bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectra and structure of organogermanes. XXII. Microwave, infrared, and Raman spectra of methylgermyl cyanide
Authors: Durig J. R. , Mohamad A. B. , Attia G. M. , Li Y. S. , Cradock Stephen
Published: July 2002

The microwave spectra have been recorded from 18.0 to 26.5 GHz for 12CH3GeH212C14N, 13CH3GeH212C14N, 12CH3GeH212C15N, 12CH3GeH213C14N, 12CH3GeD212C14N, and 12CD3GeH212C14N of the four naturally occurring isotopes of germane: 70Ge, 72Ge, 74Ge, and 76Ge. Only a-type transitions were observed and R-branch assignments have been made for all the isotopic species in the ground vibrational state from which the rotational constants were determined. From these data the complete structural parameters were determined to be r0(CME–Ge)=1.933±0.002 Å, r(Ge–CCN)=1.927±0.004 Å, r(C≡N)=1.155±0.004 Å, r(Ge–H)=1.521±0.001 Å, r(C–Hs)=1.092±0.005 Å, r(C–Ha)=1.092±0.005 Å, ∢CGeC=107.36°±0.60°, ∢HGeH=111.41°±0.04°, ∢HGeCMe=112.92°±0.08°, ∢HsCGe=109.5°±2.0°, and <HaCGe=109.3°±0.8°, where all of the parameters are rs values except for the carbon–hydrogen distances and angles. The dipole moment components were determined from the Stark effect to be ‖μa‖=4.20±0.14, ‖μb‖=0.39±0.03, and ‖μt‖=4.22±0.14 D. From the microwave splitting method, the threefold barrier to internal rotation was determined to be 1148±23 cal/mol (402±8 cm−1). The infrared (3200 to 50 cm−1) and Raman spectra (3200 to 10 cm−1) of gaseous and solid CH3GeH2CN, CH3GeD2CN, and CD3GeH2CN have been obtained. Additionally, the Raman spectra of the liquids have been recorded and qualitative depolarization values have been obtained. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and isotopic shifts. These results are compared to similar quantities in some related molecules.
On the use of semiclassical dynamics in determining electronic spectra of Br2 in an Ar matrix
Authors: Thirumalai D. , Bruskin E. J. , Berne B. J.
Published: July 2002

A semiclassical method to simulate the electronic absorption and emission spectra of Br2 in an Ar matrix is presented. The appropriate time correlation functions are evaluated in the Condon approximation using Gaussian wave packet dynamics. Several approximate methods are used to propagate the wave packet and their relative merits in obtaining high resolution spectra are discussed. The dynamics using a single Gaussian wave packet is inadequate for obtaining spectra in highly anharmonic systems, frequently encountered in condensed phase problems. Where possible, the results of the simulation are compared in detail with available experiments. The simulated electronic absorption spectra confirm the physical picture advanced by Bondebey and Brus. It is shown that from the spectral analysis of the emission spectra one can infer some features of the guest–host interaction when the guest is in the electronically excited state.
The relative motion of ions in solution. III. An NMR relaxation study of repulsive ions in water at low ionic strength
Authors: Fries P. H. , Rendell J. , Burnell E. E. , Patey G. N.
Published: July 2002

In this paper we describe a nuclear magnetic resonance study of the relative motion of the repulsive ion pairs PF−6/⋅ON(SO3)−22 and F−/⋅ON(SO3)−22 in dilute D2O solutions. From measurements at 188.217 MHz of the spin-lattice relaxation time of the 19F nuclei on PF−6 and F−, we have obtained the experimental interparticle dipolar relaxation rate of these nuclei due to their interaction with the electronic spins of the paramagnetic anion ⋅ON(SO3)−22. The underlying reduced spectral density j̄2(ωFτ), which depends on the relative motion of the interacting ions, is simply proportional to the interparticle relaxation rate. The experimental spectral densities are compared with theoretical calculations for different model electrolytes over a range of ionic strengths. It is found that the agreement between theory and experiment is generally good for both molecular and continuum solvent models which take the electrostatic ionic repulsion into account. The PF−6/⋅ON(SO3)−22 results are in somewhat better agreement with the molecular solvent picture and provide some evidence for a solvent induced minimum in the short-range behavior of the ion–ion potential of mean force.
The far-infrared spectrum of ice VIII
Authors: Tay S. P. , Klug D. D. , Whalley Edward
Published: July 2002

The far-infrared spectrum of ice VIII as recovered at ∼100 K and zero pressure has translational and rotational bands at 162 and 455 cm−1, respectively. They are undoubtedly due to the Eu translational vibration in the x, y plane and the Eu rotational vibration about the x, y axes. The correlation of the vibrations of ice VIII with the zone-center and -boundary vibrations of the centrosymmetric phase ice X is summarized.
Theory of phonon inelastic atom–surface scattering. I. Quantum mechanical treatment of collision dynamics
Authors: Choi B. H. , Poe R. T.
Published: July 2002

