bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical spin–rovibronic 2A1(Πu)–2B1 spectrum of the H2O+, HDO+, and D2O+ cations
Authors: Brommer Matthias , Weis Bernhard , Follmeg Bernd , Rosmus Pavel , Carter Stuart , Handy Nicholas C. , Werner Hans-Joachim , Knowles Peter J.
Published: July 2002

Based on extensive ab initio multiconfiguration reference configuration interaction (MRCI) electronic structure calculations, three-dimensional potential energy functions for the A 2A1(Πu)–X 2B1 linear/bent Renner–Teller systems of H2O+, HDO+, and D2O+ have been generated and used in beyond Born–Oppenheimer calculations of the spin–rovibronic energy levels by a variational approach. The effects of anharmonicity, rotation–vibration, electronic angular momenta, and spin coupling effects have been accounted for. For H2O+, HDO+, and D2O+ vibronic band origins for the bending levels to Ka=5 and for selected stretching and combination levels are given. Almost all experimentally known data have been reproduced with an accuracy better than 10 cm−1 after minor modifications of the ab initio potential energy functions. Our calculated values will facilitate further experimental assignments. A consistent interpretation of the photoelectron spectra for the different isotopes of water is given. Previous assignments of the bending modes in the A 2A1 state have been revised by two quantum numbers.
Absolute transition probability measurement of nondipole valence-shell (7–70 eV) electronic transitions of SF6 by angle-resolved electron energy loss spectroscopy
Authors: Ying J. F. , Daniels T. A. , Mathers C. P. , Zhu H. , Leung K. T.
Published: July 2002

A coplanar electron spectrometer has been constructed for angle-resolved electron energy loss measurements. The experimental arrangement and performance of the spectrometer are described. Absolute absorption transition probabilities (or generalized oscillator strengths) of valence-shell (7–70 eV) electronic transitions of SF6 have been determined as a function of energy loss and momentum transfer at an impact energy of 2.5 keV. New nondipole transitions have been observed at nonzero momentum transfer. Together with the term values obtained from previous dipole electron energy loss and photoabsorption measurements, the momentum transfer dependence of the transition probabilities can be used to provide tentative assignments of the observed nondipole valence-shell transitions of SF6. Despite the complexity of the electronic structure of SF6, the present work demonstrates the feasibility of angle-resolved electron energy loss spectroscopy for investigating nondipole phenomena and related electron-induced excitation processes.
A generalized multistructural wave function
Authors: Hollauer Eduardo , Nascimento Marco Antonio Chaer
Published: July 2002

A generalized multistructural (GMS) wave function is presented which combines the advantages of the self-consistent-field molecular orbital (SCF-MO) and valence bond (VB) models, preserving the classical chemical structures but optimizing the orbitals in a self-consistent way. This wave function is particularly suitable to treat situations where the description of the molecular state requires localized wave functions. It also provides a very convenient way of treating the electron correlation problem, avoiding large configuration interaction (CI) expansions. The final wave functions are much more compact and easier to interpret than the ones obtained by the conventional methods, using orthogonal orbitals. Applications of the GMS wave functions to a variety of systems are presented.
Ab initio group model potentials: Application to the study of intermolecular interactions
Authors: Romero José Antonio Mejías , Sanz Javier Fernández
Published: July 2002

A method for the determination of ab initio group model potentials within the Hartree–Fock framework is reported. The theoretical formulation is based on the core model potential approach and allows the replacement of a group by a polycenter model potential having a local molecular symmetry. Model potentials for the Ne atom and the HF molecule are obtained and used in Hartree–Fock calculations of the NeHF and (HF)2 complexes using a triple-zeta plus polarization (TZP) basis set. Radial and angular properties of these model potentials are analyzed and compared with all electron reference calculations. The method is free of basis set superposition error and gives binding energies in agreement with Hartree–Fock calculations. The suitability of the method for the study of intermolecular interactions of larger systems is discussed.
Molecular dynamics simulation of a polymethylene chain confined in cylindrical potentials. II. Dynamics of the trans planar segments
Authors: Yamamoto Takashi , Kimikawa Yuichi
Published: July 2002

We here simulate, up to 100 ps, the conformational motion of a polymethylene molecule constrained by a cylindrical potential; the molecule is here made of 60 CH2 groups and has variable bond lengths, bond angles, and dihedral angles. We investigate the dynamical structure of the trans planar segments, especially the dihedral angle dynamics and the related twisting and out-of-plane bending motions, as a function of temperature and constraint, and we find the following: Under strong constraint, the twisting motions are dominant in the trans planar segments; the out-of-plane bending motions, however, increase with decreasing constraint resulting in a frequent generation of the gauche bonds. The dihedral angle waves propagate along the trans segments with a velocity of about 5000 m/s in two distinct series over even and odd bonds, which is a reflection of weak interactions between the adjacent dihedral angles. The overall twisting of the chain is made of local sharp twists at gauche bonds and broad gradual twists in the trans segments; the former twists are stationary, while the latter can propagate along the chain. The velocity of the latter twist is about 5000 m/s and it is almost independent of constraint. The out-of-plane bending motions, on the other hand, reflect the constraint rather sensitively. With decreasing constraint, the high frequency bending motions are greatly diminished and the velocity of the bending wave shows an appreciable slowdown. The apparently coherent annihilation and creation of the gauche bonds are often observed; the annihilation and creation occur as if they are mutually related through a dihedral angle wave.
A mean-field theory of a localized excess electron in a polar fluid
Authors: Zhu Jianjun , Cukier R. I.
Published: July 2002

