bluuebunny/crossref_metadata_2025 huggingface dataset:
Shock waves in basalt rock generated with high-powered lasers in a confined geometry
Authors: Bolger J. A. , Montross C. S. , Rode A. V.
Published: July 2002

An investigation was made into the interaction of Q-switched Nd: glass laser pulses, with energies of 5–100 J and intensities in the range of 1–15 GW cm−2, with thin slab samples of basalt, in a geometry in which the laser–matter interaction was enhanced by confining the laser-induced plasma and vapor against the rock surface with a thin water layer. The high pressure generated during expansion of the ablated material produced a shock wave which traveled into the rock. On basalt samples of thicknesses of up to 18 mm, reflection of the shock wave at the rear surface of the sample caused a thin rock layer to be spalled. Examination of the structure within the interior of the rock after laser exposure revealed extensive grain disruption near to the laser spot, due to the shock-wave passage. Rock specimens of thickness 50 mm were tested via Vickers hardness microindenter. The laser exposure was shown to produce a hardness decrease within the rock which was up to 50%, and extended to depths of 25 mm beneath the target position. An empirical model accounting for shock-wave formation and propagation was developed and compared to the experimental results.
Essential role of impedance in the formation of acoustic band gaps
Authors: Kee Chul-Sik , Kim Jae-Eun , Park Hae Yong , Chang K. J. , Lim H.
Published: July 2002

We investigate acoustic band gaps (ABGs) in a two-dimensional lattice of cylinders for the cases of constant impedance, Z, and constant velocity, v. ABGs become wider for the case of constant v (varying Z), and become smaller, eventually disappearing in the opposite case. As the volume fraction increases, the upper (bottom) edge of the stop band increases (decreases) and then decreases (increases) in composites with impedance variation only, so that the midgap frequency changes very little and a larger ABG can be created. The upper (bottom) edge of the stop band increases (decreases) when the impedance ratio increases, so that the midgap frequency decreases slightly and the size of the ABG increases.
Excitation spectroscopy of jet-cooled ozone: The Huggins system
Authors: Sinha Amitabha , Imre Dan , Goble James H. , Kinsey James L.
Published: July 2002

Excitation spectra of rotationally cooled ozone have been taken in the Huggins band (λ∼325 nm). The extreme rotational cooling, achieved by supersonic expansion (Trot ∼3 K) has allowed partial resolution of rotational structure in the (501) and (600) band systems. Rotational contour analysis of these bands has led to the conclusion that the excited state responsible for the Huggins system is the 1B2 state (the same as for the Hartley system). From the contour analysis we also obtain an upper bound on the lifetime of 3.6 ps for this state and approximate inertial constants of A≊2.1 cm−1, B̄≊0.45 cm−1.
A study of nonrigid aromatic molecules by supersonic molecular jet spectroscopy. I. Toluene and the xylenes
Authors: Breen P. J. , Warren J. A. , Bernstein E. R. , Seeman Jeffrey I.
Published: July 2002

Dispersed emission and time of flight mass spectra are presented for jet-cooled toluene, and o-, m-, and p-xylene. The spectra exhibit features, typically within 100 cm−1 of the S1⇄S0 origins, which are assigned to transitions associated with the internal rotation of the ring methyl groups. A model is developed which treats this methyl motion as that of a one-dimensional rigid rotor. The spacings of the peaks in the spectra are used to solve for the rotational constant B of the methyl rotor, and for the size and shape of the n-fold barrier to rotation (i.e., V3, V6, etc.) within this model. For toluene and p-xylene, the barrier is found to be small in both the ground (S0) (V6∼10 cm−1) and excited (S1) (V6∼25 cm−1) electronic states. For m-xylene, the ground state is again found to have a low barrier (V6∼25 cm−1), but the excited state has a potential barrier of V3=81 cm−1, V6=−30 cm−1. The barrier to rotation of the ring methyl groups is observed to be the highest for o-xylene. In this case the ground state is found to have a rather large barrier V3=425 cm−1, V6=18 cm−1 which changes to V3∼166 cm−1, V′3 ∼−25 cm−1, and V6∼0 cm−1 in the excited state. The V3 term represents a potential cross term between the two methyl rotors. The use of a kinetic energy cross term with a weighting coefficient of 0.72 in the Hamiltonian is also required for an accurate description of the excited state of this isomer. Empirical force field (EFF) calculations are performed for toluene and the three xylenes using a molecular orbital–molecular mechanics (MOMM) algorithm. The EFF-MOMM calculations are in essential agreement with the spectroscopic results and the one-dimensional rigid rotor model.
The C–H overtone spectra of acetylene: Bend/stretch interactions below 10 000 cm−1
Authors: Smith Brian C. , Winn John S.
Published: July 2002

High resolution FT-IR spectra of C2H2 have been obtained from 1800 to 10 000 cm−1. Over 1600 rovibrational transitions belonging to 30 vibrational bands were assigned and analyzed yielding band origins and rotational constants. Band centers from this and other studies were used to calculate various stretch/stretch and bend/stretch anharmonic coupling constants. Several resonances of the type between ν3 and (ν2+ν4+ν5)0 were analyzed, and an unperturbed value of ν3 was determined. This was combined with other data to obtain ωm, xm, and λ in the local mode basis. x–K relations were imposed to yield the normal mode constants for ν1 and ν3. Treatment of Darling–Dennison resonances in the first two overtone manifolds led to reassignments of previously reported spectra and to the discovery of a Fermi resonance involving overtones of ν3 and states containing ν1, ν2, and 2ν04. Calculated rotational constants and intensity data confirm the assignments and interactions treated in the vibrational analysis. Treatment of these resonances significantly improved the agreement between calculated and observed transition wave numbers, providing a coherent picture of acetylene C–H stretch dynamics below 10 000 cm−1.
A state-selected study of the unimolecular decomposition of C2H+2(Ã,B̃) using the photoion–photoelectron coincidence method
Authors: Norwood K. , Ng C. Y.
Published: July 2002

