bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical study of the dissociation energy of BH using quadratic configuration interaction
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

An ab initio molecular-orbital study of the dissociation energy of boron monohydride of (BH) using quadratic configuration interaction and a large basis set is presented. The results suggest that the commonly accepted value for the dissociation energy Do of BH of 78.9 kcal/mol may be low by about 3– 4 kcal/mol.
A test of recently proposed He–N2 interactions: Angular distributions and rotationally inelastic collisions
Authors: Gianturco F. A. , Venanzi M. , Faubel M.
Published: July 2002

Three recently proposed anisotropic potential energy surfaces (PES) for the He+N2 (1Σg) system are employed to compute differential scattering cross sections, total and state-to-state rotationally inelastic, at two collision energies for which accurate molecular beam experiments are already available. The experiments have provided scattering angular distributions which resolved, in one case, rotational energy losses (at 27.7 meV) and which, in the other case, were able to yield only total differential cross sections (at 70.4 meV). Rigorous close-coupling (CC) calculations at both energies are compared with IOSA (infinite order sudden approximation) results and with the experimental data. The ensuing discussion shows that only one PES, the recently proposed M3SV form, appears to have the correct anisotropic behavior in agreement with all experimental data.
Renormalization-group approach to the metal–insulator transition in doped semiconductors
Authors: Bauer J. D. , Logovinsky V. , Skinner J. L.
Published: July 2002

In order to calculate the critical concentration for the metal–insulator transition in doped semiconductors, we study a model of randomly positioned interacting hydrogenic atoms within the one-electron approximation. We calculate approximate eigenfunctions for the system with the standard linear combination of atomic orbital variation method, considering explicitly the nonorthogonality of hydrogenic 1s orbitals. We then compute the correlation length using the concept of quantum connectivity, which we developed to study the localization transition in other disordered quantum-mechanical models. Finally, we employ a finite-size scaling analysis to determine the critical impurity concentration nc. If the isolated impurities have a Bohr radius a, then we find that Rc≡n1/3ca=0.250±0.011, which is in good agreement with experiment (Rc=0.26±0.05).
A complete set of elastic constants of crystalline anthracene by Brillouin scattering
Authors: Dye R. C. , Eckhardt C. J.
Published: July 2002

All 13 elastic constants of a vapor grown, uncut, anthracene single crystal were determined from acoustic phonon velocities obtained by the Brillouin scattering method. The phonon velocities are plotted for three crystallographic planes containing the crystal axis. The relationships between phonon velocities and lattice dynamics are discussed. A minimizing procedure is introduced for converting phonon velocities of low symmetry systems into elasticity coefficients. This is shown to have several advantages over previous methods used. The results are compared with those obtained by previous studies of anthracene elastic constants.
Dynamics of pair diffusion in hard sphere fluids
Authors: Evans Glenn T. , Kumar Bijoy
Published: July 2002

Kinetic theory is applied to the analysis of the position and frequency dependence of the friction coefficient describing pair diffusion of hard spheres in their parent fluid. The high frequency friction is found to increase monotonically with internuclear separation from its contact value (at R=σ) up to a maximum at R=2σ, whereupon the friction attains its Enskog value. For low frequencies, the pair friction is a sum of the background high frequency part and caging and hydrodynamic contributions. Both caging and hydrodynamic effects exhibit different R dependences, the former an increasing function of separation (up to R=2σ) and the latter a decreasing function (as σ/R for R>σ).
Two-reagent reactions of iron clusters with ammonia and deuterium: Saturated compositions and the kinetics of reactions of deuterium with ammoniated clusters
Authors: Hoffman W. F. , Parks E. K. , Riley S. J.
Published: July 2002

Studies of two-reagent reactions of iron clusters with ammonia and deuterium are described. The compositions of clusters saturated with both ammonia and deuterium have been determined, and indicate that the ammonia binds nondissociatively to iron clusters and to different sites than the deuterium. Rate constants for the reaction of the first deuterium molecule with ammoniated clusters have been measured. In general, low ammonia coverage (one to three molecules) has either little effect on cluster reactivity, or in the case of unreactive bare clusters, increases reactivity dramatically. Clusters saturated with ammonia show general reactivity decreases, although again the least reactive bare clusters show dramatic increases in reactivity when covered with ammonia molecules. Interesting cluster activation and deactivation effects are seen. Possible mechanisms for these reactions are discussed.
Comparison of Raman scattering from local and coupled surface phonon modes of small uniformly coated spheres
Authors: Mamiche Afara Suzanne , Dignam Michael J.
Published: July 2002

Building on our recent paper on the optical properties of molecules forming a uniform, uniaxial layer on a small isotropic sphere, the present paper extends the treatment to include Raman scattering from both local modes and the coupled or surface phonon modes of such a system. As with the earlier paper, the treatment is restricted to the large wavelength approximation. While Raman inactive molecular modes can result in Raman active phonon modes, the present paper considers only those Raman active phonon modes that are connected with Raman active molecular modes. For such phonon modes, the field enhancement factors for the scattered wave are identical in form to those for the incident wave, derived in our recent paper, but differ significantly from those for local modes. Calculations are presented for CO and benzene on silver particles.
Electronic polarizabilities and dipole moments for alkali halides
Authors: Sangachin A. A. S. , Shanker J.
Published: July 2002

