bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational rainbow oscillations in He–Na2 collisions: Comparison between coupled states and infinite order sudden approximations
Authors: Schinke R. , McGuire P.
Published: February 2003

Differential and integral cross sections for the rotational excitation of Na2 in collisions with He at 0.1 eV are calculated in the coupled states and the rotationally sudden approximation. The final rotational state distribution at fixed scattering angle exhibits an interference structure similar to the usual rainbow pattern in potential scattering, namely, a broad main rainbow maximum for high Δj transitions and ’’supernumerary’’ oscillations on the bright side of the rotational rainbow for lower Δj transitions. This rainbow effect is explained in the stationary phase limit of the sudden approximation by interferences between’’trajectories’’ from different atom–molecule orientations. It is shown that the oscillations in the rotational distribution at fixed angle are equivalent to the oscillations vs scattering angle for a single rotational transition as reported by Schinke [Chem. Phys. 34, 65 (1978)]. The sudden cross sections are in satisfactory accord with the coupled-states results, when the average rotational state ? is choosen to reflect energy conservation.
X-ray diffraction study of ferric chloride solutions and hydrated melt. Analysis of the iron (III)–chloride complexes formation
Authors: Magini Mauro , Radnai Tamas
Published: February 2003

A series of concentrated ferric chloride solutions, both neutral and acidic, as well as the hydrated melt FeCl3⋅6H2O have been studied by x-ray diffraction. The changes in the radial distribution curves by varying iron concentration or Cl−/Fe3+ ratio and least squares fitting of the structure functions for various solute models lead to an unambiguous conclusion with respect to the dominant species which are present in the solutions. The existence of monochloro, dichloro, trichloro, and tetrachloro complexes, predicted by literature information in dilute solutions, has been established in the different solutions investigated. Octahedral complexes, mainly Fe(H2O)4Cl+2 and Fe(H2O)3Cl3, are the principal species occurring in neutral and acidic solutions while in the hydrated melt tetrahedral FeCl−4 predominates. Species of the type FeCl5(H2O)2− or FeCl3−6 as well as polymeric network are excluded. Hydration water molecules around Cl− anions has been found for both free chloride and chloride ions bonded to iron atoms. Second order interactions around Fe3+ ions are strongly suggested by the presence of a large peak at about 4.2 Å in the distribution curves.
Convergence of multibody expansions for alkaline earth metals. Contrast between magnesium and beryllium clusters
Authors: Daudey J. P. , Novaro O. , Kol/os W. , Berrondo M.
Published: February 2003

An ab anitio SCF–LCAO–MO study of nonadditive three- and four-body interaction energies in Mg3 (equilateral and linear) and Mg4 (tetrahedral) clusters is presented. The relative importance of these few-body terms to the interaction energy is at most 39% for three- and 8% for four-body terms. This lies in marked contrast with the case of Be clusters where these terms can have a relative weight of up to 76%, and 31.6%, respectively. The reason for such radically different nonadditive effects is discussed. Some new results for Be3, Be4, and Be5 clusters are presented confirming this is a case where multibody expansions are expected to be very slowly convergent. Finally these results provide an explanation for the poor convergence of the chemisorption energy of an H atom as a function of the size of Ben clusters.
Monte Carlo trajectory study of Ar+H2 collisions: Thermally averaged vibrational transition rates at 4500 °K
Authors: Duff James W. , Blais Normand C. , Truhlar Donald G.
Published: February 2003

Quasiclassical trajectory calculations of vibrational transition rates in Ar+H2 collisions have been carried out. A realistic potential energy surface has been used, and the rates are averaged over rotational–translation distributions at 4500 °K. The same transition rates are calculated by eight distorted-wave-based theories which have been used by others for various applications. The present calculations provide a critical test of these theories, especially for high vibrational quantum numbers where data has been scarce. We also discuss dissociation rates, the rotational component of vibrational energy transfer, and a surprisal analysis of the vibrational transition rates.
A dipole interaction treatment of the polarizabilities and low energy π–π* transitions of amides of formic acid and acetic acid
Authors: Applequist Jon
Published: February 2003

Group dipole interaction theory is used to calculate polarizabiltiy tensors and Kerr constants at 589.3 nm and π–π* transition wavelengths, oscillator strengths, and polarization vectors for eight amides (formamide, acetamide, N-methylformamide, N-methylacetamide, N-t-butylformamide, N-t-butylacetamide, N,N-dimethylformamide, and N,N-dimethylacetamide). The polarizability tensor of the NC′O group is optimized to fit selected mean polarizabilities, anisotropies, and Kerr constants at 589.3 nm. The wavelength, oscillator strength, and polarization vector of the π–π* transition in the isolated NC′O group are optimized to fit selected molecular absorption wavelengths and oscillator strengths. Somewhat different NC′O parameters are found for locations of the polarizability at the NC′O mass center and the NC′ bond center. The molecular properties predicted with the mass-centered parameters give a better overall fit to molecular properties at 589.3 nm, but the bond-centered parameters lead to better Kerr constants and π–π* transition wavelengths, oscillator strengths, and polarizations.
A full polarizability treatment of the π–π* absorption and circular dichroic spectra of α-helical polypeptides
Authors: Applequist Jon
Published: February 2003

