bluuebunny/crossref_metadata_2025 huggingface dataset:
Nematics of linear assemblies in two dimensions
Authors: van der Schoot Paul
Published: July 2002

A density functional theory is presented describing the isotropic-nematic transition in dispersions of self-assembled linear objects in two dimensions. It is shown that the transition is second order. The auto-correlation function of order parameter fluctuations is also calculated on both sides of the transition point. The relevant correlation lengths diverge, as expected, at the critical point. The influence of director fluctuations, suppressed in the microscopic description, is discussed.
Dynamics of electrolyte solutions at finite wave vectors: Theoretical results for ions in a molecular solvent
Authors: Mahajan Kanishk , Chandra Amalendu
Published: July 2002

A molecular theory of the dynamics of ions and solvent molecules in electrolyte solutions is presented. The theory properly includes ion–ion, ion–solvent, and solvent–solvent molecular correlations through intra- and interspecies static structure factors and direct correlation functions. Both diffusive and nondiffusive (such as inertial) modes of relaxation of ions and solvent molecules are included in the theory. Explicit results are obtained for the time dependence of ion–ion, ion–solvent, and solvent–solvent van Hove functions at zero and finite wave vectors for solutions of varying ion concentration and dipolar strength. Frequency- and wave vector-dependent dynamic response functions of electrolyte solutions are also calculated by employing linear response theory. It is found that the dynamic response of ions and solvent molecules at finite wave vectors can be very different from that at zero wave vector (or at long wavelength). An application of the theory developed in this work is also discussed, where we have investigated the dynamics of ion solvation in electrolyte solutions by employing the frequency- and wave vector-dependent dynamic response functions.
The ClO4 radical: Experiment versus theory
Authors: Van Huis Timothy J. , Schaefer III Henry F.
Published: July 2002

The chlorine tetroxide radical and its anion have been investigated with unrestricted Hartree–Fock, density functional and hybrid Hartree–Fock/density functional methods, perturbation theory, and coupled cluster methods. It is predicted that the minimum on the ClO4 radical potential energy surface corresponds to the 2B2 ground electronic state, a C2v-symmetry structure. The C3v structure identified experimentally lies a few kcal mol−1 higher energetically. A Cs symmetry structure lies very close energetically to the C2v minimum at some levels of theory. Adiabatic electron affinities, equilibrium geometries, harmonic vibrational frequencies, infrared intensities, isotopic shifts, and dipole moments are presented for all methods. Comparisons with recent experimental vibrational data for the radical are made. It is predicted that the C2v equilibrium geometry of the ground state is Re(Cl–O1)=1.502 Å, Re(Cl–O2)=1.424 Å, θe1=93.6° and θe2=114.2°. The adiabatic electron affinity of the ClO4 radical is predicted to lie near 5.2 eV.
Calculation of magnetic properties. VI. Electron correlated nuclear shielding constants and magnetizabilities for thirteen small molecules
Authors: Cybulski Sl/awomir M. , Bishop David M.
Published: July 2002

The theory of relaxed density matrices has been developed for the calculation of second-order response properties at third-order Mo/ller–Plesset (MP3) and linearized coupled cluster double excitation (L-CCD) levels of theory. The ensuing algorithm is applied to the determination of the isotropic and anisotropic nuclear magnetic shielding constants and magnetizabilities for thirteen molecules (H2, N2, F2, HF, CO, HCN, HNC, H2O, NH3, H2O2, HCHO, CH4, and HCCH). The method uses conventional gauge-variant orbitals but, by using large basis sets, produces results which are equivalent to those found with gauge-including orbitals. In general the L-CCD values of the magnetizabilities are consistent with those obtained with multiconfigurational self-consistent-field (MCSCF) methods for molecules which have been treated by this method. For the nuclear shieldings, when there is a difference between L-CCD and MP3, the former gives results closer to the coupled-cluster singles and doubles level treatment augmented with a perturbation correction for connected triple excitations [CCSD(T)] which is our reference point. Again the results for the shieldings at the L-CCD level are quite good. We also use the paramagnetic components of the shieldings and magnetizabilities to determine the spin-rotation constants and rotational g tensors, respectively. These quantities are important since they may be compared more directly with experiment than the magnetizabilities and shieldings.
The Rydberg spectrum of CaF and BaF: Calculation by R-matrix and generalized quantum defect theory
Authors: Arif M. , Jungen Ch. , Roche A. L.
Published: July 2002

R-matrix theory combined with generalized quantum defect theory is used to calculate the electronic spectrum of the CaF and BaF molecules from the ground state up near the ionization limit. The approach, an effective one-electron method similar in spirit to the ligand-field model of Rice, Martin, and Field [S. F. Rice, H. Martin, and R. W. Field, J. Chem. Phys. 82, 5023 (1985)] and to the electrostatic polarization model of Törring, Ernst, and Kändler [T. Törring, W. E. Ernst, J. Kändler, J. Chem. Phys. 90, 4927 (1989)] removes many of the limitations inherent in the previous work. The resulting level energies (effective principal quantum numbers) are in good agreement with the available experimental data and constitute the first quantitative theoretical calculation of the full electronic spectrum of CaF and BaF. Limitations and possible extensions of the theory are discussed, and quantum defects of high orbital angular momentum states are predicted.
One-dimensional nutation nuclear quadrupole resonance spectroscopy for measurement of the electric field gradient tensor-asymmetry parameter
Authors: Robert H. , Pusiol D.
Published: July 2002