The photoion–photoelectron coincidence spectra for C2H+ and C2H+2 have been measured in the wavelength range of 645–765 Å. The C2H+2(Ã 2Ag,B̃ 2∑+u) ions prepared with internal energies above 17.39 eV are found to dissociate completely into C2H++H in the temporal range <12 μs. An upper bound of 17.33±0.05 eV is determined for the appearance energy of the process C2H2+hν→C2H++H+e− at 0 K.
A band-Lanczos method for computing matrix elements of a resolvent
Authors: Meyer H.-D. , Pal S.
Published: July 2002

The band-Lanczos algorithm is used to compute a few matrix elements of a resolvent, i.e., to compute 〈ψk‖(ω−H)−1‖ψl〉. Here H denotes a Hermitian matrix of large dimension. It is shown that the proposed method which is based on the band-Lanczos algorithm is more efficient than a method based on simple-Lanczos algorithm, e.g., than the residue algebra method. The performance of the Lanczos algorithm (both simple and band) for evaluating resolvents or other functions of H is discussed in some detail.
Wave functions of helium clusters
Authors: Rama Krishna M. V. , Whaley K. B.
Published: July 2002

The ground and several l=0 breathing mode vibrational excited state wave functions of HeN clusters are determined for N=3–5, 20, 70, and 240, using the variational Monte Carlo method. These wave functions incorporate one-, two-, and three-particle correlation effects and give binding energies, density profiles, and vibrational excitation energies accurately. The larger clusters have liquid-like structure, characterized by a pair distribution function showing approximately two coordination shells. The smallest clusters (N=3, 4) have extensively delocalized structures, which on average are equilateral triangular and tetrahedral, respectively. The N=5 cluster has a totally symmetric average structure, which can only be described in terms of a quantum liquid. No molecular structure, whether rigid or floppy can be assigned in this case. The relative importance of various correlation effects in clusters of different sizes is analyzed and discussed. These wave functions are completely analytical and are convenient as importance functions in diffusion and Green’s function Monte Carlo calculations.
Rotational spectrum and internal rotation of a methane–HCl complex
Authors: Ohshima Yasuhiro , Endo Yasuki
Published: July 2002

Rotational spectra of CH4⋅HCl and CD4⋅HCl have been studied using a pulsed-nozzle Fourier-transform microwave spectrometer. The K=0 and K=1 components of the J=1–0,2–1, and 3–2 transitions have been detected in 4–18 GHz and assigned through the Cl-nuclear quadrupole hyperfine structure characteristic for a symmetric top. The Cl-isotopic dependence of the determined rotational constants is consistent with a methane...HCl geometry, i.e., methane acts as a proton acceptor and hydrogen chloride as a donor, respectively. The K=0 transitions were observed to be split into a doublet while only one component was detected for K=1. This anomaly has been explained as it is due to two-dimensional internal rotation of methane. An analysis considering the angular momentum coupling between the internal rotation and overall rotation of the complex gives a correlation between a free-internal rotor and a rigid symmetric top. The two observed K=0 and one observed K=1 components correspond to the three lowest states with different symmetries, A, F, and E, which correlate respectively to the j=0, 1, and 2 states of the freely rotating methane.
High-pressure Raman spectroscopic study of sodium tetrasilicate (Na2Si4O9) glass
Authors: Wolf George H. , Durben Dan J. , McMillan Paul F.
Published: July 2002

The Raman spectrum of sodium tetrasilicate (Na2Si4O9) glass has been obtained as a function of pressure at 298 K to approximately 50 GPa. The scattering intensity of bands associated with silica tetrahedra containing nonbridging oxygens (Q3 species) decreases with increasing pressure and these bands are absent above 20 GPa. The spectral changes observed over this pressure interval are consistent with the formation of high-coordinate Si species at the expense of nonbridging oxygens. Above 33 GPa, the mode of change of the Raman spectrum with pressure changes abruptly. The main Raman band markedly broadens and weakens, accompanied by a twofold increase in the pressure derivative of the peak maximum. These spectral changes indicate the onset of a second densification mechanism operative at higher pressures. We conjecture that above 33 GPa, high coordinate silicon species are formed through the involvement of bridging oxygens (i.e., formation of IIIO species). The changes in the Raman spectrum associated with this latter mechanism are found to be reversible on decompression. However, spectral changes observed at lower pressures (<20 GPa) are not fully reversible and decompressed glass samples do not completely relax to their normal 1 atm state. Although silica tetrahedral species containing nonbridging oxygens hysteretically reform at low pressures on the decompression path, a comparison of the Raman spectra of pressure-quenched glass samples with that of normal sodium tetrasilicate glass indicates a change in distribution of these species. In particular, there is an increase in the relative abundance of Q2 species (silica tetrahedra containing two non-bridging oxygens) in samples decompressed from high pressures. We emphasize that Q2 species are not formed under pressurization but are formed on the decompression path and may be due to the reversion of high-coordinate silicon species to tetrahedral species under pressure release.
Reactions of silicon cluster ions, Si+n (n=10–65), with water
Authors: Ray Urmi , Jarrold Martin F.
Published: July 2002

