bluuebunny/crossref_metadata_2025 huggingface dataset:
The rotating wave approximation, including the incorporation and importance of diagonal dipole moment matrix elements, for infrared multiphoton excitations
Authors: Nakai Sachiko , Meath William J.
Published: July 2002

The incorporation of diagonal dipole moment matrix elements into the rotating wave approximation, for the interaction of many-level molecules with a continuous wave laser, is illustrated with particular emphasis on the multiphoton transitions characteristic of IR molecular excitations. Generalized rotating wave approximation (GRWA) and exact Floquet calculations are carried out for the resonance profiles, and the time evolution of the molecular states, associated with such transitions for the interaction of a laser with two, eight-level anharmonic oscillators modeled on the relatively harmonic, and the very anharmonic HCl and HeH+ molecules, respectively. The calculations are carried out for intensities ranging from 1011 to 1014 W/cm2 and for a wide range of frequencies. These results are used to investigate the validity of the ‘‘new’’ GRWA and the importance of diagonal dipole matrix elements for IR multiphoton excitations as a function of frequency and laser intensity. The discussion also includes examples of the use of the analytic expressions for the molecule-electromagnetic field couplings, in the GRWA, to help interpret/predict the effects of the diagonal dipole moment matrix elements on the resonance profiles, and the dynamics, relative to the results corresponding to these matrix elements being zero.
Anisotropic interactional potentials for HeCH4, HeCH3Cl, HeCH2Cl2, HeCHCl3, and HeCCl4 from molecular beam scattering
Authors: Henkel M. , Pfeil B. , Seidel W.
Published: July 2002

Differential cross section (DCS) measurements are reported for scattering of a He atomic beam by crossed beams of CHnCl4−n (0≤n<≤4). Damping of the DCS diffraction oscillations is used to extract isotropic and anisotropic intermolecular potentials and to compare between the molecules and their symmetry. A common value for the well depth and the well location of bound state chlorine has been found (εCl=4.34 meV and RCl=300 pm).
Improved expression of mean-square radius of gyration. I. Vinyl polymers
Authors: Zhou Zhiping , Yan Deyue
Published: July 2002

Taking account of the effect of large side groups, the mean-square radius of gyration of vinyl polymer chains has been deduced by the method of matrix algebra. According to previous approaches to the computation of conformational energy, every substituent of vinyl polymer is confined to an orientation in which the conformational energy is minimum, i.e., its plane is approximately perpendicular to the one defined by the skeletal bonds flanking the Cα atom. This model facilitates the derivation of mean-square radius of gyration. Neglecting the effect of side groups, the expression for mean-square radius of gyration given in this work reduces to be that reported by Flory. Numerical calculations indicate that for tactic polypropylene, polystyrene, poly(methyl acrylate), and poly-1,2-butadiene the dependence of mean-square radius of gyration on molecular weight can be expressed as 〈S2〉=aMb, which is in agreement with the experimental data.
Transport of macromolecules in porous media
Authors: Sahimi Muhammad
Published: July 2002

Three models of flow and diffusion of macromolecules in a porous medium are developed. The first model is fully deterministic and is, for given pore space and molecular configurations, exact. It can take into account the effect of both the convection and diffusion of the macromolecules, is valid for both steady and transient transport, and can yield all the important quantities characterizing the transport process. The second model is stochastic in which the macromolecules execute a random walk in the pore space, the transition probability of which takes into account the effect of molecular and pore sizes, and the flow field in the medium. The third model is based on an effective-medium approximation and can be used for both steady and transient diffusion of the molecules in the pore space. The porous medium is represented by a two- or three-dimensional network of interconnected cylindrical pores, in which the effective pore sizes are selected from an experimentally measured pore size distribution. The macromolecules are represented by hard spheres of a given hydrodynamic radius. Detailed calculations are carried out with the three models, and the advantages and disadvantages of each model are discussed. The predictions are compared with the experimental data on polymer diffusion in a porous catalyst, and in silica glasses. Good agreement between the predictions and the data is found in both cases. The extension of the models to more complex molecular and pore shapes is also discussed.
Transient electric birefringence of colloidal particles immersed in shear flow. I. Approximate calculations from low fields to high fields under the action of a rectangular pulse and a reversing pulse
Authors: Matsumoto Mitsuhiro
Published: July 2002

The rotational diffusion equation for rotational ellipsoidal particles in solution has been approximately solved when the solute particles oriented by a shear flow are acted on by a rectangular and reversing electric pulse along the direction of the velocity gradient of flow. The angular distribution function is obtained up to the fourth order for an electric field and/or a hydrodynamic field, and it is used in order to calculate the birefringence and the extinction angle. A similar calculation is performed for the process in which the solute is returning to a steady state influenced by a shear flow after an electric field is suddenly removed. Furthermore, the equations for the intensities of light transmitted through the analyzer are shown in two cases. One is when the polarizer is oriented at 0°, and the other is when the polarizer is oriented at 45° with the direction of the electric field. The intensity at low fields in the latter case (45°) is expressed by the sum of a term of the square of the hydrodynamic field and a term of the square of the electric field. The intensity in the former case (0°), to the first order, is proportional to a hydrodynamic field; to the second order, it is proportional to the product of the hydrodynamic field and the square of the electric field. The initial slope of the intensity change for the buildup, reverse, or decay process in the former case (0°) is a function of the axial ratio and the electric property of the solute, even at low fields.
Infrared laser spectroscopy of jet-cooled methyl manganese pentacarbonyl
Authors: Gang Jun , Pennington Mark , Russell Douglas K. , Davies Paul B. , Hansford Graeme M. , Martin Nicholas A.
Published: July 2002

