bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of H2 elimination from cyclohexadiene
Authors: Cromwell E. F. , Liu D.-J. , Vrakking M. J. J. , Kung A. H. , Lee Y. T.
Published: July 2002

A comprehensive study of the dynamics of H2 elimination from 1,4 and 1,3 cyclohexadiene is reported. Rotational and vibrational quantum state distributions as well as translation energy distributions for the H2 product are measured. State specific detection of H2 is accomplished with a transform limited vacuum ultraviolet–extreme ultraviolet laser system via (1+1) resonance enhanced multiphoton ionization (REMPI). Rate constants for the H2 elimination and 1,4 to 1,3 isomerization reactions are derived. A (v,J) correlation for H2 with v∥J primarily is observed from anisotropy in the Doppler profiles. A clear picture of the transition state configuration of 1,4 cyclohexadiene is provided from the information obtained.
N,N′-dicyanoquinonediimines as a molecular constituent of organic conductors: Vibrational behavior and electron–molecular vibration coupling
Authors: Lunardi G. , Pecile C.
Published: July 2002

The results of a vibrational analysis of neutral N,N′-dicyanoquinonediimine (DCNQI) and of 2,5-X,Y-DCNQI (X■Y■Cl; X■Cl, Y■CH3; X■Y■CH3 or CD3) are presented. A sound assignment of the in-plane fundamental vibrational modes is discussed also in terms of a normal coordinate analysis (NCA) based on a modified valence force field. For light and deuterated 2,5-dimethyl-dicyanoquinonediimine (DMDCNQI) the vibrational analysis is extended to the corresponding radical anion. The comparison of the vibrational assignment of neutral and fully ionized species, together with the results of the NCA, leads to the identification of a strong Duschinsky effect, as well as of the frequency shifts relative to the ionization process. The buν47 fundamental mode (quinoid C■N stretch) is recognized as diagnostic of the degree of charge transfer (CT) for DMDCNQI component units of CT systems. The relative values of the linear electron–molecular vibration (e–mv) coupling constants for the neutral and ionized DMDCNQI molecular structure, evaluated by a spectroscopic version of the complete neglect of differential overlap (CNDO/S) calculation associated with the eigenvectors coming from the NCA, are also reported.

The infrared vibronic features of the Ba(DMDCNQI)2 salt are sorted out by exploiting their intensity evolution with the temperature and are interpreted by applying Rice’s model for a dimerized linear chain together with the calculated e–mv coupling constants. Furthermore, the visible absorption spectrum of the DMDCNQI radical anion is analyzed and its vibronic structure is attributed to a coupling with the three vibrational modes agν′5, 1585; ν8, 1290; and ν′18, 395 cm−1. The dependence of the vibrational frequencies on ionicity and the vibronic features of the optical spectra are discussed with a view to their application in the investigation of the physical properties and the temperature and/or pressure-dependent phase transitions of conductors based on the DCNQI structure, particularly in the case of the open questions raised by the physical properties of the highly conducting Cu (DMDCNQI)2 salt.
Scaling theory for radial distributions of star polymers in dilute solution in the bulk and at a surface, and scaling of polymer networks near the adsorption transition
Authors: Ohno Kaoru , Binder Kurt
Published: July 2002

Monomer density profiles ρ(r) and center–end distribution functions g(rCE) of star polymers are analyzed by using a scaling theory in arbitrary dimensions d, considering dilute solutions and the good solvent limit. Both the case of a free star in the bulk and of a center-adsorbed star at a free surface are considered. In the latter case of a semi-infinite problem, a distinction is made between repulsive walls, attractive walls—where for large arm length l the configuration of the star is quasi-(d−1) dimensional—, and ‘‘marginal walls’’ where for l→∞ the transition from d-dimensional structure occurs. For free stars, ρ(r) behaves as r−d+1/ν for small r, where ν is the exponent describing the linear dimensions of the star, e.g., the gyration radius Rgyr∼lν. For center-adsorbed stars at repulsive or marginal walls, ρ(r∥,z) behaves as ρ(r∥,0) ∼r−d+λ( f )∥ and ρ(0,z)∼z−d+1/ν, where r∥ and z denote the distances parallel and perpendicular to the surface, respectively; the new exponent λ( f ) depends explicitly on the number of arms  f  in general. For center-adsorbed stars at attractive walls, ρ(r∥,z) behaves as ρ(r∥,0)∼r−(d−1)+1/ν(d−1)∥, ν(d−1) being the exponent describing (d−1)-dimensional stars, while ρ(0,z) decays exponentially.

On the other hand, the center–end distribution function at short distances is described by nontrivial exponents. For free stars with  f  arms, g(rCE)∼(rCE)θ( f ) for small rCE, where θ( f ) is expressed in terms of the configuration-number exponent γ( f ) and the exponent γ of linear polymers as θ( f ) =[γ−γ( f+1) +γ( f )−1]/ν. For center-adsorbed stars, at repulsive or marginal walls gs(rCE∥,ze) behaves as gs(rCE∥,0) ∼(rCE∥)θ∥( f ), gs(0,zE) ∼(zE)θ⊥( f ) with θ∥( f ) =[γ1−γs( f+1) +γs( f )−1]/ν and θ⊥( f ) =[γ−γs( f+1) +γs( f )−1]/ν, γ1 being the exponent of a linear polymer with one end at the surface. The scaling theory of general polymer networks at the adsorption transition is also presented.

