bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental study of the dynamics of the H+D2 → HD+D reaction at collision energies of 0.55 and 1.30 eV
Authors: Gerrity Daniel P. , Valentini James J.
Published: February 2003

We report here experimental measurement of the nascent HD product quantum state distributions for the H+D2 → HD+D reaction. Pulsed laser photolysis of HI in an HI/D2 gas mixture produces hydrogen atoms giving H+D2 collision energies of 0.55 and 1.30 eV. Nanosecond-time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to record rotationally and vibrationally resolved spectra of the HD reaction product under nearly single-collision conditions. The spectra are analyzed to determine the nascent, single-collision HD product quantum state distributions. These distributions are compared to the results of related experiments by E. E. Marinero, C. T. Rettner, and R. N. Zare and to the results of recent quasiclassical trajectory calculations of N. C. Blais and D. G. Truhlar. Our results are in qualitative agreement with those of Marinero et al., but there are some quantitative differences. The trajectory calculations yield HD quantum state distributions which are very close to those we have measured. We find that our HD product quantum state distributions are fairly well summarized by a linear surprisal analysis with a rotational surprisal of 3.0 and a vibrational surprisal of 2.6. The quantum state distributions indicate that 71%±1% of the energy available to the products appears in translation, while 19%±2% is in HD rotation, and 10%±2% is in vibration. These results are discussed in terms of the dynamics of the reaction.
Quantum dynamics of energy transfer between bonds in coupled Morse oscillator systems
Authors: Hutchinson John S. , Sibert Edwin L. , Hynes James T.
Published: February 2003

The quantum dynamic flow of energy between coupled anharmonic bonds is calculated and analyzed for ABA triatomics with a massive central atom. The time dependence of quantum ‘‘normal mode’’ states is found to be classical in nature, but for states which are classically local, purely quantum energy flow between bonds occurs. We show that the mechanism for this quantum energy flow between local modes can be understood as an indirect state-to-state flow of probability, involving normal mode intermediary states. In particular, there is a regime in which rapid energy flow—which is classically forbidden—occurs from nonclassical states in the quantum case. Criteria are presented for the existence of purely quantum, and classically impossible, flow of energy between oscillators. Initially prepared states are accordingly classified as normal mode states, quantum local mode states, or nonclassical states, according to the extent and rate of bond–bond energy flow, the sensitivity of the process to small asymmetries, and the participation of intermediary states in the probability flow. Implications for intramolecular energy flow in large molecules and the connection with ‘‘dynamic tunneling’’ are discussed.
Electron correlation effects on the N2–N2 interaction
Authors: Hay P. J. , Pack R. T , Martin R. L.
Published: February 2003

A b initio self-consistent field, configuration interaction, and many-body perturbation theory methods are used to calculate the intermolecular potential between two nitrogen molecules. The emphasis is placed on the repulsive region important at the temperatures and pressures encountered in detonations. In addition, electron gas calculations are employed to fit and extend the ab initio data. We also generate effective spherical potentials which fit dilute gas virial, viscosity, and differential scattering data while being constrained by Hugoniot or ab initio data in the repulsive region. Finally, we discuss the roles of electron correlation and of many-body effects on the N2–N2 interaction. Comparisons are also made to the Ar2 potential where similar ab initio calculations are compared to an accurate empirical potential.
Intermolecular potentials for CH4, CH3F, CHF3, CH3Cl, CH2Cl2, CH3CN, and CO2
Authors: Böhm H. J. , Ahlrichs R. , Scharf P. , Schiffer H.
Published: February 2003

Intermolecular potentials are determined using ab initio and experimental data. The strongly structured contributions are obtained from molecular parameters, which are determined by probing the molecular shape and the static potential. The remaining attractive terms, which are more isotropic, are then evaluated through a fit to experimental data such as second virial coefficients B(T). The method is applied to CH4, CH3F, CHF3, CH3Cl, CH2Cl2, CH3CN and CO2 and the corresponding site parameters are reported. The resulting potentials are compared with others proposed in the literature.
6
1Ni Mössbauer spectroscopy of magnetic hyperfine interaction in nickel spinels
Authors: Gütlich P. , Hasselbach K. M. , Rummel H. , Spiering H.
Published: February 2003

The sign and magnitude of the magnetic hyperfine field in the Ni spinels NiFe2O4, NiCr2O4, NiFe0.3Cr1.7O4, NiRh2O4, and GeNi2O4 were determined by means of 61Ni Mössbauer spectroscopy in external magnetic fields. The hyperfine fields of Ni2+ ions in octahedral lattice sites are negative and of the order of −10 T, and positive for Ni2+ at tetrahedral sites ranging from +2.5 T in NiRh2O4 up to +63 T in NiFe0.3Cr1.7O4. The sign and magnitude of the hyperfine fields are explained in the frame of ligand field theory including the effects of 3d–4p orbital mixing.
A dissipative Lagrangian formulation of the network thermodynamics of (pseudo)-first-order reaction–diffusion systems
Authors: Shiner J. S.
Published: February 2003