The chemical reactions of size selected Si+n (n=10–65) with D2O have been studied using injected ion drift tube techniques between temperatures of 258 and 404 K. The only products detected were a series of Sin(D2O)+m adducts. Large variations in reactivity were observed for the smaller clusters (n<40) that diminish with increasing cluster size. Si+11, Si+13, Si+14, Si+19, and Si+23 are particularly unreactive compared to their neighbors. At room temperature the larger clusters (n>40) are a factor of ∼10–1000 (depending on the bulk surface) less reactive towards water than bulk silicon. The reaction rates for all clusters exhibit an unusually strong negative temperature dependence but are independent of the buffer gas pressure. These results suggest that the reaction mechanism probably involves two steps. In the first step, a weakly bound molecularly adsorbed Si+n⋅⋅⋅D2O adduct is produced. The second step involves rearrangement to give a more strongly bound (and probably dissociatively adsorbed) SinD2O+ product. It appears that the reaction rates for some of the smaller clusters show a faster than linear dependence on D2O pressure. One possible explanation for this unusual observation is that a second D2O molecule solvates the transition state and significantly lowers the activation barrier for dissociative adsorption.
Reaction of Sc+, Ti+, and V+ with CO. MC+ and MO+ bond energies
Authors: Clemmer D. E. , Elkind J. L. , Aristov N. , Armentrout P. B.
Published: July 2002

The reactions of Sc+, Ti+, and V+ with CO are studied as a function of translational energy in a guided-ion-beam tandem mass spectrometer. Formation of both metal-carbide and metal-oxide ions are observed and rationalized by a direct atom abstraction mechanism. At high energies, the ScC+ and ScO+ cross sections exhibit additional features that are unusual but can be explained by an impulsive pairwise mechanism and formation of excited-state product ions, respectively. Thresholds of the reaction cross sections are interpreted to give the 0 K bond energies (in eV) D0(ScC+)=3.34±0.06, D0(TiC+)=4.05±0.24, D0(VC+)=3.87±0.14, D0(ScO+)=7.11±0.08, D0(TiO+)=6.88±0.07, and D0(VO+)=5.81±0.17. Additional studies are used to help verify the bond energy for ScO+ and yield a recommended value of 7.14±0.11 eV. The nature of the bonding in MO+ and MC+ is discussed and compared for these three metal ions.
Cold molecular beam electronic spectrum of C6 and C7
Authors: Haufler R. E. , Chai Y. , Chibante L. P. F. , Fraelich M. R. , Weisman R. B. , Curl R. F. , Smalley R. E.
Published: July 2002

The electronic spectra of supersonically cooled C60 and C70 have been observed in a molecular beam by two-photon ionization spectroscopy in the regions 375 to 415 nm and 595 to 640 nm. Clear spectral features are observed in both regions for C60 and in the long wavelength region for C70. Neither molecule is responsible for the diffuse interstellar bands.
Band shape and vibrational structure in Auger spectra: Theory and application to carbon monoxide
Authors: Cederbaum L. S. , Campos P. , Tarantelli F. , Sgamellotti A.
Published: July 2002

A time-dependent approach to Auger spectra is presented and used to derive simple working equations for computing the vibrational broadening and the vibrationally induced shift of the peaks in the spectrum. The formulas give the explicit dependence of the vibrational envelope on the local details of the electronic potential energy surfaces of the intermediate and final states, providing interesting general insights which we discuss in detail. It is shown that, in polyatomic molecules, relevant interaction terms among different nuclear modes arise. The theory applies as well to other processes which involve a core ionized or core excited intermediate state like, for instance, x-ray emission or resonant Auger decay. As a test application, the double ionization spectrum of CO is computed by the Green’s function method, and the new equations, together with a two-hole population analysis of the pole strengths, are used to obtain theoretical Auger spectra. The experimental spectral profiles, characteristically shaped by the varying vibrational broadening and substantial energy shifts, are accurately reproduced, giving most peak positions to within a few tenths of eV. The results present very different vibrational effects for the carbon and oxygen spectra, showing the general inadequacy of interpretations based on vertical transition energies only. Hole localization in the dicationic states is discussed in the light of the population analysis results and used to estimate the electronic Auger transition rates.
Pressure-induced crystalline-to-noncrystalline transformations of barium fluorozirconates: A probe of the medium range order of noncrystalline solids
Authors: Serghiou G. C. , Hammack W. S.
Published: July 2002

