bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of charge-transfer complexes by computer simulation. I. Iodine in benzene solution
Authors: Danten Y. , Guillot B. , Guissani Y.
Published: July 2002

In this article we report molecular-dynamics (MD) simulation results for the thermodynamic, structural, and dynamic properties of a dilute solution of iodine in liquid benzene (Bz). The molecules of the simulated solution (10I2+246Bz) interact through site–site intermolecular potentials for which the benzene molecules are modeled by 12 atomic sites and the iodine molecules by four sites (two sites coinciding with the iodine atoms plus two extra sites). The site–site potentials contain isotropic terms to describe the electrostatic, dispersion, and repulsion interactions between molecules. The potential parameters associated with benzene molecules are taken from the literature, whereas those for iodine molecules are determined from a Monte Carlo simulation of the iodine–benzene dimer. The conclusion of our MD calculations is twofold. First, neither the microscopic structure [as evaluated from site–site distribution functions gij(r) where i, j=I,C,H] nor the thermodynamics give any evidence of a specific interaction between iodine and benzene molecules when iodine is dissolved in liquid benzene. Next, the evaluation of the far-infrared absorption spectrum associated with the iodine solution shows that the induction mechanism which takes into account the dipole moments induced on each molecule by the charge distribution of nearby molecules is unable to account for the experimental spectrum over a large domain of frequency. In contrast, the introduction of overlapping dipoles between iodine and benzene molecules permits a satisfying reproduction of the observed spectrum.
O(3P) atom lifetimes and mobilities in xenon matrices
Authors: Krueger Herman , Weitz Eric
Published: July 2002

The concentration of O atoms in xenon matrices at 32 and 40 K is followed by monitoring 736 nm emission from XeO* excimers. Emission is induced by excitation of O atom Xe pairs with 193 or 248 nm radiation. O atoms are initially produced via UV photolysis of N2O. At 32 K, approximately 50% of the atoms present after the initial probe pulse persist after eight days. The most likely mechanism for O atom decay is O+O recombination. At both temperatures, the time dependence of the concentration of O atom decay cannot be fit to a single bimolecular decay. It can, however, be fit to two (or more) bimolecular decays. Based on this mechanism, diffusion coefficients for the longer time scale diffusion process are 5.4×10−18 and 2.0×10−17 cm2/s at 32 and 40 K, respectively. Other possible explanations for the observed O atom time dependence are discussed.
Structure and energetics of hydrogen clusters. Structures of H+11 and H+13. Vibrational frequencies and infrared intensities of the H+2n+1 clusters (n=2–6)
Authors: Farizon Michel , Chermette Henry , Farizon-Mazuy Bernadette
Published: July 2002

A b initio self-consistent-field (SCF) Hartree–Fock and configuration interaction (CI) calculations have been carried out for H+2n+1 (n=1–6) clusters using a triple-zeta plus polarization basis set. Fully optimized structures and energies of H+11 and H+13 are presented. These structures can be thought as the addition of H2 molecules to a deformed H+9. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Mori. Nevertheless, our energy results on the biggest clusters suffer from the lack of size consistency of CI with single and double substitutions (CISD) calculations. Analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies and infrared intensities at the two levels of theory SCF (n=1–6) and CISD (n=1–4) both with triple-zeta polarization basis sets. Of special interest are the new vibrational modes of H+11 and H+13, which have no counterpart in the H+9 cluster. Our predicted frequencies compare fairly well with the experimental results of Okumura, Yeh, and Lee.
Spin uncoupling in 3A2 CS2 studied by high-resolution Zeeman and hyperfine quantum beat spectroscopy
Authors: Cramb D. T. , Bitto H. , Huber J. Robert
Published: July 2002

The magnetic properties of the evsB2 spin component of the 3A2 state in the molecule CS2 have been investigated with a resolution of ∼100 kHz by quantum beat spectroscopy in a supersonic jet. Zeeman-level tuning in 12CS2 and 13CS2, providing Landé g factors, and measurements of the 13CS2 nuclear hyperfine structure were carried out, particularly to investigate their J dependence. Based on these results, it is proposed that both the magnetic moment and the hyperfine splitting are dominated by spin-uncoupling interactions, described in the rotational Hamiltonian by B(J−S).2 In addition, the g factors and rotational state lifetimes were used to provide spectroscopic assignments and to elucidate details of the state mixing.
Endohedral effect in inclusion complexes of the C6 cluster
Authors: Cioslowski Jerzy , Nanayakkara Asiri
Published: July 2002

An approximate theory of bonding in endohedral complexes that are formed by atoms, ions, and molecules trapped inside the C60 cage is formulated. It is found that the electrostatic potential at the cage center (the endohedral potential) determines such electronic properties of the endohedral complexes as the complexation energy, ionization potentials, and the stability with respect to internal electron transfer. The origins of the endohedral effect, i.e., the positive valuedness of the endohedral potential, are investigated with the help of the Thomas–Fermi theory. The theoretical predictions are demonstrated to be in good agreement with the numerical results of ab initio electronic structure calculations.
Resonance effects in the 5σ−1 photoionization channel of CO
Authors: Siggel M. R. F. , Hayes M. A. , MacDonald M. A. , West J. B. , Dehmer J. L. , Parr A. C. , Hardis J. E. , Iga I. , Tiit V.
Published: July 2002

