bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonequilibrium molecular dynamics calculation of self-diffusion in a non-Newtonian fluid subject to a Couette strain field
Authors: Cummings P. T. , Wang B. Y. , Evans D. J. , Fraser K. J.
Published: July 2002

Molecular dynamics simulations have been performed on a simple fluid in the non-Newtonian regime in order to study the nature of diffusion in the presence of shear stress. The nonequilibrium molecular dynamics (NEMD) sllod algorithm is used to generate a homogeneous boundary driven Couette flow. The elements of the tensor of diffusion coefficients have been evaluated by three different routes: through Einstein relations with the corresponding elements of the tensor of molecule displacements, through Green–Kubo expressions involving tensor of momentum autocorrelation functions and cross-correlation functions, and by utilizing an NEMD color field method. All three routes to the tensor of diffusion coefficients are shown to yield consistent results. A simple spherically symmetric Lennard-Jones fluid at its triple point was used in the simulation study. We find that in a Couette strain field of sufficiently large strain rate, diffusion in all directions is enhanced substantially. We propose and verify a new asymptotic relationship between the diagonal elements of the tensor of diffusion coefficients and the strain rate.
Diode laser spectroscopy of the ν2 fundamental and hot bands of NH+3
Authors: Lee Szetsen Steven , Oka Takeshi
Published: July 2002

The nondegenerate ν2←0 and 2ν2←ν2 bands of the ammonia cation NH+3 have been observed using tunable diode lasers combined with the velocity modulation technique. The ion was produced in a water-cooled 6 kHz ac glow discharge cell with a gas mixture of [He]:[N2]:[H2]=9:0.06:0.3 (Torr). From the observed spectra, the band origins were determined to be ν1←0=903.3898 cm−1 and ν2←1=939.771 cm−1. Vibration–rotation interaction constants and potential constants were derived. The missing of K=0 R-branch transitions with N=odd in the ν2←0 band and N=even in the 2ν2←ν2 band clearly shows the characteristics of a D3h planar structure with a 2A″2 ground electronic state, just like its isoelectronic case CH3 radical. Spin–rotation doublets were well-resolved in R-branch transitions and spin–rotation interaction constants of the ground, ν2, and 2ν2 states were determined. A potential function consisting of a quadratic term and a quartic term was used for modeling the out-of-plane bending vibration of NH+3. In comparison with the methyl radical, the ammonia cation is more rigid and closer to a harmonic oscillator.
Vibrational relaxation of ozone in O3–O2 and O3–N2 gas mixtures from infrared double-resonance measurements
Authors: Ménard-Bourcin F. , Ménard J. , Doyennette L.
Published: July 2002

The vibrational relaxation of ozone in O3–O2 and O3–N2 mixtures has been studied by means of a time-resolved infrared double resonance technique using two CO2 lasers. The collisional deexcitation of the first excited states of the stretching modes including the V–V transfer processes to the bending mode has been directly observed allowing the rate constants to be measured as a function of the O3 mole fraction in the mixtures. Finally, the rate coefficients for the intermode transfer (ν1, ν3)→ν2 and for the deexcitation from the ν2 state have been derived.
Unimolecular dissociation of hydrogen peroxide from single rovibrational states near threshold
Authors: Luo X. , Rizzo T. R.
Published: July 2002

Infrared-optical double resonance excitation of hydrogen peroxide using the ν3+ν5 and ν2 + ν5 combination bands as intermediate levels prepares reactant molecules in single rotational states near the OO dissociation threshold. The band origins of the ΔvOH = 4 vibrational overtone transitions originating from these combination bands provide information on the anharmonicities between the OH stretch and the OO stretch and OOH bend, respectively. At low resolution the vibrational overtone transitions are clearly parallel bands of a near prolate symmetric top and can be assigned to zeroth-order J and K quantum numbers. At 0.1 cm−1 resolution the individual features in the vibrational overtone spectra appear as clumps of sharp lines centered at the frequencies of the zeroth-order symmetric top transitions. The number of components within a clump appears to be less than the total number of available vibrational states. The narrowest feature observed has a linewidth of 0.12 cm−1 and sets a lower bound of ∼35 ps on the lifetime of the dissociating molecule. As J increases from 1 to 21, the clumps of lines coalesce into a smooth Lorentzian envelope. The overall clump width decreases with J, reaching an asymptotic value of 0.67 cm−1 at J≂17. A model incorporating a J dependent density of bath levels that couple to the zeroth-order bright state predicts the observed changes in the vibrational overtone transitions with J. The rotationally resolved vibrational overtone spectra provide information on the time scale for the conservation of K of the dissociating molecule.
Equilibrium, dynamic, and trapping properties of an excess electron in dense helium
Authors: Sheu Sheh-Yi , Cukier R. I.
Published: July 2002