Infrared absorption spectra of jet-cooled CH3Mn(CO)5 and CD3Mn(CO)5 in the region of the two strongly allowed CO vibrations have been measured using diode lasers. Analysis of the spectra yielded values for the band centers and rotational constants for parallel bands of both isotopic species, and for the perpendicular band of CH3Mn(CO)5; the latter also provided an estimate of the Coriolis constant for this vibration. The spectra were consistent with essentially free rotation of the methyl group. The rotational constants were in close agreement with those derived from electron diffraction data: however, the B-rotational constants provide a revised estimate for the axial Mn–CO bond length.
Molecular dynamics at the glass transition: One dimensional and two dimensional nuclear magnetic resonance studies of a glass-forming discotic liquid crystal
Authors: Leisen J. , Werth M. , Boeffel C. , Spiess H. W.
Published: July 2002

The molecular dynamics of the aromatic core as well as the hydrocarbon side chains have been studied in a new kind of glass-forming discotic liquid crystal, based on a triphenylene core. It serves as a model compound in which only a limited number of degrees of freedom is activated at the glass transition. Slow motion with non-Arrhenius temperature dependence (α process) is associated with the axial motion of the discs around the column axis. The geometry of this rotation is found to be neither a simple threefold jump as suggested by the pseudo threefold symmetry of the substituted triphenylene core nor does it correspond to small-step rotational diffusion, established as a prominent feature of the rotational motions of polymers at the glass transition. Instead ill-defined angular displacements with pseudo threefold symmetry are detected by two-dimensional exchange NMR. A simple model combining small-step diffusion and rotational jumps is presented that quantitatively accounts for the angular displacements observed. The motional behavior and the conformational order of the side chains were also studied. This shows that the high frequency β process detected previously by dielectric spectroscopy indeed can be associated with side chain motions. Moreover evidence is presented that the axial motion of the discs is a cooperative process, where adjacent molecules within the column rotate in a correlated fashion without changing the conformation of the side chains in the immediate neighborhood of the core.
Integral cross sections for the reaction N+2+X2→N2X++X(X=H,D): A low energy crossed-beam experiment
Authors: Tosi Paolo , Dmitrijev Oleg , Bassi Davide
Published: July 2002

Using a high-resolution crossed-beam apparatus, we have measured the integral cross section for the title reactions in the collision energy range from 0.025 to about 2 eV. Structure in the energy dependence of the cross section has been observed for both reactions. However, features for H2 and D2 appear at different collision energies. This has been attributed to the successive opening, as collision energy increases, of new adiabatic reactive channels via different vibronic states of the intermediate charge transfer complex N2+X2+(v) (X=H,D). Calculations of model potential energy surfaces show that the reactants’ surface does not cross the vibronic ground state. Therefore reactions proceed only via excited vibronic states resulting, in agreement with data from previous experiments, in vibrationally excited products.
The association constant of a flexible molecule and a single atom: Theory and simulation
Authors: Groot Robert D.
Published: July 2002

In the calculation of a complexation constant by molecular simulation, an arbitrary range of coordinates is usually introduced that defines when two molecules are to be considered as ‘‘associated,’’ and when they are to be considered as ‘‘free.’’ The arbitrariness of this definition leads to an ill-defined association constant. In practice, this does not lead to a less-accurate prediction of the association constant, as long as the complexation is strong. However, when the association constant is so small that the complex has a finite probability to dissociate during the simulation, the arbitrariness in the definition of the complex leads to a large error in the predicted pK. To solve these (conceptual) problems, the statistical thermodynamics of the complexation of a single atom by a larger molecule are reanalyzed. From this analysis a rigorous definition of the complexation constant emerges, based on a thermodynamic argument. The connection between potential of mean force measurements and the free-energy perturbation method (based on a thermodynamic cycle) is illucidated. It is concluded that the former method is the most natural way to obtain the complexation constant. To calculate the potential of mean force, the use of an umbrella sampling technique for the radial distribution function at infinite dilution is crucial. This sampling technique can also be used when a solvent is present explicitly, which requires the use of a well-chosen coupling-constant integration path. Some example calculations are given for simple linear and cyclic chain molecules.
Rotationally resolved near-threshold photoionization of the 1b1 valence orbital of H2O and D2O
Authors: Lee M.-T. , Wang Kwanghsi , McKoy V.
Published: July 2002

Results of theoretical studies of rotationally resolved ion distributions for near-threshold photoionization of the 1b1 valence orbital of H2O and D2O are reported and compared with measured spectra. Agreement between the calculated and measured spectra is very encouraging. The calculated and measured spectra reveal both type a and type c transitions in contrast to type c transitions only expected in an atomiclike picture. Type a transitions arise from odd (mainly p wave) angular momentum components of the photoelectron matrix elements which are due to l mixing in the electronic continua. These type a transitions are quite molecular in origin and are similar to nonatomiclike transitions seen in resonance enhanced multiphoton ionization of excited states of diatomic molecules. Useful rotational selection rules are also obtained.
Solvent effects on the thermal reaction of photochromic dihydroindolizines: Friction or polarizability effect within the alkanes
Authors: Dernbecher K. , Gauglitz G.
Published: July 2002