Vibrational branching ratios and photoelectron angular distributions are reported for the 5σ−1 photoionization channel of CO in the range 16 eV<hν<45 eV. Striking non-Franck–Condon effects are observed in both the branching ratios and angular distributions as a result of various autoionizing states and a σ shape resonance that lie in this spectral range. The goal of the present measurement was to observe definitive evidence for the σ shape resonance via its non-Franck–Condon effects on the vibrational ionization channels. Guided by recent calculations [Smith, Lynch, and McKoy, J. Chem. Phys. 85, 6455 (1986)], we examined the broad structure in the vibrational branching ratios and angular distributions in the range 25 eV <hν<40 eV. There, we found clear evidence for the σ shape resonance in the quantities β(v+=0 and 1) and σ(v+=2)/σ(v+=0). Substantial differences between theory and experiment for the σ(v+=1)/σ(v+=0) branching ratio, however, serve to define the limitations of the current single-channel picture for this process.
Rydberg states of the Ar2 molecule
Authors: Kane D. J. , Kim S. B. , Shannon D. C. , Herring C. M. , Eden J. G. , Ginter M. L.
Published: July 2002

Extensive spectra attributable to transitions from the 4sσ a 3Σ+ metastable state of Ar2 to excited Rydberg states have been observed by intracavity absorption spectroscopy and by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. Of these the most extensive and best resolved were the laser induced fluorescence spectra. Most of the spectra can be assigned to vibronic transitions in the nfλ (3Πg,3Σ+g) and npλ (3Πg,3Σ+g)←a 3Σ+u series. The 5pπ3Πg←a 3Σ+u and 7pσ 3Σ+g←a 3Σ+u transitions observed near 19 823 and 19 529 cm−1, respectively, exhibit many bands with v′≠0 in their (v′–v″) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0–0) transitions. Rotational structure is partially resolved in a few bands of the 7pσ 3Σ+g–a system, but most of the spectra observed appear to be either rotationally unresolved or made up of blended collections of rotational lines. Above the 7pσ, 5pπ pair, the (n+2)pσ 3Σ+g and npπ 3Πg members of the npλ series rapidly coalesce, indicating a rapid onset of decoupling of the electronic orbital angular momentum, L̂, from the internuclear axis.

Such decoupling leads to np-complex formation at n values much lower than observed in the npλ series of the lighter dimers: He2 and Ne2. From the data for the observed series, the lowest ionization limit of Ar2 (relative to a 3Σ+u,v=0 ) was determined to be 29 373±3 cm−1. Vibrational intervals ΔG(v+1/2) for a 3Σ+u(v≤4) and 7pσ 3Σ+g(v≤2) yield the vibrational constants ωe=296 and 282 cm−1 and xωe=2.5 and 17 cm−1, respectively.
Molecular adsorption of NH3 on MgO(001) and hydrogen abstraction from NH3 on gaseous LiO and Li-doped MgO(001). A computational study
Authors: Bo/rve Knut J.
Published: July 2002

Accurate ab initio quantum-mechanical calculations are presented for the molecular and dissociative adsorption of ammonia on the pure and Li-doped MgO(001) surface, and also for the hydrogen abstraction from ammonia on gas-phase LiO. The surface is described in terms of embedded clusters. In its most stable physisorbed state the ammonia is found to be doubly coordinated, with the electrostatic attraction to a cation being the important feature for bonding. Heterolytic dissociation of the N–H bond is neither found to be feasible on the pure MgO(001) surface nor on the Li-doped surface. Likewise, no hydrogen abstraction from ammonia takes place on the perfect (001) surface. On the Li-doped surface, partial optimization of the transition state renders a low reaction barrier of 7 kcal/mol for the dissociative adsorption of ammonia. The gas-phase hydrogen abstraction starting from free reactants may take place with no barrier to a product LiOH–NH2 complex. However, the reactants may be trapped in a stable complex in which ammonia is doubly coordinated to the oxide, and from this state the reaction barrier is 13 kcal/mol. The end-on abstraction, having as products gas-phase LiOH and NH2, has a barrier of 11 kcal/mol.
Low-energy electron scattering by C3H6 isomers
Authors: Winstead Carl , Sun Qiyan , McKoy Vincent
Published: July 2002

We report cross sections for the elastic scattering of electrons by the C3H6 isomers propene and cyclopropane from 5 to 40 eV. These results were calculated within the static-exchange approximation, using the Schwinger multichannel method as implemented on the Touchstone DELTA distributed-memory parallel computer. The integral elastic cross sections so obtained support recent observations of a substantial isomer effect in the total scattering cross section. Partial-wave analysis indicates the presence of an unusual A′2 shape resonance in cyclopropane.
Evolution of the electronic structure of lithium clusters between four and eight atoms
Authors: Blanc J. , Bonačić-Koutecký V. , Broyer M. , Chevaleyre J. , Dugourd Ph. , Koutecký J. , Scheuch C. , Wolf J. P. , Wöste L.
Published: July 2002

Absorption spectra of lithium clusters containing four to eight atoms have been measured using depletion spectroscopy. Few intense transitions are observed, always located in two predominant spectral regions, ∼480 and 680 nm. The spectra are interpreted using ab initio configuration interaction (CI) calculations, leading to a complete characterization of the excited states and a straightforward determination of the ground state geometrical structure. Intense transitions are explained by interference effects in the transition amplitude and symmetry considerations. Comparisons with semiclassical models, in which an effective mass correction is introduced, are also presented.
Ionization potentials of cobalt–sodium bimetallic clusters (ConNam)
Authors: Hoshino Kuniyoshi , Naganuma Takashi , Yamada Yoshiyuki , Watanabe Katsura , Nakajima Atsushi , Kaya Koji
Published: July 2002

