bluuebunny/crossref_metadata_2025 huggingface dataset:
On the binding of electrons to nitromethane: Dipole and valence bound anions
Authors: Compton R. N. , Carman H. S. , Desfrançois C. , Abdoul-Carime H. , Schermann J. P. , Hendricks J. H. , Lyapustina S. A. , Bowen K. H.
Published: July 2002

Conventional (valence) and dipole-bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe(nf ) Rydberg atoms with CH3NO2 exhibit a pronounced maximum at an effective quantum number of n*≊13±1 which is characteristic of the formation of dipole-bound anions [μ(CH3NO2)=3.46 D]. However, the breadth (Δn≊5, FWHM) of the n-dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ‘‘doorway’’ dipole anion state. Studies of the electric field detachment of CH3NO−2 formed through the Xe(nf ) reactions at various n values provide further evidence for the formation of both a dipole-bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 12±3 meV. Photodetachment of CH3NO−2 and CD3NO−2 formed via a supersonic expansion nozzle ion source produces a photoelectron spectrum with a long vibrational progression indicative of a conventional (valence bound) anion with a substantial difference in the equilibrium structure of the anion and its corresponding neutral. Assignment of the origin (v′=0, v″=0) transitions in the photoelectron spectra of CH3NO−2 and CD3NO−2 yields adiabatic electron affinities of 0.26±0.08 and 0.24±0.08 eV, respectively.
Theoretical study of the valence ionization energies and electron affinities of linear C2n+1 (n=1–6) clusters
Authors: Ohno M. , Zakrzewski V. G. , Ortiz J. V. , von Niessen W.
Published: July 2002

The valence level hole spectral functions of linear C2n+1 (n=1–6) clusters are calculated by the ab initio third order algebraic diagrammatic construction [ADC(3)] Green function method and the outer-valence Green function (OVGF) method using an extended basis set. The vertical electron affinities of linear C2n+1 (n=1–6) clusters are also evaluated by the same methods. With an increase of the number of carbon atoms, the KT energy levels become more closely spaced and start to form quasi-continua. The original spectral strength of the main line becomes distributed over several lines of comparable intensity. With an increase of the number of carbon atoms, the one-electron (or even quasi-particle) picture of the ionization breaks down because of the interaction between the initial single hole level and the final two-hole-one-particle levels. The spectral intensity of the first four ionization levels remains fairly constant independent of the number of carbon atoms. The agreement of the affinities of C2n+1 (n=1–6) with experiment is in general very good. Two anionic states are found to be bound for C9, C11 and C13.
Origins and modeling of many-body exchange effects in van der Waals clusters
Authors: Chal/asiński Grzegorz , Rak Janusz , Szczȩśniak Mal/gorzata M. , Cybulski Sl/awomir M.
Published: July 2002

We analyze the many-body exchange interactions in atomic and molecular clusters as they arise in the supermolecular SCF and MP2 approaches. A rigorous formal setting is provided by the symmetry-adapted perturbation theory. Particular emphasis is put on the decomposition into the single exchange (SE) and triple exchange (TE) terms, at the SCF and correlated levels. We also propose a novel approach, whereby selected SE nonadditive exchange terms are evaluated indirectly, as differences of the two-body SAPT corrections arising between the components of the trimer treated as a complex of a dimer and a monomer (pseudodimer approach). This provides additional insights into the nature of various nonadditive effects, an interpretation of supermolecular interaction energies, and may serve as a viable alternative for the calculation of some SE terms.
Monte Carlo simulation of the adsorption from a nonselective solvent of symmetric triblock copolymers with sticky end blocks
Authors: Haliloglu Turkan , Stevenson Daniel C. , Mattice Wayne L.
Published: July 2002

The adsorption of symmetric triblock copolymers from a nonselective solvent on an impenetrable surface has been studied by a lattice Monte Carlo simulation method. The triblock copolymers are ANABNBANA with NA=10, 5⩽NB⩽40, and surface interaction parameters −1.5⩽ε⩽−0.5, where A is the adsorbing block and B is the nonadsorbing block. The concentration, expressed as the volume fraction, is varied over the range 0.012⩽c⩽0.143. This report describes the adsorption isotherms and the kinetics of the adsorption in the simulations. At short times, the adsorption is diffusion controlled. Good approximations to the Langmuir adsorption isotherm are observed for the equilibrium adsorbed amount, Γ and surface coverage of the adsorbing block, θA. The results are compared with recent simulations for diblock copolymers. The adsorbed amount is less for the diblock than for the triblock copolymers at low concentration. The surface coverage by the adsorbing block is the same for both cases at weak surface adsorption. The surface density profiles are also obtained and compared. The adsorbed number of chains per unit area (σ) and surface coverage for triblock copolymers under weak adsorption conditions are found, respectively, to scale according to the relationship σ, θA∼1/β2, where β=(NB/NA)3/5, which implies the importance of the nonadsorbing block size for the adsorption features of triblock copolymers. These results confirm the experimental observation reported recently by Dorgan et al. [Macromolecules 26, 5321 (1993)].
The effect of dissociative chemisorption on the diffraction of D2 from Ni(110)
Authors: Bertino Massimo F. , Hofmann Frank , Toennies J. Peter
Published: July 2002

