bluuebunny/crossref_metadata_2025 huggingface dataset:
The pulsed field ionization spectrum and lifetimes of the states at the S1 origin of pyrazine
Authors: Hillenbrand Susan , Zhu Langchi , Johnson Philip
Published: July 2002

The pump–probe pulsed field ionization (PFI) spectrum through the origin of S1 of pyrazine reveals three distinct types of peaks. At low excess energy above the lowest ionization potential (I.P.) are sharp peaks with Franck–Condon factors representative of the singlet intermediate state. At ionization wavelengths <208 nm, the Franck–Condon overlap allows for the observation of primarily the triplet component of the pumped superposition state. At this energy, some resolved structure has a measured lifetime >10 μs and is determined to be the result of ionization of relaxed triplet states populated through vibrational dissociation of van der Waals clusters. A diffuse structure in the same region is the result of ionization of monomer intermediate states with lifetimes that vary with the ionizing wavelength. From this variation, it can be surmised that there is an inhomogeneous sample of intermediate states, possibly due to a mixture of unrelaxed molecular eigenstates and decoupled triplets populated by an intramolecular vibrational relaxation within the triplet manifold.
Molecular beam studies of the photolysis of allene and the secondary photodissociation of the C3Hx fragments
Authors: Jackson W. M. , Anex Deon S. , Continetti R. E. , Balko B. A. , Lee Y. T.
Published: July 2002

Angle resolved time of flight (TOF) measurements of the fragments produced when allene is photolyzed at 193 nm are described. The two primary processes that have been identified from these measurements are the H+C3H3 and the H2+C3H2 channels. The quantum yields for these first steps are 0.89 and 0.11, respectively. Subsequent photolysis of the C3H3 radical produces H2+C3H, C3H2+H, and C2H2+CH, while the C3H2 produces C3+H2, C2H+CH, and C2H2+C. The translational energy distributions for each one of these steps have been derived using the forward convolution technique. These energy distributions reveal the exit barriers and other constraints on the potential energy surfaces that lead to the above stated products.
Chemical shift correlation spectroscopy in rotating solids: Radio frequency-driven dipolar recoupling and longitudinal exchange
Authors: Bennett A. E. , Griffin R. G. , Ok J. H. , Vega S.
Published: July 2002

We present a new method of performing chemical shift correlation spectroscopy in solids with magic angle spinning (MAS). Its key feature is a longitudinal mixing period of π pulses that recouples the dipolar interaction. We discuss experimental results for triply-13C-labeled alanine and a theory combining MAS and π pulses.
The equilibrium structure of HCN
Authors: Carter Stuart , Mills Ian M. , Handy Nicholas C.
Published: July 2002

The equilibrium structure of HCN has been determined from the previously published ground state rotational constants of eight isotopomers by using (B0-Be) values obtained from a variational calculation of the vibration–rotation spectrum. The results are re(CH)=1.065 01(8) Å, and re(CN)=1.153 24(2) Å.
Ab initio molecular dynamics with correlated molecular wave functions: Generalized valence bond molecular dynamics and simulated annealing
Authors: Hartke Bernd , Carter Emily A.
Published: July 2002

We present an ab initio molecular dynamics algorithm at the generalized valence bond level. It does not need a precalculated potential energy surface or model Hamiltonian; instead the nuclei move according to first principles forces derived from the electronic wave function which in turn follows the movement of the nuclei. This technique includes the dominant static electron correlations, it can treat ground and excited many-electron states, and it can describe chemical bond formation and breaking qualitatively correctly. We apply the method to Na4, as a generic test example for small metal clusters, and show spin-dependent free dissociation dynamics as well as geometry optimization by simulated annealing. The latter involves novel boundary conditions to prevent dissociation and mass scaling to enhance performance.
Matrix isolation spectroscopy of metal atoms generated by laser ablation. II. The Li/Ne, Li/D2, and Li/H2 systems
Authors: Fajardo Mario E.
Published: July 2002

Results of experiments on lithium-doped cryogenic solids (Ne, D2, and H2) prepared by laser ablation of solid lithium are presented, including near UV-visible absorption spectra and thermal annealing studies. Li atoms are found in more than one type of trapping site in each of the systems studied. In Li/Ne matrices deposited at T=5 K, the main optical absorption feature is a so-called ‘‘triplet’’ absorption with peaks near 628, 641, and 650.5 nm. This observation extends this familiar result from heavier alkali/rare gas matrices to the lightest M/Rg matrix. In Li/D2 and Li/H2 matrices deposited at T=3 K, the main absorptions show an ‘‘asymmetrical doublet’’ pattern with peaks near 655 and 672 nm and 650 and 671 nm, respectively. The successful isolation of Li atoms in such light matrix host systems supports previous conjectures about the microscopic mechanism for trapping fast laser ablated atoms. Several highly symmetrical hypothetical Li atom trapping site structures are proposed and discussed in conjunction with the results of the classical Monte Carlo simulations reported in the following article of this journal. The observed absorption line shapes are best explained in the context of a Jahn–Teller effect caused by dynamic distortions of the trapping sites by lattice vibrations, with negligible matrix modification of the Li atom spin–orbit splitting.
Raman spectroscopic investigation of the dynamics of urea–water complexes
Authors: Hoccart Xavier , Turrell George
Published: July 2002

