bluuebunny/crossref_metadata_2025 huggingface dataset:
A lattice model for self-avoiding polymers with controlled length distributions. II. Corrections to Flory–Huggins mean field
Authors: Bawendi M. G. , Freed Karl F. , Mohanty Udayan
Published: July 2002

We continue the analysis of the lattice spin-field theory which is introduced in paper I and which formally gives the exact entropy for a set of completely flexible self-avoiding polymers on a lattice. The model allows for arbitrary chain length distributions and arbitrary polymer volume fractions. Use of random fields produces a field theory from which Flory–Huggins results are recovered in the mean field limit. Here we recast the mean field formulation to allow for the rigorous and systematic evaluation of corrections by means of a cluster expansion. We then calculate corrections to the mean field directly up to fourth order and provide a systematic diagrammatic method for evaluating general order corrections. Our results illustrate the polymer concentration dependence of the corrections to the Flory–Huggins mean field results as well as provide the origins for the entropic contribution to the Flory χ parameter and its concentration dependence. Generalizations to treat rods and semiflexible polymers will be given elsewhere.
Internal states distributions of NO thermally desorbed from Pt(111): Dependence on coverage and co-adsorbed CO
Authors: Mantell David A. , Cavanagh Richard R. , King David S.
Published: July 2002

The distribution of population in the internal energy levels of nitric oxide thermally desorbed from Pt(111) has been probed using laser excited fluorescence. The observed rotational distributions have been found to follow the Boltzmann distribution function, independent of NO coverage or the presence of pre- or post-adsorbed CO. Under all conditions of NO desorption, the observed NO was characterized by a temperature (0.95±0.05) times the surface temperature. No evidence of a preferred alignment of the rotational angular momentum vectors was observed, nor was there any difference between the two spin-orbit multiplets beyond that associated with the rotational temperature.
Generalized theory of nucleation kinetics. IV. Nucleation as diffusion in the space of cluster dimensions, positions, orientations, and internal structure
Authors: Ziabicki Andrzej
Published: July 2002

The theory of nucleation has been extended to cover phase transitions involving complex, asymmetric, and anisotropic clusters in external potential fields. The theory, considering a flux of clusters in the space of cluster dimensions, orientations, positions, and variables describing internal structure, predicts several new effects. Nucleation—production of clusters capable of spontaneous growth—can result not only from the growth of subcritical clusters (embryos), as assumed in the classical, one-dimensional theory, but also from translational, or rotational diffusion of subcritical clusters in an external field.
Rovibrational averaging of nuclear shielding in MX6-type molecules
Authors: Jameson Cynthia J. , Jameson A. Keith
Published: July 2002

Calculations of the mean M–X bond displacements in octahedral MX6 molecules by the L tensor and Bartell methods using anharmonic force fields for SF6, SeF6, TeF6, WF6, PtCl=6, and PtBr=6 are compared with electron diffraction data in SF6, and are used in the interpretation of 19F, 77Se, 125Te, and 195Pt chemical shifts in these molecules. The temperature and mass dependence of M and X chemical shifts can be written in terms of 〈ΔrMX〉, and the two together provide a critical test of anharmonic force fields. The direct proportionality of the isotope shifts to the mass factor (m′−m)/m′ is found to be a direct consequence of the calculated linear dependence of 〈Δr〉−〈Δr〉′ and 〈(Δr)2〉−〈(Δr)2〉′ on this mass factor. The observed isotope shifts and temperature dependent chemical shifts in the zero pressure limit can be used to determine the sensitivity of the nuclear magnetic shielding to bond extension.
Polarized emission from the products of oriented reactants: The Ba+N2O→BaO*+N2 reaction
Authors: Jalink H. , Parker D. H. , Stolte S.
Published: July 2002

The polarization of the BaO* chemiluminescence from the reaction of oriented N2O with Ba has been studied as a function of the relative collisional orientation at Etr=0.16 eV in a crossed beam apparatus. The dependence of the alignment of BaO* product angular momentum vector Ĵ′ upon the initial angle of attack γ0 has been determined. Perfectly oriented reactants (cos γ0=1) produce a strong alignment 〈P2(Ĵ⋅v̂r)〉=−0.3 which becomes practically zero for unfavorable geometries (cos γ0≤−0.5).
Intramolecular dephasing and relaxation in the molecule butynal (CH3–C≡CCHO)
Authors: Mühlbach J. , Huber J. Robert
Published: July 2002

The butynal molecule (CH3–C≡C–CHO), possessing an intermediate S1–T1 level structure, has been investigated in a pulsed supersonic expansion of Xe or Ar. Employing a ∼5 ns dye laser pulse (Δν̃coherence) ≂0.006 cm−1, Δν̃spectral =0.04–0.2 cm−1) the vibrational state dependence of the coherent and incoherent decay of the first excited singlet state has been explored in the excess vibrational energy range Eexcess =0–1400 cm−1 where the density of vibronic states is changing from ρvib ∼80 to ∼1200 states/cm−1. The analysis of the ‘‘biexponential’’ decay behavior, which is characteristic in intermediate molecules, shows in case of butynal that the fast component is consistent with the intramolecular dephasing of S1–T1 mixed molecular eigenstates, and that the slow component represents the incoherent decay of these eigenstates given by their lifetimes. Based on this picture and in conjunction with molecular information obtained from a previous spectroscopic study on butynal [J.Chem. Phys. 84, 3014 (1986)], a simulation of the biexponential decay has been successfully performed.
Photodetachment studies of metal clusters: Electron affinity measurements for Cux
Authors: Zheng L.-S. , Karner C. M. , Brucat P. J. , Yang S. H. , Pettiette C. L. , Craycraft M. J. , Smalley R. E.
Published: July 2002

