bluuebunny/crossref_metadata_2025 huggingface dataset:
Trnasition Dipole Moments in Extreme Renner Effect Molecules, with Application to the Visible A1 ↔ B1 Bands in CH2, NH2, and BH2
Authors: Brus L. E.
Published: February 2004

The theoretical structure of transition dipole moments in extreme Renner effect molecules is considered. Pople and Longuet-Higgins vibronic wavefunctions, which explicitly correlate nuclear and electronic motion, are employed. Constructive and destructive interference of wavefunctions causes the transition moment to vary, as a function of K and Λ, between the molecular b̂ and ĉ axes. A few unassigned lines in the CH2(1A1 →1B1) spectrum are tentatively identified as type b transitions. It is concluded that in the dihydride radicals CH2, NH2, and BH2, l is decoupled from Λ at the large bending angle where A1 ↔ B1 transitions occur.
ESR Study of OH Radicals Originating from Water of Crystallization of Carboxylic Acids. II. Role of Hydrogen Bonding in Selective Formation of OH Radicals
Authors: Iwasaki Machio , Minakata Kayoto , Nunome Keichi , Tagami Kohji
Published: February 2004

The selective and stereospecific formation of OH radicals from water of crystallization by ionizing radiation has been studied using an irradiated single crystal of dipotassoium fumarate · 2H2O. The ESR results show that the major OH radicals produced with high selectivity are formed from a scission of the O–H bond having the shortest hydrogen bond distance among the four O–H bonds in the two inequivalent water molecules in an unit cell. In addition, the evidence has been obtained for the existence of a proton transferred from the water molecule to the neighboring carboxyl oxygen atom when the OH radical is formed. The crystal of dipotassium fumarate · 2 D2O gives OD radicals together with a large amount of OH radicals from H2O contained as impurity, indicating the marked isotope effect on the formation of OH radicals. These results lead to the conclusion that proton tunneling through hydrogen bond plays an important role when radicals are formed from H2O+ by ionizing radiation. The minor OH radical, which differs in g anisotropy but has nearly the same orientation, is assigned to the unrelaxed form of the major OH radical, since the minor one having a larger g anisotropy and consequently interacting with surroundings more weakly was found to be less stable and to convert into the major species. At 77°K these OH radicals undergo small amplitude oscillation around the coordination bond between the radical oxygen atom and K+.
Error Bounds to Long-Range Three-Body, and Relativistic Interactions between Atoms
Authors: Starkschall George , Gordon Roy G.
Published: February 2004

Dipole oscillator strength distributions, constructed to reproduce known information about atomic polarizabilities, oscillator strengths, and photoionization cross sections, are used to calculate rigorous upper and lower bounds to long-range three-body, and relativistic interactions between atoms.
BEBO Calculations. II. Arrhenius Parameters and Kinetic Isotope Effects for the Reactions of CH3 and CF3 Radicals with NH3
Authors: Arthur N. L. , McDonell J. A.
Published: February 2004

The three-atom BEBO model has been used to calculate Arrhenius parameters, tunneling corrections and kinetic isotope effects for the reactions of CH3 and CF3 radicals with NH3. The theoretical values are compared with the corresponding experimental results, and the polarity of the substrate, the effect of variation of the Pauling constant, the role of tunneling, and the rates of the reverse reactions are examined.
Electron Scattering by Molecules with and without Vibrational Excitation. IV. Elastic Scattering and Excitation of the First Vibrational Level for N2 and CO at 20 eV
Authors: Truhlar Donald G. , Trajmar Sandor , Williams Walter
Published: February 2004

Normalized experimental differential cross sections for scattering angles in the range 20°–85° are presented for elastic scattering and excitation of the fundamental vibration of N2 and CO by electrons with impact energy 20 eV. From three of the four differential cross sections we are able to estimate the integral cross sections. The cross sections for N2 are in good agreement with those of Pavlovic et al. The elastic scattering cross sections are compared to the predictions of the polarized Born approximation using five model potentials (including polarization) which have been previously proposed for N2 or CO. The model of Crawford and Dalgarno is in good agreement with experiment and the model of Sampson and Mjolsness can be put in good agreement by changing an empirical ``cutoff parameter'' by about 0.5a0. The vibrational excitation of N2 appears to be dominated by resonance scattering, as originally suggested by Pavlovic et al. We show that the vibrational excitation of CO also appears to be dominated by resonance scattering.
Gas-Phase Infrared Spectra of HOF and DOF
Authors: Appelman Evan H. , Kim Hyunyong
Published: February 2004

The infrared spectra of HOF and DOF in the gas phase have been studied. From the analysis of rotational subbands, the fundamental frequencies are determined to be 3578.5, 1354.8, and 889.0 cm−1 for HOF; and 2643.5, 1003.9, and 891.1 for DOF. The respective values of the constants (A″ − B̄″) and B̄″ are 18.37 and 0.86 cm−1 for HOF, and 9.79 and 0.81 cm−1 for DOF.
Collision Broadening of C2H4O Lines. I. Broadened by Polar Molecules
Authors: Pandey Prem Chand , Srivastava Shyam Lal
Published: February 2004

