bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic circular dichroism studies of matrix-isolated atoms: Excited state spin-orbit coupling constant reduction of copper in the noble gases
Authors: Vala Martin , Zeringue Kyle , ShakhsEmampour Jalal , Rivoal Jean-Claude , Pyzalski Robert
Published: February 2003

The absorption and magnetic circular dichroism spectra of the 2P←2S transition of matrix-isolated copper atoms have been measured in krypton and xenon matrices. A dramatic reversal in the MCD 𝒞 term pattern is observed. From a spectral band moment analysis it is shown that: (1) noncubic lattice cage vibrational motions are more important than cubic ones in contributing to the observed bandwidths and (2) the Cu excited state spin-orbit coupling constant (λ) in krypton is reduced to 57% of its gas phase value, whereas in xenon it is reduced to −14% of its gas phase value. Calculations using the Moran model show that the excited state geometry of the metal/rare gas cage is distorted via the Jahn–Teller effect in opposite ways in krypton and xenon matrices. The negative sign of λ in xenon reveals a reversal in the order of the 2P state spin-orbit multiplets. Neither this reversal nor the reduction of λ in krypton can be accounted for by the Jahn–Teller effect. We attribute the spin-orbit constant reduction to the overlap of the Cu 4p orbital with the np of the rare gas matrix cage atoms. The calculated spin-orbit reduction factors (in Ar, Kr, and Xe) agree well with the experimental ones if a contracted Cu 4p orbital is used. In the same way the ground state g factor shift with matrix atom type, observed by Kasai and McLeod, is semiquantitatively accounted for by calculation of the overlaps between the Cu 4s orbital and the np orbitals of the rare gas cage atoms.
Short time dipole autocorrelation function and infrared line shape of gaseous nitrous oxide
Authors: Lacome Nelly , Levy Armand , Boulet Christian
Published: February 2003

A quasistatic treatment has been used to calculate the short time behavior of the autocorrelation function of the dipole moment for pure gaseous nitrous oxide, while the long times have been treated within the impact approximation. The shape of each vibration–rotation line has been derived by Fourier transforming this function. The profile of entire vibration–rotation bands has been calculated, for pressures up to 15 atm, by summing the so-obtained line profiles. Comparison between experimental and theoretical profiles of the 4000–0000 and 3200–0000 transitions has shown good agreement for all pressures studied.
Infrared matrix isolation spectra of the H2F2 dimer
Authors: Redington Richard L. , Hamill Delphia F.
Published: February 2003

Infrared spectra of the H2F2 dimer isolated in Ar, N2, CO, and several mixed matrices, are reported and values for all six fundamental vibration frequencies are given for the Ar-isolated dimer. Evidently due to molecular nonrigidity, the relative intensity of ν1 and ν2 of the H2F2 dimer depend strongly on the matrix environment. The HF stretching fequencies for X---HF bimolecules isolated in Ar, with X=Xe, N2, CO, and H2O are observed. Matrix isolated cyclic trimer and higher oligomers of HqFq are observed to have their HF stretching bands in the 3430 to 3175 cm−1 region. The discussion includes a vibrational predissociation lifetime correlation for the vapor phase dimer.
Laser magnetic resonance rotational spectroscopy of 2Σ radicals: Ethynyl (CCH)
Authors: Saykally Richard J. , Veseth Leif , Evenson Kenneth M.
Published: February 2003

The first terrestrial measurement of the free ethynyl radical (CCH), made by far-infrared laser magnetic resonance, is described. The N=6→7 rotational transition was observed for the lowest vibrational level of the 2Σ+ ground state. Because of the very weak spin coupling in this state, the LMR spectrum is complex and badly overlapped. A theoretical formalism for the prediction and analysis of such weakly coupled 2Σ states is presented, in which frequencies, linewidths, and intensities of all transitions are computed as a function of magnetic flux density, and the total absorption coefficient is computed at each field point in order to simulate the magnetic resonance spectrum. This formalism is used to analyze the LMR spectra of CCH. A combined least squares analysis of existing microwave, astronomical, and LMR data was carried out to determine an improved set of molecular parameters for this important interstellar molecule.
Vibronic coupling in polyenes: High resolution optical spectroscopy of a simple Schiff base
Authors: Girard Mark , Arvidson Eric , Christensen Ronald
Published: February 2003

High resolution fluroescence and fluroescence excitation spectra of a model polyene Schiff base have been obtained. 4.2 K spectra of N-(2, 4, 6, 8, 10-dodecapentaenylidine)butyl amine in n-tetradecane exhibit well-resolved electronic spectra (ππ*) from a single mixed crystal site. The major vibronic features, in common with those of polyene hydrocarbons, are due to combinations of symmetric carbon–carbon stretching vibrations. These are dominated by a double bond mode which decreases from 1589 to 1544 cm−1 upon electronic excitation. This decrease contrasts the 150–200 cm−1 increases seen in analogous polyene hydrocarbons. This difference is discussed in terms of vibronic coupling in a model previously applied to the hydrocarbons.
Stimulated emission pumping of acetylene: Evidence for quantum chaotic behavior near 27 900 cm−1 of excitation?
Authors: Abramson Evan , Field Robert W. , Imre Dan , Innes K. K. , Kinsey James L.
Published: February 2003

