bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental observations of excited dissociative and metastable states of H3 in neutralized ion beams
Authors: Gellene Gregory I. , Porter Richard F.
Published: February 2003

Electron transfer reactions for a fast beam of H3+ ions with Mg and K atoms have been investigated by neutral beam scattering techniques. Reactions with Mg and K targets form H3 molecules in the dissociative 2p 2E′ ground state and predissociative 2s 2A1′ and 2p 2A2″ excited states, respectively. Fragmentation energies, obtained from beam scattering measurements, allow the scaling of these electronic states of H3 with respect to their dissociation products. A metastable form of H3 observed in the H3+/K reaction is identified as the nonpredissociating, nonrotating molecule in the 2p 2A2″ electronic state. The cross section for the state-to-state process H3+(X 1A1′, N=1, K=0)+K(g)→H3*(2p 2A2″, N=K=0)+K+ for a 6 keV ion beam is 7.0±1.0 Å2. Total ion beam attenuation cross sections for the species H3+, H2D+, D2H+, and D3+ with K targets are in the relative order 1.0, 0.59, 0.58, 0.53. The higher cross section observed for the H3+/K reaction is partially accounted for by an usually high cross section for the near resonant process H3+(X 1A1′, N=1, K=0)+K(g)→K++H3 (predissociative states). The branching ratio for the formation of metastable/predissociative states from the H3+(X 1A1′, N=1, K=0)/K reaction is about 0.012. The relative intensity of metastable D3 neutrals is at least an order of magnitude lower than that of H3 for identical experimental conditions. This effect is discussed in reference to the Pauli principle and the statistical weights of nuclear spin states of D3+(D3) and H3+(H3). The importance of angular momentum restrictions on electron transfer cross sections is also discussed.
Specific rate constants k(E, J) for unimolecular bond fissions
Authors: Troe J.
Published: February 2003

The specific rate constants k(E, J) for unimolecular bond fission reactions are calculated explicitly by a simplified statistical adiabatic channel model. The input data are reactant and product parameters, together with the ratio α/β of a global looseness parameter α and a Morse parameter β characterizing the intermediate part of the potential surface. Angular momentum coupling restrictions are being accounted for. The approach includes rigid RRKM and loose phase space theory as limiting cases for α/β=0 and α/β≳1, respectively.
Formaldehyde: A
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o MCSCF+CI transition state for H2CO → CO+H2 on the S surface
Authors: Dupuis Michel , Lester W. A. , Lengsfield B. H. , Liu B.
Published: February 2003

A b initio multiconfiguration self-consistent field (MCSCF) and configuration interaction (CI) calculations have yielded an activation energy of 80.9±3.0 kcal/mol for the dissociation of formaldehyde to H2 and CO on the ground state potential energy surface. The error limits are estimates based on an analysis of the effects of one-particle basis set, electron correlation, and transition state structure on the activation energy. Accurate structures and harmonic frequencies are presented for H2CO (X 1A1) and the transition state.
An approximate universal energy functional in density functional theory
Authors: Ludeña Eduardo V.
Published: February 2003

Using Harriman’s orthonormal set, a closed expression for the reduced first order density operator γ1(1,2)=ρ(1)1/2ρ(2)1/2G(1,2) is obtained in the context of the independent particle approximation. It is shown that G(1,2) is given by G(1,2)=(1/n)exp{i[(n+1)/2]F(1,2)}[sin  1/2 nF(1,2)]/sin  1/2 F(1,2)], where F(1,2)=f(r2)−f(r1). Using this representation of γ1(1,2), an approximate universal functional of the energy which is given solely in terms of ρ is constructed. In particular, closed analytic expressions for the kinetic and exchange energies are explicitly derived. The simplifications brought about in these expressions by spherical symmetry are also discussed.
Solvent effects on molecular vibrations: Distortion and relaxation of the solute potential energy surface
Authors: Vicharelli Pablo A.
Published: February 2003

The solvent-induced changes in the vibrational energy levels of dissolved molecules are examined from a distortion–relaxation point of view. In this model, one considers the modified force field experienced by the atoms of a gas phase molecule when placed in a solvent. In response to this distorted force field, the electronic charge distribution readjusts until a new equilibrium is reached. This motion or relaxation of the electronic charge distribution is accompanied by a readjustment of the interatomic distances. By examining the potential energy surface as it distorts and then relaxes to a new minimum energy configuration, expressions are derived for the solute–solvent intermolecular force field responsible for the modifications. In an application of the theory, numerical values of the interaction constants for the hydrogen halides dissolved in rare gas matrices are presented.
The extramolecular contributions to the photoelectron and soft x-ray photon chemical shift in solid and liquid benzene
Authors: Ågren Hans , Karlström Gunnar
Published: February 2003

The origin of the phase transition shifts (PTS) in photoelectron and x-ray photon energies in solid and liquid benzene is investigated from a sudden, time-independent standpoint. Supermolecular dimer and perturbational induction energy calculations are undertaken. The role of localization of ionization, of reaction field due to intramolecular relaxation, and of partitioning the extramolecular polarizabilities into local bond and atom components are explored. The structure dependence of the phase transition shift is investigated and it is found that the structure of liquid benzene is sufficiently different from that of the solid to predict a significant shift in the photoelectron spectrum.
The rotational Zeeman effect in the OCO–HF, OCO–DF, OCO–HCl, and SCO–HF complexes
Authors: Shea J. A. , Read W. G. , Campbell E. J.
Published: February 2003