We present a systematic formulation of the atom–surface scattering dynamics which includes the vibrational states of the atoms in the solid (phonons). The properties of the total scattering wave function of the system, a representation of the interaction potential matrix, and the characteristics of the independent physical solutions are all derived from the translational invariance of the full Hamiltonian. The scattering equations in the integral forms as well as the related Green functions were also obtained. The configurational representations of the Green functions, in particular, are quite different from those of the conventional scattering theory where the collision partners are spatially localized. Various versions of the integral expression of scattering, transition, and reactance matrices were also obtained. They are useful for introducing approximation schemes. From the present formulation, some specific theoretical schemes which are more realistic compared to those that have been employed so far and at the same time capable of yielding effective ab initio computation are derived in the following paper. The time reversal invariance and the microscopic reversibility of the atom–surface scattering were discussed. The relations between the in and outgoing scattering wave functions which are satisfied in the atom–surface system and important in the transition matrix methods were presented. The phonon annihilation and creation, and the adsorption and desorption of the atom are related through the time reversal invariance, and thus the microscopic reversibility can be tested by the experiment.
A comparison of quantum, classical, and semiclassical descriptions of a model, collinear, inelastic collision of two diatomic molecules
Authors: Page M. , Oran E. S. , Boris J. P. , Miller D. , Wyatt R. E. , Rabitz H. , Waite B. A.
Published: July 2002

The collinear dynamics of a model diatom–diatom system is investigated. The collision partners are harmonic oscillators for which the masses and force constants are chosen to correspond to those of the nitrogen and oxygen molecules. The interaction between the molecules arises from a Lennard-Jones 6-12 potential acting between the inside atoms in the collinear system. Quantum mechanical close coupled calculations are performed for several collision energies ranging from 1.0 to 2.25 eV. The state-to-state transition probabilities which are extracted from these calculations are then used as a benchmark for comparison. Semiclassical calculations are performed within the framework of a classical path approximation. A simple scheme to modify the classical path to reflect energy exchange between the collision coordinate and the internal degrees of freedom is found to improve the results. On the whole, the agreement between the semiclassical and the quantum mechanical results is surprisingly good. The classical trajectory calculations correctly display many of the qualitative features of the collisions but the numerical agreement is not as close. Unexpectedly, the classical results do not appear to be improving as the collision energy is increased.
The potential energy surface for the F+H2 reaction as a function of bond angle in the saddle point vicinity
Authors: Schwenke David W. , Steckler Rozeanne , Brown Franklin B. , Truhlar Donald G.
Published: July 2002

We report large-basis-set CASSCF/MR-CISD/SEC (complete active space self-consistent-field orbitals used for multireference configuration interaction with all single and double excitations and scaled external correlation) and MP4 (Mo/ller–Plesset fourth order perturbation theory) calculations of the FH2 potential energy surface for collinear and bent geometries in the vicinity of the F---H--H saddle point. These calculations indicate that higher order correlation effects become much more important as the generalized transition states are bent, and that the unrestricted saddle point for this reaction is noncollinear. This means that the sterically allowed cone of reactive configurations is much broader than either previously available ab initio calculations or the present lower-order ones would predict.
Induced dipole–induced dipole interaction: A numerical calculation
Authors: Breymann W. , Pick R. M.
Published: July 2002

We present the results of a numerical study of the effects of the induced dipole–induced dipole interaction on the line shape of the internal ν1 and ν3 modes of NF3 in the plastic phase. Comparisons are made both with the experiment and with the results of a previous theoretical analysis of the effect.
Carbon near edge x-ray absorption fine structure (NEXAFS) studies of ethylene adsorption on Ni(111) and Ni(110) surfaces: Implication for surface morphology insensitive reactions
Authors: Sham T. K. , Carr R. G.
Published: July 2002

The NEXAFS of C2H4 adsorbed on Ni(111) and Ni(110) surfaces have been measured at room temperature under similar conditions. It is found that the structure and bonding of the surface species on both surfaces are essentially the same as that reported previously for C2H4/Ni(111). The surface species has a ‘‘C2’’ framework, which lies flat on the surface with a relaxed carbon–carbon bond. A di-σ configuration is proposed for the adsorbate which would have a twofold orientation symmetry on Ni(110). The implications of the twofold orientation symmetry in the case of C2H4/Ni(110) for the azimuthal dependence of NEXAFS and the significance of the overall observation for the surface chemistry of these crystals are discussed.
Order–disorder transitions in Langmuir–Blodgett films. II. IR studies of the polymerization of Cd–octadecylfumarate and Cd–octadecylmaleate
Authors: Rabe J. P. , Rabolt J. F. , Brown C. A. , Swalen J. D.
Published: July 2002

The two isomeric compounds, octadecylfumaric acid and octadecylmaleic acid (C18H37OCOCH=CHCOOH) have been synthesized, and Langmuir–Blodgett films of their cadmium salts have been studied by means of transmission and grazing incidence reflection infrared spectroscopy. These measurements provided information about the orientation of molecular segments relative to the substrate surface as well as about the lateral packing of the chains in the unit cell. A structural change of the maleate film was revealed at elevated temperatures. Polymerization of the films was brought about by either UV irradiation or thermal treatment and was followed spectroscopically as a function of exposure time. By comparison with films polymerized on the water surface of a Langmuir trough, it was concluded that the molecular structure is independent of where the polymerization occurs. Interestingly, upon polymerization on the water surface the fumarate undergoes an expansion at constant surface pressure whereas the maleate contracts. A mixture of fumarate and maleate in the ratio of 1 to 3, however, forms a two dimensional solution and preserves its area upon polymerization, a critical feature required for the fabrication of crack free Langmuir–Blodgett films polymerized on solid substrates.
A study of the ferrimagnetic ordering of the complex bimetallic salt hexaaminechromium (III) hexachloroferrate (III), [Cr (NH3)6] [FeCl6]
Authors: Helms Jeffrey H. , Hatfield William E. , Kwiecien Michael J. , Reiff William M.
Published: July 2002

