bluuebunny/crossref_metadata_2025 huggingface dataset:
Strongly bound excited states of heteronuclear diatomics: BeC
Authors: Wright James S. , Kolbuszewski Marcin
Published: July 2002

The theoretical study of strongly bound doubly excited electronic states, discussed by the authors for a series of homonuclear diatomic molecules, is extended to the case of heteronuclear diatomics. Using BeC as a prototype, it is shown that the significant bond shortening and strengthening shown in the isoelectronic B2 also persists in BeC, in spite of the much greater configuration mixing in BeC due to the lower symmetry. In particular, excitations of the form σ→π cause a predictable increase in (local) bond strength. Potential curves and spectroscopic constants are given for the low-lying states correlating up to the sixth dissociation channel. Compared to the ground state X 3Σ− with Re=3.20 bohr and ωe=905 cm−1, the doubly excited state 3 1Σ+ has a local minimum at Re=2.87 bohr where ωe=1294 cm−1, a significant bond strengthening comparable to the homonuclear diatomics. The contribution to the total (CI) wave function for each state is analyzed in terms of the dominant contributions from the important configurations. SCF potential curves for the pure configurations and MO contour maps are used to understand the relationship with the homonuclear case. Vertical transition moments for the most important transitions are given to help in identification of the (as yet unobserved) electronic absorption spectrum.
Angular momentum reorientation in CO(A 1Π)–He rotational energy transfer studied by optical–optical double resonance multiphoton ionization spectroscopy
Authors: Sha Guohe , Sun Weizhong , Jiang Bo , Hintsa Eric , Zhang Cunhao
Published: July 2002

An optical–optical double resonance multiphoton ionization (OODR-MPI) technique has been developed for measuring the angular momentum reorientation in CO(A 1Π)–He inelastic thermal collisions. In this scheme, two-photon pumping of CO(A 1Π) by using a circularly polarized laser creates a highly anisotropic oriented angular momentum distribution in CO(A 1Π). A second counterpropagating circularly polarized laser probes the oriented CO(A 1Π) via 1+1 photon resonance ionization. A depolarization factor (D), which can be calculated from the measured intensity ratios between R and P branches in the OODR-MPI spectrum, is introduced to characterize the amount of collisional reorientation in the J→J′ rotational energy transfer (RET). This method has the advantage of both high sensitivity and simplicity in data processing. The experimental results show the propensities that the depolarization factor D increases with ΔJ but decreases with increasing initial J. The D’s are asymmetric with respect to ±ΔJ. For comparison, theoretical D’s have been computed by an irreducible tensor formalism under the infinite-order-sudden approximation by Alexander and Davis [J. Chem. Phys. 78, 6754 (1983)] which is further simplified by using an exponential energy gap term to represent the dynamic effects. The computed curves of D vs ΔJ/J reproduce the experimental propensities. The best fit between theory and experiment is obtained with an average impact parameter b=2.9 Å, corresponding to a cross section of 26.4 Å2, which is close to the experimental total cross section of CO(A 1Π)–He rotational energy transfer (∼28 Å2).
Inter-ring exchange interactions in the excited electronic state of bridged diaryl compounds: Exciton splitting in 9,10-dihydroanthracene and dibenzosuberane
Authors: Zgierski Marek Z. , Chakraborty Tapas , Lim E. C.
Published: July 2002

The fluorescence excitation spectra of jet-cooled 9,10-dihydroanthracene and dibenzosuberane, in the region of their lowest-energy absorption band, exhibit two close-lying electronic origins that can be identified as the two exciton components of the lowest-energy benzene transition. The measured band positions and intensities of the two exciton components are in good agreement with the results of CNDO/S+CIS calculations based on the optimized QCFF/PI+CIS ground-state geometry.
Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid nitrogen/carbon monoxide mixtures
Authors: Schmidt S. C. , Moore D. S. , Shaw M. S. , Johnson J. D.
Published: July 2002

Vibrational spectra of liquid nitrogen/carbon monoxide mixtures, shock compressed to several high-pressure/high-temperature states, were obtained using single-pulse multiplex coherent anti-Stokes Raman scattering (CARS). The experimental spectra were compared to synthetic spectra calculated with a semiclassical model for CARS intensities and using best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths. Up to a maximum shock pressure of 9.3 GPa, both the N2 and CO vibrational frequencies were found to increase monotonically with pressure but depended strongly on the nitrogen/carbon monoxide mixture ratio. An empirical fit of the Raman frequency shifts incorporating previously published neat nitrogen and carbon monoxide data, using a functional form dependent on pressure, temperature, and mixture ratio, accurately describes both the N2 and CO shifts. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures and the vibrational temperatures obtained were used to improve the potential function used to calculate equation-of-state pressures and temperatures. The measured linewidths were different for N2 and CO and suggest that the vibrational dephasing time decreased to a few ps at the highest pressure shock state.
The electric field within a nematic droplet: Molecular orientation and dielectric properties
Authors: Bucci Pietro , Golemme Attilio
Published: July 2002