Cobalt–sodium bimetallic clusters (ConNam, n=3–48) were produced by a two independent laser-vaporization method. Ionization potentials of the ConNam clusters were measured up to m=3 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. In general, the ionization potentials decrease monotonically with the number of sodium atoms, and the ionization potentials of ConNam+1 decrease by 0.2–0.8 eV compared to those of the corresponding ConNam cluster. However, the amount of IP decrement by the Na doping is relatively large at n≤17 whereas it is small and constant at n≥18. This feature can be explained by a geometric rearrangement; at small n, the Na doping induces a large geometric rearrangement of the cluster, but at large n, the geometric change is small. Reactivity of ConNam cluster toward H2 was also measured and the effect of the Na doping was studied. The reactivity also suggests the geometric change induced by the Na doping. Moreover, we examined the anticorrelation between IP and reactivity of the ConNam clusters, and no anticorrelation between them could be revealed.
Infrared studies in free standing crystals: N2O-doped Xe and Ar
Authors: Lawrence W. G. , Apkarian V. A.
Published: July 2002

Infrared studies in N2O-doped free-standing crystals of Xe and Ar are reported. N2O isolates as monomers in Xe; however, it segregates by clustering in Ar. An anomalous temperature dependence of monomeric absorption intensities is observed in Xe, suggesting a strong guest-host electronic interaction.
Molecular population transfer, alignment, and orientation using chirped pulse absorption
Authors: Band Y. B. , Julienne P. S.
Published: July 2002

We describe a technique for totally transferring the population of a molecular vibronic level to another optically accessible level by chirped pulse absorption. The method can be applied even for nonvanishing angular momentum states where the Rabi frequencies for the transitions depend on the magnetic quantum number M of the states. The population of a large J initial state can be totally transferred to a J′=J+1 state by R branch transitions with linearly or circularly polarized light pulses with negative or positive frequency chirp if the bandwidth of the chirped pulse is sufficiently small. For an initial state with small J, having P(Q) and R transitions that are closely spaced in energy and within the chirped pulse bandwidth, the situation is complicated by the presence of transitions involving additional nearby J states which can participate in transitions induced by the chirped pulse. For positive frequency chirp, population can be totally transferred to the excited electronic state, whereas for the negative frequency chirp case the population distribution depends on the details of the energy levels and laser pulse. The technique can be employed to align and orient states in which not all the M states of the initial level are transferred to the same branch, but each M state is totally transferred from the initial state. Criteria for this type of population transfer and alignment and orientation are described and the conditions under which adiabatic passage is maintained are discussed.
Vibrational circular dichroism intensities: Ab initio vibronic coupling theory using the distributed origin gauge
Authors: Yang Danya , Rauk Arvi
Published: July 2002

The distributed origin (DO) gauge of Stephens has been implemented in the ab initio vibronic coupling theory (VCT) of Nafie and Freedman for the calculation of vibrational circular dichroism (VCD) intensities. With this implementation, direct comparison is possible between results obtained with the DO gauge and the common origin (CO) gauge. In the VCT implementation, the VCT–DO and VCT–CO results are different even in the Hartree–Fock limit. The origin of the nonequivalence and the accuracy of VCD intensities by VCT–DO and VCT–CO when smaller basis sets are used are discussed. Detailed comparison is carried out for 13 chiral molecules. Experimental data are available for eight chiral oxiranes and thiiranes. A range of basis sets, of both the ‘‘conventional’’ and ‘‘polarized’’ varieties, is used for NHDT, H2O2, D2O2, DOOH, and oxaziridine. The results are compared with the results of Stephens’s magnetic-field perturbation theory wherever the data are available. Calculated VCD intensities are basis-set and gauge dependent. Improvement of calculated rotational strengths for most molecules studied here is observed by introducing the DO gauge to the VCT formulation.
Identification of the nd Δ and Σ states and the 1,3Φ←←X 3Σ−g transition of O2 by resonant multiphoton ionization
Authors: Yokelson Robert J. , Lipert Robert J. , Chupka William A.
Published: July 2002

Spectra of the 3d Rydberg state region of O2 have been obtained by two-photon resonant ionization of the ground electronic state. By varying the rotational distribution and radiation polarization, all observed bands were identified and attributed to excitation of Σ, Δ, and Φ states. Earlier assignments were corrected. The Δ and Φ assignments are complete while the Σ assignments are so far incomplete.
Spectroscopy and nuclear dynamics of tetracene–rare-gas heteroclusters
Authors: Ben-Horin Narda , Even Uzi , Jortner Joshua , Leutwyler Samuel
Published: July 2002

In this paper we report on the electronic spectroscopy of mass-resolved tetracene⋅Arn (n=1–26) and tetracene⋅Krn (n=1–14) heteroclusters, utilizing two-photon, two-color near-threshold ionization in conjunction with mass-spectrometric detection. The spectra of the S0 → S1 transition and the ionization threshold of these heteroclusters were monitored. The structured spectral features of the S0 → S1 transition of small- and medium-sized (n=1–8) heteroclusters were attributed to the electronic origins of structural isomers and to their intermolecular vibrations. The S0 → S1 spectra of large (n≥9) heteroclusters are broad and were assigned to inhomogeneous broadening due to the coexistence of isomers, with the spectral feature(s) of each distinct isomer being homogeneously broadened. Isomer-specific inhomogeneous line broadening was interrogated by the observation of isomer-specific ionization potentials for medium-sized (n=6–7) heteroclusters and of the dependence of the relative intensities of the spectral features on the conditions of the supersonic expansion.