A novel one-dimensional nuclear quadrupole resonance experiment to measure the asymmetry parameter of the electric field gradient tensor in polycrystalline specimens using nutation spectroscopy is reported. The complete data set, necessary for the reconstruction of the powder nutation lineshape, is recorded in a single scan provided by a train of short and intense on-resonance radiofrequency pulses. A modified scheme is presented to deal with multiple well-resolved resonances to provide a frequency selective nutation method. The technique is successfully demonstrated for the two 35Cl resonances in cyanuric chloride. A reduction of 1 or 2 orders of magnitude in the data acquisition time, relative to the usual two-dimensional method, is achieved.
A theoretical study of solid hydrogens doped with atomic oxygen
Authors: Li Zhiming , Apkarian V. A. , Harding Lawrence B.
Published: July 2002

Structure and reaction dynamics in solid H2/D2 doped with O(3P, 1D, 1S) is investigated through simulations based on accurate ab initio potential energy surfaces. The ab initio calculations are performed at MCSCF level, with neglect of spin–orbit interactions. The dynamical simulations rely on nonadditive effective potentials, taking into account the anisotropy of the open shell atom by using diabatic representations for the globally fitted potential energy surfaces of O–H2. The ground state of the doped solid is well described as O(3P) isolated in para-H2(J=0) since the atom–molecule interaction anisotropy is not sufficient to orient H2. O(3P) atoms radially localize the nearest-neighbor shell, and lead to a linear increase in the density of the solid as a function of impurity concentration. The doped solid is stable at cryogenic temperatures, with a free energy barrier for recombination of next nearest-neighbor O(3P) atoms of 120 K. The solid state O(1D)+H2 reaction is considered in some depth. While in high symmetry sites the reaction is forbidden, even at 4 K, thermal fluctuations are sufficient to promote the insertion reaction.
Ab initio heats of formation for chlorinated hydrocarbons: Allyl chloride, cis- and trans-1-chloropropene, and vinyl chloride
Authors: Colegrove Brenda Thies , Thompson Tyler B.
Published: July 2002

Ab initio molecular energies at several levels of theory (MP4/6-311G**//MP2/6-31G*, MP4/6-311+G**//MP2/6-31G*,G1, and G2) are used to determine the heats of formation of several chlorinated hydrocarbons (allyl chloride, cis- and trans-1-chloropropene, and vinyl chloride) from atomization and isodesmic reactions. More than one isodesmic reaction was investigated for each molecule. Inconsistencies between the results from isodesmic reactions for a given molecule indicated possible errors in the experimental heats of formation for some of the chlorinated molecules used as references in the isodesmic reactions (in particular 1-chloropropane and 2-chloropropane). To further examine this possibility we did a multivariate regression for the G2 calculated reaction enthalpies for the 30 isodesmic reactions. In the regression, the heats of formation of the hydrocarbons and CH3Cl were fixed at the experimental values. The heats of formation of all the other chlorinated hydrocarbons were varied. The heats of formation determined using this method were: ΔHf298(CH2Cl2)=−22.6 kcal/mole, ΔHf298(CHCl=Cl2)=5.0 kcal/mole, ΔHf298(CCl2=CH2)=−0.2 kcal/mole, ΔHf298(CH2Cl–CH3)=−27.0 kcal/mole, ΔHf298(c-CHCl=CH–CH3)=−3.1 kcal/mole, ΔHf298(t-CHCl=CH–CH3)=−2.8 kcal/mole, ΔHf298(CH2=CClCH3)=−5.4 kcal/mole, ΔHf298(CH2=CH–CH2Cl)=−0.8 kcal/mole, ΔHf298(CH2Cl–CH2–CH3) =−32.2 kcal/mole, ΔHf298(CH3–CHCl–CH3 )=−35.9 kcal/mole. The calculated heats of formation were used to derive the following Benson group enthalpy values: C–(Cl)(H)2(Cd)=−15.6 kcal/mole, Cd–(Cl)(H)=−1.3 kcal/mole, and cis-halogen-alkyl =−0.3 kcal/mole.
Covariance images of the primary response from rare gas cluster ions to photoexcitation
Authors: Jukes P. , Buxey A. , Jones A. B. , Stace A.
Published: July 2002

The photoexcitation and fragmentation of rare gas cluster ions can yield large numbers of neutral products which, in turn, exhibit considerable variation in their kinetic energies. In order to interpret such events, a coincidence technique has been used to correlate the arrival times of neutral photofragments at a detector following the photoexcitation of Arn+ and Krn+, for n⩽10. By collecting data from approximately 105 photodissociation events for each type of cluster ion, covariance images have been derived which clearly demonstrate that the initial response to photoexcitation, is the ejection of a single rare gas atom which carries with it between 30% and 60% of the excess energy.
Hyperfine and nuclear quadrupole coupling in chlorine and fluorine dioxides
Authors: Fernández Berta , Christiansen Ove , Jo/rgensen Poul , Byberg Jo/rgen , Gauss Jürgen , Ruud Kenneth
Published: July 2002

