bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical model of short alcohol monolayers: Chain conformations and melting transition
Authors: Rieu J. P. , Vallade M.
Published: July 2002

A mean field statistical model is used to calculate the chain organization and the thermodynamics behavior of an amphiphilic monolayer at the surface of water. The configurations of a single chain are generated with the rotational isomeric state model chains. In order to obtain semiquantitative results for comparison with experiments on short alcohol monolayers, the model uses as an external input a relation between the average volume per CH2 and the molecular area experimentally determined in recent reflectivity measurements. Various properties associated with the chain conformations are calculated and compared with molecular dynamics simulations. The model exhibits a phase transition which is identified with the melting transition of short alcohol monolayers and which clearly underlines the major role of the conformational energy.
Dicarbocyanine dyes in methanol solution probed by depolarized Rayleigh and hyper-Rayleigh light scattering
Authors: Song Ok-Keun , Wang C. H.
Published: July 2002

The hyper-Rayleigh scattering (HRS) intensity of two symmetric carbocyanine dyes (1122 DEDC and 1144 DEDC, full names given in the text) in methanol is measured as a function of dye concentration. These dye molecules at equilibrium show a negligible permanent dipole moment. The low concentration data showing that the HRS intensity is proportional to the dye concentration are used to determine the first hyperpolarizability for each of these dyes. However, above a concentration ρb=0.1×10−3 M, the HRS intensity shows an anomalous concentration dependence. Above ρb, the HRS intensity shows a saturation behavior and it even decreases with increasing concentration at high dye concentration. The depolarization ratio of the HRS intensity is also measured as a function of dye concentration. At lowest concentration, the depolarization ratio is 0.18. As the dye concentration increases, the depolarization ratio also rapidly increases but the increase quickly saturates as the concentration exceeds ρb. The concentration dependence of the HRS intensity and depolarization ratio are interpreted as due to formation of molecular aggregates. The depolarized Rayleigh scattering (DRS) intensity is also measured as a function of dye concentration. The result of DRS corroborates well with that found in HRS.
Two-color multiphoton transitions in molecular beam electric resonance studies: Rotating wave versus Floquet, and on- versus off-resonance, calculations
Authors: Kondo A. E. , Meath William J.
Published: July 2002

A nonzero difference, d, between the diagonal dipole moment matrix elements, μjj, of two molecular states involved in either one- or two-color multiphoton transitions, can have substantial impact on the temporal evolution and spectral behavior of the states. The effects of d≠0 are investigated in this paper for two-color transitions in a two-level system previously studied in one-color molecular beam electric resonance (MBER) experiments on symmetric top molecules. The calculations suggest a two-color analog to the one-field experiments, where the flexibility furnished by the field parameters of the two continuous wave electric fields, including relative phase, can be used to advantage. Both exact Floquet calculations and the rotating wave approximation (RWA) are used in this study. Analytic RWA expressions for the one- and two-color molecule-laser(s) couplings are particularly useful in helping to interpret and/or predict the effects of d≠0. The novel aspects of two-color laser-molecule interactions, relative to the one-field case, are emphasized. In addition to investigations related to MBER studies, this work contributes to the more formal aspects of two-color laser-molecule interactions. It is shown that very useful analytical two-level RWA solutions for the on-resonance temporal behavior of the molecular states are available, even in the presence of competing resonances, whereas off-resonance numerically useful analytical results are available only when one multiphoton resonance dominates a transition.
Models for statistical decomposition of metal clusters: Decay on multiple electronic states
Authors: Shvartsburg Alexandre A. , Frederick John H. , Ervin Kent M.
Published: July 2002

The inclusion of electronic degrees of freedom in the statistical unimolecular decomposition of metal clusters is considered using Rice–Ramsperger–Kassel–Marcus (RRKM) theory. All vibronic states are included in the sums and densities of states in both adiabatic and nonadiabatic treatments of the electronic states. To model the metal cluster electronic structure, a simple semiclassical treatment is used to calculate energy levels of electrons in a variable-size spherical cavity based on the Woods–Saxon radial potential. The potential parameters are adjusted to mimic the average level spacings and electron detachment energies of realistic metal clusters of various charge states. Direct permutations of the electron occupancies of these cluster electron energy levels generate a set of cluster electronic states, which are then used in numerical calculations of the vibronic sums and densities of states for the RRKM rates. The electronic corrections to the RRKM dissociation rates of the ground electronics states are presented for several examples to show trends with respect to cluster size, charge state, number of delocalized valence electrons, and dissociation energy. Inclusion of electronic degrees of freedom always reduces the cluster dissociation rate, by a few percent to a factor of 20 or more depending on cluster size, charge state, metal properties, and the excitation energy. The adiabatic and nonadiabatic approaches give quantitatively similar rates, except for the smallest cluster sizes.
Theoretical study of bath-induced coherence transfer effects on a time- and frequency-resolved resonant light scattering spectrum. II. Energy mismatch effects
Authors: Ohtsuki Y. , Fujimura Y.
Published: July 2002