The ionization thresholds of the tetracene⋅An (A=Ar,Kr) reveal a linear (or superlinear) n dependence, being qualitatively different from the sublinear n dependence of the spectral shifts. These different patterns of the size dependence can be traced to the different intermolecular interactions which govern excitation and ionization and to the difference in the charge distribution in S0 and in the positive ion. The experimental spectroscopic data for the spectral shifts and the spectral linewidths were simulated in terms of the first and second moments of the classical line shape, which were obtained from Monte Carlo (MC) constant temperature simulations, in conjunction with a two-parameter fit of the excited-state tetracene–rare-gas potential. The Monte Carlo simulations of the structural fluctuation parameters identified several isomerization phenomena, i.e., correlated restricted and unrestricted surface motion, adcluster isomerization, surface melting and side crossing, and characterized the size dependence of the temperature onsets of these processes for small and medium sized n=2–20 clusters. These isomerization processes could not be interrogated by the investigation of the size dependence of the spectral shifts and linewidths. The size dependence and the isomer specificity of the spectral shifts are well accounted for by the MC simulations. The homogeneous spectral linewidths of small (n<8) clusters pertain to the spectroscopy of ‘‘static’’ isomers, while the line broadening of large (n≥20) clusters manifests inhomogeneous line broadening due to the coexistence of wetting and nonwetting isomers. The temperature dependence of the spectral shifts and inhomogeneous linewidths of large (n≥20) clusters provides means for internal cluster thermometry.
High resolution photoelectron spectroscopy: The vibrational spectrum of the 2-aminopyridine cation
Authors: Kim Byungjoo , Thantu Napoleon , Weber Peter M.
Published: July 2002

We describe a new time-of-flight photoelectron spectrometer that combines molecular beam techniques with two-photon ionization by a high repetition rate laser. The instrument routinely achieves a resolution of 5 meV. In a first application we studied the vibrational spectrum of the 2-aminopyridine ion. By tuning the laser to various vibrational states of the intermediate S1 resonance we were able to assign the vibrational frequencies of modes 6a, 12, I2 and 1. Other vibrational lines could be assigned by comparison with similar molecules. The ionization potential was found to be 8.099±0.003 eV, which differs from the literature value by 0.124 eV. The discrepancy might be explained by a fast intersystem crossing to a nearby triplet state.
Compact formulation of multiconfigurational response theory. Applications to small alkali metal clusters
Authors: Fuchs C. , Bonačić-Koutecký V. , Koutecký J.
Published: July 2002

A new Green’s functions oriented formulation of multiconfigurational response theory is presented. The consistent use of Fourier integrals simplifies the formalism considerably with respect to previous derivations and leads to compact expressions for (exact and) approximate response functions of arbitrary orders. Applications of multiconfigurational linear response (MCLR) theory to the interpretation of depletion spectra of Li4, Na4, and Na6 clusters are shown. The results are compared to those obtained from large-scale configuration interaction calculations, illustrating that MCLR yields an adequate description of optically allowed transitions and oscillator strengths provided an appropriate active space has been chosen.
Nonlinear response in dielectric relaxation including the induced polarization of polar molecules
Authors: Déjardin Jean-Louis
Published: July 2002

The steady-state nonlinear dielectric response is calculated where the applied electric field consists of an alternating field superimposed on a constant field. Both permanent and induced dipolar moments are considered. This gives rise to three harmonic terms in ω, 2ω, and 3ω which are time-dependent and to a stationary frequency-dependent term, if we restrict ourselves to the third-order response. These results are obtained from the Smoluchowski equation by using a perturbation method and are therefore complementary to those previously derived in Kerr effect relaxation. Some analytic expressions are presented which could be checked experimentally.
Variation of the electronic wave function with internuclear separation: High-resolution spectroscopy of the A 3Π(1) state of I 35Cl near the dissociation limit
Authors: Western Colin M. , Slotterback Timothy J. , Johnson Jeffrey R. , Pratt David W. , Janda Kenneth C.
Published: July 2002

Fluorescence excitation spectra of the A 3Π(1)−X 1Σ+ electronic transition of I 35Cl were acquired using a single-frequency dye laser and a well collimated molecular beam. Data were collected for eleven vibrational levels in the range v′=11–34. Of these, ten were well enough resolved for the hyperfine structure of both nuclei to be observed. For the highest vibrational level studied, v′=34, only the splitting due to the iodine nucleus could be resolved. The measured hyperfine constants for the A state were found to vary significantly with vibrational energy, and were found to be inconsistent with an analysis based on a single electronic configuration [linear combination atomic orbital/molecular orbital (LCAO/MO)]. Instead, it is found that the unpaired electron on the iodine atom is primarily oriented perpendicular to the bond axis for large internuclear separations and only achieves the orientation predicted by the LCAO/MO description for r<3.0 Å. The chlorine orientation is roughly in accord with the LCAO/MO prediction. These conclusions were reached by analyzing the hyperfine constants in a separated atom basis set. Additionally, a perturbation was found in the spectra for the v′=27 level. Evidence is presented that the cause of this perturbation is an interaction with the weakly bound a 3Π1 electronic state of ICl.
Shell structure in photoionization spectra of large aluminum clusters
Authors: Pellarin M. , Baguenard B. , Broyer M. , Lermé J. , Vialle J. L. , Perez A.
Published: July 2002

