bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarization relaxation, dielectric dispersion, and solvation dynamics in dense dipolar liquid
Authors: Bagchi Biman , Chandra Amalendu
Published: July 2002

A unified treatment of polarization relaxation, dielectric dispersion and solvation dynamics in a dense, dipolar liquid is presented. It is shown that the information of solvent polarization relaxation that is obtained by macroscopic dielectric dispersion experiments is not sufficient to understand dynamics of solvation of a newly created ion or dipole. In solvation, a significant contribution comes from intermediate wave vector processes which depend critically on the short range (nearest-neighbor) spatial and orientational order that are present in a dense, dipolar liquid. An analytic expression is obtained for the time dependent solvation energy that depends, in addition to the translational and rotational diffusion coefficients of the liquid, on the ratio of solute–solvent molecular sizes and on the microscopic structure of the polar liquid. Mean spherical approximation (MSA) theory is used to obtain numerical results for polarization relaxation, for wave vector and frequency dependent dielectric function and for time dependent solvation energy. We find that in the absence of translational contribution, the solvation of an ion is, in general, nonexponential. In this case, the short time decay is dominated by the longitudinal relaxation time but the long time decay is dominated by much slower large wave vector processes involving nearest-neighbor molecules. The presence of a significant translational contribution drastically alters the decay behavior. Now, the long-time behavior is given by the longitudinal relaxation time constant and the short time dynamics is controlled by the large wave vector processes. Thus, although the continuum model itself is conceptually wrong, a continuum model like result is recovered in the presence of a sizeable translational contribution. The continuum model result is also recovered in the limit of large solute to solvent size ratio. In the opposite limit of small solute size, the decay is markedly nonexponential (if the translational contribution is not very large) and a complete breakdown of the continuum model takes place. The significance of these results is discussed.
Direct and precursor dynamics in dissociative hydrogen chemisorption on Ni(100)
Authors: Zhu X.-Y. , Castro M. E. , White J. M.
Published: July 2002

We report evidence for the coexistence of direct and precursor dynamics in the dissociative chemisorption of H2 on Ni(l00). Hydrogen and deuterium uptakes on Ni(l00) were measured at various surface temperatures by following the secondary ion ratios, Ni2 H+/Ni+ and Ni2 D+/Ni+ , which are proportional to surface hydrogen and deuterium coverage on both clean and carbon-covered Ni(100). Between 100 and 200 K on clean Ni(l00), the initial sticking coefficient of hydrogen decreases, but only slightly, as the surface temperature increases. The decrease is more pronounced both in the presence of predosed carbon and for deuterium adsorption on Ni(100). This is interpreted as due to the involvement at low temperatures of a molecular precursor which mediates dissociative adsorption at low temperatures. The precursor probably involves surface defects and sites formed in the presence of carbon. In addition to the precursor channel, a direct dissociation channel also operates, and dominates for T≥200 K.
Observation of UF−6 in Xe matrices
Authors: Sherrow S. A.
Published: July 2002

Infrared spectra of UF6 isolated in water-doped xenon matrices exhibit a new absorption after UV photolysis. The absorption is attributed to UF−6 with a self-trapped hole as the counterion. The species is not produced when UF6 in pure xenon matrices is irradiated. Because direct participation of water is not observed, it is proposed that the water impurity affects the photoprocesses occurring in pure xenon.
Effect of elongational flow on the orientational order phase transitions and viscosity of hard rod fluids
Authors: Wang Shi-Qing , Gelbart William M.
Published: July 2002

We consider the generalized Smoluchowski equation for the orientational distribution of interacting hard rod-like particles. Using the Onsager theory of excluded volume in these systems, we solve for steady-state solutions to this equation in the special case of elongational flow. Accordingly, we obtain explicit predictions concerning the effect of such flow on the suppression of orientional ordering phase transitions and on the nonlinear behavior (dependence on velocity gradient) of the viscosity.
Interatomic potentials for van der Waals complexes of group 13 metal atoms: AlAr, AlKr, and AlXe
Authors: Callender C. L. , Mitchell S. A. , Hackett P. A.
Published: July 2002

Interatomic potential parameters for the ground X1,2 2Π1/2,3/2 and excited B 2Σ+1/2 states of jet-cooled van der Waals complexes of Al atoms with rare gases have been determined from fluorescence excitation and emission spectra. Vibrational numbering in the B states is established from isotopic and rotational broadening of fluorescence excitation line shapes. Bond distances are estimated by comparison with analogous states of alkali–rare gas molecules, and by modeling observed relative emission intensities with calculated Franck–Condon factors. Morse potentials are found to adequately describe the data for the ground states, but for the excited states there are indications of departures from Morse functions at large internuclear distances. Dissociation energies De are estimated from Birge–Sponer extrapolations. Multiplet splittings in Al–rare gas complexes are compared to those in In–rare gas complexes and are discussed in terms of an empirical treatment of spin–orbit coupling, which assumes that the spin–orbit coupling constant for the molecule is the same as that for the atom. The ground X1 state of each Al–rare gas molecule is more stable than that of the analogous In complex, whereas the spin–orbit excited X2 state is less stable. This is explained by the greater mixing between the X1 state and the dissociative A 2Σ+1/2 state for indium over aluminum, caused by the greater spin–orbit coupling constant for the heavier metal.
Rotational predissociation of H2O(C 1B1) studied by multiphoton ionization spectroscopy in a supersonic free jet
Authors: Kuge H.-H. , Kleinermanns K.
Published: July 2002