The magnetic susceptibility of a powdered sample of [Cr (NH3)6][FeCl6] has been measured in the temperature range 1.85 to 110 K in applied magnetic fields of 100 and 10 000 Oe. A magnetic phase transition near 2.8 K is apparent in the low field data set. The transition is confirmed by zero field Mössbauer spectroscopy data. The magnetic data above the phase transition may be fit to the Néel hyperbola for a ferrimagnet: χ−1=(T−θ)/C−ζ/(T−θ′) with best-fit values of C=6.58 emu K, θ=−14.1 K, θ′=2.22 K, and ζ=10.1 emu K. Assuming a structure similar to [Co(NH3)6] [FeCl6] and mean field theory, the intrasublattice coupling constants are 0.12 cm−1, and the intersublattice coupling constant, Jab, is −0.21 cm−1. The value of Jab is confirmed by a change in slope of the magnetization data near 2000 Oe.
Analysis of dielectric or conductive system frequency response data using the Williams–Watts function
Authors: Macdonald J. Ross , Hurt Robert L.
Published: July 2002

The electrical, optical, and mechanical behavior of many materials, particularly polymers and glasses, have been analyzed using the Kohlrausch–Williams–Watts stretched exponential relaxation function in both the time and frequency domains. This function is currently of considerable experimental and theoretical interest. Unfortunately, no relatively simple and accurate approximation representing the small-signal frequency response of stretched exponential relaxation has been available. Thus it has been impractical to obtain accurate parameter estimates from fitting of frequency response data or to discriminate well between Williams–Watts response and that of other similar response models. Here we develop such an approximation for both dielectric systems and for intrinsically conducting ones (e.g., defect hopping materials). It is in complex form and allows fitting of both real and imaginary parts of all the data simultaneously (e.g., by complex nonlinear least squares) or of either part separately. For appropriate data, which need not be electrical, fitting with the new approximation can yield parameter estimates accurate to about 0.1%. Comparison of the results of the present fitting method to those of a more approximate one are presented.
Nonadiabatic ionic–covalent transitions. Exponential-linear model for the charge exchange and neutralization reactions Na+H ⇄ Na++H−
Authors: Errea L. F. , Méndez L. , Mó O. , Riera A.
Published: July 2002

A previous study of charge exchange processes taking place through ionic–covalent transitions is extended to the case of Na+H and Na++H− collisions. A five-state molecular expansion, with the inclusion of two-electron translation factors, is employed to calculate the charge exchange and neutralization cross sections. Transitions at the first two pseudocrossings between the energy curves, practically determine the cross sections in the energy range 0.16–5 keV amu−1. We also show that the widely used multichannel Landau–Zener theory is totally inadequate, to treat these transitions.
The magnetic circular dichroism of 1-azabicyclo[2,2,2]octane (ABCO) in the n→3p region
Authors: Gedanken Aharon
Published: July 2002

An MCD and absorption spectra measured in the energy region of the second excited state of 1-azabicyclo[2,2,2]octane (ABCO) reveal large positive A terms. Simple calculations of the magnetic moment of a 1E excited state yield also a positive sign for the A terms. A small negative B term is observed 355 cm−1 below the origin of the 1E state. Based on B term calculations we assign it as the 1A1→1A1 transition (n0→3pz). The results are compared with a recent MPI study in the same energy range.
The effective mobility of fine-texture dispersions flowing in porous media
Authors: Hatziavramidis Dimitri T.
Published: July 2002

An effective medium theory for predicting the mobility of fine-texture dispersions (i.e., foams and emulsions) flowing in porous media is presented. The dispersions are viewed as random arrays of mobile spheres of uniform radius, smaller than the pore radii, imbedded in a fluid of different mobility, and their flow is approximated as a diffusion-limited process. Using a multiple scattering approach, analytic expressions for the mobility of the dispersion in terms of the ratio of the mobilities of the individual phases and the volumetric fraction of the dispersed phase are generated.
The mutual diffusion constant of binary, isotopic hard-sphere mixtures: Molecular dynamics calculations using the Green–Kubo and steady-state methods
Authors: Kincaid John M. , Erpenbeck J. J.
Published: July 2002

The mutual diffusion constant of a binary mixture of equal diameter hard spheres is estimated using the method of molecular dynamics. The mixture considered is equimolar, with a species mass ratio of ten to one in a volume that is three times the close-packed volume. Two molecular dynamics methods are used: the standard Green–Kubo technique based on the evaluation of equilibrium velocity correlation functions, and a nonequilibrium method that generates a steady diffusive flow along a composition gradient by imposing special boundary conditions on two opposing faces of the cubic volume. We find that both methods yield, within about 3%, the same value of the diffusion coefficient.
Statistical mechanics of electrolytes and polyelectrolytes. IV. A theory of higher valence electrolytes
Authors: Ramanathan G. V. , Jensen Anna-Lise
Published: July 2002

