bluuebunny/crossref_metadata_2025 huggingface dataset:
Local mode excitation and direct unimolecular reaction rate measurements in tetramethyldioxetane
Authors: Cannon B. D. , Crim F. F.
Published: February 2003

Direct excitation of overtone vibrations combined with time-resolved detection of product chemiluminescence produces both overtone vibration excitation spectra and directly measured unimolecular decay rates of tetramethyldioxetane. The spectra show increasingly pure local mode character in higher vibrational levels and exhibit splittings which arise from nonequivalent sites occupied by methyl hydrogens. The temporal evolution of the signal reflects the unimolecular decomposition rate of the highly vibrationally excited molecule, and comparing the observed behavior to Rice–Ramsperger–Kassel–Marcus theory calculations shows that they adequately describe the decomposition if properly averaged over the thermal vibrational energy content of the molecule.
Interchange of vibrational energy between molecules in collision in the presence of laser light in the infrared region
Authors: Mohan Man
Published: February 2003

A semiclassical calculation of the transition probabilities for the interchange of vibrational energy between two molecules during their impact in the presence of laser light in the infrared region is performed. The results obtained are compared with those obtained in the absence of an electromagnetic field.
Collisional quenching of He2 molecules in the a 3Σ+u level by impurity gases
Authors: Parker John W. , Anderson L. W. , Fitzsimmons W. A. , Lin C. C.
Published: February 2003

Measurements in a fast pulsed discharge of the room temperature rate constant for the quenching of He2 molecules in the metastable a 3Σ+u level by Ar, N2, O2, H2, and CO2 are reported at He pressures of 200 and 300 Torr and for various pressures of the impurity gas. The concentration of He2 molecules in the a 3Σ+u level is monitored by using a pulsed dye laser to excite the Q5 line of the e 3πg←a 3Σ+u transition and by observing the subsequent d 3Σ+u→b 3πg fluorescence. The results for N2, Ar, and CO2 are combined with previous data of others to obtain both bimolecular and termolecular rate constants for the quenching of the a 3Σ+u level.
Chemical kinetic functional sensitivity analysis: Derived sensitivities and general applications
Authors: Demiralp Metin , Rabitz Herschel
Published: February 2003

This paper considers possible applications of sensitivity analysis to kinetics problems defined in the space–time domain. At the outset it is assumed that elementary sensitivity functional densities δui/δαj are available, where ui is the ith chemical species concentration and αj is the jth system parameter function. Emphasis is placed on the use of functional calculus to manipulate the elementary sensitivities for addressing a variety of physical questions. In this fashion, a family of derived sensitivities are generated from the elementary set. An extensive list of sensitivity applications is presented along with a discussion as an aid to future work.
The photodissociation of some alkyl iodide cations
Authors: Goss S. P. , McGilvery D. C. , Morrison J. D. , Smith D. L.
Published: February 2003

Photodissociation of the molecular ions CH3I+, CD3I+, C2H5I+, and n-C3H7I+ and of the radical ion ⋅CH2I+, has been studied using a tandem quadrupole mass spectrometer and a flashlamp pumped tuneable dye laser. Predissociation of CH3I+ and CD3I+ results in very detailed spectra, which are assigned, and which permit measurement of, vibrational frequencies for the ? states of these ions. The spectra for C2H5I+, n-C3H7I+, and ⋅CH2I+ consist of broad and structureless bands. In the case of C2H5I+ this is believed to be due to the very short lifetime of a predominating ? state, while in n-C3H7I+ the dissociation is thought to arise from direct transitions between the two lowest states of the ion which are analogous to the spin–orbit components in CH3I+. In the case of the ⋅CH2I+ radical, the lack of structure is attributed to the formation of the primary ion with a wide spread in internal energies.
Raman scattering from polymerizing styrene. I. Vibrational mode analysis
Authors: Sears W. M. , Hunt J. L. , Stevens J. R.
Published: February 2003

The frequencies and intensities of the Raman spectrum of thermally polymerizing styrene were measured in order to confirm (or correct) the mode assignments in the literature and to observe the effect of polymerization on the Raman active modes. The spectra were measured over a period of three weeks while an originally highly purified sample of monostryrene was kept at about 100 °C. The polymer vibrations are identified with their monomer ’’origins’’ which are in turn compared to known benzene (or benzene derivatives) and ethylene (or vinyl group) vibrations.
Quadrupole effects in the magic-angle-spinning spectra of spin-1/2 nuclei
Authors: Zumbulyadis Nicholas , Henrichs P. Mark , Young Ralph H.
Published: February 2003

Magic-angle spinning fails to eliminate the effect of dipolar coupling on the spectrum of a spin-1/2 nucleus (e.g., 13C ) coupled to a quadrupolar nucleus (e.g., 14N). A quantitative theory of this phenomenon, based on an adiabatic approximation, is presented, together with numerical simulations of spectra. The spectrum is sensitive to the sign and magnitude of the electric-field gradient at the quadrupolar nucleus, the angles between principal axes of the dipolar and quadrupolar interaction tensors, and the orientation of the spinning axis with respect to the external magnetic field.
EPR study of Mn2+ in single crystals of lithium ammonium tartrate monohydrate and determination of symmetry and domain structure of ferroelectric phase
Authors: Jain Akhilesh Kumar , Upreti G. C.
Published: February 2003