Photoionization mass spectrometry experiments, performed on an extensive size range of aluminum clusters produced by laser vaporization technique, are reported. Ionization potential values are deduced from individual photoionization efficiency curves for the smaller AlN clusters (N=36–112). Our results confirm and complete those previously published. The mass spectra of larger clusters (N≊250–1400) reveal a regular signal oscillation. Several additional experiments give proof that this striking pattern originates from size-dependent ionization threshold effects. This structure exhibits exact periodicity as a function of N1/3 or Ne1/3 (Ne the number of valence electrons).
Unimolecular decomposition in a spherically symmetric potential
Authors: Klots Cornelius E.
Published: July 2002

Unimolecular decompositions are considered where the interaction between the separating fragments can be described by a function solely of their radial separation. It is shown how previous treatments, confined to idealized models, can be generalized. Kinetic energy distributions and rate constants may be calculated accordingly when the angular momentum is low. Applications to evaporation from van der Waals molecules are noted. A limit to the interaction parameters is then found where these properties become ‘‘thermodynamic,’’ despite the absence of angular momentum. This result is used to obtain a general form for the effects of angular momentum on a rate constant. It reduces to traditional ideas in the hard-sphere limit.
State specific reactions of Ba(1S) and Ba(1D2) with water and methanol
Authors: Davis H. Floyd , Suits Arthur G. , Lee Yuan T. , Alcaraz Christian , Mestdagh Jean-Michel
Published: July 2002

The reactions of Ba(6s2 1S0) and Ba(6s5d 1D2) with water and methanol were studied under single collision conditions using crossed molecular beams. Reaction of ground state Ba(1S)+H2O led to dominant formation of BaO+H2 at all collision energies studied (12–30 kcal/mol). Although the reaction Ba(1S)+H2O→BaOH+H could also be observed at collision energies above the endoergicity of the reaction (13 kcal/mol), it remained a minor channel even at the highest collision energy. Electronic excitation of the Ba atom reactant to the metastable 1D2 state led to a large enhancement in reactivity, but the products were BaOH+H rather than BaO+H2. The dominance of BaO+H2 from ground state Ba(1S) even at collision energies nearly equal to the Ba(1D) excitation energy of 11 395 cm−1 (∼32 kcal/mol) indicates that the observed reaction state specificity results from participation of different potential energy surfaces for reactions of ground and excited state atoms. Collisions of Ba(1S) and Ba(1D) with methanol led only to formation of BaOCH3+H. We observed no BaOH+CH3 or BaO+CH4, despite the fact that they are the more thermodynamically favorable channels. Although reactivity with methanol was strongly enhanced by electronic excitation of the incident Ba atom, no reaction was observed from Ba(1S) or Ba(1D)+dimethyl ether (CH3OCH3). These observations strongly suggest that the reactions leading to radical formation involve H-atom migration.
K-shell spectroscopy of Ar clusters
Authors: Rühl E. , Heinzel C. , Hitchcock A. P. , Schmelz H. , Reynaud C. , Baumgärtel H. , Drube W. , Frahm R.
Published: July 2002

Total ion yield spectra of a supersonic beam of Ar clusters have been recorded in the Ar K-shell (1s) spectral region using synchrotron radiation. The spectrum of the cluster component of beams containing argon clusters of mean size N̄≊400 atoms is similar to that of the x-ray absorption spectrum of solid Ar. Analysis of the Ar 1s extended x-ray absorption fine structure (EXAFS) indicates an average first shell Ar–Ar distance in the cluster similar to that of the solid, but there is an increased Debye–Waller (disorder/thermal motion) term as well as changes in the higher coordination shell signals.Time-of-flight mass spectra and photoion–photoion coincidence (PIPICO) spectra of argon cluster beams are reported and the charge separation mechanisms of multiply charged argon clusters are discussed. The results are compared to those from recent studies of the Ar 2p spectra of Ar clusters.
The two-state linear curve crossing problems revisited. III. Analytical approximations for Stokes constant and scattering matrix: Nonadiabatic tunneling case
Authors: Zhu Chaoyuan , Nakamura Hiroki
Published: July 2002

Based on the exact solution of the linear curve crossing problems reported in the first paper of this series, approximate analytical solution is discussed here for the opposite sign of slopes of the two diabatic potentials (nonadiabatic tunneling case). Two new compact analytical formulas for reduced scattering matrix are derived for ‖b2‖≥1, where b2 represents the effective collision energy. The whole range of the two parameters a2 (effective coupling strength) and b2 is divided into five regions, in each one of which the best recommended formulas are proposed. This analysis provides a complete picture of the nonadiabatic tunneling problem, for the first time. The new formulas proposed here are simple and explicit functions of the two parameters, and thus useful for practical applications. In the case of ‖b2‖≤1, certain fitting formulas are proposed for the Stokes constant on the basis of a comparison with the exactly solvable differential equation with special quartic polynomial as coefficient.
Structural transitions in benzene–argon clusters: Size and temperature effects
Authors: Schmidt Martin , Le Calvé Jacques , Mons Michel
Published: July 2002

