bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-photon excitation of the D 1Δ–X 1Σ+ transition in carbon monoxide
Authors: Kittrell Carter , Cameron Stewart , Butler Laurie , Field R. W. , Barrow R. F.
Published: February 2003

A frequency doubled pulsed dye laser is used to promote two-photon excitation of the D1Δ−X1Σ+ transition in carbon monoxide. Collisional transfer to A1Π leads to ultraviolet fluorescence which is detected with a solar blind photomultiplier. Rotationally resolved structure belonging to bands of three isotopically substituted molecules has been measured relative to the visible spectrum of iodine. Preliminary values of molecular constants are given.
Rydberg states of 7Li2 by pulsed optical–optical double resonance spectroscopy: Molecular constants of 7Li2+
Authors: Bernheim R. A. , Gold L. P. , Tipton T.
Published: February 2003

Three Rydberg series of electronic states of 7Li2 have been characterized by pulsed optical–optical double resonance spectroscopy. The observed Rydberg states, which include the previously reported E 1Σ+g and G 1Πg states, have been identified as 3–10sσ 1Σ+g, 3–10dσ 1Σ+g, and 3–15dπ 1Πg. The molecular constants for several of the upper members of each of the above series have been used to deduce the ionization potential of 7Li2 and molecular constants for the X 2Σg+ state of 7Li2+. The former was determined to be T0(∞)=41 496±4 cm−1. The latter were found to be in good agreement with recent ab initio calculations.
Synthesis and magnetic properties of (Mn1−xTax)3B4
Authors: Ishii T. , Shimada M. , Koizumi M.
Published: February 2003

Solid solution of (Mn1−xTax)3B4 (0≤x≤1) with Ta3B4-type structures were synthesized under high pressure–temperature conditions, and their crystallographic and magnetic properties were examined. From the results of magnetic measurments, the solid solutions were found to be ferromagnetic in the range of 0.1≤x≤0.4 and its Curie temperature changed from 530 to 780 K. The Curie temperature of (Mn1−xMx)3B4 (M=Cr, Mo, Ta) changes linearly with unit cell volume.
Energy and momentum accommodation coefficients on platinum and silver
Authors: Shields F. Douglas
Published: February 2003

Acoustical measurements of energy (EAC) and tangential momentum accommodation coefficients (TMAC) indicate that stable and reproducible Pt and Ag surfaces are obtained by heating in a vacuum. EAC and TMAC values are given for He and Ne on the heat-cleaned Pt and Ag surfaces and for these gases on Pt and Ag surfaces after exposure to various adsorbent gases. AC values are also given for the various adsorbent gases on themselves on the heat-cleaned Pt and Ag surfaces.
Reorientation of small molecules and anions in solution studied by resonance enhanced dynamic Rayleigh scattering
Authors: Stanton Susan G. , Pecora R. , Hudson Bruce S.
Published: February 2003

Resonance enhanced dynamic depolarized Rayleigh scattering is used to measure reorientation times of two nitrophenolate anions in millimolar aqueous and ethanolic solutions. Reorientation times of the corresponding neutral molecules in 0.01 to 0.1 M solutions are also reported. The reorientation time of 3,4-dinitrophenolate anion has a linear viscosity dependence when the viscosity is varied by varying temperature. The ratio of reorientation time to viscosity C is the same for neutral 3,4-dinitrophenol and its anion in water to within experimental error. In ethanol the C values for both neutral molecule and anion are much larger than the corresponding water values and the anion C value depends upon counterion. This final observation suggests ion pairing occurs in these solutions. The spectra of picrate salts in ethanol clearly show the presence of a second species (ion pair). The picrate anion C value in ethanol determined from the broad component of a two Lorentizain fit is slightly larger than the corresponding water value. The results from this work are compared with reorientation times of small anions reported in the literature, most of which were obtained using picosecond absorption and fluorescence techniques. Ion pairing has not been properly accounted for in many of these experiments, preventing futher conclusions concerning solute–solvent interactions. Some suggestions for futher experiments are given.
On the interaction operator in the optical spectroscopies
Authors: Lee Duckhwan , Albrecht A. C.
Published: February 2003

The appropriate form for the interaction operator used to formulate electric-dipole spectroscopies of any order is discussed within the gauge-invariant formulation previously developed by K-H. Yang [Ann. Phys. (NY) 101, 62 (1976)]. Both ‘‘active’’ (absorptive and emissive) and ‘‘passive’’ (coherent wave-mixing) spectroscopies are examined. Though formally it is the ‘‘position’’ operator which is the correct one when describing radiation-matter interaction in terms of the ‘‘dark’’ eigenstates of material, it is found that under certain limiting constraints the ‘‘momentum’’ operator leads to equivalent results. The demonstration proceeds without recourse to the commutation relation normally used to relate the position and momentum expressions for the spectroscopic parameters, and it applies to both active and passive spectroscopies at any order. However, for any of the multiply resonant (active or passive) spectroscopies, or for detailed line shape considerations in the active spectroscopies, damping effects must be introduced through the density matrix formalism. It appears that as long as damping is important, there are no circumstances under which the momentum operator expressions can match the correct, gauge-invariant, position operator expressions for coupling the dark eigenstates. Finally, if the Hamiltonian defining the dark eigenstates is an approximate one, containing a nonlocal potential (such as a Hartree–Fock Hamiltonian), the momentum form of the interaction operator must be modified to include additional (well-defined) terms before the equivalency with the position operator is established.
Spin-polarized atomic nitrogen and the 7Σ+u state of N2
Authors: Ferrante Robert F. , Stwalley William C.
Published: February 2003