Absolute scattering probabilities of nearly monoenergetic D2 and He beams are compared for the highly reactive clean Ni(110) surface at a surface temperature Ts=700 K along the more corrugated [001] direction. At incident energies between 20 and 110 meV the total reflectivity of D2 is about a factor 200 smaller than for He, whereas the first order diffraction intensities relative to the specular peak are a factor 7 larger. The D2 angular distributions also show clear evidence of rotationally inelastic diffraction peaks. The diffraction intensities of both He and D2 can be accounted for by a conventional hard wall model with reasonable values of the corrugation amplitudes of 0.060 Å for He and 0.091 Å for D2 without including a lateral variation in the probability for chemisorption. The reflectivity results when extrapolated to Ts=0 indicate that for He only 33% of the incident atoms are coherently reflected. For D2 only 9% are coherently scattered and approximately 24% are chemisorbed. The coherently scattered fraction is attributed to D2 molecules with orientations not sufficiently parallel to the surface plane to permit chemisorption to occur.
Ab initiostudy on the low-lying excited states of retinal
Authors: Merchán Manuela , González-Luque Remedios
Published: July 2002

Ab initio results for the electronic spectrum of all-trans-retinal and its truncated model 3-methyl-all-trans (10-s-cis)-2,4,6,8,10-undecapentaen-1-al are presented. The study includes geometry determination of the ground state. Vertical excitation energies have been computed using multiconfigurational second-order perturbation theory through the CASPT2 formalism. The lowest singlet excited state in gas phase is predicted to be of nπ* character. The lowest triplet state corresponds, however, to a ππ* state. The most intense feature of the spectrum is due to the strongly dipole-allowed ππ* transition, in accordance with the observed maximum in the one-photon spectra. The vertical excitation energies of the Bu- and Ag-like states are found close, the latter ≈1 eV higher than the maximum in the two-photon spectra. Solvent effects and nonvertical nature of the observed maximum in the two-photon spectra are invoked in rationalizing the deviation with respect to the best present estimate for the Ag-like state. In addition, qualitative aspects of the one-bond photoisomerization about the C11=C12 double bond of retinal are considered. The overall isomerization picture from 11-cis into all-trans-retinal, as taking place mainly along the triplet manifold, agrees with experimental evidence.
The structure of a random heteropolymer in a disordered medium: Ensemble growth simulation
Authors: Bratko D. , Chakraborty A. K. , Shakhnovich E. I.
Published: July 2002

We use Monte Carlo simulations to study the thermodynamics and structural behavior of random heteropolymers immersed in a disordered medium. Simulation results pertain to isolated heteropolymer chains with renormalized intrachain interactions determined by analytic averaging over the realizations of the external medium. Two situations are considered. In the first scenario, the random heteropolymer is such that segments of different type have a propensity to segregate, while in the second situation different types of segments prefer to be adjacent to each other. Polymer configurations are generated by the ensemble-growth Monte Carlo method wherein the slow-down effects that typically hinder dynamic Monte Carlo simulations of compact polymer states with strong interactions are alleviated. Our simulations show that for the case where the bare heteropolymer prefers to form segregated patterns, below a certain temperature, the random heteropolymer “folds” into a few dominant conformations with a segregated pattern of contacts. In the case where the bare heteropolymer favors mixing of different types of segments we find richer behavior. Here, our simulations show the existence of two frozen phases separated by a reentrant phase as temperature (or strength of the external disorder) is scanned. The heteropolymers in the two frozen phases are “folded” in different patterns (high T, mixed pattern; low T, segregated pattern). The physical reasons for these phenomena are elucidated based on our simulation results for structural features of the polymer. The results agree remarkably well with a simple mean-field theory, a fact that may be of some general consequence. Specific experiments are suggested to test the phenomena that we predict.
The ultraviolet photolysis of acetyl and propionyl radicals studied by infrared emission spectroscopy
Authors: Li Hongzhi , Li Qiang , Mao Wentao , Zhu Qihe , Kong Fanao
Published: July 2002

The photodissociation of acetyl and propionyl radicals at 248 and 266 nm has been studied by time-resolved Fourier transform infrared spectroscopy. A vibrationally excited product CO(v⩽8) was observed in the emission spectra. The vibrational temperatures of the nascent CO products were about 7400 K for the acetyl radical and 6930 K for the propionyl radical. The vibrational energy partitioning of the CO fragments fits a soft impulsive model.
Structure, dynamics, and thermodynamics of benzene-Arn clusters (1⩽n⩽8 and n=19)
Authors: Dullweber Andreas , Hodges Matt P. , Wales David J.
Published: July 2002

We use a combination of molecular dynamics, Monte Carlo and geometry optimisation techniques to study benzene-Arn clusters for 1⩽n⩽19, with particular emphasis on BzAr19. In particular, we discuss the difficulties which arise in the accurate simulation of heterogeneous clusters due to problems of ergodicity and nonadditive contributions to the energy. The sensitivity of minima, transition states and reaction pathways to parameters of the potential and the induction energy is also considered. An efficient integration scheme with adaptive step size due to Bulirsch and Stoer is employed in the molecular dynamics simulations. Both geometry optimisation and molecular dynamics are considered to evaluate the usefulness of the Jump–walking Monte Carlo method proposed by Frantz, Freeman, and Doll. This approach improves the ergodicity of canonical simulations using data from different temperatures which we achieved using multiple parallel runs. We then apply a multiple histogram method to calculate the relative number of states in phase space and various thermodynamic properties covering the full temperature range in the canonical and the microcanonical ensembles. The Monte Carlo and molecular dynamics simulations result in a consistent picture of transitions between minima and escape times. Finally, we consider the rate of side-crossing by Ar atoms in BzAr1 and BzAr19 and compare statistical theories with rates obtained from simulations.
A molecular theory of the Laplace relation and of the local forces in a curved interface
Authors: Lovett R. , Baus M.
Published: July 2002