A mean-field theory of a localized, excess electron in a classical, nonpolar fluid, presented in an earlier paper [J. Zhu and R. I. Cukier, J. Chem. Phys. 99, 1288 (1993)], is extended to polar fluids. The mean-field potential, and the effective potential, characterizing the electron–solvent interaction, are both modified by the addition of a long-ranged, attractive term arising from the charge–dipole interaction between the electron density and the solvent dipoles. The attractive part of this effective interaction is similar to that of an anion–dipole interaction, which makes possible the closure of the Ornstein–Zernike equations, characterizing the solvent–solvent and electron–solvent structure, by a suitably modified form of the mean spherical approximation, familiar from ion–dipole theories. The theory is compared with simulations of an excess electron in water, carried out with a new electron–solvent pseudopotential, designed to mimic the potentials of the mean-field theory. Agreement between the theory and simulation is good. Both theory and simulation predict that the solvation structure around the electron is weak. We find that the repulsive part of the force is dominant in the electron’s localization, and the long-ranged force serves only to contract the electron. We explore the contrast between the electron–solvation structure in water and a polar liquid with a more ideal dipole, to confirm that the deficiencies of the mean-field theory, constructed on the basis of an ideal-dipole fluid, are associated with the nonideality of the water dipole.
Photodissociation of ICl at 235–248 nm
Authors: Tonokura Kenichi , Matsumi Yutaka , Kawasaki Masahiro , Kim Hong Lae , Yabushita Satoshi , Fujimura Shikao , Saito Ko
Published: July 2002

Photodissociation of iodine monochloride has been investigated at 235–248 nm by studying Doppler profiles of (2+1) resonance enhanced multiphoton ionization spectra of fragment chlorine atoms and two-photon laser induced fluorescence spectra of iodine atoms. At 235.3–237.8 nm, a measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] is 0.68±0.10. At 248 nm, the [I*(2P1/2)]/[I(2P3/2)] ratio is 0.71±0.27. From the measured Doppler profiles and the ab initio calculation, the photoprepared states leading to Cl and Cl* production are assigned to the 3Π0+ (and possibly 3∑−0+) and the 1Π1 states, respectively. Nonadiabatic couplings between the potential curves have been discussed.
The inversion barrier in NF+⋅3
Authors: Yates Brian F. , Schaefer Henry F.
Published: July 2002

Ab initio molecular orbital theory has been used to investigate the barrier to inversion in NF+̇3. Complete geometry optimizations have been carried out with a variety of basis sets and electron correlation methods. It is shown that for this problem the use of single-point energy calculations needs to be treated with caution. Discrepancies amongst previous theoretical values are shown to be due to the use of inappropriate geometries and inadequate inclusion of electron correlation. With a variety of quantum chemical methods, the barrier to inversion is predicted to be in the range 11.3–13.3 kcal/mol. We have also predicted the vibrational splittings for the NF+̇3 potential energy surface and attempted to reconcile the differences with experiment.
The copper trimer and its argon van der Waals complex: An anomalous shift in the Ã←X̃ photodissociation spectrum
Authors: Knickelbein Mark B.
Published: July 2002

The photodissociation spectra of Cu3 and Cu3Ar have been recorded from 500 nm to 550 nm. The spectrum of Cu3 shows a number of resolved vibronic bands in 522–538 nm range, in accord with the previous observations of Morse et al. [J. Chem. Phys. 79, 5316 (1983)]. By contrast, the spectrum of Cu3Ar shows no resolved vibronic structure and is displaced over 20 nm to the blue of the Cu3 Ã←X̃ origin at 539.8 nm. In addition, the oscillator strength of the Cu3Ar absorption band is a factor of six times larger than that of Cu3. These results are discussed in light of the known photophysical and spectroscopic properties of Cu3.
Slit jet infrared spectroscopy of hydrogen bonded N2HF isotopomers: Rotational Rydberg–Klein–Rees analysis and H/D dependent vibrational predissociation rates
Authors: Nesbitt David J. , Lindeman Theodore G. , Farrell John T. , Lovejoy Chris M.
Published: July 2002

High resolution IR laser direct absorption spectra in a slit jet are presented and analyzed for nitrogen (15N14N–HF, 14N15N–HF, 15N15N–HF), and deuterium (14N14N–DF) substituted N2HF isotopomers. Both 14N15N–HF and 15N14N–HF isomers are observed, indicating a sufficiently deep minimum in the hydrogen bonding potential energy surface to quench internal rotation of the N2. The vibrationally averaged stretching potentials for each substituted species are recovered from rotational Rydberg–Klein–Rees (RKR) analysis. Features of the one-dimensional (1D) potential surface such as hydrogen bond length (RH-bond), harmonic force constant (kσ), and well depth (De) are then tested for isotopic invariance by direct comparison of the different isotopomers. Agreement among the various N substituted species for HF based complexes for either vHF=0 or 1 is excellent, and provides effective 1D potentials for the stretching coordinate between 3.39 and 3.75 Å. There is a 43 cm−1 (∼10%) strengthening of the hydrogen bond upon HF vibrational excitation, as quantitatively reflected in the experimental redshifts and the shape of the RKR potentials for vHF=0 and 1.

