bluuebunny/crossref_metadata_2025 huggingface dataset:
Mass spectra of Ca–O and Ba–O clusters
Authors: Martin T. P. , Bergmann T.
Published: July 2002

Mass spectra of metal-rich, singly and doubly ionized BanOx clusters are characterized by high peak intensities for n=13, 19, 23, 26, 29, 32, and 35 independent of the number of oxygen atoms (x=1,2,3,4). These values are identical to the magic numbers found for Ar clusters. The mass spectra of CanO show a similar behavior. Mass spectra of stoichiometric, doubly ionized [Ca(CaO)n]++ clusters are characterized by low peak intensities for n=14, 24, 32, 38, 40, 50, and 64. These values correspond to unstable clusters formed by the addition of one Ca atom or one CaO molecule to a closed rectangular block of atoms with rock salt structure.
Homogeneous nucleation in metal tetrachloride vapors: Tin and titanium tetrachlorides
Authors: El-Shall M. Samy
Published: July 2002

The upward thermal diffusion cloud chamber was used to measure the critical supersaturations for homogeneous nucleation of droplets from the supersaturated vapors of tin and titanium tetrachlorides. The measured supersaturations are in good agreement with the predictions of the classical theory of nucleation. Trends in free energy barrier are correctly predicted by the classical theory for carbon, silicon, tin, and titanium tetrachlorides. However, the classical barrier does not reflect the binding energies in the critical clusters. According to the classical theory, the size of the nucleus of SiCl4 ranges from 60 to 120 molecules while for TiCl4 it varies from 35 to 65 molecules in the temperature range of 240–310 K. The classical theory predicts a linear log J vs 1/T relationship. Corresponding states correlations are used to locate general trends and correlate them with molecular properties. Titanium tetrachloride deviates from the ‘‘simple fluid’’ behavior observed for CCl4, SiCl4, and even SnCl4. This deviation must reflect a modified intermolecular potential for TiCl4 as compared to a simple fluid potential.
Low temperature phase transformation in superionic conductors: A molecular dynamics study of silver sulfide
Authors: Ray John R. , Vashishta P.
Published: July 2002

Upon lowering the temperature below 450 K the superionic solid Ag2 S undergoes a structural phase transformation from a bcc sulfur lattice with the space group Im3m to a monoclinic lattice with the space group P21 /c. We have successfully modeled this transformation using the variable size–shape form of molecular dynamics and a pair potential constructed earlier to model structural and transport properties in the superionic phase. Very good agreement is found between the partial pair distribution functions for the transformed crystal and the distribution functions calculated using the atomic positions obtained in x-ray studies. We also find good agreement between the three-body angular distributions calculated for the transformed system and distributions calculated for the structure based on the x-ray positions.
Infrared-multiphoton excitation and wave packet motion of the harmonic and anharmonic oscillators: Exact solutions and quasiresonant approximation
Authors: Marquardt Roberto , Quack Martin
Published: July 2002

The motion of the probability density of the harmonic and anharmonic quantum oscillators under IR-multiphoton excitation with a classical laser field is studied both by means of exact solutions and in the quasiresonant and Floquet approximations. The validity of the quasiresonant approximation has been studied for the first time in an infinite level problem (harmonic oscillator) without truncation. Different régimes of quasiclassical motion of the harmonic and anharmonic oscillator and nonclassical spreading of the probability density with regular and irregular dynamics are identified.
Influence of intramolecular vibrational dynamics on state-to-state photodissociation: T
r
a
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s DONO (Ã) versus HONO (Ã)
Authors: Shan J. H. , Vorsa V. , Wategaonkar S. J. , Vasudev R.
Published: July 2002

The state-to state fragmentation of the Ã state of trans DONO and HONO are compared. All the optically accessible vibrations (00, 2n) of the excited state are photoselected with a polarized tunable laser, and the dissociation dynamics is probed through measurements on the ejected OD and OH photofragments. DONO’s N=O stretching (ν2) vibration leaves a significant fingerprint on the OD fragment’s rotational energy distribution, and rotational and orbital anisotropy. On the other hand, the effect of ν2 on OH photoejected by HONO is very small. These differences in the photochemistry of the isotopic cousins are attributed to the coupling in DONO between the –N=O coordinate and the DON bending motion ν3, leading to ‘‘V→V transfer.’’ The in-plane ν3 oscillation transforms predominantly into the high-energy wing of the OD fragment’s rotational distribution, so that increasing the number of ν2 quanta in the parent leads to higher product rotational temperature and anisotropy. The ν2 motion in HONO, by contrast, is not coupled to the ν3 coordinate and, consequently, increasing the ν2 content in this case does not result in profound changes in fragment properties.
Low energy electron attachment to SF6 in N2, Ar, and Xe buffer gases
Authors: Hunter S. R. , Carter J. G. , Christophorou L. G.
Published: July 2002

