bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational structure and predissociation dynamics of the methyl 4p
z(v=0) Rydberg state investigated by resonance enhanced multiphoton ionization spectroscopy
Authors: Black John F. , Powis Ivan
Published: July 2002

The vibrationless bands of the methyl 4p 2A″2 ←X̃ 2AA″2 0–0 Rydberg transition have been recorded by mass selected (2+1) REMPI spectroscopy using photolytically generated CH3 and CD3 radicals. A least-squares analysis of the spectra determines values for the band origin and the rotational constants B′, D′N, DNK, and (C′−C″). Linewidths in the CH3 spectrum show a pronounced rotational level dependence and by means of a spectral simulation a heterogeneous predissociation is identified and shown to be induced by a perpendicular Coriolis coupling mechanism.
Infrared line shapes of ethylidyne on the Pt(111) surface
Authors: Malik Igor J. , Agrawal Vijay K. , Trenary Michael
Published: July 2002

We have measured the temperature dependence of the infrared line shapes of three fundamental vibrations of the ethylidyne species, ≡C–CH3, on the Pt(111) surface. Each of the three line shapes possesses distinctly different properties. The C–C stretch shows the strongest temperature dependence. The full width at half-maximum (FWHM) varies quadratically with temperature from 6 cm−1 at 82 K to 10 cm−1 at 350 K. The frequency of the C–C stretch shows a linear shift from 1125 cm−1 at 82 K to 1117 cm−1 at 350 K. The symmetric C–H stretch shows a temperature independent frequency of 2883 cm−1 and only a small change in full width from 4.1 cm−1 at 82 K to 5.1 cm−1 at 300 K. The symmetric CH3 bend has an unusually narrow full width of 1.2 cm−1 at 82 K which increases to 2.4 cm−1 at 350 K while its frequency remains constant within the experimental error. The results are discussed in terms of current theories of vibrational line broadening for molecules adsorbed on metal surfaces.
Molecular dynamics of liquid butane
Authors: Toxvaerd So/ren
Published: July 2002

Molecular dynamics simulations have been used to investigate the sensitivity of liquid butane to different intra- and intermolecular potential functions. The calculations show that the thermodynamic data and the self-diffusion constant for liquid butane at room temperature are only weakly dependent on the intramolecular rotational potential. Calculations of pressure and distribution functions for systems with and without intermolecular attractive forces show that there is an increased orientational ordering in systems with purely repulsive forces. This effect must be included in perturbation theories for these molecular systems.
Reply to a ‘‘Comment on: ‘The vibrational relaxation of highly excited molecules’ ’’
Authors: Gordon Robert J. , Beck Kenneth M.
Published: July 2002

Lenzi et al’s comments on the number of photons absorbed by sulfur hexafluoride, and the subsequent transfer of energy to argon atoms are answered. It is suggested that the optoacoustic and the IR absorption measurements should be complimentary to each other.(AIP)
An interaction site approach to clustering and percolation phenomena in systems of nonspherical particles
Authors: Lupkowski M. , Monson P. A.
Published: July 2002

We present a new approach to the theory of clustering and percolation phenomena in assemblies of nonspherical particles. The method is based on an interaction site formalism. By treating each particle in the assembly as a collection of interaction sites we are able to formulate the connectivity problem in terms of a site–site pair connectedness function, Pαβ(r). Through adaptation of existing results in the theory of pair correlations in interaction site systems the cluster expansion of Pαβ(r) has been obtained and two Ornstein–Zernike-like integral equations are developed through which Pαβ(r) may be calculated. As an illustration of the approach results are presented for a system consisting of dumbbells randomly distributed in a matrix.
Extraction of ground state properties by discretized path integral formulations
Authors: Kono H. , Takasaka A. , Lin S. H.
Published: July 2002

We offer a way of determining the temperature range in which a path integral (PI) formulation of the quantum partition function works well and a way of calculating the ground state properties without employing extremely low temperatures (in order to elude the awkward problem that the quantities calculated by the PI formulation become inaccurate with decreasing temperature owing to unavoidable truncation of an infinite number of path integral variables). The fact that the PI energy, specific heat, etc. behave in a low temperature range against physical laws makes it possible to locate the ‘‘marginal’’ temperature at which the PI specific heat begins to grow infinitely and to estimate the lowest temperature at which the PI formulation functions well (the ‘‘threshold temperature’’). Whether or not the threshold temperature is low enough to extract only the ground state properties can be answered by either checking if the PI energy and free energy are equal at the threshold temperature or checking if the PI specific heat is relatively small thereat. If the system is in the ground state at the threshold temperature obtained, it is recommended to calculate the ground state properties at this temperature. This scheme can be executed by Monte Carlo methods, being open to many-particle systems. Using the discretized PI formulations, we apply the above procedure to a harmonic oscillator and a double-well potential. It is concluded that this scheme is successful at least as long as the potential is a slowly varying function of coordinates.
Photodissociation of doubly excited states of H2: Emission of Balmer lines
Authors: Glass-Maujean M.
Published: July 2002

