bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of thermodynamic properties and phase separation of micellar solutions with lower consolute points
Authors: Thurston George M. , Blankschtein Daniel , Fisch Michael R. , Benedek George B.
Published: July 2002

A recently developed thermodynamic theory is extended to study the single-phase properties and the phase separation of micellar solutions which exhibit lower consolute points. Analytical results for the location of the critical point, shape of the coexistence curve, micellar size distribution, and osmotic compressibility along the critical isochore are presented. These results are in excellent agreement with experimental findings in micellar solutions of the nonionic amphiphile n-dodecyl hexaoxyethylene glycol monoether and water.
Stabilization calculations of resonance energies for the coplanar reactions H+FH and H+H2
Authors: Lauderdale Jack G. , Truhlar Donald G.
Published: July 2002

We propose that the second moments of a generalized radius be used as a criterion to select the ‘‘best’’ resonance wave function and energy from a series of stabilization calculations performed as a function of a real scale parameter. The method is tested for the collinear H+FH → HF+H reaction and the coplanar H+H2 → H2+H reaction and is then used to predict 12 resonance energies for the coplanar H+FH → HF+H reaction. All calculations are performed on model potential energy surfaces to allow for comparison with previous related theoretical studies.
Calculated static and dynamic properties of solid α-F2
Authors: Kirin Davor , Etters R. D.
Published: July 2002

The equilibrium structure and orientations of soid fluorine are calculated at zero temperature and pressure by completely optimizing a general monoclinic lattice with two molecules per unit cell. In addition to the lattice parameters and orientations, the molar volume, sublimation energy, and gas phase second virial coefficients are calculated. The k=0 libron and phonon frequencies are calculated using a lattice dynamics procedure, based upon the optimized structure. Both the proposed C2/c and C2/m structures are found for which nearly identical sublimation energies are calculated. Consequently, we are unable to resolve the controversy concerning which structure should be physically realized, although the C2/c structure gives slightly better overall agreement with experiment. The predicted results are found to be very sensitive to the magnitude of the quadrupole moment θ, which is taken to be 0.65×10−26 esu cm2 in this work.
Temperature dependence of the rate constants for deexcitation of Ar(1P1) and Ar(3P1) by SF6 and N2
Authors: Ukai Masatoshi , Koizumi Hitoshi , Shinsaka Kyoji , Hatano Yoshihiko
Published: July 2002

The temperature dependence of the deexcitation rate constants of the resonance states of Ar, 1P1 and 3P1, by SF6 and N2 has been measured in the temperature range from 133 to 300 K using a pulse radiolysis method, thus obtaining the collisional energy dependence of the deexcitation cross sections. The results of the cross sections for SF6 are compared with the Watanabe–Katsuura theory and good agreement is obtained. The results for N2 agree with predictions of the cross sections for a nonresonant case. It is concluded that a long-range dipole–dipole interaction is important in the deexcitation processes of Ar(1P1) and Ar(3P1) by SF6, but that a short-range interaction with curve crossing dominates in the deexcitation of Ar(1P1) and Ar(3P1) by N2.
Time-resolved measurements of OH(v=1) vibrational relaxation on SiO2 surfaces: Isotope and temperature dependence
Authors: Casassa M. P. , Heilweil E. J. , Stephenson J. C. , Cavanagh R. R.
Published: July 2002

Picosecond infrared spectroscopy was used to measure the vibrational energy relaxation time T1 of OH(v=1) and OD(v=1) groups chemisorbed on silica surfaces over the temperature range 100≤T≤800 K. The observed T1 times and their temperature dependencies are discussed in terms of a multiphonon relaxation mechanism. Limiting low temperature lifetimes are T1=220±20 ps (1σ) for OH(v=1) and T1=149±10 ps for OD(v=1).
State-to-state vibrational excitation of I2 in collisions with H2 and D2
Authors: Hall Gregory , Liu Kopin , McAuliffe Michael J. , Giese Clayton F. , Gentry W. R.
Published: July 2002

We have measured the energy dependence of cross sections for I2 vibrational excitation in collisions with H2 and D2 over the kinetic energy range from threshold to about 0.46 eV for D2 and 0.29 eV for H2. D2 is generally less efficient than He in exciting I2, probably because of a softer repulsive wall in the effective interaction potential. An exception is the low-energy cross section for v=0→2 excitation, which might be enhanced for D2 collisions because of an accidental resonance in energy with the D2 ( j=4→2) rotational transition. The v=0→1 cross sections for H2 collisions show a sharp and as yet unexplained feature at an energy 0.014 eV above threshold.
Probability oscillations in single pass curve crossings: Semiclassical predictions of nonmonotonic dependence on crossing velocity
Authors: Nesbitt David J. , Hynes James T.
Published: July 2002