An EPR study is performed on Mn2+ doped single crystals of lithium ammonium tartrate monohydrate (LAT), both in the paraelectric and ferroelectric phases. The Mn2+ EPR spectra in the paraelectric phase are attributed to Mn2+ substituting for NH+4 (1) and associated with a neighboring Li+ vacancy. The phase transition at Curie temperature (∼98 K) is found to be of the second order. From the fourfold increase in the number of magnetically distinguishable complexes on going from the paraelectric to the ferroelectric phase and subsequent data for the monodomain crystal, the existence of four kinds of domains is confirmed. The present EPR results on Mn2+ are combined with the previously reported EPR results on Cr3+, and the symmetry of the ferroelectric phase which was believed to be monoclinic (P21) until now is concluded to be triclinic. The triclinic symmetry explains the previously reported but unexplained anomalies in the dielectric constant εaa and elastic compliance SE44 at Tc. The triclinic ferroelectric phase allows for four kinds of domains, viz., I, Ia, Ic, and Ib. The domains Ia and Ic are related to domain I by a twofold rotation about the a and c axes, respectively, and the domains I and Ib are related by a twofold rotation about the b axis. The polarization reversal process involves the necessary destruction of one of the two kinds of domains I and Ib (or Ia and Ic).
Charge transfer of Cl+ in Xe
Authors: Neynaber R. H. , Tang S. Y.
Published: February 2003

Absolute and relative cross sections for charge transfer of Cl+ in Xe have been measured by merging-beam technique. A wide range of collision energy is used, from 1 to 900 ev. The implication of lasing action in mixtures of HCL and Xe using this reaction are discussed. (AIP)
100 eV electron impact study of 1,3-butadiene
Authors: Doering J. P. , McDiarmid Ruth
Published: February 2003

Electron impact speclia of 1.3 batadiene taken at 100 ev incident energy and scattering angles of 3° and 12° are presented. Various valence and Rydberg transitions observed are discussed. (AIP)
Reply to ‘‘Comment on the ground state of iron (III) in the mixed crystal system Fe[S2CN(C2H5)2]2ClxBr1−x’’
Authors: DeFotis Gary C. , Cowen J. A.
Published: February 2003

NMR and magnetization measurements on the Fe[S2CN(C2H5)2]2Cl0.98Br0.02 are discussed and compared with moessbauer measurements mentioned in the comment by kostikas et al 1,2. (AIP)
Monte Carlo study of freely jointed ring polymers. II. The writhing number
Authors: Chen Yi-Der
Published: February 2003

Recently, the study of writhing in ring polymers has attracted considerable attention in relation to the study of torsional stiffness of DNA molecules in solution (see Refs. 6 and 10). In the first paper of this series we proposed an efficient dimerization method of generating ring polymers of the freely jointed model. In this paper we calculate the writhing numbers and their statistical distribution functions for the rings generated by this method. The results are compared with those obtained by LeBret (Ref. 10) and Vologodskii et al. (Ref. 6), in which the ring polymers were generated by a method completely different from out dimerization method. It seems that the writhing numbers may not be very sensitive to the ring generation procedure, but rather sensitive to the polymer model and its parameters.
A comparison of the two-state approximation and multistate treatments for vibration–vibration energy exchange in molecular collisions
Authors: Shin H. K.
Published: February 2003

We have solved the time-dependent Schrödinger equation based on a semiclassical collision to examine deviations of the two-state approximation from multistate treatments for vibration–vibration energy exchange processes. For a specific case of 41→50 in D2+D2, the two-state calculation of energy exchange probabilities fails at high collision energies (E≳2h/ω, where ω is the frequency of the oscillator). A thermal average shows that the approximation leads to large deviatons above 2000 °K.
Ionization of gases by a pulsed electron beam as studied by self-focusing. I. Monatomic gases
Authors: Arai Hidehiko , Hotta Hiroshi
Published: February 2003

Ionization of He and Ar by a pulsed high-intensity electron beam from a Febetron 706 has been analyzed self-consistently with an electronic computer by using related data in references at pressures between 1 and 300 Torr. The computational results indicate that the net current Inet at pressures below 30 Torr of He and 10 Torr of Ar keeps a nearly constant value after a certain time which is comparable to the gas-breakdown time assumed previously. As the pressure is further increased, Inet increases gradually with time and E/p induced by the pulsed beam decreases with increasing pressure. Comparing the dose observed on the beam axis with the computational result has revealed that the increase of the dose in the higher pressure region is attributed to the increase of Inet due to the decreasing Iback induced by the pulsed beam. When the doses are compared at a certain pressure among monatomic gases, the larger dose is given by a gas with the larger mean ionization time ti or 1/(αw) for secondary ionization by E/p (α is the first Townsend ionization coefficient and w is the electron drift velocity). The data on self-focusing can be interpreted by this law.
Picosecond infrared dynamics of electron trapping in polar liquids
Authors: Huppert D. , Kenney-Wallace G. A. , Rentzepis P. M.
Published: February 2003

We report new data on the formation and decay of the infrared absorbing, trapped electron (e−t) formed during the picosecond laser photoionization of TMPD in ethanol, propanol, butanol, and decanol at 295 K. The transient absorption at 1300 nm, formed ?3 ps, appears to decay on a time scale comparable to the picosecond formation time of the visible absorbing, solvated electron (e−s) in each alcohol. There is no discernible delay between the loss of e−t and the initial appearance of e−s. The results are discussed in the context of current models of electron solvation.
Hydrogen bonding of phosphodiesters to water, methanol, and methylamine as studied by the CNDO/2 method
Authors: Gay Richard , Vanderkooi Garret
Published: February 2003

