bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron spin resonance matrix isolation studies of Al2716,17O, Ga69,7116,17O and In11516,17O: Observed hyperfine interactions compared with ab initio theoretical results
Authors: Knight Lon B. , Kirk Thomas J. , Herlong John , Kaup John G. , Davidson E. R.
Published: July 2002

Electron spin resonance (ESR) studies are reported for Al16,17O, Ga16,17O, and In16,17O isolated in neon matrices at 4 K. Except for Al16O, no previous ESR measurements have been reported for these X 2Σ diatomic radicals. The pulsed laser vaporization of the metals in the presence of O162 and O172 produced high quality ESR spectra of these metal oxide radicals whose nuclear hyperfine interactions (A tensors) were fully resolved for both the metal and oxygen nuclei. An analysis of the experimental spin densities in combination with different types of theoretical calculations provided detailed information concerning the electronic structure trends going down this metal oxide group. Increased p-orbital spin density on oxygen was observed for the heavier metal oxide radicals. Nonrelativistic ab initio calculations with an extended basis set and the UB3LYP method reproduced the trends in the isotropic and dipolar hyperfine interactions. All-electron CI calculations, restricted open-shell Hartree–Fock (ROHF) wave functions, and unrestricted Hartree–Fock wave functions gave results very different from experiment and from each other for the isotropic interaction. All calculations were in fair agreement with each other for the dipolar interaction and provided an assignment of the sign for that term.
Surface free energy of electrolytes near curved interfaces
Authors: Groenewold Jan
Published: July 2002

The excess free energy per unit area of an uncharged curved interface separating a phase with a low dielectric constant from a solution containing electrolyte is considered. The contribution due to the electrolyte to the excess free energy per unit area is expanded in the curvature, and the coefficient of the linear term or spontaneous curvature is calculated. Its sign implies a tendency for an interface to curve toward the nonaqueous phase. The zeroth order term in this expansion is an earlier result obtained by Onsager and Samaras. The expansion parameter is the curvature times the Debye screening length. The opposite regime of large curvature will also be considered. To obtain these results the Gibbs adsorption equation has been used.
Resonant two-photon ionization and fluorescence excitation studies of o, m-difluorobenzene⋯Ar. Spectral shifts and intermolecular vibrations
Authors: Lu Wenyun , Hu Yihua , Yang Shihe
Published: July 2002

Resonant two-photon ionization (R2PI) spectra of the van der Waals (vdW) complexes 1,2-difluorobenzene⋯Ar (o-DFB⋯Ar) and 1,3-difluorobenzene⋯Ar (m-DFB⋯Ar) through the S1←S0 transition revealed a number of bands near the origin 000. All these bands are assigned to the low-frequency intermolecular vibrations. Quantum calculations using the method of linear-combination of three-dimensional harmonic oscillator products (LCHOP) were carried out to derive the intermolecular vibrational energy levels. Detailed assignments of the intermolecular vibrational modes are accomplished with the help of the quantum calculations and the comparison with the available data on related systems, e.g., C6H6⋯Ar and p-DFB⋯Ar. Fluorescence excitation method was used to derive the spectral shifts of intramolecular bands (S1←S0) of o, m-DFB upon complexation with Ar.
The effect of chain stiffness on the phase behaviour of isolated homopolymers
Authors: Doye Jonathan P. K. , Sear Richard P. , Frenkel Daan
Published: July 2002

We have studied the thermodynamics of isolated homopolymer chains of varying stiffness using a lattice model. A complex phase behaviour is found; phases include chain-folded “crystalline” structures, the disordered globule and the coil. It is found, in agreement with recent theoretical calculations, that the temperature at which the solid-globule transition occurs increases with chain stiffness, whilst the θ-point has only a weak dependence on stiffness. Therefore, for sufficiently stiff chains there is no globular phase and the polymer passes directly from the solid to the coil. This effect is analogous to the disappearance of the liquid phase observed for simple atomic systems as the range of the potential is decreased.
The coupled channel density matrix method for open quantum systems: Formulation and application to the vibrational relaxation of molecules scattering from nonrigid surfaces
Authors: Pesce Lorenzo , Saalfrank Peter
Published: July 2002

The recently proposed coupled channel density matrix (CCDM) method for nondissipative dynamics [L. Pesce and P. Saalfrank, Chem. Phys. 219, 43 (1997)], is extended to open quantum systems. This method, which is the density matrix analogue of the coupled channel wave packet (CCWP) method in Schrödinger wave mechanics, allows for the solution of nuclear Liouville–von Neumann equations in more than one dimension including unbound modes. A semiphenomenological, Markovian, and trace-conserving dissipative model within the dynamical semigroup approach is suggested, and efficient numerical schemes for its implementation are presented. Using a two-mode model, we apply the dissipative CCDM method to the problem of vibrationally excited gas-phase hydrogen molecules, relaxing during the scattering from a cold, metallic, and nondissociative surface. The significance of a relaxation mechanism based on electron-hole pair creation in a metallic substrate is addressed. The dependence of the survival probability of the vibrationally excited molecules on the dissipative model parameters, on their initial translational energy, and on isotopic substitution is examined and rationalized on the basis of a simple classical kinetic model.
Activated chemistry in the presence of a strongly symmetrically coupled vibration
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