A double modulation microwave spectrograph has been used to measure the linewidths of four rotational lines of C2H4O at 300°K broadened by three different polar molecules C2H4O, OCS, and CH3CN having widely different dipole moments. The linewidths have been calculated theoretically on the basis of Anderson's theory as well as Murphy's and Boggs' theory of pressure broadening taking only the dipole-dipole interaction. The width parameters calculated by Murphy and Boggs theory are smaller than that of Anderson's theory but are larger than the experimental values.
ESR and NMR in Aqueous and Methanol Solutions of Copper(II) Solvates. Temperature and Magnetic Field Dependence of Electron and Nuclear Spin Relaxation
Authors: Poupko R. , Luz Z.
Published: February 2004

ESR line-shape measurements of Cu(ClO4)2 solutions in methanol and water at X- and Q-band frequencies are reported. In both frequencies, above room temperature, the over-all linewidth increases with increasing temperature, while at low temperatures the slopes level off and (at Q band) even change signs. The results are quantitatively interpreted in terms of two main electronic relaxation mechanisms, viz., modulation of the spin rotation and anisotropic g-tensor interactions. From the magnetic parameters of the complexes determined in frozen solution and the field dependence of the linewidth, the correlation times are determined. It is found that in both solvents the anisotropic g tensor is modulated by the fast hopping of the complex distortion axis rather than by the molecular tumbling of the complex. Assuming a random jump of the distortion axis between the three octahedral principal directions, the kinetic parameters of this hopping process are calculated to be τi(25°C)=1.6× 10−11 sec−1, ΔEi=1.2 kcal/mole for the aqueous complex and τi(25°C)=1.2× 10−11 sec−1, ΔEi=1.1 kcal/mole for the methanol complex. NMR relaxation measurements of Cu(ClO4)2 solutions in methanol between −90°C and 100°C are also reported and are interpreted in terms of kinetic and magnetic parameters of the Cu2+ solvation complex. Since there is fast intramolecular inversion of the complex, only weighted averaged parameters over the axial and equatorial ligand molecules are obtained. The pseudo-first-order rate constant for the exchange of solvation shell molecules is 1/ τM(25°C)=7.4× 107 sec−1 with an activation energy ΔE=6.6 kcal/mole; the molecular tumbling time is τR(25°C)=4.6× 10−11 sec with ΔER=3.0 kcal/mole; the isotropic ligand hyperfine interaction constants are AOH/h=8.8× 105 Hz, ACH3/h=10.6× 105 Hz for the protons and AO/h=2.9× 107  Hz for 17O. Previous NMR results in aqueous solutions are discussed and reinterpreted.
Molecular Ions in Gas Photocells Irradiated with Pulsed Light
Authors: Pack Sung P. , Salomon R. E.
Published: February 2004

The dispersion in the phase angle of the alternating current in a gas photocell irradiated with pulsed light has been analyzed in terms of various ionic processes. An experimental apparatus and a procedure which tests the analysis is described. Data on the positive ion mobility and first Townsend coefficient for gaseous methanol, ethanol, n-propanol, and benzene are presented. Evidence for the ionization of gases adsorbed on the anode by very low energy electrons as well as measurements of the efficiency of this process are described.
Classical Dynamics of the Reaction of Fluorine Atoms with Hydrogen Molecules. III. The Hot-Atom Reactions of 18F with HD
Authors: Muckerman James T.
Published: February 2004

Excitation functions for the various product channels in reactions of hot 18F atoms with ground-state HD are calculated using the Monte Carlo quasiclassical trajectory technique and an optimized semi-empirical potential energy surface. The production of H18F and D18F and dissociation into F+H+D18 are reported as a function of the center-of-mass collision energy over the range 0.1–65.0 eV. The calculated excitation functions for H18F and D18F cross at ∼ 7 eV indicating an inversion in the intramolecular isotope effect with increasing collision energy. Features of these excitation functions and the calculated product energy distributions are discussed in terms of simple mechanistic models. The trajectory results, on the average, correlate well with the predictions of the spectator stripping model from epithermal collision energies up to the limiting energy where this model would lead only to dissociation. The high-energy tail of the abstraction excitation functions, however, is shown to be attributable to a mechanism which is the antithesis of spectator stripping. In these high-energy collisions, the nearer atom of the HD collides violently with the fluorine atom and recoils away. The training ``abandoned atom'' is left behind in the vicinity of the fluorine atom, with which it subsequently reacts. Information about a computer-animated film of trajectories selected from those calculated in this work illustrating these mechanistic interpretations will be supplied on request.
Calculation of the Dielectric Correlation Factor of Cubic Ice
Authors: Gobush William , Hoeve Cornelius A. J.
Published: February 2004

A systematic approximation scheme for the estimation of the dielectric correlation factor of the cubic structure of ice is presented. The graphical series approximation utilized here is analogous to the two spin Ising model calculation of the zero field susceptibility. Accounting for all graphs that have twelve edges or less, we estimate a value of 2.99584 for the correlation factor, g, of cubic ice. The ratio of successive coefficients in the series and a physical argument are given to estimate the magnitude of the remaining terms in the series.
Rotational Relaxation of Molecules in Fluids for Reorientations of Arbitrary Angle
Authors: Cukier Robert I. , Lakatos-Lindenberg Katja
Published: February 2004

A theory of rotational motion of molecules in liquids is presented. The traditional Debye diffusion model is generalized so as to include arbitrary reorientation angles by means of a nonlocal master equation. Random walk models are a special case of this generalization. We have obtained the conditional probabilities for the orientation of a body and of a vector fixed in the body for arbitrary body geometry. These results are then specialized to specific symmetries. From these conditional probabilities we calculate correlation functions which describe a large number of spectroscopic experiments such as NMR, ESR, NQR, dielectric relaxation, and infrared and Raman spectroscopy. We find that the form of these results for the correlation functions depend only on medium isotropy and body geometry. The same number of decaying exponentials are found as in the diffusion model regardless of the details of the reorientation mechanism. Only the amplitudes and relaxation times depend on these details.
Isotope Effect on Vibrational Excitation Induced by Collision of H2 and D2 Molecules
Authors: Bergeron Guy , Chapuisat Xavier
Published: February 2004