The cross section for the formation of H(n=3) or H(n=4) by photodissociation of H2 has been measured for the first time, using synchrotron radiation between 500 and 300 Å. The comparison with computed cross section curves allows the identification of the doubly excited molecular dissociating states. The importance of the diabatic behavior of these states is discussed.
The HeNe interatomic potential from multiproperty fits and Hartree–Fock calculations
Authors: Keil Mark , Danielson Laurie J. , Buck Udo , Schleusener Jens , Huisken Friedrich , Dingle Thomas W.
Published: July 2002

New high-resolution differential scattering cross sections are reported for the HeNe interaction. These experimental results are combined with Hartree–Fock calculations in constructing a highly accurate interatomic potential. The new potential is capable of reproducing all available experimental data judged to be sufficiently reliable. This includes properties that are highly sensitive to the very weak attractive well and its outer bowl, in addition to the weakly repulsive wall. The potential is compared to those previously proposed for HeNe, particularly to one obtained by direct inversion of differential cross section data of similarly high quality. The potential crosses through zero at σ=2.699 Å; its minimum occurs at rm=3.029 Å with a depth of ε=1.83 meV.
Studies of the photoionization cross sections of CH4
Authors: Braunstein M. , McKoy V. , Machado L. E. , Brescansin L. M. , Lima M. A. P.
Published: July 2002

We present cross sections and asymmetry parameters for photoionization of the 1t2 orbital of CH4 using static-exchange continuum orbitals of CH+4 to represent the photoelectron wave function. The calculations are done in the fixed-nuclei approximation at a single internuclear geometry. To approximate the near-threshold behavior of these cross sections, we assumed that the photoelectron spectrum is a composite of three electronic bands associated with the Jahn–Teller components of the distorted ion. The resulting cross sections reproduce the sharp rise seen at threshold in the experimental data and are in good agreement with experiment at higher energy. The agreement between the calculated and measured photoelectron asymmetry parameters is, however, less satisfactory.
Electronic and structural characterization of a charge transfer crystal with strong electronic correlations through infrared and Raman spectroscopy: TMPD–TCNQF4
Authors: Meneghetti Moreno , Bozio Renato , Bellitto Carlo , Pecile Cesare
Published: July 2002

The electronic and structural characterization of the title charge transfer (CI) complex is carried out mainly by the exploitation of the infrared powder spectrum and its temperature dependence from room temperature to 15 K as well as of the Raman powder spectrum at 15 K. The infrared spectrum shows that the complex has a fully ionic electronic ground state (ρ≂1) and the analysis of the spectroscopic effects due to the radical electron–intramolecular vibration interactions reveals a structure built up of segregated stacks of TMPD and TCNQF4. Moreover, this analysis shows that the TCNQF4 stacks are nonregular in the whole studied temperature range, whereas, by lowering temperature, the TMPD stacks undergo a phase transition from a regular to a nonregular structure with onset at about 180 K. The effective on-site electronic correlation parameter, the hopping integral, and the magnetic gap of both stacks in the nonregular phase are estimated by adopting a cluster model made up of a dimeric unit to describe the stacks and by comparison with known materials. On the same basis, the temperature dependence of the hopping integrals is also estimated. The collected information, considered in the light of the Peierls–Hubbard model, suggests that TMPD–TCNQF4 can be considered a compound with strong electronic correlations.
High-pressure Raman study of the hydrogen-bonded crystalline formamide
Authors: Shimizu H. , Nagata K. , Sasaki S.
Published: July 2002

The Raman spectra of hydrogen-bonded crystalline HCONH2 and HCOND2 have been measured up to 10 GPa at 300 K in a gasketed diamond anvil cell. The frequencies of the in-plane intermolecular stretching mode which characterizes the hydrogen bond along the zig–zag chain increase with increasing pressure with a slope (dν/dP) of 11.1 cm−1/GPa, and its slope becomes steeper, to 19.2 cm−1/GPa, at about 5.0 GPa (=Pc). The frequencies of the intramolecular ND2 stretching mode decrease with increasing pressure at all pressure range except around Pc where they show an increase of 20 cm−1. It turns out from these results that (1) the hydrogen (deuteron) bonds along the chain direction become stronger in response to the pressure, and their increasing strength are more developed above Pc, and (2) the other type of hydrogen (deuteron) bonds which bridge the chains to make their planar hydrogen-bonded network are weakened around Pc. The pressure-induced phase transition at Pc is discussed by focusing on the behavior of two types of hydrogen bondings and the effect of the interplanar interaction.
The spectroscopy and molecular dynamics of the high frequency ν16 intermolecular vibrations in HCN---HF and DCN---DF
Authors: Wofford B. A. , Jackson M. W. , Lieb S. G. , Bevan J. W.
Published: July 2002

Gas phase rovibrational analysis of the high frequency intermolecular hydrogen bonded bending overtone 2ν06 [ν0=1132.4783(2) cm−1] in HCN---HF and its corresponding perdeuterated fundamental ν16 [ν0=409.1660(2) cm−1] are reported. Evaluated rovibrational parameters provide the basis for quantitative modeling of the molecular dynamics associated with this vibration. A quantum mechanical calculation permits determination of the quadratic and quartic force constants K66=537(17) and K6666=4.98(12) cm−1 which in turn are used to estimate the pertinent cubic band stretching interaction constants K466=−149.3(50) cm−1 and account for the unexpected behavior in the rotational constant B16. Second order expansion of the vibrational term energies, give X46=−21.61(2), X67=−7.694(1), X66=−14.84(90), g66=−31.04(90) cm−1, neglecting corrections for Fermi resonance. The common isotopic species equilibrium rotational constant Be is evaluated to be 3681.1(11) MHz.
Fragmentation of molecular adsorbates by electron and ion bombardment: Methoxy chemistry on Al(111)
Authors: Basu P. , Chen J. G. , Ng L. , Colaianni M. L. , Yates J. T.
Published: July 2002