In the gas phase, tunneling reaction rates can be significantly enhanced if the reaction coordinate is symmetrically coupled to a harmonic oscillation, as has been emphasized by Benderskii and co-workers [Adv. Chem. Phys. 88, 1 (1994)]. This is due to the fact that the symmetric coupling leads to modulation of the barrier height. Similar effects have been observed in reactions in model condensed phase studies, as in the Hamiltonians that have been studied by Borgis and Hynes [J. Chem. Phys. 94, 3619 (1991)] and Suarez and Silbey [J. Chem. Phys. 94, 4809 (1991)]. All of these works assume that tunneling proceeds from the ground state. In this paper, we use the exponential resummation technique that we used in our recent work on the quantum Kramers problem, to study the case when there can be excitations to higher states and activated transmission over a barrier. We present a general methodology to exactly include direct coupling between the reaction coordinate and the symmetrically coupled promoting vibration and find that the rate of reactions in condensed phases is enhanced as in the case of tunneling from the ground state. This effect, however, is strongly modulated by loss of coherence induced by the condensed phase environment.
Contribution of capillary waves to the interfacial correlation function
Authors: Stecki J.
Published: July 2002

New contributions of capillary waves to the density–density correlation function H(z1,z2,k⊥) are calculated in the Gaussian approximation and compared with those found earlier “experimentally” in a MD simulation. The k⊥ dependence of the correlation function or of the scattering factor is examined and compared with the dominant 1/k⊥2 form. General equations valid to all orders in the gradients of the density profile are given and the k⊥ dependence of the structure factor (in the Born approximation) is found. The latter may be different from the 1/k⊥2 form. Regularization of the integrals is examined with two procedures applied.
Towards an accurate gold carbonyl binding energy in AuCO+: Basis set convergence and a comparison between density functional and conventional methods
Authors: Dargel Thomas K. , Hertwig Roland H. , Koch Wolfram , Horn Hans
Published: July 2002

The binding energy of the ground-state AuCO+ molecule has been systematically investigated using quantum chemical methods such as various density functionals and correlated wave function based approaches like second order Mo/ller–Plesset perturbation and the coupled cluster ansatz with perturbative treatment of triple excitations. These were combined with a total of 14 relativistic effective core potential/valence and all-electron basis sets of increasing flexibility for gold and CO, respectively. Special emphasis is paid to the role of the basis set superposition error and the relevance of different ways to improve the one particle basis sets. A significant effect on the basis set superposition error is observed in the density functional schemes upon enlarging the valence basis sets.
Competitive events in fifth order time resolved coherent Raman scattering: Direct versus sequential processes
Authors: Ulness Darin J. , Kirkwood Jason C. , Albrecht A. C.
Published: July 2002

Higher order time resolved nonlinear optical processes can often be obscured by sequential lower order processes that compete with the direct event and give similar time domain behavior though they probe different dynamics. This is true for a certain fifth order coherent anti-Stokes Raman scattering (CARS) spectroscopy designed to probe overtone vibrational dynamics. The homodyned intensity from the two competing processes is calculated and it is shown how only the direct fifth order polarization probes overtone dephasing.
Effect of composition fluctuations on tracer diffusion in symmetric diblock copolymers
Authors: Kannan Rangaramanujam M. , Su Jie , Lodge Timothy P.
Published: July 2002

We report forced Rayleigh scattering measurements of the tracer diffusion of poly(ethylenepropylene) (PEP) homopolymers in nearly symmetric poly(ethylenepropylene)-b-poly(ethylethylene) (PEP-PEE) block copolymer matrices, both above and below the order–disorder transition. Measurements of PEP-PEE copolymer diffusion in the same matrices have been reported previously. The results indicate that fluctuations suppress diffusion of both homopolymer and copolymer tracers, as predicted by recent theories. However, in the quenched ordered state, diffusion suppression is considerably stronger for block copolymer tracers than for homopolymer tracers, which appears to be counter to theoretical expectation. Varying the tracer molecular weight has little effect on the suppression of homopolymer diffusion, whereas it has a substantial effect on copolymer tracer diffusion. These differences are attributed to differential chain localization or confinement: an entangled copolymer tracer tends to be pinned near the interface between microdomains, and has to pay a significant enthalpic penalty for diffusion in any direction, as the reptating chain has to drag one block through the other microdomain. On the other hand, although a homopolymer tracer tends to be located within one microdomain, it can presumably diffuse across several grains without penalty, by reptating predominantly within a single lamella.
Primary and secondary processes in the 193 nm photodissociation of vinyl chloride
Authors: Blank David A. , Sun Weizhong , Suits Arthur G. , Lee Yuan T. , North Simon W. , Hall Gregory E.
Published: July 2002

We have investigated the photodissociation of vinyl chloride (H2CCHCl) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used vacuum ultraviolet synchrotron radiation for product photoionization. We have observed five primary dissociation channels following an initial π*←π excitation. The majority of Cl atoms originate from an excited-state dissociation. The remaining dissociation channels are consistent with competition on the ground electronic state following internal conversion from the optically prepared state. These channels include atomic and molecular hydrogen elimination, HCl elimination, and a translationally slow Cl elimination channel. We have also identified and characterized two secondary decomposition channels: (1) the elimination of Cl from chlorovinyl radicals following the primary atomic hydrogen elimination channel, and (2) hydrogen atom elimination from vinyl radicals following the primary atomic Cl elimination. By measuring the truncation in the translational energy distribution for C2H2Cl products from primary atomic hydrogen elimination we deduce a barrier for the reverse reaction of Cl+acetylene of 11±2 kcal/mol. Since Cl is known to add rapidly to acetylene with no activation barrier, we conclude that H loss primarily forms the ClCCH2 isomer, and that the observed 11 kcal/mol barrier pertains to a concerted addition/rearrangement path to form the α-chlorovinyl radical. Finally, we report low-resolution photoionization spectra for the nascent vinyl radical and HCl photoproducts, in which redshifts in the ionization onsets can be related to the internal energy content.
Sensitive detection of nitric oxide using seeded parametric four-wave mixing
Authors: Fernée Mark J. , Barker Peter F. , Knight Alan E. W. , Rubinsztein-Dunlop Halina
Published: July 2002

