the statistical properties of instantaneous cluster transitions, mainly fusions and fissions for a homogeneous vapor at equilibrium. The method yields the way to obtain mean lives, branching ratios and, to some extent, transition rates as well. To the best of our knowledge branching rations in cluster decays have not been measured before (simulationally or experimentally). An application of this method to a model system provides a critical reassessment of the standard Homogeneous Nucleation Theory (HNT). Our own simulations show that transitions different from absorbing or evaporating a monomer are quite important, representing in some cases 50% of all decay events. Our method also shows unequivocally that the decay processes involving clusters classified by size alone are not Markovian.
High dimensional anharmonic potential energy surfaces: The case of methane
Authors: Venuti E. , Halonen L. , Della Valle R. G.
Published: July 2002

The overtone vibrational spectra of all Td symmetry isotopomers of methane have been analyzed simultaneously. A Hamiltonian expressed in internal curvilinear coordinates expanded to the fourth order has been employed, with a nine-dimensional basis of harmonic oscillator wave functions in symmetry coordinates. Near-resonant anharmonic interactions are treated to first order, while weaker interactions are handled as second order perturbations. A set of optimized Born–Oppenheimer force constants is obtained, which reproduces the observations up to 9500 cm−1 and shows an excellent agreement with the results of ab initio calculations.
Structures of large Morse clusters
Authors: Smirnov B. M. , Strizhev A. Yu. , Berry R. S.
Published: July 2002

The binding energy of atoms interacting via pairwise Morse potentials is calculated by several methods for large clusters of face-centered cubic (fcc) and icosahedral structures: by numerical calculations for bulk particles, by direct numerical calculations for numbers of atoms n=561–923, and by analytical methods using perturbation theory. A phase diagram is constructed for a wide range of cluster sizes, which links the adjustable range parameter of the Morse potential to the cluster energy along the curve corresponding to the set of conditions for which the energies of the fcc and icosahedral structures are equal.
Stirring and mixing effects on oscillations and inhomogeneities in the minimal bromate oscillator
Authors: Dutt A. K. , Menzinger M.
Published: July 2002

Stirring and mixing effects on the oscillations and inhomogeneities in the bromate–bromide–cerous system (minimal bromate oscillator) have been investigated in a continuously fed stirred tank reactor (CSTR). A movable microelectrode is used to monitor the inhomogeneities inside the CSTR in an oscillating phase. The results are explained in terms of the theory of imperfect mixing.
The A 3Π state of SO
Authors: Elks John M. F. , Western Colin M.
Published: July 2002

Laser induced fluorescence and 1+1 resonance enhanced multiphoton ionisation spectra of the A 3Π–X 3Σ− transition of SO radicals prepared by an electric discharge in a supersonic jet expansion are presented. Rotational constants are given for A state vibrational levels with v′=0–13, extending to within 190 cm−1 of the A state dissociation limit. The Rydberg–Klein–Rees curve derived from these constants shows significant anharmonicity, even around the equilibrium geometry. In addition, several small local perturbations of the rotational structure are observed. Collision free fluorescence lifetimes are determined for the complete range of vibrational states, and are found to fall smoothly from 29.5 μs for v′=0 to 6.45 μs for v′=12. Combining these data with earlier measurements leads to a better determination of the A–X transition dipole moment over the range 1.4–2.0 Å.
Experimental and theoretical determination of the magnetic dipole transition moment for the Br(4p5)(2P1/2←2P3/2) fine-structure transition and the quantum yield of Br(2P1/2) from the 193 nm photolysis of BrCN
Authors: He G. , Seth Michael , Tokue I. , Macdonald R. Glen
Published: July 2002

The integrated-absorption coefficients of several hyperfine lines of the magnetic dipole allowed transition of the bromine atom, Br, center at 3685.2 cm−1 were measured, and a value for the square of the magnetic dipole transition moment of the Br atom was determined. A theoretical calculation for the magnetic dipole transition moment was also carried out using a relativistic ab initio atomic structure formulation. The theoretical value was in excellent agreement with the value predicted assuming pure LS coupling, and in reasonable agreement with experiment. The Br atom was generated in equal concentration with the cyano radical (CN) by the 193 nm photolysis of cyanogen bromine, BrCN. The CN radicals were titrated by the rapid reaction with C3H8 to generate HCN and a small amount of HNC. Both time-resolved and frequency-scanned infrared absorption spectroscopy were used to monitor the Br, HCN, and HNC species. The photolysis of BrCN at 193 nm produced both the ground state Br(2P3/2) and the spin-orbit excited Br(2P1/2) atoms, and the yield for the production of Br(2P1/2) atoms was measured to be 0.31±0.01. The rate constants for the quenching of Br(2P1/2) by BrCN and C3H8 at 293 K were also determined.
On the use of algebraic approximants to sum divergent series for Fermi resonances in vibrational spectroscopy
Authors: Goodson David Z. , Sergeev Alexei V.
Published: July 2002