A formulation of the equilibrium and dynamic properties of reaction–diffusion systems is developed which is analogous to the Lagrangian formulation of dissipative mechanical systems and electrical networks. Since the formulation uses ‘‘kinetic’’ forces for the chemical reactions, not thermodynamic forces, the Lagrangian and dissipation function are only apparent quantities. Nonetheless, the Lagrangian determines equilibrium properties completely, and the dissipation function contains all dissipative effects. The formulation shows that no modulation of the potentials in diffusional pathways is caused by the presence of the chemical reactions, as was previously claimed, if the same scale is used for all reactions and diffusion steps in the system. Furthermore, the formulation renders the system in a network form which guarantees symmetry in the force-flow relations at the stationary state. This is shown explicitly for the special case of King–Altman–Hill systems. Unfortunately, the Lagrangian approach using kinetic forces seems to be limited to systems with only first-order or pseudo-first-order reactions. To treat higher order reactions, the thermodynamic forces must be used.
Dynamics in nonentangled concentrated polymer solutions. I. The full dynamic multiple scattering approach to first order in concentration
Authors: Perico Angelo , Freed Karl F.
Published: February 2003

The concentration dependence of flexible polymer relaxation times in concentrated nonentangled solutions is studied by a full dynamic multiple scattering approach. In the absence of macroscopic entanglements effects the screening of the hydrodynamic interaction is the dominant phenomenon governing this concentration dependence. The leading concentration contribution to the relaxation times is used to introduce the screening constant through its effect on the hydrodynamic interaction matrix. The exact Langevin equation for the normal coordinates of the polymer is derived and analyzed through leading order in concentration. It is shown that there is a concentration-dependent memory kernel which emerges in the exact Langevin equation for the polymer chain dynamics in concentrated solutions, a departure from the infinite dilution situation where the (delta function) kernel is instantaneous. This implies that the relaxation matrix is frequency dependent. The general conditions for having an effective relaxation spectrum in concentrated solution are considered, and the structure of the relaxation of the normal coordinate time correlation functions is given. In general this relaxation is not described by a single exponential decaying term, but by a combination of many exponentially decaying and oscillatory exponentially decaying terms; however, the latter oscillation rates are generally very slow compared to the overall decay rates. The accompanying paper considers model calculations of the concentration dependent relaxation times.
Dynamics in nonentangled concentrated polymer solutions. II. Model calculations to first order in concentration
Authors: Perico Angelo , Freed Karl F.
Published: February 2003

Model calculations are presented to illustrate general features of polymer dynamics to first order in concentration c. The harmonic spring and three bead chain are treated because they are exactly solved to order c and because they verify some of the general dynamical behavior outlined in paper I. The time autocorrelation function Z1(t) for the slowest normal coordinate is found for these two models (as well as for chains with any numbers of beads n) to have a single exponential decay with a relaxation frequency that is reduced from its infinite dilution limit. The next mode autocorrelation function Z2(t) for n=3, however, has a linear combination of three exponentially decaying contributions. The steady state limit of the leading concentration dependence of the polymer relaxation times is evaluated numerically for chains with n≤20 under partial and nondraining limits, and extrapolations to n→∞ are estimated. The results provide a concentration dependent relaxation spectrum with the order c terms behaving in theta solutions as a−1/2 for small a/n (where a is the mode number) and then becoming constant at large a/n. These conclusions are in general agreement with recent optical flow birefringence measurements. This effective relaxation spectrum provides approximation to the Huggins coefficient and to the hydrodynamic screening constant. Finally, an exhaustive analysis is made of the errors incurred by invoking the ‘‘static’’ or equal time approximation to the mean square intersegment dynamical distances gi j(t). This approximation produces errors of greater than a factor of two for the order c contribution to both the friction coefficient and the relaxation frequencies. This demonstrates the strong sensitivity of dynamical quantities to gi j(t); therefore, the full dynamical expression for gi j(t) must be used to quantitatively describe the dynamics of polymer solutions.
The onset of elastic coupling in polymer liquids
Authors: Baumgärtner A. , Ronca G.
Published: February 2003

The modified Rouse equation of motion recently introduced for the study of multiple elastic interactions of polymer chains in concentrated liquids is applied to investigate the onset of entanglement coupling. For undiluted systems of ring polymers we conjecture a possible dynamic transition at M=3.938 Me, preceded and induced by strong alterations of the mechanical spectrum. Extrapolation to linear polymers localizes the onset of entanglement coupling at M≂1.97 Me.
Influence of temperature on elementary excitations in solid oxygen by Raman studies
Authors: Bier K. D. , Jodl H. J.
Published: February 2003

Detailed Raman investigations on gaseous, liquid, and solid oxygen in its α, β, and γ phases were made. Some new features observed in the spectra are a temperature dependent splitting of the vibrational line in γ-O2 and a vibron–phonon combination in α- and β-O2. In the low energy range we measured the librational modes; one of them shows a characteristic soft mode behavior near the α–β phase transition. The influence of temperature on the librational linewidth is explained in terms of the anharmonic interaction. These data give new information about the assignment of some of the modes.
Variation of nuclear magnetic shielding with intermolecular interactions and rovibrational motion. VIII. 19F in CF3X
Authors: Jameson Cynthia J. , Jameson A. Keith
Published: February 2003