The thermal reaction to a photochromic dihydroindolizine was studied in six alkane solutions. The reaction rate in the branched iso-octane compared to that in n-alkanes shows a distinct effect of the molecular shape of the solvent. This effect could be rationalized both by solvent friction as well as by solvent polarizability effects. The rates in all the alkanes, however, form a linear correlation to the π* polarity parameter of Kamlet and Taft, indicating, that the change of polarizability within the alkanes is the ‘‘true’’ reason for the observed solvent dependence. The well-known fact that macroscopic properties are not able to reflect microscopic influences (like molecular shape, e.g.) is valid for friction (when comparing shear viscosity with rotational relaxation times) as well as for polarity (when comparing the Onsager reaction field with π*). The reaction shows a negative temperature trend of the Arrhenius activation energy. Attempts are made to explain this by a composed mechanism, including a partial rotation prior to the final ring closure.
The effect of solvation on molecular Rydberg states: Dioxane clustered with nonpolar solvents
Authors: Moreno P. O. , Shang Q. Y. , Bernstein E. R.
Published: July 2002

One color 2+1 mass resolved excitation spectroscopy is employed to obtain molecular Rydberg 3s←n transition spectra of 1,4-dioxane clustered in a molecular beam with nine nonpolar solvents. The solvents are Ar, Kr, CH4, CD4, CF4, SiH4, Si(CH3)4, ethane, n-propane, cyclohexane-h12, and cyclohexane-d12. Spectral results are interpreted in terms of cluster size, isotope effects, and model calculations. A Lennard-Jones–Coulomb 6-12-1 potential is used to model the intermolecular interactions and predict minimum energy cluster geometries, cluster binding energies, and intermolecular force constants which are used to calculate van der Waals vibrational frequencies. The results show that for simple solvents (i.e., Ar, CH4) the calculations offer a simple interpretation of the observed spectra in terms of multiple cluster geometries with distinct transition energies; however, as the solvent becomes more complex, the cluster spectra also become more complex, and the number of calculated minimum energy cluster geometries increases. Complex spectra are interpreted as a distribution of cluster geometries with similar transition energies. For all of the clusters, the electronic origins are blue shifted with respect to the bare 1,4-dioxane origin. This observation is consistent with a model in which the excited state intermolecular potential becomes more repulsive due to the increased radial distribution of a nonbonding electron upon excitation into the 3s Rydberg state.
Influence of stochastic environments of Gaussian chains on dynamic shear and bulk properties
Authors: Schulz M. , Alig I.
Published: July 2002

A theory of dynamic bulk and shear viscosity in polymer systems is presented. To overcome the neglect due to the preaveraged hydrodynamic interaction in the theory of dynamic viscoelastic properties of polymers, a perturbation term, which includes the influence of external environments coupled to the velocity field, was introduced in the dynamic equation. The influence of this additional perturbation term to both dynamic bulk and shear properties was studied. This nonthermodynamic contribution modifies the Rouse spectrum. It was shown that the relaxation time spectrum is the same for the dynamic bulk ηV and shear viscosity ηS, whereas the ratio between the both quantities was found to exceed the value ηV/ηS=2/3 for pure viscous drag. The theoretical results are compared with experimental data.
Rotational hindering for H35Cl trapped in binary rare gas matrices. I. Experimental evidence from Fourier-transform infrared spectroscopy
Authors: Laroui B. , Perchard J. P. , Girardet C.
Published: July 2002

The infrared spectra of H35Cl isolated in solid mixtures of two rare gases, RG and RG*, the first one in large excess with respect to the second, are presented. The influence of temperature, RG*/RG molar ratio, and relative weight is also reported and discussed. At low RG*/RG molar ratio, new features in the R(0), Q, and P(1) regions are identified as arising from the presence of one RG* atom in the first matrix shell surrounding the molecule. The induction of a Q line, intensity of which strongly varies according to the RG*/RG relative weight, indicates the existence of more or less hindered molecular rotation. For increasing values of the RG*/RG molar ratio, several Q lines in the vibrational gap grow up successively and are assigned to nonrotating H35Cl with several RG* atoms in nearest neighbor position. Their shifts with respect to the vibrational frequency in pure RG solid do not vary linearly with the number of RG* partners, which suggests the preeminent role played by anisotropic forces for inducing the spectral perturbations (rotational quenching as well as vibrational shift).
HCl–rare gas van der Waals dimers trapped in rare gas matrix: Interpretation of the vibration–orientation spectrum
Authors: Laroui B. , Damak O. , Maillard O. , Girardet C.
Published: July 2002