Clumps of narrowly spaced lines (∼10 lines per cm−1 in a 2 cm−1 wide clump) appear in the stimulated emission pumping spectrum of the acetylene X̃ 1∑g+ state near 27 900 cm−1 of vibrational excitation above the vibrationless level. All of the lines in a given clump arise from transitions into levels with the same lower state J″ value. Thus, the fine structure within each clump is of a purely vibrational nature. Rotationally resolved and assigned spectra at such high vibrational excitation have not been previously reported for a polyatomic molecule. The unique qualities of these spectra make it possible for the first time to apply one of the suggested criteria of quantum chaos to a real molecular spectrum. Analysis of the spacings between adjacent lines in several of these clumps shows that they approximately follow a Wigner distribution rather than a Poisson distribution, a characteristic which is expected for chaotic behavior within the molecule.
Time-resolved spectroscopy of intramolecular energy transfer in a rigid spiran
Authors: Maki A. H. , Weers J. G. , Hilinski E. F. , Milton S. V. , Rentzepis P. M.
Published: February 2003

The kinetics of intramolecular energy transfer has been studied in the rigid spiran, spiro[9,10-dihydro-9-oxoanthracene-10,2′-5′,6′-benzindan], using picosecond time-resolved spectroscopy. The rate constant for triplet–triplet energy transfer from the donor (anthrone) to the acceptor (naphthalene) chromophore is found to be ∼3×1010 s−1. Low temperature microwave-induced delayed phosphorescence measurements on the spiran show that triplet–triplet energy transfer from the anthrone populates the sublevels of the naphthalene acceptor state randomly, rather than the Tz (z=long axis) sublevel selectively. The latter prediction arises from the known sublevel selectivity for intersystem crossing in anthrone and the geometrical arrangement of the chromophores in the rigid spiran. The randomness of the triplet–triplet energy transfer process is consistent with the rate constant for energy transfer which limits the donor lifetime to a value below the spin precession period in the donor triplet state. The results are discussed in terms of the Dexter theory for triplet–triplet energy transfer.
Environment of Fe3+ at the M2 and Si sites of forsterite obtained from EPR
Authors: Gaite J. M. , Hafner S. S.
Published: February 2003

Electron paramagnetic resonance of a forsterite Mg2SiO4 crystal doped with 57Fe revealed two strong Fe3+ centers which can be unambiguously assigned to the M2 and Si sites. All the fourth order terms of the spin Hamiltonian were determined for the two centers. It is shown that the local environment of Fe3+ at M2 is distorted without any apparent pseudoaxial symmetry, while the environment of Fe3+ at the Si position exhibits a strong axial distortion along the crystallographic a direction. There is no immediate local charge compensation for the Mg2+ and Si4+ substitution by Fe3+ in the crystal.
Lattice dynamics and crystal stability of bromine
Authors: Gamba Z. , Halac E. , Bonadeo H.
Published: February 2003

The lattice dynamics and crystal stability of bromine have been studied using an interaction potential which includes atom–atom and Coulombic terms. It was found that in addition to high order electrostatic terms, a specific interaction of the form −D/r8 between nearest neighbor atoms, which is identified as an induced dipole-induced quadrupole term, is needed to stabilize the observed orthorhombic structure and to give a good fit with other static and dynamical observables.
The coexistence curve for a mixture of hard-particle lattice gases
Authors: Poland Douglas
Published: February 2003

Exact density expansions are given for the coexistence curve for a mixture of hard-particle lattice gases. The expansions are analogous to low-temperature expansions in lattice gases with attractive interactions and reflect the surface properties of clusters of particles.
The pressure dependence of rotational states in (NH4)2 PdCl6
Authors: Prager M. , Press W. , Heidemann A. , Vettier C.
Published: February 2003

The pressure dependence of the ground-state tunnel splitting and the energy of transition to the first excited librational state of the NH+4 ion in (NH4)2 PdCl6 has been measured with inelastic neutron scattering. A Grüneisen parameter for the tunneling transitions γtun=−13 has been obtained. The relation between the rotational states and the intermolecular interaction is based on a model theory of Hüller and Raich. The influence of additional higher order terms in the rotational potential is discussed. An exponent n=11±2 of the r-dependent interaction V∼r−n is derived.
Magnetic field effects on the luminescence of Ln2[Pt(CN)4]3⋅yH2O single crystals
Authors: von Ammon W. , Hidvegi I. , Gliemann G.
Published: February 2003

Magnetic field effects on the E⊥c-polarized emission of Ln2[Pt(CN)4]3⋅yH2O single crystals (LnCP) with Ln=Lu, Yb, Tm, Er, Ho, Dy, Tb, Gd, Eu, Ce, and La at 1.9 K are reported. Compared to the MCP’s (M≠Ln), the influence of the magnetic field on lifetime, intensity, and spectral position of the LnCP emission is rather weak—with the exception of LuCP, which behaves very similar to the MCP’s (M≠Ln). The behavior of the other LnCP’s can be explained assuming an admixture of (Ln, 4f) orbitals to the E⊥c-emitting Pt-complex state A′1u which competes with the magnetic field induced admixture of the (Pt, Eu ) state, and thereby partially anticipating the magnetic field effects. The strong localization of the A′1u is confirmed by magnetic field induced effects on the emission of Yb3+, Tb3+, Gd3+, and Eu3+ doped LuCP.
Polymorphism in vapor deposited amorphous solid water
Authors: Mayer Erwin , Pletzer Rudolf
Published: February 2003