Equilibrium in a two-phase system, a liquid in equilibrium with its vapor, for example, is characterized by the constancy throughout the system of the temperature and the chemical potential and a relation between the pressures of the phases, the Laplace relation of macroscopic thermodynamics. We give a molecular expression for this latter relation in terms of a local pressure defined as the thermodynamic response to a local volume change and we show that this local pressure is the same as the local force distribution in an interface. The macroscopic characteristics of the two-phase system, including the bulk pressures, the surface tension and the location of the surface of tension are all determined by this local pressure function. As shown in a companion paper this function can easily be determined by numerical simulation.
Photoion spectra of C60 molecules at resonance excitation and ionization energies near the C 1s edge
Authors: Karvonen J. , Nõmmiste E. , Aksela H. , Aksela S.
Published: July 2002

Photoionization dynamics of gas-phase C60 molecules after selective excitation with synchrotron radiation has been studied with time-of-flight mass spectrometry. The energy level scheme deduced from previous photoelectron and Auger electron spectra is used to analyze the photoion data. At the vicinity of π* pre-edge resonances the fragmentation of multiply-charged ions varies strongly as a function of photon energy. The C60+ ion yield decreases slowly after 1s threshold indicating that the slow photoelectrons can get recaptured. Somewhat further above the threshold the enhanced relative fragmentation of the parent C60+ ions suggests excitation of the giant plasmon resonance due to 20 eV photoelectrons.
Rate constants for the reactions of O+ with N2 and O2 as a function of temperature (300–1800 K)
Authors: Hierl Peter M. , Dotan I. , Seeley John V. , Van Doren Jane M. , Morris Robert A. , Viggiano A. A.
Published: July 2002

We have studied the rate constants for the reaction of O+ with N2 over the temperature range 300–1600 K and the reaction of O+ with O2 over the range 300 to 1800 K. The results are in good agreement with previous measurements made up to 900 K. The rate constant for the O+ reaction with N2 shows a minimum in the temperature range 1100–1300 K. The increase above this temperature is due to N2 v=2 becoming populated. The rate constant for O++O2 shows a minimum in the 800–1100 K range. Comparing to previous drift tube measurements allows the rate constant for O2 (v>0) to be derived. The v>0 rate constant is approximately five times larger than the v=0 rate constant.
Interaction potentials and diffusion coefficients between sodium and rare gases
Authors: Li P. , Tang K. T.
Published: July 2002

Diffusion coefficients of Na in He, Ne, and Ar are calculated from the molecular Chapman–Enskog theory. For the Na–Ne system, only Patil’s potential [J. Chem. Phys. 94, 8089 (1991)] among all proposed potentials predicts a temperature dependence of the coefficients that is in agreement with the available measurements. Besides providing reliable diffusion coefficients, present results validate Patil’s theoretical method which has some unique and far-reaching features.
New statistical mechanical treatment of systems near surfaces. I. Theory and principles
Authors: Gujrati P. D. , Chhajer Mukesh
Published: July 2002

We present a new theoretical framework for a statistical mechanical and thermodynamic description of any general inhomogeneous system (not necessarily polymeric) in the presence of surfaces. The framework is an extension of a lattice theory recently developed for a homogeneous system and requires approximating the original lattice by a recursive lattice which, for simplicity, we take to be a modified tree structure (see Fig. 4), TM as described in the text. The tree is formed recursively by two basic elements, the main tree 𝒯 and the surface tree T¯. The model is solved exactly using a recursion technique. The technique allows us to account for connectivity, architecture, excluded-volume effects, interactions, etc. exactly. The resulting description goes beyond the random-mixing approximation used in most mean-field theories. We consider a general model of a multicomponent system and its exact solution on the modified tree TM provides us with an approximate theory of the inhomogeneous system on the original lattice. We provide a general discussion of the theory and principles involved. Our method produces results similar to those of Monte Carlo simulations but can even be applied to cases where Monte Carlo simulations are not possible. We also obtain surface free energy and the surface entropy that is not easily obtained in a Monte Carlo simulation. Our method is more reliable than the mean-field method of Scheutjens and Fleer, whose predictions are, in many cases, in direct contradiction with the Monte Carlo simulations. Our method is fast by at least three orders of magnitude compared to rival methods.
Vibrational predissociation spectra of size selected hydrazine clusters: Experiment and calculations
Authors: Beu T. A. , Buck U. , Ettischer I. , Hobein M. , Siebers J. G. , Wheatley R. J.
Published: July 2002

