bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytic evaluation of energy gradients at the coupled-cluster singles and doubles level using quasi-restricted Hartree–Fock open-shell reference functions
Authors: Gauss Jürgen , Stanton John F. , Bartlett Rodney J.
Published: July 2002

Analytic formulas for the evaluation of energy gradients at the coupled-cluster singles and doubles (CCSD) level for a special class of open-shell quasi-restricted Hartree–Fock (QRHF) reference functions are presented and their computational implementation is discussed. The reference functions considered here are those which are formed from a closed-shell determinant for the molecule in a different ionization state by either adding or removing a single electron. Due to the approximate invariance of the CCSD model with respect to the choice of orbitals, results obtained with QRHF-CCSD closely approximate those from CCSD calculations using unrestricted Hartree–Fock (UHF) or restricted open-shell Hartree–Fock (ROHF) reference functions in cases where the latter references are well behaved. However, in cases where spin contamination (UHF) or symmetry breaking (UHF and ROHF) effects are important, QRHF-CCSD offers an attractive alternative since the resulting wave function is a projected eigenfunction of the spin operator Ŝ2 and transforms as a pure irreducible representation of the molecular point group.
Two-dimensional deuteron nuclear magnetic resonance of a polymer dispersed nematic liquid crystal
Authors: Dolinšek J. , Jarh O. , Vilfan M. , Žumer S. , Blinc R. , Doane J. W. , Crawford G.
Published: July 2002

The two-dimensional nuclear magnetic resonance (NMR) spectra and relaxation rates of polymer dispersed 4′-pentyl-4-cyanobiphenyl droplets deuterated in the β position of the hydrocarbon chain have been measured in the isotropic and nematic phases. A typical Pake power line shape pattern characteristic for a system of ‘‘bipolar’’ droplets with a random distribution of their symmetry axes in the external magnetic field has been found in the ω2 domain whereas the homogeneous linewidth was observed in the ω1 domain. The existence of a weakly orientationally ordered surface layer above the nematic–isotropic transition has been demonstrated. Whereas the deuteron T1/T2≂4.3 in the isotropic phase, T1/T2≂188 in the nematic phase for confined droplets. This effect, which is absent in the bulk, shows that T2 is here mainly determined by translationally induced rotation in the nonuniformly oriented director field inside the cavity. T1, on the other hand, is determined by the slowing down of local molecular reorientations at the surface of the confined droplet as well as by local molecular fluctuations and rotations within the randomly oriented droplets.
Delayed electron emission from photoexcited C60
Authors: Wurz Peter , Lykke Keith R.
Published: July 2002

Delayed emission of electrons is observed after absorption of several photons in the wavelength range from 212.8 to 532 nm. This phenomenon is explained by thermionic emission of electrons from hot C60 molecules. Photoelectrons and photoions are recorded under the same experimental conditions for further evidence.
The inversion mechanism for the reaction H+CD4→CD3H+D
Authors: Chattopadhyay Arun , Tasaki Shintaro , Bersohn Richard , Kawasaki Masahiro
Published: July 2002

The reaction H+CD4→CHD3+D is shown to take place by an inversion mechanism. The evidence is as follows. When the H atom has an anisotropic (perpendicular) velocity distribution, the D atom velocity distribution is also perpendicular. For a relative energy near 2 eV, the reaction cross section for H+CD4 is 0.084±0.014 A2 and for H+CH3D is 0.040±0.015 A2. At the same H atom energy, when CH3CD3 is substituted for CD4, no D atoms can be detected. Finally, around 80% of the initial H atom kinetic energy is released as kinetic energy of the D atom showing that the reaction is nearly vibrationally adiabatic.
Surface modes on polymer solutions by surface light-scattering techniques
Authors: Cao B. H. , Kim M. W. , Schaffer H. , Cummins H. Z.
Published: July 2002

By using the surface heterodyne spectroscopy, we study the surface modes on polymer solutions of polyisobutylene/decane at different concentrations. For each concentration, we measure the power spectra at various wave numbers (110.0–600.0 cm−1). We find that the relation between the frequency shift ωk of the power spectrum and wave number k changes continuously from ωk∝k3/2 to ωk∝k when concentration of the polymer solution is changed from 0.5 g/dl to 8.0 g/dl. For the low-concentration polymer solutions, the surface modes are capillary modes (ωk∝k3/2). For the high-concentration polymer solutions, the surface modes are elastic modes (ωk∝k). This result is compared with the theory [J. L. Harden, H. Pleiner, and P. A. Pincus, J. Chem. Phys. 94, 5208 (1991)].
Classical path surface-hopping dynamics. II. Application to Ar+3
Authors: Kuntz P. J. , Hogreve J. J.
Published: July 2002

A classical path surface-hopping trajectory approach is applied to the study of the dissociation of singly charged argon trimer-ions produced in the ionization process Ar3→Ar+3→Ar+2+Ar. Production of the ions in each of the first three adiabatic states is investigated with a view to describing the dynamics of the newly produced ion from its formation up to the time it dissociates. In the ground state, the motion typically involves several sequences of a vibration of the charged pair in a dimer subsystem Ar+2, followed by charge transfer to form a different dimer subsystem. In the second and third states, there is concerted motion until passage of a nonadiabatic region leads to a transition to the ground state, from whence dissociation occurs via a sequence of pair collisions. In the third state, the motion is more erratic, the charge being delocalized a large fraction of the time. The different dynamical mechanisms are interpreted in terms of charge migration and nonadiabatic effects. A few trajectories for Ar+n for n=3, 4, and 5 exhibit similar charge migration behavior, suggesting the applicability of these dissociation mechanisms to small clusters.
Charge separation in core excited argon clusters
Authors: Rühl E. , Schmale C. , Jochims H. W. , Biller E. , Simon M. , Baumgärtel H.
Published: July 2002