Hydrogen bonding of dimethylphosphate anion (DMP) and dimethyl phosphoric acid (DMPH) with water, methanol, and methylamine was studied using the CNDO/2 molecular orbital method. The computed lengths of O–H⋅⋅⋅O hydrogen bonds involving the phosphate group were significantly shorter than were the computed values for nonphosphate O–H⋅⋅⋅O bonds, in agreement with experiment. The lowest energy for the monohydration of DMP or DMPH was obtained for the linear hydrogen bond in which the atoms of water were coplanar with the anionic oxygens of the phosphate group. The hydrogen bond strengths of methanol and water with DMP or DMPH were of comparable magnitude, but the N–H⋅⋅⋅O bond involving methylamine and DMP was about 25% weaker. The C–H⋅⋅⋅O interaction between the methyl group of methanol or methylamine and the phosphoryl oxygen was also examined and found to have all the characteristics (bond strength, charge transfer, and atomic polarization) of a hydrogen bond. The computed bond strength for HOCH3⋅⋅⋅DMP was −7.1 kcal/mole, as compared to −21.8 kcal/mole for a conventional CH3OH⋅⋅⋅DMP hydrogen bonding interaction.
Theoretical study of the dipole moment function of the A 1Σ+ state of LiH
Authors: Partridge Harry , Langhoff Stephen R. , Stwalley William C. , Zemke Warren T.
Published: February 2003

A theoretical study of the A 1Σ+ 7LiH dipole moment function is reported. A new theoretical A 1Σ+ potential energy curve, obtained by accounting for essentially all of the electron correlation within a large flexible Slater basis set, is reported here for the first time. The calculated equilibrium geometry (Re) and dissociation energy (De) of this new potential are in close agreement with experimental values (given in parentheses): 4.825 (4.883) bohr and 8653 (8682) cm−1, respectively. The vibrational spacings (ΔGv+(1/2)) for all levels (0⩽v⩽26) are also in agreement with experiment. The dipole moment function derived from this potential, as well as the previous theoretical dipole moment functions of Partridge and Langhoff and of Docken and Hinze, are used to obtain vibrationally averaged dipole moments μv for all of the vibrational levels of the A 1Σ+ state of 7LiH. Of particular interest is μ5, the dipole moment for the v = 5 level, measured recently by two separate experimental methods. Experimentally, the sign of μ5 is uncertain, but these theoretical results strongly suggest that μ5 is negative (i.e., Li−H+).
Vibrational energy relaxation (V–T) of highly compressed n-H2 gas in the 77–293 K temperature range
Authors: Chatelet M. , Oksengorn B. , Widenlocher G. , Marteau Ph.
Published: February 2003

Experimental measurements of the vibrational relaxation time of normal hydrogen have been carried out in the dense gas phase over a wide range of pressures (0–300 MPa) for three different temperatures (77, 196, and 293 K). We analyze our results in terms of different high density fluid relaxation theories which generally consider only binary collision processes. These theories were used with success by several authors to explain the variation of their experimental results on liquid hydrogen and oxygen; unfortunately, they disagree with our measurements. Nevertheless, an experimental linear relationship between the vibrational relaxation time and the self-diffusion coefficient was found as suggested by Zwanzig.
Energies of solute molecules from an atom charge–dipole interaction model with a surrounding dielectric: Application to Gibbs energies of proton transfer between carboxylic acids in water
Authors: Felder Clifford E. , Applequist Jon
Published: February 2003

An expression is derived for the electrostatic energy of an array of charged, polarizable atoms representing a solute molecule in a cavity in a dielectric continuum, using a generalization of the theory of the reaction field. Explicit formulas are obtained for the case of a spherical cavity. The formulas are used to calculate Gibbs energies and entropies of proton transfer between carboxylic acids in water, using atom polarizabilities from previous studies and atom charges on the COOH and COO− groups which are optimzed to fit experimental Gibbs energies of proton transfer between acyclic carboxylic acids involving monovalent anions. A cavity radius increment is likewise optimized. The optimized fit is good, and the parameters are reasonably consistent with data from other sources (charges from molecular orbital theory, experimental dipole moments, and partial molal volumes). Calculations for reactions involving divalent anions or bicyclo[2.2.2]octane-1-carboxylic acids yield correct Gibbs energies with the same atom charges, but small adjustments are required in the cavity radius increment. Entropies for the reactions involving acyclic acids are calculated assuming that the entropy change arises solely from changes of bulk solvent properties with temperature. The results are in fair agreement with experiment in some cases, but show large discrepancies in others, due in part to a large sensitivity of the entropy to the parameters of the model. Overall, the results support the view that variations in ionization energies of carboxylic acids arise largely from electrostatic interactions.
Cluster formation of a seed component in rare gas free jet expansions
Authors: Vasile M. J. , Stevie F. A.
Published: February 2003

Mass spectrometric measurements of dimer content in supersonic Ar molecular beams are in excellent agreement with previous measurements, and in very good agreement with the concentration of dimers predicted by Knuth’s sudden freeze analysis. Dimer concentrations in krypton and xenon expansions from a 0.0025 cm orifice also scale according to Knuth’s equation, but results of expansions of these gases through a 0.005 cm orifice do not scale with orifice dimension. Dimerization of XeF6 seeded into the rare gas carreers does not follow the hard sphere ternary collision rates. The XeF6 dimer concentration from the expansions apparently depends directly upon the degree of condensation of the carrier gas. Similarly, condensation of (XeF6)n, n≳2, also depends directly upon the extent of rare gas carrier condensation, and these experimental results are compatabile with a two body displacement reaction mechanism for the production of XeF6 oligomers.
The mixed alkali effect in (K, Na)–β-gallate fast ion conductor
Authors: Foster L. M. , Anderson M. P. , Chandrashekhar G. V. , Burns G. , Bradford R. B.
Published: February 2003