The absorption and circular dichroic spectra associated with the π–π* transition near 190 nm are calculated for the right-handed α-helical forms of (Gly)n, (l-Ala)n, and (d-Ala)n, n?12, using NC′O polarizability and transition parameters found for simple amides and isotropic polarizabilities of all other atoms in a partially dispersive normal mode analysis. Good agreement with experiment is found when NC′O parameters centered on the NC′ bond, with a π–π* oscillator strength twice that found for amides, are used. The calculated CD spectrum for (Gly)12 shows a negative peak at 204 nm and a positive peak at 191 nm; the negative peak usually predicted near 180 nm, but not found experimentally, is absent from these results. The effects of methyl side chains in (Ala)n are found to be small. The hypochromicity is calculated to be 0.26–0.40 (exptl. 0.30–0.36) and the splitting of the π–π* band is 4080 cm−1 (exptl. 4370 cm−1). When the n–π* transition at 222 nm is incorporated with an oscillator strength of 0.0025, a rotational strength of −0.10 DBM is calculated for the transition (exptl. −0.24 DBM).
A systematic preparation of new contracted Gaussian type orbital set. I. Transition metal atoms from Sc to Zn
Authors: Tatewaki Hiroshi , Huzinaga Sigeru
Published: February 2003

The method proposed by Tavouktsoglou and Huzinaga for determining contracted Gaussian type orbital basis sets is improved and is applied to the transition metal atoms from Sc through Zn. Two basis sets are generated. They both give satisfactory valence shell orbital energies, although they are minimal type basis sets. One of the two gives valence shell orbital energies which are quite close to those of DZ.
Rare gas dependence of vibration–vibration energy transfer processes: A diagnostic technique. Applications to CH2D2 and CH3F
Authors: Apkarian V. A. , Weitz Eric
Published: February 2003

The rare gas dependence of V–V rates can be used as a diagnostic technique to identify different mechanisms of vibrational energy transfer and determine the rate constants for individual kinetic steps. The method is especially useful for the identification and measurement of rates of resonant vibrational energy transfer processes. Analytical and numerical solutions of pertinent model equations are presented and their range of applicability is discussed. The technique is applied to CH2D2 and CH3F. In CH2D2 results of studies on ν9, [ν1, ν6] and states in the 2000 cm−1 region are presented where the application of the technique has made it possible to identify the pathways leading to population of these states and to assign rate constants to some of the steps involved. In CH3F, by studying the Ar dependence of the V–V rates of the [ν2, ν5] and [ν1, ν4] states it has been possible to construct a complete map of energy transfer pathways which can explain all experimental observations for this system, to date. The general applicability of the technique and its potential application to other systems is also considered.
Nonadiabatic interactions in unimolecular decay. IV. Transition probability as a function of the Massey parameter
Authors: Desouter-Lecomte M. , Lorquet J. C.
Published: February 2003

The nonadiabatic transition probability 𝒫 between two electronic states is studied in the semiclassical approximation. Extensive use is made of the concept of global function introduced in the diabatic representation by Delos and Thorson (their t (s) function) and by ourselves in the adiabatic representation (the Massey function, T (S)). Consideration of the shapes of these global functions results in a classification of the different models which have been proposed earlier by Landau and Zener, by Rosen and Zener, by Delos and Thorson, by Nikitin, and by Bandrauk. The relationships which exist between all of these models are displayed. For all of them, the transition probability 𝒫 can be expressed as a simple analytic expression involving two parameters, ξ and ϑo. ξ is the Massey parameter. ϑo measures the initial decoupling, i.e., the component of the adiabatic states on the basis set of the diabatic states at the initial time (corresponding to the initial excitation in a photochemistry experiment). For a given value of ϑo, the different models predict relationships between 𝒫 and ξ that do not differ greatly, so that a kind of ’’universal function’’ relating 𝒫 and ξ can be defined. The previous considerations are applied to three specific examples of nonadiabatic coupling encountered in the case of H2, N2+ and CH2+. For each of them, the global functions t (s) and T (S) are calculated ab initio. The relationship between their shape and the nonadiabatic transition probability is discussed.
Period of homogeneous oscillations in the ferroin-catalyzed Zhabotinskii system
Authors: Smoes M-L.
Published: February 2003

This work is primarily an experimental investigation of the ferroin-catalyzed Zhabotinskii system [malonic acid (MA), sulfuric acid, sodium bromate, traces of bromide ions, and ferroin]. We measure the transmittance as a function of time at 546 nm. The emphasis is placed on the properties of the period of homogeneous oscillations. It is found that T depends on the initial concentrations of reactants: T (s) = C [BrO−3]−1.60 [H2SO4]−2.70 [MA]−0.270 s (mol dm−3)4.57. The period is not perfectly constant. We investigate the conditions which favor irregularities: Low concentrations of BrO−3, H2SO4, or MA; high concentrations of catalyst or reaction products. The irregularities in the period of homogeneous oscillations are of direct interest for our understanding of the formation of waves in the distributed system. Also, we find bifurcations in which a transition from steady-state to full-blown oscillations is observed. Such bifurcations may be due to an unstable limit cycle or to a saddle-node transition. We discuss the saddle-node case in terms of the Bautin model. Finally, two steady states are observed. One of them has not been reported previously: In this state, the catalyst is mostly oxidized (blue). The oxidized steady state occurs at low [MA]0. Under the same conditions of low [MA]0, the distributed system exhibits both oxidizing (blue) and reducing (red) waves. This interesting new observation shows how much the ferroin-catalyzed Zhabotinskii system is still underinvestigated.
Angular momentum constraints in radiationless processes: The symmetric top molecule
Authors: Novak Frank A. , Rice Stuart A.
Published: February 2003

