bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of resonance emission: Microscopic origins of stochastic modulation
Authors: Rhodes William
Published: February 2003

The nature of resonance emission of light, which appears as a mixture of (coherent) resonance scattering and (incoherent) resonance fluorescence, is considered. The basic conceptual elements which distinguish the scattering and fluorescence components for the linear excitation limit are developed in terms of a simple formulation whereby a molecule interacts with a coherent Lorentzian light pulse which increases exponentially with time for t<0 and is turned off at t = 0. These results together with the ideas, currently existing in the literature, that stochastic modulation of a molecule by its medium is essential for the appearance of fluorescence provide the basis for the further formulation of a theory which does not require the ad hoc introduction of a random potential. The theory developed considers the microscopic origins of manifest stochastic modulation effects in terms of a bath potential for the molecule–medium (bath) degrees of freedom. This potential, termed the sudden potential, is associated with the change in interaction with the medium that a molecule experiences upon intramolecular state transitions which are induced by external interactions (e.g., by light). Development of the reduced molecule–field density operator leads naturally to a separation of terms which describes scattering and (the propensity for) fluorescence. Critical to the theory is a four-time correlation function for the bath (sudden) potential. The difficulties inherent in defining absorption (spectrum) resonances and the corresponding distinctions between resonance and relaxed emission are discussed. Implications are considered for the further understanding of experimental results on molecular excitation dynamics in the gas phase and in supersonic jets. Sources of stochastic modulation other than material media are mentioned. In particular, the possibility is suggested of the pervasive existence of stochastic modulation through interaction of a molecule with the vacuum state of the radiation field. This raises questions concerning the fundamental relations between quantum mechanics and thermodynamics.
Triplet state properties of pyridine derivatives
Authors: Motten Ann G. , Kwiram Alvin L.
Published: February 2003

The phosphorescence and zero-field splittings have been systematically investigated for a number of methyl, amino, and cyano derivatives of pyridine in both the neutral and protonated forms. In pyridinium and the methylpyridines, decay from the triplet state appears to have mechanisms similar to those in benzene. Interaction of lowest ππ* triplets with low-lying 3nπ* states appears to have little effect on zfs, but nonradiative decay rates are distinctly affected. Cyanopyridines and aminopyridines offer less of a comparison to pyridinium. An nπ*-type configuration may contribute to large D′s for protonated cyanopyridines. Charge-transfer effects are large in 4-aminopyridine, although the 2- and 3-aminopyridines apparently do not show significant charge-transfer character.
The spectroscopy of a benzil–bibenzyl mixed crystal system. Lifetime and optical spin orientation studies
Authors: Kohler Bryan E. , Loda Richard T.
Published: February 2003

Phosphorescence decay curves for three isotopic derivatives of a benzil–bibenzyl mixed crystal have been measured as a function of temperature over the range from 2–310 K. Optical spin orientation occurs between 40 and 4.2 K for all of the three isotopic derivatives. The results are consistent with the model where benzil has one active zero-field spin sublevel radiatively connected to the ground state. The other two sublevels are, to a good approximation, nonradiative. Effective spin–lattice relaxation rates, measured at 16 and 30 K, were found to increase with increasing temperature.
NMR study of alloys exhibiting a reentrant nematic phase
Authors: Dong Ronald Y.
Published: February 2003

Proton T1 was measured at 30 MHz as a function of temperature in three binary mixtures (27.7, 25.2, and 22.9 wt. % 60CB in 80CB) exhibiting a reentrant nematic phase. Proton T1 and T1D were also measured at 7.07 and 30 MHz, respectively, in 27.6 wt. % 60CB. The angular dependence of T1 was studied in the SA phase of 22.9 wt. % 60CB at 30 MHz and in the SA phase of 27.6 wt. % 60CB at 7.07 MHz. It was found that T1 and T1D show different behavior in the high temperature nematic and reentrant nematic phases. Nematic orientational order director fluctuation (ODF) and/or molecular self-diffusion are used to account for the relaxation rates in various mesophases. ODF seems to be an important relaxation mechanism in the reentrant nematic phase, while molecular self-diffusion appears to be responsible for spin relaxation in the high temperature nematic phase. It would appear that molecular dynamics for nematic and reentrant nematic phases are tied closely to the viscosity of the sample.
The spectroscopy of xanthione in xanthone: An example of spin–orbit coupling dominated zero-field splitting
Authors: Burland Donald M.
Published: February 2003

The phosphorescence, absorption, and excitation spectra of xanthione in xanthone are presented. The extraordinarily large zero-field splitting found for xanthione is confirmed and shown to be in reasonable agreement with calculations assuming that spin–orbit coupling dominates the splitting. In addition, an unusual crystal site dependence of the oscillator strength in the xanthione lowest singlet state is reported. Along with these results we also report the crystal structure of the host xanthone.
Cross relaxation dispersion and temperature dependence of nuclear spin dipolar and rotating frame Zeeman energies: A study of ammonium halides
Authors: Sharp A. R. , Pintar M. M.
Published: February 2003