The electron attachment rate constants ka for SF6 have been measured in dilute mixtures of SF6 in high pressure (>1 atm) N2, Ar, and Xe buffer gases at room temperature (T≊300 K) over a wide E/N range (electric field strength to gas number density ratio), corresponding to mean electron energies 〈ε〉 from near thermal electron energies (≊0.04 eV) to 〈ε〉≊4.3 eV. Particular attention has been paid to the effects of space charge distortion, molecular impurities, and changes in the electron energy distribution function on the measured electron attachment rate constant values at the lower E/N values in these mixtures. The present measured thermal electron attachment rate constants in SF6/N2 and SF6/Xe gas mixtures are in excellent agreement with recent accurate measurements of these parameters in several SF6/buffer gas mixtures. At higher 〈ε〉 values, the present SF6/N2 measurements are in fair agreement with previous measurements, while no previous measurements using Ar and Xe buffer gases have been published. These measurements have been used in numerical two term, spherical harmonic Boltzmann equation analyses of the electron motion in these gas mixtures to obtain the low energy (<10 eV) nondissociative and dissociative electron attachment cross sections for SF6. The present derived electron attachment cross sections are compared with recently measured and derived nondissociative and dissociative electron attachment cross sections for SF6. The primary value of the present results is in the large and overlapping 〈ε〉 ranges of the present ka measurements for the three buffer gases compared with that for SF6/N2 gas mixtures alone, which in turn, makes these measurements ideal for testing cross-section sets in SF6 for use in many applied studies.
New model of polymer entanglement: Localized Gauss integral model. Plateau modulus G
N, topological second virial coefficient Aθ2 and physical foundation of the tube model
Authors: Iwata Kazuyoshi , Edwards S. F.
Published: July 2002

The localized Gauss integral (LGI) model, which has been proposed recently by the authors [Macromolecules 21, 2901 (1988)], is discussed in detail. Plateau modulus GN is computed as a function of polymer concentration c and the topological interaction parameter γ̄. Parameter γ̄ is determined for various polymers using their bulk GN data; for polystyrene (PS), γ̄ determined from GN agrees well with that determined from the topological second virial coefficient, Aθ2. Using γ̄ determined from GN, Aθ2 of various polymers other than PS are computed numerically and it is found that Aθ2 of most polymers are much larger than that of PS. It is further shown that power α to the concentration dependence of GN is estimated to be 1.97–2.12 in agreement with experimental value 2.0–2.3 and that GN/c2lkBT is proportional to Aθ2, where cl is the number concentration of ‘‘localized chains.’’ Finally, LGI model is compared with the Doi–Edwards model and the two basic hypothesis of the latter model, the tensile force along the tube and the rubber-like expression of stress, are examined in view of the present model.
Rotation–vibration spectra of icosahedral molecules. I. Icosahedral symmetry analysis and fine structure
Authors: Harter William G. , Weeks David E.
Published: July 2002

Icosahedral symmetry analysis is developed for analyzing eigensolutions of rovibrational tensor Hamiltonians for molecules such as B12H12−2, C20H20, and C60. Simplified asymptotic formulas and procedures are developed for obtaining rotational spectral fine structure for high angular momentum. J=100 eigenlevels for sixth- and tenth-rank icosahedral tensors are discussed using different approximations and visualization schemes.
Spectral and ionization energy shifts in van der Waals clusters
Authors: Kim Hye-Young , Cole Milton W.
Published: July 2002

A model calculation is presented of the spectral shift (δν) and ionization energy shift (δI) for small van der Waals (vdW) clusters consisting of an aromatic molecule and a rare-gas atom. The change of the vdW dispersion interaction associated with excitation or ionization is included explicitly. This new term is found to play a very important role and improve the previous work on δI by Jortner et al. The results show consistent trends and order of magnitude agreement in comparison with experimental data.
Rotationally resolved spectra of the 61 and 6111 band of benzene in a moderately cold molecular beam: Spectral and dynamical analysis
Authors: Riedle E. , Knittel Th. , Weber Th. , Neusser H. J.
Published: July 2002

Rotationally resolved spectra of the 610 and 610110 band of benzene, C6H6, in a supersonic molecular beam at rotational temperatures between 8 and 50 K are reported. The spectra consist of lines of 85 MHz linewidth. An unexpectedly low saturation intensity of ≊104 W/cm2 is found for the observed one-photon transitions, and it is shown that the saturation intensity differs for different rotational lines within one vibronic band. The rovibronic line spectra are analyzed within the framework of a rigid symmetric top model and highly precise values of the rotational constants are determined. In addition, the rotationless transition frequencies ν00 are obtained with high precision. The spectrum of the 610 band shows no signs of rotational perturbations, while the 610110 band at higher vibrational excess energy shows indications of perturbations for lines with K′ above 10. The decay times of single rotational states within the 6111 vibronic state are reported and no rotational dependence of the decay time is found in agreement with the statistical limit character of the interstate nonradiative process.
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i
o calculations of polarizability and second hyperpolarizability in benzene including electron correlation treated by Mo/ller–Plesset theory
Authors: Perrin E. , Prasad P. N. , Mougenot P. , Dupuis M.
Published: July 2002

We present an ab initio calculation of polarizability and second hyperpolarizability for the benzene molecule including electron–electron correlation. The finite field method is used. For each selected strength of the applied electric field the energy of the benzene molecule is calculated using the self-consistent field method (SCF) as well as with its Mo/ller–Plesset correction in the second order (MP-2). Then the microscopic optical nonlinear responses are calculated by fitting both the SCF energy and the MP-2 energy to a polynomial in the field strength. We find that electron correlation significantly enhances the second hyperpolarizability. For the polarizability, our computed value shows an excellent agreement with the experimentally measured value. For the second hyperpolarizability, the computed value using MP-2 energy shows a reasonable agreement with that reported by the electrical field induced second harmonic (EFISH) generation but a poor agreement with the result of degenerate four wave mixing (DFWM). Finally, we also compare our ab initio results with those previously reported using semiempirical methods.
Phase transition behavior observed in small silicon cluster ions
Authors: Beck S. M. , Andrews J. M.
Published: July 2002