The predissociation of cold water molecules (17–75 K) is investigated in a supersonic free jet after excitation to the C 1B1 state using simultaneous absorption of three laser photons in the range 369–374 nm. Absorption of a further photon in this wavelength range ionizes the water molecule (X 1A1 (0,0,0)→C 1B1 (0,0,0)→H2O+(X); (3+1)-multiphoton ionization spectrospcopy). The line half-widths and intensities in the rotationally resolved spectrum show a pronounced dependence on the parent rotational excitation in the C state. Supersonic cooling simplifies the spectra drastically and reduces overlap of lines, so that a considerable number of individual linewidths could be measured. The linewidth measurements and spectrum simulations show excitation of a-axis rotation to be dominant. a-axis parent molecular rotation couples the excited C 1B1 state to an unbound state of A1 electronic symmetry. The linewidth measurements point to a weaker Coriolis coupling between the C 1B1 state and a 1A2 state of H2O via rotation around the c axis. According to ab initio calculations, a water molecule in this 1A2 state dissociates essentially without barrier to O(1D)+H2. This process may explain production of nearly 10% O(1D) atoms in the wavelength range 123–125 nm.
Body frame close coupling wave packet approach to gas phase atom–rigid rotor inelastic collisions
Authors: Sun Y. , Judson R. S. , Kouri D. J.
Published: July 2002

The close coupling wave packet (CCWP) method is formulated in a body-fixed representation for atom–rigid rotor inelastic scattering. For total J>jmax the computational cost of propagating the coupled channel wave packets in the body frame scales approximately as N3/2 where N is the total number of channels. For large numbers of channels, this will be much more efficient than the space frame CCWP method previously developed which scales approximately as N2 under the same conditions. Timing results are reported for a model system for 25, 64, 144, 256, and 969 channels. The calculations were run with total J=30 and with parameters corresponding to a heavy diatom. The results for some representative transitions are compared to the identical transitions obtained using the space frame formalism. For all values of N, the body frame computations ran faster than the corresponding ones using the space frame, with the ratio increasing to a value of 4.5 for 969 channels. This is a significant improvement which will continue to increase with N, encouraging us to believe that the body frame CCWP method will prove practical for calculating scattering information for more realistic inelastic and reactive systems.
Atom–diatom reactive scattering. I. Quantum theory
Authors: Webster Frank , Light J. C.
Published: July 2002

We describe a time-independent quantum theory for atom–diatom reactive scattering using a procedure based upon a division of configuration space into three distinct chemical channels. Within each chemical channel, coordinate systems defined as transformations of Jacobi coordinates are described which allow the use of R-matrix propagation in solving for the unbound motion along a scattering coordinate. The remaining five degrees of freedom are treated variationally, making use of the efficient discrete variable representation to describe the vibration–rotation interaction. The scattering information from the three chemical channels is matched on the common boundary, and the full S matrix is obtained at fixed total energy, angular momentum, and parity. In the second paper of this series, accurate results for the reactions of H+H2 and its isotopomers will be presented.
The application of electrochemistry to the measurement of selected intrinsic properties of polyacetylene
Authors: Kaner R. B. , Porter S. J. , Nairns D. P. , MacDiarmid A. G.
Published: July 2002

Electrochemical techniques are applied to a study of selected intrinsic physical properties (independent of dopant) of polyacetylene, (CH)x. Electrochemical voltage spectroscopy (EVS) is used to characterize the energies of charge injection and removal for both cis-(CH)x and trans-(CH)x. The difference in charge injection and ejection potentials is shown to give a direct measure of the approximate semiconductor band gap and the results are compared with similar data from optical measurements. Charge injection and ejection potentials are used to define the redox potentials of (CH)x and are used in conjunction with equilibrium potential measurements at varying oxidation or reduction levels of polyacetylene to present a unifying concept to the electrochemistry of (CH)x.
Structural incompatibilities and liquid–liquid phase separation in molten binary silicates: A computer simulation
Authors: Kieffer John , Angell C. Austen
Published: July 2002

We have carried out a series of ion dynamics simulations on binary silicate systems ranging from MnO–SiO2, which shows a large liquid–liquid miscibility gap, to K2O–SiO2, which does not unmix even below the stable liquidus. We have used Coulomb attractions plus Born–Mayer–Huggins repulsive pairwise-additive potentials, evaluating the energies with a full Ewald summation. Energies of the systems were determined at a constant pressure using the Andersen algorithm. In the simulated systems (which are too small to permit phase separation) results show that in the MnO–SiO2 system the heats of mixing are indeed positive, and greater than any conceivable T⋅ΔS value, in the composition range 0.5<XSiO2 <1.0 in which phase separation occurs in practice. In K2O–SiO2, by contrast, ΔHm is always negative. Analysis of angular correlations as a function of temperature using new ‘‘circular distribution functions,’’ allows the origin of the unfavorable mixing energy to be determined in terms of structural/geometrical incompatibilities. At high silica contents the incorporation of smaller divalent cations, which seek four coordination, can only be realized by distorting the network from its lowest energy topology to provide the requisite nonbridging oxygens. On cooling, the divalent cations are seen to move out of these unfavorable sites as a first step towards phase separation on a macroscopic scale. In the case of large low-charge-density cations, a greater degree of bridging is retained by forming large gaps which can easily host the cations and even allow them to diffuse freely.
The positive muon in the intermetallic hydride ZrV2Hx: A muon tracer study supplemented by differential thermoanalysis, neutron vibrational spectroscopy, and quasielastic neutron scattering
Authors: Hempelmann R. , Richter D. , Hartmann O. , Karlsson E. , Wäppling R.
Published: July 2002