The hydration of urea via hydrogen-bond formation was found to have a significant effect on the observed Raman band shapes. An analysis indicates that, on the average, one molecule of water is carried along with a solute molecule during its reorientational motion (τR≊2.5 ps). Vibrational dephasing of the ν4 vibration (τω≊0.2 ps) is independent of concentration and appears to decrease slightly with increasing temperature. No evidence of urea dimer formation was detected.
Structure and dynamics of selenium chain melts: A molecular dynamics study
Authors: Almarza N. G. , Enciso E. , Bermejo F. J.
Published: July 2002

A molecular dynamics (MD) study of liquid selenium modeled by 16 linear chains of 40 monomers each is presented. The simulated thermodynamic state corresponds to the experimental density of 3570 Kg m−3 at 873 K. The structural and force constant data of the chains were obtained from previous studies of neutron diffraction experiments, lattice dynamics, and first principles calculations. The computed structural properties show a good agreement with available neutron scattering data. The flexibility of the chains and the high temperature thermodynamic state of the liquid enabled the observation of fast torsional motions and different spatiotemporal dynamic ranges, which can be described by the Rouse model for dense polymer solutions. We identify the crossover from an atomic to an intermediate or ‘‘universal’’ chain regime, and subsequently to global chain behaviors. The dynamics of the system is discussed in terms of time and space-dependent transport coefficients. The generated MD trajectory thus provides information on the single particle motions, the collective dynamics of one chain, and the dynamics of the global system. This separation is useful for understanding the low frequency collective motions which can be measured by inelastic neutron scattering. The spectra are interpreted in terms of existent dynamical models, which imply a degree of trapping of the atoms in some spatial regions of the liquid (‘‘chain cages’’) defined by atomic crosslinks, plus a slow diffusive process which modifies the shape of the cage according the renewal of the atomic crosslinks.
Metal coated fullerene molecules and clusters
Authors: Martin T. P. , Malinowski N. , Zimmermann U. , Näher U. , Schaber H.
Published: July 2002

Coevaporation of C60 and an alkali metal in a gas aggregation cell yields a distribution of clusters with composition (C60)nMx with 0≤x<150, n=1,2,3, and M=Li, Na, and K. For singly ionized clusters the mass peaks are strong for odd values of x but only after reaching the composition C60M7+. It is suggested that the onset of even–odd alternation marks the end of electron transfer between metal and C60 and the beginning of metal–metal bonding.
Reaction path analysis of the rate of unimolecular isomerization
Authors: Jang Soonmin , Rice Stuart A.
Published: July 2002

We show that a reaction path Hamiltonian can be used, with the basic concepts of the Davis–Gray analysis of unimolecular reaction rate, to generate an accurate description of the dynamics of a model isomerization reaction.
Numerical study of semiclassical initial value methods for dynamics
Authors: Kay Kenneth G.
Published: July 2002

We present numerical tests of five related semiclassical techniques for computing time-dependent wave functions. These methods are based on integral representations for the propagator and do not require searches for special trajectories satisfying double-ended boundary conditions. In many respects, the computational techniques involved resemble those of conventional quasiclassical treatments. Three of these methods result in globally uniform asymptotic approximations to the wave function. One such method, the treatment of Herman and Kluk, is found to be capable of especially high accuracy and rapid convergence.
Self-association of monohydric alcohols in water: Compressibility and infrared absorption measurements
Authors: D’Angelo M. , Onori G. , Santucci A.
Published: July 2002

The infrared absorption spectra of aqueous methanol, ethanol, 1- and 2-propanol, t-butanol, and n-butoxyethanol in the region of asymmetric and symmetric C–H stretching modes (3200–2800 cm−1) have been measured at 25 °C as a function of alcohol concentration in the whole cosolvent mole fraction region. Measurements of adiabatic compressibility were also performed on the same water–alcohol solutions at 25 °C. The two sets of experimental data are compared and discussed in terms of a possible mechanism of molecular aggregation in the various regions of alcohol concentration.
Infrared line shapes of clusters and microcrystals: Vibrational modes mixed by dipole interactions
Authors: Ovchinnikov Mikhail A. , Wight Charles A.
Published: July 2002

Mixing of near-degenerate vibrational modes by long-range dipole–dipole interactions can lead to complicated band shapes in the infrared spectra of clusters and microcrystals, particularly when the bands are inhomogeneously broadened as a result of distributions of crystal shapes and orientations in the sample. For the special case in which the clusters have isotropic crystal structures, the equivalence of amplitude vectors for all IR-active vibrational phonon modes permits the band shapes of dipole-mixed modes to be predicted from those of isolated modes. This analysis is successfully applied to experimental infrared spectra of thin films containing weakly interacting microcrystals of CO2 or N2O, as well as to CO2 clusters prepared by condensation in the gas phase. The results prove that these gas-phase clusters of CO2 have an isotropic crystalline structure.
The development of the 3p and 4p valence band of small aluminum and gallium clusters
Authors: Cha Chia-Yen , Ganteför G. , Eberhardt W.
Published: July 2002

