bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic properties and relaxation processes of impurity molecules in solids. I. Rotational spectra
Authors: Kono H. , Lin S. H.
Published: February 2003

The spectroscopic properties and relaxation processes of impurity diatomic molecules are studied theoretically. In order to treat the eigenvalue problem and the relaxation process consistently, we propose a unified theory based on the adiabatic approximation for the system of a diatomic molecule in monoatomic crystal. The adiabatic approximation is used to separate the total number of degrees of freedom of the system into the intramolecular vibrational, rotational, and lattice vibrational degrees of freedom. In this paper, we shall report the calculation of the rotational level shifts for HCl in rare gas matrices, using the M5 potential proposed by Hutson and Howard. The effect of lattice vibration is appropriately taken into consideration by means of the Green’s function method for point defects developed by Maradudin et al. The agreement between our theory and experimental results is good. It will be shown that the coupling between the molecular rotation and lattice vibration is important, which improves the idea of the cell model in which the effect of host atoms is completely neglected.
Convective instabilities of binary mixtures with fast chemical reaction in a porous medium
Authors: Steinberg V. , Brand H.
Published: February 2003

The instabilities which can occur when a layer of a binary mixture with fast chemical reaction in a porous medium is heated from below or from above are investigated. We find that both a stationary instability or an oscillatory instability can occur as the first bifurcation depending on the sign and the magnitude of the heat of reaction.
Generation and recombination of charge carriers in p-terphenyl crystal. Photoionization of singlet exciton
Authors: Morikawa Eizi , Isono Yasuo , Kotani Masahiro
Published: February 2003

With two-color photoionization, by using a nitrogen laser and a dye laser, it is shown that photoionization of singlet excitons in p-terphenyl crystal occurs with a photoionization cross section of (5.2±1.6)×10−19 cm2 with 590 nm photons (at 5×104 V/cm). Volume recombination of electrons and holes sets in at higher excitation intensities. The recombination rate constant γeh is found to be (1.3±0.4)×10−6 cm3 s−1. The understanding of the transient current pulse shape allows an in situ determination of the penetration depth of the exciting light, with a consequent improvement in the reliability of the constants determined.
Optical potential for laser induced dissociation
Authors: Leforestier Claude , Wyatt Robert E.
Published: February 2003

A new computational approach to molecular multiphoton dissociation is presented. Use of an optical potential allows the continuum channels to be represented as a set of pseudobound states coupled to the actual bound states of the molecule. The evolution of the system is thus amenable to the standard problem of laser induced transitions between bounds states. The long time behavior is then efficiently computed by using Floquet analysis. Results for a nonrotating diatomic are discussed.
Quasiclassical trajectory (and variational transition state theory) study of the rates and temperature-dependent activation energies of the reactions Mu+H2 (completely thermal) and H, D, and Mu+H2 (v=0, j=2)
Authors: Blais Normand C. , Truhlar Donald G. , Garrett Bruce C.
Published: February 2003

We report forward and backward quasiclassical trajectory calculations of the rate constant for the reaction Mu+H2→MuH+H at 444–2400 K. The accuracy of the calculations is assessed by comparison to variational transition state theory, both with and without tunneling contributions, and to experiment. We find the backward trajectory calculations are more accurate, as would be expected since the dynamical bottleneck occurs towards the products in the Mu+H2 reaction. Both the reverse trajectory calculations and the variational transition state theory calculations including tunneling agree with the experimental rate constants within a factor of about 2, which is considered very good agreement. We also report trajectory calculations and variational transition state theory calculations for the temperature-dependent activation energy, which increases by about 4 kcal/mol over the temperature range studied. To aid in the interpretation of this temperature dependence and that found in previous studies for H+H2 and D+H2, we also report temperature-dependent activation energies for the three state-selected rate constants H(D,Mu)+H2(v=0, j=2) →H2(HD,HMu)+H.
The thermal analog of state-selected unimolecular reactions
Authors: Stratt Richard M. , Adams John E.
Published: February 2003

A direct Monte Carlo approach that uses microcanonical transition state theory to evaluate RRKM reaction rates for molecules prepared in specific states is shown to be generalizable to a canonical version. This generalization requires the definition of an artificial temperature but nonetheless turns out to be both accurate and useful. The method is first studied with an idealized (Slater) model of a unimolecular reaction and then applied to the experimentally interesting case of the vibrational predissociation of the van der Waals molecule Ar ⋅ BCl3.
Comparison of the predictions of literature intermolecular potentials for Ar–Xe and Kr–Xe with experiment: Two new potentials
Authors: Aziz R. A. , van Dalen A.
Published: February 2003

Literature intermolecular potentials for the Ar–Xe and Kr–Xesystems are analyzed and critically assessed as to their abilities to predict dilute gas bulk and microscopic properties. Two new potentials of the HFD-C form (HFDAX1 and HFDKX1) are proposed for the Ar–Xe and Kr–Xe interactions, respectively, which accurately predict a large number of properties.
An improved potential for Ar–Kr
Authors: Aziz R. A. , van Dalen A.
Published: February 2003

