bluuebunny/crossref_metadata_2025 huggingface dataset:
An a
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o study of low-lying 5f→5f excitations in PuF6
Authors: Wadt Willard R.
Published: July 2002

The excited electronic states of PuF6 below 3 eV arise from 5f→5f transitions. These states have been investigated using ab initio wave functions based on a relativistic effective core potential (RECP) for plutonium and nonrelativistic ECP for the fluorines and double-zeta quality basis sets. The effects of electron correlation were included by configuration interaction, and the effects of spin-orbit coupling were included by diagonalizing the correlated wave function over an effective spin-orbit operator. The only previous calculations on PuF6 that include both the effects of configuration interaction (CI) and of spin-orbit (SO) coupling are the semiempirical ligand field calculations of Kugel et al. The agreement between the present work and that of Kugel et al. is truly remarkable, with the average difference in excitation energies being less than 0.03 eV for the nine lowest transitions. Both sets of results are in general agreement with recent gas-phase and matrix spectroscopic studies of PuF6.
Dynamics of single spin systems under arbitrary amplitude modulated fields
Authors: Campolieti G. , Sanctuary B. C.
Published: July 2002

Using a spherical tensor operator basis a general method of solution to the time evolution of the spin density matrix for a spin of arbitrary magnitude I is given. Rather than using time ordering techniques, we present an integral equations approach for calculating the effects of arbitrary pulse shapes. The method is shown to provide a rapidly converging perturbation expansion which is useful in explaining many pulse shapes. In particular, a simple recipe for the calculation of the magnetization results from this technique. The successive terms in the perturbation expansion avoid multiple commutators such as are encountered in the Magnus expansion of the propagator. These terms are given simply as integrals of the pulse shape function. The examples of the Bloch–Siegert problem and the exact dynamics of a spin interacting with a rotating rf field combined with an isotropic relaxation and a regeneration mechanism are presented in the context of the method of transformations in Liouville space.
Sum rules for nuclear attraction integrals over hydrogenic orbitals
Authors: Koga Toshikatsu , Matsuhashi Toshiyuki
Published: July 2002

Based on the property of two functions Snlmn′l′m′(R) and Wnlmn′l′m′(R) introduced in the Fock representation of the momentum-space Schrödinger equation for one-electron diatomics, we derive various relations and sum rules for the one-, two-, and three-center nuclear attraction integrals over (bound state) hydrogenic orbitals with a common exponent.
On the electrostatic interaction in macroionic solutions and suspensions
Authors: Overbeek J. Theodoor G.
Published: July 2002

It is shown that the theory of Sogami and Ise leading to an attraction between particles with charges of the same sign contains errors. When the errors are corrected the calculated attraction disappears.
Molecular dynamics in hydrogen-bonded interactions: A preliminary experimentally determined harmonic stretching force field for HCN---HF
Authors: Wofford B. A. , Lieb S. G. , Bevan J. W.
Published: July 2002

Observation of the 2ν1 overtone band in the hydrogen-bonded complex HCN---HF permits evaluation of the anharmonicity constant X11=−116.9(1) cm−1 and determination of the anharmonicity corrected fundamental frequency ω1. This information, and available data from previous rovibrational analyses in the common and perdeuterated isotopic species of HCN---HF, offer an opportunity for calculation of an approximate stretching harmonic force field. With the assumptions f12=f24=0.0, the remaining force constants (in mdyn/Å) are evaluated as: f11=8.600(20), f22=6.228(9), f33=19.115(40), f44=0.2413(39), f13=0.000(13), f14=0.0343(2), f23=−0.211(6), f34=0.000(2). These compare to f11=9.658(2) in the HF monomer and f11=6.244(3) and f33=18.707(16) in the HCN monomer. These results provide the information necessary to quantitatively assess the applicability of the Cummings and Wood approximation in this hydrogen-bonded complex and also give an estimate of Dej, the equilibrium distortion constant in the harmonic limit. Comparisons of these experimentally determined parameters with the predictions of ab initio molecular orbital calculations at several levels of approximation are presented.
Temperature effects on the vibration–rotation spectrum of a physisorbed diatomic
Authors: Adams John E.
Published: July 2002

We report the results of a study of temperature influences on the vibration–rotation line shape of a prototypical physisorption system, HC1/Ar (111). Two particular features of the problem are examined, the first being the effect of surface motion (phonons) on the line shapes deriving from several different rotational transitions within the fundamental vibrational band. On the whole, phonon contributions are found to be significant at the lowest and highest rotational energies but negligible between the two limits. The second feature of interest is the effect of changing the system temperature on the line shape arising from a single transition. Results obtained in this investigation are consistent with experimentally determined temperature dependences for other systems where dephasing is thought to represent an important line broadening mechanism.
Analysis of gas–surface reactions by surface temperature modulation: Theoretical formulation
Authors: Engstrom J. R. , Weinberg W. H.
Published: July 2002

The theoretical formulation of a frequency response technique, based on surface temperature modulation, for the study of surface reaction dynamics is described in detail. The formalism is developed for analyzing adsorption, desorption, and both unimolecular and bimolecular surface reactions. The evaluation of the activation barriers for the elementary surface reactions is straightforward even in the presence of strong nonlinearities and requires no knowledge of the composition or configuration (i.e., structure) of the adlayer. This capability provides a distinct advantage over competitive techniques such as modulated molecular beam reactive scattering. Although the evaluation of the preexponential factors of the rate coefficients is complicated by the presence of nonlinear effects (e.g., coverage-dependent rate parameters or a bimolecular reaction), in many cases the preexponential factors may also be evaluated in a straightforward manner.
Vibrationally state-selected reactions of ammonia ions. II. NH+3(v)+CH4
Authors: Conaway William E. , Ebata Takayuki , Zare Richard N.
Published: July 2002