The reversible pressure-induced transformation of crystalline barium fluorozirconates to noncrystalline solids is reported. The transformation is observed by in situ high pressure Raman spectroscopy. Since the reported crystal-to-noncrystalline transformations occur reversibly, the medium range order (MRO) of the noncrystalline solid formed can be determined; there are very few experimental methods for determining the MRO of amorphous materials. Specifically, it is reported that crystalline β-BaZr2F10 becomes noncrystalline at 35–45 kb, α-BaZrF6 at 65–75 kb, and β-BaZrF6 at 100–120 kb. The medium range order for the noncrystalline phases formed at high pressures is as follows: those formed from β-BaZrF6 crystals consist of kinked chains of zirconium fluoride; noncrystalline materials formed from crystals of α-BaZrF6 contain chains which are connected in many directions forming a ‘‘net’’ of zirconium fluoride polyhedra; and crystals of β-BaZr2F10 form a solid composed of distorted layers of zirconium fluoride polyhedra. More succinctly, the high pressure noncrystalline phase of β-BaZrF6 is one dimensional, of α-BaZrF6 is somewhat three dimensional, and the solid formed from β-BaZr2F10 is fully two dimensional. This is the first report of a crystalline-to-noncrystalline transformation for a material which can be formed at ambient pressure as both a crystalline solid and as a melt-quenched glass. The relationship between the pressure-induced noncrystalline phases and their ambient pressure melt-quenched analogues is discussed.
Site specificity of solvent shifts as revealed by ionization threshold in aniline–(argon)n clusters
Authors: Douin Stéphane , Hermine Patrice , Parneix Pascal , Brechignac Philippe
Published: July 2002

We report ionization threshold measurements for different isomers of Aniline–(argon)n clusters (n=1–3). The data are well represented by simple additivity rules of ‘‘site-specific’’ solvent shifts, which can be generalized to other systems.
He2Cl2 and He3Cl2 van der Waals clusters: A quantum Monte Carlo study
Authors: Bac̆ić Z. , Kennedy-Mandziuk M. , Moskowitz J. W. , Schmidt K. E.
Published: July 2002

The results of the first variational and Green’s function Monte Carlo calculations of the vibrational ground states of He2Cl2 and He3Cl2 van der Waals (vdW) clusters are presented in this paper. The quantum dynamics of all internal degrees of freedom are treated exactly. The ground state wave function of He2Cl2 is characterized by means of the probability distribution functions of the intermolecular degrees of freedom, which reveal an exceptionally fluxional vdW complex. A simple model for the ground state of HenCl2 vdW clusters was developed. The zero-point energies of He2Cl2 and He3Cl2 predicted by this model are in remarkable agreement (to within 0.6%) with the accurate results.
Monte Carlo simulation of diamond growth by methyl and acetylene reactions
Authors: Frenklach Michael
Published: July 2002

A dynamic Monte Carlo technique was applied to gas-surface reactions simulating diamond growth under chemical vapor deposition. A combined methyl-and-acetylene reaction mechanism was assumed, where the additions of methyl radicals and acetylene molecules are allowed to occur only when no steric interferences arise. The sterically resolved computations demonstrate nonlinear kinetic coupling: methyl and acetylene additions occur simultaneously and interdependently on each other−adsorption of CH3 creates sites for C2H2 addition, and addition of C2H2 creates sites for CH3 adsorption. It is also shown that the incorporation of acetylene by three-center additions only, irreversible on physical grounds, is capable of explaining the rate of diamond growth, thus dismissing the argument of reaction reversibility advanced against our proposed mechanism of acetylene addition.
Nearly monodisperse spherical domain morphology in polycarbonate/poly(methylmethacrylate) blends following spinodal decomposition coupled with an interchange reaction
Authors: Rabeony M. , Hseih D. T. , Garner R. T. , Peiffer D. G.
Published: July 2002

The morphological development in blends of bisphenol-A polycarbonate (PC) and poly(methylmethacrylate) (PMMA) blends during isothermal annealing above 200 °C has been investigated where competition between liquid–liquid phase separation by spinodal decomposition and interchange reactions take place. Interchange reactions between PC and PMMA occurs at temperatures above 200 °C and leads to the formation of in situ graft copolymers from an ester–ester interchange reaction. During spinodal decomposition, graft copolymers are produced mainly at the interface region between the interconnected microphase domains. Instead of the usual ‘‘coarsening’’ process which is characteristic of the late-stage of spinodal decomposition, the mixture exhibits nearly monodisperse spherical domains as revealed by optical microscopy. This phenomenon is further studied through extensive small angle light scattering measurements. Resonance peaks up to fourth order are noted, a rare observation. The result clearly demonstrates that graft copolymers are formed in situ and can act as very effective ‘‘surfactants’’ in polymer blends. Furthermore, an attempt is made to analyze the angular dependence of the scattering intensity from this morphology with the Percus–Yevick hard sphere liquid theory. These results are believed to be general and therefore applicable to a wide variety of blends containing one or more components capable of an interchange reaction.
Photodissociation spectroscopy of NbnArm complexes
Authors: Menezes Warren J. C. , Knickelbein Mark B.
Published: July 2002

The optical absorption spectra of niobium clusters containing 7 to 20 atoms have been measured from 336 to 634 nm by way of photodissociation action spectroscopy of the corresponding van der Waals complexes with argon atoms: Nbn Arm → hν Nbn + m Ar. The clusters in this size range do not display discrete absorption bands characteristic of molecular behavior, but rather absorption cross sections which increase monotonically with decreasing wavelength. This behavior is in qualitative accord with the absorption behavior predicted by the spherical Mie model for small niobium spheres, however, the measured cross sections are 2–5 times larger than predicted over this wavelength range, with the smallest clusters displaying the largest deviations. Interpreted within the classical electrodynamic framework, these observations suggest that the absorption spectra derive oscillator strength from an incipient surface plasmon, redshifted from its predicted resonance frequency in the vacuum ultraviolet.
Energy partition in collisions of C60+ ions with diamond (111) and graphite (0001) surfaces
Authors: Busmann H.-G. , Lill Th. , Reif B. , Hertel I. V. , Maguire H. G.
Published: July 2002