Laser photodetachment studies have been performed on silver and copper cluster negative ion beams extracted from a supersonic metal cluster source. This source involves the use of laser vaporization within a pulsed nozzle to prepare the neutral cluster jet, followed by ArF excimer irradiation of the nozzle orifice as the copper clusters emerge into the expansion. Photoelectrons ejected from the aluminum nozzle by the excimer radiation are entrained in the supersonic flow and attach to the neutral copper clusters, producing negative ions which stabilize and extensively cool in the subsequent supersonic expansion. Laser photolysis of mass-selected negative copper cluster ions extracted from this source reveals efficient photodetachment to produce a free electron in the absence of measurable fragmentation. The laser fluence dependence of the photodetachment process at different photon energy permits a rough experimental determination of the electron affinities of copper metal clusters as a function of cluster size for the first time.
The Hückel model for small metal clusters. I. Geometry, stability, and relationship to graph theory
Authors: Wang Youqi , George Thomas F. , Lindsay D. M. , Beri A. C.
Published: July 2002

The relative stabilities of alkali-like metal clusters, Mn and M+n with 2≤n≤9, are calculated within the framework of the simple Hückel model. With the aid of graph theory, the binding energies for all possible Hückel structures are determined. With the exception of M+5 and M+6 , the Hückel model gives minimum energy structures which are the same as those predicted by recent local-spin-density and configuration interaction calculations. Since the Hückel method is independent of the mechanical details of the bonding, a close connection is inferred between a cluster’s stability and its topology. In the paper following this one, the Hückel results are extended to include absolute atomization energies and ionization potentials. In addition, it is shown that cluster energies may be quantitatively extrapolated to the bulk phase.
Doubly charged argon clusters and their critical size
Authors: Scheier P. , Märk T. D.
Published: July 2002

The electron impact ionization of neutral Ar clusters results in the formation of stable doubly charged Ar2+n only if their size exceeds a critical value. The observed minimum cluster size for argon is Ar2+91 . This result is compared with theoretical predictions.
Purely rotational coherence effect and time-resolved sub-Doppler spectroscopy of large molecules. II. Experimental
Authors: Baskin J. Spencer , Felker Peter M. , Zewail Ahmed H.
Published: July 2002

In this paper we describe the results of picosecond fluorescence polarization (sub-Doppler) experiments designed to determine the role of purely rotational coherence in two jet-cooled molecules: trans-stilbene and anthracene. Observations of the manifestations of purely rotational coherence in t-stilbene are reported. The relationship of purely rotational coherence to molecular parameters (excited state rotational constants and transition dipole directions) is confirmed by comparison of our measurements with the results of the theory described in paper I [P. M. Felker and A. H. Zewail, J. Chem. Phys. 86, 2460 (1987)]. The sum of rotational constants B′ and C′ of the t-stilbene S1 electronic state is determined with a precision of better than 1 part in 700 (B′+C′=0.5132±0.0007 GHz). The influence of molecular beam expansion conditions and fluorescence detection conditions on our measurements is also investigated and compared with the theroretical findings of paper I. Also measurements of time-resolved and polarization-analyzed fluorescence as a function of excess vibrational energy in the S1 electronic states of both t-stilbene and anthracene are reported. We are able to distinguish the contribution of purely rotational coherence from the contributions of purely vibrational (or rovibrational) coherence to the evolution of fluorescence from the vibrationally excited molecule. The results are first analyzed on the basis of a model in which strict separability of vibrational and rotational motion is assumed. This provides a test of the extent of coupling of these motions and its influence on intramolecular vibrational energy redistribution (IVR).
Photoionization mass spectrometric studies of SiHn (n=1–4)
Authors: Berkowitz J. , Greene J. P. , Cho H. , Ruščić B.
Published: July 2002

A photoionization mass spectrometric study of SiH4 at T=150 K reveals the presence of SiH+4 with an adiabatic threshold at 11.00±0.02 eV. The implications for the structure of this Jahn–Teller split state are discussed. The appearance potentials of SiH+2 and SiH+3 are 11.54±0.01 eV and ≤12.086 eV, respectively. The reaction of F atoms with SiH4 generates SiH3 (X 2A1), SiH2 (X 1A1 and a 3B1), and SiH (X 2Π) in sufficient abundance for photoionization studies. The measured adiabatic ionization potentials (eV) are: SiH3, 8.01±0.02; SiH2 (X 1A1), 9.15±0.02 or 9.02±0.02; SiH2 (a 3B1), 8.244±0.025; SiH, 7.91±0.01. The singlet–triplet splitting in SiH2 is either 0.78±0.03 or 0.91±0.03 eV. The dissociation energy of SiH is 2.98±0.03 eV. A Rydberg series is observed, converging to SiH+ (a 3Π) at 10.21±0.01 eV. Heats of formation of the various neutral and ionic species are presented, as are the stepwise bond energies of SiH4.
The role of cluster ion structure in reactivity and collision-induced dissociation: Application to cobalt/oxygen cluster ions in the gas phase
Authors: Freas Royal B. , Dunlap Brett I. , Waite Boyd A. , Campana Joseph E.
Published: July 2002