The rotational Zeeman effect has been measured in the hydrogen-bound complexes OCO–HF, OCO–DF, OCO–HCl and SCO–HF. The zero-field rotational spectra of the isotopic species SCO–DF, 34SCO–HF, O 13CO–HF, 18OC 18O–HF, O 13CO–HCl, and 18OC 18O–HCl have also been measured. The above studies were carried out on a pulsed Fourier-transform microwave spectrometer employing a supersonic nozzle and Fabry–Perot cavity. The following spectroscopic constants were obtained:  The Zeeman parameters are compared with values predicted by a set of equations which project the values of the individual subunits. The Zeeman parameters allow calculation of the quadrupole moment of each complex. With an estimated bulk magnetic susceptibility, diamagnetic susceptibilities, and the second moments of the electronic charge distribution are also calculated.
A coupled oscillator analysis of the pseudorotation-radial modes in cyclopentane
Authors: Smithson T. L. , Wieser H.
Published: February 2003

The CH2 scissoring band of cyclopentane is remeasured with improved resolution, and corrected and additional band positions for the pseudorotation combination sequences are reported. The pseudorotation separations in the ground state and the first excited state of the radial mode are fitted in a least-squares fashion for the first time with an appropriate two-dimensional Hamiltonian.
Trajectory studies of hot atom reactions. III. On the information content of recoil experiments
Authors: Valencich Trina
Published: February 2003

Simulation of nuclear recoil studies on T+CH4 using cross sections obtained from the empirically calibrated potential energy surface of part II, yields agreement with experimental measurements. The nuclear recoil product yields predicted by several trajectory studies are compared. The relative value of various kinds of experimental information in hot atom chemistry is discussed.
Unimolecular decomposition of nitromethane in a molecular beam with resolution of fragment recoil velocities and internal states: Dynamical evidence for an exit channel barrier
Authors: Rockney Bennett H. , Grant Edward R.
Published: February 2003

We report the application of multiphoton ionization (MPI) to a study of the dynamics of the unimolecular decomposition of nitromethane. CH3 and NO2 products from the CO2 laser-induced infrared multiphoton dissociation of CH3NO2 in a skimmed supersonic molecular beam are detected with time, space, and quantum state resolution by resonant multiphoton ionization. An analysis of angular and time-of-flight distribution of products characterizes their center-of-mass recoil energy distribution. Spectroscopic analysis of NO2 and CH3 MPI spectra determine the distributions of rotational states populated by the decomposition. Data are interpreted within the framework of a direct count phase-state theory approach which is used to calculate statistical distributions over product states within the constraint of explicitly conserved angular momentum. Phase-space theory rotational state distributions fit well with experiment, but experimentally observed translational energies are nearly an order of magnitude greater than expected. We conclude that energy release associated with an exit channel barrier for C–N scission dynamically alters fragment recoil energy distributions.
Thermodynamic approach for laser action and (vibrational) multiphoton excitation
Authors: de Hemptinne X.
Published: February 2003

Using the framework of statistical mechanics, the laser is treated as a thermodynamical machine which converts heat into electromagnetic energy. Conditions are defined for smooth emission and for spontaneous pulsing (relaxation oscillation). The treatment allows precise predictions about the phase fluctuations of the produced radiation. The results are applied to the action of laser radiaton of molecular oscillators. The role of phase fluctuations on the process of multiphoton absorption of energy is stressed.
Quantum-mechanical theory for electronic-vibrational-rotational energy transfer in atom–diatom collisions: Analysis of the Hamiltonian
Authors: Bellum J. C. , McGuire P.
Published: February 2003

We investigate forms of the molecular system Hamiltonian valid for rigorous quantum-mechanical treatments of inelastic atom–diatom collisions characterized by exchange of energy between electronic, vibrational, and rotational degrees of freedom. We analyze this Hamiltonian in terms of various choices of independent coordinates which unambiguously specify the electronic and nuclear positions in the context of space-fixed and body-fixed reference frames. In particular we derive forms of the Hamiltonian in the context of the following four sets of independent coordinates: (1) a so-called space-fixed set, in which both electronic and nuclear positions are relative to the space-fixed frame; (2) a so-called mixed set, in which nuclear positions are relative to the body-fixed frame while electronic positions are relative to the space-fixed frame; (3) a so-called body-fixed set, in which both electronic and nuclear positions are relative to the body-fixed frame; and (4) another mixed set, in which nuclear positions are relative to the space-fixed frame while electronic positions are relative to the body-fixed frame. Based on practical considerations in accounting for electronic structure and nonadiabatic coupling of electronic states of the collision complex we find the forms of the Hamiltonian in the context of coordinate sets (3) and (4) above to be most appropriate, respectively, for body-fixed and space-fixed treatments of nuclear dynamics in collisional transfer of electronic, vibrational, and rotational energies.
Reactivity and structure of NO+2 produced by the N+3+O2 reaction
Authors: Matsuoka Shingo , Nakamura Hirone , Tamura Takaaki
Published: February 2003