The effects of vibrational excitation of the ammonia ν2 inversion mode on the reaction of NH+3(X̃,v=0 to 9) with methane have been measured in a tandem quadrupole mass spectrometer over the center-of-mass collision energy range 1.5 to 10 eV. The hydrogen abstraction channel is enhanced by nearly a factor of 2 at nine quanta of vibrational energy relative to the v=0 level of the ion. Added vibration in the ammonia ion umbrella-bending mode facilitates the transition to the NH+4 product ion geometry. Protonated methylamine is formed at lower kinetic energies by attack of the ion at the methane carbon center, but with increasing vibrational excitation of the ammonia ion, the protonated methylamine decomposes by 1,2-elimination of molecular hydrogen and by C–N bond scission.
Near-equilibrium instabilities in closed chemical systems under nonideal conditions
Authors: Dutta Chitra , Dasgupta Anjan , Das Jyotirmoy
Published: July 2002

The stability behavior of a binary system in the mass-closed condition is analyzed using an integrated form of second law. The integrated change in free energy is plotted against the reaction coordinate. The trajectory provides a distribution of extremal points as a function of system parameters. As the parameters of the system exceed the critical values implying marginal stability condition, the plot undergoes a sharp change in qualitative behavior. The minima and maxima of the plot reflect locally stable and unstable equilibria, respectively, and when multiple stable critical points coexist for same set of parameter values, this plot helps to predict the selection of a particular asymptotic regime by the system as a function of initial conditions. If the system is initially in the vicinity of an unstable equilibrium separating two stable regimes, a small bias or asymmetry in the perturbation will dictate its evolution to a particular final state.
Novel a
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o self-consistent-field approach to molecular solids under pressure. I. Theory
Authors: Raynor Susanne
Published: July 2002

A fully ab initio wave-function based procedure for the study of molecular solids under high pressure is presented. The method includes all many-body interactions, takes maximum advantage of symmetry to reduce computation, and is designed to allow for the direct study of reactions occuring in molecular crystals placed under high pressure. Furthermore, the approach is completely general, allowing for its application at either semiempirical or exact levels of calculation. As an aside to this work, we also demonstrate the efficacy of using an algebraic approach, instead of the conventional series expansion, to calculate the elements of the inverse overlap matrix for the crystal.
The effect of parent internal motion on photofragment rotational distributions: Vector correlation of angular momenta and C2v symmetry breaking in dissociation of AB2 molecules
Authors: Levene Harold B. , Valentini James J.
Published: July 2002

A semiclassical calculation is presented that fully accounts for the angular momentum disposal in photodissociation of triatomic molecules. Rotational distributions are calculated for O2(3Σ−g) produced by the visible photolysis of ozone, O2(1Δg) by the UV photolysis of ozone, and OH by the 157 nm dissociation of water, to illustrate the effects of parent internal motion on fragment rotational distributions in the dissociation of C2v geometry molecules. A simple, but realistic, impulsive model of the energy release is used to describe the dissociation dynamics. The calculations are carried out for parent molecules at room temperature, as well as at the low temperatures characteristic of molecular beams. The contributions to the diatomic fragment rotational distribution from both parent triatom rotation and zero-point bending vibration are computed. Comparison of the calculated distributions with experimentally measured distributions indicates that the spread in rotational and bending vibrational angular momenta of the parent molecule can account for all or nearly all of the spread in final J of the diatomic photofragment. However, the rotational distributions of the diatomic photofragment reveal a strong vector correlation between the diatom angular momentum produced by the dissociative energy release, and the angular momentum associated with the in-plane rotation. The correlation is such that only half of all the photofragment states allowed by energy and angular momentum conservation are actually produced with appreciable probability. Of two energy degenerate photofragment states, corresponding to breaking of one or the other nominally equivalent bonds in the AB2 molecule, the one with the smaller orbital angular momentum/recoil linear momentum is strongly favored. This is explained by larger Franck–Condon overlap in the photoexcitation for the state of lower recoil angular momentum. The correlation involves selection of which of the two nominally equivalent bonds will break in the photodissociation of an AB2 triatom of C2v geometry, and thus represents a symmetry breaking mechanism in such a photodissociation.
Intramolecular dephasing and vibrational redistribution in the dispersed fluorescence of ultracold molecules: Application to anthracene
Authors: Shan Kaiyu , Yan Yi Jing , Mukamel Shaul
Published: July 2002

A reduced description for spectral line shapes in isolated ultracold polyatomic molecules is developed. The vibrational degrees of freedom are partitioned into a system and a bath. The system modes are treated using a Green function eigenstate-free procedure. The bath modes induce intramolecular vibrational redistribution (IVR) and in addition are responsible for intramolecular dephasing resulting from their thermal fluctuations. The effects of the bath are introduced statistically via correlation functions. Application is made to the dispersed fluorescence of ultracold anthracene in a supersonic beam, which shows progression of narrow lines (Raman) and broad redistributed fluorescence. The observed red shift (with respect to the absorption) and line broadening in the emission indicate that the frequencies of the bath modes are on the average 6.4% lower in the electronically excited state compared with the ground state. We further obtain the IVR rates in the high energy region.
A dynamical ‘‘white spot’’ on the potential energy surface: The close interaction region of the collinear hydrogen transfer reaction F+DBr→FD+Br
Authors: Gertitschke P. L. , Kiprof P. , Manz J.
Published: July 2002

