bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of the semicontinuum potential model to deduce localized electron trapping sites in single crystals of d-sorbitol
Authors: Kevan Larry , Schlick Shulamith , Narayana P. A. , Feng Da-Fei
Published: February 2003

The semicontinuum model of electron solvation [J. Am. Chem. Soc. 95, 1398 (1976)] has been used to calculate the stability of the trapped electron in single crystals of D-sorbitol at 4.2 K. Experimental ENDOR results suggest an environment of two OH dipoles, with the protons at equal distances from the electron. The geometry of the most probable site in a lattice undistorted by irradiation results in an unbound electron state. This arises because the dominant term in the total energy is the dipole–dipole interaction which is attractive for that geometry. If the oxygen atoms remain fixed at crystal lattice positions but the protons are allowed to rearrange so as to point towards the electron, a minimum in the total energy is obtained, but the electron–proton distance is much shorter than the experimental value. However, good agreement with experiment is obtained for configurations of two or four OH dipoles pointing towards the trapped electron when lattice rearrangement is allowed. These calculations suggest that dipole rearrangement occurs during electron trapping in single crystals even at 4.2 K.
The water monomer on the prism face of ice and above a four layer ice basal face ledge: An effective pair potential model
Authors: Hale Barbara N. , Kiefer Jerry , Ward Carolyn A.
Published: February 2003

A previous study of the water monomer on the basal faces of ice is extended to consider the interaction of the water molecule with a rigid prism face of ice and with an ice basal face ledge. The effective central force H2O–H2O potentials of Stillinger and Rahman are used to generate maximal binding energy surfaces for the H2O adsorbed on the sample substrates. The results indicate that the prism face of ice binds the water molecule more strongly than the basal faces, and the step on the basal face serves to expose high binding sites on the prism face and multiple bonding configurations at the base of the ledge. Bonding on all the substrates is preferred at sites not directly above water molecules in the first or second layer. Average maximal binding energies on the prism and ledge surfaces are 9 kcal/mol compared to a value of 8 kcal/mol on the (unpolarized) basal face. Optimal bonding configuration for the adsorbed water moment are also presented. Barriers to diffusion between maximal binding sites are 2.5 and 3.0 kcal/mol on the basal and prism faces, respectively.
Dielectric relaxation in dipolar mixtures
Authors: Sharma Ashok K. , Agarwal Vinod K. , Mansingh Abhai
Published: February 2003

Dielectric constants and losses have been measured at several microwave frequencies in the range 2.4–25 GHz for two binary mixtures: bromoform +1-bromonaphthalene (dipole moment ratio is about 1:1), and nitrobenzene +1-bromonaphthalene (dipole moment ratio is about 4:1). The dielectric data of each binary system have been analyzed in terms of two superimposed Debye regions. This analysis has been carried out to study the effect of internal field on the numerical values of the relative weights or amplitudes C1 and C2 in the dipolar mixtures. It is observed that the discrepancy between the numerical values of weight factors obtained from relaxation data and dipole moment and mole fractions of the binary mixture cannot be attributed to the internal field. For the bromoform +1-bromonaphthalene system, it has been found that both the Budo and Cole–Cole equation represent the system equally well, but for nitrobenzene +1-bromonaphthalene system the Cole distribution fits better than Budo’s equation.
Ultrafast intramolecular vibrational relaxation: The overtone spectrum of supercooled tetramethyldioxetane
Authors: West G. A. , Mariella R. P. , Pete J. A. , Hammond W. B. , Heller D. F.
Published: February 2003

A pulsed beam of TMD molecules was produced by expansion through a supersonic nozzle. The vibrational spectra was studies using a pulsed alexandrite laser beam. (AIP)
Location and energetics of transition states for the reactions H+ClF, H+FCl, H+F2, and H+Cl2
Authors: Eades Robert A. , Dunning Thom. H. , Dixon David A.
Published: February 2003

The potential energy surfaces of the reactions H+ClF→Hcl+F, H+FCl→HF+Cl, H+F2→HF+F, and H+Cl2→HCl+Cl have been investigated using ab initio molecular orbital theory. Calculations were done using polarized double zeta basis sets as the SOGVB+POL–CI level. The barrier heights uncorrected for zero-point effects for the above reactions are 3.2, 9.6, 5.2, and 2.9 kcal/mol, respectively. The transition states for the H+F2 and H+FCl reactions are found to be collinear, while the transition states for the H+FClF and H+Cl2 reactions are found to be bent. The potential surface for a hydrogen migrating from Cl to F in the region of the transition state for H+ClF was found to be very repulsive.
Rational fraction representation of diatomic vibrational potentials. V. The 3dσg state of H+2
Authors: Sonnleitner Stephanie A. , Beckel Charles L. , Colucci Anthony J. , Scaggs E. Rodney
Published: February 2003

The vibrational potential of the 3dσg electronic state of H+2 is represented by rational fractions in nuclear separation R over the range R = 1 to 90 a.u. The long range form of the potential is reproduced by using either one or three constraints on the denominator polynomial. The best overall fit achieved over the range (1–90 a.u.) has an rms of 1.54×10−7. However, fits over restricted ranges (1–28 and 5–90 a.u.) each achieve an order of magnitude smaller error.
Quantum test of quasiclassical calculations on atom–triatom collisions
Authors: Clary D. C.
Published: February 2003