K–β-gallate is a potassium/gallium isomorph of the two-dimensional fast ion conductor β-alumina. It exhibits a strong ’’mixed alkali conductivity effect’’ when part of the potassium is replaced by sodium. At ?20 at. % Na the conductivity at room temperature goes through a minimum about 1/5000th of that expected from extrapolation between the conductivities of the single alkali end members. This conductivity minimum is due to a corresponding maximum in the activation energy in the exponent of the conductivity relation, which overrides the opposing effect of a maximum in the pre-exponent. The effect is attributed to local ordering of the alkali ions—three sodiums are bound in a triplet of interstitial sites around a vacancy, and the potassiums are distributed between normal lattice sites and pairs bound to interstitial oxygens. Raman spectroscopy reveals only one prominent low energy mode that can be related to the alkali ions. This vibrational frequency cannot be related in any obvious way to the mixed alkali effect, however.
The absence of photochromic switching in semiconducting films of thioindigo
Authors: Dahlberg S. C. , Reinganum C. B.
Published: February 2003

Thioindigo is reported to undergo a photochromic cis/trans conformation change in both liquids and in epoxy resins:           This characteristic could be be useful for the writing and erasing of information with visible light. In this paper, solid films of thioindigo and indigo are examined and they are shown to be organic semiconductors with an observable photovoltage on the order of 50–100 mV. The spectral dependence of the photovoltage indicates, however, that it is originating from T–T transitions undergone by an excited triplet state and there is no evidence for photochromic switching between the trans and cis singlet electronic states.
Study of the corrosion inhibitor formamide in the aluminum–carbon tetrachloride system using IETS
Authors: Ellialtiog̃lu R. , White H. W. , Godwin L. M. , Wolfram T.
Published: February 2003

Inelastic electron tunneling spectrosocpy (IETS) has been used to determine the molecular species formed on aluminum oxide exposed to formamide. Small concentrations of formamide are known to inhibit the corrosion of aluminum by carbon tetrachloride. IETS spectra were obtained for formamide and its deuteroderivatives and for dilute solutions of formamide in carbon tetrachloride. The spectra show that formamide in the presence of carbon tetrachloride is preferentially adsorbed on the aluminum oxide surface. The results indicate that the formamide surface species are chemisorbed at oxygen vacancy sites which have exposed aluminum atoms. A model is developed to describe the interaction of the formamide molecule with the oxide surface, and show how the formation of an aluminum–nitrogen bonded formamide surface species could inhibit the corrosion of aluminum by carbon tetrachloride.
Free ion and crystal field parameters for REOCl : Eu3+
Authors: Hölsä Jorma , Porcher Pierre
Published: February 2003

The fluorescence spectra of Eu3+ activated rare earth oxychlorides REOCl : Eu3+ (RE = Y, La, Gd) were recorded and analyzed at 4.2, 77, and 300 K. For each compound more than 100 transitions between the Stark levels of the 4f6 configuration were observed and assigned. The energy level schemes established from the spectra show a relatively strong crystal field effect. The five nonzero crystal field parameters for the C4v point site symmetry occupied by the rare earth ion were determined, yielding a mean square deviation of a few cm−1. The free-ion parameters were also determined with the aid of excitation spectra. The effect of truncation of the basis set of the 2S+1LJ levels on the energy level simulation of the excited levels was also studied.
Two-color photoionization of naphthalene and benzene at threshold
Authors: Duncan M. A. , Dietz T. G. , Smalley R. E.
Published: February 2003

We observe the resonant two-photon ionization (R2PI) spectrum of naphthalene and benzene at and just above the first ionization potential in a supersonic molecular beam. For naphthalene, the spectrum consists of direct ionization step functions indicating the relative Franck–Condon factors for transitions into the ground state ion. The spacings observed between these steps measure the ν8̄ vibrational frequency in the naphthalene ion. The electric field dependence of these steps is found to be consistent with a field ionization red shift of the thresholds. In benzene we observe similar direct ionization. Superimposed on the threshold steps, however, are broad features (∼100 cm−1) due to vibrational autoionization of Rydberg states at this same energy. Implications of this data on the observation of high Rydbergs in such large organic molecules are discussed.
Microwave spectra, quadrupole coupling constants, and structure of germyl bromide
Authors: Durig J. R. , Mohamad A. B. , Trowell P. L. , Li Y. S.
Published: February 2003

The microwave spectra from 26.5 to 39.0 GHz of 74GeH2D79Br, 74GeH2D81Br, 74GeHD2 79Br, 74GeHD2 81Br, 74GeD3 79Br, and 74GeD3 81Br have been measured and assigned. From these data, along with the data obtained earlier for the ’’light’’ compound, the rs structural parameters were determined to be r(Ge–H) = 1.527±0.001 Å, r(Ge–Br) = 2.297±0.001   Å, and ∢HGeBr = 106.3±0.1 °. From a diagnostic least-squares adjustment to fit 16 rotational constants, the following r0 structural parameters were obtained: r(Ge–H) = 1.526±0.002 Å, r(Ge–Br) = 2.299±0.001 Å, and ∢HGeBr = 106.2±0.1 °. The centrifugal distortion and quadrupolar coupling constants have been obtained and compared to those previously reported. The results are compared to similar quantities in related molecules.
The effect of vibration and translational energy on the reaction dynamics of the H+2 +H2 system
Authors: Anderson Scott L. , Houle F. A. , Gerlich D. , Lee Y. T.
Published: February 2003