Ion/molecule reaction products of cobalt cluster ions have been characterized using mass spectrometric techniques. Atomic and bare-metal cluster ions were desorbed from foils by particle bombardment within a high-pressure (0.1–0.2 Torr) ion source. Sputtered metal cluster ions react with O2 to produce abundant stoichiometric or nearly stoichiometric cobalt(II) oxide cluster ions. The positive cluster product ions consist of three types: oxygen-deficient [Co(CoO)x]+ clusters, oxygen-equivalent [(CoO)x]+ clusters, and (in less abundance) metal-deficient [(CoO)xO]+ clusters. Tandem mass spectrometry and collision spectroscopy provide structural information about the more abundant cobalt cluster product ions. A major collision-induced fragmentation pathway for the oxygen-equivalent [(CoO)x]+ clusters is the loss of a CoO moiety to form [(CoO)x−1]+ fragments. A major collision-induced fragmentation pathway for the oxygen-deficient [Co(CoO)x]+ clusters is the loss of a cobalt atom to yield [(CoO)x]+ fragments. Geometric structures of the cobalt/oxygen cluster ions were calculated using a Coulomb plus Born–Mayer pair-potential model. The oxygen-equivalent cluster structures were found to be ‘‘globular’’ cages, rings, or ladders. The oxygen-deficient cluster structures were found to be strained and ‘‘angular’’ with protruding cobalt atoms. The structures are discussed in terms of the observed collision-induced fragmentations. The fragmentations are rationalized using an ‘‘instantaneous’’ dissociation model of the collision-induced dissociation of the cluster ions. Preliminary trajectory calculations using classical dynamics support the use of this instantaneous dissociation model. The role of cluster ion structure in reactivity and collision-induced dissociation is discussed in terms of the experimental data and theoretical structures.
Electronic states and geometries of small Be clusters
Authors: Marino Maria M. , Ermler Walter C.
Published: July 2002

A b initio calculations were carried out on all regular geometric configurations of beryllium clusters containing three through seven atoms. Restricted Hartree–Fock, unrestricted Hartree–Fock, and fourth-order Mo/ller–Plesset perturbation theory (MP4) using a 6-31G basis set were used to study singlet, triplet, and quintet spin states of each cluster. The septet state of the most stable nuclear configuration of Be7 was also examined using unrestricted Hartree–Fock theory. In addition, Be4 was examined as a prototype for larger clusters, for which accurate calculations may not be computationally feasible. The most stable symmetric nuclear configuration and electronic state of Be4 was determined at increasingly improved levels of approximation. Hartree–Fock, configuration interaction, and Mo/ller–Plesset perturbation theory calculations were carried out using 6-31G, 6-311G, 6-31G*, and 6-311G* basis sets. Binding energies for the most stable configuration of each cluster were improved by MP4 calculations in a 6-31G* basis set. Be–Be internuclear separations for the lowest electronic state of each cluster range from 1.9 to 2.5 Å. MP4 calculations predict singlet ground states for Be3, Be4, and Be5, whereas Be6 and Be7 are found to have quintet ground states.
On the spectral profiles of optical transitions to the autoionizing 7d′ and 9s′ levels and configuration interaction in 7d′ levels of Kr
Authors: Wada Akihide , Adachi Yukio , Hirose Chiaki
Published: July 2002

Spectral line shapes of the optical transitions from the 5p′ to the autoionizing 7d′ or 9s′ levels of Kr are found by optogalvanic spectroscopy to be symmetric, in contrast to the asymmetric profiles of the ordinary autoionization lines and the widths of some of the observed lines which are varied by changing the pumped level in an optogalvanic double-resonance spectroscopy. We conclude from the analysis of this behavior that the direct photoionization which introduces the asymmetric profile by interference with the autoionizing transition is negligibly weak and that the autoionizing levels which are close to each other are mixed by a second-order configuration interaction through the continuous levels. Based on this analysis, the accurate term values of the autoionizing 9s′ and 7d′ levels are determined.
The H+5 potential energy hypersurface: Characterization of ten distinct energetically low-lying stationary points
Authors: Yamaguchi Yukio , Gaw Jeffrey F. , Remington Richard B. , Schaefer Henry F.
Published: July 2002

A b initio molecular electronic structure theory has been used in an attempt to characterize the low-lying stationary points on the H+5 potential energy hypersurface. Three distinct levels of theory have been used: the self-consistent-field (SCF) method, configuration interaction (CI) including all single and double excitations, and full configuration interaction. Four different basis sets were used: double zeta (DZ), double zeta plus polarization (DZP), an extended basis set designated H (6s2p/4s2p), and a second extended basis set designated H (8s3p/6s3p). The higher levels of theory are in agreement that the only minimum for H+5 is a C2v structure, with three other stationary points (of D2d, C2v, and D2h symmetries) lying less than 1 kcal/mol higher in energy. The predicted dissociation energy D0 is 5.5 kcal/mol, which is estimated to be about 1 kcal/mol less than the exact D0. Furthermore, there are six other stationary points lying less than 8 kcal/mol above the minimum. Vibrational frequencies, dipole moments, and infrared intensities for each of the ten stationary points have been predicted at several different levels of theory. From the perspective of quantum chemistry, the H+5 system is very attractive as a candidate for the study of the vibrational dynamics of weakly bound systems.
Absolute cross sections for fluorine 3p→3s line emissions following single electron impact on NF3, CF4, and SF6
Authors: Blanks K. A. , Tabor A. E. , Becker K.
Published: July 2002