The equilibrium, dynamic, and trapping properties of an excess electron in dense helium are simulated. An adiabatic simulation method is used whereby the Schrödinger equation for the electron in the presence of a fixed, classical solvent configuration is solved. The solvent configuration is advanced by molecular dynamics with the force on a particular helium atom arising from the classical helium–helium potential and the expectation value of the electron-helium potential. The equilibrium properties of the electron are contrasted with those obtained by Coker and Berne [D. F. Coker and B. F. Berne, J. Chem. Phys. 89, 2128 (1988)] using a different procedure for generating helium configurations. The diffusion coefficient of the electron is obtained and, for ρ*=ρσ 3=0.9, is De=5.0×10−3 cm2 s−1. This is an order of magnitude greater than the diffusion coefficient of the helium atoms and corresponds to a very mobile electron. The distribution of times for an electron to move between donor and acceptor sites inserted in the system is obtained and shown to yield an average diffusion coefficient consistent with that obtained from the mean square displacement. The ability of the electron to move between the donor and acceptor sites by electron transfer is assessed by evaluating the reorganization energy of the solvent and using conventional electron transfer theory. If the sites are sufficiently far apart, then electron transport via detrapping from the donor site followed by transport to the acceptor site can be competitive with electron transfer as a charge transport mechanism.
J coupling in chemically equivalent spin pairs as studied by solid-state nuclear magnetic resonance
Authors: Challoner Robin , Nakai Toshihito , McDowell Charles A.
Published: July 2002

The presence of J coupling was observed in the solid-state nuclear magnetic resonance (NMR) spectrum of the 31P spin pair in polycrystalline 1,2-bis (2,4,6-tri-tert-butylphenyl)-diphosphene (TBPDP) using the magic-angle spinning (MAS) technique. This represents the first example of J coupling in a spin-pair system with nuclei having identical isotropic chemical shifts. Average Hamiltonian theory is used to derive the time-independent eigenstates for the system. It is shown that a second-order perturbation term, which is dependent on the difference between the chemical-shift tensor orientations in addition to the strength of the dipolar coupling, allows the recoupling of the J interaction for the two chemically equivalent 31P nuclei. In the absence of the perturbation term, the system is reduced to an A2 case in solution-state NMR for which, of course, no J coupling may be observed. The magnitude of the J coupling, namely, 580±20 Hz obtained from two-dimensional (2D) J-resolved experiments was found to be in excellent agreement with the value of 577±15 Hz estimated using the abovementioned theory and experimental data from the one-dimensional (1D) spectra measured as a function of the rotor spinning frequency.
Gas phase reaction rates of N+2 with CH4, O2, and n-H2 at very low temperatures
Authors: Randeniya L. K. , Smith M. A.
Published: July 2002

Using a free jet flow reactor, we have measured the bimolecular rate coefficients for the reactions N+2 with CH4, O2, and n-H2 at temperatures below 15 K. The reaction of CH4 proceeds at a rate which is only slightly faster than the rates at 70 and 300 K, but shows no apparent temperature dependence between 8–15 K. The measurements for the reaction of N+2 with O2 near 10 K indicate a marked increase of the rate coefficient from the room temperature value. The kinetic energy dependence of the rate coefficient for the reaction N+2 with n-H2 resembles the peculiar behavior shown by the reactions of NH+3 and C2H+2 with H2 where a minimum is observed in the rate coefficient. Possible mechanistic implications of such distinct temperature dependences are discussed.
Harmonic vibrational frequencies and infrared intensities from analytic fourth-order many-body perturbation theory gradients
Authors: Stanton John F. , Watts John D. , Bartlett Rodney J.
Published: July 2002

Recently developed fourth-order many-body perturbation theory [MBPT(4)] analytic gradient techniques are applied to a study of the harmonic vibrational frequencies and infrared intensities of prototype small polyatomic molecules. Results are presented for the complete fourth-order model as well as for the SDQ-MBPT(4) scheme, which neglects the contribution of triple excitations. These results are also compared to frequencies and intensities obtained at both higher and lower levels of theory. Differences between the fourth-order results and those obtained with the infinite order coupled-cluster (CC) counterparts of SDQ-MBPT(4) and MBPT(4) [CCSD and CC models including triple excitations, the latter approximated here by CCSDT-1 and the UCC(4) model] are found to be negligible for all bending modes and stretches involving single bonds. However, the infinite order effects included in CC models are important for describing stretching modes of multiple bonds. Therefore, iterative CC calculations are needed to accurately characterize these regions of the potential surface.
The effect of pressure-induced conformational changes on the longitudinal acoustic mode in perfluoro-n-alkanes
Authors: Song Kigook , Hallmark Vickie M. , Rabolt John F.
Published: July 2002

An investigation of the longitudinal acoustic mode (LAM) in a series perfluoro-n-alkanes under pressure has been carried out using Raman spectroscopy in conjunction with a diamond anvil cell. Perfluoro-n-alkanes are shown to undergo a phase transition from a helical to a planar zigzag structure at 5 kbar. This conformational change with pressure results in an abrupt increase in the LAM frequency. Since the chain axis elastic modulus of a planar zigzag molecule is larger than that of a helical one, such a conformational change leads to stiffening of the backbone giving rise to the observed change in the LAM frequency.
The dielectric behavior of vapor-deposited amorphous solid water and of its crystalline forms
Authors: Johari G. P. , Hallbrucker Andreas , Mayer Erwin
Published: July 2002

The dielectric permittivity and loss of vapor-deposited amorphous solid water (ASW) have been measured for fixed frequencies of 1 and 10 kHz from 80 K to its crystallization temperature. Similar measurements have also been made on the cubic ice formed after the crystallization of ASW and the hexagonal ice formed on heating the cubic ice. The loss tangent shows a broad sub-Tg relaxation peak centered at about 100 K and an approach towards a plateau value which appears as a shoulder. The peak is attributed to thermally activated rotation of H2O molecules with one or two dangling OH groups on the surface of the pores of the microporous sample, and the shoulder to localized motions within the network structure. Sintering of the samples on thermal cycling between 77 and ≊120 K in vacuo causes the broad peak to vanish. With increase in temperature, above Tg, the loss tangent shows the emergence of the expected α-relaxation peak of a liquid at T>Tg, whose completion is terminated by the onset of crystallization to cubic ice at about the same temperature for 1 kHz and for 10 kHz measurements. Thereafter, crystallization becomes slower, thus allowing further observation of the low-temperature part of the α-relaxation peak.