The less known properties of cross relaxation between detectable spins I and fast relaxing spins S, such as the two types of H1 dispersion and the occurrence of two minima of the rotating frame Zeeman relaxation time T1x(I), when γH1T1(S)min<1, or the occurrence of two minima of the dipolar relaxation time T1d(I), when γHL, eff(I) T1(S)min<1 and T1(S)min≪T2(IS), are briefly discussed in the framework of the Blinc–Pirš–Žumer theory. The experimental results on cross relaxation between detectable spins I (protons or deuterons) and fast relaxation spins S (bromine, iodine, and chlorine) in the cubic phases of solid NH4Br, NH4I, and NH4Cl, and their deuterated analogs, are presented and explained.
Photoion–fluorescence photon coincidence study of radiative and dissociative relaxation processes in VUV photoexcited SO2. Fluorescence of SO+2, SO+, and SO
Authors: Dujardin Gérald , Leach Sydney
Published: February 2003

Line photoexcitation sources in the range 10.2–21.2 eV are used to produce excited electronic states of SO2 and/or SO+2. Photoion–fluorescence photon coincidence measurements show that SO+2 states in the 16 eV region fluoresce at λ<370 nm, with a quantum yield φF(? 2B2+? 2A1)∼6×10−5, in competition with predissociation. The products of predissociation of excited states of SO+2 are identified by their TOF mass spectra and/or dispersed optical emission. SO+(A 2Π–X 2Π), SO(B 3Σ−–X 3Σ−), and SO(A 3Π–X 3Σ−) emissions are observed when the excited products are formed with ionic or (super) excited states of SO2. The measured lifetime of the SO+A 2Π state is 2.4±0.4 μs. The results are discussed in relation to photoion–photoelectron measurements and in the context of unimolecular dissociation models.
Effect of atomic reagent approach geometry on electronic state branching: The Ca(1P1) + HCl reaction
Authors: Rettner Charles T. , Zare Richard N.
Published: February 2003

The influence of the alignment of the laser-excited Ca(1P1) reagent on the chemiluminescent reaction Ca(1P1)+ HCl→CaCl(A2∏,B2Σ+) + H is discussed. (AIP)
Observation of magnetic vibrational circular dichroism
Authors: Keiderling Timothy A.
Published: February 2003

Magnetic circular dichroism for a fundamental vibrational excitation of an achiral molecule has been measured for the first time. The observed spectra for the CH stretching region of CH3I, (CH3)4Si, (C5H5)2Fe, Ch3CN, C6H6, and C6H12 are dominated by low intensity, single-signed features analogous to MCD B terms. These results differ from what would be expected based on previously published theoretical models and may imply that the observed vibrational MCD is due to mixing of electronic states.
The hydrogen bond formation theory of surface activity. Aqueous solutions of nonelectrolytes at the liquid–air interface
Authors: Garfias F. J.
Published: February 2003

The hydrogen bond formation (HBF) theory of surface activity is postulated to explain the surface behavior of aqueous solutions of nonelectrolytes. When a surface water molecule in the shell configuration is displaced by a solute molecule, the solute takes the chair conformation and the result is that there is an extra hydrogen bond made by each solute molecule adsorbed. In the HBF theory, an adsorbed film is depicted as a mixed monolayer, one fraction of the surface being made of water molecules in the shell configuration, and the other of solute molecules in the chair one. The general thermodynamic treatment of Belton and Evans is applied to the proposed model. The following expressions are obtained for the solute surface mole fraction x1s and the solute bulk activity a1: x1s = 1−exp(−ΠΣ2/kT), a1 = exp[Σ1(γ1−γ2)/kT] {exp(ΠΣ1/kT) −exp[Π(Σ1−Σ2)/kT]}. The molecular surface area per solute molecule adsorbed A1 is also calculated, and the same result is obtained, as the one derived from applying Gibbs’ adsorption equation to the Π–a1 relationship. The equation of state obtained is A1 = Σ1−Σ2+Σ2[1−exp(−ΠΣ2/kT)]−1. Reported surface pressures for several alcohols (C5 to C11) are compared with predicted ones, finding a sufficiently good agreement to render the HBF theory useful in studying the properties of adsorbed films for aqueous solutions of nonelectrolytes at the liquid–gas interface.
Infrared multiphoton energy deposition in CF2HCl
Authors: Duperrex R. , van den Bergh H.
Published: February 2003

Energy absorption by CF2HCl from an intense CO2 laser pulse is measured as a function of the laser energy fluence, the temperature, the pressure of CF2HCl, and the pressure of Ar bath gas. For collision free multiphoton absorption the average number of photons absorbed per molecule increases from two to neary 12 when the fluence is increased from 1.6 to 10.9 J cm−2 at 298 K. Comparison of absorption and dissociation data measured under identical conditions indicates relatively wide internal energy distributions produced by the multiphoton excitation. At 3 J cm−2 the average number of photons absorbed per molecule increases with increasing temperature betwen 300 and 450 K, whereas in the same temperature range at 10.7 J cm−2 a pronounced maximum is found in the absorption near 370 K. In the presence of 1 atm of added argon, an average of as many as 100 photons per molecule of CF2HCl can be absorbed as compared to the 18 photons necessary for collisionless dissociation.
Pressure dependence of fluorescence quantum yields and collision-induced rotational relaxation of single rotational levels of H2CO(Ã 1A2, 41)
Authors: Fairchild Paul W. , Shibuya Kazuhiko , Lee Edward K. C.
Published: February 2003