The configuration-number exponent γ𝒢 for a polymer network 𝒢 with nh h functional units in the bulk, n′h h-functional units at the surface and totally composed of  f  linear polymers with the same length is given by γSB𝒢 =α−1−f+ν +∑∞h=1[nhΔh +nhΔSBh]. Δh and ΔSBh are related, respectively, to the exponents of star polymers as γ( f )=α−1+(γ−α)f/2+Δf and γSBs( f ) =α−1+ν+(γ−α)f/2 +ΔSBf, with α given by α=2−νd. The exponent γSBs( f ) is evaluated by means of the renormalization-group ε=4−d expansion to the first order.
Scaling theory for radial distributions of star polymers in dilute solution in the bulk and at a surface. II. ε expansion for monomer densities
Authors: Ohno Kaoru , Binder Kurt
Published: July 2002

The behavior of monomer density profiles of a star polymer in a d-dimensional good solvent, which was predicted in an earlier paper using scaling arguments, is now derived by using the renormalization group ε=4−d expansion method. Both the case of a free star in the bulk and of a center-adsorbed star at a free surface are considered. In the latter case of a semi-infinite problem, a distinction is made between repulsive walls, attractive walls—where for large arm length l, the configuration of the star is quasi-(d−1)-dimensional—and ‘‘marginal walls,’’ where for l→∞ the transition from d-dimensional to (d−1)-dimensional structure occurs. For free stars, ρ(r) behaves as r−d+1/ν for small r, where ν is the exponent describing the linear dimensions of the star, e.g., the gyration radius Rgyr∼lν. For center-adsorbed stars at repulsive or marginal walls, ρ(r∥,z) behaves as ρ(r∥,0)∼r−d+λ( f )∥ and ρ(0,z)∼z−d+1/ν, where r∥ and z denote the distances parallel and perpendicular to the surface, respectively; the new exponent λ( f ) depends explicitly on the number of arms f in general. We calculate this exponent λ( f ) to first order in ε=4−d; then λ( f ) is obtained to be (f−1)ε/4+𝒪(ε2) for repulsive walls and 2−ε/4+𝒪(ε2) for marginal walls.
Accurate a
b i
n
i
t
i
o potential energy computations for the H4 system: Tests of some analytic potential energy surfaces
Authors: Boothroyd Arnold I. , Dove John E. , Keogh William J. , Martin Peter G. , Peterson Michael R.
Published: July 2002

The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry, as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely, collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. Accurate ab initio energies were computed for 6046 conformations of H4, using a multiple reference (single and) double excitation configuration interaction (MRD-CI) program. Both systematic and ‘‘random’’ errors were estimated to have an rms size of 0.6 mhartree, for a total rms error of about 0.9 mhartree (or 0.55 kcal/mol) in the final ab initio energy values. It proved possible to include in a self-consistent way ab initio energies calculated by Schwenke, bringing the number of H4 conformations to 6101. Ab initio energies were also computed for 404 conformations of H3; adding ab initio energies calculated by other authors yielded a total of 772 conformations of H3. (The H3 results, and an improved analytic PES for H3, are reported elsewhere.) Ab initio energies are tabulated in this paper only for a sample of H4 conformations; a full list of all 6101 conformations of H4 (and 772 conformations of H3 ) is available from Physics Auxiliary Publication Service (PAPS), or from the authors.

The best existing analytic PESs for H4 are shown to be accurate only for pairs of H2 molecules with intermolecular separations greater than about 3 bohr (1.6 Å). High energy collisions (such as might lead to direct collisional dissociation) cannot be well represented by such surfaces. A more general analytic PES for H4 is required, which will be accurate for compact (high-energy) conformations and for conformations that cannot be subdivided into a pair of H2 molecules. Work in progress on devising such a surface (fitted to the 6101 conformations of this work) will be reported in a forthcoming paper.
Solvent rotational mobility in polystyrene/aroclor and polybutadiene/aroclor solutions. II. A photon correlation spectroscopic study
Authors: Rizos A. , Fytas G. , Lodge T. P. , Ngai K. L.
Published: July 2002

Dynamic depolarized light scattering has been utilized to probe the rotational motion of the solvent Aroclor (A1248) in solutions containing either polystyrene (PS) (cPS=0, 0.086 or 0.146 g/cc) or 1,4-polybutadiene (PB) (cPB=0.062, 0.098, or 0.16 g/cc) near the solution glass transition. The orientational time correlation functions cVH(t) for the solvent A1248 were recorded as a function of temperature (−13 to −31 °C) over the time range 10−6 –1 s by photon correlation spectroscopy. A unimodal relaxation time distribution describes cVH(t) at temperatures near Tg and the breadth of the distribution of solvent relaxation time increases upon addition of the polymer. The average orientation time 〈τ〉 increases in PS/A1248 solutions and decreases in 1,4 PB/A1248 solutions, and the influence of 1,4 PB is temperature dependent. These main experimental results, which provide further support for the notion of polymer-induced modification to the solvent dynamics, are discussed in terms of the Adam–Gibbs description modified according to the coupling relaxation model.
Depolarized dynamic light scattering studies of ortho-terphenyl dynamics above T
g
Authors: Steffen W. , Patkowski A. , Meier G. , Fischer E. W.
Published: July 2002

Depolarized Rayleigh spectra of ortho-terphenyl (OTP) were measured in the temperature range from Tg to Tg+190 K. Two samples prepared with and without ‘‘clusters’’ were used for the measurement. Four different Fabry–Perot interferometers covering the time range from about 0.5 ps to 100 ns were employed. Two relaxation modes were observed: a slow mode with all its characteristics of the α process, and a fast mode with a constant, temperature independent relaxation time of about 3 ps. The fast mode has not been reported heretofore. The intensity of the fast mode vanishes at about the Vogel–Fulcher–Tamman temperature T0. The relaxation times of these two processes are found to be identical for both OTP with and without clusters. The temperature dependence of the relaxation time of the α process is Arrhenius at high temperatures, but shows a Williams–Landel–Ferry (WLF) behavior in the range from Tg to Tg+80 K for both samples with and without the long-range density fluctuations (i.e., cluster).
Anomalous branch intensities in the threshold photoionization of HCl
Authors: Tonkyn Russell G. , Wiedmann Ralph T. , White Michael G.
Published: July 2002

