bluuebunny/crossref_metadata_2025 huggingface dataset:
Intensity analysis of the torsional spectrum of CH3CH3
Authors: Moazzen-Ahmadi N. , McKellar A. R. W. , Johns J. W. C. , Ozier I.
Published: July 2002

The far-infrared spectrum of gaseous CH3CH3 has been measured in the region of the torsional fundamental under relatively large pressure-path length conditions. Observations were made at resolutions of 0.08, 0.02, and 0.016 cm−1 and under various experimental conditions using two different Fourier transform spectrometers. In addition, a very high resolution (0.0014 cm−1) spectrum of the lowest lying degenerate fundamental ν9 was obtained. From a frequency analysis of the far-infrared spectrum and 737 lower state combination differences in ν9 and its associated hot band ν9+ν4−ν4, an improved set of torsion–rotation parameters was obtained. An intensity analysis of the torsional bands was carried out to obtain torsional dipole moments. The torsional dipole components μ⊥ and (μ∥−μ⊥ ) were determined to be 9.37(70) and −31.7(2.0) μD, respectively. These experimental values are in good agreement with calculated values of μ⊥=10.13 μD and (μ∥−μ⊥ )=−35.10 μD. The shape of the torsional bands can be reproduced in detail with the model adopted here.
Spectral diffusion in liquids
Authors: Stein Alan D. , Fayer M. D.
Published: July 2002

Spectral diffusion of an electronic transition of solute chromophores in liquid solutions is investigated experimentally and theoretically through its influence on electronic excited-state transfer (EET). Observation of dispersive EET in liquids (the EET rate depends on the excitation wavelength) demonstrates that absorption lines are inhomogeneously broadened on a nanosecond time scale in the systems studied although the time scale for homogeneous dephasing is tens of femtoseconds. A theory is developed that relates the rate of spectral diffusion to the wavelength dependence and temperature dependence of EET. Time-resolved fluorescence depolarization measurements are used to measure EET in the systems rhodamine B (RB) in glycerol and propylene glycol as a function of wavelength and temperature from room temperature (298 K) to 200 K. Comparison with theory permits the rates of the solvent fluctuations responsible for spectral diffusion to be determined for the two solvents at several temperatures. Measurements are also made of the rates of solvent relaxation about the excited RB and of RB orientational relaxation. The results demonstrate that the mechanism for spectral diffusion is solvent orientational relaxation which causes the initial (time of optical excitation) dipolar field, produced by the solvent at the chromophore, to randomize.
Microwave spectrum of benzene⋅SO2: Barrier to internal rotation, structure, and dipole moment
Authors: Taleb-Bendiab Amine , Hillig Kurt W. , Kuczkowski Robert L.
Published: July 2002

The microwave spectrum of the benzene⋅SO2 complex was observed with a pulsed beam Fourier-transform microwave spectrometer. The spectrum was characteristic of an asymmetric-top with a- and c-dipole selection rules. In addition to the rigid-rotor spectrum, many other transitions were observed. The existence of a rich spectrum arose from torsional–rotation interactions from nearly free internal rotation of benzene about its C6 axis. Transitions from torsional states up to m=±5 were observed. The principal-axis method (PAM) internal rotation Hamiltonian with centrifugal distortion was used to assign the spectrum. Assuming six-fold symmetry for the internal rotation potential, the barrier height was determined as V6=0.277(2) cm−1. The spectrum of C6D6⋅SO2 was also assigned. Analysis of the moments of inertia indicated that the complex has a stacked structure. The distance Rcm separating the centers of mass of benzene and SO2, as well as the tilt angles of the benzene and SO2 planes relative to Rcm were determined. The values obtained were Rcm=3.485(1) Å, θC6H6=±12(1)° and θSO2=44(6)°. While SO2 is certainly tilted with the sulfur end towards benzene, the sign of the benzene tilt angle could not be unambiguously determined. The dipole moment of C6H6⋅SO2 was determined as μa=1.691(2) D, μc=1.179(2) D, and μT=2.061(2) D.
Quantum simulation of the electronic structure of diatomic molecules
Authors: Subramaniam Ravi P. , Lee Michael A. , Schmidt Kevin E. , Moskowitz Jules W.
Published: July 2002

The domain Green’s function Monte Carlo (GFMC) method with the fixed-node approximation is used to calculate the binding energies and correlation energies of most of the first row diatomic molecules. Our results show that the many-electron correlations are significant and that GFMC corrections to Hartree–Fock wave functions produce 80% to 90% of the correlation energy. Our GFMC calculations have an accuracy comparable to or better than standard methods in quantum chemistry.
Spectroscopic characterization of the X 2Π1/2, B 2Σ1/2, and H 2Σ1/2 states of AlKr
Authors: Fu Zhenwen , Massick Steven , Kaup John G. , Benoist d’Azy Odile , Breckenridge W. H.
Published: July 2002

The AlKr van der Waals complex was characterized by laser-induced fluorescence spectroscopy in a supersonic free jet. Rotationally resolved spectra were recorded for eight bands of the AlKr(B 2Σ+←X 2Π1/2) transition and for seven bands of the AlKr(H 2Σ+←X 2Π1/2) transition. Rydberg–Klein–Rees potential-energy curves were determined for the B 2Σ+ and H 2Σ+ states, using vibrational and rotational spectroscopic constants derived from the spectra. The interaction between the X 2Π1/2 ground state and the low-lying repulsive A 2Σ+ state was characterized by analysis of the Λ doubling in the ground state. Equilibrium bond lengths were obtained for the X, B, and H states, and dissociation energies for the B and H states. The fact that the more strongly bound H 2Σ+ state has a larger re value than the B 2Σ+ state was rationalized by postulating an avoided potential curve crossing between the H 2Σ+ state and a repulsive valence 2Σ+ state correlating with Al(3s23d)+Kr.
Resonant four-wave mixing spectroscopy of transient molecules in free jets
Authors: Butenhoff Thomas J. , Rohlfing Eric A.
Published: July 2002