Bath-induced coherence transfer effects on a time- and frequency-resolved resonant light scattering spectrum is theoretically investigated using the Markoff master equation. According to Eberly and Wódkiewicz, a general expression for an experimentally observable spectrum in terms of a molecular response function is derived within the density matrix formalism. To generalize our previous results of the bath-induced coherence transfer which were derived based on a displaced harmonic oscillator model [Y. Ohtsuki and Y. Fujimura, J. Chem. Phys. 91, 3903 (1989)], an eigenstate basis is used to represent a relevant system for investigating characteristics of the transfer. By the present model, we clarify the dependence of the bath-induced coherence transfer on the energy-level structure of the intermediate states associated with the transfer, i.e., energy mismatch effects. It is shown that if the energy mismatch of these states is smaller than dephasing rates, the bath-induced coherence transfer occurs resonantly. In the other cases, the energy mismatch brings about a modulation in the time evolution of the superposition state created by the bath-induced coherence transfer, which strongly diminishes the efficiency of the transfer. The resonance condition is derived analytically and is confirmed by numerical calculations of quantum beats induced by the bath-induced coherence transfer. The possibility of very rapid dephasing of a quantum beat signal which cannot be explained in terms of dephasing rates is also shown, when the transition moments have such values that give π-phase-shifted quantum beats in bath-induced fluorescence.
Infrared absorption of cyclic- and trans-NaNO2 and KNO2 in solid argon
Authors: Lo Wen-Jui , Shen Min-yi , Yu Chin-hui , Lee Yuan-Pern
Published: July 2002

Sodium nitrite dispersed in solid argon exhibited infrared absorption at 1293.0, 1222.8, and 825.6 cm−1, corresponding, respectively, to symmetric and asymmetric stretching, and bending, modes of the NO−2 moiety in a bidentate planar cyclic structure of C2v symmetry (designated cyclic-NaNO2). After irradiation of the matrix sample with emission from the KrF laser at 248 nm, new lines at 1446.2, 1159.1, and 787.1 cm−1 were recorded. A sample containing Na15NO2 absorbed at 1272.1, 1198.2, and 821.7 cm−1 before irradiation; new lines at 1419.6, 1136.9, and 782.8 cm−1 were observed after irradiation. New lines observed after irradiation are assigned to sodium nitrite with a monodentate planar trans structure (designated trans-NaNO2). Ab initio calculations according to density functional theory at Becke3LYP/6-311+G* level predicted line positions, isotopic shifts, and relative IR intensities similar to those observed experimentally for both isomers. Similar results were obtained for cyclic-KNO2 at (1313.4, 1310.8), (1217.3, 1207.9), and (807.2, 809.5) cm−1 before irradiation, and for trans-KNO2 at (1412.4, 1413.2), (1171.0, 1178.2, 1176.4), and 783.1 cm−1 after irradiation at 248 nm; the numbers grouped in parentheses reflect line splitting.
Chemical wave propagation in Hele–Shaw cells and porous media
Authors: Vasquez Desiderio A. , Wilder Joseph W. , Edwards Boyd F.
Published: July 2002

Chemical waves induce density gradients in fluids which may lead to convection. This paper studies the convective effects on chemical waves propagating in porous media or in fluids confined between two parallel vertical walls. Chemical waves in the iodate–arsenous acid system are modeled with a one variable reaction-diffusion equation. The fluid flow is modeled using Darcy’s law. A linear stability analysis on convectionless fronts shows a transition to convection. The full nonlinear equations describing the convective front are solved numerically on a vertical slab. Convective fronts propagate faster than convectionless fronts. Near the onset of convection, the fronts are raised on one side of the slab and lowered on the opposite side. Away from the onset, the fronts are raised at the middle of the slab with the opposing sides at the same height.
The Auger spectra of CF4 in the light of foreign imaging
Authors: Gottfried F. O. , Cederbaum L. S. , Tarantelli F.
Published: July 2002

The fluorine and carbon Auger spectra of CF4 are investigated by computing very many dicationic states in the valence region up to 120 eV with the Green’s function method. An analysis of the double hole density in the correlated states of CF4++ proves that pronounced hole localization phenomena at the fluorine atoms take place in almost all the final states of the Auger decay. We discuss how these phenomena are at the origin of the observed fluorine and carbon Auger spectral profiles and, in particular, how they provide a complete and conclusive interpretation of the spectra. The intra-atomic nature of the Auger process allows us, by a simple convolution of appropriate (localized) one-site components of the computed two-hole density distribution, to obtain line shapes which are in close agreement with experiment. To show the general validity of the presented arguments we also compare the results for CF4 to the Auger spectra of BF3. The central atom spectrum of these molecules can be understood in the light of the recently introduced foreign imaging picture of Auger spectroscopy.
Laser-induced desorption of NO from NiO(100): Ab initio calculations of potential surfaces for intermediate excited states
Authors: Klüner T. , Freund H.-J. , Freitag J. , Staemmler V.
Published: July 2002

