bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear effects in polar fluids: A molecular theory of electrostriction
Authors: Rasaiah Jayendran C. , Isbister Dennis J. , Stell George
Published: February 2003

A molecular theory of electrostriction arising from the study of dipolar adsorption at a wall in the presence of an electric field is described. The quadratic hypernetted chain (QHNC) approximation for the wall–particle closure is the first of the hierarchy of approximations generated by the hypernetted chain equation (HNC) to predict electrostriction; the mean spherical and linearized hypernetted chain fail to do so. It is found that the simplest bridge diagrams which are ignored in the HNC (and QHNC) approximations must be included if quantitative agreement with the thermodynamic theory for electrostriction as described by Kirkwood and Oppenheim is to be obtained. These bridge diagrams have been evaluated analytically resolving the above discrepancy in the term of O(E2), where E is the local electric field. The statistical mechanical approach has also been extended to evaluate a few of the contributions of O(E4) in electrostriction. Conditions under which the linear constitutive relation between the polarization density P(∞,E) and the electric field E is recovered are discussed; its extension to include nonlinear terms in E is also considered.
Observation of the ν2 band of CH2 by laser magnetic resonance
Authors: Sears Trevor J. , Bunker P. R. , McKellar A. R. W.
Published: February 2003

The transitions 202←111, 101←110, and 202←211 were obtained from the ground 3B1 state of CH2. (AIP)
The long time behavior of the velocity autocorrelation function of a one-dimensional Lennard-Jones fluid
Authors: Bishop Marvin
Published: February 2003

Molecular dynamics calculations have been done for a low density (0.20), high temperature (9.49) state of a one dimensional repulsive Lennard-Jones fluid. The normalized velocity autocorrelation function was shown to decay as -t−2.89±0.06, in reasonable agreement with the theoretical power law of -t−3.00. (AIP)
State-to-state relaxation processes for XeCl(B, C)
Authors: Dreiling T. D. , Setser D. W.
Published: February 2003

The XeCl (B–X) and (C–A) emission spectra obtained from reaction of Xe (3P2 or 3P1) with Cl2, CCl4, and COCl2 in the presence of He, Ne, Ar, Kr, and N2 bath gases were used to study the vibrational relaxation and transfer between the B and C states of XeCl. By using the different Cl donors, different ranges of vibrational energy were emphasized. The bound–free emission spectra were simulated for various pressures of bath gas to obtain vibrational distributions. Numerical modeling of the XeCl(B) and XeCl(C) vibrational populations and the B/C intensity ratio as a function of pressure gave rate constants for vibrational relaxation and transfer, as well as the model for the state-to-state processes. For Ar as the bath gas, vibrational relaxation can be characterized by an exponential gap model Pij ∝ e−0.1ΔE/kT, with rate constants of (1–6), (6–12), and (20–30) × 10−11 cm3 molecule−1 sec−1 for the v ranges of 0–30, 30–70, and 70–130, respectively. The rate constants for electronic state transfer are (3–11), (11–15), and (15–15) × 10−11 cm3 molecule−1 sec−1 for the same v ranges. The vibrational energy loss upon electronic state transfer was best described by a Poisson-type function displaced to lower energy from the initial energy. These basic models also describe the relaxation in the other gases with He and Ne being less efficient and Kr and N2 more efficient than Ar. The magnitudes of the rate constants and the models are discussed.
Microwave spectra of the HSO and DSO radicals
Authors: Endo Yasuki , Saito Shuji , Hirota Eizi
Published: February 2003

The microwave spectra of the HSO and DSO radicals in the ground state were observed in glow discharges in a mixture of H2S or D2S and O2. Both a-type and b-type transitions were assigned and analyzed. The hyperfine coupling constants of the hydrogen nucleus of HSO were determined for the first time. These constants resulted in a conclusion that the ground electronic state of HSO is 2A″. The rotational constants, centrifugal distortion constants, and spin–rotation coupling constants including their centrifugal distortion terms for both the species were also determined with good precision. The diagonal components of the magnetic hyperfine interaction for the deuterium nucleus of DSO were not obtained because most of the hyperfine splittings were not resolved or resolved partially. However, the off-diagonal component Tab was determined from an analysis of a local perturbation between the 312 J = 7/2 and 404 J = 7/2 levels connected by the εab+εba term.
Kinetics and mechanism of oxidation of basal plane on graphite
Authors: Yang Ralph T. , Wong Chor
Published: February 2003

The kinetics and mechanism of oxidation of carbon atoms exposed at monolayer steps on graphite surface have been studied with the etch-decoration and transmission electron microscopy technique and two pieces of direct evidence are shown for the importance of surface diffusion in the overall kinetics. It is found that the rate of carbon removal, or the turnover frequency, depends on the population density of these edge carbon atoms, being substantially higher on low-density surfaces. In the argon flush experiment, it is found that surfaces with low edge carbon densities continue to ’’burn’’ for prolonged periods of time after O2 is cut off from the gas phase. Two independent mechanisms are revealed by the experimental results: (1) reaction resulted from direct collision of O2 on the edge carbon, and (2) reaction of the edge carbon with the migrated oxygen which is first chemisorbed on the basal carbon. Furthermore, from the results of the argon flush experiments, the following results can be calculated: surface diffusion coefficient  = 5×10−12 cm2/s (for O/C = 0.34) and 2×10−12 cm2/s (for O/C = 0.06); rate constant for the edge carbon reacting with the migrated O atom  = 4.5 s−1, all at 650 °C. The amount of chemisorption on the basal plane can also be obtained from the data of the argon flush technique as demonstrated in this work.
Direct summation over vibrational levels: Λ doubling in HF+
Authors: Hutson Jeremy M. , Cooper David L.
Published: February 2003