The effects of rotational motion on the rate of intramolecular radiationless relaxation in polyatomic molecules is investigated in symmetric tops. The molecule is assumed to have the same symmetry in both the initial and final states so that vibration and rotational degrees of freedom remain separated during the relaxation. The effect of intermanifold Coriolis interaction on an internal conversion process between two singlet states is discussed in detail. The effect of Coriolis interaction on the intersystem crossing rate is also formulated; however a detailed calculation is not performed.
Sum rule expressions for bounds on van der Waals coefficients.
Authors: Luyckx R. , Coulon Ph. , Lekkerkerker H. N. W.
Published: February 2003

We present a simple matrix solution for the moment equations that occur in recently discovered bounds on van der Waals coefficients. Using this matrix solution it is possible to express these new bounds directly in terms of the oscillator strength sum rules of the interacting systems.
Nonstatistical behavior in van der Waals photochemistry: Tetrazine–Ar
Authors: Kenny Jonathan E. , Brumbaugh Donald V. , Levy Donald H.
Published: February 2003

S-tetrazine–Ar was prepared by free jet expansion of dilute mixture. Vibrational levels were excited by dye laser radiation and the resulting emission spectra analyzed. (AIP)
Molecular spectra, Fermi resonances, and classical motion: Example of CO2
Authors: Heller Eric J. , Stechel Ellen B. , Davis Michael J.
Published: February 2003

The effective Hamiltonian is quantized and the Frank–Condon spectra calculated for CO2. (AIP)
Mobilities of C+, S+, and F− ions in helium
Authors: Dotan I. , Fehsenfeld F. C. , Albritton D. L.
Published: February 2003

The ion mibilities of atomic ions C+, S+ and F− in helium have been measured. The mobilities may be used to deduce the ion-atom interaction potential at collision energies than previously possible (AIP)
Photoinduced reaction of UF6 with SiH4 in a low temperature SiH4 matrix
Authors: Jones Llewellyn H. , Ekberg Scott A.
Published: February 2003

Results are reported that contradict the interpretation of Catalano et al.2 for observing photoinduced reactions of UF6 with SiH4 in a low temperature SiH4 matrix. It was found that the reactions occur due to near uv irradiation in the UF6 A band (25 000–29 000 cm−1) rather than the 613 cm−1 ν3 absorption of UF6.(AIP)
Comments on the concept of universal intensities in local mode theory
Authors: Burberry Mitchell S. , Albrecht A. C.
Published: February 2003

Tetramethylsilane (TMS) Spectra were obtained with a Cary 14-spectropholometer. The intensities in the primary C-H LM overtones and the neighboring sidebands of TMS are given and discussed in terms of the Universal cconcept of intensities. (AIP)
An application of information theory to Compton profiles
Authors: Gadre Shridhar R. , Sears Stephen B.
Published: February 2003

The information theoretic technique of entropy maximization has been applied to Compton profile (CP) data, employing single distribution moments <p≳ and <p2≳ (average momentum and energy respectively) as constraints. Two relations between various CP parameters, viz., J (0) <p≳=N2 and J2(0) ‖ E ‖=3N3/4π, have thereby been obtained. The momentum constraint predicts J (0) and the half-width, q0.5, of the profiles quite well. A comparison of these results to some earlier empirically established correlations between J (0), q0.5, and molecular energy has been made.
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o SCF and CI calculations of the dipole moment function of ozone
Authors: Curtiss L. A. , Langhoff S. R. , Carney G. D.
Published: February 2003

The constant and linear terms in a Taylor series expansion of the dipole moment function of the 1A1 ground state of ozone are calculated with Cartesian Gaussian basis sets ranging in quality from minimal to double zeta plus polarization. Results are presented at both the self-consistent field and configuration–interaction levels. Although the algebraic signs of the linear dipole moment derivatives are all established to be positive, the absolute magnitudes of these quantities, as well as the infrared intensities calculated from them, vary considerably with the level of theory.
The role of shallow traps on the mobility of electrons in liquid Ar, Kr, and Xe
Authors: Ascarelli G.
Published: February 2003

It is shown that the saturation of the drift mobility observed in liquid Ar, Kr, Xe and other insulating liquids is not a hot electron effect but can instead be understood as an effect due to shallow traps. Estimates of the binding energy (B.E.) of such traps in Ar indicate a value ∼90 meV. The effect of the addition of impurities can be understood as enhancing a Schottky ionization process.
G sum rule applications for the vibrational intensities of the hydrocarbons
Authors: de Barros Neto B. , Bruns Roy E.
Published: February 2003