Some effects of the rotational motion of a vibrating rotator on its collisionally induced vibrational excitation are studied for H2 and D2 molecules initially at rest colliding with a He atom. Whatever the collision energy, the vibrational energy transfers are almost equal, due to cancellation between two dynamically opposed effects (in terms of collisional adiabaticity): the rotational modulation effect on the vibrational coordinate favors the vibrational energy transfer in H2, the frequency effect in D2. Numerical results (derived from computed trajectories) are presented to confirm this interpretation.
Electronic Energy Transfer Phenomena in Rare Gases
Authors: Gedanken Aharon , Jortner Joshua , Raz Baruch , Szöke Abraham
Published: February 2004

In this paper we present the results of an experimental study of electronic energy transfer in xenon-argon, krypton-argon, and xenon-krypton gaseous mixtures excited by a pulsed electric discharge. Spectroscopic evidence for electronic energy transfer is based on the decrease in the intensity of the vacuum ultraviolet emission of the excited diatomic homonuclear rare gas molecules in the presence of small amounts (10–1000 ppm) of a foreign rare gas atom, while the visible emission spectrum of the host gas is parctically unmodified under these conditions. The relative contributions of two energy transfer mechanisms involving atom-atom and molecule-atom energy transfer were established by a kinetic analysis of the dependence of the energy transfer efficiency on the host pressure. We have determined the cross sections for energy transfer from the lowest metastable Ar and Kr excited states, and from the lowest excited state of Ar2* and Kr2* to ground state Xe, and from metastable excited Ar and from Ar2* to ground state Kr. The molecule-atom energy transfer process is characterized by large ∼ 10−14 cm2 cross sections. A simplified theoretical treatment of excited molecule-ground state atom collisions provides a proper rationalization of these large cross sections in terms of long range dipole-dipole coupling.
Dielectric Tensor Components of Phenanthrene (C14H10) and Chrysene (C18H12) Derived from Electron Energy Loss Experiments
Authors: Venghaus H.
Published: February 2004

Electron energy loss spectra in the range from 0 to 40 eV are taken with 60 keV electrons on phenanthrene and chrysene single crystals. Different structure of the loss functions depending on the direction of the transferred momentum is taken into account. The orientation of the x and z axis of the dielectric tensor (not determined by crystal symmetry) is evaluated experimentally and discussed in relation to dipole transition moments. By means of a Kramers-Kronig transformation the main axis elements of the dielectric tensor are computed from the energy loss functions. The results obtained are interpreted in comparison with the energy and polarization of excited molecular states.
Projected States of Open Shell Molecules: The Pi-Electron States of the Cyclopentadienyl Cation
Authors: Phillips Donald H. , Schug John C.
Published: February 2004

Wavefunctions of pure spin and spatial symmetry were projected from the unrestricted pi-electron wavefunction of the cyclopentadienyl cation. Since the M=1 triplet functions for this molecule are forced eigenfunctions of S2 and the molecular symmetry operators, the calculations were carried out using a broken symmetry M=0 function for the starting point of the semiempirical unrestricted treatment. This procedure has the advantage of providing an unrestricted wavefunction for the lowest singlet state along with the unrestricted triplet wavefunction. The energy lowering produced by the unrestricted treatment followed by projection, as compared to that found using the molecular orbitals determined by symmetry, was greatest for the E2′ singlet state. The energy lowering produced by projecting the A2′ spatial component of the triplet was as great as the energy lowering produced by spin projection. The major results were only slightly affected by varying the semiempirical parameters by a fraction of their initial values. The A2′ triplet was the lowest energy state for all of the values used for these parameters.
Time Dependence of Saturation in Microwave Spectroscopy
Authors: Schwendeman R. H. , Pickett H. M.
Published: February 2004

Previously derived expressions for the absorption coefficient for microwave spectroscopy under conditions of partial saturation are critically examined and extended to times shortly after the onset of the radiation. It is found that the absorption coefficient varies with time as an exponentially damped sine wave. It is shown that the absorption coefficient and the population difference do not have the same time dependence and the discrepancy is explained as being the result of population changes caused by collisions. A semiclassical argument is given for the validity of the differential equations obtained from the concept of strong collisions. The relation between the time constant of the exponential damping and the conventional linewidth is discussed.
Reactions of Hot (n, γ)-Produced 80Br with CH3Br. Comparison with the Reactions between Hot 38Cl and CH3Cl, and Calculation of the Excitation Functions
Authors: Alfassi Zeev B. , Amiel Saadia , Baer Michael
Published: February 2004

The reactions of hot (n, γ)-produced 17.6 m 80Br with methyl bromide were studied in the presence of He and Kr as moderators. The survival probability of the reaction products was found to be the same in both moderators, suggesting that no observable decomposition took place upon moderation (within a 15% error). The reactivity integral for halogen displacement in the reaction was found to be equal to that in the system Cl + CH338Cl, which seems reasonable since the activation energies and steric factors are almost equal. The ratio of hydrogen displacement to halogen displacement was smaller in the case of 80Br than in that of 38Cl, as can be predicted on the basis of the translational inertial factor. From the reactivity integrals and the activation energies the excitation functions for halogen displacement were calculated by the energy-dependent hard potential model of Baer and Amiel.
Semiclassical Collinear Atom-Oscillator Collisions: An Average over the Thermal Vibrational Distribution
Authors: Morse Robert I.
Published: February 2004