Charge separation in core excited argon clusters is reported. Neutral argon clusters have been prepared in a supersonic molecular beam. Photoionization with monochromatized synchrotron radiation in the L3/L2 regime (240–260 eV) initiates various single and double ionization processes. The photoion–photoion-coincidence (PIPICO) technique is applied to measure dissociative double ionization processes in core excited argon clusters. Three series of charge separation channels are observed: (i) Ar+/Ar+n, (ii) Ar+2/Ar+n, and (iii) Ar+3/Ar+n. Kinetic energy releases from charge separation reactions as well as the relative intensities of the PIPICO signals are discussed in relation to fragmentation mechanisms, resonant Auger spectra, and properties of cluster dications, such as cluster dication fragmentation energies and charge separation distances.
Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. V. CF4→F−+CF+3
Authors: Mitsuke Koichiro , Suzuki Shinzo , Imamura Takashi , Koyano Inosuke
Published: July 2002

Ion-pair formation from photoexcitation of CF4 has been studied by negative-ion mass spectrometry using synchrotron radiation in the 11–31 eV photon energy range. Negative F− ions have been observed. The appearance energy in the F− photodissociation efficiency curve is about 2 eV higher than the thermochemical threshold for the formation of the ground-state ions F−(1Sg) and CF+3(X̃ 1A′1). The peak features observed in the spectrum are interpreted as resulting from transitions to Rydberg states with the 1T2 symmetry. Assignments of the peaks have been made on the basis of the previous work on photoabsorption cross-section curves of CF4. Repulsive Rydberg states converging to CF+4(X̃ 2T1, Ã 2T2) give the strong and broad peaks in the F− efficiency curve. Conversion to the ion-pair state is considered to occur through the avoided potential surface crossings along the dissociation coordinate of the CF3—F bond. In contrast, the npt2 Rydberg states (n≥4) converging to CF+4(C̃ 2T2) show long vibrational progressions. The radiative and nonradiative decay pathways for these Rydberg states are discussed.
Monte Carlo study of impurities in quantum clusters: H2 4HeN, N=2–19
Authors: Barnett R. N. , Whaley K. B.
Published: July 2002

Variational Monte Carlo techniques are employed in studying 4He clusters, with and without an H2 impurity. We find that a novel, yet simple, analytic nuclear wave-function form, derived from a numerical H2He wave function, yields high accuracy in computed ground-state energies of 4HeN. For the clusters studied here, three to twenty atoms, energies range from 94% to 90% of the exact values. Density profiles and distributions of particle separation are also computed. For reasonable computational cost (e.g., <20 Cray/X-MP14 minutes for the largest cluster), density profiles are determined for the first time to high statistical accuracy to within 0.5 Å or less of the cluster center. The density profile of He3 is found to possess a uniquely pronounced peak at the cluster center resulting from contributions of near-collinear atomic arrangements. We also study the effect of substituting an He by H2, using modified wave functions containing products of pairwise He–H2 terms. For all cluster sizes studied, we find a lowering of the total energy upon exchanging an He for an H2. The exchange energy increases in magnitude with increasing cluster size, yet is still well below bulk estimates at N=20. Size comparisons with the pure helium clusters show very little change upon He/H2 exchange, e.g., the rms radii differ by ≤2% for N>3. Density profiles and bond distributions show noticeable differentiation between H2 and He. For N≥4, the peak in the H2 density profile is not at the cluster but does remain inside the cluster. This peak is most pronounced for H2He13 implying an enhanced resistance to H2 penetration for He13.
Gas/surface complex coordinate scattering theory: HD/Ag(111), HD/Pt(111) rotationally inelastic transition intensities
Authors: Peskin Uri , Moiseyev Nimrod
Published: July 2002

The method developed by Engdahl–Moiseyev–Maniv [J. Chem. Phys. 94, 1636 (1991)] for computing the scattering intensities of atomic beams from periodically corrugated solid surface is presented in more general form for scattering of molecules from solid surfaces. The method is numerically exact. By complex scaling of the Hamiltonian the full Green operator is calculated, using techniques that were originally developed for bound states. There is no need to impose specific boundary conditions on the scattering states. The method is used to calculate the rotationally inelastic transition intensities vs normal incident beam energy for HD scattering from Ag(111) and Pt(111) surfaces. Our results are in a very good agreement with theoretical scattering transition probabilities previously obtained by Whaley and Light and by Schinke.
Relativistic calculations on platinum hydride using effective core potentials and first-order perturbation theory
Authors: Gropen Odd , Almlöf Jan , Wahlgren Ulf
Published: July 2002