An analysis of electronic polarizabilities of ions in alkali halides has been performed by making use of the Wilson–Curtis–Coker model for describing the variation of polarizabilities in going from free state to a bonded state. More accurate values of free state polarizabilities for anions have been obtained making use of photoelastic data. These polarizabilities are found to be larger than Pauling’s values but smaller than Coker’s values. The free state polarizabilities obtained in the present work are then used to estimate the molecular state polarizabilities for 20 alkali halides. The dipole moments and their derivatives with respect to internuclear distances are then calculated and compared with experimental data in order to make a critical test of the polarizability data.
Chemiluminescent reactions of electronically excited alkaline-earth atoms. I. Energy dependence in Ba(6s6p1P1)+CO2→BaO(A,A′)+CO
Authors: Cuvellier J. , de Pujo P. , Mestdagh J. M. , Meynadier P. , Visticot J. P. , Berlande J. , Binet A.
Published: July 2002

The visible chemiluminescence from the reaction between a barium atom excited in the 6s6p1P1 resonance state and a CO2 molecule has been studied as a function of the collision energy. The emission originates from a BaO molecule excited in the A 1∑+ and A′ 1Π levels. The population ratio of the electronic A and A′ states appears to be statistical. The internal energy of the electronically excited BaO product is also statistically distributed at low collision energy (0.12 eV) and starts to deviate from statistics when the collision energy is increased, the mean internal energy of BaO remaining almost constant. The cross sections for chemiluminescence are of the order of a few Å2 and depend only weakly of the collision energy. The reaction of Ba (1P) with CO2 to form BaO (A or A′) is interpreted as a direct reaction without a barrier.
Theory of photoinduced intramolecular electron transfer in condensed media
Authors: Lin S. H.
Published: July 2002

In this paper, we are concerned with the theoretical treatment of intramolecular electron transfer (ET). In particular we are interested in the effect of the chemical groups connecting between donor and acceptor groups on electron transfer. We treat two cases. Type 1 refers to the situation where the energy difference between the LUMO of the connecting groups and the LUMO of the donor group is much larger than vibrational energies. Type 2 refers to the situation where the energy difference between the LUMO of the connecting groups and the LUMO of the donor group is comparable to vibrational energies. We show that Type 1 ET is similar to the Raman scattering while Type 2 ET is similar to the resonance Raman scattering.
The potential energy function for O2 X(3∑−g ) and the transition dipole moment of the Schumann–Runge band near X-state dissociation
Authors: Yang X. , Wodtke A. M.
Published: July 2002

Emission spectra were analyzed for transitions of O2 (B 3∑−u, v′→X 3∑−g, v″) with v″ as high as 35. The X 3∑−g potential energy function, Vx (r), was determined nearly to the dissociation limit with an estimated accuracy of ±50 cm−1. Vx (r) is compared to the highest level ab initio calculation available. The transition dipole moment function, μ(r), for the O2 Schumann–Runge band is also extended almost to the dissociation limit. The O2 molecule now provides one of the best testing grounds for ab initio theories of dissociation and radiation.
Brownian dynamics study of the shape of star and linear polymers in different regimes
Authors: Bishop Marvin , Clarke Julian H. R.
Published: July 2002

Brownian dynamics computer simulations are used to investigate the shape of linear and star polymers in three regimes: (1) excluded volume, (2) θ, (3) collapsed. It is found that the mean-square radius of gyration 〈S2〉 displays a different power law exponent in the different regimes: 〈S2〉∼(1) (N−1)1.2 (2) (N−1)1.0, and (3) (N−1)2/3 where N is the number of chain units. The pair-correlation function using the central vertex bead as the origin reveals the polymer structure. In the collapsed regime the structure resembles face-centered-cubic (FCC) packing. The g ratio of star/linear chains as well as the asphericity of the polymers have been determined for the three regimes.
A selected ion flow tube study of the reactions of the PH+n ions (n=0 to 4) with several molecular gases at 300 K
Authors: Smith David , McIntosh Bruce J. , Adams Nigel G.
Published: July 2002

The reactions of ions in the series PH+n (n=0 to 4) have been studied at 300 K with CH3NH2, PH3, NH3, CH3CCH, H2S, C2H4, CH3OH, COS, C2H2, O2, H2O, CH4, HCN, CO2, CO, and H2 using a selected ion flow tube (SIFT) apparatus. The majority are fast binary reactions, the measured rate coefficients, k, being close to the respective collisional values, and for reactions involving polyatomic reactant molecules multiple product ions are observed. Most reaction mechanisms are represented in this large number of reactions including charge transfer, proton transfer, atom abstraction and especially evident is phosphorus atom insertion in which product ions such as HnPN+, HnPO+, HnPS+ and HnCmP+ are formed. The reactions of the PH+n ions (n=0 to 2) with CO2, CO, and H2 proceed by the process of ternary association except for the P+ reaction with CO2, in which PO+ is formed in a fast binary reaction. PH+4 reacts only with CH3NH2 and NH3 by fast proton transfer, a process which is always rapid when it is exothermic. Upper limits to the heats of formation of HPO+, H2PO+, PC2H2+ and HCP have also been deduced from their observation as products of some of the reactions.
Photochemistry of CCl3F and CCl2F2 in the 106–200 nm region
Authors: Ibuki Toshio , Hiraya Atsunari , Shobatake Kosuke
Published: July 2002