Velocity distributions and fragmentation of C60 molecules after collision with type IAa diamond (111) and graphite (0001) surfaces are studied for primary energies up to 500 eV at a scattering angle of 140° using time-of-flight mass spectrometry. The absolute scattering yield is much lower for diamond due to polishing induced surface corrugation. However, the final velocity distribution of C60+ and its rate of metastable fragmentation after collision are similar for scattering off the two targets, and is interpreted by their similar lateral atomic structure. At a particular impact energy, the final internal energy of C60+, as monitored by the rate of metastable fragmentation, increases with final kinetic energy, but the energy transferred to the target decreases. These results are understood when considering the collision as a two-step mechanism, in which the impact energy is transformed into thermal and deformational energy of the target and C60. This latter energy is subsequently transformed to final kinetic and further thermal energy.
Theoretical study of small Ar*n clusters in low-lying electronic states
Authors: Last Isidore , George Thomas F.
Published: July 2002

Rydberg excited rare gas atom clusters are treated by the diatomics-in-ionic-systems (DIIS) method and by a simple model approach. According to the results of the calculations, metastable clusters Ar*n (n≥2) are not formed, most probably, from the lowest excited state (4s), but can be formed from higher excited states such as 4p. DIIS calculations of small Arn van der Waals clusters provide the excitation energy dependence on the number n of atoms. The model calculation shows that in the Ar13 van der Waals clusters the excited electron is pushed away from the inner cluster region to the outside region.
Ridge method for finding saddle points on potential energy surfaces
Authors: Ionova Irina V. , Carter Emily A.
Published: July 2002

A new method is proposed for locating saddle points on potential energy surfaces. The method involves walking on the ridge separating reactants’ and products’ valleys toward its minimum, which is a saddle point in coordinate space. Of particular advantage for ab initio calculations, the ridge method does not require evaluation of second derivatives of the potential energy. Another important feature of the method is that no assumptions about the transition state geometry are needed, and it is easy to impose linear constraints on the molecular structure. The ridge method is supplemented by a heuristic detour algorithm, which enables one to deal with unfortunate choices of reactants’ and products’ coordinates. Both algorithms are illustrated by several examples where the complexity of the potential energy surface ranges from a simple analytical formula to a numerical many-body ab initio potential.
Study of Si4 and Si4− using threshold photodetachment (ZEKE) spectroscopy
Authors: Arnold Caroline C. , Neumark Daniel M.
Published: July 2002

The threshold photodetachment (ZEKE) spectrum of Si4− is presented. Although no transitions to the ground state of Si4 are observed, we obtain detailed information on the anion and several of the low-lying excited states of neutral Si4. The spectrum shows a long progression of well-resolved transitions between the D2h 2B2g rhombus anion and ν2 vibrational levels of the first excited D2h 3B3u neutral. The length and spacing of the progression is consistent with ab initio calculations performed by Rohlfing and Raghavachari [J. Chem. Phys. 96, 2114 (1992)], but some of the sequence bands observed within the progression are not. We also observe transitions to the Si4 1B3u state which is found at a lower excitation energy than predicted. The perturbed vibrational structure in this band is attributed to vibronic coupling to a nearby electronic state which is ‘‘dark’’ with respect to ZEKE spectroscopy. The ZEKE spectra are compared to the previously obtained photoelectron spectra of Si4− as well as ab initio calculations on Si4− and Si4.
Annealing and dissociation of carbon rings
Authors: Hunter Joanna M. , Fye James L. , Jarrold Martin F.
Published: July 2002

Laser vaporization of graphite generates carbon clusters which possess a number of different geometric structures ranging from planar ring systems to fullerenes. We have employed a multicollision excitation scheme combined with injected ion drift tube techniques to examine the dissociation and annealing of carbon cluster ions. In this paper, we focus on clusters containing 30–50 atoms. Nonfullerene cluster ions in this size range can be annealed in the gas phase to just two dominant isomers—the fullerene and an isomer which appears to be a large monocyclic ring. Fragmentation by loss of C2 units is associated with annealing into the fullerene. This process occurs at low injection energies and rapidly becomes more important with increasing cluster size. However, relatively few clusters follow this path in the size range examined here (less than 3% for C40+). Most of the nonfullerene isomers anneal into a large monocyclic ring which (for clusters in the size range examined here) does not appear to convert into the fullerene. At higher injection energies, a second fragmentation route is observed. This second dissociation process seems to be associated with cleavage of the large monocyclic rings into two roughly equal fragments. Activation energies for these processes are estimated, and the possible role they play in the gas phase synthesis of fullerenes is discussed.
Ground state configurations of model molecular clusters
Authors: Clarke A. S. , Patey G. N.
Published: July 2002