A b initio relativistic calculations have been performed for the two lowest electronic states of the PtH molecule, with relativistic effects accounted for by means of relativistic core potentials and first-order perturbation theory. Electron correlation has been treated at the multireference configuration interaction level. The 2Σ and 2Δ states are close in energy, with the Σ state somewhat lower, according to the core potential calculations. It is concluded that a method which gives a correct atomic description is imperative to describe the spectroscopy of the molecule.
Structure and energetics of model metal clusters
Authors: Uppenbrink Julia , Wales David J.
Published: July 2002

A Finnis–Sinclair-type many-body potential and a combined Lennard-Jones (two-body) plus Axilrod–Teller (three-body) potential are employed to study the structural properties of model clusters in the size range 13 to 309 atoms. For the first potential, parameter values for the fcc transition metals as proposed by Sutton and Chen are used, while for the latter potential a least-squares fitting routine is employed to fit the parameters to bulk data. For suitable parameter ranges, the relative energies of optimized decahedral, icosahedral, and face-centered-cubic (fcc) cuboctahedral clusters are compared in the light of experimental observations for these metals. Variation of the parameters in the Finnis–Sinclair potential, which govern the range of the repulsive two-body term and the attractive many-body term, influences the size at which the icosahedra and fcc cuboctahedra cross over in energy. Similarly, an increase in the three-body contribution in the LJ+AT clusters leads to a decrease in the crossover size from several thousand to a few hundred atoms. For appropriate parameter ranges, a convex curvature of the facets as well as a contraction of the clusters is observed for both potentials. These effects are compared with experimental observations for the fcc transition metals copper, silver, gold, nickel, palladium, and platinum.
Comment on: Molecular simulation of water along the liquid–vapor coexistence curve from 25 °C to the critical point
Authors: Strauch H. J. , Cummings P. T.
Published: July 2002

In an earlier paper [J. Chem. Phys. 93, 7355 (1990)], de Pablo et al. reported Gibbs ensemble Monte Carlo (GEMC) simulations of SPC water with long-range intermolecular interactions computed using the Ewald summation method. The coexistence curve of SPC water was obtained from 25 to 300 °C. We report new simulations with a slight modification of the SPC potential to reflect the lower effective dipole moment in the vapor phase. The vapor–liquid coexistence curve is found to agree better with experiment.
Demixing and mixing of binary hard-core Yukawa mixtures
Authors: Malescio G.
Published: July 2002

Demixing and mixing of a model binary mixture of hard spheres interacting through a Yukawa tail are investigated using the hypernetted-chain and Percus–Yevick integral equations. The dependence of the local composition of the mixture on the interaction potential and its relationship with the phase stability is studied.
Vibrational state-to-state calculations of H++O2 charge transfer collisions
Authors: Sizun M. , Grimbert D. , Sidis V. , Baer M.
Published: July 2002

A comprehensive theoretical investigation of vibrational excitation and vibronic charge transfer in the H++O2 collision at ECM=23 eV is reported. The calculations of differential and integral scattering observables are undertaken within both the quantal infinite order sudden (QIOS) and the vibronic semiclassical (VSC) approximations. They involve 2×15 vibronic expansions associated with the diabatic states determined by Grimbert et al. [Chem. Phys. 124, 187 (1988)] using a so-called effective model potential (EMP) method. A quadripartite comparison involving experimental data of Noll and Toennies [J. Chem. Phys. 85, 3313 (1986)], results of a QIOS treatment of Gianturco et al. [Phys. Rev. A 42, 3926 (1990)] based on DIM potentials and the present QIOS, and VSC results is presented. From the comparison of the theoretical and experimental results we find that the present calculations based on the EMP reproduce much better the experimental data than those based on the DIM potentials. Though differences are found between the present QIOS and VSC results, the experimental data do not help deciding between the two theories. The relative merits of the QIOS and VSC approaches are put forward and discussed. By scrutinizing intermediate results of lengthy calculations we have been able to bring out elementary models in them. Pure vibrational excitation is found to be a prominent process in this system and charge transfer substantially affects it.
Structures of small metal clusters. II. Phase transitions and isomerization
Authors: Vlachos D. G. , Schmidt L. D. , Aris R.
Published: July 2002

The binding energy, atom coordination numbers, bond lengths, surface restructuring, and bulk melting behavior of small clusters versus temperature are compared for the Lennard-Jones (LJ) potential and embedded atom (EA) potential using the Monte Carlo method with parameters fitted to Ni. We find that EA clusters are more thermally stable than LJ clusters with regard to evaporation. For small clusters whose minimum energy structure is polyicosahedral, a smooth change of physical properties with temperature is observed for both potentials. However, for clusters whose minimum energy structure is quasicrystalline, a structural phase transition analogous to a first order transition of bulk materials can be found for both potentials. This structural phase transition is manifested by discontinuous changes of atom coordination numbers and bond length, and in some cases, of energy. Implications of this transition in catalysis are discussed. Isomerization between minima of each one of the two potential hypersurfaces along with its dependence on temperature are examined. The many body nature of the EA potential results in lower isomerization probabilities between EA isomers as contrasted to LJ isomers.
Vibrational band oscillator strengths and dipole transition moment of the A 1Π–X 1Σ+ system of CO
Authors: Eidelsberg M. , Rostas F. , Breton J. , Thieblemont B.
Published: July 2002

