bluuebunny/crossref_metadata_2025 huggingface dataset:
A quantum Monte Carlo calculation of the ground state energy of the hydrogen molecule
Authors: Traynor Carol A. , Anderson James B. , Boghosian Bruce M.
Published: July 2002

We have calculated the ground state energy of the hydrogen molecule using the quantum Monte Carlo (QMC) method of solving the Schrödinger equation, without the use of the Born–Oppenheimer or any other adiabatic approximations. The wave function sampling was carried out in the full 12-dimensional configuration space of the four particles (two electrons and two protons). Two different methods were employed: the diffusion quantum Monte Carlo (DQMC) method and the Green’s function quantum Monte Carlo (GFQMC) method. This computation is very demanding because the configurations must be evolved on the time scale of the electronic motion, whereas the finite nuclear mass effects are resolved accurately only after equilibration on the much slower time scale of the nuclear motion. Thus, a very large number of iterations is required. The calculations were performed on the CM-2 Connection Machine computer, a massively parallel supercomputer. The enormous speedup afforded by the massive parallelism allowed us to complete the computation in a reasonable amount of time. The total energy from the DQMC calculations is −1.163 97±0.000 05 a.u. A more accurate result was obtained from the GFQMC calculations of −1.164 024±0.000 009 a.u. Expressed as a dissociation energy, the GFQMC result is 36 117.9±2.0 cm−1, including the corrections for relativistic and radiative effects. This result is in close agreement with accurate nonadiabatic-relativistic dissociation energies from variational calculations (corrected for radiative effects) in the range of 36 117.9–36 118.1 cm−1 and with the best experimentally determined dissociation energy of McCormack and Eyler 36 118.1±0.2 cm−1.
Ultrafast optical Kerr dynamics studied with incoherent light
Authors: Hattori Toshiaki , Kobayashi Takayoshi
Published: July 2002

Femtosecond optical Kerr dynamics in various transparent liquids were measured using incoherent light with a 60 fs autocorrelation width. From the measurement of the optical Kerr effect (OKE) of binary mixtures of CS2 and various liquids, the contribution of the intermolecular interaction-induced polarizability change to the OKE was found to be affected remarkably by the femtosecond molecular dynamics of CS2. Especially, data from diluted solutions of CS2 in nonviscous solvents composed of molecules with a low molecular weight are consistent with the binary collision model in free space. An oscillatory feature, which was attributed to an intermolecular vibrational mode, was found in the OKE dynamics of neat benzene and several benzene derivatives. A theoretical expression for the delay-time dependence of the signal intensity was also derived with no restriction on the statistical properties of the incoherent light. It is expressed in terms of the autocorrelation function of the intensity fluctuation of the incident incoherent light which was utilized as both the pump and the probe.
A generalized intermediate picture of quantal time evolution using operator algebraic methods. Application to translational–vibrational energy transfer in molecular collisions
Authors: Echave Julián , Fernández Francisco M. , Micha David A.
Published: July 2002

A method for solving the time-dependent Schrödinger equation in an intermediate picture defined by an effective time-dependent Hamiltonian is discussed. The state function in this intermediate picture is written as a linear combination of time-independent basis functions with time-dependent coefficients. It is shown how to use Lie algebraic methods to build the interaction potential in the intermediate picture and calculate physical observables. The construction of the interaction potential is explicitly discussed in the case of one degree of freedom with a classical coordinate analog. The effective Hamiltonian is obtained by expanding the potential energy function around a reference value of the coordinate. Effective Hamiltonians for He–H2 collisions obtained for equilibrium and average reference values of the vibrational coordinate of H2 are compared and the expansion convergence in the resulting intermediate pictures are discussed.
Single-particle kinetic energies of noble-gas solids and liquids: Xenon, krypton, and argon
Authors: Peek D. A. , Simmons R. O.
Published: July 2002

Direct measurements have been made on the average single-particle kinetic energies, 〈Ek〉, of a series of solid and liquid specimens of heavy noble gases near melting. To do this, the distributions of energy losses of incident eV neutrons, scattered at angles ranging from 87 to 136 deg, were analyzed on the assumptions that atoms in these specimens have a Gaussian momentum distribution n(p) and that for these experimental conditions the atoms scatter independently of one another. It is known that the solids, Ar, Kr, and Xe show considerable differences in such characteristics as (a) their respective ratios of triple point to Debye temperatures, (b) the extent of multibody force contributions to cohesion, and (c) phonon anharmonicity. Regardless, it is found for all three solids near their respective triple points that 〈Ek〉 is equal to the equipartition value (3/2)kT well within the various estimated experimental uncertainties of 3% to 7%. Moreover, liquid Kr near its triple point yields the same result, as does liquid Xe at temperature up to 1.8 times its triple point temperature. Because all these systems are expected to be classical at these temperatures, these results demonstrate applicability of this neutron recoil method to determine 〈Ek〉 even of heavy atoms in suitable cases where a value may not be known in advance.
Preparation and spectroscopic characterization of a thin polymer film in ultra high vacuum: 2,5-distyrylpyrazine
Authors: Abraham M. , Dütting J. , Schreck M. , Lege R. , Reich S. , Oelkrug D. , Göpel W.
Published: July 2002