The physical properties of vapor deposited amorphous solid water, (H2O)as, depend strongly on a parameter not investigated before: the state of the vapor prior to deposition on a cryoplate. Supersonic flow conditions give (H2O)as desposits with a very rugged surface, whereas breakup of supersonic flow by a baffle gives transparent, glass-like deposits. These two types of (H2O)as deposits devitrify in the presence of adsorbed gas quite differently: (H2O)as prepared by baffling supersonic flow gives according to DTA on heating only one exothermic peak as expected, whereas (H2O)as prepared by condensing water vapor with supersonic flow characteristics devitrifies at higher temperatures in several steps, the prominent peaks occurring at 160, 164 and 177 K. We have investigated the influence of parameters controlling supersonic flow and condensation in the beam on the devitrification pattern of (H2O)as and conclude that supersonic flow conditions cause the formation of water clusters already in the gas phase. We further suggest that condensation of gas phase clusters on the cryoplate gives a highly porous solid with a network of pores or voids of various sizes, whereas condensation of monomers from baffled flow gives a nonporous solid. The role of adsorbed gas on the devitrification of (H2O)as can be interpreted by assuming first that surface-induced nucleation is delayed in voids or pores by adsorbed gas, and second that macroscopic fractions of the amorphous samples have different void sizes and therefore gas occupancies. In addition, we have found evidence for preferential adsorption of O2. Since (H2O)as is usually prepared by condensation of water vapor flowing out of capillaries, supersonic flow and cluster formation might have been a general feature of (H2O)as preparations. Implications are discussed with respect to reports on properties of (H2O)as. Preliminary experiments with other vapor deposited amorphous molecular solids show that the effect of stepwise devitrification is not restricted to (H2O)as. To relate the state of the vapor to the structure of the amorphous solid, we suggest as a working hypothesis that the structure of (H2O)as prepared by condensation of supersonic cluster beams should better be described in terms of a discontinuous model such as the amorphous cluster model, whereas the structure of (H2O)as prepared by condensation of monomers is as usual described best by the continuous random network model. Intermediate stages between these two extremes depending on the preparative details can be envisaged easily. This hypothesis would imply polymorphism for (H2O)as, and is discussed with respect to other amorphous solids prepared by vapor deposition.
A detailed analysis of the Raman enhancement mechanisms associated with the interaction of a Raman scatterer with a resonant metal cluster: Results for Lin–H2
Authors: Pandey Prabhat K. K. , Schatz George C.
Published: February 2003

The results of time dependent Hartree–Fock calculations of frequency dependent polarizability derivatives for H2–Lin clusters (n=2, 4, 6) are presented and analyzed in terms of Raman enhancement mechanisms associated with interaction between H2 and the metal clusters. In these calculations, the width factors associated with unoccupied Hartree–Fock molecular orbitals are chosen so that the cluster polarizabilities are the same as those of bulk metal spheroids of the same size. The calculations are otherwise ab initio, and predict polarizability derivatives whose squares are enhanced by 103–104 for H2–Lin clusters at equilibrium geometries when irradiated at frequencies corresponding to metal cluster excitation energies. These enhancements were found to be reasonably independent of cluster size and vary by factors of 2–4 with H2 adsorption location. The enhancements were found to vary as the inverse fourth power of the excited state widths, and to decrease with increasing adsorbate–metal separation with a complicated functionality which reflects the varying contributions of three different enhancement mechanisms. At relatively large separations, induced polarization effects (i.e., electromagnetic interactions) make a large contribution to the enhancement factor. This is also important near equilibrium, but also important at this location (as well as at large separations) is a mechanism which involves the modulation of the metal orbital energies by the adsorbate vibrational motions. At smaller separations, corresponding to the repulsive region of the H2–Lin interaction potential, a third mechanism becomes dominant which involves the enhanced dipole matrix element derivatives which arise from charge transfer between adsorbate and metal. Symmetry of the electronic states giving rise to the cluster resonances was also found to influence enhancements, with the largest enhancements obtained for excited cluster states which have large overlap with the adsorbate HOMO and LUMO. Extension of these mechanistic interpretations to the description of surface enhanced Raman scattering (SERS) is considered.
A crossed beam study of the charge-transfer reaction of Ar+ with N2 at low and intermediate energies
Authors: Friedrich B. , Trafton W. , Rockwood A. , Howard S. , Futrell J. H.
Published: February 2003

The charge-transfer reaction Ar+(N2,Ar) N+2 was studied in a crossed beam experiment at 1.73 and 4.01 eV collision energies. A weak plasma ion source was used to generate a nearly pure Ar+ (2P3/2) ion beam, which is crossed at 90° by a He-seeded supersonic jet N2 beam. Detailed analysis of the results demonstrates that the reaction proceeds cleanly to generate N+2(X Σ+g, v=1) with a distribution of rotational states. A direct mechanism, qualitatively described as electron jump with negligible momentum transfer, is followed at collision energies of 1.7 and 4 eV.
An evaluation of the role of internal energy and translational energy in the endothermic proton transfer reaction of N2H+ with Kr
Authors: Villinger H. , Futrell J. H. , Saxer A. , Richter R. , Lindinger W.
Published: February 2003