Rotationally integrated vibrational absorption cross sections of the A 1Π−X 1Σ+ (v′−0) bands of CO have been measured in the wavelength range 125–155 nm using monochromatized synchrotron radiation as background light source. Absorption cross sections for a few singlet-triplet transitions are also obtained. Vibrational band oscillator strengths are determined for 0≤v′≤12 and compared to other experimental and theoretical values. It is found that the new data are well reproduced by extrapolating to shorter internuclear distances the parabolic dipole transition moment function determined by De Leon [J. Chem. Phys. 89, 20 (1988); 91, 5859 (1989)]. Deperturbed lifetimes for levels 0≤v′≤13 are calculated from the parabolic dipole moment function and, except for v′=13, are found in good agreement with available experimental results.
Direct simulation of continuous wave electron paramagnetic resonance spectra from Brownian dynamics trajectories
Authors: Robinson B. H. , Slutsky L. J. , Auteri F. P.
Published: July 2002

A method is developed for the direct simulation of continuous wave electron paramagentic resonance (CW-EPR) line shapes from molecular trajectories. The method is applied to the cases of an axially symmetric nitroxide spin label undergoing two different types of motion (i) two-site jumping motion and (ii) isotropic rotational Brownian motion. Probabilistic models are developed that generate trajectories stable for large numbers of evolution time steps. Spectra at both 10 (X band) and 35 (Q band) GHz of satisfactory quality are generated from 40 trajectories of 214 steps sampled at 0.05 ns intervals. The resulting calculations illustrate that this method can be used as a basis of CW-EPR spectral simulation for systems undergoing arbitrarily complex dynamics using trajectories generated by classical Brownian dynamics.
Ab initio calculations for the adsorption of small molecules on metal oxide surfaces. I. Cluster calculations for carbon monoxide CO on nickel oxide NiO(100)
Authors: Pöhlchen Martin , Staemmler Volker
Published: July 2002

Quantum chemical ab initio calculations for the adsorption of CO on the NiO(100) surface have been performed at different levels of accuracy: self-consistent field (SCF), complete active space self-consistent field, and coupled electron pair approximation. Basis sets of double zeta and triple zeta + polarization (TZP) quality have been used. The NiO(100) surface is represented by a cluster containing one Ni2+ ion and the five adjacent O2− ions. The charge neutrality of the cluster and the saturation of the dangling bonds is achieved by adding eight protons, which gives the total composition Ni(H2O)3(OH)2. Alternatively, the Ni2+(O2−)5 unit is embedded in a lattice of point charges which correctly represent the half-infinite ionic crystal. In the most favorable configuration, CO is adsorbed linearly in the on-top position on the Ni2+ ion, with the C atom pointing toward the surface. The binding energies at the SCF level are rather small, only 0.08 eV and 0.03 eV for CO and OC bound to the cluster (TZP basis set, counterpoise correction for the basis set superposition error included). For the two configurations the equilibrium Ni–C and Ni–O distances are 5.40 and 5.50 a0, respectively. Electron correlation does not change these values markedly. Estimating the errors in our calculation we arrive at binding energies of 0.10±0.05 and 0.05±0.05 eV, respectively, for CO and OC on NiO(100). This is in agreement with the experimental estimates. The bonding is attributed predominantly to electrostatic and inductive forces. No genuine ‘‘chemical’’ bond (overlap, charge transfer) exists between CO and the NiO(100) surface, i.e., CO is only physisorbed to this ionic surface. The harmonic vibration frequencies for the Ni–C stretching and the Ni–C–O bending vibrations are estimated to 52 and 139 cm−1, respectively.
The effect of vibrational excitation (3≤v″≤19) on the reaction Na2 (v″)+Cl→NaCl+Na*
Authors: Dittmann P. , Pesl F. P. , Martin J. , Coulston G. W. , He G. Z. , Bergmann K.
Published: July 2002

The variation of the total integral cross section for the chemiluminescent channel with the vibrational excitation 3≤v″≤19 is investigated using laser-induced fluorescence or coherent population transfer, a new method employing delayed interaction with two cw lasers to populate vibrationally excited levels. The cross section increases with v″ by about 0.76% per vibrational level. The results are in agreement with the results of phase-space calculations, as well as with those from simple curve crossing models.
State-to-state rotationally inelastic scattering of ND3 on a graphite (0001) surface
Authors: LaVilla Michael E. , Ionova Irina V. , Ionov Stanislav I.
Published: July 2002

State-selected molecular beams of deuterated ammonia, ‖JKMε≳‖inversion≳=‖1111≳‖−≳ or ‖222−1≳‖−≳ and ‖3331≳‖−≳ states in the proportion 2.3:1, are produced via hexapole electrostatic focusing and then scattered at near-normal incidence on a graphite (0001) surface at Ts=130 K. The post-collisional rotational distribution of directly scattered molecules is monitored by laser photoionization spectroscopy. It is observed that the state-to-state rotational transitions in the surface scattering of ND3 are strongly coupled to the inversion symmetry change, as predicted by Corey and Alexander [J. Chem. Phys. 89, 790 (1988)]. This coupling may be viewed as a constructive and destructive interference between the ‘‘left’’ and ‘‘right’’ inversion configurations of the molecule on the surface potential. The selection rules prohibiting para to ortho conversion, which have been predicted by Corey and Alexander for rotationally inelastic scattering of ammonia, are observed to be violated, perhaps, due to ND3 distortion at the impact.
Exploiting quantum interference effects for the determination of the absolute orientation of an electronic transition moment vector in an isolated molecule
Authors: Plusquellic David F. , Pratt David W.
Published: July 2002