We consider dynamical curve crossing behavior for one dimensional intermolecular potentials of arbitrary curvature. Our focus is threefold. (1) A semiclassical WKB formalism yields simple analytic expressions for time dependent transition probabilities throughout the crossing region. For low velocities v, our results reduce to the required quasistatic unitary transformation between diabatic and adiabatic representations. At higher v, the WKB solutions exhibit time dependent oscillations (‘‘beating’’ between the stationary adiabatic states) in excellent quantitative agreement with exact numerical solutions. (2) The WKB analysis indicates that an oscillatory dependence of curve crossing probability Pcr on v occurs for potential crossings of sufficient curvature, in qualitative disagreement with the extensively used Landau–Zener expression. These probability oscillations can be understood and predicted from simple considerations of potential curvature and coupling strengths. (3) We explore this oscillatory behavior for realistic curve crossing geometries. The results indicate that a nonmonotonic dependence of Pcr on velocity may occur in real molecular systems and could be experimentally observed in favorable albeit special circumstances. Even in the absence of oscillations, however, pronounced departure from Landau–Zener predictions can be anticipated.
Elastic light scattering by randomly oriented macromolecules: Computation of the complete set of observables
Authors: McClain W. M. , Ghoul W. A.
Published: July 2002

We present a calculation of all of the angle dependent observables that characterize completely the elastic scattering of light from solutions of macromolecules of arbitrary size and shape. The algorithm embodies the result of an exact analytic average over all orientations of the macromolecule, which may be large compared to the scattered wavelength. Our results are directly comparable to experiment; they are presented as a four-by-four Perrin matrix linking the four Stokes parameters of the incident light to the four Stokes parameters of the scattered light. The Perrin elements which we previously identified as retardation elements behave as they should: When we assume instantaneous electromagnetic interaction among the subunits of the macromolecule, they vanish; when the interaction is assumed to proceed at the speed of light, they are finite and may have easily measurable magnitude. Our algorithm captures all of the physical effects present in the Mie solution for scattering by spheres; but unlike the Mie solution it may be applied to scattering bodies of any shape, and with local variations in anisotropy.
Transient x-ray scattering calculated from molecular dynamics
Authors: Bergsma J. P. , Coladonato M. H. , Edelsten P. M. , Kahn J. D. , Wilson K. R. , Fredkin D. R.
Published: July 2002

With the continued development of pulsed x-ray sources, it may in time be possible to use transient x-ray diffraction to follow the molecular dynamics of chemical reactions in the liquid and solid states. To explore this possibility from the theoretical side, we have calculated, using classical molecular dynamics, the picosecond time-resolved x-ray scattering of a simplified model for a liquid state chemical reaction of substantial interest: the photodissociation of I2 molecules in rare gas and hexane solvents. The time scale of the separation of the I atoms and the effect of the solvent on their motion are observed in the computed transient x-ray diffraction patterns, and such effects might also be observed in a suitably designed experiment. This illustrates that transient x-ray diffraction might be an experimental tool for discovering the molecular dynamics of chemical reactions, with the advantage over transient optical spectroscopies such as infrared, electronic, and Raman that the connections between dynamics and diffraction can be precisely computed without uncertainties due to imprecise knowledge of dipole moments, transition dipole moments, and polarizabilities.
Mean field approach to orientational transitions in charge–transfer molecular crystals
Authors: Luty Tadeusz , Kuchta Bogdan
Published: July 2002

The mean field approach to orientational transitions in two-component molecular crystals has been formulated and illustrated by numerical calculations for two model systems: complexes of tetracyanobenzene (TCNB) with naphthalene (N–TCNB) and anthracene (A–TCNB). The study had addressed the characterization of the orientational disorder and mechanism of phase transitions in the crystals by calculating single particle and coupled rotational susceptibilities. It is concluded that the orientational instability in A–TCNB crystal is of the displacive type and a soft librational phonon mode at the M point of the Brillouin zone is predicted. In the N–TCNB crystal, an order–disorder type of the instability is predicted. For both systems the orientational phase transitions are found to be predominantly two dimensional.
Fourier transform photoelectron spectroscopy: The correlation function and the harmonic oscillator approximation
Authors: Ruščić Branko
Published: July 2002

The correlation function describes the time development of the wave packet placed by photoabsorption or photoionization onto the potential surface of an upper electronic state. The function can be obtained as a Fourier transform of the electronic band, and gives information about the features of the final state. The analytical expressions for the correlation function within the harmonic oscillator approximation are presented. Because of some unique properties of the correlation function, the expressions can be used to obtain accurate geometric details of the final state from experimental data. The approach is tested on some photoelectron spectra of diatomics and compared to known data. The method yields the equilibrium internuclear distance with an accuracy of ±0.0025 Å, and resolves the sign uncertainty present in the conventional harmonic Franck–Condon analysis. The comparison of the experimental data with the predictions of the harmonic model gives a deeper insight into the behavior of a wave packet in an anharmonic potential.
Rovibrational energy levels and expectation values for perturbed Kratzer oscillators
Authors: Requena A. , Zúñiga J. , Fuentes L. M. , Hidalgo A.
Published: July 2002

A perturbative treatment for perturbed Kratzer potentials is carried out. Recursion expressions for the energy and expectation values of powers of the internuclear separation are derived by using conmutation relations. An application to the Simons, Parr, and Finlan potential is made. Numerical results are compared with those derived by direct numerical integration.
Simple interpretation of dephasing in absorption and resonance Raman theory
Authors: Harris Robert A. , Mathies Richard A. , Pollard Walter T.
Published: July 2002