The experiments have been performed with a time-resolved atmospheric pressure ionization mass spectrometer (TRAPI). The NO+2 ion produced by the reaction of N+3 with O2 was found to decay to NO+ by a unimolecular thermal decomposition reaction having a rate constant represented by 5.6×107 exp(−4500/RT) s−1. NO+2 was also found to react rapidly with C2H6 (k=8.7×10−10 cm3 s−1), although ONO+ being produced by the charge exchange between O+2 and NO2 (nitrogen dioxide) showed undetectable reactivity (k<2×10−12 cm3 s−1) at the same experimental conditions. It is concluded from the present results that NO+2 has not the usual structure of ONO+ but has a structure of NOO+. The initial product ion ratio of NO+2/NO+ in the N+3+O2 reaction was measured to be 1.72±0.2 at 303 K.
The kinetics of reaction of photolytically generated Fe(CO)x (x=2,3,4) with CO
Authors: Ouderkirk A. J. , Weitz Eric
Published: February 2003

Coordinatively unsaturated iron carbonyls were generated by gas phase UV photolysis of iron pentacarbonyl. Transient infrared absorptions of these carbonyls were probed by a cw CO laser. The reaction rate of CO with Fe(CO)4 was measured to be 3.5±0.9×1010 cc mol−1 s−1. Reaction of CO with Fe(CO)2 was more rapid than reaction of CO with Fe(CO)3. Both reactions were over two orders of magnitude faster than the reaction of CO with Fe(CO)4. Coordinatively unsaturated metal carbonyl absorptions do not appear immediately following Fe(CO)5 photolysis. Possible explanations for this phenomenon are discussed.
Comparison between the convergence of perturbation expansions in one-dimensional square and triangle-well fluids
Authors: Bishop Marvin , Boonstra Martin A.
Published: February 2003

The convergence of perturbation theory in the isobaric and canonical ensembles is investigated for one-dimensional square and triangle-well fluids. It is found that for the square-well fluid the canonical expansion is in much better agreement with the exact results than the isobaric expansion, but that both expansions are in very good agreement with the exact results for the triangle-well fluid.
Naphthalene and azulene: Theoretical comparison
Authors: Haddon R. C. , Raghavachari Krishnan
Published: February 2003

Ab initio molecular orbital theory has been used to calculate the structure of naphthalene and it has been shown that the inclusion of electron correlation preferentially favors azulene over naphthalene by 10 kcal/mol—the final results are in excellent agreement with the available experimental data. (AIP)
Infrared spectroscopic investigation of the rhodium gem-dicarbonyl surface species
Authors: Yates John T. , Kolasinski Kurt
Published: February 2003

The infrared spectrum of the Al2O3—supported surface species RhI(CO)2 has been measured for systematic isotopic substitution of CO18 for CO16. Six infrared bands are observed for the three possible isotopic species, confirming that the species is RhI(CO)2(C2v). Force constants for the C–O stretching motion and for CO–CO interaction between the two CO ligands on RhI(CO)2 have been evaluated, and used to predict the spectrum of RhI(CO16) (CO18) which is in good accord with the experimental observations. Agreement of these force constants with the work of Knozinger et al. on various anchored RhI(CO)2 complexes provide further confirmation of the spectral assignment. Kinetic and infrared intensity measurements of the exchange process at 200 K indicate that the exchange occurs under nonequilibrium conditions, caused most likely by the participation of a slow CO diffusion process which is rate controlling.
Dissociation of CO2+2 ions into CO+ and O+ fragments
Authors: Shah A. V. , Wood R. M. , Edwards A. K. , Steuer M. F. , Monce M. N.
Published: February 2003

Dissociative ionization of CO2 is further studied by the bombardment of 1-MeV He+ ions. The coincident detection of CO+ and O+ fragment pairs confirms the dissociative channel CO22+→CO++O++6.0 eV which is the principal contributor to the ion yield. (AIP)
Comment on ‘‘Configuration interaction studies on low-lying valence and Rydberg states of PO’’
Authors: Lefebvre-Brion H. , Grein F.
Published: February 2003

Grein and Kapur’s assignment of the third 2π state of PO is found to be incorrect. New assignment for this valence state of PO are considered. (AIP)
EPR and optical studies in pseudotetrahedral tetramethyl ammonium copper (II) bromide
Authors: De D. K.
Published: February 2003

EPR studies of single crystal copper tetramethyl ammonium bromide, T2CuBr4[T=(CH3)4N] in the temperature range 77–300 K indicate the largest covalency in this compound compared to similar halogenated pseudotetrahedral copper compounds. The angle cos−1γ between the c axis and the ionic g∥ axis is 12° and 65° at 250 and 220 K, respectively. The change is continuous down to 200 K. The g shift between 300 and 200 K indicates an increase in the strength of the ligand field with lowering of temperature. The magnetic susceptibility ellipsoid and the g-tensor ellipsoid do not coincide in this material. Optical spectra in T2(Cu:Zn)Br4 have been assigned. The nonobservation of an EPR signal in T2(Cu:Zn)Br4 and Cs2CuBr4 indicates that ligand field properties in these pseudotetrahedral compounds are quite different from that of T2CuBr4.
Two photon resonance enhanced multiphoton ionization spectroscopy and state assignments of the methyl radical
Authors: Hudgens Jeffrey W. , DiGiuseppe T. G. , Lin M. C.
Published: February 2003