Photoelectron spectra of Al−n and Ga−n clusters in the size range n=1–15 are presented. Using 5.0 and 3.68 eV UV light (KrF excimer and nitrogen laser, respectively), electrons from molecular orbitals corresponding to the 3p and 4p orbitals of the atoms are detached. The spectra reveal a rich fine structure not observed in earlier experiments. The data are compared with the results of quantum chemical calculations. A change in the pattern of the spectra near n=6 can be interpreted as a transition from planar to compact 3D structures. The spectrum of Al−13 agrees with the icosahedral structure predicted for this particle. The data do not agree with jellium model predictions. Differences between Al and Ga data can be correlated with the larger binding energy of the Ga 4s band.
Interaction between the ion dipole and the ion-induced dipole in reactions of the polar ion ArH+3
Authors: Praxmarer C. , Hansel A. , Lindinger W.
Published: July 2002

The reactions of the highly polar ion ArH+3 with the nonpolar neutrals Xe, Kr, CH4, CO, NO, O2, and N2 and with the polar neutrals SO2, H2O, NH3, CH3I, and C2H5I have been investigated in a selected ion flow drift tube experiment at near thermal conditions. The rate coefficients obtained have typically 15%–35% larger values than the collisional limiting values kL and kADO obtained by the Langevin and ADO (average dipole orientation) theory, respectively. These data are explained on the basis of the interaction between the dipole of ArH+3 and the ion-induced dipole and the permanent dipole, respectively, of the neutral reactants. Rate coefficients for the reactions of H+3 with the above neutrals were also obtained for comparison, as well as the zero field mobility of ArH+3 in helium being (18.5±1.5) cm2/V s. The ion KrD+3 (KrH+3) was detected unambiguously for the first time. Its dissociation energy is larger than 0.29 eV which is the lower limit of the dissociation energy of ArH+3.
Role of the interaction range in the shaping of phase diagrams in simple fluids. The hard sphere Yukawa fluid as a case study
Authors: Lomba Enrique , Almarza Noé G.
Published: July 2002

We have investigated the influence of the interaction range on the shape of the phase diagram in a system of particles that interact via a hard-sphere potential plus an attractive Yukawa interaction. The gas–liquid phase diagram is evaluated both by Gibbs ensemble Monte Carlo simulation and reference hypernetted chain integral equation theory. An estimate of the liquid–solid transition line was computed using a recently proposed one-phase entropic criterion with thermodynamic and structural input from the integral equation results. The overall agreement between theory and simulation is remarkable. Our results clearly indicate that the range of temperatures for which the liquid is stable shrinks as the interaction range decreases. At the same time, the gas–liquid phase diagram tends to flatten out, in particular on the liquid side. For sufficiently short interaction ranges the system has no stable liquid phase.
Numerical Hartree–Fock energies of singly charged cations and anions with N≤54
Authors: Koga Toshikatsu , Tatewaki Hiroshi , Thakkar Ajit J.
Published: July 2002

Numerical Hartree–Fock (NHF) calculations have been performed for the singly charged cations Li+–Cs+ and anions H−–I− in their ground states. For the cations, the NHF values in the literature are found to be insufficiently accurate, while for the anions our NHF study covers the whole series H−–I− (except Sc− and Pd−) for the first time. Atomic ionization potentials and electron affinities are computed and compared with experiment.
High resolution study of methane’s 3ν1+ν3 vibrational overtone band
Authors: Boraas Kirk , Lin Zhen , Reilly James P.
Published: July 2002

Methane’s 3ν1+ν3 vibrational overtone spectrum has been recorded at room temperature, 100 K, and approximately 15 K using laser intracavity photoacoustic and molecular beam techniques. Although rotational congestion renders the room temperature spectrum undecipherable, the 100 K data suggest possible rovibrational assignments that are confirmed in the 15 K spectrum by their individual temperature dependences. Molecular beam Stark spectroscopy is used to unambiguously identify the E symmetry components of the R(2), P(2), and Q(2) transitions. These results indicate the presence of several other bands whose intensities are derived through interactions with the 3ν1+ν3 vibrational overtone transition. Quantitative analysis of these interactions suggests a bright state origin of 11 277.0 cm−1.
How accurate is the Rice–Ramsperger–Kassel–Marcus theory? The case of H+3
Authors: Berblinger Michael , Schlier Christoph
Published: July 2002

The classical Rice–Ramsperger–Kassel–Marcus formula is tested at an accuracy level of a few percent by comparing results of numerical phase space integration with lifetimes deduced from trajectory calculations. The test object is HD+2; the calculation has been done for total energies of 0.5, 1.0, and 1.5 eV above dissociation, and for total angular momenta of 0–60ℏ. Presupposing that the trajectory calculations show the true classical dynamics, we find systematic deviations of up to 40% of the RRKM results. They can be fully explained by the influence of ‘‘direct trajectories,’’ a special kind of nonergodic behavior of the system. After correction for this phenomenon, both methods agree to within the accuracy of the calculations, which is about 3%. We also verified that the discrepancy vanishes when the energy approaches the dissociation energy.
Pseudospectral contracted configuration interaction from a generalized valence bond reference
Authors: Murphy Robert B. , Friesner Richard A. , Ringnalda Murco N. , Goddard William A.
Published: July 2002

A multireference configuration interaction method is presented based upon pseudospectral integration and a novel generalized valence bond referenced contraction procedure. The combination of these approaches is shown to allow for unprecedented multiconfiguration self-consistent-field calculations on large molecules.
Dynamics of block-copolymer chains near the microphase separation transition
Authors: Genz U. , Vilgis T. A.
Published: July 2002