The hyperfine and nuclear quadrupole coupling tensors have been calculated for the two chlorine dioxide isomers OClO and ClOO and for fluorine dioxide FOO. The coupled-cluster singles and doubles (CCSD) approach with a perturbative treatment of triple excitations [CCSD(T)] has been used and basis saturation has been investigated. For the symmetric isomer OClO close agreement is obtained with the accurate and detailed experimental data. For FOO a geometry optimization as well as a comparison of calculated and experimental hyperfine coupling tensors suggest a shorter F–O bond length than that obtained experimentally. For the isomer ClOO, calculations have been carried out at the theoretical equilibrium geometry determined by Peterson and Werner and at the geometry proposed by Byberg for the matrix isolated molecule. The hyperfine coupling tensors obtained at these two geometries are substantially different, but the estimated accuracy of the calculations is not high enough to allow a determination of the geometry of ClOO from the hyperfine data.
Avoiding long propagation times in wave packet calculations on scattering with resonances: A new algorithm involving filter diagonalization
Authors: Kroes Geert-Jan , Wall Michael R. , Pang John W. , Neuhauser Daniel
Published: July 2002

We present a new and more efficient implementation of a hybrid approach to computing the solution of scattering problems affected by resonances. In the computationally expensive part of the calculation, wave packet propagation is used to obtain the time-dependent wave function Ψ(t) up to some time τ at which direct scattering is over. This part is made efficient by using a recently introduced modification for the absorbing boundary conditions evolution operator which allows the use of real operator algebra if the initial wave function is chosen real. In the second part of the calculation, filter diagonalization is used to efficiently obtain the energies, widths, and expansion coefficients of resonances needed to describe the long time behavior of the scattering wave function. This part is made efficient by using a recently introduced algorithm which avoids the storage of energy-dependent basis functions. We demonstrate the application of the method to a two-dimensional reactive scattering problem.
Monte Carlo simulation of the solid to superliquid phase transition of Langmuir monolayers. II. Characteristics of phase transition
Authors: Gibson M. D. , Swanson D. R. , Eckhardt C. J. , Zeng X. C.
Published: July 2002

The restricted-to-free rotator phase transition of fatty acid monolayers has been modeled using a potential which represents the amphiphiles as planar cross sections of fourfold symmetry. Using much larger system sizes than paper I [D. R. Swanson, R. J. Hardy, and C. J. Eckhardt, J. Chem. Phys. 99, 8194 (1993)], Monte Carlo simulations of the isobaric–isothermal ensemble of model systems with varied number of objects were undertaken to study the effect of system size on the characteristics and order of the phase transition. A peak in the specific heat vs temperature curve was observed near the transition. For each system size, the maxima of the peak showed a linear dependence on the area, which is characteristic of a first order transition. The latter is further confirmed from the observation of a small “van der Waals’’ loop by increasing the volume at a constant temperature. The result of this simulation matches the qualitative behavior of experimental observations of monolayers, which undergo a weakly first order transition. Additional similarity with experiment was found by the determination that the simulated superfluid phase has the short range translational order and quasilong range bond orientational order characteristic of a hexatic phase.
The vibrational spectra of molecular ions isolated in solid neon. XIV. CH3F+, CH3Cl+, CH3Br+, and their ylidion isomers
Authors: Lugez Catherine L. , Forney Daniel , Jacox Marilyn E. , Irikura Karl K.
Published: July 2002

When a Ne:CH3X (X=F, Cl, Br) sample is codeposited at approximately 5 K with neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit includes prominent absorptions which can be assigned to cation products. It has previously been established that the ylidion (H2CXH+) isomers have stable potential minima and that H2CFH+ is lower in energy than CH3F+. The identification of the new absorptions is aided by experimental studies on isotopically substituted Ne:CH3X samples and by ab initio calculations of the structures and vibrational fundamentals of both CH3X+ and H2CXH+. In each of the three systems, a prominent absorption which is intermediate in frequency between the gas-phase (and neon-matrix) absorptions of HX and HX+ can be assigned to the ylidion, as can be several other absorptions. Still other absorptions can be assigned to CH3Cl+ and CH3Br+. The behavior of the product absorptions on exposure of the deposit to filtered visible and ultraviolet radiation is consistent with the proposed assignments.
Polarization of emission in asymmetric rotors. I. The effects of elastic collisions, electron and nuclear spins
Authors: Truhins Kaspars , Al Wahabi Zeyad T. , Auzinsh Marcis , McCaffery Anthony J. , Rawi Zaid
Published: July 2002

We report measurements of the linear and circular polarization ratios as a function of rotational state for the asymmetric rotor NH2. This molecule displays fine structure splitting from its unpaired electron and hyperfine structure from coupling with the nuclear spins. We present a theory of polarized emission for this molecule which includes the effects of fine and hyperfine interactions. These have a marked effect on the polarization ratios and are well described by a theory in which the effect of electron and nuclear spin are introduced as time-independent perturbation coefficients. We find that theory predicts different values of polarization ratio according to the manner of coupling of the proton nuclear spins. The best fits to experimental data are obtained when the coupling follows a physically intuitive scheme rather than that usually adopted. When all intramolecular couplings due to electron and nuclear spins are properly accounted for; there is no depolarization that may be attributed to the effect of elastic collisions. Thus, as in the case of diatomic molecules, orientation and alignment show a marked stability to change by collision.
Dipole polarizabilities of Na and long-range coefficients for various molecular states of Na2
Authors: Mérawa Mohammadou , Rérat Michel
Published: July 2002