The photoabsorption cross sections and fluorescence excitation spectra of CCl3 F and CCl2 F2 were measured using synchrotron radiation at 106–200 nm. The observed absorption bands were accounted for as Rydberg transitions. The emitters produced from the CCl3 F and CC12F2 photodissociative excitations were attributed to CClF(Ã 1A″←X̃1  A′) and CF2 (Ã 1B1 →X̃ 1A1 ) transitions, respectively, and their radiative lifetimes were determined to be 626 ± 28 and 58 ± 2 ns. The emission from the CClF(A) starts at 140 nm and increases to 9.1 Mb at 106 nm, and that for CF2 (A) was deduced to be 0.9 Mb at 104.8 nm. The onsets of the fluorescence suggest that the electronically excited CClF and CF2 radicals are formed by the atomic Cl elimination but not by molecular Cl2 releasing process.
Computer simulations of two-dimensional flexible polymers in the dense phase
Authors: Tuthill G. F.
Published: July 2002

Using Monte Carlo simulation we study the frequency distribution for the number of chain extremities in the dense phase of two-dimensional noncrossing polymer models. For a system in which cyclization is forbidden we find consistency with recent predictions, namely, a Poisson form raised to a fractional power. Permitting cyclization restores the Poisson distribution. We also examine the Flory exponent ν for noncyclic chains in the dense phase for each case, and find trends consistent with 1/2 (cyclization forbidden) and the recently proposed value of 4/7 (cyclization allowed).
Distribution function of the radius of gyration for Gaussian molecules
Authors: Wei Gaoyuan
Published: July 2002

An analysis of the distribution function of the radius of gyration for Gaussian molecules of arbitrary complexity imbedded in d-dimensional space is carried out. An explicit expression for the coefficients in an exact series expansion of the distribution function is obtained as finite sums of terms involving only the eigenvalues of the connectivity matrix. For odd d, a recursive method is developed for evaluating the distribution function as a sum of simple integrals. For even d, a finite-sum representation exists. The leading term of the asymptotic series for the distribution function for large values of the argument is analytically evaluated for any Gaussian molecule of arbitrary length. Moments of the distribution are given in terms of either the eigenvalue spectrum or the coefficients of the eigenpolynomial of the connectivity matrix. For a special class of molecules, the limiting distribution functions are found to share a generic form which can be represented by a double series. The length-dependence of the 3D distribution function for linear chains and the dimensionality effect on the breadth of the limiting distributions for both circular and linear chains are investigated numerically. It is found that higher dimensionality tends to broaden the distribution.
The permanent electric dipole moment of iron monoxide
Authors: Steimle Timothy C. , Nachman David F. , Shirley Jeffrey E. , Merer Anthony J.
Published: July 2002

The Stark effect of the R(2), Q(2) and P(3) lines of the Ω′=2−X 5 Δ2 subband of the ‘‘orange’’ system of iron monoxide at 5919 Å has been measured. Values for the permanent electric dipole moments μ of 4.7±0.2 and 2.6±0.2 D for the ground and excited electronic states, respectively, were derived from the analysis of the Stark shift. The results are discussed in terms of the various bonding models and compared with theoretical estimates.
Mechanism of and alignment effects in spin–changing collisions involving atoms in 1P electronic states: Ca(4s5p 1P)+noble gases
Authors: Alexander Millard H. , Pouilly Brigitte
Published: July 2002

We present a detailed study of the mechanism of spin–changing transitions between 1P and 3P electronic states corresponding to the nsn′p Rydberg state of an alkaline earth atom in collision with a structureless, spherical partner. This type of process takes place on four potential curves and involves collisional coupling between the electronic angular momentum L of the electronically excited atom, the spin S of the atom in the final 3P state, and the orbital angular momentum l associated with the relative motion of the collision partners. We review the use of Hund’s case coupling schemes in describing the collision and present a careful analysis of the initial state preparation under conditions of laser excitation in a crossed beam experiment. This underlies the development of a new statistical model for the dependence of the spin–changing cross section on the alignment of the pump laser. Finally, we describe a new technique to follow the redistribution of flux onto locally adiabatic states as the initially excited atom is approached by the closed-shell partner. This is used to show that dynamical corrections to our simple statistical model are small.
The eikonal approximation to molecular photodissociation: Application to CH3I
Authors: Swaminathan P. K. , Stodden C. D. , Micha D. A.
Published: July 2002

The selfconsistent eikonal treatment is applied to the photodossociation of CH3 I by near ultraviolet light. A model based on two potential-energy surfaces is used as in previous studies, to compare our results with other treatments. State-to-state transition probabilities are calculated from eikonal wave functions constructed from classical trajectories and their divergence, using a recently developed procedure which avoids the generation of trajectory bundles. Numerical results have been obtained for cross sections versus light wavelength and for each vibrational state of the methyl fragment. Our results are in very good agreement with wave packet calculations. In addition, we have calculated these cross sections for several initial vibrational states of CH3 I, for some of which the total cross sections show several maxima.
A unified theory of electron transfer and internal conversion based on solitary electronic states
Authors: Knapp E. W. , Fischer S. F.
Published: July 2002