A two-region theory is proposed for the study of higher valence electrolytes. In the inner region, which is close to an ion and resembles a boundary layer where ion association occurs, the gradient of the counterion radial distribution function is assumed to be large. In terms of this large parameter, asymptotic solutions are obtained in the inner and outer regions for symmetrized versions of the Born–Green–Yvon and the Poisson–Boltzmann equations, respectively, and these solutions are matched at the boundary. The theory is applied to the charged hard sphere model and contact values, internal energy and osmotic coefficient are calculated. They are compared with HNC and Monte Carlo results available in the literature. The agreement is good for 1–1, 1–2, 1–3, and 2–2 electrolytes over a wide range of concentrations. The degree of association and the extent of the inner region are also calculated. The latter is found to be considerably smaller than the Bjerrum length for higher valence electrolytes.
The second Born approximation for electron scattering. II. The high energy limit for small angle dipole allowed inelastic scattering from atoms
Authors: Bonham R. A. , Goruganthu R. R.
Published: July 2002

The exact second Born amplitude for direct electron scattering from atoms in the LS coupling approximation is evaluated by use of an analytic model for the transition charge densities for ns→mp excitations. Coupling to all channels is treated rigorously for intermediate s, p, and d state coupling in the limit as the momentum transfer approaches zero. Exchange is considered approximately by using a high energy exchange approximation. Expressions for the slope of the second order cross section as a function of momentum transfer are developed for the limit as the momentum transfer approaches zero. It is shown that the first Born result is achieved in the limit as K→0 of the effective generalized oscillator strength (Lassettre’s theorem) only if the limit is taken off the energy shell. The on energy shell limit as K→0 yields a second Born correction to the Lassettre’s theorem result. On the two off energy shell paths for taking the limit of the effective GOS as K→0 that were investigated the leading correction term is found to be proportional to K. For the on energy shell path, in addition to the second Born correction to the first Born limiting value the first order correction was found to be proportional to K2. The expansions in powers of K of the effective GOS investigated here showed no anomalies in contrast to findings in a previously reported investigation. We can only surmise, in view of the extreme sensitivity of the result to the choice of the path in the limit process K→0, that these previous results were due to a different choice of the limit path than those investigated here.
Unimolecular diffusion-mediated reactions with a nonrandom time-modulated absorbing barrier
Authors: Bashford D. , Weaver D. L.
Published: July 2002

A diffusion-reaction model with time-dependent reactivity is formulated and applied to unimolecular reactions. The model is solved exactly numerically and approximately analytically for the unreacted fraction as a function of time. It is shown that the approximate analytical solution is valid even when the system is far from equilibrium, and when the reactivity probability is more complicated than a square-wave function of time. A discussion is also given of an approach to problems of this type using a stochastically fluctuating reactivity, and the first-passage time for a particular example is derived.
A simple kinetic theory model of reactive collisions. II. Nonrigid spherical potential with angle-dependent reactivity
Authors: She Richard S. C. , Evans Glenn T. , Bernstein Richard B.
Published: July 2002

The kinetic theory model of Evans et al. (1985), which dealt with reactive collisions of rigid nonspherical molecules with angle-dependent reactivity, is extended to a nonrigid (but spherical) potential model, also with orientation dependent reactivity. An explicit formula is obtained for the differential orientational cross section in terms of the angle-dependent barrier (threshold) energy and an angle-dependent critical interparticle separation. The rigid nonspherical model and the soft, spherical model are combined heuristically; together, they account for molecular nonsphericity and nonrigidity (i.e., potential softness). The results of the combined treatment reduce properly to the steric opacity models of Smith (1982) and Levine and Bernstein (1984) in the appropriate limits.
A new threshold energy for NO+O3→NO*2 +O2
Authors: Kahler Carol C. , Kowalczyk Marta
Published: July 2002

The threshold energy for the chemiluminescent reaction between NO and O3 to give NO*2 and O2 has been measured using a photon detection crossed molecular beam machine. A supersonic NO beam was crossed at 90° by a quasieffusive ozone beam allowing better collision energy resolution than in previously published work. The data were fit using a cross section function of the form σ ∝ [1−Ethr/E]1.5 where Ethr=2.0±0.2 kcal/mol. Taking reactant internal energy into account, the pure translation energy threshold is calculated to be 2.6±0.2 kcal/mol. In addition, a low resolution chemiluminescence spectrum is reported for four collision energies between 4.5 and 8.6 kcal/mol.
Graph theoretical approach to the evaluation of matrix elements of one-body operators in S
U(n)
Authors: Kent R. D. , Schlesinger M.
Published: July 2002

A graph theoretical formulation of SN adapted irreducible representations (irrep) of SU(n) is presented. Closed form expressions involving only SN and SU(n) irrep labels are derived for the matrix elements of one-body operators in the unitary group approach based on the group generators.
Unimolecular reactions near threshold: The overtone vibration initiated decomposition of HOOH (5νOH)
Authors: Ticich T. M. , Rizzo T. R. , Dübal H.-R. , Crim F. F.
Published: July 2002