Vibrational predissociation spectra of hydrazine (N2H2)n clusters have been measured from the dimer to the tetramer using a linetunable, isotopically substituted CO2-laser in order to fill the frequency gap between 990 and 1010 cm−1. The clusters are size selected in a scattering experiment with helium atoms. The large blue shifts of the asymmetric NH2 wag mode at 937 cm−1 are completely interpreted by calculations based on a recently determined systematic model potential. The gross shifts of 60 cm−1 for the dimer, 80 cm−1 for the trimer, and 110 cm−1 for the larger clusters are explained by the different structures: Cyclic arrangements with two hydrogen bonds per molecule for the dimer, rings with one hydrogen bond per molecule for the trimer, and three-dimensional structures for the larger ones. The peaks in the spectra are caused by characteristic vibrations to which more than one isomer contributes.
Dynamic properties and third order diffusion coefficients of ions in electrostatic fields
Authors: Koutselos Andreas D.
Published: July 2002

Velocity correlation functions and third order diffusion coefficients of ions moving in a buffer gas under the influence of an electrostatic field are determined via molecular dynamics simulation. For the closed shell system of K+ in Ar using a universal interaction model potential, the general form of the third order correlation functions is found to be monotonically decaying in time except in the cases of 〈ΔvZ(0)ΔvX(t)2〉, 〈ΔvZ(0)ΔvY(t)2〉, and 〈ΔvZ(0)ΔvZ(t)2〉, with Δv(t)=v(t) −〈v(t)〉 and the field in the z direction. These functions acquire positive slope at short times showing enhancement of correlations between instantaneous vz components of the ions and their future kinetic energies or velocity measures. This feature is shown to quantify the dynamics of correlations between velocity components suggested in the past by Ong, Hogan, Lam and Viehland [Phys. Rev. A 45, 3997 (1992)] in order to explain the form of an ion velocity distribution function calculated through a Monte Carlo simulation method. In addition, within a stochastic analysis which establishes a relation between velocity correlation functions and third order diffusion coefficients, only two independent components of the diffusion tensor, Q∥ and Q⊥, are predicted. We thereby calculate the Q⊥ component, which has not been determined so far, over a wide field range. The magnitudes of the resulting third order diffusion coefficients indicate that their contribution to the ion transport in usual drift-tube measurements should be very small.
Analytical theory of benzene diffusion in Na-Y zeolite
Authors: Auerbach Scott M.
Published: July 2002

We have developed an analytical expression for the diffusion coefficient of benzene in Na-Y at infinite dilution in terms of fundamental rate coefficients, which has been confirmed by extensive kinetic Monte Carlo simulations. This model assumes that benzene jumps among SII and W binding sites, located near Na+ ions in 6-rings and in 12-ring windows, respectively. Our diffusion theory is based on D=16ka2 where a≅11 Å is the intercage length and k is the cage-to-cage rate coefficient. We have determined that k=k(SII→W)⋅12⋅3[1+k(W→W)/k(W→SII)], a finding that has resolved discrepancies between theory and simulation and has suggested new interpretations of benzene diffusion in Na-Y. When α(T)≡k(W→W)/k(W→SII) is between 0 and 1, the factor 3[1+α(T)] counts the number of thermally allowed target sites for cage-to-cage motion. Alternatively, when α(T)≫1, benzene mobility is interpreted as interstitial diffusion, wherein k is controlled by the probability of W site occupancy multiplied by the rate of W→W jump processes. This limit is expected to arise with benzene loadings of four molecules per supercage.
Photochemistry of adsorbed molecules. XVIII. Photodissociation and exchange reaction in CH3Br/MgO(001) at 193 nm
Authors: Garrett S. J. , Heyd D. V. , Polanyi J. C.
Published: July 2002

Methyl photofragments from the 193 nm photodissociation of CH3Br adsorbed on MgO(001) were studied by angularly resolved time-of-flight mass spectrometry. The translational energy distributions of the photofragments showed evidence of three pathways for the release of CH3. The pathways were termed “direct” [DIR], “indirect(1)” [IND(1)], and “indirect(2)” [IND(2)]. The DIR methyl translational energy distribution, P(ET′) peaked at 2.3 eV, with a full width at half-maximum (FWHM) of 0.65 eV, corresponding roughly to that reported for gas phase CH3 (peak=2.5 eV, FWHM=0.5 eV). The P(ET′) of the DIR pathway was consistent with CH3 escaping directly from the topmost molecular layer without collisions. For these DIR methyls, as previously demonstrated, the angular distribution reflected the prior Br–C bond direction. In the higher coverage range, 1.5–10 monolayers (ML) angular distributions, P(Θ), for the DIR methyls peaked at 22° to the normal with FWHM of 20°. At low coverage, 0.75 ML, the DIR peak shifted to ∼40° with doubled FWHM. The IND(1) methyls, despite the loss of 1.2 eV in a strong inelastic encounter, exhibited the same angular distributions and FWHM as the DIR component: 22° peak, 20° FWHM at high coverage; 40° peak, 40° FWHM at low coverage. The mechanism attributed to IND(1), which accounts for the retention of direction of methyl with concurrent substantial energy loss, is one proposed in a prior theoretical study [Barclay et al., J. Phys. Chem. 97, 12541 (1993)]: an exchange reaction favored by the adsorbate geometry; in the present case CH3+BrCH3′→CH3Br+CH3′. The second indirect channel, IND(2), exhibited broader translational energy distribution than DIR or IND(1) peaked at 0.6 eV lower energy than IND(1), and a broad angular distribution (cos2 Θ) peaked at the normal, characteristic of strongly inelastic encounters in which memory of the initial CH3 recoil direction is lost.
Geometrical effects on folding of macromolecules
Authors: Caliri A. , da Silva M. A. A.
Published: July 2002

