bluuebunny/crossref_metadata_2025 huggingface dataset:
Excimer laser photolysis studies of translational-to-vibrational energy transfer in collisions of H and D atoms with CO
Authors: Wight Charles A. , Leone Stephen R.
Published: February 2003

Translational-to-vibrational excitation of carbon monoxide is observed as a result of collisions of high energy H and D atoms with CO. The fast atoms are produced by excimer laser photolysis of H2S, D2,S, HCl, HBr, or HI at 193 or 248 nm. Detection of time and wavelength-resolved infrared fluorescence is used to quantify the CO vibrational state excitation. The CO (v=1−6) state distribution from H (H2S, 193 nm, Ecm=2.3 eV) +CO collisons is 0.74±0.15, 0.15±0.01, 0.08±0.01, 0.01±0.01, 0.02±0.01, and 0.01±0.01. The corresponding state distribution from D (D2S, 193 nm, Ecm=2.2 eV)+CO is 0.79±0.19, 0.13±0.01, 0.05±0.02, 0.02±0.02, 0.01±0.01, and 0.01±0.01. Rotational excitation is at least as significant as vibrational excitation, based on estimates of the total energy transfer. Measurements of the relative vibrational excitation efficiency as a function of initial H atom energy show that the fraction of translational energy converted to CO vibration increases by more than 300% as the initial H atom energy is increased from 1.0 to 3.2 eV. Good qualitative agreement is found between the experimental results and classical trajectory calculations carried out for collinear collisions of H and D atoms with CO using a simple repulsive interaction potential.
Rotational and vibrational-rotational relaxation in collisions of CO2(0110) with He atoms
Authors: Clary D. C.
Published: February 2003

Rotational and vibrational-rotational relaxation of CO2(0110) in collisions with He atoms is studied theoretically. Cross sections and rate coefficients have been calculated using a vibrational close-coupling, rotational infinite-order sudden method, together with an ab initio potential energy surface. Comparisons with previous calculations and experiments on rotational relaxation in He+CO2(0001) are made. The rotational relaxation cross sections are found to be insensitive to the vibrational dependence of the He–CO2 potential. Transitions between even and odd rotational states of the (0110) level have relatively small cross sections. Interesting oscillating structures are predicted for the rotational dependence of the cross section distributions for transitions involving the (0110) level.
Time-resolved resonance fluorescence studies of O(1D2) yields in the photodissociation of O3 at 248 and 308 nm
Authors: Greenblatt G. D. , Wiesenfeld J. R.
Published: February 2003

The yield of electronically excited oxygen atoms O(1D2) following excimer laser photolysis of ozone at 248 and 308 nm was directly measured using time-resolved resonance fluorescence emission at ≊130 nm. The result obtained at 248 nm is in good agreement with a previous resonance fluorescence measurement in which the yield of ground-state O(3PJ) was determined in the presence of gases which either quenched or reacted with O(1D2). The relative yield determined at 308 nm 0.79±0.02 is the first such determination in the region of weak Hartley band absorption near the thermochemical threshold for production of O(1D2)+O2(a1Δg). When taken in conjunction with the previously measured relative yields of O(1D2) at this wavelength, this result suggests that the yield of electronically excited oxygen atoms at 300 nm is 0.96±0.02.
Atom–molecule collisions at very low energies: A correlation function approach
Authors: Cerjan Charles , Lipkin Michael , Rice Stuart A.
Published: February 2003

An extension of the neutron diffraction formalism to low energy atom–molecule scattering including all recoil effects is presented. The nature of the atom–molecule interaction suggests that the diffraction model should be qualitatively correct in the low collision energy regime for vibrational relaxation. The customary impulsive collision assumptions which lead to the use of the Born approximation are replaced by more general transforms which account for possible distortion of the wave function by the potential. Model calculations for a simple vibrational deexcitation process are given.
Electron impact excitation of methane
Authors: Vuškovic̀ L. , Trajmar S.
Published: February 2003

Electron energy-loss spectra covering the elastic and inelastic region up to 15.0 eV were obtained at 20, 30, and 200 eV impact energies at scattering angles ranging from 8° to 130°. Relative differential elastic scattering cross sections were determined at each impact energy and then, normalized to the absolute scale by utilizing the 100° differential cross sections of Tanaka et al. (private communication) at 20 and 30 eV and to theory [Dahl et al. J. Phys. B 12, 2727 (1979)] at 200 eV. The inelastic region of the energy-loss spectrum was divided into five ranges. From the integrated scattering intensities over these ranges with respect to the elastic scattering and from the normalized elastic cross section, the inelastic cross sections were calculated.
Energy levels and photopredissociation of the He–I2 van der Waals complex in the IOS approximation
Authors: Segev Eli , Shapiro Moshe
Published: February 2003

Theory for bound states and photodissociation of triatomic molecules based on the IOSA is developed. The theory is appled to the energy levels and photopredissociation of the He–I2 van der Waals complex. The observed vibrational excitations are shown to be due to the librational mode and the two peaks observed experimentally (X′=1 and W′=1 excitations) are shown to arise from interaction between librations about skewed and near collinear configuration. A small librational barrier (∼1 cm−1) is postulated to exist. Interference effects between direct photofragmentation and the resonance enhanced (predissociation) process, as well as interferences between two angular ranges leading to rotational rainbow type phenomena, are predicted to exist.
Collisional effects on the rovibrational transitions of the HD fundamental band
Authors: Nazemi S. , Javan A. , Pine A. S.
Published: February 2003