The dielectric loss of amorphous solid water at 80 K is nearly 20 times that of the cubic ice formed on its crystallization after heating to 193 K, and nearly 3.5 times higher at Tg. The dielectric loss of the cubic ice formed on crystallization tends towards a plateau value prior to rapidly increasing with increase in temperature, an evidence for a low temperature relaxation which vanishes on conversion to hexagonal ice. This relaxation indicates a remnant topologically disordered structure of intergranular water and/or stacking faults in cubic ice. Samples sintered by thermal cycling in vacuo to ≊120 K crystallized in one step, whereas those without prior thermal cycling crystallized in two steps to cubic ice. Annealing of the ASW at 130 K completely removes the broad peak at ≊100 K and reduces the magnitude of the β-relaxation process. Partial crystallization of ASW completely removes the β-relaxation process, which indicates either preferential crystallization at the local regions of high mobility in the amorphous matrix or a change in the structure of the amorphous phase during crystallization.
Bond-length and bond-angle distributions in coarse-grained polymer chains
Authors: Laso M. , Öttinger H. C. , Suter U. W.
Published: July 2002

The joint distribution pN(ω,R1,R2) of the lengths of the vectors (R1,R2) and the angle ω between the vectors joining the center of mass of a block or bead in a chain with the centers of mass of the preceding and succeeding beads of a coarse-grained chain is investigated. Somewhat unexpectedly, the a priori and the conditional distributions for the connector length pRN(R1) and for the interconnector angle pωN(ω) can be determined in closed form for the Gaussian random walk for any number of monomers N per bead. Additionally, pωN(ω) is also obtained for an unperturbed polymethylene chain by a Monte Carlo scheme. In general, pωN(ω) is asymmetric and shifted to values greater than π/2 for all bead sizes, implying that the chain made up of the centers of mass of the beads tends to be locally more extended or spread out than a random walk. Convergence to the (analytically known) asymptotic distribution pω∞(ω) for the Gaussian random walk is given analytically and turns out to be very nearly quadratic in 1/N. For the polymethylene chain convergence is linear. The a priori and the conditional distributions are used to formulate a Monte Carlo scheme for the generation of the coarse-grained chain. The bias of pωN(ω) to angles greater than π/2 should be taken into account by any model that attempts to represent a linear chain polymer by lumping its detailed structural information in coarser units at a larger length scale.
Paramagnetism of high nuclearity metal cluster compounds as derived from local density functional calculations
Authors: Rösch Notker , Ackermann Lutz , Pacchioni Gianfranco , Dunlap Brett I.
Published: July 2002

The electronic structure of high nuclearity carbonylated Ni clusters [Ni32C6(CO)32]n− and [Ni44(CO)48]n− (n=0–6) has been investigated by means of all-electron calculations within the local density functional approach. The transition from the molecular to the metallic state was studied by determining the magnetic properties of bare and carbonylated clusters. The appearance of the metallic magnetic behavior is connected with the presence of metal atoms with bulklike coordination. The effect of interstitial impurity atoms in quenching the magnetic moment is also discussed.
The nature of the interactions of poly(methyl methacrylate) oligomers with an aluminum surface
Authors: Shaffer J. Scott , Chakraborty Arup K. , Tirrell Matthew , Davis H. Ted , Martins Jose L.
Published: July 2002

Polymer–metal interfaces are of increasing technological importance in a variety of applications. These interfaces are characterized by specific interactions between functional groups of the organic polymer and the metallic substrate. In order to study the structure of these interfaces at the molecular level, the energetics of the segment–surface interactions must be well characterized. We have used density functional theory to investigate the interactions of poly(methyl methacrylate) (PMMA) oligomers with aluminum surfaces. The aluminum surface is represented by the simple jellium model. The energetics of the interactions between the organic molecules and the aluminum surface are calculated as a function of the orientation with respect to the surface of the organic molecule and various internal degrees of freedom. The computed energy hypersurfaces exhibit a rich structure characterized by several energy minima and barriers. Implications of such an energy hypersurface for the architecture of long chain molecules at the interface are discussed. The energy hypersurfaces also show that the rotational conformational statistics of PMMA interacting with an aluminum surface should be quite different from that of bulk PMMA. The reported energy hypersurfaces have been used to construct an empirical force-field for future molecular simulations of long chains subject to the proper potentials.
Excited electronic states of protonated acetylene
Authors: Gianturco F. A. , Schneider F.
Published: July 2002