The rotationally resolved threshold photoionization spectrum of HCl has been observed for ionization into both spin-orbit components of the ground electronic state of HCl+. The data indicate an extreme asymmetry in the apportioning of angular momentum between the escaping photoelectron and the ion core. Observed transitions in which the core rotation decreases are found to be heavily favored compared to those where the core rotation increases by an equal amount. A mechanism of increased negative branch intensity due to field- or dipole-induced mixing of Rydberg series converging to higher ion rotational levels is proposed as a possible explanation. Direct observation of the transition HCl (X 1Σ+,J″=0)→HCl+ (X̃ 2Π3/2,J+= (3)/(2) ) yields an ionization potential of 102 802.8±2 cm−1 for HCl.
Polarization effects in resonance Raman scattering from coupled optically bright states
Authors: Harris Robert A. , Wedlock Michael R. , Butler Laurie J. , Freed Karl F.
Published: July 2002

This paper extends the McClain formulation of two-photon absorption to show what observables can be independently determined in an emission experiment for which incident and scattered photon polarizations are resolved. Interelectronic state coupling contributions and quantum interference can be extracted from the polarized emission spectrum by determining the three linearly independent contributions to the scattering intensity. This enhances the value of polarized emission spectroscopy as a tool for understanding transitions between coupled electronic states that occur during dissociation dynamics. We demonstrate that the three linearly independent contributions to the polarized emission spectrum can be determined by performing three polarization experiments, at least one of which must involve circularly polarized light. Furthermore, the three polarization experiments must be performed at a scattering angle other than π/2. We also present sample calculations on a model of the methyl iodide system which serve as an illustration of the general theory and demonstrate the sensitivity of this technique to the details of the potential-energy surfaces.
Infinitesimal differential diffusion quantum Monte Carlo study of diatomic vibrational frequencies
Authors: Vrbik Jan , Rothstein Stuart M.
Published: July 2002

We show how to extend the formalism of infinitesimal differential diffusion quantum Monte Carlo to the case of higher derivatives of the ground-state energy of a molecule with respect to the molecular geometry. We use LiH as an example, but the technique can be extended to more complicated, nonliner molecules as well. We obtain good agreement with experimental values for the energy derivatives and for the harmonic and anharmonic frequencies of LiH and LiD, despite using a compact single-determinant wave function.
Formation of highly excited oxygen atoms O(2p33p,5P) and O(2p33s,5S) in electron irradiation of mixtures of rare gases and O2
Authors: Mori Tomoo , Kanou Kouichiro , Ishikawa Yo-ichi , Arai Shigeyoshi
Published: July 2002

The 777 nm emission and absorption observed for electron-irradiated He and Ne containing O2 are attributed to the optical transition between O(2p33p,5P) and O(2p33s,5S). He*2(2s 3Σ+u) produces these excited oxygen atoms in its reaction with O2 and the rate constant is (4.2±0.4)×10−10 cm3 s−1. In irradiated Ne and irradiated Ne containing O2, we found the absorption due to Ne*2 (3p 3Πg, v′=1←3s 3Σ+u, v″=0, v′=2←v″=1, and v′=3←v″=2) and the sharp absorption due to O(2p33s,5S) at ∼777 nm, together with the emission of O(2p33p,5P) at the same wavelength.

The vibrational constants of Ne*2 for the 3p 3Πg state are ωe=647 cm−1 and ω′exe=28 cm−1. Ne*2(3s 3Σ+u) may produce O(2p33s,5S) in its reaction with O2. On the other hand, the reactions of excited Ne atoms with O2 appear to be responsible for the emission. The formation of O(2p33s,5S) in irradiated Ar containing O2 is explained in terms of the mechanism involving highly excited Ar** (E≥14.3 eV) and O2. The rate constant for the reaction of Ar** with O2 and the deactivation constant of Ar** by Ar were determined to be (3.2±0.2)×10−10 and (8.9±1.3)×10−15 cm3 s−1, respectively. O(2p33s,5S) disappears by its reaction with O2 in He, Ne, and Ar. The rate constants were found to be (1.8±0.2)×10−10 cm3 s−1 in He, (1.7±0.2)×10−10 cm3 s−1 in Ne, and (1.7±0.2)×10−10 cm3 s−1 in Ar. Ar deactivates O(2p33s,5S) at a rate constant of (2.9±0.2)×10−15 cm3 s−1. Similar deactivation rate constants of O(2p33s,5S) by He and Ne are (1.8±0.1)×10−15 and (3.4±0.3)×10−15 cm3 s−1, respectively. The rough estimation of the radiation chemical yield for O(2p33s,5S) gives G=1×10−2 for the mixture of 7600 Torr He and 0.1 Torr O2.
Spectroscopy and excited state dynamics of the HNF (DNF) molecule
Authors: Chen Jing , Dagdigian Paul J.
Published: July 2002