Dynamic polarizabilities are calculated for Na in its 3s2S, 3p 2P and 4s 2S states with a time-dependent gauge-invariant (TDGI) method. Coulombic long-range interactions are deduced for various states of Na2 and compared to previous calculated and experimental results.
Collapse of a ring polymer: Comparison of Monte Carlo and Born–Green–Yvon integral equation results
Authors: Taylor Mark P. , Mar James L. , Lipson J. E. G.
Published: July 2002

The equilibrium properties of an isolated ring polymer are studied using a Born–Green–Yvon (BGY) integral equation and Monte Carlo simulation. The model polymer is composed of n identical spherical interaction sites connected by universal joints of bond length σ. In particular, we study rings composed of up to n=400 square-well spheres with hard-core diameter σ and well diameter λσ (1⩽λ⩽2). Intramolecular site–site distribution functions and the resulting configurational and energetic properties are computed over a wide range of temperatures for the case of λ=1.5. In the high temperature (good solvent) limit this model is identical to a tangent-hard-sphere ring. With decreasing temperature (worsening solvent) both the radius of gyration and the internal energy of the ring polymer decrease, and a collapse transition is signaled by a peak in the single ring specific heat. In comparison with the Monte Carlo calculations, the BGY theory yields quantitative to semiquantitative results for T≳Tθ and is qualitatively accurate for T≲Tθ, where Tθ is the theta temperature. The thermal behavior of an isolated square-well ring is found to be quite similar to the behavior of an isolated square-well chain. The BGY theory indicates that rings and chains have comparable theta and collapse transition temperatures. In the low temperature limit (collapsed state) the microscopic structure of rings and chains becomes nearly identical.
Diffusion influenced reversible transfer of electronic excitation energy in liquid solution by long-range interaction
Authors: Bandyopadhyay T.
Published: July 2002

An approach to donor–acceptor kinetics in the presence of diffusion and reversible energy transfer due to microscopic interaction is described. A set of coupled dynamic equations for the reduced distribution functions of reactant molecules in forward and reverse steps is presented on the basis of a hierarchical system of many-body Smoluchowski equations. With a view toward treating long-range dipolar interaction mediated energy transfer kinetics, this formalism is generalized to incorporate unimolecular decay pathways. Numerical calculations of this set of equations for the excited donor decay after a delta pulse show the dependence of the result on acceptor concentration, forward and back transfer distance-dependent rates, donor and acceptor lifetime, and on their diffusive motion. Comparison with the corresponding forward and irreversible reactive dynamics provides detailed insights into the time dependence of flow of electronic excitation in donors and acceptors. The irreversible Smoluchowski limit is only recovered when both the forward and the reverse reactive systems are undergoing fast molecular motion (slow reaction). In the donor–donor case the temporal profile of the excited donor decay and the evolution of ground state donor (that receives energy from the initially excited donor) is presented, along with the total excitation that resides in these two kind of donors. The total excitation in this situation is found to follow a monotonic exponential decay with its characteristic natural lifetime, as expected in a lifetime measurement experiment.
High resolution laser spectroscopy of the Cs2 C 1Πu state: Perturbation and predissociation
Authors: Kasahara Shunji , Hasui Yasushi , Otsuka Kyoko , Baba Masaaki , Demtröder Wolfgang , Katô Hajime
Published: July 2002

High resolution spectra of the C 1Πu(v′,J′)−X 1Σg+(v″,J″) transitions of the Cs2 molecule were measured by the technique of Doppler-free optical–optical double resonance polarization spectroscopy. A number of C 1Πu(v⩾13,J) levels were found to be strongly perturbed. Transitions to the perturbing levels were observed, and the energy shifts of e and f levels of the C 1Πu state were studied and identified as originating from perturbations between the C 1Πu and (2)3Π1u states. Molecular constants of the C 1Πu(v=7–18) and (2)3Π1u(v=v0–v0+6) levels were determined. Line splittings were observed for the transitions to the C 1Πu(v=13 and 14,Jf) levels which were identified as hyperfine splittings originating from additional interaction with the (2)3Σu+ state. Remarkable line broadenings were observed in the transitions to the C 1Πu(v>15,Je and f) levels, and were identified as originating from the predissociation through the dissociative (2)3Σu+ state. The repulsive potential curve of the (2)3Σu+ state was estimated to cross that of the C 1Πu state around the outer turning point of the C 1Πu(v=16 or 17) level.
Oriented molecule photoelectron angular distributions in the vicinity of a photoionization shape resonance: Continuum multiple scattering-Xα calculations for valence ionization of CF3Cl
Authors: Powis Ivan
Published: July 2002

The potential use of fixed molecule photoelectron angular distributions (PADs) as a detailed probe of photoionization dynamics is explored using Continuum Multiple Scattering-Xα calculations for the molecule CF3Cl. A number of dynamical phenomena (various shape resonances and a Cooper minimum) encountered in the vuv photoionization of CF3Cl up to 70 eV are examined. Cross-sections and asymmetry parameters are also reported and the nature of the low energy shape resonances is examined using eigenchannel plots. These show a clear correspondence between the present scattering calculations and the alternative description of shape resonances as antibonding virtual valence orbitals. PADs from fixed in space molecules are predicted to show a pronounced orientation, favoring electron emission from one end of the molecule. After passing through a shape resonance this orientation can flip direction as a consequence of resonant phase shifts in the interfering partial waves. Resonant electron-ion interactions can also result in greater alignment of the electron’s orbital angular momentum with the molecular axis; the corollary is that the PAD becomes less strongly aligned or oriented along this direction at the resonance energy.
Vibrational spectroscopy of CH/NH stretches in pyrrole: An algebraic approach
Authors: Lubich L. , Oss S.
Published: July 2002