We present the first self-consistent field (SCF) calculation of the repulsive 7Σ+u state of molecular nitrogen. This calculation is used with attractive dispersion to provide a model potential for electron spin-polarized atomic nitrogen N↑ appropriate in the range 2.5≤R≤20a0. Potential parameters obtained from the calculation are used to provide estimates of equilbrium thermodynamic properties of N↑ in the quantum theorem of corresponding states framework. Consideration is given to increased stability of N↑ with respect to single electron spin-flip processes, as compared to H↑.
Nonlinear programming approach to locally constrained variational calculations
Authors: King Frederick W. , Dalke Brian D.
Published: February 2003

The standard variational problem is modified so that the local behavior of the wave function is indirectly incorporated into the optimum energy calculation. The minimum energy is determined subject to certain inequality constraints, formulated so that the reduced local energy is bounded above and below by a certain error, at different points in configuration space. This modified formulation of standard variational theory constitutes a problem in nonlinear programming. The gradient projection method has been employed to solve this problem. Calculations are carried out using the Hartree–Fock formalism for the ground state of the helium atom.
Theoretical studies of surface diffusion: Self-diffusion in the fcc(100) system
Authors: McDowell H. K. , Doll J. D.
Published: February 2003

Surface self-diffusion constants for the fcc(100) Lennard–Jonesium system were calculated using classical molecular dynamics methods. Self-diffusion Arrhenius parameters calculated using pair potentials derived from bulk thermodynamic data were found to be in good agreement with reported Rh(100) experimental values.
Generalized Langevin dynamics simulations with arbitrary time-dependent memory kernels
Authors: Berkowitz M. , Morgan J. D. , McCammon J. Andrew
Published: February 2003

An algorithm is described that allows dynamical simulations to be performed based on generalized Langevin equations with arbitrary, time-dependent memory kernels. Test simulations show that good results are obtained for kernels with distinctly different forms (e.g., exponential and Gaussian).
Microstructure of two-phase random media. II. The Mayer–Montroll and Kirkwood–Salsburg hierarchies
Authors: Torquato S. , Stell G.
Published: February 2003

It is shown that the Mayer–Montroll (MM) and Kirkwood–Salsburg (KS) hierarchies of equilibrium statistical mechanics for a binary mixture under certain limits become equations for the n-point matrix probability functions Sn associated with two-phase random media. The MM representation proves to be identical to the Sn expression derived by us in a previous paper, whereas the KS representation is different and new. These results are shown to illuminate our understanding of the Sn from both a physical and quantitative point of view. In particular rigorous upper and lower bounds on the Sn are obtained for a two-phase medium formed so as to be in a state of thermal equilibrium. For such a medium consisting of impenetrable-sphere inclusions in a matrix, a new exact expression is also given for Sn in terms of a two-body probability distribution function ρ2 as well as new expressions for S3 in terms of ρ2 and ρ3, a three-body distribution function. Physical insight into the nature of these results is given by extending some geometrical arguments originally put forth by Boltzmann.
Photochemical formation and dissociation of molybdenum–dinitrogen complexes in krypton matricesa)
Authors: Foosnaes T. , Pellin M. J. , Gruen D. M.
Published: February 2003

Three molybdenum–dinitrogen complexes [MoN2,Mox(N2) and Mo(N2)3] have been synthesized in Kr matrices and characterized by means of the effects of isotopic substitution on the dinitrogen stretching frequencies. The Mo–N force constants calculated from these spectra are found to be 17.21±0.02 mdyn/Å for MoN2 and fr=18.72 mdyn/Å and frr=0.36 mdyn/Å for Mo(N2)3. The Mo complexes appear to be unique in that rapid formation during co-deposition with N2 occurs only upon photoexcitation of the Mo 7S–7P atomic resonance transition situated at 355 nm in Kr matrices. Although minor amounts of the dinitrogen complexes are formed during deposition in the absence of radiation, their concentration is not increased by thermal annealing. Each of the complexes gives rise to its own electronic absorption spectrum. Irradiation into the electronic absorption bands of individual complexes induced selective photodissociation. Photoinduced growth and decomposition studies allowed correlations to be made between the vibrational and corresponding electronic spectra.
Production of negative ions by dissociative electron attachment to SO2
Authors: Orient O. J. , Srivastava S. K.
Published: February 2003

Dissociative electron attachment cross section measurements for the production of O−, S−, and SO− have been performed utilizing a crossed target SO2 molecule beam–electron beam geometry. The relative flow technique is employed to determine the absolute values of cross sections. The attachment energies corresponding to various cross section maxima are: 4.30 and 7.1 eV for O−/SO2; 4.0, 7.5, and 8.9 eV for S−/SO2; and 4.7 and 7.5 eV for SO−/SO2.
Intersystem collisional transfer of excitation in low altitude aurora
Authors: Benesch William
Published: February 2003