The hydrogen bond is further strengthened by D/H isotopic substitution; this is a result of reduced vibrational averaging over DF vs HF bending motion, yielding a more linear, and hence stronger, hydrogen bond geometry. In contrast to the nearly apparatus-limited linewidths (Δνprediss∼7 MHz) observed for each of the N2HF isotopomers, the N2DF complexes yield significantly broadened lines with 73±9 MHz homogeneous linewidths due to vibrational predissociation. This tenfold increase in predissociation rates upon deuteration is in contrast to previous measurements in other HF/DF containing complexes, and indicates the importance of a near resonant vibrational channel to form N2(v=1)+DF(v=0). The energetic accessibility of this V→V channel would suggest an upper limit on the N2DF binding energy of D0≤547 cm−1, which is also consistent with upper limits on D0 from the rotational RKR analysis.
Toward a working theory of polarized light scattering from helices
Authors: Shapiro D. B. , Hull P. G. , Shi Y. , Quinby-Hunt M. S. , Maestre M. F. , Hearst J. E. , Hunt A. J.
Published: July 2002

The first Born approximation is used to calculate all 16 Mueller scattering matrix elements for a single, continuous helix at any orientation with respect to the incoming light. The results are compared to those from a helix characterized by point-polarizable subunits using the Born approximation and the coupled-dipole approximation. The number of point-polarizable groups necessary to describe the helix is investigated by comparing the calculations using the continuous helix to those using the helix made of discrete subunits. It is found that for large helices, many subunits are necessary, so in these cases, the continuous model may be more applicable. Conditions for the necessity of including dipolar interactions are established as a function of the dielectric constant of the material.
Calculation of vibrational fundamental and overtone band intensities of H2O
Authors: Kjaergaard Henrik G. , Henry Bryan R. , Wei Hua , Lefebvre Sébastien , Carrington Tucker , Sonnich Mortensen O. , Sage Martin L.
Published: July 2002

Vibrational intensities are calculated for the fundamental and overtone transitions of H2O up to approximately 18 000 cm−1. The intensities are determined from a dipole moment function expanded in the three internal bond coordinates. The expansion coefficients are computed ab initio at the second-order Mo/ller–Plesset level of theory with a 6-311G** basis set. Vibrational wave functions are calculated either from a three-dimensional harmonically coupled anharmonic oscillator (HCAO) model which uses Morse oscillators to represent both the stretches and the bend of H2O, or from a variational calculation employing the best available potential energy surface and an exact kinetic energy operator. To obtain the most meaningful vibrational intensities we define dipole moment components using the Eckart embedding. Both the HCAO and the variational intensities agree quite well with the experimental results, which span eight orders of magnitude. From the calculations we predict that it may be possible to detect as yet unobserved vibrational transitions of H2O.
Critical dynamics and diffusion behavior of polymer blend solutions of polystyrene/poly(methylmethacrylate)/d6-benzene
Authors: Miyashita Naoshi , Nose Takuhei
Published: July 2002

The dynamic critical behavior of ternary mixtures of polystyrene (Mw =3.55×105)/poly(methylmethacrylate) (Mw=3.27×105)/deuterated (d6-) benzene was investigated by quasielastic light scattering, pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) and shear viscosity measurements. The total polymer concentration c was fixed at c≂12.9⋅c* (c* is the overlap concentration), where polymer chains were weakly entangled. The relaxation rate ΓI of polymer composition fluctuations was separated to the critical part Γc and the background part Γb. Γc was in good agreement with the Kawasaki function modified by Siggia, showing that the hydrodynamic backflow dominated over the diffusion as in binary fluids of small molecules. The experimental mode-coupled region was much wider than the recent theoretical prediction by Roby and Joanny. The self-diffusion of constitutional polymers did not show any influence by critical fluctuations in the present experimental time and length scales.
Electron-momentum-specific valence-shell electronic structure of and many-body effects in cis-dichloroethylene: Ionic-state-resolved orientationally averaged electron momentum density measurement by symmetric noncoplanar (e,2e) spectroscopy
Authors: Mei L. , Chuaqui M. , Mathers C. P. , Ying J. F. , Leung K. T.
Published: July 2002

The ionization energy (IE) spectra and electron momentum distributions (MDs) of the valence shell of cis-dichloroethylene have been obtained using symmetric noncoplanar (e,2e) spectroscopy. The IE spectra were found to be in good accord with earlier photoelectron data and in qualitative agreement with a literature pole-strength spectrum generated by a Green’s function (GF) calculation. In particular, extensive many-body features were observed above ∼22 eV in the IE spectra and found to be consistent with the pole-strength splitting of the three innermost valence states (7a1)−1, (6b2)−1, and (6a1)−1, as predicted by the GF calculation. The measured MDs were compared with ab initio calculations using self-consistent-field wave functions of 4-31G, 6-31G, and 6-31++G** basis sets. The general lack of quantitative agreement between experiment and calculations indicated the need for improved wave functions beyond the 6-31++G** level for the outer-valence orbitals. Moreover, the experimental MDs have provided an unambiguous assignment of the ordering of the D(8b2)−1 and E(2b1)−1 ionic states. A new satellite feature at ∼21 eV was found to have a MD similar to that of the H(7b2)−1 state, confirming the GF result. Furthermore, the many-body features above 26 eV were dominated by s-type angular dependence and could be ascribed predominantly to ionization of the innermost valence orbital 6a1. Finally, the bonding morphology of the valence orbitals was found to be different from other related ethylene derivatives due to ligand substitution effects.
(3+1) resonance enhanced multiphoton ionization photoelectron spectroscopy on nf Rydberg states of carbon dioxide
Authors: Dobber M. R. , Buma W. J. , de Lange C. A.
Published: July 2002