Exact quantum calculations are performed to examine the accuracy of recent quasiclassical results1,2 for the energy transfer in collinear Kr+CO2 collisions. (AIP)
Electron spin echo study of Nd3+ solvation in methanol glasses
Authors: Janakiraman R. , Kevan Larry
Published: February 2003

Electron spin echo modulation studies have been carried out for Nd3+ in methanol glasses at 4.2 K. By using the partially deuterated methanols (CH3OD and CD3OH), analysis of deuterium modulation in three-pulse electron spin echo decay curves gives the distances and numbers of Nd–D(OD) and Nd–D(CD3) interactions. It is found that Nd3+ is coordinated by nine equivalent methanol molecules, with distances of Nd–H (OH) of 3.1 Å and Nd–H (CH3) of 4.0 Å. These distances establish that the molecular dipole of methanol is oriented toward Nd3+.
Studies of chemiluminescence in boron atom reactions with O2, SO2, N2O, NO2, and H2O2
Authors: DeHaven James , O’Connor Michael T. , Davidovits Paul
Published: February 2003

Chemiluminescence has been observed in the single collision reaction of boron atoms with O2, SO2, N2O, NO2, and H2O2. The experiments were performed in a beam-gas apparatus using photon counting to detect the optical signals. The electronically excited A 2π state of BO was observed in all the reactions. In addition, the BO2(A 2πu) state was observed in the reaction of boron with SO2. Cross sections for the production of electronically excited molecules were determined. These cross sections for the reactions with O2, SO2, N2O, NO2, and H2O2 are, respectively, 0.048, 0.0078, 0.01, 0.023, and 0.005 Å2. In the reactions of boron with SO2 the branching ratio was obtained for the two observed chemiluminescent reaction channels. The vibrational distributions in the electronically excited state of BO were determined in the reactions where the spectra were adequately resolved. Some implications of the results are discussed.
Observation of selective desorption of one-phonon inelastically scattered He atoms from a LiF crystal surface
Authors: Brusdeylins G. , Doak R. Bruce , Toennies J. Peter
Published: February 2003

Measurements of angular distributions for scattering of low energy (≊20 meV) monoenergetic (Δv/v = 0.8% FWHM) He beams from a LiF (001) crystal along the 〈100〉 and 〈110〉 directions and from a NaF (001) crystal along the 〈100〉 direction reveal a large number of mostly small maxima and minima between the specular and diffraction peaks. Typical intensities of these features are about 10−3 of the specular peak. Time-of-flight spectra of the scattered atoms at the incident angles corresponding to the maxima reveal that the atoms were inelastically scattered by single phonons. From the known bound state energies for both systems He–LiF and He–NaF, we find that most of the maxima can be explained by a two step process in which the atoms are first elastically selectively adsorbed by diffraction into an in-plane bound state and then are subsequently desorbed as a result of a single phonon inelastic process. From the observed half-widths the lifetimes of atoms in the trapped states are found to depend on the vibrational quantum number and for the weakest bound state is as large as 60×10−12 sec. Evidence for minima involving selective adsorption into out-of-plane states is also presented.
An ESR and ENDOR study of tunneling rotation of a hindered CH3 group in C2H5 radicals trapped in xenon matrices at 4.2 K: Environment effect on internal rotation
Authors: Toriyama Kazumi , Iwasaki Machio , Nunome Keichi , Muto Hachizo
Published: February 2003

Two kinds of ethyl radicals have been observed in Xe matrices depending upon the reactions of radical formation. The CH3 group in C2H5(I) formed from H abstraction by H atoms at temperatures below 50 K exhibits an ESR hyperfine pattern characteristic of tunneling rotation in a threefold hindering potential, whereas that in C2H5(II) formed from homolytic scission of the C–H bond exhibits a conventional spectrum typical of free or random hopping rotation with a small barrier. The g and hyperfine coupling tensors indicate that both the ethyl radicals possess a conventional planar or nearly planar structure. It is concluded that the difference in the internal motion arises from an environmental effect which lowers the rotational symmetry from a six- to threefold potential. The tunneling splitting has been determined to be 450 MHz for C2H5(I). From this the barrier to internal rotation is estimated to be about 1 kcal/mol. In estimating the barrier height the influence of the interaction of the CH2 group with the surroundings is taken into consideration. The contribution of a zero-point torsional amplitude in the averaging process of the β-proton coupling constant is also discussed.
Acetic acid under pressure: The formation below 0 °C, x-ray powder diffraction pattern, and far-infrared absorption spectrum of phase II
Authors: Bertie John E. , Wilton Richard W.
Published: February 2003

This paper presents the phase diagram of acetic acid below 0 °C and 2 kbar, and the x-ray powder diffraction pattern and the far-infrared absorption spectrum of acetic acid II at ∼100 °K and atmospheric pressure. The x-ray powder diffraction pattern of acetic acid I and the far-infrared absorption spectra of acetic acid I and propanoic acid have been remeasured and are presented for comparison. The transition from acetic acid I to II becomes very slow below −20 °C, so that acetic acid II can not be readily prepared below 1050 bar. The extrapolation of the equilibrium line suggests that phase II is the stable phase at atmospheric pressure below about −60 °C. Acetic acid II does not convert rapidly to acetic acid I at atmospheric pressure below −34 °C. The transition pressure at 0 °C is 1135±18 bar, compared with previous values of 1106 and 1064 bar. The x-ray powder pattern of phase II has been indexed on the monoclinic cell a = 13.30 Å, b = 10.39 Å, c = 8.59 Å, β = 86.1°, Z = 16. The x-ray data and the far-infrared spectra, which can not be fully interpreted, both suggest that phase II, like phase I, contains chains of hydrogen-bonded monomers which are packed in a slightly different way in the two phases.
Pulsed molecular beams: A lower limit on pulse duration for fully developed supersonic expansions
Authors: Saenger Katherine L.
Published: February 2003