Pressure dependent values of SRL fluorescence quantum yields Φf(J′, K′) in the 1–120 mTorr range are reported for 75 rotational levels (E′rot = 37–1127 cm−1) of S1 H2CO (41). High lying rotational levels tend to show an increase in Φf with increasing pressure of S0 H2CO, whereas low lying rotational levels tend to show a decrease. For some levels, collision-induced ΔJ′ transitions occur 5–10 times faster than the gas kinetic collision rate but probably with a much slower rate for ΔK′ transitions. A ΔK′-constrained, collision-induced rotational relaxation model involving mainly ΔJ′ = ±1 transitions is proposed to explain the observed pressure dependence of Φf(J′, K′).
Features of the H2CO potential energy hypersurface pertinent to formaldehyde photodissociation
Authors: Goddard John D. , Yamaguchi Yukio , Schaefer Henry F.
Published: February 2003

In the light of intense current experimental and theoretical interest in the photodissociation of formaldehyde, several features of the H2CO potential surface have been explored using recently developed analytic configuration interaction (CI) gradient techniques. Employing a double zeta plus polarization basis set and CI including all valence shell single and double excitations, the transition state for the molecular dissociation H2CO→H2+CO has been precisely located and characterized by its vibrational frequencies. These results support previous, less sophisticated, theoretical predictions that the activation energy for this process is ∼87 kcal/mol. A similar theoretical treatment for the H2CO→HCOH isomerization suggests an activation energy of ∼84 kcal/mol for this process. Both transition states have also been examined at several simpler levels of theory and the comparisons are instructive. For the dissociative process, the reaction pathway has been mapped out in the vicinity of the saddle point. At the most complete level of theory, the structures and energetics of cis- and trans-hydroxycarbene were determined along with some information concerning their (yet unobserved) electronic spectra. The lowest singlet–singlet vertical electronic transition is predicted to lie at ∼3.2 eV.
A simple equilibrium statistical mechanical theory of dense hard sphere fluid mixtures
Authors: Andrews Frank C. , Ellerby H. Michael
Published: February 2003

The equilibrium statistical mechanical theory of Andrews for a hard core fluid is reformulated generally from simple, rigorous probability theory and is shown to apply to multicomponent systems. A simple expression is rigorously derived for the third virial coefficient of a mixture of nonadditive hard spheres. In the cases of additive potentials or binary systems, the expression is a great simplification of the previously known result. The exact theory of mixtures of not necessarily additive one-dimensional hard lines is developed simply for uniform mixing. Expressions for the reciprocal of the activity and the pressure in mixtures of additive hard spheres are determined approximately by similar methods. Calculations for binary mixtures with diameter ratios of 1.1 to 1, 1.667 to 1, and 3 to 1 are compared with computer experiments. The theory is significantly better than either the equation of state obtained from the Percus–Yevick equation or from the scaled particle theory. Indeed, it is about as accurate as the best empirical equation of Mansoori, Carnahan, Starling, and Leland. The physical origin of the decrease in pressure with diameter ratio at constant volume is discussed. Equations are listed, permitting calculation of various excess functions and excess functions of mixing for hard spheres.
Statistical mechanics of dielectric fluids in electric fields: A mean-field treatment
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

A previously developed theory of dielectric fluids in the presence of electric fields is extended and applied to a mean-field model to yield explicit results, which are compared to Monte Carlo results of Pollock and Alder.
Fluorescence decay studies of anisotropic rotations of small molecules
Authors: Barkley Mary D. , Kowalczyk Andrzej A. , Brand Ludwig
Published: February 2003

The fluorescence decay and the decay of the emission anisotropy of perylene and 9-aminoacridine in glycerol have been investigated at several excitation wavelengths over the temperature range 10 to 40 °C. Under these conditions the fluorescence lifetime of both compounds is essentially constant: about 4.6 ns for perylene and 12.8 ns for 9-aminoacridine. The decay of the fluorescence emission anisotropy could be analyzed in terms of a double exponential function with rotational correlational times independent of excitation wavelength and dependent on temperature. The pre-exponential terms are independent of temperature but do depend on the excitation wavelength as expected from geometrical considerations. The anisotropic character of the rotation of perylene is quite pronounced. Excitation of an absorption dipole perpendicular to the emission dipole gives rise to decay curves for the emission anisotropy showing unusual oscillatory behavior; these are predicted by theory. The rotational dynamics of perylene are consistent with those of a disk with the slipping boundary condition. The ratio of the diffusion coefficients about the axes perpendicular and parallel to the plane of the disk is 10±1, in agreement with previous estimates. The anisotropic character of the rotation of 9-aminoacridine is less striking than that of perylene, but is clearly revealed by measurements at several excitation wavelengths. The rotational dynamics can be interpreted in terms of a hydrodynamic prolate ellipsoid of revolution with a ratio of diffusion coefficients about the major and minor axes of 1.4±0.1 and an axial ratio of about 1.5. Experimental criteria for detecting anisotropic rotations by means of polarized fluorescence are discussed.
NMR relaxation dispersion in an aqueous nitroxide system
Authors: Borah Babul , Bryant Robert G.
Published: February 2003