A simple closed form expression for the energy transfer in a collinear atom-oscillator collision averaged over a thermal distribution of initial oscillator states is given. The expression is evaluated for a He–W atomic oscillator in a semiclassical approximation. The necessity of including multiquanta transitions is shown.
On the Schrödinger-Langevin Equation
Authors: Kostin M. D.
Published: February 2004

It is shown that the Heisenberg-Langevin equation can be used to derive a Schrödinger equation for a Brownian particle interacting with a thermal environment. The equation derived is iℏ(∂ψ/∂ t)=−(ℏ2/2m)∇2ψ+Vψ+VRψ+[(ℏ f/2im) ln(ψ/ψ*)+W(t)]ψ(r, t),          W(t)=−(ℏ f/2im)∫ψ* ln(ψ/ψ*)ψ dr,where f is the friction constant and VR is a random potential.
Dielectric and Pressure Virial Coefficients of Imperfect Gases. V. Octopole Moments of CH4 and CF4
Authors: Bose T. K. , Sochanski Jerzy S. , Cole R. H.
Published: February 2004

Dielectric second virial coefficients varying inversely with temperature have been determined by cyclic expansion measurements at 6.6, 49, and 100°C in the pressure range 1–100 atm. Attributing the temperature dependent contribution to dipole moments induced in one molecule by the octopole moment of a second gives 3.0× 10−34 and 4.4× 10−34 esu for the molecular octopole moments of CH4 and CF4.
Normal Stress Effects in Cholesteric Mesophases
Authors: Erhardt P. F. , Pochan J. M. , Richards W. C.
Published: February 2004

Normal forces have been found in rheological studies on cholesteryl chloride-cholesteryl oleyl carbonate liquid crystal mixtures over the composition range 0–27 wt.% cholesteryl chloride. These normal forces appear to be associated with the homeotropic texture and are evident in the transition region for the shear induced focal conic to homeotropic transition. The Kotaka model for normal stresses arising from hydrodynamic interactions of rotating rods has been applied. The results are consistent with a model of the homeotropic texture of the cholesteric mesophase comprised of rodlike helical arrays of molecules rotating in an applied shear field. The calculated rod lengths are less than the characteristic pitch of the material.
Mixture-Model Approach to the Theory of Classical Fluids. II. Application to Liquid Water
Authors: Ben-Naim A.
Published: February 2004

The possibility of viewing liquid water as a mixture of various species is examined within the framework of classical statistical mechanics. In the first part of the paper the application of the so-called ``Two Structure Model'' to liquid water is examined. Two novel features of the present treatment are: (1) An exact classification procedure is employed to transform a one-component system into a mixture of two components. (2) No assumption as to the nature of the mixture, such as ideality, is introduced. The second part of the paper is devoted to a more generalized mixture-model formalism. Two representative examples are discussed in some detail. The first demonstrates that the volume of the Voronoi polyhedron (or Dirichlet region) of a given molecule may be viewed as the partial molar volume of certain species. The second one treats the binding energy of a molecule as part of the partial molar energy of certain other species.
Mass Spectrometric Study of the Gas-Phase Hydration of the Monovalent Lead Ion
Authors: Tang I. N. , Castleman A. W.
Published: February 2004

Thermodynamic properties of hydrated monovalent lead ions in the gas phase were measured over the temperature range 2–144°C using high-pressure mass spectrometry. Clusters of up to eight water molecules about a lead ion were observed. The hydration enthalpies and entropies for the clusters, Pb+(H2O)m, m≤ 6, are derived from appropriate van't Hoff plots. The implication of the data in terms of the bonding of the small clusters and the distribution of lead ions in the atmosphere are discussed.
Rejection of the Devonshire Theory of Accommodation Coefficients in Gas-Surface Interactions (Including a Correction for Bound State Transitions)
Authors: Goodman Frank O. , Gillerlain Joseph D.
Published: February 2004

The main object of the paper is to reject the one-dimensional (1D) quantum theories, in particular the Devonshire theory, of accommodation coefficients (ACS). Effects of transitions of a gas atom into bound states at the solid surface are included in the calculations. It is shown that the 1D theory cannot be made to fit the modern experimental data on the ACS of He–W for any realistic interaction parameters. The procedure of dividing the 1D theoretical results by a factor is rejected as a basis for calculation of the ACS. Results from the 1D theory are compared with corresponding preliminary results from a new 3D theory and with the experimental data. Reasons why the 3D fits are so much better than the corresponding 1D fits are discussed. The authors conclude that the 1D models must now be rejected.
Flash Desorption and Decomposition of Benzene from Clean and Hydrogen or Carbon Contaminated Polycrystalline Tungsten
Authors: Cratty Leland E. , Hansen Robert S.
Published: February 2004