Two semiempirical potentials are used to interpret the vibration–orientation spectrum of the hydrogen chloride–RG* dimers trapped in RG matrices (RG*, RG=Ar, Kr, Xe). The geometries of these dimers are calculated by including matrix relaxation and compared to those observed in the gas phase. It is shown that dimers in matrices can have radial and angular configurations very different from those reported in the gas phase, which have been used to fit potential parameters. The main features of the experimental spectrum of the trapped dimers are satisfactorily interpreted on the basis of one or the other potential. But none can lead to a universal interpretation of the infrared spectra of all the dimers. Arguments are developed on the main causes of discrepancies.
The reaction dynamics of sodium with ozone
Authors: Covinsky Michael H. , Suits Arthur G. , Davis H. Floyd , Lee Yuan T.
Published: July 2002

The reaction of sodium atoms (Na) with ozone (O3) has been studied using crossed molecular beams at a collision energy of 18 kcal/mole. The only product observed was NaO. The reaction occurs via an electron-transfer mechanism that yields NaO which is forward scattered with respect to the sodium beam. The translational energy distribution peaks near the forward stripping limit but extends out to the maximum allowed by energy conservation. The fragmentation ratio of NaO upon electron bombardment ionization I(NaO+)/[I(NaO+)+I(Na+)] as a function of product translational energy has been measured for an electron energy of 200 V.
Anharmonic interactions and intramolecular vibrational energy redistribution in S1 benzene
Authors: Rashev S.
Published: July 2002

The involvement of anharmonic interactions in the vibrational level mixing and IVR in S1 benzene at excess vibrational energy up to ∼5000 cm−1 was theoretically studied. The employed procedure is based on certain generally valid regularities governing the magnitude of anharmonic coupling matrix elements combined with detailed analysis of vibrational density computations. Definite conclusions concerning the rule of purely anharmonic interaction mechanisms in the vibrational mixing and redistribution behavior of S1 benzene have been arrived at.
Photoinitiated H- and D-atom reactions with N2O in the gas phase and in N2O–HI and N2O–DI complexes
Authors: Böhmer E. , Shin S. K. , Chen Y. , Wittig C.
Published: July 2002

Reactions of H atoms with N2O have two product channels yielding NH+NO and OH+N2. Both channels were observed via NH A 3Π←X 3∑ and OH A 2∑←X 2Π laser-induced fluorescence spectra. Photoinitiated reactions with N2O–HI complexes yield a much lower [NH]/[OH] ratio than under the corresponding bulk conditions at the same photolysis wavelength. For hot D-atom reactions with N2O, this effect is somewhat more pronounced. These results can be interpreted in terms of entrance channel geometric specificity, namely, biasing hydrogen attack toward the oxygen. Another striking observation is that the OH and OD rotational level distributions (RLD) obtained under bulk conditions differ markedly from those obtained under complexed conditions, while the NH as well as the ND RLD are similar for the two environments. In addition, OH Doppler profiles change considerably in going from bulk to complexed conditions, while such an effect is not observed for NH. The changes observed with the OH RLD are most likely due to OH–halogen interactions and/or entrance channel specificity. Under bulk conditions, the Doppler shift measurements indicate a large amount of N2 internal excitation (i.e., ∼25 000 cm−1) for the OH (v=0) levels monitored. This is consistent with a reaction mechanism involving an HNNO° intermediate. The hot hydrogen atom first attaches to the terminal nitrogen of N2O and forms an excited HNNO° intermediate having a relatively elongated N–N bond compared with N2O. Then the H atom migrates from nitrogen to oxygen and exits to the N2+OH product channel, leaving N2 vibrationally excited. A simple Franck–Condon model can reconcile quantitatively the large amount of N2 vibrational excitation.
A photoselection study on the fluorescence of pyrenebutyric acid in solution
Authors: Hennecke Manfred , Wadle Armin
Published: July 2002

Measurements with linearly polarized exciting and fluorescent light show that the excimerlike fluorescence of pyrenebutyric acid in alkaline solution is polarized. The degree of polarization r depends on temperature and, very pronounced, on the wavelength of excitation. This occurs in the spectral region where the excitation spectrum of the excimer differs from the corresponding monomer spectrum. The value of r, after correction of rotational depolarization, is 0.36 and indicates that single transition moments of absorption and emission coincide nearly in their direction. These results, together with the fluorescence time decay laws, strongly support the conclusion that, at least in an hydrophilic surrounding, not excimers but ground-state stable dimers of pyrene are formed.
Nascent product states in the photoinitiated reaction of O3 and H2O
Authors: Sauder Deborah G. , Stephenson John C. , King David S. , Casassa Michael P.
Published: July 2002

The rotational, vibrational and fine-structure state distributions for the reaction 16O(1D)+ H218O →16OH +18OH, triggered by 266 nm photolysis of O3, have been measured under conditions where less than 1% of the nascent fragments experienced collisions prior to detection. The distributions are qualitatively different than those reported earlier for 266 nm photolysis, which were evidently affected by collisions. The rotational and vibrational state distributions are similar to recent 248 nm photolysis experiments, but with differences attributed to collisional and/or energetic effects in those experiments. The ‘‘new’’ 16OH is formed with vibrational populations in the ratio 0.39(v=0):0.29(v=1):0.3(v≥2). Gaussian rotational energy distributions peaked near N=12 give average rotational energies of 〈Erot〉 = 3440 and 2780 cm−1 for 16OH v=0 and v=1, respectively. The ‘‘old’’ 18OH is much colder with vibrational populations 0.94(v=0):0.06(v=1) and a 18OH v=0 Gaussian rotational energy distribution characterized by 〈Erot〉 = 1920 cm−1.