The characteristics of auroral optical emission as a function of altitude are exploited as a source of data for the investigation of the intersystem collisional transfer (ICT) of excitation in the nitrogen molecule. The procedure is based on a recently proposed model for the generation of the red lower border of type B auroras as resulting from the effect of the increased collision frequency at lower altitudes on the distribution of population within the excited molecules. In particular, the 85 km turn-on altitude for the red lower border of auroral arcs provides an indicator for the determination of the density and pressure required for the onset of the ICT process at mesopause temperatures. Values are obtained for collision cross sections and rate constants for the coupling of neighboring vibrational levels in adjacent electronic states in nitrogen. The results are compared with the findings of several laboratory studies on similar collisional transfers among nested electronic states. A two-part process is suggested to account for the observed features of intersystem collisional transfer phenomena.
Field induced and surface catalyzed formation of novel ions : A pulsed-laser time-of-flight atom-probe study
Authors: Tsong T. T. , Kinkus T. J. , Ai C. F.
Published: February 2003

High electric field induced formation of novel ions such as H+3, AuH+2, RhHe2+, and PtHe2+2 on metal surfaces has been studied in the pulsed-laser time-of-flight atom-probe field ion microscope. From the fractional abundances, and the high resolution mass spectra and energy distributions measured for these ions, several conclusions can be drawn. Field desorption of hydrogen below the evaporation field of the substrate often results in formation of H+3 ions. This formation depends not only on the applied field strength, but surprisingly also on the material and the atomic structure of the substrate. Plenty of H+3 ions can be found from the high index planes and the lattice steps of Mo, W, and Au surfaces in the field range of 2 to 3 V/Å. Few are found from Ir and Ni surfaces, and none are found from the densely packed W(110) plane. Formation of H+3 can therefore be considered a surface catalyzed and field induced chemical reaction. Using H2–D2 mixed gases, we find little correlation between the ionic species obseved in field desorption and the chemisorption states of the gases. The field desorbed ions are formed directly from the field adsorbed state. Field adsorption occurs mostly in the diatomic molecular form. On highly protruded atomic sites of some materials, field adsorption occurs also partly in the triatomic molecular form. Field evaporation of metals in hydrogen or helium often results in the formation of metal hydride and metal helide ions. These ions are formed right at the metal surface by a polarization binding. A fraction of them dissociates in the high field region near the surface. The ‘‘dissociation zone’’ is found to be only several Å in width. From this width, the average lifetime of these complex ions in a field of 3 to 4.4 V/Å is estimated to be on the order of 5×10−13 s.
The phases of insoluble monolayers: Comparison between the surface pressure-molecular area (Π-A) diagram and shear modulus measurements
Authors: Miyano K. , Abraham B. M. , Ketterson J. B. , Xu S. Q.
Published: February 2003

Simultaneous measurements of the surface pressure-molecular area (Π-A) diagrams and the shear modulus of a series of long chain aliphatic alcohols C18OH, C20OH, C21OH, and C22OH spread on water indicate that the conventional phase assignments based on the compressibility are not reliable.
Scattering activities and symmetry of skeletal stretching vibrations of long chain molecules by Raman spectroscopy
Authors: Harrand M.
Published: February 2003

Calculation of Raman scattering activities for skeletal stretching vibrations of hydrocarbon chains has been carried out by means of a simple process. Consequently, the anomaly observed with DPPC (and polyethylene or other phospholipids) has been explained, i.e., the νs(C–C) and νa(C–C) lines are respectively stronger in the ZZ and ZX polarizations (Z average axis of sample), contrarily to what is expected when the motions of the carbon atoms are considered.
Entropy of vaporization at the boiling point
Authors: Shinoda Kōzō
Published: February 2003

The entropy of vaporization of liquid (whose cohesive energy is totally due to London force) at the boiling point ΔSvb is not constant as required by Trouton’s rule, but increases with the logarithm of vapor volume, i.e., at constant pressure, with logarithm of temperature. A relation ΔSvb≊const +R ln TbK−1, i.e., ΔSvv −R ln TbK−1≊const (38 J K−1 mol−1) holds. Namely, the entropy of vaporization at the boiling point to the same volume is constant which is essentially equivalent to the Hildebrand rule.
Comments on ‘‘Scaling theory and enthalpy of mixing for binary fluids’’
Authors: Morrison Graham
Published: February 2003

The stability criterion from the second law of thermodynamics is emplasized for the enthalpy of mixing of a binary mixture near a critical solution temperature. Brinke and Karasz1 results are discussed regarding a scaling model. (AIP)
The electronic structure of DNA related periodic polymers
Authors: Otto P. , Clementi E. , Ladik J.
Published: February 2003

Results of ab initio LCAO Hartree–Fock crystal orbital calculations using a minimal atomic basis set are reported for the single stranded periodic B-DNA models of cytosine, thymine, adenine, and guanine stacks and two polynucleotides with adenosine and thymidine as repeating unit, respectively. Further, the energy band structures of two poly(base pairs), poly(adenine–thymine), and poly(guanine–cytosine), representing a simple model of the B-DNA double helix are discussed.
Topological elements of the magnetically induced orbital current densities
Authors: Gomes J. A. N. F.
Published: February 2003