Dynamical properties of an individual diblock copolymer are investigated. A generalized Rouse equation is derived for polymer chains composed of several blocks differing with respect to their monomeric units. This equation takes into account explicitly the dynamics of the block copolymer melt surrounding the probe chain. In general, the structure of this generalized Rouse equation differs from the corresponding result for a multicomponent homopolymer blend. It is applied to diblock copolymers close to the microphase separation, which is characterized by strong composition fluctuations and the critical slowing down of their dynamics. Emphasis is laid on the role of composition fluctuations in the dynamics of individual diblock chains. A slowing down with respect to the center-of-mass motion as well as the internal relaxations is expected. Results for the Gaussian model and the Brazovskii theory are compared. They show significant differences.
Electron transfer in the Marcus inverted region: Experiment and adiabatic tunneling mechanism
Authors: Dakhnovskii Yu. I. , Doolen Robert , Simon John D.
Published: July 2002

This paper presents an adiabatic tunneling model for electron transfer reactions in the Marcus inverted region. The model is used to address the role vibrational modes in determining the magnitude of the reaction rate constants for the inverted charge transfer reaction: (benzene+)Cl–O−→(benzene)Cl–O in acetonitrile solution. Two frequency ranges for the accepting vibrational mode were studied; 1500 cm−1 representative of the stretching mode of the aromatic ring, and 208 cm−1, used to model the intermolecular stretching mode of the donor–acceptor complex. The temperature dependence is different for the two calculated cases. In the high-frequency case the tunneling rate constant is independent of temperature (270–330 K) while the activated rate constant changes by one order of magnitude. For the low-frequency mode, the situation is more complicated. The temperature dependence is a function of the magnitude of the matrix coupling element, H12. The calculated temperature behavior for a tunneling mechanism using either class of vibrational modes is consistent with experiment. While electronic coupling and reorganization energy are not available from experiment or quantum chemical calculation, the approach used allows for the determination of the range for these parameters that fit the experimental values.
Thermal rate constants for the Cl+H2 and Cl+D2 reactions between 296 and 3000 K
Authors: Kumaran S. S. , Lim K. P. , Michael J. V.
Published: July 2002

Rate constants for the Cl+H2 and D2 reactions have been measured at room temperature by the laser photolysis-resonance absorption (LP-RA) technique. Measurements were also performed at higher temperatures using two shock tube techniques: laser photolysis-shock tube (LP-ST) technique with Cl-atom atomic resonance absorption spectrometric (ARAS) detection, over the temperature range 699–1224 K; and higher temperature rates were obtained using both Cl-atom and H-atom ARAS techniques with the thermal decomposition of COCl2 as the Cl-atom source. The combined experimental results are expressed in three parameter form as kH2( ± 15%) = 4.78 × 10−16 T1.58 exp(−1610 K/T) and kD2( ± 20%) = 9.71 × 10−17 T1.75 exp(−2092 K/T) cm3 molecule−1 s−1 for the 296–3000 K range. The present results are compared to earlier direct studies which encompass the temperature ranges 199–1283 (H2) and 255–500 K (D2). These data including the present are then used to evaluate the rate behavior for each reaction over the entire experimental temperature range. In these evaluations the present data above 1300 K was given two times more weight than the earlier determinations.

The evaluated rate constants are: kH2( ±14%)=2.52×10−11 exp(−2214 K/T) (199≤T<354 K), kH2(±17%)=1.57×10−16 T1.72 exp(−1544 K/T) (354≤T≤2939 K), and kD2(±5%)=2.77×10−16 T1.62 exp(−2162 K/T) (255≤T≤3020 K), in molecular units. The ratio then gives the experimental kinetic isotope effect, KIE ≡ (kH2/kD2). Using 11 previous models for the potential energy surface (PES), conventional transition state theoretical (CTST) calculations, with Wigner or Eckart tunneling correction, are compared to experiment. At this level of theory, the Eckart method agrees better with experiment; however, none of the previous PES’s reproduce the experimental results. The saddle point properties were then systematically varied resulting in an excellent model that explains all of the direct data. The theoretical results can be expressed to within ±2% as kH2th = 4.59 × 10−16 T1.588 exp(−1682 K/ T) (200≤T≤2950 K) and kD2th=9.20×10−16 T1.459 exp(−2274 K/T) cm3 molecule−1 s−1 (255≤T ≤3050 K). The KIE predictions are also compared to experiment. The ‘‘derived’’ PES is compared to a new ab initio calculation, and the differences are discussed. Suggestions are noted for reconciling the discrepancies in terms of better dynamics models.
Electronic properties and geometric structures of Li4H and Li9H from optical absorption spectra
Authors: Vezin B. , Dugourd Ph. , Bordas C. , Rayane D. , Broyer M. , Bonačic-Koutecký V. , Pittner J. , Fuchs C. , Gaus J. , Koutecký J.
Published: July 2002

Optical absorption spectra of Li4H and Li9H clusters have been recorded by depletion spectroscopy in the visible range. From comparison with ab initio calculations, geometries of both clusters are identified. The hydrogen atom assumes a peripheral position bridging two and three Li atoms in the planar and three-dimensional structures of Li4H and Li9H, respectively. Na4F and Na9F clusters are also theoretically studied and it is shown how the strong electronegativity of the F atom leads to different geometries than in lithium hydrids. Finally, the metallic character of these clusters is discussed and in both cases, the hydrogen or fluorine atom localizes one valence electron. However, the optical absorption spectra are much broader than in pure Lin and Nan clusters due to the lower symmetry.
Theory of resonance Raman scattering for finite and infinite polyenes
Authors: Jin Bih-Yaw , Silbey Robert
Published: July 2002