ZrV2 is an intermetallic compound with energetically different interstitial sites, which upon hydrogenation are successively filled with hydrogen (thermodynamic distribution). When a positive muon is added as a microscopic probe, it is thermalized at random and thus at low temperatures statistically distributed over the available (unblocked) interstices. By varying the preloading we scan the accessible hydrogen sites and find that at high hydrogen content three kinds of interstices are occupied, in contradiction to literature diffraction data. This surprising result is confirmed by a detailed neutron vibrational spectroscopy study on ZrV2Hx which is presented as well. The temperature dependence of the μ+ relaxation rate is textbook-like in empty ZrV2: static muon at T<50 K, complete motional narrowing at T>200 K. In ZrV2Hx, however, the relaxation rate exhibits an intermediate plateau which we attribute to a relocation of the muon. The motion of the muon above 150 K is interpreted as being directly related to the hydrogen diffusion which we have separately measured by means of quasielastic neutron scattering. In addition, we investigated the ZrV2/H phase diagram by differential thermoanalysis.
On the electronic structure and spectrum of CuF2 and CuCl2
Authors: Bauschlicher Charles W. , Roos Björn O.
Published: July 2002

The ground and low-lying states of CuF2 and CuCl2 have been studied using the single and doubles configuration interaction (SDCI) and coupled pair functional (CPF) methods in a large Gaussian basis set including up to g-type functions. The results include a determination of the bond distances for all the ligand field states (the three states with one hole in the 3d shell) and charge transfer (CT) states, and force constants for the ligand field states. Overall the SDCI (with or without the Davidson correction for higher excitations) and CPF results are in good agreement. The CPF results, which we believe to be the most accurate, can be summarized as follows. The computed value for the asymmetric stretching frequency in the 2Σ+g ground state of CuF2 is 740 cm−1, compared to the experimental value of 765 cm−1. The d–d transition energies for CuF2 are 2500 and 10 800 cm−1. The two lowest charge transfer states are at 31 200 and 33 900 cm−1, respectively. CuCl2 has been found to have a ground state which is an almost equal mixture of 2Σ+g and 2Πg when an estimate of the spin–orbit interaction is included. Three d–d transitions are found: 1600 cm−1 ( J=1/2), 7500 cm−1 (J=3/2), and 9700 cm−1 (J=5/2). The lowest charge transfer states have been computed to lie at 16 700 and 19 600 cm−1. Two bands have been found in the gas-phase spectrum of CuCl2 at 9000 and 19 000 cm−1, in good agreement with the present results. The computed  f value for the CT band is 0.017, to be compared to the experimental value of 0.014.
An absolute proton affinity scale in the ∼130–140 kcal mol−1 range
Authors: Adams N. G. , Smith D. , Tichy M. , Javahery G. , Twiddy N. D. , Ferguson E. E.
Published: July 2002

The dependences on temperature of the rate coefficients for the endothermic proton transfer reactions of HBr+ with CO2 and CH4 have been obtained in a variable-temperature selected ion flow tube. The measurements have been used to determine the 300 K proton affinity of CO2, P.A.(CO2), =128.5±1.0 kcal mol−1, utilizing the literature value of P.A.(Br)=131.8 kcal mol−1, obtained from the dissociation energy of HBr+, as a primary standard. The proton affinity difference between CO2 and CH4 has been substantiated by equilibrium constant measurements as a function of temperature for proton transfer between CO2 and CH4. Similar equilibrium constant measurements have been used to determine the proton affinities of HCl, N2 O, HBr, and CO, giving a proton affinity ladder ordered (in kcal mol−1) as CO(141.4), HBr(138.8), N2 O(137.3), HCl(133.0), Br(131.8), CH4 (130.0), and CO2(128.5). Proton affinities have also been determined for Br2(140.0), NO(127.0), and CF4(126.5), the last two values being obtained from selected ion flow drift tube measurements. An upper limit to P.A.(SF6) of 127 kcal mol−1 has been inferred from the instability of SF6 H+ towards dissociation into SF+5 and HF.
The vibrational relaxation of highly excited Freon-22 molecules
Authors: Tosa Valer , Labuda Serghei , Bruzzese Riccardo , de Lisio Corrado , Solimeno Salvatore
Published: July 2002