The (3+1) resonance enhanced multiphoton ionization spectrum of carbon dioxide has been investigated for the (2Πg3/2,1/2)nf Rydberg states. The excitation spectrum has been obtained for the n=4–13 members, while photoelectron spectra with a resolution of 15 meV have been measured for the n=4–7 members. The photoelectron spectra demonstrate that a delicate balance exists between direct and indirect ionization processes as evidenced by the large differences in vibrational activity observed in the spectra obtained via various members of the (2Πg3/2,1/2)nf Rydberg series. Strong vibrational activity, which would be forbidden by simple selection rules, is found and interpreted to arise in part from vibronic coupling between various ionic states. Long vibrational progressions, mainly involving the symmetric stretch vibration of the X 2Πg state of the ion, indicate further severe deviations from Franck–Condon behavior. Two-color excitation of the (2Πg3/2,1/2)nf Rydberg states provides strong evidence that the superexcited states accessible at the four-photon level have a profound influence on the vibrational progressions and distributions in the photoelectron spectra. Finally, resonances associated with multiphoton excitation of atomic carbon and oxygen have been observed. The processes which might be responsible for the occurrence of these atoms are discussed.
Site-dependent binding of methoxy on Cu(111): Cluster model studies
Authors: Witko M. , Hermann K.
Published: July 2002

Model calculations on CunOCH3 clusters with n ranging from 1 to 25 are carried out using ab initio Hartree–Fock and configuration interaction (CI) methods. The Cun clusters are chosen to simulate the substrate near the (111) surface and various adsorbate geometries modeling different lateral adsorption sites (top, bridge, fcc hollow, hcp hollow) at the Cu(111) surface are considered. A comparison of the different adsorption sites identifies the face-centered-cubic (fcc) hollow site as energetically preferred for OCH3 binding (D=2.8 eV at the CI level) in agreement with recent experimental results. Correlation contributions account for about 0.9 eV of the binding energies and do not depend on the binding site nor on cluster size. The OCH3–Cu interaction is characterized by metal to methoxy charge transfer filling partially occupied 2e (O 2p lone pair) radical orbitals. This leads to a negatively charged adsorbate such that ionic binding contributions become important. The binding scheme is qualitatively similar to that of other lone pair radical adsorbates like OH and influences the results of adsorbate core and valence ionization in the clusters. The present model results are consistent with experimental data concerning adsorbate geometries and binding as well as photoemission.
The use of the current density in the analysis of molecular photodissociation
Authors: Miller Steven M. , Alexander Millard H.
Published: July 2002

We have performed time-independent quantum mechanical calculations within the driven equation approach to photofragmentation processes. By extending our previous work on the analysis of photofragment flux, we calculate the quantum mechanical current density and related quantities from the time-independent wave function. We apply the method to two model problems: CH3I, where the dissociation dynamics are dominated by a crossing of two excited state potentials, and CH3ONO, where a local minimum in the excited state potential, which is located near the Franck–Condon region, gives rise to a series of strong vibrational resonances in the absorption spectrum.
Alignment of photoions far from threshold
Authors: Das Romith , Wu Chuanyong , Mihill A. G. , Poliakoff E. D. , Wang Kwanghsi , McKoy V.
Published: July 2002

We present results of measurements and calculations of the alignment for CO+(B 2Σ+) photoions over an extended energy range (0≤Ek≤210 eV). The polarization of CO+(B 2Σ+→X 2Σ+) fluorescence indicates that the photoions retain significant alignment even at high energies. Agreement between the measured and calculated polarization of the fluorescence is excellent.
Rydberg excited iodine–argon van der Waals complexes studied by resonance enhanced multiphoton ionization spectroscopy
Authors: Cockett M. C. R. , Goode J. G. , Maier R. R. J. , Lawley K. P. , Donovan R. J.
Published: July 2002

Several new gerade Rydberg states of the I2–Ar van der Waals complex are reported in the region 53 000–69 000 cm−1 above the electronic ground state, using mass resolved (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectra all exhibit anharmonic vibrational progressions in the I2...Ar van der Waals stretching mode, ν3, having typical ωe values in the range 49–62 cm−1, depending upon the principal quantum number of the Rydberg state involved. The observed increase in ωe and spectral red shift which accompanies the increase in principal quantum number is consistent with a progressive strengthening of the van der Waals bond, resulting from less effective shielding of the ion core by the Rydberg electron. The apparent good Franck–Condon overlap in the ν3 mode between the ground state and the Rydberg states together with the absence of any clear evidence of bending mode progressions in the spectra, indicates that the I2–Ar complex retains a T-shaped geometry in all the Rydberg excited states observed.
First observation of the v=3 level of the B 1Σ+ Rydberg state of CO
Authors: Baker Jacob , Tchang-Brillet W.-Ü. Lydia , Julienne Paul S.
Published: July 2002