We derive an expression for Δtmin, the mimimum pulse duration (’’valve open time’’) required if a pulsed nozzle is to produce a supersonic beam comparably ’’cold’’ to that obtained from a continuous source.(AIP)
Measurement of the mobilities of Cl−, NO2−⋅H2O, NO3−⋅H2O, CO3−⋅H2O, and CO4−⋅H2O in N2 as a function of temperature
Authors: Eisele F. L. , Perkins M. D. , McDaniel E. W.
Published: February 2003

The zero-field mobilities of Cl and the first water clusters of NO2,-NO3,-CO3,-and COu-in N2 are measured. The measurements are perfromed in a high-pressure drift tube mass spectrometer. (AIP)
The product channels in the quenching of O(1S) by O2(a 1Δg)
Authors: Slanger T. G. , Black G.
Published: February 2003

The discovery that quenching of O(1S) by O2(a 1Δg) is a very rapid process, and thus significant in both atmospheric and afterglow systems, makes it important to determine the products of this reaction. Using chemiluminescence and resonance fluorescence techniques, it has been found that 17% of the interaction proceeds through an O(1D)+O2(b 1Σ+g) product channel, whereas 19% results in fragmentation, yielding three ground state atoms. The remainder gives a ground state atom and presumably a triplet O2, most probably in either the A 3Σ+u or A′ 3Δu state. Through detailed balancing, the reverse of the fragmentation process leads to a partial rate coefficient of 1×10−37 cm6 molecule−2 s−1 for the Chapman mechanism for generating thermospheric O(1S).
Unified semiclassical perturbation and infinite order sudden approximation, with application to the reaction path Hamiltonian model
Authors: Miller William H. , Shi Shenghua
Published: February 2003

It is shown how two popular approximate dynamical models—the semiclassical perturbation (SCP) approximation and the infinite order sudden (IOS) approximation—can be combined in a consistent way that includes the correct features of both. Application of this unified SCP–IOS approximation to the reaction path Hamiltonian model of Miller, Handy, and Adam [J. Chem. Phys. 72, 99 (1980)] leads to extremely simple, explicit formulas for the reactive S matrix, product state distrubutions, etc., which can be readily applied to polyatomic systems. Initial numerical tests on a simple model problem indicate that the model is of useful accuracy.
Application of an approximate double substitution coupled cluster (ACCD) method to the potential curves of CO and NeHe: Higher order correlation effects in chemically and weakly bonded molecules
Authors: Bachrach Steven M. , Chiles Richard A. , Dykstra Clifford E.
Published: February 2003

The recently introduced approximate double substitution coupled cluster (ACCD) method has been applied to two diatomics: CO and NeHe. The ACCD method, using an electron pair matrix operator approach, is computationally efficient, has the orbital invariance properties of the complete CCD treatment, and goes to the proper pair description in the limit of separated electron pairs. ACCD and CCD results for the He2 potential curve are virtually identical. The calculations on CO and NeHe are among the first tests of ACCD for more than a four-electron system. Application to these two diatomics provides an opportunity to compare higher order correlation effects in chemically bonded and weakly attractive species. It is found that ACCD agrees quite well with CCD for both systems. However, the higher order correlation effects seem much less important in NeHe than in CO, where CCD reduces the error in the prediction of the vibrational frequency by a factor of 3 over a variational treatment that accounted only for singly and doubly substituted correlating configurations.
A self-definining a
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i
t
i
o procedure modeling the (100) face of diamond
Authors: Lopez Jesus P. , Fink William H.
Published: February 2003

The (100) face of diamond is investigated within the cluster approach. The edge effects introduced by the use of a cluster of small size are minimized with the help of an ab initio model calculation. The model is implemented by two techniques: (1) a modified Hartree–Fock SCF wave function for the cluster, which approximately includes a representation of the extended solid, and (2) a projection procedure. For comparison, full SCF calculations are also performed. A Gaussian lobe basis set is used in all of the cases. The electronic structure for a carbon on the (100) face is presented. Results of Mulliken population analysis are reported and interpreted in terms of electronic density contour maps of the difference between the model and SCF calculations. The results show that the model gives a more uniform distribution of the charge across the surface than the SCF.
Large scale contracted MC–CI calculations on acetylene and its dissociation into two CH(2Π) radicals
Authors: Siegbahn Per E. M.
Published: February 2003