The electric field present in an inisotropic medium confined in a spherical cavity within an isotropic medium has been calculated. We considered a nematic liquid crystal with a cylindrically symmetrical director configuration with the symmetry axis along the direction of an applied a.c. electric field. The mesophase and the surrounding medium are considered as having different conductivities and dielectric constants. A lattice of points is defined in the droplet volume and on the boundary surface and the field is obtained at such points. The field is calculated via an iteration process in which the charge densities and the field are calculated alternatively until convergence. The field is obtained by calculating its components, both in phase and in quadrature with respect to the applied field. Results are discussed for various director configurations, relative values of conductivity and dielectric constant, and frequencies of the applied field.
Graphical description of the symmetries of potential energy surfaces
Authors: Nefedova Veronika V. , Boldyrev Alexander I. , Simons Jack
Published: July 2002

Shapes of the potential energy surfaces (PES) of ‘‘floppy’’ molecules can be represented by graphs in which vertices denote global minima and edges connecting the vertices describe energetically accessible paths connecting such global minima. Such graphical representations of PESs are characterized by (i) the number of vertices; (ii) edges that connect (or do not) various vertices; and (iii) the symmetry of the PES, which may be obtained from the graphical figure. These characteristics are shown in this paper to be reflected in the vibration–rotation wave function’s symmetry. Such a symmetry analysis reduces to that provided by the conventional point group for rigid molecules, but provides a useful tool for characterizing vibrations of floppy molecules. In particular, it is shown that for so-called connected graphs, the nonrigid group of the molecule is its full permutation-inversion group. PESs with connected graphs having three and six global minima are examined as examples, and relevant character tables are provided. PESs with disconnected graphs (which characterize species containing some inaccessible barriers), are also considered. The flexible molecules ArH3+, C2H3+, CH4+, and LiBH4 are considered as examples of the tools presented here.
Ionization potentials of small carbon clusters
Authors: Ramanathan Ragulan , Zimmerman Jeffrey A. , Eyler John R.
Published: July 2002

Ionization potentials (IP’s) of small carbon clusters (Cn, n=3–6) have been bracketed with an uncertainty of a few tenths of an eV by charge-transfer methods in a Fourier transform ion cyclotron resonance mass spectrometer. Values obtained are compared to previously determined experimental and theoretical values. Several interesting reactions other than charge-transfer were observed between the carbon cluster ions and the charge-transfer reference compounds. The products of these reactions are reported and discussed.
Ab initio calculation of the potential surfaces and the electronic transition moments for the valence and Rydberg doublet electronic states of HCO
Authors: Lorenzen-Schmidt Heike , Perić Miljenko , Peyerimhoff Sigrid D.
Published: July 2002

Results of large-scale configuration interaction calculations of the bending and C–O stretching potential curves and the corresponding electric transition moments for the low lying doublet electronic states of HCO are presented. Special attention is paid to the study of interaction between valence- and Rydberg-type species.
Deexcitation cross sections of Ne(3P2, 3P1, and 3P) by molecules containing group-IV elements
Authors: Yoshida Hiroaki , Kitajima Masashi , Kawamura Hideo , Hidaka Koji , Ukai Masatoshi , Kouchi Noriyuki , Hatano Yoshihiko
Published: July 2002

Deexcitation cross sections of Ne(3P2), Ne(3P1), and Ne(3P0) by CH4, SiH4, GeH4, CF4, and SiF4 have been measured at a mean collisional energy corresponding to room temperature (295 K) and compared systematically with those by other molecules to understand general features of the cross section values dependent on both the electronic states of excited neon atoms and the target molecules. A small difference in the cross section values between metastable atoms, Ne(3P2) and Ne(3P0), and a resonant atom, Ne(3P1), has indicated that Penning ionization by Ne(3P1) is mainly governed by an electron exchange interaction rather than a dipole–dipole interaction. A spatial electron distribution of the outermost orbital of a target molecule is especially of great importance in the determination of the absolute cross section values in the deexcitation of excited neon atoms due to an electron exchange interaction.
Second virial coefficients for seven fluoroethanes and interaction second virial coefficients for their binary mixtures with helium and argon
Authors: Bignell C. M. , Dunlop Peter J.
Published: July 2002

Using an accurate method which has been described in detail previously, data are reported for the second virial coefficients of seven fluoroethanes and for interaction second virial coefficients for their binary mixtures with helium and argon. All of the fluoroethanes have large dipole moments and, as expected, have virial coefficients which are much smaller than those of ethane and hexafluoroethane.
Analysis of water self-diffusion in polycrystalline lamellar systems by pulsed field gradient nuclear magnetic resonance experiments
Authors: Coppola L. , La Mesa C. , Ranieri G. A. , Terenzi M.
Published: July 2002

A study of the water self-diffusion in polycrystalline lamellar systems, by pulsed field gradient spin-echo (PFG-SE) nuclear magnetic resonance (NMR) experiments, is reported here. Our analysis confirmed the validity of previous equations for bidimensional water self-diffusion in lamellar lyomesophases and clarified their applicability limits. Depending on the diffusion experimental time range, two different procedures can be built up; for wide amplitude windows, the experimental diffusion coefficients are correlated to the fraction of bound water, whereas for short times it has been possible to point out the ‘‘obstruction factor’’ to diffusion. Experimental support for the theoretical findings was inferred by investigation of the lamellar mesophase of the binary system water/sodium-bis-2-ethylhexyl sulfosuccinate (AOT). It confirmed the existence of some defects in the lamellae at water concentration higher than 70 wt. %.
CO product distribution from metastable levels of HCO X̃ 2A′ prepared by stimulated emission pumping
Authors: Neyer D. W. , Luo X. , Houston P. L. , Burak Itamar
Published: July 2002