When different electronic states of a system are mixed by a small perturbation, expressions for molecular properties often involve summations or integrals over vibrational energy levels. A recently developed method for evaluating these expressions exactly from potential curves is tested, and is found to be numerically accurate and computationally convenient. The method does not require vibrational wave functions for the perturbing electronic state, and works for both bound and unbound perturbing states. Λ doubling in HF+ is re-examined using the new method, and results significantly different from previous calculations are obtained. The new calculated parameters are believed to be more reliable than experimental quantities from high resolution electronic spectroscopy.
Gradient expansion of the classical Coulomb energy of an atomic charge distribution
Authors: Bartolotti Libero J. , Parr Robert G.
Published: February 2003

Let ρ be the Hartree–Fock electron density for the ground state of an atom or ion with N electrons. Earlier studies (Refs. 1 and 2) have established that a good approximation to the repulsion between ρ and itself, the quantity J[ρ] = 1/2 ℱ ℱ{[ρ(1)ρ(2)]/r12}dτ1dτ2, is provided by the formula JL = B0N2/3ℱρ4/3 dτ, where B0 is a universal constant. An improved approximation to J is established: JNL = 5/3(B0LN2/3)ℱρ4/3 dτ −(4/243π)(B0LN2/3)2 ℱ(∇ρ ⋅ ∇ρ)/ ρ4/3 dτ, where B0L = 0.7543 is a parameter the value of which was earlier established in local density-functional theory. For 1785 atoms and ions, the standard error in J/N2/3ℱρ4/3 dτ as computed from this formula is 1.59%.
An XPS study of intensity borrowing in core ionization of free and coordinated CO
Authors: Freund H. J. , Plummer E. W. , Salaneck W. R. , Bigelow R. W.
Published: February 2003

It is shown experimentally that the pronounced satellite peaks (shakeup) seen in the core level spectra of transition metal carbonyls borrows intensity from the main line. The core level spectra of free uncoordinated molecules like CO, N2, O2, NO, and CO2 and transition metal carbonyls like Fe(CO)5 can be understood using simple intensity sum rules based on the sudden approximation. By applications of the ’’Manne–Aberg theorem,’’ we are able to explain binding energy shifts that occur upon coordination of a molecule as effects due to relaxation in the ion rather than to initial state shifts. The importance of the shakeoff continuum is addressed. Finally, a comparison of the satellite energies and positions is made to the excitation spectra of the neutral molecule. A CNDO/2–CI calculation is used to support the conclusions based upon this comparison.
Infrared diode laser spectroscopy of the fundamental band of NF(a 1Δ)
Authors: Davies P. B. , Hamilton P. A. , Okumura M.
Published: February 2003

Thirty-one lines of the fundamental vibration–rotation band of the NF free radical in its a 1Δ state have been detected in absorption near 8.6 μm using a tunable infrared diode laser. Linewidths were Doppler limited and several transitions were accompanied by resolved hyperfine structure due to fluorine and nitrogen nuclear moments. Wave number calibration using accurately determined N2O lines yielded ν0 = 1165.952±0.001 cm−1 for the band center. Rotational and centrifugal disortion constants for both v = 0 and 1 states have also been determined.
On highly excited electronic states of the NO molecule reached by multiphoton spectroscopy
Authors: Dressler K. , Miescher E.
Published: February 2003

A recent multiphoton experiment by Wallace and Innes involves an energy level of the NO molecule that belongs to a very interesting sequence of rovibronic states in which an np Rydberg complex (C 2Π+D 2Σ+, v = 6) and a regular and inverted valence state (B 2Πr, v = 24 and L 2Πi, v = 5) are strongly mixed with each other. This multistate interaction, which is typical for highly excited molecular states reached in multiphoton spectroscopy, is discussed as an unique example of a nontrivial case of Rydbergization of valence states. Gallusser’s vibronic coupling calculation and new interpretations of local perturbations in three different isotopes are used to establish the absolute vibrational numbering and the deperturbed rovibronic and spin–orbit constants of the L 2Πi state which had been poorly known previously.
ESR studies of irradiated single crystals of alkali halides doped with ferrocyanide ion
Authors: Viswanath A. Kasi , Rogers Max T.
Published: February 2003

Single crystals of RbCl, RbBr, and KCl doped with [Fe(CN)6]4− were grown from solution and ESR spectra of both x- and γ-irradiated crystals were recorded over a wide temperature range. A number of Fe(I) radicals were obtained and identified by their ESR spectra. Thermally treated crystals showed the presence of halogen-substituted radicals. Rotations were made in the crystallographic planes and the spectra analyzed using a suitable spin Hamiltonian including, where appropriate, nitrogen and halogen ligand superhyperfine interactions. Powder spectra were also recorded and used as a cross check on the single crystal analyses. The g and superhyperfine splitting tensors are shown to be consistent with having the odd electron in an antibonding orbital of a1g symmetry made up largely from the metal dz2 orbital. The spin densities in the ligand s and p valence orbitals and the p/s ratios were computed where possible. Structures have been proposed for the various radicals based on the observed ESR parameters.
Photon echoes stimulated from an accumulated grating: Theory of generation and detection
Authors: Hesselink Wim H. , Wiersma Douwe A.
Published: February 2003