The resonant two-photon ionization spectra of the first benzene–Arn (n≤8) clusters are interpreted in the light of a model calculation, including local energy minima determinations and Monte Carlo simulations. Based on spectral shift information, two types of structures are distinguished: the argon cluster either only solvates one side of the substrate molecule or covers simultaneously both sides. The ‘‘wettest’’ structures are assigned to sandwich-type forms for n≤4 and to clamshell-type forms beyond. Such a 2D–3D transition in the n=5–8 size range is shown to be specific to small substrate size. In the one-sided forms, the argon cluster geometry is strongly influenced by the presence of the Bz molecule since it adopts a quasiplanar geometry, more or less tightly bound to the substrate depending on its size: the small argon clusters exhibit two types of complexation sites (c and s forms) which are simultaneously visited (surface decoupling) even at low temperature (∼7 K). As the size increases, the c form becomes prevalent and the surface decoupling becomes inhibited. At n=8 a 2D–3D transition is observed: beyond the n=7 species, characterized by a caplike close-packed structure, the one-sided conformers disappear in favor of bridged forms. Additional results on the ionization behavior of the one-sided species show regular variation of the ionization potential with the cluster size.
A polarizable, dissociating molecular dynamics model for liquid water
Authors: Halley J. W. , Rustad James R. , Rahman A.
Published: July 2002

We describe a molecular dynamics model for dissociable, polarizable water. The model, which describes both the static and dynamic properties of real water quite reasonably, contains the following features: Self-consistent local fields are calculated in an extension of an earlier algorithm in which the dipole moments of the water are treated as dynamical variables. An intramolecular three-body potential assures that the molecular properties of water are in agreement with experiment. Ewald methods are used to take account of monopole–dipole and dipole–dipole as well as monopole–monopole interactions. The model was optimized using a Monte Carlo procedure in the parameter space which is described.
Relativistic effects in reactions of the coinage metal dimers in the gas phase
Authors: Lian Li , Hackett Peter A. , Rayner David M.
Published: July 2002

Reactions of Ag2 and Au2 with several small molecules have been studied in the gas phase, under thermalized conditions at room temperature, in a fast-flow reactor. Ag2 was observed to react only with NH3, implying bimolecular rate constants of less than 5×10−15 cm3 s−1 at 6 Torr He for reaction with O2, N2O, N2, H2, CH4, CO2, CO, and C2H4. The complex, Ag2NH3, was found to be in equilibrium with Ag2 and NH3 with an equilibrium constant of 4.6±0.3×10−15 cm3. Third law modeling of this equilibrium constant leads to a value of 58±8 kJ mol−1 for the binding energy of Ag2NH3. Au2 was not observed to react with O2, N2O, N2, and CH4, again implying bimolecular rate constants of less than 5×10−15 cm3 s−1 at 6 Torr He. Au2 reacted with CO, NH3, and C2H4 in a manner characteristic of association reactions in their low pressure limit. Limiting low-pressure, third-order, rate constants were found to be 2.6±0.9, 22±4, and 230±50×10−30 cm6 s−1, respectively. These rate constants are 3 to 20 times larger than those for corresponding Cu2 reactions. Modeling studies of these rate constants imply that the Au2 complexes are bound by 20–40 kJ mol−1 more than their Cu2 counterparts, if both the complexes of both dimers have similar molecular structure. In addition, Au2 exhibits a unique reaction amongst the coinage metal dimers in that it reacts with hydrogen via what appears to be an insertion reaction with a pressure independent rate constant of 1.4±0.3×10−14 cm3 s−1 at 297 K. Trends in the reactivity of coinage metal dimers revealed by this work and our earlier studies of Cu2 dimer reactions are discussed in terms of the role relativistic effects play in metal cluster chemistry. In particular, we remark that the anomalous chemical behavior of Au2 is consistent with other anomalies credited to relativistic effects in the physics and chemistry of gold.
Reaction between Ba and N2O in large Arn clusters
Authors: Lallement A. , Mestdagh J. M. , Meynadier P. , de Pujo P. , Sublemontier O. , Visticot J. P. , Berlande J. , Biquard X. , Cuvellier J. , Hickman C. G.
Published: July 2002