It is shown that the effects of ‘‘pure dephasing’’ in vibronic absorption and resonance Raman scattering may be simply derived by beginning with the Fermi Golden Rule. The medium introduces electronic dephasing by modulating the electronic energy gap of the molecule. In both absorption and Raman scattering the cross section depends upon a medium correlation function φ(t) whose form is identical to one that appears in vibrational relaxation theory. Explicit equations for the resonance Raman cross section are derived for an exponential medium correlation function. Also, φ(t) is derived for medium models which consist of a collection of classical and quantum mechanical oscillators.
Search for hexadecapole interaction in KTaO3 by 181Ta-NMR
Authors: Doering Elana B. , Waugh J. S.
Published: July 2002

A single-crystal 181Ta-NMR experiment was performed in KTaO3 in an unsuccessful attempt to observe the splitting pattern characteristic of electric hexadecapole coupling. A linewidth of about 12 kHz was observed, and assigned to residual first-order quadrupole couplings arising from imperfections in this nominally cubic crystal. Experimental difficulties prevented the removal of the electric quadrupole (and magnetic dipole–dipole) couplings by multiple-pulse linenarrowing methods.
Water–hydrocarbon interactions: Structure and internal rotation of the water–ethylene complex
Authors: Peterson K. I. , Klemperer W.
Published: July 2002

The rotational spectra of C2H4–H2O and C2H4–D2O were measured using the molecular beam electric resonance technique. The rotational and centrifugal distortion constants obtained for C2H4–H2O are: B+C=7274.747 (24), B−C=371.103 (8), A=25 858.4 (36), ΔJ=0.0279 (17), ΔJK=1.7352 (66), and δJ=0.002 99 (22) MHz. The dipole moment for both isotopic species is 1.094 (1) D. The structure derived from an analysis of the rotational constants and dipole moment is nonplanar with Cs symmetry. The water molecule is singly hydrogen bonded perpendicular to the plane of the ethylene; i.e., into the π system. The plane of the water bisects the C–C bond. The hydrogen bond length is 2.48 Å. Splittings are observed in the rotational transitions of C2H4–H2O but not in C2H4–D2O. These are assigned to excited torsional levels of the hindered internal rotation of the water with respect to the ethylene. The barrier height is estimated to be V2=1.0±0.2 kcal/mol which is surprisingly high for this weakly bound complex.
The infinite-order sudden approximation for collisions involving molecules in Π electronic states: A new derivation and calculations of rotationally inelastic cross sections for NO(X 2Π)+He and Ar
Authors: Corey Gregory C. , Alexander Millard H.
Published: July 2002

A new derivation is presented of the infinite-order sudden (IOS) approximation for rotationally inelastic collisions of a diatomic molecule in a Π electronic state with a closed-shell atom. This derivation clearly demonstrates the connection between the two sudden S functions for scattering off the adiabatic potential surfaces of A′ and A″ symmetry, which would arise from an ab initio calculation on an atom+Π-state molecule system, and the S-matrix elements in a diabatic basis, which are required in the quantum treatment of the collision dynamics. Coupled-states and IOS calculations were carried out for collisions of NO(X 2Π) with He and Ar, based on the electron gas potential surface of Nielson, Parker, and Pack [J. Chem. Phys. 66, 1396 (1977)], at total energies of 63, 150, and 300 meV. The IOS approximation is not reliable for collisions of NO with Ar, even at the highest collision energy considered here. However, for collisions with He at 150 and 300 meV, the IOS approximation is nearly quantitative for transitions both within and between the Ω=1/2 and Ω=3/2 manifolds.
Electrohydrodynamic stability of a fluid layer. II. Effect of a normal electric field
Authors: Mohamed Abou El Magd A. , Shehawey El Sayed F. El , Dib Yusry O. El
Published: July 2002

The electrodynamic stability of two interfaces separating three fluids is studied. A linear model allowing general surface deformations is applied. The case where there are no surface charges present at the interfaces showed a destabilizing influence of the electric field. When there are surface charges on the interface, it is found that the field is still destabilizing but this effect is partially shielded in some situations, which shows some analogy with the nonlinear stability of a single interface.
The breakdown of Hund’s rule in systems of fourfold symmetry
Authors: Gallup G. A.
Published: July 2002

The lowest energies of molecular systems with fourfold symmetry can violate Hund’s multiplicity rule. We derive relationships between two electron integrals that in favorable cases allow one to predict when this will happen. These relationships result in a fairly simple inequality as a criterion. The systematics of the phenomenon are illustrated with a series of calculations on model systems consisting of hydrogen atoms. These are rings of three, four, five, six, seven, and eight members as well as octahedral and square planar arrangements. These show how the results and simple ideas of molecular orbital theory can be qualitatively modified when various levels of configuration interaction are allowed in the theoretical discription.
Collective proton motions in H2O/H2O2 mixtures: Evidence for defects and network reconstruction
Authors: Green J. L. , Lacey A. R. , Sceats Mark G.
Published: July 2002

Measurements of the in-phase collective OH stretching band in the Raman spectrum of H2O/H2O2 mixtures have been carried out over a wide range of compositions and temperatures. At low mole fractions (<0.08) H2O2 introduces defects into the tetrahedral network of the water molecules, whereas at higher concentrations the behavior is interpreted in terms of a reduction of the vibron bandwidth resulting from reconstruction of the hydrogen bond network to one of lower average coordination.
Bifurcation from excitability to limit cycle oscillations at the end of the induction period in the classical Belousov–Zhabotinsky reaction
Authors: Noszticzius Zoltan , Wittmann Maria , Stirling Peter
Published: July 2002