A description of electron transfer processes is presented which makes use of the concept of solitary states for the construction of the initial and the final state wave functions. Even though this concept is closely related to an adiabatic description in the sense that it provides partial delocalization of these states it also assures localization for a symmetric donor–acceptor system. The theory can be applied to arbitrary values of the internal parameters characterizing the system, that is the electronic donor–acceptor coupling V, the free energy change ΔE, and the coupling of the electron to nuclear motions represented by a Stokes shift S. This way the theory of nonadiabatic electron transfer processes is put on equal footing with the theory of adiabatic transitions and even more interestingly the same concept unifies the common electron transfer theory and the theory of internal conversion processes if ‖ΔE‖ and (or) ‖V‖ exceed S. It is shown analytically how the results of the conventional treatments for adiabatic, nonadiabatic, and internal conversion processes fall out of this theory as special cases. It is further examplified how the system develops from the so-called normal regime with ‖ΔE‖S as function of ΔE for fixed parameters of V and S and as function of V for fixed parameters of ΔE and S.
Statistical mechanics of small chain molecular liquids. II. Structure and thermodynamic properties of modeled n-butane liquid
Authors: Enciso E. , Alonso J. , Almarza N. G. , Bermejo F. J.
Published: July 2002

The theory of molecular liquids based on the extended reference interaction site model (RISM) equation is used to describe the intermolecular structure of a small hydrocarbon chain liquid like n-butane. The model treats the methyl and methylene groups as single interaction centers with pair interactions modeled by a Lennard-Jones (12:6) function and located at the position of the carbon nuclei in the molecule. The theory assumes that the equilibrium internal states distribution of nonpolar flexible molecules are determined primarily by the short range, hard repulsive portions of the intermolecular potential (see preceding paper). The molecular structure factor has been calculated from the theory, and the results are in good agreement with recent x-ray scattering experiments. From this structural information a perturbative scheme to obtain the thermodynamic properties is employed. The theoretical predictions compare favorably with recent simulation results, giving quantitative agreement with the equation state and the intermolecular energy. The effective energy difference between trans and gauche states is obtained from Van’t Hoff plots, giving a value of 0.486 kcal/mol. Also, the effective difference of molar volumes between conformers is calculated from ln K vs P plot, obtaining a value of −3.1 cm3/mol, the same order of magnitude than the experimental value observed on liquid 1,2-dichloroethane.
Time-dependent, diffusion-controlled reactions: The influence of boundaries
Authors: Loewenberg M. , Gavalas G. R.
Published: July 2002

A configurational averaging procedure is presented for the analysis of diffusion-controlled reactions in a medium containing a hard-sphere distribution of spherical sinks. The hierarchy of equations produced from the averaging procedure is truncated assuming low concentration of sinks. The method is shown to reproduce previous results for the decay of an initially uniform distribution of reactant in an unbounded medium and is subsequently applied to determine the reactant flux on the boundary of a semiinfinite domain. In the latter problem, the boundary has a significant effect which cannot be accounted for by the usual rate coefficient.
Selective particle clustering and percolation in binary mixtures of randomly centered spheres
Authors: Wu GuoHong , Chiew Yee C.
Published: July 2002

The clustering and percolation of particles in binary mixtures of randomly centered spheres are examined based on a selective particle connectivity criterion in which only particles of different species are allowed to form directly connected bonds. This problem is different from the usually studied ‘‘simple’’ percolation problem in which pairs of particles form directly connected bonds as long as they are separated by a distance σ or less. The percolation threshold and pair-connectedness function of the binary mixture are determined based on the connectivity Ornstein–Zernike integral equation in the Percus–Yevick (PY) approximation. It is shown that, within the PY closure, the present system can be mapped into the Widom–Rowlinson model in the theory of liquid state. The percolation thresholds and the pair–connectedness functions of the particles are numerically computed for a wide range of particle densities and number fractions. It is found that their percolation densities differ considerably from those found in the simple percolation problem for a binary mixture of randomly centered spheres. To our knowledge, this is the first study of selective particle clustering and percolation in multicomponent mixtures of particles.
Large computer simulations on elastic networks: Small eigenvalues and eigenvalue spectra of the Kirchhoff matrix
Authors: Shy L. Y. , Eichinger B. E.
Published: July 2002

Computer simulations of the formation of trifunctional and tetrafunctional polydimethyl-siloxane networks that are crosslinked by condensation of telechelic chains with multifunctional crosslinking agents have been carried out on systems containing up to 1.05×106 chains. Eigenvalue spectra of Kirchhoff matrices for these networks have been evaluated at two levels of approximation: (1) inclusion of all midchain modes, and (2) suppression of midchain modes. By use of the recursion method of Haydock and Nex, we have been able to effectively diagonalize matrices with 730 498 rows and columns without actually constructing matrices of this size. The small eigenvalues have been computed by use of the Lanczos algorithm. We demonstrate the following results: (1) The smallest eigenvalues (with chain modes suppressed) vary as μ−2/3 for sufficiently large μ, where μ is the number of junctions in the network; (2) the eigenvalue spectra of the Kirchhoff matrices are well described by McKay’s theory for random regular graphs in the range of the larger eigenvalues, but there are significant departures in the region of small eigenvalues where computed spectra have many more small eigenvalues than random regular graphs; (3) the smallest eigenvalues vary as n−1.78 where n is the number of Rouse beads in the chains that comprise the network. Computations are done for both monodisperse and polydisperse chain length distributions. Large eigenvalues associated with localized motion of the junctions are found as predicted by theory. The relationship between the small eigenvalues and the equilibrium modulus of elasticity is discussed, as is the relationship between viscoelasticity and the band edge of the spectrum.
Density dependence of the ground-state energy of excess electrons in liquid methane
Authors: Plenkiewicz B. , Plenkiewicz P. , Jay-Gerin J.-P. , Jain Ashok K.
Published: July 2002