The complete vibrational spectroscopy of pyrrole is addressed within the one-dimensional algebraic framework. Particular attention is devoted to the study of the CH/NH stretching levels, including their possible anharmonic resonance mechanisms with other ring modes.
Angle-energy distributions of Penning ions in crossed molecular beams. III. Angle-energy coupling in He*(2 1S,2 3S)+Ar→He+Ar++e−
Authors: Longley E. J. , Siska P. E.
Published: July 2002

Relative doubly differential cross sections for the Penning ionization of Ar by spin-state-selected metastable He (1s2s) are reported at four center-of-mass collision energies covering the range E=1.8–6.3 kcal/mol in a crossed supersonic beam experiment employing a rotatable mass spectrometer detector. The measurements are sufficiently dense in velocity space as to avoid having to functionalize the differential cross sections in order to transform the intensities into the c.m. The Ar+ product is scattered sharply forward with respect to the incident direction of Ar at all energies for both spin states, increasingly so at higher E, but with a persistent backward-hemisphere component. On the average the products have lost energy upon recoil, mean recoil energy 〈E〉<E, except for 1S at high E, but E is found to depend strongly on c.m. recoil angle θ, with forward scattering yielding 〈E(θ)〉≈E and sideways up to 50% energy loss. Extensive comparison is made with predictions based on previously determined He*+Ar optical potentials and spectroscopic ion exit channel potentials. Agreement is generally good for both spin states, except for a significant discrepancy in backscattered intensity for 1S that grows from a factor of 2–6 from lowest to highest E, theory falling below experiment. An alteration to the repulsive part of the 1S potential suggested by previous work fails to improve the comparison, but does show that the angle-energy coupling is uniquely sensitive to the form of the repulsion. The results appear to indicate nonexponential behavior of the discrete-continuum coupling (imaginary part of the optical potential) at short range.
Phase coexistence in clusters: An “experimental” isobar and an elementary model
Authors: Vekhter Benjamin , Berry R. Stephen
Published: July 2002

This work examines the temperature dependence of the coexistence ratios of phaselike forms of atomic clusters of two kinds, a homogeneous cluster bound by pairwise Lennard–Jones forces that simulates Ar55, and a binary, ionic cluster simulating (KCl)32. Two methods have been used: isothermal molecular dynamics and a simple analytical model based on highly simplified densities of states. The phase behavior of the alkali halide cluster is well represented by a two-level density of states, a nondegenerate lower level, and a highly degenerate upper level. The phase behavior of the argon cluster needs an intermediate level of moderate degeneracy to reproduce the three-phase behavior of the simulations.
Ultrafast dielectric relaxation response of polar liquids
Authors: del Castillo L. F. , Dávalos-Orozco L. A. , Garcı́a-Colı́n L. S.
Published: July 2002

In this paper we calculate the complex dielectric constant of polar liquids for microwave and far-infrared range of frequencies. We use the formalism of extended irreversible thermodynamics (EIT) to obtain the autocorrelation function of the polarization vector. To this end, we introduce the local variation of the polarization flow as an independent variable, which allows for the description of collective interacting effects into the formalism. The results obtained here are compared with previous work in the so-called three-variable theory based on the Mori-Zwanzig projection operator technique. This comparison provides an interpretation of EIT in terms of a microscopic theory from which the role of the new independent variable can be established. Using the result of the complex dielectric constant presented here, we show that the fitting of experimental data in polar fluids can be improved in the far infrared frequencies.
A slow-turning method for measuring large anisotropic interactions in inhomogeneously broadened nuclear magnetic resonance spectra
Authors: Hill Edward A. , Yesinowski James P.
Published: July 2002

A new NMR method is proposed for measuring anisotropic spin-Hamiltonian parameters, STEAMER (slow turning echo amplitude modulation and echo reduction). It involves slow turning of a powdered sample about an axis perpendicular to the external magnetic field. We have theoretically analyzed and experimentally demonstrated this technique for the specific case of an axially symmetric second-rank tensor interaction. The method relies upon a slow continuous rotation of the sample that changes the orientation of the principal-axis systems of interaction tensors with respect to the external magnetic field and renders the spin Hamiltonian time dependent. As a consequence a conventional Hahn spin-echo pulse sequence yields imperfect refocusing and altered echo amplitudes. Two principal advantages of STEAMER are that only a small portion of the entire powder pattern need be observed at a single frequency and that anisotropic interactions can be distinguished from distributions of isotropic interactions. The N14 (I=1) selectively excited π/2-π spin-echo NMR signal in KNO3 provides a good test case for the effects of slow turning (rotation at 1 rpm) since it has a first-order quadrupole interaction from a N14 nuclear quadrupole coupling constant (NQCC) of 0.75 MHz and a small asymmetry parameter (0.022). Theoretical simulations of the echo amplitude as a function of the pulse interval for an axially symmetric second-rank tensor agree well with the experimental results at a chosen frequency. These numerical simulations are compared to an analytical approximation derived in terms of the zeroth-order Bessel function. In addition, a simple graphical method for obtaining NQCC values from STEAMER data is developed. Other aspects and possible extensions of the STEAMER experiment are discussed.
Photodissociation spectroscopy of CaCH4+
Authors: Chen J. , Cheng Y. C. , Kleiber P. D.
Published: July 2002