The collision between a Ba atom and an Arn cluster carrying N2O molecules has been investigated under crossed molecular beam conditions. The argon cluster acts as a solvent for the Ba+N2O reaction, which is monitored through its chemiluminescent channel forming electronically excited BaO. The effects of cluster size and the number of N2O molecules per cluster have been investigated systematically as have the effects of extra molecules present upon the cluster (CH4). It has been shown that (i) the BaO reaction product either stays solvated in the cluster or is lost from the cluster; (ii) the reaction probability between Ba and N2O is approximately unity for the clusters considered here; (iii) the chemiluminescence quantum yield decreases as the number of N2O molecules per cluster is increased. The effect of a thermal bath (the argon cluster) on the dynamics of the well studied gas phase reaction Ba+N2O is discussed.
Collision-induced dissociation of Cr+n (n=2–21) with Xe: Bond energies, dissociation pathways, and structures
Authors: Su C.-X. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the collision-induced dissociation (CID) of Crn+ (n=2–21) with xenon is studied by using a guided ion beam mass spectrometer. Examination of the general dissociation behavior over a broad collision energy range shows that chromium cluster ions dissociate primarily by sequential atom loss with a few exceptions, most notably Cr5+. Bond energies of chromium cluster ions D(Crn−1+–Cr) are determined from measurements of the CID thresholds. The cluster size dependence of chromium cluster bond energies shows that odd-sized clusters are more stable than even clusters for smaller clusters (n≤9) and local maxima at n=13, 14, and 20 for larger clusters. The even–odd alternation in the stability of small chromium clusters suggests that these cluster cations are bound mainly by the 4s electrons. The pattern of stability for the larger clusters, in particular, the observation that the 14- and 20-atom clusters are relatively stable, is consistent with clusters built around a dimer core.
Collision-induced dissociation of Vn+ (n=2–20) with Xe: Bond energies, dissociation pathways, and structures
Authors: Su C.-X. , Hales D. A. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the collision-induced dissociation (CID) of Vn+ (n=2–20) with xenon is studied by using a guided ion beam mass spectrometer. Examination of the general dissociation behavior over a broad collision energy range shows that vanadium cluster ions dissociate primarily by sequential atom loss, although dimer loss is also observed for several clusters, notably V4+. Bond energies of vanadium cluster ions, D(Vn−1+–V), are determined from measurements of the CID thresholds. Bond energies for neutral vanadium clusters are derived by combining these ionic bond energies with literature ionization energies for Vn. The bond energies of cationic and neutral vanadium clusters are very similar and show an even–odd oscillation below n=11 with even-sized clusters being more stable. For larger clusters, prominent stable clusters occur at n=13 and 15. These patterns in stability are found to correlate well with the reactivities of the neutral and cationic clusters, such that stable clusters are the least reactive. Possible electronic and geometric structures consistent with these stability and reactivity patterns are discussed.
Properties of atoms in molecules: Magnetic susceptibilities
Authors: Bader Richard F. W. , Keith Todd A.
Published: July 2002

The molecular magnetic susceptibility tensor χ is expressible as a sum of atomic or group contributions. An atomic contribution consists of a basin and a surface component; the former is given by the integral of a magnetization density over the basin of the atom, and the latter, by the integral of the flux in the position weighted current density through the interatomic surfaces that the atom shares with its bonded neighbors. The surface component is obtained as a consequence of the atomic hypervirial theorem defining the average of the velocity operator. Magnetic properties are determined by the observable electron current density, and the atomic behavior of this field has been correlated with corresponding behavior of the electron density. Thus the importance of the magnetization within an atomic basin relative to the flux in current through its interatomic surfaces parallels the extent to which the electron density is localized within the individual atomic basins. For example, 77% of the pronounced anisotropy in benzene arises from the flux in current through the interatomic surfaces of the ring atoms induced by a field applied perpendicular to the ring surface. The methyl and methylene group contributions to χ̄ are found to be transferable in the homologous series of hydrocarbons and to equal Pascal’s group increments within experimental error.
What is the global minimum energy structure of the water hexamer? The importance of nonadditive interactions
Authors: Mhin Byung Jin , Kim Jongseob , Lee Sik , Lee Jin Yong , Kim Kwang S.
Published: July 2002

The global minimum energy structures of the water hexamer predicted by widely used analytic water potentials are very different from each other, while the cyclic hexamer does not appear to be a low-lying energy structure. However, high levels of ab initio calculation predict that a number of low-lying energy conformers including the cyclic conformer are almost all isoenergetic due to the balance of two-body and nonadditive interactions. For modeling of water potentials, we suggest that the binding energy of the dimer be between −5.0 and −4.7 kcal (mol dimer)−1, while the three-body corrections be taken into account to a large extent.
Evidence for a size-dependent melting of sodium clusters
Authors: Martin T. P. , Näher U. , Schaber H. , Zimmermann U.
Published: July 2002

Geometric shell structure in the mass spectra of sodium clusters was found to disappear as the clusters were heated. The exact temperature at which the shells disappeared was dependent on the size of the clusters. These observations are interpreted as evidence for a size-dependent melting. Clusters containing 1000 atoms appear to melt at 288 K, clusters containing 10 000 atoms at 303 K. Both values lie well below the bulk melting temperature of 371 K.
Analysis and extension of a model for constraining zero-point energy flow in classical trajectory simulations
Authors: Peslherbe Gilles H. , Hase William L.
Published: July 2002