G sum rule graphical solutions for the atomic effective charges, ξα, of the hydrocarbons, acetylene, ethylene, ethane, propane, and cyclopropane, are reported and compared with the numerical solutions. The importance of experimental error uncertainties in the effective charge values is emphasized. Polar tensor and effective charge values have been calculated for all the possible sign combinations of the ∂p/∂Qi’s of ethylene and ethane. ξα values calculated from the G sum rule and the polar tensors show excellent agreement indicating that normal coordinate and band separation approximations introduce little error into the effective charge values. Although ξH is relatively constant for all the hydrocarbons, save acetylene, ξC shows large variations with changes in molecular environment. The empirical relation nCξC = nHξH holds surprisingly well for most hydrocarbons, although the effective charges for methane provide a striking exception to this rule.
Fluorescence quantum yields and radiative and nonradiative lifetimes of glyoxal (1A
u)
Authors: MacDonald Bruce G. , Lee Edward K. C.
Published: February 2003

Fluorescence quantum yields (ΦF) from single vibronic levels (SVL’s) of glyoxal have been measured between 1 and 12 mTorr. The zero-pressure extrapolated values of ΦF decrease nearly monotonically from 0.58 for the zeroth vibrational level (00) to 0.22 for the 2181 level at Evib=2126 cm−1. From our values of ΦF and the fluorescence decay times (τF) measured earlier by Beyer, Zittel, and Lineberger (BZL), the collision-free values of radiative (τR) and nonradiative (τNR) lifetimes have been obtained. The observed value of τR for the 00 level is 4.1 μsec, as compared to the calculated value of 15 μsec estimated from integrated absorption by BZL. Since the observed values of ΦF are considerably greater than what had been estimated earlier, the new values for the nonradiative rate (kNR) are correspondingly smaller. The out-of-plane deformation mode (ν′8) is most effective as an inducing and promoting mode.
Electric field-induced fluorescence quenching in organic photoconductors
Authors: Popovic Zoran D. , Menzel E. Roland
Published: February 2003

The influence of electric fields on the fluorescence of several organic photoconductors was studied. The materials investigated were two cyan pigments, the X and β polymorphs of metal-free phthalocyanine (H2Pc), one magenta pigment (NRB), and one yellow pigment (DFC). In both H2Pc polymorphs and NRB, fluorescence quenching was observed and attributed to carrier generation with the first excited singlet state as a direct precursor. In DFC, fluorescence enhancement, rather than quenching, was observed which is believed to be due to a strong coupling of the external electric field to the dipole moment in the excited state. For DFC, no definite conclusion about which state is a precursor to photogenerated carriers could be made. It was found that the fluorescence quenching data could not be fitted well by theories based on an isotropic continuum approximation. A simple empirical formula which fits the experimental data remarkably well is proposed.
State-to-state cross sections for electron impact on N2. Close coupling and polarized Born calculations for rotational and vibrational excitation and pure elastic scattering at nonresonant energies
Authors: Onda Kunizo , Truhlar Donald G.
Published: February 2003

Calculations of cross sections for elastic scattering, rotational excitation, and vibrational excitation of ground state N2 by electrons with impact energies of 10 and 50 eV have been performed using realistic static-exchange-plus-polarization interaction potentials, rotational close coupling, and the vibrational sudden approximation. The effect of vibrational averaging on the elastic scattering is found to be small. The calculated integral cross sections for pure elastic scattering and for pure rotational (summed over j′≠0), pure vibrational (j′=0,v’=1 and 2), and mixed rotational–vibrational (summed over j′≠0 for v′=1 and 2) excitation are 34.4, 18.6, 2.70×10−2, and 4.13×10−2 a02, respectively, at 10 eV and 19.4, 11.7, 1.28×10−2, and 6.92×10−2 a02, respectively, at 50 eV. The sum of the differential cross sections for pure vibrational and mixed rotational–vibrational excitation for v=0→v′=1 can be compared to experiment and agreement is good at both energies except for scattering angles ϑ?30°. The pure rotational excitation differential cross section has not been resolved experimentally but we predict that it is dominated by j′=2 at 10 eV and for ϑ?130° at 50 eV and j′=4 for larger ϑ at 50 eV. The calculated state-to-state cross sections are also compared to results calculated by the polarized Born approximation with simplified model potentials based on the long-range forces.
Relative ’’dynamic stabilities’’ of stationary states in pumped chemical systems
Authors: Noyes Richard M.
Published: February 2003

When a chemical system is pumped with a continuous flux of radiant energy or high energy chemicals, it usually relaxes to a stationary state that remains invariant as long as the flux remains constant. Although the stationary state composition is often a unique function of pumping conditions, some systems can exhibit two or more stationary states within a considerable range of pumping rates. It two different stationary states can be created by identical pumping conditions, one test of relative ’’dynamic stability’’ is to determine which state becomes dominant when they are mixed. Applicability of this criterion has been examined by model calculations on reactions in a continuously stirred tank reactor (CSTR). If the chemical process involves a single reactant and product, the pumping rate generating ’’dynamic equistability’’ is that at which probabilities are equal as to which of two different identically pumped systems will become dominant as a result of mixing. An argument based on the Gibbs free energy function provides a method for treating mixing of more complicated pumped systems. A purely kinetic treatment of such systems is reserved for a later manuscript. Comparison of different computational procedues indicates it is not possible to define relative dynamic stabilities of stationary states in identically pumped systems even though relative thermodynamic stabilities can be unequivocally assigned to unpumped systems.
Electron paramagnetic resonance in ferrous fluosilicate at submillimeter wavelengths
Authors: Rubins R. S. , Fetterman H. R.
Published: February 2003

