bluuebunny/crossref_metadata_2025 huggingface dataset:
An experimental study of the formation and reactivity of ionic hydrogen clusters: The first observation and characterization of the even clusters H+4, H+6, H+8, and H+10
Authors: Kirchner Nicholas J. , Bowers Michael T.
Published: July 2002

A specially designed coaxial drift tube type ion source has been utilized to generate ionic hydrogen clusters. The cluster spectrum observed is dominated by odd clusters (H+5, H+7, H+9, and H+11 ) in line with past studies on hydrogen ion clusters. However, for the first time even hydrogen cluster ions are formed in a high pressure ion source, with relative intensities (H+6≫H+8>H+10>H+4) . The observation of a relatively intense H+6 peak (H+6/H+5 =0.05) was unexpected. The peak at nominal mass 6 was confirmed to be H+6 (and not H4D+) by high resolution studies. A number of possible formation mechanisms for H+6 are discussed. The most likely mechanism is reaction of odd hydrogen cluster ions (H+3, H+5, H+7,... ) with metastable electronically excited odd hydrogen cluster neutrals (H*3, H*5, H*7,...) . The H*3 species has been experimentally characterized by other research groups (Herzberg; Kuppermann) and is probably formed by electron recombination with H+3 (v≠0), H+5 or H+7 in our source. The H+6 ion is observed to lose both H atom and H2 by metastable decomposition in a ratio of H/H2≂4. The kinetic energy distribution for H loss is broad and bimodal with an average release of 38 meV and a maximum release of 140 meV. Collision induced decomposition studies are utilized to characterize both the even and odd clusters. A 50/50 mixture of H2 and D2 yielded a large number of isotopically mixed clusters that were identified by high resolution mass spectrometry.
Dynamics of the endothermic reaction He+H+2 →HeH++H on an accurate a
b i
n
i
t
i
o potential-energy surface
Authors: Joseph Tomi , Sathyamurthy N.
Published: July 2002

We report herein a successful analytic fit of the ab initio potential-energy surface of McLaughlin and Thompson for the ground state HeH+2 system and also the results of a three-dimensional quasiclassical trajectory study of the exchange reaction over a wide range of vibrational states and relative translational energies of the reactants. While there is good agreement between theory and experiment in many respects, there are some quantitative discrepancies remaining with respect to some of the experimental results.
Complex-forming reactions in neutral noble gas clusters
Authors: Levandier D. J. , McCombie J. , Pursel R. , Scoles G.
Published: July 2002

The complex-forming reaction between CH3F and HCl imbedded in medium–large argon clusters has been monitored using molecular beam infrared spectroscopy. A ‘‘pickup’’ technique has been used which consists of exposing the (CH3F)mArn clusters, formed by expansion of a dilute mixture of CH3F in Ar from a supersonic nozzle, to a cross flux of HCl molecules. The clusters are characterized, with and without the reactant flow, by bolometric photoevaporation IR spectroscopy using line tunable CO2 lasers. Our data show that the products can be clearly distinguished from the reactant species. A plot of reactivity vs nozzle pressure, which is directly related to cluster size, indicates that, within our range of measurements, the HCl diffusion into the clusters is completed in less than 100 μs.
Accurate a
b i
n
i
t
i
o calculations which demonstrate a 3Πu ground state for Al2
Authors: Bauschlicher Charles W. , Partridge Harry , Langhoff Stephen R. , Taylor Peter R. , Walch Stephen P.
Published: July 2002

The spectroscopic parameters and separations between the three low-lying X 3Πu, A 3Σ−g, and a 1Σ+g states of Al2 are studied as a function of both the one-particle and n-particle basis sets. Approximate correlation treatments are calibrated against full CI calculations correlating the six valence electrons in a double-zeta plus two d-function basis set. Since the CASSCF/MRCI 3Πu–3Σ−g separation is in excellent agreement with the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/[6s5p3d2f] Gaussian basis. Including a small correction for relativistic effects, our best estimate is that the 3Σ−g state lies 174 cm−1 above the 3Πu ground state. The 1Σ+g state lies at least 2000 cm−1 higher in energy. At the  CPF level, inclusion of 2s and 2p correlation has little effect on De, reduces Te by only 26 cm−1, and shortens the bond lengths by about 0.02 a0. Further strong support for a 3Πu ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as 3Πu→3Πg. The (2)3Πg state is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorter r values, such that the strongest experimental absorption peak probably corresponds to the 0→2 transition.
CASSCF/CI calculations of low-lying states and potential energy surfaces of Au3
Authors: Balasubramanian K. , Liao M. Z.
Published: July 2002

Complete active space MCSCF (CASSCF) and second-order configuration interaction (SOCI) calculations of low-lying electronic states [2B2,2A1] of Au3 as well as the 1Σ+g state of Au2 are carried out. The bending potential energy surfaces of 2A1 and 2B2 states are also presented. A barrier is found in the potential energy surface of the 2A1 state in moving from the linear to bent structure. Two nearly-degenerate structures are found for the ground state. The 2Σ+u state arising from the linear structure with an Au–Au bond length of 2.66 Å is only 3.2 kcal/mol below the 2A1 bent state. The equilibrium geometry of the 2A1 state is an isosceles triangle with an apex angle of 54°. The Au3 cluster is found to be more stable than the gold dimer. The effect of d correlation is studied on Au2 by carrying out MRSDCI (multireference singles and doubles CI) calculations on the 1Σ+g state of Au2 which include excitations from the d orbitals.
High resolution zero kinetic energy photoelectron spectroscopy of benzene and determination of the ionization potential
Authors: Chewter L. A. , Sander M. , Müller-Dethlefs K. , Schlag E. W.
Published: July 2002