Small silicon clusters produced by laser vaporization supersonic expansion are heated to different internal temperatures during a subsequent photoionization step, depending upon the ionization laser wavelength. Following ionization and mass selection the cluster is photofragmented by a 532 nm pulse from a Nd:YAG laser, and the charged fragments are collected and mass analyzed by a time-of-light mass spectrometer. It is found that the fragmentation patterns for clusters larger than Si+10 have a dependence upon the cluster temperature. This dependence is ascribed to clusters which are fairly rigid structures at low temperatures, undergoing a transition to a fluxional state at higher temperatures. Such a transition is analogous to a phase transition in larger systems.
Production and properties of singly and multiply charged Kr clusters
Authors: Lezius M. , Scheier P. , Stamatovic A. , Märk T. D.
Published: July 2002

Kr clusters produced in a supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry. Mass resolved spectra (with n up to 180) show two homologous series consisting of Kr+n and Kr2+n ions. The distribution of Kr+n ions shows distinct magic number effects, the observed abundance anomalies being very similar to the ones observed in Ar and Xe. This confirms the superior stability of closed-shell and -subshell icosahedral structures. Moreover, we have found evidence for the occurrence of Kr3+n and Kr4+n ions. It was possible to determine appearance sizes of these multiply charged cluster ions (yielding n2=69, n3=156, and n4=264), and to study the electron energy dependence of singly and doubly charged cluster ions (yielding a linear threshold law). These results are discussed in view of various theoretical considerations and previous results where available.
Ni clusters: Structures and reactivity with D2
Authors: Raghavan K. , Stave Mark S. , DePristo Andrew E.
Published: July 2002

We report the results from a theoretical investigation of the structures and reactivity of various isomers of Ni clusters in the size range from 4 to 13 atoms. The geometries of the clusters were optimized using binding energy values calculated by the corrected effective medium (CEM) theory. Two different potential energy surfaces were used to describe the interaction between D2 and the Ni clusters. The first used the form and parameters that were determined in the study of H2 dissociative chemisorption on Ni surfaces, while both used atomic positions appropriate to the clusters, the second used the same form but determined the parameters by comparison to CEM values of the H/Ni13 interaction. Using these PES, we investigated the dissociation dynamics of D2 on NiN (N=4–13) clusters by classical trajectory techniques. We found that: (1) for clusters of size less than Ni9 , the rate constants varied strongly with cluster size; and (2) for all size clusters, the rate constants were very sensitive to different isomers. This isomeric variation of the rate constant is discussed in terms of various structural features in different isomers, a number of which do not have any analogy in the dissociative chemisorption on low Miller index surfaces.
A photoion–photoelectron coincidence study of (N2)2 and (N2)3
Authors: Norwood K. , Luo G. , Ng C. Y.
Published: July 2002

The photoion–photoelectron coincidence (PIPECO) spectra for (N2)+2 in the wavelength range 650–866 Å have been measured at different nozzle stagnation pressures. The formation of stable (N2)+2 from fragmentation of excited (N2)+n cluster ions initially produced by photoionization of (N2)n, n≥3, is efficient. For nozzle expansion conditions which minimize the production of (N2)n, n≥3, the intensities for the N+2(Ã,B̃)⋅N2 PIPECO bands are found to be negligibly small compared to that of the N+2(X̃)⋅N2 PIPECO band, indicating that the electronically excited N+2(Ã,B̃)⋅N2 dimer ions are dissociative in temporal ranges <42 μs. Assuming that the radiative lifetimes for N+2(Ã,B̃) and N+2(Ã,B̃)⋅N2 are identical, we estimate that the dissociative lifetimes for N+2(Ã)⋅N2 and N+2(B̃)⋅N2 are ≲10 μs and ≲60 ns, respectively. The ionization energy for (N2)2 is determined to be 14.50±0.08 eV (855±5 Å), suggesting that N+2(X̃)⋅N2 is bound by 1.09±0.08 eV. The PIPECO data for (N2)+2 presented here and those for (CO)+2 reported previously support the perturbed monomer ion model for the photoionization of a van der Waals cluster. Namely, the formation of N+2⋅(N2)n−1 by photoionization of (N2)n, n≥2, can be viewed as a photoionization process of N2 perturbed by the presence of other N2 molecules in the clusters. We suggest that the rapid dissociation of electronically and vibrationally excited dimer ions is a general mechanism for the suppression of autoionization features in the photoionization efficiency spectrum for an ionized van der Waals dimer.
Finding saddle points for clusters
Authors: Wales David J.
Published: July 2002

The Cerjan–Miller eigenvector-following method is highly successful in finding saddle points for a variety of clusters whose potential energy surfaces are well described using simple analytical pair potentials. Examples are given for argon clusters, including Ar33 and Ar55, potassium chloride clusters and clusters of ions held in an ‘‘ion trap.’’ The method reveals a number of important patterns and opens the way to a more detailed understanding of the relation between energetics, reactivity, and potential energy surfaces for these and other systems.
Ionization potentials of large van der Waals molecules
Authors: Horin Narda Ben , Even Uzi , Jortner Joshua
Published: July 2002