The classical structure of protonated C2H2, and the nonclassical, bridge arrangement for the same species, have been studied along the rearrangement pathway with the intention of throwing new light on the role played by the electronically excited state of the full system in the collisions between a proton and the C2H2 molecule. Furthermore, the potential energy on one possible dissociation path into acetylene and proton has been obtained. Extensive, multiple reference double-excitation configuration-interaction (MRD-CI) calculations have been carried out for the first time with the specific aim of locating energies and geometries of the ground and of the lower-lying excited states of the C2H+3 species as it goes from one structure to the other and from the C2V geometry to separated C2H2+H+ or C2H+2+H fragments. The results show an interesting behavior of the excited states and provide a preliminary understanding of the full potential energy surfaces thought to be involved in charge-transfer (CT) inelastic collisions at low and intermediate energies (E≤100 eV).
Analysis of the crystal-field spectra of the actinide tetrafluorides. I. UF4, NpF4, and PuF4
Authors: Carnall W. T. , Liu G. K. , Williams C. W. , Reid M. F.
Published: July 2002

An interpretation of the low-temperature absorption spectra of AnF4 (An=U, Np, Pu) is presented. Using an effective operator Hamiltonian with orthogonalized free-ion operators and initializing crystal-field parameter values based on a superposition model calculation for An4+ sites with C2 symmetry, good agreement between the model calculations and experimentally observed absorption band structure could be obtained. Correlations with published magnetic and heat capacity measurements are discussed.
On the N2–He potential energy surface
Authors: Beneventi L. , Casavecchia P. , Volpi G. G. , Wong C. C. K. , McCourt F. R. W. , Corey G. C. , Lemoine D.
Published: July 2002

A detailed comparison of the predictive powers of two recently determined empirical and two recently proposed theoretical potential energy surfaces for the N2–He interaction has been carried out. In particular, the following properties have been tested: at the microscopic level, total and state-to-state differential cross sections and absolute total integral cross sections, while at the macroscopic level, interaction second virial, diffusion, viscosity, and thermal conductivity coefficients, as well as the rotational relaxation time, depolarized Rayleigh spectral collision broadening, and shear viscosity and thermal conductivity field-effect data in N2–He mixtures. Exact calculations have been employed, from the close-coupling method for treating scattering data at low energies to the classical trajectory method with second-order corrections to compute the effective cross sections that determine the bulk transport and relaxation phenomena.

The empirical exponential-spline–Morse-spline–van der Waals surface [J. Chem. Phys. 85, 7011 (1986)], closely followed by the model Bowers–Tang–Toennies surface [J. Chem. Phys. 88, 5465 (1988)], gives better simultaneous agreement with the scattering data, the second virial coefficient data, the bulk transport data, and the depolarized Rayleigh collision-broadening data, which are properties sensitive to the spherical component of the interaction and to the anisotropy of the low repulsive wall. None of the potential surfaces examined here includes a dependence upon the vibrational stretching coordinate of the N2 molecule, since none of the data employed in the fitting is sensitive to this coordinate. The two theoretical model potentials, especially that based upon an earlier Hartree–Fock plus damped dispersion model surface [J. Phys. Chem. 88, 2036 (1984)], gives better agreement with the rotational relaxation and field-effect data, which are properties sensitive to the anisotropy of the high-repulsive wall. It is established that the exponential-spline–Morse-spline–van der Waals and Bowers–Tang–Toennies surfaces are on the whole the more reliable of the empirical and model surfaces examined, respectively. It is concluded that the optimum N2–He potential energy surface should be a blend of the empirical exponential-spline–Morse-spline–van der Waals and of the two model surfaces.
Doubly charged valence states of formaldehyde, acetaldehyde, acetone, and formamide studied by means of photon excited Auger electron spectroscopy and a
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Authors: Correia N. , Naves de Brito A. , Keane M. P. , Karlsson L. , Svensson S. , Liegener C-M. , Cesar A. , Ågren H.
Published: July 2002

The doubly ionized states in formaldehyde, acetaldehyde, acetone, and formamide have been studied by means of x-ray excited core (C1s, N1s, O1s)–valence–valence Auger electron spectroscopy. Assignments of the spectra have been made using ab initio Hartree–Fock, Green’s function, and configuration interaction (CI) calculations. A molecular orbital analysis has been carried out for the high kinetic energy part of the spectra. The breakdown of the single particle picture is found to be important over a large energy interval in the spectra. The results obtained illustrate the usefulness of Auger electron spectroscopy in characterizing the doubly ionized states even in the case of large molecular systems. The first double ionization energies for the four molecules presented have been determined to be 33.8, 30.3, 28.0, and 30 eV, respectively.
The influence of adsorbent microstructure upon adsorption equilibria: Investigations of a model system
Authors: Kaminsky R. D. , Monson P. A.
Published: July 2002

We present a theoretical study of the influence of the microstructure of a porous adsorbent upon associated adsorption behavior. A model is developed which describes the interactions of adsorbed molecules with an adsorbent treated as a matrix of particles each of which is a continuum of interaction centers. The model leads to an analytic expression for the adsorbate-adsorbent particle potential which is an analog of the 9-3 potential model for adsorption on planar solid surfaces. To illustrate the utility of the approach, an application to methane adsorbed in a microporous silica gel is presented. Several adsorbent microstructures are investigated, including a variety of crystal lattices as well as structures derived from equilibrium configurations of hard spheres. Adsorption in these structures is studied through calculation of Henry’s law constants and by using grand canonical Monte Carlo simulation to determine adsorption isotherms and the structure of adsorbed fluids. The results obtained are related to details of the adsorbent microstructure.
Spectroscopic detection of the A(v=2) level of 4HeD
Authors: Tokaryk D. W. , Brooks R. L.
Published: July 2002