(High resolution) electron energy loss spectroscopy [(HR)EELS] has been used successfully to provide direct spectroscopic evidence regarding details of the molecular fragmentation of methoxy (CH3O) on Al(111) caused by energetic electron and ion beams. Chemisorbed methoxy on Al(111) is produced by heating of adsorbed CH3OH. Irradiation of CH3O(a) by either energetic (∼300 eV) electrons or Ar+ ions results in C–O and C–H bond scission with simultaneous formation of Al–O and Al–C bonds. During electron stimulated desorption the CH3O(a) species undergo sequential fragmentation first to CHx groups that are captured by the surface and in the final decay process to adsorbed carbon. C–O bonds in CH3O(a) are depleted preferentially compared to C–H bonds in CHx(a) species. The electron induced sequential fragmentation of the parent CH3 group (from methoxy) to resultant CHx(a) occurs with an efficiency ∼3 orders of magnitude greater then the subsequent process of CHx(a) →C(a). Cross sections for various bond scission processes in electron and ion bombardment have been estimated.
The role of surface reactions in monosilane pyrolysis
Authors: Scott B. A. , Estes R. D. , Jasinski J. M.
Published: July 2002

Monosilane pyrolysis has been studied by measuring the rate of silicon deposition from SiH4–Ar mixtures in a flow reactor at temperatures between 400–500 °C. By varying gas residence time, SiH4 partial pressure and total pressure, the process could be separated into experimental regimes dominated by either homogeneous or heterogeneous reactions. In the surface regime at low total pressures and short residence times, silicon deposition is due to SiH4 alone, follows Langmuir–Hinshelwood kinetics, and saturates at high SiH4 partial pressures. The rate-determining step is most likely hydrogen desorption, and the overall activation energy is 37.5 kcal/mol. Film deposition in the homogeneous limit at high total pressures and long residence times is dominated by Si2H6 and higher silane products of the gas phase decomposition. The ratio of surface to gas phase decomposition rate has been obtained from the surface rate data and RRKM calculations. The contribution to the initial pyrolysis rate of direct (SiH4 only) surface channels at 430 °C, e.g., is >50% below 10 Torr, but becomes negligibly small for pressures exceeding 100 Torr. In experiments which follow the time evolution of the pyrolysis, Si deposition rates increase as the homogeneous reaction accelerates into the Purnell–Walsh ‘‘later time,’’ and film growth is soon superseded by homogeneous nucleation of silicon powder.
Vibrational level dependence of Na2–Ne scattering cross sections at very low collision energy
Authors: Gaubatz U. , Becker M. , Bergmann K.
Published: July 2002

A significant increase in the total integral cross section in (Na)2−Ne collisions with the vibrational level v’’ of the sodium molecule in the electronic ground state, at a collision energy of the order of 1 mev is reported. (AIP)
A
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n
i
t
i
o investigation of the bound rovibrational states in the electronic ground state of HeN+2
Authors: Miller Steven , Tennyson Jonathan , Follmeg Bernd , Rosmus Pavel , Werner Hans-Joachim
Published: July 2002

The two-dimensional interaction potential of the N+2(X 2Σ+g) –He(X 1S) system has been calculated using highly correlated ab initio MCSCF-CI wave functions for a fixed value of the N+2 bond length (2.110 a0). It is found to have a minimum about 140 cm−1 below the N+2 +He dissociation limit, significantly deeper than the minimum in the neutral system N2–He. This well depth is enough to give rise to a cluster-like, bound structure with a considerable number of vibration–rotation levels in the electronic ground state. The well depth is almost independent of the N2–He angle, which leads to large amplitude bending motions. Rovibrational calculations have been performed on the surface for J=0, 1, and 2. Values for the rotational constant B, the fundamental stretching frequency νs, and its first and second overtones, and the rotational constant Cn in each nνs manifold, have been obtained from the energy levels computed. It is found that B=1.879 cm−1, 1νs =55.231 cm−1, 2νs =83.404 cm−1, 3νs =92.491 cm−1, C0=0.477 cm−1, C1=0.370 cm−1, and C2=0.254 cm−1. Estimates of the accuracy of these parameters are made.
Potential of mean force between polar molecules adsorbed onto a wall: The exact long-range behavior
Authors: Badiali J. P.
Published: July 2002