Studies of the flash decomposition of benzene on clean and hydrogen-predosed polycrystalline tungsten filaments show that benzene is adsorbed rapidly and irreversibly on such surfaces. Displacement stoichiometries and surface area considerations strongly favor a dissociative adsorption model (δ-bonded phenyl group) for this process. Preadsorbed hydrogen is displaced preferentially from high temperature sites, indicating that these sites are preferentially occupied by the chemisorbed phenyl groups. The irreversibly chemisorbed benzene undergoes neither exchange nor hydrogeneration with preadsorbed or gas-phase hydrogen, and is therefore not an important intermediate in the catalysis of these reactions by tungsten. Flash decomposition of chemisorbed phenyl groups involves sequential loss of hydrogen atoms from phenyl group to surface and their subsequent desorption. The transfer of hydrogen to the surface is the rate limiting step except at very small coverages, in which case desorption of hydrogen from high-temperature hydrogen sites is rate limiting. Two low temperature molecular desorption peaks are also observed when the filament is heavily dosed with benzene at 95°K. A strong molecular desorption of benzene is observed on a carbon-contaminated filament, and this state may be an important intermediate in catalytic hydrogenation and exchange reactions.
Flash Photochemical Study of the Reaction OH+NO+M Using Resonance Fluorescent Detection of OH
Authors: Stuhl F. , Niki H.
Published: February 2004

The kinetic behavior of OH radicals was studied in the presence of a large excess of NO at 5–82 torr He. Hydroxyl radicals were produced by pulsed vacuum-uv photolysis of H2O and were monitored directly by a resonance fluorescence method. The rate constant for the reaction OH+NO was nonlinearly dependent on the concentration of diluent He in this pressure range. The high pressure limit of the bimolecular rate constant was determined to be (2± 1)× 10−12cm3molecule−1· sec−1 at 300°K.
Temperature Dependence of O(1S) Deactivation by CO2, O2, N2, and Ar
Authors: Atkinson R. , Welge K. H.
Published: February 2004

The collisional deactivation of O(1S) by CO2, O2, N2, and Ar has been investigated as a function of temperature in the range from 200 to 450°K. O(1S) was produced in a pulsed mode by vacuum ultraviolet flash photolysis of CO2. The decay of O(1S) was observed by the 5577 Å emission as a function of time after the production pulse. Deactivation rate constants were derived from the dependence of the decay rate on the concentrations of added gases. Arrhenius plots yielded the following results: kCO2=3.3× 10−11 exp(−2630± 200/RT) cm3molecule−1· sec−1,kO2=4.9× 10−12 exp(−1700± 200/RT) cm3molecule−1· sec−1,kN2<5× 10−17cm3molecule−1· sec−1 (independentoftemperature),kAr<5× 10−17cm3molecule−1· sec−1 (independentoftemperature).In the case of N2 and Ar the collisional enhancement effect of the O(1S) emission was observed and was found to be independent of temperature.
Zwanzig Model for Liquid Crystals
Authors: Straley J. P.
Published: February 2004

The restricted-orientation model for liquid crystals is discussed. Results for models with dodecahedral and icosahedral geometry are presented. It is concluded that these models are not useful for the discussion of real liquid crystals.
NMR of Some Nitroxide Radicals: 13C Coupling Constants
Authors: Hatch G. F. , Kreilick R. W.
Published: February 2004

We have investigated the 13C NMR spectra of a series of nitroxide radicals to determine the sign and magnitude of the electron-carbon coupling constants. Couplings from the α–γ carbons have been observed. These couplings are discussed in terms of the relative geometry of the various radicals.
EPR Studies of the uv-Induced Free Radical Conversion in γ-Irradiated Amino Acid Crystals
Authors: Fujimoto M. , Saito S.
Published: February 2004

Ultraviolet energy absorbed by γ-irradiated α-amino acids (glycine, L-alanine, and methylalanine) at 77°K was found to induce a conversion of the trapped radical to another species, which was identified as (NH2CRR′CO2H)−(R,R′=H,CH3). The latter radical was stable in the temperature range 77–250°K, but converted irreversibly back to the former, on raising the temperature beyond 250°K. At room temperature, uv irradiation induced no particular effect except for a noticeable loss in EPR intensity. It was interpreted that the uv excitation in the trapped radical caused a subsequent electron transfer to an adjacent host molecule. During the thermal process above 250°K, most of the captured electrons are believed to return to the original radicals.
Nuclear Quadrupole Resonances in 1,3-Dimethylurea and Tetramethylurea
Authors: Dinesh  , Rogers Max T.
Published: February 2004

Pure NQR frequencies and nuclear quadrupole coupling constants have been determined as functions of temperature for 14N in 1,3-dimethylurea and tetramethylurea. The values of |; e2qQ | are larger than that reported for urea indicating that there is a decrease in the population of the nitrogen 2p orbital perpendicular to the molecular plane on going from urea to tetramethylurea. Since CNDO calculations indicate that the reverse would be expected for the free molecules, the observed trend is attributed to decreasing hydrogen bonding in the solid state with increasing methyl substitution.
On the Isotopic Shift for Large-Amplitude Vibrations
Authors: Redding Rogers W.
Published: February 2004

The isotopic frequency ratio νH/νD is calculated for a linear molecular model O–H–O where the hydrogen atom moves in a double-minimum potential. The results show that the isotopic ratio deviates considerably from the harmonic value but never becomes less than unity.
Thermodynamics of Curved Boundary Layers
Authors: Vieceli J. J. , Reiss H.
Published: February 2004