In order to interpret the experimental results of the state resolved UV-laser-induced desorption of NO from NiO(100) (rotational and vibrational populations, velocity distributions of the desorbing NO molecules, etc.), we have performed ab initio complete active space self-consistent field (CASSCF) and configuration interaction (CI) calculations for the interaction potential between NO and the NiO(100) surface in the electronic ground state and for those excited states which are involved in the desorption process. The NiO(100)–NO distance and the tilt angle between the NO axis and the surface normal have been varied. A cluster model containing a NiO8−5-cluster embedded in a Madelung potential has been used for representing the NiO(100) surface. The excited states which are important for the desorption process, are charge transfer states of the substrate–adsorbate system, in which one electron is transferred from the surface into the NO-2π-orbital. The potential curves of these excited charge transfer states show deep minima (4 eV–5 eV) at surface/NO distances which are smaller than that in the ground state. The angular dependence of these potentials behaves similar as in the ground state. A semiempirical correction to the calculated excitation energies has been added which makes use of the bulk polarization of NiO. With this correction the charge transfer states are considerably stabilized. The lowest excitation energy amounts to about 4 eV which is in reasonable agreement with the onset of the laser desorption observed experimentally at about 3.5 eV. The density of the NO−-like states is rather high, so that probably several excited states are involved in the desorption process. The potential energy curves for all of these states are quite similar, but the transitions from the ground state into different excited charge transfer states show strongly differing oscillator strengths, which are also strongly dependent on the surface/NO distance. This fact is important for the dynamics of the deexcitation process in the sense of a selection criterion for the states involved. The magnitude of the oscillator strengths is large in comparison with the excitation of NO in the gas phase, which might be an indication for the possibility of optical excitation processes. One dimensional wave packet calculations on two potential energy curves using fixed lifetimes for the excited state in each calculation have been performed and enable us to estimate the mean lifetime of the excited state to be 15 fs≤τ≤25 fs. This implies that the dynamics of the system is dominated by the attractive part of the excited state potential.
Semiclassical molecular dynamics simulations of low-temperature clusters: Applications to (Ar)13; (Ne)13; (H2O)n, n=2,3,5
Authors: Fredj E. , Gerber R. B. , Ratner M. A.
Published: July 2002

Semiclassical molecular dynamics simulations are developed as a tool for studying anharmonic clusters and solids at energies near the zero point. The method employs the time-dependent self-consistent-field approximation, that describes each mode as moving in the mean dynamical field of all other modes. The method further describes each mode by a semiclassical Gaussian wave packet. The scheme is carried out in normal modes. The method is restricted to systems of moderate anharmonicity at low temperatures. It is, however, computationally efficient and practically applicable to large systems. It can be used for the dynamics of nonstationary states as well as for stationary ones. Structural, dynamical and a variety of spectroscopic properties can easily be evaluated. The method is tested for thermal equilibrium states of (Ne)13, (Ar)13 against ‘‘numerically exact’’ quantum Feynman path integral simulations. Excellent quantitative agreement is found for the atom–atom pair distribution functions. The method is also applied to (H2O)n clusters. Good agreement is found with experimentally available fundamental stretch-mode frequencies.
On the energetics of the lower excited states of N-methylpyrrole
Authors: McDiarmid R. , Xing X.
Published: July 2002

An absorption, fluorescence excitation, and (2+1) resonantly enhanced multiphoton ionization spectroscopic investigation was carried out on the lowest excited state of N-methylpyrrole (NMP). Transitions to and from this state were shown to give rise to the structured bands observed around 240 nm. An energy-dissipative channel was found that was only slightly higher in energy. Possible assignments of this channel in NMP and its relation to the states of pyrrole are discussed.
Density dependence of structure of supercritical carbon dioxide along an isotherm
Authors: Ishii Ryo , Okazaki Susumu , Okada Isao , Furusaka Michihiro , Watanabe Noboru , Misawa Masakatsu , Fukunaga Toshiharu
Published: July 2002

Neutron diffraction experiments for supercritical CO2 have been carried out over a wide range of Q (0.018≤Q≤30 Å−1) at ρ*=ρ/ρc=1.5, 1.2, 0.77, and 0.34 along an isotherm at 310 K (T*=T/Tc=1.02). The measurement enabled us to obtain quantitatively reliable radial distribution functions of the fluid including both short-ranged structure and long-ranged density fluctuation. The structure factor and radial distribution function showed the structural change from the attraction-predominant gaslike structure to the repulsion-predominant liquidlike one with increasing fluid density. With respect to the long-ranged structure, almost linear Ornstein–Zernike–Debye plots were obtained for S(Q) of the fluids at all densities. A plot of correlation length against reduced density seems to have a maximum at the critical density. A ratio, α(r), of the density fluctuation produced by the correlation within r to that to the infinity presented a new aspect of the density fluctuation. Molecular dynamics simulation has also been performed in order to investigate a molecular basis of the experimental radial distribution functions. The calculated GN(r)’s were in good agreement with experimental ones at each density. In laboratory space, a variety of orientational coordination structures are almost equally found in the fluid at all densities examined, while T-shaped structure is preferentially present in polar-angle space.
Phonon, π electron localization and size of the charge carrier in para-phenylenevinylene oligomers and polymer: A spectroscopic study
Authors: Zerbi G. , Galbiati E. , Gallazzi M. C. , Castiglioni C. , Del Zoppo M. , Schenk R. , Müllen K.
Published: July 2002

The vibrational infrared and Raman spectra of a series of t-butyl substituted oligomers of para-phenylenevinylene and of the corresponding polymer in the pristine and potassium-doped states have been recorded for the first time and interpreted on the basis of the theory of the effective conjugation coordinate. The Raman spectra of selectively deuteriated materials have also been analyzed. Evidence is found of strong phonon confinement. The information derived from the spectra is that the charge carrier is confined over a few monomer units and that when the polymer is doped with electron donors the benzene ring is preferentially perturbed. The data collected in this work are compared with the results from UV spectroscopy and ESR experiments which indicate a larger dimension of the charge carrier. The information which can be derived by using in a complementary way the data from these spectroscopies are presented and discussed.
High resolution infrared spectra of H2–Ar, HD–Ar, and D2–Ar van der Waals complexes between 160 and 8620 cm−1
Authors: McKellar A. R. W.
Published: July 2002