We have determined the effects of intermolecular interactions on the 19F nuclear shielding for the series of fluorinated methanes CF3X, X=H, F, Cl, Br, I, and the pseudohalide CN. The second virial coefficient of nuclear shielding, obtained from measurements of the 19F nuclear resonance frequency in low density gas samples, corrected for bulk susceptibility effects, are: (σ1−σ1b)=−6.76, −8.65, −15.70, −17.19, −21.11, and −30.1 ppb/amagat at 300 K varying systematically in the sequence X=H, F, Cl, CN, Br, and I. On the other hand, the temperature dependence of the 19F shielding in the zero-pressure limit shows that at 300 K, dσ0/dt=−5.60, −5.01, −6.78, −7.45, −7.75, and −9.25, for X=H, F, Cl, CN, Br, and I, respectively, which are not in any systematic order.
A more reliable oxygen-17 absolute chemical shielding scale
Authors: Wasylishen Roderick E. , Mooibroek Sandra , Macdonald J. Bruce
Published: February 2003

For the first time, oxygen-17 chemical shifts together with experimental details are reported for CO, CO2, OCS, N2O, and OF2. An absolute 17O shielding scale, based on a recent redetermination of the 17O spin-rotation constant in C 17O is proposed: σ (C 17O)=−42.3±17.2 ppm. Absolute shielding constants derived from 17O NMR chemical shifts are compared with those calculated from recent molecular orbital studies. Also scalar coupling constants involving 17O are reported for OF2 and CO. In OF2, J(F,O)=300±30 Hz; in carbon monoxide, J(O,C) is less than 5 Hz.
Relaxation dynamics of photoexcited benzene–rare gas van der Waals complexes
Authors: Stephenson Thomas A. , Rice Stuart A.
Published: February 2003

The vibrational relaxation/predissociation dynamics of various benzene–rare gas van der Waals complexes have been investigated. These studies indicate that the pathways of vibrational redistribution within the complex are highly selective and involve only a small subset of the energetically accessible levels. While in general energy gap law behavior is observed, i.e., large benzene vibrational energy changes are disfavored, the sensitivity of the relaxation dynamics to this constraint varies with the vibrational level initially excited. This behavior is ascribed to varying degrees of sensitivity to the details of the atom–molecule potential and is related to the scheme of nuclear displacements involved in the benzene normal modes. Some, but not all, of the relaxation dynamics can be understood in terms of a very simple model that expresses the atom–molecule potential as a sum of atom–atom interactions and assumes a rigid geometry for the complex. A kinetic analysis that assumes only first order processes allows crude vibrational relaxation-predissociation rate constants to be extracted. The variation of the rate constants with rare gas and vibrational level is consistent with the energy gap law.
Velocity dependence of rotationally inelastic collisions: 7Li*2(A 1Σ)+Ne, Ar, and Xe
Authors: Smith Neil , Scott Thomas P. , Pritchard David E.
Published: February 2003

We present velocity-selected measurements of the cross sections for rotationally inelastic collisions of 7Li*2(A 1Σ) with Xe, Ar, and Ne, with various pairs of initial and final rotational levels, ji and jf: Li*2(vi=9, ji) +X→Li*2(vf=9, jf=ji+Δ)+X. For each target gas measurements were made with ji=8, 22, and 42 for approximately eight values of Δ with an overall range of −20 to +24; a total of 69 cross sections were measured. Velocity selection is accomplished by a Doppler-based laser-selection technique combined with further numerical inversion methods. We achieve a measurable relative velocity range of ≂0.5→3×105 cm/s, corresponding to kinetic energies of 120 to 3500 K. In contrast to the few previous similar studies, our data show a dramatic and varied velocity dependence. We have observed a few cross sections which increase substantially with velocity, some which rise faster than v−1r at low velocity, some that fall like v−2r at high velocity, and numerous examples of peaks in endoergic cross sections near threshold. We have seen two distinct types of rotationally adiabatic reduction in certain cross sections: ‖Δ‖=2 in Li*2–Xe and ji=42, Δ≲−16, in all three systems, particularly in Li*2–Ne. The velocity dependent behavior of these cross sections is sensitive to the long-range and short-range interaction potential, respectively. Adiabatic effects in the latter case result in the existence of classical ‘‘dynamical threshold velocities’’ for these large −Δ collisions. Some analytical results of a quantitative investigation into these rotationally adiabatic effects are also given.
Rate constants for energetic atoms with competitive reactions
Authors: Gorensek M. B. , Kostin M. D.
Published: February 2003

The effect of a source of energetic reactant atoms on the bimolecular rate constant is investigated by considering the Boltzmann equation for a dilute homogeneous reacting gas. A differential form of the Boltzmann equation is used to find approximate analytical expressions for the rate constants of three competitive reactions with different threshold energies. Under certain conditions it is shown that the rate constants of the three competitive reactions have an Arrhenius dependence on temperature, but they all have the same apparent energy of activation, which is equal to the lowest threshold energy. The validity of the approximations is verified by comparison with exact numerical solutions.
On the nature of the bonding of lone pair ligands to a transition metal
Authors: Bagus Paul S. , Hermann Klaus , Bauschlicher Charles W.
Published: February 2003