A mass resolved multiphoton ionization spectroscopy study of CH3 and CD3 has revealed two new electronic Rydberg series in these radicals. Simultaneous absorption of two photons prepared the excited states. Absorption of a third photon from the excited state generated the parent ions of methyl radical (m/z 15; CH3 and m/z 18; CD3) which were detected in a quadrupole mass spectrometer. The np 2A″2 Rydberg series was detected and identified by a rotational assignment of the 3p 2A″2 state of CD3 and by a polarization study of the np states. The quantum defect of this series was 0.6. The rotational constants determined for the ground and 3p 2A″2 excited states were B″0=4.798 cm−1 and B′0=4.768 cm−1. A vibrational analysis shows that both the symmetric stretch and out-of-plane bending mode are active in the 3p 2A″2 state. A second Rydberg series with a quantum defect of zero was detected and tentitively assigned to the nf 2E′ series.
On quantum molecular dynamics and electrodynamics
Authors: Witkowski Andrzej
Published: February 2003

A new approach to the quantum mechanical fundamentals of molecular physics is presented. We start by introducing the interaction representation for molecular systems in which the interaction between molecular fermions and bosons leads to the time evolution of molecular harmonic oscillators. Such a linear harmonic oscillator determines the metric tensor in the two-dimensional Riemann space and its time evolution occurs on a self-time curve which is a geodetic in the Riemann space. The molecular bosons in this self-time representation have the energy dispersion and the superfluid properties. The molecular spectra in the self-time representation acquire the line shape which results from nonlinearity of the geodetic and are related to dephasing between the nuclear and electronic motions. Dephasing between the nuclear and electronic motions leads to the retarded electrodynamic interactions in the form of the Liénard–Wiechert potentials. In the limit of the Born–Oppenheimer approximation, the electrodynamic interactions reduce to the purely electrostatic ones and the molecular line shape function to the Dirac δ function.
A transition state theory of nonadiabatic unimolecular reactions controlled by a conical intersection. Application to the C2H+4 ion
Authors: Desouter-Lecomte M. , Sannen C. , Lorquet J. C.
Published: February 2003

We propose an extension of the RRKM method to nonadiabatic reactions. The basic ideas were suggested by Zahr, Preston, and Miller in the simpler case of a spin-orbit interaction for which the coupling matrix element remains constant over the configuration space. These ideas have been adapted to the more complicated case of a conical interaction. The rate constant is then expressed as a product of a RRKM rate term times an average probability of staying in a given electronic state which is evaluated from the Landau–Zener–Nikitin linear model. The method is applied to the dissociation of the C2H+4 ion, whose reaction mechanisms have been previously determined by ab initio calculations, and for which convincing experimental observations suggest that the system can be represented by a microcanonical ensemble. The relevant rate constants are calculated as a function of energy and fragmentation yields derived therefrom. This gives rise to a mass spectrum calculated from first principles. Agreement with experiment is satisfactory.
Molecular properties from MCSCF-SCEP wave functions. I. Accurate dipole moment functions of OH, OH−, and OH+
Authors: Werner Hans-Joachim , Rosmus Pavel , Reinsch Ernst-Albrecht
Published: February 2003

Potential energy and dipole moment functions for the ground states of OH, OH−, and OH+ have been calculated from MCSCF, MCSCF-SCEP, and SCEP-CEPA electronic wave functions. The stability of the dipole moments with respect to the number of configurations (up to 598) and orbitals (up to 14) simultaneously optimized in the MCSCF procedure and the number of reference configurations (up to 11) in the MCSCF-SCEP wave functions (up to 69 830 configurations) has been investigated. The dipole moment functions obtained from the best electronic wave functions are more accurate than all previously calculated ones. The deficiencies of the former calculations have been critically analyzed. The OH− and OH+ ions are predicted to be stronger IR emitters than the neutral OH radical. The rotationless rates of spontaneous emission A10 for the fundamental transitions are calculated to be 12.2, 137, and 263 s−1 for OH, OH−, and OH+, respectively. The calculated dipole moments in the vibrational ground states are 1.65, 1.04, and 2.32 D, respectively.
Density fluctuation in a ‘‘Theorist’s ideal glass’’
Authors: Roe Ryong-Joon
Published: February 2003

The density fluctuation in glassy materials can be considered to consist of dynamic and quasistatic contributions, the dynamic part being related to the compressibility of the glass determined on the same experimental time scale as the measurement of the density fluctuation. A justification for such an assumption, and a clarification of the meanings of the terms dynamic and quasistatic density fluctuation are obtained through the concept of ‘‘theorist’s ideal glass’’ which was originally described by Edwards for a canonical ensemble but now extended for a grand canonical ensemble.
Kinetic models for stilbene photoisomerization: Comparison of theory and experiments
Authors: Ladanyi Branka M. , Evans Glenn T.
Published: February 2003

Torsional correlation times for trans to cis isomerization of stilbene in solution are calculated using Fokker–Planck and diffusional models for the barrier crossing process. Hydrodynamic interactions and the solvent-dependent potential of mean force are incorporated into the calculations. Both convective effects and hydrodynamic interactions are found to have a significant impact on the isomerization rate of stilbene in alkane solvents near room temperature. It is found that the potential barrier between the trans and 90° states definitely has to be below 1200 cm−1, the latest experimental value of this quantity, in order to explain the observed photoisomerization rate. The dependence of the rate on friction is calculated and various improvements to our model and their relative importance are discussed.
Rotational isomerism and ultrasonic relaxation in 1,2-dibromoethane
Authors: Takagi K. , Choi P.-K. , Seki W.
Published: February 2003