Vibrational and rotational distributions have been measured for the CO produced following dissociation of HCO prepared in metastable ground-state levels by stimulated emission pumping. The metastable levels are characterized by high vibrational excitation in the HC–O stretching coordinate. The CO (v=2) rotational distribution following dumping of population on the QR0(5) transition to the (0, 7, 0) level is rotationally cold and its distribution is in very good agreement with the theoretical prediction by Dixon. The linewidths for stimulated emission pumping transitions to the (0, 7, 0) and (0, 8, 0) levels are 1.0 and 1.6 cm−1, respectively. Since it is possible to prepare a wide variety of metastable HCO levels whose wave functions sample different regions of the HCO potential energy surface, measurement of the dissociation dynamics of HCO so prepared should provide a detailed picture of the potential surface at energies from 0–10 000 cm−1 above the dissociation limit.
Statics and dynamics of self-assembled dipolar rods
Authors: Drye T. J. , Cates M. E.
Published: July 2002

We study theoretically the effect of applied fields on self-assembled rods. The rods are assumed to be rigid, semidilute and to consist of repeat units of variable number, with each repeat unit carrying a small dipole moment. The static nonlinear susceptibility of the system is calculated, and found to be quite similar to that of a system of monodisperse rigid-rod polymers of the same linear susceptibility. The dynamical response of the system is also studied. Our model couples the rotational diffusion of the rods with a description of their reversible exchange reactions. (These reactions are presumed to involve only collinear rods, and presumed to be frequent on the time scale of rotation of a rod of the average length.) In the linear response regime, we show that the relaxation time for the net polarization greatly exceeds that for the angular reorientation of any individual dipole in the system. For the nonlinear regime (strong fields) an equation of motion is presented and is solved numerically to determine the relaxation of the polarization under switch-on, switch-off, and field-reversal conditions. In this regime, highly nonexponential relaxation is found. The extension of the model to the case of rods with induced (rather than permanent) dipole moments is briefly described, and the structure factor S(q) of such rods calculated under various field conditions.
A Born–Green–Yvon integral equation treatment of compressible lattice mixtures
Authors: Lipson J. E. G. , Brazhnik P. K.
Published: July 2002

The statistical thermodynamics of compressible multicomponent lattice mixtures is developed using the Born–Green–Yvon (BGY) integral equation technique. Expressions for the internal energy, Helmholtz free energy, chemical potential and equation of state are derived, and the routes to the volume, Gibbs energy, and noncombinatorial entropy are discussed. We use these results to study the phase separation behavior of a theoretical polymer blend which exhibits both an upper and lower critical solution temperature, focusing primarily on the spinodal. The effects of changing the pressure, polymer chain length, lattice coordination number, and interaction energies are also investigated.
Direct atomic orbital based self-consistent-field calculations of nonlinear molecular properties. Application to the frequency dependent hyperpolarizability of para-nitroaniline
Authors: Ågren Hans , Vahtras Olav , Koch Henrik , Jo/rgensen Poul , Helgaker Trygve
Published: July 2002

We outline a method for the calculation of nonlinear properties such as dynamic hyperpolarizabilities for self-consistent-field (SCF) wave functions. In this method, two-electron integrals are only addressed in the evaluation of Fock matrices and Fock matrices with one-index transformed integrals. These matrices are determined directly in terms of integrals evaluated in the atomic orbital basis, avoiding expensive integral transformations between atomic and molecular orbital bases as well as storing and retrieving the two-electron integrals. The method is double direct—direct in the sense of constructing Fock matrices from atomic integrals, and direct in the sense of solving the response equations iteratively using direct linear transformations of a generating matrix times trial vectors. Applications can be performed on species of the same size as in direct SCF. The cost of evaluating a single nonlinear molecular property is comparable to that of optimizing the wave function. Additional properties can be obtained at little extra cost by solving all response equations simultaneously. As a demonstration, we calculate the static and dynamic hyperpolarizabilities of para-nitroaniline.
A lattice Monte Carlo study of long chain conformations at solid–polymer melt interfaces
Authors: Bitsanis Ioannis A. , ten Brinke Gerrit
Published: July 2002

In this paper we present a comprehensive lattice Monte Carlo study of long chain conformations at solid–polymer melt interfaces. Segmental scale interfacial features, like the bond orientational distribution were found to be independent of surface–segment energetics, and statistically identical with Helfand’s predictions for the full-occupancy, infinite chain length limit. Conformational statistics of chains longer than 5–6 statistical segments followed the predictions of the Scheutjens–Fleer theory and the same power laws as a single ideal chain at the critical value of the surface–segment adsorption free-energy. Our simulations tested the predictions of random walk next to a ‘‘reflective’’ surface statistics for the spatial variations of chain dimensions and chain center of mass density. It was found that these statistics furnish the correct long chain limit, independently of surface–segment energetics. The random walk next to a ‘‘reflective’’ boundary predictions for the ‘‘adsorbed’’ amount and the distributions of tail, loop, and train number, as well as tail and loop size were in quantitative agreement with the simulation data. The correspondence between random walks and real (or simulated) chains required the knowledge of a single, microscopic parameter, the number of chemical segments per statistical segment, a∞. This quantity was very close to the average length of adsorbed sequences (trains), in the long chain limit. Our simulations tested thoroughly and established firmly the validity of ‘‘reflective’’ boundary statistics in the melt. The inevitability of these statistics has broad implications on ‘‘desorption’’ kinetics, chain mobility, and chain relaxation, which are currently under study.
An ab initio study of the OH stretching frequencies in ice II, ice VIII, and ice IX
Authors: Knuts Sören , Ojamäe Lars , Hermansson Kersti
Published: July 2002