In previous work a method, based on an instantaneous normal mode analysis and time reversal of the momentum of any local normal mode whose energy falls below its quantum harmonic zero-point value, has been proposed for controlling zero-point energy flow during classical trajectory simulations. From the previous work it is not clear that this ZPE constraint method, which was developed for nonrotating systems, conserves linear and angular momenta. The projection of infinitesimal rotations and translations from the Cartesian force constant matrix, during the instantaneous normal mode analysis, results in specific orthogonality relations for the vibrational eigenvectors. Since the vibrating molecular system is defined to obey the instantaneous Eckart conditions, it is shown that these orthogonality relations have a form which preserves the linear and angular momenta as constants of motion when the ZPE constraint is applied. Based on this property, an extended ZPE constraint scheme is proposed for molecular systems with nonzero angular momentum. This extended scheme is successfully tested in a study of vibrationally and rotationally excited Al3 and C2H6. Descriptions are given of the effect of the ZPE constraint on Hamiltonians that (1) are separable in normal mode coordinates and momenta, (2) have ergodic dynamics, and (3) have tori which are not described by normal mode coordinates and momenta. The ZPE constraint scheme, when applied to a normal mode Hamiltonian, does not affect the trajectories which satisfy the ZPE condition. For ergodic Hamiltonians, preliminary analyses indicate the scheme gives microcanonical unimolecular rate constants which are in agreement with the quantum values. This suggests that the scheme may be a practical approach for evaluating quantum microcanonical unimolecular rate constants for anharmonic and vibrationally/rotationally coupled Hamiltonians with ergodic dynamics. However, more studies need to be completed to determine whether it is a general conclusion. For the latter Hamiltonian, the ZPE constraint may induce transitions between tori and between tori and chaotic trajectories.
One- and two-color two-photon resonance enhanced multiphoton ionization spectroscopy of the d 1Σ+ state of NH
Authors: Wales N. P. L. , de Beer E. , Westwood N. P. C. , Buma W. J. , de Lange C. A. , van Hemert M. C.
Published: July 2002

The ionization and dissociation processes occurring after two-photon excitation of the d 1Σ+ state of NH have been investigated using one- and two-color multiphoton ionization spectroscopy in combination with mass-resolved ion and kinetic energy-resolved electron detection. The photoelectron spectra obtained for ionization of the molecule via the d 1Σ+ state (v′=0,1) reveal a one-photon ionization process to all energetically accessible vibrational levels of the X 2Π ground ionic state, at variance with a one-configuration description of the two states. Moreover, electrons are observed that derive from a one-photon ionization of excited neutral nitrogen and hydrogen atoms. Two-color excitation experiments of the d 1Σ+ state show that the influence of discrete states at the overall three-photon level on the dissociation dynamics is insignificant. Using the results of ab initio calculations it is concluded that these experimental observations can be interpreted consistently if two competing processes are assumed to take place from the d 1Σ+ excited state: a molecular one-photon ionization which is forbidden in zeroth order, and a two-photon nonresonance enhanced excitation to neutral (pre)dissociative states.
Exact quantum Monte Carlo calculations of the potential energy surface for the reaction H+H2→H2+H
Authors: Diedrich Drake L. , Anderson James B.
Published: July 2002

We report ‘‘exact’’ quantum Monte Carlo calculations of the potential energy surface for the reaction H+H2→H2+H. The method used is free of systematic error. The statistical or sampling error was reduced to ±0.10 kcal/mol for several hundred points distributed across the surface, to ±0.02 kcal/mol for the minimum energy approach of H to H2, to ±0.02 kcal/mol near the saddle point, and to ±0.01 kcal/mol at the saddle point. The upper and lower surfaces in the region of the Jahn–Teller cusp were determined with a statistical error of ±0.2 kcal/mol.
Experiment versus molecular dynamics simulation: Spectroscopy of Ba–(Ar)n clusters
Authors: Visticot J. P. , de Pujo P. , Mestdagh J. M. , Lallement A. , Berlande J. , Sublemontier O. , Meynadier P. , Cuvellier J.
Published: July 2002

This work presents a quantitative comparison between experiment and molecular dynamics simulations for the excitation spectra of large van der Waals clusters. The emission and excitation spectra of mixed Ba(Ar)n clusters have been obtained for average cluster sizes ranging between 300 and 4000. The simulation is performed by using classical dynamics and pairwise additive potentials for two cases corresponding to the barium atom at the surface or inside the argon cluster. A very good agreement with the experiment is found when the barium atom is at the surface.
Theory of one- and two-photon dissociation with strong laser pulses
Authors: Shapiro Moshe
Published: July 2002

The theory of one- and two-photon dissociation processes with strong laser pulses for slowly varying multiple continua (‘‘direct’’ dissociation) is developed. Closed form expressions for the state preparation and evolution during and after the excitation pulse are derived. We show how saturation of the photodissociation process as a function of the laser power sets in. We also show that for direct dissociation, the fragment state distribution is independent of the laser power. The dependence of spontaneous emission during dissociation (continuum Raman and resonance fluorescence) on the pulse intensity is studied. The formulation is extended to treating resonantly enhanced two-photon dissociation with strong laser pulses. Closed-form expressions for slowly varying pulses and slowly varying continua are derived. Using these expressions, the existence of adiabatic passage to the continuum (APC) by which a system executes a complete population transfer from an initial bound state to the continuum by following two guiding light pulses is established. A simple iterative scheme for going beyond the adiabatic approximation is introduced.
New pseudospectral algorithms for electronic structure calculations: Length scale separation and analytical two-electron integral corrections
Authors: Greeley Burnham H. , Russo Thomas V. , Mainz Daniel T. , Friesner Richard A. , Langlois Jean-Marc , Goddard William A. , Donnelly Robert E. , Ringnalda Murco N.
Published: July 2002

We describe improved algorithms for carrying out pseudospectral Hartree–Fock calculations; these algorithms are applicable to other ab initio electronic structure methodologies as well. Absolute energies agree with conventional basis set codes to within 0.25 kcal/mol, and relative energies agree to better than 0.1 kcal/mol for a wide variety of test molecules. Accelerations of CPU times of as large as a factor of 6.5 are obtained as compared to GAUSSIAN 92, with the actual timing advantage increasing for larger basis sets and larger molecules. The method is shown to be highly reliable and capable of handling extended basis sets.