There is negligible rotational alignment of the 16OH fragments [βμJ′ = β20(02) = 5A(0)2/4 = 0.06± 0.09], which is significantly less than expected for fragment rotations aligned with respect to the O+H2O relative velocity vector. The spin-orbit propensities deviate slightly from the statistical expectation and are characterized by [F2,N]/[F1,N]=(0.89±0.06) ×N/(N + 1). The Λ-doublet distributions for 18OH (all N) and 16OH (low-to-moderate N) fragments conform to an unconstrained-dynamics prior distribution. A slight propensity for Π(A′) in excess of this expectation is seen for the 16OH (high N) fragments. These new results are discussed in terms of possible insertion and abstraction mechanisms for the reaction.
The dynamical Renner–Teller effect. II. Rotationally inelastic scattering of NCO(X̃ 2Π,0010)+He
Authors: Macdonald R. Glen , Liu Kopin
Published: July 2002

State-resolved integral cross sections for the inelastic scattering of the Hund’s case (a) radical NCO(X̃ 2Π) by He have been measured in a crossed-beam apparatus. Two different types of measurements are reported: (1) state-resolved integral cross sections at fixed collision energies from 0.94–3.05 kcal mol−1 and (2) the collision energy dependence of state-specific product states for both 2Π3/2 and 2Π1/2 spin–orbit states of NCO(X̃ 2Π,0010). As in a preliminary account of this work for E0=3.74 kcal mol−1 [J. Phys. Chem. 91, 9630 (1991)], herein referred to as part I, dramatically different rotational level distributions were found for spin–orbit conserving or changing collisions. It is argued that this behavior and many other observations on this system are generic to the inelastic scattering of Hund’s case (a) 2Π radicals. A conceptual framework to understand these results is elucidated in detail. The direct connection between the experimental observables and the Hund’s case classification is made for the inelastic scattering of radicals. This connection can be viewed as a dynamical consequence of the Renner–Teller effect.
Partial widths and time-dependent dissociation dynamics by the optical potential method
Authors: Jolicard Georges , Billing Gert Due
Published: July 2002

A procedure to obtain partial widths and branching ratios for half and full collision experiments is developed by using square-integrable wave functions in the framework of the optical potential theory. Stable and precise results are obtained with the two-channel Noro–Taylor model Hamiltonian by calculating the asymptotic quantum flows. This paper discusses the relationships between the long-time limit relaxation induced by the resonance states and the preliminary stage of the dissociation process and shows that, when a simplified scheme is used, parameters as (i) the time at which the flows produced by the resonance are analyzed, (ii) the radial points where this analysis is made, or (iii) the norm of the resonance state, cannot be independently fixed.
A spectroscopic view of internal conversion in a small polyatomic molecule: Sub-Doppler intracavity dye laser spectroscopy of thioformaldehyde
Authors: Clouthier Dennis J. , Huang G. , Merer A. J.
Published: July 2002

Intracavity dye laser spectroscopy has been used to obtain sub-Doppler spectra of selected rotational lines in the Ã 1A2–X̃ 1A1 410 band of thioformaldehyde with very high resolution and sensitivity. Many of the spectra show extra lines due to perturbations involving high vibrational levels of the ground state. Most of the perturbations observed for K′a = 0 and 4 are found to correlate well with previous observations of anomalously long single rotational level fluorescence lifetimes and reduced quantum yields [J. Dunlop and D. J. Clouthier, J. Chem. Phys. 93, 6371 (1990)]. S1–S0 interaction matrix elements of 0.001–0.006 cm−1 are found for levels involved in simple two level perturbations. The large number of small random perturbations by levels of the ground state is indicative of the first stages of the onset of quantum chaos in a small molecule. Some larger perturbations in the rotational structure are also observed; these are caused by additional local interactions with levels of the nearby triplet state.
Multiquantum vibrational energy transfer into adsorbates on solid surfaces by atomic collisions: A semiclassical treatment based on dynamical correlations
Authors: Vilallonga Eduardo , Rabitz Herschel
Published: July 2002

A semiclassical treatment for vibrational excitation of adsorbates on surfaces by atomic collisions in the superthermal energy regime (0.5≲E≲5 eV), which was introduced previously in one dimension [Vilallonga and Rabitz, J. Chem. Phys. 85, 2300 (1986)], is here extended to three dimensions. The projectile motion is represented in the limit of short de Broglie wavelengths, i.e., by classical trajectories and their associated phases, whereas adsorbate–surface vibrations are treated quantum mechanically. Using the Feynman-path integral representation of the transition operator, this limit is approached in a flexible way that does not require a priori assumptions about the gas–surface potential and allows for strong surface corrugation, e.g., due to molecules adsorbed at low surface coverage. Distributions of transferred energies are approximated nonperturbatively by algebraic methods using time-correlation functions of the semiclassical transition operator. A large number of energetically open states are thus incorporated as well as the thermal average over initial vibrational states. The treatment is well suited for investigating multiquantum transitions of adsorbate modes and lattice phonons. The differential (in final angles and energy) scattered intensity is given in a form that is convenient for numerical calculations, since it requires integration of the Hamilton equations for the projectile, plus a straightforward sequence of fast Fourier transforms. This result is analyzed further in terms of adsorbate-localized vibrations and of lattice phonons in order to investigate how adsorbates can influence the structure of collisional energy-loss spectra. Particular attention is paid to the role of surface temperature, which can lead to different distributions for the energies going into adsorbates and into phonons. Comparisons are made with Born-type approximations and with impulsive treatments.
Glass formation and crystallization of liquid and glass rubidium: A constant-pressure molecular-dynamics study
Authors: Lu J. , Szpunar J. A.
Published: July 2002