In studies of adsorption from solutions, the potential of mean force between solute particles is a key quantity from which we may derive the thermodynamics of adsorption via the osmotic partition function. We consider the case of a polar solute in a polar solvent. The long-range part of the potential of mean force is obtained via a statistical mechanical derivation. The result contains a direct and an image potential as it is expected from a dielectric continuum approach. We show rigorously that the contribution of the dipole–dipole interaction comes essentially from a region far beyond the interfacial region where all electrostatic properties can be described in terms of dielectric constant. The new result consists in showing that the dipole moment of the solute particle must be replaced by an effective dipole moment which depends on the position of the molecule relative to the wall. We give the exact relation between the effective dipole moment and the short-range part of the solute–solvent pair correlation function, thus generalizing a previous result established in the context of the theory of dielectric constant. By using a simple model for the interfacial structure we discuss the magnitude of the effective dipole moment for a particle located onto the wall. The possibility of interpreting the lateral interactions in terms of local dielectric constant is analyzed.
Electronic and structural characteristics of five poly membered heterocycles (polythiophene, polypyrrole): An ultraviolet and x-ray photoelectron spectroscopy study
Authors: Tourillon G. , Jugnet Y.
Published: July 2002

The electronic and structural properties of electrochemically synthetized five poly membered heterocycles [polypyrrole (PP), poly-N-methylpyrrole (PNMeP), polythiophene (PT), polybithiophene (PBT), and poly-3-methylthiophene (PMeT)] in their doped conducting and undoped semiconducting states have been determined by x-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS). A systematic evolution of the π bonding orbitals is observed by going successively from thiophene to bithiophene and to terthiophene which leads finally to the formation of an intrinsic π bonding band for the undoped polythiophene, located at 1.05 eV below the Fermi level Ef. This π bonding band is also detected with PP, PBT, and PMeT in good agreement with theoretical band structure calculations and implies a long range order along the polymeric chains. In contrast, PNMeP valence spectra exhibit broad features connected with the existence of structural defects and/or torsion angle between the monomeric units. Shake-up satellites are generally observed on the high binding energy side of the C1s, S2p (or N1s) core levels related to π→π* transitions. The S/C (or N/C) ratio is always very close to the predicted theoretical value which means that the structural architecture of the polymeric backbone is very similar to the monomer one. Upon ClO−4 doping, the π bonding band is extended up to ∼0.2 eV below Ef.

Simultaneously, the bonding band intensity decreases, and the C1s, S2p, (or N1s) core level peaks become broad and asymmetric (PP, PMeT, PT). These evolutions are completely reversible during the doping–undoping processes, independent of the anion used (BF−4,SO3CF−3) and of the thickness of the film (20–1000 Å). These modifications are dramatically reduced with PNMeP although its doping level is as high as 30%. These UV photoelectron spectroscopy and x-ray photoelectron spectroscopy results combined with the near edge x-ray absorption fine structure (NEXAFS) characteristics give insight into the conduction mechanism inside these conducting polymers: Appearance of a metallic-like behavior due to first the extraction of electrons from the π bonding band, and second, an extension of the π and π* bands towards Ef narrowing the band gap from 2.1 eV in the undoped state to 0.3–0.4 eV in the doped form.
Kinetic analysis in thermally anisotropic systems: Application to supersonic free jet expansions
Authors: Mazely T. L. , Smith M. A.
Published: July 2002

Expressions for gas phase reaction rate coefficients in a thermally anisotropic medium are presented. These are related to homogeneous, equilibrium rate coefficients applicable to an effective temperature, averaged over the density profile present in the medium. Experimental procedures for determining these bulk rate coefficients at the ultracold temperature within a free jet expansion are discussed. Depending on the specific intermolecular force between the colliding species, data analysis of the experiments is influenced heavily by the flow region in which the measurements are taken.
Structure-based analysis of the initial electron transfer step in bacterial photosynthesis: Electric field induced fluorescence anisotropy
Authors: Lockhart David J. , Goldstein Robert F. , Boxer Steven G.
Published: July 2002

The fluorescence intensity from randomly oriented, immobilized Rb. sphaeroides reaction centers at 77 K increases in the presence of an externally applied electric field. We have proposed that this increase is due to a net decrease in the rate of the forward electron transfer reaction which competes with the prompt fluorescence. This decrease results from the change in the free energy difference between the reactant and very dipolar product state in the presence of the electric field. Because the free energy change and thus the electron transfer rate for a given reaction center depends on its orientation relative to the field, the intensity of the competing fluorescence likewise becomes orientation dependent. An expression is derived relating the degree of this electric field induced fluorescence anisotropy to the angle ζet between the fluorescence transition moment and the effective dipole moment whose interaction with the field results in the change in the rate of the electron transfer reaction which competes with fluorescence. ζet is determined to be about 69°. This angle can be estimated from the x-ray crystal structure coordinates for possible identities of the initial electron acceptor. The results are inconsistent with a two-step hopping mechanism in which the bacteriochlorophyll on the L side is the initial electron acceptor whose formation competes with fluorescence. Effects of an electric field on the electronic coupling for a superexchange mechanism are discussed. The theoretical and experimental approach should be generally applicable for studying long-range electron transfer reactions.
Grüneisen gamma and acoustic velocity for soft sphere fluids
Authors: Shaner J. W.
Published: July 2002

New experimental and computational methods make possible measurement or simulation of the thermodynamic properties of fluids over wide ranges of temperature and density. We use a model for fluid thermodynamics based on the (1/r)n repulsive potential to show qualitatively what behavior one might expect for measurements of Grüneisen’s gamma and acoustic velocity for compressed and expanded fluid metals and molecular fluids. We find limited regions of validity for such empirical rules as ργG =constant and linear density dependence of sound velocity. These results are compared with experimental data for fluid metals, and with N2 simulations presented in the accompanying paper.
Rydberg fluorescence of NO trapped in rare gas matrices
Authors: Chergui M. , Schwentner N. , Chandrasekharan V.
Published: July 2002