The appearance of full blown oscillations at the end of the induction period in the classical BZ reaction (substrate: malonic acid) is usually explained by a subcritical Hopf bifurcation. Our perturbation experiments have revealed that the classical BZ reaction is excitable in its induction period and the threshold of excitability is decreasing gradually to zero during that period. Thus the sudden appearance of limit cycle oscillations at the end of the induction period can be explained by a saddle-node infinite period (SNIPER) bifurcation as well.
Variation of the electron attachment to n-C4F1 with temperature
Authors: Datskos P. G. , Christophorou L. G.
Published: July 2002

The total electron attachment rate constant ka(〈ε〉) for n-C4F10 has been measured using an electron swarm technique in the mean electron energy, 〈ε〉, range 0.76 to 4.81 eV and over the temperature, T, range 300 to 750 K. The measured ka(〈ε〉,T) and the total electron attachment cross section σa(ε,T) determined from these by unfolding are reported. The ka(〈ε〉) first decrease slowly with T between 300 and 400 K, then decrease precipitously between 400 and ∼500 K, and subsequently increase for T>500 K. The overall variation of ka with T depends on 〈ε〉. From the measured dependence of ka(〈ε〉) on the total gas number density and T we determined the contribution of dissociative and nondissociative attachment to ka as a function of 〈ε〉; the contribution of nondissociative attachment decreases with T and the contribution of dissociative attachment increases with T. The ratio Rd/t of the dissociative to the total electron attachment is small at low 〈ε〉 and T, but it increases with both 〈ε〉 and T and reaches unity for T>500 K and 〈ε〉>1.7 eV. These results are discussed in relation to similar earlier work on C2F6 and C3F8.
High pressure equation of state for solid argon from interatomic potentials
Authors: Barker J. A.
Published: July 2002

The pressure–volume isotherm for solid argon at 298 K is calculated by the Monte Carlo method using the pair potential of Barker, Fisher, and Watts (BFW) together with the Axilrod–Teller (AT) three-body interaction. Comparison is made with recent experimental data of Ross and co-workers extending up to 800 kbar. Agreement with experiment is good though not perfect. Comparison is made with the pair potentials of Aziz and Chen and Koide, Meath, and Allnatt and with the exp-6 ‘‘effective’’ pair potential of Ross.
The effect of vibrational excitation on the 225 nm absorption spectrum of OCS: A non-Condon analysis
Authors: Joens Jeffrey A.
Published: July 2002

A model for the calculation of the electronic spectrum of a triatomic molecule for the case of a forbidden transition that becomes allowed due to vibronic coupling is presented. The model represents the upper electronic state potential in terms of a small number of variables that can be determined from experimental measurements, and takes into account the dependence of the transition moment on the geometry of the molecule. The model is used to analysis the 225 nm system of OCS. In general, there is good agreement between available experimental information on the absorption spectrum and the results obtained from the model. There are however some limitations in the model, particularly in the way in which it treats higher order and coupling terms in the upper state stretching modes of OCS. The model is useful for predicting the effect of vibrational excitation on a molecular electronic spectrum. The model can be used to analyze the spectra of a number of other molecules besides OCS, and provides a starting point for more sophisticated photodissociation models.
C
i
s-glyoxal in effusive and supersonic beams. Spectroscopy, selective pumping, and c
i
s–t
r
a
n
s interconversion
Authors: Butz Kirk W. , Johnson Jeffrey R. , Krajnovich Douglas J. , Parmenter Charles S.
Published: July 2002

S 1–S0 fluorescence excitation and dispersed fluorescence spectra from molecular beams containing both trans- and cis-glyoxal have been used to extend the characterization of the 1A1 (S0) and 1B1 (S1) states of cis-glyoxal. Explorations using both effusive and supersonic beams with rotational temperatures ranging from 350 to 30 K have revealed no conditions where cis can be pumped (S1←S0) without simultaneous excitation of trans. Selective cis excitation at low beam temperatures is hampered by highly efficient cis→trans conformational interconversion in the molecular beam expansions. Under conditions of optimal cis:trans contrast (cool expansions with Ar carrier gas), four new S1–S0 cis absorption bands (510,520,610, and 720 ) are identified, yielding cis frequencies ν′5 =303 cm−1, ν6 =713 cm−1, and 2ν′7 =688 cm−1. Single vibronic level fluorescence spectra have been obtained from the levels 00, 51, and 61 of cis-glyoxal, from which values of two cis S0 fundamentals are newly established: ν4 =826 cm−1 and ν′′6 =1049 cm−1. Previous assignments of ν4 and ν′′8 are shown to be incorrect and