A detailed topological analysis is given, both for the standard orbital current density and for the complete orbital current density. The concepts developed here provide a very useful instrument for the description of the vector field of the magnetically induced currents and should be very helpful for the understanding of the contributions that different regions of a molecule give to its magnetic properties. The ingredients for this topological analysis are (1) the symmetry properties of the electronic Hamiltonian in the presence of the magnetic field and (2) the nature of certain singular points. Two categories of singular points are considered: (i) Nodal vortices which have associated a circulation around a nodal line and may be of the axial or toroidal types as previously considered by Hirschfelder; (ii) Stagnation points where the current vanishes without the vanishing of the charge density. There is a considerable variety of stagnation points and, under certain conditions basically on the magnetic field, they may have associated a vortical circulation of currents in their immediate neighborhood. For the complete orbital current (defined as the sum of the standard orbital current and the orbital exchange current), the stagnation points may occur isolated or they may form stagnation lines. When the regime of the circulation around this line changes between vortical and normal, a new type of stagnation point, the transition point, is found.
An ab initio study of the reaction Be(3P) +H2(1Σg+) → BeH(2Σ+)+H(2S)
Authors: Poirier Raymond A. , Peterson Michael R. , Menzinger Michael
Published: February 2003

The lowest triplet potential energy surface for the reaction of metastable Be(3P) with H2 has been studied by ab initio SCF–MO and CI calculations. The stationary points along the reaction coordinate were obtained by automatic gradient optimization. Broadside (C2v) attack is found to be favored over the collinear C∞v approach and a stable intermediate of C2v symmetry is found for 3BeH2. We discuss experimental implications for the dynamics of this interesting exothermic process.
Smectic behavior and molecular structure: On the mesomorphism of a series of hydrocarbons
Authors: de Jeu W. H. , Eidenschink R.
Published: February 2003

The polymorphism is discussed of a series of hydrocarbons that show, besides a smectic-A phase, a strong tendency towards the formation of smectic-B. The powder x-ray diffraction patterns show at large Bragg angles an unusual double reflection with a very small separation, which indicates a 3D hexagonal closed packing. It is shown that, quite generally, nonpolar molecules form orthogonal smectic phases. The implications are discussed for the relationship between molecular structure and smectic behavior.
Proton Zeeman and dipolar relaxation study of the ferroelectric liquid crystal DOBAMBC
Authors: Dong Ronald Y. , Sandeman J.
Published: February 2003

The proton Zeeman spin-lattice relaxation time T1 and the proton dipolar spin-lattice relaxation time T1D of a chiral compound p-decyloxybenzylidene p′-amino-2-methyl butyl cinnamate are studied as a function of temperature and sample rotation in a magnetic field at 30 MHz. In addition, proton T1 is measured in the smetic A, ferroelectric smectic C* and H* phase at 15.1 and 45.5 MHz. The spin relaxation mechanisms for the proton T1 and T1D in the smectic A and C* phase are ‘‘diffusion-induced’’ orientational order director fluctuations and molecular self-diffusion. In contrast with the achiral compound TBBA, the T1 angular dependence in the smectic C* and C phase are different, while the T1D angular dependence appears to be similar in these smectic phases.
Transitions and molecular packing in highly purified 1,2-dipalmitoyl- phosphatidylcholine–water phases. I. Transitions, improved phase diagrams, method of packing analysis, molecular structures of diglyceride, and polar regions
Authors: Albon Norman
Published: February 2003

Results of extensive studies of phases prepared from highly purified 1,2-dipalmitoyl-sn-phosphatidylcholine and 1,2-dipalmitoyl-sn -glycerol are presented. The methods used included x-ray diffraction and thermal analysis and both temperature and water content were varied. Details of the many well-defined phases obtained, all with bilayer structures, are given in tables. An improved phase diagram was obtained from thermal data which showed well-defined transitions, and from discontinuities in a plot of bilayer repeat spacings with water content. For analysis of molecular packing the bilayer was divided into diglyceride and phosphorylcholine regions. The properties of the diglyceride regions of both compounds are discussed. Estimates of the size and shape of the polar groups are made and the general principles involved in their packing and the influence of water are discussed. Among new features reported, are the existence of the 0⊥ close chain packing for the crystalline dipalmitoyl glycerol but not in any lecithin phase. Three single crystal lecithin phases with different water contents were prepared which are more stable than the hexagonal chain phases usually reported in the same regions. These hexagonal phases slowly recrystallize to more stable forms, but are usually obtained on cooling melts or higher temperature phases. However, the single crystal phases only exist over restricted composition ranges while phases with hexagonal, square, and disordered chain packing can accommodate a variation in water content by a tilt of the chain axes and changes in bilayer thickness. Transition widths vary and are extremely sensitive to impurities.
The rate of absorption of Brownian particles by a sphere
Authors: Harris S.
Published: February 2003

We consider the solution of the Fokker–Planck equation for the case where a uniform dilute system of Brownian particles filling the volume r≳R is absorbed at the boundary r=R for times t≳0. An approximate solution is obtained for this time-dependent problem by using the lowest order terms in a half-range moment expansion of the Brownian particle distribution function. The solution obtained is used to examine the validity of Fick’s law and to find the rate coefficient for the absorption process. This latter result is compared with the result found using the diffusion equation solution together with Fick’s law. We find that the rate coefficient can be substantially smaller than the result predicted by the diffusion equation; this conclusion is in general agreement with earlier results for a related steady-state problem.
Difference frequency laser spectroscopy of the ν1 band of the HO2 radical
Authors: Yamada Chikashi , Endo Yasuki , Hirota Eizi
Published: February 2003