An accurate intermolecular potential of the HFD-C form is proposed which appears to be the best representation of the Ar–Kr interaction. The potential, which possesses the correct long range behavior, predicts a wide variety of macroscopic and microscopic data. It is consistent with accurate second virial, viscosity, and diffusion coefficients. Differential scattering and low as well as high energy total cross section data are accurately predicted.
Reaction-path interpolation models for variational transition state theory
Authors: Truhlar Donald G. , Kilpatrick Nancy J. , Garrett Bruce C.
Published: February 2003

We present interpolation formulas for estimating parameters in the reaction-path Hamiltonian from known data in three vicinities—reactants, saddle point, and products. Whereas conventional transition state theory calculations are usually based on a quadratic expansion of the potential energy surface at the saddle point, the present approach attempts to estimate the variational transition state theory rate constant by also using selected cubic and quartic force constants of the saddle point or by using quadratic force constants determined over a small range of the reaction path, not necessarily including the variational transition state. The method is illustrated and the formulas are tested for four collinear classical reactions.
Pulse radiolysis of crystalline D2O at 6 K
Authors: Wu Zhennan , Gillis Hugh A. , Klassen Norman V. , Teather George G.
Published: February 2003

In experiments on the pulse radiolysis of crystalline D2O ice at a nominal temperature of 6 K, yields of e−vis, e−ir and the OD radical have been measured as a function of accumulated dose. The yield of e−ir shows a large effect of dose, increasing from Gε=1.03×104 (G≂0.27) for a previously unirradiated crystal to Gε=4.1×104 (G≂1.08) for a sample that had received 300 krad. The effect of dose is decreased to about half by annealing at 12.5 K, but can only be completely eliminated by annealing above 100 K. The effect is interpreted as being due to the scavenging, by D and OD, of D2O+ which otherwise recombines with e−qf. The G of OD at 6 K for a previously unirradiated crystal was measured as 3.8; this is much higher than the previously reported G of OH from γ-irradiated H2O ice at 4 K.
A correlated one parameter momentum space function for the ground state of helium
Authors: Lombardi John R.
Published: February 2003

The integral equation for the ground state of the helium atom is iterated to produce a correlated one parameter wave function in momentum space. This new function has second order poles at iζ and a first order pole at 2iζ, where ζ is retained as a variational parameter. The function may be written φ(p1p2)=√2/13 [(2ζ)4/π] [1/(p1 −iζ)2(p2−iζ)2(p1+p2−2iζ)] with resulting energy expressed as E=(14/13) ζ2+(32/13) [(619/48) −20 ln 2] ζ+(16/13)ζ. Optimizing gives ζ=1.6391 and E=−2.8933 a.u. within 0.0104 a.u. of the exact energy.
Electron correlation described by an extended group function model: The EXGF1 model
Authors: Ro/eggen I.
Published: February 2003

A scheme for obtaining approximate solutions of the electronic Schrödinger equation is proposed in terms of an extended group function model. The model is formulated within the context of the method of moments. The model is size consistent, and it has additive accuracy since the correction to a pure antisymmetrized product function is obtained as additive terms describing intragroup and intergroup correlation corrections. In a test calculation on the ground state potential energy curve for the F2 molecule, the advocated model yields a dissociation energy of 1.63(1.66) eV and an equilibrium internuclear distance of 1.392(1.412) Å. Experimental values are within parenthesis.
Angular momentum projection operators and molecular bound states
Authors: Blanco Mario , Heller Eric J.
Published: February 2003

A new method is presented which fully adapts a Cartesian basis set (via projection techniques) to rotational symmetry. The method is intended as a practical tool to make use of the convenience of a Cartesian basis (simplicity of the Hamiltonian, matrix elements, etc.) while at the same time enjoying the advantages of rotational symmetry adaptation.
Optical spectroscopy and electron paramagnetic resonance of Pa4+ in ThBr4 and ThCl4 crystals
Authors: Krupa J. C. , Hubert S. , Foyentin M. , Gamp E. , Edelstein N.
Published: February 2003

Polarized absorption and emission spectra of Pa4+ in ThCl4 and ThBr4 host matrices were recorded at different temperatures from 4.2 to 300 K in the infrared range. The seven crystal field levels of the 5f1 configuration were determined from the optical data and fit by a parametrized model assuming a D2d site symmetry for the Pa4+ ion. However, at low temperature both matrices undergo a displacive phase transition and the low temperature structure is incommensurate and modulated. The halide position modulation reduces the site symmetry from D2d to D2 for some Pa4+ ions, and, consequently, the shapes of some absorption bands are characterized by edge singularities. EPR experiments performed on these doped crystals provided g values for the ground state of Pa4+ which fit satisfactorily with the values calculated from the optical data.
Microwave optical double resonance spectroscopy of ND2 in (000) of X̃ 2B1
Authors: Cook J. M. , Hills G. W.
Published: February 2003

Microwave optical double resonance spectroscopy has been used to observe in excess of 200 microwave transitions involving 21 rotational levels in the (000), X̃ 2B1 state of ND2. The observed transitions have been fitted to two model Hamiltonians. In the first fit, accurate values for the spin rotation constants and 2D hyperfine constants were determined. The 2D hyperfine constants are in excellent agreement with those calculated from 1H parameters in NH2, (000) X̃ 2B1. In the second fit, only the constants of the Hamiltonian describing the spin-rotation effect were determined. The derived spin-rotation parameters for ND2 (As, Bs, Cs, ΔsK, ΔsN, ΔsNK + ΔsKN, δsK, and δsN) are compared to the appropriate scaled constants of NH2.
Rotational structure and vibrational predissociation in the HF stretching bands of the HF dimer
Authors: Pine A. S. , Lafferty W. J.
Published: February 2003