A new near infrared emission spectrum of 4HeD has been recorded at 4.2 K during proton irradiation of a mixed sample of helium gas and solid deuterium. The upper state of the transitions is an admixture of the accidentally degenerate D 2Σ+(v=0) and C 2Σ+(v=3) levels. The lower state is the previously unobserved A 2Σ+(v=2) level. The observation of this level allows for evaluation of the rotational constants B″2 and D″2 for the A 2Σ+ state, and for a better estimate of the vibrational parameters ω″e and ω″eχ″e.
State-to-state dynamics of atom + polyatom abstraction reactions. I. The H+CD4→HD(v′,J ′)+CD3 reaction
Authors: Germann Geoffrey J. , Huh Young-Duk , Valentini James J.
Published: July 2002

We report measurement of the HD product quantum state distributions and absolute cross section for the H+CD4→HD(v′,J′)+CD3 reaction at a collision energy of 1.5 eV. The total reaction cross section is small, 0.14±0.03 Å2, making the experimental measurements difficult. The HD quantum state distribution peaks at low J′ in both v′=0 and v′=1, the only vibrational states in which product is observed. Very little of the 1.5 eV available energy appears as internal excitation of the HD product molecule, 7% in HD product vibration and 9% in rotation. However, linear surprisal analysis shows that this limited internal energy disposal in the HD product in some ways exceeds that expected statistically, since two of the best-fit surprisal parameters (Θr=2.9±0.6 for v′=0, Θr=−1.9±0.5 for v′=1, λv=−2.2±0.6 ) are negative. The HD rovibrational state distribution shows an anomalous positive correlation of product vibrational and rotational excitation. Those molecules formed in the vibrationally excited state, v′=1, have significantly more rotational energy (〈Erot〉=0.17 eV) than those molecules formed in the vibrational ground state, v′=0 (〈Erot〉=0.13 eV). This behavior runs counter to the otherwise universal behavior for direct bimolecular reactions—a negative correlation of product vibrational and rotational excitation. We speculate as to the source of this anomalous energy disposal.
A combined quantum-classical dynamics method for calculating thermal rate constants of chemical reactions in solution
Authors: Truong Thanh N. , McCammon J. Andrew , Kouri Donald J. , Hoffman David K.
Published: July 2002

We present a combined quantum-classical-stochastic dynamics method based on the flux–flux correlation function for calculating the thermal rate constants of chemical reactions in solution or in biological systems. The present method is an extension of an earlier method by Metiu and co-workers [J. Chem. Phys. 88, 2478 (1988)] to include stochastic dynamics. The method is tested by applying it to a simple model of hydrogen atom transfer reaction in solution. We also examine the behavior of the flux–flux correlation function and the rate constants as functions of viscosity.
Electron thermalization processes in the bicomponent systems of Kr and some simple molecular gases
Authors: Okigaki Shigeyasu , Suzuki Etsuhito , Hayashi Kouji , Kurashige Kazuhisa , Hatano Yoshihiko
Published: July 2002

The time evolution of electron mean energy (Te) in the bicomponent systems of Kr and some simple molecules: H2, D2, N2, CO, and CO2, at room temperature (Tg) has been observed by means of a pulse-radiolysis microwave-conductivity method. The excess electron temperature, Te–Tg, in a near thermal energy region decays exponentially with time. An observed decay lifetime is defined as an electron thermalization time. From the effect of the addition of a simple molecular gas to Kr on an electron thermalization time, the product (in unit of 1014 μs⋅cm−3) of a gas number density and a thermalization time for each pure additive molecule is obtained to be (3.3±1.2)×102 for H2, (3.4±1.2)×102 for D2, (2.3±0.3)×102 for N2, 8.9±3.0 for CO, and 2.5±0.3 for CO2, respectively.
CO2 laser Stark spectroscopy of the ν4 band of SiHF3: The C rotational constant and vibrationally induced dipole moment
Authors: Harada Kensuke , Akao Syuji , Miyachi Kazumi , Tanaka Keiichi , Tanaka Takehiko
Published: July 2002

The ν4 (SiF3 degenerate stretch) fundamental band of SiHF3 has been investigated by the CO2 laser Stark spectroscopy. The vibrationally induced dipole moment was found to give remarkable perturbations of the type (Δl,ΔK)=(2,−1) in the ν4 vibrational state under intense electric field. Several perturbation-induced rovibrational transitions obeying the Δ(K−l)=±3 selection rules were observed in the Stark spectrum. The C0 rotational constant, which is otherwise hard to obtain for a symmetric top molecule, was determined to be 4134.73(71) MHz. The vibrationally induced dipole moment μr in the ν4 state was derived as ±0.034 38(17) D, the sign being experimentally indeterminable. The band origin, the Coriolis constant Cζ4, and the vibration–rotation constant αC4 are 999.581 076(30) cm−1, 2445.59(71) MHz, and 9.243(22) MHz, respectively. The dipole moment is 1.359 20(22) D in the ground state, and increases by 0.026 73(10) D on excitation of the ν4 vibration. The uncertainties given in the parentheses correspond to three standard deviations. The present results are compared with our previous results of a similar analysis for the deuterated species. The dipole moment in the ground vibrational state decreases by 0.002 01(28) D on deuteration. The vibrationally induced dipole moments μr and the vibrational changes δμ4 of the dipole moment for both isotropic species are discussed theoretically. We introduce the coefficient of the mean square amplitude dependence for the bond moment, which is useful in the discussion.
Comparison of bending, C–C stretching, and collision energy effects on the reaction of C2H+2 with D2
Authors: Chiu Yu-hui , Yang Baorui , Fu Hungshin , Anderson Scott L. , Schweizer Martin , Gerlich Dieter
Published: July 2002