The UHV-preparation and -characterization of thin, highly ordered polymer films of 2,5-distyrylpyrazine is reported. Monomer films were evaporated from a Knudsen cell. Subsequent photo-oligomerization occurs by illumination with UV light. The desorption behavior of the monomers and the lower oligomers was studied by temperature-programmed desorption mass spectrometry (TDS). The photoreaction was monitored in situ by high resolution electron energy loss spectroscopy (HREELS) in the range of electronic transitions. The information obtained from the loss spectra are compared to those from absorption and fluorescence spectroscopy. The results are discussed in the framework of the calculated electronic structure of related compounds.
Observation of very small doubly charged homogeneous para difluorobenzene clusters
Authors: Martrenchard S. , Jouvet C. , Lardeux-Dedonder C. , Solgadi D.
Published: July 2002

Double ionization of para difluorobenzene (PDFB) has been observed following irradiation of a supersonic cluster beam at 260 nm. PDFB++n are observed for n ≥ 3: The anomalously small critical size for stability cannot be explained by a simple liquid-drop model, but is more probably due to specific structures of these clusters.
A kinematical model of homogeneous phase-transition kinetics involving the simultaneous growth of multiple phases
Authors: Markworth Alan J.
Published: July 2002

A model is developed for homogeneous phase-transition kinetics in a system for which multiple secondary phases are simultaneously growing from a supersaturated primary phase. The approach is entirely kinematical in nature, with no account being taken of the actual physics of the nucleation/growth process. Randomly dispersed nuclei are assumed to act as growth centers for each of the respective phases, which grow at time-dependent rates that are generally different for different phases. Expressions are derived for the time-dependent volume fraction occupied by all the phases as well as by each individual phase. The results are illustrated by application to a simple example.
Comment on: Electron scattering in dense atomic and molecular gases
Authors: Freeman Gordon R.
Published: July 2002

A recent correlation between electron scattering lengths of molecules and their polarizabilities [Rupnik et al., J. Chem. Phys. 92, 2303 (1990)] raises questions about the zero energy limits of electron scattering cross sections of polyatomic molecules. A new approach to the theory of scattering very low energy (<10−21 J) electrons by polyatomic molecules is needed.
Intramolecular vibrational redistribution in alkylbenzenes. I. Normal modes and their energy distribution
Authors: Gruner Daniel , Brumer Paul
Published: July 2002

Normal modes for the ground and first excited singlet state are computed for ethylbenzene, pentylbenzene, and hexylbenzene and are assigned in terms of zero-order mode descriptors. The energy distribution, i.e., the partitioning of energy between the ring and chain, is computed for the normal modes and the concept of ‘‘system modes,’’ i.e., modes localized on the ring, is examined. Several of the basic assumptions made in traditional interpretations of intramolecular vibrational redistribution in molecules with a ‘‘ring+tail’’ structure are examined in light of the computed normal mode characteristics.
The branching ratio in the F+HD reaction: An experimental and computational study
Authors: Johnston G. W. , Kornweitz H. , Schechter I. , Persky A. , Katz B. , Bersohn R. , Levine R. D.
Published: July 2002

The dynamical stereochemistry of the reaction of hot F atoms with HD is discussed with reference to the measured branching ratio using both exact and model classical trajectory computations. It is argued that the dominant effects are due to the shift of the center of mass from the center of charge. In particular this leads to enhanced reactivity of the D end of the molecule due both to reorientation of the molecule and to the recrossing of the barrier. The observed preference for reaction at the H end is attributed to HD rotational excitation reflecting however the shape of the potential energy surface and not the longer arm of the H atom about the center of mass. Measuring the reaction cross sections for rotationally cold HD will provide a critical test of our understanding of the dynamics.
A combination band at 1699.8 cm−1 for C4 trapped in Ar
Authors: Shen L. N. , Withey P. A. , Graham W. R. M.
Published: July 2002

An absorption has been detected at 1699.8 cm−1 in the midinfrared spectrum of the vacuum ultraviolet photolysis products of 1,3-butadiene (C4H6) trapped in Ar at 10 K, which on the basis of extensive13 C-substitution experiments is shown to belong to the C4 carbon chain. The behavior of the new band correlates closely with the ν2, C■C stretching vibration of linear C4 at 1543.4 cm−1 identified in an earlier paper. It is argued that the 1699.8 cm−1 absorption is a combination band involving the C■C stretching vibration and a bending mode of C4, with a cis-bent structure.
Quadrupole hyperfine structure of v=3←0, 6←0 overtone band spectral lines of HI observed with near infrared diode lasers
Authors: Matsushima Fusakazu , Kakihata Shinya , Takagi Kojiro
Published: July 2002

Diode lasers in the near infrared region are used to observe overtone band transitions (v=3←0, and 6←0) of the HI molecule. From spectral line shape analysis of low J transitions, quadrupole coupling constants eqQ in the v=3, and 6 vibrational states are determined to be −1948±5 and −2066±5 MHz, respectively.
Direct observation of orbit rotation predissociation in the O2 Schumann–Runge system
Authors: Yang X. , Wodtke A. M. , Hüwel L.
Published: July 2002