A new diffuse vuv band of 12C16O at 92 800 cm−1 has been observed in absorption and assigned to the B–X(3–0) transition. The assignment is based on the excellent agreement found between the observed band and a calculated spectrum of the B–X(3–0) transition, where a previously optimized two channel close-coupling model of the B 1Σ+–D′ 1Σ+ Rydberg–valence predissociation interaction has been used to calculate the B(v=3) state molecular constants, J dependent predissociation widths and the J dependent B–X(3–0) vibronic transition moments. The relative absorption cross-section for this transition has also been calculated.
The geometric vector potential in molecular systems with arbitrarily many identical nuclei
Authors: Kendrick Brian , Mead C. Alden
Published: July 2002

It is known that Born–Oppenheimer electronic wave functions (in systems in which electronic spin plays no role) can acquire a double-valuedness as functions of nuclear coordinates unless the real wave function is multiplied by a phase factor that cancels the sign change that occurs when the nuclear coordinates traverse a closed path enclosing an intersection between two electronic energy levels. We show how such phase factors can be obtained in principle for systems of arbitrary complexity by making use of the cofactors of the adjusted Hamiltonian matrix H−Ej, where Ej is an eigenvalue. The method makes no direct reference to the location of the intersection, and therefore can be used when one is interested in paths that go around the intersection without approaching it closely, bypassing the necessity of costly electronic calculations near the intersection. With appropriate choice of basis functions, the phase factor will cause the electronic wave function to be not only single-valued but invariant under permutations of identical nuclei. Some simple examples are discussed.
The kinetics of H2 dissociative chemisorption: The role of transients
Authors: Chakravarty Charusita , Metiu Horia
Published: July 2002

The Lee–DePristo model for the dissociative chemisorption of H2 on Ni(100) has an interesting behavior. The potential energy surface has a minimum corresponding to a molecular, physisorbed state. This minimum is shallow and at 300 K the lifetime of physisorbed H2 is extremely short. One is then justified in treating the kinetics as a direct dissociation and ignoring physisorption. At 100 K physisorbed H2 has a long lifetime and one is forced to consider a two-step kinetics: H2 is first physisorbed and then dissociated. While chemical kinetics describes easily these two limiting cases, it offers little guidance for the intermediate temperature. We show here how the correlation function theory deals with this situation by providing equations which cover all temperature regimes. The theory is general and can be used in all cases when intermediates with shallow wells participate in the reaction mechanism.
Ab initio studies of critical conformations in ethane, methylamine, methanol, hydrazine, hydroxylamine, and hydrogen peroxide
Authors: Chung-Phillips Alice , Jebber Kimberly A.
Published: July 2002

Ab initio calculations on conformations corresponding to the torsional energy minima and maxima of ethane, methylamine, methanol, hydrazine, hydroxylamine, and hydrogen peroxide were carried out with geometry optimizations using second-order Mo/ller–Plesset perturbation theory (MP2) in the 6–311+G(3df,2p) basis. Compared with experiments the MP2/6–311+G(3df,2p) calculations yield absolute average deviations of 0.128 kcal/mol for 6 rotational barriers, 0.009 Å for 18 bond lengths, 0.7° for 16 bond angles, 0.5° for 2 dihedral angles, and 0.17 D for 5 dipole moments. Three smaller basis sets, 6–31G(d), 6–31+G(d,p), and 6–311G(d,p), were also used to study variations in the total energies and barrier heights as a result of basis set expansions. Several single-point calculations were performed to estimate effects of electron correlation enhancement from MP2 to quadratic configuration interaction [QCISD(T)]. Simulating a high level calculation with lower level calculations in a procedure similar to the recent G2(MP2) theory was found very successful.
Concentration fluctuation effects on chain orientation dynamics of polymer blends in the critical region
Authors: Rizos Apostolos K. , Fytas George , Semenov Alexander N.
Published: July 2002

Dielectric relaxation spectroscopy has been employed to investigate the normal mode relaxation of polyisoprene I chains in binary polystyrene/polyisoprene (S/I) blends over a broad temperature range in the one and two phase region. The normal mode relaxation time in the homogeneous phase near the transition is faster than expected on the basis of the average bulk composition and the distribution of relaxation times becomes bimodal in the transition region. A concentration correlation model is proposed to account for the new experimental findings in the framework of Rouse chain dynamics and mean-field static behavior. The computed normal mode relaxation times compare well with the experiment for an effective composition averaged over a microscopic scale O(nm).
Extended assignments of the 3ν1+ν3 band of methane
Authors: Tsukamoto Takeo , Sasada Hiroyuki
Published: July 2002