We present a theoretical description of the generation and detection of photon echoes, stimulated from an accumulated frequency grating in the inhomogeneous distribution. We show that detection of the echoes takes place via interference in the sample between a properly phased and directed probe pulse and the echo polarization. In this description the echo phenomenon emerges as an induced transparency of the sample rather than a burst of coherent radiation. By solving simultaneously the optical Bloch and Maxwell equations, an expression is obtained which correlates quantitatively the echo intensity to the decay parameters, pulse intensity, and the transition moment. As an application of the theory we present results on the intersystem crossing of pentacene in naphthalene at 1.5 K.
Pressure and temperature Raman study of reorientational relaxation of propyne in hydrogen bonded complexes
Authors: Perry S. , Żerda T. W. , Jonas J.
Published: February 2003

A Raman band study of propyne in the liquid phase and in solutions with inert and active solvents as a function of temperature (−80 to+80 °C) and pressure (1–4000 bar) is reported. An increase of the reorientational correlation times τϑ with increasing pressure was found. It is shown that the theoretical correlation functions for diffusion-type models (the extended diffusion theory and the Fokker–Planck–Langevin friction theory) can reproduce the experimental correlation functions of propyne only in the absence of hydrogen bonding. It is also shown that a relatively free rotation of the propyne symmetry axis in acetone–propyne complexes takes place even at the highest pressures, and two simple pictures of the motion are proposed.
Relation of dissociation energy, internuclear distance, and overlap integral for hydrogen halides to atomic ionization potentials
Authors: March N. H. , Mucci J. F.
Published: February 2003

Correlation between dissociation energy, interatomic distances and electron affinites in hydrogen halides are explored. (AIP)
Ultrasonic wave velocity in some normal paraffins
Authors: Sachdeva V. K. , Nanda V. S.
Published: February 2003

Ultrasonic wave velocity has been measured in four n-paraffins by using a variable path interferometer. The liquids studied are n-pentane, n-hexane, n-heptane and n-decane.
Cation electric field gradients in metaperiodates with the Scheelite structure
Authors: Segel S. L.
Published: February 2003

The electric quadrupole coupling constants have been measured for the cations of meta-periodate salts with the Scheelite structure. These values are as follows: 14NH4—0.013(2) MHz; 23Na—0.048(2) MHz; 39K—0.102(5) MHz; and 87Rb—1.00(5) MHz. Using a new value for the 14N antishielding factor, the resulting electric field gradients are smaller than the values for the isomorphous perrhenate salts by an order of magnitude.
Electronic spectra of polyatomic molecules with resolved individual rotational transitions: Benzene
Authors: Riedle E. , Neusser H. J. , Schlag E. W.
Published: February 2003

The density of rotational transitions for a polyatomic molecule is so large that in general many such transitions are hidden under the Doppler profile, this being a fundamental limit of conventional high resolution electronic spectroscopy. We present here the first Doppler-free cw two-photon spectrum of a polyatomic molecule. In the case of benzene, 400 lines are observed of which 300 are due to single rotational transitions, their spacing being well below the Doppler profile. The resolution so achieved is 1.5×107. Benzene is a prototype planar molecule taken to have D6h symmetry in the ground as well as in the first excited state. From our ultra-high resolution results it is found that benzene in the excited S1 state is a symmetric rotor to a high degree. A negative inertial defect is found for the excited state. The origin of this inertial defect is discussed.
Quasiclassical trajectory and statistical studies of atom exchange and dissociation in Ar+Ar2, Ar+Ne2, and Ar+ArKr collisions
Authors: Thompson Donald L. , Raff L. M.
Published: February 2003

Rare gas atom–diatom collisions of Ar+Ar2, Ar+Ne2, and Ar+ArKr have been investigated to determine the general features of exchange and dissociation reactions involving van der Waals molecules. Quasiclassical trajectory calculations on pairwise additive potential surfaces show that metastable dimers with internal energy above the dissociation limit of the dimer but below the rotational barrier play a major role in both the exchange and dissociation dynamics. Lifetime distributions for such metastables, obtained from WKB calculations, suggest that they will be of importance in crossed molecular beam studies of such systems as well as in nucleation studies. Computed branching ratios are found to approach statistical behavior at higher energies, but deviate significantly from such predictions at lower energies where the endothermic pathway involving exchange with the lighter atom is unexpectedly found to be favored. Product rotational state distributions are found to be governed largely by statistical factors. The distribution for metastable Ar2* dimers is found to be trimodal in character. Although such a result is predicted by a density of quantum states model, no classical explanation was found.
The structure of CO on Ni(111)
Authors: Netzer Falko P. , Madey Theodore E.
Published: February 2003

Electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and temperature programmed thermal desorption (TPD) have been used to study the adsorption of CO on Ni(111) in the temperature range 80−300 K. For low coverages, the CO layer is disordered; a c(4×2) pattern appears at coverages ϑ∼0.5, the maximum coverage at 300 K. At temperatures 220–240 K, a well-ordered (√7/2×√7/2)R 19° LEED pattern forms at saturation (ϑ∼0.57). At 80 K, the CO saturation layer is characterized by a ’’complex’’ LEED pattern. Only one binding state is seen in TPD for ϑ≲0.40 (peak temperatures 450–430 K); species having lower desorption temperatures populate at higher coverage. At 300 K adsorption the only ESD ion observed is O+, with desorption centered about the direction perpendicular to the surface. The O+ ion yield shows a maximum at intermediate coverages. CO+ ions are also observed at adsorption temperature < 300 K at higher coverages. The ESDIAD patterns for saturation coverage in the range 80–260 K indicate off-normal ion emission in addition to the normal component. The data suggest that for ϑ≲ 0.5, CO is adsorbed in multiply coordinated sites with the molecular axis perpendicular to the surface. At temperatures < 300 K, a fraction of CO can adsorb in singly coordinated sites as well. Upon heating the CO layer formed at 80 K (ϑ?0.57) the CO+ ion yield maximizes at ∼240 K when the (√7/2×√7/2)R 19° LEED pattern is well defined. The CO+ yield therefore reflects the ordering behavior in the adsorbate layer at temperatures below the onset of desorption and indicates a shift of a fraction of CO molecules from multiply coordinated to singly coordinated sites. Finally, a model is proposed involving resonance charge exchange between adsorbed CO molecules to account for the observed variations of the O+ yield with surface coverage.
Rotational distributions in the photodissociation of bent triatomics: H2S
Authors: Hawkins W. G.
Published: February 2003

A laser-induced fluorescence technique is used to study the SH fragment rotational distribution following dissociation of H2S at 193 nm. (AIP)
Relativistic effects on R
e and D
e in AgH and AuH from all-electron Dirac–Hartree–Fock calculations
Authors: Lee Yoon S. , McLean A. D.
Published: February 2003

All-electron relativistic self-consistent-field calculations on AgH and AuH show bond length concentrations of 0.08 and 0.25 Å and dissociation energy increases of 0.08 and 0.42 eV from nonrelativistic values. With correlation estimates from nonrelativistic calculations, the experimental Re and De of AgH are completely accounted for. In AuH, this is not the case, and we conclude that relativistic correlation effects are markedly different from nonrelativistic in this molecule. Comparison of effective core potential calculations on AuH with the present all-electron calculations shows good agreements for De’s but significant discrepancies for Re’s.
Electronic energy transfer from pyrazine to a silver(111) surface between 10 and 400 Å
Authors: Whitmore P. M. , Robota H. J. , Harris C. B.
Published: February 2003

The phosphorescence lifetime of pyrazine molecules above a Ag(111) surface has been measured as a function of molecule–metal seperation between 10 and 400 Å. In contrast to the previous work on the pyrazine/Ag system, we find that the lifetime decreases monotonically as the pyrazine approaches the metal surface. The distance dependence of the phosphorescence lifetime is described quantitatively using the classical treatment of nonradiative energy transfer of Chance, Prock, and Silbey.
Profiles of hydrogen stretching IR bands of molecules with hydrogen bonds:  A stochastic theory. II. Strong hydrogen bonds
Authors: Bratos S. , Ratajczak H.
Published: February 2003

A theory is proposed to describe the shape of hydrogen stretching IR bands of complexes with strong hydrogen bonds in disordered condensed phases. The theory uses stochastic arguments and represents a generalization of that proposed earlier to study weak and medium-strong hydrogen bonds. The cases of strong asymmetrical and of strong symmetrical or nearly symmetrical bonds are treated separately. The profiles are generated by three essential band shaping mechanisms: the anharmonic coupling between the hydrogen stretching and the bond stretching modes, the Fermi resonance between the singly excited hydrogen stretching and various singly or doubly excited harmonic levels perturbed by hydrogen bonding and the anharmonicity of the hydrogen stretching mode itself. The anomalous isotope effect on the band position and on the integrated intensity is discussed in detail; its existence does not necessarily imply the potential energy surface to have two low energy minima.
Some properties of the EPR spectrum of paramagnetic centers on β decay of tritium incorporated into glycine
Authors: Legasov V. A. , Usatyi A. F. , Ibragimov R. A. , Myasoedov N. F.
Published: February 2003

Paramagnetic centers arising from radioactive β decay of tritium incorporated into glycine amino acid were investigated by the method of EPR in a frozen aqueous solution (H2O/D2O). Tritium-labeled alanine and glycyl–glycine dipeptide were also studied in H2O solutions. In the cases of glycine and glycyl–glycine, the EPR spectra specific solely to this process were obtained (compared to EPR spectra formed by the outer ionizing radiation). The analysis of the accumulation kinetics of this signal, its temperature and photochemical dependences, and the character of the hyperfine structure lead to the concept that at temperatures below 130 °K a He-containing paramagnetic complex (C–3He+ bond) may exist within the labeled glycine radiolysis product or its ion radical. Basing on this concept the activation energy of the hypothetical product dissociation is determined.
Infrared spectra of matrix-isolated gaseous ternary oxides M2XO4
Authors: Bencivenni L. , Gingerich K. A.
Published: February 2003

The vapors above heated Rb2XO4 (X = S, Cr, Mo, and W), Cs2CrO4, In2CrO4, In2WO4, and Tl2WO4 have been studied by IR matrix isolation spectroscopy. All these compounds show similar spectra in the X–O stretching region, consistent with that expected from a M2XO4 molecule. 18O substitution in the K2WO4 molecule shows the IR isotopic pattern expected for a bidentate binding in XO4=. The splitting of the ν3(F2) vibration of the tetrahedral XO4= anion by passing from Td to D2d symmetry and the effect of the coordination of the cation are discussed.
Non-Born–Oppenheimer density functional theory of molecular systems
Authors: Capitani Joseph F. , Nalewajski Roman F. , Parr Robert G.
Published: February 2003