Padé summation of large-order perturbation theory can often yield highly accurate energy eigenvalues for molecular vibrations. However, for eigenstates involved in Fermi resonances the convergence of the Padé approximants can be very slow. This is because the energy is a multivalued function of the perturbation parameter while Padé approximants are single valued, and Fermi resonances occur when a branch point lies close to the physical value of the parameter. Algebraic approximants are multivalued generalizations of Padé approximants. Using the (200) state of H2S and the (400) state of H2O as examples of Fermi resonances, it is demonstrated here that algebraic approximants greatly improve the summation convergence.
Dissociative model of water clusters
Authors: Corrales L. René
Published: July 2002

A model of water is introduced that dissociates into either ionic or covalent molecular fragments. The model is based on a semiempirical calculation embedded in a molecular mechanics approach. It contains electronic degrees of freedom that allow the calculation of partial charges that vary smoothly from the bound state to the dissociated states. The hydrogen bond is modeled classically with Coulomb interactions, and shows significant improvement in predicting cluster behavior by adding directional character via an overlap between a lone pair orbital on oxygen and the hydrogen of an adjacent molecule. An implicit bond polarization is present due to stretching bonds and charge transfer when the hydrogen bonds are formed. The energetic and geometric properties of water dissociation products, molecular clusters, and ionic clusters are presented.
Self-diffusion in single-file zeolite membranes is Fickian at long times
Authors: Nelson Peter H. , Auerbach Scott M.
Published: July 2002

We have developed a theory for self-diffusion in single-file Langmuirian zeolites of finite extent, which has been validated by open system kinetic Monte Carlo simulations. Our theory is based on a two-stage, Fickian diffusion mechanism, wherein a vacancy must traverse the entire file length to produce particle displacements of one lattice spacing. For times shorter than the vacancy diffusion time, tc, particle transport proceeds via the nonFickian, single-file diffusion mode, with mean-square displacements increasing with the square-root of time. For times longer than tc, however, we find that self-diffusion in single-file systems is completely described by Fick’s laws. We find that the fraction of time in the single-file diffusion mode scales inversely with file length for long files, suggesting that Fickian self-diffusion dominates transport in longer single-file zeolites. Through correlations among the particle movements, the single-file self-diffusivity is sensitive to sorption limitations for short files, and scales inversely with file length for long files. Experimental verification of the theory by pulsed field gradient NMR and tracer zero-length column experiments is discussed.
Linear response theory for the polarizable continuum model
Authors: Cammi Roberto , Mennucci Benedetta
Published: July 2002

We present the extension of the linear response formalism to the polarizable continuum model (PCM) used to study solvent effects on molecular electronic structures and properties. The most relevant formal aspects of the theory are discussed and numerical applications at the random phase approximation (RPA-PCM) level for the calculation of static and dynamic polarizabilities and magnetic shielding tensors, are also presented. In particular, the nuclear magnetic shielding tensors are obtained by a generalization of the localized orbital/local origin (LORG) approach.
Green’s function calculation of electron spin polarization. III. Freely diffusing radical pairs
Authors: Doktorov A. B. , Neufeld A. A. , Pedersen J. Boiden
Published: July 2002

A general analytic expression is derived for the electron spin polarization (CIDEP) generated in neutral radicals pairs in liquid solutions at high magnetic fields. It incorporates arbitrary reactivities of the singlet and triplet states of the radical pair complex, additional dephasing processes at the contact distance, and arbitrary values of the singlet–triplet mixing parameter. The exchange interaction is assumed to decrease exponentially with the separation between the radicals and is characterized by a decay length lx and a maximum value J0. The only condition for the applicability of the expression is that the exchange interaction is locally weak (J0lx2/D)2≪1, but it may be integral strong (D is the relative diffusion constant for the radicals). By comparison with numerical results, it is shown that the expression is very accurate; no deviations were found. The expression is obtained from our previously derived general expression by explicitly evaluating all the fundamental quantities from the Green’s function of the free diffusive motion. It is shown that the previously published formulas correspond to special cases of the present expression.
Enrichment of surfaces in contact with stable binary mixtures: The case with long-ranged surface fields
Authors: Frisch Harry L. , Puri Sanjay , Nielaba Peter
Published: July 2002

We consider the experimentally relevant problem of a stable binary mixture in contact with a surface which has a preference for one of the components of the mixture. In particular, we focus on the dynamics of surface enrichment resulting from a surface field turned on at zero time. We analytically solve this problem in the linearized approximation and thereby obtain the asymptotic behavior of various characteristics of the enrichment profiles. Our numerical results indicate that some of the important predictions of linearized theory are valid even in the strongly nonlinear regime.
Electron stimulated desorption from solid CO2 and a hydrogen containing CO2 matrix: A comparative study
Authors: Šiller L. , Hedhili M. N. , Le Coat Y. , Azria R. , Tronc M.
Published: July 2002