Large scale MC–CI calculations with up to 178 000 configurations have been performed on acetylene and its dissociation into two CH(2Π) radicals with a newly developed contracted CI scheme. The geometry obtained for acetylene was RC–C = 1.208 Å (exptl. = 1.203 Å) and RC–H = 1.061 Å (1.060 Å). The dissociation energy into two CH(2Π) was De = 10.00 eV (10.26 eV). The barrier along the linear dissociation path recently predicted by Raimondi et al. was confirmed but their value of 0.33 eV was raised to 0.57 eV and the location of the barrier was moved from 6.0 to 5.0 a.u. The origin of the barrier is an avoided crossing between the states dissociating into two CH(2Π)—and two CH(4Σ−)—radicals. The energy difference between the states at the point of the avoided crossing was computed to be 0.19 eV from second root contracted CI calculations. The energy barrier was increased in going from MCSCF to CI and this unusual behavior is explained by the much larger amount of correlation energy in CH(2Π) than in CH(4Σ−). The minimum energy path is however found to be nonlinear and has no energy barrier. A simple molecular orbital argument is given for why this should be so. The potential surface for acetylene is further found to exhibit irregular regions with double minima for bending which were not predicted in the surface given recently by Carter et al. All the presently performed calculations were done on a minicomputer VAX-11/780.
Diffusion in a medium with static traps
Authors: Bixon Mordechai , Zwanzig Robert
Published: February 2003

The problem of diffusion in the presence of a random distribution of spherical traps is solved to the first order in density. The resulting time dependent solution has a structure (long time tail) that cannot be obtained from a simple effective diffusion equation. On the other hand, such an equation can describe the steady state situation.
Shear viscosity in a moderately dense hard sphere gas
Authors: Eder O. J. , Lackner T.
Published: February 2003

The three contributions to the shear viscosity in a moderately dense gas—namely, the kinetic, the cross, and the potential part—are calculated separately and the intimate interrelationship between those coefficients and the corresponding correlation functions is shown. The starting point is the calculation of the relevant correlation functions within the ’’successive uncorrelated binary collision’’ model. We prove that while the cross-correlation is strictly proportional to the kinetic correlation function, we obtain two contributions for the potential part of the correlation function one proportional to the kinetic part and the second proportional to a delta function, due to the instantaneous transfer of momentum in hard sphere collisions.
Line tension effects in heterogeneous nucleation theory
Authors: Navascués G. , Tarazona P.
Published: February 2003

The classical theory of heterogeneous nucleation is revised including line tension effects. Contrary to what is believed, the uniqueness of the critical nucleation contact angle obtained from experimental data is shown. Besides other physical effects, the critical nucleation contact angle and nucleation barrier are drastically modified by the line tension effect. Experimental data previously used to discard the classical approach can be now satisfactorily explained.
Auger spectra of methyl cyanide and related compounds
Authors: Rye R. R. , Houston J. E.
Published: February 2003

The gas phase, electron excited Auger spectra of CH3CN, C2H2, N2, and CH4 are presented and analyzed. The C(KVV) spectrum of CH3CN, which contains two carbon atoms with different local environments, is shown to be consistent with independent contributions from an acetylenic-like, sp hybridized carbon (the cyanide carbon) and from a methyl-like, sp3 hybridized carbon (the terminal methyl carbon). The cyanide N(KVV) spectrum is shown to be similar in shape to that of N2 but shifted in energy. Analysis of the CH3CN spectrum on a two-hole binding energy scale shows that the Auger process leads to the same set of doubly ionized final states starting from different core–hole sites in the molecule, but with intensities modulated by the polarization of individual molecular orbitals, i.e., the variation of the electron density local to the various atomic sites represented by a particular molecular orbital. In addition, it is shown that the interaction of the two final-state holes resulting from the Auger transition gives rise to shifts of the spectrum towards lower kinetic energies with respect to that predicted for noninteracting holes, and that these shifts vary in magnitude across the Auger feature.
Matrix effects in the optical spectra of alkali atoms trapped in Ar, Kr, and Xe matrices: A pseudopotential calculation
Authors: Ossicini Stefano , Forstmann Frank
Published: February 2003

By a pseudopotential method the perturbation of the outer electronic s and p levels of Na and K by rare gas matrices is calculated. It is concluded that the stable trapping site consists of four vacancies, except for a substitutional site for Na in Xe. A metastable site of three vacancies gives the ’’red triplet’’ in the absorption spectrum.
Photodissociation of ClN3 at 193 and 249 nm
Authors: Coombe R. D. , Patel D. , Pritt A. T. , Wodarczyk F. J.
Published: February 2003