We studied the dissociation of the fluorine-containing molecules NF3, CF4, and SF6 by controlled electron impact in a crossed electron beam–gas beam apparatus. Absolute emission cross sections and appearance potentials were determined for the most intense 3p→3s atomic fluorine lines in the visible range of the optical spectrum between 6200 and 7800 Å. These lines are emitted by atomic fluorine fragments that are formed in the various excited states associated with the 1s22s22p43p electron configuration. The energy dependence for all cross sections indicated that optically allowed, electric dipole excitation of the parent molecule plays the leading role. For a particular line the absolute emission cross sections were found to decrease from NF3 to CF4 to SF6. The measured onset potentials indicated that in all cases total fragmentation of the molecule is a dominant process. NF3, CF4, and SF6 are the main constituents of etchant gas plasmas that are widely used in various areas of semiconductor processing. An attempt is made to relate our single collision results to results obtained in optical studies of rf processing plasmas.
A perturbed electron droplet model for the electronic structure of small aluminum clusters
Authors: Upton T. H.
Published: July 2002

The results of first principles total energy calculations for optimized structures of Aln(n=2–6) are presented in which the evolution of cluster orbital occupation with cluster size is found to be different than that predicted by the ‘‘electron droplet’’ or electronic shell model. We find that these variations may be understood in terms of an extension to the droplet model that introduces the structure of the exact electron–nuclear attraction potential as a perturbation on the smooth spherically symmetric potential of the droplet model. We discuss the limits in which this effect is significant, finding that its importance should diminish with increasing cluster size. The detailed calculations produce cluster ionization potentials that are almost all larger than for the Al atom (5.8 to 6.6 eV), in agreement with experiment. Electron affinities are larger as well, ranging from 0.1 to 2.1 eV. Bond lengths increase from 2.51 Å for Al2 to 2.81 Å for Al6, but are highly sensitive to cluster electronic state. Cohesive energies increase with cluster size, but do not tend to the bulk value with the conventional n−1/3 dependency. Neutral (and ionic) clusters assume predominately low-spin states, also in agreement with experiment. Bonding is essentially from 3p atomic orbital combinations, and the cluster bonding orbitals are delocalized. Interaction with nonbonding 3s states significantly affects the bonding 3p orbital occupation and produces three-dimensional cluster geometries.
Systematics of liquid helium clusters
Authors: Stringari S. , Treiner J.
Published: July 2002

The properties of 3He and 4He clusters are investigated in the framework of a density functional approach. We find that 4He clusters are always bound and do not exhibit any shell effect. Conversely, 3He clusters require a minimum number of atoms (Nmin≂30) to ensure binding. 3He clusters are found to exhibit harmonic oscillator type shell effects associated with the magic numbers N=1/3 (m+1)(m+2)(m+3) (with m integer)=40,70,112,168,... and single particle energy gaps of the order of 0.2–0.4 K.
The UV absorption spectrum of C60 (buckminsterfullerene): A narrow band at 3860 Å
Authors: Heath J. R. , Curl R. F. , Smalley R. E.
Published: July 2002

The absorption spectrum of the special C60 cluster buckminsterfullerene has been studied in a supersonic beam by laser depletion of the cold van der Waals complexes of C60 with benzene and methylene chloride. Both complexes were found to display a single, isolated absorption band in the near ultraviolet superimposed on a structureless absorption continuum. For the methylene chloride complex this feature is centered at 3860 Å, and is roughly 50 cm−1 wide. In the benzene van der Waals cluster, the corresponding feature is located at 3863 Å, and has a similar width. This spectrum is tentatively assigned to the 0–0 band of the lowest 1T1u←1Ag (LUMO+1←HOMO) transition of a truncated icosahedral carbon shell structure, broadened by coupling to the underlying quasicontinuum of ground state vibrational levels.
Photoelectron spectroscopy of the negative cluster ions NO−(N2O)n=1,2
Authors: Coe J. V. , Snodgrass J. T. , Freidhoff C. B. , McHugh K. M. , Bowen K. H.
Published: July 2002

We have recorded the photoelectron (photodetachment) spectra of the gas-phase negative cluster ions NO−(N2 O)1 and NO−(N2 O)2 using 2.540 eV photons. Both spectra exhibit structured photoelectron spectral patterns which strongly resemble that of free NO−, but which are shifted to successively lower electron kinetic energies with their individual peaks broadened. Each of these spectra is interpreted in terms of a largely intact NO−subion which is solvated and stabilized by nitrous oxide. For both NO−(N2 O)1 and NO−(N2 O)2, the ion–solvent dissociation energies for the loss of single N2 O solvent molecules were determined to be ∼0.2 eV. Electron affinities were also determined and found to increase with cluster size. The localization of the cluster ion’s excess negative charge onto its nitric oxide rather than its nitrous oxide subunit is discussed in terms of kinetic factors and a possible barrier between the two forms of the solvated ion.
Electronic structure of GaAs2
Authors: Balasubramanian K.
Published: July 2002

Complete active space MCSCF (CASSCF) followed by multireference singles + doubles configuration interaction (MRSDCI) calculations are carried out on the electronic states of GaAs2. Three electronic states, namely, 2B2, 2A1, and 2B1 are found among which the 2B2 state is the lowest state. The 2A1 state is about 29.4 kcal/mol above the 2B2 state while the CASSCF splitting of the 2B1 state with respect to the 2B2 state is 41 kcal/mol. The equilibrium geometries of all the three states are isosceles triangles. The 2B2 ground state exhibits greater As–As bonding and ionic character than the 2A1 state, which shows enhanced Ga–As bonding.
FT-ICR probes of silicon cluster chemistry: The special behavior of Si+39
Authors: Elkind J. L. , Alford J. M. , Weiss F. D. , Laaksonen R. T. , Smalley R. E.
Published: July 2002