Three different ultrasonic techniques, pulse echo, HRB (high-resolution Bragg reflection), and Brillouin scattering were used to measure the sound velocity and absorption in 1,2-dibromoethane at 11, 20, and 30 °C over the frequency range from 3 MHz and 6 GHz. The observed results showed a typical spectrum for a single relaxation process, from which the relaxation frequency and strength were determined. The temperature dependence of the relaxation frequency and strength suggested that the activation energy for the gauche molecule is 3.0±0.5 kcal/mol and that the energy difference is 1.3±0.2 kcal/mol, respectively. The magnitude of the relaxation strength was successfully described by taking the effect of the volume change into account, and ΔV/V was estimated to be 0.05.
A microscopic model for ordered chain systems
Authors: Baram A. , Goldhirsch I.
Published: February 2003

A microscopic model for a system of ordered chains is presented. The statistics of a single chain is calculated in a quadratic mean field potential that approximates the steric repulsions between the hydrocarbon chains. The partition function of the problem that contains long range interaction between segments, is transformed to a mathematically tractable Markovian form. Statistical averages and correlations are calculated showing a constant chain alignment near the interface that decays only towards the end of the chain.
Two-photon NMR on spins with I=1 in solids
Authors: Zur Yuval , Vega Shimon
Published: February 2003

The NMR excitation of the double quantum (DQ) transition of a spin system with I=1 in a solid by double-frequency irradiation fields is studied. The efficiency of the DQ excitation by the two irradiation fields applied simultaneously at the two sides of the single quantum frequency spectrum of this system is calculated. The theory of Shirley for periodical time-dependent Hamiltonians is extended to describe these NMR experiments. The two-photon character of the DQ excitation is discussed and the two-photon resonance conditions are derived. The effective rf irradiation intensity on the DQ transition is calculated to be ω1ω2{(ωQ−ωT)−1 +(ωQ+ωT)−1}, where ω1 and ω2 are the intensities of the two rf irradiation fields, 2ωT the frequency distance between these rf fields and ωQ the quadrupole frequency of the spin system. This result is valid for all ωQ values as long as ω1, ω2< ‖ ωQ−ωT ‖. Accurate computer calculations are performed to examine the exact time evolution of the density matrix of the spin systems. Experimental results from 2D-NMR measurements on an oriented single crystal of deuterated malonic acid are compared with the theory.
Trajectory-surface-hopping study of Na(3p 2P) +H2 → Na(3s 2S)+H2(v′, j′, θ)
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2003

Trajectory-surface-hopping calculations involving the three lowest-energy 2A′ potential surfaces are reported for Na(3p 2P) collisions with H2(v=0, low j) at 0.9 kcal/mol relative translational energy. In addition to the total quenching cross section, we report distributions of final translational energy, final vibrational and rotational quantum numbers, internal energy, scattering angle, and collision time for the quenching collisions. We also report the opacity function, the correlation of scattering angle with impact parameter, and separate product translational spectra for the forward and backward scattered halves of the quenched ensemble. These results provide a detailed picture of the chemical dynamics of a typical quenching system proceeding through a quasibound intermediate configuration with large ionic character.
Nonequilibrated energy distribution in polyatomic molecules: The infrared MPD of cyclobutanone
Authors: John Phillip , Humphries Mark R. , Harrison Robert G. , Harper Philip G.
Published: February 2003

The TEA CO2 infrared MPD of cyclobutanone, on irradiation of the ν24 fundamental mode, has been investigated as a function of incident fluence Fi, wavelength and pressure. Dissociation proceeds via two molecular routes; the first yields ethylene plus ketene CH2CO, and the second cyclopropane and CO plus minor amounts of propylene. The branching ratio R≡[C2H4]/[C3H6] increases dramatically at pressures less than ∼3 Torr from the high pressure thermal limit. This nonthermal feature is most pronounced at low Fi. The average number of quanta absorbed per molecule 〈n〉, measured by pyroelectric and optoacoustic detection, is independent of cyclobutanone pressure between 0.1 and 10.0 Torr. 〈n〉 varies linearly with Fi in the range investigated 0.2–2.0 J cm−2. An explicit formulation of the infrared MPD process is presented based on a truncated s-fold degenerate harmonic oscillator approach. Vibrational energy transfer is incorporated via v–v bimolecular collisions. The predicted dependence of R on pressure is in good agreement with the experimental data presented herein. Moreover, the observed linear dependence of 〈n〉 with Fi, and its independence with respect of pressure, is also borne out by this theory.
Thomas–Fermi theory in a weak, slowly varying vector potential
Authors: Harris Robert A. , Cina Jeffrey A.
Published: February 2003

The gauge invariant kinetic energy of an inhomogeneous electron gas in the presence of a weak, slowly varying vector potential is determined by a generalization of Schwinger’s method. The kinetic energy is a function of the charge density and current density. The resulting energy should prove useful in determining the linear responses of closed shell systems to magnetic fields.
Characteristic function of momentum density distribution. II
Authors: Koga Toshikatsu
Published: February 2003