The ν1 (O–H stretching) band of the HO2 radical was observed in absorption by using a difference frequency output obtained from an Ar ion laser and a cw dye laser as a source, combined with a multiple-reflection discharge cell. Zeeman modulation was employed to improve the sensitivity. The HO2 radical was generated directly in the cell by a dc discharge in a mixture of allyl alcohol and oxygen. About 280 lines were observed and analyzed to precisely determine the rotational constants, centrifugal distortion constants, and spin-rotation interaction constants in the ν1=1 state. The band origin was determined to be 3436.1951(4) cm−1, which is 22 cm−1 higher than the value previously obtained by a matrix isolation study.
Fluctuations of cross sections in chemical processes: Infrared photoquenching of fluorescent states in thiophosgene
Authors: Grover J. R. , Brenner D. M.
Published: February 2003

Pronounced fluctuations have been observed in the cross section for the quenching of the fluorescence of the different members of the vibrational manifold belonging to the Ã 1A2 state of thiophosgene. This phenomenon is reminiscent of fluctuations that occur for nuclear reactions in regions of high level density, and that can be interpreted in terms of reduced transition amplitudes that are approximately normally distributed. The analysis of the fluctuations gives an effective number of independent reaction channels; in general, the greater the number of channels, the narrower is the range of the fluctuations. A knowledge of the number of independently contributing channels can be useful in the interpretation of experimental results, and sometimes lends important insight into reaction processes. The quenching cross sections for thiophosgene were therefore examined to see whether an analysis of their fluctuations would be useful. It was found necessary to develop a form of analysis that circumvents a serious obstacle in the chemical data, namely, the existence of significant correlations among the cross sections. The resulting formalism is quite simple to apply however, and, after suitable generalization, should be widely applicable to chemical experiments in which resolved cross sections that exhibit fluctuations are measured. Application of this formalism to the thiophosgene quenching cross sections shows that two hypotheses of the quenching mechanism must be rejected, but provides no reason to reject a third, that quenching is via absorption by states in a ‘‘bottleneck’’ region that is reached by rapid photon transitions from the fluorescent target states, which transitions cannot be statistically distributed. The analysis also reveals the existence of an important, experimentally determined constraint that must still be accounted for by any successful hypothesis, viz. the number of channels increases approximately as the square root of the peak power of the quenching infrared radiation.
Auger spectra of tetrahedral halides and hydridesa)
Authors: Rye R. R. , Houston J. E.
Published: February 2003

Auger electron spectra are presented for the gas-phase molecular species CF4, SiF4, CCl4, and SiCl4 and discussed in terms of the effect of the interaction of the two final-state holes on the spectral line shapes. The highly polar character of the bonding for this series leads to systematic behavior with respect to this hole–hole interaction, showing rare gas-like localized halide spectra for SiF4 and a delocalized component of increasing intensity as we proceed through the series to SiCl4, CF4, and CCl4. The central atom spectra show a composite line shape resulting from the sum of two versions of the same one-electron final states but with different values of U (the hole–hole interaction energy). The Auger process occurs as a result of the electron density available at the central atom site but, since the one-electron orbitals are highly polarized towards the halide, the two holes principally appear around the halide sites. With both holes on the same halide site a rather large U results, but with the two holes on separate halide sites one obtains a reduced value of U. The relative intensity of the smaller U component appears to increase as we proceed through the series SiF4, SiCl4, CF4, and CCl4. This tendency in both the halide spectra and in the central atom spectra depends on the strength of the intersite interaction. The molecular parameter which seems most reasonable to predict this division between localized and delocalized behavior is the ratio of the halide–halide distance to the halide radius. In addition, we demonstrate that for a rather extended series of molecular species the trends in the value of U can be predicted surprisingly well on the basis of an experimental value of U for Ne and the known atomic and molecular dimensions. The implications of this simple predictive capability to other molecular systems is discussed.
Laser induced fluorescence excitation spectrum of jet-cooled benzoic acid dimers
Authors: Poeltl David E. , McVey Jeffrey K.
Published: February 2003

Intermolecular hydrogen bonding in gas phase dimers of benzoic acid is investigated following preparation in a pulsed free-jet expansion. Laser induced fluorescence excitation spectra are analyzed to provide information regarding the vibrational assignments of the 1Lb excited state. In agreement with studies in low temperature mixed crystals, the electronic excitation is essentially localized on one monomer unit in the dimer. A 58 cm−1 band is associated with the ν3 hydrogen bonding stretch+bend mode and is prominent in combination bands involving internal motion in the carboxyl groups. The free jet technique has been shown to be particularly advantageous in studies of intermolecular hydrogen bonding.
Site-selection spectroscopy of Eu3+-doped germinate glass
Authors: Gang Xu , Boulon Georges , Powell Richard C.
Published: February 2003

A tunable dye laser was used to selectively excite Eu3+ ions in nonequivalent types of crystal field sites in a germinate glass host. The results demonstrate the existence of three major types of sites in this host. In addition, time-resolved spectroscopy results show the presence of energy transfer between ions in these different types of sites.
Semiclassical determination of adiabatic barriers on a three-dimensional potential energy surface
Authors: Pollak Eli , Wyatt Robert E.
Published: February 2003