Geometrical effects on folding of macromolecules are investigated using linear chains with tetrahedral structure and hard-core interactions among its monomers; extra self-avoidance, namely, nontopological neighbor, is also considered. Our results were obtained by exact calculations using chains with small number N of monomers (up to 16) and by Monte Carlo simulation, using the ensemble growth method (EGM), for larger N. For some cases we provide a comparative study using two types of lattice and three different models. The original number of angle choices, ζ=3 (coordination number), is shown to be effectively reduced to ζeff=2.760, and the radius of gyration and end-to-end distance, for finite chains (N⩽140), scales with the number of monomers as Nν, where ν≅2/3. This is significantly larger than the corresponding value for the self-avoiding walk model, ν≅0.6. The relative frequency of monomer pair contacts was obtained by the exact Gibbs ensemble, involving all possible configurations. The same calculation using the EGM reveals ergodic difficulties; its significance on the setting up of pathways for folding of macromolecules is discussed.
Spectroscopy of ionic alkali rare gas excimers in matrices
Authors: Moeller S. , Gürtler P.
Published: July 2002

Using a clean preparation technique of doping alkali atoms in argon matrices, VUV emissions of the charge-transfer states of the ionic xenon–alkali excimers (XeA)+ have been measured. With synchrotron radiation as an excitation source, the ionic species were generated in the matrix and energy and time-resolved spectroscopy has been performed with synchrotron radiation as an excitation source. The emissions of the charge-transfer reaction: (Xe+A)→(XeA+)+hν have been observed for all ionic (XeA)+ excimers with energies in the VUV range between 6.05 and 7.4 eV. The excitation spectra revealed a primary formation process through direct ionization of the xenon; however, neutral xenon resonance states are also contributing. As obtained from lifetime measurements, the bands are assigned to the 0I−→0+ transition of the fine-structure components which account for the long decay time. An increase of the xenon concentration favors the formation of ionic trimers (Xe2A)+ whose emissions have also been identified.
Ab initio study of the O2(X 3Σg−)+Ar(1S) van der Waals interaction
Authors: Cybulski Slawomir M. , Kendall Rick A. , Chalasinski Grzegorz , Severson Mark W. , Szczesniak Malgorzata M.
Published: July 2002

A potential energy surface for the Ar(1S)+O2(X 3Σg−) interaction is calculated using the supermolecular unrestricted Mo/ller–Plesset (UMP) perturbation theory and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the T-shaped geometry, around 6.7 a0. Our UMP4 estimate of the well depth of the global minimum is De=117 cm−1 and the related ground state dissociation energy obtained by diffusion Monte Carlo calculations is 88 cm−1. These values are expected to be accurate to within a few percent. The potential energy surface also reveals a local minimum for the collinear geometry at ca∼7.6 a0. The well depth for the secondary minimum at the UMP4 level is estimated at De=104 cm−1. The minima are separated by a barrier of 23 cm−1. The global minimum is determined by the minimum in the exchange repulsion in the direction perpendicular to the O–O bond. The secondary, linear minimum is enhanced by a slight flattening of the electron density near the ends of the interoxygen axis.
Simulation of atomic zinc luminescence in rare gas solids: A localized pair potentials approach
Authors: McCaffrey John G. , Kerins Paul N.
Published: July 2002

The luminescence spectroscopy of atomic zinc isolated in the solid rare gases (Zn/RG) is compared with theoretical predictions obtained from the sum of diatomic Zn⋅RG and RG⋅RG pair potentials. In particular the existence of pairs of emission bands, both of which are assigned to the same gas phase electronic transition, is examined with the use of diatomic pair potentials to simulate the potential energy surfaces of the Jahn–Teller active vibrational modes of Zn in the solid rare gases Ar, Kr, and Xe. Simulations of the solid state Zn/RG luminescence are developed from a consideration of the excited state Zn(1P1)⋅RGn van der Waals cluster species in the gas phase. The maximum binding energy of the Zn(1P1)⋅RGn clusters is found in the Zn⋅RG4 cluster having a square planar structure at the energy minimum. Based on the results of the cluster calculations, lattice distortions which led to a dominant interaction between the Zn atom and four of its host atoms were sought to simulate the solid state luminescence. Two such vibronic modes were identified; one a lattice mode in which four rare gas atoms contract on a single plane toward the Zn atom, referred to as the waist mode, and the other a motion of the Zn atom toward an octahedral interstitial site of the lattice, the body mode. Energy calculations of these modes were carried out for rigid and relaxed rare gas lattices allowing identification of the high energy emission bands in the Zn/RG systems as arising from the waist mode, while the lower energy bands are associated with the body mode. The model also rationalizes the differences exhibited in the time-resolved behavior of the pairs of singlet emission bands in the Zn/Ar and Zn/Kr systems, whereby the lower energy band of a given system shows a risetime of a few hundred picoseconds while the higher energy band exhibits direct feeding. The steep gradient calculated on the waist mode, feeding the high energy band, and the flat gradient found on the body mode, feeding the lower energy emission, are consistent with the existence of a risetime in the latter and its absence in the former. The close agreement found between theory and experiment indicates the validity of using pair potentials in analysis of matrix zinc spectroscopy and thereby indicates that the luminescence is controlled by localized guest–host interactions.
Bond selective dissociation of CH3SH+ and CH3CH2SH+ via collisional activation
Authors: Chen Y.-J. , Fenn P. T. , Stimson S. , Ng C. Y.
Published: July 2002