Laser fluorescence excitation has been employed to detect HNF and its isotopomer DNF in the F/HN3(DN3) system. The observation of this molecule in the F+HN3 reaction has confirmed that this reaction proceeds to form HNF+N2, as well as the well-known HF+N3 products. Laser fluorescence excitation scans were taken for a number of HNF and DNF Ã 2A′(0,v′2,0)–X̃ 2A″(0,0,0) bands. For DNF, excitation of the Ã (0,2,1) and (0,3,1) levels were also detected. A partial rotational analysis of the DNF bands was carried out. With the derived A rotational constants and previously determined HNF rotational constants, it was possible to derive ground and excited state vibrationally averaged geometries. The K structure of the bands was observed to become simpler with increasing v2, reflecting the reduction in the highest K′ levels observable by fluorescence excitation. Decay lifetimes for a variety of HNF and DNF Ã 2A′ excited levels were determined. It was found that the decay rate, scaled approximately by the ν3 factor, increases abruptly at an energy of 23 800±500 cm−1 above the HNF(X̃ 2A″) zero-point level. This threshold is tentatively assigned to the onset of a predissociation channel. The ground and excited states of HNF form a Renner–Teller pair, whose energies become degenerate at linear geometries. The excited state dynamics of HNF (DNF) is compared with the dynamics of the well-studied Renner–Teller molecules HCO and HNO.
The role of tunneling in precursor mediated dissociation: Alkanes on metal surfaces
Authors: Luntz A. C. , Harris J.
Published: July 2002

While all experimental results for methane dissociation on metals are compatible with a direct on-impact tunneling mechanism, some larger alkane-metal systems display both direct and indirect dissociation channels, the latter involving a molecularly adsorbed intermediate. To date, decay of such intermediates via dissociation or desorption has only been treated within transition-state theory, i.e., in terms of classical ‘‘over the barrier’’ processes. We show here that an alternative mode of dissociation, tunneling, can compete effectively with desorption even when barriers to dissociation are relatively high. We discuss several experiments involving alkane dissociation on metals in light of this and show that they can be understood solely in terms of tunneling, both for the direct on-impact process and for precursor mediated processes. This interpretation resolves an apparent contradiction that arises when these experiments are interpreted strictly within transition-state theory.
Interference of reaction channels for highly exothermic electron transfer reactions
Authors: Burshtein A. I. , Frantsuzov P. A. , Zharikov A. A.
Published: July 2002

The interference of electronic transitions to different vibronic states of the product is considered. The restrictions on the additivity of their rates are established. The upper limit of the full rate is found for the case when interference is significant. It cannot exceed the maximum rate in the fastest channel.
Rotational analysis of v=1 level of n=8∼10 Rydberg states of CO by triple resonant multiphoton spectroscopy
Authors: Ebata Takayuki , Hosoi Nobuki , Ito Mitsuo
Published: July 2002

A detailed analysis of the rotational structure of high Rydberg states of CO converging to the ionic state is presented. These states were pumped from the 3sσB 1Σ+ state using triple resonant multiphoton excitation. The observed states were n=8∼10 of the s-, p-, d-, and f-Rydberg states in the vibrational levels with v=1. The observed spectra were rotationally well resolved and the term values, the quantum defects, and the rotational constants were obtained for these states. p- and f-Rydberg states were described by the Hund’s coupling case (d). On the other hand, only the σ component was observed for the d-Rydberg states and its linewidth was very broad. For the nf-Rydberg state, the long range force model was used to obtain the ionization potential of CO(v=1), the polarizability and the quadrupole moment of the CO+ core. The mixing among Rydberg states seems to be large, as suggested by anomalous values of the rotational constants for several states. In particular, in the nf-Rydberg states, a rotational branch that should not appear for a pure Rydberg state was observed, and was explained by mixing with nearby (n+1)s-Rydberg states.
The continuum approximation in nucleation theory
Authors: Wu David T.
Published: July 2002

The continuum approximation in nucleation theory is reconsidered. It is shown that a minor change in indexing the discrete flux leads naturally to an approximation which is both simple and accurate. More complicated schemes are introduced using the formalism of spectral density (weighting) functions. Optimization of these functions produces additional approximations that minimize the errors in either the rate equation or the nucleation current. These new continuum approximations are compared to the traditional Frenkel form [Kinetic Theory of Liquids (Oxford University, Oxford, 1946)] and to the alternatives proposed by Goodrich [Proc. R. Soc. London, Ser. A 227, 167 (1964)] and Shizgal and Barrett [J. Chem. Phys. 91, 6505 (1989)]. Results show that the new forms are more accurate. Generalization to multipath kinetics (clustering or association) is also discussed. Finally, it is shown that within the continuum approximation, nucleation is mathematically equivalent to position-dependent diffusion.
Nonlinear mass and momentum transport in a dilute gas
Authors: Garzó V. , Santos A.
Published: July 2002

Far from equilibrium particle and momentum transport in a binary mixture subject to uniform shear flow is analyzed. Particles of each species are labeled by a ‘‘color charge.’’ Mutual diffusion is created by the action of an external field that accelerates particles of different species along opposite directions. For a dilute gas of Maxwell molecules, the set of two coupled Boltzmann equations is seen to be solvable by the moment method. The color conductivity tensor and the shear viscosity coefficient are obtained as nonlinear functions of the shear rate and the color field. The usual choice of the external color field [Cummings et al., J. Chem. Phys. 94, 2149 (1991)] yields a zero-field limit of the color conductivity tensor different from the self-diffusion tensor. In order to avoid the above discrepancy, a different form of the external field is proposed.
Dynamical simulations of the photodissociation of CH3Br on a LiF (001) surface
Authors: Huang Zhi-Hong , Guo Hua
Published: July 2002