We report the effects of vibrational excitation and collision energy on the cross sections and branching ratios for reaction of acetylene cations with D2, using two different guided-ion-beam instruments. Two major reaction channels are observed, both of which are nearly thermoneutral. Hydrogen atom exchange is slightly exoergic due to zero point energy, and is inhibited by both collision and vibrational energy. Formation of the two isotopic ‘‘C2H+3 ’’ products is enhanced by collision energy and C–C stretching vibration, but not by bending vibration. The branching ratios at low collision energies are consistent with reaction via an intermediate complex, and Rice–Ramsberger–Kassel–Marcus (RRKM) analysis is used to extract further information. At collision energies above 1 eV, D-atom transfer by a direct mechanism is also observed as a route to C2H2D+ production. Comparison of our results using both the Stony Brook and Freiburg instruments is made with the state-selected experiments of Honma, Kato, Tanaka, and Koyano [J. Chem. Phys. 81, 5666 (1984)], who previously studied both the C2H+2+D2 and C2D+2+H2 isotopic reactants. Our results for C2H+2+D2 are consistent with their C2D+2+H2 data for all collision energies and with their C2H+2+D2 data at 0.2 eV collision energy. We do not reproduce the anomalous vibrational effects they reported for 1 and 2 eV collision energies.
Vibrational relaxation of acetylene produced by the photolysis of vinyl bromide
Authors: Murray K. K. , Morter C. L. , Curl R. F.
Published: July 2002

Vibrational relaxation of the ν2 and 2ν2 CC stretch vibrational states in the ground electronic state of acetylene has been investigated using infrared kinetic spectroscopy. Excited vibrational states of acetylene are produced by 193 nm ArF excimer laser photolysis of vinyl bromide in a room-temperature helium buffer gas. Vibrational state populations are probed by direct absorption with the continuous single-frequency output of a color center laser operating in the 3 μm region. The vibrational transitions observed were the ν2+ν3−ν2 and the 2ν2+ν3−2ν2 combination bands. Rate constants for He vibrational relaxation are 1.5(3)×10−14 and 2.1(7)×10−14 cm3 molecule−1 s−1 for ν2 and 2ν2, respectively.
Forward and reverse rate coefficients in equilibrating isomerization reactions
Authors: Green Nicholas J. B. , Marchant Philip J. , Perona Michael J. , Pilling Michael J. , Robertson Struan H.
Published: July 2002

Three models for the relaxation kinetics of a reversible unimolecular isomerization reaction are formulated and analyzed: a generalization of the simple Lindemann–Hinshelwood scheme, a detailed model with the strong collision approximation, and a master equation solution. For such systems the use of a classical relaxation analysis has been questioned. In each case it is found that the relaxation analysis does not give forward and reverse rate constants appropriate to the pure irreversible reactions, but that the rate constants so obtained can be interpreted in terms of irreversible schemes which allow for back reaction before collisional stabilization. The accuracy of this decomposition is linked with the applicability of the steady-state approximation for the populations of the reactive states, as is demonstrated analytically under the strong collision approximation, and numerically with the full master equation. An alternative approach using perturbation theory is shown to be unacceptably inaccurate.
Periodic motion around the crossing point in a two-level system
Authors: Lopez-Castillo J.-M. , Filali-Mouhim A. , Jay-Gerin J.-P.
Published: July 2002

We study a two-level system described by a Hamiltonian which exhibits a time-periodic variation of its diagonal matrix elements in the small coupling regime. We investigate the transition probability, the Bloch–Siegert shift, and the Rabi frequency of the multiquantum transition as functions of the amplitude and frequency of the oscillations. These quantities all exhibit a very distinct behavior depending upon whether or not the amplitude of the oscillations allows for the crossing of the levels. This behavior is monotonic for small amplitudes while it presents oscillations as a function of the frequency for larger amplitudes. These oscillations are unambiguously attributed to the Landau–Zener–Stueckelberg interference effects between the crossing point and the turning point. A discussion of the relevance of these oscillations to possible experimental observations, notably in electron-transfer reactions, is presented.
Natural bond orbitals in multiconfigurational expansions: Local treatment of electron correlation in molecules
Authors: Nemukhin A. V. , Weinhold F.
Published: July 2002

We describe a localized treatment of electron correlation in terms of complete active-space wave functions derived from natural bond orbitals, closely related to the localized transferable units of molecular structure. We find that in many cases the procedure leads to solutions which retain the essentially localized character of their ‘‘parent’’ orbitals. Such localized solutions lead to an extremely selective treatment of electron correlation in molecules, allowing one to readily extend the concepts of electron correlation in diatomic molecules (‘‘left-right,’’ ‘‘in-out,’’ ‘‘angular,’’ etc.) to the general polyatomic case using a familiar chemical language. We illustrate the procedure with applications to various diatomic (Li2, CO), triatomic (H2O, symmetric and unsymmetric dissociation), and polyatomic (formaldehyde ground and excited states; malonaldehyde) species, in order to demonstrate the important advantages of this procedure with respect to numerical behavior (improved convergence, reduced dimensionality, easy implementation with standard program packages) and conceptual utility (selectivity, transferability, description in a language common to all chemists).
Guiding-function optimization in biased-selection Monte Carlo calculations
Authors: Alexander S. A. , Coldwell R. L. , Morgan John D.
Published: July 2002