Strong preference is observed for the C–S bond scission process, leading to the formation of CH3++SH (CH3CH2++SH), in the collision induced dissociation (CID) reaction of CH3SH++Ar (CH3CH2SH++Ar). Since the dissociation energy of 81.4 kcal/mol (45.2 kcal/mol) for the CH3+–SH (CH3CH2+–SH) bond is significantly higher than that of 48 kcal/mol (33.9 kcal/mol) for the H–CH2SH+ [H–CH(CH3)SH+] bond, this observation indicates that the CID process is nonstatistical. The high yield for the C–S bond breakage process is attributed to the more efficient translational to vibrational energy transfer for the C–S stretching mode than for C–H and S–H stretching modes via collisional activation, and to weak couplings between the low frequency C–S and high frequency C–H and S–H stretching vibrational modes of CH3SH+ andCH3CH2SH+.
Multiphonon He atom scattering from Xe overlayers on Cu(111) and Cu(001) surfaces
Authors: Braun J. , Fuhrmann D. , Bertino M. , Graham A. P. , Toennies J. P. , Wöll Ch. , Bilić A. , Gumhalter B.
Published: July 2002

We have studied the scattering of He atoms from ordered monolayers of Xe atoms adsorbed on Cu(111) and Cu(001) surfaces. Both Xe overlayers, the commensurate (√3×√3)R30° on Cu(111) at substrate temperature above 47 K and the incommensurate one on Cu(001), exhibit weak diffraction. On both surfaces the Xe overlayers sustain Einstein-like, vertically polarized Xe vibrations which can be multiply excited and annihilated even at low He atom incident energies and substrate temperatures. In spite of the anharmonic Xe-substrate potentials, the energies of multiphonon excitations are not found to exhibit any noticeable anharmonic shifts in either system. The results of the measurements are compared to theoretical energy and lateral momentum resolved scattering distributions, which were calculated by using the recently developed formalism for treating multiphonon scattering in the collision regimes, in which both the motion of the scattered particle and surface vibrations must be treated quantum mechanically. We can interpret the multiphonon scattering spectra and obtain good agreement with experiments by assuming linear He atom–phonon coupling, delocalized phonons in Xe adlayers, and by employing the one-phonon interaction parameters determined from the He–Xe interaction potentials. Effects of the quantum recoil on the motion of the scattered He atom, which give rise to a difference between phonon emission and absorption probabilities, and thereby non-Poissonian scattering distributions, were assessed.
Low-energy dissociative electron attachment to CFCl3, CF2Br2, and 1,1,1- and 1,1,2-C2Cl3F3: Intermediate lifetimes and decay energetics
Authors: Finch C. D. , Parthasarathy R. , Akpati H. C. , Nordlander P. , Dunning F. B.
Published: July 2002

Dissociative electron attachment to CFCl3, CF2Br2, and 1,1,1- and 1,1,2-C2Cl3F3 is studied by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity-selected K(np) Rydberg atoms. The data are analyzed with the aid of a Monte Carlo collision code that models the detailed kinematics of the reactions. Measurements with CFCl3 and CF2Br2 show that the mean lifetimes of the CFCl3−* and CF2Br2−* intermediates initially formed by Rydberg electron capture are ∼3.5 and 4.5 ps, respectively, and that, upon dissociation, only a small fraction of the excess energy of reaction appears as translational energy of the fragments. The data for 1,1,1-C2Cl3F3 indicate that reaction proceeds by two channels: in the first, essentially all the excess energy appears in translation, suggesting that the electron is captured directly into an antibonding orbital; in the second, limited conversion of the excess energy to internal motions occurs, pointing to an intermediate lifetime of ∼0.1 to 1 ps. The data for the 1,1,2 isomer suggest that again only limited energy transfer to internal motions occurs, but the particular reaction channel(s) operative could not be identified. Possible reaction channels for the C2Cl3F3 isomers are discussed with the aid of ab initio calculations.
Wavelength and intensity-dependent transient degenerate four-wave mixing in pseudoisocyanine J-aggregates
Authors: Gadonas R. , Feller K.-H. , Pugzlys A. , Jonusauskas G. , Oberlé J. , Rullière C.
Published: July 2002