A sensitive near-resonant four-wave mixing technique based on two-photon parametric four-wave mixing has been developed. Seeded parametric four-wave mixing requires only a single laser as an additional phase matched “seeder” field is generated via parametric four-wave mixing of the pump beam in a high gain cell. The seeder field travels collinearly with the pump beam providing efficient nondegenerate four-wave mixing in a second medium. This simple arrangement facilitates the detection of complex molecular spectra by simply scanning the pump laser. Seeded parametric four-wave mixing is demonstrated in both a low pressure cell and an air/acetylene flame with detection of the two-photon C 2Π(v′=0)←X 2Π(v″=0) spectrum of nitric oxide. From the cell data a detection limit of 1012 molecules/cm3 is established. A theoretical model of seeded parametric four-wave mixing is developed from existing parametric four-wave mixing theory. The addition of the seeder field significantly modifies the parametric four-wave mixing behaviour such that in the small signal regime, the signal intensity can readily be made to scale as the cube of the laser pump power while the density dependence follows a more familiar square law dependence. In general, we find excellent agreement between theory and experiment. Limitations to the process result from an ac Stark shift of the two-photon resonance in the high pressure seeder cell caused by the generation of a strong seeder field, as well as a reduction in phase matching efficiency due to the presence of certain buffer species. Various optimizations are suggested which should overcome these limitations, providing even greater detection sensitivity.
Relationships between bridging oxygen O17 quadrupolar coupling parameters and structure in alkali silicates
Authors: Vermillion Karl E. , Florian Pierre , Grandinetti Philip J.
Published: July 2002

We have performed ab initio calculations on the model cluster (OH)3Si–O–Si(OH)3 with lithium or sodium cations coordinated to the central oxygen in order to refine the relationships between O17 quadrupolar coupling parameters and the local structure around the bridging oxygen in the case of alkali silicates. We have also used a point charge model to derive approximate expressions to describe the dependence of the O17 electric field gradient tensor on the orientation of the alkali cation-bridging oxygen internuclear vector(s). From these calculations we predict that the previously established trend in O17 quadrupolar coupling constant, Cq, with Si–O–Si angle is systematically shifted to lower magnitudes with increasing number and field strength of coordinating alkali cations and that Cq will be relatively insensitive to variations in the alkali cation-bridging oxygen internuclear vector orientation. The previously established trend in O17 quadrupolar coupling asymmetry parameter, ηq, with Si–O–Si angle is systematically shifted to higher values by the presence of one coordinating alkali cation, and only slightly shifted to higher values by the presence of two coordinating alkali cations. As with the quadrupolar coupling constant, the magnitude of the shift in asymmetry parameter increases with increasing field strength of the coordinating alkali cation(s). In contrast to Cq, ηq is additionally dependent on variations in the alkali cation-bridging oxygen internuclear vector orientation. With two or more coordinating alkali cations, however, the magnitude of these variations are reduced, and for certain alkali cation configurations are eliminated. Finally, the parametric dependence of Cq on ηq via the Si–O–Si angle can be used to distinguish between bridging oxygen environments with differing number of coordinating alkali cations.
Kinetic theory of bimolecular reactions in liquid. II. Reversible reaction A+B⇄C+B
Authors: Yang Mino , Lee Sangyoub , Shin Kook Joe
Published: July 2002

A theoretical formalism based on the fully renormalized kinetic theory is applied to a diffusion-influenced pseudo-first order reaction kinetics of reversible bimolecular reaction A+B⇄C+B including unimolecular decay processes. Linear response of the system, initially at equilibrium, to a thermal perturbation is examined and a rate kernel equation for the reactant concentrations is derived. The rate kernel has a hierarchical structure and the propagator appeared in the kernel expression is truncated by a disconnected approximation. When the unimolecular reactions are turned off, the response of the system is found to be the effective irreversible survival probability. In this way, many-body complications inherent to the history of reactive pair creation are properly implemented in the description of the reversible kinetics. We compare the present theory with the other existing theories such as the rate equation, the superposition approximation, and the convolution approaches. In some limiting cases, results obtained from the present theory can be reduced to those from the existing theories. For the present reaction scheme, we found that the description of the above many-body complications in the present theory lead to the equivalent result as in the rate equation approach.
Comment on “Normal, single-file, and dual-mode diffusion of binary adsorbate mixtures in AlPO4-5” [J. Chem. Phys. 107, 4384 (1997)]
Authors: Hahn K. , Kärger J.
Published: July 2002

Some aspects of “dual-mode diffusion” as introduced by Sholl and Fichthorn are commented. The concentration dependence of the larger molecules in dual-mode diffusion coincide with that of single-file diffusion. Generally, the experimentally observed mobility factors are significantly larger than the simulated ones.
The intermolecular potential between an inert gas and a halogen: Prediction and observation of transitions between the linear and T-shaped isomers of HeClF
Authors: Higgins Kelly , Tao Fu-Ming , Klemperer William
Published: July 2002