We report an analysis of the interaction of the closed shell, lone pair ligands CO, NH3 and PF3 with a Cu surface. The surface is represented by one and by five atom clusters. The interaction is analyzed with the constrained space orbital variation, CSOV, method which leads to a division of the interaction into several steps and permits an unambiguous assignment for the energetic importance of each step. The importance of the σ repulsion between the ligand lone pair and the metal is demonstrated and the significance of the dipole moment of the ligand is shown. The charge rearrangements are divided into intra-unit polarization and inter-unit donation between the units. Formation of covalent chemical bonds is not particularly important for NH3; the bonding can be viewed as electrostatic and arising because of the large NH3 dipole moment. For CO and PF3, covalent bonds are formed by metal to ligand π donation; for CO, the relative importance of dπ and valence 4pπ for this bonding is examined.
Excess electrons in simple fluids. I. General equilibrium theory for classical hard sphere solvents
Authors: Chandler David , Singh Yashwant , Richardson Diane M.
Published: February 2003

In this paper we develop a theory for an excess electron in simple fluids. It is based upon the path integral formulation of quantum theory which maps the behavior of the electron on to that of a classical isomorphic polymer. The influence functional, i.e., the solvent induced potential between different sites on the polymer, is estimated from the RISM integral equation. The functional is not pair decomposable and the resulting polymer problem is not trivial to solve. The evaluation of the electron partition function and correlation functions is pursued with two approximations: (i) a mean field approximation which neglects the role of polymer fluctuations on the solvent induced interactions: (ii) a polaron approximation which is a linear approximation in the sense that it neglects large fluctuations in the polymer structure itself. The theory brings to light a new approach to the computation of multiparticle correlation functions in fluids, and the theory provides what appears to be a practical scheme for attacking a number of other problems including the analysis of polymer conformations in liquid environments. While this paper focuses on the role of packing forces in these systems, the theory can be generalized to include polarization and charge interactions as well.
Bond-bending-force shell model for phonons in diatomic cubic crystals
Authors: Kushwaha Manvir Singh
Published: February 2003

To study the crystal dynamics of ionic and partially ionic crystals a bond-bending-force shell model (BBFSM) is developed. The BBFSM incorporates the bond-bending forces in the treatment of the second-neighbor shell model with both the ions polarizable. The basic parameters appropriate to such a model are: 14 for NaCl structure, 13 for CsCl structure, and 14 for zinc-blend structure. Seven of the parameters, in each case, were determined by using some constraints [e.g., crystal stability conditions, elastic constants, critical-point phonon frequencies, lattice constants, dielectric constants, and piezoelectric constants (in zinc-blende structure)] and the rest of them were obtained by means of a least-squares fit to the measured dispersion relations. The adequacy of the BBFSM has been investigated by applying it to compute the phonon dispersion relations in NaCl, CsCl, and GaAs compounds which crystallize, respectively, in NaCl structure, CsCl structure, and zinc-blende structure. The comparison of the theoretical results with the available experimental data due to neutron scattering measurements appears to be encouraging and gives us confidence to regard it (BBFSM) as a useful ramification of the shell model and to extend its application to study the crystal dynamics of other compounds belonging to the aforesaid crystal structures. The merits of BBFSM are discussed in detail.
A Monte Carlo approach to chemical bonding in condensed phases
Authors: Chiles R. A. , Jongeward G. A. , Bolton M. A. , Wolynes P. G.
Published: February 2003

Chemical bonding can be markedly affected by a condensed phase environment. This arises from a solvent induced electron–electron attraction. This effect can be taken into account using path integral techniques. The problems that face usual path integral approaches arising from the antisymmetry of the wave functions are ameliorated by performing the path sum with intermediate states that are already antisymmetric. The path sums are evaluated via a Monte Carlo method. Because of the use of a discrete basis this is the path integral analog of a configuration interaction calculation including the influence of the solvent. Illustrative calculations for the dissociation curves of H2 in model environments are shown, illustrating the possibility of dissociation to ions characteristic of condensed phase potential curves.
Phase transition at 434 K, independent strain coupling in second transition at 400 K, and thermal expansivity in ferroelastic K2TeBr6
Authors: Abrahams S. C. , Ihringer J. , Marsh P. , Nassau K.
Published: February 2003

X-ray diffraction and specific heat measurements have identified two independent strain-coupled processes in a broad phase transition at about 400 K and a normal lambda-type phase transition at 434 K in K2TeBr6 between 20 K and decomposition at 590 K. Phase I is cubic Fm3m with four K2TeBr6 per unit cell, a435=10.7456(14) Å, and expansion coefficients to third-order of α=0.134(3)×10−3 Å K−1, β=0.198(17)×10−6 Å K−2, and γ=0.162(27)×10−9 Å K−3: it transforms on cooling to phase II at 434 K with an entropy change of 1.3 J mol−1 K−1. Phase II is tetragonal, P4/mnc, with two K2TeBr6 per unit cell, a410=7.5755(10) and c410=10.7702(22) Å, with expansion coefficients to third-order for the tetragonal a axis identical to those of the cubic a axis, and linear coefficient αc=−0.103(12) ×10−3 Å K−1: it transforms on cooling to phase III at about 400 K with an entropy change of 2.9 J mol−1 K−1. Phase III is monoclinic, P21/n, with two K2TeBr6 per unit cell, a295=7.4908(10), b 295=7.5492(7), c295=10.6984(11) Å, β 295=90.307(6)°, with monoclinic a axis expansion coefficients to third-order identical to those of the cubic a axis, linear coefficient αb =−0.308(6)×10−4 Å K−1, second-order coefficients αc=0.711(14) ×10−4 Å K−1, βc =0.290(99)×10−7 Å K−2. The monoclinic angle follows a Landau critical power law with β−90∝(400−T)1/2, whereas a−b∝(359−T). Group theoretical analysis of the phase II symmetry operators predicts two independent soft Eg modes for the K and Br atom special positions at the transition to phase III, leading to independent proportionality constants for the thermal variation of β and a−b, as observed. By contrast, the transition from phase I to phase II has only a single soft A2g mode, leaving the K atom special position unchanged. A simple model predicts 1.52 J mol−1 K−1 for the phase transition at 434 K and 2.64 J mol−1 K−1 for that at 400 K. Dynamical short-range tetragonal order may set in above 359 K. Ferroelastic transformation in phase III at room temperature is shown to be accompanied by atomic displacements no larger than about 0.6 Å as the a and b axes are exchanged under applied compressive stress.
Measurement of N+4 recombination rate vs electron temperature in a proton beam created plasma
Authors: Fitaire M. , Pointu A. M. , Stathopoulos D. , Vialle M.
Published: February 2003