The Innsbruck SIDT (selected ion drift tube), now modified to include an octupole guided beam region, has been utilized to investigate the title reaction using both virbationally excited and ground state N2H+. Rate constants measured for three distinguishable proton transfers at a field strength of 40 Td are 1.1×10−9, 2.7×10−10, and 1.3×10−11 cm3 s−1. These are assigned to the exothermic proton transfer reaction N2H+ (X,v=2)+Kr→KrH++N2 and the endothermic reactions of N2H+ (X,v=1) and N2H+ (X,v=0), respectively. This assignment is supported by studies of the kinetic energy dependence of the rate coefficients and the collisonal relaxation of excited N2H+ ions with N2. The latter process is characterized by a large rate coefficient kr=3×10−10 cm3 s−1 approximately one-third the collision rate coefficient.
Nuclear and electron dynamics in the photodissociation of water
Authors: Andresen Peter , Ondrey G. S. , Titze B. , Rothe Erhard W.
Published: February 2003

The photodissociation of water in its first absorption band is studied by photolyzing H2O at 157 nm with an excimer laser. This dissociation proceeds directly to produce the electronic ground states of H and OH. Both nascent internal state distributions and alignment of the product OH (2Π) are probed by laser induced fluorescence. This is done with both warm (300 K) and cold (∼10 K) water. About 88% of the excess energy is translation, 10% vibration, about 2% rotation. The first three vibrational levels 0, 1, 2 have population ratios 1:1:0.15, respectively. The rotational distributions depend strongly upon the H2O temperature and are very different for the upper and lower energy components of the Λ doublets, which are measured via Q and P, R lines, respectively. For Q lines, the distributions can be described by rotational temperatures which are 930 K for warm and 475 K for cold water, a surprising difference. For P,R lines strong deviations from Boltzmann behavior are found for cold H2O. The spin distribution is almost statistical. A strong J dependent Λ-doublet population inversion is found from cold H2O, but there is no inversion from warm H2O. The inversion provides a possible pump mechanism for the astronomical OH maser and is simply explained by approximate symmetry conservation. The orientation of the unpaired pπ lobe in OH in the upper Λ-doublet state is measured to be perpendicular to the OH rotation plane. The J dependence of the inversion is explained by Λ-doublet mixing in OH and quantitatively described in terms of the singly occupied pπ-lobe in the excited water and the orientation of the corresponding singly occupied pπ-lobe in OH. The alignment of OH is measured by polarizing both lasers. The large polarization effects are strongly dependent upon J and also upon the temperature of H2O. It is shown that the dependence is related both to Λ-doublet mixing and hyperfine structure of OH. For the cold H2O the data indicate, despite the strong J dependence of both polarization and Λ-doublet inversion, a completely planar dissociation process. It is shown that due to Λ-doublet mixing the transition moment of Π molecules has a J dependent angle relative to the OH rotation plane which approaches the high J limit at the same rate that the molecule shifts from Hund’s case (a) to case (b). The model for the J dependence of the Λ-doublet population and the polarization is important for chemical reactions, surface scattering and other processes where Π molecules are analyzed with LIF.
The electronic structure of alkali bihalide molecules MX2. I. Ground states of LiF2, NaF2, LiCl2, and NaCl2 systems
Authors: Maessen Bärbel , Cade Paul E.
Published: February 2003

The ground electronic states of the triangular alkali bihalide molecules LiFad2, NaFad2, LiClad2, and NaClad2 are investigated theoretically at the restricted−Hartree−Fock level using large Gaussian basis sets. It is argued that these MXad2 systems are stable species on the M+X+X potential energy surface and LiFad2, NaFad2, and NaClad2 are stable entities with respect to dissociation into the MX+X systems. Potential energy curves are obtained for the (X−−−X)− and M+−−−(x2)− motion in the MXad2 systems and comparison with experiment is given in terms of calculated spectroscopic properties. The vibrational assignments of Howard and Andrews for thee MX2 systems are supported by these calculations and a study is made of the influence of the M+ cation on the spectra of the constituent X2−(X 2Σu+) molecular anions.
The shell structure of atoms and ions in momentum space
Authors: Simas Alfredo M. , Westgate William M. , Smith Vedene H.
Published: February 2003

The radial momentum distributions for the ground states of the first 92 atoms from hydrogen to uranium, and of singly positive ions from helium to barium and from lutetium to radium, constructed from nonrelativistic SCF wave functions, are investigated. These distributions display a number of maxima that, as is shown, depend strongly on the electronic structure and can be assigned each to a particular shell—the first maximum corresponding to the outermost shell. Four maxima is the most that appear for each atom or ion studied implying that in some cases only the outermost shells are evident. This result is similar to the fact that for some atomic systems in position space, only the innermost shells lead to maxima in the radial density. As a general conclusion, if both momentum and position space radial electronic densities are considered, then all shells become evident for the atomic cases studied.
Analytic energy second derivatives for general MCSCF wave functions
Authors: Hoffmann Mark R. , Fox Douglas J. , Gaw Jeffrey F. , Osamura Yoshihiro , Yamaguchi Yukio , Grev Roger S. , Fitzgerald George , Schaefer Henry F. , Knowles Peter J. , Handy Nicholas C.
Published: February 2003