FT-ICR techniques were used to probe the surface chemistry of isolated silicon cluster ions in the 7–65 atom size range. Dissociative chemisorption reactions with NH3 were observed to proceed with rates which varied widely with cluster size. One particular cluster, Si+39, was found to be remarkably inert. Clusters with 20, 25, 33, and 45 atoms were found to be unreactive as well, while those with 18, 23, 30, 36, 43, or 46 atoms were quite reactive. Similarly oscillating reaction patterns were observed with CH3OH, whereas highly reactive free radical scavengers such as O2 and NO showed little selectivity. These results suggest the silicon clusters in this size range have well-defined structures which vary in ability to catalyze dissociative chemisorption at the surface.
Small Al clusters. II. Structure and binding in Aln (n=2–6, 13)
Authors: Pettersson Lars G. M. , Bauschlicher Charles W. , Halicioglu Timur
Published: July 2002

The structure and stability of aluminum clusters containing up to six atoms have been studied using correlated wave functions and extended basis sets. The lowest energy structure is planar for Al4 and Al5, but three dimensional for Al6. The icosahedral, hcp, fcc, and two planar structures of Al13 were considered at the SCF level. The lowest energy structure is the icosahedron, but the planar structures are fairly low lying even in this case. A simplified description using two- and three-body interaction potentials is found to agree well with the ab initio structures and binding energies.
The near-infrared spectrum of ONNHF—direct evidence for geometric isomerism in a hydrogen bonded complex
Authors: Lovejoy Christopher M. , Nesbitt David J.
Published: July 2002

The near-IR spectrum and equilibrium structure of a novel hydrogen bonded complex between nitrous oxide and hydrogen fluoride is described. In contrast to a previously reported structure, in which the HF bonds to the oxygen end of NNO, the present structure has the HF bonded to the nitrogen end of NNO. The structure is unambiguously confirmed by isotopic substitution. The identification of the two stable, spectroscopically distinct structures represents the first demonstration of geometric isomerism in a hydrogen-bonded complex.
Line shape cross sections of HD immersed in HE and H2 gas. I. Pressure broadening cross sections
Authors: Schaefer Joachim , Monchick Louis
Published: July 2002

The R(0) and R(1) line shapes of HD are of interest because observation of these lines in the atmosphere of the outer planets should lead to unambiguous values of the D/H ratio. The electronic potential energy surfaces used to calculate HD–He and HD–H2 line shape cross sections have been well validated in previous publications of this series. The results of two additional tests of the electronic potential energy surface of hydrogen-like systems are reported here: the second virial coefficient of para-hydrogen at low temperatures and HD–D2 elastic and inelastic differential scattering cross sections. Formulas are given for Dicke narrowing as well as for the line broadening and shift cross sections calculated here. Close coupling calculations were carried out for the R(0) and R(1) transitions of HD immersed in He and for the R(0) transition of HD immersed in H2. These cross sections are anticipated to be quite reliable at temperatures above 50 K, i.e., where clustering can be ignored.
Fluids in micropores. I. Structure of a simple classical fluid in a slit-pore
Authors: Schoen M. , Diestler D. J. , Cushman J. H.
Published: July 2002

Equilbrium properties of a rare-gas fluid contained between two parallel fcc(100) planes of rigidly fixed rare-gas atoms were computed by means of the grand-canonical ensemble Monte Carlo method. The singlet distribution function ρ(1), and the pair-correlation function g(2) in planes parallel to the solid layers, indicate that the structure of the pore fluid depends strongly on the distance h between the solid layers. As the separation increases from less than two atomic diameters, successive layers of fluid appear. The transitions between one and two layers and three and four layers are especially abrupt and are accompanied by changes in the character of g(2) from dense fluid-like to solid-like. Long-range, in-plane order in the fluid layers diminishes with increasing h, but is still evident in the contact layer (i.e., that nearest the solid layer) at h=16.5 atomic diameters, the largest separation considered. The structure of the contact layer reflects the solid-layer structure and differs significantly from the adjacent inner fluid layers, whose g(2) resembles that of the corresponding bulk fluid. Decreasing the density of atoms in the solid layers blunts the peaks in ρ(1) and g(2), although even for the least dense layer considered the contact layers of fluid evince long-range, in-plane order. Replacing the discrete pairwise fluid–solid interactions with the mean field resulting from smearing the solid atoms over the plane of the solid layer destroys the ‘‘phase transitions’’ and the associated long-range, in-plane order.
On the necessity of f basis functions for bending frequencies
Authors: Simandiras Emmanuel D. , Rice Julia E. , Lee Timothy J. , Amos Roger D. , Handy Nicholas C.
Published: July 2002

The calculation of out-of-plane bending vibrations for π-bonded systems appears to be extraordinarily sensitive to the choice of a one-particle basis set. Ab initio predictions are reported for acetylene, an extreme example, at the self-consistent field (SCF), singles and doubles configuration interaction (CISD), nth order Mo/ller–Plesset perturbation theory (MPn,n=2–4), coupled-pair functional (CPF), and singles and doubles coupled cluster (CCSD) levels of theory. It is found that the addition of a set of f  basis functions to the carbon atom changes the value of the SCF πg frequency by +45 cm−1, and the value of all correlated πg frequencies by more than +100 cm−1. Evidence is presented that this behavior is present in other π-bonded systems. It is concluded that basis sets consisting of triple zeta plus two sets of polarization functions plus one set of f functions (TZ2P+f ) can predict highly accurate (∼1% average error) harmonic frequencies with the MP2, CPF, and CCSD methods, for a large number of molecules.
The polyene 2 1A
g state in polyacetylene photoinduced absorption and thermal isomerization
Authors: Kohler Bryan E.
Published: July 2002