Rigorous relations between the moments 〈plxpmypnz〉 and 〈pn〉 and the characteristic function B(r) of the momentum density distribution ρ(p) are here generalized to the case of negative l, m, and n. A simple application is given which illustrates that the present results enable us to obtain the moments of momenta without referring to any momentum-space quantity.
Corrections to the HNC equation for associating electrolytes
Authors: Bacquet Russell , Rossky Peter J.
Published: February 2003

The accuracy of the HNC approximation and modifications based on a simple approximation for the sum of bridge graphs has been studied for a charged soft sphere model for dilute aqueous 2-2 electrolytes. The accuracy is judged based on extensive Monte Carlo (MC) simulation for three representative concentrations: 0.005, 0.0625, and 0.2 M. The MC simulations find monotonically increasing like-charge correlation functions in agreement with the modified HNC results but in contrast with the HNC prediction of a local maximum at low concentration. Nevertheless, HNC provides the best overall quantitative match with MC except at very low concentration; at 5 mM the modifications are a significant improvement. For unlike charge correlations, there is good agreement among all methods; at the ionic contact peak, however, HNC underestimates the peak height and the modifications provide improved accuracy.
Molecular Zeeman effect measurements on the ethylene–HCl complex
Authors: Kukolich S. G. , Aldrich P. D. , Read W. G. , Campbell E. J.
Published: February 2003

The molecular g values and magnetic susceptibility anisotropies for the ethylene–HCl complex were measured using pulsed Fourier transform microwave spectroscopy in magnetic fields of 25 and 30 kG. Values obtained are gaa=0.0383(5), gbb=−0.0072(3), gcc=−0.0082(2), 2χaa−χbb−χcc =−4.27(0.06), and 2χbb−χaa−χcc=1.05 (0.08)×10−9 MHz/G2. Quadrupole moments for the complex are Qaa=−2.30(24)×10−26 esu cm2, Qbb=2.40(38) ×10−26 esu cm2, and Qcc=−0.10(35)×10−26 esu cm2. From this data molecular quadrupole moments and other Zeeman parameters are derived for ethylene.
High resolution absorption spectroscopy of N+2X(v″=0)→ A(v′=4) using charge exchange detection
Authors: Hansen J. C. , Kuo C. H. , Grieman F. J. , Moseley J. T.
Published: February 2003

The optical absorption spectrum of the N+2 (4,0) Meinel band has been studied at high resolution (0.1 cm−1) using an ion flow tube in the cavity of a ring dye laser. The absorptions are observed by detecting the Ar+ products of the charge exchange reaction N+2 (A)+Ar→N2(X)+ Ar+. Improved molecular constants are obtained for the N+2A 2Πu(v=4) state. The charge exchange detection technique should prove generally useful for studying absorptions to long lived excited states.
L edge chemical shift and bond length difference of the mixed oxidation Ce3+/Ce4+ redox couple in solution
Authors: Sham T. K.
Published: February 2003

L edge absorption spectra of the Ce3+/Ce4+ redox couple in solution have been recorded with synchrotron radiation. It is found that the L2 and L3 near edges exhibit large chemical shifts (7.3–9.0 eV) between the intense resonance (‘‘white line’’) of Ce3+ and Ce4+ ions in solution. This shift is interpreted in terms of atomic and chemical effects and is correlated with thermodynamic parameters. Analysis of the L3 edge EXAFS gives an average Ce3+–H2O distance of 2.50±0.02 Å. The lower bound of the Ce–H2O bond length difference between Ce(H2O)4+n and Ce(H2O)3+n in solution is estimated to be 0.07±0.02 Å.
Mössbauer effect study of tight spin coupling in oxidized chloro-5,10,15,20-tetra(mesityl)porphyrinatoiron(III)
Authors: Boso Brian , Lang George , McMurry Thomas J. , Groves John T.
Published: February 2003

Mössbauer spectra of a polycrystalline form of oxidized chloro-5,10,15,20-tetra(mesityl)porphyrin- atoiron(III) [Fe(TMP)Cl], compound A, were recorded over a range of temperatures (4.2–195 K) and magnetic fields (0–6 T). These spectra of compound A exhibit magnetic features which are markedly different from those of the analogous protein complexes, horse radish peroxidase compound I (HRP-I) and compound ES of cytochrome c peroxidase, even though chemical evidence and optical spectroscopy indicate that compound A is similar to the others in comprising a Fe(IV) complex within a porphyrin cation radical. We interpret the data by employing a spin Hamiltonian model in which the central Fe(IV) complex, with S=1, is tightly coupled to a S=1/2 system of the oxidized porphyrin to yield a net S=3/2 system as suggested by the susceptibility measurements. The theoretical treatment yields information on the d-electron energies which is similar to that more directly available in the peroxidase spectra. The strength of the axial crystal field is found to increase progressively in the series HRP-I, ES, compound A, while the spin coupling in A is the strongest of the three by several orders of magnitude. The good fits to experimental data confirm the efficacy of the theoretical treatment which may be applicable to a variety of other coupled systems.
Vibronic coupling activity in the resonance Raman spectra of alkyl benzenes
Authors: Ziegler L. D. , Hudson B. S.
Published: February 2003

The Raman spectra of the alkyl benzene derivatives toluene, para-xylene, and mesitylene (1,3,5,-trimethylbenzene) have been obtained with 212.8 nm laser radiation. This radiation is resonant with the 1La excited state of these benzenoid aromatics. The dominant dipole forbidden character of this electronic transition is reflected in the resonance Raman spectra. The additional substituent-induced dipole allowed character is clearly evident in the scattering spectra of the D2h and C2v derivatives as well. These results illustrate the validity of the vibronic theory of Raman scattering.
Rotational relaxation of OCS in n-alkanes: Collective and collisional effects
Authors: Goulay A. M.
Published: February 2003