Expressions for the determination of analytic energy second derivatives for general MCSCF wave functions are presented. Equations for two distinct approaches: (1) direct differentiation of the energy expression and associated Lagrangian condition; and (2) power series expansion of the Hamiltonian and exponential-i-lambda transformation of the wave function, are developed. The problem of the nonzero nullity of the Hessian, and the resultant existence of redundant variables in the coupled perturbed multiconfiguration Hartree Fock (CPMCHF) equations, is discussed and a straightforward solution proposed. The viability of the methods presented in this paper are illustrated by a sample calculation on formaldehyde, using a double zeta (DZ) basis set and including 325 MCSCF configurations in the state space.
Collision-induced dipoles of rare gas mixtures
Authors: Birnbaum George , Krauss Morris , Frommhold Lothar
Published: February 2003

New ab initio calculations of the collision-induced dipole moment of the rare gas systems He–Ar, Ne–Ar, Ne–Kr, and Ar–Kr are obtained on the basis of a molecular Hartree–Fock treatment. With these and recent potential functions the spectral moments and line shapes of collision-induced absorption spectra are computed. Reasonable agreement with existing measurements is observed for the first time for the system Ne–Ar and Ne–Kr.
Potential energy curves and adiabatic corrections of weakly bound states: Application to the LiH B 1Π state
Authors: Vidal C. R. , Stwalley W. C.
Published: February 2003

For the B 1Π state of the 7LiH and the 7LiD molecule the Dunham-type coefficients, their adiabatic corrections and the Λ doubling constant are given. Using all measured term values of both isotope combinations, we obtain the potential energy curve and its adiabatic correction using a quantum mechanical variational method. The dissociation energies of the B, A, and X states are reanalyzed.
The effects of local modes and Morse oscillator potentials on the nonradiative ‘‘communicating states’’ rate calculations in benzene
Authors: Hornburger H. , Schröder H. , Brand J.
Published: February 2003

By means of ‘‘communicating states’’ model calculations it is shown that the sharp increase of nonradiative transitions with excess vibrational energy in the first excited electronic state S1(B2u) of benzene has to be interpreted as an internal conversion process. Intersystem crossing S1uT1 cannot explain the disappearance of fluorescence quantum yields. This remains true even if a Morse potential for the ν2(a1g) vibration or for all C–H stretching modes in a local mode description is assumed. The sharp increase of the rate in the ‘‘channel-three’’ excess energy region is still governed by the ν4(b2g) out-of-plane mode. In addition, the excess energy dependence of the nonradiative rates is found to be extremely nonexponential.
Quantum mechanical treatment of the F+D2→DF+D reaction
Authors: Abusalbi N. , Shoemaker C. L. , Kouri D. J. , Jellinek J. , Baer M.
Published: February 2003

Reactive infinite order sudden (RIOS) approximation calculations for the F+D2→DF+D reaction using the Muckerman 5 potential are reported. Primitive γ-dependent state resolved reaction amplitudes and probabilities, γ-averaged probabilities, vibrational state resolved angular distributions, total integral cross sections, and vibrational branching ratios are presented. The results are compared against results of other methods and against similar RIOS results for the F+H2→HF+H reaction. The accuracy of the RIOS results is discussed and in addition, qualitative comparison with experimental vibrational state resolved angular distributions is made. We find that the RIOS F+D2 results are similar to RIOS results for the F+H2 system. Similarly, experimental results for these two systems are in qualitative agreement with one another. In the case of comparison of RIOS and experimental vibrational state resolved angular distributions, qualitative agreement is obtained for the vf =3,2,1 DF product states. However, the RIOS results for DF (vf=4) are strongly backward scattered while most recent experiments show strong forward peaking. Analogously, the RIOS results for HF (vf=3) are strongly backward scattered while the most recent experimental results show strong forward peaking. Detailed analysis of the RIOS results using Smith’s lifetime matrix approach to characterizing resonances has been carried out. Clear evidence supporting the existence of resonances in the F+D2 system is found. The implications of all these results for the potential surface are discussed.
Self-consistent molecular orbital methods 25. Supplementary functions for Gaussian basis sets
Authors: Frisch Michael J. , Pople John A. , Binkley J. Stephen
Published: February 2003

Standard sets of supplementary diffuse s and p functions, multiple polarization functions (double and triple sets of d functions), and higher angular momentum polarization functions (f functions) are defined for use with the 6-31G and 6-311G basis sets. Preliminary applications of the modified basis sets to the calculation of the bond energy and hydrogenation energy of N2 illustrate that these functions can be very important in the accurate computation of reaction energies.
Correlated states in linear polyenes, radicals, and ions: Exact PPP transition moments and spin densities
Authors: Ramasesha S. , Soos Z. G.
Published: February 2003

The low-lying eigenstates ψn of the Pariser–Parr–Pople (PPP) model for polyenes with N≤11 carbons are found exactly as linear combinations of nonorthogonal covalent and ionic valence-bond (VB) diagrams. We extend diagrammatic VB theory to normalize ψn efficiently and to evaluate exactly transition moments, spin densities, and other matrix elements within subspaces of fixed total spin S. Charge orthogonality in the VB basis results in a block-diagonal overlap matrix whose evaluation is rapid for linear combinations of over 105 diagrams. We obtain S=0 and 1 states of all-trans decapentaene (N=Ne =10), S=1/2 states of nonatetraenyl (N=Ne =9), S=0 states of its anion and cation, and spin densities through undecapentaenyl (N=Ne =11), all with standard molecular PPP parameters. Correlation effects on excitation energies, on transition moments, and on spin densities are contrasted with one-electron Hückel results and compared with data on finite polyenes and polyacetylene. Standard PPP parameters successfully describe the 2 1Ag and 1 1Bu excitations of alternating even polyenes, their reduced oscillator strengths, the 1 1A↔1 1B transition of regular odd anions, and the nearly uniform negative and positive spin densities of regular polyene radicals.
Theory of phase transitions in charge-transfer complexes: Anthracene–tetracyanobenzene
Authors: Luty T. , Munn R. W.
Published: February 2003