The effect of collisions with self and buffer gases, Ar, Kr, and Xe, on the linewidth, line shape, and frequency of the rovibrational transitions of HD in its ground electronic state are studied with a high-resolution tunable laser difference-frequency spectrometer. Collisional narrowing, manifested in sub-Doppler width of the transitions, is observed (for the first time in HD) in both P and R branches of the infrared fundamental band. The measured line profiles and the pressure dependence of their widths are analyzed in the limits of strong and weak velocity-changing collisions. It is shown that both models fit the data equally well in the low pressure range (below about 300 Torr). The strong collision model, however, is found to be the best fit for HD/HD collisions in the entire 10–760 Torr pressure range. In the case of HD/buffer gas collisions, a model incorporating correlations between the velocity and state-changing collisions appears to be more appropriate. The dependence of collisional narrowing and broadening parameters on the rotational quantum number is reported and discussed.
An improved analysis of the OH stretching spectrum of amorphous solid water
Authors: Nielson Glen , Rice Stuart A.
Published: February 2003

We report a calculation of the OH stretching region of the infrared and Raman spectra of amorphous solid water. The configuration space distribution of water molecules employed as a realization of the amorphous solid is a modified scaled version of the Henderson model of amorphous Ge. The molecular dynamics includes, in addition to interaction between OH oscillators, local variation of hydrogen bending strength, anharmonicity of the OH stretching motion, and Fermi resonance between the overtone of the HOH bending motion and the OH stretching motion. A comparison with available experimental data is made and is used to analyze some of the shortcomings of the configuration space model and the dynamical analysis.
Variation of dipole moments with triplet state energy spacings in substituted benzaldehydes
Authors: Winkler I. C. , Hanson David M.
Published: February 2003

Recent studies of the Stark effect on bands in the laser induced phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde, isolated in a durene single crystal, revealed that the dipole moment associated with the most intense feature in the spectrum, believed to arise from the T2(nπ*) state, was the same within experimental uncertainty as that for the T1(ππ*) origin. This result may indicate that the T2(nπ*) state is strongly coupled to a set of nearby T1(ππ*) vibronic levels. In order to evaluate the effect of the density of ππ* vibronic levels on the dipole moment of the T2(nπ*) state, we have made similar measurements on 2,4- and on 2,5-dimethylbenzaldehyde. We have found that as the T1-T2 energy spacing decreases, the dipole moment of the second triplet state becomes more characteristic of an nπ* state.
Gas bubble formation in nonequilibrium water–gas solutions
Authors: Kwak Ho-Young , Panton Ronald L.
Published: February 2003

This paper concerns the nonequilibrium situation that is created by suddenly reducing the pressure of a liquid which contains a large amount of dissolved gas. A theoretical model for the resulting formation of gas bubbles is presented. For a given temperature, the model predicts the amount of pressure reduction required for the threshold of bubble formation and also the reduction required for massive bubble formation. With a few extra assumptions the model will also describe the temperature dependence of these pressure reductions. Results from the model show good agreement with experiments for various gases dissolved in water solutions.
Ionic salt limit in graphite–fluoroarsenate intercalation compounds
Authors: Milliken J. W. , Fischer J. E.
Published: February 2003

Starting with C8nAsF5 intercalation compounds of stage n=1 and 2, we have recorded changes with fluorine uptake in basal plane conductivity and its temperature variation, reflectivity and C(1s) core level x-ray photoemission spectra. The data are all consistent with an ionic salt limit C+20 (0.05 electrons per C atom extracted by oxidation) beyond which direct fluorination takes place. Defect scattering from the resulting C–F bond sites is shown to be the conductivity-limiting factor, rather than π charge localization around AsF6− sites. The present results also support our previous conclusions that the oxidation of the initial compounds by AsF5 goes only 40% and 52% to completion for stages 1 and 2, respectively.
Magnetic susceptibility calculations from crystal field theory for high spin ferric complexes of tetragonal charactera)
Authors: Labarta A. , Viñas X. , Tejada J.
Published: February 2003

An analytical expression for the magnetic susceptibility of ferric-high spin complexes with high tetragonal character has been obtained. Our analytical expression agrees closely with published data.
Electron mobilities and electron–ion recombination rate constants in solid, liquid, and gaseous methane
Authors: Nakamura Yoshio , Shinsaka Kyoji , Hatano Yoshihiko
Published: February 2003

Electron mobilities and electron–ion recombination rate constants have been measured in methane by an x-ray pulse conductivity technique over the density region from 2×1020 to 1.9×1022 molecules/cm3 including the critical region and the liquid–solid phase change. The observed electron mobility abruptly increases on the transition from liquid to solid, which is probably due to the change of isothermal compressibility of methane on the phase change. The observed electron–ion recombination rate constant is in good agreement with the value calculated by the reduced Debye equation in solid methane but not in liquid and gaseous methane. The deviation from the reduced Debye equation is larger in the gas phase than in the liquid phase, which is probably caused by the difference in the efficiency of excess-energy loss of electrons within the reaction radius of electron–ion recombination.
A self-consistent field approach to atomic scattering: Application to low energy positron–hydrogen scattering and annihilation
Authors: Horbatsch M. , Darewych J. W. , McEachran R. P.
Published: February 2003