A general thermodynamic theory of curved boundary layers in one-component, one-phase fluids is presented; final results are displayed as the boundary analogs of the Gibbs adsorption equation and the Gibbs-Tolman-Koenig-Buff equation. The derivation does not rely on the conventional cone or wedge shaped container of interfacial thermodynamics and therefore avoids the difficulty associated with these containers when dealing with a microscopic boundary surface as in scaled particle theory. The method of derivation and results, although of particular importance to scaled particle theory, are of general interest because of the fundamental distinction between boundary and interfacial effects.
Anharmonic Effects on the Rate of Relaxation of Vibrational Energy in Rapidly Expanding Flows
Authors: Center R. E. , Caledonia G. E.
Published: February 2004

Detailed numerical results are presented for the time dependent vibrational relaxation process in the rapid expansion of anharmonic oscillators in nonequilibrium nozzle flows. Calculations for the expansion of CO, CO/Ar, and CO/N2 mixtures from high stagnation temperatures, in the range 2000–4500°K, are compared with previously reported experimental results. It is concluded that the effects of the anharmonicity of the molecules are unlikely to lead to an increase of more than a factor of 2 in the vibrational energy relaxation rate. A simple analytical result is derived in terms of the molecular constants and the expansion temperature ratio to predict the degree of distortion of the low level ``vibrational temperature'' which results from the anharmonicity.
Vibrational Excitation and Relaxation of HCl (V=2) State
Authors: Hopkins Barbara M. , Chen Hao-Lin
Published: February 2004

Vibrational fluorescence from V=2 to V=1 state is observed following excitation by a pulsed HCl chemical laser. The time dependence of the fluorescence is used to determine the rate of near-resonant vibration-to-vibration energy transfer, HCl(V=1)+HCl(V=1)⇄HCl(V=2)+HCl(V=0)+Δ E=104 cm−1. The cross section for this reaction in the exothermic direction at room temperature has been found to be 0.73± 0.15× 10−16 cm−2.
Model for Locally Compensated Cubic Sites of Tripositive Rare Earth Ions in Fluorite Crystals
Authors: O'Hare J. M.
Published: February 2004

The existence of a separate cubic phase RF3:3(MeF2), which provides a cubic symmetry site with local charge compensation for a tripositive rare earth ion in a fluorite lattice, is examined in order to explain the increase in cubic sites with increasing rare earth impurity concentration. An expression for the minimum concentration at which such a phase would be stable is presented. For the case of SrF2:Ge3+ or SrF2: (Gd3+, Ce3+) a calculation estimates that such a phase could form at 0.36% total rare earth impurity concentration.
Gas-Phase Electron Paramagnetic Resonance Absorption in a N 16O, N 17O, and N 18O Mixture
Authors: Ashford Nicholas A. , Jarke Frank H. , Solomon Irvine J.
Published: February 2004

The X-band EPR spectra of a gaseous mixture of 14N16O, 14N17O, and 14N18O are presented. Statistically significant differences in the spectra indicate that a unified theory of isotopic effects is derivable from the data. Further interpretation will be presented in a second paper. The oxygen-17 hyperfine interaction was found to be ≈ 57.4 G and a large oxygen-17 quadrupole interaction is indicated. The s character of the 2pπ electron on oxygen in nitric oxide is estimated to be 2.0%, in good agreement with that estimated for the isoelectronic molecule O2−17.
Characteristic Energy Losses by Slow Electrons in Organic Molecular Thin Films at 77°K
Authors: Hiraoka Kenzo , Hamill William H.
Published: February 2004

Characteristic energy losses by slow electron impact have been measured for ⪖102 Å films of several aromatic compounds as well as alkanes, alkenes, alcohols, acetone and ice at 77°K. Resonances are clearly indicated by the electron current I transmitted by the dielectric as a function of the modulated thermionic emitter voltage V, displayed as dI/dV vs V. Energy losses are measured directly by retarding potential analysis of backscattered electrons. Measurements at several incident electron energies provide the V-dependence of the cross section. The method is particularly useful for measuring small losses (∼1 eV). There is no potential barrier for injecting electrons into the dielectrics examined, and the evidence indicates that they have positive bulk electron affinities. All systems show resonances at 0.5–1 eV which are attributed to transient negative ions. Acetone, n-hexane and ice were examined in some detail for 0–4 eV losses by retarding potential analysis. The results indicate that electrons are also inelastically scattered by higher transient negative ion states. All compounds examined show resonances in the 3–4 eV range, which may be due to low lying triplet states. The alkanes also have considerable oscillator strength in the 4–7 eV range, approximating that in the 7–10 eV range.
Characteristic Energy Losses by Slow Electrons in Thin Films of Alkali Halides
Authors: Hiraoka Kenzo , Hamill William H.
Published: February 2004

The potassium halides and the alkali iodides have been examined by electron impact in the range 0–50 eV. Both electron transmission and backscattering currents (It, Ib) have been measured while sweeping the modulated accelerating or retarding potentials (Vi, Vr). The characteristic energy loss spectra are presented as dIt/dVi vs Vi or dIb/dVr vs Vr. The principal events, in order of increasing Vi or decreasing Vr, are: electron trapping by cations to ns 2S and np 2P alkalis, and possibly higher; usually three resonances attributed to low-lying states of halide ions, possibly 3P2, 1, 0 among others (triplet exciton); the fundamental singlet exciton; the vacuum level; the higher fundamental singlet exciton and vacuum level involving halogen ns 4P. The crystal electron affinities have been evaluated by two methods.
Multicomponent Cluster States in Dilute Mixed Molecular Crystals, with Application to 1B2u Naphthalene Excitons
Authors: Hong Hwei-Kwan , Kopelman Raoul
Published: February 2004