The phase transition in anthracene tetracyanobenzene (A-TCNB) is identified as of mixed order–disorder and displacive character. This arises from coupling between the anthracene orientation, treated as a pseudospin, and librational phonons of Bg symmetry at the M point in the high-temperature Brillouin zone. Suitable coupling is provided by the orientational dependence of charge-transfer interactions, enhanced by dielectric effects due to the highly polarizable TCNB; an expression is deduced for the electrostatic energy in the disordered structure. This coupling raises the critical temperature in the Landau mean field treatment, but further coupling with totally symmetric acoustic phonons makes the transition of first order at a still higher temperature. The transition temperature, pseudospin ordering, and orientational displacements are derived. Strong coupling favors a displacive transition, weak coupling an order–disorder transition. Similar considerations should apply to phase transitions in other charge-transfer crystals.
Statistical thermodynamics of polydisperse fluids
Authors: Briano J. G. , Glandt E. D.
Published: February 2003

The statistical mechanical formalisms for dealing with mixtures containing an infinite number of components are discussed. The partition function and the thermodynamic properties can be easily generalized from the corresponding results for pure substances when the polydisperse fluid is studied in a four-dimensional space. The chemical potential field acts along the additional coordinate: the composition axis. A perturbation theory for narrow distributions is developed. A small parameter measuring the width of the chemical potential function is used within a semigrand ensemble. An equivalent, hybrid expansion expresses the results in terms of the variance of the composition distribution.
Generation of multiple attractors and nonequilibrium phase transitions
Authors: Rehmus Paul , Vance William , Ross John
Published: February 2003

We develop and discuss the generation of new limit cycles in chemical systems by external periodic perturbations. The deterministic analysis is based on prior work by Loud. We choose a model derived from experiments, the conversion of 2,3 epoxy-1-propanol to glycerine. The autonomous system of equations which govern the dynamics of this system has a limit cycle arising from a normal Hopf bifurcation, and a stable focus, and a limit cycle associated with an inverted Hopf bifuraction. For specific choices of periodic perturbations a prescribed number of periodic attractors can be generated from a single periodic attractor (normal Hopf bifurcation); and two periodic attractors, a periodic and biperiodic, or two biperiodic attractors can be generated from a stable focus and periodic attractor (inverted Hopf bifurcation). Noise is then imposed on the attractors of the autonomous system and the multiple attractors of the externally forced system. Noise imposed on the autonomous system with a stable focus and limit cycle causes random transitions, analogous to first order phase transitions. This behavior has some resemblance to intermittency noted in some biological systems. From the results of noise imposed on systems with different numbers of isoperiodic limit cycles we suggest, after a limited number of observations, the trend that as the number of attractors increases, the lines in the power spectrum narrow and the area in the phase plane within contours of equal probability increases.
Hydrogen bonding in liquid methanol and ethanol determined by x-ray diffraction
Authors: Narten A. H. , Habenschuss A.
Published: February 2003

The x-ray diffraction patterns of liquid methanol and ethanol have been measured at 20 °C. The data are analyzed to yield the molecular structures, and the distinct structure functions Hd(k) are analyzed to obtain the hydrogen bonding in these alcohols. The data show clearly that hydrogen-bonded hydroxyl groups occur in methanol and ethanol with an OH⋅⋅⋅OH distance of 2.8Å, and that each hydroxyl group has 1.8±0.1 nearest neighbors at this distance.
Nonuniform concentration profiles of dilute macromolecular solutions in rotational viscometric flows
Authors: Brunn Peter O.
Published: February 2003

Some dilute solution kinetic theory results for dumbbell model macromolecules in nonuniform flows are obtained. Concentrating on rotational flows, it turns out that the flow induced cross-stream migration of the dissolved macromolecules is (a) due to the curvilinearity of the streamlines and (b) directed strictly towards the axis of rotation. For the special case of a Gaussian spring, explicit results are obtained and compared to a continuum approach using the constitutive equation for a dilute Hookean dumbbell solution. As long as wall effects are neglected, identical results are obtained. This fact enables us to immediately obtain explicit results for Rouse model macromolecules as well.
On the isotropic-liquid crystal phase separation in a solution of rodlike particles of different lengths
Authors: Lekkerkerker H. N. W. , Coulon Ph. , Van Der Haegen R. , Deblieck R.
Published: February 2003

The Onsager theory for the isotropic–anisotropic phase separation in a solution of rodlike particles is extended to the case of mixtures of such particles with different lengths. The concentration, composition, order parameters, and orientation dependent thermodynamic quantities of the coexisting phases are calculated for the case of a mixture of rods of two different lengths for different length ratios. It is found that there is a significantly higher mole fraction of the longer rods in the anisotropic phase than in the isotropic phase. The order parameter of the longer rods is higher than in the one component case, whereas the order parameter of the shorter rods first increases and then decreases as the mole fraction of the longer rods is increased. All these features are accentuated as the length ratio of the two kinds of rods increases.
Geminate recombination in liquid argon
Authors: Freeman Gordon R.
Published: February 2003