Laser induced fluorescence detection of the fragments from the unimolecular dissociation of hydrogen peroxide initiated by direct excitation of the fourth overtone of the OH stretching transition (5νOH) is a means of measuring both vibrational overtone predissociation spectra and populations of individual quantum states of the OH products. Because the excitation adds insufficient energy to break the OO bond, the measurements detect reactions of molecules that initially possess substantial thermal vibrational–rotational energy (≥1100 cm−1). The spectroscopic data, in which hot band and high energy transitions are particularly prominent, are fit by a vibrational–torsional model that adiabatically separates the low frequency torsional vibration from the higher frequency OH and OO stretching vibrations. The spectroscopic analysis shows that the effective trans barrier in the torsional potential increases by 75 cm−1 for each quantum of OH stretching excitation but decreases by 50 cm−1 upon excitation of the OO stretching vibration. The product state population distributions demonstrate that the lowest rotational level is preferentially populated in the unimolecular decomposition. This result is inconsistent with phase space theory, which predicts substantially more rotational excitation, but agrees with the statistical adiabatic channel model. The predictions of the latter model agree with the data for HOOH (5νOH) as well as with previous results for HOOH (6νOH). The statistical adiabatic channel model contains one parameter in addition to the ones required in phase space theory, and the value of this parameter that provides the best agreement with the measurements is consistent with an independent analysis of thermal recombination data. The experiment serves to test statistical calculations critically because it explores reactions near the threshold energy, where the predicted product state distributions are most sensitive to the details of the theory.
Radiationless transitions in gas phase phenol and the effects of hydrogen bonding
Authors: Sur Abha , Johnson Philip M.
Published: July 2002

Quantum yields and rates of intersystem crossing and internal conversion are obtained for the lowest excited singlet state of collisionless phenol. It is found that internal conversion is the dominant decay channel, even at the origin of S1, in contrast to previously studied molecules. Also, triplet lifetimes measured as a function of vibrational energy are an order of magnitude shorter than in other isoelectronic aromatics. Hydrogen bonding by water and in a dimer is found to lengthen the singlet lifetime and decrease the quantum yield of internal conversion. This behavior parallels the effects, in solution, of going from a nonpolar to a polar solvent, indicating that substantial solvation effects are caused by a single hydrogen bond.
Microwave-optical double resonance of HNO. II. Rotational spectrum in Ã 1A′′(020)
Authors: Takagi Kojiro , Saito Shuji , Suzuki Tetsuo , Hirota Eizi
Published: July 2002

Rotational transitions in the Ã 1A″(020) state of HNO were observed in the frequency range between 0.4 MHz and 154 GHz using the technique of microwave-optical double resonance (MODR). An anomalous behavior in K-type doubling frequencies observed for Ka=2 in Ã 1A″(020) has been attributed to the Coriolis interaction with Ka=1 in Ã 1A″(100). The laser excitation spectrum has been observed for the (020)–(000) Ka=2–1 subband, (100)–(000) Ka=1–0 subband, and some other lines in the Ã 1A″–X̃ 1A′ band system. The Q-branch microwave transitions between the two vibrational states (020)J2,J−2–(100)J1,J−1 in Ã 1A″ were observed by the MODR method at J=13 and 14, where the two rovibrational levels come closest. The MODR spectra for (020) and (100) together with the LEF spectrum for the (020)Ka=0,1, and 2 states and for the (100)Ka=0 and 1 states have been analyzed and the rotational constants, the centrifugal distortion constants, and a Coriolis coupling constant have been determined for Ã 1A″(020) and Ã 1A″(100). The K-type doublet transition of Ka=3 in Ã 1A″(020) and Ka=2 and 3 in Ã 1A″(100) observed in the radio frequency region are locally perturbed and most of them show magnetic activity.
Geometry in classical statistical thermodynamics
Authors: Levine R. D.
Published: July 2002

A Euclidean geometry for classical thermodynamics is discussed. The central physical idea is that it is useful to characterize the system in terms of a number of mean values of ‘‘relevant’’ observables. These mean values are written, as usual, as an expectation ∑iAi pi over a (classical) probability distribution. The expectation value is then interpreted as a scalar product between vectors belonging to dual spaces. A metric is introduced via the transformation from one space to another. In terms of the metric, the scalar product of two vectors belonging to the same space (e.g., two probability distributions or two observables) can be defined. In the space of all states the metric does not depend on the state of the system and the curvature tensor vanishes, i.e., the space is Euclidean.
A problem of gauge for matter–radiation interaction in calculations of intensities of vibrational transitions
Authors: Sobolewski A. L.
Published: July 2002

A theory of absolute intensities of molecular vibrational transitions is formulated in terms of the T matrix formalism of scattering theory. It is shown that the problem of the selection of the ‘‘velocity’’ (Ā ⋅ p̄) gauge or the ‘‘length’’ (Ē ⋅ r̄) gauge for description of the matter–radiation interaction in vibrational transitions is determined by the choice of the description of molecular eigenstates. The resulting consequences for radiationless transitions theory in polyatomic molecules are indicated.
Optical spectroscopy of Mn2SiO4 crystals
Authors: Powell Richard C. , Elouadi Brahim , Xi Lin , Loiacono G. M. , Feigelson Robert S.
Published: July 2002

The temperature dependences of the fluorescence intensities and lifetimes of Mn2SiO4 and the results of time-resolved spectroscopy measurements on this material are reported. A model is developed to explain the results in terms of thermally activated migration of Mn2+ excitons terminating on deep trapping centers. Fiber growth techniques are shown to decrease the concentration of fluorescence quenching centers in this material, and dilution with Mg or Ca ions is found to decrease the efficiency of the quenching process. Four-wave mixing results suggest the presence of laser-induced photoionization processes in these crystals.
Photoacoustic spectroscopy and energy relaxation processes in matrix isolated CF3NO
Authors: Bozlee B. J. , Nibler J. W.
Published: July 2002