The intermolecular potential surface of He and ClF is calculated with a large basis at the fourth-order Mo/ller–Plesset level. The rotation–vibration levels calculated from the intermolecular potential surface serve as an excellent guide for finding the experimental spectra. Pure rotational transitions are observed for the lowest linear Σ0 state and for an excited T-shaped K=0 Σ1 state of He35ClF and He37ClF. Direct transitions between the linear ground state and the T-shaped state are observed for He35ClF. The observed energy difference between the J=0 level of the linear state and the J=0 level of the T-shaped state is 2.320 cm−1. In addition, transitions into the two J=1 levels and one J=2 level of the K=1 T-shaped state, Π1, are observed for He35ClF. The He–ClF complex is highly nonrigid, undergoing large amplitude oscillation in both angular and radial coordinates. The effect of zero-point oscillation is seen in the large difference, 22.9 cm−1, between the calculated potential energy minima of −58.1 (linear) and −35.2 cm−1 (T-shaped) and the measured value (including zero-point energy) of 2.320 cm−1. The potential surface is poorly represented as a sum of spherical atom–atom interactions. At both minima the He–Cl distance is shorter than the sum of van der Waals radii. The ab initio potential is too shallow since an appreciably better fit of the spectral transitions is obtained by uniformly increasing the magnitude of the interaction potential by 10%. Bound states calculated for a potential with the T-shaped minimum removed show significant differences from experiment, indicating that the T-shaped minimum does indeed exist. Spectroscopic constants for He35ClF are obtained in a fit to experimental data. For the ground state, Σ0, B=5586.8312(34), D=1.6595(10) MHz, H=36.472(93) kHz, μa=0.8780(14) D, and eqeff Q(J=1)=−133.659(18) MHz. For the T-shaped state, Σ1, ν=69 565.023(35), B=7056.161(17), D=6.9523(24) MHz, μa=0.620(12) D, and eqeff Q(J=1)=−39.936(92) MHz. For the T-shaped Π state, Π1, ν=100 302.239(46), B=7430.338(32), ql=1380.622(46) MHz, μa=0.5621(99) D, and eqeff Q(Π1−J=1)=−45.15(87) MHz. The large change in geometry between the Σ0 and Σ1 states is evidenced by the difference in rotational constants, dipole moments, and quadrupole coupling constants for each state. In addition, these values are consistent with a T-shaped Σ1 state rather than an antilinear Σ1 state.
Formation of XeBr*, Xe*, and Br* by the Xe+(2P1/2)/Br−/He and Xe+(2P3/2)/Br−/He three-body ionic-recombination reactions in a helium flowing afterglow
Authors: Tsuji Masaharu , Ide Masahiro , Oda Erika , Nishimura Yukio
Published: July 2002

Three-body ionic-recombination reactions of Xe+/Br−/He leading to XeBr* excimers and their predissociation products (Xe* and Br*) have been studied by observing emission spectra in a He flowing afterglow. The branching ratios of XeBr(B):XeBr(D):Xe(6s 3P1,6p 1D2):Br(5p) in the Xe+(2P1/2)/Br−/He reaction are 0.65:0.077:0.087:0.19, while those of XeBr(B):XeBr(C):XeBr(D):Xe(6s 3P1):Br(5s) in the Xe+(2P3/2)/Br−/He reaction are 0.87:0.12:0.0040:0.0014:0.0067, respectively. The large relative formation rate of XeBr(B) in the Xe+(2P1/2)/Br−/He reaction indicated that a breakdown of the ion configuration of Xe+ occurs significantly in the Xe+(2P1/2) reaction. The XeBr(B) state is probably formed through multiple collisions with third-body He atoms, which cause a trajectory change from the entrance V[Xe+(2P1/2),Br−] potential to exit V[Xe+(2P3/2),Br−] ionic potentials via predissociation V(Xe*,Br) potentials. The large relative formation rates of XeBr(B,C) in the Xe+(2P3/2)/Br−/He reaction indicated that the ion configuration of Xe+ is conserved well in the Xe+(2P3/2) reaction. The electronic state distribution of Br(5p) in the Xe+(2P1/2)/Br−/He reaction is nonstatistical, providing any preference for doublet, quartet, or for particular J states. It was concluded that most of Xe* and Br* are formed via predissociation of vibrationally excited XeBr* at long internuclear distances above 20 Å.
Electronic excitation of gaseous pyrrole and pyrazole by inner-shell electron energy loss spectroscopy
Authors: Duflot D. , Hannay C. , Flament J.-P. , Hubin-Franskin M.-J.
Published: July 2002

The spectroscopy of the unoccupied molecular orbitals of gaseous pyrrole and pyrazole has been studied by electron impact under electric dipole scattering conditions (2 KeV, small angle) and the inner-shell electron energy loss method. The core-excitation spectra have been recorded at the C1s and N1s edges with 0.2 eV resolution, resolution allowing to observe separately the transitions from the carbon atoms with different chemical environment. Ab initio calculations, using the equivalent core model, were performed to help in the interpretation of the spectral features. The spectra are dominated by transitions to the π* and σ* molecular orbitals. The calculations confirm that the 3b1(π*) orbital is the lowest-energy unoccupied one in pyrrole. In several cases, the intensity of the Rydberg excitations is enhanced by an important valence σ* character of the Rydberg orbitals.
Effect of solid-state interactions on the vibrational hyperpolarizability of all-trans polyacetylene chains from ab initio Hartree–Fock calculations
Authors: Champagne Benoit , Kirtman Bernard
Published: July 2002