Ab initio studies of the uncoupled, anharmonic OH and OD stretching frequency shifts in the three proton-ordered ice phases known, ice II, ice VIII, and ice IX, are presented. The ice structures are simulated by (H2O)5 supermolecules surrounded by point charges representing the correct crystal potentials. The calculations include electron correlation at the MP2 (DZP) level. For the eight different OH (OD) vibrators studied, the crystal environment leads to a downshift of the anharmonic OD frequency in the range 195–265 cm−1, in good agreement with experimental values (222–281 cm−1) when corrections are made for the limited supermolecular size (∼−45 cm−1), and, for ice VIII, also for the effects of the nonhydrogen bonded network (∼+75 cm−1). Also the agreement between absolute experimental and theoretical OD frequencies is good when errors due to basis set limitation (∼−75 cm−1) are taken into account. The calculations suggest a reassignment of two of the experimental OD bands in ice II and all three experimental OD bands in ice IX. Calculations for charge-embedded (H2O)9 and (H2O)13 ice clusters show that at least a nonamer is needed to avoid boundary effects from the size of the supermolecule. Theoretical correlation curves between H-bond parameters–R(O...O), ν(OH), re(OH), and infrared absorption intensity—are presented for the three ice phases and are compared to liquid water computations.
Correlated static–exchange interaction calculation for e−+N2 scattering using the coupled cluster techniquea)
Authors: Ghose Keya Basu , Pal Sourav , Meyer H.-D.
Published: July 2002

The correlation contribution to the static–exchange interaction potential (ΔVSE) appears as an important quantity in the electron molecule scattering calculations. We have shown that the single and double excitation configuration interaction (CISD) method does not give reliable results for ΔVSE. Only properly chosen multireference CISD calculation or Green’s function based calculations were found to be reliable. In this paper we carry out the coupled cluster (CC) calculations to test whether good results are obtained or not. The CC method will also help us to identify the important contributions to ΔVSE and to understand why a low order CI performs poorly.
Local superdirect self-consistent field
Authors: Peng Chunyang , Boggs James E.
Published: July 2002

A ‘‘local superdirect self-consistent field (SCF)’’ method has been developed which reduces the computational cost of a typical Hartree–Fock procedure from N4 to N2 more quickly than conventional SCF for a given molecular size. The program is based on the authors’ modifications of density matrix formalism, the local space approximation, and a direct SCF method. Special features include more efficient integral upper-bound formulas as integral prescreening techniques and ‘‘superdirect’’ prescreening of the projected Fock matrix elements.
Ab initio molecular orbital study of electronic and geometrical structures of MCH+2 and MSiH+2 complexes (M=Co, Rh, and Ir)
Authors: Musaev Djamaladdin G. , Morokuma Keiji , Koga Nobuaki
Published: July 2002

Complete active space self-consistent field (CASSCF) and internally contracted single and double configuration interaction from the CASSCF reference functions (MR-SD-CI-CASSCF) methods have been used to calculate electronic and geometrical structures of low-lying 3A1, 3A2, 3B1, 3B2, and 1A1 states of methylene, MCH+2, and silylene, MSiH+2, complexes for M=Co+, Rh+, and Ir+. The ground state of CoCH+2, IrCH+2, and all considered MSiH+2 complexes is the nearly degenerate 3A1 and 3A2 states. For RhCH+2 the ground state is 1A1, with 3A1 and 3A2 states lying only 4–5 kcal/mol higher. The M+=CH2 bond is ∼10–20 kcal/mol stronger than the M+=SiH2 bond. Our best calculated binding energies are 80.3 for CoCH+2, 78.3 for RhCH+2, 113.1 for IrCH+2, 61.5 for CoSiH+2, 69.3 for RhSiH+2, and 98.7 kcal/mol for IrSiH+2, which are in general in good agreement with experimental values. By using the MC/LMO/CI technique has been shown that all present silylene complexes have mostly Fischer-type character with donor-acceptor σ and π bond. In contrary, the analogous methylene complexes are mostly Schrock-type complexes with covalent σ and π bond, except 3A1, 3B2, and 3B1 states of RhCH+2, which have mostly Fischer-type character.
The dependence of the potential of mean force on the solvent friction: Consequences for condensed phase activated rate theories
Authors: Haynes George R. , Voth Gregory A.
Published: July 2002

The curvature of the potential of mean force along the reaction coordinate in condensed phase chemical reactions depends on the zero-time magnitude of the reaction coordinate friction. The consequences of this dependence are examined within the context of the dynamical correction factor to the classical transition state theory rate constant. These consequences are found to be quite significant.
Photoelectron angular distribution and ion vibrational branching ratio: The (2+1)-photon ionization of E 1Π-state-aligned CO molecules
Authors: Sha G. , Proch D. , Kompa K. L.
Published: July 2002