It is shown that the Hohenberg–Kohn–Levy density functional theory of molecular structure is not restricted by the Born–Oppenheimer approximation. The existence of the corresponding ground-state density functionals for the case of the exact nonadiabatic, nonrelativistic, field-free Hamiltonian of a molecular system, in terms of the one-particle electronic and nuclear densities, is proven and the associated Euler equations are discussed. Extensions to the case of the system in an external electric field and to the bound excited states, are briefly examined. As an example the non-Born–Oppenheimer Hartree–Fock theory of Thomas is discussed from the density functional viewpoint. Possible applications of the theory in the analysis of molecular structure and chemical reactivity are identified.
Structural phase transition of benzil
Authors: Terauchi Hikaru , Kojima Toshiro , Sakaue Kiyoshi , Tajiri Fumiko , Maeda Hironobu
Published: February 2003

The temperature dependence of order parameters such as spontaneous strains and atomic displacements in benzil (C6H5COCOC6H5) has been obtained by the x-ray diffraction method. The diffuse x-ray scattering has also been examined just above the transition temperature (Tc = 83.5 K). It has been found that the primary order parameter is a spontaneous strain ezx and the benzil crystal is ferroelastic below Tc.
Sound attenuation in critically slowed-down chemically reactive mixtures
Authors: Hentschel H. G. E. , Procaccia I.
Published: February 2003

The attenuation and dispersion of sound are examined theoretically as a potential probe of the recently reported theoretical predictions of slowing down of chemical reactions near thermodynamic critical points. A mode-coupling calculation of the complex sound attenuation coefficient in a chemically reactive binary mixture near the liquid–gas critical point reveals a divergent contribution, with critical exponents that are determined by the chemical rather than the diffusional pathway for composition relaxation. Possible experiments are discussed.
Energy and angle resolved SIMS studies of CO on Ni(001)
Authors: Gibbs R. A. , Holland S. P. , Foley K. E. , Garrison B. J. , Winograd N.
Published: February 2003

Energy and angle resolved secondary ion mass spectra (SIMS) for CO chemisorbed on Ni(001) have been examined in detail. This system has been chosen as a model since it provides intense secondary ion yields and since the original surface geometry of the adsorbed CO is known by other techniques. Theoretical curves for the ejected atomic and molecular species have been generated using a classical dynamics procedure for computing the momentum dissipation of the 1000 eV incident Ar+ ion. We found that for Ni+ ion ejection the results agreed well with calculated spectra of the neutral particles where the CO is placed in a linear bonded site, if the neutral atom trajectories were modified by inclusion of an image force. The agreement was excellent for polar angle, azimuthal angle, and secondary particle kinetic energy distributions. Similar agreement was found for Ni2+ and NiCO+ species although the statistical reliability of the calculated curves was not as high as for the Ni+ species. The results provide convincing evidence that the classical dynamics model can provide a semiquantitative insight into the SIMS process. In addition, the presence of a relatively strong image force indicates that the ion must be formed very close to the surface. Finally, since agreement between theory and experiment was found over a wide range of conditions, the results suggest that the ionization probability of the ejecting particle is isotropic and only weakly dependent on particle velocity. These criteria impose a number of constraints on possible theories of ionization mechanisms.
Optical density effects in photon echo experiments
Authors: Olson R. W. , Lee H. W. H. , Patterson F. G. , Fayer M. D.
Published: February 2003

The effects of optical density (OD) on photon echo experiments is examined both theoretically and experimentally. The theoretical treatment involves coupled Maxwell-Optical Bloch equations, and the experiments are picosecond photon echo studies of the mixed molecular crystal, pentacene in naphthalene. Under high power excitation (π/2, π) in high OD samples, the echo decays are predicted and observed to be faster than decays arising solely from the intrinsic molecular T2. The increase in the echo decay rate vanishes in the limit of low power or low OD, permitting the intrinsic molecular T2 to be measured. Previously reported concentration effects are shown to actually be optical density effects. The experimentally determined intrinsic molecular T2 of pentacene in naphthalene is shown to be independent of concentration and temperature (below ∼2 K) and T2≠2T1. It is suggested that the difference between T2 and 2T1 is due to the static multiplet level structure arising from the electronic state-phonon coupling.
Combination rules for intermolecular potential parameters. I. Rules based on approximations for the long-range dispersion energy
Authors: Diaz Peña M. , Pando C. , Renuncio J. A. R.
Published: February 2003

Combination rules for intermolecular potential parameters based on different approximations for the long-range dispersion energy are derived and applied to the Lennard-Jones (12–6) and Kihara potential functions. The resulting group of rules is given by the expressions σ12 = <σ≳j, and ε12 = <εσ6γ≳i/‖<σ≳6j<γ≳k ‖, where the i,j, and k subscripts may adopt the values a, g, or h indicating the type of mean (arithmetic, geometric, or harmonic, respectively) to be taken for the magnitude within the brackets. Expressions for γ depend on the approximation chosen for the van der Waals coefficient c6. This group of rules includes most of those previously proposed and others which are new. Experimental values of the interaction virial coefficient and unlike-pair potential parameters obtained from viscosity data are used to test the validity of the rules. Six related rules are shown to be satisfactory for both potential functions and for accurate correlation of virial and viscosity data as well.
Collision processes in Ne–O2 afterglows
Authors: Kuehn D. G. , Chanin L. M.
Published: February 2003