The V–T relaxation from Freon-22 molecules has been studied by using an interferometric technique. By raising the excitation energy from 500 cm−1 up to 3000 cm−1 (about three photons) a decrease of the effective relaxation time followed by a leveling off, has been observed. In agreement with this behavior, the average energy 〈ΔE〉 transferred per collision was found to depend linearly on 〈E〉 for extremal values of the energy absorbed, and stronger than linearly for intermediate energy values. This behavior parallels that observed in SF6 and other polyatomic molecules, and confirms the role played by the sparse and quasicontinuum regions of the vibrational spectrum.
Infrared spectra of diatomic polar molecules in rare-gas liquids. I. Spectral theory
Authors: Pérez J. , Luis D. , Hernández A. Calvo , Velasco S.
Published: July 2002

A theoretical model for the infrared (0–1) band of dilute solutions of diatomic polar molecules in nonpolar rare-gas liquids is presented. The model is based on decomposition of the rotational motion of the diatomic molecule into two limiting cases, according to the Bratos model: quasifree rotation and rotational diffusion. Contribution to the infrared absorption coefficient due to quasifree rotation is analyzed within a non-Markovian formalism using a stochastic directing intermolecular field (DIF) model to describe the diatomic molecule–solvent interaction. The P and R branches appear as a consequence of the quasifree contribution, which is also important in the Q-branch region. The contribution due to rotational diffusion is calculated making use of the Debye model and is mainly significant in the Q-branch region.
Spherically inhomogeneous fluids. I. Percus–Yevick hard spheres: Osmotic coefficients and triplet correlations
Authors: Attard Phil
Published: July 2002

For fluids in a spherically symmetric external field, it is shown that the Ornstein–Zernike convolution integral becomes a simple algebraic equation upon Legendre transformation. Applying the usual closure relations, the full inhomogeneous pair correlation functions become available. A discrete orthogonal transform pair is also derived, which, in conjunction with the Legendre factorization, makes computations feasable for these generic systems. The general method is here applied to a bulk uniform fluid of hard spheres, using the pair Percus–Yevick closure, but at the triplet level in the hierarchy of distribution functions. Consequently, the pair distribution function and the osmotic coefficient are better than the known analytic results. The method enables the accurate calculation of the triplet correlation function, and several examples are given.
Gaussian functions in Hylleraas-configuration interaction calculations. II. Potential curves for the b 3Σ+u and the e 3Σ+u states of hydrogen
Authors: Frye D. , Lie G. C. , Clementi E.
Published: July 2002

The Hylleraas-CI method with Cartesian Gaussian basis functions has been used to calculate the potential energy curve of the repulsive b 3∑+u state of the hydrogen molecule at internuclear distances R ranging from 1.0 to 12.0 bohrs. At small R, sizable improvements over the best previous calculations using elliptical coordinates are reported. The potential curve for the bound e 3∑+u state is also reported, as a by-product since it compares very well with the best conventional CI results.
Correlation of molecular valence- and K-shell photoionization resonances with bond lengths
Authors: Sheehy J. A. , Gil T. J. , Winstead C. L. , Farren R. E. , Langhoff P. W.
Published: July 2002

A theoretical study of the relationship between interatomic distances and the spectral positions of valence- and K-shell σ* photoionization resonances is reported for a selected series of molecules. Three-dimensional graphical representations of the occupied and virtual-valence σ-symmetry orbitals of these compounds reveal their striking similarity to the wave functions of a particle in a cylindrical well, substantiating qualitative notions long employed in free-electron molecular orbital (FEMO) approximations. Accordingly, the molecular potential along the symmetry axis in these compounds is modeled after a finite square well, with a depth approximately equal to the energy of the lowest σ-symmetry valence molecular orbital and a width determined from analogies to FEMO theory. Calculated minimal-basis-set molecular-orbital energies for both occupied and virtual states are seen to correlate accurately with the simple square-well energy level formula (π2 /2)(n2/l 2 ) when measured in Hartree atomic units from the bottom of the well. The calculated σ* orbital energies are furthermore in excellent agreement with experimentally and theoretically determined valence-shell photoionization resonance positions, the latter consequently also satisfying the square-well correlation formula. A similar situation obtains for experimentally and theoretically determined K-shell resonance positions, although energy shifts from minimal-basis values are evident in these cases. These circumstances are clarified quantitatively on basis of Feshbach–Fano considerations, in which minimal-basis-set virtual-valence σ* orbitals play the roles of zeroth-order states subject to modification by interactions with nonresonant background continua. Concluding remarks contrast and compare molecular-orbital and square-well approaches to photoionization resonances with those based on multiple-scattering and barrier models. The present results appear to clarify the origins of recently reported empirical correlations of bond lengths with resonance positions, and help to determine their range of applicability.
Kinetic energy dependence of partial cross sections for the collisional ionization of H2O, H2S, O2, and Ar with He(2 3S) metastable atoms
Authors: Mitsuke Koichiro , Takami Tomohide , Ohno Koichi
Published: July 2002

The kinetic-energy dependences of partial cross section σ(E) for the collisional ionization of H2O, H2S, O2, and Ar with He(2 3S) metastable atoms are measured in the energy range of 40–300 meV by collision-energy-resolved electron spectroscopy. The σ(E) functions are markedly dependent on the system and, in some cases, on final ionic states because of the anisotropy of incoming interaction potential surfaces or potential surface crossings with other potentials. Negative energy dependences of σ(E) are obtained for H2O+ (X 2B1, A 2A1, B 2B2) and H2S+ (X 2B1, A 2A1, B 2B2), whereas positive energy dependences for O+2 (a 4Πu, b 4Σ−g) and Ar+ (2P3/2). The σ(E) curves for O+2 (X 2Πg, A 2Πu) produced by Penning ionization via the ion-pair potential of He+(2S)+O−2(2Πg) are almost independent of the collision energy. The results are interpreted in terms of the semiclassical scattering theory of Penning ionization with assuming appropriate function forms to the potential and the transition rate.
Rotational polarization produced by molecule/LiF (001) collisions in a Knudsen particle flow between 78 and 695 K
Authors: Horne R. , Hermans L. J. F.
Published: July 2002