ν8 now joins the list of unknown frequencies. The 1B1–1A1 system of cis-glyoxal contains forbidden transitions, vibronically induced by Δv=±1 changes in the a2 mode ν6. A remeasurement of the cis–trans energy separation in the ground electronic state gives ΔH=1350±200 cm−1, matching to within experimental uncertainty a previous experimental determination. As an aside, the trans-glyoxal fundamental ν′′3 =1352 cm−1 has been obtained from observations of the trans 301 and 301510 transitions. With this addition, all trans S0 fundamentals have now been directly measured.
Resonantly enhanced two-photon spectroscopy of HCl and DCl in the 77 000–87 000 cm−1 region
Authors: Callaghan Robert , Arepalli Sivaram , Gordon Robert J.
Published: July 2002

The two-photon spectra of HCl and DCl were studied using the technique of resonantly enhanced multiphoton ionization. Rovibrational bands were observed for the X 1Σ+→V 1Σ+, E 1Σ+, F 1Δ2, d 3Π1, f 3Δ2, and g 3Σ− transitions. The results are compared with the single photon spectra of Ginter, Douglas, and co-workers and with the molecular orbital calculations of Bettendorf et al.
Spectroscopic studies of cryogenic fluids: Benzene in nitrogen
Authors: Nowak R. , Bernstein E. R.
Published: July 2002

Energy shifts and bandwidths for the 1B2u←1A1g optical absorption of benzene in supercritical nitrogen are presented as a function of pressure, temperature, and density. The pressure and density dependence of energy shifts of room temperature emission of benzene in nitrogen fluid is also reported. Both absorption and emission spectra exhibit shifts to lower energy as a function of density, whereas almost no spectral shifts are observed if the density is kept constant and temperature and pressure varied simultaneously. Thus, density is the fundamental microscopic parameter for energy shifts of optical transitions in supercritical nitrogen. This result is analogous to the findings for the liquid benzene/propane system and can be interpreted qualitatively in terms of changes occurring in the intermolecular potential; however, in the benzene/supercritical nitrogen system an additional small density independent temperature effect on the transition energy has been identified. Experimental results are compared to dielectric (Onsager–Böttcher and Wertheim) and microscopic quantum statistical mechanical (Schweizer–Chandler) theories of solvent effects on solute electronic spectra. Reasonably good agreement between experiment and theory is found. The results demonstrate that liquid state theory can be used to describe the supercritical nitrogen fluid.
Intramolecular energy transfer in the HNC/HCN isomerization reaction: Quasiclassical state specific isomerization rates controlled by localized potential features
Authors: Smith R. Scott , Shirts Randall B. , Patterson Chris W.
Published: July 2002

A model of the HCN molecule with two degrees of freedom, consisting of the H–NC stretch and the H–NC bend with the C≡N distance held fixed, was studied. The semiclassical technique of adiabatic switching was used to obtain trajectories at eigenvalues of metastable states of the system. Quasiclassical results for the isomerization rate from HNC to HCN at various stretch and bend excitations are given. Results show that the isomerization rate is highly dependent on the bend excitation and less so on the total energy of the system in contradiction to predictions of statistical theories. Examination of individual trajectories and surfaces of section reveal the development of chaos near the Nbend =6 level. The development of chaos is apparently responsible for the energy transfer between the modes and thus the isomerization rate. Examination of the potential surface reveals a distinct topological change near the Nbend =6 level. The region responsible for energy transfer is identified, and the possible mechanisms for this transfer are enumerated.
A
b i
n
i
t
i
o studies of the structures and energies of the H−(H2O) and H−(H2O)2 complexes
Authors: Chal/asinśki Grzegorz , Kendall Rick A. , Simons Jack
Published: July 2002

Accurate calculations for the H−(H2O) complex with extended basis sets are reported at the restricted Hartree–Fock (RHF) through the fourth-order Mo/ller–Plesset (MP) perturbation levels of theory. In the equilibrium geometry of the H−(H2O) complex the H− anion is found to lie almost along one of the H–O bond directions. The H–H− distance proved to be very sensitive to electron correlation effects: it is 1.8 and 1.4 Å at the RHF and MP2 levels, respectively. The interaction energy between H− and H2O at the MP4 level including conterpoise corrections for basis set superposition error, depending upon the basis set used, is found to range from 16.2 to 16.9 kcal/mol, and the electron correlation is responsible for one-third of this value. The enthalpy of formation of H−(H2O) is estimated to be from −15.2 to −16.0 kcal/mol compared with the experimental value of −17.3±1.2 kcal/mol. The vibrational frequencies of H−(H2O) are also reported. The H−(H2O)2 complex is also studied by using a polarized double zeta basis set. The geometry of the H−(H2O)2 complex is partly optimized at the MP2 level and the energetic effects of the addition of the second water to H−(H2O) are analyzed.
Diatomics-in-molecules models for H2O and H2O−. II. A self-consistent description of the 1A′, 1A″, 3A′, and 3A″ states of H2O
Authors: Polak R. , Paidarova I. , Kuntz P. J.
Published: July 2002

This paper describes a small (6 to 9 basis functions) model for the potential energy surfaces relevant to the chemical reaction O(1D)+H2→OH(X 2Π)+H. The model is optimized with respect to the 1A′ and 1A″ states correlating with the reagents and products of this reaction; this is done in such a way as to simultaneously provide a qualitatively correct description of the 3A′ and 3A″ states of H2O. In this sense the model is self-consistent. In agreement with other semiempirical work, the results indicate that two 1A′ surfaces and one 1A″ surface are pertinent for a dynamical study of this reaction. The model adequately represents the most important features of H2O potential energy surfaces and is at the same time small enough to be used directly in a trajectory calculation of the reaction cross section.
Ultraviolet and visible spectroscopy of poly(paraphenylene vinylene)
Authors: Obrzut Jan , Karasz Frank E.
Published: July 2002