Optical emission and absorption spectroscopy has been used to study stationary afterglows in Ne–O2 mixtures. The quenching rate constants for Ne(3P0) and Ne(3P2) by O2 were found to be (2.1±0.3)×10−10 cm3 s−1 and (1.9±0.3)×10−10 cm3 s−1, respectively. O i lines corresponding to the 3 p3P → 3 s3S, 3 p5P → 3 s5 S, 4 p3P → 3 s3S, and 4 p5P → 3 s5S transitions plus the forbidden [O i] line due to the 2p4 1S0 → 2p4 1D2 transition were all apparent during the afterglow period. From the decay rate of the latter line were determined both the diffusion constant DP0 = 330±10 Torr cm2 s−1, and the two-body quenching rate k = (2.1±0.3)×10−16 cm3 s of O(1S0) by Ne atoms.
Comparison of electron gas and a
b i
n
i
t
i
o potentials for the N2–N2 interactions. Application in the second virial coefficient
Authors: van Hemert M. C. , Berns R. M.
Published: February 2003

A detailed potential for the interaction between two rigid N2 molecules is given in the form of a spherical expansion. The interaction energy is found as the sum of the so-called Hartree–Fock part of the electron gas expression including the Rae correction and the ’’ab initio’’ dispersion energy in the multipole expansion. Potential surface cuts computed with this expansion agree to a large extent with a similar potential completely based on ab initio calculations. Comparison of the experimental second virial coefficient curve with the curves obtained from a four dimensional quadrature using both ab initio and electron gas potentials demonstrates the usefulness of these potentials, and underlines the importance of the anisotropic contributions.
Collision energy dependence of the reactive scattering of Br with HAt
Authors: Grover J. R. , Malloy D. E. , Mitchell J. B. A.
Published: February 2003

Laboratory angle-differential cross sections were measured for the reaction Br+HAt→HBr+At at collision energies from 4.2 to 11.7 kcal/mol. Transformation of these data into the center-of-mass system revealed that the mean energy in product translation is extraordinarily small, rising from 1.5±0.2 to 3.3±0.6 kcal/mol. The ratio of products scattered forward to products scattered backward is about 1.0 to 1.5 at all collision energies studied, but more detailed angular information could not be obtained. The total reaction cross section is in the range of 1.5 to 2.0 Å2 from 4.2 to 8.3 kcal/mol of collision energy, and rises to 2.9±0.34 Å2 at 11.7 kcal/mol. (The uncertainties cited for the cross section values represent random errors only, systematic errors contribute an additional ±15%.) These results confirm the observations made earlier at a collision energy of 5.8 kcal/mol. Phase space calculations show that for collision energies greater than about 7 kcal/mol the total reaction cross section is approaching the maximum value that the small average energy in product translation permits it to have, which means that energy barriers and geometric restrictions are of secondary importance. For collision energies less than 5 kcal/mol the total reaction cross section becomes significantly smaller than the permissible maximum, and some constraint(s) must therefore be effective; for example, the data are consistent with an entrance-channel barrier of 2 to 3.5 kcal/mol. The mean energy in product translation is always far less than the collision energy, and this means that neither direct reactions nor completely statistical reactions are important, a conclusion supported by the forward/backward ratio near unity in the angular distributions. A model intermediate between direct and statistical is proposed in which the rapidly-moving hydrogen atom can possess too much energy to permit separation of HBr and At, and at the same time too little energy to permit spearation of Br and HAt, so that the bromine and astatine atoms are temporarily trapped in one another’s vicinity. This behavior is associated with potential hypersurfaces characterized by a pronounced widening in the neighborhood of the transition state. Such surfaces give a high probability of snarled trajectories, broad angular distributions of products, and small energies in product translation. The results of preliminary calculations are presented to support the model.
On two Sturmian alternatives to the LCAO method for a many-center one-electron system
Authors: Duchon Ch. , Dumont-Lepage M. Cl. , Gazeau J. P.
Published: February 2003

Two Sturmian techniques for determining the energy eigenvalues of a many-center one-electron system are presented and compared. The first one was originally used by Novosadov and recently revisited by Monkhorst and Jeriorski. The second one is closer to the usual LCAO method and displays an excellent convergence rate when applied to the H2+ ion problem.
Universal scaling relations for free and bonded atoms
Authors: Tal Yoram , Bartolotti Libero J. , Bader R. F. W.
Published: February 2003

Scaling relations, relating the energy and expectation value of r−1 in free and bonded atoms, are studied. In the limit of a large number of electrons these relations are functions of the ratio q = N/Z, where N is the number of electrons and Z is the nuclear charge. It is shown that the q dependence of such relations may be rather accurately predicted from the Thomas–Fermi and local density functional theories even for small values of N. The validity of the theoretical prediction is demonstrated by a comparison between the theoretical and the calculated Hartree–Fock values for a large number of atoms and ions. The same comparison is made for bonded atoms as defined by the theory of quantum topology. It is found that both free and bonded atoms obey the proposed relations to a similar degree of accuracy.
Theoretical study of electrophilic addition: O(3P)+C2H4
Authors: Dupuis M. , Wendoloski J. J. , Takada T. , Lester W. A.
Published: February 2003