NMR relaxation rate measurements are reported for protons in aqueous nitroxide solutions over the frequency range from 0.01 to 20 MHz. A complete relaxation dispersion curve is observed that is analyzed in terms of dynamical models for the interaction between the solvent and solute. Both translational and rotational motions contribute to the relaxation behavior, although the translational contribution is dominant. Dipole–dipole interactions are sufficient to account for the data over this frequency range. At 298 K translational motions account for 75% for the relaxation, and are characterized in the present analysis by correlation time of 73 ps and 2.5 Å as the distance of closest approach. The small rotational term requires a 22 ps correlation time and an interdipole separation of 2.1 Å.
The vibrationally adiabatic distorted wave method for direct chemical reactions: Application to X+F2(v = 0, j = 0)→XF(v′, j′, m
j′)+F(X = Mu, H, D, T)
Authors: Clary D. C. , Connor J. N. L.
Published: February 2003

The computational feasibility of the vibrationally adiabatic distorted wave (VADW) method is examined for the prediction of vibration–rotation product distributions of atom–diatomic molecule chemical reactions when there are a large number of open product states present. Application is made to the chemical laser reactions X+F2(v = 0, j = 0)→XF(v′, j′, mj′)+F (X = Mu, H, D, T) in three dimensions (3D). Over 1000 product vibration–rotation TF states are open in the T+F2 reaction. The best extended LEPS no. II potential energy surface of Jonathan et al. is used. The relative vibrational product distribution P(v′) for the H+F2 reaction at a translational energy of 0.106 eV peaks at v′ = 6. It agrees well with thermal experimental measurements and with previous 3D quasiclassical trajectory, accurate 1D quantum, and 1D→3D information theoretic calculations. The P(v′) for the Mu, D, and T reactions are found to peak at v′ = 1, 9, and 12, respectively, which is in good agreement with the results of the 1D→3D method. The average available energy present in product vibration is found to increase with increasing mass of X, in agreement with the 1D→3D results and the light atom anomaly concept. The relative rotational product distribution for H+F2 agrees well with thermal experimental results, with very low rotational excitation. The average available energy present in product rotation is very low for all four reactions and decreases slightly as the mass of X increases. The product differential cross sections for the H, D, and T reactions are distributed in both the forward and backward directions, while the Mu reaction is purely backward scattered. In every case the cross section for reaction into the state v′, j′,‖mj′‖ is greater than that for reaction into v′, j′,‖mj′‖+1. The VADW technique is computationally inexpensive; only 20 min of CDC 7600 computer time are required to compute the full vibration–rotation product distributions for the H+F2 reaction at one energy.
Excitation transfer collisions and electron seeding processes in a resonantly excited sodium vapor
Authors: Krebs D. J. , Schearer L. D.
Published: February 2003

A dense sodium vapor in a high pressure buffer of argon has been simultaneously excited by short (4 ns) laser pulses from two lasers: the first tuned to one of the D line transitions at 589 nm and the second tuned to the photoionization threshold of the 3p states near 406 nm. The temporal evolution of the system was studied with and without the photoionizing laser pulses. At early times (∼100 ns) excited state populations are determined by energy transfer collisions between two laser-excited 3p atoms while the ion/electron density is controlled by superelastic heating of ’’seed’’ electrons followed by electron impact ionization of excited state atoms. At late times (∼1 μs) excited state populations are controlled by collisional–radiative recombination processes. Excitation transfer rates into the 4d, 5d, 6d, and 6s levels are measured.
Intramolecular and intermolecular interactions in methyl substituted pentanes as revealed by high energy CH-stretching overtone spectra
Authors: Henry Bryan R. , Mohammadi M. Ali , Thomson James A.
Published: February 2003

The overtone spectra of pentane and 12 methyl substituted pentanes are measured in the liquid phase in the region of the pure local mode overtones corresponding to ΔvCH = 3, 4, 5, and 6. The local mode structure of the spectra is discussed. The local mode frequencies ωCH and diagonal local mode anharmonicities XCH for the methyl oscillators are obtained from an analysis of the spectra. The change in ωCH and XCH with increased methyl substitution is related to a barrier to large amplitude vibrational motion arising from steric crowding. The height of this barrier is obtained from dissociation energies calculated from ωCH and XCH by taking the local methyl CH stretching potential to be a Lippincott–Schroeder potential. The methyl steric interactions are described in terms of nonbonded Lennard-Jones and Morse potentials. The parameters for these nonbonded potentials are calculated from the barrier heights and structural information obtained from molecular models for their least strained conformations. These parameters are used to identify a positional dependence in methyl steric hindrance. Force constants evaluated from the nonbonded potentials are correlated with bulk liquid properties.
Theory of photon echoes from a pair of coupled two level systems: Impurity dimers and energy transfer in molecular crystals
Authors: Skinner James L. , Andersen Hans C. , Fayer M. D.
Published: February 2003

We present the theory of the photon echo and stimulated photon echo from a four level system that results from a pair of weakly coupled two level systems. This model describes an impurity dimer in a molecular crystal. We include the effect of relaxation processes, focusing on energy transfer between delocalized states. We find that both the photon and stimulated echoes contain information about static level splittings and dynamic energy transfer processes. Moreover, these observables show a dramatic dependence on the excitation power. We discuss implications of our results for recent experiments on concentration dependent dephasing of impurities in molecular crystals. Our analysis casts considerable doubt on the intermolecular interaction interpretation of these experiments.
Single-mode versus multimode calculations of Raman intensities of cytochrome c
Authors: Champion P. M. , Albrecht A. C.
Published: February 2003