Experiments directed toward elucidating the nature of fragments produced by the photodissociation of ClN3 have been performed. The utlraviolet absorption spectrum of this molecule exhibits three features between 300 and 170 nm, with λmax = 250, 205, and <170 nm. The shortest wavelength feature (whose maximum was not resolved) was the strongest of the three, with ε170 = 5000 ℓ mol−1 cm−1. Photolysis of ClN3 at 193 nm produces N2 and NCl fragments in both the singlet and triplet manifolds. In the singlet manifold of fragments, emission from electronically excited NCl(b 1Σ+) was observed. The NCl(b 1Σ+) molecules are also vibrationally excited, and emission from levels as high as v′ = 10 was recorded. Time resolution of the NCl(b 1Σ+) emission indicated that the higher vibrational levels were quenched by the gas mixture (∼3% ClN3 in helium) much more rapidly than the lower vibrational levels. The rate constant for quenching of the low v′ levels (v′ = 0–2) of NCl(b 1Σ+) was determined to be kQ = (1.72±0.05)×103 s−1 Torr−1; the rate constant for quenching of the higher levels (v′?8) was found to be kQ = (3.8±0.2)×104 s−1 Torr−1. The spontaneous emission rate for NCl(b 1Σ+), v′ = 0–2, was measured to be A = 1590±160 s−1. In the triplet manifold of fragments, photolysis at 193 nm produces excited N2(A 3Σ+u) metastable molecules. The presence of this species was determined from the observation of N2(C 3Πu)→(B 3Πg) emission produced by energy pooling processes, and from the excitation of NO by energy transfer from the N2 metastables. Photolysis of ClN3 produces at 193 nm excited singlet and triplet fragments with comparable probability. Photolysis at 249 nm also generates both excited singlet and triplet NCl and N2 fragments. Emission from vibrationally excited NCl(b 1Σ+) was again observed, and the initial vibrational distribution appeared to be similar to that found for photolysis at 193 nm. The yields for production of NCl(b 1Σ+) at 193 and 249 nm are approximately equal. Photolysis at 249 nm also produces N2(A 3Σ+u) fragments, although in substantially smaller yield than found at 193 nm.
Saddle point model for atom transfer reactions in solution
Authors: Grote Richard F. , Hynes James T.
Published: February 2003

A simple classical model for a collinear atom transfer (exchange) reaction in solution is investigated. The reaction is analyzed in terms of reactive and nonreactive modes in a saddle point region. Dynamical solvent effects are treated at the generalized Langevin equation level. The reaction rate constant k is determined and compared to the transition state theory prediction kTST. It is found that, for typical reactions governed by sharp potential barriers, the solvent is not very effective in the saddle region in reducing k much below kTST. In addition, solvent-induced coupling of the reactive mode to nonreactive modes of motion typically has only a small influence on k. Some limitations of the model are discussed.
The effect of vibrational excitation of H2 and of OH on the rate of the reaction H2+OH→H2O+H
Authors: Glass G. P. , Chaturvedi B. K.
Published: February 2003

The reaction between the hydroxyl radical and H2 has been studied at room temperature using a discharge flow apparatus equipped for EPR detection of small, free radicals. Vibrationally excited H2 was generated using electric discharge techniques. The rate of the reaction H2+OH→H2O+H was shown to be enhanced by a factor of 155 when H2 was excited into its first vibrational state. Earlier work had shown that the rate of this reaction was increased by less than 50% when OH was excited to either its first or second vibrational state.
The dynamics of the reaction Sr+HF(v = 1)→SrF+H: Effect of rotation
Authors: Man Chiu-Kwan , Estler Ron C.
Published: February 2003

In order to gain insight into the effects of rotation in chemical reactions, the Sr+HF(v = 1)→SrF+H reaction has been studied under single collision conditions as a function of reagent HF rotation. The HF rotational state is selected via HF laser excitation. Product internal state distributions and relative total reactive cross sections have been measured using laser-induced fluoresence. For each initial rotational level examined (J = 1→3), a nearly statistical distribution of product vibrational energy is observed. For each initial HF rotational state, the vibrational distribution peaks at v = 0, decreasing monotonically thereafter, while increasing the initial rotational state of HF increases the population of the higher vibrational states of SrF product. Cross section measurements indicate that the reaction is inhibited by HF rotation: the cross section decreases to less than one half its value on comparing Sr+HF(v = 1, J = 1) to Sr+HF(v = 1, J = 3). A large uncertainty in the J = 4 measurement could place this cross section either above or below the cross section measured for J = 3. These results are compared to those predicted by phase space theory and implicatons of the results are discussed with regard to the potential energy surface of the reaction.
Polarized photofragment emission following two-photon dissociation of XeF2 at 266 nm
Authors: Loge Gary W. , Wiesenfeld John R.
Published: February 2003

The polarization anisotropy of molecular photofragment emission following two-photon induced dissociation of triatomic molecules is calculated in the high J limit. This theoretical treatment is applied to the two-photon induced photodissociation of XeF2 at 266 nm from which the XeF B→X emission displays a polarization anisotropy of 3.5%±1.5%. The XeF2 dissociating state is shown to be non-totally symmetric by measurement of the relative intensities of B→X emission excited by linearly and circularly polarized actinic radiation. Dissociation proceeds via a 1πu Rydberg state.
On finding transition states
Authors: Cerjan Charles J. , Miller William H.
Published: February 2003

An algorithm for locating transition states on a potential energy surface is described. The most important feature of the algorithm, which makes explicit use of the second derivative matrix of the potential surface, is that it is able to ’’walk uphill’’ from the minimum on a potential surface to the transition state essentially automatically. The method is illustrated by application to a two dimensional model problem, to the vinylidene–acetylene rearrangement (H2C = C:↔HC≡CH), and to the dissociation and rearrangement of formaldehyde (H2CO↔H2+CO, HCOH). The algorithm is also seen to provide an improved way of following a reaction path from a transition state down to reactants or products.
Dissociative photoionization of H2 at 26.9 and 30.5 eV
Authors: Kirby K. , Uzer T. , Allison A. C. , Dalgarno A.
Published: February 2003