The rotational structure in the HF stretching bands of the HF dimer has been recorded with nearly Doppler-limited resolution using a tunable difference-frequency laser spectrometer and a long-path cell held at low temperatures and pressures. Two bands are observed; the higher frequency band, corresponding principally to the ‘‘free’’ hydrogen stretch, has the appearance of a B-type or perpendicular band with two prominent Q subbranches RQ0 and PQ1; the lower band is an A-type or parallel band arising primarily from the ‘‘bonded’’ hydrogen vibration. The K=0 subbands of both vibrations have been fully assigned and fit with polynomial expansions in J(J+1) to yield ground state constants in excellent agreement with a previous microwave resonance molecular beam study of the HF dimer. The subbands exhibit doubling and 10:6 intensity ratios for alternate J indicative of the internal rotation tunneling motion proposed in the microwave study. A pressure-independent broadening of the lines to about twice the Doppler width, attributed to vibrational predissociation, is observed for the bonded hydrogen stretching band.
Spectroscopic and line-narrowing properties of resonant sum and difference frequency generation
Authors: Dick Bernhard , Hochstrasser R. M.
Published: February 2003

Three wave mixing is investigated with respect to its spectroscopic applications under fully resonant conditions. A theoretical study of the appropriate second order susceptibility χ(2) reveals interesting features both for sum and difference frequency generation. For difference frequency generation two different mechanisms are possible one of which allows the observation of resonances between excited states and the study of dephasing induced coherent emission. The coherence decay rate between the two excited states may be measured and also its pure dephasing rate. The line-narrowing characteristics of the fully resonant χ(2) are studied with respect to correlation between the inhomogeneous distribution for different levels. The line-narrowing capabilities are found to be partly complementary for the sum—and the two difference frequency resonance configurations. This leads to the suggestion of new line-narrowing techniques which could provide information about type and amount of correlation in inhomogeneously broadened systems.
Pure electronic, vibronic, and two-particle surface excitons on the anthracene crystal. I. General theoretical basis, experimental study, and analysis of the (0,0) region
Authors: Orrit M. , Bernard J. , Turlet J. M. , Kottis Ph.
Published: February 2003

These two papers (papers I and II) present a complete study—theoretical and experimental—of the first singlet–singlet electronic and vibronic transitions of the anthracene crystal. We interpret the new observed structures in terms of pure electronic, vibronic, and two-particle site shift surface exciton states. The first paper (I) begins by a model description of the bulk and surface excitons in a rigid lattice. We then describe the experimental study of the anthracene crystal reflectivity and surface emission excitation at low temperatures with and without nitrogen coating of the crystal surface. For the first time, we clearly established the following results: (1) The existence and the accurate position of the upper a polarized Davydov component for the pure electronic surface exciton; (2) The equality within our experimental accuracy (≊2 cm−1), of the surface and bulk excitonic Davydov splitting (Δ̄=223 cm−1) and the translational equivalence (δ̄=206 cm−1) of the surface structures with their bulk counterparts; (3) A variation of the surface Davydov splitting when the crystal surface is coated with frozen nitrogen.
Theory of conductance and related isothermal transport coefficients in electrolytes
Authors: Altenberger A. R. , Friedman Harold L.
Published: February 2003

The transport coefficients are calculated from consideration of the fluctuations in local ion concentrations in view of a recent advance in the theory of inhomogeneous systems at equilibrium. The theory is applicable to mixed electrolytes of arbitrary charge type and to frequency dependent phenomena, although the example worked out here is for the zero frequency coefficients for a binary electrolyte.
Direct simulation of the chemical potential
Authors: Fixman Marshall
Published: February 2003

An algorithm that combines several known devices for reducing the variance of Monte Carlo simulations was developed for the direct simulation of the chemical potential. An application to the hard sphere fluid indicated that the method is practical for all fluid densities. A strong sensitivity to the size of the system and to boundary conditions was exhibited in the phase transition region.
Distribution of internal energy in CO and CO2 vibrationally excited by a hot platinum surface
Authors: Mantell D. A. , Ryali S. B. , Haller G. L. , Fenn J. B.
Published: February 2003

Changes in the internal state of CO and CO2 molecules scattered by a hot platinum surface have been characterized by Fourier transform infrared emission spectroscopy. The only molecules which can be detected are those that are vibrationally excited for CO and excited to the asymmetric stretch mode for CO2. The distribution of rotational energy in these molecules is Boltzmann for CO2 but not for CO. In both cases, the rotational energy is substantially less than the equilibration values based on the surface temperature. The accommodation coefficient for the asymmetric stretch vibrational mode of CO2 drops from 0.22 at 700 K to 0.16 at 1500 K. Very few of the molecules which are vibrationally excited in the asymmetric stretch mode are also excited in either the bending or symmetric stretch mode. For CO, the vibrational accommodation coefficient is about 0.7. However, the fraction of the molecules excited to the v=2 vibrational level compared to the v=1 level indicates complete vibrational accommodation at the surface temperature. This implies a partitioning of CO molecules between those that completely accommodate vibrationally and those that do not accommodate at all.
Intramolecular vibrational relaxation from C–H stretch fundamentals
Authors: Stewart G. M. , McDonald J. D.
Published: February 2003