Glass formation and crystallization processes in supercooled metallic liquid and the crystallization processes in glass are investigated for a rubidium system with 500 atoms by means of a constant-pressure molecular-dynamics technique. Calculations of thermodynamic and structural properties associated with the phase transitions are presented. The processes of structure transformation are analyzed by means of the atomic pair concepts, polyhedra, volume, energy, and pair distribution function. The results demonstrate a feature of structure transformation and show a clear picture of structure transitions during the cooling and heating processes. The glass formation has no sharp variations in the volume, the energy, or the structure, with temperature. However, the crystallization of both the supercooled liquid and the glass do have sharp variations. The results confirm that the transitions associated with these crystallization processes are first-order-like transitions, and that if there is a relaxation of the glass before the crystallization, the heating process which follows makes the glass more stable at low temperature. It is also confirmed that the cooling and heating rates have a strong effect on the transitions.
Dynamic scaling behavior of spinodal decomposition in a critical mixture of 2,5-hexanediol and benzene
Authors: Kubota K. , Kuwahara N. , Eda H. , Sakazume M. , Takiwaki K.
Published: July 2002

Time-resolved light scattering was used to study the time evolution of phase separation in a critical mixture of 2,5-hexanediol and benzene in the spinodal region. The early stage behavior of spinodal decomposition where the linearized theory of Cahn–Hillard is valid has been observed. The wave number and the interdiffusion coefficient characteristic of the early stage were well described by three-dimensional Ising model values, and the symmetric law of critical point universality was ascertained. A shoulder was observed at the higher scattering angle region in the structure factor and its wave number dependence was well expressed by the Porod law in the later stage of spinodal decomposition. The scaled behavior of the scattering function relating to the time evolution of phase separation was examined. The present result is in good agreement with the recent dynamic scaling theory.
Long time tails in canonical ensemble unimolecular decay
Authors: Jain Shashi , Dumont Randall S.
Published: July 2002

Decay of a canonical ensemble of noninteracting metastable reactant species is investigated with respect to the effects of nonergodicity due to energy and angular momentum conservation. Cases of energy conservation alone, and both energy and angular momentum conservation, are considered separately in analytic treatments. The result is the prediction of algebraic asymptotic decay of the form, O[t−1/(n−1)]. In the case of E and J conservation, n=3N−6, where N is the number of reactant atoms. Otherwise, n is the number of reactant degrees of freedom. Numerical computations of T-shaped Ar3 inversion exhibit O(1/t) long time tails in population decays, in agreement with theory.
A spectroscopic puzzle in ArHF solved: The test of a new potential
Authors: Lovejoy C. M. , Hutson J. M. , Nesbitt D. J.
Published: July 2002

The perturbed (v,b,K,n)=(1210),(1113)←(0000) band of ArHF is observed in a tunable laser/slit supersonic expansion spectrometer. The (1210) level correlates with j=2 rotation of the HF within the complex and therefore provides a test of high-order terms in a Legendre expansion of the intermolecular potential. Transitions to (1113) are observed due to intensity sharing with (1210), induced by a strong homogeneous (J-independent) perturbation that is analyzed quantitatively. The (1113) level has three quanta of Van der Waals stretch and thus probes the radial dependence of the potential close to the dissociation limit. The vibrational and rotational assignment is made possible by predictions based on the new H6(4,3,2) intermolecular potential of Hutson [J. Chem. Phys. 96, 6752 (1992)], which agree nearly quantitatively with experiment.
Vibrational splitting in Si 2p core-level photoelectron spectra of silicon molecules
Authors: Sutherland D. G. J. , Bancroft G. M. , Tan K. H.
Published: July 2002

High-resolution (∼0.1 eV) Si 2p gas phase photoelectron spectra of the following twenty-three silicon compounds have been recorded: SiHXD4−X; Si(CH3)X(OCH3)4−X; Si(CH3)X[N(CH3)2]4−X; SiHX[Si(CH3)3]4−X (where x=0–4), and SiH3–CH3, SiH3–SiH3, SiH3–SiH2–SiH3, Si(CH3)3–Si(CH3)3, Ge[Si(CH3)3]4, and [Si(CH3)2]6. Vibrational fine structure has been resolved in the Si 2p spectra of the five SiHXD4−X (x=0–4) compounds, methyl silane (H3C–SiH3), disilane (H3Si–SiH3), and trisilane (SiH3–SiH2–SiH3). For the five mixed hydrogen/deuterium compounds and methyl silane, the Si 2p vibrational structure is determined by the totally symmetric Si–H, Si–D, or Si–C stretching vibrational mode. In contrast, the spectra of disilane and trisilane are dominated by the nontotally symmetric Si–H bending vibrations—the first example of this in core-level photoelectron spectroscopy. This unusual vibrational effect is interpreted in terms of vibronic coupling that results from core-hole localization in the ion states of molecules such as disilane and trisilane which have equivalent cores. In the remaining compounds the vibrational effects are not well resolved. However, the Si 2p peak widths increase in the order Si(CH3)4≤Si[Si(CH3)3]4≤Si[N(CH3)2]4≤Si(OCH3)4≤SiF4 showing that the size of the vibrational manifold increases in the same order. The Si 2p photoelectron spectra of the series SiHx[Si(CH3)3]4−x mimic the chemical shift effects of zero, one, two, and three hydrogens bonded to a silicon surface. Vibrational effects must now be considered for adsorbate systems such as H adsorbed on a silicon surface. The Si 2p spectra are also used to predict the overall C 1s linewidth of organic analogs in the gas phase and in polymers.
Photolysis of nitric acid in solid nitrogen
Authors: Chen Whei-Jen , Lo Wen-Jui , Cheng Bing-Ming , Lee Yuan-Pern
Published: July 2002