The rotational relaxation time τlR of OCS in liquid n-dodecane is determined at several temperatures by means of infrared and isotropic Raman measurements. The results are compared with data on OCS in other n-alkanes and are interpreted in terms of hydrodynamic theories as well as of the theory of Hynes et al. The latter theory explains well our results and shows the predominance of collisional over collective effects. In this case, a simple cage model accounts for all results and displays the dominant role of the relative free volume.
A refined potential surface for the X̃ 3B1 electronic state of methylene CH2
Authors: Bunker P. R. , Jensen Per
Published: February 2003

In this paper we use the nonrigid bender Hamiltonian to calculate the rotation–vibration energy levels of 12CH2, 12CD2, and 13CH2 in the X̃ 3B1 ground state. We adjust the shape of the potential surface used in the calculation in order to optimize the fit of the calculated energy level separations to the 61 available experimental values for these three isotopes of methylene. In the fit we vary nine parameters and determine, among other things, that the equilibrium bond length is 1.0748±0.0004 Å, the equilibrium bond angle is 133.84°±0.05°, the height of the barrier to linearity is 1940±80 cm−1, and the optimum bond length for the molecule when it is linear, at the saddle point in the potential surface, is 1.060±0.005 Å. From our results we predict the stretching frequencies in 12CH2 to be ν1=2950±40 cm−1 and ν3=3080±40 cm−1.
Exponential time-dependent perturbation theory in rotationally inelastic scattering
Authors: Cross R. J.
Published: February 2003

An exponential form of time-dependent perturbation theory (the Magnus approximation) is developed for rotationally inelastic scattering. A phase-shift matrix is calculated as an integral in time over the anisotropic part of the potential. The trajectory used for this integral is specified by the diagonal part of the potential matrix and the arithmetic average of the initial and final velocities and the average orbital angular momentum. The exponential of the phase-shift matrix gives the scattering matrix and the various cross sections. A special representation is used where the orbital angular momentum is either treated classically or may be frozen out to yield the orbital sudden approximation. Calculations on Ar+N2 and Ar+TIF show that the theory generally gives very good agreement with accurate calculations, even where the orbital sudden approximation (coupled-states) results are seriously in error.
Microstructure of two-phase random media. III. The n-point matrix probability functions for fully penetrable spheres
Authors: Torquato S. , Stell G.
Published: February 2003

We examine the n-point matrix probability functions Sn (which give the probability of finding n points in the matrix phase of a two-phase random medium), for a model in which the included material consists of fully penetrable spheres of equal diameter (i.e., a system of identical spheres such that their centers are randomly distributed in the matrix). Exploiting the special simplicity of the model we give an explicit closed-form expression for S3 as well as sharp bounds on S3 and S4. Our best lower bound on S3 and our corresponding upper bound on S4 satisfy certain asymptotic forms (for both small and large separation of points) that are satisfied by the exact S3 and S4 for impenetrable as well as penetrable spheres, even though the bounding properties of our expressions can only be guaranteed for penetrable spheres. These expressions (and the resulting approximation for S4 in terms of S1 and S2 obtained from them) are thus highly appropriate approximants for both systems to be used in composite-media transport-coefficient expressions that involve integrals over the Sn. The S3 expression has in fact been suggested some time ago by Weissberg and Prager; our methods here provide further justification for this expression as well as one means of systematically generalizing it to Sn for higher n.
Monte Carlo study of dynamics of the multichain polymer system on the tetrahedral lattice
Authors: Koliński Andrzej , Romiszowski Piotr
Published: February 2003

Diffusion of the chain molecules in the concentrated solutions was studied by means of the computer simulation method. The computations were made for various chain lengths and polymer concentrations. It was observed that the rate of diffusion of the polymer chains strongly depends on the chain length according to the relation D∝n−b. It was found that the value of exponent b increases with the polymer concentration.
Extended x-ray absorption fine structure (EXAFS) of highly dispersed rhodium catalysts
Authors: Via G. H. , Meitzner G. , Lytle F. W. , Sinfelt J. H.
Published: February 2003

An investigation was conducted of the extended x-ray absorption fine structure (EXAFS) associated with the K-absorption edge of the rhodium in two catalysts, Rh/SiO2 and Rh/Al2O3, which contained 1 and 0.5 wt. % rhodium, respectively. Hydrogen chemisorption data indicated that the rhodium atoms were predominantly surface atoms in both catalysts. However, the EXAFS data indicated that the average number of nearest neighbor rhodium atoms about a given rhodium atom was much lower for the Rh/Al2O3 catalyst than for the Rh/SiO2 catalyst, 1.5 compared to 9. These values compare with a value of 12 for metallic rhodium. Although the detection of at least some Rh–Rh coordination for the Rh/Al2O3 catalyst indicates that the rhodium is not present exclusively as single atoms bound to the alumina, the EXAFS data do not exclude the possibility that rhodium in this form could be present along with rhodium clusters.
The influence of steps on the desorption kinetics of NO from Pt(111)
Authors: Serri J. A. , Tully J. C. , Cardillo M. J.
Published: February 2003