Quantum interference effects have been observed in the O00 band of the fully resolved S1←S0 fluorescence excitation spectrum of the hydrogen bonded complex of trans-2-naphthol with ammonia (t-2HNA). The effects owe their origin to the simultaneous existence of (1) ab hybrid band character in the spectrum; (2) perturbations in the spectrum that are produced by a coupling of overall rotation with the hindered internal rotation of the attached NH3 group; and (3) low barriers along the torsional coordinate in both electronic states. Analysis of the resulting interference patterns makes possible an unambiguous determination of both the magnitude and the sign of the orientation of the transition moment vector with respect to the a inertial axis in t-2HNA, θTM=−66±2°. Comparison of this result with that for the S1←S0 transition of the bare molecule shows that the orientation of its transition moment in the molecular frame is unaffected by complex formation. Additionally, symmetry-based arguments are given that may be used to predict the existence of quantum interference effects of this type in other systems.
Reactive scattering from oriented molecules: The three-center reaction K+ICl→KI+Cl, KCl+I
Authors: Loesch H. J. , Möller J.
Published: July 2002

In a crossed molecular beam experiment, we have measured the angular and time-of-flight (TOF) distributions of the products KCl and KI formed in the reaction K+ICl→KI+Cl, KCl+I at an elevated collision energy of Etr=1.64 eV. Employing the brute force method, we have prepared an oriented ICl beam and studied in addition also the orientation dependence of these distributions. The results are (i) KCl is the dominant product, but also KI is substantially formed with a branching ratio of 4:1; (ii) the double differential reaction cross section in the center-of-mass frame (contour maps) indicates that all products are preferentially forward scattered and constrained to the forward hemisphere; (iii) the KCl flux consists of two distinct components which differ markedly in kinetic energy and dependence on the ICl orientation; there are also indications of the existence of two components of KI; (iv) 65%, 84%, and 64% of the available energy is vested into the internal degrees of freedom for the fast, slow component of KCl and KI, respectively; (v) the existence of two components can be rationalized on the basis of the harpooning mechanism where the jumping electron accesses the ground state or one of the low excited states of the ICl− ion and triggers the subsequent explosion of the ion with more or less kinetic energy of the fragments depending on the initially populated state; (vi) the energies released during dissociation of ICl− in the 2Σ ground state and the first 2Π state are ≤0.19 and ≤1.2 eV, respectively; (vii) the fast KCl component features a negative steric effect suggesting favorable product formation for attacks of K to the I end of ICl, the steric effect of the slow KI component is positive, i.e., attacks to the Cl end form products favorably; the other components exhibit no significant steric effect; (viii) the steric effects can be quantitatively rationalized using the same model as mentioned above; (ix) the magnitude of the steric effect suggests a rotational temperature of the low-lying states of the ICl beam of 14±1 K.
Nonclassical structures in silicon-containing molecules: The monobridged isomer of Si2H2
Authors: Cordonnier Michel , Bogey Marcel , Demuynck Claire , Destombes Jean-Luc
Published: July 2002

A monobridged isomer of the disilyne Si2H2, the planar bridged-2 form Si(H)SiH, has been detected through its millimeter- and submillimeter-wave rotational spectrum. The molecules are produced in a low pressure, low power plasma in a mixture of SiH4 and Ar cooled at liquid nitrogen temperature. From the analysis of a total of 148 lines, a set of accurate rotational constants have been derived. Using the geometry predicted by ab initio calculations (see the preceding paper), the spectrum of Si(D)SiD has been predicted, observed, and analyzed. From the two sets of rotational constants, a preliminary molecular structure has been derived by fixing the Si1–Si2–bridged H angle to the ab initio value of 52.5°: Si1–Si2=2.119 Å; Si2–bridged H=1.629 Å; Si2–terminal H=1.474 Å; Si1–Si2–terminal H=157.5°.
Calculation of barriers to proton transfer using multiconfiguration self-consistent-field methods. I. Effects of localization
Authors: Luth Karl , Scheiner Steve
Published: July 2002

The usefulness of multiconfiguration self-consistent-field (MCSCF) calculations in computing correlated proton transfer potentials is investigated for the systems HF2−, H7N2+, H3O2−, and H5O2+. In deciding whether to include particular molecular orbitals, it is important to consider the balance of electron density between the donor and acceptor groups and the interactions that are incorporated in the orbitals. Only orbitals which have the proper symmetry to interact with the transferring hydrogen need be included in the MCSCF active space. Reasonable transfer barriers are obtained when the orbitals are balanced and only interactions relevant to the transfer process are allowed in the MCSCF active space. Equivalent barriers are determined, but the criteria are more easily met, if the canonical molecular orbitals are first subjected to a localization. Only the two localized molecular orbitals that contain the F, N, or O interaction with the transferring hydrogen are needed, which reduces the difficulty of eliminating unproductive interactions. In addition, the localization allows additional virtual orbitals to be included without producing an undesirable correlation.
A two-color laser-induced grating technique for gas-phase excited-state spectroscopy
Authors: Buntine Mark A. , Chandler David W. , Hayden Carl C.
Published: July 2002