We show that the guiding function, which is used to generate the Monte Carlo integration points in a biased-selection calculation, can be optimized so as to minimize the statistical error of an expectation value. For excited states we introduce an orthogonalizing part into the minimization functional to prevent ground and excited states from mixing.
Large-order dimensional perturbation theory for two-electron atoms
Authors: Goodson D. Z. , López-Cabrera M. , Herschbach D. R. , Morgan John D.
Published: July 2002

An asymptotic expansion for the electronic energy of two-electron atoms is developed in powers of δ=1/D, the reciprocal of the Cartesian dimensionality of space. The expansion coefficients are calculated to high order (∼20 to 30) by an efficient recursive procedure. Analysis of the coefficients elucidates the singularity structure in the D→∞ limit, which exhibits aspects of both an essential singularity and a square-root branch point. Padé–Borel summation incorporating results of the singularity analysis yields highly accurate energies; the quality improves substantially with increase in either D or the nuclear charge Z. For He, we obtain 9 significant figures for the ground state and 11 for the 2p2 3Pe doubly excited state, which is isomorphic with the ground state at D=5 by virtue of interdimensional degeneracy. The maximum accuracy obtainable appears to be limited only by accumulation of roundoff error in the expansion coefficients. The method invites application to systems with many electrons or subject to external fields.
Polarization dependence of the ac Stark effect in multiphoton transitions of diatomic molecules
Authors: Girard Bertrand , Sitz Greg O. , Zare Richard N. , Billy Nicolas , Vigué Jacques
Published: July 2002

The (2+2) resonance-enhanced multiphoton ionization (REMPI) of N2 via the a 1Πg(v=1,J) levels shows a strong dependence on the polarization of the laser beam causing this process. This behavior is attributed to the ac Stark effect produced by the near resonance of the N2  o31Πu(v=0,J) levels with the sum of the first three photons. The multiphoton transitions are broadened and asymmetric in appearance; one level is even split. The line profiles change markedly as the polarization of the laser beam is varied from linear to circular. A general theory is presented for the ac Stark effect in a diatomic molecule undergoing a multiphoton transition. When the sum of the photon energies is resonant with an allowed transition, a splitting of the line is observed (Autler–Townes effect). Off resonance, the magnetic sublevels are shifted by different amounts, causing the line profile to be broadened and distorted. This theoretical treatment is able to explain in a satisfactory manner the observed behavior of (2+2) REMPI of N2 via the a 1Πg–X 1Σ+g transition and the two-photon laser-induced fluorescence of CO via the A 1Π–X 1Σ+ transition.
Infinite order-sudden cross sections for excitation of overall and internal rotation in CH3OH–He collisions
Authors: Davis Stephen L.
Published: July 2002

Inelastic scattering cross sections are reported within the infinite-order sudden (IOS) approximation for excitation of overall and internal rotation in CH3OH by collisions with He at an average energy of 500 cm−1, using a self-consistent field (SCF) plus damped dispersion interaction potential. The sudden approximation was used for the overall as well as the internal rotation, which was described using the zero-order internal axis method (IAM). The torsionally elastic cross sections are dominated by ΔK=0 transitions, followed by ΔK=±3. However, torsional excitation is significant only for K-changing collisions, particularly ΔK=±3. These latter cross sections approach zero as ρ (the ratio of the internal rotor moment of inertia to that of the entire molecule about its symmetry axis), approaches unity. Cross sections were also calculated for the high-barrier and zero-barrier limits to internal rotation. The high-barrier limit values are close to those for the actual barrier of 373 cm−1. The zero-barrier cross sections show strong propensities toward particular K→K′, which are quite sensitive to the value of ρ. These propensities are weaker in the actual barrier cross sections. The results also show a propensity for A±↔A± over A±↔A∓ in ΔK=0 transitions.
Understanding the origin of rotational distributions in triatomic photodissociation: A k–j wave packet study of ICN
Authors: Qian Jiwen , Williams Carl J. , Tannor David J.
Published: July 2002