We have determined the ground state configurations of model molecular dipolar clusters using Monte Carlo simulations. Cluster sizes ranged from N=3 to N=20. The molecular models consisted of spherically symmetric hard sphere or Lennard-Jones interactions, as well as dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. Molecular polarizability was also included in some calculations. We first found the ground state configurations of dipolar hard sphere clusters, and then systematically increased the quadrupole moment and molecular polarizability from zero up to typical values for small molecules. The relevance of our results to real molecular clusters is discussed.
Collision-induced dissociation of Co+n (n=2–18) with Xe: Bond energies of cationic and neutral cobalt clusters, dissociation pathways, and structures
Authors: Hales David A. , Su C.-X. , Lian Li , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of collision-induced dissociation (CID) of Co+n (n=2–18) 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 cobalt cluster cations dissociate exclusively by loss of single atoms (cluster ‘‘evaporation’’), with no evidence found for elimination of molecular cluster fragments. Bond dissociation energies for cobalt cluster cations, Co+n (n=2–18), are determined from measurements of the CID thresholds. Bond energies for neutral cobalt clusters, Con (n=4–18), are derived by combining these cationic bond energies with ionization energies for Con from the literature. The dependence of binding energy on cluster size is similar to that observed for iron clusters, and inspires some speculation regarding cluster ion structures.
The thermal stability and fragmentation of C60 molecule up to 2000 K on the milliseconds time scale
Authors: Kolodney E. , Tsipinyuk B. , Budrevich A.
Published: July 2002

A method of generating effusive molecular beams of C60 with vibrational temperatures up to 2000 K is presented. Direct measurement of the thermal stability and fragmentation kinetics of C60 in the range of 1100–1970 K, on the milliseconds time scale, provides thermal rate constants k(T)=10–300 s−1 (for T=1720–1970 K, respectively) and activation energy of E0=4.0±0.3 eV.
Chemical binding and electron correlation effects in x-ray and high energy electron scattering
Authors: Wang Jiahu , Tripathi Awadh N. , Smith Vedene H.
Published: July 2002

The total, inelastic and elastic intensities for x-ray and high energy electron scattering from the ten-electron molecules CH4, NH3, H2O, and HF have been calculated with configuration interaction wave functions. The probability density of the interelectronic distance or the radial intracule density is extracted from the intensities. The importance of basis sets, chemical binding, and electron correlation to the scattered intensities and the radial intracule density has been examined. It has been found that scattered intensities predicted with correlated wave functions agree well with experiment. Based on the electron pair distribution, the exchange and correlation holes in molecular systems have been briefly discussed.
Rotational analysis of the 7pσ 3Σ+g←a3Σ+u system of the Ar2 molecule
Authors: Herring C. M. , Kim S. B. , Eden J. G. , Ginter M. L.
Published: July 2002

Rotationally resolved transitions from the metastable 4sσ a3Σ+u state of Ar2 to the 7pσ 3Σ+g state have been observed by laser excitation spectroscopy in a pulsed corona discharge. Laser induced fluorescence spectra for the 7pσ 3Σ+g←a3Σ+u transition observed near 19 530 cm−1 have led to the first resolved triplet splittings and rotational analyses for Rydberg–Rydberg transitions in Ar2. Analyses for the (0–0), (0–1), (1–0), and (1–1) bands of 7pσ←a are presented here. Bands with v′=2 are observed for this system but exhibit no rotational structure and bands with v′≳2 are not observed, indicating that the predissociation lifetimes of these upper levels fall rapidly for v′≳1. Data presented lead to rotational energies and molecular constants for the a3Σ+u and 7pσ 3Σ+g states, including the effective rotational constant Be which is determined to be 0.1412 cm−1 and 0.1345 cm−1 for the 4sσ a3Σ+u and 7pσ 3Σ+g states, respectively. Results are compared to ab initio and other calculations, and combined with existing data to produce experimental estimates of D0 and De for the a3Σ+u state of 5700±200 cm−1 and 5850±200 cm−1, respectively.
Coupling between diffusion and deformation of gels in binary solvents: A model study
Authors: Yoshimura Ken-ichi , Sekimoto Ken
Published: July 2002

We study theoretically the dynamics of chemically crosslinked gels in binary solvents on the basis of the Flory-type model and nonequilibrium thermodynamics. We find with the aid of numerical integration of the evolution equation that an instability occurs in the gel upon changing the composition of the surrounding solution. This instability is caused by a marked difference of timescales between quick mutual diffusion of solvent molecules and slow deformation of the gel network. Profiles of solvent composition and network deformation are also described in some details.
Theoretical study of the ultraviolet photodissociation of ozone. Comparison with experiments
Authors: Leforestier Claude , LeQuéré Frédéric , Yamashita Koichi , Morokuma Keiji
Published: July 2002

We present a fully ab initio study of the photodissociation of the ozone molecule in the Hartley band. New ab initio potential energy surfaces PESs of the ground and B (1B2) states of ozone have been calculated with the complete-active space self-consistent field-single excitation configuration interaction double zeta+polarization method to describe the three-dimensional photodissociation process. The dissociation energy of the ground state and the vertical barrier height of the B PES are obtained to be 0.88 and 1.34 eV, respectively, in better agreement with the experimental values than previous calculations. The photodissociation process has been studied within the time-dependent wave-packet formulation, using hyperspherical coordinates. The calculations reported correspond to a total angular momentum value J=0. Detailed comparisons to experimental results are presented for the photodissociation spectrum, the autocorrelation function (AC), and the O2(1Δg) fragment rovibrational distributions. The recurrences observed in the AC function have been associated to a family of Feschbach resonances between the symmetric stretch and the bending motions.
Charge-transfer reaction of 14,15N+(3P
J)+N2(1Σ+g) from thermal to 100 eV. Crossed-beam and scattering-cell guided-ion beam experiments
Authors: Freysinger W. , Khan F. A. , Armentrout P. B. , Tosi P. , Dmitriev O. , Bassi D.
Published: July 2002