The results of transient degenerate four-wave mixing (DFWM) and “pump-probe” spectroscopy in aggregates of 1,1′-diethyl-2,2′-cyanine (pseudoisocyanine, PIC) chloride at 300 K are reported. Spectral dispersion of DFWM efficiency within the J-band and near exciton resonance has been measured. Time response of both transient absorption changes and DFWM signal is found to be strongly dependent on pump photon fluence and wavelength. This behavior is qualitatively explained within a physical model of nonlinear optical dynamics in aggregate domains which accounts for exciton annihilation, and the effect of nonthermal phonons produced as a result of decay of two-exciton states. Intensity-dependent evolution of excess dynamic disorder due to nonthermal phonons manifests itself in pump-probe experiments as dispersion-type differential spectrum appearing at high pump intensity. The third-order nonlinear susceptibility of PIC aggregates has been calculated supposing highest density packing. The nonlinear figure of merit of aggregates was evaluated which is intensity dependent due to exciton annihilation and associated subsequent processes.
The torsional conformations of butane: Definitive energetics from ab initio methods
Authors: Allinger Norman L. , Fermann Justin T. , Allen Wesley D. , Schaefer III Henry F.
Published: July 2002

The torsional potential function for butane was probed theoretically using increasingly complete basis sets (up to 840 functions) and treatments of electron correlation [up to the CCSD(T) method] until it was shown that the sequence of relative energies approached convergence. The Schrödinger limit in the Born–Oppenheimer approximation was thus estimated. The equilibrium energies relative to the anti conformation (ΔEe) as obtained by this focal-point extrapolation were 0.62, 3.31, and 5.51 kcal mol−1 for the geometrically optimized stationary points having carbon backbone torsion angles of 64.8°, 119.6°, and 0°, respectively. The final prediction of the anti–syn difference is 5.40±0.15 kcal mol−1. Some consequences of this result are discussed.
Structure, vertical electron-detachment energy, and O–H stretching frequencies of e+(H2O)12
Authors: Kim Jongseob , Park Jung Mee , Oh Kyung Seok , Lee Jin Yong , Lee Sik , Kim Kwang S.
Published: July 2002

The first comprehensive ab initio study is performed on an excess electron bound to the water dodecamer to find out if this wet electron can be regarded as a precursor of the fully solvated electron. Various structures of the wet electron are explored using ab initio calculations. Among a number of possible geometries categorized as unbounded, surface, internal, and partially internal excess-electron states, the lowest-energy conformer is predicted to be a structure of a partially internal state. The predicted vertical electron-detachment energy of this structure is in good agreement with the experimental value of Coe et al. [J. Chem. Phys. 92, 3980 (1990)]. This indicates that in the experiment the partially internal excess-electron state structure would have been detected. The electronic structure, interactions between the excess electron and dangling H atoms (e⋯ H interaction), and red-shifts of the O–H stretching frequencies with strong IR intensities are discussed.
Electron paramagnetic resonance study of the concentration of spin-label molecules in hydrated glass pores
Authors: Cevc P. , Choi C. , Holly R. , Pintar M. M.
Published: July 2002

Dehydration of porous glass hydrated with a 5 mM solution of the spin-label molecules 2,2,6,6-tetramethyl-4-acetamido-piperidine-1-oxyl (TMAPO) was studied with electron paramagnetic resonance at 9 GHz. The spin-label molecules’ concentration remains constant, i.e., it follows the Langmuir isotherm when glass pores of 292±20 nm diameter are dehydrated. When the pore diameter is only 127±11 nm, or less, the Langmuir isotherm appears not to apply despite the fact that in smaller pores the surface-to-volume ratio is larger. The reason for this apparent failure could be the establishment of a water cluster phase which does not dissolve TMAPO, and/or it could be a kinetic effect of the spin-label molecules. In this report the possibility that the failure of the Langmuir isotherm is the consequence of ordered water clusters is examined. To account for the lower concentration of TMAPO in smallest pores, as much as 70% of total water volume has to consist of such water clusters, in agreement with an earlier proposal.
Penetration depth of photomobilized F atoms in Ar layers from a sandwich experiment
Authors: Bressler C. , Dickgiesser M. , Schwentner N.
Published: July 2002

The mean value and the distribution of the penetration depth of F atoms is determined from samples composed of three layers with controlled thickness in the monolayer range and with a test of the compactness of the films via the intensity of surface excitons. F atoms with an average kinetic energy of 4.3 eV are generated in the top layer (Ar doped with F2) by photodissociation of F2 with 10.15 eV in a spin forbidden repulsive state. The F atoms are injected into an Ar spacer layer of variable thickness. Those reaching the interface to the Kr bottom layer are monitored via the intensity of the Kr2F fluorescence at a wavelength of 444 nm, which allows one to discriminate between F in Ar (439 nm), in Kr (453 nm), and at the Kr/Ar (444 nm) interface. The F content at the interface is kept below 1/20th of a monolayer to suppress recombination, and the detection sensitivity is increased to 1/1000th of a monolayer by excitation via Kr exciton energy transfer. The probability for F atoms to penetrate the Ar spacer layer decreases exponentially with increasing thickness down to 10% for a thickness of 23 monolayers, and an average penetration depth of 10 monolayers is derived. These very large penetration depths exceed those of F+ and F− ions by more than one order of magnitude. They are consistent with those molecular dynamics calculations, which predict a rather rectilinear motion in channels of the Ar lattice. An average length of travel of up to 27 monolayers with a mean-free path (large angle scattering) up to four monolayers is compatible with the results.
A density functional especially designed for hydrogen-only systems
Authors: Chermette H. , Razafinjanahary H. , Carrion L.
Published: July 2002