The excitation energy and isomerization properties of the 2 1 Ag state are known for discrete linear polyenes. These properties can be extrapolated to polyenes of arbitrary chain length to give a consistent and quantitative accounting of the photoinduced absorption and thermal isomerization observed for polyacetylene films.
Morphology of medium-size silicon clusters
Authors: Phillips J. C.
Published: July 2002

Periodic oscillations in the reactivity of Si+n clusters (14≤n≤47) suggest that the morphology of some of these clusters may be cylindrical.
Photofragmentation of antimony and bismuth cluster cations at 248 nm
Authors: Geusic M. E. , Freeman R. R. , Duncan M. A.
Published: July 2002

Antimony and bismuth cluster cations are produced by laser vaporization in a pulsed nozzle source and probed with mass spectroscopy and mass selected photofragmentation at 248 nm. Mass spectral abundances are interpreted to reflect cation stability patterns. Fragmentation products and branching ratios are consistent with the production of stable neutral and cation molecular fragments, suggesting a statistical dissociation mechanism.
Reactivity-structure correlations in ion clusters: A study of the unimolecular fragmentation patterns of argon ion clusters, Ar+n, for n in the range 30–200
Authors: Lethbridge P. G. , Stace A. J.
Published: July 2002

A high resolution double focusing mass spectrometer in association with a pulsed supersonic nozzle has been used to study the unimolecular fragmentation of Ar+n clusters for n in the range 30–200. All ion clusters within this range are observed to undergo the reaction Ar+n→Ar+n−1+Ar; with some ions also exhibiting further fragmentation involving the loss of two and three argon atoms. A systematic study of the above reaction as a function of n reveals that the relative intensities of the fragment ions fluctuate considerably. In many cases, these fluctuations coincide with depleted or enhanced ion intensities in the normal ion cluster mass spectrum. Through the use of a model based on unimolecular reaction rate theory (RRKM), it has been possible to reproduce many of the features observed in both the fragmentation spectra and the normal mass spectrum. Of the two principle variables involved in the model, reaction symmetry number and dissociation or binding energy, we have been able to show that only the binding energy is important in determining the fragmentation pattern. Through the calculations, the experimental data are interpreted in terms of ‘‘stable’’ and ‘‘unstable’’ ion clusters. Stable ion clusters have enhanced intensities in the normal mass spectrum, whereas unstable ions produce both depletions in the normal mass spectrum, and enhanced fragment ion intensities. It is proposed that unstable argon ion clusters consist of an atom weakly bound to an underlying stable cluster. A series of hard-sphere structures are presented to account for some of the observed stable ion clusters. Each of the structures has icosahedral symmetry and accommodates a central Ar+2 ion. It is concluded that, because the clusters are forced to accommodate a charge-carrying unit, the positive charge appears to have a structural influence which extends well beyond the electrostatic limit.
Numerical solution of Poisson’s equation in polyatomic molecules
Authors: Becke A. D. , Dickson R. M.
Published: July 2002

We have developed a completely numerical scheme for solution of Poisson’s equation in multicenter systems. We are thus able to numerically calculate the Coulomb potential of arbitrary charge distributions in polyatomic molecules. The method is based on a decomposition of the multicenter Poisson problem into independent single-center problems, each of which is solvable in standard spherical coordinates. In combination with our multicenter numerical integration scheme reported previously, completely numerical evaluation of arbitrary two-electron Coulomb integrals is possible. Test calculations on the classic two-electron Coulomb and exchange integrals of H2 and the Coulomb interaction energies of several model polyatomic systems indicate that the scheme is both practical and accurate.
Laser-induced molecular desorption from size-selected nickel cluster carbonyl ions: Kinetics of the adsorption–desorption equilibrium
Authors: Brechignac C. , Brechignac Ph. , Fayet P. , Saunders W. A. , Wöste L.
Published: July 2002

Gas phase reactivity of size-selected nickel cluster ions with carbon monoxide has been studied in a triple quadrupole mass spectrometer. Intensity distributions of the various nickel carbonyl ions are measured. Irradiation with a cw Ar+ laser beam demonstrates that efficient detachment of CO ligands occurs following single photon absorption by the metal cluster core. Laser-induced photodesorption is used to counterbalance the adsorption processes, in order to control the molecular coverage of the metal cluster. Time-resolved measurements of the kinetics of these two competing processes have been performed. A rate equation model is proposed which brings a detailed understanding of the kinetics and allows derivation of numerical values for the chemisorption rate constant and for the photoabsorption cross section.
Electronic structure of metal-free phthalocyanine: A valence effective Hamiltonian theoretical study
Authors: Ortí E. , Brédas J. L.
Published: July 2002

We present a valence effective Hamiltonian (VEH) nonempirical investigation of the electronic properties of metal-free phthalocyanine. The valence one-electron energy levels are related to those of the phthalocyanine components: benzene, pyrrole, and isoindole. From the electronic structure standpoint, phthalocyanine has to be viewed as formed by joining four benzene moieties to the central carbon–nitrogen ring rather than by combining four isoindole units through nitrogen bridges. Comparison of the VEH density-of-valence-states curves with the experimental ultraviolet photoelectron spectroscopy (UPS) data is quantitatively excellent and allows for a complete interpretation of the experimental spectra.
The visible photoabsorption spectrum of Ar+3
Authors: Levinger Nancy E. , Ray Douglas , Murray Kermit K. , Mullin Amy S. , Schulz C. P. , Lineberger W. C.
Published: July 2002