We report vibrational level channeling and photoelectron angular distributions which characterize the single-step photoionization of the E 1Π Rydberg state of CO. Particular attention is paid to the influence of the intermediate state’s rotational preparation and its spatial alignment. The two-photon excitation is tuned to populate the lowest possible J values of E, which permits us to isolate a single magnetic sublevel of this intermediate state. Vibrational branching is found to be insensitive to the choice of the excitation line and does not match the predictions suggested by the Franck–Condon factors for a CO E 1Π(v1)→CO+ X 2Σ+(v+) direct ionization. A competitive autoionization from the three-photon-energy state S1 may be responsible for this anomalous behavior. The phenomenological pattern of photoelectron distributions can in all cases be rationalized on the basis of the intermediate state alignment. A separate chapter of the discussion is devoted to P(2) pumping, since it provides a particularly simple situation where all excited molecules are oriented with their axes parallel to the light vector E, which makes this case tractable to a quantitative calculation. Its results show qualitative agreement with the experimental observations. The remaining discrepancy can be attributed to the autoionization which has not been considered in the calculation.
Triplet exciton formation due to interaction between singlet excitons in polydiacetylene
Authors: Ichimura K. , Yoshizawa M. , Matsuda H. , Okada S. , Ohsugi M. M. , Nakanishi H. , Kobayashi T.
Published: July 2002

The time-resolved spectroscopy in polydiacetylene single crystals (PDA-MADF {poly-1-[3-(methylamino)phenyl]-4-[3,5-bis(trifluoromethyl)phenyl]-1, 3-butadiyne}) on femtosecond and picosecond time scales was performed. The time dependence of the photoinduced reflectance change in PDA-MADF exhibits three components with lifetimes ∼200 fs, 1–2 ps, and ≫200 ps. These components are assigned to the free 1Bu excitons, self-trapped excitons, and triplet excitons. From the excitation intensity dependence of the reflectance change, a significant role of bimolecular interactions between singlet excitons for the formation of triplet excitons is clarified. A model of the mechanism of triplet exciton formation after the creation of singlet excitons is proposed.
A general analysis of reactive ion transport in dynamic equilibrium
Authors: Iinuma K. , Sasaki N. , Takebe M.
Published: July 2002

In a drift-tube analysis, an asymptotic solution for the transport of inter-reacting multiple ion swarms approaching to dynamic equilibrium is proposed. The solution describes a set of similar Gaussian profiles with two common ‘‘equilibrium’’ transport coefficients. Although all the profiles apparently aggregate to the same location, there exists a slight peak difference between arbitrary two spectra. The difference holds even in the equilibrium. A generalized equilibrium constant is defined by the quotient of two reaction-frequency determinants. Those four kinetic constants are uniquely determined from the kinetic coefficients of all ion species without any influence of the initial conditions. The discrepancy between the thermal equilibrium and dynamic equilibrium is briefly discussed.
Magic-angle spinning nuclear magnetic resonance spectra of second-order two-spin systems in the solid state
Authors: Wu Gang , Wasylishen Roderick E.
Published: July 2002

Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of second-order two-spin (AB) systems are investigated. Using average Hamiltonian theory (AHT), general expressions for the positions and relative intensities of the four allowed transitions are derived. In principle, correction terms to any order of the average Hamiltonian may be applied; however, terms up to and including third order appear to be adequate in reproducing the most important experimental features. In general, both the positions and relative intensities of the peaks are sensitive to the sample spinning frequency. Only at the high MAS frequency extreme do the MAS NMR spectra of two-spin (AB) systems in solids correspond to those predicted using formulas derived for solution samples. Under slow MAS conditions, MAS NMR spectra of AB spin systems deviate considerably from the corresponding AB spectra in solution NMR studies. Three general types of MAS NMR spectra are identified and their characteristic features are discussed. The theoretical expressions derived here are applied to reproduce the observed 31P MAS NMR spectra of a phosphole tetramer and cis-1,2-bis(diphenylphosphino)ethylene. It is shown that correction terms higher than first order must be considered in order to reproduce the anomalous spinning-frequency dependencies in MAS NMR spectra. The importance of carrying out measurements at two different applied fields is also demonstrated in the case of the phosphole tetramer.
Spectroscopic studies of jet-cooled AlNi
Authors: Behm Jane M. , Arrington Caleb A. , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy has been used to interrogate diatomic AlNi produced by laser vaporization of a 1:1 alloy target in a supersonic molecular beam of helium. Although a large density of states in this molecule prohibits a concise elucidation of its electronic structure, the presence of discrete transitions has allowed several bands to be rotationally resolved. From the analysis of these bands the ground state has been determined as X 2Δ5/2, originating from the 3sAl23dNi9σ2 configuration, and the bond length has been measured as 2.3211±0.0007 Å. The dissociation energy and ionization potential of AlNi have also been determined as D0○(AlNi)=2.29±0.05 eV and I.P.(AlNi)=6.95±0.09 eV, respectively.
Momentum space and Coulomb correlation: A two-particle density analysis for the H2(1Σ+g) molecule
Authors: Banyard K. E. , Sanders J.
Published: July 2002