Coupled perturbed Hartree–Fock/6-31G finite oligomer cluster calculations are carried out, which show that solid-state interactions lead to a large decrease in the vibrational hyperpolarizability of all-trans polyacetylene. The vibrational property behaves similarly to the corresponding electronic property with increasing cluster size. Low-frequency collective and interchain modes are very significant for small oligomers, but the high-frequency vibrations that are most important for a single chain become dominant as the chains are lengthened. Since the solid-state interactions do not modify the geometry of the individual chains the decrease of the vibrational and electronic hyperpolarizabilities in polyacetylene is due to direct electronic effects.
Solving the Poisson–Boltzmann equation to obtain interaction energies between confined, like-charged cylinders
Authors: Ospeck Mark , Fraden Seth
Published: July 2002

We numerically solve the nonlinear Poisson–Boltzmann equation for two cylinders confined by two parallel charged plates. The repulsive electrical double layer component of the cylinder pair potential is substantially reduced by confinement between like-charged plates. While the effective cylinder surface charge is increased by the confinement, the effective interaction screening length is reduced, this effect being dominant so that the repulsive confined cylinder–cylinder interaction potential is reduced.
On the mechanism of antiphase electron spin resonance spectra of radical pairs
Authors: Neufeld A. A. , Pedersen J. Boiden
Published: July 2002

We have investigated the origin of the antiphase structure of the mw-induced ESR spectrum of a radical pair in a micelle. By applying an accurate numerical method we have examined various specially designed cases that clearly distinguish between the proposed mechanisms for the production of antiphase structure. It is demonstrated that neither of the previously proposed mechanisms can explain the observations. We argue that the only possible mechanism is a coherent, reencounter type mechanism involving two consecutive steps similar to the well established S–T0 mechanism.
Storage ring measurements of the dissociative recombination and excitation of the cyanogen ion CN+(X 1Σ+ and a 3Π, ν=0)
Authors: Le Padellec A. , Mitchell J. B. A. , Al-Khalili A. , Danared H. , Källberg A. , Larson Å. , Rosén S. , af Ugglas M. , Vikor L. , Larsson M.
Published: July 2002

Absolute cross sections and rate coefficients were measured for the dissociative recombination of CN+(X 1Σ+ and a 3Π, ν=0) with electrons using the heavy ion storage-ring CRYRING at Stockholm University. The a 3Π state that lies only 0.08 eV above the X 1Σ+ ion ground state is most certainly populated in the ion beam injected into CRYRING, as this is a long-lived excited state. Over the 1 meV–3 eV energy range that was investigated, two energy dependencies were found for the cross sections, namely E−1.05 below 0.1 eV and E−1.41 above 0.4 eV. The energy domain 0.1–0.3 eV exhibits a resonant structure. Branching fractions over the final atomic product states were determined using a position sensitive imaging detector. Thermal rate coefficients are deduced from the measurements and the efficiency displayed by the dissociative recombination mechanism is typical of that of a diatomic ion (3.4×10−7 cm3 s−1 at room temperature). Cross sections for the dissociative excitation process are also reported and their magnitude at the plateau are unusually large. This may be explained by the number of potential curves which can be populated at moderate collision energy (<10 eV) for the CN+ ion.
Nonlinear optical response functions for a chromophore with linear and quadratic electron–vibration coupling
Authors: Toutounji Mohamad , Small Gerald J. , Mukamel Shaul
Published: July 2002

Two models for the third-order response function of a two-electronic level chromophore are investigated. The first assumes an excited state vibrational Hamiltonian whose phonon modes exhibit both linear and diagonal quadratic electron–phonon coupling. Impulsive stimulated photon echoes are calculated for this model. The second assumes linear electron–phonon coupling including vibrational relaxation and pure electronic dephasing. Impulsive two-pulse photon echo signals and their dependence on temperature and electron–phonon coupling strength are calculated for this model. The initial fast nonexponential (free-induction) decay due to all multiphonon transitions, quantum beats and the slow decay component due to the zero-phonon line (pure electronic dephasing) are identified and correlated with features of the single-site absorption spectrum whose relationship to the hole burned spectrum is well understood. Pure electronic dephasing associated with the zero-phonon line contributes to the decay of the quantum beats. This contribution may be non-negligible at high temperatures in certain systems. An application is made to the special pair absorption band of the bacterial reaction center.
Hysteresis in entropy production in a model chemical reaction exhibiting subcritical Hopf bifurcation
Authors: Dutt A. K.
Published: July 2002

In the reversible Oregonator, a model exhibiting oscillations in the Belousov–Zhabotinskii reaction, a bistability in the entropy production (free energy dissipation) between a steady state and an oscillatory state is reported in the neighborhood of a subcritical Hopf bifurcation. We identified this region in the past by a perturbation method. The numerical results present a thermodynamic formulation of multistable structures in a nonlinear dissipative system.
Luminescence spectroscopy of matrix-isolated atomic cadmium
Authors: Healy Brendan , McCaffrey John G.
Published: July 2002