Infrared fluorescence measurements have been used to determine the presence or absence of intramolecular vibrational relaxation (or more precisely state mixing) from an excited C–H stretch in 23 representative molecules varying in size from methane to nonbornene. Included are aliphatic hydrocarbons, aromatic molecules, ethers, and ketones, including cyclic molecules. The rate of resonance fluorescence, calculated relative to a nonrelaxing molecule, was used as the criterion of relaxation. Small rates imply state mixing and hence in the large molecule limit, relaxation. The primary correlation with the presence of mixing is with state density; about ten states per wave number (cm−1) are needed to insure mixing. The spread in threshold densities reflects variations in anharmonicity. For some molecules true relaxed (non C–H stretch) fluorescence is observed. These experiments were done in a molecular beam, using a pulsed optical parametric oscillator for excitation and a circular variable filter for spectral resolution.
Local modes and cross-correlation functions
Authors: Muckerman J. T. , Noid D. W. , Child M.S.
Published: February 2003

It is shown how the use of cross-correlation functions and their Fourier transforms provides a convenient probe of the division of the classical phase space for stretching vibrations of H2O into local, normal, and stochastic fractions. Resulting classical estimates for the numbers of total and local states, N(E) and Nl(E), respectively, as functions of energy are in excellent agreement with previous quantal and semiclassical results. The classical estimate of the number of stochastic states Ns(E) as a function of energy is a new result. The present method is readily generalizable to polyatomic molecules with many vibrational degrees of freedom.
Comparison of quantal, classical, and semiclassical behavior at an isolated avoided crossing
Authors: Noid D. W. , Koszykowski M. L. , Marcus R. A.
Published: February 2003

The quantal and classical/semiclassical behavior at an isloated avoided crossing are compared. While the quantum mechanical eigenvalue perturbation parameter plots exhibit the avoided crossing, the corresponding primitive semiclassical eigenvalue plots pass through the intersection. Otherwise, the eigenvalues agree well with the quantum mechanical values. The semiclassical splitting at the intersection is calculated from an appropriate Fourier transform. In the quasiperiodic regime, a quantum state near an avoided crossing is seen to exhibit typically more delocalization than the classical state. However, trajectories near the ‘‘separatrix’’ display a quasiperiodic ‘‘transition’’ between two zeroth order classical states.
The nature of hydrogen bonding in the NN–HF, OC–HF, and HCN–HF complexes
Authors: Benzel Mark A. , Dykstra Clifford E.
Published: February 2003

The nature of hydrogen bonding in the complexes NN–HF and OC–HF, as revealed from detailed ab initio electronic structure calculations, is unlike the bonding in the isoelectronic HCN–HF complex. The strong dipole–dipole attraction in HCN–HF is absent in the other two complexes. Competing effects that are possible in all hydrogen bonds are unmasked in NN–HF and OC–HF. Calculations have been performed with a number of large basis sets, extensive treatment of electron correlation, energetic partitioning, and evaluation of molecular properties. The potential energy surfaces that were generated demonstrate the linearity of the equilibrium structures, and show roughly comparable flexibility for bending, thus refining somewhat the experimentally inferred picture. In NN–HF and OC–HF, less than one-half of the hydrogen bond strength of each is due to just the interaction of the unrelaxed charge distributions of the monomers; while polarization of monomer electron distributions accounts for most of the remaining attraction. There is consequently a substantial electric dipole moment enhancement. The total correlation effect is responsible for about one-third of the well depth in NN–HF and OC–HF. Charge transfer is unimportant in all three complexes.
Electronic correlations and midgap absorption in polyacetylene
Authors: Soos Z. G. , Ducasse L. R.
Published: February 2003

Electronic correlations are important when they lift degeneracies of Huckel models. Standard Pariser–Parr–Pople (PPP) parameters split the degenerate midgap absorption of neutral solitons in undoped trans polyacetylene (CH)x. The dipole intensity goes entirely to the upper state close to the optical gap Eg. PPP correlations lower slightly the nondegenerate transition at Eg/2 of charged solitons, which consequently provide all the E∼Eg/2 intensity. Exact symmetries of correlated states and finite-chain computations are exploited in associating the midgap absorption of neat and lightly doped (CH)x entirely with charged solitons.
Interactions between neutral dissociation and ionization continua in N2O
Authors: Guyon Paul M. , Baer Tomas , Nenner Irene
Published: February 2003

Autoionized Rydberg states converging to the Ã 2Σ+ and B̃ 2Π states of N2O+ are shown to be predissociated into neutral fragments. The decay to excited neutral fragments is observed by their fluorescence in the visible and ultraviolet regions of the spectrum. These decay channels are in competition with autoionization, which was shown in a previous study to yield unexpectedly large numbers of low energy electrons. On the basis of the striking similarity between the fluorescence excitation and the threshold photoelectron spectra, models are presented which connect dissociative channels to the production of threshold electrons. Ionization of dissociating neutral states and Rydberg states converging to the ground electronic state of N2O+(X̃ 2Π) are suggested as being responsible for the production of low energy electrons. The distribution of photoelectron energies resulting from such autoionization channels are calculated.
Multiphoton dissociation of OCCl2 at 193 nm: Formation of electronically excited Cl2
Authors: Wilson M. W. , Rothschild M. , Rhodes C. K.
Published: February 2003