We apply two different resonant four-wave mixing techniques to transient molecules that are produced by laser vaporization into a supersonic expansion. Degenerate four-wave mixing (DFWM) excitation spectra are obtained for C3 and SiC2 and two-color laser-induced grating spectroscopy (LIGS) is used to produce background-free stimulated-emission-pumping spectra of SiC2.
Ab initio pseudopotential study of Yb and YbO
Authors: Dolg Michael , Stoll Hermann , Flad Heinz-Jürgen , Preuss Heinzwerner
Published: July 2002

Large-scale self-consistent-field configuration interaction calculations with single and double excitations, multireference configuration interaction calculations, and coupled pair functional calculations using nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials for Yb, together with extended basis sets, have been performed for thirteen low-lying electronic states of YbO in the ΛS coupling scheme. Spin–orbit coupling was treated by means of energy-adjusted ab initio spin–orbit operators in connection with quasirelativistic double group configuration interaction calculations and configuration interaction calculations with iterative perturbative selection of the zeroth-order wave function for the twenty-five corresponding Ω states in the intermediate coupling scheme. Controversial assignments for the YbO ground-state electronic configuration derived from experimental facts interpreted within a ligand field model or previously published quasirelativistic pseudopotential calculations are discussed. Despite excellent results for YbH and YbF, the molecular constants of the lowest experimentally observed state of YbO (Re = 1.807 Å, D0 = 4.29 eV, ωe = 699 cm−1) recently assigned to be a 4f14σ 2σ 2π4 1Σ+ (Ω = 0+) ground state by McDonald et al. [J. Chem. Phys. 93, 7676 (1990)], show only limited agreement with the results obtained in the present work for this state (Re= 1.895 Å, D0 = 2.47 eV, ωe = 653 cm−1). In our most extensive calculations the lowest state arising from the 4f13σ 2σ 2π4σ 1 configuration (Ω = 0−) is still found to be 0.93 eV lower in energy than the 4f14σ 2σ 2π4 1Σ+ (Ω = 0+) state. The importance of differential relativistic and correlation effects in the energy separation between states arising from configurations with differing 4f occupation is demonstrated and discussed.
Nonstatistical inversion dynamics of T-shaped Ar3
Authors: Dumont Randall S. , Jain Shashi
Published: July 2002

Numerical computations reveal nonstatistical characteristics of microcanonical T-shaped Ar3 inversion at energies associated with strongly chaotic dynamics. Nonstatisticality is most pronounced at higher energies where internal relaxation time scales are comparable to the inversion time. At such energies, population decay curves exhibit damped oscillations about the equilibrium population. At energies just above the inversion threshold, where inversion is very slow, near statistical nonoscillatory behavior is observed. The ‘‘absorbing barrier method’’ of Straub and Berne [J. Chem. Phys. 83, 1138 (1985)] is shown to provide a reasonable model for observed population decays. Characteristics of corresponding gap distributions are described in terms of an adapted ‘‘delayed lifetime gap model.’’ Analysis of the model which combines the absorbing barrier method and the adapted delayed lifetime gap model provides insight into the observation of both oscillatory and nonoscillatory population decays. Specifically, the analysis describes the observations in terms of an ‘‘underdamped’’ or ‘‘overdamped’’ harmonic oscillator, respectively.
Relativistic effects in physics and chemistry of element 105. II. Electronic structure and properties of group 5 elements bromides
Authors: Pershina V. , Sepp W.-D. , Fricke B. , Kolb D. , Schädel M. , Ionova G. V.
Published: July 2002

Relativistic self-consistent charge Dirac–Slater discrete variational method calculations have been done for the series of molecules MBr5, where M=Nb, Ta, Pa, and element 105, Ha. The electronic structure data show that the trends within the group 5 pentabromides resemble those for the corresponding pentaclorides with the latter being more ionic. Estimation of the volatility of group 5 bromides has been done on the basis of the molecular orbital calculations. According to the results of the theoretical interpretation HaBr5 seems to be more volatile than NbBr5 and TaBr5.
Temperature dependence of nuclear shielding and quadrupolar coupling of noble gases in liquid crystals
Authors: Lounila Juhani , Muenster Oliver , Jokisaari Jukka , Diehl Peter
Published: July 2002

A theoretical model for nuclear magnetic resonance (NMR) observables of noble gas atoms in nematic and smectic A liquid crystal environments is developed. It is used to account for the behavior of the nuclear shielding of 129Xe (σ) and the quadrupolar coupling of 21Ne (B) measured in 1-butyl-c-4-(4′-octylbiphenyl-4-yl)-r-1-cyclo-hexan-carbonitrile (NCB 84). The analysis provides detailed information on the orientational and positional order of the liquid crystal solvent and of the solute atoms. In the nematic phase, the decrease of σ with decreasing temperature is attributed to the increase of the density and orientational order parameter S of the solvent. The former factor determines the variation of the isotropic part of σ, while the latter governs the change in the anisotropic part (the anisotropy of the shielding tensor Δσ is of the order of −10 ppm). The prominent increase of σ in the smectic A phase is ascribed to the tendency of the atoms to occupy the interspaces between the smectic layers rather than their interiors (the deviation from the uniform distribution of the atoms is of the order of 5%). The main result is that the isotropic part of σ increases as the density of the solvent in the immediate neighborhood of the solute decreases. In the case of the quadrupolar coupling, the effect of the redistribution of the atoms is small, as there is no isotropic contribution to B. The behavior of the quadrupolar coupling of 21Ne in the nematic phase indicates that there are two distinct contributions to B. One is due to the distortion of the electron cloud from the spherical symmetry and the other arises directly from the external electric field gradient produced by the neighboring molecules. The temperature dependence of the latter contribution is not determined by the variation of S alone.
Effect of temperature on the attachment of slow (≤1 eV) electrons to CH3Br
Authors: Datskos P. G. , Christophorou L. G. , Carter J. G.
Published: July 2002