We report the direct high-resolution determination of the ionization potential of benzene and the observation of rotational structure in the vibronic ground state of the benzene ion. We employ a new technique of two-color high-resolution zero kinetic energy photoelectron spectroscopy (ZEKE-PES) via selection of an intermediate resonant (neutral) state. The ZEKE-PES method allows for the investigation of rotationally resolved photoionization dynamics and the direct high-precision determination of ionization potentials, the precision being derived directly from the laser wavelength calibration. For benzene we determine an IP of 74 555.0±0.4 cm−1. We compare this method to ionization potential measurements from ion yield measurements under various conditions.
Double vacancies in the core of benzene
Authors: Cederbaum L. S. , Tarantelli F. , Sgamellotti A. , Schirmer J.
Published: July 2002

Because of the six equivalent carbon atoms in benzene, there is only one type of single core vacancy, but four different types of double core vacancies. The binding energies of single and double core vacancies are calculated and analyzed. Particular attention is paid thereby to reveal the relation between the single and double hole states. The π and σ electrons are found to contribute differently to the different types of double vacancies. It is generally stressed that double core ionization probes the bonding properties much more sensitively than single core ionization.
Binding and correlation effects in nitrogen and oxygen, and the correlation effects in neon, as studied by gas-phase x-ray diffraction
Authors: Nishikawa Keiko , Iijima Takao
Published: July 2002

Total (coherent and incoherent) x-ray scattering intensities of nitrogen, oxygen, and neon have been measured by the use of the energy-dispersive method with an accuracy of about 0.5%. A sealed-off tube with a tungsten target was used as the source of white x rays. Comparison of the measured intensities with the Hartree–Fock independent-atom calculations revealed the binding and correlation effects. Further comparison with advanced theoretical calculations in literature has been made and the experimental values have been found to be consistent with the theoretical predictions at the CI level.
Infrared spectroscopic observations of hydrogen bonding and Fermi resonance of adsorbed methyl chloride on alumina surfaces
Authors: Crowell J. E. , Beebe T. P. , Yates J. T.
Published: July 2002

The physical adsorption of methyl chloride onto Al2O3 surfaces containing surface OH groups has been studied using transmission infrared spectroscopy. Methyl chloride reversibly bonds via hydrogen bonding to the surface hydroxyl groups with a spectroscopically measured heat of adsorption of −3.37±0.38 kcal mol−1. Physisorption of methyl chloride results in a significant reduction in the intensity of the rotational wings of the methyl chloride absorption bands relative to the gas phase, a small downward frequency shift in their band centers, and substantial effects on the ν(OH) region of the alumina spectrum due to hydrogen bonding of surface OH groups with methyl chloride. Fermi resonances are observed for adsorbed methyl chloride and result in the observation of the overtones of both the symmetric and asymmetric methyl deformation modes. It is postulated that the hydrogen-bonded CH3Cl species possess rotational (or other) degrees of freedom which lead to a high entropy in the adsorbed layer, compared to the liquid.
Modeling Fe3+ in quasitetrahedral surroundings using the Newman superposition model
Authors: Mombourquette M. J. , Weil J. A.
Published: July 2002

The Newman superposition model, using certain empirical ‘‘intrinsic’’ parameters, is commonly used in attempts to obtain information about the local environment of impurity ions in crystals from experimental data such as electron paramagnetic resonance (EPR) spin-Hamiltonian data. Using this model, we have developed equations relating the positions of four near-neighbor ions relative to a central paramagnetic ion to EPR crystal-field parameters eBl,m. We have explored the restrictions and hazards involved in using these equations in a fitting procedure. A set of experimental eBl,m parameters, measured for Fe3+ in quartz, has been used here in an attempt to determine the coordinates of four oxygen ions thought to be surrounding the iron located in a silicon site. We find that the result of fitting to the second-order parameters eB2,m alone is consistent with the substitutional site model for the iron in quartz, but is unreliable in that there is actually an infinite number of fits to these parameters. Using the second-order and fourth-order parameters together gives a unique fit but one which seems physically unreasonable. Fitting to the second-order and fourth-order parameters using the intrinsic parameter B4(R0) as a fitting parameter along with the coordinate parameters gives a unique fit which strongly supports the substitutional model.
Alignment and orientation of N2 scattered from Ag(111)
Authors: Sitz Greg O. , Kummel Andrew C. , Zare Richard N.
Published: July 2002

The measurement of the alignment and orientation of nitrogen scattered from Ag(111) is reported. Pulsed supersonic beam of nitrogen is scattered off a clean Ag(111) crystal and the scattered nitrogen is detected by two−photon resonant four−photon ionization.(AIP)
New simulation method for the equation of state of lattice chains
Authors: Dickman Ronald
Published: July 2002

A new method for determining the pressure in Monte Carlo simulations of lattice chains is described, and preliminary results are presented. The method, in which the pressure is related to the density of segments at a repulsive wall, is applicable over a wide range of densities and chain lengths. Flory–Huggins theory is in good agreement with simulation results at high densities.
The Hückel model for small metal clusters. III. Anion structures and HMO electron affinities
Authors: Lindsay D. M. , Chu Lin , Wang Youqi , George Thomas F.
Published: July 2002