We investigate the resonance Raman scattering of finite and infinite polyenes. The connection between the vibronic picture of Raman scattering and the third order perturbation approach normally used in solid state physics for infinite systems is clarified. Starting from the Kramers–Heisenberg–Dirac formula for the Raman scattering, we derive expressions for the Condon and Herzberg–Teller terms for chains from N=2 (a simple two-level system) to N=∞ (a two-band system) by using the standard vibronic picture. Close to resonance the Condon term dominates and converges to the usual solid state result. In the off-resonance region, the Herzberg–Teller term is comparable to the Condon term for both the small molecule and the solid state system.
Simulation of collisional relaxation of trapped ion clouds in the presence of space charge fields
Authors: Parks J. H. , Szöke A.
Published: July 2002

A statistical formulation of ion-neutral interactions in rf Paul traps is presented which provides a basis for calculating the collisional relaxation of a trapped ion cloud. These calculations describe the ion cloud as a statistical distribution in space and energy and treat collisions by Monte Carlo methods. Ion motion is calculated in the presence of space charge fields in a mean field approximation which allows each ion to be treated independently. This formulation provides an adequate description of the ion cloud dynamics for ion densities ≤108 cm−3. Frequency shifts induced in the ion cloud ensemble by space charge are characterized. Energy relaxation of a C60+ ion cloud initially at a temperature of 5000 K by collisions with a background He gas at 300 K is calculated in the presence of space charge.
Vibrational wave packets in the C 1Πu state of Cs2: Two color pump–probe experiments
Authors: Rodriguez G. , John P. C. , Eden J. G.
Published: July 2002

Two color, pump–probe laser experiments on the ∼100 fs time scale have been applied to examining the dynamics of vibrational wave packets in the C 1Πu state of Cs2. Wave packets consisting of more than 20 C state vibrational levels are produced with an initial pulse having a central wavelength between ∼620 and 680 nm. The temporal history of the wave packet is inferred from the time and energy-integrated photoelectron signal produced when the excited state is photoionized by a time-delayed pulse centered at 605, 610, 615, 617, or 620 nm. Because of the difference in equilibrium internuclear separations for the Cs2(C) and Cs+2(X) states (ΔRe≊0.75 Å), wave packets are readily observed (signal-to-noise ratio ≳10) without the need to resort to mass selection techniques. Frequency analysis of the wave packet data shows a dominant (fundamental) component that decreases from 29 to ∼28.3 cm−1 as the pump wavelength is tuned from 627 to 641 nm. Other spectral modes at ∼23.5 and ∼34 cm−1 and a weaker term at twice the fundamental frequency are also observed and quantum mechanical calculations of the wavepacket motion are in accord with the experimental results.
A new class of free stable doubly negative systems: Three Si2O2−5 isomers and their interconversion
Authors: Sommerfeld T. , Scheller M. K. , Cederbaum L. S.
Published: July 2002

The free dianionic Si2O2−5 system is studied by standard ab initio methods. We have located three distinct isomers in the Si2O2−5 system. All three Si2O2−5 isomers are found to be thermodynamically stable with respect to dissociation into monoanionic fragments. However, only two of them are predicted to be clearly stable with respect to vertical electron autodetachment. Minimal energy pathways which interconnect the three isomers have been studied together with the binding energy of the excess electron along them. The results give rise to an unusual nuclear dynamics problem. In particular, the associated barriers have been found to be high and wide enough to predict long lifetimes for two free dianionic species.
Reaction path Hamiltonian based on a reaction coordinate and a curvature coordinate
Authors: Taketsugu Tetsuya , Gordon Mark S.
Published: July 2002

We propose a reaction path-based Hamiltonian in terms of the reaction coordinate, the curvature coordinate, the remaining 3N−8 transverse normal coordinates (whose directions are orthogonal to the path tangent and curvature vectors), and their conjugate momenta, for an N atomic reaction system. The 3N−8 transverse vibrational modes are independent of the motion along the reaction path, although they have coupling terms with the curvature direction in the harmonic approximation. A two-dimensional plane determined by the path tangent and curvature vectors is termed the ‘‘reaction plane.’’ We introduce a function that estimates changes of the reaction plane along the reaction path, and analyze the reaction path based on this function for an abstraction reaction, CH3+H2→CH4+H. The scheme proposed here should be effective when a reaction path has a sharply curved region.
A generator coordinate version of the closed-shell Dirac–Fock equations
Authors: Jorge F. E. , da Silva A. B. F.
Published: July 2002

A generator coordinate version of the Dirac–Fock equations for relativistic closed-shell atoms is presented. The integration of the Dirac–Fock equations is performed through the integral discretization technique so as to preserve the continuous character of the generator coordinate formalism. With the new formalism we generate a universal Gaussian basis set for relativistic closed-shell atoms with d and f orbitals (zinc up to nobelium). The results obtained with the universal Gaussian basis set for Dirac–Fock–Coulomb self-consistent-field energies are compared with numerical-finite-difference results and Dirac–Fock–Coulomb energies obtained by using other Gaussian basis sets. The excellent performance of our universal Gaussian basis set is attributed to the integral discretization technique of the generator coordinate Dirac–Fock method, as with it we are capable of generating Gaussian-type function exponents that are able to represent properly the relativistic kinematics of an electron inside the nucleus.
Cooperative optical bistability in the dimer system Cs3Y2Br9:10% Yb3+
Authors: Hehlen Markus P. , Güdel Hans U. , Shu Qize , Rand Stephen C.
Published: July 2002