The simple single-mode model for Raman scattering is examined with regard to the relative Raman intensity ratios of cytochrome c at the 0–0 and 0–1 transition frequencies of S1 and S2. Interference between A- and B-term amplitudes as well as nonadiabatic effects are considered. The model is found to generate the experimentally observed intensity ratios only under the condition that the damping factors in S2 (Soret band) and S1 (Q band) are nearly equal. This condition is at variance with observation unless multimode effects are included to account for the large bandwidth of S2 in cytochrome c. Use of the multimode model generates the observed Raman intensity ratios in S2 without the need for A- and B-term interference effects.
Coverage-dependent surface enhanced Raman spectra of halide ions on colloidal silver
Authors: Garrell Robin L. , Shaw Keith D. , Krimm Samuel
Published: February 2003

Surface enhanced Raman spectra of halide ions on colloidal silver as a function of coverage is obtained. (AIP)
Spin–orbit coupling and Λ doubling in NaAr
Authors: Cooper David L.
Published: February 2003

A b initio values of spin-orbit coupling constants and Λ-doubling parameters have been obtained for the A 2Πr state of NaAr. Values are in reasonable agreement with experiment and, in particular, suggest that calculated potentials may be used to account for the Λ-doubling, provided that a reasonable model is used. The interest in NaAr arises from the fact that the Λ-doubling parameters in the A 2Π state give direct information about the Na+Ar interaction in the B 2Σ+ state. The success of the fine structure calculations presented here encourages application of similar methods to other van der Waals species.
Ion mobility test of Li+–Ar potentials
Authors: Gatland I. R.
Published: February 2003

Theoretical potentials for the interaction of lithium ions with argon atoms have been developed by Kim and Gordon, and by Gordon and Waldman, using electron gas–Drude model calculations and by Olson and Liu using both SCF and CI calculations. In this paper the ion mobilities are obtained for each of these potentials and are compared with experimental data. The CI potential gives the best agreement with experiment but still differs significantly in the low E/N region. A potential which is directly determined to fit the experimental data only departs from the CI potential for internucleus separation distances larger than 6 a.u. and leaves the potential well (around 4.5 a.u.) almost unchanged.
Instabilities of the Hartree–Fock solution and the vibrational mode in a molecule
Authors: Deguchi K. , Nishikawa K. , Aono S.
Published: February 2003

The Hartree−Fock instability and the force constant formula for a normal coordinate of a molecule are discussed. (AIP)
Matrix reactions of silver atoms with ozone molecules
Authors: Tevault D. E. , De Marco R. A. , Smardzewski R. R.
Published: February 2003

Matrix reaction of silver atoms with ozone molecules is studied. (AIP)
Laser photofragment spectroscopy of NO+. I. Predissociation of the 2 3Π state
Authors: Cosby P. C. , Helm H.
Published: February 2003

NO+ ions, produced by electron impact on NO, are observed to predissociate into O+(4S0)+N(4S0) when irradiated by a dye laser at wavelengths between 6600–5650 Å. The highly structured wavelength dependence of the photofragments reflects absorptions from three vibrational levels in a long-lived NO+ electronic state to 20 vibrational levels of a predissociated electronic state. The transitions are tentatively identified as 2 3Π(v′ = 0–19)←b′ 3Σ−(v″ = 8–10). The weakly bound 2 3Π state is found to lie 0.698–0.925 eV above the lowest separated atom limit of NO+ and to adiabatically correlate to the N+(3P)+O(3P) limit. This state is also found to be homogeneously perturbed, most likely by the 3Π state arising from the O+(4S0)+N(2D0) limit, and to be predissociated by the α 5Σ+ state. The predissociation lifetimes for all rotational levels in v′ = 0–8 are ≥1.6 ns. Perturbations in b′ 3Σ−(v = 9) are discussed.
Pressure-induced multiphoton ionization of krypton–argon mixtures at 248 nm
Authors: Rothschild M. , Gornik W. , Zavelovich J. , Rhodes C. K.
Published: February 2003

Pressure-induced three-photon ionization of Kr atoms at 248 nm has been examined in mixtures involving Ar and Ne. This density-dependent effect is attributed to the role of a two-photon near resonance, which, although forbidden by parity in the free atom, acquires an allowed character in the two-center molecular system. The response of Ar/Kr mixtures was greater than that observed in pure krypton. It was found that these effects reveal both the detailed structure of the near-resonant molecular states and the variation of the participating matrix elements with internuclear separation. It was also determined that the three-body recombination of Kr* to Kr*2 when assisted by Ar, is governed by a rate constant whose value is (3.0±0.7) ×10−32 cm6/sec.
Trajectory studies of model H–C–C→H+C = C dissociation. II. Angular momenta and energy partitioning and their relation to non-RRKM dynamics
Authors: Hase William L. , Wolf Ralph J.
Published: February 2003