The electron paramagnetic resonance spectrum of FeSiF6⋅6H2O was studied at 4.2 K in the parallel orientation using the 1 mm radiation from a CSF carcinotron, the 337 and 311 μm radiation from an HCN laser and the 195 and 190 μm radiation from a DCN laser. In addition, the submillimeter spectrum was measured at zero-field. A good fit of the data was obtained using the spin-Hamiltonian, ℋ=DSz2+E(Sx2−Sy2)+ g∥βHSz, with D=11.88 cm−1, E=0.67 cm−1, and g∥=1.96. The fit of the two highest field lines could be improved slightly by assuming that ϑ, the angle between the z axis of a magnetic site and the crystallographic c axis, was about 4°.
Ferroelectric–ferroelastic Tb2(MoO4)3: Room temperature crystal structure of the transition-metal molybdates. VII
Authors: Svensson C. , Abrahams S. C. , Bernstein J. L.
Published: February 2003

Beta-terbium molydbate, Tb2(MoO4)3, is a coupled ferroelectric–ferroelastic crystal with Curie temperature of 436 K. Crystallizing in the orthorhombic system with space group Pba2 and four formulas per unit cell, Tb2(MoO4)3 has lattice constants at 298 K of a=10.344 43±0.00005,b=10.386 23±0.00005, and c= 10.654 85±0.00004 Å.The integrated intensities 19 406 reflections were measured with an Enraf–Nonius CAD-4 diffractometer, using monochromatic MoKα radiation, from a small single-domain crystal. The crystal structure was refined by the method of least squares, using the coordinates of isomorphous β-Gd2(MoO4)3 as the initial model, with 7505 symmetry independent Fmeas. The final agreement factor R is 0.0485, based on anisotropic thermal vibrations for all atoms. The average value of the Mo–O distance in the three independent (MoO4)2− tetrahedra is 1.759±0.005 Å. The Tb3+ ion is seven-coordinated with Tb-O distances ranging from 2.256±0.006 to 2.429±0.004 Å: The average distance is 2.336±0.016 Å. The measured spontaneous polarization in this three-dimensional ferroelectric crystal is 0.19×10−2 C m−2: a point-charge model gives 0.09×10−2 C m−2. A negative charge is generated on (001) with increasing temperature. The largest atomic displacement involved in ferroelastic transformation at room temperature is 0.67 Å for two oxygen atoms: all other displacements are less than 0.35 Å.
Electronic absorption spectrum of CH3O
Authors: Wendt H. R. , Hunziker H. E.
Published: February 2003

The ? 2A1-? 2E electronic transition of CH3O, which is qualitatively analogous to the ? 2Σ+−? 2Πi transition of OH, has been detected in absorption. It consists of a long series of vibronic bands described by ν= (31 538±9)+v′3(676±5)−v′23(5.7±0.6 cm−1), where ν is the transition wavenumber at the band intensity maxima, and v′3 is the upper state C–O stretching vibrational quantum number. The estimated oscillator strength of the transition, f= (4±2) ×10−4, has the same magnitude as in OH. It is shown that the observed C–O vibrational frequencies and vibronic intensities are consistent with the ab initio SCF geometry of CH3O, and with Franck–Condon factors derived from it. Earlier difficulties in detecting the absorption spectrum are attributed to a low absorption coefficient, and to the marginal thermodynamic stability of the CH3O ground state.
A theory for magnetic resonance measurements of diffusion in anisotropic fluids
Authors: Berner Bret
Published: February 2003

Electron spin resonance (ESR) spin exchange and the enhancement of nuclear spin relaxation frequencies due to the presence of paramagnetics can be used to study diffusion in liquid crystals and membranes. Previous analyses of the diffusion data obtained in this manner assumed isotropic diffusion. In this paper derivations are given for the dependence of the relaxation frequencies measured in these two experiments on the diffusion constants parallel and perpendicular to the director, D∥ and D⊥, respectively.
Transmission of 0–15 eV monoenergetic electrons through thin-film molecular solids
Authors: Sanche Leon
Published: February 2003