A kinetic model is presented which describes thermal desorption of molecules from surfaces and incorporates trapping at steps. The model is applied to desorption of NO from Pt(111). The results of three molecular beam experiments, which differ substantially from each other, are accounted for quantitatively by the model using one set of rate parameters. It is demonstrated that steps, even when present in small concentrations on accurately cut crystals, can play a dominant role in thermal desorption. The magnitude of the effect depends on the difference between the binding energies on the terrace and at steps, which for NO on Pt(111) is large (25±2 kcal/mol and 34±2 kcal/mol, respectively). Furthermore, it is shown that the rate parameters governing terrace desorption, escape from steps, and surface diffusion can all be extracted from modulated molecular beam studies carried out for a suitable range of conditions.
Vibrational relaxation of NO+ (v) ions in neutral collisions
Authors: Dobler W. , Federer W. , Howorka F. , Lindinger W. , Durup-Ferguson M. , Ferguson E. E.
Published: February 2003

Vibrational relaxation of NO+ (v) by collisions with NO, N2, CO2, CH4, NO2, He, Ar, and Kr has b een measured from thermal energy up to 1 eV. NO+ (v) is vibrationally relaxed at near the collision frequency by resonant charge transfer with NO. N2 is quite efficient due to near resonant V → V transfer. CO2, CH4, and NO2 are efficient, Ar, Kr, and especially He are not. The quenching rate constants for N2, CO2, and CH4 decrease with KE, those for NO and NO2 do not.
The simulation of spinodal decomposition in two dimensions: A comparison of Monte Carlo and Langevin dynamics
Authors: Meakin Paul , Metiu H. , Petschek R. G. , Scalapino D. J.
Published: February 2003

Langevin dynamics and Monte Carlo methods have recently been developed for the simulation of spinodal decomposition. In this paper we show, using the central limit theorem, that the Monte Carlo method (in the limit of small step sizes) is equivalent to Langevin dynamics (in the high friction limit). In this limit an exact relationship is established between the number of trials in a Monte Carlo simulation and Langevin dynamics time. The relationship between the Monte Carlo and Langevin dynamics methods is demonstrated for practical simulations and the effects of deviations from the small step size limit in the Monte Carlo simulation is a linear distortion of the effective time scale, provided the step size is not too large. The equivalence of the Monte Carlo and Langevin dynamics methods (in the appropriate limits) are demonstrated for the case of spinodal decomposition in two dimensions. However, our theoretical analysis is quite general and is not restricted to spinodal decomposition or two-dimensional systems.
Response of liquid carbon disulfide to shock compression: Equation of state at normal and high densities
Authors: Sheffield S. A. , Duvall G. E.
Published: February 2003

Liquid carbon disulfide (CS2) is known to exhibit unusual behavior when it is shock compressed to pressures of approximately 6.0 GPa. This behavior has been explained by either a phase transition to a ‘‘black solid CS2 ’’ or a decomposition chemical reaction. In order to learn more about the anomalous behavior as it relates to a shock-induced chemical reaction, it was necessary to develop equations of state for both the unreacted liquid and the reaction products. This paper describes the development of these equations of state along with comparisons of calculated Hugoniot information to measured data available in the literature. Both equations are based on the use of a Murnaghan isotherm relation along with assumed forms for specific heat and the Gruneisen parameter. Methods employed to match the calculations to the data in order to establish the values of various adjustable parameters are detailed. Temperature calculations are included although there is some uncertainty as to their accuracy because of a lack of experimental temperature data.
Molecular shape effect on electron transport in fluids: Density and temperature studies in mixtures of n-pentane and neopentane
Authors: György István , Gee Norman , Freeman Gordon R.
Published: February 2003

Electron mobility μ0 in nonpolar and slightly polar (D≤0.5 D) hydrocarbon fluids is strongly affected by the external shape of the molecules. The mobility is large in a liquid of spherelike molecules such as neopentane. Addition of rodlike molecules such as n-pentane decreases μ0, but has little effect on the value of the threshold field for electron heating. A liquid of spherelike molecules forms a conduction band that has relatively small undulations in its lower potential surface. Addition of rodlike molecules provides orientational disorder, anisotropic polarizability, and restricted rotation, which in combination produce potential fluctuations that are durable enough to localized electrons. The maximum in μ0 that occurs in pure liquid neopentane at 3.5×1027 molecule/m3 progressively decreases in magnitude and shifts to lower densities as n-pentane is added; it occurs at the critical density in n-pentane. A plot of log μ0 against mol % n-pentane for liquid mixtures of n- and neo-pentane at constant T/Tc is slightly S shaped. A plot of the Arrhenius temperature coefficient Eμ against mole % n-pentane is S shaped in the opposite direction. At T/Tc=0.999 the log μ0 plot is not S shape and the slope is much smaller than at T/Tc≤0.92; the molecular shape effect is minimal in the critical fluids and dense gases, 1.0≥n/nc≥0.3. Quasilocalization occurs to a similar extent in all gases at the same n/nc and T/Tc. In the low density gases the molecular shape effect is the opposite of that in dense liquids; at electron energies <0.10 eV the spherelike neopentane has a larger scattering cross section than does its nonspherelike isomer n-pentane. In an equimolar mixture the stronger scatterer dominates the electron behavior, which means n-pentane in the liquid and neopentane in the normal gas.