The photoacoustic, fluorescence excitation, and absorption spectra in the visible region are compared for CF3NO isolated in argon. Results are related to possible photodissociative and nonradiative decay channels available to the molecule in the S1 excited state. Vibrational relaxation occurs in less than a few nanoseconds in this state and the quantum yield for fluorescence from the v′=0 level is 0.015. Subsequent relaxation is likely to involve internal conversion to the ground state, as in the vapor. In contrast to gas phase results, the matrix data give no indication of a direct, ‘‘second channel,’’ predissocation path near 627 nm. No evidence is seen for transient escape of the CF3 and NO photofragments from the matrix cage and all nonradiative processes appear to be complete in less than one microsecond. The time evolution of the photoacoustic pulse and the detection limits of these matrix measurements are discussed briefly.
Configuration of the SO=4 ion in the cubic phase of Li2SO4 from band shape studies of the ν3 mode
Authors: Cazzanelli Enzo , Frech Roger
Published: July 2002

A Raman spectroscopic study of the molecular ν3 band in the plastic phase of Li2SO4 is presented. The analysis of the band shape and the depolarization ratio vs frequency suggests a prevalence of C3v configurations around the sulfate tetrahedra. Spectroscopic evidence from the pretransitional region in the monoclinic phase and a similar analysis of the ν3 band in the melt reinforce this hypothesis. These data are compared with structural results and a computer modeling of the possible orientational distribution of the SO2−4 tetrahedra in the plastic phase.
Criterion for phase change in terms of oscillation of the pair distribution function
Authors: Liboff Richard L. , Schenter Gregory K.
Published: July 2002

A new criterion for gas–liquid phase change is presented based on the existence of purely oscillatory modes of the radial distribution function in the asymptotic domain of large interparticle separation. Working with a recently derived linear equation for the radial distribution function this criterion is found to be T̄=(9/4)n̄. Barred parameters represent the ratio of variables to their respective critical values. This finding is in very good agreement with results of the van der Waal theory of fluids in the low-density regime.
Comment on the Gibbs–Bogoliubov inequality for molecules
Authors: MacGowan David
Published: July 2002

A recently proposed variational procedure aimed at optimizing spherical reference systems for molecular liquids [Goldman and Kumar, J. Chem. Phys. 82, 4276 (1985)] is shown to be useless.
Enantiomeric phase separation in a lattice gas model: Guggenheim approximation
Authors: Huckaby Dale A. , Shinmi Masato , Ausloos Marcel , Clippe Paulette
Published: July 2002

We consider a lattice gas in which the two enantiomeric forms of a tetrahedral molecule, consisting of a central carbon atom bonded to four different groups A, B, G, and H, are adsorbed onto a triangular lattice, such that the carbon atom is above a lattice site, the three bonds to A, B, and G point toward neighboring lattice sites, and the bond to H points perpendicular to and away from the plane of the lattice. For a certain choice of intermolecular interactions, such as may exist between the zwitterion forms of an amino acid, the phase diagram was investigated using a Guggenheim approximation with two order parameters. Enantiomeric phase separation into two symmetric condensed phases occurs at low temperatures. These condensed phases become a single racemic condensed phase at a critical line, and they are in equilibrium with a racemic gas phase along a line of triple points. These two lines coincide at a critical endpoint. The racemic condensed and gas phases are in equilibrium along a two phase coexistence line which begins at the critical endpoint and ends at a critical point. No tricritical point was found in the model for the special choice of interactions studied.
C+(2P)+H2(D2,HD)→CH+(CD+)+H(D). II. Statistical phase space theory
Authors: Ervin Kent M. , Armentrout P. B.
Published: July 2002

Statistical phase space theory is applied to the reactions of carbon(1+)ions with isotopic molecular hydrogen (H2, D2, and HD) to form methyliumylidene (CH+ and CD+). Reaction cross sections and isotope effects as a function of kinetic energy are compared to recent experimental results. The important features of the potential energy surfaces for the C+(2P)+H system are reviewed. The treatment of multiple electronic potential energy surfaces and excited state product channels in phase space theory is discussed. Phase space theory accurately reproduces the relative behavior of the experimental cross sections in the threshold region. Also, the cross section magnitude is reproduced if only those potential energy surfaces without energy barriers in the entrance channel are included in the phase space model. Phase space theory fails, however, to predict quantitatively the kinetic isotope effects or the shape of the cross section at higher energies where excited state CH+ production may be important.
Numerical calculation of nuclear attraction three-center integrals for arbitrary orbitals
Authors: Talman James D.
Published: July 2002

The problem of calculating a nuclear attraction three-center integral by expanding each of the factors about a common center is studied. An analysis is made of the convergence rate for various choices of the expansion center. The optimum choice for the expansion center from the point of view of the convergence rate is found. Several numerical examples of the convergence are described and results for a number of three-center integrals are given.
On the photodissociation of uranium hexafluoride in the B band
Authors: Menghini M. , Morales P. , Dore P. , Schisano M. I.
Published: July 2002