In single crystals of the dimer compound Cs3Y2Br9:10% Yb3+ below 31 K, both visible (VIS) and near-infrared (NIR) luminescence intensities were found to exhibit hysteresis as a function of incident NIR intensity and temperature. The optical bistability is intrinsic to Cs3Y2Br9:10% Yb3+ and not a result of an external feedback. Lowering the temperature to 11 K strongly enhances the all-optical switching behavior. The switching on VIS cooperative upconversion and NIR luminescence transitions occurs simultaneously and with opposite polarity reflecting the competition of both emission processes. On/Off switching ratios of up to 4.8 and 1.7 were observed for VIS and NIR luminescence intensities. Using NIR luminescence spectroscopy, differences in the internal sample temperature of up to 7 K were found between the upper and lower branches of the hystereses. A two-level density-matrix model is developed which includes ground- and excited-state interactions and shows that the intrinsic bistability due to a local field different from the external field is strongly amplified by the nonlinear cooperative upconversion process. Alternatively, a rate-equation model is presented which takes the multilevel nature of the ions into account but is more phenomenological in nature. Formally, the two models are shown to be equivalent, and they qualitatively explain all major experimental observations. It is found both theoretically and experimentally that increasing the coupling within Yb3+ dimers and/or decreasing energy migration through the Yb3+ lattice enhances switching and renders it easier to observe intrinsic optical bistability.
Orientational ordering in the nematic phase of a thermotropic liquid crystal: A small angle neutron scattering study
Authors: Hamley I. W. , Garnett S. , Luckhurst G. R. , Roskilly S. J. , Sedon J. M. , Pedersen J. Skov , Richardson R. M.
Published: July 2002

The long range orientational order in the nematic phase of 4,4′-dimethoxyazoxybenzene (PAA) has been studied using small angle neutron scattering (SANS) from mixtures of protonated and perdeuterated molecules. Unlike spectroscopic methods, the determination of the complete set of orientational order parameters is, in principle, possible using this technique. The orientational order parameters were extracted from the anisotropic single molecule scattering isolated at small scattering angles. These order parameters were compared to those extracted from previous neutron scattering measurements, to second rank order parameters extracted from NMR, and to those predicted from the Maier–Saupe theory of nematic ordering. It was found that only the second and fourth rank terms in the expansion of the single molecule scattering in a basis of Legendre functions are statistically significant, which means that the order parameters P2̄ and P4̄ only could be obtained. For the semirigid nematogen PAA, the Maier–Saupe theory is found to underestimate P2̄ and its dependence on the reduced temperature T/TNI where TNI is the nematic–isotropic (NI) transition temperature. The theoretical values of P4̄ are, however, in remarkably good agreement with the measured values. Order parameters extracted from NMR are found to be slightly lower than those determined from the SANS experiment, and we also make comparisons with earlier neutron scattering results. Singlet orientational distribution functions are reconstructed from the measured order parameters using the maximum entropy method.
Density functional study of the global potential energy surfaces of the [H,C,N,O]+ system in doublet and quartet states
Authors: Luna A. , Mebel A. M. , Morokuma K.
Published: July 2002

The global potential energy surfaces of the [H,C,N,O]+ system both in doublet and quartet states have been rather exhaustively studied with the B3LYP density functional method, with special attention to cover nearly all intermediate and transition states. Ionization potentials, energies of reactions, and proton affinities of fragments are calculated and compared with experiments to assess the reliability. In the doublet state, all six chain isomers and three cyclic structures exist, and rearrangements among them take place over high barriers mainly via the 1,3-H shift and chain–cycle transformation. Pathways for fragmentations of the isocyanic acid cation HNCO+ and fulminic acid cation HCNO+ have been identified and compared with the experiments. In the quartet state, there exist ion–molecule complexes between fragments as well as trans and cis forms of chain isomers and cyclic and open branched isomers, and the potential surfaces for interconversion and fragmentation are much more complicated. The potential energy profiles for the reaction of O+(4S)+HCN have been examined and pathways for production of experimentally observed HCN+, NO+, HCO+, and HOC+ have been identified to go through long-lived chain intermediates HCNO+ and HOCN+ and open branched intermediate NCHO+, while production of CO+ is more complicated. The crossing seam minimum with the doublet has been found right at the quartet intermediate HCNO+, and intersystem crossing and production of the stable doublet fulminic acid cation HCNO+ is likely to be an efficient process.
The reflection of predissociation dynamics in pump/probe photoelectron distributions
Authors: Braun Michael , Meier Christoph , Engel Volker
Published: July 2002