An experimental method for studying electron–ion recombination process is described. It uses RF pulse heating of a plasma created by a proton beam in high pressure Ne:N2 mixture. The recombination coefficient of N+4 ion has been determined to be (2.3±0.5)10−6 (Te/300)−(0.53±0.14) cm3 s−1 for electron temperature Te between 2000 and 11 000 K.
Photodissociation of 1,2-chloroiodoethane at 248 and 266 nm: The enthalpy of formation of CH2ClCH2I
Authors: Minton Timothy K. , Felder Peter , Brudzynski Richard J. , Lee Yuan T.
Published: February 2003

The technique of photofragmentation translational spectroscopy has been utilized to study the photodissociation of CH2ClCH2I at excitation wavelengths of 248 and 266 nm. The primary photofragments are C2H4Cl and either I*(2P1/2) or I(2P3/2). A fraction of the chloroethyl radical product contains enough internal excitation after the primary process to undergo secondary dissociation into C2H4 and Cl. By observing the threshold for this secondary process, the reaction enthalpy for CH2ClCH2I→C2H4+Cl+I was determined, which leads to: ΔH0f,0 (CH2ClCH2I)=−7.8±1 kcal/mol. The c. m. translational energy distribution of the photofragments was derived for both the I* and I channels, yielding I*/I branching ratios of 1.5 at 248 nm and 3 at 266 nm. The translational energy distribution also revealed that about 50% of the excess energy went into translation. The angular distributions of dissociation products with respect to the laser polarization indicate that the primary photodissociation process for the ground and excited state iodine channels at both wavelengths proceeds via a parallel transition, i.e., the transition moment must be nearly parallel to the C–I bond.
Semiclassical adiabatic theory of resonances in chemical reactions: Application to 3D H+H2 and F+H2
Authors: Pollak Eli , Wyatt Robert E.
Published: February 2003

Spectroscopic properties of resonances in the 3D H+H2 and F+H2 reactions are predicted through application of a semiclassical adiabatic theory. The theory is based upon an assumed time scale separation between translation-vibration, bending, and overall rotational motions. In the first step, bending and rotational coordinates are frozen, and translation-vibration periodic orbits are semiclassically quantized. In the second step, the quantized translation-vibration energy (parametrized by the bending angle) serves as an effective potential for the slower bending motion, which is also semiclassically quantized. Finally, average rotational constants are derived from the bending and translation-vibration periodic orbits and the total rotational energy is quantized. A novel result is the prediction of bend level structure for excited resonance states.
The effects of wavelength, temperature, and trifluoromethane-H buffer gas pressure on the infrared multiphoton decomposition of trifluoromethane-D
Authors: O’Neill James A. , Robins Jeffrey R.
Published: February 2003

The isotopically selective infrared multiphoton dissociation of trifluoromethane-d (CF3D) has been studied as a function of trifluoromethane-h (CF3H) buffer gas pressure at three temperatures and two wavelengths. The measurements revealed that the isotopic selectivity in dissociation (ratio of probabilities of CF3D and CF3H dissociation) was high even under conditions where many collisions took place between CF3D and CF3H molecules during the laser pulse. For example, this selectivity was >220 under conditions where approximately 50 gas kinetic collisions would occur. It was also found that the laser fluence required to cause CF3D dissociation was a relatively slow function of collision number. The number of collisions at room temperature required to double the fluence required for dissociation, calculated assuming a simple threshold model for the dependence of yield on fluence, was ∼60 and ∼140, respectively, at the CO2 laser transitions R(26) and R(12). The implications of these results on economic production of deuterium are discussed.
A
b i
n
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t
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o calculations of potential energy curves of Hg2 and TlHg
Authors: Celestino Karen Calvert , Ermler Walter C.
Published: February 2003

Potential energy curves for electronic states of Hg2 and TlHg are presented and analyzed. They are derived using large scale configuration interaction procedures for the valence electrons, with the core electrons represented by ab initio relativistic effective potentials. The effect of spin-orbit coupling are investigated for the low-lying excimer states. It is determined that neither system possesses strongly bound electronic states for which transitions to the repulsive ground states are optically allowed.
An efficient reformulation of the closed-shell self-consistent electron pair theory
Authors: Pulay Péter , Saebo/ Svein , Meyer Wilfried
Published: February 2003