A new excited−state spectroscopic method is reported. It is a two−color laser−induced grating tecnique for detecting optical transitions of rovibronically excited molecules in the gas phase. (AIP)
Orientation and molecular orbital dependence in the spin changing collision C2H2O2(S1)+Ar→C2H2O2(T1)+Ar: A study through van der Waals complexes
Authors: Lapierre Luc , Dai Hai-Lung
Published: July 2002

The orientation dependence in the spin-changing collision C2H2O2(S1)+Ar→C2H2O2(T1) +Ar has been examined by time-resolved laser induced fluorescence studies of the intersystem crossing rates in the glyoxal⋅Ar complexes with isomeric structures. Dramatic variation of intersystem crossing rates with complex structures and van der Waals excitations indicate strong orientation dependence in this process. Experimental results show that Ar interaction with the glyoxal n(O) orbital is two orders of magnitude more effective in inducing intersystem crossing than Ar interaction with the π* (CO) orbital.
The silicon–carbon symmetric stretching fundamental ν1 of Si2C: Nonintuitive theoretical behavior
Authors: Bolton Evan E. , DeLeeuw Bradley J. , Fowler Joseph E. , Grev Roger S. , Schaefer Henry F.
Published: July 2002

There are two experimental values for ν1(Si2C), namely 658 and 839 cm−1. Previous theoretical studies suggest a third value slightly below 800 cm−1. Here, elaborate theoretical studies using very large basis sets confirm that the ν1=839 cm−1 experiment of Presilla-Marquez and Graham is correct. The contributions of both higher order correlation effects (as gauged by coupled cluster methods) and f functions are contrary to previous experience.
Reaction path potentials in internal coordinates: Application to the dissociation of HCOH.+
Authors: Ma Ngai Ling , Collins Michael A.
Published: July 2002

A method for transforming an intrinsic reaction path potential into an internal-coordinate path potential is presented. The form of the resulting potential is suitable for use in classical trajectory simulations. The method is applied to dissociation of HCOH.+ (and deuterated analogs) via a direct cleavage pathway. Kinetic energy release values are compared with experimental results and previous calculations.
Electron attachment to Cn clusters (n≤30)
Authors: Carman Howard S. , Compton Robert N.
Published: July 2002

Low-energy electron attachment to neutral Cn clusters (n≤30) has been studied using the Rydberg electron transfer technique. The mass spectra of Cn− ions obtained suggest that certain carbon clusters (e.g., C5, C10, C12, C16, C18) have much larger low-energy electron attachment cross sections than other clusters.
Melting of Yukawa systems: A test of phenomenological melting criteria
Authors: Stevens Mark J. , Robbins Mark O.
Published: July 2002

The melting transition of particles interacting via a Yukawa potential is determined from the evolution of systems containing coexisting solid and fluid phases. There is no nucleation barrier, and the stable phase expands rapidly to fill the system. Results for the phase boundary agree with recent free energy calculations, and are used to test the Lindemann and Hansen–Verlet criteria for melting. A third criterion related to the Debye–Waller factor of the smallest reciprocal lattice vector is also identified.
Reactions of pulsed laser produced boron and nitrogen atoms in a condensing argon stream
Authors: Andrews Lester , Hassanzadeh Parviz , Burkholder Thomas R. , Martin J. M. L.
Published: July 2002

Reactions of pulsed laser produced B and N atoms at high dilution in argon favored diboron species. At low laser power with minimum radiation, the dominant reaction with N2 gave BBNN (3Π). At higher laser power, reactions of N atoms contributed the B2N (2B2), BNB (2Σu+), NNBN (1Σ+), and BNBN (3Π) species. These new transient molecules were identified from mixed isotopic patterns, isotopic shifts, and ab initio calculations of isotopic spectra.
Structure and photodissociation spectra of mixed ethene–acetone clusters
Authors: Buck U. , Hobein M. , Schmidt B.
Published: July 2002

Infrared photodissociation spectra of the mixed complexes C2H4–CH3COCH3 and C2H4–(CH3COCH3)2 have been observed after size selecting them by scattering from a helium beam combined with mass spectrometric detection. The excitation of the ν7-out of plane symmetric wagging mode of ethene near its gas phase frequency at 949.3 cm−1 with a cw-CO2 laser leads to a characteristic depletion of the cluster beam. The dissociation spectrum of the 1:1 complex can be explained by two peaks at 950.8 and 961.6 cm−1. Calculations of minimum energy configurations and band shifts based on an empirical site–site potential show that these frequencies can be attributed to the absorption of two different isomers. They correspond to the two different binding patterns of the H atoms of ethene to the O atom of acetone and those of acetone to the C–C group of ethene, respectively. For the 1:2 complex, a large peak at 958.5 cm−1 and a smaller one around 940.5 cm−1 are found which can be explained in a similar manner by several isomers found in the structure calculations.
Gaussian type orbital basis sets for the calculation of continuum properties in molecules: The photoionization cross section of H2
Authors: Cacelli Ivo , Moccia Roberto , Rizzo Antonio
Published: July 2002

The now well-established L2 techniques applied to the description of the electronic continuum of small molecules and generally implemented with suitable oscillating basis sets are here adapted and used in connection with Gaussian type orbital (GTO) functions. As a test, a K-matrix approach associated with extended sets of GTOs is employed to compute with satisfactory accuracy partial wave phase shifts and both the integral and differential photoionization cross sections of the hydrogen molecule. It is apparently possible to greatly extend the range of applicability of the usual L2 techniques beyond the limited set of molecular systems amenable to the one center expansion approximation.
Electrostatic interactions in slabs of polarizable particles
Authors: Clercx H. J. H. , Bossis G.
Published: July 2002