The strong FDBr interaction region located in the corner of the LEPS model potential energy surface is diagnosed as dynamical white spot. Although energetically accessible, it is dynamically unapproachable by the vast majority of reactive classical trajectories, quantum waves represented by S-matrix propagation, or resonance wave functions. The quantum and classical derivations and explanations yield important consequences of the dynamical white spot: In particular, quantum chemical evaluations of this region of the potential energy surface are rendered unneccessary because its dynamical effects on bi- and unimolecular reactivity are insignificant.
A threshold study of the classical dynamics of collision-induced dissociation in collinear H+H2
Authors: Grice M. Edward , Andrews Burton K. , Chesnavich Walter J.
Published: July 2002

In this paper we report the results of a classical trajectory study of collinear H+H2 on the PK2 surface at fixed total energies slightly above the dissociation threshold. Two energies, 4.85 and 5.00 eV, were investigated. The method of Andrews and Chesnavich was used to resolve the reagents phase space into nonreactive, reactive, and dissociative regions. A striking feature of the phase space banding structure is that it is remarkably simple. Also, all trajectories exhibit regular and smooth behavior with respect to changes in initial condition. No evidence was found of either periodic trajectories or of ‘‘snarled’’ trajectories. Some general properties of the dynamics are discussed, including a suggested explanation as to why all trajectories in one of the reactive bands cross the symmetric stretch line three times. Some suggestions for future work, including model studies which could provide information on phase space banding in three-dimensional systems, are also given.
Vibronic energy distribution of H2O+ produced in charge transfer scattering of D+ by H2O
Authors: Friedrich Bretislav , Niedner Gereon , Noll Martin , Toennies J. Peter
Published: July 2002

A crossed beam study of the charge exchange reaction between deuterons and water molecules is reported. Individual vibronic states of the triatomic molecular ion produced in collisions are resolved.(AIP) e
Rovibronic state to rovibronic state reaction dynamics: O(3P)+HCl(v=2,J)→OH(v′,N′)+Cl(2P)
Authors: Rakestraw D. J. , McKendrick K. G. , Zare R. N.
Published: July 2002

We report an experimental investigation of the reaction O(3P)+HCl(v=2,J)→OH(v′=0 and 1,N′)+Cl(2P), where HCl is prepared in a single rovibronic level using a tunable optical parametric oscillator and the rovibronic levels of the OH reaction product are monitored using laser-induced fluorescence. The nascent rotational state distributions in OH(v′=0 and 1) have been measured as a function of the initial rotational state J of HCl(v=2). These distributions are substantially rotationally excited and exhibit a shift towards higher N′ as J is increased. Comparable branching into OH(v′=0) and OH(v′=1) is observed. The reaction cross section is found to increase markedly with reagent rotation: the increase is approximately a factor of 3 over the range J=1–10 of HCl(v=2), but is not necessarily monotonic for low J.
Multidimensional quantum eigenstates from the semiclassical dynamical basis set
Authors: Frederick John H. , Heller Eric J.
Published: July 2002

A new method for obtaining molecular vibrational eigenstates using an efficient basis set made up of semiclassical eigenstates is presented. Basis functions are constructed from a ‘‘primitive’’ basis of Gaussian wave packets distributed uniformly on the phase space manifold defined by a single quasiperiodic classical trajectory (an invariant N-torus). A uniform distribution is constructed by mapping a grid of points in the Hamilton–Jacobi angle variables, which parametrize the surface of the N-torus, onto phase space by means of a careful Fourier analysis of the classical dynamics. These primitive Gaussians are contracted to form the semiclassical eigenstates via Fourier transform in a manner similar to that introduced by De Leon and Heller [J. Chem. Phys. 81, 5957 (1984)]. Since the semiclassical eigenstates represent an extremely good approximation of the quantum eigenstates, small matrix diagonalizations are sufficient to obtain eigenvalues ‘‘converged’’ to 4–5 significant figures. Such small diagonalizations need not include the ground vibrational state and thus can be used to find accurate eigenstates in select regions of the eigenvalue spectrum. Results for several multidimensional model Hamiltonians are presented.
Hartree–Fock energy derivatives with respect to basis set exponents. Integral derivatives using Rys polynomials
Authors: Tonachini Glauco , Schlegel H. Bernhard
Published: July 2002

The Rys polynomial method has been used to evaluate the first derivatives of the two electron integrals and the nuclear electron attraction integrals with respect to the Gaussian exponents. Gradients for the sulfur 6-31 G(d) basis set in HS−, H2S, H3S+, H2S2, H3CSH, H2CS, CS, H2SO, SO2 and SO3 have been evaluated to illustrate the effect of charge, multiple bonding and hypervalency on the basis set exponents.
Model studies for rovibronic IVR with applications to benzene
Authors: Dietz W. , Fischer S. F.
Published: July 2002