The ground-state energy (V0) of excess electrons in liquid methane is calculated as a function of liquid density. In the calculation, we use a high-precision molecular potential which satisfactorily describes the scattering of low-energy electrons in the gas phase. Our results are found to reproduce very well the experimental V0 values.
A model for impulsive mode–mode energy transfer in highly vibrationally excited molecules
Authors: Muller Richard P. , Hutchinson John S. , Holme Thomas A.
Published: July 2002

The classical dynamics of mode–mode vibrational energy transfer are investigated for a coupled Morse oscillator system by explicitly integrating Hamiltonian’s equations in action–angle coordinates. This method permit the identification of specific oscillator phase relationships which dictate the extent and timing of large-scale, nonperiodic energy transfer; we term this ‘‘impulsive energy transfer.’’ In previous classical trajectory studies, such impulsive events have been related to the dynamics of isomerization immediately prior to reaction. A qualitative understanding of the required phase relationships for impulsive energy transfer is gained via the Chirikov hindered rotor analysis, usually applied to resonant energy flow. The implications of the model are illustrated in a three mode isomerization system.
Radio frequency and microwave spectroscopy of the HCCH–CO2 and DCCD–CO2 van der Waals complexes
Authors: Muenter J. S.
Published: July 2002

Molecular beam electric resonance spectroscopy has been used to study HCCH–CO2 and DCCD–CO2, giving the following results for HCCH–CO2: A=8875 MHz, B=2861.45 MHz, C=2156.25 MHz, DJ =12.4 kHz, DJK =36.7 kHz, and d1=−3.25 kHz. The permanent dipole moment is 0.1611 D. The equilibrium geometry has the monomers parallel to one another in a configuration having C2v symmetry. Deuterium eQq data provide information on rms vibrational amplitudes and also gives an estimate of 40 cm−1 for the out of plane bending mode. The monomer–monomer stretching vibration is estimated to be 75 cm−1 from the DJ measurement.
A laser-induced fluorescence study of product rotational state distributions in the charge transfer reaction: Ar+(2P3/2)+N2→Ar+N+2 (X) at 0.28 and 0.40 eV
Authors: Sonnenfroh David M. , Leone Stephen R.
Published: July 2002

The nascent rotational state distributions of N+2 produced in the charge transfer reaction of Ar+ (2 P3/2 ) with N2 at 0.28 and 0.40 eV are remeasured by laser-induced fluorescence. A supersonic expansion is used to reduce the initial rotational angular momentum of the N2 . The N+2 product rotational distributions, in both v″=0 and v″=1, have low and high energy components. For ease of reference, we describe each distribution as a summation of two Boltzmann distributions. At a relative collision energy of 0.28 eV, the Boltzmann temperatures are 100±20 K and 745±120 K for N+2 (v″=0) and 80±10 K and 680±30 K for N+2 (v″=1). Adiabatic potential energy curves for the lowest vibronic states are calculated and a simple curve hopping model is presented. Applying this model to the production of N+2 (v″=1), for example, those reactants that charge transfer on the outgoing leg of a reactive trajectory interact with a deep potential well in the entrance channel for collinear geometry. We postulate that rotationally excited products result. In comparison, reactants that charge transfer on the ingoing leg (or in perpendicular geometry) do not sample the collinear potential well and the resulting products are less rotationally excited.
Rotationally inelastic scattering of N2 from clean and hydrogen covered Pd(111)
Authors: Lykke Keith R. , Kay Bruce D.
Published: July 2002

Quantum-resolved molecular beam–surface scattering from clean and hydrogen covered Pd(111) is shown to be a sensitive dynamical probe of changes in the gas–surface interaction potential brought about by the presence of chemisorbed surface hydrogen. In particular, the specular scattering of N2 from hydrogen covered Pd(111) is appreciably less inelastic rotationally than scattering from the clean surface. The specularly scattered rotationally elastic channel is utilized as a nonperturbative probe of the surface chemisorbed hydrogen coverage during a temperature programmed desorption (TPD) experiment.
Relaxation dynamics following transition of solvated electrons
Authors: Barnett R. B. , Landman Uzi , Nitzan Abraham
Published: July 2002

Relaxation dynamics following an electronic transition of an excess solvated electron in clusters and in bulk water is studied using an adiabatic simulation method. In this method the solvent evolves classically and the electron is constrained to a specified state. The coupling between the solvent and the excess electron is evaluated via the quantum expectation value of the electron–water molecule interaction potential. The relaxation following excitation (or deexcitation) is characterized by two time scales: (i) a very fast (∼20–30 fs) one associated with molecular rotations in the first solvation shell about the electron, and (ii) a slower stage (∼200 fs), which is of the order of the longitudinal dielectric relaxation time. The fast relaxation stage exhibits an isotope effect. The spectroscopical consequences of the relaxation dynamics are discussed.
Gaussian-like quadrature rules for quantum mechanical calculations
Authors: Linderberg Jan , Öhrn Yngve , Padkjær So/ren B.
Published: July 2002