The transmission coefficient for monoenergetic electrons (0.04 eV FWHM) passing through thin films (∼100 Å) of molecular solids is measured as a function of electron energy in the range 0–15 eV. The visibility of sharp maxima in the transmission spectra is enhanced by measuring the negative value of the second energy derivative of the transmitted current with respect to electron energy. In benzene, fluorobenzene, pyridine, benzaldehyde, furan, cyclohexene and 1,3- and 1,4-cyclohexadiene, the spectra exhibit from six to eight maxima, whereas in pyrrole and thiophene four structures are resolved. With the exception of the first two features in fluorobenzene and the first feature for the other solids investigated, all other maxima correlate within at least ±0.5 eV with the gas-phase electronic transitions. This correlation is achieved by increasing the energy scale of the transmission spectra relative to the gas-phase electronic energy levels. These findings corroborate previous experiments on solid organic films by Hamill and co-workers who attributed peaks in the first energy derivative of the transmitted current to energy-loss events. It is shown that the transmission features can arise either from an increase in the overall transmission coefficient due to the production of inelastically scattered electrons or from the formation of an electron–exciton complex through the temporary capture of an injected excess electron by field of a Frenkel exciton. The first low-energy feature in each spectrum may possibly arise from an enhancement of vibrational excitation related to the energy dependence of the structure factor.
Test of mean-field behavior by light scattering in three phases of a fluid mixture near its tricritical point
Authors: Kim M. W. , Goldburg W. I. , Esfandiari P. , Levelt Sengers J. M. H.
Published: February 2003

Measurements are reported of the angular dependence of intensity and spectral width of light scattered quasielastically from three coexistent liquid phase α, β, γ near a tricritical point. From the data, the susceptibilities χ and correlation lengths ξ in each of the phases were calculated. Two predictions from mean-field theory were tested, namely ’’Griffiths’ first sum rule’’: χα1/2+χγ1/2 =χβ1/2 or ξα+ξγ=ξβ, and ’’Griffiths’ second sum rule’’: χα−1/2+χγ−1/2−χβ−1/2∝ (T−Ttr), or ξα−1+ξγ−1−ξβ−1 ∝ (T−Ttr). The second sum rule is found to hold within the accuracy of our data. The first sum rule is violated for the correlation lengths. It is found to hold for the susceptibilities, but only after the scattered intensities are properly corrected for differences, between phases, in the derivative of the dielectric constant with respect to order parameter. This derivative was obtained from model calculations of Kaufman and Griffiths. The viscosities of the coexisting phases were measured; combined with the spectral width and correlation length data, they confirmed, with no adjustable parameters, Kawasaki’s equation for the Rayleigh linewidth of a critical fluid.
High magnetic field Mössbauer studies of deoxymyoglobin, deoxyhemoglobin, and synthetic analogues: Theoretical interpretations
Authors: Kent T. A. , Spartalian K. , Lang G.
Published: February 2003

Mössbauer spectra of deoxymyoglobin, deoxyhemoglobin, and the synthetic analogues, iron (ii) 2-methylimidazole meso-tetraphenylporphyrin, and iron (ii) 1,2-dimethylimidazole meso-tetraphenylophorphyrin, observed in high magnetic fields and over a wide range of temperature, have been examined theoretically in an effort to correlate them with known susceptibility data, and a search for a satisfactory model within the 5D term has been carried out. It is shown in a general way that any solution within 5D would demand the use of magnetic hyperfine coupling constants about half as large as the smallest previously observed. Even allowing this anomaly we have failed to find a solution within 5D. An interesting result is that the myoglobin appears to have a singlet ground state while the 2-methyl substituted model has a nearly degenerate ground doublet, approximately describable as Sz=±2. Yet the hyperfine fields of these materials are very similar at elevated temperatures. It appears that the introduction of low lying states of different spin multiplicity will be needed to accommodate this behavior and to provide the observed magnitude of magnetic hyperfine coupling.
Intramolecular rate processes in highly vibrationally excited benzene
Authors: Bray Robert G. , Berry Michael J.
Published: February 2003

Intracavity cw dye laser techniques incorporating photoacoustic detection have been used to measure visible absorption spectra of gas phase benzene (C6H6 and deuterated analogues C6H5D, p-C6H2D4, and C6HD5). The prominent spectral features are attributable to CH stretch overtones that correspond to highly localized (’’bond-selective’’) excitations of individual CH oscillators. These features have broad (∼100 cm−1 FWHM) Lorentzian line shapes that narrow as a function of increasing vibrational energy for most isotopic species. A simple model is presented to interpret the broad line shapes and their energy dependence as a manifestation of intramolecular V→V′ (vibration-to-vibration) energy redistribution and/or dephasing processes that occur rapidly (on 5×10−14 sec time scales), but nonstatistically, in isolated benzene and other polyatomic molecules.
A study of the ground states and ionization energies of H2, C2, N2, F2, and CO molecules by the variational cellular method
Authors: Brescansin Luiz M. , Leite J. R. , Ferreira L. G.
Published: February 2003

Self-consistent calculations for the ground state potential curves and ionization energies are reported for the molecules H2, C2, N2, F2, and CO, using the recent proposed variational cellular method. For all these molecules the results are quite good. The calculated binding energies, equilibrium interatomic distances, and energy ionization spectra are in excellent agreement with the available experimental findings.
Resonance fluorescence as a probe to elucidate mechanisms of intramolecular relaxation processes
Authors: Chen Kuo-mei , Yeung Edward S.
Published: February 2003