Spectra of weakly bound hydrogen–argon complexes have been studied at high spectral resolution (0.04–0.10 cm−1) using a long-path (154 m), low-temperature (77 K) absorption cell and a Fourier transform infrared spectrometer. The observations cover a wide spectral range from the far-infrared D2–Ar S0(0) band at 180 cm−1 to the near-infrared H2–Ar S2(1) band at 8600 cm−1. Compared to earlier studies, the new results have considerably improved resolution and accuracy. They also extend to new regions, namely the first overtone band of H2 and the pure rotational band of H2 and D2, and they include weak transitions involving excitation of the van der Waals stretching motion. These data serve as a basis for determining a greatly improved three-dimensional intermolecular potential energy surface for the hydrogen–argon system in the following paper.
Mobilities of Li+ in Ne and in N2 and Na+ in SF6: Effect of inelastic energy loss
Authors: Iinuma K. , Iizuka M. , Ohsaka K. , Satoh Y. , Furukawa K. , Koike T. , Takebe M.
Published: July 2002

Ion mobility measurements were made for Li+ in Ne and in N2 and Na+ in SF6 using a variable-temperature drift tube. The measurements were made by two different methods: variable-E/N method at constant T and variable-T method at constant E/N, where E, N, and T are the electric field strength, the gas number density, and the gas temperature, respectively. Two datasets were compared at the same effective temperature Teff, as calculated from the Wannier equation. For Li+ in Ne, the two datasets are put on a single curve in the range 150 K<Teff<10 000 K, indicating that the scaling rule holds well. However, for Li+ in N2 and Na+ in SF6 there exists a mobility difference between the two datasets, which is attributed to inelastic energy loss in the variable-E/N measurement. Using the modified Wannier equation developed by Viehland et al., we determined the inelastic energy loss factor for Na+ in SF6.
Can chirp enhance cumulative pre-resonant impulsive stimulated Raman excitation?
Authors: Hiller Eric M. , Cina Jeffrey A.
Published: July 2002

Simple arguments are presented and numerical calculations are performed which show that frequency chirp increases the amplitude of vibrational motion induced in the ground state of I2 by optimized sequences of ultrashort, pre-resonant optical pulses. Sequences with a variety of constraints on pulse chirp rates and pulse center frequencies were generated by time local optimization and are compared. A sequence with pulse-by-pulse variable chirp and variable center frequency is shown to be considerably more effective than the most effective sequence with a fixed negative chirp rate and constant pulse center frequency, which is in turn much more effective than a sequence of unchirped fixed-center-frequency pulses considered previously. The best sequence of pulses induces a 0.4 Å circuit in the expectation value of the I2 stretch from cumulative pre-resonant impulsive stimulated Raman scattering.
Fourier transform infrared spectroscopy of the ν2 vibration of BC2 in Ar at 10 K
Authors: Presilla-Márquez J. D. , Larson C. W. , Carrick P. G. , Rittby C. M. L.
Published: July 2002

The products of vaporization of boron/carbon mixtures around 3000 K were trapped in argon matrices at 10 K and their Fourier transform infrared spectra were measured. Analysis of the spectra combined with the predictions of density functional theory (DFT) calculations have resulted in the assignment of a previously observed vibration at 1194.4 cm−1 to the ν2 fundamental of cyclic BC2, which is effectively symmetric. The assignment is supported by 10B, 11B, and 13C isotopic data and is in good agreement with the theoretical predictions.
Microscopic view of glass transition dynamics: A quasielastic neutron scattering study on trans-1,4-polychloroprene
Authors: Kanaya T. , Kawaguchi T. , Kaji K.
Published: July 2002

We have studied the glass transition dynamics of trans-1,4-chloroprene from microscopic view points using a quasielastic neutron scattering technique in a time range of ∼4×10−13 to ∼4×10−10 s. It was found that the so-called fast process of picosecond order appears at around the Vogel–Fulcher temperature T0, similarly to cis-1,4-polybutadiene having no large side groups [J. Chem. Phys. 98, 8262 (1993)]. It is considered that the onset temperature at around T0 must be characteristic to polymers having no large side groups or no large internal degrees of freedom. In addition to the fast process, the slow process of subnanosecond order sets in at around the glass transition temperature Tg and the activation energy of the relaxation time was found to be ∼2.5 kcal/mol. The nature of the slow process is discussed in terms of conformational transition near Tg.
Photodynamics of charge transfer and ion-pair states of Cl2:Xe complexes in liquid Ar
Authors: Hill M. H. , Apkarian V. A.
Published: July 2002

Sequential two photon studies of Xe:Cl2 complexes in liquid Ar are reported. The first photon prepares the excited state complex Xe:Cl2(3Πu), which radiatively relaxes with a lifetime of 10±1 μs (22±2 μs for the uncomplexed Cl2(3Πu)). A second photon accesses either the molecular ion pair state, Xe:Cl+Cl−, or the contact charge transfer transition, Xe+Cl−2. The latter dissociates to Xe+Cl−+Cl even though the product is formed at threshold. The former undergoes charge rearrangement and decays radiatively via Xe+Cl−2 (2Σu)→XeCl2(1Σg). Analysis of the excitation and emission spectra of the key intermediate in laser induced harpoon reactions, Xe+Cl−2, leads to the conclusions that the complex is linear in both ionic and neutral states, and allows a direct contrast between dissociation dynamics on ionic versus neutral surfaces.
Conservation of the Kr+(2P1/2) state in the reactive quenching of Kr(5s′[1/2]) atoms by halogen-containing molecules
Authors: Zhong D. , Setser D. W. , Sobczynski R. , Gadomski W.
Published: July 2002