Intramolecular vibrational energy redistribution (IVR) is treated. Nearly isoenergetic rigid-rotor harmonic-oscillator states are rovibronically coupled by the interplay of anharmonic and Coriolis forces. The theory predicts linewidth distributions of rovibronic states and explains the selective supression of rotational spectral lines. Applications to sub-Doppler spectra of the 1410120 band of benzene allow to extract values for effective intramolecular interactions. These results are supported by direct calculations based on the force field.
Direct observation of Ba+ velocity distributions in a drift tube using single-frequency laser-induced fluorescence
Authors: Dressler Rainer A. , Meyer Henning , Langford Andrew O. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

First results are presented on velocity distribution measurements of Ba+ ions drifted in helium using a well-characterized drift tube and single-frequency laser-induced fluorescence detection. Reduced mobilities of Ba+ in helium are obtained for E/N values between 7.74 and 23.2 Td. The velocity distributions that are observed parallel to the electric field fit a displaced Maxwell–Boltzmann distribution, in agreement with theoretical predictions. However, the temperatures parallel and perpendicular to the field are not in good agreement with theoretical treatments.
Theory of laser-stimulated surface processes. III. Desorption through vibrational excitation by an IR laser
Authors: Beri A. C. , George Thomas F.
Published: July 2002

Desorption of an adatom from a solid surface through vibrational excitation by an IR laser is studied using a generalized master equation approach and the isomnesic approximation. A sharp transition from no desorption to almost instantaneous desorption is seen as the bound-to-continuum population transfer rate is varied. The critical rate increases linearly with laser power.
Collisional dynamics of the IF B 3Π(0+) state. III. Vibrational and rotational energy transfer
Authors: Wolf Paul J. , Davis Steven J.
Published: July 2002

Rate constants at T=300 K for collision induced vibrational and rotational energy transfer within the IF B 3Π(0+) state have been determined using both cw and pulsed laser induced fluorescence. State-to-state vibrational energy transfer rate coefficients have been measured for IF(B; v′) collisions with He, Ne, Ar, Kr, Xe, N2, O2, and F2. Vibrational energy transfer has also been studied as a function of the initially excited vibrational level. Steady-state fluorescence emission from v′=3 has been analyzed to yield total rotational removal rate coefficients with the noble gases and N2 as collision partners. Vibrational energy transfer is generally inefficient; the thermally averaged cross sections for single quantum transfer (Δv=−1) from v′=3, σ(3,2), decrease from 1.2% to 0.3% of the gas kinetic cross sections, σg, for the periodic series He to Xe. For diatomic collision partners, the respective values of σ(3,2)/σg for N2, O2, and F2 are 0.014, 0.033, and 0.020. The Δv=−1 cross sections for the noble gases monotonically decrease with μ1/3 while no clear dependence on the collision reduced mass is observed with N2, F2, and O2. The vibrational transfer cross sections for the noble gases, N2, and O2 scale linearly with vibrational quantum number. The results also reveal that ‖Δv‖=1 transfer is at least an order of magnitude more probable than that for ‖Δv‖=2. Rotational energy transfer is the most efficient kinetic process in IF(B). The estimated efficiencies for total rotational energy transfer from J′=22 in v′=3 with the rare gases and N2 are typically 20 to 200 times greater than those for vibrational transfer. The rate coefficients range from (9.7±1.8)×10−11 cm3 molecule−1 s−1 for He to (1.1±0.1)×10−10 cm3 molecule−1 s−1 for Xe. The rotational transfer rate coefficients show a smooth dependence on both the collision reduced mass and the polarizability of the collision partner. The qualitative results of these experiments are discussed in relation to traditional energy transfer models.
Photochemical reactions of c
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s-1,2-dichloroethene adsorbed on Pd(111) and Pt(111)
Authors: Grassian Vicki H. , Pimentel George C.
Published: July 2002

The photochemical behaviors of cis- and trans-1,2-dichloroethene (DCE) adsorbed on Pd(111) and Pt(111) surfaces have been studied using electron energy loss spectroscopy (EELS). For multilayer coverage on either metal surface, irradiation of physisorbed DCE at 110 K with broad band irradiation (λ>200 nm) results in photoisomerization, cis–trans. For monolayer coverage on Pt(111) at 110 K, photolysis of chemisorbed DCE causes loss of the two chlorine atoms to form a single hydrocarbon product, chemisorbed acetylene. Apparently, for λ>237 nm, the chlorine atoms remain bound to the platinum surface whereas for shorter wavelengths, λ>200 nm, the chlorine atoms leave the surface. These results are interpretable in terms of singlet excitation of the chemisorbed alkane followed by chlorine elimination on an excited singlet reaction surface. This study indicates that photochemistry of molecules chemisorbed on a metal surface is possible despite the proximity of the conducting surface. It shows that energy relaxation processes connected with this proximity are not prohibitively fast.
Infrared spectra of the (N2)2 and N2–Ar van der Waals molecules
Authors: McKellar A. R. W.
Published: July 2002