We consider a formalism for light-particle scattering by atoms, in which the wave function is written as a sum of two parts Φ0+Φc. The first Φ0 is written as a usual eigenfunction expansion sum (real and/or pseudostate) that contains all the open channels. Φc is taken to be a self-consistent Slater-type wave function which is used to describe the system when all particles are in proximity of the nucleus, including possibly correlation effects. The method is tried on low energy e+–H scattering. The calculated phase shifts and Zeff are compared to those of other calculations. The calculated phase shifts are close to the ‘‘exact’’ results.
Orientation dependence in the reaction of Xe* with photodissociation polarized IBr
Authors: de Vries Mattanjah S. , Srdanov Vojislav I. , Hanrahan Ciaran P. , Martin Richard M.
Published: February 2003

The reaction of metastable Xe* with IBr to produce XeI* and XeBr* excimers was studied in crossed molecular beams. The IBr beam was rotationally polarized by using laser photodissociation to selectively remove most of the M state distribution. The reaction cross section was found to be largest when the Xe* approaches parallel to the plane of rotation of the IBr, and smallest when the Xe* approaches perpendicular to the plane of rotation. Reaction models for excimer formation are discussed, and it is concluded that the observed steric effect results from the anisotropy of the ionic Xe+/IBr−(2Π) potential surface, involving the first excited state of IBr−, which is the intermediate state in the formation of XeI*.
The valence-bond self-consistent field method (VB–SCF): Theory and test calculations
Authors: van Lenthe J. H. , Balint-Kurti G. G.
Published: February 2003

A new and very general form of valence-bond theory is described. In this theory the molecular wave function is written as any desired linear combination of valence-bond structures, and the nonorthogonal orbitals used in the construction of the valence-bond structures are allowed to distort to their optimal shapes. The orbital optimization is achieved through successive transformations of an orbital basis. The theory of the method is based on an extension of the ‘‘generalized Brillouin theorem’’ which is presented in the text. The new VB–SCF method is a generalization of the molecular orbital MC–SCF method to permit the use of nonorthogonal orbitals. It, therefore, encompasses the MC–SCF method as a restricted subclass. Several different forms of restriction may be imposed on the orbitals and on the optimization procedures. One of these only allows orbitals centered on the same atom to mix during optimization and in this way generates optimal hybrid orbitals, which it is expected will prove to be of particular interest in providing simple qualitative descriptions of the chemical bond. Test calculations are presented for LiH, He+2, and for the ground-state potential energy curve of OH. The VB–SCF wave function and optimized orbitals may be used as a starting point for the construction of a larger multistructure valence-bond wave function which we term a VB-CI function. The VB–SCF and VB–CI results presented in the paper show that the method is capable of yielding very accurate molecular potential energy curves.
The multistate vibronic coupling problem
Authors: Cederbaum L. S.
Published: February 2003

The interaction of several closely lying electronic states through a vibrational mode is discussed. Several examples are studied which show that this interaction may lead to strong nonadiabatic effects not present in a two-state vibronic coupling problem. Special attention is paid to the vibronic coupling problem in a manifold of randomly distributed electronic states with random coupling. A large number of computations have been performed for such randomly determined electronic systems. In spite of the randomness of the energies and coupling constants, the calculated spectra show some joint trends and regularities.
Ab initio studies of weakly bound complexes between some nonpolar molecules and hydrogen fluoride
Authors: Sapse A. M.
Published: February 2003

The complexes formed by nonpolar molecules and HF are studied with 6-31G, 6-31G*, and 6-31G** Gaussian basis sets. Geometries and binding energies are determined and an estimation of the correlation effects is provided using the Moller–Plesset perturbation theory (MP2). The binding energy dependence on the polarizability of the nonpolar molecules is discussed.
Ab initio studies of the CO2–HF and N2O–HF complexes
Authors: Sapse A. M. , Howell J. M.
Published: February 2003

Ab initio calculations are used to confirm the geometries found experimentally by Klemperer et al. for the CO2–HF and N2O–HF complexes.
Low-temperature magnetism of some alkali metal uranates(V) and alkaline earth neptunates(IV). Examples for ferrimagnetism in mixed actinide oxides
Authors: König E. , Rudowicz C. , Desai V. P. , Kanellakopulos B.
Published: February 2003

The conventional molecular field (MF) approach is extended to apply to ferrimagnetically coupled f electron systems. The observed curvature of the inverse molar magnetic susceptibility 1/χm as a function of temperature is attributed to the sizable effect of the TIP contribution. It is demonstrated that the magnetic behavior of LiUO3, NaUO3, KUO3, Li3UO4, BaNpO3, and SrNpO3 is consistent with that of a two-sublattice ferrimagnet. The calculated 1/χm data depend on the Curie constants CA, CB, the TIP contributions τA, τB, and the intrasublattice MF constants α, β. Excellent numerical fits of the experimental magnetic data are obtained above the transition temperature Tc, whereas below Tc the intersublattice MF constant γ has to be increased slightly. Due to the number of parameters involved (CA, CB τA, τB, α, β), the emphasis is concentrated on the qualititative rather than the quantitative agreement with the applied model.
On the interaction operator in the optical spectroscopies with spina)
Authors: Lee Duckhwan , Albrecht A. C.
Published: February 2003