The second metastable state of Kr, 5s′[1/2]0, was generated from the first metastable state, 5s[3/2]2, by optical pumping in a flow reactor. Under optimum conditions, the entire Kr(5s[3/2]2) concentration could be removed with more than 40% conversion to the Kr(5s′[1/2]0) state, which is stable in He carrier gas. The Kr(5s′[1/2]0) and Kr(5s[3/2]2) states have the Kr+(2P1/2) and Kr+(2P3/2) ion cores, respectively, as do the Kr+X−(D) and Kr+X−(B,C) states. The reactions of a series of fluorine-, chlorine-, and bromine-containing molecules, RX, with the Kr(5s′[1/2]0) and Kr(5s[3/2]2) atoms were studied by measuring the total quenching rate constants and by observing the KrX(B,C,D) product emission spectra. In contrast to the Kr(5s[3/2]2) atoms, which give KrX(B and C) products, the Kr(5s′[1/2]0) atoms have a high propensity to give KrX(D) plus a lesser amount of KrX(B), depending on the reagent, as products. Discrimination against KrX(C) formation by reactions of Kr(5s′[1/2]0) atoms is severe. The reactions with F2, NF3, and N2F4 exhibit the highest conservation of the Kr+(2P1/2) core, and these Kr(5s′[1/3]0) reactions give ≳70% KrF(D). The total quenching constants of Kr(5s′[1/2]0) atoms generally are equal to those for Kr(5s[3/2]2), but the branching fractions for KrX* formation from Kr(5s′[1/2]0) atoms generally are smaller than for Kr(5s[3/2]2) atoms. A correlation diagram based on conservation of Kr+ ion-core state and Ω=0− is developed to discuss these trends and the reactions of the Xe(6s′[1/2]0 and 6s[3/2]2) atoms. Due to the absence of KrX(C–A) emission from the Kr(5s′[1/2]0) atom reactions, the KrX(B–A) and Kr(D–A) transitions could be observed and the radiative branching ratios to the X and A states were assigned.
Layer-thinning transitions near the smectic-A-isotropic transition in liquid-crystal films
Authors: Kranjc T. , Žumer S.
Published: July 2002

A qualitative explanation of a novel melting phenomenon, a layer-by-layer thinning transition, recently discovered in free-standing partially fluorinated liquid-crystal films, is looked for using a phenomenological Landau free energy density formalism. In the thinning process, the number of layers in a particular film-thickness jump depends on the initial number of layers; the temperature at which a jump occurs also depends on the initial thickness and increases with decreasing number of initial layers. The results of the calculation are in qualitative agreement with the experimental studies of the layer-thinning transitions above the bulk Sm-A-isotropic transition.
Spectroscopic constants and potential energy surfaces for silanone (H2SiO), hydroxysilylene (HSiOH), the hydroxysilylene dimer, and the disilynyl radical (Si2H)
Authors: Ma Buyong , Allinger Norman L. , Schaefer Henry F.
Published: July 2002

Ab initio quantum mechanical methods were employed to study the spectroscopic constants and potential energy surfaces of H2SiO, HSiOH, the HSiOH dimer, and the Si2H radical. Consideration of the spectroscopic constants of silanone, cis- and trans-HSiOH and Si2H began with the TZ2P SCF level of theory. We predict a strongly bonded cis-HSiOH dimer. The structure of the cis-HSiOH dimer was optimized at the DZP SCF, DZP CISD, DZP+diff CISD and DZP MP2 levels. The hydrogen bond energy of the dimer is 14.8 kcal/mol at the DZP MP2 level and 12.0 kcal/mol at the DZP CCSD/DZP CISD level. The vibrational frequency of one Si–O bond stretch in the HSiOH dimer is 967 cm−1 at the DZP MP2 level, close to the 951 cm−1 and 986 cm−1 fundamentals observed experimentally for HxSiyOz aggregates. Therefore, it is possible that the HSiOH dimer has been observed in matrices. The potential energy surface of the Si2H radical was studied initially at the DZP CISD level. We found a bent Cs 2A″ Si2H structure which is 10.8 kcal/mol higher in energy than the C2v 2B1 structure. The C2v Si2H structures were optimized at the TZ2P (f,d) CCSD level. The 2B1 state is predicted to lie ΔE0=1.6 kcal/mol lower in energy than the 2A1 state of Si2H radical.
An ab initio study of the internal conversion rate from the first singlet excited state to the ground state in formaldehyde
Authors: Nakajima Tohru , Kato Shigeki
Published: July 2002