Spectra of the weakly bound van der Waals molecules (N2)2 and N2 –Ar have been studied in the 2250–2450 cm−1 region corresponding to the N2 monomer vibrational frequency. The experiments were performed on equilibrium gas samples at low temperatures (77–90 K) using a long path length (154 m) cell and a Fourier transform infrared spectrometer. The spectra appear as relatively weak fine structure on top of the broad, diffuse, collision-induced fundamental band of nitrogen. The structure due to (N2)2 is extremely weak, and resembles that seen in earlier low-resolution work by Long et al. [Chem. Phys. 2, 485 (1973)], except that a new Q branch feature is detected here near the band center. On the other hand, the structure due to N2 –Ar is much more prominent, varied, and sharp than that of (N2)2, showing far greater detail than did a previous low-resolution study by Henderson and Ewing [Mol. Phys. 27, 903 (1974)]. Existing calculations are not adequate to utilize the information carried in the N2 –Ar spectrum, but in the future it should be possible to refine our knowledge of the intermolecular potential for this system by using these results.
Small angle scattering of bicontinuous structures in microemulsions
Authors: Vonk C. G. , Billman John F. , Kaler Eric W.
Published: July 2002

We present a model of bicontinuous microemulsion structure based on a disordered lamellar structure. The scattering predicted by this model is compared with measured small angle neutron scattering curves from a family of microemulsions formed with sodium 4-(1′-heptylnonyl) benzenesulfonate, iso-butanol, D2O brines, and the even alkanes from octane to hexadecane. The model, with two parameters, fits the data on an absolute scale well. Disordered lamellae spacings and distortion lengths vary from approximately 850 and 570 Å, respectively, for microemulsions formed with dodecane to 430 and 400 Å for those formed with hexadecane. A distribution of lamellae thicknesses can be introduced to account for scattering curves that display a maximum in intensity scattered at zero scattering vector.
ℒ2 golden rule method to calculate partial widths for the decay of resonance states
Authors: Tucker Susan C. , Truhlar Donald G.
Published: July 2002

We have calculated resonance energies and partial widths for two two-dimensional models of van der Waals molecule predissociation. We use a general method involving only Hamiltonian and overlap integrals in a square integrable (ℒ 2) basis set containing a scale parameter. We use a stabilization method with a compactness criterion to find the resonance energies, and a decoupled golden rule method to find the partial widths. The results are compared to accurate energies and partial widths obtained by fitting solutions of the close-coupling equations to multichannel Breit–Wigner expressions. We studied resonances having two open channels for two sets of potential parameters, and in each case we obtained an accuracy of 16% or better for both partial widths by the ℒ 2 method.
A computer and analytic study of the metallic liquid–glass transition
Authors: Li D. H. , Moore R. A. , Wang S.
Published: July 2002

The present work examines two aspects of metallic liquid–glass transitions. First, a computer experiment, consisting of a temperature-quench Monte Carlo calculation for the liquid–glass transition of Ca0.7Mg0.3, is carried out. A comparison with such calculations for monatomic metals indicates that there is a significant difference in the Wendt and Abraham criterion for monatomic metals and metallic alloys. In addition, a comparison with laboratory experiments indicates that the discrepancies between the Wendt–Abraham parameters obtained in these laboratory experiments and in the computer experiments can be ascribed to different cooling rates. Second, an analytic study of the pair distribution function near the transition temperature is presented and reproduces the linear temperature dependence of the Wendt–Abraham parameter. This gives a physical picture for this parameter and indicates that the liquid–glass transition is more likely to be a first-order transition than a second-order one.
Vibrational relaxation of guest and host in mixed molecular crystals
Authors: Hill Jeffrey R. , Chronister Eric L. , Chang Ta-Chau , Kim Hackjin , Postlewaite Jay C. , Dlott Dana D.
Published: July 2002

Vibrational relaxation (VR) of dilute impurity molecules (naphthalene, anthracene) in crystalline host matrices (durene, naphthalene) is studied with the ps photon echo technique. The results obtained by echoes on vibrations in the electronically excited state are compared to previous ps time delayed coherent Raman studies of ground state vibrations of the pure host matrix. The relaxation channels for guest and host, and the effects of molecular and crystal structure on VR rates are determined.
Vibrational spectroscopy of diatomic molecules in static electric fields: An a
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o study of HF
Authors: Malik David J.
Published: July 2002

For the specific case of the hydrogen fluoride molecule, a detailed determination of the vibrational effects on molecular electrical properties has been made for the lowest four vibrational states. Electrical properties at fixed H–F distances obtained from correlated and SCF-level ab initio treatments were used along with an ab initio potential energy curve to describe the vibrating system. Using numerical vibrational wave functions, the electrical properties of the low-lying vibrational states were rigorously calculated by finite differences and these are compared with values obtained with certain approximate means. One and two-photon, dipole, and quadrupole absorption cross sections were calculated in the same manner. The extensive incorporation of electrical properties and the detailed treatment of the vibrational states provides certain fundamentally new information on (i) the role of vibrational motion in dipole and other types of transitions, and (ii) the effect of external electrical fields on vibrational effects.
Investigation of soliton-like excitations in t
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s-polyacetylene
Authors: Reid R. D. , Schug J. C. , Lilly A. C. , Dwyer R. W.
Published: July 2002