The rate of photodissociation of uranium hexafluoride is measured for the first time as a function of wavelength in the B band. The experimental technique used tests the collision-free behavior and, with the addition of a buffer gas, the collisional effects on molecular relaxation. In both cases, the dissociation yield is strongly nonuniform. A qualitative interpretation of these results is attempted. Their relevance to the subject of laser isotopic separation and more generally of selective photochemistry is outlined.
Reply to the ‘‘Comment on ‘Remarks on the mutual diffusion coefficient of Brownian particles’ ’’
Authors: Yoshida Nobuo
Published: July 2002

It is suggested that the Comment by Altenberger and Tirrell may be inadequate. The present author, nevertheless, admits that his previous treatment of mutual diffusion of Brownian particles is also in error and here presents an argument showing that the diffusion coefficient derived from the dynamic structure factor by use of the generalized Smoluchowski equation is in fact identical with the one as would be obtained by conventional gradient diffusion experiments, contrary to his previous contention.
Hyperfine structure and Λ doubling in the A 2Δ state of CH: Observation of an internal hyperfine perturbation
Authors: Ubachs Wim , van Herpen W. M. , ter Meulen J. J. , Dymanus A.
Published: July 2002

We report the observation of a resonance between hyperfine states, due to the crossing of rotational ladders at N=10 of Δ5/2 and Δ3/2 spin-doublet states in the excited A 2Δ state of CH. Accurate values for the hyperfine constants a, b, and c in the A 2Δ state were obtained. From measurements of the Λ-doublet splittings we estimate an upper limit to the Λ doubling in the A 2Δ state.
Toward optimizing the sphericalized reference state in molecular perturbation theory: The Lennard-Jones diatomic fluid
Authors: Kumar Bijoy , Goldman Saul , Smith William R.
Published: July 2002

We present a molecular perturbation theory based on an isotropic reference state, and apply it to a Lennard-Jones diatomic model of liquid nitrogen. The approach involves an additional degree of freedom made manifest through a parameter (α) that was previously absent from such theories. α controls both the form of the expansion functional and the reference state potential. We use a free-energy annulment criterion to objectively select a value for this parameter. We give working expressions and numerically evaluate the thermodynamic, quasithermodynamic and structure-related properties of the molecular fluid for our model fluids. We find, by comparisons with Monte Carlo simulations, that this approach compares very favorably against all competing sphericalization procedures. The only exception is in its prediction of the mean-squared torque (and the underlying spherical harmonic coefficients for this function) for which the so-called RAM perturbation theory is better.
Further simplification in the numerical determination of chemical equilibrium
Authors: White W. B.
Published: July 2002

A highly simplified method is presented for the numerical determination of chemical equilibrium in complex mixtures. Problems of convergence in cyclic computations are discussed and eliminated.
Milne problem with a grey boundary
Authors: Naqvi K. Razi , Waldenstro/m S.
Published: July 2002

The stationary version of the linear Boltzmann equation (the Lorentz model) is studied in the half-space x>0, x=0 being a grey surface of reflection coefficient κ (0≤κ≤1). The spherical harmonics method and Marshak-type boundary conditions are used to find expressions for n(K)(x), the particle density in the so-called PK approximation (K=1,3,5). The results given by the P5 approximation accord very well with recent numerical computations [R. G. Cole, T. Keyes, and V. Protopopescu, J. Chem. Phys. 81, 2771 (1984)]. The desideratum in this context, an accurate expression for the particle density, is produced by grafting the desirable features of n(5)(x) on an almost exact expression derived, for the special case of a black surface (κ=0), by J. LeCaine [Phys. Rev. 72, 564 (1947)].
The butterfly geometry: A Jahn–Teller view
Authors: Ceulemans Arnout
Published: July 2002

The butterfly geometry of 62-electron tetranuclear metal clusters is explained as a special solution of the (T1+T2)×(e+t2) Jahn–Teller equation in a tetrahedral cluster. General expressions for the minimal roots of this equation are derived and represented on an O’Brien sphere. The results are related to the epikernel principle and a parallel is drawn with the coordination chemistry of the isosymmetric Fe(CO)4 fragment.
Entropy of thin solid layers
Authors: Bruch L. W.
Published: July 2002

The entropies of thin adsorbed solid layers and of three-dimensional solid are compared for a model of harmonic nearest-neighbor adatom coupling and harmonic first layer coupling to the substrate. The competing effects of the constraining substrate coupling and of geometry are illustrated for one-layer to four-layer films. The entropy difference at layer condensation relative to the three-dimensional solid is calculated: the values for monolayer condensation are at the lower end of the range of values derived from experiment; the values at further layer condensations are much smaller than many experimental values.
Infrared–microwave double resonance spectroscopy of the ν3 band of BF3 using a tunable diode laser
Authors: Yamamoto Satoshi , Kuchitsu Kozo , Nakanaga Taisuke , Takeo Harutoshi , Matsumura Chi , Takami Michio
Published: July 2002

Pure rotational transitions allowed by vibrationally induced dipole moment are observed in the ν3 state (E′) of 10BF3 by infrared–microwave double resonance using a tunable diode laser. The observed rotational transitions are analyzed simultaneously with the infrared vibration–rotation transitions and pure rotational transitions in the ground state to obtain the molecular constants for the ground and ν3 states. The C0 and DK constants are determined to be 0.172 165 83(8) and 0.3526(14)×10−6 cm−1, respectively. Rotational transitions in the ν3 state of 11BF3 reported previously are also analyzed with infrared vibration–rotation transitions including the Fermi resonance with 3ν4.
Experimental determination of the moments of the generalized oscillator strength distribution of He
Authors: Ketkar S. N. , Bonham R. A.
Published: July 2002