The nonradiative transition rates from the single vibronic levels of the first singlet excited state to the ground state were estimated using a time-dependent method based on Fermi’s golden rule. In the present method, the initial wave packet is constructed with the use of the nonadiabatic coupling matrix elements calculated by ab initio molecular orbital method. The wave packet dynamics calculation is carried out using the reaction path Hamiltonian. The vibrational relaxation on the ground state surface is treated by introducing the effective Hamiltonian. The parameters required to construct these Hamiltonians were obtained with the complete active space self-consistent field wave function and the electronic matrix elements of nonadiabatic coupling between the ground and first singlet excited states were calculated with the state-averaged complete active space self-consistent field wave function analytically. The calculated rate constants were in good agreement with the experimental ones. It is found that vibrational relaxation in the ground electronic state is an important factor in obtaining the nonradiative transition rate constants.
Phase space compression and entropy of nonequilibrium steady states
Authors: Baranyai András
Published: July 2002

We show that the variational principle of Evans and Baranyai (Phys. Rev. Lett. 67, 2597 (1991)] can be derived from a macroscopic entropy definition of nonequilibrium steady states. This definition claims that the entropy of nonequilibrium steady states is the sum of the equilibrium entropy of the system under the same external conditions (fixed internal energy, E, volume, V, and number of particles N), and an additional term which can be measured by the minimum excess work performed on the system by the external field, Fe, by bringing it from its equilibrium state to the final steady state.
Collision-energy/electron-energy resolved two-dimensional study of Penning ionization of Ar by He metastable atoms 23Sand 21S
Authors: Ohno Koichi , Yamakado Hideo , Ogawa Tetsuji , Yamata Toshiaki
Published: July 2002

Selection of collision energies by a time-of-flight method has been combined with Penning ionization electron spectroscopy by utilizing a multichannel scaler and 2 MB random access memory. This technique provides a two-dimensional Penning ionization electron spectrum (2D-PIES) in which the Penning electron intensity is observed as a continuous function of both collision energies and electron energies. The observed 2D-PIES for Ar/He* (23S and 21S) was compared with calculated 2D-PIES obtained by classical trajectory calculations based on interaction potentials and transition probabilities.
Hydrogen abstraction from methane by Li doped MgO: A periodic quantum mechanical study
Authors: Orlando R. , Corà F. , Millini R. , Perego G. , Dovesi R.
Published: July 2002

The catalytic abstraction of hydrogen from methane on the (001) surface of Li-doped MgO has been studied at the ab initio level within the periodic unrestricted Hartree–Fock approximation. The catalyst is modeled with an infinite slab, periodic in two dimensions and of finite thickness. Active sites for the reaction are surface O− radical ions resulting from coupling with the Li+ dopants; in the periodic model they are regularly distributed upon the surface. At 0 K the energy difference between reactants and products is close to zero, but a potential barrier of 18 kcal/mol is found to reach the transition state. Relaxation of ions in the zone of the Li+/O− couple stabilizes the reaction products, and plays a fundamental role in the energy balance.
The infrared spectroscopy and dynamics of OCO–HCl and SCO–HCl: An example of mode specific intermolecular energy transfer
Authors: Oudejans L. , Olson D. , Miller R. E.
Published: July 2002

Optothermal near infrared laser spectroscopy has been used to study the OCO–HCl and SCO–HCl complexes by exciting the H–Cl stretch using an F-center laser. In both cases, the two isotopic forms associated with H35Cl and H37Cl have been observed. All of the observed spectra are consistent with and analyzed in terms of a linear structure. Vibrational predissociation is observed to be abnormally fast in the OCO–HCl complex, considering the rather weak coupling between the intermolecular degrees of freedom and the H–Cl stretch suggested by the small vibrational frequency shift associated with complex formation. Comparisons are made between the two systems studied here that indicate that this anomalous rate is due to a near resonance between the energy available to the fragments after the photodissociation of the complex and the asymmetric stretch of the CO2 fragment.
Resonance Raman spectrum of HOCl—A time-dependent quantum dynamical treatment
Authors: Offer Alison R. , Balint-Kurti Gabriel G.
Published: July 2002

The emission spectrum of the dissociating HOCl molecule, following the 2 1A′←X̃ 1A′ excitation, is computed using a rigorous time-dependent quantum mechanical method. The theory needed to include all the polarization and angular momentum effects within the time-dependent framework is developed and applied for the first time. Detailed predictions are made of the emission or resonance Raman spectrum of the dissociating molecule both ‘‘on resonance,’’ near the peak of the first absorption band and ‘‘off resonance,’’ in the tails of the band. If the exciting radiation is linearly polarized, and the direction of polarization of the emitted radiation is also measured, then four different ‘‘cases’’ or geometric arrangements of the two polarization directions relative to each other can be identified. The different signals which result are computed and discussed. Expressions are given for the Raman amplitudes as a product of a dynamic and a geometric factor.
Dissociation energies of the molecules Bcl and Bcl+
Authors: Hildenbrand D. L.
Published: July 2002

The molecule Bcl was generated by chlorination of B(s) with both MgCl2 and SrCl2 in a heated effusive beam source, and identified by mass spectrometry. Gaseous equilibria in the B–Mg–Cl and B–Sr–Cl systems were studied in the regions 1350–1600 K and 1760–1830 K, respectively, and the dissociation energy D0°(Bcl)=121.3±1.1 kcal mol−1 (5.26±0.05 eV) was derived from thermochemical analysis. When compared with a dissociation limit derived from a short upper state extrapolation, this result indicates a potential hump of about 6 kcal mol−1 in the A 1Π state of Bcl. Electron impact measurements yielded an ionization potential of 10.03±0.10 eV for Bcl, which can be used to evaluate D0°(Bcl+)=81.4±2.6 kcal mol−1 (3.53±0.11 eV). Other values in the literature, some of which differ considerably from the new results, are discussed.
Molecular dynamics study of Na+ and Cl− in methanol
Authors: Sesé G. , Guàrdia E. , Padró J. A.
Published: July 2002