The 3ν1+ν3 band of methane has been investigated using a titanium sapphire laser (TSL) spectrometer. This band has recently been studied by others, and 31 transitions were assigned [Boraas et al., J. Chem. Phys. 100, 7916 (1994)]. In this work, the tone-burst modulation spectrum at liquid nitrogen (77 K) and room temperature (295 K) has been recorded to determine the rotational quantum number J″ of the lower level of each transition. Stark modulation spectra were also recorded at room temperature by applying a 40 kV/cm electric field to identify the E symmetry and other species. Among the 269 transitions observed in the 77 K tone-burst modulation spectrum, we have assigned 215 transitions with respect to J″, of which 49 are identified up to their tetrahedral components. We found a congested band structure that results from the large fraction of dark vibrational states that derive their intensity from a single bright state as a result of Coriolis interactions.
The hybrid rotational components in the 000 origin band of the ÃA″(S1)←X̃A′(S0) transition in acetaldehyde
Authors: Liu H. , Lim E. C. , Judge R. H. , Moule D. C.
Published: July 2002

The 000 origin band of the S1←S0, electronic transition that results from n→π* electron promotion has been observed under molecular beam conditions with a pulse amplified ring laser. At low temperatures, ∼0.7 K, the spectrum consisted of 11 lines that originated from Ka″=0 and J″=0 or 1 rotational levels. A rotational analysis revealed that the transition between the a1–a1 torsional levels gives rise to a c-type band, whereas the e–e levels are connected by a hybrid transition that has components along the a, b, and c principal axes. The fluorescence emission from the e levels was greatly reduced at temperatures above 3 K. The interpretation of this photophysical effect requires an intermolecular collision within the molecular beam that quenches the fluorescence from the S1 state.
Direct calculation of time delays and eigenlifetimes for the reaction He+H+2■HeH++H
Authors: Darakjian Zareh , Pendergast Phil , Hayes Edward F.
Published: July 2002

The Parker and Pack method for calculating accurate three-dimensional reactive scattering information uses adiabatically adjusting, principal axes hyperspherical (APH) coordinates to reduce the three-dimensional Schrödinger equation to a set of coupled equations in the hyperradius ρ. Solution of these coupled equations in the usual manner produces the scattering S matrix for the three-atom system of interest. To obtain these coupled equations it is necessary to solve a series of two-dimensional Schrödinger equations on the surface of a hypersphere defined by the hyperspherical polar and azimuthal angles θ and χ, respectively. In this paper, the computational advantages of the direct method for obtaining the energy derivatives of the S matrix are further documented using both the discrete variable representation and the analytical basis method of Pack and Parker for obtaining surface functions. Detailed studies of the title reaction are used to explore various operational criteria to assure that the predicted scattering results such as state-to-state transition probabilities and time delays are converged to the extent desired. It is also shown that the Hermitian property of the Smith lifetime matrix Q, which is accurately produced with the direct energy derivative method, is often not preserved when numerical energy derivatives are employed.
Theoretical investigation of the relation of hole-burning properties and the electronic structure of chemisorbed dyes
Authors: Frank Irmgard , Grimme Stefan , Peyerimhoff Sigrid D. , Sauter Bernd , Bräuchle Christoph
Published: July 2002

The ground and first excited states of the organic dyes quinizarin (Q) and tetrahydroxyphenylporphyrin (TOHP) chemisorbed on γ-Al2O3 were investigated with semiempirical PM3 and multireference CI methods. Q/γ-Al2O3 and TOHP/γ-Al2O3 represent promising materials for frequency domain optical data storage (FDOS) based on persistent spectral hole burning (PSHB), since it is possible to detect spectral holes at high temperatures (77 K). To find out whether differences in the electronic structure between the free and the adsorbed molecules account for the hole-burning properties, we performed calculations of Q–Al complexes with various coordination spheres. The calculated vertical excitation energies for the first electronically excited state of the investigated structures are in good agreement with experimental data. Analysis of the molecular orbitals shows that complexation of the investigated molecules with Al affects the electronic structure of the dyes only little. Furthermore, the character of the first excited state is conserved. The change of the dipole moment between the ground and the first excited states is about the same for the free and for the complexated systems. Therefore, it is concluded that hole burning of dye molecules chemisorbed on γ-Al2O3 at high temperatures is not due to a change of the electronic structure of the adsorbed dye molecules compared to the free dye molecules, but has to be attributed to the special interactions of Q with the γ-Al2O3 surface.
Nonequilibrium molecular dynamics of liquid crystal shear flow
Authors: Sarman Sten
Published: July 2002

We have calculated the viscosities of a nematic liquid crystal phase of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by using nonequilibrium molecular dynamics simulation methods. The calculations are facilitated by applying a Gaussian constraint method that makes it possible to fix the orientation of the director. The viscosity is a fourth rank tensor. In an isotropic fluid it has got three independent components whereas it has got seven components in an axially symmetric liquid crystal. Our estimates of the shear viscosities and the twist viscosities agree with the equilibrium fluctuation results of a previous study [S. Sarman and D. J. Evans, J. Chem. Phys. 99, 9021 (1993)]. We have also found that the streaming angular velocity is different from zero even though the angular velocity of the director is constrained to be zero thus demonstrating that these two angular velocities are different quantities. Finally we have calculated the irreversible entropy production due to the symmetric traceless strain rate as a function of the alignment angle. We have found it to be minimal near the preferred alignment angle. This is in agreement with the principle of minimum entropy production of linear irreversible thermodynamics.
Relativistic effects on sixth group monohydrides
Authors: Pisani L. , Clementi E.
Published: July 2002