The UV–visible spectrum of 200 nm thick films of poly(p-phenylene vinylene) (PPV) shows three absorption bands, with maxima at 6.12, 5.06, and 3.09 eV. The first two absorption bands are related to localized molecular states while the band at 3.08 eV has a more complicated character with a 1.5 eV bandwidth, a 2.49 eV band edge and a maximum absorption coefficient of 1.5×105 cm−1. Semiempirical MO calculations at the INDO and PPP levels and VEH techniques have been applied in an interpretation of the electronic structure and excitations in this electrically conducting polymer. A description of the lower energy excited states of PPV has been achieved using an exciton model in which the polymer chain is treated as an ordered array of identical molecules. The 3.08 eV polymer absorption band is then attributed to an electron excitation spread over several monomer units strongly perturbed by electronic–vibrational coupling.
On the role of density fluctuations in the entropy of a fluid
Authors: Wallace Duane C.
Published: July 2002

The phase-space region occupied by a dense fluid is characterized by significant multiparticle correlations, and by small density fluctuations. On the basis of this characterization, a statistical-mechanical formula is constructed, to represent the entropy of an equilibrium dense fluid. For the example of fluid sodium, the statistical entropy agrees well with the measured thermodynamic entropy.
Triplet exciton fusion at dislocations in anthracene
Authors: Yokoi K. , Ohba Y.
Published: July 2002

The fusion reaction of triplet excitons trapped at edge dislocations (001)[010] was studied in anthracene crystals by observing delayed fluorescence. The fusion rate obtained by kinetic analysis which includes the exciton-trapping effect is about unity measured in cm3 s−1, which is much greater than the fusion rate of free excitons (≅4×10−12 cm3 s−1), and also greater than an estimation based on the diffusion transport of trapped excitons along the dislocation line. The fusion mechanism of a pair of trapped excitons must be dominated by some long-range interaction.
Theoretical studies of electron transfer in metal dimers: XY+→X+Y, where X, Y=Be, Mg, Ca, Zn, Cd
Authors: Cave Robert J. , Baxter David V. , Goddard William A. , Baldeschwieler John D.
Published: July 2002

The electronic matrix element responsible for electron exchange in a series of metal dimers was calculated using ab initio wave functions. The distance dependence is approximately exponential for a large range of internuclear separations. A localized description, where the two nonorthogonal structures characterizing the electron localized at the left and right sites are each obtained self-consistently, is found to provide the best description of the electron exchange process. We find that Gaussian basis sets are capable of predicting the expected exponential decay of the electronic interactions even at quite large internuclear distances.
A frequency weighted sum rule for vibrational circular dichroism intensities
Authors: Polavarapu P. L.
Published: July 2002

It is shown that the sum of vibrational circular dichroism (VCD) intensities of fundamental bands, with each VCD band intensity weighted by the inverse square of vibrational frequency, of isotopic molecules are related through inverse force constant matrix elements. This relation, similar to that for the absorption intensities, is useful for determining the force constants from the experimental data.
A Stieltjes imaging method for molecular shake-off continua. Application to the core photoelectron spectrum of water
Authors: Ågren Hans , Carravetta Vincenzo
Published: July 2002

A Stieltjes imaging method for molecular shake-off continua is devised and numerically applied to the core photoelectron spectrum of water. Assuming the independent particle approximation, the full core photoelectron spectrum corresponding to an energy range of 0–100 eV, including main peak, shake-up peaks, and shake-off continua, is constructed in one ansatz. The shake-off cross sections are obtained in both length and velocity gauges and the effect of discrete and continuum interaction between channels with different spin coupling is investigated. The variation of the dipole transition moment vs excitation energy for the primary photoelectron is explicitly calculated, which gives the corrections to the sudden approximation also in the high-energy limit of x-ray excitation. By comparison, the plane wave approximation for the primary photoelectron is found appropriate only for relative cross sections in the shake spectrum, while for the absolute photoionization cross section it significantly deviates from results obtained from the variationally optimized photoelectron functions. The form of the variationally optimized continuum waves close to the oxygen nucleus is compared with the corresponding plane waves and Coulomb waves of different effective charges.
An optical–optical double resonance spectroscopy of Br2: Observation of the 1g(3P2) ion-pair state
Authors: Ishiwata Takashi , Hara Takefumi , Obi Kinichi , Tanaka Ikuzo
Published: July 2002

We report the first observation of the 1g(3P2) ion-pair state of Br2 by the optical–optical double resonance technique. This state is accessible from the ground state stepwisely in the pump–probe excitation scheme, 1g(3P2) –A 3Π(1u) –X 1Σ+g. The double resonance transitions were interpreted on the basis of the optical selection rules. Emission spectra of the 1g(3P2) –A 3Π(1u) system were interpreted to establish the absolute v′ numbering of the ion-pair state from the Franck–Condon factor calculations. Observations were made on the v′=0 through v′=10 levels of the 79Br2 isotope species. The molecular parameters obtained are Te =49 390.364(16), ωe =151.704(10), ωexe =0.4169(24), ωeye =4.7(16)×10−4, Be =0.042 1840(73), αe =1.5209(61)×10−4, De =1.32(10)×10−8, and q=4.91(19)×10−5 (all in cm−1 with 3σ in parentheses).
Algebraic time-dependent variational approach to dynamical calculations
Authors: Shi Shenghua , Rabitz Herschel
Published: July 2002