The most stable structures for the alkali-like clusters M−3–M−8 are calculated within the framework of the simple Hückel model. The Hückel geometries are, on average, slightly ‘‘less compact’’ than those of the neutral and cation clusters, a phenomenon which may be related to the additional electronic kinetic energy of the anions. Cluster compactness is quantified by an estimation of ‘‘soft sphere’’ volumes, which also allows for a comparison of classical and experimental polarizabilities. The Hückel model gives electron affinities which compare favorably with the experimental results for Cu2–Cu8. To our knowledge, the Hückel results in this paper represent the first systematic search for the stable structures of small alkali-like anion clusters.
The a2g mode in ground state benzene
Authors: Berman Joel M. , Goodman Lionel
Published: July 2002

Harmonic ground state frequencies for the a2g mode, ν3, in C6H6, C6D6, and 13C6H6 have been determined by application of the Teller–Redlich product rule to b2u mode frequencies obtained from two-photon fluorescence excitation experiments for the D6h symmetry molecules and a′2 frequencies from 1,3,5-C6H3D3 determined at rotational contour resolution. The a2g harmonic frequency values are 1366.6 cm−1 for C6H6, 1063.0 cm−1 for C6D6, and 1355.4 cm−1 for 13C6H6. The anharmonic correction to ν3 in C6H6, estimated from the difference in product rule and infrared frequencies is −17 cm−1. The b2u mode frequencies, ν14 and ν15, respectively, have also been significantly modified from those obtained from lower resolution spectra to 1309.4 and 1149.7 cm−1 in C6H6, 1286.3 and 827.9 cm−1 (harmonic value) in C6D6, and 1269.8 and 1138.1 cm−1 in 13C6H6. The anharmonic correction for ν15 in C6D6 is estimated as −4 cm−1. The B2u←A1g electronic origin of 13C6H6, has been determined for the first time as 38 105.5±0.3 cm−1. The B2u←A1g electronic origin of 1,3,5-C6H3D3 has been redetermined to be 38 187.5±0.3 cm−1, 3.5 cm−1 higher than previously reported.
Spectroscopic study of the E 1Σ+g ‘‘shelf’’ state in 7Li2
Authors: Bernheim R. A. , Gold L. P. , Tomczyk C. A. , Vidal C. R.
Published: July 2002

The E 1Σ+g ‘‘shelf ’’ state of the 7Li2 molecule was investigated using a pulsed optical optical double resonance technique. The measurements cover the vibrational levels in the range 0≤v≤29 including the shelf region around 13≤v≤15. Molecular constants have been determined. Using an inverted perturbation approach, an effective potential energy curve has been generated within the adiabatic approximation whose quantum mechanical energy eigenvalues reproduce all the measured term values to within 0.113 cm−1 for the range 0≤v≤23 and 0≤J≤47.
Focusing and state selection of NH3 and OCS by the electrostatic hexapole via first- and second-order Stark effects
Authors: Gandhi Suketu R. , Bernstein Richard B.
Published: July 2002

Neither the linear molecule OCS nor the rapidly inverting pyramidal NH3 have conventional first-order Stark effects (SE) and so are not ideally focusable by an electrostatic hexapole field. However, strong focusing and state selection of these molecules by such a hexapole field has been achieved using the UCLA apparatus, consisting of a pulsed supersonic beam source, 3 m hexapole rods, and a mass spectrometer detector. For NH3, a single intense peak in the focused intensity curve I(V0) is obtained (where V0 is the ‘‘rod voltage’’). From the observed linear velocity dependence of V0, characteristic of focusing via second-order SE, the peak is assigned as the ‖JKMJ〉=‖11±1〉 state (the upper state of the JK=11 inversion doublet with para nuclear symmetry), previously quadrupole focused by Kay and Raymond (1986). In contrast, ND3 focuses like a symmetric top, as expected, with a quadratic velocity dependence characteristic of first-order SE. For OCS, the first three peaks in the I(V0) curve, showing quadratic velocity dependence, are assigned to the ‖JlMJ〉 states ‖111〉, ‖212〉, and ‖313〉, focused via first-order SE for the l-doubled excited bending vibrational state, analogous to that found by Stolte et al. (1986) for N2O. An intense peak at higher V0 is due primarily to the ‖JMJ〉=‖10〉 state of the ground vibrational state, i.e., the linear OCS focused by second-order SE, following the theory of English (1971).
Feshbach and pseudopotential theories. A useful analogy
Authors: Martín F. , Mó O. , Riera A. , Yáñez M.
Published: July 2002

An analogy is drawn between the well-known Feshbach theory on one hand, and on the other the reputedly cruder pseudopotential technique for two-electron atomic and molecular systems. We show how both formalisms can be treated on an equal footing by using a generalized Phillips–Kleinman formalism for the second. Among other benefits stemming from this analogy, the simplicity and versatility of pseudopotential methods can be used to considerably simplify the calculation of positions (couplings) and widths involving two-electron autoionizing atomic (molecular) states.
A detailed study of the reactions between size selected aluminum cluster ions, Al+n (n=3–26), and oxygen
Authors: Jarrold Martin F. , Bower J. Eric
Published: July 2002