A quantal resonance formalism is applied to the calculation of the cross sections for the absorption of photons of energies 26.9 and 30.5 eV into a 1Σ+u resonance Rydberg state of H2. The 1Σ+u state undergoes autoionization into the vibrational continuum of the 2Σ+g state of H+2, producing energetic H+ ions. The calculations reproduce state of H+2, producing energetic H+ ions. The calculations reproduce the maxima in the measured cross sections at H+ energies near 3.2 eV for overestimate the cross sections below 2.5 eV. An arbitrary reduction in the autoionizing widths by a factor of 2 restores agreement over the measured energy range, though more elaborate calculations of the widths suggest that values should be increased.
Atomic carbon emission produced through photodissociative excitation of CO
Authors: Wu C. Y. R. , Judge D. L.
Published: February 2003

Atomic carbon fluorescence at C i 1561, 1657, and 1931 Å has been produced through photodissociative excitation of CO. Absolute partial fluorescence cross sections have been measured from threshold to ∼400 Å (31 eV) by using a line emisson source. The fluorescence excitation functions show that the observed peaks correlate with the asymptotes of C(3D0, 3P0, and 1P0)+O(3P, 1D, and 1S). The maximum yield for producing the presently measured C i lines and the O i (1304 Å), from earlier work, photodissociative excitation of the CO molecule is found to be ∼3% at a primary photon wavelength of 572 Å.
Energy partitioning in the reaction 16O(1D)+H2 18O→16OH+18OH. IV. Microscopic probabilities for 16OH+18OH coincident pairs
Authors: Comes F. J. , Gericke K.-H. , Manz J.
Published: February 2003

The reaction 16O(1D)+H2 18O→16OH(16f)+18OH( 18f) yields product molecules with internal [i.e., rotational (j), electronic (Ω), and vibrational (v)] levels 16f and 18f. The corresponding populations 16P(16f) and 18P(18f) are determined experimentally by photochemical preparation of reactants and detection of products after a 10 ns delay. Using this information, we predict the joint probabilities P(18f,16f) of observing the product pair simultaneously. For this purpose, we exploit that (a) summations of the less resolved joint vibrational probabilities P(18v,16v) over 18v or 16v yield exactly the observed vibrational populations 16P(16v) or 18P(18v), respectively; (b) a fraction of the nonvibrational energy of the (18v,16v) pair is released as rotational energy; and (c) the product molecules rotate with approximately opposite angular momenta 16j≊−18j due to the kinematics of light atom transfer. Imposing the constraints (a)–(c), information theory is used to determine P(18f,16f) such that the summations over 18f or 16f yield optimal agreement with the experimental 16P(16f) or 18P(18f), respectively. The results show that (a) simultaneous vibrational excitation of the product pair is negligible, i.e., vibrational energy release is very specific; (c) specific rotational energy release increases with vibrational excitation; and (c) the internal distribution of vibrationally excited OH radicals also contributes to the internal distributions of their v = 0 partners.
Theory of diffusion-controlled reactions on spherical surfaces and its application to reactions on micellar surfaces
Authors: Sano Hisatake , Tachiya M.
Published: February 2003

Diffusion-controlled reactions of a pair of particles on a spherical surface are studied. An analytical expression for the pair survival probability is derived in the case of a pair of freely diffusing particles. The decay of the pair survival probability is found to be well approximated by a single exponential. A simple approximate method for calculating the decay time which characterizes the exponential is presented. The decay time of a pair of particles interacting with each other by an arbitrary force is derived by use of this method. The results are applied to analyze available experimental data on reactions occurring on micellar surfaces, including the dismutation reaction of Br2− on the surface of cationic micelles and that of Ag2+ on the surface of anionic micelles, and the diffusion coefficients of these species on the micellar surfaces are estimated.
Quantum calculations on the collisions of nonlinear triatomic molecules with atoms: Vibrational excitation in He+SO2(v1v2v3)
Authors: Clary D. C.
Published: February 2003

The combined vibrational close-coupling, rotational infinite-order sudden quantum technique is extended to the calculation of vibrational excitation cross sections for the three-dimensional collisions of atoms with nonlinear triatomic molecules. The method has been incorporated into a general computer program which should be applicable to many atom–nonlinear triatomic systems having realistic potential energy surfaces. Calculations on the He+SO2(v1v2v3) system are reported for four different potential energy surfaces. Various vibrational transition propensities are observed. The results are used to examine the validity of assumptions made by Siebert and Flynn on the dominant collision-induced energy transfer pathways contributing to the fluorescence rate of the v3 mode of SO2.
A study of CO chemisorption on the stepped Cu(311) surface by angle resolved photoemission spectroscopy
Authors: Shinn N. D. , Trenary M. , McClellan M. R. , McFeely F. R.
Published: February 2003

The binding geometry and equilibrium adsorption thermodynamics of CO chemisorbed on Cu(311) have been studied by angle resolved photoemission spectroscopy employing Heii (40.8 eV) radiation. Under saturation conditions, three distinct CO orientations are found; two of these are with the molecular axis nearly perpendicular to either the (100) or (111) microsurface. A third geometry, characterized by a lower heat of adsorption with complex coverage dependence, is seen in which the molecule lies flat on the (311) surface, parallel to the step direction. Local chemisorption sites are proposed and their significance in catalytic processes is considered.
The virial theorem for systems subjected to sectionally defined potentials
Authors: Fernández Francisco M. , Castro Eduardo A.
Published: February 2003