A new experimental technique combining molecular beams, photoionization, and guided beam ion optics has been used to study several isotopic H2+ +H2 reactions. The technique is described. By using this method we are able to observe the effects of both reagent translational and vibrational energy. Cross sections are reported for charge transfer, H3+ formation, and collision induced dissociation. Evidence is seen for competition betwen the channels, charge hopping in the reaction entrance channel, potential energy surface hopping, reaction on excited potential surfaces, and isotopic scrambling. A model for the reaction which takes into account the multisurface nature of the systems seems to explain the results satisfactorily.
Fluorescence of S2(B–X) excited by fixed frequency ultraviolet lasers
Authors: Smith Allan L. , Hopkins John B.
Published: February 2003

Fluorescence spectra from rotational levels in the B 3Σ−u state of disulfur excited by pumping the ground state with a cw helium–cadmium laser (325 nm) and with a pulsed nitrogen laser (337 nm) have been studied. The He–Cd laser excites the transition (v′ = 3, N′ = 24, J′ = 25) ← (v″ = 3, N″ = 25, J″ = 26), or (3,3)P1(25). A comparison of high resolution spectra of the nitrogen laser and of S2 shows that several transitions in the (2,4) band could be excited: R3(23), P2(13), R2(21), and R3(25).
Measurement of product alignment in beam–gas chemiluminescent reactions
Authors: Prisant Michael G. , Rettner Charles T. , Zare Richard N.
Published: February 2003

A procedure is developed for determining the product rotational alignment in the center of mass frame from polarization measurements of the chemiluminescent atom–diatom exchange reaction A+BC→AB+C under beam–gas conditions. The degree of product alignment with respect to the initial relative velocity reaches a maximum when all reagent orbital angular momentum appears as product rotational angular momentum. For beam–gas chemiluminescence, this implies a limiting degree of polarization of the product emission referenced to the beam axis about which the initial relative velocities are cylindrically symmetric. Calculations are carried out to determine this limiting chemiluminescent polarization for a wide range of beam-gas reaction conditions. Averages over initial conditions are performed by Monte Carlo sampling. These calculations show that under realistic conditions the degree of beam–beam polarization does not exceed twice that of beam–gas polarization. Product polarization is measured in the beam–gas chemiluminescent reaction Ca(1D)+HCl→CaCl(B 2Σ+)+H and found to be greater than 20%. Because of kinematic constraints, this value closely approaches the calculated limiting polarization.
Charge transfer excitation in low energy collisions between rare gas ions and cadmium atoms
Authors: Barrett J. L. , Mlynczak M. G. , Leventhal J. J.
Published: February 2003

Excited state production in collisions between rare gas ion and cadmium atoms has been studied in a crossed beam apparatus at kinetic energies as low as 2 eV. It was found that charge transfer excitation cross sections resulting from impact of metastable argon ions were considerably larger than those with other rare gas ion reactants, either ground or excited state. The selective population of CdII (6s 2S) in these low energy Ar+/Cd interactions suggests that, under the proper conditions, laser action at 257.3 and/or 274.9 nm might be possible.
Ionization of gases by a pulsed electron beam as studied by self-focusing. II. Polyatomic gases
Authors: Arai Hidehiko , Hotta Hiroshi
Published: February 2003

In order to analyze data on the self-focusing of a pulsed electron beam in polyatomic gases, the net current Inet in H2, N2, and CH4 was computed self-consistently as functions of time in the pressure range between 5 and 300 Torr of these gases by using swarm parameters. The computational result indicates that the larger dose Dobs, observed by a piled dosimeter on the beam axis, is attributed to the larger Inet, which is mainly determined by a mean ionization time t1 for secondary ionization by the electric field induced by the pulsed beam. When values of Dobs for different gases are compared at the same pressure, the larger Dobs is given by the larger ti. This relationship is demonstrated for several polyatomic gases by estimating ti from various parameters in a function of secondary electron energy or E/p such as the electron drift velocity, the first Townsend ionization coefficient, the ionization cross section, and so on. For the short pulse duration of a Febetron 706, electron–ion recombination processes scarcely affect Inet except at high pressures of some polyatomic gases, while the effect of electron-attachment processes is appreciable in SF6, CCl2F2, and N2O.
Energy disposal in the reactions O(1D)+NH3→OH+NH2 and O(1D)+ND3→OD+ND2
Authors: Córdova José F. , Rettner Charles T. , Kinsey James L.
Published: February 2003

Further investigations of the reaction O(1D)+NH3→OH+NH2 and the first results on the reaction O(1D)+ND3→OD+ND2 are reported. The OH and OD rotational distributions have been found to be statistical. Hotter than statistical vibrational distributions are measured. The spin state distribution is statistical, with a strong preference to populate the energetically lower Λ component in both spin states. A preliminary study of the NH2 product shows very little rotational excitation. An analysis of this radical’s vibrational hot bands was not carried out due to lack of detailed spectroscopic information. The total energy is found to be partitioned according to 〈fR(OH)〉?0.1, 〈fV(OH)〉?0.25, 〈fR(NH2)〉?0.04, and 〈fT〉?0.25.
Atom scattering from an augmented corrugated hard wall surface
Authors: Foster J. J. , Masel R. I.
Published: February 2003