The model alkyl dissociation reaction H–C–C→H+C = C has been studied on a potential energy surface derived from an analytic potential energy function for ethyl radical dissociation. Nonrandom excitation of H–C–C is simulated by the chemical activation reaction H+C = C→H–C–C, and different initial relative translational, rotational, and vibrational energies are investigated. Comparisons are made between the unimolecular dynamics of nonrandomly excited H–C–C radicals and those excited randomly. These two types of excitation yield strikingly different unimolecular lifetime distributions, each non-RRKM. However, if angular momentum constraints are propertly included, the partitioning of product energies is independent of the excitation process. For total energies slightly in excess of the dissociation energy the energy distributions at the dissociation barrier are in excellent agreement with the RRKM predictions, and the nonstatistical product energies arise from the preferential release of potential energy in the exit channel to relative translation. As the total energy is increased, the relative translational energy at the dissociation barrier exceeds the RRKM prediction. This results from the system remaining on the same effective potential energy curve before and after crossing the dissociation barrier, which nonstatistically apportions energy to relative translation. A dynamical model based upon angular momentum constraints and conservations yields a quantitative fit to the partitioning of angular momenta and energy in the products.
Picosecond dynamics of electronic energy transfer in condensed phases
Authors: Millar D. P. , Robbins R. J. , Zewail A. H.
Published: February 2003

Energy transfer between donor and acceptor molecules randomly distributed in condensed phases is investigated by time-resolved spectroscopy on the picosecond and nanosecond time scales. The effects of translational diffusion and excitation transfer among the donors is experimentally observed and used to test theoretical models based on a diffusion equation for the donor excitation. The time-resolved data demonstrate that the Förster dipole–dipole model is valid in the cresyl violet (donor):azulene (acceptor) system from 1 ps to at least 10 ns after excitation, and over a 1000-fold range of acceptor concentration. The critical transfer distance obtained from the transient experiments (26.6 Å) is in excellent agreement with the value obtained from the spectral overlap (27.8 Å) at all acceptor concentrations. In fluid solutions the donor decay agrees very well with the approximate solution of the diffusion equation including a sink term for energy transfer. The deviations observed at high donor concentrations suggest that donor–donor excitation transfer is nondiffusive on the picosecond time scale.
Piezomodulation spectroscopy of molecular crystals. II. The pyromellitic dianhydride–anthracene electron donor–acceptor complex
Authors: Merski J. , Eckhardt C. J.
Published: February 2003

Piezomodulated reflection spectra are presented for the crystalline complex of pyromellitic dianhydride–anthracene. Two differently polarized charge transfer transitions are found and their strain sensitivities are obtained. The role of the charge transfer interaction in determining the magnitude of the piezomodulation is assessed. The piezomodulation spectra are found to have the response expected for an oriented gas. Significant enhancement of structure is also observed and is correlated with structure observed at 20 K.
A study of the possible effects of an external magnetic field upon the reduction of NiO, Fe2O3, and Co3O4 by hydrogen
Authors: Gallagher P. K. , Gyorgy E. M. , Jones W. R.
Published: February 2003

The reduction of NiO, Fe2O3, and Co3O4 by H2 was followed using evolved gas analysis of the H2O formed during the process. This technique is insensitive to the presence of an external magneteic field. Samples of these oxides were heated at several rates in flowing H2 both with and without the application of an external field of 4–7 kOe. There were no differences observed in the reduction behavior due to the presence of this external field, contrary to some previous reports.
Comparison of modified infinite-order sudden theory with experimentally measured state-to-state cross sections for R↔T energy transfer in Ar+HF
Authors: Viswanathan R. , Raff L. M. , Agrawal P. M.
Published: February 2003

The results of an infinite-order sudden theory modified by the inclusion of an explicit exit-channel velocity dependence of the scattering cross section (IOSAM) are compared with standard IOSA results having a scattering cross section independent of exit-channel velocity and with the experimental data reported by Barnes et al. for the HF–Ar system. Calculations of σ(j′←1) and σ(j′←2) are reported at three relative translational energies using two different potential-energy surface formulations, a pairwise additive Lennard-Jones (12,6) potential, and an SCF ab initio surface augmented by an attractive van der Waals term. In general, the IOSAM results on the LJ (12,6) surface are found to be in good to excellent accord with experiment for transitions with ΔJ≳0. For transitions with Δj<0, the agreement between theory and experiment is poor. The results obtained on the unadjusted SCF surface have the correct qualitative trends, but the degree of agreement with experiment is significantly less than that obtained on the LJ (12,6) surface. For virtually all state-to-state transitions, the IOSAM cross section ratios are found to be in better accord with experiment than is the case for IOSA calculations. It is therefore concluded that the improvement in the infinite-order sudden theory achieved by the inclusion of an explicit exit-channel velocity dependence into the scattering cross section is a general feature of the theory that is not dependent upon the detailed topography of the potential-energy surface.
Negative ion–molecule reactions of SF4
Authors: Babcock Lucia M. , Streit Gerald E.
Published: February 2003

A study of some negative ion–molecule reactions involving SF4 has been carried out by the flowing afterglow technique at ambient temperature. By examining a series of charge exchange reactions of SF4 and SF4−, the electron affinity of SF4 has been determined to be 2.35±0.1 eV. Rate coefficients for the charge exchange reactions of HS−, S−, OH−, and O− with SF4 and of SF4− with Cl2 and NO2 are reported. In addition, the fluoride transfer reactions of SF4− and SF6− with SF4 to produce SF5− have been examined. That both reactions proceed indicates that the fluoride affinity of SF4 is greater than that of SF3 or SF5. A lower limit of 3.7 eV for the electron affinity of SF5 may also be deduced from the fluoride transfer reactions. The two body addition of halide ions (X−) to SF4 to form the adduct SF4X− proceeds at near the collision limit (k = 9.7×10−10 cm3 molecule−1 s−1) for F−, very slowly (k = 2.6×10−11 cm3 molecule−1 s−1) for Cl−, and not at all within experimental limits (k<2×10−12 cm3 molecule−1 s−1) for Br−.
Ionization of gases by a pulsed electron beam as studied by self-focusing. III. He, Ar, and O2 mixtures
Authors: Arai Hidehiko , Hotta Hiroshi
Published: February 2003