A b initio Hartee–Fock (HF) and multiconfiguration Hartee–Fock (MCHF) calculations have been carried out to characterize the reactants, transition state, and products of the electrophilic addition of O(3P) to the π bond of ethylene. The results show that the diradical product CH2CH2O is stable with respect to the reactants. The transition state has Cs symmetry, not C2v, with the oxygen atom localized on one of the two double-bond C atoms.
Photoionization cross sections and photoelectron asymmetries of the valence orbitals of NO
Authors: Southworth S. , Truesdale C. M. , Kobrin P. H. , Lindle D. W. , Brewer W. D. , Shirley D. A.
Published: February 2003

Partial photoionization cross sections and photoelectron asymmetry parameters were measured for the valence orbitals of NO over the photon energy range 16–31 eV by photoelectron spectroscopy, using synchrotron radiation. The results are compared with previous measurements and with theoretical curves based on the multiple-scattering model. Maxima observed in the partial cross sections of the 2π, 5σ, and 4σ orbitals are attributed to continuum shape resonances. The multiple-scattering model calculations predict the observed photoelectron asymmetries of the 5σ and 1π orbitals very well, but the calculated asymmetry curves are shifted strongly from the measured asymmetries produced by the 2π and 4σ orbitals. The π molecular orbitals are observed to produce qualitatively different photoelectron asymmetry curves than do the σ orbitals. We discuss the double-angle-time-of-flight method for the measurement of photoelectron angular distributions. This technique offers the advantages of increased electron collection efficiency and the elimination of certain systematic errors.
Intermolecular effects on 1H, 13C, and 15N nuclear magnetic shielding in HCN
Authors: Jameson Cynthia J. , Jameson A. Keith , Oppusunggu D. , Wille S.
Published: February 2003

We have observed the temperature dependence of the 1H, 13C, and 15N NMR spectra in HCN in the liquid phase and also in the saturated vapor. Concentrations observed include 0.05 to 0.30 mol/l for the vapor and 21–27 mol/l for the liquid at temperatures ranging from 230 up to 380 K. The gas-to-liquid shifts and the chemical shift between the dimer and the monomer have been determined for 1H, 13C, and 15N nuclei. A dichotomy in the 15N shifts in nitrogen nuclear environments capable of hydrogen bonding is noted. 15N in HCN appears to be an archetype of nitrogen nuclear environment which is in the same class as pyridine and acetonitrile (and which behaves in a manner opposite to that of 15N in NH3 and saturated amines), such that the effect of intermolecular interactions is to lead to an unusual increase rather than the usual decrease in shielding observed for all other nuclei.
Excited state geometry of uracil from the resonant Raman overtone spectrum using a Kramers–Kronig technique
Authors: Chinsky Laurent , Laigle Alain , Peticolas Warner L. , Turpin Pierre-Yves
Published: February 2003

The Kramers–Kronig transform technique of Blazej and Peticolas is extended to permit the calculation of overtone and combination bands in a resonant Raman spectrum relative to the fundamentals. The method is applied to the ultraviolet (257 nm) resonant Raman spectrum of uracil which exhibits five fundamental bands and numerous overtone and combination bands. The method permits the determination of Δej the shift of the excited state potential energy minimum along the jth normal mode for each of the five resonant Raman active modes. The Δej values obtained are in reasonable agreement with those obtained earlier from the absorption spectrum and excitation profiles of the fundamentals. However, no internal standard for the experimental determination of Raman intensities is required nor is it necessary to assume any value for Γ the vibronic linewidth. The shape of the absorption band and relative intensities of the harmonics to the fundamental are sufficient to obtain the value of each Δej.
The effect of rhodium precursor on Rh/Al2O3 catalysts
Authors: Worley S. D. , Rice C. A. , Mattson G. A. , Curtis C. W. , Guin J. A. , Tarrer A. R.
Published: February 2003

The infrared spectra of the CO/Rh/Al2O3 system have been examined following several different means of preparing the supported rhodium catalyst. Rhodium precursor materials including RhCl3 ⋅ 3H2O, Rh(NO3)3 ⋅ 2H2O, Rh6(CO)16, [Rh(OCOCH3)2]2, and Rh2(SO4)3 have been compared as to their tendencies to produce upon reduction the various CO/Rh/Al2O3 species generally attributed to this catalytic system. The nitrate and carbonyl precursors are most easily reduced to rhodium metal. The acetate and sulfate anions poison the Rh/Al2O3 surface through decomposition during reduction leading to very minimal CO adsorption. The catalytic properties of Rh/Al2O3 could be quite dependent upon the nature of rhodium precursor chosen.
Molecular motions in liquidlike pockets of frozen solutions: Electron spin relaxation study of semiquinones in DMSO and DMSO–ethanol mixtures
Authors: Krishnamoorthy G. , Prabhananda B. S.
Published: February 2003

The ESR spectra of semiquinones in frozen DMSO show motional averaging of anisotropic ESR parameters. The following observations support the liquidlike pocket model for the environment of semiquinones giving such spectra in these systems even up to 50° below the freezing points of the solutions: (i) The temperature dependence of linewidths and spin lattice relaxation times T1 of semiquinones in frozen DMSO are characterized by an activation energy ΔE?3.2 kcal mol−1. This value is close to the ΔE associated with the viscosity η[ = η0 exp (ΔE/RT)] of DMSO in the liquid state. (ii) The dependences of linewidths and T1 on the sizes of semiquinones are significantly different. Also, T1 calculated from linewidth data using the Hubbard relation do not agree with the experimentally observed values of T1. These behaviors are similar to that seen for semiquinones in liquid alcohols. (iii) There is good agreement between the η0 estimated in this model from the linewidth data or T1 data of semiquinones of different sizes in frozen DMSO. Similar conclusions can be drawn about semiquinones in frozen DMSO–ethanol mixtures. The dominant contributions to T1 comes from spin rotation interaction, and that to linewidths comes from the modulation of anisotropic ESR parameters, in this temperature range.
High pressure luminescence studies of energy transfer in rare earth chelates
Authors: Hayes A. V. , Drickamer H. G.
Published: February 2003