We report on studies of the structure and dissociation of CaCH4+ and its isotopomer CaCD4+, using photodissociation spectroscopy. Molecular absorption bands are observed to the red of the Ca+ (4p 2P←4s 2S) resonance transition. The photodissociation action spectrum shows evidence for spin–orbit doubling and complex rovibrational structure. No reactive quenching product is observed. The transition is assigned as 2E←2A1 in C3v geometry. The spin–orbit constant in the upper state is found to be Aso=111±4 cm−1. The action spectrum shows a strong progression in the Ca+–CH4 (CD4) intermolecular stretch in the 2E state with a fundamental stretch frequency of ωs′=270±16(244±4) cm−1. Weaker intermolecular bending vibration is also apparent, with a fundamental bending frequency of ωb′=112±16(92±14) cm−1 for Ca+–CH4(CD4). The resonances show evidence for predissociation broadening. These results differ markedly from our previous results on the analogous MgCH4+ system.
Ab initio direct dynamics study of OH+HCl→Cl+H2O
Authors: Steckler Rozeanne , Thurman Glenn M. , Watts John D. , Bartlett Rodney J.
Published: July 2002

We report the first-principles computation of rate constants for atmospheric reactions, by combining variational transition state theory (VTST) and high-level electronic structure theory. Extending the direct dynamics approach, the rate constants for OH+HCl were computed, directly, using ab initio electronic structure theory at the second-order many-body perturbation theory [MBPT(2)] and coupled-cluster singles-and-doubles with a perturbative triples correction [CCSD(T)] levels and variational transition state theory including tunneling. The computed room-temperature rate constant, 7.03×10−13 cm3 molecule−1 s−1, is in good agreement with experiment (8.0×10−13). The rate constant was computed over the temperature range 138–1055 K. The methods developed in this work are quite general, and can be applied to obtain rate constants for many reactions of interest in atmospheric and combustion chemistry, in the absence of experiment.
Response to “Comment on ‘Relaxation of collective excitations in LJ-13 cluster’’[J. Chem. Phys. 106, 5296 (1997)]
Authors: Salian Umesh A. , Behera S. N. , Ramamurthy V. S.
Published: July 2002

The qualitative change in response (at a kinetic temperature of about 7.0 K) of the 13 atom Lennard-Jones cluster to monopole excitation is discussed. It is argued that the phenomenon is neither due to a classical-potential-barrier-crossing process nor due to the breakdown of the harmonic regime.
Ab initio study of NO2. VI. Vibrational and vibronic coupling in the X̃2A1/Ã2B2 conical intersection up to 16 000 cm−1
Authors: Leonardi Erminia , Petrongolo Carlo
Published: July 2002

Following our previous work [J. Chem. Phys. 105, 9051 (1996)], we have investigated the vibrational and vibronic mixings in the X̃2A1/Ã2B2 conical intersection spectrum of NO2. By analyzing the expansion coefficients of the nonadiabatic states, we have discussed the X̃2A1 vibrational resonances above 4600 cm−1 and the interplay among the X̃2A1 vibrational and X̃2A1/Ã2B2 vibronic couplings in the energy range 8900–12 000 cm−1. The assignment and the vibrational mixing of the nonadiabatic bands have been studied by comparing two-dimensional contour plots of Born–Oppenheimer and of generalized vibrational functions. A Dunham fit to the Ã2B2 vibrational bands has extended our previous assignment and has shown that Fermi, Darling–Dennison, and two new Ã2B2 vibrational resonances contribute to several nonadiabatic states above 12 000 cm−1. We have finally reported the Ã2B2 vibrational bands up to 16 000 cm−1 and the corresponding clusters of nonadiabatic levels, which have been compared with some experimental results.
Geometric and electronic properties of small vanadium clusters: A density functional study
Authors: Grönbeck Henrik , Rosén Arne
Published: July 2002

The geometric and electronic properties of vanadium clusters in the range from V2 to V8 have been investigated using the density functional theory, and an LCAO approach for the expansion of the electronic wavefunctions. The optimized low energy isomers are found to be three dimensional for clusters larger than the tetramer, and the evaluated bond dissociation energies, ionization potentials and electron affinities are in good agreement with experimental results. All cluster sizes are found to possess low magnetic moments as ground states, which is in contrast with previous suggestions. In the case of V3− and V4−, a comparison with photo-electron spectra is done by computing the self consistent excitation spectra.
Free jet absorption millimeter wave spectrum and van der Waals potential energy surface of the 2,3-dihydrofuran-argon adduct
Authors: Maris Assimo , Melandri Sonia , Caminati Walther , Favero Paolo G. , Makarewicz Jan
Published: July 2002

The free jet millimeter wave spectrum of the 2,3-dihydrofuran-argon molecular complex was investigated in the 60–78 GHz frequency range. Only one of the two possible conformers due to the puckered configuration of isolated 2,3-dihydrofuran was observed. Estimates of the geometry and of the van der Waals interaction potential, and their relation to the ring puckering motion, are given.
High-order commensurate structures of CF4 on Cu(110) from interaction potential calculations
Authors: Marmier A. , Hoang P. N. M. , Ramseyer C. , Girardet C. , Diercks V. , Zeppenfeld P.
Published: July 2002