We present results of a study of electrostatic interactions in slabs consisting of L layers with polarizable spheres on a two-dimensional Bravais lattice in an externally applied electric field. A multipole expansion to describe N-particle electrostatic interactions, introduced by the authors in a simultaneously published paper, has been used to calculate the induced dipoles on particles in the respective layers of the slab. The conclusion is that the induced dipoles differ from their bulk values if situated near the surface, and these differences depend strongly on the internal structure of the slabs. With these results the surface energy has been calculated. Moreover, the local electric field has been studied as a function of the distance from the surface of the slab. These results are compared with a mean field approximation where each particle is considered to be surrounded by a homogeneous polarization. The differences between both models are striking.
Non-Rice–Ramsperger–Kassel–Marcus dynamics and the statistics of reaction rates in chaotic systems
Authors: Gentile Ann C. , Schofield Sarah A. , Wolynes Peter G.
Published: July 2002

Recrossings of the transition state are normally thought of as modifying the dynamical prefactor (transmission coefficient) which corrects the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Here, a semiclassical path analysis is used to show that recurrences to parts of phase space from which reactive transitions can occur also lead to fluctuations of the reaction rate about its microcanonical average. This result leads to a simple formula for the power spectrum of the rates.
Electron-temperature dependence of the recombination of H3O+(H2O)n ions with electrons
Authors: Johnsen R.
Published: July 2002

The electron-temperature dependence of the recombination of H3O+(H2O)n cluster-ions with electrons has been measured in an afterglow experiment in which the electrons were heated by a radio-frequency electric field. The recombination coefficients were found to vary with electron temperature Te as ∼Te−1/2, in better agreement with theoretical expectations than earlier results of microwave-afterglow measurements.
Hyperfine interaction and structure of a gallium arsenide cluster: Ga2As3
Authors: Van Zee R. J. , Li S. , Weltner W.
Published: July 2002

Vapor species produced by laser heating of gallium arsenide crystals were allowed to aggregate in a relatively high pressure of argon or krypton before condensation of the matrices at 4 K. Electron-spin resonance (ESR) identified the spectrum of the S=1/2 Ga2As3 cluster via the 69,71Ga (I=3/2) and 75As (I=3/2) hyperfine structure as containing two equivalent Ga and three equivalent As nuclei. Its geometry is therefore trigonal bipyramid and its ground state probably 2A2″, as obtained by Lou et al., from local-spin-density theory. The observed hyperfine parameters indicate that the unpaired spin is confined almost entirely to the two axial gallium atoms.
CH2+ is bent
Authors: Graber T. , Kanter E. P. , Vager Z. , Zajfman D.
Published: July 2002

The Coulomb explosion imaging (CEI) technique has been applied to a beam of vibrationally cold CH2+ molecular ions and the distribution of bond angles was extracted. From these data, we are able to demonstrate conclusively that the electronic ground state potential of this molecule has a C2v minimum and that the zero-point energy lies below the potential barrier to linearity.
Hot bands in overtone absorption of pyrrole, methyl isocyanide and isobutane: Photoacoustic spectra at 140 C
Authors: Hassoon Salah , Snavely Deanne L.
Published: July 2002

The intracavity laser photoacoustic spectra of the 4νN–H and 4νC–H overtones of pyrrole, the 5νC–H of methyl isocyanide and acetonitrile, the 5νC–H and 6νC–H of isobutane, and the 6νC–H of cyclohexane were measured at temperatures up to 140 C (T140) and compared to the room temperature (Trt) spectra. Hot bands were identified by their temperature dependence in pyrrole, methyl isocyanide, and isobutane. From the temperature dependence of the relative intensities of the hot band compared to the main band, the following average wave numbers of the thermally populated states were deduced: 340, 300, and 208 cm−1 for pyrrole, isobutane, and methyl isocyanide, respectively. These values compare well to the bending motion frequencies in these molecules. Large redshifts of the hot bands from the main peak were observed, as expected for high overtone transitions due to large anharmonicities. The anharmonic constants determined from these shifts are −25 cm−1 for the N–H in pyrrole and −20 cm−1 for the C–H in methyl isocyanide and isobutane. In the case of the N–H absorption the hot band was completely separated from the main transition. In the C–H overtone spectra these hot band absorptions were not separated and result in an apparent increased width of the overtone transition.
A mean-field theory of a localized excess electron in a classical fluid
Authors: Zhu Jianjun , Cukier R. I.
Published: July 2002

A mean-field, density-functional theory for a ground-state, localized excess electron in a classical solvent is presented. We obtain a Schrödinger equation for the electron’s wave function, with a mean-field potential dependent on the local density of the solvent, and an integral equation for the electron–solvent correlation function, with an effective (averaged over the electron density) electron–solvent interaction potential. We show that this effective interaction is weak and use this feature to suggest closures of the integral equation characterizing the electron–solvent correlations. The coupled system of the Schrödinger and integral equations are solved self-consistently, using an iterative method. The results are in good agreement with path-integral and time-dependent self-consistent-field simulations of an excess electron in supercritical helium. We show that these two simulation methods should agree when the electron is essentially always in its ground electronic state, as is the case for an electron in sufficiently dense helium.
Multiconfiguration self-consistent field models for gas–surface scattering
Authors: Jackson Bret
Published: July 2002