In this paper we provide an extension of the electrostatic model for the structure and energetics of large van der Waals ions consisting of an aromatic cation bound to rare-gas atoms. We consider the effects of intermolecular electronic overlap on the charge-induced dipole interactions. Model calculations for tetracene⋅R+n (n=1–3 and R=Ar, Kr) and for benzene⋅R+ provide a reasonable account for the ionization potentials of these vdW ions.
Geometrical picture of reaction in enzyme kinetics
Authors: Nguyen An Hoang , Fraser Simon J.
Published: July 2002

A generalization of the steady-state and equilibrium approximations for solving the coupled differential equations of a chemical reaction is presented. This method determines the (steady-state) reaction velocity in closed form. Decay from rapidly equilibrating networks is considered, since many enzyme mechanisms belong to this category. In such systems, after transients have died away, the phase-space flow lies close to a unique trajectory, the slow manifold M. Locating M reduces the description of the system’s progress to a one-dimensional integration. This manifold is a solution of a functional equation, derived from the differential equations for the reaction. As an example, M for Michaelis–Menten–Henri mechanism is found by direct iteration. The solution is very accurate and the appropriate boundary conditions are obeyed automatically. In an arbitrary mechanism, at vanishing decay rate, the slow manifold becomes a line of equilibrium states, which solves the functional equation exactly; it is thus a good initial approximation at finite decay rate. These ideas are applied to a mechanism with both enzyme–substrate and enzyme–product complexes.
Mode-coupling theory of the dynamics of polymer liquids: Qualitative predictions for flexible chain and ring melts
Authors: Schweizer Kenneth S.
Published: July 2002

The qualitative predictions of the mode–mode-coupling (MMC) theory developed in the preceding paper are determined for various transport properties and time correlation functions. The degree of polymerization N dependence of the self-diffusion constant D of long flexible chain and rigid rod melts are found to be in agreement with the reptation/tube model scaling predictions. Ideal ring polymer liquids also follow a D∝N−2 law, but for collapsed non-Gaussian rings a stronger power law dependence is obtained. The viscoelastic properties of chain melts are derived from a linear generalized Langevin equation (GLE), which at long times consists of the usual Rouse terms plus a chain length and internal normal mode-dependent frictional contribution.

The latter novel term gives rise to of order N slow internal modes, and hence a plateau modulus, and a shear viscosity and recoverable compliance which scale as N3 and N0, respectively. All the predictions are derived from the MMC dynamic memory function by neglecting end effects, and without a priori invoking the existence of static entanglements, a confining tube, nor curvilinear diffusion. The molecular weight dependent renormalizations arise from time-dependent intermolecular force fluctuations on the radius of gyration and longer length scales. An alternative mathematical approximation for the viscosity memory function leads to an asymptotic scaling law of N3.5 for chain melts. This nonreptation prediction follows from the physical assumption that the fluctuating repulsive forces exerted by the surrounding matrix on the internal modes of a probe polymer can fully relax only after a time proportional to the probe translational diffusion time. The qualitative form of the mode-coupling GLE in the terminal relaxation regime can be approximately interpreted in terms of curvilinear reptation à la the Doi–Edwards formulation. However, this physical interpretation is not uniquely established, and the alternative possibility of isotropic, but highly cooperative, motion is not precluded. The viscoelastic properties of flexible ring melts are found to be qualitatively similar to their chain polymer counterparts.

A general microscopic approach for including the effects of matrix polymer mobility is formulated in terms of the collective dynamic structure factor of the melt. The qualitative implications for self-diffusion and crossover phenomena are studied, and a self-consistent theory for the shear viscosity is derived.
Vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 clusters
Authors: Nimlos M. R. , Young M. A. , Bernstein E. R. , Kelley D. F.
Published: July 2002

The first excited electronic state (S1) vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 van der Waals (vdW) clusters have been studied using molecular jet and time resolved emission spectroscopic techniques. The rates of intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) as functions of vibrational energy are reported for both clusters. For vibrational energy in excess of the cluster binding energy, both clusters are observed to dissociate. The dispersed emission spectra of these clusters demonstrate that aniline(Ar)1 dissociates to all energetically accessible bare molecule states and that aniline(CH4)1 dissociates selectively to only the bare molecule vibrationless state. The emission kinetics show that in the aniline(Ar)1 case, the initially excited states have nanosecond lifetimes, and intermediate cluster states have very short lifetimes. In contrast, the initially excited aniline(CH4)1 states and other intermediate vibrationally excited cluster states are very short lived (<100 ps), and the intermediate cluster 00 state is observed. These results can be understood semiquantitatively in terms of an overall serial IVR/VP mechanism which consists of the following: (1) the rates of chromophore to vdW mode IVR are given by Fermi’s golden rule, and the density of vdW vibrational states is the most important factor in determining the relative [aniline(Ar)1 vs aniline(CH4)1] rates of IVR; (2) IVR among the vdW modes is rapid; and (3) VP rates can be calculated by a restricted vdW mode phase space Rice–Ramsberger–Kassel–Marcus theory. Since the density of vdW states is three orders of magnitude greater for aniline(CH4)1 than aniline(Ar)1 at 700 cm−1, the model predicts that IVR is slow and rate limiting in aniline(Ar)1, whereas VP is slow and rate limiting in aniline(CH4)1. The agreement of these predictions with the experimental results is very good and is discussed in detail.
Coverage-dependent dipole coupling for carbon monoxide adsorbed at ordered platinum(111)-aqueous interfaces: Structural and electrochemical implications
Authors: Chang Si-Chung , Weaver Michael J.
Published: July 2002