A recently proposed method, based on periodic orbits, for finding vibrationally adiabatic barriers and wells in collinear collisions is generalized to the full three-dimensional case. The main idea is a consistent use of the adiabatic approximation—one first solves for the fast vibrational motion to obtain an effective Hamiltonian for the slower bend motion which in turn is solved to obtain an effective Hamiltonian for the overall rotation. The method is applied to the hydrogen exchange reaction. We find the bend-vibration adiabatic barrier levels for the H2(v=1) state. The zero point motion in the bend degree of freedom is found to be substantial (0.1 eV) and is a source for nonnegligible discrepancies between approximate theories such as the infinite order sudden and quasiclassical trajectory approach and exact quantal scattering computations. Having found the barrier levels we are able to evaluate the collision cross section. Our analysis points out that differences between experimental cross sections and theoretical predictions may be due to inaccuracy in the potential energy surfaces. The available surfaces probably overestimate the adiabatic barrier height.
Quantum theory of vibrational, rotational, and translational energy exchange in collisions of polyatomic molecules. Application to methyl halides
Authors: Miklavc Adolf
Published: February 2003

We derive a quantum theory of inter- and intramolecular transfer of vibrational, rotational, and translational energy in collisions of polyatomic molecules, in the case that short range forces between the collision partners are of prevailing importance. For this purpose, we first transform the Hamiltonian into the form which enables us to eliminate the coordinates which are cyclic in the limit of zero range forces and can thus be expected to be approximately ignorable when forces are of short, but finite, range. Instead of six rotational and three translational coordinates, we are then left with one single relevant generalized coordinate. The corresponding mass can easily be calculated; it depends on the usual reduced mass of the colliding molecules, on their moments of inertia, and on the geometry of the potential surface. The motion of the only relevant generalized coordinate is then quantized. To calculate the probabilities for the collision induced vibrational-rotational transitions, one can make use of any of a large number of the approximation schemes which have been developed in scattering theory. In the present work, we use the distorted wave expansion, for its simplicity and also because it leads, in the corresponding limit, directly to the results obtained recently by the semiclassical theory. As an application, we study vibrational deactivation probabilities of methyl halides colliding with inert gases: He, Ne, Ar, Kr, and Xe. Calculations are performed for two intermolecular potentials: exponential repulsive and Morse potential. The results obtained by the two potentials are very similar and agree well with the measured vibrational transition probabilities.
Photodissociation of NH3 at 106–200 nm
Authors: Suto Masako , Lee L. C.
Published: February 2003

Cross sections and quantum yields of emissions from the photofragments of NH3 were measured in the 106–200 nm region using synchrotron radiation as a light source. The emission systems of NH2 (Ã 2A1→X̃  2B1), NH(c 1Π→a 1Δ, b 1Σ+), and NH(b 1Σ+→X 3Σ−) were observed from the NH3 photodissociation. The dependences of emission intensities on the NH3 and/or He (buffer gas) pressures were studied. The NH(c→a,b) and NH(b→X) emissions were observed when the NH2(Ã2A1) species was quenched by 10 Torr of He. The NH(b→X) and NH(c→a) emissions start to appear at 182.3±1.5 and 127.9±0.9 nm, respectively. From these emission thresholds the threshold energy for photodissociation of NH3 into NH(X 3Σ−)+H2(X 1Σ+) is determined to be 4.17±0.06 eV, and the upper limit for the heat of formation of NH is 3.81±0.06 eV. For excitation wavelengths longer than 128 nm the NH(b 1Σ+) species is produced by a primary photodissociation process, and at shorter excitation wavelengths the NH(b 1Σ+) may be partly produced by a cascading process following the NH(c→b) transition. Contrary to previous theoretical interpretations, all the vibronic levels of the NH3 (B̃ and C̃) states produce the NH2(Ã→X̃) emission. The quantum yield for the C̃ state is lower than that of the B̃ state. The vibronic levels of the C̃ state are renumbered in accordance with their quantum yields.
The hydrogen chloride dimer energy from the second virial coefficient
Authors: Powles J. G. , Wojcik M.
Published: February 2003

The enthalpy of dimer formation for HCl is evaluated from the recent accurate data for the pressure send virial coefficient for Hcl over a wide temperature range. (AIP)
Simultaneous TOF spectra of sputtered and scattered neutrals and ions from 3 keV Ne+, Ar+, and Ar2+ bombardment of CsBr
Authors: Rabalais J. Wayne , Schultz J. Albert , Kumar Ranjit , Murray P. Terrence
Published: February 2003