Two-quantum excitation of OCCl2 with 193 nm radiation, although corresponding to an energy above the ionization limit, is observed to produce a yield upon dissociation into excited neutral fragments; namely, CO in the v=7 of its X 1Σ+ state and Cl2 in the first electronically excited 3Πg and 1Σ+u states of ionic character. Strong emissions originating from Cl*2 were observed at 199 and 258 nm together with a weaker band at 244 nm. The radiative lifetime tr of the Cl2(3Πg) state was determined to be tr=23±2 ns. Quenching rate constants of the Cl2(1Σ+u) level by rare gases were also measured. Strong enhancement of the 258 nm radiation was observed in the presence of He and Ne, while intense emissions from KrCl* and XeCl* indicate the role of atomic displacment reactions of the type Cl*2+(Kr/Xe)→(Kr/Xe)Cl*+Cl.
Anisotropies of the Na2–rare gas interaction potentials from angularly resolved inelastic scattering
Authors: Jones P. L. , Gottwald E. , Hefter U. , Bergmann K.
Published: February 2003

Rotational rainbow angular positions in the inelastic differential scattering cross sections of argon, krypton, and xenon from Na2 are reported. The observed transitions for small rotational quantum number change (Δj≤16) are analyzed in terms of the classical model of hard ellipsoid scattering which relates the rainbow angle to the anisotropy ε of the potential, where ε is the difference in length of the long and short axes. The analysis yields ε=0.67±0.05, 0.79±0.09, and 0.66±0.05 Å for the systems Na2–Ar, Kr, and Xe, respectively. From earlier work, the anisotropies for Na2–He and Na2–Ne are known to be ε=0.70 (calculated) and 0.72±0.05 Å. The data demonstrate the similarity within this model for the anisotropies of all the Na2–rare gas interaction potentials for low effective collision energies.
Oscillating reactivity of collinear symmetric heavy+light–heavy atom reactions
Authors: Hiller C. , Manz J. , Miller W. H. , Römelt J.
Published: February 2003

The oscillatory reaction probability (as a function of energy) of collinear heavy+light–heavy systems (e.g., I+HI→IH+I) that has been seen in earlier quantum mechanical reactive scattering calculations is shown to be described quantitatively by a semiclassical WKB model. Because these reactions are highly vibrationally adiabatic they reduce to a two-state symmetric resonance system (analogous to symmetric charge transfer, e.g., H++H→H+H+) that involves only the phase shifts of the one-dimensional g (symmetric) and u (antisymmetric) combinations of the two states. Comparisons of the semiclassical and quantum mechanical reaction probabilities over a wide range of energy for the cases I+MuI→IMu+I and I+HI→IH+I show almost perfect agreement. The vibrationally adiabatic symmetric exchange problem is also solved classically (analytically) and is seen to have an interesting relation to the quantum/semiclassical result. The classical reaction probability is also an oscillatory function of energy, although the structure of the oscillations is different from the quantum/semiclassical ones (‘‘saw-tooth’’ rather than sinusoidal) and the phase of the classical oscillations is only approximately the same as that of the quantum/semiclassical case. (In the high energy limit, the phase of the oscillations increases roughly as the square root of collision energy.) Thus, though the classical (light atom hopping between two heavy atoms) and quantum mechanical (resonance interference of g and u collision channels) interpretations of the oscillatory reactivity seem at first to be quite different, they are seen in fact to be essentially the same.
Observation of levels near dissociation in the X 1Σ+g state of 7Li2
Authors: Verma K. K. , Koch M. E. , Stwalley William C.
Published: February 2003

The fluorescence spectrum of 7Li2 excited by a Kr+ (5682 Å) laser has been observed and analyzed. The laser simultaneously excites the A1Σ+u and the B1Πu states of the 7Li2 molecule. The analysis reveals two very interesting things: (a) the A–X spectrum consists of the transitions involving v″ levels of the entire ground state potential well (5≤v″≤36) and thus we have made the very first observation of the levels near the dissociation limit in the ground state of 7Li2, which suggests that D″e=8521.5±4 cm−1; and (b) the B–X spectrum is actually enhanced by the fluorescence from the A state which is an interesting single frequency analog of ‘‘modulated population spectroscopy’’ or ‘‘optical–optical double resonance’’ which we call ‘‘coupled laser induced fluorescence series’’ (CLIFS).
Rydberg–Rydberg transitions of NO using an optical–optical double resonance multiphoton ionization technique
Authors: Cheung Wan Yee , Chupka William A. , Colson Steven D. , Gauyacq Dolores , Avouris Ph. , Wynne James J.
Published: February 2003