Potential energy calculations are performed in order to interpret the high-order commensurate (13×18) and (2×18) structures of the CF4 layer adsorbed on Cu(110) determined from helium diffraction experiments. We find that the most stable geometry at 0 K is a low order commensurate (2×2) phase containing two CF4 molecules which are dipod oriented in the Cu troughs. However, several other configurations close to this stable structure with tilted dipod and tripod orientations yield total binding energies per molecule which are only 10 meV (i.e., less than 5%) weaker. Due to the lack of additional information on the potential accuracy, we find that the minimization procedure has difficulty discriminating unequivocally between structures with a large number of molecules per unit cell and for which changes in molecular orientations do not sensitively modify the total energy in the cell. In order to recover the experimental structure we propose a (2×18) phase with 18 CF4 molecules per unit cell, deduced from the optimization calculation and leading to an electronic corrugation above the surface which is compatible with the measured helium diffraction profile.
Circumventing the pathological behavior of path-integral Monte Carlo for systems with Coulomb potentials
Authors: Müser M. H. , Berne B. J.
Published: July 2002

The imaginary-time discretized path integral for systems interacting through Coulomb potentials and/or bounded by hard walls behaves pathologically. In this paper we give an analysis of the pathologies. A new method is used to calculate the path integral for the Coulomb system within the primitive algorithm. This is done by constructing an effective potential that implicitly depends on Trotter number P. The procedure makes possible the treatment of Coulombic potentials by means of Monte Carlo at surprisingly low Trotter numbers and without the need of computing three-body potentials, necessary when high-order approximants for the density matrix are employed.
Yield determination of OH(v=0,1) radicals produced by the electron-ion recombination of H3O+ ions
Authors: Gougousi Theodosia , Johnsen Rainer , Golde Michael F.
Published: July 2002

A flowing afterglow apparatus in conjunction with laser induced fluorescence (LIF) diagnostics has been used to determine the yield of OH(v=0,1) produced by the dissociative recombination (DR) of H3O+ ions with electrons at 300 K. The yield for v=0 radicals (0.48 ± 0.07) was determined by two different methods: (1) by comparing it to the known OH yield of the ion-molecule reaction Ar++H2O and (2) by comparing it to that of the reaction of metastable Ar atoms (Ar*) with H2O. The yield of vibrationally excited OH(v=1) (0.12± 0.02) was obtained relative to that in v=0 by comparing LIF spectra. The results corroborate earlier experimental work which determined the yields of OH in v=0 and in unspecified vibrationally excited states v>0.
The NO(a 4Π) state: Spectroscopy of the a 4Πi–X 2Πr 11–0 and 14–0 bands
Authors: Cosby Philip C. , Copeland Richard A. , Williamson David G. , Gaudin Gilles , Dyer Mark J. , Huestis David L. , Slanger Tom G.
Published: July 2002

Improved spectroscopic measurements have been made on the NO(a 4Πi–X 2Πr) 11–0 band, and new measurements on the 14–0 band, bringing the explored region of the a 4Π state to within 0.3 eV of the NO dissociation limit. Doubling and mixing of dye laser radiation is used to access the 200–210 nm region with an ultraviolet linewidth of 0.15 cm−1. The a 4Π molecules collisionally convert their energy to the B 2Π state, from which radiation is detected. Almost 300 lines have been observed in each of the two bands, and accurate spectroscopic constants are presented. New information is provided on the relative positioning of perturbed levels in the a 4Π and B 2Π states.
Instability scenarios for doped He4 clusters
Authors: Gatica S. M. , Hernández E. S. , Barranco M.
Published: July 2002

In the frame of finite range density functional (FRDF) theory, we review the systematics of energetic features of HeN4 clusters doped with atomic or molecular impurities, as well as the instability scenario of their collective motions. It is shown that for drops up to N=500, the predictions of FRDF theory are coincident with those arising from microscopic many-body calculations that employ variational or diffusion Monte Carlo methods. For larger drops, the present description predicts a smooth approach to zero of the collective energies, a scenario that appears to support the most recent experimental data and microscopic calculations that demonstrate that the impurity is located within the bulk of the drop.
Agmon–Hopfield kinetics in the slow diffusion regime
Authors: Pechukas Philip , Ankerhold Joachim
Published: July 2002

“Agmon–Hopfield kinetics” refers to a first-order process whose rate depends on a control variable that changes with time by diffusion. We analyze the transition from short-time behavior, accurately described by the static approximation D=0, to long-time behavior, determined by competition between diffusion and the slow tail of the rate function. The analysis is asymptotic in D, increasing in accuracy as D→0.
Theoretical study of the dipole-bound anion (HF)2−
Authors: Gutowski Maciej , Skurski Piotr
Published: July 2002

It has long been assumed that dipole-bound anionic states possess two properties; (i) the electron binding energy is dominated by the electrostatic electron–dipole interaction, and (ii) the geometrical distortion of the polar molecule upon attachment of a distant electron is negligible. Our results indicate, however, that the dispersion interaction between the loosely bound electron and the electrons of the neutral dimer is as important as the electrostatic electron–dipole stabilization. In addition, the hydrogen bond in (HF)2 is susceptible to a deformation upon attachment of a distant electron. This deformation enhances both the electrostatic and dispersion components of the electron binding energy. The calculated Franck–Condon factors indicate that neutral dimers formed in electron photodetachment experiments may be vibrationally excited in both stiff intramolecular HF stretching modes and soft intermolecular modes. The predicted value of the adiabatic electron detachment energy for (HF)2− is 396 cm−1 and the theoretical photoelectron spectrum is in remarkable agreement with the experimental data of Bowen et al. J. H. Hendricks, H. L. de Clercq, S. A. Lyapustina, and K. H. Bowen, Jr. [J. Chem. Phys. 107, 2962 (1997)].
Waterlike anomalies from repulsive interactions
Authors: Speedy Robin J.
Published: July 2002