The electron attachment rate constant ka(〈ε〉, T) for CH3Br has been measured in a buffer gas of N2 using a high pressure electron swarm apparatus, within the mean electron energy range from thermal (∼0.046 eV) to 0.87 eV and over the temperature range, T, 300–700 K. At room temperature, CH3Br attaches low energy electrons weakly but as T is raised from 300 to 700 K the total electron attachment cross section increases by more than 2 orders of magnitude. At T=300 K the electron attachment cross section exhibits a peak at 0.38 eV which shifts progressively to lower electron energies as T is increased. The thermal value of ka is 1.08×10−11 cm3 s−1 at 300 K and 3.28×10−9 cm3 s−1 at 700 K.
Photodissociation of CH2. I. Potential energy surfaces of the dissociation into CH and H
Authors: Beärda Robert A. , van Hemert Marc C. , van Dishoeck Ewine F.
Published: July 2002

The photodissociation processes of CH2 into CH and H have been studied using ab initio multireference configuration-interaction methods. Two-dimensional potential energy surfaces of the ten lowest triplet states correlating with the seven lowest states of CH have been calculated as functions of bond angle and one C–H bond distance, keeping the other C–H distance fixed at the equilibrium CH2 value. Transition dipole moments connecting the excited states with the ground state have been obtained as well. It is shown that efficient photodissociation of CH2 into CH (X 2Π)+H can occur by absorption from the ground X̃ 3B1 (1 3A″) state into the 1 3A1 (1 3A′) state at about 6.3 eV. Photodissociation into excited CH (a 4Σ−)+H can take place through the 1 3A2 (2 3A″) and 2 3B1 (3 3A″) states, although in a more complex manner since several avoided crossings occur along the reaction path. The 1 3A2 state is a so-called low-angle state, which has an equilibrium bond angle of less than 60° and correlates directly with C(3P)+H2. At 180°, when the molecule has D∞h or C∞v symmetry, interesting crossings between the ground and low-lying surfaces are found. Altogether, these crossings and correlations are predicted to lead to complicated dissociation dynamics for most of the states. The higher-lying states of CH2 can photodissociate either directly into excited states of CH, or they can be predissociated by the repulsive 1 5A2 (1 5A″) state, which correlates with CH (a 4Σ−)+H.
Complex generalized minimal residual algorithm for iterative solution of quantum-mechanical reactive scattering equations
Authors: Chatfield David C. , Reeves Melissa S. , Truhlar Donald G. , Duneczky Csilla , Schwenke David W.
Published: July 2002

A complex GMRes (generalized minimum residual) algorithm is presented and used to solve dense systems of linear equations arising in variational basis-set approaches to quantum-mechanical reactive scattering. The examples presented correspond to physical solutions of the Schrödinger equation for the reactions O+HD→OH+D, D+H2→HD+H, and H+H2→H2+H. It is shown that the computational effort for solution with GMRes depends upon both the dimension of the linear system and the total energy of the reaction. In several cases with dimensions in the range 1110–5632, GMRes outperforms the LAPACK direct solver, with speedups for the linear equation solution as large as a factor of 23. In other cases, the iterative algorithm does not converge within a reasonable time. These convergence differences can be correlated with ‘‘indices of diagonal dominance,’’ which we define in detail and which are relatively easy to compute. Furthermore, we find that for a given energy, the computational effort for GMRes can vary with dimension as favorably as M1.7, where M is the dimension of the linear system, whereas the computer time for a direct method is approximately proportional to the cube of the dimension of the linear system.
Velocity relaxation of S(1D) by rare gases measured by Doppler spectroscopy
Authors: Nan G. , Houston P. L.
Published: July 2002

Velocity relaxation of S(1D) by He, Ar, and Xe has been monitored by measuring the Doppler profile of the S(1D) for variable collision partner pressures at a fixed time delay following creation of S(1D) by pulsed laser photolysis of OCS at 222 nm. The nascent S(1D) has a mean speed about three times that at room temperature and an effective anisotropy parameter of β=0.5. A calculation assuming elastic hard-sphere collisions is performed to model the process. The data are in qualitative agreement with the model in that the angular distribution relaxes more rapidly with collision number as the mass ratio between the collision partner and sulfur approaches infinity, whereas the speed distribution relaxes more rapidly as the mass ratio approaches one. Helium behaves as predicted by the hard-sphere model with a collision cross section of σcol=26±2 Å2. However, the cross sections for argon and xenon are found to depend on the collision energy. The dependence allows an estimation of the following Lennard-Jones parameters: for argon σ=3.6±0.5 Å and ε=2.5±0.5 kJ/mol; for xenon σ=3.9±0.5 Å and ε=3.9±0.8 kJ/mol.
Thermodynamical, structural, and clustering properties of a microemulsion model
Authors: Skaf Munir S. , Stell George
Published: July 2002