The luminescence spectroscopy of atomic cadmium isolated in the solid rare gases is recorded using pulsed synchrotron radiation excitation of the 5p 1P1←5s 1S0 resonance transition. Steady-state and time-resolved analysis of the ultraviolet (UV) emission bands recorded in the Cd/Ne, Cd/Ar, and Cd/Kr systems allows identification of the associated 227.3, 233.4, and the 241/262 nm bands to the singlet fluorescence of atomic cadmium. The origin of the pair of singlet emission bands at 241 and 262 nm in the Cd/Kr system is ascribed to the coexistence of two nondegenerate minima on the 5p 1T1u surface. The weak band present in the Cd/Kr system at 326 nm and the intense pair at 324.4 and 329.6 nm in Cd/Xe all have decay times longer than 1 μs and are associated with the triplet transitions of atomic cadmium. Line shape analysis of the near-UV emission pair in Cd/Xe allows a tentative assignment of the narrow 329.6 nm band to the 5p 3P0→5s 1S0 transition. The intensity of the triplet state emission was observed to be enhanced in the heavier rare gases, being completely absent in Ne and Ar, weak in Kr, and the only emission observed in Xe. The efficiency of intersystem crossing in the Cd/RG systems is very similar to that exhibited by the Zn/RG matrix systems. However, the presence of the 5p 3P0→5s 1S0 emission in Cd/Xe resembles the Hg/RG matrix systems, a reflection of the larger spin-orbit splitting in atomic cadmium compared with zinc.
Infrared spectrum of matrix isolated ClSiO and ab initio calculations
Authors: Junker Markus , Schnöckel Hansgeorg
Published: July 2002

Cl atoms and SiO molecules are trapped in a solid Ar matrix at 16 K. The formation of the as yet unknown ClSiO molecule has been followed via its IR spectrum. The stretching frequencies of the isotopic isomer 35Cl28Si16O are observed at 1160.9 and 509.4 cm−1. Experiments with the 18O isotopic isomers were performed in order to confirm the assignment of the absorptions and to characterize the force field. With the help of quantum chemical calculations (DFT) the optimized ClSiO bond angle is obtained at 125.2°. The computed bond lengths are determined to be 153.6 pm for d(Si–O) and 207.8 pm for d(Si–Cl). The SiCl bond is weak in comparison with that of Cl2SiO (203.4 pm) which is in line with a decrease in the corresponding Si–Cl force constant. The calculation of its thermodynamic data, ΔfH0(298)=−167.2 kJ/mol; ΔfS0(298)=+279.1 J/(mol⋅K), is of high importance for high temperature gas phase reactions of industrial processes, e.g., the combustion of SiCl4 by O2.
Contraction of the model for the Bray–Liebhafsky oscillatory reaction by eliminating intermediate I2O
Authors: Čupić Željko , Kolar-Anić Ljiljana
Published: July 2002

The existing model for the Bray–Liebhafsky reaction aimed at simulating the largest possible number of various self-organization phenomena observed experimentally, is reduced herewith with the purpose of removing the intermediate I2O preserving the main characteristics of the parent model. The stoichiometric network analysis is applied.
Pressure dependence of spin–orbit levels of Co2+ in KZnF3
Authors: Freire P. T. C. , Lemos V. , Pilla O. , Vieira N. D.
Published: July 2002

Low temperature emission measurements were performed at atmospheric and high pressures in KZnF3:Co2+. Comparing the zero-phonon line positions and using the phonon dispersion relation of the host material, the fine structure in the emission spectrum was identified. Pressure coefficients were obtained for several among the emission lines from which the values for zero-phonon lines were deduced. Analysis shows that the spin–orbit parameter changes with pressure are negligible in this material.
Clustering and continuum percolation of hard spheres near a hard wall: Monte Carlo simulation and connectedness theory
Authors: Pugnaloni Luis A. , Vericat Fernando
Published: July 2002

The effect of a hard wall on the clustering and continuum percolation of a hard spheres fluid is studied using Monte Carlo simulations and connectedness theory. We calculate an averaged pair-connectedness function ρ†(r;z) which is the probability density of finding two particles in the same cluster and separate by a distance r under the assumption that one of them is fixed at a distance z from the wall. We also obtain the mean size S for the cluster containing the fixed sphere and the critical percolation density ρc at which it becomes macroscopically large. Monte Carlo results allow us to conclude that, for given number density and connectedness distance, the wall causes the decrease of S and the increase of ρc in comparison with those found for the bulk in the absence of the wall. Both effects diminish with increasing z. The simulation data also show that, in the presence of the wall, the clusters are eccentric with cylindrical symmetry, slightly flattened in the region of contact with the wall. The theoretical calculations involve the solution for ρ†(r;z) of an integral equation. It is derived from the one proposed some time ago by Giaquinta and Parrinello to obtain the average of the ordinary pair correlation function in the presence of the hard wall [J. Chem. Phys. 78, 1946 (1983)]. Integrating the pair-connectedness function over r we have S whose divergence determines the theoretical critical density. The results so obtained are in satisfactory agreement with Monte Carlo data.
High resolution electronic spectroscopy and an empirical potential energy surface for Ne⋅SH/D
Authors: Carter Christopher C. , Miller Terry A. , Lee Hee-Seung , McCoy Anne B. , Hayes Edward F.
Published: July 2002