The photodissociation dynamics of CH3Br adsorbed on LiF(001) is investigated using a classical molecular dynamics method. The adsorbate is modeled as a pseudodiatomic molecule and the LiF surface is represented by a cluster surrounded by static walls. All the particles involved are allowed to move in all three dimensions. It is found from Monte Carlo simulations that CH3Br binds to the LiF surface perpendicularly with a binding energy of approximately 0.3 eV. Both methyl up and Br up orientations of the adsorbate are found to be stable. The results from the dynamical simulations of the photodissociation process indicate that the experimental data for the CH3Br/LiF(001) photodissociation can only be explained on the basis of one dissociation channel open for the CH3Br photodissociation, in contrast to dual channels in the gas phase. When the methyl is up, its translational energy distribution resembles the corresponding gas phase result. In the case of methyl down and Br up, however, the dissociating methyl rebounds from the surface and collides with Br. With only a single collision, the kinetic energy of Br can be increased up to 1 eV, in agreement with experiment.
Vibration–rotational Einstein coefficients for HF/DF and HCl/DCl
Authors: Arunan E. , Setser D. W. , Ogilvie J. F.
Published: July 2002

The experimentally based dipole-moment functions have been combined with the best Rydberg–Klein–Rees potentials to calculate the vibration–rotational Einstein coefficients for HF, DF, HCl, and DCl. Calculations were done for the Δv=1, 2, and 3 transitions for v≤6 for HF and v≤7 for HCl, which are in the range of the internuclear distance, r, for which the dipole moment functions are valid. The calculations were done for J≤25 for each v level. The higher v levels of HF were investigated using a Padé extrapolation of the experimental dipole function and a recently published ab initio function. Our Δv=1 Einstein coefficients for HF agree closely with those from an earlier experimentally based dipole function and with the new ab initio results for v≤6. Our results for HCl, however, represent a significant improvement over the Einstein coefficients currently in the literature. The isotopic ratio of Einstein coefficients for the Δv=1 transitions, ADX/AHX, were not changed significantly. Also, the changes in the ratios of the Einstein coefficients, Av,v−1/A1,0, were less significant than the changes in the absolute magnitudes. The effect of high rotational energy on the Einstein coefficients for HF(v,J) is discussed.
Photofragment spectroscopy of resonances in the A 1Πu state of N22+
Authors: Larsson M. , Sundström G. , Broström L. , Mannervik S.
Published: July 2002

A photopredissociation spectrum of the A 1Πu–X 1Σ+g (8,2) transition in N2+2 is reported. The spectrum was recorded by detecting the photofragment signal as a function of excitation wavelength in a merged laser-ion beam apparatus. Line positions for the (8,2) band were measured and reduced to molecular constants. The natural linewidths, and hence predissociation rates, of all observed v′=8 levels were measured. The predissociation rates were found to be e/f symmetry dependent and, in particular, for the f levels, also J dependent. Somewhat surprisingly it appears as if tunneling does not contribute to the observed predissociation; rather, the v′=8 level of the A 1Πu state is depleted by coupling to the continua of the 1Σ−u, 3Σ+u, and 3Δu states.
Rotationally resolved ultraviolet–ultraviolet double resonance study of the nonplanar Ẽ state of acetylene
Authors: Lundberg James K. , Jonas David M. , Rajaram Bhavani , Chen Yongqin , Field Robert W.
Published: July 2002

In centrosymmetric molecules such as acetylene, one expects one-photon and two-photon spectroscopy to be mutually exclusive by the g↔u electric dipole selection rule. An ultraviolet–ultraviolet double resonance (UVUVDR) spectrum of the 74 000–78 000 cm−1 region of acetylene, recorded using the Ã←X̃ transition for the first step in double resonance consists almost entirely of transitions terminating on levels of the Ẽ state identified in one-photon vacuum ultraviolet (vuv) spectroscopy. Evidence for a nonplanar, noncentrosymmetric structure of the Ẽ state obtained by rotationally resolved fluorescence dip detected UVUVDR is presented.
A study of the S(3P2,1,0;1D2) production in the 193 nm photodissociation of CH3S(X̃)
Authors: Hsu C.-W. , Liao C.-L. , Ma Z.-X. , Tjossem P. J. H. , Ng C. Y.
Published: July 2002

The dynamics of S(3P2,1,0;1D2) production from the 193 nm photodissociation of CH3SCH3 has been studied using 2+1 resonance-enhanced multiphoton ionization techniques. The 193 nm photodissociation cross section for the formation of S from CH3S initially prepared in the photodissociation of CH3SCH3 is estimated to be 1×10−18 cm2. The branching ratio for S(3P)/S(1D) is found to be 0.15/0.85. The fine-structure distribution observed for product S(3P2,1,0) is nearly statistical. Possible potential energy surfaces involved in the 193 nm photodissociation of CH3S(X̃) have been examined theoretically along the CH3–S dissociation coordinate in C3v symmetry. These calculations suggest that predissociation of CH3S(C̃ 2A2) via the repulsive CH3S(Ẽ 2E) surface is most likely responsible for the efficient production of S(1D). For vibrationally excited CH3S(X̃), a viable mechanism for the dominant production of S(1D) may involve direct dissociation via the CH3S(Ẽ 2E) state formed in the 193 nm photoexcitation.
Ligand-field theory applied to diatomic transition metals. Results for the d
A9d
B9σ2 states of Ni2, the dNi9dCu10σ2 states of NiCu, and the dNi8(3F)dCu10σ2σ*1 excited states of NiCu
Authors: Spain Eileen M. , Morse Michael D.
Published: July 2002

A ligand-field theory has been developed for transition-metal diatomics having electronic configurations of dA9dB10σ2, dA9dB9σ2, and dA8(3F)dB10σ2σ*1. The theory treats each atom as a point charge and includes spin–orbit interactions. No contributions due to d-orbital chemical bonding are included. Since the d orbitals are quite small compared to the bond lengths in these molecules, the only inputs to the theory are the ligand charges (ZA and ZB), the radial expectation values <rA2≳nd, <rB2≳nd, <rA4≳nd, and <rB4≳nd, the atomic spin–orbit parameters ζA and ζB, and the bond length, R. Calculations employing no adjustable parameters (setting ZA, B =+1.0, and using radial expectation values and spin–orbit parameters from atomic tables) provide essentially quantitative agreement with ab initio results on the dNi9dCu10σ2 manifold of states in NiCu, and on the dA9dB9σ2 manifold of states in Ni2.