New Rydberg–Rydberg transitions of 14N 16O have been observed by the technique of optical–optical double resonance multiphoton ionization (OODR-MPI). The analysis of S 2Σ+–A 2Σ+ (0,0), (1,1) and 4f–A 2Σ+ (1,1) bands are reported. Molecular constants of the S 2Σ+ v=0,1 levels are in good agreement with those derived from absorption data. The structure of the 4f(v=1) level of the 14N 16O isotope can be fully investigated for the first time since the OODR-MPI spectrum is free from any overlap, in contrast to the corresponding absorption spectral region. No detectable spectral perturbation was observed for levels up to N=10, although the non-Rydberg G 2Σ−(v=8) level lies at the same energy as the 4f(v=1) level. The 4f(v=1) rotational levels are described by an intermediate case (b)/case (d) coupling scheme and present a similar pattern to that of the 4f(v=0) rotational levels previously studied from absorption data. Rotational constants B1=1.968 cm−1 and αe=0.020 cm−1 have been derived from our analysis and the B0 value already known from absorption work. The constant k, which is related to the quadrupole moment of the vibrating molecular core, has also been derived for the 4f(v=1) level. Our observations suggest the presence of a ‘‘significant’’ amount of l=2 character in the lowest member of the ns series, (3sσ) A 2Σ+ state, and confirm the pure l=3 character of the 4f Rydberg level.
Subpicosecond time resolved multiphoton ionization: Excited state dynamics of cis-stilbene under collision free conditions
Authors: Greene B. I. , Farrow R. C.
Published: February 2003

Subpicosecond time resolved multiphoton ionization is demonstrated to be a useful technique for probing unimolecular dynamics in vapor phase samples. Vapor phase cis-stilbene molecules are observed to decay out of their initially prepared S1 state with a single exponential time constant of 0.32 ps. This decay, thought to correspond to a rapid twisting of the molecule about the ethylenic double bond, occurs in roughly one c=c double bond torsional vibrational period.
On the nature of the bonding in Cu2—a Comment
Authors: Pauling Linus
Published: February 2003

Bauschlicher, Walch, and Siegbahn’s1 calculations on the structure of Cu2 molecule are commented on. Semi−empirical arguments are suggested to support the thesis that the bond in this molecule is a triple bond and not a single bond as suggested by Bauschlicher, Walch, and Siegbahn in their calculations. (AIP)
On the nature of the bonding in Cu2—an ab initio viewpoint
Authors: Bauschlicher Charles W. , Walch Stephen P. , Siegbahn P. E. M.
Published: February 2003

Calculations are performed on Cu2 using the basis functions from Ref. 1. As a result of present calculations, there is evidence for the existence of both a single bond and a triple bond( from Pauling’s semi-classical results). (AIP)
Electrophoretic light scattering studies of poly(L-lysine) in the ordinary and extraordinary phase. Effects of salt, molecular weight, and polyion concentration
Authors: Wilcoxon Jess P. , Schurr J. Michael
Published: February 2003

Electrophoretic light scattering (ELS) is employed to determine the electrophoretic mobilities μE and apparent diffusion coefficients DELS obtained from ELS linewidths for three samples of poly(L-lysine)HBr (PLL) of degree of polymerization n=406, n=946, and n=2273 as a function of salt (Cs) and polyion (Cp) concentration. Dynamic light scattering (DLS) is used to determine apparent diffusion coefficients Dapp of these same PLL samples in the absence of the applied field. The polyelectrolyte system exhibits an ordinary to extraordinary phase transition at low salt concentrations that is manifested by a more than 50-fold decrease in Dapp as well as a twofold decrease in scattered intensity. In the extraordinary phase, the DELS values are typically very small, though still somewhat (frequently twofold) larger than Dapp. When the salt concentration is raised to enter the ordinary phase DELS remains small, less than one tenth of Dapp, in the low-salt end of the ordinary phase, but increases with increasing Cs to become comparable to, though still about twofold smaller than, Dapp in 0.1 M NaBr. The fluctuations in polyion density responsible for the observed ELS signal are evidently relaxed very slowly under low-salt conditions, regardless of which phase prevails. In the ordinary phase μE increases rapidly with decreasing Cs, but in the extraordinary phase μE either remains nearly constant for (n=946), or actually decreases (for n=2273), as Cs decreases further to 0. The present μE data disagree with a previous empirical prediction of Schurrr and rule out the particular interpretation of the diffusion virial coefficient upon which that was based. The theory of Manning predicts a value of the tracer mobility μp that is quite close to the value of μE observed for (1ys)406 at Cs=0.1 M, but predicts much too large an increase in μp with decreasing Cs, and fails to predict the significant molecular weight dependence of μE observed in the intermediate Cs range near 0.02 M. It is pointed out that incorporation of electrolyte friction into Manning’s theory could improve the agreement with experiment.
Vibrational predissociation and intramolecular vibrational relaxation in electronically excited s-tetrazine–argon van der Waals complex
Authors: Brumbaugh Donald V. , Kenny Jonathan E. , Levy Donald H.
Published: February 2003

The van der Waals complex tetrazine–argon formed in a supersonic expansion has been investigated by means of laser-induced excitation and fluorescence spectroscopy. Excitation spectra reveal the band shift D′′0−D′0=−23 cm−1, the tetrazine–argon stretch frequency νw=44 cm−1, and a lower limit to the dissociation energy D′′0≳254 cm−1. The band shift is vibrationally dependent in the case of two low frequency out-of-plane modes of tetrazine. The absence of additional broadening of the excitation linewidths due to the presence of argon shows that the dynamical processes in the complex are slower than the photodissociation of tetrazine itself (0.5–1.4 ns) which serves as an internal timing mechanism. At five single vibronic levels of tetrazine–argon above the S1 zero-point level, the rates of vibrational predissociation and intramolecular vibrational relaxation are ∼108 s−1. The V→V propensities in dissociation and relaxation are selective and different from collision-induced propensities. Among the V→V channels in dissociation, we find the lowest upper limit to D′0 is 381 cm−1. Considerable rotational excitation of the tetrazine fragment is observed following dissociation which may reduce the estimate of D′0 to as low as 306 cm−1. Although redistribution of vibrational energy during dissociation is selective, no mode selective effects could be attributed to the initial level of excitation, despite variations in the energy gap, number of quanta, total energy, or the dynamics of the vibrational motion. The van der Waals potential is virtually identical in the two electronic states.
Intramolecular vibrational relaxation: Effects on electronic nonradiative relaxation rates
Authors: Morse M. D. , Puiu A. C. , Smalley R. E.
Published: February 2003

The effects of intramolecular vibrational relaxation (IVR) on radiationless transition rates are investigated. IVR has previously been suggested as an explanation of the observed saturation of the decay rate in tetracene S1→S0 internal conversion and in toluene T1→S0 intersystem crossing at high vibrational energies. Explicit calculations for transition rates under weak coupling in the statistical limit are presented which demonstrate that when IVR is operative a slow exponential increase of the nonradiative decay rate with vibrational energy is expected, in contrast to the experimentally observed rapid initial increase followed by saturation at higher vibrational energies. Speculations of alternative decay mechanisms are presented, and new, simple approximate formulas are derived for the relative nonradiative decay rates of single vibronic levels.
Spin decoupling of fragment states in valence-bond theory: Application to the diatomics-in-molecules method
Authors: Radlein Desmond St. A. G.
Published: February 2003

An ‘‘elimination’’ algorithm for efficiently carrying out the spin decoupling of fragment states in diatomics-in-molecules (DIM) calculations, using valence-bond types of basis sets, is described. More generally, it is applicable to the determination of fragment valence states and the calculation of overlap integrals; the sole restriction being that the underlying atomic orbital basis should be orthogonal. The algorithm is given as a set of elimination rules which are reduced by applying second quantization operator techniques.
Bounds for the atomic electronic density: The near nuclear region
Authors: King Frederick W.
Published: February 2003

Some upper bound estimates for the atomic electron density are derived using Block’s inequality. A rough bound for the density is obtained in terms of the number of electrons, the nuclear charge, and the electronic energy. The bounds derived in this work are tested for some members of the helium isoelectronic series using electronic densities derived from 20 term Hylleraas type wave functions. A comparison is also made with some previously derived upper bound estimates for the electronic density. The bounds presented do not diverge in the near nuclear region like r−n, a feature which characterizes presently available bounds.
An orthogonal orbital MCSCF plus CI treatment of molecular symmetry breaking. I. The lowest 3B2 and 3A2 states of CO2
Authors: Engelbrecht L. , Liu B.
Published: February 2003

A general, orthogonal orbital, multiconfuguration self-consistent field (MCSCF) plus configuration interaction (CI) procedure has been developed for treating open shell molecules subject to symmetry breaking (doublet instability, etc.) in the restricted Hartree–Fock approximation. The chemical origin of symmetry breaking was analyzed using a simple valence bond picture and the lowest 3A2 and 3B2 states of CO2 as examples. This analysis provided the necessary information to design MCSCF wave functions which possessed the symmetry of the nuclear frame, and changed smoothly between high and low symmetry nuclear configurations. Near qualitative potential energy surfaces near the potential minima of the two lowest triplet states of CO2 were obtained by CI calculations using the MCSCF orbitals and reference configurations. Both states were found to have C2v potential minima with the 3A2 state lying approximately 0.9 eV above the 3B2 state. The equilibrium CO bond lengths and OCO bond angles are 1.24 Å, 118° and 1.26 Å, 127° for the 3B2 and 3A2 states, respectively.
Delocalized magnetic currents in benzene
Authors: Gomes J. A. N. F.
Published: February 2003

Results of ab initio calculations of the magnetic properties of cyclic, conjugated hydrocarbons are analyzed for the delocalized current. They fit well into earlier models for localized and delocalized currents, suggesting that an important delocalized current exists, this being due, particularly, to the π electrons. A rigorous definition of delocalized current is proposed. This is achieved by considering the current contribution from each natural orbital, which is conserved (divergenceless, in a stationary problem) when the exchange currents are included. It is argued that these conserved orbital currents should be preferred for the analysis of the induced current density. The delocalized current now defined may be related to the ring current introduced in the context of semiemperical theories. The results of this paper appear to give substance to the conventional ring current ideas, a conclusion which is at variance with the most recent interpretations of the new ab initio calculations.
The chemical and physical properties of pyrrole-based conducting polymers: The oxidation of neutral polypyrrole
Authors: Pfluger P. , Krounbi M. , Street G. B. , Weiser G.
Published: February 2003

A series of experiments on the physics and chemistry of polymers derived from pyrroles show that the oxidation of neutral insulating polypyrrole (PP0) to its conducting counterpart (PP+) is a multistep process. In particular, the conductivity of the polymers increases only in the early stages of oxidation, whereas significant changes in the optical and EPR properties occur in later stages of the oxidation when no further changes in the conductivity take place. The early stages of oxidation lead to an ionic (PP+ anion−) polymer and the later stages of oxidation result in chemistry at the nitrogen atoms of the pyrrole rings. Similar behavior is observed for all the oxidized pyrrole polymers independent of the method of oxidation.
Nonequilibrium effects in the energy distribution function
Authors: Burns George , Cohen L. Kenneth
Published: February 2003

The relative nonequilibrium energy distribution function, in the steady state for the irreversibly reacting Br2 in an argon system at 3500 K, is calculated. It is based upon 44 400 classical 3D trajectories, and uses the single uniform ensemble method [H. D. Kutz and G. Burns, J. Chem. Phys. 72, 3562 (1980)]. Although the raw data display a considerable scatter, they clearly indicate a depletion from the equilibrium distribution function over a wide energy range. A careful statistical study of the data is performed. It is found that their histograms can be described over the entire possible energy range by a simple analytical function with only one adjustable parameter. The best fitting procedure yields a surprisingly narrow goodness of fit. However, an apparent deviation of the fit from the data is observed in the energy region where the reaction channel opens. To that extent, this work sheds a new light on the nature of the steady state in an irreversible reaction.
On classical paths and the Wigner path integral
Authors: McLafferty Frank
Published: February 2003

We ask and answer the question: How do we derive the approximation of Heller (that the Wigner function propagates along classical paths) from the path integral formulation of the Wigner kernel of Kitahara?.
Franck–Condon analysis of the 1 1A−g→1 1B+u transition of 1,3-butadiene from absorption and Raman intensities
Authors: Hemley R. J. , Dawson J. I. , Vaida V.
Published: February 2003

Vibronic activity in the 1 1A−g→1 1B+u electronic transition of 1,3-butadiene is analyzed using both the observed structure in the absorption spectrum and Raman band intensities measured at eight excitation frequencies between 23 522 and 37 026 cm−1. The Raman spectra are dominated by the fundamental of ν4, and ag carbon–carbon stretching mode. From an analysis of the preresonance Raman excitation profiles, a total of six ag modes are found to be active in the electronic transition. The distortion of the 1 1B+u excited state is calculated in terms of displacements in these six normal coordinates by fitting the Raman relative intensities and the absorption Franck–Condon envelope first assuming no excited state mode mixing. A significantly improved fit of the absorption spectrum is obtained when the two most active modes ν4 and ν7 are allowed to mix in the excited state. The geometry of the 1 1B+u state is calculated with and without excited state mode mixing, and both analyses predict bond inversion of the ground state single and double bonds together with an overall in-plane skeletal expansion.
Quantum mechanical treatment of the F+H2→HF+H reaction
Authors: Baer Michael , Jellinek Julius , Kouri D. J.
Published: February 2003

In this paper is presented a quantum dynamical study of the F+H2 reaction within the infinite order sudden approximation for the energy range Etot=0.28–0.50 eV. Results at various stages of the calculation are given ranging from the most detailed phases and S matrices to the total integral cross sections. The accuracy of the IOS is assessed by comparisons of the average l-labeled quantal IOS results with exact classical, initial-l labeled classical IOS, and l-initial labeled quantum IOS results. Comparison with experiment indicates that the qualitative state-to-state angular distributions are reproduced within this method. On the other hand, vibrational branching ratios for the product HF molecule are only partially reproduced. The main part of the discussion in the paper is devoted to the recent hypothesis concerning the existence of a superposition of resonances which strongly influence the angular distributions as a function of final vibrational state of the HF product.
Comparison of photon stimulated dissociation of gas phase and chemisorbed CO
Authors: Ramaker David E.
Published: February 2003

Theoretical interpretation of electron–electron and electron–ion coincidence data for CO in the gas phase indicates one hole, two hole, and two hole–one electron excited states (e.g., the 3σ−1, 5σ−2, and 5 σ−2 6σ states) contribute to molecular dissociation. Detailed comparison with O+ ion yield data for photon stimulated desorption from CO/Ru (001) at both valence and core level excitation energies reveals many similarities but also significant adsorbate-substrate interaction effects; the most important are due to charge transfer from the substrate which screens the core or valence holes in the adsorbate. Appreciation of the competitive decay rates of the excited states is critical to an understanding of which excited states of either CO gas or the CO/metal system may be expected to lead to dissociation. Resonant decay to the continuum and Auger decay are important decay mechanisms in both systems. Adsorbate–adsorbate interactions on the surface are seen to provide an additional decay mechanism. The excited states responsible for O+, CO+, and CO desorption from the surface are assigned for each of the threshold energies. The two hole–one electron states are the most important for desorption from the surface.
Chemical waves in a two-dimensional oscillating system
Authors: Walgraef D. , Dewel G. , Borckmans P.
Published: February 2003

The early stages of the spontaneous desynchronization of chemical oscillations of the limit cycle type in a two-dimensional medium are investigated. It is shown in the framework of a reaction-diffusion kinetics that the behavior of the system is governed by the phase dynamics. Target patterns are radiated from centers which are spontaneously emitted by the system. They have various characteristic frequencies which are always larger than the frequency of the bulk oscillations. The statistical distribution of these centers is shown to depend on the wavelength of the corresponding waves and on the parameters of the limit cycle. Rotating spiral waves which correspond to vortexlike phase fluctuations have a vanishingly small probability and have to be initiated.