A lattice version of the microemulsion model introduced by A. Ciach, J. Ho/ye, and G. Stell [J. Phys. A 21, L111 (1989)] (CHS) is studied within a mean-field approximation. In the absence of (orientational) surfactant–surfactant interactions, an exact integration of the amphiphiles’ orientational degrees of freedom in the CHS model yields an effective spin-one Hamiltonian with multibody, temperature-dependent interactions between particles, closely resembling the model introduced by M. Schick and W. H. Shih [Phys. Rev. Lett. 59, 1205 (1987)] and subsequently studied by Gompper and Schick. The phase diagram for the CHS effective Hamiltonian on a two-dimensional lattice is calculated at a mean-field level. Comparisons with selected results from Schick’s model are then discussed. The calculated structure functions are in qualitative agreement with experimental results, showing a structural evolution from water-in-oil, to bicontinuous, to oil-in-water microemulsions as the water-to-oil concentration ratio is varied. The symmetric (ρW=ρO) subspace of the disordered phase of both models is then investigated using a percolation theory previously introduced by the authors. In both models the bicontinuous microemulsion phase is identified as a region of the phase diagram where the three molecular species are simultaneously percolating. Finally, the percolation threshold lines are investigated, for both models, as functions of their energy couplings. We find, again, similar behavior for the CHS effective Hamiltonian and Schick Hamiltonian. However, the thresholds are found to be more sensitive to the amphiphilic strength of the surfactant in the former.
An algebraic variational approach to dissociative adsorption of a diatomic molecule on a smooth metal surface
Authors: Sheng Jia , Zhang John Z. H.
Published: July 2002

A general theoretical treatment employing a time-independent algebraic variational method (S-matrix Kohn variational method) for dissociative adsorption of a diatomic molecule on a smooth metal surface is presented. The time-independent scattering treatment using the S-matrix Kohn variational method naturally enables one to obtain accurate state-to-state reaction probabilities for dissociative adsorption of molecules on surfaces. In this paper, the S-matrix Kohn variational method is adapted to the 3D dissociative adsorption of H2 on Ni(100) surface treated as a flat surface, and the state-to-state transition probabilities are obtained. The dependence of the dissociation probabilities on the initial rovibrational state of H2(vjm) is examined. As a result of flat surface approximation, which conserves the rotation quantum number jz=m, the exchange symmetry of H2 has an important consequence on the vibrations of the adsorbates. Specially if the rotational state of H2 satisfies the condition j+m=odd, the vibrational quantum number of two adsorbed hydrogen atoms must be different. The orientational effect of rotation is such that the in-plane rotation (m=j) is more favorable for molecular dissociation on surface than the out-of-plane rotation (m=0).
A Gaussian-2 ab initio study of CH2SH, CH2S−, CH3S−, CH2SH−, CH3SH−, CH3+, and CH3SH+
Authors: Chiu See-Wing , Li Wai-Kee , Tzeng Wen-Bih , Ng Cheuk-Yiu
Published: July 2002

Using the Gaussian-2 (G2) theoretical procedure, we have examined the molecular structures and total energies for CH2SH, CH2S−, CH3S−, CH2SH−, CH3SH−, CH3+, and CH3SH+. Contrary to the relative stabilities of CH3S+(C3v;3A2) and CH2SH+(Cs;1A′), the methylthio radical CH3S(Cs;2A′) and the methylthio anion CH3S−(C3v;1A1) are predicted to be more stable than the mercaptomethyl radical CH2SH(C1;2A) and the mercaptomethyl anion CH2SH−(Cs;1A′) by 9.2 and 38.0 kcal/mol, respectively. The CH2SH−(Cs;1A′) anion may exist in the cis configuration or the less stable trans structure. Combined with the results of previous G2 calculations, this calculation yields predictions for the adiabatic ionization energies (IE) of CH3 (9.79 eV), CH2SH (7.41 eV), and CH3SH (9.55 eV), which are in accord with the experimental IEs of 9.84 eV for CH3, 7.536±0.003 eV for CH2SH, and 9.440 eV for CH3SH. The G2 values for the adiabatic electron affinities (EA) of CH2S, CH2SH to trans-CH2SH−(Cs;1A′), CH2SH to cis-CH2SH−(Cs;1A′), and CH3S are 0.38, 0.52, 0.61, and 1.86 eV, respectively. The EA(G2)’s of CH2S and CH3S also agree with the respective experimental values of 0.465±0.023, and 1.861±0.004 eV. We find that CH3SH− is unstable with respect to the electron detachment channel CH3SH+e−.
Low-energy electron attachment to molecules studied by pulse-radiolysis microwave-cavity technique combined with microwave heating
Authors: Shimamori Hiroshi , Tatsumi Yoshitsugu , Ogawa Yuji , Sunagawa Takeyoshi
Published: July 2002

A new experimental technique has been developed to study electron-energy dependence of the electron attachment to molecules. Time dependence of electron density is measured by conventional pulse-radiolysis microwave-cavity method, and a microwave heating technique is additionally applied in order to vary the mean electron energy from thermal to several electronvolts. The calibration of the mean electron energy is made by analyzing the time profile of microwave conductivity signals for thermalizing electrons produced by pulsed x rays in gaseous Xe which shows the Ramsauer minimum in the momentum-transfer cross sections in collisions with electrons. Presented are rate constants for electron attachment to SF6, CCl4, CHCl3, CFCl3, CF3I, CF3Br, 1,1,1-C2F3Cl3, and 1,1,1,-C2H3Cl3 measured in the electron-energy range from thermal to about 2 eV. The data are discussed in conjunction with previous data obtained by different experimental methods.
The complex coordinate scattering theory and the Kohn variational method: A general formulation and application to long range potentials
Authors: Peskin Uri , Moiseyev Nimrod
Published: July 2002