The photodissociation cross section of Ar+3 was measured at a number of wavelengths between 1064 and 320 nm. A single broad and featureless band was observed peaking near 520 nm with a width of ≈2600 cm−1 and a peak cross section of ≈10−16 cm2. Consideration of the electronic structure of Ar+3 indicates that the measured spectrum is equivalent to the photoabsorption spectrum. Two ionic products, Ar+ and Ar+2, were observed in the photodissociation of Ar+3, indicative of at least two exit pathways and suggestive of two electronic transitions.
Production and fragmentation of antimony and bismuth cluster ions
Authors: Ross Mark M. , McElvany Stephen W.
Published: July 2002

Cluster ions of antimony and bismuth were generated by direct laser vaporization in a Fourier transform mass spectrometer (LV/FTMS) and in a time-of-flight mass spectrometer (LV/TOFMS), and by particle bombardment (secondary ion mass spectrometry, SIMS) in triple quadrupole and sector instruments. The LV/FTMS and LV/TOFMS yielded Sb+x and Bi+x with x=1 to 5, whereas SIMS generated similar Sb+x species but Bi+x with x=1–14. The anomalous regions in the ion abundance distributions (enhanced abundances at x=3, 5, 7, 10, and 13) and the extent of the distributions reflect differences in these cluster ion production and detection methods and in the stabilities of the cluster ions. Low- and high-energy collision-induced dissociation of both homogeneous and mixed (Bix Sb+y ) cluster ions provide insight into the fragmentation processes. The dissociations of Bi+x and Sb+x are dominated by the losses of stable neutral dimers and tetramers.
Time- and state-resolved measurements of nitric oxide dimer infrared photodissociation
Authors: Casassa Michael P. , Stephenson John C. , King David S.
Published: July 2002

Picosecond and nanosecond lasers and pulsed molecular beam techniques have been used to measure the infrared photodissociation spectra, the product state distributions, and the predissociation lifetimes of vibrationally excited nitric oxide dimer (NO)2 . Results for the ν1 (v=1) symmetric NO stretching mode and the ν4 (v=1) antisymmetric NO stretching mode are presented. Predissociation lifetimes are determined by time-resolved laser induced fluorescence probing of the NO monomer product appearance rate. A dramatic mode dependence of the predissociation lifetimes is observed with the higher energy ν1 mode decaying in approximately 1 ns, and the lower energy ν4 mode decaying in approximately 40 ps. The mode dependence is independent of which product state is probed. The product state distributions show that 75% to 80% of the available energy is channeled into relative translational energy of the fragments for both modes. Rotational state distributions are Boltzmann-like with temperatures ranging from 71 to 112 K depending on both the initially excited mode and on the NO product spin–orbit state. Predissociation from ν1 produces NO fragments in the 2Π1/2 and 2 Π3/2 states with equal probability. Predissociation from ν4 exhibits a propensity for producing the lower energy 2 Π1/2 spin–orbit state. The observations are discussed in terms of various vibrational predissociation mechanisms, including vibrational potential coupling and electronically nonadiabatic predissociation.
Generalized Gaussian wave packet dynamics, Schrödinger equation, and stationary phase approximation
Authors: Huber Daniel , Heller Eric J. , Littlejohn Robert G.
Published: July 2002

We have recently published a new semiclassical method, generalized Gaussian wave packet dynamics, which extends Gaussian wave packet dynamics into complex phase space. Although we were able to give an accurate formulation of the method, we had at the time of writing that paper only an intuitive, heuristic understanding of the deeper causes which make the method work. A more mathematical understanding was needed. To close this gap we show in this paper the equivalence of the new method with a first order expansion of ℏ of the Schrödinger equation. We further prove that the new method is equivalent to the stationary phase approximation, using the usual WKB formula for the propagator. The latter equivalence enables us to show that all the symmetry properties of time-dependent quantum mechanics also hold in the new semiclassical theory. Finally, we provide some elaboration of the method, and clarify several issues that were not discussed before. With this new insight we are able to formulate a simple rule for the calculation of semiclassical wave functions that contain contributions from more than one branch. This corrects for the divergence of semiclassical wave functions near caustics, a problem that we encountered in the preceding paper.
Reactive etching of GaxAs−y by HCl
Authors: Reents W. D.
Published: July 2002

The gas phase reactions of HCl with anionic gallium arsenide clusters, GaxAs−y, containing two to six atoms are presented. Reaction rates and product distributions for the primary, secondary, and tertiary reactions are tabulated. HCl etches GaxAs−y by loss of GaCl to form Gax−1AsyH−. These products are also etched by HCl through loss of GaCl to form Gax−2AsyH−2. Those clusters which do not contain gallium either react to lose AsCl (As2H− and As3H−) or react to abstract H+ and form Cl−(AsH−2). Three gallium-containing clusters (GaAs−4, Ga2AsH−, and GaAs2H−2) react by proton abstraction to form Cl− rather than lose GaCl. Two clusters (Ga3As−2 and Ga2As−3) have an additional reaction pathway open to form two neutrals (GaCl and As2) plus a smaller anionic cluster. Formation of GaCl and As2 mimics the etching of bulk gallium arsenide by HCl at elevated temperatures. Five clusters (GaAs−4, Ga2As−3, GaAs4H−, Ga4As−2, and Ga2As−4) exhibit dual populations that can be differentiated kinetically. There is an inverse correlation between bare cluster reactivity and its initial abundance from laser desorption. The only exception is Ga2As−3 which has ∼50% population of a very reactive species that is the most abundant cluster initially desorbed.
A photoion–photoelectron coincidence study of Arn (n=2–4)
Authors: Norwood K. , Guo J.-H. , Ng C. Y.
Published: July 2002