We have studied the charge-transfer reaction of ground-state N+(3PJ) ions with N2(1Σ+g) from thermal to 40 and 100 eV in the center-of-mass frame with the Utah guided-ion beam mass spectrometer and the Trento crossed-beam guided-ion beam experiment. Although the formation of N+2(X 2Σ+g) is endothermic by only 1.043 eV, the charge-transfer process to yield N+2 is not observed until approximately 2.8 eV. Studies using isotopically labeled 15N+ ions demonstrate that electron- and atom-transfer channels start at the same energy and have similar energy dependencies until about 6 eV, a result that implies a symmetric but acyclic N+3 intermediate. We discuss the mechanism for this reaction by means of state correlation diagrams that demonstrate that the charge-transfer reaction must involve strong nonadiabatic effects.
Global orientational order in model polar clusters
Authors: Lavender Holly B. , Iyer Karthik A. , Singer Sherwin J.
Published: July 2002

Overall rotational anisotropy is found to be generic in compact clusters of model dipolar particles. This broken rotational symmetry, or ‘‘global’’ orientational order, is pronounced in zero temperature configurations. It arises from the tendency of dipolar particles to form head-to-tail chains which encircle the cluster, giving rise to a circulating pattern. In many cases, the zero temperature arrangement of dipolar particles is chiral. Expansion of the dipole density field in vector spherical harmonics provides order parameters to quantify broken orientational symmetry, and facilitates analysis of inversion and reflection symmetry. Finite temperature simulations of a 13-particle cluster with Stockmayer (Lennard-Jones+point dipole) interactions indicates that the global orientational order persists to higher temperatures, even above the point of translational melting. The 13-particle Stockmayer cluster remains orientationally ordered up to the onset of frequent evaporation from the cluster. Ramifications of this effect are discussed.
Dissociation dynamics of Ar+n (n=3–16) in collision with He and Ne
Authors: Hirokawa Jun , Ichihashi Masahiko , Nonose Shinji , Tahara Tahei , Nagata Takashi , Kondow Tamotsu
Published: July 2002

Dissociation dynamics of Ar cluster ions, Ar+n (n=3–16), in collision with He and Ne atoms was investigated by measuring the absolute dissociation cross sections and the branching fractions of the product ions at the collision energies of 0.1–10 eV in the center-of-mass frame. The cross sections and the branching fractions were successfully explained in terms of collisional excitation of the parent cluster ion and subsequent dissociation of it; the collisional excitation was treated by a hard sphere collision model with the aid of the molecular dynamics (MD) and the diatomics-in-molecules (DIM) calculations reported previously, and the subsequent dissociation was explained by the Rice–Ramsperger–Kassel (RRK) theory.
Infrared spectra of ice surfaces and assignment of surface-localized modes from simulated spectra of cubic ice
Authors: Rowland B. , Kadagathur N. S. , Devlin J. P. , Buch V. , Feldman Tova , Wojcik M. J.
Published: July 2002

The use of a new method of preparing micron-thick deposits of nanocrystals of ice for Fourier transform infrared sampling, with the nanocrystals supported on a vertical infrared window, has greatly improved the signal-to-noise levels of the spectra available for large ice clusters. High quality spectra of modes of the surface molecules are reported, even for regions that underlie the intense bands of the bulk ice modes. These experimental features are most clearly displayed through the use of difference spectra. For example, the difference between spectra obtained for nanocrystals, before and after an annealing cycle that significantly increases the average cluster size, reflects the decrease in number of surface groups and the corresponding increase in number of interior molecules. Similarly, differences between spectra of bare and adsorbate-covered nanocrystals, obtained at the same temperature for the same ice sample, show the significant shifts of ‘‘surface-localized’’ ice modes caused by the adsorbate molecules. These difference spectra, and similar spectra for amorphous ice, are rich with information about the (three) distinct types of ice surface water molecules and their interactions with small adsorbate molecules. The extraction of that information has been initiated by comparison of the experimental difference spectra from two sizes of D2O cubic ice nanocrystals with simulated difference spectra for a relaxed cubic ice surface compared to bulk cubic ice.

From these comparisons specific experimental features have been assigned to modes of the three categories of surface D2O(HDO) molecules: (a) three-coordinated molecules with dangling-D—2725 (2713) cm−1; (b) three-coordinated D2O molecules with dangling-O—2645 (∼2600) cm−1; (c) relaxed four-coordinated molecules—∼2580 (∼2550) cm−1. Also, information has been obtained on the approximate positions (cm−1) of other modes of surface molecules: (a) D-bonded part of dangling-D(H) molecules; ∼2350; (b) dangling-O molecules; ∼2500; (c) four-coordinated molecules; 2300–2500. The computations also indicate that, of the various modes of the surface molecules, only the higher frequency modes of the dangling-D and dangling-O are strongly localized; and only the dangling-D mode is localized on individual surface molecules.
2Σ+ potential energy curves of the (Na19–Na)+ jellium-cluster-ion system
Authors: Guissani M. , Sidis V.
Published: July 2002