Recently the formalism describing the interaction of light with matter has been examined by Yang from the point of view of gauge invariance. Here this subject is discussed at the electric-dipole approximation for spin ‘‘forbidden’’ optical transitions between molecular eigenstates. It is shown how the ‘‘position’’ form of the interaction operator is the correct form, just as in the case of the spin-free spectroscopies. Furthermore, direct (allowed) optical transition between states with different spin angular momenta is possible (even at the electric-dipole approximation to the field), through the interaction of the spin magnetic moments with the magnetic field that is generated by the orbital motion of the electrons within the sinusoidally varying electric field of the incident light. However, the magnitude of its contribution to a spin forbidden transition is at the level of the direct magnetic-dipole transition between pure spin states; it can be expected to be relatively weak for systems containing atoms from the second period and beyond where the usual spin-orbit coupling mechanisms dominate in making spin forbidden transitions allowed. Cross terms between transition amplitudes from different mechanisms are found to vanish after appropriate orientational averaging. Spin forbidden transitions in benzene are reexamined in detail.
Emission and laser excitation spectra of rotationally cooled cations: Ã 2Πu↔X̃ 2Πg band systems of X(C≡C)2 X+, X=Cl, Br
Authors: Klapstein Dieter , Maier John P. , Misev Liubomir
Published: February 2003

The Ã 2ΠΩ,u↔X̃ 2ΠΩ,g, Ω=3/2, 1/2, electronic transitions of rotationally and vibrationally cooled dichlorodiacetylene and dibromodiacetylene cations in the gas phase have been studied by emission and laser excitation spectroscopies. The emission spectra were obtained by electron impact excitation of the molecules seeded in a supersonic helium free jet and the excitation spectra by laser-excited fluorescence of cations cooled to liquid nitrogen temperature. The narrowing of the vibronic bands in the spectra allow the vibrational frequencies for these cations in the X̃ 2ΠΩ,g and Ã 2ΠΩ,u states to be inferred to within 1–2 cm−1. Complementary use of both sets of data enable the spin-orbit components to be unambiguously identified and many spectral features to be interpreted. The vibrational frequency trends are also discussed.
Vibrational spectroscopy at high pressures. XL. A Raman study of manganese rhenium decacarbonyl
Authors: Adams David M. , Ekejiuba Isaiah O. C.
Published: February 2003

MnRe(CO)10 undergoes two first-order phase transitions under hydrostatic pressures, at ∼7 and ∼13 kbar.The Raman spectra have been followed through these transitions and are interpreted in terms of a change of molecular geometry from staggered to eclipsed and back to staggered with increase of pressure. Comments are offered on the assignment of the vibrational spectra of this material.
ESR effects of singlet–triplet mixing in radical pairs. Determination of the individual g tensors and the exchange coupling constant from spectral shifts due to off-diagonal Zeeman terms
Authors: Byberg J. R. , Bjerre Nis , Lund A. , Samskog P.-O.
Published: February 2003

ESR spectra of exchange-coupled radical pairs exhibit shifts in the fine structure splitting proportional to the square of the microwave frequency ν0 and shifts in the hyperfine splittings proportional to ν0, provided the g tensors of the members of the pairs are nonequivalent. These shifts stem from electronic Zeeman terms in the full spin Hamiltonian of the pairs, which tend to mix the singlet and triplet electron spin states. Expressions for the shifts are derived from a second order perturbation calculation based on the conventional triplet description. The results are employed to obtain the individual g tensors and the exchange coupling constant for a radical pair formed by x irradiation at 26 K of pentaerythritol crystals.
A photoelectron spectroscopic study of (3+1) resonant multiphoton ionization of NO and NH3
Authors: Achiba Yohji , Sato Kenji , Shobatake Kosuke , Kimura Katsumi
Published: February 2003

In the present work, we have measured energy spectra and angular distributions of photoelectrons emitted by three-photon resonant four-photon ionization of NO and NH3 in the gas phase. The following conclusions have been obtained: (1) Ionization of NO through the Rydberg F and H (H′) states at v′=0 and 1 gives rise to the ground-state ion with v=0 and 1, respectively, by a Δv=0 transition; (2) Ionization of NH3 through the Rydberg C′ states with v′=0–5 gives rise to the ground-state ion with v=0–5, respectively, by Δv=0 transitions; (3) Photoelectron angular distributions obtained for the (3+1) processes may well be interpreted in terms of cosine-square distributions. This fact strongly indicates that the ionization step takes place by one-photon direct ionization from the three-photon resonant states.
Photodissociation of triatomic molecules: Rotational scattering effectsa)
Authors: Heather R. W. , Light J. C.
Published: February 2003