The closed-shell SCEP method is reformulated in terms of pairwise nonorthogonal configuration state functions with particle-hole singlet coupling. The use of a single type of external pair function instead of the usual singlet and triplet coupled pairs results in the elimination of the internal coupling coefficients in the residual vector formulas, simplifying the latter considerably. The new CI coefficients are represented as unsymmetrical quadratic matrices which are naturally adapted to the description of long-range correlation between spatially distant localized orbitals. The number of matrix multiplications required for the interorbital pair coupling terms is reduced by half. This should lead to a reduction in the total computational effort by about 30%, except for very small molecules where intraorbital correlation dominates the computation.
Basis set dependence of a
b i
n
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t
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o SCF elastic, Born, electron scattering cross sections for C2H4
Authors: Xie Shang-de , Fink M. , Kohl D. A.
Published: February 2003

The results of ab initio Hartree–Fock calculations of the orientationally averaged, elastic electron scattering cross section of C2H4 with six different basis sets are reported. The averaging and Fourier transform were calculated by the approach of Kohl, Pulay, and Fink. Six different basis sets, ranging from 6-31G to 6-311 G4*, were employed in the calculations. The improvement in the calculated Born cross section parallelled the lowering of the energy as the basis was varied. For C2H4, a calculation at the 6-311G** level provides a good description of the cross section at a modest expenditure of computational time.
Incoherent scattering factors
Authors: Thakkar Ajit J.
Published: February 2003

Waller–Hartree calculations of molecular incoherent scattering factors S( μ) for CH4, NH3, and H2O are reported to provide an unambiguous comparison with recent calculations of S( μ) for these molecules made by integration of impulse approximation Compton profiles. It is found that the latter approach leads to both qualitative and quantitative errors at small momentum transfer μ. Moreover, it is pointed out that Waller–Hartree calculations of S( μ) require very little additional effort when one is calculating elastic cross sections. An illustrative calculation for diborane is used to suggest that even self-consistent field calculations of the Waller–Hartree S( μ) can be useful in the analysis of experimental data provided that some caution is used and that the calculations are approximately corrected for electron correlation effects.
Exact treatment of linear non-Condon terms in the theory of T≠0 K multimode resonance Raman scattering and optical absorption
Authors: Chan C. K.
Published: February 2003

Non-Condon terms derived from expanding the electronic transition dipole matrix elements to terms linear in the normal coordinates are included in the time-correlator theory of resonance Raman scattering. Exact T≠0 K multimode model expressions for the optical absorption and first-order resonance Raman profiles are derived, and Tehver’s approximate absorption→profile transform relation is rederived from these expressions. Numerical calculations show that for realistic model parameters for a wide class of systems, this approximate transform relation yields sufficiently accurate first-order RR profile line shapes. On the other hand, it is shown that the approximations necessary for this transform relation are unnecessary for numerical modeling of the optical absorption and resonance Raman profiles, because the exact model expressions are very convenient for computation.
Q-band EPR study of Mn2+ doped single crystals of NH4I
Authors: Chand Prem , Upreti G. C.
Published: February 2003

EPR of Mn2+ doped ammonium iodide single crystals has been studied at room temperature (∼300 K) and at Q band (∼35.25 GHz). An axial complex has been observed. The spin-Hamiltonian analysis has been carried out and the obtained parameters reported. From the analysis of observed forbidden hyperfine transitions, the quadrupole coupling constant Q has been estimated.
Excited state dynamics of NaF:Cu+
Authors: Goldberg Andrea B. , Payne Stephen A. , McClure Donald S.
Published: February 2003

We have previously used one- and two-photon spectroscopy to characterize the vibronic structures observed for NaF:Cu+ in the excited states of the d 9s configuration. We were able to resolve the zero-phonon lines and a progression of the first three or four quanta of the t1u vibration (involving the off-center motion of Cu+). In the present work we model the line positions and intensities with a mixed quadratic–quartic spherical potential. The spherical potential along with a perturbative treatment of the crystal field describes the members of the 1T2g state progression well. We also semiquantitatively treat the Jahn–Teller effect, operative in the 1Eg state only. The fits indicate the existence of a radial barrier in the potential for the off-center motion of Cu+ in the 1T2g and the 1Eg states of approximate magnitude 1.9 and 73 cm−1, respectively. We also find that the Cu+ ion moves preferentially toward the fluorines rather than between them in the 1T2g state. The derived potential energy surfaces are discussed in terms of the dynamical mixing of the excited 4s and 4p orbitals of Cu+.
Excited-state torsional dynamics of c
i
s-stilbene from resonance Raman intensities
Authors: Myers Anne B. , Mathies Richard A.
Published: February 2003

Resonance Raman spectra of cis-stilbene have been obtained using excitation at 356, 299, 282, and 266 nm. Intense Raman lines at 403 and 560 cm−1 are observed in the frequency region characteristic of double bond torsions. Resonance spectra of trans-stilbene show no Raman lines of comparable intensity which can be assigned to double bond torsions. These results suggest that electronically excited cis-stilbene twists about its central double bond much more rapidly than does the trans isomer. To quantitate these observations, the Raman lines of cis-stilbene at 165, 403, 560, and 963 cm−1 are assigned to specific out-of-plane normal modes by comparison with QCFF/PI calculations. We then model the excited state surface as a linear dissociative potential along the 560 cm−1 double bond torsion and a harmonic potential along all other modes. The experimental Raman intensities are used to determine the slope of the excited state torsional potential and the excited state origin displacements in the harmonic modes. Wave packet propagation on this excited state surface gives a 25° torsion about the central double bond of cis-stilbene in only 20 fs. The differences in excited state torsional dynamics between cis- and trans-stilbene are attributed to ground state conformational distortions due to the severe steric hindrance in the cis isomer.
Spectroscopic study of conformational dynamics of 1,1′-binaphthyl in a jet
Authors: Jonkman Harry T. , Wiersma Douwe A.
Published: February 2003