Surface infrared spectra in the terminal C–O stretching region are reported for various 12CO/13CO mixtures adsorbed on an ordered Pt(111) electrode in 0.1 M HClO4 as a function of CO coverage θ (0.05≲θ≲0.6) and electrode potential E. Contributions to the coverage-induced shift in the C–O stretching frequency νtCO arising from dynamic dipole–dipole coupling ΔνD, and from static dipole and ‘‘chemical’’ effects ΔνC, are obtained from the mixed isotope spectra. Similar to the corresponding ultrahigh vacuum (UHV) system, the former provides the largest contribution, ΔνD being 30–35 cm−1 for saturated CO layers (θ≊0.6). Substantial differences in the νtCO -θ (and ΔνD -θ) dependencies were observed for CO adlayers formed by dosing from suitably low solution concentrations (∼2×10−5 M), and from saturated irreversibly adsorbed layers by means of partial electrooxidative stripping.

Using the latter ‘‘stripping’’ procedure, only small (≲5 cm−1) decreases in νtCO and ΔνD were observed as θ is decreased over the range 0.1≲θ<0.6, whereas the former ‘‘dosing’’ procedure yielded substantially larger νtCO -θ and ΔνD -θ dependencies, especially in the presence of coadsorbed hydrogen (at −0.25 V vs SCE) rather than coadsorbed water (at 0.1 V). These behavioral differences are interpreted in terms of coverage-dependent CO island formation. For electrooxidative stripping conditions, extensive island formation survives to low CO coverages, providing direct evidence that CO electrooxidation occurs via nucleation and growth at the island edges. For dosing conditions, smaller and more coverage-dependent CO patches are apparently obtained in the presence of coadsorbed hydrogen. These undergo slightly more facile electrooxidation than the CO islands formed at intermediate coverages by prior stripping. The coverage-dependent frequencies and intensities of the bridging CO form (νbCO ≊1800–1850 cm−1) vs the terminal CO form (νtCO ≊2020–2075 cm−1) are also examined in comparison with corresponding data for related UHV systems. In contrast to Pt(111)/CO in UHV, the bridging site is occupied even at low dosed coverages (θ≳0.05): this is also suggestive of the presence of CO islands having high local coverages. Similarities are noted with vibrational spectra obtained for Pt(111)/CO with coadsorbed water and hydrogen in UHV.
Melting and freezing of small argon clusters
Authors: Wales David J. , Berry R. Stephen
Published: July 2002

An investigation of melting, freezing, and coexistence phenomena is presented for small clusters using Ar7, Ar8, Ar13, and Ar14 as specific representative examples. Combining the results of molecular dynamics simulations, especially short-time kinetic energy averages and quenching, with accurate calculations of the local minima and transition states illuminates the relationship between the potential energy surface and dynamical processes. The results are consistent with a recent general defect theory of melting.
State-to-state dynamics of the H+p-H2→o,p-H2+H reaction: Feshbach resonances and vibrational spectroscopy of the transition state
Authors: Nieh Jong-Chen , Valentini James J.
Published: July 2002

We describe the results of an extensive series of state-to-state dynamics experiments on the hydrogen exchange reaction, H+p-H2(v=0, j=0,2)→o,p-H2(v′, j′)+H, for which we have measured the v′, j′ product state distributions and partial reaction cross sections at 22 collision energies between the v′=1 and v′=2 thresholds at 0.7 and 1.1 eV. A tunable ultraviolet laser photodissociates HI to make H atoms and select the collision energy. Coherent anti-Stokes Raman scattering (CARS) is used to detect the reaction products with state selectivity under single-collision conditions. We observe sharp oscillations in the v′=1 partial cross sections as the collision energy is varied, with three well-defined maxima at 0.70, 0.93, and 0.98 eV, and possibly another at 0.83 eV. These are energies at which Feshbach dynamical resonances have been predicted theoretically, and we interpret the peaks we observe in the energy-dependent yield of v′=1 product to be such resonances. These resonances result from excited vibrational states of the H3 transition state, and their observation amounts to vibrational spectroscopy of the transition state. In particular, the three clear resonance maxima we observe are assigned to the (1,0,0), (1,20,0), and (1,22,0) excited vibrational states of H3. Our experimental observations are compared with the results of recent quantum scattering calculations for this reaction. Although resonance features very similar to those we observe are evident in calculations for individual partial waves and for cross sections summed over small partial waves, fully converged calculations do not show the sharp resonances that we observe. This discrepancy implies that either the experimental or theoretical results are in error, and we discuss possible sources such error in both the experiments and the theory.
Cavity functions and association in models for weak electrolytes and sticky hard spheres
Authors: Rasaiah Jayendran C. , Zhu Jianjun
Published: July 2002