The extended Onsager model for geminate neutralization is supported by the field dependence of the ionization yield in liquid argon irradiated by high energy electrons or x rays. Attempts to employ the model fail unless the distribution of initial separation distances between the thermalized electrons and their sibling ions (secondary electron thermalization ranges) is included. Data of Scalettar and co−workers are reanalyzed.
Comparison of calculated and experimental electric dipole intensity parameters for europium t
r
i
s(ethylsulfate) enneahydrate
Authors: Reid Michael F. , Richardson F. S.
Published: February 2003

Calculations of 4f–4f electric dipole intensity parameters are reported for Eu(C2H5SO4)3⋅9H2O. The calculations include contributions from both the static-coupling and dynamic-coupling mechanisms. Comparisons with empirical parameters lead to the same conclusions as previous studies by the present authors: dynamic coupling makes the dominant contribution to all Aλλ+1,p parameters. Intrinsic intensity parameters, defined within the context of the superposition model, are derived from the empirical parameters.
Influence of a velocity gradient on small angle neutron scattering of a sheared solution with rodlike micelles
Authors: Thurn H. , Kalus J. , Hoffmann H.
Published: February 2003

A solution consisting of 20 mM cetylpyridiniumsalicylate (CPS) and 20 mM sodiumsalicylate (NaSal) in which rodlike micelles are present, was sheared and studied in a small angle neutron scattering (SANS) experiment. Without a velocity gradient the scattering curves of constant intensity were circles which reflect an isotropic distribution of the orientation of the scatterers. If a velocity gradient is applied to the solution the curves of constant scattering intensity show a characteristic deviation from the circles due to the partially ordering of the rods. From that deviation the rotational diffusion coefficient can be obtained. A theoretical treatment of this problem is given and applied to the experimental data.
The rotational spectrum and structure of NH3–HCN
Authors: Fraser G. T. , Leopold K. R. , Nelson D. D. , Tung A. , Klemperer W.
Published: February 2003

The microwave spectrum of H3N–HCN has been measured using the molecular beam electric resonance technique. A symmetric top spectrum is observed and the following spectroscopic constants were obtained::[RW2:B0(MHz):3016.756 1(24)]
π electron spin density distributions and g values in semiquinone cations
Authors: Felix Christopher C. , Prabhananda B. S.
Published: February 2003

Intuitive arguments suggest that the approximate linear relation between g and the π electron spin density on the oxygens observed in semiquinone anions must also be applicable to the corresponding cations. This has been substantiated by redetermining the spin density distribution in p-benzosemiquinone cation and by demonstrating the linear relationship between g and the 1H hyperfine splittings aHOH associated with the hydroxyl protons of the cations. Our estimates of the spin density on the oxygens and QHOH are substantially different from the estimates reported in the literature. The 17O hyperfine splitting aO can be accounted for by using an additional parameter QOOH (∼50 G). Such a parameter was needed to account for the aO of semiquinone anions in hydrogen bonding solvents.
Photodissociation of bromine azide. II. Production of Br2(D′ 3Π2g)
Authors: Coombe R. D. , Lam C. H.-T.
Published: February 2003

The quenching of N2(A 3Σ+u) metastables by Br2(X 1Σ+g) results in the generation of Br2(D′ 3Π2g)→A′(3Π2u) emission. Although this energy exchange is near resonant, its rate constant is only (5±2)×10−13 cm3 molecule−1 s−1. Comparison of this result with the known rate for N2(A 3Σ+u) quenching by I2 indicates the importance of Franck–Condon factors in these energy transfer processes. In light of this result, the generation of intense Br2 D′→A′ emission from the UV photolysis of BrN3 is modeled as a Franck–Condon favored process in which Br2(A′ 3Π2u or A 3Π1u) is excited to the D′(3Π2g) state by collisions with N2(A 3Σ+u) metastables. Br2 (A or A′) is generated as a product of the reaction of bromine atoms, produced as a photodissociation fragment, with BrN3. The rate constant for this process is k5=(3.0±0.5)×10−11 cm3 molecule−1 s−1. The rate constant of the energy transfer process is inferred to be k6≥1.5×10−10 cm3 molecule−1 s−1. A second mechanism producing Br2(D′ 3Π2g) is evident in the time profile of the D′→A′ emission. This mechanism, which produces prompt D′→A′ emission, does not involve excitation by collisions with N2(A).
Intramultiplet mixing collisions of chromium 3d54p 7P
J with gases. Absolute cross sections and velocity dependence
Authors: Parson J. M. , Ishikawa T.
Published: February 2003

An atomic Cr beam has been excited with a cw dye laser to the 7P°4 and 7P°3 states in a beam-gas study of intramultiplet mixing transitions between these states and to the 7P°2 state. Absolute cross sections for these transitions have been found for the collision partners He, Ar, H2, D2, N2, CO, O2, N2O, NH3, and C2H4 by comparison of fluorescence intensity of laser excited and collisionally excited states. Cross sections are large in all cases, being larger than gas kinetic for NH3, and comparable to gas kinetic for N2, CO, and C2H4. Smaller cross sections for the others may be due to competing reactions. Time-of-flight measurements using a mechanically chopped Cr beam have yielded the velocity dependence of cross sections. Very little variation with relative velocity is found. Comparisons have been made for Cr scattering with He and Ar of experimental relative cross sections and velocity dependence with predictions of radial and Coriolis coupling models for nonadiabatic processes. Neither model alone can explain the observations. An application of the principle of microscopic reversibility to the N2 and CO time-of-flight results indicates that product energy partitioning in translation and rotation in a thermal experiment is quite similar to that of the reactants.
Chemical chaos in the chlorite–thiosulfate reaction
Authors: Masel/ko Jerzy , Epstein Irving R.
Published: February 2003