An exact quantum method for calculating the state-to-state direct photodissociation probabilities for arbitrary (linear or bent) triatomic molecules is presented. It is shown that this method, which requires the accumulation of Franck–Condon overlap integrals between the initial state and a scattering state satisfying arbitrary scattering boundary conditions, is equivalent to the solution of the driven equations of Band, Freed, and Kouri. For both HCN and ICN the effects of variation of the final state potential on the vibrational and rotational distributions of the CN fragments are examined. Agreement of our exact calculations with the approximate calculations of Morse and Freed is very good.
A new multiple (mass ratio) scale analysis of atom–diatom collisions
Authors: Chang B. , Rabitz H.
Published: February 2003

Atom–diatom collisions are investigated within the framework of a general mass weighted mixed coordinate system. It is found that physically distinct parts of the Hamiltonian are controlled by functions that depend on the mass weighted mixing coefficients of the new coordinates. In principle, the mixing coefficients can be derived variationally from a stationary functional for the T matrix. The mixed coordinate system developed in this paper can also be used to investigate a quantum mechanical decoupling approximation by parametrizing the mixing coefficients of the new coordinates in terms of the approximation’s decoupling parameter. The goal of this paper is to examine the case when the mixing coefficients are expressed in terms of the ES decoupling parameter for vibrational transitions. A brief discussion is also given to the application of this technique to the investigation of the ES method for rotational transitions and vibrational transitions. This approach offers a practical avenue for improving ES results for collision systems having very small mass ratios. It is found in an example that when the collision system strongly satisfies the mass scaling criterion, the method gives uniformly accurate results for all energies and all transitions. When the collision system weakly satisfies the criterion, the method improves only the energy sudden results for multiple quantum transitions at energies which are close to the threshold limit.
Analytical and numerical calculations of the transient and the steady-state electric birefringence for a sudden application of a sinusoidal electric field of sufficient strength
Authors: Watanbe Hiroshi , Morita Akio
Published: February 2003

The transient and the steady-state electric birefringence for a sudden application of a sinusoidal electric field E(t)=E0 cos ωt of sufficient field strength was studied. An analytical expression which is valid up to the fourth power of E was obtained. For stronger electric fields, the electric birefringence was calculated numerically by making use of the Runge–Kutta–Gill method. The effect of the field strength on the dispersion of the birefringence was demonstrated for several ratios of the contribution of the permanent dipole moment and the induced dipole moment. The contribution of harmonics in the birefringence signals in the nonlinear region is expressed as the Fourier components of the signals.
Phase transition of lattice polymer systems
Authors: Baumgärtner A. , Yoon D. Y.
Published: February 2003

Monte Carlo simulations of a lattice polymer system of chains of N=20 segments each, comprising a total of 21 chains on a 21×21 square lattice, are carried out in order to clarify recent controversies concerning statistical thermodynamics of lattice models of polymers. Assuming an energy ε≳0 for every right-angle step, we find a first-order phase transition of disordered phase to an ordered state at kBTc/ε≊0.93, as demonstrated by discontinuities in the internal energy (conformational order) and the orientational order parameter. Comparison of our results with the predictions of the mean-field lattice theory of Flory and the implications of these findings on phase transitions of real polymer systems are discussed.
Enhanced nuclear spin lattice relaxation of deuterium in solids by sample rotation
Authors: Eckman Richard
Published: February 2003

The deuterium powder NRM specta of glycine-d5 are measured at room temperature. It is demonstrated that a very slow sample rotation enhances the spin lattice relaxation of rigid sites, when there are neighbouring fast relaxing sites. (AIP)
The influence of the well width on the convergence of perturbation theory for one-dimensional square-well fluids
Authors: Bishop Marvin , Boonstra Martin A.
Published: February 2003

It is found that when perturbation theory is appled to a one-dimensional square-well fluid it converges more slowly as the well-width is increased. (AIP)
Bound and Efimov states for 4He2 and 4He3 and Bose–Einstein condensation
Authors: March N. H.
Published: February 2003

The existence of a Bose–Einstein condensate at absolute zero is disputed. Superfluidity without a condensate is implied. Bound state of helium dimers and trimers are discussed. (AIP)
Comment on ‘‘The overtone spectrum of acetylene: A rotational analysis based on a local model description’’a)
Authors: Lehmann Kevin K. , Scherer George J. , Klemperer W.
Published: February 2003

Many serious errors in the recent article ‘‘The overtone spectrum of acetylene: A rotational analysis based on a local model description’’ by Bryan R. Henry, M. Ali Mohammadi, and Allan W. Tarr [J. Chem. Phys. 77, 3295 (1982)] are discussed.
Interpretation of infrared intensity changes on molecular complex formation. I. Water dimer
Authors: Zilles Barbara A. , Person Willis B.
Published: February 2003