The high resolution, laser-induced fluorescence Ã 2Σ+↔X̃ 2Π spectra of the Ne⋅SH van der Waals complex and its deuterated analog are reported. The rotational analyses provide information about the fine, hyperfine, spin–rotation, and parity interactions in these two complexes. These results are used in conjunction with measured vibrational intervals to develop an empirical potential energy surface for Ne⋅SH/D.
Solvation of cyclopentadienyl and substituted cyclopentadienyl radicals in small clusters. II. Cyanocyclopentadienyl with polar solvents
Authors: Yao J. , Fernandez J. A. , Bernstein E. R.
Published: July 2002

Clusters of the cyanocyclopentadienyl (CNcpd) radical and several polar solvent molecules (e.g., CF2H2, CF3H, CF3Cl, CH3Cl, ROH, H2O) created in a supersonic jet expansion are studied by laser induced fluorescence and hole burning spectroscopies. Lennard-Jones–Coulomb atom–atom potential energy calculations are employed in combination with ab initio calculations to aid in the interpretation of the observed spectra and to understand the nature of the radical polar solvent solvation behavior. The calculations predict quite reasonable cluster binding energies and structures, but are less accurate in predicting van der Waals vibrational mode energies and cluster spectroscopic shifts. The limitations of the atom–atom potential energy surface model in dealing with the more subtle aspects of CNcpd–polar solvent intermolecular interactions are discussed. Some possible causes of inadequacies of the approach are presented.
Adsorption of PF3 on Cu(001): Ordered overlayer structures and frustrated translational modes
Authors: Braun J. , Bishop G. G. , Ermakov A. V. , Goncharova L. V. , Hinch B. J.
Published: July 2002

The adsorption of PF3 on Cu(001) was studied by means of high resolution helium atom scattering (HAS) and SPALEED. PF3 adsorbs at surface temperatures below 210 K and forms a lattice gas at exposures less than 0.8 L. Saturation is reached for exposures greater than 3 L. At these coverages PF3 forms an ordered c(4×2) layer for surface temperatures above 145 K. For temperatures below 145 K an incommensurate, hexagonal PF3 layer was observed that necessarily excludes uniquely on-top site PF3 molecule to surface coordination. A new vibrational mode in the thermal energy regime corresponding to the frustrated translation parallel to the surface was identified; the energies of excitation were 3.3 meV and 3.5 meV, respectively, for the isolated molecules and the c(4×2) structure. No dispersion of the frustrated translation in the ordered c(4×2) phase was found. The results are discussed in terms of adsorbate–adsorbate and adsorbate–substrate interactions.
Classical trajectory study of argon–ice collision dynamics
Authors: Bolton Kim , Svanberg Marcus , Pettersson Jan B. C.
Published: July 2002

Classical trajectory simulations have been used to study Ar–ice Ih collisional energy transfer, trapping coefficients and scattering distributions for initial Ar kinetic energies between 0.1 and 2.0 eV, incident angles between 0 and 70° and surface temperatures between 0 and 300 K. Collisional energy transfer is extremely efficient due to substantial transfer of energy from the Ar atom to the ice surface over typically 2–4 gas-surface encounters, and the rapid dissipation of this energy away from the collision center, preventing energy transfer back to the Ar atom. This leads to large trapping coefficients over this range of Ar collision energies, incident angles and surface temperatures. Scattered gas atoms lose most of their initial kinetic energy and have broad angular distributions. The large trapping coefficients obtained for the Ar–ice collisions are expected to be found for similar reactions under stratospheric conditions (e.g., HCl–ice, HOCl–ice and ClONO2–ice).
Microcanonical rates for the unimolecular dissociation of the ethyl radical
Authors: Gilbert Thomas , Grebner Thomas L. , Fischer Ingo , Chen Peter
Published: July 2002

We report results on the photodissociation dynamics of the ethyl radical, C2H5. A beam of internally cold radicals is generated by supersonic jet flash pyrolysis of ethyl iodide or n-propylnitrite, respectively. Upon excitation into the A 2A13s Rydberg state ethyl dissociates, presumably on the ground-state surface, into ethylene and a hydrogen atom that is detected by time-resolved photoionization. By varying the excitation energy between 264 and 245 nm, microcanonical rates are obtained as a function of excitation energy. The reaction rates are on the order of 107 s−1, several orders of magnitude slower than expected from simple Rice–Ramsperger–Kassel–Marcus calculations.
Vibrational time recurrences in a model of acetylene C212H2
Authors: van Ede van der Pals P. , Gaspard P.
Published: July 2002

The time-dependent intramolecular vibrational dynamics of acetylene is investigated with the vibrogram method for the effective Hamiltonian obtained by Abbouti Temsamani and Herman [J. Chem. Phys. 102, 6371 (1995)]. The quantum recurrences evidenced by the vibrogram are shown to be in correspondence with emerging periodic orbits, especially, of the bending type. In this model, we find different kinds of bifurcations and, in particular, a birth of bending local modes as well as further bifurcations causing the destabilization of bending periodic orbits. This destabilization leads to a faster damping of the corresponding recurrences and to an increase in the intramolecular vibrational relaxation. A comparison with monodeuterated acetylene is also carried out.
Transient spectral hole burning and hydrogen-bond breaking determined in different solutions of ethanol in deuterated ethanol
Authors: Laenen R. , Rauscher C. , Simeonidis K.
Published: July 2002