Density matrix formalism is used to study resonance fluorescence processes of isolated gaseous molecules. The Wangsness–Bloch–Fano theory is applied to the irreversible intramolecular relaxation process. In the present treatment, it is assumed that the rotational angular momentum J is a constant of motion in the zeroth order approximation. Thus intramolecular interactions can be classified as tensor operators with respect to J. The fluorescence decay patterns depend on excitation bandwidths and monitoring conditions. Under narrow band excitation, the population (monopole), orientation (dipole), and alignment (quadrupole) of the excited state density matrix ρe can be prepared. For a sharply prepared angular momentum state under pulsed excitation, the population does not undergo radiationless decay and the resonance fluorescence intensity is a sum of three exponentials in general. For cw experiments, where lifetime limited linewidth or polarized fluorescence intensities are measured, the population can decay via an intramolecular relaxation channel providing the orientation component is initially prepared. Explicit J dependences of lifetimes are given for pulsed experiments. Under broad band excitation, quantum beats due to the coherence of ρe are derived from the Liouville equation. Most important of all, the population can decay under pulsed, broad-band excitation conditions. The scalar part of the nonadiabatic coupling becomes an additional contributor to the intramolecular relaxation process in this last case.
Action-angle variables in quantum mechanics
Authors: Augustin Stuart D. , Rabitz Herschel
Published: February 2003

Conventional quantum mechanical treatments of many systems have worked with coordinates and momenta that are not canonically conjugate. In this work it is shown how the quantum expressions may be reformulated in terms of the canonical set of action-angle variables, and specific examples of the harmonic oscillator, linear rotor, and triaxial rotor are presented. When expressed in these terms, the quantum mechanics take on a form which can be directly related to analogous results from classical mechanics. In addition, it becomes possible to express the Hamiltonian in the minimum number of coordinates. It is also shown that the common assumption of an exponential form for the overlap of canonical coordinate and momentum eigenstates is false for an asymmetric rotor. This has important implications for the quantization rules applicable to nonseparable systems.
Interaction potentials and their effect on crystal nucleation and symmetry
Authors: Hsu C. S. , Rahman Aneesur
Published: February 2003

Molecular dynamics technique has been used to study the effect of the interaction potential on crystal nucleation and the symmetry of the nucleated phase. Four systems, namely rubidium, Lennard-Jones, rubidium-truncated, and Lennard-Jones-truncated, have been studied each at reduced density 0.95. Two types of calculations were performed. Firstly, starting from a liquid state, each system was quenched rapidly to a reduced temperature of ∼0.1. The nucleation process for these systems was monitored by studying the time dependence of temperature and the pair correlation function, and the resulting crystalline structure analyzed using among other properties the Voronoi polyhedra. Only in the case of rubidium was a b.c.c. structure nucleated. In the other three cases we obtained a f.c.c. ordering. Secondly, we have studied the effect of changing the interaction potential in a system which has already achieved an ordered state under the action of some other potential. After establishing a b.c.c. structure in a rubidium system, the change in the symmetry of the system was studied when the pair potential was modified to one of the other three forms. The results from both types of calculations are consistent: the rubidium potential leads to a b.c.c. structure while the other three potentials give an f.c.c. structure. Metastable disordered structures were not obtained in any of the calculations. However, the time elapse between the moment when the system is quick-quenched and the moment when nucleation occurs appears to depend upon the potential of interaction.
Anisotropic hyperfine interaction in the electron spin resonance spectrum of the methyl radical trapped in CH3COONa⋅3D2O crystal at low temperatures
Authors: Kubota Shozo , Iwaizumi Masamoto , Ikegami Yusaku , Shimokoshi Kazuo
Published: February 2003

The electron spin resonance spectrum of the methyl radical produced by γ irradiation of CH3COONa⋅3D2O single crystal at 77 K has been measured at low temperatures. The spectrum observed at below 24 K consisted of eight lines with approximately equal intensities, instead of the four lines (1:3:3:1) at higher temperatures, due to the quantum tunneling of the rotational states. The transitions at low temperatures were angular dependent upon the applied field. From the analyses of the angular dependence of the spectrum with the aid of the theoretical calculation of the transition field for the tunneling methyl radical, the following anisotropic hyperfine tensor for the proton was determined (MHz): Axx=−31.4, Ayy=−93.7, Azz=−60.3, a (iso) =−61.8. These parameters provide the first experimental assessment of the well-known theoretical results on the proton hyperfine anisotropy in the free methyl radical.
Time-dependent theory of Raman scattering
Authors: Lee Soo-Y. , Heller E. J.
Published: February 2003

A time-dependent picture of vibrational Raman scattering in the weak field limit is presented. From this viewpoint we can separate the static effects, due to the coordinate dependence of the electronic transition dipole, from the dynamic effects that arise from wave packet propagation on the Born–Oppenheimer surfaces. Away from resonance, the energy uncertainty relation gives the propagation time necessary to obtain the cross section as being inversely proportional to the mismatch of the excitation frequency with the excited surface. The wave packet, given by the initial vibrational wave function times the transition dipole, hardly moves on the excited surface when the excitation frequency is far from resonance. As the excitation frequency is tuned closer to resonance, the propagated wave packet samples a larger portion of the surface. Using the short time approximation to the propagator, we obtain formulas for the cross section that are applicable for Raman scattering by polyatomics. The short time approximation is expected to be good away from resonance independent of the nature of the surface, and also on resonance with a repulsive surface. For an attractive surface, the approximation gives the average resonant cross section useful in the case when the vibrational structures cannot be observed.
A semiclassical model for vibrational energy relaxation in simple liquids and compressed fluids
Authors: Delalande C. , Gale G. M.
Published: February 2003