Electron correlation in momentum space is assessed for the ground-state of H2 by examining changes in the two-particle probability density evaluated over preselected momentum planes. The associated radial and angular ‘‘planar Coulomb shifts,’’ ΔR(p12) and ΔA(εp), are also calculated. This study complements an earlier analysis for H2 in position space. The natural expansion of the correlated wave function used previously was converted into momentum space by means of a Dirac–Fourier transform. As before, correlation effects are measured with respect to the description provided by the first natural configuration. Characteristics of the three components of Coulomb correlation for a linear molecule are highlighted by specific choices for the momentum of a ‘‘test’’ or ‘‘reference’’ electron. Unlike position space, it was found that one component worked in distinct opposition to the other components, a feature analogous to atomic studies in momentum space. The trends observed throughout a series of probability changes in momentum space for H2 were rationalized and linked to the correlation behavior in position space.
Theoretical characterization of the reaction NH2+O→products
Authors: Walch Stephen P.
Published: July 2002

The potential energy surface for NH2+O has been characterized using complete active space self-consistent field (CASSCF)/derivative calculations to determine stationary point geometries and frequencies followed by internally contracted configuration interaction (ICCI) calculations to determine the energetics. The calculations predict a NO bond strength of 85.8 kcal/mol for NH2O. The barrier for isomerization of NH2O to trans-HNOH is predicted to be 48.0 kcal/mol and the barriers for H+HNO forming NH2O and NHOH are predicted to be 2.1 and 8.3 kcal/mol, respectively (all corrected for zero-point energy). The computed heats of formation for NH2O and cis- and trans-HNOH reported by Soto, Page, and McKee are in good agreement with the present results. Our results also agree with those obtained by Melius and Binkley to within their stated ±3 kcal/mol error limits. The barrier for H+HNO→H2+NO is computed to be ≊0.3 kcal/mol, which is about 0.7 kcal/mol lower than the value computed by Soto and Page (without correction for zero-point energy).
The excess thermodynamic properties of liquid (CH4+CD4)
Authors: Calado J. C. G. , Jancsó G. , Lopes J. N. C. , Markó L. , da Ponte M. Nunes , Rebelo L. P. N. , Staveley L. A. K.
Published: July 2002

The vapor pressure differences between a mixture of (CH4+CD4) and CH4 and between CD4 and CH4 were measured simultaneously with the CH4 vapor pressure. This was done at 29 temperatures between 97 and 122 K, and for six different mixtures, of compositions 0.150, 0.250, 0.375, 0.500, 0.625, and 0.750 mole fraction in CD4. These mixtures exhibit very small positive deviations from Raoult’s law. Calculated excess Gibbs energies for equimolar mixtures were 0.60 J mol−1 at 100 K and 0.42 J mol−1 at 120 K. These values of GE are 2 to 3 orders of magnitude smaller than those typically found in binary mixtures of simple nonisotopic species. The molar excess enthalpy, calculated from the temperature dependence of GE, is HE(x=0.5)=(1.5±0.2) J mol−1. The experimental results were used to test three theoretical models: the vdW-1 fluid theory, 1cLJ perturbation theory, and the theory of isotope effects in mixtures. While the first two proved to be inadequate, the isotope effect theory agrees well with experiment.
Microwave spectroscopic investigation of the mixed rare gas van der Waals trimers Ne2–Kr and Ne2–Xe
Authors: Xu Yunjie , Jäger Wolfgang , Gerry M. C. L.
Published: July 2002

Pure rotational spectra of six isotopomers of the rare gas trimer Ne2–Kr and five isotopomers of the trimer Ne2–Xe have been measured in their ground vibrational states using a Balle–Flygare-type cavity pulsed microwave Fourier transform spectrometer. Rotational constants have been evaluated, from which the geometries of the complexes have been derived. In the case of Ne2–Kr it has been possible to obtain centrifugal distortion constants and to carry out a force field analysis. The magnitudes of the induced dipole moments of these trimers have been estimated from the ‘‘π/2 condition.’’ The nuclear quadrupole hyperfine patterns due to 83Kr and 131Xe have been resolved, and the corresponding quadrupole coupling constants have been obtained. Observation of the spectra of these fundamental trimers has made it possible to compare their properties to those of their constituent rare gas dimers. The effects of three-body nonpairwise additive forces have been discussed in light of the structures obtained, the estimated induced dipole moments, and the nuclear quadrupole coupling constants.
Molecular dynamics of thermal dissociation in liquid N2O4
Authors: Katō Toshiko , Hayashi Soichi , Oobatake Motohisa , Machida Katsunosuke
Published: July 2002