This demonstrates that the ligand-field model has some validity for metal molecules containing nickel, primarily because of the compact nature of the 3d orbitals in this element. Similar calculations of the dA9dB9σ2 manifold of states in Pt2 and the dNi9dPt9σ2 manifold of states in NiPt are presented for comparison to future ab initio or experimental measurements, although the possibility of d-orbital contributions to the bonding in these species makes the ligand-field model less favorable in these examples. The dNi8(3F)dCu10σ2σ*1 excited electronic states of NiCu, which are well known from resonant two-photon ionization spectroscopy, are also investigated in the ligand-field model. As a final example, the dNi8(3F)σ2σ*1 excited electronic states of NiH are also examined using the same treatment as that employed for the dNi8(3F)dCu10σ2σ*1 excited manifold of NiCu.
Pressure effects on reorientational dynamics of molecular liquids in confined geometries
Authors: Xu Shu , Zhang Jing , Jonas J.
Published: July 2002

For the first time the NMR relaxation behavior of polar molecular liquids confined to porous glasses was studied at pressures up to 5 kbar. Analysis of the relaxation data by the two-state, fast exchange model allowed us to determine the pressure effects on the dynamics of the surface layer liquid.
Quantum-mechanical calculations of solvation free energies. A combined ab initio pseudopotential free-energy perturbation approach
Authors: Vaidehi Nagarajan , Wesolowski Tomasz Adam , Warshel Arieh
Published: July 2002

A practical ab initio quantum-mechanical approach for calculations of free energies of molecules in solutions is developed. This approach treats the solute molecules by an explicit ab initio self-consistent-field approach while representing the solvent molecules by a pseudopotential. The solvation energies are evaluated by a free-energy perturbation approach that uses the distribution function associated with a classical force field as a reference state for the quantum-mechanical calculations. The performance of the method is examined by evaluating the solvation energy of an Li+ ion. It is found that the calculation times are not much longer than that of the corresponding classical free-energy perturbation calculations.
Kinetic theory calculations of pressure effects of diffusion
Authors: Pan Shaqing , Storvick Truman S.
Published: July 2002

The pressure effects of diffusion in an isothermal capillary tube are calculated. The linearized Boltzmann equation, the third-order Gross–Jackson model for the collision term, and Maxwell’s diffuse-specular boundary condition at the wall are used to compute the dimensionless component mean velocities for binary mixtures using numerical methods. The pressure effects are obtained by mass conservation. With the accommodation coefficients as parameters, the calculations reproduce the experimental data for the systems of nitrogen–hydrogen, argon–nitrogen, and carbon dioxide–argon quantitatively and the sign change in pressure effects observed for the system of ethylene–neon. Independent assignment of the accommodation coefficients is required for a predictive theory.
Electronic excitation of CH4 by low-energy electron impact
Authors: Winstead Carl , Sun Qiyan , McKoy Vincent , Lino José L. S. , Lima Marco A. P.
Published: July 2002

We report cross sections for excitation of the (1t2→3sa1) singlet and triplet states of methane by low-energy electron impact. The cross sections for these dissociative states were obtained using the Schwinger multichannel variational method with up to seven channels (three physical states) coupled. Aspects of the convergence with respect to channel coupling are discussed. A preliminary comparison with the recently measured CH2 fragment production cross section [T. Nakano, H. Toyoda, and H. Sugai, Jpn. J. Appl. Phys. 30, 2912 (1991)] shows fair agreement.
Close encounters of the coil kind: Evidence for shrinkage and for collisions in dilute solutions of polymer coils
Authors: Schaller J. K. , Devanand K. , Selser J. C.
Published: July 2002

The concentration dependence of the characteristic frequency, ΔΓ1(c), for the overall intra- coil relaxation of linear polystyrene in good and theta solvents was determined using photon correlation spectroscopy. This dependence was unusually strong in good solvent and absent in theta solvent. In good solvent, coil–coil collisions, whose frequency depended on concentration, contributed to coil shrinkage and to the interruption of overall intracoil relaxations resulting in an unusually strong concentration dependence for ΔΓ1(c). In marked contrast to the good solvent behavior, coils interpenetrated readily at theta interacting only very weakly and as a result ΔΓ1(c) exhibited little, if any, concentration dependence.
Translational spectroscopy of metastable fragments produced by dissociative excitation of carbon disulphide
Authors: Mason N. J.
Published: July 2002

The production of the S(5s) metastable state by electron impact dissociative excitation of carbon disulphide is reported. Kinetic energy distributions of the sulphur fragments determined by the method of time-of-flight spectroscopy are also presented.
Reaction dynamics of C(1D)+H2(v)→CH(X 2Π)+H
Authors: Mikulecky Klaus , Gericke Karl-Heinz
Published: July 2002