A set of time-dependent basis states is obtained with a group of unitary transformations generated by a Lie algebra. Applying the time-dependent variational principle to the trial function subspace constructed from the linear combination of the time-dependent basis states gives rise to a set of ‘‘classical’’ equations of motion for the group parameters and the expansion coefficients from which the time evolution of the system state can be determined. The formulation is developed for a general Lie algebra as well as for the commonly encountered algebra containing homogeneous polynominal products of the coordinate Q and momentum P operators (or equivalently the boson creation a° and annihilation a operators) of order 0, 1, and 2. Explicit expressions for the transition amplitudes are derived by virtue of the cannonical transformation properties of the unitary transformation. The applicability of the present formalism in a variety of problems is implied by two illustrative examples: (a) a parametric amplifier; (b) the collinear collision of an atom with a Morse oscillator.
CO binding to hemoglobin: Spin-tunneling temperature dependence, Fe electronic states, and electronic effects on nonadiabatic dynamics
Authors: Gerstman Bernard S.
Published: July 2002

The electronic spin change of the iron in heme proteins that occurs upon the binding of carbon monoxide is investigated theoretically. A group theoretic approach is taken to determine an Fe electronic level scheme that is consistent with the experimentally observed temperature dependence of the rate of binding of CO. We show that a nearly degenerate ground state orbital doublet used with the standard theory for the spin transition involving second-order spin–orbit coupling gives a temperature dependence consistent with the experimental data. Measurements, using techniques independent of the recombination experiments, have produced evidence for this ground state orbital doublet, and we describe additional experimental methods for testing this model. The effect of this electronic transition on the nonadiabaticity of the reaction dynamics as a function of temperature is also calculated.
Comment on: The Magnus expansion for the harmonically driven harmonic oscillator
Authors: Fernández Francisco M.
Published: July 2002

The exponential time-evolution operator for the harmonically driven harmonic oscillator is exactly obtained from the equations of motion for the coordinate and momentum in the Heisenberg representation. The convergence of the Magnus expansion is discussed.
Kerr effect. II. Contributions from hyperpolarizability and anisotropic polarizability
Authors: Ho/ye J. S. , Stell G.
Published: July 2002

Contributions to the Kerr effect in a polar polarizable fluid, glass, or cubic crystal resulting from anisotropic polarizability and hyperpolarizabilities (due to the nonlinear response to the applied static field) are derived from a statistical mechanical microscopic theory for a classical molecular model with fluctuating molecular polarizability. These contributions are embodied in an expression that involves the dielectric constant and the indices of refraction parallel and perpendicular to the applied field. Either theoretically derived or experimental values of these quantities can be used in the quantitative evaluations of the expression, which is to be added to the contribution arising from distortion of the pair correlation function resulting from the applied field, derived earlier [J. S. Ho/ye and G. Stell, J. Chem. Phys. 81, 3200 (1984)].
Nonuniversality of the single-chain rod-to-coil transition scaling functions: Exact results for directed walks
Authors: Privman V. , Frisch H. L.
Published: July 2002

We report exact calculations of several thermodynamic and correlation quantities for the turn-weighted self-avoiding walk models on the directed square, partially directed square, and partially directed triangular lattices. The conformational transition from the coil-to-rod-like chains, obtained as the statistical weight per turn goes to zero, obeys scaling laws. The scaling functions, however, are not universal for the different models considered.
Classical dynamics of an internal Coriolis-induced isomerization of t
r
a
n
s-diimide
Authors: Spears L. Gene , Hutchinson John S.
Published: July 2002

The isomerization dynamics of a model of vibrationally excited trans-diimide are reported. Energy transfer into the torsional reaction coordinate does not proceed through the resonance processes described in a companion paper. Rather, a Coriolis coupling of the NH stretches to the torsion dominates the reaction when appropriate relative phase relationships are satisfied. Large amounts of energy can be transferred in this process; thus isomerization is observed in as few as three NH stretching periods. Furthermore, the phase relationships appropriate for isomerization are very restrictive, so that a surprisingly well defined average behavior exists for several vibrational periods both prior to and after the reactive event. We have identified a specific dynamic mechanism responsible for trans/cis isomerization and for trapping the reactive trajectories in the cis-configuration. Despite the complexity of this six degrees of freedom model system, the reactive dynamics can be described in a simple manner.
Generating function methods for macromolecules at surfaces. II. One molecule between two planes
Authors: Eichinger B. E. , Jackson David M. , McKay Brendan D.
Published: July 2002