The complex coordinate scattering theory for the calculation of T-matrix elements, as was introduced by Engdahl, Moiseyev, and Maniv [J. Chem. Phys. 94, 1636 (1991)] and by Peskin and Moiseyev [J. Chem. Phys. 96, 2347 (1992)], is shown to satisfy the complex version of the Kohn variational principle introduced by Nuttall and Cohen [Phys. Rev. 188, 1542 (1969)]. This theory and the related S-matrix version of the Kohn variational principle, developed by Zhang, Chu, and Miller [J. Chem. Phys. 88, 6233, (1988)] are combined to formulate a generalized variational basis set approach for quantum scattering calculations. In this approach the Kohn variational procedure to optimize the linear parameters in the T matrix is followed by an optimization of the complex nonlinear parameters. This enables the application of the complex coordinate analytical continuation of the T matrix to the calculation of scattering probability amplitudes for long range potentials. Illustrating numerical applications to short and long range potentials are given.
Critical behavior in a binary polymer blend as studied by static and dynamic light scattering
Authors: Meier G. , Momper B. , Fischer E. W.
Published: July 2002

We report static and dynamic light scattering experiments of an almost symmetric polymer mixture made up from poly(dimethylsiloxane) (PDMS), N=260, and poly(ethylmethylsiloxane) (PEMS), N=340, with N being the degree of polymerization, both below the entanglement molecular weights. The mixture exhibits an upper critical solution temperature Tc≂57 °C. The measurements were performed at the critical composition φc,PEMS = 0.465 in a broad temperature range in the one phase region above the spinodal point. The main results for the static case are: the temperature dependence of the static structure factor S(q=0) can be described by a mean field behavior. For T close to Tc, a crossover to an Ising behavior is observed according to a modified Ginzburg criterion. From the angular dependence of S(q), the static correlation length ξ is determined via an Ornstein–Zernike plot. Our experimentally determined values for limT→∞S(0) and limT→∞ ξ, respectively, are in agreement with theoretical predictions. For the dynamic case, the main results are summarized as follows: as expected, the mutual diffusion coefficient D̃, accessible by quasielastic light scattering, shows a critical slowing down for T→Tc.

For qξ≥1, we observe that the q scaling of the Rayleigh linewidth Γ changes from a q2 to a q3 behavior, which is in agreement with mode coupled expressions. This occurs in a relatively broad temperature range, due mainly to the fact that polymer mixtures exhibit a larger ξ0 ∝√N, on the contrary to any other systems known, which allows us therefore to reach the region qξ≥1 even with light scattering easily. From the separation of the measured Rayleigh linewidth into a critical part and a background part, we have estimated the crossover between mode coupled to nonmode coupled dynamics. It is governed by the coil size. The scaling predictions for the critical part and the background part of the linewidth are in agreement with the predictions of the mode coupled theory by Kawasaki and subsequently by Fredrickson. We find that the mode coupled dynamics reaches far into the mean field regime which is not yet understood by theory. Furthermore, we can show that the critical part of the linewidth data is well represented by the Kawasaki shape function including the viscosity correction. Finally, we have estimated a segmental mobility W0∝D̃⋅S(q=0) which can be interpreted being a segmental quantity only down to characteristic lengths ξ(T)≂Rg. For ξ larger than the coil dimensions, W0∝ξ as predicted by mode coupled dynamics.
Study of Renner–Teller, spin–orbit, and Fermi-resonance interactions in X̃ 2Π (v1v20) levels of NCO by stimulated emission pumping spectroscopy
Authors: Wu Ming , Northrup F. J. , Sears Trevor J.
Published: July 2002

We report extensive measurements of rovibronic levels in the 020/100 and 040/120/200 Fermi-resonant polyads of NCO. The levels were accessed by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of bands in the Ã 2Σ+−X̃ 2Π spectrum of the radical. The data were analyzed in terms of an effective Hamiltonian which explicitly included reference to all the possible vibronic levels of 2Π symmetry in the polyad under consideration. The effects of levels outside these were treated by perturbation theory. The model was successful in fitting the data close to the experimental measurement precision and the resulting parameters were interpreted in terms of the harmonic and anharmonic vibrational terms. The effective Renner–Teller coupling parameter in NCO varied with vibrational level; however, parameters describing the Fermi-resonance interaction were found to be constant for all the levels investigated.
The structure of Ar–C2H4 from high resolution infrared spectroscopy and ab initio theory: The twofold barrier to C2H4 internal rotation
Authors: Block P. A. , Pedersen L. G. , Miller R. E.
Published: July 2002

The optothermal detection method has been used to obtain high resolution infrared spectra for both the ν9 and ν11 vibrational bands of Ar-ethylene. Analysis of these spectra reveals that the argon atom lies essentially in the plane of the ethylene, situated next to the carbon–carbon double bond. Although the rigid structure that best fits the measured rotational constants actually places the argon atom 15° out of the ethylene plane, the experimental and theoretical results are more consistent with a planar equilibrium geometry and wide amplitude in-plane and out-of-plane motion of the ethylene sub-unit. In both spectra the transitions appear as doublets, due to the fact that the ethylene sub-unit undergoes internal rotation between equivalent frameworks. Of the possible paths, only internal rotation about the C=C axis is consistent with the data. The barrier to internal rotation is estimated from the tunneling splitting to be 76 cm−1, which is compared with the results of ab initio calculations.
Methyl rotor effects on acetone Rydberg spectra. II. The 1B2(3s←n)←1A1 transition
Authors: Philis John G. , Goodman Lionel
Published: July 2002