The CH(X 2Π,v,J,Ω,Λ) product state distribution from the reaction C(1D)+H2(v)→CH+H was determined by laser-induced fluorescence (LIF) where the B 2Σ–X 2Π transitions were probed. Most of the available energy is released as translation. A nearly thermal rotational distribution is obtained for CH(v=0,1). Only a small fraction, 4.1×10−4, of the CH products is formed in the vibrationally excited state. A higher propensity for the production of CH in the symmetric Π(A′) Λ sublevels is evident. For studying the influence of vibrational excitation on the reaction dynamics, H2 was excited to its first vibrational state via stimulated Raman pumping (SRP). H2(v=1) increases the reaction rate and enhances the population of higher rotational states, but diminishes the Λ selectivity. The vibrational population ratio P(v=0)/P(v=1) of the CH product remains unaltered. Insertion of the C(1D) atom into the H2 bond is the major reaction mechanism, but the probability for an abstractive process seems to increase when H2(v=1) is reacting with C(1D).
A systematic theoretical study of the harmonic vibrational frequencies for polyatomic molecules: The single, double, and perturbative triple excitation coupled-cluster [CCSD(T)] method
Authors: Thomas J. Russell , DeLeeuw Bradley J. , Vacek George , Schaefer Henry F.
Published: July 2002

Analytic gradient methods have been used to predict the harmonic vibrational frequencies and the infrared (IR) intensities of HCN, HNC, CO2, CH4, NH4+, C2H2, H2O, H2CO, and NH3 at the single, double, and perturbative triple excitation coupled-cluster [CCSD(T)] level of theory. All studies were performed using a double zeta plus polarization (DZ+P) basis set with a contraction scheme of (9s5p1d/4s2p1d) for C, N, and O and (4s1p/2s1p) for H. The results of previous studies using the same basis set with self-consistent field (SCF), single and double excitation configuration interaction (CISD), and the single and double excitation coupled-cluster (CCSD) methods are included to allow a detailed comparison. The theoretical harmonic frequencies of all nine molecules are compared to the 28 (out of a total of 35 possible) known experimental harmonic vibrational frequencies. IR intensities are also compared to known experimental values. The absolute average error in frequencies for the CCSD(T) method with respect to experiment was 2.4%. Values of 9.1%, 3.7%, and 2.3% have been reported previously for the SCF, CISD, and CCSD methods, respectively. The CCSD(T) method is the only one for which a significant number of the theoretical vibrational frequencies lie below experiment. If the absolute errors for two frequencies that are known to be described inadequately with the DZP basis set are not included in the averages, they become 8.6% (SCF), 3.6% (CISD), 1.7% (CCSD), and 1.5% [CCSD(T)]. The establishment of an average error for the DZP CCSD(T) method should improve the reliability in the prediction of unknown experimental frequencies.
Configurational entropy in a microemulsion: The error implicit in the use of a phenomenological lattice model
Authors: Sturgeon Kathy S. , Reiss Howard
Published: July 2002

Lattice models of complex fluids (micellar solutions, microemulsions, etc.) have been an effective tool for the study of both the internal structures of various complex phases and the corresponding phase diagrams. In some approaches, e.g., the random-mixing model of a microemulsion, the lattice parameter is many times the dimension of a single molecule and has been used as a length scale for computing the configurational entropy (entropy of mixing) of the microemulsion. By investigating a one-dimensional model that can be solved exactly in the continuum, we show that the lattice scale is grossly inappropriate for the evaluation of the mixing entropy because of the omission of a very large number of configurations available in the continuum. Indeed, the proper length scale is of the order of the cube root of the free volume per molecule or much smaller. The lattice parameter is, however, an appropriate scale for the amplitude of interface roughness or persistence length. Since the proper scale leads to a much larger value of the mixing entropy, the microemulsion could be stable against a larger (nonzero value) of interfacial tension than is assumed at present.
Dynamics of surface-aligned photochemistry. III. A quantum mechanical study of the photodissociation of HBr(ad)/LiF(001)
Authors: Barclay V. J. , Polanyi J. C. , Zeiri Y. , Kosloff R.
Published: July 2002

A quantum mechanical probe of surface and adsorbed layer structure is presented based on the photodissociation of ordered hydrogen halide adsorbates. The photolysis of the adsorbate molecule released atomic hydrogen which scattered from the surface as well as from neighboring adsorbed molecules. The coherent character of the hydrogenic wave function formed in the photodissociation process proved to be a revealing probe of the surface and of nearby adsorbates by way of localized atomic scattering (LAS). The dynamics of the H-atom motion were modeled by the time-dependent Schrödinger equation. This study includes two-dimensional simulations of the photodissociation of a single HBr molecule adsorbed on LiF(001) as well as photodissociation in full mono-layer and bilayer systems. It was found that for photolysis of an isolated adsorbate molecule, depending on the separation of the adsorbate from the surface, the angular distribution showed either a single specular peak (small z) or diffraction (larger z), while for higher coverage a series of diffraction peaks as well as peaks which corresponded to trapped unstable periodic orbits were observed. The quantization rules of these periodic orbits were identified and served as a link between the observed angular distribution and the structure of the adsorbed layer(s).
The X̃ AlOH–X̃ HAlO isomerization potential energy hypersurface
Authors: Vacek George , DeLeeuw Bradley J. , Schaefer Henry F.
Published: July 2002