We develop the theory of Frenkel excitons for multicomponent cluster states in medium-dilute mixed molecular crystals. This theory can be applied to both secondary host traps induced by a single impurity and to multicompositional, chemical, or isotopic clusters. A Green's function technique, which is a generalization of the Koster-Slater method, is developed and utilized. Symmetry properties of such clusters are discussed, with emphasis on interchange equivalent sites in nonsymmorphic crystals. The optical spectra of naphthalene-h8 in naphthalene-d8 can now be further analyzed, with the help of some numerical calculations on multicomponent cluster states. Using our recently acquired dispersion relation for the 1B2u naphthalene exciton state we fit satisfactorily both the fine structure and the ``hyperfine'' structure, without any additional parameters, except for the experimentally known trap-depths of a few isotopic impurities. This corroborates both the importance of the exciton superexchange effect and the validity of the exciton dispersion formula.
Collision Matrix Elements near a Pseudocrossing of Potential Energy Curves
Authors: Oppenheimer M.
Published: February 2004

The behavior of the matrix element of the collision operator, d/dR, between adiabatic states with variations in R, the internuclear separation, is studied qualitatively for diatomic molecules. These matrix elements often have a maximum or a zero near the pseudocrossing of the two adiabatic potential energy curves. Effects of these extremes of behavior on cross sections are considered and limits on the validity of approximations which use R-independent orbitals are developed. Finally, the effect of a second pseudocrossing on these matrix elements is analyzed.
Proton Impact Double Ionization of Noble Gas Atoms
Authors: Roy B. N. , Tripathi D. N. , Rai D. K.
Published: February 2004

The cross sections for proton impact double ionization of noble gas atoms have been calculated using Gryzinski's semiclassical formula. The calculated cross sections are in good agreement with experimental results. Values of the ratio σ+/σ++ (single ionization cross section/double ionization cross section) have also been calculated for all noble gas atoms. It is found that the calculated variation of σ+/σ++ with incident proton energy agrees well with experimentally observed variation in the case of helium atom.
Vibrational Nonequilibrium Effects in the (H2, H2) Reactive System
Authors: Shizgal B.
Published: February 2004

The decrease in the forward rate of the (H2, H2) reaction from its equilibrium value due to the perturbation of the vibrational distribution function by reaction is calculated from a numerical solution of a non-linear Master equation. The inelastic and reactive cross sections that are required for this calculation were obtained from a Monte Carlo average of many quasiclassical collision trajectories. At 3200°K, the highest temperature studied, the decrease in the equilibrium rate of reaction was 0.21%. The vibrational relaxation time for H2 in the absence of reaction was also calculated and found to obey a Landau-Teller temperature dependence (Pτ)calc=62× 10−9 exp (148/T1/3) atm·sec. over the range 1200–3200°K.
Diffusion and Reorientations in Crystalline Acetylene
Authors: Albert Shmuel , Ripmeester John A.
Published: February 2004

Crystalline acetylene (HC≡CH) in its two solid modifications was studied by means of temperature dependent static and rotating frame spin-lattice relaxation time measurements, T1 and T1ρ respectively, between 188–122°K. The presence of molecular motions was established in both phases. Two types of motions were observed in the high temperature cubic phase. One of the motions was associated with a minimum in the T1ρ curve below the melting point and also gave rise to a marked increase in the length of the free induction decay and was assigned to the onset of self-diffusion. The other motion gave a minimum in the T1 plot near 170°K and a corresponding decrease with temperature in the T1ρ curve. Calculations of the longitudinal relaxation time behavior, as well as second moment considerations, for models of twofold flipping (assumes independent motion with two correlation times τc and τc/2) and spherical rotations (assumes a single correlation time τc) of the acetylene molecule, are both in agreement with the experimental data. However, the T1ρ results are in better agreement with the spherical reorientation model which, actually, may approximate to a good degree a twofold flipping motion about two mutually perpendicular axes. Discontinuities in the T1 and T1ρ plots were observed at the phase transition. In the low temperature phase relatively slow twofold flips or, less likely, spherical reorientation occurs. Motional parameters such as activation energies, pre-exponential factors, etc., were deduced from our data, and it was found that they vary only slightly at the transition. In light of the results obtained in this study it is evident that the solid phase of acetylene stable immediately below the melting point is of a plastic crystalline nature.
Microwave Absorption and Molecular Structure in Liquids. LXXIX. Dielectric Behavior and Association of Some Normal Alcohols in n-Heptane Solution
Authors: Glasser L. , Crossley J. , Smyth C. P.
Published: February 2004

Dielectric constants and losses at 25°C of 1-butanol, 1-hexanol, 1-heptanol, and 1-decanol, each at six concentrations from 0.03 to 0.3 mole fraction in n-heptane, have been measured at wavelengths from 2 mm to 600 m. At the lower concentrations, the dielectric absorption may be resolved into two dispersion regions, characterized by relaxation times τ3 (≈ 3 psec and almost concentration independent) and τ2 (ranging from 25 to 100 psec and concentration dependent). The latter dispersion increases its contribution relative to the former with increasing alcohol concentration. At the higher concentrations a final process with a long relaxation time, τ1 (strongly concentration dependent), appears. This third process corresponds to the principal low-frequency dispersion already well characterized in pure primary normal aliphatic alcohols. The high-frequency process, because of its very short relaxation time, is attributed to rotation of the hydroxyl group about its C–O bond, the intermediate process is related to the reorientation of the monomer and possibly small multimers or a polymer chain segment, such as an -OR group rotating about its bond, while the low-frequency process involves hydrogen-bonded structures of some considerable size.
Laser-Excited Electronic Fluorescence: Collision-Induced Radiationless Transitions in Propynal
Authors: Thayer C. A. , Yardley James T.
Published: February 2004