Molecular angular momentum polarization produced in molecule–surface interaction is investigated by studying the magnetic field effect on a Knudsen flow at temperatures between 78 and 695 K. The effects for H2, D2, HD, N2, and CH4 scattered from LiF (001) are found to be 2–3 orders of magnitude larger than previously observed for other surfaces. This is attributed to the large corrugation of the LiF crystal face, since the polarization type involved depends entirely on the existence of in-plane forces. The production matrix elements are found to be on the order 10−1–10−2. Both ‘‘alignment’’ and ‘‘orientation’’ type of polarization are produced. The data indicate that the production of alignment primarily stems from purely reorienting collisions (i.e., ΔmJ transitions with ΔJ=0) while orientation tends to be produced from rotationally inelastic collisions (i.e., transitions with ΔJ≠0). The off-center rotation of HD is found to play only a minor role in the nonspherical part of the interaction producing these polarizations.
Wetting and drying at a solid–fluid interface
Authors: Velasco E. , Tarazona P.
Published: July 2002

We have studied the wetting and drying transitions in a solid–fluid interace with truncated Lennard-Jones interactions, for which there have been reported molecular dynamics results by Sikkenk et al. [Phys. Rev. Lett. 59, 98 (1987)]. We consider in detail the differences resulting from the use of a ‘‘real’’ solid substrate instead of the ‘‘inert wall’’ model which has been used in all previous calculations.
A hole burning study of excitonic states of chain molecules in glasses
Authors: Hirschmann R. , Friedrich J.
Published: July 2002

Thermal broadening of spectral holes burnt into excitonic states of long chain molecular aggregates of pseudoisocyanine iodide is investigated over a temperature range from 350 mK to 80 K. The results differ very much from the usual behavior of small molecules in glasses. We found that extended states are almost completely decoupled from the amorphous host lattice and spectral diffusion effects play a minor role. The homogeneous linewidth is independent of temperature below 10 K. Above, thermal broadening occurs in two steps: there is a weak onset around 15 K and a strong onset around 65 K. The data can be fitted by a superposition of two exponentials.
Water dimer Coriolis resonances and Stark effects
Authors: Hu T. A. , Dyke T. R.
Published: July 2002

E ± 2 symmetry states for K=0, 1 of (H2O)2 are found to be perturbed and a Coriolis resonance model is used to give a satisfactory treatment of the observations. The effective b-type Coriolis coupling constant ζ is 1409.4 MHz, implying appreciable vibrational angular momentum since ζ/2C≊1/8. The spacing Δ between the upper K=0 levels and the lower K=1 levels is 10 719 MHz. The rotational constants from the deperturbation analysis are now found to be in substantially better agreement with those from tunneling states of other symmetries and with the structure of the dimer. The Stark effects of the perturbed states have been analyzed. The a component of the electric dipole moment is well determined; the c component appears to be small, but the results are not completely consistent.
Dicke narrowing of the polarized Stokes–Raman Q branch of the v=0→1 transition of D2 in He
Authors: Blackmore Robert , Green Sheldon , Monchick Louis
Published: July 2002

A b initio calculations were carried out for several Stokes–Raman Q branch line shapes of the v=0→1 transition of D2 in helium in the low pressure regime where Dicke narrowing is observed. Five different approximations were studied: a generalization of Hess’s method, which is based on the equivalent of a Bhatnagar–Gross–Krook model of the Waldmann–Snider kinetic equation; two high pressure asymptotes of Hess’s method; a moment method of the Chapman–Cowling type due to Corey and McCourt; and a collision kernel method that has found extensive use lately in rarefied gas dynamics and other highly nonequilibrium states. Using generalized cross sections incorporating S-matrices calculated in the close coupled approximation with an ab initio potential energy surface, most of the methods agreed moderately well with the most recent line width determinations at most densities. Of the five methods, the generalized Hess and collision kernel methods agreed best with experiment over the whole range. At the lowest densities where the collision kernel method might be expected to have convergence problems none were observed, and at high densities the ab initio generalized Hess method may not be flexible enough.
The dependence of Fourier transform nonlinear Raman spectroscopies on the temporal characteristics of the excitation fields
Authors: Hartland Gregory V. , Henson Bryan F. , Felker Peter M.
Published: July 2002