Laser induced predissociation fluorescence spectroscopy was performed with a tunable Argon fluoride laser in the Schumann–Runge band of O2 in an open atmospheric flame. Simple rotationally resolved spectra of the 10←2 and 11←2 bands were obtained when the fluorescence was dispersed through a monochromator. Line widths of these transitions, which predominantly reflect state specific predissociation, were measured. It was found that the orbit rotation interaction between the B 3Σ−u state and the 3Πu continuum must be included in addition to the spin–orbit interaction between B 3Σ−u and 5Πu, 1Πu, 3Σ+u, and 3Πu in order to fit the data. The general coupling formulas for orbit–rotation interaction for the case of intermediate coupling between Hund’s case a and b were derived and orbit–rotation interaction constants for v=10 and 11 were estimated. This is the first experimental evidence of the orbit rotation interaction in the O2 Schumann–Runge system.
Interrupted separatrix excitability in a chemical system
Authors: McCormick William D. , Noszticzius Zoltan , Swinney Harry L.
Published: July 2002

Models of excitability fall into two broad categories: one with a sharp threshold, the other with a narrow but finite threshold region. In an attempt to clarify the notion, we have studied the application of two models of excitability to experiments on a variant of the Belousov–Zhabotinskii reaction in a well-stirred reactor. An interrupted separatrix model, which has a sharp threshold, is found to describe our experimental results better than a simple nullcline model, which has a finite threshold region.
Use of the reference interaction site model perturbation method to describe the dissolution of nonpolar gases in molecular liquids
Authors: Laria Daniel , Fernandez-Prini Roberto
Published: July 2002

We have used the interaction site model in order to calculate the thermodynamics of dissolution of simple nonpolar gases in molecular liquids. The calculations were carried out for the liquids benzene and n-alkanes, from butane to heptane, and for inert gases, methane, and ethane as solutes using a first-order perturbation method. It is shown that this procedure is much better than the sphericalization of the anisotropic intermolecular potentials since it explicitly incorporates the details of the structure of the different fluids. The proposed procedure is extremely sensitive to the values used for the intermolecular parameters. This feature coupled to the possible inaccuracies of the simple Lorentz–Berthelot combining rule, is a weakness of the calculation procedure when it is used to predict the behavior of real systems. For this purpose it is recommended that the procedure be used semiempirically adjusting the values of the intermolecular parameters with experimental solubility data.
Observation of pair absorption and self-broadening in Ba vapor
Authors: Ueda K. , Sotome H. , Sato Y.
Published: July 2002

Reduced absorption coefficients γBaBa were measured for the far-wing absorption of the self-broadened Ba6s2 1S0–5d6p 1P1 line, Ba(6s2 1S0) +Ba(6s2 1S0)+ℏω → Ba*(5d6p 1P1)+Ba(6s21 S0)and the pair absorption Ba(6s2 1S0)+Ba(6s2 1 S0) +ℏω→Ba*(6s6p 1P1 +Ba*(6s5d 1D2). The self-broadening shows the red-shaded asymmetric profile: γBaBa on the red wing is proportional to Δν−3/2, where Δν is the detuning from the line center. The pair absorption shows a broad asymmetric peak having a blue tail and a blue satellite. The estimated γBaBa, which is based on the quasistatic line-broadening theory and the long-range dipole–dipole interaction between the relevant atomic excited levels, shows reasonable agreement for the red wing of the self-absorption whereas it gives larger values by a factor of 3 for the blue tail of the main peak for the pair absorption.
A model for dense colloidal systems with deformable, incompressible particles
Authors: Pakula Tadeusz
Published: July 2002

A model for computer simulation of dense systems of colloidal particles which is aimed to reflect static and dynamic properties is proposed. The objects representing particles are considered as incompressible and deformable. Various structures and dynamic states ranging from liquid-like to crystalline ones are demonstrated as a function of the deformability parameter and particle concentration. Changes of both these parameters appeared equivalent in the influence on the order and dynamics of the simulated systems.
Autoionization spectra of CH3I in dense gaseous methane, ethane, and propane observed by photoconduction
Authors: Meyer J. , Reininger R. , Asaf U. , Steinberger I. T.
Published: July 2002

The influence of light alkane perturbers (up to densities 5×1020 cm−3 for methane and ethane and 2×1020 cm−3 for propane, respectively) on the autoionization spectrum of CH3I nd′(2E1/2) Rydberg series was investigated by photoconductivity excitation spectroscopy. In the experiments, we recorded photocurrents due to electronic transitions from the absorber molecule to the conduction state of each perturber. The absorption maxima of the perturbed autoionization Rydberg states appeared as maxima in the photocurrent spectrum. The asymptotic pressure shift of the Rydberg lines equals the shift of the second vertical ionization potential I(2E1/2) of the perturbed absorber. The energy shift also provides the electron scattering length A in methane, ethane, and propane. The data obtained are compared with results in the literature on photoinjection, electron beam, and swarm experiments in pure alkanes.
Raman-imaging and -spectroscopy with surface plasmon light
Authors: Knobloch Harald , Knoll Wolfgang
Published: July 2002

Plasmon surface polariton field-enhanced Raman scattering is combined with an optical multichannel detection scheme in order to increase the sensitivity for vibrational spectroscopy with Langmuir–Blodgett–Kuhn monolayer and multilayer assemblies of cadmium arachidate. Moreover, the imaging capabilities of the employed (CCD-) TV camera are used to spatially resolve (in one dimension) the Raman-spectral information of heterogeneous samples with a lateral materials contrast.
Vacuum ultraviolet resonance Raman studies of the valence excited electronic states of benzene and benzene-d6: The E1u state and a putative A2u state
Authors: Sension Roseanne J. , Brudzynski Richard J. , Hudson Bruce
Published: July 2002