High pressure luminescence studies have been made on several tris rare earth betadiketonates. From this information the pathway for intramolecular energy transfer between ligand and rare earth ion localized levels is clearly established. The emission from europium dibenzoylmethide (EuDBM) and europium thenoyl-triflnoroacetylacetonate (EuTTF) is examined in depth. Further studies are presented on other Eu chelates as well as on samarium and terbium complexes. A brief discussion of the effects of pressure on processes internal to the rare earth ion or the ligand is also presented.
Absorption and MCD spectra of Cs3UO2F5
Authors: Görller-Walrand C. , Colen W. , Dao Nguyen Quy
Published: February 2003

In this paper a D5h coordination symmetry of UO++2 is investigated. MCD spectra and absorption spectra of Cs3UO2F5 are taken at liquid helium temperature. The interpretation is mainly based on the presence of three progressions that exhibit a negative A term in MCD. They are identified as Γg←1Σ+g transitions. The Γg states are issued from the σ+uφu and π3uφu configurations.
Microwave optical double resonance spectrum of NH2. VII. Hyperfine coupling constants (14N and 1H) in Σ(0, 9, 0) and Π (0, 10, 0) of a Ã2A1
Authors: Hills G. W. , Brazier C. R. , Brown J. M. , Cook J. M. , Curl R. F.
Published: February 2003

Magnetic dipole allowed transitions NKaKc J = N−1/2 ↔ J = N+1/2 have been observed in two vibrational states Σ(0,9,0) and Π (0,10,0) of ? 2A1 NH2 using microwave optical double resonance spectroscopy. Extensive hyperfine structure involving both ortho and para rotational states has been analyzed to yield 14N and 1H magnetic hyperfine coupling constants for the Π (0,10,0) vibronic state; the Σ (0,9,0) data were not sufficient alone to determine all these constants. The hyperfine structure of Σ (0,9,0) and Π (0,10,0) could not be fitted simultaneously with a single set of hyperfine coupling constants. However, contrary to our expectations and some previous analyses, no significant difference between the Fermi contact terms for 14N in the Σ (0,9,0) and Π (0,10,0) could be identified. Instead the discordance between the two states appears to arise from different magnetic dipole–dipole coupling constants. The fine and hyperfine structure clearly reflect the presence of perturbations which limit the precision attainable for the derived hyperfine coupling constants. The coupling constants obtained for Π (0,10,0) are aN = 153.0(13), TaaN = −39.5(33), TccN = −37.4(19), ξaaN = 1.4(10), ξccN = −7.4(10), aH = 52.2(14), TaaH = 59.5(34), and TccH = −50.0(20), where all constants are in MHz and the uncertainties listed are 1 standard deviation. The magnetic constants for 14N are quite consistent with a simple sigma molecular orbital picture for the unpaired electron.
The concentration dependence of the vibrational linewidth and shift in liquid binary mixtures: An analytical model
Authors: Knapp E. W. , Fischer S. F.
Published: February 2003

The concentration dependence of the vibrational line shape of molecules in liquid binary mixtures is explained with an analytical model [E. W. Knapp and S. F. Fischer, J. Chem. Phys. 74, 89 (1981)] which accounts simultaneously for the static and dynamical aspect of concentration fluctuations at the reference molecule. The influence of the diffusion dynamics is small and is responsable for line narrowing with rising temperature. According to this model the concentration dependence may be classified with respect to the total line shift ΔΩ and the total change of the line width ΔΓ of the pure substance versus infinite dilution. The agreement with available experiments is good. The model makes also specific predictions upon the concentration dependence of the frequency shift and asymmetry of the spectral line profile in terms of the interaction with nearest neighbors and their diffusion dynamics.
Raman and far-infrared studies of lattice modes of piezoelectric crystal of trioxane
Authors: Kobayashi Masamichi , Furumi Kazuko
Published: February 2003

Optically active lattice vibrations of trioxane (CH2O)3 crystal with the piezoelectric space group of R3c-C63v were investigated by means of Raman and far-infrared spectroscopy. As predicted by the group theory, one A1 [referred to as (A1)L, at 70 cm−1] and three E(EL1, EL2, and EL3 at 95, 90, and 65 cm−1, respectively) bands appeared with distinct dichroism in the polarized far-infrared spectra taken on a uniaxially oriented thin film cooled at liquid nitrogen temperature. Right-angle Raman scattering was measured on single-crystal specimens of various orientations at temperatures from 280 to 40 K. The lowest EL3 mode exhibited clear LO–TO doubling with a frequency gap of 6 cm−1, owing to the piezoelectric or polar character of the crystal. The angular dependence of the Raman frequency was also observed. The broad Raman band around 90 cm−1 was found to separate into two components of different polarization, assignable to the EL1 and EL2 modes. At a temperature as low as 60 K, one component (EL2) split further into a doublet due to the LO–TO splitting with a frequency gap of 4.1 cm−1.