Molecular dynamics simulations were performed for the dissociation and association (D/A) reactions N2O4■2 NO2 in the gas phase and in liquid N2O4. The trajectory was initialized from an equilibrium distribution of all variables in liquid N2O4, except the reactive mode, the NN distance of a reactant NO2 pair, was excited above the dissociation limit of the Morse-like potential between NO2 fragments, and the dynamics were calculated for 500 fs both forward and backward in time. Characteristics of the translational and vibrational energy relaxations of the reactant were studied in detail. Energy ERT, which is defined to be the sum of the potential and kinetic energies of interfragment motion, is found to play a key role in the D/A dynamics; a reactant pair is associated when ERT<0 and the pair is dissociated when ERT≳0. The transition state to the D/A reactions is hence defined by the last associated phase curve ERT=0 in the phase space. Energy transfer between intrafragment vibrational modes and the interfragment translational mode, which occurs at the inner turning point of the interfragment potential, is found to be the dominant prompter of the D/A reactions. The vibration–translation (V–T) energy transfer is found to excite the relative translational motion between fragments or gives rise to dissociation, and T–V energy transfer often causes deactivation of the relative translational motion or association in both the gas and liquid phases. In minor cases, the D/A reaction is found to occur by an energy transfer between reactant relative translational mode and solvent modes. The reaction rates are determined essentially by the rates of energy transfers among relative translational mode, intrafragment vibrational modes, and solvent modes.
Virial coefficients and equation of state of hard alkane models
Authors: Vega C. , Lago S. , Garzón B.
Published: July 2002

The virial coefficients up to the fifth of hard conformers of alkanes have been evaluated numerically. The conformers were selected by using the rotational isomeric state approximation. The effect of branching and length of the chain on the anisotropy of alkanes is discussed and a relation between the nonsphericity parameter α and the acentric factor ω for this kind of molecule is proposed. We modified the equation of state for tangent hard spheres first proposed by Wertheim [M. S. Wertheim, J. Chem. Phys. 87, 7323, (1987)] to allow for overlapping of the constituting hard spheres. This modified version predicts with high accuracy the virial coefficient of the hard conformers of alkanes. Moreover comparison of this equation of state with simulation results of repulsive models of n-alkanes reveals very good agreement. The effect of condensed phase effect on the conformational equilibria of n-alkanes up to n-octane is analyzed.
Entropy of buckminsterfullerene at 0 K
Authors: Johari G. P.
Published: July 2002

The entropy of C60 has been determined from its heat capacity data. It is 4.7±0.5 J/K/mole at 0 °K. The configurational thermodynamics analysis shows that C60 loses a considerable enthalpy when isothermally kept below 90 K.
Theoretical studies of He(1S)+CH(X 2Π). II. Fully ab initio cross sections for the inelastic scattering and comparison with experiment
Authors: Alexander Millard H. , Kearney William R. , Wagner Albert F.
Published: July 2002

We report a series of full close-coupling calculations of integral cross sections for fine structure resolved, rotationally inelastic transitions of CH induced by collisions with He. These calculations use the necessary two CH(X 2Π)+He potential energy surfaces as determined by a variety of ab initio techniques described in the preceeding paper. The calculated N=1→N=2−7 cross sections confirm a previous prediction of preferential population of final state levels in which the electronic wave function of the CH molecule is antisymmetric with respect to reflection in the plane of rotation of the molecule. A generally good overall agreement is found between the energy-dependent cross sections determined in earlier experiments of Macdonald and Liu and appropriate averages of the calculated cross sections. However, small, systematic, qualitative discrepancies persist between theory and experiment. Diagnostic calculations were carried out to identify the cause of these discrepancies. These calculations explored the influence of restrictions in basis set, configuration interaction, and functional forms used in fitting the potential energy surfaces. They also explored the consequences of the dynamical approximations of CH as a rigid rotor and the independence of the CH spin–orbit constant on the approach of the He partner. All these diagnostic calculations generally confirm the anticipated marginal influence of these approximations.
Unimolecular reactions in a spherically symmetric potential. II. Some tractable potentials
Authors: Klots Cornelius E.
Published: July 2002

Collision cross sections derived from a long-range potential can be made more realistic through the addition of a short-range component. It is shown how this can be achieved with no loss of simplicity, and in agreement with experiment. Upon addition of a Coulombic term, improved expressions for ion–electron neutralization and thermionic emission are then obtained. These developments permit the transition between the small and large molecule regimes to be followed explicitly.
Many-body perturbation theory electronic structure calculations for the methoxy radical. II. Hyperfine coupling coefficients
Authors: Bent Gary D.
Published: July 2002

The methoxy radical undergoes the dynamic Jahn–Teller effect which gives rise to diagonal and off-diagonal hyperfine coupling constants. The off-diagonal constants are between the degenerate vibronic wave functions of the lowest spin–orbit level. The experimental hyperfine coupling constants were measured in terms of spherical tensors. In this paper the spherical tensors are expanded in Cartesian matrix elements. These expansions are used to calculate the hyperfine coupling constants, both the isotropic constants (also known as the Fermi contact terms) and the anisotropic constants (also known as the dipolar terms). The diagonal hyperfine coupling constants could be calculated by standard methods, but the off-diagonal hyperfine coupling constants had to be calculated between nonorthogonal orbitals. The calculated dipolar hyperfine terms, both diagonal and off-diagonal, are in fair agreement with the known experimental values. The calculated diagonal Fermi contact terms are in good agreement with experiment, while the calculated off-diagonal Fermi contact terms are in terrible agreement with experiment.
Reactions of thermal hydrogen atoms in ethane and propane at 10 K: Secondary site selectivity in hydrogen abstraction from propane
Authors: Adrian Frank J. , Bohandy Joseph , Kim Boris F.
Published: July 2002