Dirac–Fock and Hartree–Fock calculations have been performed for the ground state of the HO, HS, HSe, HTe, and HPo molecules. Equilibrium geometries, atomization energies, and ionization potentials, with both methods, are evaluated, compared, and discussed. Calculations on the molecules H2M (M=O, S, Se, Te, and Po) have been already published [L. Pisani and E. Clementi, J. Chem. Phys. 101, 3079 (1994)], therefore, the results of the two series of molecules are compared. The effects of electronic correlation have been estimated by using the k-functional technique [L. Pisani, L. De Windt, and E. Clementi, Int. J. Quantum Chem. (in press)]. The agreement with the experimental data, available for low Z, is satisfactory.
Hard-sphere bridge function calculated from a second-order Percus–Yevick approximation
Authors: Henderson Douglas , Sokol/owski Stefan
Published: July 2002

A procedure for calculation the bridge function for a fluid is developed. The method uses the theory of inhomogeneous fluids to yield a higher-order Percus–Yevick or hypernetted chain (or other closure) approximation. The method is applied to the hard sphere fluid. The inhomogeneous Percus–Yevick approximation gives particularily good results.
Photoinduced Rydberg ionization spectroscopy
Authors: Taylor David P. , Goode Jon G. , LeClaire Jeffrey E. , Johnson Philip M.
Published: July 2002

A new method for the high-resolution electronic spectroscopy of cations with mass resolution. Optical transitions from Rydberg states converging on the ground state of the cation to states with electronically excited cores are detected by their rapid autoionization. The spectrum of these transitions provide ionic information because the photoabsorption cross-section of a molecule in a high-n Rydberg state closely mirrors the spectrum of the ion core. The technique presented here, called photoinduced Rydberg ionization (PIRI) spectroscopy, is applied in this case to record the B̃ 2E2g→X̃ 2E1g transition of the benzene cation via the vibrational origin of the X̃ 2E1g ionic state.
Origins of molar volume isotope effects in hydrocarbon systems
Authors: Lacks Daniel J.
Published: July 2002

Lattice dynamics calculations are carried out on the hydrogenated and deuterated forms of crystalline polyethylene to elucidate the origins of the molar volume isotope effect. These calculations focus on the consequences of molecular packing on the vibrational free energy, rather than the differences between the C–H and C–D bond lengths arising from differences in the vibrational amplitudes which has been the standard theory for the molar volume isotope effects. It is found that a significant molar volume isotope effect occurs even in the absence of C–H and C–D bond length differences, and is due to differences in intermolecular separations (i.e., H...H or D...D nonbonded distances) rather than intramolecular bond lengths. This effect arises primarily from the volume dependence of the C–H/D stretching frequencies. The total molar volume isotope effect will include both the intermolecular effects described here, plus the intramolecular C–H/D bond length effects described previously. These results show that the standard theory for the molar volume isotope effect, based on the differences in the C–H and C–D bond lengths, is not complete.
Characterization of solvent clusters in a supercritical Lennard-Jones fluid
Authors: Martinez Hernan L. , Ravi R. , Tucker Susan C.
Published: July 2002

We have developed a new methodology for characterizing the solvent cluster structures which occur in a pure supercritical fluid in its compressible regime. This methodology takes advantage of the time scale separation which exists between collective-cluster and individual-solvent–atom motions in order to classify atoms according to their ‘‘instantaneous’’ local environments. The resultant picture is of a fluid having density inhomogeneities on a mesoscopic length scale—i.e., clusters and cavities. Calculation of partial radial distribution functions shows that atoms residing in different density domains have very different equilibrium structural properties, information which is not available from the usual total radial distribution function. For example, for a 2-dimensional Lennard-Jones fluid at a reduced temperature Tr=1.06 the nearest neighbor coordination number in a high density domain is 4.2, whereas in a low density domain it is only 1.0. We have also found that, for such clustering fluids that in a finite volume system there is an ensemble independent [within terms of 𝒪(1/N)], nonstructural long-range correlation which arises from an excluded volume effect. This long range correlation enables us to determine the average domain size, volume fraction and density for both the high and low density domains.
Intensities and rates in the spectral domain without eigenvectors
Authors: Gruebele M.
Published: July 2002

An exact quantum-mechanical formula is presented which relates energy level fluctuations of a complex multidimensional quantum system to its spectrum. The formalism presented here can be used to efficiently calculate the spectrum and linewidth (rate), without recourse to eigenvectors of the Hamiltonian matrix, and yields insights into the relation between level fluctuations and spectral features.
An efficient approach to estimating thermodynamic properties of fluid mixtures in molecular simulation
Authors: Wu Xiong-Wu , Sung Shen-Shu
Published: July 2002

To efficiently estimate the composition-dependent thermodynamic properties of fluid mixtures by molecular simulation, a method that adopts a thermodynamic path along a composition profile was proposed. In this method the relative values of both entropy-related properties, such as free energy, and the ensemble average properties, such as enthalpy, are evaluated as a sum of differences between neighboring composition states along the thermodynamic path. Correspondingly, a simulation with particle transitions, called the particle transition simulation, was used to sample all the composition states along the thermodynamic path. On such paths, the sampling sizes for the calculation are increased significantly because of the statistical equivalence among the particles of each component. The relative thermodynamic properties of all the composition states along the path can be calculated by this approach. This method has been used to estimate excess properties of binary and ternary Lennard-Jones mixtures in the NVT or NPT ensemble. Good agreement with other reports was found.
Resonance enhanced multiphoton ionization spectroscopy of the SnF radical
Authors: Pearson Jonathan , Dixon Richard N. , Hudgens Jeffrey W. , Johnson Russell D.
Published: July 2002