In tetravalent network materials like quartz or ice the main interactions are the strong bonds that connect each vertex to four others and the repulsive forces that keep nonbonded vertices further apart. Computer simulations, in which those interactions are modeled by square wells, show that they are sufficient to account for the following behavior: The stability of icelike crystals, with tetrahedral bond angles, at low temperature; endothermic melting of the crystal to a denser waterlike amorph at intermediate temperatures; exothermic collapse of the supercooled crystals at high pressure, to a glass, without breaking any bonds; expansion on cooling of both the crystal and the amorph; and a line of spinodal instabilities in the waterlike supercooled amorph where the compressibility, heat capacity, and negative expansivity diverge. The spinodal is associated with correlated low-density, low-energy clusters, characterized by an excess of near tetrahedral bond angles.
Polarization and probe delay effect on degenerate four wave mixing of pyrazine
Authors: Li Hongzhi , Kong Wei
Published: July 2002

Degenerate four wave mixing (DFWM) in supersonically cooled pyrazine was investigated using different polarization combinations of the three input beams. A common feature of these spectra was the strong Q branch, stronger than the corresponding spectra for a single photon process. The ratios for all the rotational branches demonstrated strong dependence on the polarization combination. The YYXX combination showed similar intensities for the P and R branches, while the YXYX combination had a much stronger R branch. The Q branch, relative to the P branch, was the strongest for the YYYY combination. All experiments probed for the same electronic transition with the same Hönl–London factor; therefore the variation in the rotational branching ratios was solely an effect of the polarization combinations. This polarization dependence of DFWM can be traced back to the selectivity in the magnetic quantum number for this multiphoton process. Based on the theoretical framework by Williams et al. [J. Chem. Phys. 101, 1072 (1994)], these spectra were simulated successfully. The calculations and analysis of the experimental conditions revealed that among the twelve gratings included in the original theory, only three of them had observable contributions to the experimental spectra, i.e., the gratings formed through the ground-state molecules via the two forward beams. The other nine gratings either were washed out or decayed in the collision free environment (supersonic expansion) under an excitation pulse of 7 ns. This argument was further confirmed by another experiment using a time delay between the backward probe beam and the two forward beams. The spectra recorded with a 15 ns delay in the probe beam (longer than the pulse duration of each input beam) were similar to those without delays. These results implied that (1) the backward beam did not participate in the grating formation process even when it arrived at the interaction region simultaneously with the two forward beams. (2) Relaxation from any of the three contributing gratings was not observable. (3) The fast intersystem crossing, known to exist in pyrazine, had no effect on the grating formation and relaxation processes. Although complex in nature, degenerate four wave mixing offers versatility in experimental arrangements, providing both dynamics and spectroscopy information.
Cluster isolated chemical reactions: Reactivity of Ba atoms and small Ba clusters with SF6 and CO2 molecules on large argon clusters
Authors: Gée C. , Gaveau M. A. , Sublemontier O. , Mestdagh J. M. , Visticot J.-P.
Published: July 2002

The cluster isolated chemical reaction technique was used to investigate the reactivity of the Ba/CO2 and Ba/SF6 systems in the environment of Ar≈6000 clusters. The method was extended to document several aspects of the reactivity. Notably, mass spectrometry gives insight into the full reactivity of the system deposited on the clusters. Laser induced fluorescence (LIF) and chemiluminescence are also used as detection tools. Unexpectedly, we found that a single barium atom neither reacts with CO2 nor with SF6 at the cluster temperature (32 K). In contrast, the LIF results suggest the formation of a weakly bound covalent Ba…CO2 complex. Finally, Ba2 and larger barium aggregates react with CO2, and Ba3 and larger aggregates react with SF6. The chemiluminescent products are Ba2O in the first case, and BaF in the second. These observations are rationalized on the ground of the harpoon model.
Comment on “Experimental determination of a longitudinal phonon dispersion curve in a quasi-two-dimensional system” [J. Chem. Phys. 106, 6194 (1997)]
Authors: Bruch L. W.
Published: July 2002

The isothermal compressibility of a monolayer solid of Xe/Cu(001) is estimated from structural data of Graham et al. [J. Chem. Phys. 106, 6194 (1997)] and is in the range inferred for three other xenon monolayer solids. This analysis does not support an assignment of the anomalous excitation observed by Graham et al. to be the longitudinal acoustic branch of the xenon solid.
Monte Carlo studies of effects of substrate size on water–substrate interaction energy and water structure
Authors: Fang J. X. , Marlow W. H. , Lu J. X. , Lucchese R. R.
Published: July 2002

Water on a planar substrate and on spherical substrates of radii r=10, 15, and 20 Å are studied at temperatures of 200, 240, 270, and 290 K by Monte Carlo simulations. The average water–water potential energies and the average water–substrate interaction energies are obtained by use of the single point charge model for the water–water interaction and the nonsingular van der Waals potential plus the Born-type repulsive potential for the water–substrate interaction. Water structure and water–substrate interaction energy are shown to be sensitive to substrate size.