Fluorescence spectra from the Rydberg A 2Σ+ (v=0) level of NO trapped in Ne, Ar, Kr, and Xe matrices have been obtained for two different sites. The main site is characterized by broad A(0,v″) bands (FWHM≂80 meV), blue gas-to-matrix shifts of 70 to 300 meV, and by absorption–emission Stokes shifts of 300 meV in Xe to 800 meV in Ne matrices. The line shapes and Stokes shifts are treated within a configuration coordinate model by a moment analysis. A linear and quadratic coupling is invoked with relaxation energies ranging for the excited state from 160 meV in Xe to 540 meV in Ne and for the ground state from 150 meV in Xe to 280 meV in Ne and an increase of the cage radius of 3% in Xe to 15% in Ne. The ‘‘red’’ site fluorescence shows emission bands with matrix shifts of −140 meV in Xe to 80 meV in Ne and absorption–emission Stokes shifts of 210 meV in Xe to 830 meV in Ne. Red site ‘‘hot’’ A (v=1) fluorescence is also observed. Red sites are attributed to local disorder around the molecule in Ar, Kr, and Xe matrices and to hexagonal close packed (hcp) pockets within the face centered cubic (fcc) lattice in the case of Ne matrices.
Periodic orbit analysis of the photodissociation spectrum of H+3
Authors: Gomez Llorente Jose M. , Pollak Eli
Published: July 2002

The photodissociation spectrum of the hydrogen−3 cation is explained. The regular structure in the coarse grained spectrum is contended to be due to rotational transitions.(AIP)
Perturbations of the A 1Σ+u and b 3Πu states of Na2 and the effects on the transition intensity and the line splitting
Authors: Katô Hajime , Otani Mitsuhiko , Baba Masaaki
Published: July 2002

Doppler-free polarization spectra of the A 1Σ+u (v=8, J=0–15) and b 3Π0u (v=14, J=0–15) states were observed. By analyzing the energy shifts, the spin–orbit interaction between the A 1Σ+u (v=8) and b 3Π0u (v=14) states was studied. The magnitudes of the hyperfine splittings were different in corresponding lines of the two states. In addition to the energy shifts, the line intensities and the line shapes were found to change remarkably near the perturbation. These are useful to study the mixing of the perturbing states.
Epitaxial growth of CO on NaCl(100) studied by infrared spectroscopy
Authors: Chang Huan-Cheng , Richardson Hugh H. , Ewing George E.
Published: July 2002

Vibrational spectra of CO physisorbed onto well defined NaCl(100) surfaces were studied using a Fourier transform infrared interferometer. Structures of CO starting from the monolayer to multilayers were explored. At 31.5 K and a CO pressure of 1×10−6 mbar only the monolayer is formed. Polarization measurements confirm our earlier study that the monolayer CO molecules are aligned perpendicular to the NaCl(100) surface. Increasing the CO pressure to 7×10−6 mbar produces multilayer adsorption. The multilayer spectra closely resemble that of α-CO absorption previously reported. The near perfect match of crystal structures and lattice constants of α-CO and NaCl is reasoned to force the epitaxial growth of single crystal multilayers in our experiments. At 22 K the monolayer absorption is at 2155.01 cm−1 with a bandwidth (FWHH) of 0.26 cm−1. The two prominent features in the multilayer spectra at 22 K are assigned to the longitudinal optical (LO) mode at 2142.54 cm−1 and the transverse optical (TO) mode at 2138.51 cm−1. Their frequency separation is a consequence of the lowering of the cubic symmetry of the bulk α-CO crystal by the shape, in the form of thin slabs, of our multilayer samples. Their bandwidths depend on the thickness of the sample and are characterized by a bandwidth parameter of 0.25 cm−1 for the LO mode and 0.85 cm−1 for the TO mode. The relative absorbances of these modes depend on the polarization of the infrared radiation. Theoretical formalism to account for the band splitting and absorption profiles of the infrared absorption is reviewed and applied to our measurements. While many features of our data can be explained by the present theory, further work is required to account for all the experimental results.
On the use of scaled quantum mechanical force fields for predicting quartic centrifugal distortion constants
Authors: Császár A. G. , Fogarasi G.
Published: July 2002

The reliability of scaled quantum mechanical (SQM) force fields to predict centrifugal distortion constants has been studied on some prototype molecules. Quartic constants have been calculated for formaldehyde, thioformaldehyde, formamide, ethylene, t-acroleine, pyrazole, pyrrole, pyridine, and benzonitrile. Experimental values are reproduced within about 10%, which is quite satisfactory as these constants represent small correction terms in the rotational Hamiltonian.
The S1–S(1B2u–1A
g) transition of p-difluorobenzene cooled in a supersonic free jet expansion. Excitation and dispersed fluorescence spectra, vibrational assignments, Fermi resonances, and forbidden transitions
Authors: Knight Alan E. W. , Kable Scott H.
Published: July 2002