The correct formulation of the virial theorem for systems subjected to sectionally defined potentials is derived. It is shown that a trial wave function with an optimized scaling factor according to variational principles satisfies the virial theorem. Formal results are tested for a model reported by Ley-Koo and Rubinstein [J. Chem. Phys. 71, 351 (1979)] and results are analyzed in a comparative fashion.
Analytic configuration interaction (CI) gradient techniques for potential energy hypersurfaces. A method for open-shell molecular wave functions
Authors: Osamura Yoshihiro , Yamaguchi Yukio , Schaefer Henry F.
Published: February 2003

A formalism is presented for the determination of analytic energy first derivatives for the most common types of open-shell correlated wave functions. These are the cases for which the electronic structure is qualitatively understood in terms of a set of molecular orbitals which are eigenfunctions of Roothaan’s restricted Hartree–Fock (RHF) operator. Using these RHF orbitals, configuration interaction (CI) wave functions of broad generality are included in the formalism. The method has been implemented in conjunction with the loop-driven graphical unitary group approach. Application to the vibrational frequencies of methylene suggests that the triplet state has 0.5 kcal more zero-point vibrational energy than does the lowest singlet state.
Frozen Gaussians: A very simple semiclassical approximation
Authors: Heller Eric J.
Published: February 2003

A new and convenient semiclassical method is proposed. It relies only upon classical trajectories and Gaussian integrals. It seems to work very well for the model molecular vibrational spectra investigated here. It should be applicable to a wide variety of processes and can be variationally improved if necessary.
Correlated prolate–spheroidal wave functions for the excited states 1,3Σg+(1sσg, 2sσg) of H2 and He22+
Authors: Le Sech C. , Jorus A. M. , Aubert-Frécon M.
Published: February 2003

The total energy of the excited states 1,3Σg+(1sσg, 2sσg) of H2 and He22+ has been calculated for several values of the internuclear distance from a one-parameter correlated prolate–spheroidal wave function. Such diabatic results are seen to be in fairly good agreement with the best adiabatic ones outside the avoided-crossing regions.
Localized molecular orbital calculations of vibrational circular dichroism. I. General theoretical formalism and CNDO results for the carbon–deuterium stretching vibration in neopentyl-1-d-chloride
Authors: Nafie Laurence A. , Polavarapu Prasad L.
Published: February 2003

The theoretical basis and methodology for the calculation of vibrational circular dichroism (VCD) intensities using the localized molecular orbital (LMO) model are described. In this approach VCD intensities are determined from the motions of both nuclei and centroids of localized molecular orbitals for individual normal modes of vibration. The concepts and approximations inherent in the LMO model are discussed and then illustrated for the C–D stretching mode of neopentyl-l-d-chloride, where favorable comparison to experimental measurement and previous fixed partial charge calculation is found. The necessity of using localized molecular obitals is demonstrated by comparison to results obtained using nonlocalized molecular orbitals. The LMO–VCD result presented here is the first example of the use of an all-valence electron model to determine VCD intensity.
Localized molecular orbital calculations of vibrational circular dichroism. II. CNDO formulation and results for the hydrogen stretching vibrations of (+)-(3R)-methylcyclohexanone
Authors: Polavarapu Prasad L. , Nafie Laurence A.
Published: February 2003

The results of localized molecular orbital (LMO) calculations of vibrational circular dichroism (VCD) for the 12 carbon–hydrogen stretching vibrations in (+)-(3R)-methylcyclohexanone are reported. The VCD is evaluated from the motion of the nuclei and the centroids of LMO’s which are obtained by localization of canonical CNDO molecular orbitals. The detailed expressions used in our CNDO–LMO–VCD procedure are given and the approximation of retaining only the one-center integrals is discussed. Comparison of the CNDO–LMO–VCD calculations to experiment shows good agreement, thus indicating a potential utility of the LMO–VCD calculational method. The results are analyzed in terms of nuclear and orbital vibrational motion and figures depicting these displacements are provided for each of the 12 normal modes.
Cluster expansion of the wave function. Electron correlations in the ground state, valence and Rydberg excited states, ionized states, and electron attached states of formaldehyde by SAC and SAC–CI theories
Authors: Nakatsuji H. , Ohta K. , Hirao K.
Published: February 2003

The SAC (symmetry-adapted-cluster) and SAC–CI theories are applied to the study of electron correlations in the ground state, valence and Rydberg excited states, ionized states, and electron attached states of formaldehyde. The results compare well with experiments, showing the utility of the SAC and SAC–CI theories. Some new assignments of the spectra are reported.
Mechanism of extrinsic carrier photogeneration in poly-N-vinylcarbazole. II. Quenching of exciplex fluorescence by electric field
Authors: Yokoyama M. , Endo Y. , Matsubara A. , Mikawa F H.
Published: February 2003

Based upon the observation that the exciplex fluorescence of poly-N-vinylcarbazole film doped with a weak electron acceptor is partly quenched by an electric field, a model is proposed for the extrinsic carrier photogeneration mechanism involving the field-assisted thermal dissociation into free carriers of an ion pair produced from a ’’nonrelaxed’’ exciplex state. Interaction between a migrating singlet exciton and an electron acceptor produces an electron–hole pair having an interionic separation of 22 Å through the nonrelaxed exciplex state and subsequent thermalization of its excess energy. The ion pair produced dissociates into free carriers with assistance from the applied field according to the Onsager probability of escaping geminate recombination. Recombination leads to the ordinary relaxed fluorescent exciplex state. The initial separation r0 of the ion pair was determined by comparing experimental values of the field-induced exciplex fluorescence decrement and the theoretical prediction. The temperature dependence of the field-induced exciplex fluorescence quenching provides further proof for this mechanism. As the electron affinity of a series of added acceptors is increased, r0 becomes larger as expected.
Molecular flexibility and orientational statistics of liquid crystals: Raman study of 7-CB and 8-OCB
Authors: Prasad S. N. , Venugopalan S.
Published: February 2003