Atom scattering from a potential of the form V0+VHw, where V0 is a one-dimensional potential and VHw is a hard wall potential, is considered. The formalism is quite standard; the only difficulty comes in finding a Green’s function for [h/2∇2/2M+E−V0]−1. It is argued that the Green’s function used in CCGM and elsewhere is incorrect; the expansion of all scattering states that decay as Z→−∞ is not complete for potentials characteristic of atom–surface scattering. A new Green’s function is derived using a plane wave basis set. Calculations show that the effect of V0 is to make the surface seem flatter, but it makes little difference to the qualitative features of the scattering profile except near a resonance. If one had data at a single energy for scattering from a potential that approximates an augmented hard wall, one could fit them using a simple hard wall (i.e., V0 = 0) but when one does so, one calculates the wrong surface shape. As a result, if one wants to use atom–surface scattering to determine surface morphology, one has to take data at a wide range of incident energies, something that is not being done by most experimentalists.
Expansion of the classical differential cross section in a Fourier series
Authors: Kosmas Agni , Gislason Eric A. , Jorgensen Andrew D.
Published: February 2003

The procedure for expanding the classical differential cross sections I(ϑ) determined from a classical trajectory study in a Fourier sine series is developed. Simple expressions for the uncertainty in the differential cross section are presented. A smoothing technique, which utilizes a Gaussian filter, guarantees that I(ϑ) is everywhere nonnegative. This is recommended as the best way to display a differential cross section computed from a finite number of reactive trajectories. The results are applied to a model system and compared with the histogram technique.
Helix–coil transition of polypeptides on the basis of correlated walks
Authors: Fujita Shigeji , Blaisten-Barojas Estela , Torres M. , Godoy Salvador V.
Published: February 2003

The helix-coil transition of polypeptides is treated on the basis of the correlated walk model which incorporates both the physical shape (helix or coil) and the hydrogen bonding. The statistical mechanical calculations reproduce the essential features of the classic theory established by Zimm and Bragg. Moreover, the nucleation parameter σ, which represents the degree of difficulty for forming a first helical turn, is related to the probability γ′ of the model polymer making the correct turn: σ = γ′3. In the model in which the hydrogen bonding is attained only after three successive correct turns, the numerical value for the probability γ′ obtained after comparison with the optical rotation study of the poly-γ-benzyl-l-glutamate by Doty and Yang is found to be 0.010. This value is compatible with the detailed molecular calculations by Scherag and his collaborators, but it is about one-sixth of the value which results from γ′ = 3√σ with σ = 2×10−4 obtained in the truncated Z–B model. This difference arises from the more detailed representation of the phase-volume restriction for generating part of a helix and thus acquiring the hydrogen bonding in the present work.
Dynamics of hydrogen bond exchange in carboxylic acid dimers
Authors: Graf F. , Meyer R. , Ha T. -K. , Ernst R. R.
Published: February 2003

The simultaneous hydrogen bond exchange within carboxylic acid dimers is investigated by ab initio quantum chemical calculations and by computations of nuclear dynamics. A significant lowering of the potential barrier to ∼60 kJ/mol has been achieved by extensive structure optimization. The calculations suggest that the proton motion is coupled to a deformation motion of the frame of heavy nuclei. The flexible model treatment of this cooperative nuclear motion yields low vibrational and tunneling frequencies.
Ground state properties of trigonal prismatic and trigonal bipyramidal d1,9, d2,8, d3,7, and d5 systems: Effects of trigonal distortion
Authors: Hempel Judith C. , Miller Michael E.
Published: February 2003

Empirical ligand field theory has been used to calculate ground state energy space diagrams for d1,9, d2,8, d3,7, and d5 systems of D3h symmetry and allowed ground states for trigonal bipyramidal (five coordinate) and trigonal prismatic (six coordinate) D3h geometries have been identified. We demonstrate that entirely similar behavior should be expected for trigonal prismatic and trigonal bipyramidal d5 systems. Spin–orbit calculations complete within the d5 basis are used to establish that the ESR zero field parameter D for a 6A′1(D3h) ground state will normally be negative. Normalized spherical harmonic (NSH) ligand field Hamiltonians symmetry adapted to chains based on the D6h covering group are defined for all trigonal symmetries. Multiple symmetry parameter sets, which arise for trigonal symmetries other than D3h, describe the range of orbital mixing possible. Spin–orbit and Zeeman interactions reveal the effects of orbital mixing and restrict to two the number of symmetry parameter sets possible for a trigonal system. We specifically consider the magnetic properties of the five coordinate complexes of C3v symmetry [M(Me6tren)Br]Br, with M = Cu(II) and Ni(II) using calculations for C3v symmetry which incorporate spin–orbit and Zeeman interactions and are complete within the dn basis.
Padé approximants to correlation functions for restricted rotational diffusion
Authors: Lipari Giovanni , Szabo Attila
Published: February 2003

Simple closed-form approximate expressions are presented for time-correlation functions of spherical harmonics, with l = 2, m = 0, ±1, and ±2, within the framework of the diffusion in the cone model. In this model, a unit vector, with orientation specified by the polar angles ϑ and φ, is restricted to diffuse in a cone of semiangle ϑ0. By comparing with exact numerical results, it is shown that our expressions are highly accurate when m = 0, ±2 for 0°⩽ϑ0⩽90° and when m = ±1 for 0°⩽ϑ0⩽75°. The utility of our results is demonstrated in the context of modeling the effect of hindered motion of probes in membranes on the fluorescence emission anisotropy and the effect of internal motions on the nuclear magnetic resonance relaxation times of protons in nucleic acids.
Dynamics of entangled polymer melts: A computer simulation
Authors: Baumgärtner A. , Binder K.
Published: February 2003