In this paper, we analyze in detail the results of a quantum mechanical, time-dependent wave packet calculation of ICN photodissociation in the A continuum. Using the model potentials of Goldfield et al., the CN(X 2∑+) rotational final product distributions are calculated for all excitation frequencies. In a previous paper [J. Chem. Phys. 95, 1721 (1991)], we presented the details of the methodology of the calculation. The results are in good agreement with the time-independent close-coupling calculation by Guo and Schatz [J. Chem. Phys. 92, 2397 (1990)]. In the present paper, the connection between the final product state distribution and the wave packet reaction dynamics is discussed. In particular, the bimodal rotational distribution in the I(2P3/2) final products channel, with its superimposed oscillatory structure, is analyzed in detail. By focusing on the time evolution of the wave packet in k–j space, i.e., the conjugate space in the coordinate of I–CN relative translation and CN rotational motion, respectively, it is possible to determine completely the origin of these features. The bimodal distribution in the I(2P3/2) final products’ channel arises from the abrupt angle dependence of the coupling of the two surfaces. The high j distribution is enhanced relative to the low j distribution by selective recrossing of the low j amplitude to the I(2P1/2) surface at later times. The superimposed oscillations are the signature of a rotational rainbow which arises from the angular well of the bent potential surface. They are associated unambiguously with the high j portion of the distribution and are not due to the interference of the two modes of the ‘‘doorway state.’’ These observations allow for some speculation on the dynamical origin of the experimental distributions.
Muonium reaction kinetics with the hydrogen halide gases
Authors: Gonzalez Alicia C. , Tempelmann Alexandra , Arseneau Donald J. , Fleming Donald G. , Senba Masayoshi , Kempton James R. , Pan James J.
Published: July 2002

The reaction rates of the muonium (Mu) atom with HBr and HI in ∼1 atm N2 moderator have been measured over the temperature range 160–490 K using the μSR technique. While both abstraction and exchange reactions are possible, only the abstraction reaction should be observable, being moderately exothermic. Comparisons with the corresponding H(D) reactions reveal small kinetic isotope effects in both reactions, which do not vary strongly with temperature (kMu/kH≊3.5 near 300 K), consistent with the (classical) ratio of mean velocities. Surprisingly, quantum tunneling, normally facile for similarly exothermic reactions of the ultralight Mu atom (mMu/mH≊1/9), appears to be of little importance here. This despite the fact that the (temperature-independent) experimental activation energies are much less than the expected vibrationally adiabatic barrier heights (estimated to be ≊1.5 kcal mol−1) and, particularly in the case of Mu+HI, much less than the corresponding H-atom activation energy: 0.13±0.03 vs 0.70±0.3 kcal mol−1. In the case of reactions with HBr, the experimental Mu- and H-atom activation energies are much more similar: 0.51±0.03 and 0.74±0.12 kcal mol−1, respectively, over comparable temperature ranges. These data pose a conundrum in which several compensating effects related to the much lighter Mu-atom mass seem to be involved. Theoretical calculations are urgently required.

In our view the topography of the potential-energy surface(s) for H2X is poorly known, particularly in the region of the barrier. It may be that the abstraction barriers for both Mu+HI and Mu+HBr are considerably later and even smaller than current calculations indicate, resulting in a cancellation of the effects of zero-point-energy shifts and quantum tunneling at the transition state. Differences in skewing angles between Mu and H+HX could favor a shorter tunneling path for the H-atom reaction, possibly compensating for its heavier mass. Steric or rebound effects from ‘‘bottlenecks’’ on the (mass-weighted) potential surfaces for Mu reactivity may also play some role. An upper limit for the 300 K reaction rate of Mu+HCl is given as well. In contrast to both HBr and HI, this reaction is quite endothermic and hence exhibits an inverse kinetic isotope effect (kMu≪kH).
Formally exact path integral Monte Carlo calculations using approximate projection operators
Authors: Hall Randall W.
Published: July 2002

A class of approximate projection operators is used to reduce the variance in path integral Monte Carlo calculations in a formally exact manner. Paths are classified according to the projection operators, allowing the identification of paths whose contribution to the variance is negligible. The approach is applied to two canonical systems: Two like-spin electrons in a three dimensional harmonic well and the two dimensional Hubbard model, representing localized and extended electronic states, respectively. Time savings of 15–900 over straightforward Monte Carlo calculations are observed.
The study of flux redistribution during molecular photodissociation: Adiabatic and diabatic analyses and application to the dissociation of CH3I
Authors: Alexander Millard H. , Rist Claire , Manolopoulos David E.
Published: July 2002

This paper extends our new method for the study of the mechanism of molecular photodissociation. This method involves the time-independent study of the growth of photofragment flux, governed by photon absorption, and the subsequent redistribution of the flux, governed by the Hamiltonian in the excited, unbound state. The flux analysis can be carried out easily in either a diabatic (asymptotic), locally adiabatic, or fully adiabatic basis. The redistribution of the photofragment flux can be investigated in either internal state space or coordinate space at each excitation energy. Application is made to one- and two-dimensional models for the photodissociation of CH3I.
State-resolved electronic quenching of NH(a 1Δ) by Xe and CO
Authors: Patel-Misra Dipti , Dagdigian Paul J.
Published: July 2002

A crossed beam study of the quenching of NH(a 1Δ) molecules by the collision partners Xe and CO is presented. Relative cross sections for the formation of the various rovibrational/fine-structure states in the NH(X 3∑−) products are reported. Vibrational levels v=0–4 were detected, and the vibrational state distribution was found to be monotonically decreasing with increasing v. Despite the large energy gap, the rotational state distribution was found to be quite cold. Population in all three fine-structure levels of a given rotational angular momentum N was found, and the distributions among the fine-structure levels were nearly statistical. The observed fine-structure state distribution differs significantly from that observed for the spin-forbidden decomposition of HN3, for which no significant population in the F2 levels was observed. These experimental results are interpreted with a heuristic model in which the collision-induced spin–orbit coupling which enables the mixing of the initial singlet and final triplet states is derived by consideration of the polarization of the NH electronic wave functions induced by the approach of the collision partner.