Sustained aperiodic oscillation, or chemical chaos, has been observed to occur in the reaction between chlorite and thiosulfate at pH 4 in a flow reactor via two different routes. In the first, chaotic behavior is found between successive regions of increasingly complex periodic oscillation as the residence time is increased. The second route occurs only at higher [ClO−2]0. At high flow rates, a periodic low frequency oscillation is observed. On decreasing the flow rate, bursts of a second higher frequency oscillation appear, the system becomes quasiperiodic, chaotic and then finally periodic at the higher frequency. Both types of chaos are analyzed in terms of an experimentally determined cubic next amplitude map for the system.
Theory of transport processes in dense fluids
Authors: Sung Wokyung , Dahler John S.
Published: February 2003

The projection operator methods of Mori and Zwanzig are used to construct kinetic equations for the single-particle distribution functions of a fluid mixture. The particle interactions are pair-additive sums of hard-core and soft, continuous potentials. The mean field kinetic equations obtained by discarding the multicomponent memory functions are identical in form to van Beijeren and Ernst’s revised Enskog equations for a hard sphere mixture, but incorporate the radial and direct correlation functions characteristic of the composite interaction. The 13-moments methods is used to construct an approximate solution of the mean field kinetic equation for a simple fluid and to relate the associated, zero-frequency transport coefficients to those of the conventional Enskog theory. Our choice of interaction parameters optimally mimicks a Lennard-Jones 12-6 fluid and yields excellent estimates of viscosity.
Composite pulse excitation in three-level systems
Authors: Levitt M. H. , Suter D. , Ernst R. R.
Published: February 2003

It is demonstrated by theory and experiment that it is possible to excite coherence uniformly in three-level systems with a wide range of anharmonicities by using a composite pulse excitation method. The theory is based on a formal analogy between the anharmonic three-level case and the simpler system with equally spaced energy levels where composite pulse techniques are already well known to compensate for the effects of mismatch between photon energies and level separations. Computer simulations and solid-state NMR experiments on the three-level system of deuterium (spin 1) verify the uniformity of the excitation. The NMR powder spectrum of deuterated polymethylmethacrylate is faithfully reproduced even using a radio-frequency (rf) field strength weaker than the quadrupole interactions.
Relation between carbon–carbon bond length and compressibility from high pressure studies on liquid benzene
Authors: Pucci R. , March N. H.
Published: February 2003

The frequency shift of the benzene ring stretching mode has been shown experimentally to be linear with pressure P over a range of about 10 kbar. Using Wilson’s formula for the free space molecule, arguing that the important force constant change with pressure is that associated with the carbon–carbon bond length l, and employing a semiempirical relation between force constant and l, value of dω/dl is determined, ω being the stretching mode frequency. Use of equation of state data on liquid benzene enables dω/dP to be calculated, in good agreement with experiment.
Conformational kinetics of methyl nitrite. II. Phase dependence of kinetic parameters
Authors: Chauvel J. Paul , True Nancy S.
Published: February 2003

Temperature and pressure dependent exchange broadened NMR spectra of gaseous methyl nitrite are consistent with the following kinetic parameters for the syn–anti conformational interconversion: Eact (∞), 12.2(4) kcal/mol; Eact (bimolecular), 9.6(2) kcal/mol; ΔH‡, 11.7(4) kcal/mol; ΔG‡298 11.97(7) kcal/mol; and ΔS‡298 , −0.9(4) cal/mol K. Kinetic parameters for interconversion in the neat liquid and in a 1% solution in CS2 are slightly higher: Eact; 12.9(2); ΔH‡, 12.4(2) kcal/mol; ΔG‡, 11.81(8) kcal/mol; and ΔS‡, 2.6(2) cal/mol K. These remarkably small solvent effects indicate that dielectric effects on conformational dynamics in this system are nearly equal in magnitude and opposite in direction to solvent internal pressure effects.
Anisotropic van der Waals model of the liquid–vapor interface
Authors: Weeks John D. , Bedeaux D. , Zielinska B. J. A.
Published: February 2003

We study the properties of the liquid–vapor interface in a model system with anisotropic long-ranged attractive interactions, so chosen that exact calculations (using the density functional formalism) can be carried out and for which most results of the classical van der Waals theory of the liquid–vapor interface hold true without approximation. In particular there is an ‘‘intrinsic’’ interface profile of finite width, which near the critical point takes on the hyperbolic tangent form. However, contrary to approximate calculations of Evans, all pair correlations (except for trivial k=0 correlations) are short ranged. Indeed they are identical to those in a nonuniform (one phase) hard sphere system in an external field chosen to produce the desired density profile. The long-ranged anisotropic interactions suppress long wavelength (capillary wave) density fluctuations, which in a realistic system produce both long-ranged pair correlations parallel to the interface and a nonuniversal density profile.