We report here the experimental determination of the moments, S(n,K), of the generalized oscillator strength (GOS) distribution of Helium for n=−5,−4,−3,−2,−1,1,2. Our results for n=1,2 agree with the recent calculation of Sharma and Thakkar. Our results for n=−5, −4, −3, −2, and −1 agree, in the optical limit, to the earlier reported values of the moments of the optical oscillator strength distribution. We also discuss the problems encountered in using the Bethe sum rule for normalizing spectra taken at a fixed angle over a finite energy loss range.
On the question of metallic and superconducting poly(carbon diselenide)
Authors: Iqbal Z. , Khanna Y. , Murthy N. S. , Szobota J. S. , Maleysson C. , Baughman R. H. , Okamoto Y. , Choi L. S.
Published: July 2002

Polymerization of carbon diselenide, CSe2, at 5 kbar and ∼100 °C has been reported to give a metallic (CSe2)n ladder polymer that superconducts near 6 K at 220 kbar pressure. Using a variety of techniques we find that the material synthesized (showing essentially the same x-ray diffraction pattern as previously published) is not (CSe2)n but instead consists of a mixture of free, trigonal Se and an amorphous C–Se polymeric composition consistent with the formula (CSe0.5)n. Interestingly, possibly as a consequence of staged reactions during synthesis, the Se phase consists of a mixture of large, aggregated crystallites (∼600 Å in diameter) melting at ∼220 °C and nonaggregated small crystallites (∼150–250 Å in diameter) showing a broad melting transition with an endothermic maximum at ∼180 °C. Percolation of the C–Se polymeric composition probably provides the high observed electrical conductivity [σ(300 K)≊10–20 S cm−1 and σ(300 K)/σ (8 K)≊2.5] in the presently available samples. The previously reported high pressure superconductivity is probably associated with the free Se phase in the samples, since selenium is a known superconductor in the 6 K and 220 kbar range.
A theoretical analysis of the vibrational spectrum of carbon monoxide on platinum metal electrodes
Authors: Korzeniewski Carol , Pons Stanley , Schmidt P. P. , Severson M. W.
Published: July 2002

The vibrational frequencies of carbon monoxide adsorbed on a platinum metal electrode are obtained in terms of discrete interatomic interactions. We show that for linear changes in the binding energy of carbon and oxygen to the surface there is a linear dependence of the shift in CO vibrational frequency. Based on our model, we suggest that there is a direct, linear relationship between the binding energy and the applied electrostatic potential. As a result, we predict a linear dependence of the frequency on electrochemical potential in accord with experiment. Further, we calculate a Stark tuning rate of 9.0×10−9 cm−1/(V/m). We also demonstrate that the application of a relatively large external electric field (of the order of 3×109 V/m) is necessary to account for the observed vibrational frequency shift. Because the surface charge of the electrode must change in order to give rise to changes in the electric field, we conclude that a combination of chemical bonding and electric field interactions are required to account for the observed spectral shifts.
Classical limits to the sudden approximation
Authors: Cross R. J.
Published: July 2002

We have examined the classical limits to the orbital and energy sudden approximations. It is shown that at large orbital and rotational quantum numbers the transformations which diagonalize the coupling matrix in the sudden limit also diagonalize the coupling matrix in the classical limit. The eigenvalues are no longer a delta function fixed in position during the collision but become a narrow wave packet moving with the classical velocity. The result is a uniform approximation valid in both the sudden and classical limits. A key feature of the theory is the use of the discrete-variable representation which allows an accurate counting of quantum states in the sudden representation. The theory should improve the accuracy of the sudden approximation while requiring little additional computer time.
Polarized Raman spectra of thin films. I. The case of oblique incidence for uniaxially oriented samples
Authors: Harrand Monique
Published: July 2002

In the case of oblique incidence (when the incident and the scattered beams are not perpendicular) it is shown that if the electric vector is parallel to the scattering plane the Raman spectra have a ‘‘complex’’ polarization—the Raman intensities are obtained after adding the scattered amplitudes and squaring their sum. However, if the samples are uniaxially oriented around a Z axis, simplification occurs, due to the average over the φ rotation angle (and if necessary over ψ, which is the rotation around the z axis of the molecule) and now it is the intensities which can be added.
Infrared laser dissociation of CH3NH2 in a molecular beam: Initial translational and internal energy distributions, and relaxation of NH2
Authors: Xiang Tian-Xiang , Guillory William A.
Published: July 2002

The IRMPD of methylamine to generate NH2 radicals has been studied in a molecular beam. The initial relative translational energy distribution was detected by means of an optical time-of-flight spectrum. An exponential function gives the best fit to the data with the average translational energy 〈ET〉 ≂0.22 kcal/mol. The photolysis mechanism is determined to be direct C–N bond rupture. The initial rotational distribution was extensively detected in the beam by means of laser induced fluorescence. The nuclear spin components as well as rotational degrees of freedom were not equilibrated in the nascent NH2 radicals. The states with symmetric nuclear spin components were underpopulated by a factor of 2.3 compared to the antisymmetric component. The subsequent relaxation of the rotational distribution was observed and characterized by a cooling effectiveness parameter σr. The relaxation between different nuclear spin components was found to be extremely slow.