Molecular dynamics simulations of solutions made up by one ion in methanol have been performed. The ions under study have been Na+ and Cl−. Structural and dynamical data as well as the dynamics of solvation have been analyzed. Both translational and reorientational motions of solvent molecules have been studied. An analysis of the solvent response to instantaneous changes of the electrical distribution of the solute in the linear response approximation has been undertaken. Special attention has been paid to differences between solvent molecules in the first shell and in the bulk, which happen to be more important in the Na+ shell. The influence of the ionic mass on the solvent properties has also been studied.
Statistical mechanics of microemulsions: Droplet phases and macroscopic interfaces
Authors: Palmer Kirk M. , Morse David C.
Published: July 2002

We discuss the statistical mechanics of a droplet microemulsion within the context of the Canham–Helfrich model of interfacial elasticity. We focus on the behavior of Winsor I and Winsor II states in which a dilute droplet phase coexists with an excess phase of the dispersed fluid. By taking into account fluctuations of the size, position, and shape of the microemulsion droplets on an equal footing, our treatment resolves existing ambiguities regarding the correct definition of the conformational entropy within such a droplet phase, and thus allows quantitative predictions to be made for, e.g., the dependence of average droplet size and interfacial tension upon the elastic parameters of the interfaces. Our results for the droplet phase may be summarized in terms of a phenomenological model in which, if the renormalized bending energy of a spherical droplet is used as the statistical weight for droplets of constrained size and center-of-mass position, then a length scale of order of the thermal roughness of a droplet’s surface due to shape fluctuations appears as the natural measure of surface displacements used to calculate the excess entropy arising from variations of droplet radius (i.e., polydispersity) and position. The renormalized bending energy of a droplet is found to contain contributions arising from the renormalization of both the mean and Gaussian bending rigidities, and from a separate renormalization of the bending torque, or the spontaneous curvature, of the monolayer. We also reconsider the interpretation of experiments probing the fluctuations of a macroscopic interface such as that separating the microemulsion and excess phases in such systems, and discuss the effect of fluctuations upon the measured interfacial tension of systems with very low tensions. We show that the fluctuations of this interface can be described in terms of a renormalized bending rigidity κR and surface tension γ that both depend upon a crossover length ξ∼√κR/γ and upon a microscopic cutoff length, but that, in contrast to the wave number dependencies predicted in some earlier studies, are both essentially independent of the wavelength of the fluctuations.
Bounds for the zeroth-order exchange-energy functional for atomic systems
Authors: Tao Jianmin , Li Guobao
Published: July 2002

Bounds for the zeroth-order exchange-energy functional for atomic systems are derived using the generalized Carlson’s inequality. They are formulated entirely in terms of the average electron density 〈ρ〉, the expectation value 〈r−1〉, and the number of electrons N, viz., (3/4)(3/π)1/3N2/3〈ρ〉1/3⩾|K0[ρ]|=(3/4)(3/π)1/3∫ρ4/3(r)dr⩾(1/18π5)1/3〈r−1〉2/〈ρ〉1/3. The quality of the bounds presented are numerically analyzed and discussed. Atomic units are used throughout.
Control of reactant flow at the boundary: Its effect on the onset of nonequilibrium macroscopic structures in chemical reactions
Authors: Izús G. G. , Ramírez O. , Deza R. R. , Wio H. S.
Published: July 2002

In a one-dimensional, two-component excitable chemical model system (the ‘‘Brusselator’’) we investigate the effects of controlling the flow of reactants at the boundary, on the stability of the thermodynamical branch. The reflectivity of the boundary adjusts the external flux and acts as a control parameter, capable of enhancing or weakening the self-organizing processes leading to the appearance of temporal or spatial patterns.
Ab initio and scaled potential energy surfaces for Ar–C2H2: Comparison with scattering and spectroscopic experiments
Authors: Yang Moonbong , Alexander Millard H. , Werner Hans-Joachim , Bemish R. J.
Published: July 2002

New coupled-cluster ab initio potential energy surfaces (PES’s) were determined for the interaction of Ar with a rigid acetylene molecule. These PES’s were in addition modified by scaling the correlation energy. Based on both the original and scaled PES’s, close-coupled calculations of the total differential scattering cross section were carried out. Rovibrational energy levels of the Ar–C2H2 complex were computed variationally. In addition, we simulated the ir spectra corresponding to excitation of the upper diad of the ν3/ν2+ν4+ν5 excited molecular vibrational states. The comparison of all these quantities with experiment shows generally good agreement for the several scaled PES’s. In addition, the sensitivity of the PES to the experimental data are investigated by varying the scaling factor. The original and scaled PES’s are also compared with several phenomenological PES’s and a previously published ab initio PES [F.-M. Tao, S. Drucker, and W. Klemperer, J. Chem. Phys. 102, 7289 (1995)].
Vector and scalar correlations in statistical dissociation: The photodissociation of NCCN at 193 nm
Authors: North Simon W. , Hall Gregory E.
Published: July 2002