A resonance Raman study of benzene has been performed with the use of excitation wavelengths ranging from 184 to 160 nm. Excitation resonant with the 1 1E1u (ππ*) state results in Raman spectra dominated by progressions in the totally symmetric ring breathing vibration (ν1) both alone and in combination with the overtone of the e1g CH bending vibration (ν10) and the overtone of the b2g ring deformation vibration (ν4). Much weaker progressions are observed involving ν1 and fundamental and overtone transitions of the Jahn–Teller active e2g vibrations. The observation of intensity in e2g fundamental vibrations indicates that the E1u electronic state is, at least slightly, Jahn–Teller active. More significant however, is the observation of considerable intensity in many of the out-of-plane vibrations indicating that benzene is distorted from planarity in the 1 1E1u electronic state. In addition, the observation of the ν10 fundamental transition and the 2ν10, 2ν4, and 2ν8 overtone transitions in the Raman spectra obtained using 171 to 160 nm excitation is indicative of the presence of one or more valence states of A2u symmetry in the high energy tail of the strong ππ* transition.
Electronic structure of donor or acceptor substituted linear polyenes
Authors: Kohler Bryan E. , Spangler Charles W. , Westerfield Curtis
Published: July 2002

Vibrationally resolved optical spectra, fluorescence decay times, and fluorescence quantum yields have been measured for all-trans diphenyloctatetraene and all-trans diphenyloctatetraene substituted in the para position(s) of the phenyl ring(s) with an electron donor (MeO) or (and) and electron acceptor (CN). The electronic excitation energies, S0–S1 and S0–S2 transition dipoles and vibrational properties of the spectra provide a detailed picture of the effects of donor–acceptor substitution on the S0, S1, and S2 states. The electronic excitation energies are well fit by a simple parametrized quantum-mechanical model for polyene electronic structure which is used to estimate substituent-induced shifts in π-electron density in the S0, S1, and S2 states.
Light scattering by alkali halide melts: A computer simulation study
Authors: Madden P. A. , O’Sullivan K. , Board J. A. , Fowler P. W.
Published: July 2002

A model for the interaction-induced polarizability of halide ions in condensed phases has been developed from ab initio electronic structure calculations. This model is incorporated into computer simulations of molten LiF, LiCl, and NaCl and used to calculate correlation functions of the polarizability in order to simulate the light scattering spectra. The results for the line shapes, frequency dependent depolarization ratio, and absolute intensity compare extremely well with the available experimental data. The relative importance of the contributions to the interaction-induced spectra from changes in the ionic polarizability caused by short-range overlap, ionic field-induced distortion, and dipole–induced dipole interactions is assessed and their distinctive spectral characteristics identified.
Electrostatic trapping of a colloidal monolayer near a charged wall
Authors: González-Mozuelos P. , Medina-Noyola M.
Published: July 2002

The structure of a model colloidal suspension in the vicinity of a charged wall is studied in the framework of the Derjaguin–Landau–Verwey–Overbeek interaction potential and the hypernetted-chain approximation. Here we consider the case of dilute suspension of highly charged macroparticles interacting with weakly repulsive, neutral, and attractive walls. As the wall–particle electrostatic interactions become successively less repulsive, the formation of a monolayer of colloidal particles, strongly adsorbed onto the surface, is predicted by our results. This monolayer of electrostatically confined particles mimic the effect of an effective surface charge distribution adjacent to the wall which, together with the bare wall surface charge, induces on the other, nonconfined, colloidal particles the same local-concentration profile as that in front of a highly charged and repulsive wall.
A photoemission study of the surface carbonate species on Ag{110}
Authors: Ricken D. E. , Somers J. S. , Robinson A. W. , Bradshaw A. M.
Published: July 2002

The electronic structure of the surface carbonate species adsorbed on Ag{110} has been studied with angle-resolved photoemission using synchrotron radiation. Despite the low symmetry of the surface complex (Cs) and more than one azimuthal orientation of the vertical symmetry plane, it has been possible to assign the adsorbate-induced levels. Recent semiempirical quantum chemical calculations have proved to be of considerable assistance in this process. The resonant behavior observed for the 3a″ level is discussed in the context of photoelectron data for the free BF3 molecule, which is isoelectronic with CO2−3. The Cs symmetry pertaining in the present experiment is compared with the effective D3h symmetry observed in x-ray-absorption spectroscopy of the same system.
On the determination of atomic shell boundaries
Authors: Schmider Hartmut , Sagar Robin P. , Smith Vedene H.
Published: July 2002

Integrations of the charge density between the critical points of ∇2ρ(r) show that these points are not suitable as definitions of atomic shell boundaries. A comparison with the corresponding scheme based on D(r) favors the latter. Minima in D(r) and corresponding shell populations are tabulated for 55≤Z≤92.
Compensating Hamiltonian method for chemical reaction dynamics: Xe desorption from Pd(100)
Authors: Jansen A. P. J.
Published: July 2002

We report on a method that allows us to study with molecular dynamics a chemical reaction with an activation barrier that is much higher than the thermal energy. The method leads to a new Hamiltonian with which it is possible to compute reaction rate constants, and reactive trajectories even at low temperature. The method is applied to Xe desorption from Pd(100). The desorption rate constants are compared with transition state rate constants. The mechanism of the desorption is studied, and a model is given that explains the molecular dynamics results. Conversion of lateral into normal kinetic energy during the desorption process is shown to be important.
Quantum algebraic method for the calculation of model potential parameters from spectral fits. I. Diatomics
Authors: López Vicente , Hernández E. , Sepúlveda M. A. , Fairén Victor
Published: July 2002