A detailed study of the reactions between size selected aluminum cluster ions and oxygen is presented. The experiments were performed using a low energy ion beam apparatus. Measurements of product distributions and total reaction cross sections at collision energies of 1.2 and 4.2 eV for aluminum cluster ions with between 3 and 26 atoms are reported. The total reaction cross sections increase with cluster size in a way which roughly correlates with the increase in the cluster’s physical size. The main products are Al+n−4, Al+n−5, Al+n−6, and Al+. Only a very small fraction of the product ions contain oxygen. We suggest that the reaction occurs by chemisorption of O2 onto the cluster followed by rapid loss of two Al2O molecules to give Al+n−4. If the Al+n−4 fragment contains sufficient energy it will undergo further dissociation by loss of one or more aluminum atoms to give Al+n−5, Al+n−6, and Al+. RRKM theory is used to estimate the amount of energy above the dissociation threshold required to cause dissociation of the bare clusters on the experimental time scale. For the larger clusters this excess energy is remarkably large. Using this data we are able to deduce some information about energy disposal in the reaction. It is likely that the Al2O molecules carry away a substantial fraction of the exothermicity arising from chemisorption of oxygen onto the clusters.
Picosecond photofragment spectroscopy. III. Vibrational predissociation of van der Waals’ clusters
Authors: Knee Joseph L. , Khundkar Lutfur R. , Zewail Ahmed H.
Published: July 2002

This paper, last in this series, reports on the picosecond dynamics of vibrational predissociation in beam-cooled van der Waals’ clusters. Reaction rates have been measured for clusters (1:1) of phenol and cresol (p-methylphenol) with benzene by the picosecond pump–probe photoionization mass-spectrometry technique. Dissociation to form phenol (cresol) and benzene takes place from vibrational levels of the S1 state of phenol (cresol) prepared by the pump laser. The predissociation rates were measured for a number of different excess energies upto ∼2500 cm−1, and the reaction threshold was found to be 1400 cm−1 above the S1 origin for phenol–benzene and ∼1795 cm−1 for cresol–benzene, respectively. For phenol–benzene, the predissociation rates, following excitation of ring-type modes, vs excess energy vary more or less smoothly. Cresol–benzene exhibits biexponential decay, with the fast component becoming more dominant at higher energies. A non-RRKM model involving division of the vibrational phase space is discussed to explain this observation.
Excited-state dynamics of rare-gas clusters
Authors: Scharf Dafna , Jortner Joshua , Landman Uzi
Published: July 2002

In this paper we explore the dynamic implications of energy exchange in electronically vibrationally excited states of mixed rare-gas clusters. The classical molecular dynamics method was applied for the study of vibrational energy flow from electronically excited atomic Xe(3P1) states in Xe*Ar12 and Xe*Ar54, and the consequences of Xe*2 (3Σu) excimer formation in Xe*2 Ar11 and Xe*2 Ar53 clusters. We have established the occurrence of an ultrafast vibrational energy flow (∼300 fs) from local Rydberg atomic and excimer excitations into the cluster, which is accompanied by large configurational dilation around the excited state, due to short-range repulsive interactions. Size effects on cluster dynamics were elucidated, being manifested by vibrational predissociation in small clusters and by vibrational relaxation and vibrational energy redistribution in large clusters. A gradual transition from reactive molecular type relaxation in small clusters to nonreactive condensed-matter type relaxation in large clusters was documented. Qualitative and quantitative differences between relaxation of excited species initially located in the interior or on the surface of the cluster were established, being exhibited in the details of the vibrational energy flow. In the case of bulk Xe*2 Ar53, excessive local heating is manifested in cluster melting, which results in mass transport of the excimer to the cluster surface. The many facets of the dynamics of electronically excited mixed rare-gas clusters are amenable to experimental interrogations.
State-resolved photodissociation of OCS monomers and clusters
Authors: Sivakumar N. , Hall G. E. , Houston P. L. , Hepburn J. W. , Burak I.
Published: July 2002

Photodissociation of OCS in the region from 222–248 nm has been investigated by monitoring the CO and S(1D2) primary photoproducts; as well as the secondary production of S(3P2), S(3P1), and S(3P0) using fluorescence induced by a tunable vacuum ultraviolet laser source based on four-wave mixing in magnesium vapor. The quantum yield of S(3P) was found to be 0.00±0.02 at 222 nm. Thus, in contrast to our preliminary report, the present more detailed investigation shows that the sole sulfur product appears to be S(1D). The CO photofragment is produced almost exclusively in v=0 [CO(v=1)/ CO(v=0)≤0.02], but the rotational distribution is inverted and peaked at very high rotational levels. The peak shifts from J=56 for dissociation at 222 nm to J=31 at 248 nm. Doppler profiles of the CO rotational transitions reveal (1) that all observed levels are produced in coincidence with S(1D), (2) that for 222 nm photolysis the fragment recoil anisotropy shifts from a distribution characterized by β=1.9 at J=67 toward one characterized by β=0 near J=54, (3) that the CO velocity vector is aligned nearly perpendicular to its angular momentum vector, and (4) that the CO angular momentum vector is also aligned parallel to that component of the transition dipole which lies perpendicular to the recoil velocity. These results are interpreted in terms of a model for the dissociation in which excitation takes place to two surfaces of A′ and A″ symmetry derived from a bent 1Δ configuration. Dissociation of OCS clusters was also investigated and was found to produce a photochemistry completely different from that of the monomers. Rotationally cold CO as well as S2 in both the X3Σ−g and a1Δg states was observed.
Growth patterns in binary clusters of Group IV and V metals
Authors: Wheeler R. G. , LaiHing K. , Wilson W. L. , Duncan M. A.
Published: July 2002