As a model for dense polymer solutions or melts we consider an ensemble of n freely jointed chains consisting of N rigid links in a box. A Lennard-Jones interaction is taken to model repulsive and attractive interactions between the units of the chains. Dynamics is introduced into the model by allowing for stochastic rotations of the beads of the chains, where the transition probability satisfies detailed balance and only such rotations are allowed which do not lead to any intersection of chains, in order to take entanglement restrictions into account. Although the chains treated by the simulation are very short (N = 16), with increasing density and/or decreasing temperature we do find a transition from a regime of essentially Rouse-like dynamics (with decreased mobilities) to a glass-like state, where the ’’monomer’’ displacements are very small and follow different power laws of time than in the Rouse model. We also treat the case where all chains apart from the mobile one are strictly frozen in, so that the mobile chain moves through fixed obstacles. We clearly confirm the de Gennes reptation mechanism for this case, while it does not become effective in the other cases above where either ’’tube rearrangement’’ is not negligible or all motions are more or less frozen. The experimental relevance of these results is briefly discussed.
The relation between the limiting shear modulus and degree of hydrogen bonding in alcohols
Authors: Kono Ryusuke
Published: February 2003

Existing data of the limiting shear modulus at high frequency on associated liquids are found to obey the modified Hirai–Eyring equation G∞ = (2HRT/V0)exp(Eh/RT) above the glass transition temperature, where Eh is the energy required to create 1 mole of holes, V0 is the molar volume of alcohol, and H is the degree of hydrogen bonding. This semiemperical equation, deduced by considering the local interaction between molecules, should serve as a useful extrapolation formula for associated liquids and mixtures.
Magnetic and electrical properties of LaC2, CeC2, PrC2, NdC2, and SmC2
Authors: Sakai Tetsuo , Adachi Gin-ya , Yoshida Takeshi , Shiokawa Jiro
Published: February 2003

The electrical resistivities of the tetragonal CaC2-type rare-earth dicarbides RC2 (R = La, Ce, Pr, Nd, and Sm) and the magnetic susceptibilities of RC2 (R = Pr, Nd, and Sm) are reported. The dicarbide LaC2 becomes a superconductor at 1.6 K, which is in good agreement with literature values. The resistivity vs temperature curve of CeC2 indicates a small hump at 30 K, and those of PrC2 and NdC2 show a sharp change in slope at 18 and 25 K, respectively, all verifying the antiferromagnetic transitions found by neutron diffraction. The temperature dependence of resistivity of CeC2 was explained on the basis of the Γ7 components of Ce3+. The compound PrC2 becomes ferromagnetic at a zero-field Curie temperature of 7 K. This magnetic behavior differs significantly from that reported previously, suggesting an antiferromagnetic-to-ferromagnetic transition in the field of less than 700 Oe. The susceptibility of NdC2 shows a field-independent antiferromagnetic transition at 24 K. An antiferromagnetic transition in SmC2 was found anew at 21 K in both the resistivity and susceptibility data, exhibiting Van Vleck’s Sm3+ characteristics. The Sm3+ ion in SmC2 appears to be largely confined in the J = 5/2 ground state at about 21 K, since the paramagnetic Curie temperature reduced thereby and those of the other dicarbides can be correlated reasonably in the de Gennes factor plot.
Effects of van der Waals interactions on the polarizability of atoms, oscillators, and dipolar rotors at long range
Authors: Hunt K. L. C. , Zilles B. A. , Bohr J. E.
Published: February 2003

Both classical multipolar interactions and the interaction-induced changes in intrinsic polarizability associated with van der Waals forces contribute to the long-range pair polarizability of atoms, oscillators, and dipolar rotors. The frequency-dependent nonlinear polarization of an isotropic system by the field due to the randomly fluctuating multipoles of neighboring systems and by an applied field determines the change in intrinsic polarizability induced in the system by van der Waals interactions. Frequency-dependent values for the mean-square fluctuating multipoles are obtained from the fluctuation–dissipation theorem. To lowest order (R−6) the dispersion contribution to the polarizability α(ω) of a pair of atoms A and B is related to an integral over imaginary frequencies iu of the symmetrized product (1+𝒫AB)γA(ω, iu,−iu)αB(iu), where γA(ω, iu,−iu) is a linear combination of the γ-hyperpolarizability tensor components of atom A and αB(iu) is the polarizability of atom B. By using a mean-frequency approximation, the dispersion contribution to the static polarizability of inert-gas atom pairs is found in terms of the known van der Waals coefficients CA6B, the static atomic polarizabilities, and static γ hyperpolarizabilities. To order R−6, the change in intrinsic polarizability is positive for hydrogen and inert-gas atom pairs, zero for pairs of isotropic Drude-model oscillators with vanishing dipole at the equilibrium geometry, and negative for pairs of rigid dipolar rotors.
The Jahn–Teller effect in the photoelectron spectrum of iron pentacarbonyl
Authors: Hubbard John L. , Lichtenberger Dennis L.
Published: February 2003