We develop a quantum method that provides relations between adjustable parameters of model potentials and empirical spectroscopic constants and that subsequently permits the calculation of those parameters. The method is based on the comparison of the quantum normal form, obtained using the Lie transform, with the experimental fits of spectra. The well established consensus on the representation and amount of experimental data of rotovibrational spectra of diatomic molecules, not existent in the case of polyatomic molecules, makes us to specifically address, in a first step, the issues concerning them. We test the accuracy of the method by calculating quantum corrections to previously computed adjustable parameters by a similar but semiclassical method.
Exact solution of the diffusion problem for a piecewise linear barrier
Authors: Privman V. , Frisch H. L.
Published: July 2002

We report exact results for the problem of diffusion over a barrier, for a piecewise linear potential. Full solution of the diffusion problem is obtained in the Laplace-transformed time variable. The ‘‘survival probability’’ of a particle not to have escaped over the barrier at large times is studied in detail, and the results are compared with the asymptotic predictions of Kramers in the large-barrier limit.
Theoretical study of the kinetic energy flow between adspecies ions, solid ions, and electrons
Authors: Volpilhac Geneviève , Achard Francis
Published: July 2002

The adspecies–solid complex is considered as three well-defined interacting parts: the electrons, the adspecies ions, and the solid ions. The total Hamiltonian is rigorously rewritten as a sum of an adiabatic Hamiltonian which separately describes the electrons, the adspecies ions, and the solid ions and a coupling operator which correlates these three elements’ pairwise interactions. The model is applied to the study of the Li–H vibrational decay (H on a Lin chain). The relative importance of the electron–hole pair and of the phonon mechanisms are compared.
Complex quasivibrational energy formalism for intense-field multiphoton and above-threshold dissociation: Complex-scaling Fourier-grid Hamiltonian method
Authors: Chu Shih-I
Published: July 2002

We present a new complex-scaling Fourier-grid Hamiltonian (CSFGH) method for accurate and efficient determination of laser-induced (multichannel) molecular resonance states without the use of basis set expansions. The method requires neither the computation of potential matrix elements nor the imposition of boundary conditions, and the eigenvectors provide directly the values of the resonance wave functions at the space grid points. The procedure is particularly valuable for excited-state problems where basis set expansion methods face the challenge. The simplicity and usefulness of the CSFGH method is demonstrated by a case study of the intensity-dependent complex quasivibrational energy eigenvalues (ER, −Γ/2) and eigenvectors associated with multiphoton and above-threshold dissociation of H+2 ions in the presence of intense laser fields (I=1012–1014 W/cm2 ).
The intramultiplet relaxation of Cd(5 3P2) by H2 and D2
Authors: Umemoto Hironobu , Masaki Akira , Ohnuma Toshiharu , Kuwahara Kazuya , Wada Youichi , Misaizu Fuminori , Fuke Kiyokazu
Published: July 2002

The deactivation processes of Cd(5 3P2) by H2 and D2 were studied by employing pulsed laser techniques. The cross sections for the intramultiplet relaxation to produce Cd(5 3P1) and Cd(5 3P0) were determined as follows: Cd(5 3P2)+H2→Cd(5 3P1)+H2; 6.3,   Cd(5 3P2)+H2→Cd(5 3P0)+H2; 1.0, Cd(5 3P2)+D2→Cd(5 3P1)+D2; 4.4,   Cd(5 3P2)+D2→Cd(5 3P0)+D2; 0.9, in units of 10−16 cm2 at 630 K. The cross sections for the overall deactivation of Cd(5 3P2) by H2 and D2 were determined to be 9.8×10−16 cm2 and 6.9×10−16 cm2, respectively. These values are compared with the calculated results based on a semiclassical curve crossing mechanism. It is suggested that electronic-to-rotational energy transfer without sharp resonances plays an important role in the deactivation of Cd(5 3P2).
An electron–spin–echo study of the nonradiative triplet state of polyenals
Authors: Ros Marianne , Groenen Edgar J. J.
Published: July 2002

The lowest triplet state T0 of the all-trans isomers of dodecapentaenal, decatetraenal, octatrienal, and hexadienal dissolved in polyethylene films has been investigated by electron–spin–echo spectroscopy in combination with pulsed laser excitation. Microwave transitions between the spin sublevels have been observed both in magnetic field and in zero field. For these polyenals T0 is found to be of ππ* character. The direction of the principal axes of the fine-structure tensors and the frequencies of the zero-field transitions have been determined. The largest zero-field splitting varies from 2496 MHz for dodecapentaenal to 5260 MHz for hexadienal. The zero-field splitting turns out to be inversely proportional to the number of double bonds in the conjugated chain, which is found to be compatible with a description of the triplet excitation as the promotion of an electron from the highest occupied to the lowest unoccupied molecular orbital. The upper spin sublevel is preferentially populated in the intersystem crossing process and decays fastest with a lifetime of 14 μs for dodecapentaenal, of 28 μs for decatetraenal, and of 77 μs for octatrienal. This variation is adequately described by an energy-gap law, which indicates that the Franck–Condon factors play a dominant role in the decay process.
Microwave spectrum of alkali metal tetrahydroborate. I. Rotational transitions and molecular structure of NaBH4 in the ground vibrational state
Authors: Kawashima Yoshiyuki , Yamada Chikashi , Hirota Eizi
Published: July 2002