Using a simple model intermolecular potential, a density dependence of the vibrational population relaxation time in liquids is obtained which is in qualitative and quantitative agreement with experiment, over the whole range of density and temperature studied. This model is applicable to systems where the energy transfer involved is large with respect to thermal energies and describes the variation of the relaxation time as being essentially related to the variation of the collision frequency on the strongly repulsive part of the intermolecular potential. The formalism employed demonstrates clearly the role of excluded volume at high density and of bound states at low density and temperature.
New study of CO adsorption at low temperature (90 K) on Pt (111) by EELS
Authors: Baró A. M. , Ibach H.
Published: February 2003

We have performed a new study of the vibrational modes of CO adsorbed at low temperature (90 K) on Pt(111). As in previous work a sequential occupation of top and bridge sites is observed. At a certain bridge site coverage, however, a new vibrational mode with a frequency of 720 cm−1 appears. The mode is not observed at room temperature since sufficiently high coverages are not achieved there. The appearance of the 720 cm−1 mode is attributed to the C–Pt asymmetric stretching vibration becoming dipole active at higher coverages when, according to LEED results, an out-of-registry CO overlayer is formed.
Generalized sensitivity analysis in quantum collision theory
Authors: Eno Larry , Rabitz Herschel
Published: February 2003

This paper considers the sensitivity of the scattering matrix with respect to variations of input parameters including those that persist asymptotically. The analysis shows that the first order sensitivity coefficients (i.e., the partial derivatives with respect to input parameters) of the scattering matrix may be expressed in terms of the available solution of the Schrödinger equation. This work encompasses earlier results which appear as special cases. As an application sensitivity with respect to the total energy is considered and the relevance of the result to the energy behavior of observables is discussed. In a second application, we show how sensitivity analysis may be used to estimate corrections to effective Hamiltonian results and also to examine their accuracy. The coupled states and infinite order sudden methods are chosen to illustrate the approach. Finally, the scope of sensitivity analysis in quantum collision theory is discussed.
Comment on the role of constraints on the conformational structure of n-butane in liquid solvents
Authors: Chandler David , Berne B. J.
Published: February 2003

The constraints on the conformational structure of n−butane in liquid solvents are discussed. Computer simulation studies are carried out on the conformational transitions of this molecule (AIP).
A combined dynamical–statistical approach to calculating rates of complex bimolecular exchange reactions
Authors: Davis Jon P.
Published: February 2003

It is proposed that an efficient method of calculating, classically, the rates of complex bimolecular exchange reactions is a combination of exact dynamics for direct trajectories and transition state theory for complex trajectories. To test this proposal classical trajectory calculations were performed on a series of model collinear A+BA→AB+A potential surfaces. The reaction probabilities obtained by the combined method were in good agreement with the fully dynamic results. Miller’s unified statistical theory was also compared and found to give good results for two of the six potentials used.
Precise determination of differential electron scattering cross sections. III. Exchange corrections (CH4, N2, CO2, and Kr)
Authors: Fink M. , Schmiedekamp C.
Published: February 2003

It is shown that newly measured, large angle, differential cross sections for 40 keV electrons and molecular gases can be brought in very good agreement with theory. However, the calculations have to be corrected for the center of mass conversions in the inelastic channel and for the exchange in the elastic and inelastic scattering process. This result advances the reliability in the matching of our relative data to the theory. The corrections do not alter the small angle data which are used for studies of atomic correlation and binding effects.
Padé approximants for two-body multipole–multipole dispersion interaction. II
Authors: Suleman F. I. , Yousif F. N. , Shukla G. C.
Published: February 2003

The second order dispersion energy between an interacting hydrogen pair has been evaluated using up to [6,5] Padé approximants for the frequency dependent multipole polarizabiliities of atomic hydrogen, resulting in convergent values of E2(H–H) which are in harmony with earlier accurate calculations.
Magnetic field effects on the thermal conductivity of binary mixtures of O2 and NO with CO2
Authors: Bhanja Sisir , Barua A. K.
Published: February 2003

The effects of a magnetic field on the thermal conductivity of binary mixtures of the paramagnetic gases O2 and NO with CO2 have been measured differentially by using the hot-wire method, with the magnetic field applied perpendicular to the temperature gradient in the experimental gas mixture. The small SB effect in the diamagnetic partner (CO2) observed only at high values of the magnetic field-to-pressure ratio has been neglected while estimating the saturation field effect for the paramagnetic partner. The results for the O2–CO2 system have been satisfactorily explained with a suitable extension of Köhler and Raum’s theory, treating O2 as a mixture of ms=0 and ms=±1 states. The data for the NO–CO2 mixture, at room temperature, have been interpreted in terms of the theory of Köhler and Raum, using a single effective field parameter for the NO molecule. Several effective cross sections for these systems have also been calculated.