High quality photoelectron spectra of gaseous Fe(CO)5 excited by HeI, HeII, and ArI photons have been obtained. Major attention is focused on the primarily metal 3d ionizations, which occur in the binding energy region from 7 to 11 eV. Ionization to the 2E′ positive ion state (centered at 8.6 eV) clearly shows the effects of Jahn–Teller distortions in the positive ion. This ionization results in two ionization bands of approximately equal intensity and shape separated by 0.38 eV at room temperature. These bands broaden and the splitting increases to 0.47 eV at 473 °K. Ionization to the 2E″ positive ion state, centered at 9.9 eV, is much less influenced by the Jahn–Tellar effect. There is no discernable splitting of this band at room temperature. These observations are discussed in terms of the electronic structure and bonding of Fe(CO)5. Simple model calculations of the energies of the doubly degenerate electronic states in relation to the appropriate doubly degenerate normal vibrational modes are used to investigate the splitting. The magnitudes of the 2E′ and 2E″ splittings are reproduced very well, and the temperature dependence is reproduced within experimental error. It is found that the vibrational motion primarily responsible for the splitting is a low frequency OC–Fe–CO bending in the equatorial plane. This bending is found in two normal modes, one of which is closely related to the Berry pseudorotation process. The splitting of the 2E′ ionization is clear experimental evidence in the gas phase that the preferred geometry of d7 M(CO)5 is not D3h in this case.
The effects of focusing on the efficiency of coherent anti-Stokes Raman scattering
Authors: Guha Shekhar , Falk Joel
Published: February 2003

The efficiency of coherent anti-Stokes Raman scattering (CARS) is calculated as a function of the input beam sizes of the Stokes and pump laser beams. A Green’s function formalism is used to show that maximum efficiency occurs when the confocal parameters of the two input beams are unequal and when the phase mismatch between the anti-Stokes field and the nonlinear polarization is nonzero.
Multiphoton ionization of NO2: Spectroscopy and dynamics
Authors: Morrison Richard J. S. , Rockney Bennett H. , Grant Edward R.
Published: February 2003

We have obtained the multiphoton ionization (MPI) spectrum of NO2 in the region from 520 to 420 nm under conditions of moderate pressure (2 Torr) in a static cell and at low pressure with mass resolution in both a flow system and a supersonic molecular beam. Resonance enhancement due to the 3sσ Rydberg state of NO2 at the level of the third photon characterizes the spectrum above 500 nm. Below 490 nm the spectrum is altered by efficient predissociation to NO. We observe the dynamics of this process, as manifested in the MPI spectrum of nascent NO, to depend upon wavelength and pressure, as these parameters affect the partitioning between competing two-phonon photodissociation channels producing NO(X 2π)+O(3P) and NO(X 2π)+O(1D).
O− detected by ESR as a radiation-induced electron-excess defect in KClO3
Authors: Byberg J. R.
Published: February 2003

A family of ESR spectra from KClO3 crystals irradiated with x rays at 26 K are assigned to electron-excess defects consisting of O− ions trapped in different environments, which may be identified because the electron spin on O− polarizes the adjacent ions. In the primary configurations, O− is associated with ClO−2. Secondary configurations, in which O− is trapped between two ClO−3 ions, arise from a thermally activated substitution process of the Grotthuss type: leading to migration of the excess negative charge along the crystallographic axis a. The O− defects in KClO3 are structurally very similar to those observed earlier in irradiated KClO4. As in KClO4, O− reacts with O2 at ∼230 K to yield O−3.
Raman spectra of NH4Br at high pressure and the location of the IV–V phase transition
Authors: Schwake A. , Hirsch K. R. , Holzapfel W. B.
Published: February 2003

The effect of pressure on the ν5(TO) lattice mode and on the internal modes ν1, ν3, and ν2+ν4 is reported for the phase IV of NH4Br at 100 K. The IV–V transition is located by changes in the Raman spectra in the temperature range between 100 and 300 K at a constant pressure of about 65 kbar. The II–IV phase transition shows a strong decrease of its pressure dependence pointing towards a triple point of the phases II–IV–V around 350 K and 65 kbar.
Global analysis of the NaNe excimer band systems: A molecule between Hund’s cases
Authors: Gottscho Richard A. , Ahmad-Bitar Riad , Lapatovich Walter P. , Renhorn Ingemar , Pritchard David E.
Published: February 2003

We have least-squares fit 244 transitions in the mutually perturbing NaNe A 2Π–X 2Σ+ and B 2Σ+–X 2Σ+ excimer band systems using a Hund’s case ’’c’’ basis. Interatomic potentials were determined by matching computed spectroscopic parameters to 43 fitted parameters. This deperturbation analysis implies that the Na atomic angular momentum qunatum numbers—L and S—are also good molecular quantum numbers. NaNe exhibits an intramolecular transition from Hund’s case c to Hund’s case ’’a’’ angular momentum coupling as the internuclear distance is decreased. The nature of this transition is mediated by the shapes, depths, and positions of both the A 2Π and B 2Σ+ van der Waals potentials.
An inhomogeneous self-consistent reaction field theory of protein core effects. Towards a quantum scheme for describing enzyme reactions
Authors: Tapia O. , Johannin G.
Published: February 2003

A quantum chemical framework is described where the electronic properties of model subsystems inside or at the surface of globular proteins (enzymes) can be calculated. Protein and solvent surroundings are incorporated in our effective Schrödinger equation. The theory is a generalization of the SCRF of protein core effects [O. Tapia, F. Sussman, and E. Poulain, J. Theor. Biol. 71, 49 (1978)]. A test has been carried out within the CNDO–INDO approximate scheme on the proton relay system of liver alcohol dehydrogenase. The results have elicited the fundamental role played by the protein core potential and the polarization potential in stabilizing ion-pair structures against canonical H-bonded ones. The polarization field introduces a nonlinear dependency on the model system wave function via the field created by its charge density. This fact is instrumental for a proper description of a highly polarized subsystem coupled to a polarizable surrounding medium.