We present simulations of pump/probe photoionization experiments on a diatomic molecule with two nonadiabatically coupled electronic states. The NaI molecule is used as an example. The nuclear wave packet dynamics in two coupled electronic states is mapped into the kinetic energy distribution of photoelectrons which are recorded at different delay times between the pump pulse, preparing the initial wave packet and the ionizing probe pulse. In this way details of the indirect fragmentation process can be observed in real time.
A semiclassical approach to intense-field above-threshold dissociation in the long wavelength limit
Authors: Thachuk Mark , Ivanov M. Yu. , Wardlaw David M.
Published: July 2002

A new semiclassical formalism has been developed to treat Hamiltonians having explicit time dependence, with particular application to the dissociation of diatomic ions in intense laser fields. Based on this formalism, a hopping algorithm is presented which specifies how classical trajectories should be moved between coupled electronic surfaces. The theory is laid out in a rigorous, general form and an analysis is also presented for the case where only two electronic surfaces are strongly coupled. In addition, valuable physical insight into the hopping process is obtained by considering the theory in a number of physically relevant limiting cases. From this insight a number of guidelines are proposed which detail the manner in which trajectory hopping should be implemented when time-dependent potential energy surfaces are present, including the effects of phase coherence and conservation principles.
Comparison of density functional and coupled cluster methods in the study of metal–ligand systems: Sc–CO2 and Cu–NO2
Authors: Rodríguez-Santiago Luis , Sodupe Mariona , Branchadell Vicenç
Published: July 2002

The structure, binding energies, and vibrational frequencies have been determined for the 1A1 state of the η2-O,O coordination mode of Cu–NO2 and the 2A1 state of the η2-O,O coordination mode of Sc–CO2. Calculations have been done using coupled cluster methods and methods based on the density functional theory. The results obtained show that all the levels of calculation lead to very similar equilibrium geometries and vibrational frequencies, while different results are obtained for the binding energy. For Sc–CO2 density functional methods overestimate the binding energy with respect to coupled cluster, while for Cu–NO2 the density functional binding energies are lower than the coupled cluster value. In both cases the inclusion of the exact Hartree–Fock exchange into the functional leads to an improvement of the density functional result. Our best estimates for the binding energies of Sc–CO2 and Cu–NO2 are 25 and 50 kcal mol−1, respectively.
Equilibrium structure and bonding of small iron–carbon clusters
Authors: Nash Beth K. , Rao B. K. , Jena P.
Published: July 2002

The equilibrium geometries, binding energies, and electronic structure of neutral FeCn (n≤3) clusters have been calculated self-consistently using ab initio molecular orbital theory. The exchange and correlation contributions to the total energy were computed using both the density functional method (with and without nonlocal correction) as well as the Hartree–Fock–Möller–Plesset theory. All levels of theory predict the equilibrium geometries to be cyclic. The binding energies, bond lengths, and the Mulliken charges obtained from both methods of calculations are also in agreement. The results are compared with recent mobility and photoelectron spectroscopy experiments.
Surface layers overlap and effective adhesion in reverse micelles: A discussion from the adhesive spheres mixture model
Authors: Amokrane S. , Regnaut C.
Published: July 2002

Structural data on reverse micelles from the literature are analyzed from the adhesive spheres mixture model in the Percus–Yevick approximation. The solvent is modeled by a fluid of pseudoadhesive spheres with same compressibility and coordination number as a square well fluid. The attractive part of the direct intermicellar potential is taken proportional to the overlap volume of interpenetrating surface layers. The micelle–solvent coupling is characterized by a square or triangular well and a constraint on the packing fractions. A very good correlation is then found between the micelles stickiness parameters computed from the actual potentials via the equality of second virial coefficients and those determined from a fit to experiment. The change in effective adhesion observed with micelles of different surface composition and in the same solvent can then be associated to the change in the direct intermicellar interaction at nearly unchanged solvent–micelle interactions. The interpretation of the role of the overlap volume resulting from this multicomponent approach is contrasted with that based on approximate calculations of the potential of mean force.
Quantum chemical ab initio calculations of the magnetic interaction in alkalithioferrates(III)
Authors: Mödll Martin , Dolg Michael , Fulde Peter , Stoll Hermann
Published: July 2002

Quantum chemical ab initio calculations for two ternary thioferrate crystals, Na5FeS4 and Na3FeS3, have been performed. Using a cluster model approach with 250 optimized point charges for the single well-isolated complexes [FeS4]5− and [Fe2S6]6−, the electronic and magnetic properties are investigated. In particular, the weak antiferromagnetic coupling of two half-filled Fe 3d shells in the dimeric complex is discussed within the active-electron approach. The complete active space self-consistent-field (CASSCF) wave functions with 10 active electrons in 10 active orbitals are analyzed by expectation values for the local spin and the local charge fluctuations at one Fe site, allowing for a quantitative discussion of interatomic and intraatomic non dynamical correlation. With CASSCF a clear deviation from the Heisenberg Hamiltonian is found with coupling constants ranging from J(4→5)=−18 cm−1 to J(0→1)=−27 cm−1, which is a factor 4 smaller than experiment. The need of going beyond the active-electron approach and correlating all ligand orbitals is demonstrated, and an approximative scheme based on increments derived from multireference configuration interaction and averaged coupled-pair functional calculations is proposed, which yields J(4→5)=−65 cm−1.
Structure and energetics of small helium clusters: Quantum simulations using a recent perturbational pair potential
Authors: Lewerenz Marius
Published: July 2002