Ab initio molecular electronic structure theory has been used to study the AlOH–HAlO unimolecular isomerization reaction on the singlet ground state potential energy surface. Electron correlation effects were included via configuration interaction and coupled-cluster methods. Basis sets as complete as triple zeta plus two sets of polarization functions and a set of higher angular momentum functions [TZ(2df,2pd)] were employed. The classical barrier for hydrogen migration from X̃ HAlO to X̃ AlOH is predicted to be 38.4 kcal mol−1 using the TZ(2df,2pd) basis set with the coupled-cluster method including all single and double excitations with the effect of connected triple excitations included perturbatively [CCSD(T)]. After correction for zero-point vibrational energies (ZPVEs), an activation energy of 36.6 kcal mol−1 is obtained. The ΔE for isomerization is −42.2 (−40.5 with ZPVE correction) kcal mol−1 at the same level of theory. The dipole moments of HAlO and AlOH in their equilibrium geometries are 4.525 and 1.040 Debye, respectively, at the same level of theory. A comparison is also made between a theoretically predicted harmonic vibrational frequency and a recently determined experimental fundamental frequency for X̃ AlOH.
An approximate discretized real time path integral simulation method for nearly classical systems
Authors: Amini Abolfazl M. , Herman Michael F.
Published: July 2002

A new approximate technique for real time discretized path integral simulations is introduced. The technique transforms the oscillatory integrands into nonoscillatory functions for accurate Metropolis Monte Carlo evaluation. The method is applied to a system containing a Br2 diatomic in two Ar atoms. The vibrational transition probability of finding the diatomic in its ground vibrational state is calculated at time t. The diatomic is initially in its first vibrational state. Computations are carried out for two different times and the results are compared to the previously published results of full discretized path integral calculation. The convergence of the technique is tested as a function of the number of points in the discretized solvent path.
Broad band dipolar recoupling in the nuclear magnetic resonance of rotating solids
Authors: Sodickson D. K. , Levitt M. H. , Vega S. , Griffin R. G.
Published: July 2002

The effects of homonuclear dipole couplings may be reintroduced into magic angle spinning nuclear magnetic resonance (NMR) spectra by judiciously spaced π pulse trains. This recoupling phenomenon occurs over a wide range of chemical shift differences and sample rotation speeds. We present results of magnetization exchange experiments exploiting the broad band recoupling effect, and propose a theoretical description in which pulse-assisted dipolar recoupling may be understood as a form of compensated rotational resonance.
Color conductivity induced by a shear-rate dependent color field
Authors: Garzó V. , Santos A.
Published: July 2002

Recently, the color diffusion in uniform shear flow has been analyzed from the Boltzmann equation [V. Garzó and A. Santos, J. Chem. Phys. 97, 2039 (1992)]. Here, the analysis is generalized to allow for a shear-rate dependence of the color field, which yields a zero-field limit of the color conductivity tensor identical to the self-diffusion tensor.
Asymmetry in the ionic fragmentation of N2O photoexcited around the N and O K edges
Authors: Bozek John D. , Saito Norio , Suzuki Isao H.
Published: July 2002

Asymmetry parameters (β) for the fragment ions of N2O following excitation or ionization of the N and O K shell electrons have been determined from photoelectron–photoion coincidence (PEPICO) and photoion–photoion coincidence (PIPICO) spectra measured using monochromatized synchrotron radiation and a rotatable time-of-flight mass spectrometer. Profiles of the β parameters from the N+ ions with low kinetic energies (K.E. <4.5 eV) in the PEPICO spectra were found to be very isotropic (close to 0) around both the N and O K edges. β parameter profiles for the O+ ions with contributions from N+ ions with K.E. ≳4.5 eV and N2+ ions overlapped with NO+ ions show structure with negative values at the π* resonances below the N K-edges and positive values at the unresolved Rydberg resonances below the terminal N K edge and O K edge and at the shape resonance above the N K edges. β parameters from the N+–NO+ and N2+–O+ PIPICO signals were found to be considerably more anisotropic than the corresponding values from the PEPICO signals at the π* resonances below the N K edges and the shape resonance above the ionization edges. β parameters from both the PEPICO and PIPICO signals were close to zero at the π* resonance below the O K edge. The higher degree of anisotropy of ionic fragmentation observed in the PIPICO signals highlights the importance of measuring the fragmentation spectra of polyatomic molecules in coincidence in order to obtain accurate β parameters.
Thermodynamic and stochastic theory of reaction-diffusion systems with multiple stationary states
Authors: Chu Xiaolin , Ross John , Hunt P. M. , Hunt K. L. C.
Published: July 2002

The thermodynamic and stochastic theory of chemical systems far from equilibrium is extended to reactions in inhomogeneous system for both single and multiple intermediates, with multiple stationary states coupled with linear diffusion. The theory is applied to the two variable Selkov model coupled with diffusion, in particular to the issue of relative stability of two stable homogeneous stationary states as tested in a possible inhomogeneous experimental configuration. The thermodynamic theory predicts equistability of such states when the excess work from one stationary state to the stable inhomogeneous concentration profile equals the excess work from the other stable stationary state. The predictions of the theory on the conditions for relative stability are consistent with solutions of the deterministic reaction-diffusion equations. In the following article we apply the theory again to the issue of relative stability for single-variable systems, and make comparison with numerical solutions of the reaction-diffusion equations for the Schlögl model, and with experiments on an optically bistable system where the kinetic variable is temperature and the transport mechanism is thermal conduction.
Dependence of the rotational viscosity of nematic mixtures on the rotational diffusion constants of their components
Authors: Christoph G. , Stille W. , Strobl G.
Published: July 2002

The relation between the rotational viscosity γ1 of binary nematic mixtures and the different rotational diffusion constants Dri of their components was studied by a combination of director reorientation experiments with dielectric relaxation spectroscopy. Marrucci’s theory for the viscosity of single component nematics can be applied when the weighted reciprocal rotational diffusion constants are averaged arithmetically. An explanation is given by considering the sum of the molecular frictional torques.