Quadrature rules for the evaluation of matrix elements relevant for one-dimensional quantum mechanical problems are developed and tested. Nodes and weights are determined from a related problem and the general transformation between localized and delocalized basis representations is discussed.
The Ã 2E–X̃ 2A1 transition of monomethyl calcium: A rotational analysis
Authors: Brazier C. R. , Bernath P. F.
Published: July 2002

The 000 band of the Ã 2E–X̃ 2A1 transition of CaCH3 has been rotationally analyzed at Doppler-limited resolution by laser excitation spectroscopy. Observation of an internal perturbation in the excited state permitted simultaneous determination of both A′ and A″. The ground-state molecular structure was found to be RCa–C=2.349(13) Å and θH–C–H=105.6(28)° with RC–H fixed at 1.100(20) Å.
Observation of collision-induced-dipole absorption bands in strontium–rare-gas mixtures. II. Bands involving high-lying states
Authors: Ueda K. , Komatsu T. , Sato Y.
Published: July 2002

We have observed many collision-induced-dipole (CID) absorption bands arising from the transitions between quasimolecular ground and high-lying (n≤10) states in the strontium–rare-gas systems. For each absorption band, we have measured the energy shift of the absorption peak from the energy of the correlating atomic forbidden transition and the effective oscillator strength per unit perturber density fCID/Np. The shift is roughly proportional to the electron scattering length L0 for each rare-gas atom, whereas the fCID/Np is roughly proportional to L20. The shift decreases in general as the principal quantum number n increases, and increases as one goes from the s state to the d state, and to the degenerate manifold state with l≥3. These general features of the shift and fCID/Np are consistent with the predictions by a simple Fermi-potential model, suggesting the important role of the interaction between a Rydberg electron and a rare-gas atom in the CID absorption processes.
Observation of collisionally transferred spikes in NH3 by infrared–infrared double resonance
Authors: Matsuo Yukari , Lee Sang K. , Schwendeman R. H.
Published: July 2002

The shapes of spectral lines obtained by a series of four-level infrared–infrared double-resonance experiments in 15NH3 have been used to obtain information about the collisional transfer of energy in this molecule. In these experiments a CO2 laser has been used to pump a near-resonant vibration–rotation transition in the ν2 band while an infrared microwave sideband laser system scans a vibration–rotation transition in the 2ν2←ν2 band. When the k quantum numbers in the pump and probe are such that Δk=3n, where n is a positive or negative integer, the probe transition is a superposition of a Gaussian part and a transferred spike. When Δk≠3n, only the Gaussian part is observed. The widths of the transferred spike for ‖Δk‖=3 and 6 transitions, which have been observed for the first time in this work, are substantially greater than the width of the spike observed for Δk=0 transitions. Theoretical expressions are given for the line shape of the transferred spike and for the ratio of the intensities of the two components of the double resonance. The line shapes have been used to estimate the root-mean-square change in velocity upon collision for Δk=0 and ‖Δk‖=3 transitions. The results of three-level double resonance measurements in which a ν2 fundamental transition is pumped while a 2ν2←ν2 transition is probed are also reported. The widths of the three-level double resonance for copropagating and counterpropagating beams are significantly different and the results of simulations to reproduce the line shapes and their implications for collisional energy transfer are discussed.
Isothermal–isobaric molecular dynamics simulation of polymorphic phase transitions in alkali halides
Authors: Ruff Imre , Baranyai Andras , Spohr Eckhard , Heinzinger Karl
Published: July 2002

As the first application of the isothermal–isobaric molecular dynamics technique to a system with Coulombic forces, the pressure-induced face-centered-cubic to body-centered-cubic transition of some alkali halides is studied assuming Born–Mayer–Huggins potentials between the ions. The long-range forces are handled by the cubic harmonic expansion of the Ewald summation. The property that any periodic boundary condition of orthorhombic symmetry can be uniformly treated in this expansion allows the independent fluctuation of each box length in the course of the simulation of the isobaric ensemble. This reduces stresses during phase transitions from one crystalline state to another and yields less defects. By the appropriate change in the external pressure, the fcc–bcc transition takes place reversibly only in the presence of a considerable amount of defects. Otherwise, quite large hysteresis is found. It is shown that the [100], [010], and [001] directions of the original fcc lattice become the [ 1/2 1/2 1̄], [1̄01], and [111] directions in the bcc lattice, respectively. This confirms the Watanabe–Tokonami–Morimoto mechanism supported also by most of the experimental observations.
Matrix induced frequency shifts of transition metal atoms in Ar, Kr, and Xe matrices at 10 K
Authors: Jeong G. H. , Klabunde K. J.
Published: July 2002