The resonance enhanced multiphoton ionization spectrum of the free radical SnF has been recorded using excitation wavelengths in the range 540 to 360 nm. The previously documented C 2Δ, D, E 2Π, and F 2Σ+ states and two hitherto unidentified excited states, the H and I 2Φ states, were all observed as two-photon resonances. Band contour simulations yielded spectroscopic constants for the E, F, and I states.
Dynamics of NO reduction by H2 on Rh(111): Velocity and angular distributions of the N2 product
Authors: Colonell J. I. , Gibson K. D. , Sibener S. J.
Published: July 2002

The velocity and angular distributions of N2 produced from the reduction of NO by H2 on Rh(111) have been measured in the low nitrogen coverage limit as a function of surface temperature. Both the angular and velocity distributions are well fit by bimodal forms. The high energy channel has average translational energies about six times that expected for molecules accommodated at the surface temperature, an unusually sharp angular distribution, and angle dependent velocity distributions. The low energy channel is also hyperthermal, with average translational energies about twice thermal, a cosine angular distribution, and velocity distributions which are independent of angle. Application of surprisal analysis to the data shows that the high energy channel may be characterized by constraints on the normal velocity and the total energy; the low energy channel may be characterized by a single constraint on the velocity.
Generation of fullerenes and metal–carbon clusters in a pulsed arc cluster ion source (PACIS)
Authors: Lu Wenyun , Huang Rongbin , Ding Junqi , Yang Shihe
Published: July 2002

By combining a pulsed arc cluster ion source with a reflection time-of-flight mass spectrometer, we have produced and detected in situ fullerene cations as well as vanadium–carbon cluster cations. The cluster arrival time distributions favor the ‘‘fullerene road’’ mechanism for fullerene growth. The formation of fullerene cations was found to be strongly dependent on the discharge and nozzle conditions. Fullerene C+60/C+70 were preferentially produced by optimizing these conditions. Vanadium–carbon cluster cations were produced and detected by using a composite electrode (V2O5/graphite). While the metallocarbohedrene V8C+12 could be produced with a reasonable intensity, it was not dominant under our experimental conditions.
The nematic transition of bendable ellipsoids
Authors: Evans G. T.
Published: July 2002

The transition from the isotropic to nematic phase is investigated for a fluid comprised of bendable ellipsoids. When the bending amplitude is chosen such that the bodies remain convex, density functional-Onsager methods can be employed to estimate the phase transition properties. The convexity requirement places a restriction on the size of the effects; however, a trend is discernible. As the density of the fluid increases, in both the isotropic and nematic phases, the mean bending amplitude is suppressed. Furthermore, the density of the isotropic to nematic transition increases with the bending amplitude in accord with the predictions of the Khokhlov–Semenov theory.
On the generality of optimal control theory for laser-induced control field design
Authors: Gross Peter , Rabitz Herschel
Published: July 2002

Optimal control theory is argued to be the most general method for designing fields used to induce desired chemical reactions. Other ‘‘schemes’’ for field design which constrain the field form and introduce limiting dynamical assumptions are shown to be particular solutions of the more general optimal control theory (OCT).
Ab initio calculations and analysis of the torsional spectra of dimethylamine and dimethylphosphine
Authors: Senent M. Luisa , Smeyers Yves G.
Published: July 2002

In the present paper, ab initio calculations at MP2/RHF level are performed with different basis sets 6-31G(d,p), 6-311G(d,p), and 6-311(df,p) to determine the potential energy functions, the kinetic parameters, and the dipole moment components as a function of the double methyl rotation in dimethylamine (DMA) and dimethylphosphine (DMP). From the potential energy and kinetic parameters, the torsional energy levels and torsional functions are determined, and from the dipole moment variations, the far infrared spectra are synthesized by calculating both the frequencies and the intensities. The results are in relatively good agreement with experimental spectra. Calculations confirm the assignments performed with the experimental potentials fitted with only five terms. The calculations, however, allow to reassign the observed band at 239.8 cm−1 in DMA and at 177.2 cm−1 in DMP to the superimposition of two different transitions: the 03→04 third sequence and an 10→11 vibrationally excited fundamental.
Differential cross sections for rotational excitation of NH3 by collisions with Ar and He: Close coupling results and comparison with experiment
Authors: van der Sanden G. C. M. , Wormer P. E. S. , van der Avoird A.
Published: July 2002

By means of the close coupling method we have calculated state-to-state differential and integral cross sections for rotational excitation and inversion of NH3 by collisions with Ar and He. For NH3–Ar we used an empirical and a scaled ab initio potential, for NH3–He an ab initio potential. The differential cross sections for NH3–Ar obtained from the empirical potential have an angular dependence that is in closer agreement with experiment than those obtained from the scaled ab initio potential. The integral cross sections are reproduced equally well by the two potentials. Also for NH3–He the differential cross sections are in accordance with experiment. For the integral cross sections the agreement is good too, except for the very small cross sections to some of the higher rotationally excited states. For both complexes the differential cross sections show a strong dependence on energy, both in their angular dependence and in their relative magnitudes.