Binary clusters composed of Group IV and Group V metals (e.g., Sn/Bi) are prepared by laser vaporization of corresponding alloys in a pulsed nozzle molecular beam source. Relative abundances of these clusters are studied with laser photoionization mass spectroscopy at 193 and 157 nm. Mass spectral abundance patterns for all systems studied (Sn/Bi, Pb/Sb, Sn/As) exhibit highly nonstatistical combinations of component elements. Preferential stoichiometries follow common patterns throughout these cluster systems, which can be related to the bonding and stability of condensed phase cluster ions of these same metals. These patterns are predicted by a simple valence electron counting model.
Ring torsional dynamics and spectroscopy of benzophenone: A new twist
Authors: Frederick John H. , Heller Eric J. , Ozment Judy L. , Pratt David W.
Published: July 2002

The low energy portion of the high resolution S1←S0 fluorescence excitation spectrum of benzophenone recently reported by Holtzclaw and Pratt [J. Chem. Phys. 84, 4713 (1986)] is modeled here using a simple two-degree-of-freedom vibrational Hamiltonian. The Hamiltonian features a 1:1 nonlinear resonance between the two low frequency ring torsional modes of the molecule in its S1 state. Line positions and intensities of the two major spectral progressions are well reproduced using parameters similar to those derived from earlier matrix diagonalizations. The comparison of the theory and experiment results in a determination of the displacement of the S1 surface relative to the ground electronic state along the symmetric torsional coordinate and permits a calculation of the excitation spectra of various isotopically substituted molecules not yet measured in the laboratory. A clear picture of the relationship between the dynamics on the S1 surface and the spectroscopy of benzophenone is revealed by comparing a time domain analysis of the experimental data with wave packet dynamics on the model S1 surface. This comparison provides new insight into energy flow in the isolated molecule and permits a qualitative simulation of the effects of collisional quenching on the fluorescence spectrum. We also discuss, using a classical trajectory analysis, the resonance dynamics of the torsional modes and note the existence of heretofore undetected local modes in the high resolution spectrum.
Excited state dipole moments in a polyatomic molecule determined by Stark quantum beat spectroscopy
Authors: Schmidt P. , Bitto H. , Huber J. Robert
Published: July 2002

High resolution Stark quantum beat spectroscopy is demonstrated to be a powerful method to determine magnitude and orientation of excited state dipole moments in asymmetric polyatomic molecules. We measured the dipole moments μ of the vibrationless S1 state of planar propynal and αD-propynal in a pulsed supersonic jet and determined both components of the μ vector in the molecular plane: μ′a=0.874(10)D, μb=1.06(2)D, ‖μ′‖=1.374(22) D for HC≡CCHO; μ′a=0.863(10)D, μb=1.04(1) D ‖μ′‖=1.351(14) D for HC≡CCDO. The results are related with recently reported microwave data of the ground state dipole moment μ(S0) and are interpreted in terms of electron delocalization from the carbonyl group to the ethinyl moiety. The dipole moments of S0 and S1 propynal are oriented essentially along the CO bond. The applicability of the Stark quantum beat technique to polyatomic molecules is discussed.
Computer simulation of interactions of model Pt particles and films with the silica surface
Authors: Levine S. M. , Garofalini S. H.
Published: July 2002

The molecular dynamics method was used to simulate adsorption and particle growth behavior of model Pt metal on the vitreous silica surface. Analysis of depositions from single atoms through the development of a continuous film of metal were simulated, and were then compared to similar systems analyzed in the laboratory. Consistent with experimental findings, the single atom depositions showed both adsorption onto the surface of the substrate as well as penetration into the interior (subsurface) sections. Multiple Pt atom interactions resulted in more complex absorption behavior. This finding yielded better quantitative agreement with the ‘‘real’’ systems analyzed previously. Applications of this type of study are discussed.
Structure and stability of Xe+n clusters
Authors: Amarouche M. , Durand G. , Malrieu J. P.
Published: July 2002

The equilibrium structure and energy of Xe+n clusters (n≤9, n=13 and 19) have been calculated using a model (of excitonic type) hamiltonian extracted from the potential curves of Xe+2 and Xe2, and including spin–orbit coupling and three-body interactions between instantaneous induced dipoles. Due to the p nature of the valence AO, the hole delocalization proceeds along one direction but it rapidly becomes less favorable to delocalize the hole on an nth atom than to polarize a neutral atom situated around the (n−1) atoms linear cluster. Xe+3 and Xe+4 are therefore linear and generate two families of heavier clusters, obtained by polarizing neutral atoms situated on crowns around the axis of the Xe+3 or Xe+4 ionized cores. Each crown may bear up to five atoms and there are two crowns around Xe+3, three crowns around Xe+4, Xe+13, and Xe+19 appearing then as compact structures with filled crowns. Rules are given for the filling of the neutral crowns. The two families (Xe+3 supported and Xe+4 supported) are nearly degenerate in energies with numerous isomers. The calculated adiabatic ionization potential for Xe19 (10.1 eV) is in good agreement with the observed value for the bulk (9.8 eV).
Correlation energy of an inhomogeneous electron gas: A coordinate-space model
Authors: Becke A. D.
Published: July 2002

We develop a coordinate-space model for dynamical correlations in an inhomogeneous electron gas. The model treats opposite-spin and same-spin pairs separately, and it also accounts properly for correlation contributions to the kinetic energy. Furthermore, it gives identically zero correlation energy in the case of one-electron systems. Applications to the uniform electron gas and to the atoms H through Ar are reported.
A
b i
n
i
t
i
o MRD CI investigation of the optical spectra of C4 and C5 clusters
Authors: Pacchioni Gianfranco , Koutecký Jaroslav
Published: July 2002