The rotational spectrum of NaBH4 was observed in the millimeter-wave region using a high temperature absorption cell. The observed spectrum of NaBH4 showed the pattern of a symmetric top molecule: Strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C3v symmetry. The rotational and centrifugal distortion constants for the 11B and 10B species were determined. The observed rotational constants of Na11BH4 and Na10BH4, combined with the assumption that r(B–Hb)−r(B−Ht)=0.04 Å and θ (Hb–B–Ht)=111°, gave estimates for r(Na–B) and r(B–Hb) to be 2.308±0.006 Å and 1.28±0.10 Å, respectively, where the uncertainties are mainly due to those of the assumed values. This bond length obtained for Na–B is much shorter than the reported value in crystal: 3.08 Å. The bond lengths derived indicate that NaBH4 has a tridentate molecular structure with three bridging hydrogens. This result agrees with those of ab initio calculations.
Temperature effects in positronium formation and lifetime in solutions of nonpolar organic solvents
Authors: Diehl D. A. , Schrader D. M.
Published: July 2002

The temperature dependence of the positron annihilation lifetime (PAL) spectra of several solutions are examined. Four solvents are investigated: n-hexane, cyclohexane, squalane, and an equivolume mixture of n-hexane and squalane. Solutes in the study are nitrobenzene, nitrocyclohexane, and hexafluorobenzene. Arrhenius plots of lifetimes are made from 10 °C to 50 °C. The temperature dependence of the PAL spectra of nitrobenzene solutions in a magnetic field varying up to 15 kG are also presented. Anomalous behavior is observed for the solvents n-hexane and squalane, both neat and with nitrobenzene as a solute. In addition, it is found that the behavior of neat and nitrobenzene solutions of the n-hexane–squalane mixture is not intermediate between those involving n-hexane and squalane separately. We conclude that there is a change in the mechanism for positronium formation from n-hexane to squalane which is not accounted for by changes in free volume. Nitrocyclohexane is found to enhance positronium formation in squalane. This is the first example of antiinhibition by an aliphatic solute known to the authors.
Twisted intramolecular charge transfer and the torsional potential function of dimethylaminobenzonitrile
Authors: LaFemina John P. , Schenter Gregory K.
Published: July 2002

Recent experimental and theoretical studies of the twisted intramolecular charge transfer process and its interdependence with the torsional potential for dimethylaminobenzonitrile (DMABN) are discussed. It is shown that while the available experimental and theoretical evidence does not effectively discriminate between an untwisted or twisted ground state for DMABN in either the gas phase or solution, strong indirect evidence exists for a twisted DMABN ground state in the presence of a polar solvent.
Fourier transform infrared study of amorphous N2O4 solid: Destabilization with inert impurities
Authors: Givan A. , Loewenschuss A.
Published: July 2002

Amorphous N2O4 solid was formed on predeposited oxygen or krypton layers. Evaporation of the underlying layer and redeposition of the inert impurities resulted in decomposition of the ionic nitrosonium nitrate clusters within the solid nitrogen tetroxide. Infrared features indicate NO+NO−3 decomposition to occur via molecular NONO3 , with NO2 monomers as products. Evaporation of the inert impurities and recooling restored the amorphous N2O4 from which NO+NO−3 was formed again.
The rotation–vibration potential of He–H2 and its connection with physical phenomena
Authors: Smith Maureen J. , Rabitz Herschel
Published: July 2002

This paper examines the effect of infinitesimal functional variations in a three-dimensional vibration–rotation He–H2 potential surface on several different levels of physical observables: inelastic cross sections, rate constants, and energy level populations. Earlier equations for a rigid-rotor system are extended and a comparison of the current results with earlier rigid-rotor results is made. A significant difference in the sensitivity of observables to the potential components has been observed between those observables which are purely rotationally inelastic and those which are vibrationally inelastic. The region of highest sensitivity is dependent upon the energy or temperature as well as the states related by the individual observable. Significant information loss has been observed in the transition from the microscopic observables to the macroscopic ones for those observables which are vibrationally inelastic.
Spin-forbidden decay of the dication HS2+
Authors: Parlant Gérard , Senekowitsch Jörg , ONeil Stephen V. , Yarkony David R.
Published: July 2002

The lifetimes of the low-lying vibrational levels of the X2Π state of the recently identified dication HS2+ [Miller et al., Int. J. Mass Spectrom. Ion Proc. 100, 505 (1990)] are considered. The stability of this state is attributable to a barrier formed from the avoided crossing of 2Π states asymptotically characterized as H++S+ and H+S2+. As a result of this barrier, the nonrelativistic X2Π potential energy curve supports several quasibound vibrational levels that are long lived with respect to tunneling. However, this is not the principal decay mechanism. We show that the lifetimes of the low-lying vibrational levels, v=0−4, are controlled entirely by the spin–orbit induced perturbation, 14Σ−∼X2Π, and the corresponding allowed crossing of the X2Π potential energy curve by the dissociative 14Σ− potential energy curve which correlates with the ground state asymptote H++S+(4S).
The exchange energy of H+2 calculated from polarization perturbation theory
Authors: Tang K. T. , Toennies J. P. , Yiu C. L.
Published: July 2002