A perturbation theory analysis of Fourier transform (interferometric) versions of coherent Raman scattering and ionization-detected (or fluorescence-detected) stimulated Raman scattering is presented. The analysis has the principal aim of examining the information content of these spectroscopies as a function of the temporal characteristics of the excitation fields. The general conclusion of the analysis is that the information content of the techniques does not depend on excitation pulsewidths and, in particular, that spectral resolution is not limited by such pulsewidths. This is an important result because it allows for the use of short-pulse laser systems in implementing the methods. It is also shown, however, that the temporal characteristics of the excitation fields do influence the practical implementations of the spectroscopies significantly in that signal-to-noise ratios, Doppler broadening, etc. can be dependent on these characteristics.
Rotational state distributions from vibrational autoionization of H2 revisited
Authors: Pratt S. T. , McCormack E. F. , Dehmer J. L. , Dehmer P. M.
Published: July 2002

Photoelectron spectra of vibrationally autoionized (X 2Σ+g)np, v=1,2 Rydberg states of H2 were obtained by using an optical–optical double-resonance technique. The spectra were obtained by using a hemispherical electrostatic electron energy analyzer that had sufficient resolution to determine the rotational state distributions of the H+2 ions. The ionization process occurred in a magnetic and electric field-free region. All of the results are consistent with the assumptions that the photoelectrons are ejected as p waves and that spin effects are negligible. This is in sharp contrast to our earlier results obtained by using a magnetic bottle electron spectrometer, in which the ionization process occurred in a 0.5–1.0 T magnetic field [J. L. Dehmer et al., J. Chem. Phys. 90, 6243 (1989)]. It is concluded that the magnetic field can significantly modify the rotational distributions observed for vibrational autoionization. The implications of this conclusion are discussed, with particular attention to other experiments performed with the magnetic bottle spectrometer.
A simple method analyzing 2H nuclear magnetic resonance line shapes to determine the activation energy distribution of mobile guest molecules in disordered systems
Authors: Rössler E. , Taupitz M. , Börner K. , Schulz M. , Vieth H.-M.
Published: July 2002

We investigated the 2H nuclear magnetic resonance (NMR) line shape of deuterated benzene and hexamethylbenzene as guest molecules in organic glasses in the temperature range of 10–150 K. A broad distribution G(ln τ) of correlation times determines the slowing down of the molecular reorientation around the sixfold symmetry axis of the guests. The line shape is described by a superposition of temperature-dependent fractions F(T) of only two subspectra corresponding to fast and slowly rotating molecules; no spectra characteristic for intermediate mobility as found in crystal matrices are observed. Assuming a thermally activated motional process, the temperature dependence of G(ln τ) comes from a temperature-independent distribution of activation energies g(E). In this case, the derivative of the fraction dF(T)/dT yields directly the distribution g(E). Using this method an asymmetric distribution g(E) with its maximum at the low energy side is found for the glasses. While the general shape of g(E) is similar for different matrices, the mean activation energy differs significantly. We used the same approach to discuss similar ‘‘two-phase’’ spectra for the isotropic reorientation of toluene in polystyrene below the glass transition of the mixed system. Here, an alternative explanation is offered considering a distribution of glass transition points Tg for the dynamics of toluene in the mixed system.
The structures and vibrational frequencies of the NNO analogs NPO and PNO and their protonated forms
Authors: Davy Randall D. , Schaefer Henry F.
Published: July 2002

The surprising relative stability of PNO compared to NPO cannot be explained by a single configuration molecular orbital (MO) model. However the MO model can be used to understand the pattern of stability among the protonated isomers HPNO+, PNOH+, HNPO+, and NPOH+, and perhaps to explain important aspects of the general chemistry of multiple bonds between first and second row atoms.
Experimental and theoretical total state-selected and state-to-state absolute cross sections. I. The H+2(X,v′)+Ar reaction
Authors: Liao C. -L. , Xu R. , Flesch G. D. , Baer M. , Ng C. Y.
Published: July 2002

Total state-selected and state-to-state absolute cross sections for the reactions, H+2(X̃,v′=0–4)+Ar→H2(X,v) +Ar+(2P3/2,1/2) [reaction (I)], ArH++H [reaction (II)], and H++H+Ar [reaction (III)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.48–100 eV. Experimental state-selected cross sections for reactions (I) and (II) measured at Ec.m.=0.48–0.95 eV are in agreement with those reported previously by Tanaka, Kato, and Koyano [J. Chem. Phys. 75, 4941 (1981)]. The experiment shows that prominent features of the cross sections for reactions (I) and (II) are governed by the close resonance of the H+2(X̃,v′=2)+Ar and H2(X,v=0)+Ar+(2P1/2) vibronic states. At Ec.m.≤3 eV, the vibrational state-selected cross section for the charge transfer reaction (I) is peaked at v′=2.

The enhancement of the charge transfer cross section for v′=2 as compared to other v′ states of reactant H+2 increases as Ec.m. is decreased. The state-to-state cross sections for reaction (I),measured at Ec.m.≤3 eV, show that the enhancement for the charge transfer cross section for v′=2 is due to the preferential population of Ar+(2P1/2). At Ec.m.=0.48–0.95 eV and v′=2, nearly 80% of the charge transfer product Ar+ ions are formed in the 2P1/2 state. However, at Ec.m.>5 eV, the intensity for charge transfer product Ar+(2P3/2) is greater than that for Ar+(2P1/2). Contrary to the strong vibrational dependence of the cross section for reaction (I), the cross section for reaction (II) is only weakly dependent on the vibrational state of H+2. At Ec.m.≤3 eV, the cross section for the formation of ArH+ is the lowest for v′=2 compared to other v′ states, an observation attributed to the competition of the nearly resonant Ar+(2P1/2)+H2(X,v=0) charge transfer channel. The cross section for reaction (II) decreases with increasing Ec.m..