The configuration statistics for a macromolecule that is sandwiched between and either repelled or adsorbed by a pair of plane parallel surfaces are developed by means of a generating function technique. The theory is based on our previous work [J. Chem. Phys. 85, 5299 (1986)] on the problem of a single plane; it has the advantages of simplicity and flexibility. The generating functions that are constructed are used to evaluate expectation values for the lengths of loops, bridges, and tails as well as the number of bound segments. The canonical partition function for a molecule of n lattice steps is extracted from the generating function by use of numerical integration.
The Rydberg states of FH2
Authors: Petsalakis I. D. , Theodorakopoulos G. , Wright J. S. , Hamilton I. P.
Published: July 2002

The ground and first few excited electronic states of FH2 have been calculated by the MRD-CI method, in an effort to make predictions on the Rydberg spectrum of this molecule. The results show that the excited Rydberg states, up to the 4p levels (in the united atom notation), are bound and have minima at geometries similar to that of the cation FH+2 except the first excited state, which is also found to be bound but with minimum energy at a considerably longer bond length. The most intense bound–bound transitions are predicted to occur from the 3d, 4p, and 4s states to the first excited state 1 2B2(3p).
Variational studies of the importance of triple and quadruple excitations on the barrier height for F+H2→FH+H
Authors: Scuseria Gustavo E. , Schaefer Henry F.
Published: July 2002

Configuration interaction (CI) methods including all single, double, triple, and quadruple (SDTQ) excitations have been applied with several basis sets to the prediction of the barrier height for the F+H2 reaction.
Reduced equations of motion for molecule–surface scattering
Authors: Diestler D. J. , Riley M. E.
Published: July 2002

Novel reduced equations of motion for an admolecule scattering from a solid surface are proposed. The equations, derived under the assumption that the collisional force varies slowly with respect to typical periods of vibration of the solid atoms, can be cast as a set of coupled vector equations with dyadic friction coefficients that depend upon the instantaneous configuration of the system. In essence, the reduced equations describe the admolecule moving in the field of the instantly readjusting lattice. The reduced equations are applied to an idealized collision system (an adatom interacting via the Morse potential with a single atom of the simple-cubic Rosenstock–Newell lattice at T=0 K) for which numerically exact energy accommodation coefficients are available. The approximation is quite reliable over a reasonable range of system parameters, including those characteristic of realistic systems.
Theory of enhanced energy tranfer in an aerosol particle
Authors: Leung P. T. , Young K.
Published: July 2002

The enhanced tranfer of energy between donor and acceptor molecules in a micrometer size aerosol particle due to the presence of sharp morphology dependent resonances is discussed quantum mechanically. By making use of density of states and quasimode concepts, it is shown that the enhancement factor depends only on the quality factor Q of the resonances, but not on the mode functions, therefore yielding results valid for any microparticle geometry (for which the mode functions are, in general, unknown). Emphasis is also placed on the absorption dominated limit (easily reached in practice), in which the result is particularly simple.
Structure and predissociation of electronically excited HgN2 complex
Authors: Yamanouchi Kaoru , Isogai Shinji , Tsuchiya Soji , Duval Marie-Christine , Jouvet Christophe , d’Azy Odile Benoist , Soep Benoit
Published: July 2002

The structure and the predissociation dynamics of an excited van der Waals (vdW) complex HgN2 which is produced in a supersonic jet, have been investigated by means of laser-induced fluorescence (LIF) spectroscopy. The vibronic systems of the A–X and B–X transitions are analyzed in terms of the vdW bond stretching and the Hg–N2 bending modes. The predissociation process, HgN2(A or B)→Hg(3P0)+N2, in both of the A and B vibronic states has also been discussed by detecting Hg(3P0) as a predissociation product. Assignment of vibrational quantum numbers for the vdW bond stretching mode is made on the basis of the observed splittings among isotopic species for mHgN2 (m=198, 199, 200, 201, 202, and 204). Rotational contour analysis of the LIF and the 3P0 formation spectra is performed to determine (1) the branching ratio for the fluorescence and the predissociation processes as a function of vibrational and rotational quantum numbers and (2) potential functions for the A and B states along the vdW bond coordinate. The dissociation energies of the complex in the A and B states are determined to be 660(77) and 41(8) cm−1, respectively, and the equilibrium distances between Hg and c.m. of N2 are 3.08(2) and 5.37(10) Å for the respective two states. Based on the simulation of the Franck–Condon profile of the bending progressions for the A–X band, the equilibrium configurations for the X and A states are estimated to be T shaped and linear, respectively.
Electrostatic fluctuation interactions between neutral surfaces with adsorbed, mobile ions or dipoles
Authors: Attard Phil , Kjellander Roland , Mitchell D. John , Jönsson Bo
Published: July 2002

The electrostatic fluctuation interactions between two surfaces which carry charged or dipolar, laterally mobile particles, are investigated using both a formally exact theory and approximate methods. Each surface constitutes the boundary between two dielectric continua and has no net charge. The exact analysis demonstrates the interdependence of the van der Waals interaction between the dielectric media and the electrostatic image interactions of the particles. General asymptotic laws for the pressure between the surfaces, valid at large separations, are extracted from the exact equations. Even at relatively small separations, the asymptotic expressions give surprisingly good estimates of the actual interactions, as calculated using the hypernetted chain equation and Monte Carlo techniques. A simple mean field perturbation approximation is also investigated and is shown to give good results, provided accurate pair distributions are known for the corresponding isolated, single surface.