The Rayleigh–Schrödinger (polarization) perturbation theory without symmetrization is used to calculate the exchange energy of H+2 through the surface integral of Holstein–Herring. It is mathematically proven that the exchange energy series so obtained is exact in the same sense as Herring’s result is exact. It is shown that the contributions to the leading term of the exchange energy series from all orders of polarized wave functions can be calculated exactly. Furthermore, it is explicitly demonstrated that the sum of these contributions converges to the exact value. The rate of convergence is relatively fast. With the first four orders of wave function, virtually 100% of the asymptotic exchange energy is recovered. With the present theory, terms other than the leading one can also be calculated systematically. This is demonstrated by the calculation of the first three terms of the exchange energy series from the first- and second-order wave functions.
The effect of diffusion on the Hopf bifurcation in a model chemical reaction exhibiting oscillatory behavior
Authors: Dutt Arun K.
Published: July 2002

The symmetry breaking instability in the irreversible Oregonator model for sufficiently low and unequal diffusion coefficients of its three intermediate species has been investigated using the rate constants of Field and Försterling. The effects of diffusion and the control parameters on the shift of Hopf bifurcation points associated with the emergence of heterogeneous waves of short wavelengths are reported. The results are found to be in good agreement with those obtained by Prigogine et al. for Brusselator and Turing models.
Extension of microscopic theory of nucleation to macroscopic clusters
Authors: Kobraei H. R. , Anderson B. R.
Published: July 2002

A microscopic model of homogeneous nucleation is extended to apply to macroscopic clusters. The definition used for physical clusters is identical to that of Reiss, Katz, and Cohen, which has also been introduced by Lee, Barker, and Abraham. Practical approximations have been employed to obtain results in closed form. The Gibbs free formation energy and the nucleation rate are easily calculated in this approach. The extended macroscopic treatment is compared to recent experimental data for methanol and ethanol. In these cases, the calculated nucleation rate agrees well with the experimental data. The agreement in both cases is consistent and far better than the classical theory.
Quantum canonical transformation of the rotational–vibrational Hamiltonian to remove a Coriolis term
Authors: Krishnan Mangala S. , Carrington Tucker
Published: July 2002

A quantum mechanical rotational–vibrational Hamiltonian with one Coriolis term which couples vibration and rotation is transformed to eliminate the Coriolis term. This is achieved by employing a Bogoliubov–Tyablikov transformation. A closed-form energy level expression is obtained for the vibrational–rotational energy levels of a rigid symmetric top coupled to harmonic oscillators. For an asymmetric rotor the transformation introduces small off-diagonal matrix elements which couple vibrational states. Nearly degenerate vibrational states would be strongly coupled by the original Coriolis term, but the small off-diagonal matrix elements of the transformed Hamiltonian may be treated perturbatively to obtain an effective rotational Hamiltonian for each vibrational state. The new theoretical method is compared with variational calculations.
Electronic relaxation of Xe2Cl in gaseous and supercritical fluid xenon
Authors: Okada F. , Apkarian V. A.
Published: July 2002

Spectroscopic and kinetic studies of the triatomic Xe2Cl exciplex in gaseous and supercritical fluid xenon up to 150 atm are reported. Clustering of the exciplex with Xe atoms is observed and its effect on spectral shifts, radiative lifetimes, and collisional deactivation are discussed. A kinetic treatment, incorporating diffusion controlled encounter probabilities, is used for the analysis of the relaxation data throughout the studied range. The radiative lifetime of the exciplex is observed to be 328(±20) ns, the exciplex is quenched by molecular chlorine with a rate constant of 7(±1)×10−10 cm3 s−1, while quenching by Xe is imperceptible even at the highest densities, implying a rate constant ≤10−17 cm3 s−1. Observations of transient loss, and their implications with respect to condensed phase triatomic rare gas halide exciplex lasers, are discussed.
Magnetic effects on positronium formation and annihilation in organic liquids
Authors: Diehl D. A. , Schrader D. M.
Published: July 2002

It was recently found that an external magnetic field markedly enhances the orthopositronium annihilation rate in certain organic solutions above and beyond that expected for the Zeeman effect. Several solutions have been studied, but the effect has been found only for nitrobenzene in n-hexane. A mechanism for this is proposed and tested numerically with a kinetics scheme modified to include nonhomogeneous spur effects. The mechanism features the quenching of orthopositronium by a short-lived triplet solute species, the concentration of which is moderated by the applied magnetic field.
Semiclassical theory of quantum diffusion in 1D: A stochastic process for the Landauer equation
Authors: Godoy Salvador
Published: July 2002

With the help of the quantum theory of scattering, a set of simultaneous difference equations is proposed, defining a diffusive stochastic process (correlated walk), which describes a semiclassical, one-dimensional, tunneling diffusion process in a periodic lattice. The jump probabilities are just the quantum transmission coefficients of the unit cell. With this process, we prove that by quantum tunneling or scattering above the potential, the particles diffuse in a one-dimensional lattice with a diffusion coefficient given by the Landauer formula. Next, the theory is generalized to include higher dimensions, and a square lattice case is shown as an example. There we show that the Landauer formula will not be obtained for 2D or 3D cases. Finally, examples are given, in which the Landauer formula is also valid for other cases of one-dimensional Markovian correlated walks.
Photoionization of a singlet exciton in an anthracene single crystal through two-color, two-step excitation
Authors: Katoh Ryuzi , Kotani Masahiro
Published: July 2002