We have studied electron stimulated desorption of positive ions from ∼15 monolayer (ML) film of pure solid CO2 adsorbed on polycrystalline Pt surface at ∼40 K using incident electrons in the energy range 1–31 eV. The positive ions O+, C+, CO+, CO2+, and O2+ were observed and their threshold energies are discussed. Hydrogenation of the CO2 film was found to reduce the O2+ production but the threshold remained the same as for the pure CO2 matrix. After hydrogenation of the CO2 matrix additional ions—H+, H3O+, HCO+, and H2CO+—are observed. The formation of formaldehyde cations (with a threshold energy of 17±1.5 eV) in the hydrogenated CO2 matrix has been observed for the first time and is a product of electron stimulated ion—molecule reactions, possibly via a process analogue to the so-called photosynthesis. These results could be relevant to a better understanding of the atmosphere of Mars.
Rotational spectra and hyperfine constants of ZrO and ZrS
Authors: Beaton Sara A. , Gerry Michael C. L.
Published: July 2002

The pure rotational spectra of ZrO and ZrS have been recorded using cavity Fourier transform microwave spectroscopy in the frequency range 9–26 GHz. The molecules were generated by laser ablation of a solid Zr rod in the presence of 0.05% of O2 or H2S, respectively, in either argon or neon. Rotational spectra of five previously unobserved isotopomers of ZrO in the X 1Σ+ state have been measured. Spectra for all five Zr32S isotopomers and for the Zr90S34 isotopomer in natural abundance have also been measured; this is the first report of pure rotational transitions for ZrS. Transitions in several excited vibrational states were also measured for the most abundant isotopomers of both species. Atomic mass-dependent Born–Oppenheimer breakdown correction terms were determined by fitting the data obtained for each molecule to a Dunham-like expression. Values for the equilibrium bond lengths of the two species were also calculated from the results of these fits. For both the Zr91S32 and Zr91O16 isotopomers, nuclear hyperfine structure due to the zirconium nucleus was observed and values for eQq0(91Zr) and CI(91Zr) have been determined. A rotational transition in the low lying a 3Δ state of ZrS has also been observed.
Dissociation of electronically excited D3O to O+D2+D from the glancing charge transfer reaction of D3O+ with H2O
Authors: Cohen Michael H. , Seitzinger Jeffrey , Tissandier Michael D. , Coe James V.
Published: July 2002

A method based on a microchannel plate particle detector’s output pulse height distributions has been used to specify the masses of product fragments in the glancing collisional charge transfer reaction of D3O+ with H2O. The predominant product channel involves D3O→O+D2+D.
On the ground electronic state of MoO+: Upgrade density functional theory calculations
Authors: Broclawik Ewa , Piskorz Witold , Adamska Katarzyna
Published: July 2002

Density functional theory calculations for MoO+ with the new BPW91 exchange-correlation functional are reported and compared to recent experimental results regarding electronic states and bonding in the cation. In variance to the local and previous versions of nonlocal functionals, BPW91 gives proper description of the ground state of MoO+ and ionization potentials of MoO in very good agreement with the experiment.
The photodissociation dynamics of CFBr excited into the Ã(1A″) state
Authors: Knepp Pamela T. , Kable Scott H.
Published: July 2002

The dynamics of the photolysis reaction, CFBr+hν→CF+Br, have been investigated for photolysis energies in the range, ν̄=23 500–26 000 cm−1 (λ=385–435 nm). These energies correspond to excitation into the Ã(1A″) state of CFBr with 2500–5000 cm−1 of excess vibrational energy. Following dissociation of jet-cooled CFBr, the internal energy (Ω, Λ, J) of the nascent CF fragments (X 2Π, υ=0) was probed by laser induced fluorescence spectroscopy. Two distinct types of product state distributions were observed. At energies above T00+3360 cm−1 the populations of the Π1/22 and Π3/22 spin–orbit states of CF were equal, while A″ lambda doublet states were preferred over A′. These populations are consistent with a direct dissociation mechanism on the Ã state, over a barrier with a height of 3360 cm−1. The strong state mixing in the vicinity of the barrier ensures a statistical mixture of final spin–orbit states. The preference for the A″ lambda doublet states is consistent with the two lone electrons in in-plane orbitals pairing up in the final CF product, leaving one unpaired electron in an out-of-plane orbital, lying parallel to the J vector of the recoiling fragment. For excitation at energies below T00+3360 cm−1 the ground spin–orbit state of CF (2Π1/2) is preferred, while the lambda doublet populations are equal. The interpretation of these populations is that at these energies Ã state CFBr is stable with respect to dissociation over the barrier. The molecule crosses to either the X̃ or ã state where it encounters a deep attractive potential well. The subsequent slower dissociation rate allows the molecule to follow a more adiabatic pathway producing the lowest spin–orbit state of CF, and for any preference for lambda doublet states to be lost.