Both a2(ν12) and b1(ν17) methyl torsion fundamentals and their overtones are found to be active in two-photon resonance multiphoton ionization jet spectra of the acetone 1B2(3s←n)←1A1 Rydberg transition. The acetone 3s Rydberg state torsional fundamental frequencies determined from fundamental and sequence band measurements are 118 cm−1 (ν′12) and 175 cm−1 (ν′17) compared to ground state values 77.8 and 124.5 cm−1, respectively. Corresponding values in fully deuterated acetone are 83 and 132 cm−1 compared to 53.4 and 96.0 cm−1 in the ground state. Our measured frequencies differ significantly (greatly so for the gearing torsional fundamental ν′17) from published values for both acetone-h6 and -d6. The 3s state 2ν′12, 2ν′17, and ν′12 + ν′17 frequencies are also measured allowing determination of excited state methyl torsion potential constants. The important increase (∼250 cm−1) in the 3s state V3 constant from its ground state value leads to a large increase in the potential barrier height to internal rotation, i.e., ≊1250 cm−1, 50% greater than the ≊800 cm−1 ground state barrier. This result is similar to the large increase in barrier height for eclipsed–eclipsed→staggered–staggered synchronous rotation recently found for the 1A2(3px←n) Rydberg state. There are, however, important differences in potential curvature between the 3s and 3px state internal rotation potential functions.
The core-penetrating Rydberg series of the CaF molecule: At the borderline between valence and Rydberg states
Authors: Harris Nicole A. , Field Robert W.
Published: July 2002

This paper reports the results of a pulsed optical–optical double resonance study of three new low energy Rydberg states and the previously known but not rotationally analyzed F 2Π state [C. A. Fowler, Phys. Rev. 59, 645 (1941)] of the CaF molecule. The v=0 levels of these four electronic states are in the energy region 36 000–39 800 cm−1 (n*≊3-4). Molecular constants (A,p,q) or γ, and B were measured for all of the states. The F′ 2Σ+ state at 36 125 cm−1 (n*=3.177) is particularly important because it is the lowest and previously unobserved member of the ′p′ 2Σ+ Rydberg series. The assignment of the F′ state as the lowest member of the ′p′ 2Σ+ series corrects a mistaken assignment of the C′ 2Σ+ state as the lowest member of the ′p′ 2Σ+ series [J. E. Murphy, J. M. Berg, A. J. Merer, N. A. Harris, and R. W. Field, Phys. Rev. Lett. 65, 1861 (1990)].
Energy landscapes, glass transitions, and chemical reaction dynamics in biomolecular or solvent environment
Authors: Onuchic José Nelson , Wolynes Peter G.
Published: July 2002

We discuss the origin of smooth free energy surfaces in a limiting case model of electron transfer reactions on a rough energy landscape with many minima. Above a certain critical temperature, the linear response picture of Marcus is recovered, but as the glass transition is approached interesting nonlinearities are uncovered. The relation of the distribution of barriers to underlying energy landscape is considered.
Time evolution of correlations in a random sequential adsorption process
Authors: Pedersen F. B. , Hemmer P. C.
Published: July 2002

The pair correlation function C(r,t) for random sequential adsorption of particles with nearest-neighbor exclusion on a one-dimensional lattice evolves in time as C(r,t) = −1/2e−2(1−e−t)∑∞n=0{ [−2(1−e−t)]2n+r+1/(2n+r+1)!}. The distribution of the number of adsorbed particles on a lattice with N sites is shown to have a variance N(1 − e−t)e−4(1−e−t) for large N.
The vibrational spectra of molecular ions isolated in solid neon. X. H2O+, HDO+, and D2O+
Authors: Forney Daniel , Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

When a Ne:H2O≥200 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, new infrared absorptions appear close to the gas-phase band centers of the three vibrational fundamentals of H2O+. Detailed isotopic substitution studies confirm this assignment and provide assignments for all of the vibrational fundamentals of HDO+ and D2O+. When ions are present in the neon matrix, rotation of a significant fraction of the water molecules is inhibited. Electrons produced by the photodetachment of anions, which must be present to maintain overall charge neutrality of the deposit, accelerate nuclear spin equilibration of water in the matrix. As the concentration of H2O+ is decreased by capture of the photodetached electrons, the absorptions assigned to nonrotating water are also reduced in intensity. The nature of the other ionic species which may be present in the sample is considered.
Collisional energy transfer from highly vibrationally excited SF6
Authors: Lendvay György , Schatz George C.
Published: July 2002

Classical trajectory calculations based on the redistributed successive collision method and the single energy collision method were used to model the relaxation of vibrationally hot SF6 molecules in collisions with He, Ar, Xe, CO, and SF6. The average energy transfer is found to have a stronger than linear dependence on initial energy in SF6 for light colliders such as He, while heavier colliders such as Xe show a nonlinear increase at low energies, a linear part at intermediate energies, and a slower than linear part at high energies. The energy transfer is energy independent at high energies if the collider is SF6. The partitioning of energy transferred among vibration, rotation, and translation of the bath molecules is studied, and we find that for a CO bath, vibration is inactive, while for an SF6 bath, energy transfer to vibration accounts for most of the energy transfer. The formation of collision complexes facilitates energy transfer, but even for SF6+SF6, most of the collisions are impulsive. The relationship between these results and experiments for SF6 and other molecules is analyzed in detail, and certain common trends in energy transfer behavior are identified.
Stress-induced failure and melting of ideal solids
Authors: Blumberg Selinger Robin L. , Lynden-Bell R. M. , Gelbart William M.
Published: July 2002