At Ec.m.≥20 eV, the cross sections for the formation of ArH+ become negligible compared to those for Ar+. The appearance energies for the collision-induced dissociation H+2(X̃,v′=0–4) are consistent with the thermochemical threshold for reaction (III). The cross sections the formation of H+ are ≤20% of those for H+2. Theoretical state-to-state cross sections for reaction (I) at Ec.m.=19.3 and 47.6 eV calculated using the nonreactive infinite-order sudden approximation are found to be in fair agreement with experimental results.
Kinetic parameters obtained from area integration of single peak thermal desorption spectra
Authors: Cabrera A. L.
Published: July 2002

This paper presents a method to analyze thermal desorption spectra based on area integration of the desorption peaks. An expression is derived for the rate of desorption in a thermal-activated process which permits evaluation of the rate from the intensity and area values for each point of the curve. Arrhenius plots of the rate data yield the activation energy and attempt frequency for desorption. The order of the desorption process is determined from the best linear fit in the Arrhenius plots. Included are applications for analyzing thermal desorption spectra obtained with a thermal conductivity detector or a mass spectrometer. Analyses of NO desorption from KNbO3 and H2 desorption from Ni surfaces illustrate the method.
Rescaled mean spherical approximation for wall–particle correlations in colloidal suspensions
Authors: González-Mozuelos P. , Medina-Noyola M.
Published: July 2002

The idea of the rescaled mean spherical approximation (RMSA) is extended to describe the local concentration profile of a suspension of highly charged particles near a charged wall. The predictions of this essentially analytic approximation for the wall–particle correlations, under conditions that the ‘‘rescaling’’ is needed, are in excellent quantitative agreement with those of the HNC approximation. It is found that the structure of the inhomogeneous suspension (the magnitude and relative spacing of the oscillations in the local macroparticle concentration, as a function of the distance from the wall) only depends on the bulk properties of the suspension. The charge density of the wall only contributes to determine the displacement of this local structure away from the wall.
A linear 1B2 state of the water molecule
Authors: Abramson E. H. , Zhang J. , Imre D. G.
Published: July 2002

Spectra of the lowest 1B2 state of H2O and D2O have been recorded. The state is linear in its equilibrium geometry and has a bond length of 1.02 Å. The spectra exhibit vibrational bending progressions and are rotationally resolved. They were recorded from energies of 80 000 to 90 000 cm−1 via two-photon, laser-induced flourescence (LIF) and 2+1 multiphoton ionization (MPI).
The silaformyl radical HSiO and its SiOH isomer
Authors: Xie Yaoming , Schaefer Henry F.
Published: July 2002

The HSiO/SiOH system is investigated by ab initio self-consistent-field (SCF) and single and double excitation configuration interaction (CISD) methods, using a triple-ζ plus double polarization (TZ2P) basis set. All geometries were fully optimized by both methods. There exist two minima for the isomer HSiO on its SCF potential energy hypersurface, but only one minimum remains when electron correlation is taken into account. The isomer SiOH is predicted 11.6 kcal/mol lower in energy than HSiO. The energy barrier for hydrogen rearrangement from the HSiO side is predicted to be 25.7 kcal/mol after zero-point energy correction. Although this prediction could be too high by perhaps 5 kcal/mol, HSiO nevertheless should be makable in addition to the global minimum SiOH. Harmonic vibrational frequencies and their infrared intensities (which are substantial) for both minima and the transition state are also predicted at the SCF and CISD levels of theory.
The dissociation mechanism of triplet formaldehyde
Authors: Yates Brian F. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

A b initio molecular electronic structure theory has been used in conjunction with flexible basis sets to predict the barrier height to radical dissociation for the lowest triplet state (T1) of formaldehyde (3A″H2CO→H⋅+HCO⋅). Self-consistent-field (SCF), complete active space SCF (CASSCF), and configuration interaction with single and double excitations (CISD) levels of theory were employed with basis sets ranging from double zeta plus polarization (DZP) to quadruple zeta plus triple polarization (QZ3P). Complete geometry optimizations of the equilibrium structure of X̃ 1A1 H2CO, ã 3A″H2CO, the transition state, and the dissociated radical on the potential energy surface were carried out. Improved basis set, triple zeta plus double polarization with higher angular momentum polarization functions [TZ(2df,2pd)], single point methods were used to further refine relative energies. Higher correlated level, multireference CISD (MR-CISD), was employed to verify the calculations involving higher excitations. At the highest level of theory [CISD(Q) with the TZ(2df,2pd) basis set], the exit barrier height at 0 K for the T1 state is predicted to be 7.8 kcal mol−1 with the zero point vibrational energy (ZPVE) correction with an estimated error bar of 3.0 kcal mol−1, favorably comparing with the most recent and accurate experimental estimate of 2.9–6.0 kcal mol−1 by Chuang, Foltz, and Moore [J. Chem. Phys. 87, 3855 (1987)]. This study also presents the most sophisticated theoretical predictions to date on the equilibrium structure and physical properties of the lowest triplet state, ã 3A″, of formaldehyde.
Strong forces between hydrophilic macromolecules: Implications in biological systems
Authors: Ben-Naim A.
Published: July 2002