Weak electrolytes and other association reactions are modeled as sticky spheres. An analysis of the density expansion, including the bridge diagrams, of the cavity functions yAB(L) for sticky hard spheres (charged or uncharged spheres binding at a distance L) leads to an approximation which provides the degree of association α as the solution to a simple quadratic equation determined by the association constant K0 and the cavity function y0AB(L) for the reference system in which the chemical bonding between the reacting species has been turned off. Similar relations are assumed to hold when the bonding is directional and specific enough to lead only to the formation of dimers. Applications to the determination of the reference cavity functions for acetic acid and monochloro acetic acid from experimental data of the degree of association are discussed. In a discrete solvent, the approximation scheme for α remains the same, except that the reference cavity function is scaled differently. Solvent medium effects on the association constant are shown to be related to the cavity function of the undissociated dimer in a pure solvent. An exponential approximation for the reference cavity function y0AB(L) is derived when the associating species are of the same size and the bonding is spherically symmetric. Expressions for the changes in the thermodynamic functions due to association are obtained analytically in terms of the degree of association and the reference cavity functions. The magnitude of the degree of association, calculated from the exponential approximation for y0AB(L), and its effect on the thermodynamic properties are different from what was previously observed using the hypernetted chain (HNC) approximation. The thermodynamics of weak 1–1 electrolytes are discussed using the new method and a comparison is made between the new and old methods for 2–2 electrolytes.
Cluster model for the monoclinic to cubic transition in SF6 clusters
Authors: Torchet G. , de Feraudy M.-F. , Raoult B. , Farges J. , Fuchs A. H. , Pawley G. S.
Published: July 2002

Clusters made of several hundreds of SF6 molecules are produced in the free jet expansion of a Ne–SF6 mixture and studied by electron diffraction methods. Cold clusters have the monoclinic structure which has been identified recently in a polycrystalline sample at and below 85 K. Clusters adopt the body centered cubic structure of the bulk plastic phase when they are warmed up after crossing the frontal shock wave. Diffraction patterns show a progressive evolution, particularly in several line heights, from the low to the high temperature structure. In order to account for these intermediate states, a 512-molecule cluster is simulated by molecular dynamics on the DAP computer, using a rigid-molecule model with a single Lennard-Jones atom–atom potential. Warming up the model from 50 to 160 K by steps of 5 K, it is found that the structural transition occurs between 90 and 120 K. Calculated diffraction functions are in very good agreement with experimental patterns for both crystalline structures and for the intermediate stages. Intermediate experimental patterns can thus be attributed to the evolution of single clusters rather than to the coexistence of clusters with different structures. According to the model, the high temperature structure is initiated by surface molecules, which suggests that a surface effect is responsible for the temperature spread of the transition.
Strong orientation dependence of the scattering of fluoroform by graphite (0001)
Authors: Mackay R. Scott , Curtiss Thomas J. , Bernstein Richard B.
Published: July 2002

Measurements are reported of the scattering of hexapole−oriented fluoroform by the (0001) face of graphite. The experiments were conducted on the UCLA oriented−molecule beam machine. (AIP)
An approximate quantal treatment to obtain the energy levels of tetra-atomic X ⋅⋅⋅ I2 ⋅⋅⋅ Y van der Waals clusters (X,Y=He,Ne)
Authors: García-Vela A. , Villarreal P. , Delgado-Barrio G.
Published: July 2002

The structure of tetra-atomic X ⋅⋅⋅ I2 ⋅⋅⋅ Y van der Waals (vdW) clusters, where X,Y=He,Ne, is studied using an approximate quantal treatment. In this model the above complexes are treated as like diatomic molecules with the rare-gas atoms playing the role of electrons in conventional diatomics. Then a H2-like molecular-orbital formalism is applied, choosing the discrete states of triatomic systems I2 ⋅⋅⋅ X(Y) as molecular orbitals. Calculations at fixed configurations as well as including vdW bending motions restricted to the plane perpendicular to the I2 axis have been carried out for the sake of comparison with previous results. Finally, the restrictions are relaxed and the vdW bending motions are incorporated in a full way within the framework of a configuration interaction. The structure of these clusters is also studied through the probability density function.
State-specific predissociation of N2(B 3Πg) measured by laser-induced fluorescence on a molecular beam
Authors: Geisen H. , Neuschäfer D. , Ottinger Ch.
Published: July 2002

Using sub-Doppler laser induced fluorescence spectroscopy on a beam of metastable nitrogen N2(A 3∑+u), the predissociation of the N2(B 3Πg) state was studied quantitatively and under collision-free conditions. Predissociation rate constants γ in the range 0.1–200×106/s were measured (a) from the decreased fluorescence quantum yield and (b) from the increased linewidth. In method (a), coherent saturation effects were quantitatively accounted for. For the 3Πe0,3Πe,f1, and 3Πe,f2 levels, γ is generally high and independent of the molecular rotation in the range J=1–8, while for 3Πf0 levels γ is smaller and increases proportionally to J(J+1). This pattern identifies the N2(A′ 5∑+g) state uniquely as being responsible for the perturbation of N2(B). A pronounced dependence of γ on the vibrational level of N2(B) was also found in the range v′=13–18. It correlates quantitatively with calculated bound–free Franck–Condon factors between the B and A′ states. For the latter, a recent ab initio potential curve was used, with a slight (0.02 Å) radial adjustment. The B–A′ electronic coupling matrix element is on the order of 2 cm−1, much smaller than expected for a spin–orbit mechanism. This can be understood on the basis of the CI composition of the interacting states as calculated ab initio.
Torsional band assignment and intramolecular twist potential of 9,9′-bianthryl and its 10-cyano derivative in a free jet
Authors: Subaric-Leitis A. , Monte Ch. , Roggan A. , Rettig W. , Zimmermann P. , Heinze J.
Published: July 2002