A linear correlation was found for rare gas polarizabilities and matrix induced frequency shifts of Ru, Os, and Ir atoms. Nickel, Mo, Ag, and Au atoms also showed the same trend. A modified Lennard-Jones (6-12) function where many-body interactions are considered between an isolated atom (guest) and matrix atoms (host) was used to investigate the relationship between the matrix induced frequency shifts and the rare gas polarizability. It is concluded that van der Waals interaction is the major source of the frequency shift for Ru, Os, Ir as well as Ni, Mo, Ag, and Au atoms. However, Y atoms did not show such a linear correlation, and additional interaction modes are indicated.
Diffuse vibrational structures in photoabsorption spectra: A comparison of CH3ONO and CH3SNO using two-dimensional a
b i
n
i
t
i
o potential energy surfaces
Authors: Schinke Reinhard , Hennig Steffen , Untch Agathe , Nonella Marco , Huber J. Robert
Published: July 2002

We investigated the photodissociation of methyl nitrite (CH3 ONO) and methyl thionitrite (CH3 SNO) within the first absorption band (S1 ←S0 ). The calculations were based on a two-dimensional model including the O–NO/S–NO and N=O bond distances as active coordinates. The S1 -potential energy surfaces were calculated with quantum chemical methods and the dynamical calculations were performed exactly within the time-independent approach. The main emphasis is on the origin of diffuse vibrational structure in the photoabsorption spectrum of both molecules. A low potential barrier of 0.086 eV along the O–NO dissociation coordinate in CH3 ONO prevents immediate dissociation and leads to an initial state dependent lifetime for the excited complex of 100–250 fs corresponding to 3–8 NO vibrational periods. CH3 ONO decays nonadiabatically via vibrational predissociation. The absorption spectrum of CH3 ONO is dominated by narrow Feshbach-like scattering resonances which can be characterized by two quantum numbers, m and n*: m=0 and 1 specifies the quanta of excitation in the O–NO bond and n*=0,1,2,... specifies the excited vibrational level of the N=O bond. The potential barrier is absent in CH3 SNO and the dissociation is direct on the time scale of about 10 fs corresponding to only one third of a NO vibrational period. Nevertheless, the absorption spectrum exhibits diffuse vibrational structures. The shape of the individual absorption peaks is determined by the classical Franck–Condon reflection principle. The dissociation of CH3 SNO is primarily adiabatic which leads to a pronounced energy dependence of the final NO vibrational state distribution. The diffuse structures originate in both cases from excitation of the NO stretching vibration. In order to make contact with time-dependent theory we calculated the autocorrelation function of the time-dependent wave function by inverse Fourier transformation of the energy-dependent spectra. The agreement with available experimental data for both molecules is quite satisfactory. This includes the energy spacing of the vibrational structure, the overall shape of the absorption spectrum, and the

lifetime of the excited complex.
Classical dynamics of collision-initiated intramolecular energy flow between metal⋅⋅⋅diatom bonds
Authors: Shin H. K.
Published: July 2002

A classical trajectory study of energy flow between two metal-molecule bonds separated by a heavy metal atom in a collisionally perturbed complex O 3/4 C⋅⋅⋅M⋅⋅⋅C 3/4 O is presented. The left M⋅⋅⋅CO bond is excited by collision and subsequent flow of excitation is followed by classical mechanics. In the heavy metal system, the behavior of energy flow blockage by the metal atom prevails at certain total energies of the complex. The blockage causes a long time delay for energy flow between two M⋅⋅⋅CO bonds and bond dissociation. Such energy flow blockage is found to be absent in the light metal system, where the amount of energy transfer to the complex is much larger. The energy flow patterns associated with the blockage are discussed in detail. Specific systems considered are the collisional excitation of OC⋅⋅⋅Pt⋅⋅⋅CO and OC⋅⋅⋅Ni⋅⋅⋅CO by Ar.
Implications for multidimensional effects on isomerization dynamics: Photoisomerization study of 4,4′-dimethylstilbene in n-alkane solvents
Authors: Park N. S. , Waldeck D. H.
Published: July 2002

Studies of the photoisomerization of 4,4′-dimethylstilbene in n-alkane solvents are reported. As with similar studies it is possible to identify an activation barrier to isomerization and the viscosity dependence of the isomerization is not adequately explained by a one-dimensional hydrodynamic Kramers equation. Comparison of the isomerization rates with those of other stilbenes suggests that the use of a frequency dependent friction for the solvent does not consistently explain the data. These results and those of other stilbenes can be explained by a two-dimensional isomerization coordinate where the reaction trajectory is linked to the solvent viscosity.
Comparison of quasiclassical trajectory calculations to accurate quantum mechanics for state-to-state partial cross sections at low total angular momentum for the reaction D+H2→HD+H
Authors: Blais Normand C. , Zhao Meishan , Mladenovic Mirjana , Truhlar Donald G. , Schwenke David W. , Sun Yan , Kouri Donald J.
Published: July 2002

We compare the results of classical simulations to accurate quantum studies for the reaction D+H2(v=0, j=1)→HD(v′, j′)+H (where v and v′ are vibrational quantum numbers and j and j′ are rotational quantum numbers) on a double many-body expansion (DMBE) potential energy surface. The quantal results presented here are converged to three significant figures. We consider orbital angular momenta 0 and 1 and total angular momenta 0–2. The total energies considered, Etot=0.983–1.333 eV, correspond to relative translational energy Erel=0.70–1.05 eV for the v=0, j=1 vibrational–rotational state. For J=1 and odd parity, the number of channels included is up to 453 and the number of open channels is up to 177. For J=2 and even parity, the number of channels is 630 and of open channels is 147.
Probing the He–H2 potential surface with dynamical and kinetic observables
Authors: Smith Maureen J. , Shi Shenghua , Rabitz Herschel
Published: July 2002