In the study reported here, unrestricted Hartree–Fock calculations were performed on the ground state and several soliton-like defect states of the model large polyene, trans-polyacetylene. The calculations included the use of the random phase approximation to obtain screened electron repulsion integrals and projection of the UHF eigenfunctions to obtain pure spin states. The trans-polyacetylene ground state geometry is characterized here by alternating double and single bonds with lengths of 1.456 and 1.369 Å, respectively. Calculations on model soliton-like defect states yielded functional forms for the bond alternation, spin density, and charge density defects which are consistent with the soliton model for trans-polyacetylene and are similar to those reported elsewhere. The average positive and average negative spin density on chains containing neutral kink defects are 0.045 and −0.011, respectively. These values are in fair agreement with the best experimental estimates of 0.06 and −0.02.
Rare gas matrix ESR investigations of 12CH3 63,65CuF, 13CH3 63,65CuF, H63,65CuF, and D63,65CuF generated by reactive laser vaporization
Authors: Knight Lon B. , Cobranchi S. T. , Gregory B. W. , Jones G. C.
Published: July 2002

A new experimental procedure for conducting reactive laser vaporizations is described and applied to the generation of the previously unreported copper radicals, 13CH3CuF, 12CH3CuF, HCuF, and DCuF, for rare gas matrix ESR investigations. Laser vaporization was conducted on a thin film of reactant condensed on the copper metal surface. The g and A tensors for these are compared with other small copper radicals in order to determine electronic structure and bonding trends. The neon magnetic parameters (MHz) for 13CH363CuF are: g∥ =1.965(1), g⊥ =2.3626(3); A∥(63Cu) =3076(2), A⊥(63Cu) =2993(1); A∥(19F) =94(1), A⊥(19F) =68(1); A∥(13C) =19(1), A⊥(13C) <3. The H quartet hfs for CH3CuF was resolved on the perpendicular transitions, having an A value of 9.3(3) MHz. For H63CuF, the neon results (MHz) are: g∥ =1.975(1); g⊥ =2.4075(4); A∥(63Cu) =2826(3), A⊥(63Cu) =2717(1); A∥(19F) =127(2), A⊥(19F) =94(1); A∥(H) =61(3), and A⊥(H) =29(1) MHz.
Anisotropy of static and dynamic orientational correlations in N-alkanes
Authors: Bahar Ivet , Erman Burak
Published: July 2002

Static and dynamic orientational correlations in n-alkanes are analyzed using the rotational isomeric states formalism and its newly introduced adaptation to local chain dynamics. For systems in equilibrium, the anisotropy of directional correlations is investigated in relation to the respective orientations of two coordinate frames rigidly attached to the terminal bonds. The resulting static correlations exhibit a strong odd–even effect depending on the number of intervening bonds between the two frames. Dynamic correlations, on the other hand, necessitate the analysis of the time-dependent autocorrelation of a single reorienting frame rigidly embedded in the motional molecule. Expressions for both the first and second orientational correlation functions—identified according to the order of the associated Legendre polynomials—are formulated, for static and dynamic systems. Dynamic correlations are mathematically related to static correlations through the limiting values to which they asymptotically converge, as time goes to infinity. The orientational correlations are calculated to be strongly anisotropic in all cases.
Nonadiabatic divergence in barrier models of excitonic self-trapping
Authors: Wagner Max , Koengeter Andreas
Published: July 2002

The conventional adiabatic concept for excitonic self-trapping via local lattice relaxation is shown to display a fundamental divergence if the oscillatory kinetic energy operator is consistently accounted for. This outcome is due to an extreme violation of the adiabatic principle. Consequently, the often used ‘‘barrier models’’ of self-trapping lose their theoretical justification. Possible alternative concepts for ‘‘delayed’’ self-trapping are shortly indicated.
Observation of an odd/even delta-J propensity in the collisional excitation of CO2 by hot deuterium atoms
Authors: Hershberger John F. , Hewitt Scott A. , Flynn George W. , Weston Ralph E.
Published: July 2002

We report here the first experimental observation of such oscillations in the ro-vibrational excitation of CO2 (0110) by hot atoms. (AIP)
The anharmonic stretching–bending potential of CO2 from inversion of spectroscopic data
Authors: Romanowski H. , Gerber R. B. , Ratner M. A.
Published: July 2002

The full anharmonic stretching and bending potential of CO2 is determined to high accuracy from the measured vibration/rotation spectrum. The calculation consists of two stages: first, a direct explicit inversion of the data within a semiclassical self-consistent-field (SCF) treatment of vibrational dynamics and second, a refinement of this result by a first-order perturbative approach that includes corrections to the SCF approximation, but assumes that the SCF-inverted potential shows only small deviations from the true surface. The final result is tested by comparison of the energy levels calculated exactly from the determined potential against the original experimental input. Based on this criterion of accuracy in reproducing experimental frequencies, the present inverted potential appears more accurate than previous potentials obtained from empirical fitting of the data. These results indicate that the perturbatively corrected SCF inversion method is a very powerful tool for obtaining potential energy surfaces directly from experimental data, with errors not exceeding several wave numbers.
Molecular dynamics simulation for an ensemble of coupled quantum two level systems with classical degrees of freedom
Authors: Hofmann Paul , Brickmann Jürgen
Published: July 2002

An ensemble of model particles represented as quantum two level systems with induced dipole–dipole-type interactions is studied with molecular dynamics techniques. The particles are treated as oriented molecules according to the direction of the actual dipole moment. A set of equations of motion is derived on the basis of a time-dependent Hartree method, considering the classical limit for the rotational motion. From the numerical calculations, three different types of behavior for the ensemble of model particles are found. For weak interactions the system nearly behaves like an ensemble of uncoupled particles. With increasing interaction strength, the intramolecular energy starts to oscillate between the two levels, while in the strong interaction range the quantum states of individual molecules are represented by stationary linear combinations of the two states of the model quantum system for the particles. The change in behavior, when going from medium to strong interaction, occurs like a phase transition in bulk matter. The simple model presented here is well suited to describe the generic properties of an ensemble of molecules which may perform an intramolecular rearrangement which is essentially quantum mechanical. Similarities of the model system to a set of ammonia molecules (with the well-known inversion) are pointed out.
Comment on: ‘‘Total scattering, surface ionization, and photoionization of a beam of H3 metastable molecules’’
Authors: Gellene Gregory I. , Porter Richard F.
Published: July 2002