The concept of hydrophilic (HφI) interaction between small HφI molecules or groups is extended to large molecules containing such groups. The Gibbs energy change associated with pairs, triplets, and quadruplets HφI interactions is estimated. The corresponding forces derived from these interactions are shown to be stronger and of longer range compared with the HφI forces between two HφI groups. Some implications of these forces and interactions in biological systems are outlined.
Trajectory studies of SN2 nucleophilic substitution. I. Dynamics of Cl−+CH3Cl reactive collisions
Authors: Vande Linde Scott R. , Hase William L.
Published: July 2002

Classical trajectories were used to study the dynamics of the Cl−+CH3Cl→Cl−---CH3Cl association and the Cl−+CH3Cl→ClCH3+Cl− substitution reactions. Substantial deviations are found between the underlying microscopic dynamics of the reactions and the assumptions of statistical rate theories. The energy dependence of the trajectory rate constant for the majority of Cl−---CH3Cl→Cl−+CH3Cl dissociation is in accord with a model in which only the Cl−---C stretch and the two Cl−---CH3Cl bend modes are active degrees of freedom. At 300 K the trajectory rate constant for Cl−+CH3Cl→Cl−---CH3Cl association is approximately forty percent smaller than that of microcanonical variational transition state theory, with the difference increasing with an increase in temperature. For thermal conditions substitution occurs by an indirect mechanism in which the reactive system is initially trapped in the Cl−---CH3Cl potential well. The cross section for this process decreases dramatically as the reactant relative translational energy is increased. The effect of rotational energy is less precipitous. Exciting the C–Cl stretch normal mode of CH3Cl opens up a direct substitution mechanism without trapping in either of the two potential wells. There is a significant decrease in the cross section for this direct substitution when CH3Cl is rotationally excited.
Potential energy surfaces for water dynamics. II. Vibrational mode excitations, mixing, and relaxations
Authors: Ohmine Iwao , Tanaka Hideki
Published: July 2002

Dynamical behavior of liquid water is investigated by analyzing the potential energy surface involved. Multidimensional properties of the potential energy surface are explored in terms of vibrational mode excitations at its local energy minima, called inherent structures. The vibrational mode dynamics, especially mechanism of mode relaxation and structure transitions, is analyzed. It shows very strong excitation energy dependence and mode dependence. There are three kinds of vibrational coupling among modes. For excitations of energy near the room temperature, most modes (more than 90% of total modes) individually interact with only one or two other modes, and yield near recurrence of the mode energy in a few tens picoseconds (very slow relaxation). Spatially localized modes in the intermediate frequency range couple with many delocalized modes, yielding fast relaxation. The coupling is governed by atomic displacement overlaps and frequency matching. Each mode couples with nearby frequency or double frequency modes through the Fermi resonance. Lowest frequency modes almost always lead to transitions from a potential energy well to neighbor potential wells, called inherent structure transitions. In high energy excitation, some intermediate frequency modes also yield such transitions. There exist very low energy paths involving single or few water molecule displacements at almost every inherent structure, indicating that certain facile molecular movements occur even in very low temperature states. Different energy excitations of a low frequency mode result in different inherent structure transitions; transitions caused by high energy excitations involve many large molecular displacements. These inherent structure transitions are the source of the water binding structural reorganization dynamics. Significance of these vibrational mode dynamics in the water dynamics is discussed.
Molecular simulation of water along the liquid–vapor coexistence curve from 25 °C to the critical point
Authors: de Pablo Juan J. , Prausnitz John M. , Strauch Henry J. , Cummings Peter T.
Published: July 2002

Previous work has shown that the simple point-charge (SPC) model can represent the experimental dielectric constant of water. In this work, we present results of Monte Carlo simulations of SPC water in the isothermal–isobaric (NPT) ensemble and in the Gibbs ensemble. Long-range intermolecular interactions are included in these simulations by use of the Ewald summation method. When Ewald sums are used, simulated, uniphase liquid potential energies are slightly lower (in absolute value) than those obtained for a simple spherical cutoff of the intermolecular potential. The coexistence curve of SPC water is obtained from 25 to 300°C. The critical constants of SPC water are estimated by adjusting the coefficients of a Wegner expansion to fit the difference between simulated liquid and vapor orthobaric densities; the estimated critical temperature is 314 °C and the estimated critical density is 0.27 g/cm3.
Monte Carlo simulation of lattice models for macromolecules at high densities
Authors: Reiter Johannes , Edling Thomas , Pakula Tadeusz
Published: July 2002

Polymer chain models on quadratic, cubic, triangular, and face-centered cubic lattices at volume fractions near unity are simulated with a Monte Carlo algorithm which transports beads from kinks or chain ends along the chain contour to another position of the chain by a slithering motion of the intervening chain part. Special cases are the slithering snake motion where the whole chain takes part in the slithering motion and a conformation change of a kink or an end group. For dense systems it is found that this algorithm is much more efficient than the slithering snake algorithm or algorithms which use only local motions. It can be used with good efficiency even for systems at a volume fraction of unity by moving chain parts collectively (collective motion algorithm). The computed chain dimensions agree with data obtained from other algorithms and with literature data.