The reactions of hydrogen atoms, generated by ultraviolet photolysis of hydrogen iodide, with solid ethane and propane at 10 K are investigated using electron spin resonance (ESR) to identify the reaction products. Although the results are generally similar to those found by others for these tunneling hydrogen abstraction reactions in dilute solutions of the alkanes in xenon, there also are significant differences, most notably, evidence that only the secondary hydrogen of propane is abstracted in its tunneling reaction with an H atom. Also notable is the high degree of crystalline order in the ethane matrix, which is preserved in the ethyl radical product of the H atom–ethane reaction.
Suppression of sidebands in magic-angle-spinning nuclear magnetic resonance: General principles and analytical solutions
Authors: Antzutkin Oleg N. , Song Zhiyang , Feng Xiaolong , Levitt Malcolm H.
Published: July 2002

Several theoretical and experimental aspects of sideband suppression in the nuclear magnetic resonance (NMR) spectra of rotating solids are considered. The principles of sideband suppression are explored using general symmetry arguments and previous treatments are examined critically. Analytical solutions are given for sideband suppression pulse sequences employing four, five, six, and nine π pulses. The analytical solutions for four π pulses are complete. Experimental demonstrations are given.
Transient vibrational mode renormalization in dipole-coupled adsorbates at surfaces
Authors: Kuhnke K. , Harris A. L. , Chabal Y. J. , Jakob P. , Morin M.
Published: July 2002

Dipole interactions among adsorbates at solid surfaces can strongly affect the intensities, positions, and line shapes of vibrational resonances. An understanding of these effects has been important in spectroscopic investigations of surface structure. Here, the adsorbate dipole interactions are shown to create transient spectral intensity and resonance position changes when vibrational modes are excited in ultrafast pump–probe laser experiments at surfaces. The spectral changes occur because the intensities and positions of vibrational resonances are dependent upon the magnitude of interadsorbate dipole interactions, and vibrational excitation modifies the effective oscillator dynamic dipoles that determine these interactions. The vibrational modes are different (renormalized) after excitation because of the change in coupling. The effects account for unusual spectral transients observed in recent pump–probe experiments on the Si–H stretching modes of vicinal H/Si(111) surfaces [K. Kuhnke, M. Morin, P. Jakob, N. J. Levinos, Y. J. Chabal, and A. L. Harris, J. Chem. Phys. 99, 6114 (1993)]. The predicted effects serve as a novel time-resolved probe of the strength of dipolar interactions in adsorbate layers, and will arise in any adsorbate layer where the vibrational dynamic dipole interactions are large enough to cause spectral intensity borrowing among different adsorption sites or different adsorbates.
Real-time probing of two-photon absorption with phase related pulses
Authors: Kawashima Hitoshi , Nelson Keith A.
Published: July 2002

Real-time probing of two-photon absorption is formulated. A two-photon polarizability is introduced and its linear response is derived. The impulse response is shown to depend on the optical phase of incident radiation, and the nonstationary polarizability can be monitored by using a phase-related pair of pulses. Coherent molecular vibration in ‘‘dark’’ electronic states (accessible only through two-photon absorption) can be observed. The results are compared to time-domain observations of coherent vibrations initiated through one-photon absorption or impulsive stimulated Raman scattering. An estimate of the absolute signal intensity is presented.
Structure in liquid methanol from spatial distribution functions
Authors: Svishchev I. M. , Kusalik P. G.
Published: July 2002

A structural approach that employs the spatial distribution functions of atoms has been shown recently [J. Chem. Phys. 99, 3049 (1993)] to greatly improve our understanding of the local structure in liquid water. In the present study we obtain the oxygen–oxygen and oxygen–carbon spatial distribution functions, gOO(r,Ω) and gOC(r,Ω), respectively, for liquid methanol and use them to characterize its equilibrium structure. For this purpose molecular dynamics simulations with the three-site model of Haughney, Ferrario, and McDonald are carried out at a temperature of 25 °C. Using the spatial distribution functions we demonstrate that the dominant H-bonded structure in this liquid is an open, nonlinear (‘‘zig–zag’’) chain of monomers packed spatially in a tetrahedral manner. gOO(r,Ω) yields an average coordination number of 1.92 which agrees well with results from chain length analysis. There is no evidence in our structural data to support a local planar assembly of oxygen sites. We also observe features in gOO(r, Ω) which suggest that neighboring H-bonded chains in liquid methanol have a tendency for parallel arrangements.
A nonlocal density-functional theory of electric double layer: Charge-asymmetric electrolytes
Authors: Patra Chandra N. , Ghosh Swapan K.
Published: July 2002

A nonlocal density-functional theory of inhomogeneous ionic fluids proposed by us recently [J. Chem. Phys. 100, 5219 (1994)] for symmetric electrolytes is extended to study the structure of electric double layer for a charge-asymmetric (2:1) situation involving hard sphere ions of equal diameter with a continuum or neutral hard sphere model for the solvent. The hard sphere contributions to the excess free energy density and its derivatives for the inhomogeneous system are evaluated nonperturbatively through a position-dependent effective weighted density, which is also used to obtain the corresponding ionic contributions through a second-order functional Taylor expansion. The calculated results for the continuum solvent model show reasonably good agreement with the available simulation results, while the layering effect due to hard sphere exclusion and the charge inversion phenomena are some of the interesting consequences arising from the molecular nature of the solvent.