A multiconfiguration self-consistent field model is presented which accurately describes gas–surface energy transfer in the single phonon limit. The model is designed so that the results converge on the exact close-coupling results in the limit of a large number of configurations. The model is used to compute probabilities for energy transfer and sticking for He incident on a flat Cu surface, and the results are compared with those from a close-coupling calculation. The multiconfiguration approach is shown to give accurate results using few configurations, and is more stable than the close-coupling method when very few molecular basis functions are used. A model containing only two configurations, one for trapped particles and one for free particles, is discussed. It is shown to give reasonable results for sticking, and is a significant improvement over the single-configuration Hartree approach.
Nonstatistical inversion dynamics of T-shaped Ar3. II. Separatrix transition states
Authors: Jain Shashi , Bleher Siegfried , Dumont Randall S.
Published: July 2002

T-shaped Ar3 inversion population dynamics is treated in terms of separatrix transition states and flux–flux correlation formalism. Observed numerical population decays exhibit oscillations which are modeled statistically using a four species mechanism. Successful models are based on the generalized absorbing boundary method, and a statistical approximation for two of the four required gap distributions. The statistical rate constants essential to the latter approximations are obtained via Monte Carlo integration together with short time trajectory computations. The delayed lifetime gap model provided excellent model gap distributions, in contrast to previous results [R. S. Dumont and S. Jain, J. Chem. Phys. 97, 1227 (1992)] based on the primitive transition state. However, it is an essential ingredient of the model population decays.
Efficient determination of thermodynamic properties from a single simulation
Authors: Rickman J. M. , Srolovitz D. J.
Published: July 2002

A method for calculating the density of states of a system directly from its trajectory in phase space is described. As a specific example, the method is applied to the Monte Carlo simulation of a two-dimensional Ising model. The energy distribution function is calculated from the density of states and the associated Helmholtz free energy per spin is calculated for various system sizes and temperatures and shown to be in excellent agreement with the exact results.
Rotational energy dispersions for van der Waals molecular clusters: Analytic descriptions for Rg3, Rg4, and Rg6
Authors: Lohr Lawrence L. , Huben Carl H.
Published: July 2002

We have obtained analytic expressions, parametric in centrifugal displacement coordinates, which provide exact classical descriptions of the rotational energy dispersions, that is, the dependence of the combined rotational and ‘‘electronic’’ (vibrational potential) energies on the rotational angular momenta, for small molecular clusters bound by van der Waals interactions modeled by pairwise additive Lennard-Jones 6–12 potential energies. The clusters considered consist of three (equilateral triangle), four (tetrahedron), and six (octahedron) units and serve as models for small clusters of rare-gas atoms such as argon. This work represents an extension of our recently published study of analytic rotational energy dispersions for diatomic molecules bound by harmonic oscillator, Morse, or Lennard-Jones potentials [J. Mol. Spectrosc. 155, 205 (1992)]. A parallel set of studies were made using an angular momentum-conserving simulation program. The physical properties of the clusters that are addressed using our results include calculation of quartic and higher-order spectroscopic constants, location of rotational instabilities, and characterization of the ‘‘cubic’’ anisotropies for the spherical top clusters A4 and A6. Of particular interest is the result that for each of these cluster types the preferred direction of the rotational angular momentum is parallel to a molecular fourfold axis, leading to reduced symmetries of D2d for tetrahedral A4 and D4h for octahedral A6.
The isotropic to nematic liquid crystal transition for flexible nonspherical molecules
Authors: Tang Shuang , Evans Glenn T.
Published: July 2002

A torsionally flexible molecule is modeled as having a hard convex core and flexible caps in the polar regions. During an isomerization, the aspect ratio of the molecule changes from its minimum value in the cis position to its maximum in the trans conformation. Intermolecular potentials arise from hard core repulsion and repulsion from a square shoulder potential that operates only when the flexible portions overlap. When this potential is used in an Onsager-density functional theory, extended to account for the torsional degree of freedom, both the shape of the molecule and the order parameter are density and temperature dependent. As the density increases in the isotropic phase, the molecule becomes increasingly spherical. At the isotropic, nematic transition, the molecular shape abruptly elongates and this elongation continues with further increases in density. Both the increased sphericalization and the increased elongation are small effects (1%–2%) even for 5:1 and 10:1 molecules undergoing a 20% length fluctuation. The primary order parameter 〈P2〉 and the relative density change at the isotropic, nematic transition are nearly independent of the amplitude of torsional fluctuations. This study suggests that for estimates of the density changes at the I–N transition, hard body models should be sufficient.
Radiative decay of topologically disordered excitons
Authors: Wang Ningjun , Muenter Annabel A. , Mukamel Shaul
Published: July 2002

We calculate the time resolved fluorescence from a topologically disordered two-dimensional molecular aggregate smaller than an optical wavelength. The photon emission rate is expressed in terms of a configurationally averaged particle-hole Green’s function, calculated using the ladder diagram approximation. The exciton coherence size at a given energy is shown to be equal to the average oscillator strength per state, provided the superradiance decay rate is much smaller than the absorption linewidth. The variation of the coherence size with molecular density and with exciton energy across the band is explored.