The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p-difluorobenzene (000 at 36 838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been identified. The prevalent Fermi resonance between ν′5 and 2ν6 has been analyzed with the assistance of both excitation and dispersed fluorescence spectroscopy, yielding a coupling matrix element [g566<51‖Q5‖50><60‖Q6‖62>]=−1 cm−1. Thirty-one matrix elements describing cubic anharmonicity and involving a variety of vibrational modes have been estimated. The majority of the coupling matrix elements lie within the range ±2 cm−1.
Relative performances of the Kohn, Schwinger, and Newton variational principles in scattering theory
Authors: Ramachandran B. , Wei Tai-Guang , Wyatt Robert E.
Published: July 2002

Comparisons of three variational principles commonly used in scattering problems, namely those due to Kohn (KVP), Schwinger (SVP), and Newton (NVP), are presented. These comparisons are conducted by computing K-matrix elements for elastic scattering from nine different interaction potentials. We represent the KVP trial functions as expansions containing two non-L2 terms that represent the asymptotic free wave, and a set of L2 functions, while the SVP and the NVP trial functions are expansions containing only the L2 terms. Three different sets of L2 functions are used to examine the effect of changing the basis on the convergence characteristics of the three methods. We find that the rates of convergence for the Kohn, Schwinger, and Newton methods are strongly dependent on the nature of the potential and the basis set used. We also find that purely repulsive potentials are, in general, easier to converge than purely attractive potentials.
Quasidegenerate variational perturbation theory and the calculation of first-order properties from variational perturbation theory wave functions
Authors: Cave Robert J. , Davidson Ernest R.
Published: July 2002

In previous work on the treatment of correlation in molecular systems we have applied a multireference version of second-order Hylleraas variational perturbation theory. The choice made for the partitioning of H treated the interactions between the correlating functions to infinite order and gave the corrections to the wave function to first order. The method was shown to be accurate in many cases, but became less so when near degeneracies occurred between the reference energy and other eigenvalues of H0. In this article we introduce an effective Hamiltonian method that is analogous to variational perturbation theory, but which is significantly more accurate when near degeneracies are important. This quasidegenerate variational perturbation theory (QDVPT) is an explicitly multireference procedure and treats the entire reference space as a quasidegenerate space. A novel method for solving the QDVPT equations is introduced that avoids explicit construction of the effective Hamiltonian. As a result, the work involved in application of QDVPT is on the roder of that required for variational perturbation theory. We also present an approximate method for calculating first-order atomic and molecular properties based on Hylleraas variational perturbation theory, multireference linearized coupled cluster, and QDVPT wave functions. The properties are calculated as derivatives of the energy with respect to the field strength. Construction of a one-electron density matrix based on the energy derivative expression allows rapid evaluation of one-electron properties. Results are presented and compared to full and truncated CI results. Good agreement is found in the cases examined.
On an alternative form for the free energy of a single polymer chain in solution
Authors: Dayantis Jean
Published: July 2002

The free energy of an isolated polymer chain in solution is decomposed into a sum of three terms. It is shown that one of these terms is the Flory ‘‘elastic’’ contribution to the free energy; the other terms originate from chain segment–segment interactions of any order. This decomposition of the free energy in a sum of terms is valid whatever the quality of the solvent. The relative importance, as a function of temperature, of the Flory elastic and of the nonelastic terms is not presently elucidated. It may however be shown that in the two limits of the perfect nonsolvent and the ideally good solvent the elastic term vanishes. An application to the chain collapse problem is suggested.
Multichannel processes of H2O in the 18 eV region
Authors: Wu C. Y. Robert , Judge D. L.
Published: July 2002

The various absolute partial cross sections for producing the excited neutral fragments, OH(A 2Σ+), H(n=2–4), and H2(a 3Σ+g), from photoexcitation of H2O in the 15–20.5 eV region are presented. It is found that a large fraction of the neutral products which amounts to 0.35 in the 15.7–17.7 eV region can be attributed to the undetected neutral products of (1) H(n=1)+OH(X 2Π), (2) H2(X 1Σ+g)+O(1D), and (3) H2(X 1Σ+g)+O(1S). All these products will possess high kinetic energy. The present results clearly show strong competitions among processes leading to photoionization and various photodissociation processes.
A numerical stochastic approach to network theories of polymeric fluids
Authors: Petruccione F. , Biller P.
Published: July 2002

A new numerical approach is presented to exactly solve the convection equation arising in network theories. The method is based on a direct stochastic interpretation of the convection equation. We show that with this approach models can be studied extensively which are not solvable analytically. It turns out that a conceptually simple approach to network theories predicts a qualitatively satisfying rheological behavior.
Unexpected hydrogen induced displacement of chemisorbed CO from the Ni(100) surface
Authors: Shen Shikong , Zaera Francisco , Fischer Dan A. , Gland John L.
Published: July 2002