Using a pulsed supersonic beam and laser-induced fluorescence spectroscopy the torsional structure of the S0 and S1 states of 9,9′-bianthryl (BA) and its derivative 10-(cyano)-9,9′-bianthryl (CBA) was investigated. Taking into account the very different equilibrium positions of the S0 and S1 potentials which result in a nonobservable 0–0 transition within the jet spectra, a new and straightforward procedure of torsional band assignment is carried out. This is based on a characteristic pattern of Franck–Condon factors within the dispersed fluorescence spectra. The torsional potentials were determined by a fit procedure of a one-dimensional model to the experimental data. The results show that the S1 double minimum potential for BA is shallower than for CBA indicating a stronger interaction between the molecular halfs of the latter compound. The observed rotational contours of torsional bands recorded for CBA reflect the change from a symmetric top molecule (for states above the S1 torsional barrier) to an asymmetric top (for states below the barrier) and manifest the tunneling splitting of the level just below the barrier. The dispersed fluorescence spectra of CBA are discussed in terms of intramolecular vibrational redistribution (IVR) processes. The measured fluorescence decay rates as a function of excess vibrational energy of CBA reflect a saturation behavior already within the origin region in contrast to BA (saturation near 380 cm−1). This is tentatively ascribed to a low lying dark background state possibly of charge transfer character.
Photodissociation of Kr+n clusters
Authors: Chen Z. Y. , Cogley C. D. , Hendricks J. H. , May B. D. , Castleman A. W.
Published: July 2002

The photodissociation of Kr+n (3≤n≤11) cluster ions is observed for the first time in the 565 to 630 nm wavelength range. Kr+3 has a photodissociation cross section of (8.1±0.8)×10−17 cm2 at 612 nm, while Kr+4 and Kr+5 are found to have larger photodissociation cross sections than Kr+3 with a slightly red-shifted spectrum. Only Kr+ is detected as a photofragment of Kr+3, while in the Kr+n (4≤n≤7) experiments, both Kr+2 and Kr+ photofragments are seen. As size n increases from 7 to 11, Kr+ disappears and Kr+3 appears. Laser power studies show that all are single photon photodissociation processes. For higher order Kr+n clusters (4≤n≤7) the intensity ratio between the two photodissociation products Kr+2 and Kr+ is dependent on the wavelength of the laser light used in the photodissociation, but independent of the polarization direction of the laser. Translational energy analysis of the photofragments is used to investigate the photodissociation mechanisms. In contrast to the photodissociation of Kr+3, where two types of Kr+ photofragments with different values of translational energy release are observed, only one type of Kr+2 photofragment, with zero kinetic energy release is found in the Kr+4 photodissociation. These findings are discussed in terms of the dynamics of photodissociation and possible structures of these cluster ions.
Vibrationally resolved photoelectron spectra of Si−3 and Si−4
Authors: Kitsopoulos T. N. , Chick, C. J. , Weaver A. , Neumark D. M.
Published: July 2002

Photoelectron spectra of the Si−3 and Si−4 cluster anions have been obtained at 355 and 266 nm. The spectra show transitions to the ground and low-lying excited electronic states of the neutral clusters. Several of the electronic bands show resolved vibrational structure. The electronic state energies and vibrational frequencies are compared to recent ab initio calculations. The Si−4 spectrum is consistent with the prediction of a planar, symmetric rhombus for the ground state of Si4.
Hund’s case (e): Application to Rydberg states with a 2Π ionic core
Authors: Lefebvre-Brion H.
Published: July 2002

The Rydberg states of high n belonging to series converging to an ion state with a nonzero spin–orbit splitting must be described in so-called Hund’s case (e). The derivation of the wave function obtained by transition from case (a) to case (e) is given. Application is made for a 2Π ionic core. This scheme is particularly useful to understand, for example, rotationally resolved spin–orbit autoionization.
Time delay for resonant vibrational excitation in electron–molecule collisions
Authors: Gauyacq J. P.
Published: July 2002

An analysis of the time delay associated with vibrational excitation in electron–molecule collision is presented. It consists of a direct study of the time dependence of the process for three model systems. An electron wave packet, that is narrow in time, is sent on the target and the amplitudes in the different inelastic channels are studied as functions of time. The time delay is found to correspond to very different time effects: broadenings, shifts in time of the wave packet, but also complex distortions that cannot be represented by a time delay. The direct analysis of the scattered wave also provides new insights into the vibrational excitation process. It should be a useful tool to analyze complex collision processes.
Generalization of the Colle–Salvetti correlation energy method to a many-determinant wave function
Authors: Colle Renato , Salvetti Oriano
Published: July 2002

The Colle–Salvetti method for calculating the correlation energy [Theor. Chim. Acta 37, 329 (1975)] is generalized to treat cases in which the reference function not a Hartree–Fock one, but a many-determinant wave function. Through calculations on atoms and diatomic molecules it is shown that this generalized approach gives the ‘‘experimental,’’ non-relativistic electronic energy at a millihartree level of accuracy also for internuclear distances far from the equilibrium positions.
Interaction of silicon cluster ions with ammonia: The kinetics
Authors: Ray Urmi , Jarrold Martin F.
Published: July 2002