Analysis of the fluorescence excitation spectrum of 1,1′-binaphthyl in a jet shows that in the origin region there are four transitions, in an energy range of 150 cm−1, which strongly overlap through the occurrence of strong progressions in a torsional mode. The four transitions can be described in terms of excitations to levels of an interchange-equivalent naphthalene dimer in a trans and cis configuration. The dimer splittings are 50 and 63 cm−1 for 1,1′-binaphthyl-h14 and 50 and 57 cm−1 for 1,1′-binaphthyl-d14. The Franck–Condon envelope of the librational progressions indicates that the potential energy surface for states of A and B symmetry are different, which implies that the interplanar angle in each of the four excited states is different. From the intensities in the A and B progressions it is further deduced that for the origin transitions the transition moment in each subunit is oriented in the naphthalene plane at an angle of ∼50° with respect to the long axis. From the Franck–Condon envelope in the librational progressions it is further deduced that the change in torsional angle on optical excitation to the B-potential energy surface is ∼12° to the trans and 6° to the cis side. From this a maximum (state dependent) barrier of 150 cm−1 is calculated. It is further shown that, near the intense false origin, numerous modes appear which are induced through vibrational mixing. This Fermi-resonance effect is most obvious in perdeutero binaphthyl, where the vibronically induced intensity in the region of the most intense false origin is smeared out over many modes. The emission spectrum of the molecule can be interpreted by assuming that in 1,1′-binaphthyl very efficient vibrational relaxation occurs through conversion of vibrational into librational energy with molecular rotations providing the matching energy. It is also suggested that, after vibronic excitation, vibrational relaxation occurs into the energetically most stable structure. Finally, the observed red shift of the binaphthyl fluorescence in solution must be due to solvent stabilization of the excited molecule in a structure which is more planar than the one observed in the isolated molecule.
Infrared laser spectroscopy of the ν3 fundamental of HCO+
Authors: Davies P. B. , Hamilton P. A. , Rothwell W. J.
Published: February 2003

Eighteen lines in the P and R branches of the ν3 band of HCO+ have been measured by diode-laser absorption spectroscopy using a velocity modulation technique. The position of the band center is 2183.9505(51) cm−1.
The electronic emission spectrum of methylnitrene
Authors: Carrick P. G. , Engelking P. C.
Published: February 2003

The Ã 3E–X̃ 3A2ultraviolet emission spectrum of methylnitrene (CH3N) was obtained in two ways: (1) by reacting methylazide (CH3N3) with metastable N2 in a flowing afterglow; and (2) by discharging a mixture of methylazide (CH3N3) and helium in a corona excited supersonic expansion (CESE). The origin appears at T0=31 811 cm−1. Several vibrational progressions were observed leading to the determination of a number of vibrational frequencies: v″1=2938 , v■2=1350, v″3=1039, v■4=3065, and v″6=902 cm−1. Deuterium substitution confirmed the assignments of the vibrational frequencies. The X̃ 3A2 state is a normal, bound local minimum on the triplet electronic potential surface, and the upper Ã 3E state is able to support at least one quantum of vibration, assigned to v3, predominantly a C–N stretch. A comparison of flowing afterglow hollow cathode discharge sources and corona excited supersonic expansion sources shows the advantage of the CESE method of radical production for spectroscopy.
Rayleigh–Brillouin spectroscopy of simple viscoelastic liquids
Authors: Carroll P. J. , Patterson G. D.
Published: February 2003

The Rayleigh–Brillouin spectrum of 2,4-dimethyl pentane, 2,4,6-trimethyl heptane, 3,5-dimethyl heptane, 2,2,4,4-tetramethyl pentane, and 2,2,4,4,6,6,8,8-octamethyl nonane was measured as a function of temperature from well above the Brillouin linewidth maximum down to the glass transition region. Brillouin splittings and linewidths were determined at each temperature. The results were analyzed quantitatively in terms of the theory of Rytov. The spectrum of the Mountain peak due to slowly relaxing density fluctuations was obtained over the entire temperature interval where it appears. The spectrum was very non-Lorentzian at all temperatures. Near the glass transition the dynamics of the fluctuations associated with the Mountain peak were studied with photon correlation spectroscopy. The relaxation functions were observed to be highly nonexponential. The average relaxation time changed very rapidly with temperature, as expected near the glass transition. In addition to the slowly relaxing part of the Mountain peak observed with photon correlation spectroscopy, high frequency components were observed directly in the frequency domain spectra. These rapidly relaxing density fluctuations account for the finite Brillouin linewidths observed near the glass transition. A very broad distribution of relaxation times must be taken into account to explain the entire Rayleigh–Brillouin spectrum.
Generalized van der Waals theory. XIII. Curved interfaces in simple fluids
Authors: Hooper Malcolm A. , Nordholm Sture
Published: February 2003