We report the first observation of hydrogen induced displacement of chemisorbed CO from the Ni (100) surface. This displacement is unexpected since the heat of adsorption for CO is 30 kcal/mol, about 7 kcal/mol larger than the 23 kcal/mol heat of adsorption for hydrogen. These displacement studies were performed in a UHV system equipped with Auger electron spectroscopy and facilties for temperature programmed desorption. Rates of displacement were measured by integrating CO temperature programmed desorption spectra for a series of displacement times. Hydrogen pressures in the 10−3 to 10−4 Torr range cause displacement of chemisorbed CO in the 290 to 330 K temperature range in a matter of minutes. After displacement, the surface contained only undisplaced CO and adsorbed hydrogen. No surface contamination was detected following CO temperature programmed desorption. The displacement reaction is clearly positive order in CO coverage. The CO coverage data suggests two first order reaction regions with a decrease in rate with decreasing coverage. Displacement is about half-order in hydrogen pressure. For low CO coverages (below 0.3 to 0.4 monolayer) and 309 K the displacement probability is around 5×10−7 per incident H2 molecule and has an activation energy of about 12±1 kcal/mol. At high coverages the displacement probability is about 1×10−6 per incident H2 molecule and the activation energy decreases to about 8±2 kcal/mol. No CO displacement was observed in the 270 to 330 K temperature range for pressures up to 10−3 Torr of He, Ne, CH4 or N2. The CO displacement rate is also insensitive to deuterium substitution. The thermal activation energies measured indicate that the metal surface is furnishing 8 and 12 kcal/mol for the displacement of high and low coverages of adsorbed CO, respectively. In order for adsorbed CO to be removed from the surface either the heat of desorption for a portion of the adsorbed CO must be decreased to match the thermal activation energies or direct energy transfer must be occurring during the displacement process. The current results do not allow us to distinguish between these two types of molecular mechanisms.
Vibrational branching ratios following two-color excitation of autoionizing n
p Rydberg states of H2
Authors: O’Halloran M. A. , Dehmer P. M. , Tomkins F. S. , Pratt S. T. , Dehmer J. L.
Published: July 2002

Two-color resonantly enhanced multiphoton ionization-photoelectron spectroscopy (REMPI-PES) was used to determine vibrational branching ratios following autoionization of the ungerade npσ 1Σ+u and npπ 1Πu Rydberg states of H2. In this two-step experiment, one laser used to excite the two photon transition to the E,F 1Σ+g, v′=E2, J′=1 state, and a second laser was used to access the autoionizing Rydberg states near the H+2X 2Σ+g, v+=2 ionization limit. Electrons corresponding to the formation of H+2X 2Σ+g, v+=0 and 1 were collected and energy analyzed using a magnetic bottle electron spectrometer. In agreement with the well-known propensity rule for vibrational autoionization, the vibrational branching ratios strongly favor the final ionic state that corresponds to the minimum change in vibrational quantum number. In general, the branching ratio into the v+=1 channel is 94%–96%, while that into the v+=0 channel is 4%–6%; however, two major deviations from this trend were observed for Rydberg states that are perturbed by the 3pπ 1Πu, v=9 and 4pσ 1Σ+u, v=7 states. Although these low n/high v interlopers were not observed in the present work (since their ionization efficiency is near zero), interchannel coupling apparently causes their influence to be felt by nearby Rydberg states, resulting in v+=0 branching ratios as high as 18%. A number of additional studies suggested by these initial results are discussed.
Effect of pressure on the internal rotation angle of biphenyl in carbon disulfide
Authors: Katō Minoru , Higashi Mikio , Taniguchi Yoshihiro
Published: July 2002

The effect of pressure on the conformation of biphenyl in carbon disulfide was measured by using high pressure infrared spectroscopy with a diamond anvil up to 13 kbar. Pressure effects on the nonplanar vibrational modes indicate that the dihedral angle decreases at the rate of 2° kbar−1. Under high pressure the frequency of the C–H stretching vibrational modes shifts to blue so that the repulsive force becomes more predominant in the solute–solvent interactions between CH and CS2 molecule. The decrease in the dihedral angle of biphenyl induced by compression is related to the change of the intrinsic volume of the biphenyl molecule.
A variational calculation of the molecular vibrations of H2O for states with zero angular momentum
Authors: Shipsey E. J. , Kohl D. A.
Published: July 2002

A different approach for calculating the zero angular momentum, vibrational energy levels of polyatomic molecules was implemented for H2O. The special features of this approach are that, even for arbitrarily large amplitudes of vibration, all operators in the Hamiltonian are easily evaluated analytically, there are no pathological singularities, angular momentum is rigorously decoupled, and the variables are a mixture of two curvilinear and 3N−8 Cartesian coordinates. As a demonstration of the approach, a full variational calculation was done and the results are compared with a prior result based on the Watson Hamiltonian. Although the two Hamiltonians appear to be very different and although two different computational methods were employed, the results of the two independent calculations were virtually identical for the lowest 14 vibrational states of H2O. Compared to the conventional normal coordinate formalism, the kinetic energy operator contains products of anti-Hermitian operators and these ‘‘extra’’ terms are found to change the vibrational frequencies by tens of wave numbers.
Theoretical study of short-range order in supercooled liquids and amorphous solids
Authors: Stillinger Frank H. , Root Leslie J.
Published: July 2002