A spectrometer system for simultaneous mass and velocity analysis of scattered and sputtered ions and velocity analysis of fast neutrals ejected from a surface by bombardment with a pulsed ion beam is described. Combination secondary ion and scattered neutral plus ion TOF spectra and their corresponding energy spectra for 3 keV Ar+ and Ar2+ bombardment of CsBr are presented. Selective sputtering of Br down to a steady state level is observed with Ne+ bombardment with an estimated depletion cross section of 10 Å2. Scattering spectra of neutrals plus ions and neutrals alone allow determination of scattered Ar+ ion survival fractions of 0.22 and 0.13 from 3 keV Ar+ collisions with Cs and Br, respectively. This indicates preferential neutralization during scattering from Br. Ion fractions for Ar2+ and Ar+ are determined for Ar2+ scattering, illustrating the stepwise neutralization process for multiply charged ions. With 3 keV Ar2+ bombardment the fractions surviving as Ar+ for Cs and Br collisions are 0.18 and 0.08, respectively, while the fraction surviving as Ar2+ is ∼0.10 for both atomic collisions. The ion fraction data are interpreted in terms of Auger and resonant neutralization transitions between the ion and specific surface atoms.
Reduction of angular momentum coupling trees and the polarization dependence of nonlinear optical processes
Authors: Minard R. A. , Stedman G. E. , McLellan A. G.
Published: February 2003

Diagrams representing angular momentum coupling trees with vector (j=1) external labels in a Cartesian basis and with even degree (up to 8) are expressed as sums of products of Kronecker delta functions. Integrity basis theory is used to verify the possibility of such a representation, and to determine the number of independent invariants. The results are of use in the analysis of irreducible Cartesian tensors and thus, e.g., in nonlinear optics. As examples, the polarization dependences of the hyper-Raman and three-photon Raman effects are discussed.
Influence of ionic strength on the diffusion of polystyrene latex spheres, bovine serum albumin, and polynucleosomes
Authors: Schmitz Kenneth S. , Lu Mei , Gauntt Jennifer
Published: February 2003

Quasielastic light scattering methods were used to determine apparent diffusion coefficients (Dapp) for polystyrene latex spheres, bovine serum albumin, and polynucleosomes under conditions of neutral pH and low ionic strength (0.1–50 mM). Data were collected at several time intervals (Δt) at each of several scattering angles (θ) in the range 35<θ<120 and analyzed by the asymptotic analysis method (Dapp vs T, where T=NΔt is the time window for the N+1 point autocorrelation function). These data suggest that the asymptotic limit Dapp(T → 0) is independent of θ for the three systems examined, whereas the asymptotic limit Dapp(T → ∞) appears to be proportional to the reciprocal of the total light scattering intensity. These results are interpreted in terms of ‘‘free’’ and ‘‘temporal aggregate’’ domains within the polyelectrolyte solution. The critical size, hence stability, of the temporal aggregate is proposed to result from a balance between attractive forces arising from fluctuations in the small ion and polyion distributions and the disruptive Brownian forces. Under conditions of extremely low ionic strength, direct interactions between polyions tend to dominate as inferred from data in the literature. These observations suggest at least four regimes in the description of the ionic strength dependence of Dapp.
Noncrystalline structure of argon clusters. I. Polyicosahedral structure of ArN clusters, 20<N<50
Authors: Farges J. , de Feraudy M. F. , Raoult B. , Torchet G.
Published: February 2003

Small argon clusters are produced in a free jet expansion and studied by electron diffraction. Due to their very low proportion in the beam sample in comparison to the monomers, the smallest detectable clusters contain about 20 atoms. When they grow up to some 50 atoms, they present the same noncrystalline structure. This structure is identical to that of solid models constructed during a molecular dynamics calculation by cooling a liquid drop of atoms interacting through a Lennard-Jones potential. Because it is composed of icosahedra of 13 atoms, either joined one to the other either interpenetrating each other, this structure may be called polyicosahedral. The comparison between theoretical and experimental diffraction functions leads to an estimate of the cluster temperature 27±3 K and of the proportion of monomers in the beam. We discuss the similarity in the radial distribution functions of both PIC models and bulk amorphous materials, and give arguments for the stability of the clusters.
Mössbauer studies of after effects of Auger ionization following internal conversion in organo-129mtellurium compounds
Authors: Nath Amar , Sauer Ch. , Halpern A.
Published: February 2003

The after effects of Auger ionization in diphenyl[129mTe]-telluride, dibenzyl[129mTe] telluride, and their dispersions in a solvent at 4.2 K, were compared with the same systems labeled with tellurium in the ground state, viz. 129Te. The Mössbauer emission, in the former case is preceded by Auger event and β decay, while in the latter case, it is preceded only by β decay. Three species were observed in the two compounds, I−, C6H5I or C6H5CH2I, and the third one has tentatively been identified as (C6H5)2I+ or (C6H5CH2)2I+. The formation of I− represents rupture of both bonds, i.e., complete fragmentation of the molecule. The probability of fragmentation was estimated with the help of matrix isolation experiments, as the electronically excited tellurium-129 ion formed by fragmentation of the molecule, following the Auger ionization, may interact with a neighboring organo-tellurium molecule in neat compounds, and enter combination. In about 85% of the events in case of diphenyl telluride and 55% in case of dibenzyl telluride, molecules escape complete fragmentation despite the fact that about 100 eV excitation energy may be deposited during charge neutralization. A novel mode for rapid disposal of energy is discussed.
Theory of electrolyte solution interfaces at finite dilution
Authors: Wilson Michael A. , Nichols Albert L. , Pratt Lawrence R.
Published: February 2003