The ground and excited electronic states of linear and rhombic C4 and linear C5 clusters have been studied with ab initio single reference and multireference configuration interaction calculations. The spectrum of linear C4 is characterized by the existence of low-lying Π states at 1–1.5 eV above the 3Σ−g ground state. In rhombic C4, which has very similar ground state energy as the linear form, the first allowed transition is found at 2.4 eV. The optical spectrum of linear C5 exhibits some similarities with that of linear C3: in both molecules the ground state is 1Σ+g and the lowest allowed transition, 1Πu←1Σ+g, is about 3 eV above the ground state.
Diffusion-controlled reactions: Mathematical formulation, variational principles, and rigorous bounds
Authors: Rubinstein Jacob , Torquato S.
Published: July 2002

This paper is concerned with the problem of predicting the effective rate constant k associated with diffusion-controlled reactions in media composed of static and reactive traps (sinks) which are generally distributed randomly throughout a region containing reactive particles. The effective equation for diffusion-controlled reactions is derived using the method of homogenization. This leads to a rigorous definition of k. General variational principles are then formulated to obtain rigorous upper and lower bounds on k. These variational principles are applied by evaluating them for three different types of admissible fields. The upper and lower bounds which result are computed for both random and periodic arrays of equisized spherical sinks.
Excited states and Jahn–Teller interactions in the sodium trimer
Authors: Cocchini Franco , Upton Thomas H. , Andreoni Wanda
Published: July 2002

We present a complete assignment from first principles of the excited state spectrum for gas phase Na3 in the region below 2.7 eV. We find that the spectrum is complicated by the presence of numerous Jahn–Teller split states derived from both symmetry and accidental degeneracy. Our results indicate that the absorption bands observed experimentally at about 2 eV [the B and B′ bands Ref.(1)] correspond to a complicated mixing between an E′ doublet and an A′1 state, in which coupling between the A1 state and the E′ state dominates the usual coupling between E′ components. Another set of bands due to a similar mixing of E′ and A′1 states is predicted to lie at lower energy, outside the energy region explored in previous experiments. We are able to assign all four experimentally observed bands, and find a total of four new electronic states below 1.5 eV, which should result in two additional bands in the optical spectrum. All orbitals for Na3 ground and excited states can be characterized in terms of the electron droplet or shell model.
The anisotropic potentials of He–N2, Ne–N2, and Ar–N2
Authors: Bowers M. S. , Tang K. T. , Toennies J. Peter
Published: July 2002

The anisotropic potentials of He–N2, Ne–N2, and Ar–N2 are predicted using the Tang–Toennies potential model. This model damps the long-range ab initio dispersion terms individually using a universal damping function and adds to this a simple Born–Mayer repulsive term. The Born–Mayer parameters for the three systems were derived from SCF calculations. The dispersion coefficients were estimated from established combining rules using an effective multipole spectrum for the N2 molecule computed by Visser and Wormer from the time-dependent coupled Hartree–Fock approximation. The resulting potentials were used to predict the second interaction virial coefficients for each system, and they are found to be in excellent agreement with experiment. It is concluded that the spherical symmetric potentials are within 2%–3% of the true potentials. Some discrepancies with recent molecular beam experiments appear to be present, however, for the anisotropies especially in the case of He–N2. Finally, it is found that the law of corresponding states for anisotropic systems, which predicts that the reduced shapes of the potentials for a given geometrical configuration are identical, also holds for the highly anisotropic rare gas–N2 systems.
The exact multicenter multipolar part of a molecular charge distribution and its simplified representations
Authors: Vigné-Maeder Fabienne , Claverie Pierre
Published: July 2002

We study the problem of representing the molecular charge distribution in a convenient way for practical applications and we propose, instead of a single representation, a flexible procedure for building approximations with an arbitrary level of accuracy as concerns the long-range part of the electrostatic potential. We first discuss the splitting of the total electrostatic potential into a multipolar part (long-range) and a penetration part (shortrange) in connection with the usual one-center multipole expansion: at large enough distances, this expansion precisely converges towards the multipolar part. However, this representation is not practically efficient as soon as the molecule departs from a spherical shape, and we therefore consider a so-called multicenter multipole representation. In the MO-LCAO framework, the use of a basis of Gaussian atomic orbitals {χα} generates such a representation in a natural way: indeed, each elementary distribution χ*αχβ then reduces to a one-center distribution with a finite number of multipoles, hence an exact multipolar representation (corresponding to the approximate MO-LCGO wave function) involving a finite (albeit large, in general) number of centers, each one bearing a finite (small) number of multipoles. From this representation used as a reference, we then generate simplified multicenter multipole representations through a systematic procedure of reduction of the number of centers (having chosen a reduced set of new centers, we represent the multipoles of each suppressed old center through suitable new multipoles located on a few new centers closest to the old one under consideration). The number and locations of the new centers are arbitrary, hence the great flexibility of the method. We checked the accuracy of this procedure for molecules of different polarity (water, benzene, adenine), and we noticeably found that a representation involved as centers, the atoms plus one

point per chemical bond exhibited quite a satisfactory accuracy. In the conclusion, we briefly discuss the possibility of extending the method to other kinds of basis functions (e.g., Slater orbitals), in connection with the problem of representing the charge distribution associated with the exact molecular wave function, we also discuss the choice of the number centers according to the desired accuracy and we briefly describe a way of applying the method to the problem of representing a large molecule in terms of molecular fragments.
Half-collision studies of the Hg–NH3 excimer
Authors: Duval M. C. , Soep Benoit , van Zee Roger D. , Bosma Wayne B. , Zwier Timothy S.
Published: July 2002