The measurable properties of supercooled liquids and amorphous solids reflect the short-range packing geometry of the contituent particles. This paper is devoted to the description of that short-range order by the Born–Green–Yvon (BGY) local stress equation which relates pair and triplet distribution functions to the pair potential. Since metastable (but long-lived) phases are at issue, it has been necessary to identify an appropriate class of ensembles for which the BGY relation can be justified. In particular, low-temperature amorphous solids have preparation-method-dependent properties, and we propose to classify their representative ensembles by the choice of a triplet superposition correction function K. As background for such choice, we have reexamined and extended Alder’s lattice enumeration method for K in regular structures. The Kirkwood superposition approximation K≡1 has disastrous consequences for the BGY equation at low temperature; numerical pair correlation functions for the cases of hard spheres and of repelling Gaussian particles display long-range ordering that is impossible for the amorphous solid state. This failure is partially relieved by choosing a K that enhances the concentration of compact pentagonal particle groupings. Study of the inverse problem of determining K from physically reasonable pair correlation functions suggests that K must possess relatively long-ranged fluctuations about unity. These considerations highlight the desirability of accurate simulation studies of K for amorphous deposits at absolute zero.
Complex oscillations in the combustion of acetaldehyde
Authors: Harding Robert H. , Sevčikova Hana , Ross John
Published: July 2002

Aperiodic dynamics are observed experimentally in the cool flame combustion region of acetaldehyde (ACH) in a continuous stirred tank reactor (CSTR). A gradual transition is seen, with variation of exit orifice size, from limit cycle oscillation to aperiodic variations in light emission, and then back to near periodic oscillations. We analyze this transition by calculating power spectra, autocorrelation functions, phase portraits, period distributions, and Poincaré sections. The variation in peak amplitude and peak-to-peak period of the temporal variations of light emission increases during the transition. There are many initial indications of a transition to chaos. However, after an in-depth analysis, given in the following article, we ascribe the transition to the presence of a Hopf bifurcation and noise: the path traced out in the constraint space by the change in exit orifice size is nearly tangent to a Hopf bifurcation set but does not cross this set.
A potentially size-consistent multiconfiguration based coupled electron pair approximation
Authors: Hoffmann Mark R. , Simons Jack
Published: July 2002

A coupled electron pair approximation is derived and illustrative calculations are presented. The present approximation, which we refer to as the unitary coupled electron pair approximation (UCEPA), provides multiconfigurational (MC) reference capability and is as computationally tractable as multireference configuration interaction (MRCI). The method is capable of yielding size-consistent energies if the MC reference function is of the complete active space (CAS) variety. The coefficient matrix of the resultant set of simultaneous linear equations is evaluated using internal/external orbital space partitioning within a unitary group approach (UGA) treatment of the state space. We demonstrate the accuracy of the method on several small benchmark molecules for which full CI results are known, and on nontrivial studies on the singlet–triplet splitting in methylene and the electron affinity of the oxygen atom.
Light scattering with incident evanescent waves: A method for studying the properties of adsorbed polymers
Authors: Gao Jun , Rice Stuart A.
Published: July 2002

A new method for studying the properties of adsorbed polymer molecules is proposed. If polymer molecules are adsorbed on the low refractive index side of an interface between two transparent dielectric media, they can scatter the evanescent radiation generated by total internal reflection of light incident on the interface from the high refractive index medium. Analysis of the angular distribution of the scattered light yields information about the mean extension of the polymer perpendicular to the interface and its mean extension parallel to the interface. As an example, the theory is illustrated for the case of rods, anchored at one end to the interface, and with uniform angular distribution in the half space of the low refractive index medium. Both analytical and numerical analyses are presented.
Nuclear shielding tensors, atomic polar and axial tensors, and vibrational dipole and rotational strengths of NHDT
Authors: Jalkanen K. J. , Stephens P. J. , Lazzeretti P. , Zanasi R.
Published: July 2002

It was recently shown that vibrational rotational strengths of chiral molecules can be expressed in terms of two nuclear shielding tensors, γλαβ(0) and ξ̂λαβ(0). We report the first calculations of vibrational rotational strengths from these nuclear shielding tensors. The molecule studied is NHDT. Calculations are carried out at the SCF level using the random phase approximation. γλαβ(0) is calculated using length, momentum and force representations. ξ̂λαβ(0) is calculated using angular momentum and torque representations. A range of basis sets, both ‘‘conventional’’ and ‘‘polarized’’ in type, are employed. Gauge and origin dependence are also investigated.
Vibrational line broadening in molten nitrate mixtures: A static model for the coordination number and concentration fluctuations
Authors: Katō Toshiko
Published: July 2002

The isotropic Raman spectra of the ν1(A′1) mode of NO−3 are measured in molten NaNO3–TlNO3 mixtures at various concentrations (the mole fraction of NaNO3, 1≥CA≥0) at 600 K. Vibrational correlation functions in mixtures at CA≂0.8–0.5 show a considerable contribution of inhomogeneous vibrational dephasing. A model is presented which accounts simultaneously for the coordination number fluctuation and the concentration fluctuation at the reference anion. Concentration dependence of the isotropic Raman band shape (peak frequency, bandwidth, and asymmetry) is fully explained by the static model for the molten NaNO3–TlNO3 and LiNO3–RbNO3 mixtures. The bandwidths at mole fractions of about 0.5, slightly smaller than those calculated with the static model, suggest a motional narrowing due to diffusion dynamics. The model also makes predictions on the concentration dependence of the spectral line profile in terms of the interaction with the nearest neighbors and diffusion dynamics in liquid mixtures.