Photoionization of a singlet exciton has been studied in an anthracene single crystal. The photoionization spectrum and optical-absorption spectrum have been measured with two-color, two-step excitation. By comparing the two spectra, the ionization efficiency (Φ) of higher excited states could be obtained as a function of the excitation energy. It was found that Φ=2×10−4 between 5.1 and 5.5 eV, independent of excitation energy. We interpret this result to indicate that autoionization occurs after a rapid relaxation from an initially populated higher excited state. Above 5.5 eV, Φ increases sharply. This is interpreted as the onset of another ionization channel, possibly the direct ionization. Polarization study of the photocarrier generation indicates that the intermediate state before the photoionization is the relaxed lowest singlet exciton state.
The Jahn–Teller system Ag2+: NaF, an electron paramagnetic resonance and optical absorption study
Authors: Monnier A. , Gerber A. , Bill H.
Published: July 2002

The ion Ag2+ introduced into NaF shows a tetragonal electron paramagnetic resonance spectrum at 4.2 K which dynamically averages above ≂40 K. Uniaxial stress is used to show that the ground state is a strongly coupled E⊗ε Jahn–Teller state. The well-resolved superhyperfine structure due to the F− neighbors is analyzed with a linear combination of atomic orbitals picture. Optical absorption of as-grown and treated crystals is further presented. The former ones show peaks at 202, 213, 219 nm due to Ag+. The latter ones present complex absorption spectra related to silver.
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o configuration interaction study of the reaction between small lithium clusters (Li4, Li6) and H2 molecule
Authors: Przybylski K. , Koutecký J. , Bonačić-Koutecký V. , von Ragué-Schleyer Paul , Guest M. F.
Published: July 2002

A reaction path for the Li4+H2 interaction has been determined allowing for the relaxation and the reconstruction of the system. For various steps along the reaction path characterized by the given values of one or two geometrical parameters the full geometry optimization of the remaining variables has been carried out in the framework of the Hartree–Fock (HF) procedure. The configuration interaction (CI) energies for the optimal HF geometries in each step have been calculated using multireference diexcited (MRD-CI) procedure. The calculated dissociation energy for the molecular hydrogen is reduced from 103.6 to 19.6 kcal/mol in the presence of Li tetrameter. During the first stage of the reaction path referred as ‘‘dissociative chemisorption’’ the geometry of the Li tetrameter does not change substantially. After dissociation of H2 the full reconstruction of the tetrameter takes place leading to a local minimum which is separated from the lowest minimum by a moderate barrier along the chosen reaction path. The stable compound has the form of deformed rhombus with two hydrogen atoms bridging two neighboring rhombus sides. The preliminary calculations using complete active space self-consistent field procedure followed by direct CI carried out for the Li6+H2 interaction in the C2v symmetry yield fairly stable three dimensional LI6H2 compound, which can be formed after passing an energy barrier of comparable size at the present level of calculation as that found for the Li4+H2 reaction.
Calculation of the dispersion interaction energy by using localized molecular orbitals
Authors: Kapuy Ede , Kozmutza Cornelia
Published: July 2002

In this paper, we present a method for the calculation of dispersion interaction energies of molecules, which is based on the localized molecular orbitals of the supermolecule. It is shown (using various basis sets) that the energy contributions of the localized molecular orbitals, for distances around the intermolecular energy minimum, are transferable to a fairly good approximation from the noninteracting monomers to the supermolecule. The correlation energy component of the interaction energy can be separated into inter- and intraparts. We show how the basis set superposition errors can be located and separated at the correlated level. Results are presented for several systems [(H2O)2, (NH3)2, and Ne2 ] with different basis sets from the minimal one to the split shell plus polarization functions. The method applies the well-known Boys’ localization procedure and has the advantage that for the dispersion energy at a given geometry, the integral transformation should only be carried out once (from the basis functions to the localized molecular orbitals of the supermolecule).
Density matrix calculation of population transfer between vibrational levels of Na2 by stimulated Raman scattering with temporally shifted laser beams
Authors: Band Y. B. , Julienne P. S.
Published: July 2002

We compare the results of full density matrix calculations with recently reported experimental results describing the population transfer in Na2 using Raman scattering with the fundamental laser beam temporally shifted relative to the Stokes laser beam. Our calculations confirm the conclusion that almost total transfer of the population into the terminal level of the Raman transition can be achieved when the laser frequencies are tuned to resonance and the fundamental laser beam is temporally delayed relative to the Stokes shifted laser beam but still partially overlapping it. We analyze our results when the frequencies of the lasers are tuned off-resonance as a function of the Rabi frequencies and the detuning, and compare with experimental and theoretical results. The alignment of the terminal level as a function of time displacement of the Stokes and pump beams is predicted.
Observation of structure in transition state spectra of the bimolecular reactions K+XNa→KX+Na (X=Br,Cl)
Authors: Barnes Michael D. , Brooks Philip R. , Curl R. F. , Harland Peter W.
Published: July 2002

Laser excitation of the transient species [KXNa]‡ formed in reactions in crossed beams of K and NaX (X=Br,Cl) is observed. The excitation spectra display features dependent on the chemical systems, which might lead to insight into the nuclear motion during reaction.