The infrared intensities for the three fundamental vibrations of the H2O monomer have been analyzed to obtain experimental values for the atomic polar tensors (APTs) for the atoms in the monomer. These APTs have been compared with values calculated for the monomer by an ab initio quantum mechanical treatment using IBMOL-5 with a 4-31G basis set. The agreement is satisfactory enough to extend the ab initio calculation to the atoms in the linear water dimer. It is argued that the differences in the APTs from monomer to dimer (and, hence, the intensities for the intramolecular vibrations of the water dimer in terms of the corresponding intensities for the monomer) can be predicted with reasonable certainty by this treatment. The spectrum predicted for the water dimer compares very favorably with the experimental spectrum for the dimer in a N2 matrix, reported by Tursi and Nixon. The calculated APTs for the atoms in the monomer and in the dimer are subjected to a ‘‘charge–charge flux–overlap’’ (CCFO) analysis in order to see just what is responsible for the observed changes in intensity. The only significant change occurs in Pyy(CF), the charge flux contribution to the change in the dipole moment along the O–H–O hydrogen bond for motion of the hydrogen-bonded H atom along that bond. This result is analyzed in terms of contributions from the change in charge transfer that ocurs when the O–H bond stretches and contributions from the change in polarization of the EA and ED molecules. This analysis shows that the contribution from changing intermolecular charge transfer in the water dimer accounts for approximately one-half of the APT increase that gives rise to the characteristic infrared intensification observed for vibrations (particularly the O–H stretch of the hydrogen-bonded H atom) on hydrogen bond formation.
Anomalous blue shift of the resonant Raman excitation profile of hematoporphyrin IX in methanol
Authors: Cataliotti Rosario Sergio , Paliani Giulio , Poletti Antonio
Published: February 2003

A peculiar behavior of the resonant Raman excitation profile, shown by the hematoporphyrin molecule in methanolic solution, is interpreted in this article with the Kasha’s model of the molecular exciton for a van der Waals dimer, by attributing a particular role to the methanol in the stabilizing ability of the excimer.
Ab initio effective valence shell Hamiltonian calculations of Li2 potential curves
Authors: Takada Toshikazu , Sheppard Maurice G. , Freed Karl F.
Published: February 2003

Potential curves are calculated for the lithium molecule using an effective valence shell Hamiltonian formulation of quasidegenerate many-body perturbation theory and two different sets of molecular orbitals. One set utilizes the low lying valence molecular orbitals which are optimized for particular valence states, while the other set is simply taken as the union of the atomic valence sets. The shapes of the potential curves for the lowest four states (1Σ+g,3Σ+u,3Πu, and 1Σ+u) are in excellent agreement with those obtained by Olson and Konowalow from extensive MCSCF calculations and hence with experiments. The total energies are high by about 0.1 a.u. because our basis set does not describe the core electrons as well. The perturbation expansion is found to display excellent convergence properties between the second and third order calculations. A future paper will compare the bond length dependence of the effective matrix elements with those customarily assumed in semiempirical theories of bonding.
Ab initio analytical molecular gradients and Hessians
Authors: Jo/rgensen Poul , Simons Jack
Published: February 2003

Molecular gradients and Hessians have been derived for MCSCF, CI, coupled cluster, and Mo/ller–Plesset wave functions. In deriving the gradients and Hessians, atomic orbital basis set effects have been incorporated into the finite basis Hamiltonian, and unitary exponential operators have been used to determine the wave function’s configuration and orbital responses. The gradients and Hessians are expressed in terms of products of configuration and orbital responses and matrices of the same form as the gradient and Hessian matrices appearing in energy and wave function optimizations. The molecular gradients and Hessians have also been cast into forms that are computationally very tractable.
Relativistic integrals over Breit–Pauli operators using general Cartesian Gaussian functions. I. One-electron interactions
Authors: Chandra P. , Buenker Robert J.
Published: February 2003

Cartesian Gaussian functions are employed to derive general expressions for integrals over all one-electron operators of the Breit–Pauli Hamiltonian. It is shown that in atoms of higher atomic number p6, p8, ⋅⋅⋅ operators can be important in determining relativistic corrections to the kinetic energy. All other operators of this Hamiltonian can be expressed as some derivative of 1/r. Thus, a general expression is derived for the integral over the operator (∂1/∂x1) (∂m/∂ym) (∂n/∂zn) (1/r) by employing its Fourier transform. The operator and charge-distribution-dependent parts can be separately identified in the resulting expression and hence for a given charge distribution, integrals over any number of operators that can be expressed in the above form can be obtained simultaneously. In addition to nuclear attraction, these operators include the spin-orbit and Darwin terms of the Breit-Pauli Hamiltonian, as well as the electric field components and their derivatives, and other interactions over operators required in the study of magnetic shieldings. Furthermore, these expressions serve as a prototype for more difficult and numerous two-electron integrals, as discussed in the second paper in this series.
Relativistic integrals over Breit–Pauli operators using general Cartesian Gaussian functions. II. Two-electron interactions
Authors: Chandra P. , Buenker Robert J.
Published: February 2003

Expressions are derived for the multicenter integrals over all two-electron operators of the relativistic Breit–Pauli Hamiltonian employing Cartesian Gaussian atomic basis functions of any combination of angular momentum quantum numbers. It is shown that by employing the Fourier transform of the (∂1/∂x1) (∂m/∂ym) (∂n/∂zn) (1/r12), a general expression for the integral could be derived which is valid for electron repulsion, three components of electric field, and six components of the space part of the spin–spin interaction. A slight modification of this operator leads to spin-other orbit and orbit–orbit interaction integrals. Replacement of the function fL by gL and modifying the summation limits changes the expression for the integral over spin–spin interaction to that of a spin-orbit operator. It is shown in general that the charge-distribution dependence can be separated from the operator dependence of the integral in question, and thus for a given pair of charge distributions that integrals over all such operators can be derived simultaneously.
The effect of the anisotropy of the intermolecular potential on the second pressure virial coefficient of CH4
Authors: Meinander N. , Tabisz G. C.
Published: February 2003