The nature of the metastable state of hydrogen trimers described by Ga and Kuppermann (ref.1) is further discussed. The analysis is based on linewidth measurements in the emission spectrum of the trimer, the quantum mechanical selection rules, and earlier beam and photoionization measurements.(AIP)
Freezing of the hard core Yukawa fluid
Authors: Kloczkowski Andrzej , Samborski Adam
Published: July 2002

We have studied the freezing of hard spheres with a Yukawa attractive or repulsive potential in the mean spherical approximation by using the density functional theory proposed by Ramakrishnan and Yussouff and extended later by Haymet and Oxtoby. We have calculated the densities of the liquid and solid phases at the phase transition as a function of temperature for two different parameters of the range of Yukawa potential (z=1 and z=5).
Experiments on bifurcation of periodic states into tori for a periodically forced chemical oscillator
Authors: Vance William , Ross John
Published: July 2002

We study experimentally continuous transitions from quasiperiodic to periodic states for a time-periodically forced chemical oscillator. The chemical reaction is the hydration of 2,3-epoxy-1-propanol, and is carried out in a continuous stirred tank reactor (CSTR). Periodic oscillatory states are observed to arise in the autonomous system through supercritical Hopf bifurcations as either the total flow rate or the cooling coil temperature is changed. Under conditions of oscillation for the autonomous system, small-amplitude periodic variation of the total flow rate generates an attracting two-torus from the stable limit cycle. From the experiments we determine the structure of the toroidal flow, stroboscopic phase portraits, and circle maps as a function of the forcing amplitude and period. A continuous transition from the quasiperiodic to a periodic state, in which the two-torus contracts to a closed curve (Neimark–Sacker torus bifurcation), is observed as the forcing amplitude is increased at a constant forcing period, or as the forcing period is changed at a constant moderate forcing amplitude. Qualitative theoretical predictions compare well with the experimental observations. This paper presents the first experimental observation of a Neimark–Sacker torus bifurcation in a forced chemical oscillator system, and relates the bifurcation diagram of the unforced system to that of the forced system.
Experimental evidence for the formation of H<H* H+5/e− dissociative recombination
Authors: Miderski C. A. , Gellene Gregory I.
Published: July 2002

The effects of temperature and hydrogen density on the efficiency of production of H*3 Rydberg levels in a hollow cathode discharge are determined by monitoring the 3s 2A1→2p 2A″2 transition of H*3 . Kinetic modeling of the ion–molecule reactions occurring in the discharge indicates that the observed H*3 spectral intensity dependence on discharge temperature and density can be accounted for if it is assumed that H+5/e− dissociative recombination is the primary process leading to H*3 formation. The previously observed extensive Doppler-like broadening of the H*3 emission lines is explained in terms of the dissociative kinetic energy release of H**5 .
Theoretical vibrational study of the FX⋅⋅⋅O(CH3)2 hydrogen-bonded complex
Authors: Bouteiller Y. , Mijoule C. , Szczesniak M. M. , Scheiner S.
Published: July 2002

This paper presents the first ab initio attempt to reconstruct the observed band profile of the stretching fundamental vFX (X=H,D) in the FX⋅⋅⋅O(CH3)2 hydrogen-bonded system. The two-dimensional potential energy surface V(rFH,RF⋅⋅⋅O) is evaluated by means of large basis set SCF calculations. The related force constants up to the fourth order are obtained via the analytical fit to a polynomial expansion. The vibrational problem is solved by means of a variational treatment which includes the effects of mechanical anharmonicity. The side bands of the stretching fundamental vFX are described in terms of the vFX ±nvFX⋅⋅⋅O combination bands in excellent agreement with experiment.
Hamiltonian theory for vibrational dephasing rates of small molecules in liquids
Authors: Levine Alfred M. , Shapiro Moshe , Pollak Eli
Published: July 2002

A technique is developed for solving the generalized Langevin equation (GLE) describing anharmonic oscillators in the weak coupling limit. The GLE is rewritten as a Hamiltonian with a nonlinear system coupled to an infinite bath of harmonic oscillators. A normal mode transformation followed by a perturbation technique is used to obtain the fluctuating system frequency. When the method is applied to a single oscillator with cubic anharmonicity, both the classical and quantal dephasing rates are shown to be equal to the well-known result of Oxtoby. The technique is also applied to a system with more than one vibrational degree of freedom (linear triatomic molecules) to obtain the dephasing rates for the symmetric and antisymmetric normal modes. The effects of system anharmonicity on frequency shifts are investigated.
The treatment of exchange in path integral simulations via an approximate pseudopotential
Authors: Hall Randall W.
Published: July 2002