The Raman depolarization ratios of the –C≡N vibrational band of 7-CB and 8-OCB have been measured in the aligned liquid crystal and isotropic phases. The temperature dependence of the absolute orientational order parameters 〈P2〉 and 〈P4〉 have been evaluated for the mesophases of both compounds. A comparison of their 〈P4〉 values in the nematic phase with those determined by Miyano for 5-CB suggests that molecular flexibility is an importnant factor that serves to lower 〈P4〉 well below the predictions of mean field theories.
Persistent nondiffusive behavior in Brownian motion
Authors: Harris S.
Published: February 2003

We consider the relaxation of the average (over initial and final velocities) of the solution of the Fokker–Planck equation to the solution of the diffusion equation. In the case of an initial Maxwellian distribution of velocities the relaxation is exponential, as expected; however, for other possible choices of the initial velocity distribution this relaxation can be algebraic.
The cohesive energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (TTF TCNQ) as a function of charge transfer
Authors: Metzger Robert Melville
Published: February 2003

The Madelung, polarization, charge–dipole, and dispersion contributions to the theoretical lattice energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (TTF TCNQ) were computed for the uniform lattice as a function of charge transfer ρ and for the ρ = 1/2 Wigner lattice. The atom-in-molecule charges, local hybrid dipole moments, and polarizabilities used in this study were obtained from a finite-electric-field perturbation MINDO/3 calculation. Some improvements were obtained over the Madelung- only case, but the charge–dipole term tends to cancel the polarization energy term (or the Madelung energy term). The dispersion energies calculated with atom-in-molecule polarizabilities and molecular and ionic ionization energies are smaller than the dispersion energies obtained by using parametrized van der Waals coefficients for neutral hydrocarbons. The lattice energy terms at intermediate charge transfer were obtained by linear interpolation of the charges, local moments, and polarizabilities between their ρ = 0.0 values and their ρ = 1.0 values. The McConnell, Hoffman, and Metzger criterion for ionicity used along with all of the cohesive energy terms included here fails to yield the desired cohesive energy minimum at fractional charge transfer. Using the Soos estimate of the cost of ionization does provide a minimum at intermediate charge transfer, but not significantly far from the minimum obtained from using the Madelung energy alone.
Theory of quantum beats: Master equation approach
Authors: Kono H. , Fujimura Y. , Lin S. H.
Published: February 2003

Based on the master equation approach, an expression for the time dependent fluorescence intensity has been derived taking into account both collision effects resulting from the elastic and inelastic scattering from the heat bath and the statistical properties of the incident light. The degree of coherence for the molecular eigenstates is defined. The pure dephasing constant between the excited states attenuates the degree of coherence for the molecular eigenstates during and after the light pulse duration; on the other hand, the pure dephasing between the ground and excited states attenuates it during the pulse duration. A separable nonstationary correlation function for the incident radiation field is introduced to investigate the effects of the field coherence on the time dependent fluorescence. Model calculations of the quantum beats in the fluorescence from molecules with two levels in the excited state in the weak radiation field have been performed to demonstrate the effects of the dephasing constants and the field coherence as well as those of the population decay constants.
Multiphoton ionization–fragmentation patterns of tertiary amines
Authors: Parker D. H. , Bernstein R. B. , Lichtin D. A.
Published: February 2003

Multiphoton ionization (MPI)–fragmentation patterns are reported for a series of normal and caged tertiary amines. Ionization is enhanced by two-photon resonance with the 3s and 3p Rydberg states of trimethylamine, triethylamine, and the caged amines quinuclidine and triethylenediamine. Over the wavelength region λ = 400–530 nm, N(CH3)3 ionizes to the parent ion (P) and fragments only by the loss of a H atom to yield the P–H daughter ion; N(C2H5)3 ionizes to its parent ion and fragments by the loss of a methyl to form the P–CH3 ion. The branching ratio of daughter to parent ions is found to be essentially independent of laser intensity but strongly dependent on laser wavelength. The caged amines quinuclidine [N(C2H4)3CH, or ABCO] and triethylenediamine [N(C2H4)3N, or DABCO] fragment extensively over this λ range in a manner dependent on both laser wavelength and intensity. The extent of daughter ion formation in N(CH3)3 and N(C2H5)3 can be understood by consideration of the wavelength regions in which the total available energy from the initial three- or four-photon ionization event exceeds the appearance potential of the given daughter ion. For the caged amines direct observation of this mechanism is masked by fragmentation due to sequential absorption of photons (during the ∼5 ns pulse duration) by the parent and/or daughter ions. The present results show that even for molecules with broad, unstructured UV absorption and MPI spectra such as N(CH3)3 and N(C2H5)3, considerable information on photon–molecule and photon–ion interactions can still be gained by the MPI mass spectrometry technique.