A pulsed dye laser tuned to 3821 Å is used to excite vapor phase propynal molecules, making possible the observation of time resolved luminescence from the A″ states. The collision-free lifetime for the ground vibrational level of the 1A″ electronic state is 0.978 μsec. Quenching of this state by 19 different collision partners with dipole moments ranging from 0 to 3.95 D has been examined at room temperature. Cross sections ranging from 8 to 175 Å2 have been found. The larger cross sections appear to be associated with the more polar collision partners and evidently arise from long range forces. Evidence is presented that quenching is the result of a spin-allowed transition. A somewhat general theory for collision-induced radiationless transitions is proposed which appears to correlate the observations for propynal. As an example of the applicability of this model to other molecular systems, the theory is applied to earlier data on I2.
Electrons Trapped on Dislocations in Alkali Halide Crystals
Authors: Deutsche Craig W.
Published: February 2004

Experimental work performed by various groups in the past has suggested that electrons may be trapped upon edge dislocations in alkali halide crystals. Measurements of the magnetic properties of these crystals were tentatively interpreted in terms of rather large numbers of such electrons. While electrons might be trapped by the deformation potential associated with the elastic strain at the dislocation, repulsive Coulombic interactions must necessarily limit the total number of electrons which may be accomodated. Using an approximate Hamiltonian including some effects of electronic exchange a variational calculation of electronic energy was performed for several different forms of trial wavefunctions. The trapped electron density calculated under the most reasonable assumptions is less than the experimentally estimated unpaired spin density. There are, however, significant uncertainties in both these quantities. It is therefore particularly important that no conditions could be found which allowed orbital degeneracy for the trapped electrons. This fact argues strongly against a simple electronic origin of the observed magnetic moments and, consequently, other explanations are tentatively proposed.
Density Matrix Study of Atomic Ground and Excited States. II. Beryllium 1S Excited State
Authors: Olympia Pedro L. , Smith Darwin W.
Published: February 2004

An 85-configuration CI wavefunction for the first beryllium 1S excited state is reported and discussed. The total energy obtained is −14.404518 a.u. compared to the Hartree-Fock and experimental nonrelativistic energies of −14.3622 and −14.4178 a.u., respectively. The 1 and 2 matrices and geminal energies of the Be ground and first excited states are compared in detail to clarify the changes undergone by the natural states on excitation.
Infrared Spectrum of Gaseous Methyl Radical by Rapid Scan Spectroscopy
Authors: Tan Luisa Y. , Winer Arthur M. , Pimentel George C.
Published: February 2004

Gaseous methyl radical was produced through flash photolysis of methyl iodide and of dimethyl mercury and a portion of its infrared spectrum was recorded (450–740 cm−1) using a rapid scan infrared spectrometer with scan rates up to 1 cm−1/μsec. Two absorptions were observed for CH3, at 607.0 and 603.3 cm−1, two for CH2D, at 561.2 and 558.0 cm−1, and one for CD3 at 460.6 cm−1, with a second possible feature at 457 cm−1. These frequencies are close to the corresponding matrix frequencies, which permits these bands to be assigned to ν2, the out-of-plane (umbrella) mode of CH3. The possibility that the doublets are due to inversion doubling of a pyramidal molecule is considered but evidence is presented to show that the lower frequency component is due, instead, to vibrationally excited methyl radical. The out-of-plane vibrational potential function is calculated assuming CH3 is planar. If the harmonic oscillator approximation is used, k2=0.174 mdyn/Å but the CD3 frequency shows that the quartic contribution is not negligible. If a quartic term is included, the force constants obtained are k2=0.137 mdyn/Å and a=0.098 mdyn/Å3.
Viscosity of the Binary Gaseous Mixtures of Nitrogen with Argon and Krypton
Authors: Hellemans J. M. , Kestin J. , Ro S. T.
Published: February 2004

This paper presents new measurements of the viscosity of the binary mixtures N2–Ar and N2–Kr both at zero density and in the temperature range 25–500°C. The measurements are compared with extensive calculations based on the extended law of corresponding states proposed by Kestin, Ro, and Wakeham. It is found that the calculations represent all data with a standard deviation of 0.33% and a maximum deviation of less than 0.5%. This is taken as new and additional evidence for the validity of the law of corresponding states. The latter is used as a device for the interpolation and extrapolation of the data in the form of tables ranging from 0°C to 2000°C for the mole fractions 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 for each system.
Raman Spectra of α and β Cristobalite
Authors: Bates John B.
Published: February 2004

Raman spectra of polycrystalline crystobalite were measured over the frequency interval of the fundamental optical modes from 77°K to 538°C. Spectra recorded above and below the α–β transition temperature were strikingly different. The strong bands measured at 416 and 230 cm−1 with α cristobalite at 25°C were not observed from spectra of the β phase. The Raman spectrum of β cristobalite exhibited only three bands and, in contrast to crystalline quartz, a strongly temperature dependent band was not observed from measurements at temperatures near the transition point. Infrared reflection spectra of α cristobalite also were recorded, and the longitudinal optical mode frequencies were determined. Raman spectra of β cristobalite showed that the structure of this material should not be employed as a lattice model for noncrystalline SiO2.