An hybrid functional which includes a larger amount of pure exchange has been specially designed for the description of hydrogen-only systems. Both the H abstraction by H2 and the H2n+1+ clusters have been investigated. Comparison with experimental values shows that the proposed functional gives dissociation energies and vibrational frequencies better than previous ab initio calculations. The results compare favorably with those obtained by a coupled clusters method [CCSD(T)], also performed in this work for sake of reference data.
Stochastic theory of combined radiative and nonradiative transport
Authors: Berberan-Santos Mário N. , Nunes Pereira Eduardo J. , Martinho José M. G.
Published: July 2002

A stochastic theory of combined radiative and nonradiative transport is presented. The stochastic approach is physically clear and versatile, allowing the consideration of the combined effect of radiative and nonradiative transport, carried out here for the first time. The stochastic approach is formulated for delta-pulse excitation and for the photostationary state. General equations for the intensity, polarization, and anisotropy decays are derived.
A study of the ArCl2 Van der Waals complex: Ab initio-based potential energy surfaces, the relative stability of conformers, and the “hidden” microwave spectrum
Authors: Naumkin F. Y. , McCourt F. R. W.
Published: July 2002

Two new ground state potential energy surfaces have been obtained for the ArCl2 Van der Waals complex, one purely ab initio, the other constructed from empirical ArCl potentials modified in terms of ab initio data for the ArCl2 and ArCl moities. The ab initio surface has a well for the linear conformer that is slightly deeper than that for the T-shaped conformer, but inclusion of the zero-point energies reverses the relative binding in the two configurations. The microwave spectrum has been calculated using the new potential surfaces, and a series of lines that can be associated with the linear conformer have been predicted. Possible reasons for their nonobservability under the usual experimental conditions are discussed. A simple modification which employs empirical information on the ArCl potentials used in modelling the ArCl2 potential surface transforms the well for the T-shaped conformer into the global minimum, with a dissociation energy that lies within 0.5% of the experimental value, and microwave transition energies that deviate from experimental values by no more than 1.3%. A final two-parameter scaling allows reproduction of both the experimental D0 value (within experimental uncertainty) and the positions of all observed microwave lines (within 0.02%) for each of the ab initio and empirically based potential surfaces.
Theoretical evaluation of medium effects on absorption maxima of molecular solutes. I. Formulation of a new method based on the self-consistent reaction field theory
Authors: Houjou Hirohiko , Sakurai Minoru , Inoue Yoshio
Published: July 2002

A new method is formulated for evaluating solvent effects on the absorption maxima of molecules. The method is based on a self-consistent reaction field polarizable continuum model (SCRF-PCM), coupled with the configuration interaction (CI) procedure. We propose a new scheme in which the polarization of solvent, induced by the excitation of a solute molecule, is partitioned into orientational and electronic contributions, and the exact expression of excitation energy is derived with nonspecific parameters of solvents, i.e., dielectric constant and refractive index. On the basis of this result, the expression of the diagonal element of the CI matrix is derived. In addition, we show that the expression of the off-diagonal element can be obtained without losing generality under the condition that the solute wave function is expressed by the Hartree product. The present method is implemented into a semiempirical molecular orbital calculation program. It is demonstrated that the test calculation for a merocyanine dye well reproduces the solvatochromic shifts observed over a variety of solvents with wide ranges of dielectric constant and refractive index.
Ab initio theoretical study of dipole-bound anions of molecular complexes. Water trimer anion
Authors: Smith Dayle M. A. , Smets Johan , Elkadi Yasser , Adamowicz Ludwik
Published: July 2002

We present results of ab initio calculations of our study of (H2O)3−. The main conclusions of this work are as follows: the most stable cyclic structure of (H2O)3 has a dipole moment too small to form a dipole-bound state with an excess electron; the dipole-bound anion of the water trimer observed experimentally appears to be a hydrated complex of the water dimer anion, (H2O)2−, by a single water molecule. The water trimer anion, (H2O)3−, has an open shape. The calculated vertical electron detachment energy of this anion is predicted to be equal to 141 meV, which is in good agreement with the experimental value of Bowen and co-workers, equal to 142±7 meV. Although the open optimal geometry of the (H2O)3− anion obtained in the present calculations is an equilibrium structure, its energy is higher than the energy of the cyclic equilibrium structure of the neutral complex, indicating that the anion is a metastable system. Based on calculations, we predict significant differences in the IR vibrational spectra of (H2O)3 and (H2O)3−, which may be used for identification of the two species.
Volume dependence of potential energy landscapes in glasses
Authors: Malandro Dennis L. , Lacks Daniel J.
Published: July 2002

The volume dependence of the potential energy landscapes of glasses is examined. Glassy structures, corresponding to local potential energy minima, are compressed and expanded. The local minima often disappear with volume changes, after which the system becomes mechanically unstable and subsequently relaxes to a different, unrelated local minimum. Several characteristics of the potential energy surface are monitored as the volume is changed, including normal mode frequencies, barrier heights, and orientations and distances between the local minima and transition states. These characteristics are used to elucidate the disappearance of the local minimum and the ensuing relaxation to a different local minimum. The disappearance of an energy minimum becomes more probable as the number of atoms in the system increases. Implications of these results are discussed with regard to pressure-induced structural rearrangements and equation-of-state properties of glasses.