The effect of the anisotropy of the intermolecular potential energy on the second pressure virial coefficient of methane is studied. It is found that the largest contribution (≳50%) to the anisotropy correction arises from the short range repulsive potential. The most important long range contribution is due to the anisotropic dispersion potential, which involves the dipole–quadrupole polarizability A. Difficulties met in determining the magnitude of A from virial coefficient data are discussed.
Perturbation theory for the density profile of a fluid in contact with a solid
Authors: Meister T. F.
Published: February 2003

In the framework of a perturbation theory for inhomogeneous fluids, we study the density profile of a Lennard-Jones fluid in contact with its (100) solid fcc face. We neglect the motion of the solid atoms and consider them fixed at their ideal lattice sites. To be able to describe the effect of attractive forces acting on the fluid atoms, we extend the Abraham–Singh theory to a generalized van der Waals theory. We find that the attractive parts in the potential energy change the fluid profile markedly. Our numerical results are in good quantitative agreement with existing computer simulations.
Reactive dynamics in energy transfer problems
Authors: Sipp B. , Voltz R.
Published: February 2003

An approach to energy transfer due to microscopic interactions between moving acceptor and donor molecules is described. The theory is based on a set of coupled evolution equations, assuming that the displacements of the reacting particles obey a diffusion equation. A general expression for the transfer rate holding for any microscopic interaction form is derived and its asymptotic behavior at long and moderately long times is examined. Special emphasis is given to the influence of the molecular motion. In the fast diffusion limit, the transfer rate is independent of the mobility of the particles and given by the integral of the microscopic interaction form over space. In a finite diffusion limit, a characteristic transfer length is introduced; it may be interpreted as the radius of a sphere within which trapping is complete in spite of outward diffusion. Its exact expression is derived for multipolar and exponential interactions.
Transitions and molecular packing in highly purified 1,2-dipalmitoyl-phosphatidyl choline–water phases. II. The structures of DPPC phases with low water content
Authors: Albon Norman
Published: February 2003

The structures of the polar regions are established for a series of bilayer phases with less than four water molecules per lipid molecule and shown to be closely related. For this analysis, the size and symmetry of the unit cells, determined for three phases for which single crystals were grown, were compared with the size and shape of the phosphorylcholine group and the influence of the charges on the packing. The structures of two of these crystalline phases are almost identical with that of a dimyristoyl lecithin for which some details of a structure determination were reported recently. For other phases, reference is made to the chain subcells, which are obtained from x-ray diffraction data and to the polar subcells which can be identified. The polar structures can also be related, directly or indirectly, with those in the crystalline phases from evidence of the changes occurring at the transitions. A square packing of the polar groups is observed for an anhydrous phase and for a transient phase formed at higher temperatures from samples with higher water content. The choline groups are packed in a rectangular array with an hexagonal chain packing for a phase with from one to three water molecules per lipid. In this phase, the chain axes are perpendicular to the bilayer plane and the choline groups and phosphate nonester oxygen atoms are not coplanar. This arrangement is stabilized by the proximity of another bilayer. The crystalline monohydrate phase involves a rectangular packing with coplanar phosphate nonester oxygen atoms and choline groups. This very stable polar structure is found at higher temperatures for the dihydrate and at room temperature for phases with higher water contents.
Computer simulation of self-avoiding walks: Testing the scanning method
Authors: Meirovitch Hagai
Published: February 2003

The scanning method is a computer simulation technique for macromolecules suggested recently. The method is described here in detail and its applicability (in contrast to other simulation techniques) to a wide range of chain models is discussed. It is argued that for most of these models the scanning method constitutes the most efficient tool for estimating the entropy. The method is applied to self-avoiding walks (SAWs) (of N≤399 steps) on both a three-choice square lattice and a five-choice simple cubic lattice and the results for the entropy, the end-to-end distance, the radius of gyration, and other quantities of interest are found to be in very good agreement with the results obtained by other numerical techniques. In particular, our calculations support the law of divergence for the persistence length of SAWs on two-dimensional lattices suggested recently by Grassberger. However, for the simple cubic lattice, the persistence length is found by us to be constant.
The photodissociation of thiophosgene at 248 nm
Authors: Ondrey G. S. , Bersohn R.
Published: February 2003

A pulsed molecular beam of thiophosgene was photodissociated at 248 nm. The detection of CSCl and Cl2 fragments formed under collision free conditions is evidence for two decomposition pathways: (a) CSCl2 → CSCl+Cl (b) CSCl2 → CS(X 1Σ)+Cl2. An estimate for the quantum yield for (a) is calculated to be 0.8±0.1. An impulsive spectator model is consistent with the measured translational energy distribution in process (a) for which on average 16±2 kcal/mol is found as translational energy. The results imply a vibrationally excited CSCl fragment.
The collinear photodissociation of OCS at 157 nm
Authors: Ondrey G. S. , Kanfer S. , Bersohn R.
Published: February 2003