This paper examines the effect of infinitesimal functional variations in a rigid rotor He–H2 potential surface on several different levels of observables: inelastic cross sections, rate constants, and energy level populations. Equations are derived for the functional derivatives of these observables with respect to a variation in the potential surface. Sensitivities are presented with respect to the entire potential surface, as well as the individual Legendre components Vn(r). The dynamical and kinetic observables studied were found to be most sensitive to the V2(r) term in the potential with the region of highest sensitivity dependent upon the energy or temperature as well as the states related by the individual observable. Sensitivity to the entire surface tends to show more structure due to interference among sensitivities to the individual components. While the main information on the underlying potential is retained, some information loss has been observed in the transition from the microscopic observables to the macroscopic ones.
Theory of non-Markovian reversible dissociation reactions
Authors: Agmon Noam , Weiss George H.
Published: July 2002

We consider a reversible dissociation–recombination reaction in solution which is described by a distribution of waiting times rather than a single dissociation rate constant. This is a non-Markovian generalization of the backreaction boundary condition. We formulate the new boundary condition in terms of the residence time in the bound state and illustrate the theory by assuming a stable-law density for the residence time. Explicit expressions are found for the Laplace transform of the survival probability in one and three dimensions, which can be inverted analytically for special values of the stable-law parameter α and numerically for other values of α. We derive the long-time behavior of the survival probability for arbitrary α, and note that the survival probability undergoes a first-order phase transition in one dimension, in which its asymptotic value changes abruptly at α=1/2. In three dimensions it undergoes a second-order phase transition at α=1, in which only the asymptotic slope of the survival probability changes discontinuously.
The effect of reagent rotation on chemical reactivity: F+H2 revisited
Authors: Harrison Judith A. , Isakson Laura J. , Mayne Howard R.
Published: July 2002

Classical trajectory calculations on the gas phase reaction F+H2 ( j)→HF+H have been carried out. Different reactivity trends were seen depending on whether there was a chemically significant and anisotropic well in the entrance channel of the potential surface. For those in which there is no such well, rotation may decrease reactivity at low values of j, but increases it thereafter. The reaction cross section SR ( j) decreases slowly from j=0, reaching a minimum near j=6 then increases again. This behavior has been reported for several systems, including H+H2, and seems to be the ‘‘canonical’’ behavior for SR ( j) for most direct chemical reactions. For F+D2 the minimum does not occur until j=8. However, this does correspond to the same amount of rotational energy as the minimum for F+H2 . For potentials in which there is a deep anisotropic well, it is found that the j=0 results are dominated by the presence of the well, and that the SR ( j=0) is anomalously high. On such surfaces there is normally a sudden drop in cross section from j=0 to j=1, followed by an increase. The experimental findings of Lee’s group [J. Chem. Phys. 82, 3045 (1985)] that the cross section increases on going from j=0 to j=1 probably precludes the possibility of a chemically significant well in the entrance valley. The rotational product state distribution for both types of potential is dominated by kinematics away from threshold, and does not show the same trends as the reaction cross section. The mean product vibrational quantum number 〈v′〉 can decrease at low j, then increase at higher j. This occurs only at collision energies close to threshold, and on potentials which have a tight bend force constant at the transition state. The more general case for this reaction is that 〈v′〉 increases with j. For the case in which the potential has no well the differential cross section shows precisely the same trends as does SR( j). All these trends can be explained using a simple model we have recently proposed.
Multiplet-specific shape resonant features in vibrationally resolved 3σg photoionization of O2
Authors: Braunstein M. , McKoy V.
Published: July 2002

We report multiplet-specific vibrationally resolved photoionization cross sections and photoelectron angular distributions for the 3σg orbital of O2 leading to the v+=0–3 levels of the b 4Σ−g and B 2Σ−g states of O+2. These studies were motivated by recent work which shows significant nonstatistical behavior in the vibrationally unresolved spectrum at low photoelectron energies arising from the sensitivity of the kσu shape resonance to the multiplet-specific exchange potentials. In addition to the anticipated non-Franck–Condon vibrational distributions arising from the kσu shape resonance, we also find substantial nonstatistical effects in our vibrationally resolved cross sections and particularly in our photoelectron angular distributions over a broad energy range. Extensive electronic autoionization due to Rydberg levels leading to the c 4Σ−u (2σ−1u) ion makes it difficult to assess these effects in the available experimental data.
Molecular alignment from circular dichroic photoelectron angular distributions in (n+1) resonance enhanced multiphoton ionization
Authors: Dubs Richard L. , McKoy Vincent
Published: July 2002

The theory for determination of molecular alignment from circular dichroism in photoelectron angular distributions is generalized to treat the case in which the excitation polarization direction and the laboratory z axis do not coincide. A new method of data analysis is presented here. Alignment created by surface scattering or photofragmentation should be obtainable by these procedures. For studies of orientation with elliptically polarized excitation, differential cross sections at a given collection angle are found to be, to a good approximation, independent of excited-state alignment. Orientation can thus be obtained from differential cross sections by the methods developed by Kummel, Sitz, and Zare [J. Chem. Phys. 88, 6707 (1988)].