The large shifts (−30.5 and +41.7 cm−1, respectively) of the IR absorption lines of nitric acid (HONO2) in a N2 matrix in the ν1 (OH stretching) and ν3 (HON bending) vibrational modes, relative to those observed for HONO2 in an Ar matrix, indicate a strong interaction between HONO2 and N2. Photolysis of HONO2 in solid Ar produced predominantly HOONO, whereas relatively intense lines of N2O and HONO were observed after photolysis of HONO2 in solid N2 with a mercury lamp, with the relative yield of HOONO decreased by more than a factor of 10. Possible photochemical processes are discussed.
Brillouin light scattering from nematic polymer solutions
Authors: Hu Z. , Wen X. , Vanderwal J. J. , Ao X. , Walton D. , Meyer R. B.
Published: July 2002

A multipass tandem Fabry–Perot interferometer was used to measure the Brillouin light scattering in nematic polymer solutions of poly-γ-benzyl glutamate (PBG). The longitudinal acoustic phonon frequency and linewidth were determined as a function of the angle between the phonon wave vector and the nematic director. We have observed an anisotropy of 22% in sound velocity and 40% in the Brillouin linewidth in PBG nematic solutions. Our analysis indicates that elastic relaxation due to polymer concentration fluctuations is important. However, the proposed second sound wave could not be determined due to the experimental limit of the present technique.
Properties of the combination harmonic in spectra of primary electron spin echo envelope modulation of orientationally selected disordered systems. Application to aqua-oxovanadium complexes
Authors: Tyryshkin A. M. , Dikanov S. A. , Evelo R. G. , Hoff A. J.
Published: July 2002

The properties of the sum combination (να+νβ) harmonic in the primary electron spin echo envelope modulation for systems with large g factor and hyperfine interaction anisotropy are investigated. The influence of the hyperfine interaction (HFI) parameters in the weak HFI limit on the magnitude of the shift of the combination harmonic away from the double Zeeman frequency of the nucleus are examined together with the quadrupole splitting of this line for a deuterium nucleus. The theoretical results are used for the analysis of the electron spin echo envelope modulation spectra of oxovanadium-aqua complexes with H2O and D2O in frozen solution, which yield the HFI parameters of the ligand water protons. The structural information based on these HFI data are discussed in comparison with previous single crystal and powder electron nuclear double resonance results.
Cluster-induced photochemistry of CH3I at 248 nm
Authors: Fan Y. B. , Donaldson D. J.
Published: July 2002

We have carried out a systematic study of the 248 nm excimer-laser photodissociation of small methyl iodide clusters in a free jet expansion. Ground electronic state I2 is formed from the photolysis of methyl iodide dimers and detected via the laser induced fluorescence (LIF) excitation spectrum of the (B–X) transition. The internal energy of the I2 is approximately 2.5 kJ/mol and is the same for CH3I seeded in CO2, Ar, Xe, O2, and He, as well as for the neat expansion and deuterated sample. A room temperature flow cell experiment shows that the reaction channel I*+CH3I→I2+CH3I does not contribute to the measured I2 signal. The results strongly imply that a cluster-induced cooperative effect is responsible for the I2-producing chemistry.
Core x-ray photoelectron shake-up states of model molecules for polyaniline
Authors: Sjögren B. , Salaneck W. R. , Stafström S.
Published: July 2002

The shake-up features of the N1s and C1s core-level x-ray photoelectron spectra of model molecules of polyaniline are studied theoretically using the intermediate neglect of differential overlap method in combination with configuration interaction. The calculations are performed on N,N′-diphenyl-1,4-phenylenediamine and N,N′-diphenyl-1,4-benzo-quinondiimine, which are model molecules for fully reduced and fully oxidized polyaniline, respectively. The core-level spectra of these two molecules are also studied experimentally by means of x-ray photoelectron spectroscopy. Experimental and theoretical results are found to be in excellent agreement, which allows for a detailed interpretation of the observed shake-up spectra. Core ionization of imine nitrogens is shown to be accompanied by large shake-up intensity. Our studies of model molecules are compared with recently published core-level spectra of polyaniline at various oxidation levels. The increase in imine nitrogen content upon oxidation of the polymer give rise to an increase in the N1s shake-up intensity, a result which is in agreement with what we find for the model molecules.
Influence of the spatial distribution of reactive centers on diffusion controlled reactions
Authors: Burlatsky S. F. , Ivanov O. F. , Deutch J. M.
Published: July 2002