Photoion–photoelectron coincidence (PIPECO) spectra for Ar+n (n=2–4) in the wavelength region of 750–875 Å have been measured at different nozzle stagnation pressures. The ionization energies (IE) for the ground Ar+2[I(1/2)u] and Ar+3[I(1/2)u] states are determined to be 14.518±0.017 eV (854±1 Å) and 14.350±0.033 eV (864±2 Å), in agreement with the findings of previous photoionization experiments. The IE for Ar+2[II(1/2)u] is found to be ≲15.83 eV (783 Å). The observation of the Ar+2[II(1/2)u] PIPECO band supports the interpretation that Ar+2[II(1/2)u] is metastable with a lifetime longer than 47 μs, a value in accord with the calculated radiative lifetime of 90.9 μs for the II(1/2)u →I(1/2)g transition. The PIPECO spectrum for Ar+2 is dominated by autoionization structure similar to that resolved in the photoionization efficiency spectrum for Ar+2, indicating that a significant fraction of electrons produced by these autoionizing states are slow electrons with near-zero kinetic energies. Evidence is found that metastable Ar+3 and Ar+4 ions formed by photoionization of Ar3 and Ar4 at energies above the Ar+2[I(1/2)u]+Ar(1S0) and Ar+3[I(1/2)u]+Ar(1S0) dissociation limits may live longer than 58 and 66 μs, respectively. We suggest that the formation of Ar+3 in metastable electronic states may be responsible for the long dissociation lifetimes of Ar+3 observed in this experiment.
Monte Carlo simulations of helium bubble states
Authors: Murthy C. S. , Swaminathan P. K.
Published: July 2002

Excited He* (2 3S) atoms in normal liquid 4He(1 1S) environment exist inside ‘‘bubbles’’ that have sizes and shapes characteristic of the electronic state of He* as well as the thermodynamic state of the liquid. The bubbles are stabilized by the repulsive interaction of the Rydberg-like excited electron with bath He atoms. We employ classical computer simulation methods to characterize these ‘‘bubble states’’ in a high pressure (gigapascals) regime. We analyze for the presence of clusters (He*n) within the bubbles, and find the results sensitive to the electronic state involved as well as the pressure. The He*n along with the He atoms on the bubble’s inner surface behave like a single-shell solventberg at lower pressures, whereas a two-shell structure emerges in the high pressure regime. The simulated bubble radius varies between 6–8 a0 in the pressure range 14–0.5 GPa.
Vibrational branching ratios and shape resonant photoionization dynamics in N2O
Authors: Braunstein M. , McKoy V.
Published: July 2002

Vibrational branching ratios and photoelectron asymmetry parameters for alternative vibrational modes in the photoionization of N2O(7σ−1) have been studied using accurate photoelectron continuum orbitals. Earlier dispersed ionic fluorescence measurements [E. D. Poliakoff, M. H. Ho, M. G. White, and G. E. Leroi, Chem. Phys. Lett. 130, 91 (1986)] revealed strong non-Franck–Condon vibrational ion distributions for both the symmetric and antisymmetric stretching modes at low photoelectron energies. Our results establish that these features arise from a σ shape resonance which, based on its dependence on internuclear geometry, must be associated with the molecular framework as a whole and not with either of its fragments, N–N or N–O. This behavior accounts for the more pronounced deviations of the vibrational branching ratios from Franck–Condon values observed in the symmetric than in the antisymmetric mode. The σ continuum also supports a second shape resonance at higher energy which does not influence the vibrational branching ratios but is quite evident in the photoelectron asymmetry parameters around a photon energy of 40 eV. These vibrationally resolved studies of the photoelectron spectra of this polyatomic system provide an interesting example of the rich shape resonant behavior that can be expected to arise in polyatomic molecules with their alternative vibrational modes.
Theory of orbital contributions to magnetic hyperfine fields in hemoglobin derivatives—application to azidomyoglobin
Authors: Roy J. N. , Mishra Santosh K. , Mishra K. C. , Das T. P.
Published: July 2002

A variational procedure has been applied in conjunction with the linear combination of atomic orbitals (molecular orbitals) [LCAO(MO)] method for electronic structure investigations to study the unquenching of the orbital angular momentum due to the influence of spin–orbit interaction in large biomolecular systems and its influence on the magnetic hyperfine fields, especially at 57mFe nuclei. Application is made to azidomyoglobin and it is found that incorporation of spin–orbit effects leads to drastic effects on the hyperfine fields as suggested from earlier investigations based on the crystal field model. In contrast to the values of 37, 166, and 13 kG obtained without incorporation of orbital unquenching effects and with the external magnetic field applied along the a, b, and c axes of the crystal, the values we obtained when orbital unquenching effects are included are 252, 133, and 116 kG which agree considerably better with the experimental values of 185±5, 133±5, and 99±5 kG from Mössbauer measurements. Suggestions are made for bridging the remaining gap between theory and experiment.
Ionization yields, total absorption, and dissociative photoionization cross sections of CH4 from 110 to 950 Å
Authors: Samson James A. R. , Haddad G. N. , Masuoka T. , Pareek P. N. , Kilcoyne D. A. L.
Published: July 2002

Absolute absorption and photoionization cross sections of methane have been measured with an accuracy of about 2% or 3% over most of the wavelength range from 950 to 110 Å. Also, dissociative photoionization cross sections were measured for the production of CH+4 , CH+3 , CH+2 , CH+, and C+ from their respective thresholds to 159 Å, and for H+ and H+2 measurements were made down to 240 Å. Fragmentation was observed at all excited ionic states of CH4.