In order to investigate the failure of ideal solids, we perform molecular dynamics simulations of a three-dimensional defect-free crystal under uniaxial applied force/strain, using a semi-empirical many-body potential, under conditions of fixed temperature and pressure. We use a hybrid molecular dynamics/Monte Carlo computation technique that allows the periodic-boundary-condition cell to change both its shape and volume. We investigate the role of loading (or strain) rate as well as of low and high temperature. Under low temperature and slow loading conditions, we find that failure is sometimes preceded by a crystal–crystal phase transition that corresponds to a physically allowed lattice invariant (PALI) strain. At sufficiently high temperatures we observe direct failure of the system via stress-induced melting.
The microwave rotational spectrum of the Ar–CO dimer
Authors: Ogata T. , Jäger W. , Ozier I. , Gerry M. C. L.
Published: July 2002

Pure rotational spectra of four different isotopomers of the dimer Ar–CO have been investigated between 8 and 18 GHz using a pulsed beam cavity Fourier transform spectrometer. The spectra confirm that the complex is a prolate near-symmetric rotor with an essentially T-shaped structure, and that it undergoes large amplitude zero-point motion. It is shown that on the average the argon is closer to the oxygen than to the carbon. The transitions measured obey a-type selection rules with ΔJ=+1, ΔKa=0, and ΔKc=+1. For 40Ar–12C16O, transitions have been observed for Ka=0 and 1 with lower state J values of 1, 2, and 3. For 40Ar–13C16O and 40Ar–13C18O, a similar series was measured, but only for Ka=0. For 40Ar–13C17O, the 17O quadrupole hyperfine pattern was resolved in the rotational transition JKaKc = 202–101. Determinations have been made for rotational and centrifugal distortion constants, as well as for the 17O quadrupole coupling constant χaa. Effective values have been obtained for the length of the line from the center of mass of the CO subunit to the argon nucleus, and for the angle between this line and the CO bond.
Polymer reference interaction site model theory: New molecular closures for phase separating fluids and alloys
Authors: Schweizer Kenneth S. , Yethiraj Arun
Published: July 2002

The polymer reference interaction site model integral equation theory when combined with known atomic-like closure approximations is shown to be qualitatively inconsistent with classical mean field predictions for both long wavelength concentration fluctuations and the molecular weight dependence of the critical temperature of binary polymer blends. The fundamental error is shown to arise from the failure of atomic-like closures to explicitly account for strong correlations between the segments on two interpenetrating polymer coils which are close in space but widely separated in chemical sequence. A family of new ‘‘molecular’’ closures are formulated which explicitly account for chemical-bonding mediated correlations. These new closures are all qualitatively consistent with mean field scaling of the critical temperature with chain length. A detailed analytical derivation of the predictions of the new closures for thread-like symmetric blends is carried out, and the influence of density and concentration fluctuations on the effective chi parameter, small angle neutron scattering profiles, and phase behavior are determined. Qualitative agreement with recent computer simulations is demonstrated. Generalization and/or application of the new molecular closures to treat strongly interacting fluids, soft repulsive force liquids, the liquid-vapor transition, and polymer-solvent demixing are also presented.
Chlorine bonding sites and bonding configurations on Si(100)–(2×1)
Authors: Gao Q. , Cheng C. C. , Chen P. J. , Choyke W. J. , Yates J. T.
Published: July 2002

A combination of experimental methods has been employed for the study of Cl2 adsorption and reaction on Si(100)–(2×1). At 100 K, Cl2 adsorption occurs rapidly to a coverage of ∼0.7 Cl/Si. This is followed by slower adsorption kinetics with further Cl2 exposure. Two Cl adsorption states are observed experimentally. One of the adsorption states is terminally bonded Cl on the inclined dangling bond of the symmetric Si2 dimer sites, with a vibrational frequency, ν(SiCl) of 550∼600 cm−1. These bonded Cl atoms give four off-normal Cl+ ESDIAD emission beams from the orthogonal domains of silicon dimer sites. The Si–Cl bond angle for this adsorption configuration is estimated to be inclined 25°±4° off-normal. The second Cl adsorption state, a minority species, is bridge bonded Cl with ν(Si2Cl) of ∼295 cm−1 which produces Cl+ ion emission along the surface normal direction. Both adsorption states are present at low temperatures. Irreversible conversion from bridge bonded Cl to terminally bonded Cl begins to occur near 300 K; the conversion is complete near ∼673 K. LEED studies indicate that the (2×1) reconstruction for the substrate is preserved for all Cl coverages. The most probable Cl+ kinetic energy in electron stimulated desorption, ESD, is 1.1−+0.30.6 eV. A significant adsorbate-adsorbate quenching effect reducing the Cl+ ion yield in ESD occurs above a Cl(a) coverage of ∼0.5 ML (monolayer) due to interadsorbate interactions. The maximum Cl+ yield is about 4×10−7 Cl+/e at an electron energy of 120 eV. Temperature programmed desorption results show that SiCl2 is the major etching product which desorbs at about 840 K.
Structures in the energy dependence of the rate constant for ketene isomerization
Authors: Lovejoy Edward R. , Moore C. Bradley
Published: July 2002