The Hg–NH3 complex has been studied by forming the complex in a supersonic jet and probing the bound-to-bound transitions to the two excited electronic states correlated to Hg(6 3P1)+NH3. Laser-induced fluorescence and action spectroscopy have been combined with isotopic studies to map out the characteristics of these states. Both excited states are found to be bound by more than 5000 cm−1, over 20 times greater than the ground state binding energy. Extensive vibrational structure is found and interpreted in terms of a stretching progression of the Hg–NH3 bond and bending of the NH3 moiety with respect to the mercury atom. The two states show striking differences in their behavior with respect to predissociation to Hg(6 3P0). The B̃ state is not observed in fluorescence, but predissociates efficiently to Hg(3P0)+NH3, while the Ã state shows predominant fluorescence with only a minor amount of Hg(6 3P0) formation. Rotational band contour analysis has been used to assign the B̃ state as the 3E and the Ã state as the 3A1 state. Both states are characterized by a shortening in the Hg–N bond distance from 3.35 Å in the ground state to about 2.2 Å in either excited state. The rotational contour assignments show that the electronic angular momentum of the excited mercury atom is preserved in the complex despite the complex’s polyatomic nature. This allows an interpretation of the electronic relaxation in a quasidiatomic fashion. All our results are consistent with a C3v geometry for the Hg–NH3 complex in both the ground and excited states. The characteristics of the Ã and B̃ states and their couplings to the ã state correlated to 3P0 enable a comparison with the full-collision studies and has led us to postulate the Ã and B̃ states as the source of the luminescence observed in those studies.
Vacuum ultraviolet laser spectroscopy. V. Rovibronic spectra of Ar2 and constants of the ground and excited states
Authors: Herman P. R. , LaRocque P. E. , Stoicheff B. P.
Published: July 2002

Vibronic spectra of three band systems of 40Ar2 and 36Ar40Ar at 108, 107, and 105 nm were recorded and analyzed. Isotope shifts of bandheads were used to establish vibrational numbering of the observed bands. Rovibronic structure was examined at resolving powers up to 5×105, yielding band centers, rotational constants, and providing evidence of Hund’s case (c) coupling for Ar2. From band system II at 107 nm, with bands involving v′=20–27 and v″=0–5, improved constants for the ground X 0+g state were obtained: ω″e=30.68(8), ωex″e=2.42(5), ωey″e=−0.062(13), ωez″e=0.010(2), and D″e=99.2(10) cm−1, B″e=0.059 65(8) cm−1, and Re=3.761(3) Å. Similarly, constants for v′=20–27 of the B 0+u were evaluated, with estimates of T′e=88 210(400) and De=5640(400) cm−1. Band system I at 108 nm, led to constants for the v′=23–30 levels of the A1u state and to estimates of T′e=87 458(500) and De=5786(500) cm−1. Analysis of band system III at 105 nm yielded new and improved data for levels v′=0–9 of the C 0+u state, including T′e=95 033.6(3), De=465.8(4), ω′e=68.16, ωexe=4.631, ωey′e=0.0479, ωeze=0.0070, and B′e=0.065 24(20) cm−1, the latter resulting in Re=3.596(5) Å. Potential energy curves based on these data were calculated for the ground and three excited states.
Charge transfer photodynamics in halogen doped xenon matrices. II. Photoinduced harpooning and the delocalized charge transfer states of solid xenon halides (F, Cl, Br, I)
Authors: Fajardo Mario E. , Apkarian V. A.
Published: July 2002

The optically accessed charge transfer states of solid xenon doped with atomic halogens are excitonic in nature: an electron localized on the guest halogen atom and a delocalized hole centered on xenon atoms. These excitonic states are effectively self-trapped such that luminescence is observed exclusively from the localized molecular charge transfer states: the triatomic xenon halide exciplexes. The latter relax radiatively. The emission spectra of Xe+2 I−, Xe+2 Br−,Xe+2 Cl−, and Xe+2 F− are centered at 390, 480, 573, and 775 nm, and their radiative lifetimes are 130, 185, 225, and 190 ns, respectively. The charge transfer excitation spectra of the atomic solids are presented. In the case of F doped solids, the vertical transitions correspond to the diatomic XeF (B←X) and (D←X) absorptions: fluorine is bound to xenon in the ground state. The heavier halogens isolate atomically. Their excitation spectra are treated by a modified reflection approximation: reflection of the halogen–xenon radial distribution function from the hole transport potential. Ion–hole pairs separated by many lattice sites can be created by optical excitation, hence the spectra yield both the long range hole transport potential and the extended structure of the solid around the impurity site. The structure is fcc. Cl atoms generated by photodissociation of HCl or Cl2 are born at interstitial sites and convert to substitutional upon annealing of the solids. The atomic solids are prepared by two-photon induced harpoon reactions between xenon and molecular halogens: Xe+X2+2hν→[Xe+X−2 ]→Xe+X−+X. The inordinate efficiency of these reactions are attributed to the ionic potentials and the intimate participation of the polarizable cage atoms in ejecting the neutral halogen—a ‘‘negative’’ cage effect is postulated.
Electronic structure of Cs multilayer and monolayer adsorbed on Ru(001): A photoemission study
Authors: Sham T. K. , Hrbek J.
Published: July 2002