The influence of the spatial distribution of fixed reactive centers on the diffusion controlled kinetics of reagent particles is investigated on the basis of a mean-field method for the reaction rate. The reaction kinetics are analyzed in systems where the reactive centers are randomly distributed on the sphere, a line, an array of lines, and other geometrical structures imbedded in two and three dimensions. Finite-size effects influence the time dependence of the reaction rate on different time scales as a result of the competition for the diffusing particles by the fixed reactive centers.
Rotationally resolved photoelectron spectra in resonance enhanced multiphoton ionization of Rydberg states of NH
Authors: Wang Kwanghsi , Stephens J. A. , McKoy V. , de Beer E. , de Lange C. A. , Westwood N. P. C.
Published: July 2002

Results of combined theoretical and experimental studies of photoelectron spectra resulting from (2+1) resonance enhanced multiphoton ionization (REMPI) via the f 1Π(3pσ), g 1Δ(3pπ), and h 1Σ+(3pπ) Rydberg states of NH are reported. The overall agreement between these calculated and measured spectra is encouraging. Strong ΔN=N+−N′=even peaks, particularly for ΔN=0, are observed in these spectra. Low-energy Cooper minima are predicted to occur in the ℓ=2 wave of the kπ(1Σ+), kπ(1Σ−), and kπ(1Δ) photoelectron channels for the f state, the kπ(1Δ), kδ(1Π), and kδ(1Φ) channels for the g state, and the kπ(1Σ+) and kδ(1Π) channels for the h state of NH. Depletion of the d wave (ℓ=2) contributions to the photoelectron matrix element in the vicinity of these Cooper minima subsequently enhances the relative importance of the odd ℓ  waves. The observed ΔN transitions are also affected by strong ℓ  mixing in the electronic continuum induced by the nonspherical molecular potential. Interference of continuum waves between degenerate ionization channels also determines the spectral pattern observed for photoionization of the f 1Π state of NH. Photoelectron angular distributions and the angular momentum compositions of photoelectron matrix elements provide further insight into the origin of these Cooper minima.
Auger electron spectroscopy of molecules: Theory for spin polarization following photoabsorption
Authors: Chandra N. , Chakraborty M.
Published: July 2002

In this article we derive an expression for the angular distribution of spin-detected electrons emitted in the Auger decay of a vacancy created by photoabsorption in a molecule belonging to one of the 32 point groups. The geometrical factor in the final expression is identical to the one recently obtained by us for angle- and spin-resolved photoelectron spectroscopy of freely rotating, unpolarized polyatomics by using a modified definition of the angular momentum transferred from the ionizing radiation to the outgoing electron. The ‘‘reduced’’ amplitude in the present case contains, of course, both the Auger decay and photoionization matrix elements. The results derived herein, therefore, mean that identical geometrical analysis is applicable to spin-resolved Auger electron as well as photoelectron spectroscopies of molecules. We, in particular, show that the degree of spin-polarization of Auger electrons emitted following photoabsorption in C∞v and D∞h linear molecules can be completely characterized by four independent parameters. The procedure developed herein is used to predict those molecular vacancies whose Auger decay will give rise to anisotropic angular distribution provided spin of the ejected electron is also observed. The degree of spin polarization of Auger electrons in this case is completely determined by a single parameter, say αa, and does not depend upon the state of polarization of the absorbed photon. In the end, αa is predicted for some of the possible Auger transitions which may follow photoionization in 2a1 orbital of CCl4, SiCl4, and GeCl4.
Molecular differential photoionization cross-section calculations with L2 basis functions in the random-phase approximation. Results for H2O
Authors: Cacelli I. , Carravetta V. , Moccia R.
Published: July 2002

A computational procedure, based on the K-matrix technique, is presented to solve the random-phase-approximation equations for the description of excitations in the molecular electronic continuum using L2 basis functions. The proposed method has been applied to the calculation of the photoionization cross sections and asymmetry parameters of the three main valence ionization processes in H2O including the effect of the channel interaction. The comparison of the calculated quantities with the available experimental data shows a remarkable improvement for the integral cross sections as well as an amelioration of the angular distribution parameters with respect to the static-exchange results previously obtained by a similar computational technique.
Modifications for ab initio calculations of the moderately large-embedded-cluster model. Hydrogen adsorption on a lithium surface
Authors: Fukunishi Yoshifumi , Nakatsuji Hiroshi
Published: July 2002

The moderately large-embedded-cluster (MLEC) model of Grimley, Pisani, Ravenek, and others are modified so that the model is more easily applicable to ab initio calculations. We give a linewidth to each discrete energy level of a cluster for simulating the density of states of a bulk metal and for preventing from the singularity. The dependence of the calculated results on this linewidth is shown to be small. Symmetric orthogonalization of a basis set and a new convergence algorithm are adopted in writing up our ab initio program. These modifications give a rapid convergence of the density matrix in the self-consistent-field calculation. Test calculations are performed for hydrogen adsorption on a Li (100) surface with the use of several cluster and embedded cluster models.