The chemical reactions of Si+n (n=10–50) with ammonia have been studied using injected ion drift tube techniques at thermal energies (296–414 K), and low energy ion beam techniques, at a center of mass collision energy of ∼0.2 eV. Virtually all of the products arise from the adsorption of one or more ammonia molecules on to the parent cluster ion. In the drift tube experiments all clusters (except those with 11, 13, 14, 19, 22, and 23 atoms) were found to react with ammonia at close to the collision rate at room temperature. The reaction rates decrease with increasing temperature. Thermally activated desorption of ammonia from the products contributes to the negative temperature dependence. This observation suggests that unlike bulk silicon surfaces, which are known to adsorb NH3 dissociatively (and desorb H2 at ∼800 K), the silicon clusters may not be able to dissociate ammonia (at least on the time scale of our experiments). For clusters with 30–50 atoms, total cross sections for adduct formation were measured at collision energies of ∼0.2 eV. The cross sections are close to the hard sphere values and increase slowly with cluster size. In contrast to the results of Smalley and co-workers, obtained using Fourier transform ion cyclotron resonance, we do not find Si+33, Si+39, and Si+45 to be particularly unreactive. Several possible explanations for the large differences in the reactivities of these clusters (as measured by the two different experimental techniques) are discussed.
Theoretical studies of vacuum ultraviolet emission spectra of NeLi+ and ArLi+
Authors: Iwata Suehiro , Nanbu Shinkoh , Kitajima Hideki
Published: July 2002

Two of the ionic rare-gas alkali molecules, (Ne+Li)+ and (Ar+Li)+, were studied with ab initio calculations. The first and second excited singlet states 2 1Σ+ and 1 1Π of both ions form excimers. The simulated emission spectrum from 2 1Σ+ of NeLi+ peaks at 78.3 nm, and that from 1 1Π peaks at 78.5 nm, while the recently observed experimental peak is 80.3 nm. The theoretical spectral profile is also in good agreement with the experimental one. The radiative lifetimes of the corresponding excited states (v′=0) are 2.6 and 4.3 ns, respectively. For ArLi+, the theoretical spectrum from both excited states is 119.8 nm, while the experimental one is 124.7 nm. The lifetimes are 4.5 ns for 2 1Σ+ and 23 ns for 1 1Π.
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o study of the structure, infrared spectra, and heat of formation of C4
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

The lowest-lying 1Ag and 3Σ−g states of C4 have been studied using basis sets including f functions, quadratic configuration interaction, and full fourth-order Mo/ller–Plesset perturbation theory with multiple spin projection. Basis set effects on the linear-cyclic separation have been found to be fairly significant. Use of the 6-311G* and [5s3p1d] basis sets leads to qualitatively different conclusions; this problem is remedied by expansion of the polarization space. At the highest level of theory considered, the cyclic structure lies about 1 kcal/mol below the linear structure. A G1-type estimate of its heat of formation at 0 K of 249.6 kcal/mol is proposed. This value is in disagreement with the generally quoted experimental values, but in good agreement with third-law values by Drowart et al. [J. Chem. Phys. 31, 1131 (1959)] corrected for improved partition functions. Geometries and harmonic frequencies are reported. It is tentatively suggested that a matrix ir band at 1284 cm−1 may belong to cyclic C4; to aid experimentalists in confirming or rejecting this assignment, theoretical isotope shifts are reported.
Vibrational energy relaxation of a polyatomic adsorbate on a metal surface: Methyl thiolate (CH3S) on Ag(111)
Authors: Harris A. L. , Rothberg L. , Dhar, L. , Levinos N. J. , Dubois L. H.
Published: July 2002

The lifetime of the first excited level of the symmetric C-H stretching mode of methyl thiolate (CH3S) bonded to Ag(1 1 1) is measured by populating the level with a picosecond infrared pulse and probing the population by transient sum frequency generation spectroscopy. The population transient shows a biexponential decay across the experimental temperature range from 110 to 380 K. The fast decay component has a lifetime of 2.5–3 ps at all temperatures. The slow relaxation component lifetime varies from 55 ps at 380 K to 90 ps at 110 K. Neither relaxation component shows decay rates that are compatible with direct energy transfer to phonons or electron-hole pairs of the metal substrate. Both relaxation components are instead assigned to intramolecular energy transfer to excited vibrational levels of other vibrational modes of the molecule.
The structure of nickel clusters
Authors: Parks E. K. , Winter B. J. , Klots T. D. , Riley S. J.
Published: July 2002

The reactions of nickel clusters with ammonia and with water are used to probe cluster geometrical structure. Ammonia uptake experiments allow the determination of the number of preferred binding sites on cluster surfaces. This number shows pronounced minima in the 50- to 116- atom size range for many of the cluster sizes that appear as magic numbers in mass spectra of rare gas clusters. Since these magic numbers arise from closings of shells and subshells of the Mackay icosahedra, the correlation suggests that ammoniated nickel clusters in this size region also have icosahedral structure. Similar structure is found for ammoniated clusters smaller than ∼30 atoms, but is not seen for room temperature clusters in the vicinity of the third shell closing at 147 atoms. Icosahedral features do appear for the larger clusters at elevated temperatures. For many clusters above 50 atoms, prolonged exposure to ammonia causes a conversion from the icosahedral structure to some other structure that binds more ammonia molecules, and often the two structures are seen together. The equilibrium reaction of a single water molecule with the bare clusters probes the strength of the cluster–water bond. Enhanced water adsorption is often seen for clusters one atom larger than those showing minima in ammonia uptake, suggesting that these bare clusters likewise have icosahedral structure. The reasons for minima in ammonia uptake and maxima in water binding are discussed.