A prototypical description of the (Na19–Na)+ system is reported. The Na atom, in its ground and first two excited states, is treated by a one-electron pseudopotential method. The cluster is first described in the spherical jellium background model (SBJM). A numerical Hartree–Fock approach is used to calculate the electronic wave function of the cluster in its ground state configuration. Singly excited Na19* states are obtained using an improved virtual orbital technique to allow for the distortions of the cluster electron cloud during the Na19–Na approach. The matrix of the electronic Hamiltonian in a (diabatic) basis of projected valence bond configuration state functions are determined with an effective model potential method. As a first model case, the (Na19–Na)+ system is treated holding the isolated positive background of the jellium cluster unchanged. This description pertains to rapid displacements of the atom relative to the cluster. As a second case, we consider distortion and reconstruction of the positive background as the Na+-ion core approaches and penetrates into the cluster core. The latter description is appropriate for slow displacements of the atom relative to the cluster. Potential energy curves are reported for the 2Σ+ states.
Three dimensional quantum calculation of the visible absorption spectrum of Ar+3
Authors: Gadéa Florent Xavier , Quéré Frédéric Le
Published: July 2002

We present a theoretical visible absorption spectrum of Ar+3. It relies on DIM potential energy surfaces and transition dipole moments and the dynamical calculations have been performed using a full quantum treatment for each degree of freedom of the molecule. We used hyperspherical coordinates in order to describe the full symmetry of the molecule and the wave packet was developed on a grid. We computed the spectrum with the RRG method, for three different excited electronic states and found a very good agreement with experimental data and previous calculations of restricted dimensionality.
A new class of free stable doubly negative systems: First investigations on the SimO2−n series
Authors: Sommerfeld T. , Scheller M. K. , Cederbaum L. S.
Published: July 2002

Studies on several isomers of the C2O2−n and Si2O2−n series of dianionic systems are performed at the Hartree–Fock, configuration interaction, Mo/ller–Plesset perturbation theory and outer-valence Green’s function levels of theory. All examined C2O2−n systems are predicted to be unstable with respect to electron autodetachment. However, the analogues silicon systems Si2O2−n turn out to be electronically more stable than the corresponding carbon species. For Si2O2−4 which marks the onset of electronic stability in the SimO2−n series the influence of the basis set and electron correlation on the geometrical parameters as well as on the electron detachment energy are studied in some detail. The possibility of decomposition via fragmentation or adiabatic electron loss is discussed. A simple criterion to determine whether a given structure may represent a promising candidate for a stable dianion of the SimO2−n type is suggested.
Instability of nonequilibrium fluctuation in electrochemical nucleation. I. Occurrence of instability
Authors: Aogaki Ryoichi
Published: July 2002

From the viewpoint of unstable growth of nonequilibrium fluctuations, nucleation processes were examined. Electrostatic equilibrium produced by the electric double layer is always locally broken by the thermal motion of solution particles. This breakdown, i.e., fluctuation occurs not at the Nernstian equilibrium, so that we call it ‘‘nonequilibrium fluctuation.’’ In examining the mechanism of the fluctuation, it was shown that a new type of instability occurs, which takes place in the absence of the intense specific adsorption of ions on the electrode surface. Then, two different modes for the instability are possible, that is, one is the case where from the beginning, there is no specific adsorption, and another the case where the surface is specifically adsorbed by the ions at the initial stage. The former case allows the fluctuation to continuously transfer from the stable state to the unstable nucleus growth whereas in the latter case, immediately after specific adsorption ceases, nucleation starts rapidly. On the contrary, when the specific adsorption occurs at the nonspecifically adsorbed surface, nucleation will be suddenly stopped. Furthermore, the fluctuation of the second type is formed for the mass flux of depositing metal ions in the diffusion layer, which becomes always unstable, accompanied with the unstable growth of the fluctuation of the first type.
An ab initio treatment of the electronic absorption spectra of excess-electron alkali halide clusters Nan+1Cln up to Na18Cl17
Authors: Ochsenfeld Christian , Gauss Jürgen , Ahlrichs Reinhart
Published: July 2002

Electronic excitation energies of alkali-excess clusters Na2Cl, Na3Cl2, Na4Cl3, Na6Cl5, Na14Cl13, and Na18Cl17 are investigated using CIS (configuration interaction singles) and RPA (random phase approximation). The accuracy of these approximations is established for Na2Cl by equation-of-motion coupled-cluster singles and doubles calculations and by comparison to experimental results. The mode of localization of the excess electron is decisive for the electronic excitation energy. No cluster-size dependence of the excitation energy is found. The direct UHF-CIS (unrestricted Hartree–Fock-CIS) and UHF-RPA implementation within the program package TURBOMOLE is briefly described in the appendix.
Theoretical study on the excitation spectrum and the photofragmentation reaction of Ni(CO)4
Authors: Hada M. , Imai Y. , Hidaka M. , Nakatsuji H.
Published: July 2002

The ground and excited states of Ni(CO)4 are studied using the symmetry adapted cluster (SAC)/SAC-configuration interaction (SAC-CI) method. The experimental absorption spectrum is well reproduced by the present calculations. All the peaks observed in the range of 200∼350 nm are assigned to the electronic allowed 1T2 excited states. The third peak is assigned to the 3 1T2 and 4 1T2 states. Next, the potential energy curves of the ground and the low-lying excited states are calculated by the same method and utilized to clarify the mechanism of the photofragmentation reaction of Ni(CO)4 by a XeCl laser (308 nm). A reaction pathway involving several excited states is proposed for the photofragmentation reaction into the excited Ni(CO)3 and CO. The calculated emission energy from the former agrees well with the observed luminescence spectrum.