The absorption in OCS in the 160–140 nm region has considerable vibrational structure and has been assigned (Rabalais et al.) as a 1Σ ← 1Σ transition. The quantum yield for the production of S(1S0) has been found (Black et al.) to be close to one in this same region. This absorption band therefore provides an example of a collinear photodissociation. A F2 excimer laser at 157 nm was used in a time of flight experiment to measure the translational energy distribution of the fragments from which the vibrational energy distribution was inferred. It is highly inverted in spite of the fact that the CO bond distances are similar in CO and OCS. The vibrational population inversion, we believe, is related to the vibrational structure seen in absorption. As Pack has noted, a symmetric type vibration is excited during the dissociation which tends to pull both the O and S atoms away from the C atoms. The quasivibrational trajectory ends with the sulfur atom separating but in the process the CO vibration is strongly excited.
Diffusion and reversible reaction in a medium with penetrable sinks
Authors: Calef Daniel F. , Deutch J. M.
Published: February 2003

A theory of diffusion controlled reactions with randomly distributed penetrable sinks is presented. The model considered includes a reversible reaction between a diffusing species, and a fixed reactive sink. For low reactivity, both steady state and initial value problems are discussed. The dependence on the sink concentration of the rate coefficients and the diffusion coefficients is found. By considering the limit of fast irreversible reaction, the connection is made with previous work.
van der Waals forces from scattering functions. The Ne–Ne potentiala)
Authors: Wozniak S. , Raof K. F. , Linder B.
Published: February 2003

Previous work relating the long-range interaction energy to scattering functions, via the charge-density susceptibility, is extended to two interacting Ne atoms. It is shown that, within the closure approximation, the second order dispersion and polarization energies can be formulated in terms of generalized scattering functions, which for uncorrelated (e.g., Hartree–Fock) atomic functions can be expressed rigorously in terms of ‘‘orbital’’ form factors which are the Fourier transforms of the one-electron densities. We calculate the dispersion and polarization energies, using a distance-dependent mean excitation energy, which we evaluate with the aid of overlap integrals, and known values of the collision diameter and atomic polarizabilities. We obtain the total potential energy curve in two ways: (1) by adding the first order Coulomb and exchange energies to the second order dispersion and polarization energies, and (2) by adding the diatom SCF interaction energies to the dispersion energies. The potential curves so obtained are in very good agreement with previous experimental and theoretical values.
IR and UV induced photorotamerization of 2-nitroethanol in matrices. Ab initio optimization of conformers and potential energy calculations
Authors: Räsänen M. , Aspiala A. , Murto J.
Published: February 2003

Conformer interconversion of 2-nitroethanol was studied in rare gases Ar, Kr, and Xe as well as in N2 matrices. The initial conformer Gg′ is transformed into conformer Tt both by UV (selectively with wavelengths of about 280 nm) and by IR irradiation. Frequencies in the νOH region were found to be the most effective in initiating the IR induced photorotamerization. The process is reversible, i.e., irradiation at the νOH band frequencies of the product conformer Tt produces the conformer Gg′. The IR induced process obeys the first order rate law in all the matrices studied, and the rate in different hosts decreases in the order Xe≫Kr≳Ar≳N2. Upon prolonged broadband IR irradiation the process proceeds to an equilibrium; the final Tt/Gg′ conformer ratio (determined from the νOH bands) was the same, ∼0.5, in all matrices, while upon UV irradiation in Ar the final ratio was ∼1.1. The observation that in a Xe matrix at deposition temperatures above 20 K the rotamerization is almost totally inhibited, is explained in terms of the formation of guest–host interactions of van der Waals type. The photorotamerization mechanism is discussed. Our conformer assignment is based on ab initio (STO-3G and 3-21G) calculations; only five conformers were found. The calculations indicate a strong intramolecular interaction for species Gg′. However, the ΔνOH between the Gg′ and Tt species is only about 40 cm−1. The related nitroalcohols 2-nitro-1-propanol and 3-nitro-1-propanol also show relatively small ΔνOH’s in matrices. Semirigid STO-3G and 3-21G torsional potential energy surfaces were calculated and used to discuss the torsional barriers.
Energy disposal in charge transfer reactions producing NH+.3: Dependence of the NH+.3+H2O reaction on NH+.3 internal energy
Authors: Kemper Paul R. , Bowers Michael T. , Parent Denise C. , Mauclaire Gérard , Derai Renée , Marx Rose
Published: February 2003

The division of available energy between translational and internal modes has been determined for seven charge transfer reactions which produce NH+.3. The charge transfer reagent ions studied were Ar+., Kr+., Xe+., N+.2, O+.2, CO+., and CO+.2. The average kinetic energy released in the reaction was measured directly using an ion cyclotron resonance (ICR) technique. The internal energy deposited in the NH+.3 product was determined by further reacting the NH+.3 product with H2O in a tandem ICR spectrometer. The product distribution of this subsequent reaction is shown to depend strongly on the NH+.3 internal energy; and this dependence is quantitatively determined. The energy deposited in the neutral charge transfer product is calculated using energy balance. Comparisons are made between charge transfer reagent ions of similar recombination energies to show the effects of reagent ion internal modes on the division of available reaction energy.