The self-focusing of a pulsed electron beam from a Febetron 706 in He, Ar, and O2 is enhanced by 10% addition of polyatomic gases at total pressure higher than 50 Torr. The enhancement by addition of N2 to He is stronger than that expected from pure N2. The effect of additives to Ar appears abruptly from a certain pressure dependent on additives. According to the conclusion in part II, this enhancement is caused by the increasing of the mean ionization time ti of secondary ionization due to electric field E induced by the pulsed beam. Such inference is demonstrated in the present paper on the basis of the time ti for a mixture given by Eq. (1); ti for a mixture is determined by the ionization potential of additives relative to that of a host gas, the initial slope of the energy dependence of the total ionization cross section σi of additives, and the retardation of secondary electrons by additives for acceleration by E. When a gas having small σi and large effect of slowing down is added, the self-focusing is strongly enhanced due to the large increase of ti. This increase of ti is also demonstrated by the increasing of 1/(wα) by addition of additives (w is the electron drift velocity and α is the first Townsend ionization coefficient); the values of both parameters for some cases can be known either from papers of experimental data and the Boltzmann equation analysis or by the average electron method.
Force calculations in the density functional formalism
Authors: Harris J. , Jones R. O. , Müller J. E.
Published: February 2003

A force formula for an arbitrary system of electrons and nuclei is derived within the frozen core approximation to the density functional scheme. The formula has the important computational advantage over a direct application of the Hellmann–Feynman theorem in requiring only stationarity of a modified energy functional with respect to the valence charge density. The usefulness of the method is illustrated by its application to light dimers.
Multidimensional wave functions from classical trajectories
Authors: Davis Michael J. , Heller Eric J.
Published: February 2003

A new technique is developed to generate semiclassical wave functions. The method uses only information already available from a standard semiclassical quantization of a system. Linear superpositions of Gaussian coherent states that lie along quantizing classical trajectories are used, with phases given by the action integrals plus a Maslov-type correction. Wave functions generated in this way suffer from none of the problems with caustics that primitive semiclassical wave functions encounter. The semiclassical wave functions are convenient for subsequent use in applications, e.g., molecular spectra. By generating wave functions for several simple systems, we show that under most circumstances these wave functions are very accurate approximations to the true quantum states.
A theoretical analysis of the four-photon ionization spectrum of the nitric oxide molecule
Authors: Cremaschi Pietro
Published: February 2003

A theoretical analysis of the four-photon ionization spectrum of NO is performed starting from a frozen core SCF calculation with a very large basis set, using Coulomb functions for the description of the continuum and Morse oscillator functions for the evaluation of the vibrational integrals. The time-dependent perturbation theory for the evaluation of the cross sections and the kinetic approach for the estimate of the ion concentrations are able to rationalize, either by direct or indirect evidence, the relative intensities of the resonances of the first 16 Rydberg states and to suggest a partially new assignment of a few peaks.
Relationships among derivatives of the integrals in the calculation of the gradient of the electronic energy with respect to the nuclear coordinates
Authors: Kahn Luis R.
Published: February 2003

The use of the translational invariance property of molecular integrals is extended by its combination with the closely related transformation properties under infinitesimal rotations. This further reduces the number of truly linearly independent derivatives of the molecular integrals that require calculation in obtaining the gradient of the electronic energy. The detailed linear equations that yield the linearly dependent derivatives in terms of the ’’independent’’ derivatives are presented.
Tunneling states in phase III of solid partially deuterated methanes. Determination by inelastic neutron scattering and correlation with thermodynamic data
Authors: Lushington K. J. , Maki Kazuo , Morrison J. A. , Heidemann A. , Press W.
Published: February 2003

Energies of tunneling states in solid CH3D, CH2D2, and CHD3 in the range 0<ε<200 μeV were observed directly at low temperatures (T→1.5 K) by high resolution neutron scattering. An isotope effect was found which is broadly consistent with theoretical predictions. This is in contrast to the ’’view’’ obtained previously from the results of heat capacity measurements on the same solids at low temperatures: such an isotope effect was not apparent. By application of a new pocket state formalism that takes account of differences in energy for different orientations of the molecules under anisotropic fields, it is shown that the two kinds of experimental data can be correlated. The important general result is the achievement of a much deeper understanding than available heretofore of orientational and tunneling states in solid methane. Because the structure, and hence the site symmetries, of the lowest temperature phase (phase III) have not been established unambiguously, a relatively simple model is used to analyze the experimental results. It assumes equal proportions of sites of C2 and Cs symmetry and it gives a remarkably good description of the essential features of both the neutron scattering and the thermodynamic data. The heat capacities of CH2D2 and CH3D are not, however, reproduced within their probable accuracies at all temperatures. Moreover, some features of the neutron scattering spectra are broader than those predicted by the model. However, these are viewed as relatively minor imperfections in a model for what must be a structure that has a far more complex sublattice arrangement than our model assumes.
Electrical and convective polarization of the cylindrical macroions
Authors: Fixman M. , Jagannathan S.
Published: February 2003