An approximate form that includes the effects of exchange is suggested for the short time propagator used in path integral simulations. The form is inspired physically by the approximation made in Hartree–Fock treatments of atoms and molecules. The approximate propagator is used with quantitative accuracy in two systems: an ideal gas of fermions localized in a three-dimensional harmonic well and the triplet state of the sodium dimer.
Photochemistry of ClNCO
Authors: Gilbert J. V. , Coombe R. D.
Published: July 2002

The gas phase chemistry that occurs after the photolysis of ClNCO at 248 nm is characterized by the action of two excited intermediates, N(2 D) and N2 (A). Each of these reacts with ClNCO to generate NCO(A 2 Σ+ ), and blue emission from the NCO(A 2 Σ+ →X 2 Π) transition is observed. The time profile of the NCO emission consists of two distinct components, a fast component arising from the N(2D)+ClNCO reaction and a slow component arising from the N2 (A)+ClNCO reaction. The N(2 D) is an initial photofragment and the N2(A) is generated in the N(2 D)+ClNCO reaction. The value of the rate constant for N(2D)+ClNCO is determined to be 7.0±0.8×10−11 cm3 s−1. The rate constant measured from the decay of the NCO emission in a previous work (2.1±0.2×10−11 cm3 s−1) is assigned to the N2(A)+ClNCO reaction.
One- and two-color multiphoton ionization of argon
Authors: Li Leping , Wu Ming , Johnson Philip M.
Published: July 2002

A systematic explanation of the pressure-dependent multiphoton ionization spectra of argon is given in terms of the phase matching requirements of third-harmonic generation. Two-color sum-frequency generation in a highly dispersive medium and its effects on normal three-photon excitation process are analyzed, using argon as an example. Finally an anomaly in the resonance line shapes of the two laser MPI spectra of argon is reported and attributed to the displacement of the foci of the two counterpropagating laser beams.
An empirical determination of the dipole moment function of OH(X 2Π)
Authors: Turnbull D. N. , Lowe R. P.
Published: July 2002

An electric dipole moment function (EDMF) for OH has been derived by combining airglow and laboratory intensity measurements with recent high precision measurements of the dipole moments for v=0,1, and 2. This empirical EDMF shows remarkable agreement with the a priori MCSCF(7)-SCEP EDMF. However, it differs sufficiently to produce transition probabilities which are in much better agreement with observations than previously available sets.
Analysis of the heat capacity results of deutero-ammonium and ammonium solids
Authors: Smith David
Published: July 2002

The heat capacities of NH+4 and ND+4 compounds have been analyzed by subtracting the heat capacity of the NH+4 compound from that of the ND+4 compound. Theoretical models predict one maximum in this difference curve, which is due to the difference in the contribution to the heat capacity of harmonic oscillators with different librational frequencies. Heat capacity data is available so that the following compounds may be studied: NH4Cl, NH4Br, NH4IO4, NH4ReO4, NH4BF4, (NH4)2SO4, and α-NH4HgCl3. The difference curves of three of the compounds NH4BF4, NH4IO4, and NH4ReO4 do not follow the theoretical models. From the difference curves it is possible to get accurate values of the librational frequencies of both NH+4 and ND+4 in their compounds.
Electrochromism of an aggregating thiapyrylium dye
Authors: Marchetti A. P. , Scozzafava M. , Young R. H.
Published: July 2002

Electrochromism (electric-field-modulated absorption) spectra are reported for a thiapyrylium dye in a polycarbonate host. When the dye is homogeneously dissolved in the polymer, excitation to the first excited singlet state changes its dipole moment by approximately 6 D (2×10−29 C m). When the dye is aggregated with the host polymer, the structure of the aggregate is believed to consist of layer of the dye associated with layers of the polymer. The electrochromism spectrum indicates that the aggregate possesses two (or more) excitonic states separated by ∼450 cm−1, of which only the lower contributes to the absorption spectrum in the absence of an electric field; application of a field mixes and shifts these states and transfers intensity between them. The near degeneracy is believed to be a consequence of weak Davydov interactions between the exciton states of pairs (or larger stacks) of dye layers.
The electronic spectra and structure of complexes between s-tetrazine and acetylene
Authors: Morter Cheryl L. , Koskelo Aaron , Wu Yenchune R. , Levy Donald H.
Published: July 2002

Complexes between s-tetrazine and acetylene have been prepared in a supersonic free jet and their fluorescence excitation spectra have been observed. Analysis of the rotational structure in the electronic spectra of the complexes was used to determine the geometry of the complexes. Four bands due to complexes were observed at shifts of +15, +80, +201, and +208 cm−1 with respect to the origin of uncomplexed tetrazine at 18 128 cm−1. The bands at +15 and +80 cm−1 were assigned to two different isomers of a complex containing one molecule of tetrazine and one molecule of acetylene. The +15 band was due to a planar structure where the proton of acetylene forms a hydrogen bond with the nitrogen lone pair of tetrazine and the π cloud of the acetylene interacts with the hydrogen atom of the tetrazine ring. The +80 cm−1 band is due to a stacked complex with the acetylene molecule above and parallel to the plane of the tetrazine. In this isomer, the axis of the acetylene bisects the N–N bonds of the tetrazine. The band at 208 cm−1 is due to a complex consisting of two acetylene molecules and one tetrazine molecule with one acetylene above and parallel to the tetrazine ring and the other at the side of but perpendicular to the acetylene ring. The band at +201 cm−1 is assigned to a larger complex containing more than two acetylene molecules, but further analysis of this band was not possible.