bluuebunny/crossref_metadata_2025 huggingface dataset:
Yield of singlet and triplet excitons from x-ray and ruby laser excitation of anthracene single crystals
Authors: Hughes R. C. , Soos Z. G.
Published: February 2003

A new method for determining the singlet:triplet yields for x-ray and ruby laser excited anthracene crystals is described. It was found that the ratio is higher than has been reported for x-ray excitation and lower than reported for ruby laser excitation. X-ray excitation produces roughly equal numbers of triplets and singlets. Ruby laser excitation indicates that the intersystem crossing rate from the first excited singlet is 1.5±0.5×107 sec−1, which is larger than had been reported, but still smaller than for anthracene molecules in solution. The greater triplet yield produced by x irradiation is related to efficient spin–lattice relaxation in highly excited electronic states and neither spin–spin nor spin–lattice relaxation in the electron and hole ground states is important in the short time involved in geminate recombination.
Thermochemical properties of gaseous ZrO and ZrO2
Authors: Murad Edmond , Hildenbrand D. L.
Published: February 2003

The dissociation energy of gaseous ZrO, D0° (ZrO), and the heat of atomization of gaseous ZrO2 have been measured using a high temperature mass spectrometer. A number of equilibria were studied and the following results were derived: ΔfH (ZrO,g,298.15 K) =83±12 kJ/mol, D0° (ZrO) =761±15 kJ/mol) (or 7.89±0.15 eV), ΔfH (ZrO2,g,298.15K) =−298±30 kJ/mol, and D0° (ZrO2) =1386±30 kJ/mol (or 14.4±0.3 eV). The ionization potentials of ZrO and ZrO2 were derived from the measured appearance potentials. In the case of ZrO it was necessary to make corrections for the thermal populations of low-lying electronic states. The derived values are I.P.(ZrO) =6.0±0.2 eV, I.P.(ZrO2) =9.4±0.2 eV. These measurements are discussed in relation to other studies. Uncertainties in the derived thermochemical quantities owing to uncertainties in the spectral data are discussed. The chemical bonds in ZrO and ZrO2 are briefly discussed.
The excitation and quenching of N(2P)
Authors: Young R. A. , Dunn O. J.
Published: February 2003

The rate coefficients for quenching N(2P) by N, O, NO, CO, H2, O2, N2O, and CO2 have been measured by following the decay of emission from N(2P) excited in a Tesla discharge by energy transfer from N2(A 3Σ+u). Some of these results differ considerably from those in the literature.
Hartree–Fock and Gordon–Kim interaction potentials for scattering of closed-shell molecules by atoms: (H2CO,He) and (H2,Li+)
Authors: Green Sheldon , Garrison Barbara J. , Lester William A.
Published: February 2003

The Gordon–Kim (GK) electron gas model for calculating the forces between closed-shell atoms and molecules is applied to the He–H2CO and Li+–H2 systems. GK interaction energies are computed following the original theory and also including the self-energy correction suggested by Rae. GK interaction energies, neglecting correlation, are found to be in qualitative accord with Hartree–Fock (HF) interaction energies for the two systems. However, quantitative discrepancies are noted which are possible sources of error if GK potential energy surfaces are used to compute accurate scattering cross sections.
Debye temperatures and cohesive properties
Authors: Abrahams S. C. , Hsu F. S. L.
Published: February 2003

Values of crystalline-solid Debye temperatures depend both on the method and temperature of measurement. Simple relationships between Debye temperatures and such cohesive properties as compressibility and melting point were derived over 60 years ago by Madelung, by Einstein, and by Lindemann. Debye temperatures ