bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the spin-Hamiltonian parameters for Cr3+ in mirror and inversion symmetry sites of alexandrite (Al2−xCrxBeO4). Determination of the relative site occupancy by EPR
Authors: Forbes C. E.
Published: February 2003

The EPR spectra of Cr3+ in single crystals of alexandrite (Al2−xCrxBeO4, x<0.01) have been obtained at temperatures between RT and 4.2 K in the frequency range 9.5–9.1 GHz. The orthorhombic structure of Al2BeO4 has two distinct Al3+ sites in the unit cell corresponding to Cs and Ci symmetry. The EPR spectra at 35 GHz of Cr3+ in the Cs site have been previously analyzed by other investigators. This study reports the observation of Cr3+ in the Ci site and the refined spin-Hamiltonian parameters for both magnetic centers. The absolute sign of the fine structure tensor elements for the Ci site has been determined to be positive. The observed orientations of both the chromium mirror and inversion magnetic axes are consistent with the crystallographic symmetry relationships of pure Al2BeO4. A comparison of the measured Cr3+ EPR intensities for resonances from both sites indicates that for two magnetically dilute crystals, the mirror site is preferentially occupied with a relative site occupancy of 78%±3%.
Ultraviolet two-photon spectroscopy of benzene: A new gerade Rydberg series and evidence for the 1 1E2g valence state
Authors: Whetten Robert L. , Fu Ke-Jian , Grant Edward R.
Published: February 2003

In the first application of continuously tunable ultraviolet two-photon absorption spectroscopy to a study of the higher excited states of a polyatomic molecule, we have obtained the two-photon absorption spectrum of jet-cooled benzene in the energy region from 55 000 to 75 000 cm−1. The strongest features of this spectrum, seen as two-photon resonance structure in three-photon ionization, are assigned to members of a new Rydberg series. Vibronic elements of these Rydberg states show evidence of dynamical effects associated with a Jahn–Teller instability in the 2E1g ionic core toward which the series converges. Notably, the magnitude of vibronic coupling terms in these states appears quite comparable to that present in C6H6+ and the sym-halobenzene cation ground states, while differing significantly from theoretical calculations predicting ν8 (ring stretch) as the major distorting mode. In addition, we observe two other new band systems: The first, with an origin at 60 800 cm−1, has vibronic structure, isotope shifts, and dramatically increased diffuseness upon deuteration, which taken together suggest its assignment as a highly valenized 3d Rydberg state or a degenerate valence state, most likely the lower of the two 1E2g states predicted by molecular orbital theory. The second, lying in the 55 000–58 000 cm−1 region, can be at least partially assigned to the two-photon forbidden 3px,y Rydberg states.
Nonadiabatic representations of the 1Σ+u and 1Πu states of the N2 molecule
Authors: Stahel D. , Leoni M. , Dressler K.
Published: February 2003

The vibronic bands in the dipole-allowed absorption spectrum of N2 associated with the lowest three electronic 1Σ+u and the lowest three electronic 1Πu states are represented in a basis of electronically coupled diabatic states as well as in the basis of nuclear-momentum coupled adiabatic states. Parameters defining the diabatic states and their electronic coupling energies are first evaluated by fitting the eigenvalues of a vibronic interaction matrix to the observations. The coupled-oscillator equations are then solved directly by Johnson’s numerical integration method and the diabatic representation is redetermined via the matrix method and coupled equations iteratively. The fit of the experimental vibronic terms, B values, and absorption intensities achieved with R-independent electronic coupling energies in a diabatic basis of valence and Rydberg-type states (b′+c′+e′)1Σ+u and (b+c+o)1Πu is satisfactory. Comparison with the corresponding adiabatic representation shows that the nonadiabatic perturbations are larger in that basis than in the diabatic one. The vibronic intensity distributions observed in the absorption spectrum show numerous interesting examples of intensity envelopes over discrete vibronic progressions with Fano-type resonance profiles as well as with distinctly non-Fano-type profiles which can be attributed to variations of the relevant coupling terms over the widths of the vibronic resonances.
The rotational Zeeman effect in the ArOCS van der Waals complex
Authors: Shea J. A. , Read W. G. , Campbell E. J.
Published: February 2003

The rotational Zeeman effect has been measured in the weakly bound complex ArOCS. The study was carried out on a pulsed Fourier-transform microwave spectrometer employing a supersonic nozzle and a Fabry–Perot cavity. The following spectroscopic constants were obtained: Projection equations are used to relate the magnetic properties of free OCS to those of the complex. An analysis of the force field as obtained from centrifugal distortion is used to determine an average structure for ArOCS and mean square amplitudes of the van der Waals motions. These are used to augment the projection analysis. The molecular quadrupole moment of ArOCS is calculated from the Zeeman parameters. With an estimated bulk magnetic susceptibility, diamagnetic susceptibilities and the second moments of electronic charge distribution are also calculated.
Lattice theory of binary fluid mixtures: Phase diagrams with upper and lower critical solution points from a renormalization-group calculation
Authors: Walker James S. , Vause Chester A.
Published: February 2003

A generalized lattice model representing the interaction between two molecular species in a binary liquid mixture is introduced. The model interaction potential consists of a highly directionally-dependent orientational part, due to hydrogen bonding, coupled to the configurational van der Waals interaction. Closed-loop temperature vs concentration solubility phase diagrams are produced in good agreement with experimental systems. These diagrams are obtained using renormalization-group methods. The possible types of phase diagrams predicted by the phenomenological Landau theory are also discussed.
Spontaneous site reorientation in a mixed molecular crystal: Tetracene in benzoic acid
Authors: Levinsky Howard B. , Wiersma Douwe A.
Published: February 2003

Absorption and fluorescence spectra of tetracene in a benzoic acid host crystal at 1.5 K are presented. The fluorescence zero-phonon line is shifted by more than 800 cm−1 to the red of the maximum of the 120 cm−1 broad absorption origin. This shift is attributed to a lateral site reorientation of the guest upon excitation, permitted by the difference in size between the tetracene and the benzoic acid dimer it replaces. In addition, other features in the fluorescence spectrum are ascribed to proton tautomerization occurring in the host dimers in the vicinity of the guest. These features disappear upon deuteration of the host acid protons, while the magnitude of the red shift is virtually unchanged.
Comment on ‘‘A time-dependent quantum mechanical theory for multiphoton dissociation of diatomic molecules’’
Authors: Whaley K. B. , Light J. C.
Published: February 2003

The rate of dissociation of a diatomic molecule in the presence of an intense infra-red laser field is derived, using a non-perturbative method. The T-matrix elements for absorption from an initial bound state to a dissociative state were calculated. (AIP)
Theoretical studies of the electronic structure and spectra of low-lying states of NH+3
Authors: Rossi Angelo R. , Avouris Ph.
Published: February 2003

We report on ab initio calculations of the energies and geometries of the 2A″2 ground state and the 2E and 2A′1 excited states of NH+3. The Jahn–Teller distortion of the 2E is treated in detail. By global geometry optimization we determine that the Jahn–Teller stabilization energy is between 1.2 and 1.4 eV and that the 2A″ state is the lowest energy Jahn–Teller component. We find that angular distortions provide the major contribution to the stabilization energy and that the JT distortion is a static one. Nonlinear electronic-vibrational coupling appears to be important but intermode coupling is small. Finally, we discuss the second photoelectron band of NH3 and the observed photofragmentation patterns in terms of the computed state energies and geometries.
A general relation between the intrinsic convergence properties of SCF Hartree–Fock calculations and the stability conditions of their solutions
Authors: Facelli J. C. , Contreras R. H.
Published: February 2003

In this paper, the convergence criteria given by Stanton [J. Chem. Phys. 75, 5416 (1981)] for the Hartree–Fock SCF calculations of closed-shell systems are generalized for an unrestricted one-determinantal Hartree–Fock SCF calculation. Finally, these criteria are related to the stability conditions of the Hartree–Fock wave functions.
Dynamics of a molecular impurity coupled to phonons: Computer simulation
Authors: Fivez Jan , De Raedt Bart
Published: February 2003

We investigate the classical rotational motion of a molecular impurity in a crystal starting from a model which contains the dynamic coupling of the rotator to the phonons of the host crystal. We use molecular dynamics to simulate the relevant correlation functions. Depending on their symmetry these correlation functions characterize mainly the librational or reorientational motion. The corresponding spectral functions are found by direct Fourier transformation for correlations decaying rapidly in time. The relaxation times of the reorientational motion are very long and have to be estimated from the long time tails of the corresponding time correlations. We find the inverse relaxation time to satisfy an Arrhenius law when the thermal energy is below the potential barrier. This agrees with numerous experimental results and with the ansatz used to stochastic jump models. For high temperatures the inverse relaxation time is proportional to temperature corresponding to rotational diffusion.
Molecular model for nematic polymers in liquid crystal solvents
Authors: ten Bosch A. , Maissa P. , Sixou P.
Published: February 2003

A theory for the nematic-isotropic phase transition in semirigid polymer/liquid crystal solutions is proposed. The polymer is described by an elastic chain and van der Waals and steric intermolecular interactions are included in a mean field approximation. It is shown that these interactions orient the liquid crystal molecules and the polymer chains in the solution. The order parameter of the liquid crystal molecules and of the polymer are calculated as a function of temperature and concentration and the phase diagram for the first order phase transition is given. The effects of chain length and chain flexibility are discussed.
Exciton–phonon scattering and exciton trapping in one-dimensional exciton systems
Authors: Benk Hartmut , Silbey Robert
Published: February 2003

We calculate the k → k′ scattering of excitons by phonons when impurities play a role. This is important for very narrow one-dimensional exciton bands as in tetrachlorobenzene. Three mechanisms are discussed and the dependence on impurity concentration and jump size are considered for all three.
Scaling in multichain polymer systems in two and three dimensions
Authors: Bishop Marvin , Kalos M. H. , Sokal Alan D. , Frisch H. L.
Published: February 2003

The mean dimensions of multichain polymer systems are predicted to follow a scaling relation with scaling variable X=ldν−1 ρ, where l is the number of statistical segments on the chain, ρ is the segment density, d is the dimension, and ν is the critical exponent for the mean dimensions of an isolated polymer chain. The scaling laws are 〈R2〉≊A(X) l2ν for l→ ∞ with X bounded, and 〈R2〉≊B(ρ) l for l→ ∞ with X → ∞. Moreover, the critical amplitudes behave as A(X)∼X−(2ν−1)/(dν−1) as X → ∞ and B(ρ)∼ρ−(2ν−1)/(dν−1) as ρ → 0. Simulations of both continuum and lattice systems are reanalyzed and found to be consistent with these scaling relations. Previous naive use of short-chain data has led to misleading results.
HD scattering from Pt(111): Rotational excitation probabilities
Authors: Cowin James P. , Yu Chein-Fan , Sibener Steven J. , Wharton Lennard
Published: February 2003

We measured the rotationally inelastic diffractive scattering probabilities for an HD beam colliding with a smooth Pt(111) surface. These large T → R inelastic probabilities were measured as a function of incidence angle for a 110 meV beam energy. The results are in excellent agreement with a simple physical model of an eccentrically weighted sphere colliding with a hard wall, with an attractive well depth of 55±10 meV. The numerical ‘‘GR’’ method of Garcia and co-workers was found superior to an eikonal method in solving this rotational quantum boundary value problem.
Chemistry in noninteger dimensions between two and three. I. Fractal theory of heterogeneous surfaces
Authors: Pfeifer Peter , Avnir David
Published: February 2003

In this, the first of a series of papers, we lay the foundations for appreciation of chemical surfaces as D-dimensional objects where 2≤D<3. Being a global measure of surface irregularity, this dimension labels an extremely heterogeneous surface by a value far from two. It implies, e.g., that any monolayer on such a surface resembles three-dimensional bulk rather than a two-dimensional film because the number of adsorption sites within distance l from any fixed site, grows as lD. Generally, a particular value of D means that any typical piece of the surface unfolds into mD similar pieces upon m-fold magnification (self-similarity). The underlying concept of fractal dimension D is reviewed and illustrated in a form adapted to surface-chemical problems. From this, we derive three major methods to determine D of a given solid surface which establish powerful connections between several surface properties: (1) The surface area A depends on the cross-section area σ of different molecules used for monolayer coverage, according to A∝σ(2−D)/2. (2) The surface area of a fixed amount of powdered adsorbent, as measured from monolayer coverage by a fixed adsorbate, relates to the radius of adsorbent particles according to A∝RD−3. (3) If surface heterogeneity comes from pores, then −dV/dρ∝ρ2−D where V is the cumulative volume of pores with radius ≥ρ. Also statistical mechanical implications are discussed.
Electronic excited state transport and trapping as a probe of intramolecular polymer structure
Authors: Fredrickson Glenn H. , Andersen Hans C. , Frank Curtis W.
Published: February 2003

A theory is presented for the incoherent transport and trapping of electronic excitations among chromophores attached to two specific sites on a macromolecule, such as the ends of a polymer chain. Two problems are considered: (a) the donor and trap chromophores are randomly distributed on the sites; (b) each macromolecule has exactly one donor and one trap chromophore. The formalism includes the possibility of a statistical distribution of intersite distances within the macromolecule, and allows transport between chromophores on different molecules, as well as intramolecular transport and trapping processes. In the dilute limit (isolated macromolecules) the theory is exact, and for large intramolecular site separations or high densities the self-consistent formalism of Loring, Andersen, and Fayer is recovered. The theory can be applied to any transfer rate, any distribution of intramolecular intersite distances and transition moments, and is well behaved at all polymer densities and site occupancy probabilities. Sample calculations are performed for the Förster rate and a Gaussian distribution of distances between sites on the same molecule. The theory will allow fluorescence depolarization and trap fluorescence experiments to be used as a probe of the distribution of end to end distance for end-tagged polymer chains in viscous solution, or in solid noncrystalline materials. Applications to structural studies in biological systems are likely.
The ν1 fundamental band of HCO+ by difference frequency laser spectroscopy
Authors: Amano Takayoshi
Published: February 2003

The P-branch as well as the R-branch lines of HCO+ are reported. The measurements were obtained by difference frequency laser spectroscopy. The accuracy of measurements is estimated as ±0.001 cm−1. (AIP)
Nonradiative relaxation of pyridine vapor: Transient absorption studies of triplet state formation, decay, quenching, and structure
Authors: Selco Jodye I. , Holt Patrick L. , Weisman R. Bruce
Published: February 2003

Relaxation processes in pyridine vapor have been studied using high-performance transient absorption spectrometry following excitation low in the S1 manifold. The ultraviolet spectrum of the lowest triplet state has been identified and measured, and its dependence on delay time and background pressure has been investigated for a variety of collision partners. It is concluded that formation of the triplet through S1 → T1 intersystem crossing proceeds in the statistical limit. Subsequent radiationless decay of the triplet population was found to show unusual pressure-dependent kinetics which apparently reflects collisional interconversion between two forms having very different intrinsic lifetimes. A simple model is proposed to explain the nonradiative behavior, collisional quenching, and spectra of the lowest triplet in terms of strong pseudo-Jahn–Teller vibronic coupling between nearly degenerate 3ππ* and 3nπ* states that leads to a double minimum in the T1 potential surface along the out-of-plane coupling coordinate. It is suggested that vibrationally relaxed T1 pyridine is nonplanar in structure whereas the vibrationally activated form is quasiplanar, and that the nonradiative T1 → S0 decay rates of these two forms are <105 s−1 and ∼5×106 s−1, respectively. Quenching of the triplet by ground state molecular oxygen was found to follow a sequential kinetic mechanism in which a transient intermediate was spectroscopically intercepted. This species is thought to be a weak complex formed between triplet pyridine and oxygen.
Quasiclassical trajectory studies of the chlorine–hydrogen system. IV. The effect of reagent vibrational excitation and of the location of the potential energy barrier on the dynamics
Authors: Persky Avigdor , Rubin Rachamim , Broida Michael
Published: February 2003

Three dimensional quasiclassical trajectory calculations were performed for the reaction of chlorine atoms with hydrogen molecules (H2, D2, and HD). Three LEPS semiempirical potential energy surfaces, all having the same barrier heights but with slightly different saddle point locations, were used for the calculations. Surface I, the surface used in our earlier studies of this reaction for the reactants in their ground vibrational state v=0, was employed here to investigate the effect of vibrational excitation of the reactants on the dynamics of this reaction. This was done by carrying out calculations for H2(v=1–4 and v=7), HD(v=1), and for D2(v=1) and comparing the results to those obtained earlier for v=0. The two other surfaces EN and EX were used to investigate the effect of the position of the barrier on the dynamics of the reaction. These two surfaces are very similar to surface I, but in surface EN the barrier is slightly shifted in the direction of the entry valley, while in surface EX it is slightly shifted in the direction of the exit valley. The interatomic distances at the saddle points of surfaces EN and EX differ by only 0.03 to 0.05 Å from those of surface I. For surfaces EN and EX, calculations were carried out for H2(v=0 and 1), D2(v=0), and HD(v=0) and the results were compared to those obtained for surface I. The results of the calculations on surface I show that the rate of reaction is significantly enhanced by the vibrational excitation of the reagents [kr(v=1)/kr(v=0)=400 for Cl+H2 Cl+H2(v) at 300 K)] that vibrational energy of the reactants (Ev) is very efficiently transferred into vibrational energy of the products (Ev′), and that the average scattering angle of the products decreases with the increase of either translational or vibrational energy of the reactants. The results of the calculations on surfaces EN and EX show that the rate constants are very sensitive to the location of the energy barrier, being significantly higher for surface EX than for surfaces I and EN. The energy partitioning among reaction products, as well as the scattering angle at low collision energies, also depend on the position of the potential energy barrier.
Thermal lens spectroscopic studies of radiationless processes in NO2
Authors: Imasaka Totaro , Shimanoe Katsuhiro , Ishibashi Nobuhiko
Published: February 2003

The transient thermal lens signal of NO2 in air was measured using a continuous wave (cw) Ar+ laser. The observed signal intensities were 1.03±0.16 (488.0 nm) and 1.10±0.15 (514.5 nm) times of the theoretical values calculated under the assumption that the NO2 molecules immediately and completely relax from the optically allowed electronic excited state to the ground state. The quantum yield of a heat producing process by collisionally induced radiationless transition was concluded to be unity for NO2 in this wavelength region. The transient signal was also measured by using a pulsed dye laser as an exciting source. The shape of the decay curve agreed with the theoretical curve fairly well. The observed rise time was less than 1 μs, and the similar result was obtained at the reduced pressure condition (4 Torr). These results indicate that the energy relaxation takes place immediately after optical excitation and produces the heat in a very short period (<1 μs). This fact allows the use of NO2 as a standard gas for normalization of the signal intensity in photoacoustic spectroscopy.
Observation of the C(3/2)←X(1/2) transition in XeF
Authors: Helm H. , Huestis D. L. , Dyer M. J. , Lorents D. C.
Published: February 2003

XeF(Xv′′), produced by KrF laser photodissociation of XeF2, is excited by a doubled dye laser to XeF(Bv′) and XeF(Cv′), and the broadband B→A and C→A fluorescence is detected. The B and C states are distinguished by their differing decay times at low pressure. The C←X transition is observed for the first time through its fluorescence excitation spectrum. The vibrational analysis of the C–X bands locates the C(v′=0) level at 775 cm−1 below the B(v′=0) level, and yields the first experimental vibrational constants for the C state. A band shape analysis of the C–X bands suggests that the equilibrium internuclear separation of the C state lies around 2.45 Å. Vibrational relaxation and vibrationally resolved collision-induced dissociation of XeF(Xv′′) is also observed.
Classical dynamics of a coupled double well oscillator in condensed mediaa)
Authors: Fonseca Teresa , Gomes J. A. N. F. , Grigolini Paolo , Marchesoni Fabio
Published: February 2003

The influence of a deterministic coupling between the reaction coordinate and a transverse normal mode on the reaction rate is considered within the context of classical (stochastic) dynamics. Our theoretical approach is based on a scheme of elimination of rapidly relaxing (irrelevant) variables. This procedure, perturbational in nature, makes it possible to get corrections of increasing accuracy to the results of the ‘‘standard adiabatic method’’ as perturbation terms of higher and higher order are taken into account. In the large friction case, the very first significant correction to the standard adiabatic scheme is shown to picture the reaction coordinate as being driven by a diffusionlike equation exhibiting instability phenomena. Thus, our approach makes an intriguing link between the activation of a reaction process via excitation of a nonreactive normal mode and noise-induced phase transition phenomena. When the frequency of the nonreactive mode varies slowly with the reaction coordinate, the reaction threshold can be expressed by a simple analytical formula. Results of computer calculations, which allow the exploration of a still wider range of physical conditions, are also discussed.
Fluorescence quenching at high quencher concentrations
Authors: Peak David , Werner T. C. , Dennin Richard M. , Baird James K.
Published: February 2003

Chemical reactions occurring in dense media at high reactant concentrations can be described by rate ‘‘constants’’ which are actually functions of concentration. We present a theoretical model in which this so-called rate constant ‘‘renormalization’’ occurs for the specific case of fluorescence quenching in solution. We show that both the quenching and the excitation rate constants can become concentration dependent. We fit our theory to several sets of experimental data—our own and some from the literature—and show that excellent agreement is obtained by varying a single free parameter, namely, the efficiency with which a fluorophore-quencher collision leads to a quench of the excited state.
Laser excited, state detected calcium-rare gas collisional energy transfer: Ca(4s5p 1P1) spin changing and Ca(4s5p 3P1) fine structure changing cross sections
Authors: Hale Michael O. , Leone Stephen R.
Published: February 2003

Pulsed dye laser excitation along with time and wavelength-resolved fluorescence detection techniques are used to measure cross sections for state changing collisions of Ca(4s5p 1P1) and Ca(4s5p 3P1) with the rare gases He, Ne, Ar, Kr, and Xe. The Ca(4s5p 1P1)+M deactivation involves a near-resonant spin changing process which populates predominantly the Ca(4s5p 3P2,1,0) states. The total deactivation cross sections for 1P1 are 22, 25, 5, 3, 13, and 31 Å2 (±20%) for 3He, 4He, Ne, Ar, Kr, and Xe, respectively. The cross sections for Ca(4s5p 3P1)+M fine-structure changes are 38, 25, 46, 56, and 135 Å2 (±35%) for He, Ne, Ar, Kr, and Xe, respectively. Branching ratios for the electronic energy transfer Ca(4s5p 1P1)+He⇄Ca(4s5p 3Pj, 3d4p 3Fj, and 3d4p 1D2)+He are determined to be 90±3:6±4:3±2, respectively, compared to statistical values of 26:60:14. The predominant product states 4s5p3P2,1,0 are produced by a near-resonant spin changing process; the fine structure components of the 3P state are populated essentially statistically (55:33:11). Fine-structure branching Ca(4s5p 3P1)+M⇄Ca(4s5p 3P2,0)+M is also measured and gives a statistical distribution of Ca(4s5p 3P2,0) states. The radiative lifetimes of the 4s5p 1P1 and 4s5p 3P1 levels are measured to be 65±2 and 71±8 ns, respectively. The results are discussed in terms of the competing effects of velocity and interaction strengths on the curve crossing probabilities.
Dynamical effect of vibrational relaxation on electronic relaxation
Authors: Boeglin A. , Villaeys A. A. , Voltz R. , Lin S. H.
Published: February 2003

A main purpose of this paper is to develop a density matrix formalism that can treat the effect of vibrational relaxation and vibrational dephasing on electron processes in condensed phases. Master equations will be derived. It will be shown that in this formalism the memory functions involved in master equations play a very important role in describing the time evolution of the system. As an application of this density matrix formalism, we present a theory of vibrational effect on luminescence. In this case, intramolecular vibrational relaxation, lattice vibrational relaxation and dephasing, and electronic relaxation should be considered simultaneously. Numerical calculation has been performed to illustrate the theoretical results.
Vibrational relaxation of highly excited diatomics. IV. HF(v=1–7) + CO2, N2O, and HF
Authors: Dzelzkalns L. S. , Kaufman F.
Published: February 2003

Vibrational relaxation rate constants are measured for HF(v=1–4) with Q=CO2, N2O, and HF by the fast flow infrared chemiluminescence technique using four HF(v) generating reactions whose initial vibrational distributions are found be be unrelaxed. The data are combined with earlier results for v=5, 6, and 7 to provide information on v dependence and quenching mechanism. The rate constants, kQv,v−1 range from 1.2×10−12 to 4.5×10−10 cm3 s−1 and increase with power law exponents n of 2.7 to 3.0 in k∝vn for all three quenchers. The relaxation is principally V–V for CO2 and N2O, but mainly V-R,T for HF, at least for the higher v levels. The relaxation rate constants are compared with theoretical estimates and form a valuable data base for future theoretical work.
An isotropic exchange model for the magnetic behavior of tetranuclear copper complexes of the type Cu4OX6L4
Authors: Jones Donald H. , Sams John R. , Thompson Robert C.
Published: February 2003

A theory for the magnetic behavior of distorted tetrahedra of magnetically coupled spin 1/2 atoms is derived using the assumptions of (i) isotropic coupling; and (ii) a linear relationship between the coupling constants and interatomic distances. It is found that the theory provides the basis for a model of the magnetic behavior of complexes of the form Cu4OX6L4 which involves dynamic or fluxional distortions. The new model is found to be in several respects more satisfactory than either the anisotropic or the intercluster exchange models proposed previously.
Mobilities of Na+ ions in H2–D2 mixtures at room temperature
Authors: Iinuma K. , Takebe M. , Satoh Y. , Seto K.
Published: February 2003

The mobility of Na+ ions in H2–D2 mixtures has been measured at three ionic energies of 9.5, 51.5, and 127.5 Td, and compared with the values calculated from Blanc’s law. The experiment has been carried out at 309.5 K and 1.0 Torr, 308 K and 0.25 Torr, and 308 K and 0.15 Torr, for respective three ionic energies by the conventional flight-time scanning method using a continuous guard ring. Mobilities have been obtained with partial pressures of D2 in the range 0%–100% at intervals of about 10%.
Reexamination of scaling relations of athermal multichain lattice systems by computer experiments
Authors: Okamoto Hiroshi
Published: February 2003

The scaling relations in athermal multichain (n-mers) systems are reexamined by computer experiments. In the three dimensional space, it is found that the chain mean dimensions and the chain element distributions around a mass center are described by a scaling variable nνd−1φ irrespective of n and the chain concentration φ. The logarithms of the state sum per chain, a kind of the chemical potential, and the osmotic pressures divided by chain number concentrations are found as being described by another variable defined as nνd−1−δφ, where ν is the exponent in the mean square end to end distance vs chain contour length relation, d is the space dimensionality, and δ is a small value. In the two dimensional space, because of the stronger volume exclusion, the value of δ is much smaller and may be 0, as in our earlier paper. A qualitative reasoning based on the picture of the coarse graining is made to interpret the results. It leads us a conclusion that our finding is reasonable in spite of its inconsistency with the scaling theories and some of the experiments. Whether the exponent δ is universal or not remains unsolved.
Shape resonances, overtones, and electron energy loss spectroscopy of gas phase and physisorbed diatomic molecules
Authors: Gadzuk J. W.
Published: February 2003

Electron energy loss spectra of O2 and N2 physisorbed on metallic substrates showing a series of high overtone losses have recently been reported. In the case of N2, the intense overtone excitation is credited to the formation of a well-known temporary negative ion state with a resonance lifetime ∼10−15 s for gas phase N2−. The principal distinction between the gaseous and physisorbed molecule EELS spectrum is a significant depletion of the overtone intensity which has been attributed to a surface-induced decrease in the resonance lifetime. In the present work, a time dependent quantum mechanical model applicable to vibrational excitation in resonance scattering is outlined which quantitatively accounts for the observed spectra and, in particular, the surface modifications to the gas phase results. The essential feature of the model is one in which the intramolecular dynamics of the intermediate state is characterized by nuclear propagation over a harmonic potential curve spatially displaced from the ground state curve for a time duration equal to the resonance lifetime. The resulting calculated overtone spectra agree well with the experimentally observed ones. The results suggest that the physisorbed N−2 lifetime is about 40% of that of the free molecule.
Kinetics of coagulation by dynamic light scattering
Authors: Versmold H. , Härtl W.
Published: February 2003

The coagulation kinetics of an initially monodisperse colloid solution is investigated. With the aid of the Smoluchowski theory of rapid coagulation the two most important quantities the mean diffusion constant D̄0(t) and the poly-dispersity factor Q(t) for the dynamic light scattering experiment of weakly interacting particles are discussed as functions of time. The short time expansion of these quantities as well as numerical results are given. The investigation concludes with a short comparison of experimental coagulation results obtained from a solution of amphoteric latex particles with D̄0(t) as obtained from Smoluchowski’s theory. Theory and experiment are found to be in good agreement for the system under study.
Extrinsic photoconduction with reduced image forces
Authors: Prock Alfred , Das Kamalendra N. , Melman Paul
Published: February 2003

Unipolar charge injection from an electrode/fluid interface into a medium of nearly the same dielectric constant as the electrode is studied. For both photo- and dark injection, electrical image forces no longer serve as an impediment to charge detachment. The system investigated is mesotetraphenylporphin (excited in the vis-near UV) in fluorobenzene solution against tin oxide electrodes. Despite polarity of the medium, photocurrent is extrinsic, with both dark and photocurrent injection occurring at the positive electrode. Both show saturation with applied field in accordance with a model based on forced diffusion theory. In addition, interfacial charge generation and recombination rates are extractable from the data. This method shows promise as a way to study these as a function of excited state and dielectric properties of the system.
Role of initial rotation on vibrationally inelastic collisions: Enhancement and specificity in level to level rate constants for Li*2
Authors: Saenger Katherine L. , Smith Neil , Dexheimer S. L. , Engelke Charles , Pritchard David E.
Published: February 2003

Rate constants were measured by laser induced fluorescence for vibrational-rotation changing collisions of Li*2(A 1Σ+) with Xe from initial vibration level vi=4 and initial rotation levels ji=14, 28, 44 with Δv=±1, ±2 and resolved final rotation levels jf. The jf distribution of kvi,ji→vf,jf peaks dramatically at high ji: for Δv=−1 the width (FWHM) is ≂28 for ji=14 and ≂5 for ji=44. The peak rate corresponds to a level to level cross section σ(vi=4, ji=44→vf=3, jf=48)=3 Å 2. Increasing ji from 14 to 44 also increases the total rate kjivi→vf =Σkjfviji→vfjf of vibrationally inelastic collisions by a factor of about 3. The magnitude of the mean total cross sections σjivi→vf is large, ranging from 1 Å2 (Δv=+2, ji=14) to 15 Å2 (Δv=−1, ji=44).
A new electronic state of water revealed by gas phase multiphoton ionization spectroscopy
Authors: Ashfold M. N. R. , Bayley J. M. , Dixon R. N.
Published: February 2003

A new electronic state of water has been identified by three photon absorption (four photon ionization) spectroscopy. Rotational analysis of the bands associated with this new D̃′ state in both H2O (ν0=84 434±2 cm−1) and D2O (ν0=84 646±2 cm−1) show it to be of B1 symmetry. The nature of the electronic promotion giving rise to this D̃′ state is considered.
Molecular dynamics simulations of ionic crystal films
Authors: Heyes D. M.
Published: February 2003

Thin crystalline films of 6 to 18 ionic (100) planes thick are simulated by the computer simulation technique of molecular dynamics. They are found to be stable up to the normal melting temperatures. Model LiCl, NaCl, KCl, and RbCl parameters are used in a rigid ion approximation and are compared with a NaCl simulation using a point polarizable ion model. It is concluded that previous static lattice and lattice dynamics calculations have probably overestimated the influence of polarization forces in producing a surface contraction. The Coulomb point charge interactions were evaluated using isolated laminae boundary conditions. Surface ion relaxation is less than several percent of the regular lattice interplanar spacing. It is shown to be quite sensitive to the potential interaction parameters. The rigid ion mean square amplitudes of vibration are ∼20% smaller than experimental scattering data. Velocity autocorrelation functions and their derived power spectra are evaluated for motion which is parallel and perpendicular to the surface plane. They provide insights into the dynamical coupling of sublattice vibrations which are manifest, at a more resolved level, in the surface phonon spectra. Surface energies and stress decrease by 10% to 25% on equilibrium from the regular laminas and show a fascinating temperature dependence.
Crystal and molecular structure and EPR triplet spin excitons of NBP TCNQ, the 1:1 salt of 5-(1-butyl) phenazinium (NBP) with 2,2′- (2,5-cyclohexadiene-1,4-diylidene)-bispropanedinitrile (TCNQ)
Authors: Gundel Dieter , Sixl Hans , Metzger Robert M. , Heimer Norman E. , Harms Ralf H. , Keller Heimo J. , Nöthe Dietrich , Wehe Dieter
Published: February 2003

The title compound C28H21N6, Mr=441.51 crystallizes in the triclinic space group P1̄, with lattice constants a=831.5(3), b=965.7(5), c=1638.3(8) pm, α=70.54(3)°, β=71.51(3)°, γ=76.78(3)°, V=1.165(1) nm3, Z=2, dc=1.258 Mg m−3. Final refinement yielded residuals R=0.050 and Rw=0.065. The structure consists of (NBP+)2 π-bonded dimers and (TCNQ−)2 π-bonded dimers that alternate along the [101] direction. The structure of NBP TCNQ thus is very similar to that of NBP TCNQF4 (where TCNQF4 is the 2,3,5,6-tetrafluoro derivative of TCNQ) but the latter salt belongs to the space group P21/c. The NBP+ and TCNQ− ions are planar, with interplanar distances of 344(2) pm for the donor pair, and 316(3) pm for the acceptor pair. The angle between the NBP+ and the TCNQ− planes is 5.9°. In the ESR experiments a single thermally activated ‘‘quasi-immobile’’ Frenkel triplet spin exciton (TSE) is seen, along with the usual g=2 impurity line and a second set of narrower g=2 lines that are probably due to exchange-narrowed TCNQ− radicals. The thermal activation energy for the S=1 spectrum is Ea=0.13 eV. The ESR fine structure tensor orientations and fine structure constants D/hc=±0.013 09(5) and E/hc=∓0.002 0(2) cm−1 are consistent with a model calculation using a triplet (TCNQ−)2 supermolecular state.
Laser-induced fluorescence-line-narrowing studies of LaCl3:Nd3+: Static and dynamic aspects
Authors: Chan I. Y. , Goldenberg Barbara L.
Published: February 2003

We have applied the fluorescence-line-narrowing technique in investigating the interaction of trivalent neodymium ions with each other and with the surrounding lanthanum trichloride lattice. A 0.04 cm−1 linewidth dye laser was used to site-select sets of impurity Nd3+ ions within the 2 to 5 cm−1 spectral profile of the E1(4G7/2, μ=3/2) excited electronic state. High resolution detection monitored the E1 → Y3(4I11/2, μ=5/2) fluorescence characteristics. Studies were undertaken on two types of crystals. Nonradiative relaxation within the Y(4I11/2) manifold was investigated by measuring homogeneous linewidths in a dilute (0.5 mol %) crystal. The results qualitatively conform to relaxation via a direct one-phonon process. Crystals with higher Nd3+ concentrations (8% and 15%) were used to probe site variation and energy transfer dynamics. Species with short excited state lifetimes and distinct spectral shifts indicate the presence of distorted sites, most likely clusters of Nd3+ ions. This behavior exemplifies a situation where concentration fluctuation instead of crystalline strain is the major inhomogeneous line broadening mechanism. Dynamic measurements yielded a relatively short-ranged R−8 dependence on energy transfer rate, indicative of an electric dipole–quadrupole coupling. Phonon-assisted energy transfer appears to proceed via an Orbach mechanism.
Intramolecular dynamics by photoelectron spectroscopy. II. Nonadiabatic processes
Authors: Dehareng D. , Leyh B. , Desouter-Lecomte M. , Lorquet J. C. , Delwiche J. , Hubin-Franskin M. J.
Published: February 2003

The Fourier transform of an electronic spectrum leads to an autocorrelation function which provides information on the propagation of a wave packet on the potential energy surface of the electronic state reached in the transition. The formula is valid even when nonadiabatic interaction is present, i.e., when the wave packet splits at a particular surface crossing with one part branching off to another potential energy surface. An explicit expression of the correlation function is given for a model of several discrete states interacting with a continuum. Closed-form solutions are given in the case of one and two resonances. A very simple formula valid in the strong coupling limit is also derived. The method is applied to the photoelectron spectrum of state A 2Σ+ of HBr+, which is shown to correspond to the strong coupling case.
Vibrational and rotational level dependence of the S1 decay of propynal in a supersonic jet
Authors: Stafast H. , Bitto H. , Huber J. Robert
Published: February 2003

Single vibronic level and single rovibronic level decays of the first excited singlet state [ΔEvib (S1)≤1300 cm−1] of propynal (HCCCHO) and propynal-d1 (HCCCDO) have been investigated in a pulsed supersonic jet. For this purpose, the emission decay rates were measured for ∼200 rotational levels in 14 vibronic bands of HCCCHO and ∼100 rotational levels in nine vibronic states of HCCCDO. The internal conversion process, which is the main deactivation route of the S1 state in HCCCHO, was found distinctly mode dependent with the ν10 vibration (CHald wagging mode) acting as the dominant promoting mode. The nonradiative decay rates showed no simple systematic dependence of the rotational quantum numbers J and K. For a number of rotational levels an oscillatory decay behavior (quantum beats) was observed the analysis of which provided information on the S1–T1 coupling in propynal.
Multiple ranges of flow rate with bistability and limit cycles for Schlögl’s mechanism in a CSTR
Authors: Escher Claus , Ross John
Published: February 2003

Continuous flow stirred tank reactor (CSTR) conditions are imposed upon Schlögl’s trimolecular reaction mechanism A+2X ⇄ 3X and X ⇄ B. A nonzero constant flow rate (=inverse residence time) rather than constant concentrations of A,B forces the system far away from equilibrium. This change in constraints leads to some surprisingly complex features; first the system has bistability for separate ranges of flow rates, under constraints of all other parameters being fixed including the concentration of the chemical species in the input. This phenomena occurs even if the input contains A alone. Secondly, there occur limit cycle oscillations, but in that case the input has to include B or X as well as A.
Reliability of high-order phase-integral eigenvalues for single and double minimum potentials
Authors: Paulsson Rolf , Karlsson Finn , Le Roy Robert J.
Published: February 2003

For single and double minimum model potentials, energy eigenvalues are calculated using the Fröman and Fröman phase-integral approximations and compared with exact (numerical) quantum mechanical results. For the double minimum potential, results are obtained both from the correct phase-integral quantization condition including quantum effects near the barrier maximum (the σ term) up to and including the fifth-order approximation, and from the quantization condition with the σ term neglected up to and including the 13th-order approximation. When the σ term is included, the third- and fifth-order eigenvalues are accurate enough for all practical purposes, even close to the barrier maximum, but if the σ term is neglected, the phase-integral quantization condition breaks down near the barrier maximum, and this breakdown becomes more dramatic with increasing order. This property is used to discuss the question of which order of phase-integral approximation will give the optimum result. For a single minimum LJ(12,6) potential, phase-integral eigenvalues are calculated up to and including the 13th-order approximation. In this case, the phase-integral approximations appear able to yield much higher accuracy than can be practically obtained by quantal calculations using existing numerical methods. The reliability of phase-integral eigenvalues at energies near the dissociation limit is discussed, and a generalization of an earlier criterion for the onset of the breakdown of higher-order phase-integral quantization conditions near the asymptote of a potential with an attractive inverse-power r−ν long range tail (where ν≳2) is given.
Variational solution of the time dependent Coulomb problem
Authors: Horbatsch Marko
Published: February 2003

The problem of a hydrogen-like atom with a time-varying nuclear charge Z(t) is solved variationally. The evolution of the complete set of eigenfunctions is given in closed analytic form. Consequences and the relevance of the present problem for atomic collisions are discussed.
van der Waals vibrational modes of the pentacene⋅R complexes (R=Ne, Ar, Kr)
Authors: Leutwyler Samuel , Schmelzer Andreas , Meyer Rolf
Published: February 2003

The vibrational van der Waals modes of the pentacene⋅R van der Waals complexes (R=Ne, Ar, Kr) were theoretically studied. Empirical model calculations according to the method of Ondrechen et al. yielded the pentacene⋅R potential surfaces. One- and two-dimensional vibrational calculations were performed using two different numerical treatments, yielding the vibrational energies and wave functions of the intermolecular pentacene⋅R motions, as well as tunnel splittings and tunneling rates. Estimates for the three-dimensional case were derived from the results for one and two dimensions. Anharmonic coupling between the three van der Waals modes are shown to be relatively unimportant. However, partial counterrotation of the whole complex in space, balancing vibrational angular momentum, turned out to have a sizable effect on one of the modes. The calculated results were compared with available experimental data.
Comparison of the symmetrized Cartesian and angular momentum basis sets to describe vibrationally excited states in spherical top molecules
Authors: Levene Harold B. , Perry David S.
Published: February 2003

Recent analyses of the 3ν3 vibrational band in two spherical tops, SF6 and SiF4, have shown the Cartesian basis to be an appropriate description for this band in the octahedral case, while the tetrahedral case is adequately represented by the angular momentum basis. Using spectroscopic constants derived from force field analyses, the F1u/F2 subblocks of the vibrational Hamiltonian matrices of the bands of four vibrational quanta and one band of five quanta 2ν23ν3 are diagonalized in each bases set for SF6, CF4, and CH4 to extend the comparison to more general vibrational levels. Both bases fail to adequately describe CF4 and CH4. The validity of the various angular momentum designations as quantum numbers is strongly dependent on the particular set of spectroscopic parameters. SF6 is poorly described by the angular momentum basis. Within a vibrational band of SF6, only a component whose vibrational motion lies along a single Cartesian axis is well described by the Cartesian basis. It is the lowest energy component, well separated in energy from the remaining components because of the single bond anharmonicity.