The isomerization of highly vibrationally excited and rotationally cold ketene has been investigated by monitoring the 12CO and 13CO dissociation products following laser excitation of jet-cooled 12CH2 13CO, 13CH2 12CO, and 12CD2 13CO. The rate constants for the reactions 12CH2 13CO⇄13CH2 12CO and 12CD2 13CO⇄13CD2 12CO are reported as a function of energy with a resolution of 1 cm−1. The rate constants exhibit pronounced peaks as a function of energy near the reaction threshold. This structure is attributed to quasistable motion along the reaction coordinate in the vicinity of the isomerization transition state.
Stationary conditions of the electronic structures against the extension of molecular systems and their application to the elongation method
Authors: Maekawa Koji , Imamura Akira
Published: July 2002

Stationary conditions of the electronic structures against the extension of molecular systems are introduced in the Hartree–Fock approximation and a straightforward method to extract the stationary orbitals, which are molecular orbitals unaltered by the extension of the cluster, is formulated. This method can be combined with the previously proposed elongation method and applied to the cluster-extending calculation of the electronic structure of polymers at the ab initio level. The results obtained for three typical polymer systems, i.e., the molecular crystal of hydrogen fluorides, polyethylene, and three isomers of polyacetylene, make us confirm the flexibility and reliability of the present method. Especially, this method enables us to treat any orbital systems, e.g., inner and valence shells or σ and π orbitals by the same procedure and the same criterion. Moreover, through the constant cluster-extension processes, the periodicity of the electronic structure can be extracted as the stationary orbital with translational symmetry and it is represented clearly in the freezing patterns of the electron density.
Infrared laser kinetic spectroscopy in discharges: The CN radical (v
=0–7)
Authors: Fehér Miklós , Rohrbacher Andreas , Maier John P.
Published: July 2002

Infrared laser kinetic spectroscopy was extended to the study of formation and decomposition processes in a hollow cathode discharge. The processes responsible for the disappearance of the CN radical in different vibrational states (v=0–7) were investigated. From the time evolution of the signal, information on the reaction rates of vibrationally excited CN molecules with C2N2 and on their vibrational relaxation could be obtained. The measured first order reaction rate constants lie between 350–800 s−1 and the rate constants for the deactivation, which is thought to be due to V–V relaxation in collisions with C2N2, between 520–900 s−1 for the v=0–7 states of CN.
Random matrix treatment of intramolecular vibrational redistribution. I. Methodology and anharmonic interactions in 1-butyne
Authors: Perry David S.
Published: July 2002

A random matrix methodology is presented which is capable of modeling sparse through intermediate case intramolecular vibrational redistribution (IVR). A class of random Hamiltonian ensembles, called the Gaussian Poisson ensembles, is defined. These ensembles deviate from the Gaussian orthogonal ensemble (GOE) in a way that allows particular molecular spectra to be modeled, yet they can retain the desirable GOE statistical properties. The principal assumption tested by this work is that the vibrational identity of the bath states in both the calculation and in 1-butyne is sufficiently scrambled that a statistical treatment is justified. Comparison to the experimental eigenstate-resolved infrared spectra of 1-butyne is accomplished by means of four measures of IVR: the dilution factor, the interaction width, the counted level density, and the effective level density. Corrections to each of the four measures for limited experimental signal-to-noise are presented. A fit to the dilution factor and interaction width yielded the root-mean-square matrix elements for anharmonic coupling of the bright state to the bath. The values obtained, 0.010 and 0.014 cm−1, respectively, for the ν1 and ν16 bands of 1-butyne, are in close agreement with those obtained by direct deconvolution of the spectra.
Jet-cooled fluorescence excitation spectra, conformation, and carbonyl wagging potential energy function of cyclopentanone and its deuterated isotopomers in the S1 (n,π*) electronic excited states
Authors: Zhang Jian , Chiang Whe-Yi , Laane Jaan
Published: July 2002

The jet-cooled fluorescence excitation spectra of cyclopentanone and its 2,2,5,5-d4 isotopomer have been recorded in the 305–335 nm region. In addition, the spectra of d1, d2, and d3 species were obtained from isotopic mixtures. The electronic band origin of the d0 molecule for the S1 (n,π*) state of A2 symmetry occurs at 30 276 cm−1, while that of the d4 molecule is at 30 265 cm−1. More than 100 fluorescence bands were assigned for each species. These arise from combinations of ν3 (C=O stretch), ν11 (ring-angle bending), ν18 (ring twisting), ν25 (C=O out-of-plane wag), ν26 (ring bending), and ν36 (C=O in-plane wag) and their vibrational excited states. The vibrational frequencies for ν3, ν11, and ν36 are significantly lower in the S1 state than the S0 ground state. However, the out-of-plane ring modes ν18 and ν26 are only slightly shifted. A progression observed for ν26 does indicate that in the S1 state, the bent ring conformation lies about 500 cm−1 above the ring-twisting minimum and corresponds to a saddle point in the two-dimensional ring-twisting/ring-bending potential energy surface. Band progressions for ν18 can be used to calculate the ring-twisting barriers (the barriers to planarity) for the d0 and d4 isotopomers to be 1433 and 1240 cm−1, respectively. Because of limited data, however, these values may be as much as several hundred cm−1 too high. The energies for the C=O out-of-plane wagging states up to v25=9 for each isotopomer were determined for the S1 state and these were used to calculate the C=O wagging potential energy functions for each. In the S1 state, the barrier to inversion of the C=O group is 672±10 cm−1 and the wagging angle is 22°±1°.