The electronic structure of Cs adsorbed on Ru(001) at multilayer and monolayer coverages has been investigated with high resolution photoelectron spectroscopy (PES) using synchrotron radiation. It is found that at multilayer coverage, the electron distribution curve (EDC) of the Cs overlayer is essentially the same as that of the Cs metal reported previously except that, with the high resolution, a surface atom core-level (Cs 5p3/2) binding-energy shift of +0.24 eV is observed for the first time. It is also found that at monolayer coverage which exhibits a ( 7/8 × 7/8 )-R30° LEED pattern, the Cs 5p3/2 level shifts by 1.0 eV to lower binding energy relative to that of the multilayer; this shift is accompanied by a significant linewidth broadening and a nearly complete suppression of O3VV Auger emission. These results are interpreted on the basis of the volume effect and a rehybridization mechanism in which the compression of the Cs atom and the interplay of conduction and 5d electrons at the Cs site are crucial to the 5p binding energy shift, core hole lifetime, and Auger transition.
Crystallization of fcc (111) and (100) crystal-melt interfaces: A comparison by molecular dynamics for the Lennard-Jones system
Authors: Burke E. , Broughton J. Q. , Gilmer G. H.
Published: July 2002

The crystal growth rates of Lennard-Jones fcc (face-centered cubic) (111) and (100) faces into the melt have been studied as a function of undercooling by molecular dynamics. The (100) grows without activation energy barrier at rates determined by the difference in the free energies of the crystal and melt phases, and the arrival rate of atoms across a plane determined from the kinetic theory of gases. The maximum velocity occurs at approximately half the melting point and represents 80 m/s for argon. The (111), on the other hand, grows at rates two to three times lower than this; the exact rate being size dependent. The growth kinetics are now activated and resemble Wilson–Frenkel behavior. However, the step responsible for such activation is not the simple liquid diffusion of Wilson–Frenkel theory, but rather the concerted motion of atoms at the interface selecting either all fcc or all hcp (hexagonal close packed) triangular lattice sites before a layer can grow.
Negative ion formation in K(n
d)–CS2 collisions: Detection of electric-field-induced detachment from CS−2
Authors: Kalamarides A. , Walter C. W. , Smith K. A. , Dunning F. B.
Published: July 2002

Charged particle production in thermal-energy K(nd)–CS2 collisions is investigated for intermediate values of n. The data show that collisions result in the formation of relatively long-lived CS−2 ions and of free electrons. A fraction of the CS−2 ions is observed to undergo rapid electric-field-induced detachment in fields of only a few kilovolts per centimeter and this novel phenomenon is discussed.
Dynamics and kinetics of the metastable decay series: Ar3+*→Ar2+*→Ar+
Authors: Scheier P. , Stamatovic A. , Märk T. D.
Published: July 2002

Metastable decay of Ar+*3 produced by electron impact ionization of a supersonic Ar cluster beam has been investigated qualitatively and quantitatively with a double focusing sector field mass spectrometer. We present direct experimental proof that the observed metastable decay of Ar+*3 into Ar+ in the microsecond time regime proceeds predominantly via a sequential decay series Ar+*3→k3,2Ar+*2→ k2,1Ar+ involving the evaporation of a single monomer in each of these successive decays. The metastable decay rates deduced are very different, i.e., k3,2 =45 s−1 and k2,1 =680 s−1 at an ion acceleration voltage of 3 kV. Moreover, the metastable decay rates of Ar+*3 exhibit a strong dependence on time elapsed since electron impact production of the Ar+3, and on ion production parameters (i.e., size and internal excitation energy of the neutral precursor). For instance photodissociation (with visible laser light) of the neutral cluster beam leads to a dramatic increase of the observed metastable reaction rules yielding values of up to 100 000 3−1 in the μs time regime.
Heat capacity of some liquid Zintl compounds: Equiatomic alkali–lead alloys
Authors: Saboungi Marie-Louise , Reijers H. T. J. , Blander M. , Johnson G. K.
Published: July 2002

Enthalpies for equiatomic APb (A=Na, Rb, and Cs) have been measured by high-temperature drop calorimetry over a temperature range covering solid and liquid phases. Thermodynamic information on the solid state transitions and the melting process is derived. With the exception of NaPb, the heat capacities of the liquid alloys show a remarkably strong negative temperature dependence above the melting point. It is suggested that this behavior is a consequence of the dissociation of molecular entities present in the liquid state related to the Zintl structures detected in the corresponding solid compounds. Thermodynamic calculations based on the presence of polyvalently charged anions yield valuable information on the temperature dependence of the concentration of these species.
The A 2Σ+ state of rare gas–NO van der Waals molecules probed by 1+1 multiphoton ionization spectroscopy
Authors: Miller John C.
Published: July 2002

The previously unobserved bound–bound spectra of ArNO, KrNO, and XeNO have been observed slightly blue shifted from the (1,0) and (0,0) bands of the A 2Σ+←X 2Π1/2 transition of uncomplexed nitric oxide. Although the structured but incomplete spectra cannot be assigned with certainty, limits to the ground and excited state bond dissociation energies, D″0 and D0, respectively, can be estimated. For ArNO these limits are D″0 ≥89 and 54 cm−1≤D0 ≤101 cm−1. The observed fragmentation of the KrNO and XeNO molecules, coupled with earlier results for MPI via the C 2Π state, suggests that superexcited, autoionizing states of the van der Waals molecules dissociate to yield excited rare gas atoms whenever energetically possible.