The theory of dilute electrolyte solution interfaces is extended to treat higher concentration solutions. It is shown that the dominant initial higher concentration correction to the Onsager–Samaras results derive from solving the random phase approximation (RPA) for the salt concentration profiles in a satisfactorily self-consistent manner. Further, for conventional primitive solution surface models appropriate to dilute electrolyte solutions interfaces, it is shown that short-ranged non-Coulombic interionic interactions make a negligible contribution to the salt concentration profiles for bulk concentrations as high as a few tenths molar. Under these conditions the initial correction to the Onsager–Samaras absorption has a negative definite sign, so that the Onsager–Samaras surface tension will be less than the true surface tension for the primitive surface model treated. The theoretical results are compared with available computer simulation data. For the concentration range indicated, the agreement between these results is excellent. Finally, we provide a new analytical treatment of the RPA which is suitable for use at higher concentrations, and reduces to the Onsager–Samaras type results in the limit of infinite dilution. This improved treatment is obtained by analytically evaluating all integrals in the resulting RPA functional for a flexible category of troughlike trial concentration profiles. The choice of trial concentration profile is then determined by an optimization condition that the leading correction to the predicted interfacial absorption must vanish.
Electronic excited state transport and trapping in disordered systems: Picosecond fluorescence mixing, transient grating, and probe pulse experiments
Authors: Miller R. J. Dwayne , Pierre Marc , Fayer M. D.
Published: February 2003

A detailed experimental examination of the dynamics of energy transport and trapping in two component systems, using rhodamine 6G (R6G) as the donor and malachite green (MG) as the trap in both glycerol and ethanol solvents, is presented. The experiments were performed using fluorescence mixing and ground state recovery techniques providing temporal resolution of ∼50 ps. Samples ranging from high trap-low donor concentrations (the Förster limit) to the opposite regime of high donor and low trap concentrations, were studied. These results were compared with no adjustable parameters to the recent theoretical work of Loring, Andersen, and Fayer (LAF). The excellent agreement between theory and experiment over the entire donor-trap concentration range confirms the theoretical results of LAF and yields a comprehensive description of excited-state dynamics in solution. A variety of dynamic properties are calculated using the LAF theory and the measured parameters associated with R6G-MG system.
Mesoscopic kinetic theory of polymer melts
Authors: Grmela Miroslav , Carreau Pierre J.
Published: February 2003

A novel approach to phase space kinetic theory of dense polymer systems and to the study of their equilibrium and nonequilibrium properties is proposed. Phase space kinetic equations are constructed as continuity equations of the time evolution equation of a representative dumbbell subjected to an averaged field force. A thermodynamic equation of state, an expression for the pressure tensor involving the configuration space distribution function and an equation governing the time evolution of the configuration space distribution function are obtained as three consequences of a single phase space kinetic equation.
Theoretical results for aqueous electrolytes. Ion–ion potentials of mean force and the solute-dependent dielectric constant
Authors: Patey G. N. , Carnie Steven L.
Published: February 2003

The linearized hypernetted-chain theory is applied to solutions of hard spherical ions immersed in a dipole-tetrahedral quadrupole solvent with waterlike parameters. Temperatures ranging from 25 to 300 °C are considered and the ion–ion potentials of mean force and the solute-dependent dielectric constant are examined in the infinite dilution limit. For this model electrolyte the ion–ion potentials of mean force are shown to be less structured and to approach the continuum limit more rapidly than those previously obtained for simple dipolar solvents. The limiting slope which determines the behavior of the solute-dependent dielectric constant at low ionic concentrations is negative and much smaller in magnitude than that found for comparable dipolar hard sphere solvents. Finally, the theoretical limiting slope is shown to be consistent with experimental results for aqueous solutions at 25 °C and low ionic concentrations.
On the ring-to-chain ratios of radii of gyration and sedimentation coefficients of polymers of the freely jointed model: Monte Carlo calculations and the ε method
Authors: Chen Yi-der
Published: February 2003

The configurations of ring polymers of the freely jointed model (with excluded-volume effect) are generated on a computer using an efficient ring-generation method recently developed by us. The ring-to-chain ratios of radii of gyration and sedimentation coefficients are calculated using the generated configurations of chains and rings for a number of polymer sizes. The limiting ratios at infinite molecular weight are obtained by extrapolation and compared with those obtained by the ε method of Bloomfield and Zimm, Sharp and Bloomfield, and Yu and Fujita. It is found that all three treatments do not agree with the Monte Carlo results.
Deuterium relaxation of a partially oriented methyl group. III. Order director fluctuations and solute reorientation in a nematic solution of acetonitrile-d3
Authors: Jaffe David , Vold Robert L. , Vold Regitze R.
Published: February 2003

Deuteron spin-lattice relaxation rates (R1Z) and relaxation rates of quadrupolar order (R1Q) have been determined between 13 and 47 °C at 38.4 MHz, for a 10.5 mol % of acetonitrile-d3 in Merck Licristal phase 5, and selective and nonselective measurements of spin-lattice relaxation have been carried out at 4.6 and 9.2 MHz over the same temperature range. The data can be satisfactorily explained by a combination of fast, anisotropic reorientation (corrected for effects of static ordering) and hydrodynamic fluctuations in the orientation of the nematic director. Large values of the rotational anisotropy were derived from the spin-lattice relaxation data, and confirmed by 38.4 MHz measurements of the temperature dependence of the relaxation rate R2(ZQ) of a zero quantum coherence. The results obtained here are discussed in terms of recent related 13C relaxation studies of acetonitrile in nematic solution.