The calculation of the dipole moment induced in the ion atmosphere of a cylindrical macroion is divided into two parts: first, the polarization in the absence of relative motion between the macroion and the solvent, and second, the convective polarization due to the relative, electrophoretic motion. The first calculation is carried through, for longitudinal and transverse polarizations of counterions and co-ions, according to the variational procedure described in the preceding paper [M. Fixman, J. Chem. Phys. 75, 4040 (1981)]. The convective polarization is further divided into two contributions: first, the polarization that would result from a constant relative motion (equal to the macroscopic electrophoretic velocity), and second, the correction due to the decrease in relative velocity in the vicinity of the macroion surface. The second convective contribution is estimated to be relatively small unless the cylinders are long compared to the Debye length. Longitudinal polarization of the counterions is not large compared to co-ion polarization, or to transverse polarization of co-ions and counterions, unless the cylinder is long compared to the Debye length. Otherwise, the outer parts of the ion atmosphere make the dominant contributions and are quite susceptible to convective effects.
A new adiabatic ensemble with particle fluctuations
Authors: Ray John R. , Graben H. W. , Haile J. M.
Published: February 2003

The statistical mechanics of a new shell ensemble, which is the adiabatic counterpart of the grand canonical ensemble, is developed through its analogy to the microcanonical ensemble and to the isoenthalpic ensemble. The important particle fluctuation formula that is characteristic of the ensemble is derived.
Thermochemical properties of the gaseous alkaline earth monohydroxides
Authors: Murad Edmond
Published: February 2003

The dissociation energies D°0(Ca–OH), D°0(Sr–OH), and D°0(Ba–OH) were measured using high temperature mass spectrometry. By passing H2(g) over CaO(s), SrO(s), and BaO(s) in a Mo Knudsen cell, gas-phase equilibria involving CaO(g), CaOH(g), SrO(g), SrOH(g), BaO(g), and BaOH(g) were studied. These results yielded D°0(Ca–OH)≊D°0(Sr–OH) = 92±4 kcal/mol (4.0±0.17 eV) and D°0(Ba–OH) = 107±4 kcal/mol (4.55±0.17 ev). In addition, the following appearance potentials were measured: A.P.(MgOH+) = 7.5±0.3 eV, A.P.(CaOH+) = 5.55±0.2 eV, A.P.(SrOH+) = 5.1±0.2 eV, A.P.(BaOH+) = 4.35±0.3 eV, A.P.(CaO+) = 7.6±0.5 eV, A.P.(SrO+) = 7.0±0.2 eV, and A.P.(BaO+) = 6.8±0.2 eV. Other important properties, e.g., proton affinities and heats of formation, are derived from these measurements. Bonding in the alkaline earth monoxides and monohydroxides is discussed briefly.
The accuracy of the Pitzer–Gwinn method for partition functions of anharmonic vibrational modes
Authors: Isaacson Alan D. , Truhlar Donald G.
Published: February 2003

The Pitzer–Gwinn method is applied to calculate partition functions at 200–4000 K for 26 potential energy curves exhibiting various types and degrees of anaharmonicity. This method, when combined with a simple approximation to estimate the zero point energy, yields reasonably reliable results for all cases studied.
Finite ion size effects in the electrical double layer—A Monte Carlo study
Authors: Snook Ian , van Megen William
Published: February 2003

The grand canonical ensemble Monte Carlo method is applied to monovalent and divalent symmetric primitive model electrolytes between plane charged surfaces. Results for continuous and discrete surface charge distributions are compared with the Gouy–Chapman theory.
Density matrix treatment of localized electronic interactions in molecules and solids
Authors: Kirtman B. , de Melo C.
Published: February 2003

A general method is presented for determining the electronic structure changes when an extended system is disturbed by a localized interaction. Such an interaction occurs, for example, when a small molecule is chemisorbed onto a metal surface. Our treatment is based on the density matrix formulation of the electronic structure problem and is applicable within the Hartree–Fock, Xα, or any other independent particle model. In essence, we obtain an effective self-consistent-field equation for the local interaction region. This equation includes charge transfer and all other delocalization effects due to the presence of the extended system. Illustrative calculations are carried out for an artificially modified beryllium atom and for chemisorption of atomic hydrogen onto a tungsten (100) surface. In the case of Be, the exact Hartree–Fock formulas are applied whereas, in the second illustration, we utilize the Anderson–Newns approximation.
Cell cluster calculations of the entropy of cracked crystals
Authors: Carman Howard S. , Huckaby Dale A.
Published: February 2003

The harmonic contribution to the entropy of a linear crack r lattice spacings in length in a crystal on the triangular lattice can be written as ΔSr/k = σr+τ(r), where σ is the surface entropy and τ(r) contains the entropy effects due to the crack tips. Pth order cell cluster approximations σ( p) and τ( p) (r) can be constructed, where τ( p)(r) = τ( p)(∞) if r≳p–1. Calculations of σ( p) and τ( p)(∞) for p?6 were obtained. Values of σ( p)r+τ( p)(r) can be extrapolated versus p−1 to yield values for ΔSr/k which compare favorably with results obtained from calculations on small crystals.