We report accurate ground state energies and structural properties for small clusters of 4He computed with the diffusion quantum Monte Carlo (DMC) method combined with high quality trial wave functions and using the recent analytical pair potential of Tang, Toennies, and Yiu [Phys. Rev. Lett. 74, 1546 (1995)]. Calculations based on the older HFD-B(He) potential are reported for comparison. The clusters are found to be extremely floppy and to be characterized by very diffuse wave functions. The DMC results for 4He2 and 4He3 are in excellent agreement with other calculations using conventional methods. 4He3 is found to have a noticeable contribution from nearly linear geometries. The internal structure of the clusters is described by a three particle correlation function which reveals a significantly non-spherical internal cluster structure. The energies for all cluster sizes are found to be slightly higher than those obtained with the HFD-B(He) pair potential. Exploratory calculations on the helium trimer indicate that the effects of three body interactions do not exceed the uncertainty margin provided by different state of the art pair potentials.
Photofragment excitation spectrum for O(1D) from the photodissociation of jet-cooled ozone in the wavelength range 305–329 nm
Authors: Takahashi Kenshi , Kishigami Masahiro , Taniguchi Nori , Matsumi Yutaka , Kawasaki Masahiro
Published: July 2002

The photofragment excitation (PHOFEX) spectrum for O(1D) production from the photolysis of ozone under supersonic free-jet conditions was measured, scanning the photodissociation wavelength in the region of 305–329 nm and probing the O(1D) atoms by vacuum ultraviolet laser induced fluorescence at 115.2 nm. The bond dissociation energy D00(O2–O) was determined to be 101.53±0.25 kJ mol−1 from the cut-off wavelength in the PHOFEX spectrum for the photodissociation of jet-cooled ozone (Trot≈5 K) to O(1D)+O2(a 1Δg). The cut-off wavelength for vibrationally hot band excitation to the dissociative continuum of O(1D)+O2(a 1Δg) was also observed in the PHOFEX spectrum. It was found that the active mode for the hot band excitation was the antisymmetric stretching mode ν3 in the ground electronic state of ozone. Sharp peaks corresponding to vibrational bands in the Huggins system were also observed in the PHOFEX spectrum of the O(1D) atoms produced via the spin-forbidden dissociation process, O(1D)+O2(X 3∑g−). The spectral widths of the vibronic bands in the Huggins system are discussed in terms of the dissociation dynamics.
Model potentials for main group elements Li through Rn
Authors: Sakai Yoshiko , Miyoshi Eisaku , Klobukowski Mariusz , Huzinaga Sigeru
Published: July 2002

Model potential (MP) parameters and valence basis sets were systematically determined for the main group elements Li through Rn. For alkali and alkaline-earth metal atoms, the outermost core (n−1)p electrons were treated explicitly together with the ns valence electrons. For the remaining atoms, only the valence ns and np electrons were treated explicitly. The major relativistic effects at the level of Cowan and Griffin’s quasi-relativistic Hartree–Fock method (QRHF) were incorporated in the MPs for all atoms heavier than Kr. The valence orbitals thus obtained have inner nodal structure. The reliability of the MP method was tested in calculations for X−, X, and X+ (X=Br, I, and At) at the SCF level and the results were compared with the corresponding values given by the numerical HF (or QRHF) calculations. Calculations that include electron correlation were done for X−, X, and X+ (X=Cl and Br) at the SDCI level and for As2 at the CASSCF and MRSDCI levels. These results were compared with those of all-electron (AE) calculations using the well-tempered basis sets. Close agreement between the MP and AE results was obtained at all levels of the treatment.
Symmetry-adapted perturbation theory of three-body nonadditivity of intermolecular interaction energy
Authors: Lotrich Victor F. , Szalewicz Krzysztof
Published: July 2002

The many-body symmetry-adapted perturbation theory of intermolecular interactions has been extended to the interactions of three closed-shell atoms or molecules. Orbital formulas for the nonadditivity of the second- and third-order induction energies, third-order induction-dispersion energy, and third- and fourth-order dispersion energies with neglect of intramonomer correlation effects have been derived from the polarization expansion. The first-order intramonomer correlation correction to the third-order dispersion energy has also been obtained. The symmetrized Rayleigh–Schrödinger energy expansion has been used to derive orbital formulas for the first- and second-order exchange energies with neglect of intramonomer correlation effects. The first-order exchange energy was approximated by the terms proportional up to the fourth power in the intermolecular overlap integrals. The second-order exchange energy was developed through the third power of the intermolecular overlap integrals.
Li
+
solvation in ethylene carbonate–propylene carbonate concentrated solutions: A comprehensive model
Authors: Cazzanelli E. , Croce Fausto , Appetecchi Giovanni Battista , Benevelli Francesca , Mustarelli Piercarlo
Published: July 2002

Spectroscopic (Raman, NMR, impedance spectroscopy), and thermal [differential scanning calorimetry (DSC)] techniques have been used to study the solvation mechanism of lithium ions in ethylene carbonate (EC)–propylene carbonate (PC) concentrated solutions. For values of N=[Li+]/[EC+PC]⩽0.2 all the cations are solvated by ∼4 solvent molecules and interaction chiefly takes place between Li+ and the ring oxygens. For N>0.2 a part of Li+ ions begins to form complexes with two solvent molecules (sandwich configuration). At N≅0.5 nearly all cations are complexed, and a crystalline compound is formed at room temperature. For higher values of N a reassociation of the salt takes place.