We present data on time-resolved infrared spectroscopy of ethanol diluted in deuterated ethanol at two concentrations of 1 and 50 vol % and excitation within the OH-stretching band. Transient spectral holes with widths of 35–55 cm−1 and 25 cm−1, respectively, are determined as well as breaking of hydrogen bonds after vibrational excitation. The lifetime of the OH mode of associated ethanol depends on the strength of the H bond and varies between <1 ps and 1.5±0.5 ps. While at the lower concentration the thermal effect due to the deposited energy is negligible, the higher concentration results in a pronounced heating of the excited molecules. Two different relaxation mechanisms of the excited OH mode of associated ethanol are discussed.
Displacement and distortion of the ammonium ion in rotational transition states in ammonium fluoride and ammonium chloride
Authors: Alavi A. , Lynden-Bell R. M. , Brown R. J. C.
Published: July 2002

Ab initio density functional calculations have been carried out on ammonium fluoride to determine the equilibrium structure and the transition state for rotation of the ammonium ion. The calculated equilibrium geometry agrees satisfactorily with crystallographic data. Optimization of the crystal geometry in the transition state for rotation results in significant distortion and displacement of the ammonium ion within the unit cell. Upon reexamination of the rotational transition states in ammonium chloride, similar distortion and displacement of the ammonium ion are found. The rotation process is similar to a carousel motion, in which the ion moves along the rotation axis and changes shape as it rotates. These results show that rigid ion models for ammonium ion rotational dynamics in crystals are, at best, incomplete.
The probe frequency dependence of nonresonant femtosecond pump–probe nuclear responses: Undercutting vibrational inhomogeneities
Authors: Zhou Y. , Constantine S. , Harrel S. , Ziegler L. D.
Published: July 2002

Expressions are derived for the intensity of nuclear responses appearing in the frequency dispersed optical heterodyne detected (OHD) birefringence and dichroism of nonresonant materials. The dispersed birefringence of chloroform is reported and the detuning dependence of the two intramolecular modes (260 cm−1 and 370 cm−1) are compared with theory. These probe frequency dependent birefringence responses are pumped by a 40 fs pulse and probed with either a 40 fs transform limited Gaussian pulse (FWHH∼400 cm−1) or a one-sided exponential pulse (FWHH∼36 cm−1, Lorentzian). Excellent agreement with theory is found. Due to the CARS and CSRS resonances inherent to these responses, the relative magnitude of different nuclear responses which contribute to the total response of a system can be selectively enhanced in the dispersed pump–probe response when the probe spectrum is narrower than the difference between the relevant mode frequencies. It is shown how this two-dimensional (time and frequency) P(3) technique can be used as a measure of the extent of inhomogeneous broadening contributing to the impulsively excited low frequency intermolecular density of states by the appropriate choice of detection frequencies and pulse shape.
Rotational effects in six-dimensional quantum dynamics for reaction of H2 on Cu(100)
Authors: McCormack Drew A. , Kroes Geert-Jan , Olsen Roar A. , Baerends Evert-Jan , Mowrey Richard C.
Published: July 2002

We present results of six-dimensional (6D) quantum wave-packet calculations for the dissociative adsorption of (ν=0,j=4,mj) H2 on Cu(100). The potential-energy surface is a fit to points calculated using density-functional theory (DFT), with the generalized gradient approximation (GGA), and a slab representation for the surface. New aspects of the methodology we use to adapt the wave function to the symmetry of the surface, which relate to calculations for initial rotational states with odd mj (the magnetic quantum number), are explained. Invoking detailed balance, we calculate the quadrupole alignment for H2 as it would be measured in an associative desorption experiment. The reaction of the helicopter (ν=0,j=4,mj=4) state is preferred over that of the (ν=0,j=4,mj=0) cartwheel state for all but the lowest collision energies considered here. The energy dependence of the quadrupole alignment that we predict for (ν=0,j=4) H2 desorbing from Cu(100) is in good qualitative agreement with velocity-resolved associative desorption experiments for D2+Cu(111). The vibrational excitation probability P(ν=0,j→ν=1) is much larger for j=4 than for j=0, and the mj-dependence of P(ν=0,j=4,mj→ν=1) is markedly different from that of the initial-state-resolved reaction probability. For all but the highest collision energies, vibrational excitation from the (ν=0,j=4) state is accompanied by loss of rotational energy, in agreement with results of molecular beam experiments on scattering of H2 and D2 from Cu(111).
The mean spherical approximation for a dipolar Yukawa fluid
Authors: Henderson Douglas , Boda Dezső , Szalai István , Chan Kwong-Yu
Published: July 2002

The dipolar hard sphere fluid (DHSF) is a useful model of a polar fluid. However, the DHSF lacks a vapor–liquid transition due to the formation of chain-like structures. Such chains are not characteristic of real polar fluids. A more realistic model of a polar fluid is obtained by adding a Lennard–Jones potential to the intermolecular potential. Very similar results are obtained by adding a Yukawa potential, instead of the Lennard–Jones potential. We call this fluid the dipolar Yukawa fluid (DYF). We show that an analytical solution of the mean spherical approximation (MSA) can be obtained for the DYF. Thus, the DYF has many of the attractive features of the DHSF. We find that, within the MSA, the Yukawa potential modifies only the spherically averaged distribution function. Thus, although the thermodynamic properties of the DYF differ from those of the DHSF, the MSA dielectric constant of the DYF is the same as that of the DHSF. This result, and some other predictions, are tested by simulations and are found to be good approximations.