Nascent Doppler profiles of CN (X 2Σ+) fragments from the 193 nm photodissociation of NCCN have been measured using high-resolution transient frequency modulated (FM) absorption spectroscopy. This new method is highly suited for Doppler spectroscopy of nascent photoproducts. The experimental line shapes suggest an asymptotic available energy of 5300±100 cm−1 and are well described by a model in which the available energy is partitioned between a statistical reservoir (4700 cm−1) and a modest exit barrier (600 cm−1). We have determined state dependent v⋅j correlations. A trend of j becoming increasingly perpendicular to v for the higher rotational states is in accord with phase space theory, although the observed correlations are more than twice as strong. The v⋅j correlations can be quantitatively modeled by further restricting the phase space model with an approximate conservation of the K-quantum number, the projection of total angular momentum about the linear axis of NCCN. Global rotational and vibrational product distributions have also been measured. The highest accessible rotational states are underpopulated, compared to a phase space calculation. The global vibrational distribution is substantially colder than the phase space theory predictions. Vibrational branching ratios for coincident fragments have been measured as a function of the detected CN state from a close analysis of high signal-to-noise Doppler profiles. The correlated vibrational distribution, P(v1,v2), shows an excess of vibrationless coincident fragments, at the expense of dissociation to give one ground state and one vibrationally excited CN fragment. The correlated formation of two vibrationally excited CN fragments is as likely as the phase space prediction, yet the formation of v=2 is strongly suppressed. The fragment vector and scalar correlations provide a highly detailed view of the loose transition state typical for reactions well described by statistical reaction theories.
Two-color resonant four-wave mixing: Analytical expressions for signal intensity
Authors: Williams Skip , Rohlfing Eric A. , Rahn Larry A. , Zare Richard N.
Published: July 2002

We present analytical signal expressions for each two-color resonant four-wave mixing (TC-RFWM) scheme that can be used for double-resonance molecular spectroscopy in the limit of weak fields (no saturation). The theoretical approach employs time-independent, diagrammatic perturbation theory and a spherical tensor analysis in an extension of recent treatments of degenerate four-wave mixing [S. Williams, R. N. Zare, and L. A. Rahn, J. Chem. Phys. 101, 1072 (1994)] and TC-RFWM for the specific case of stimulated emission pumping [S. Williams et al., J. Chem. Phys. 102, 8342 (1995)]. Under the assumption that the relaxation of the population, the orientation, and the alignment are the same, simple analytic expressions are derived for commonly used experimental configurations. The TC-RFWM signal is found to be a product of a concentration term, a one-photon molecular term, a line shape function, and a laboratory-frame geometric factor. These expressions are intended to facilitate the practical analysis of TC-RFWM spectra by clarifying, for example, the dependence on beam polarizations and rotational branch combinations.
On the magnetic coupling in NiO
Authors: de Graaf C. , Illas F. , Broer R. , Nieuwpoort W. C.
Published: July 2002

The results are reported of ab initio calculations on the magnetic ordering in NiO, a prototype of the antiferromagnetic insulator. By analyzing wave functions for different cluster models, information is obtained about the physical effects determining the sign and the magnitude of the magnetic coupling parameter J. The role of the edge oxygens, surrounding the essential unit (Ni2O), is found to be quantitatively important but purely environmental in contrast to the role of the bridging oxygen. Furthermore, the importance of electron correlation and the usefulness of pseudopotentials in the calculations is investigated. The final result for J compares reasonably with experiment (about 50%), and possible sources for the remaining discrepancies are discussed.
Rotational–translational electrolyte friction on nonspherical particles
Authors: Hernández-Contreras M. , Alarcón-Waess O. , Medina-Noyola M.
Published: July 2002

We present and apply an approximate general theory for the effects of the direct interactions on the translational and rotational diffusion coefficients of a nonspherical Brownian particle interacting with other spherical particles diffusing around it. These results are specialized to a system formed by a charged axially-symmetric macroparticle that diffuses in an ionic solution, whose static properties are described within the Debye-Hückel approximation. In particular, we calculate the static friction coefficients of a Brownian point-dipole embedded in a hard-sphere, and of a charged hard-ellipsoidal tracer particle.
Vapor–liquid coexistence of quasi-two-dimensional Stockmayer fluids
Authors: Gao G. T. , Zeng X. C. , Wang Wenchuan
Published: July 2002

A quasi-two-dimensional (2D) Stockmayer model is developed in which the center of mass of the molecule is confined on a plane while the dipole of the molecule can rotate freely in three dimensional space. This model entails essential characteristics of systems such as dipolar molecules physisorbed on a solid surface, or a Langmuir monolayer consisting of short-chain molecules with a dipolar tail. The Gibbs ensemble Monte Carlo technique is employed to determine the vapor–liquid equilibria of the model fluids. An Ewald sum for this quasi-2D model is formulated to account for the long-range dipolar interactions. Three systems with different reduced dipole moments were studied. The critical point of each system is determined by fitting the vapor–liquid coexistence data to a 2D scaling law and the rectilinear law. We find that in general the critical temperature of the system is reduced due to the confinement and is sensitive to the strength of the dipole moment, whereas the critical density is not. The effect of reducing the dispersion part of potential on the vapor–liquid equilibria is also studied. We find the dispersion potential reduction leads to a lower critical temperature and a higher in-plane part of molecular dipole moment; however, because the reduced critical temperature is relatively small compared with that of a 3D system, disappearance of the critical point is not observed in the quasi-2D SM system within practical scope of the simulation.