Recent work has shown that a simple free energy density functional theory (the GvdW theory) can resolve the surface tension and interface width of gas/liquid interfaces in simple fluids to good accuracy. In a straightforward extension the analysis and calculations are here applied to liquid drops and gaseous cavities in simple fluids. The fluid particles are assumed to interact by a Lennard-Jones (12-6) potential and to be sufficiently heavy to allow the use of classical mechanics. The surface tension and profile width are obtained as functions of the radius R for drops and cavities at several different levels of approximation illustrating different contributing mechanisms. A significant decrease in the surface tension is found for both drops and cavities as the radii decrease to become comparable to the particle diameter.
Random systems of particles: An approach to polydisperse systems
Authors: Salacuse J. J.
Published: February 2003

The concept of a random system of particles is introduced and a probabilistic description of these types of systems is given. In addition the relationship of random systems to polydisperse systems is explored. The random systems approach to polydisperse systems is a particle as opposed to a continuum type theory and yields a number of results concerning the particle structure of polydisperse systems as well as a statistical mechanical description of polydisperse systems. As an illustration, the thermodynamic properties of a polydisperse system of hard rods are obtained from first principles. Phase equilibrium in polydisperse systems is considered in the context of the random systems approach. A set of equilibrium conditions are derived and shown to be equivalent to conditions previously given.
Measurement of small electrophoretic mobilities by light scattering and analysis of the amplitude weighted phase structure function
Authors: Schätzel K. , Merz J.
Published: February 2003

A novel signal processing scheme for the analysis of laser light scattered by small particles allows the measurement of very small collective particle motion in the presence of large diffusive motion. The technique uses a symmetric real fringe Doppler system with frequency shift. Amplitude and phase of the high pass filtered Doppler intensity signal are measured simultaneously and processed into an amplitude weighted phase structure function. An analysis of this function yields diffusion coefficient and, in a sinusoidal electric field, electrophoretic mobility. If present, a constant velocity motion, e.g., due to thermal convection, is simultaneously determined. The technique overcomes a principal limit of conventional light scattering techniques and allows the use of small fields or the measurement of small zeta potentials. Experimental results on latex systems and macroemulsions demonstrate possible improvements.
Dynamical scaling and the growth of diffusion-limited aggregates
Authors: Hentschel H. G. E. , Deutch J. M. , Meakin Paul
Published: February 2003

The validity of dynamical scaling for the growth of aggregates in a bath of particles of concentration c0 is investigated. A length scale ξ∼c0−1/(d−D) is introduced which governs the crossover from a fractal of dimension D to compact aggregate behavior. For static scaling a variable x=R/ξ is introduced where R is the radius of gyration of the aggregate. The cluster size N is found to vary as N(c0,R)=(ξ/a)DxDg(x), where a is the particle size and the static structure function is well represented by g(x)=(A+Bx)d−D. To consider dynamical scaling the asymptotic dynamics are first examined. For R≪ξ the radius of gyration and cluster size are respectively found to grow with time as R(c0,t)∼(c0t)1/(2+D−d) and N(c0,t)∼(c0t)D/(2+D−d). While for R≫ξ one finds R(c0,t)∼c01/(d−D)t and N(c0,t)∼c01+d/(d−D)td. If a scaled time variable τ=Dft/ξ2 is introduced where Df is a microscopic diffusion constant, then the previous asymptotic results can be incorporated into dynamical scaling forms for R(c0,t) and N(c0,t). These are R(c0,t)/ξ=x=τ1/(2+D−d)f1(τ) where f1(τ) is well represented by the form f1(τ)=[(A1+B1τ)]1−1/(D+2−d); while N(c0,t)=(ξ/a)DτD/(2+D−d) f2(τ) where f2(τ) can be written [A2+B2τ]d−D/(2+D−d). Using these results all concentration c0 and time t regimes can be investigated. Dynamical scaling is found to hold within the errors inherent in computer simulations due to depletion effects of finite lattices.
An improved method to estimate ionic size for calculating polyatomic ion mobility
Authors: Lande S. S. , Hagen D. F. , Seaver A. E. , Pearlman R. S.
Published: February 2003

A method is described as an alternative to the shadowgraphic method developed by Lin et al. for estimating ionic radii (RI) of protonated molecular ions of polynuclear aromatic hydrocarbons (PAH). These investigators had applied the estimated RI values to calculate the gas phase ion mobility values by means of the Langevin equation. The new method assumes that a neutral molecule and its positive molecular ion can be treated as effective hard spheres of the same radius RI which can be estimated from total molecular volume or total molecular surface area values computed using an algorithm previously described. These RI values are then used to estimate collision cross-sectional areas. The gas phase ion mobilities were calculated utilizing the Einstein–Nernst–Townsend equation as well as the Langevin equation. The ion mobility values so calculated compare reasonably well with experimental values for the 21 PAH previously used by Lin et al.
Laboratory detection of the 11←111 submillimeter wave transition of the H2D+ ion
Authors: Warner Hugh E. , Conner William T. , Petrmichl Rudolph H. , Woods R. Claude
Published: February 2003

The 110←111 transition of H2D+ was detected in a dc discharge in Ar–H2–D2 mixtures with liquid nitrogen cooling and an applied axial magnetic field. The transition frequency was determined to be 372 421.380±0.100 MHz. This transition is expected to be the most favorable one for radioastronomical detection of this very crucial ion in the interstellar medium.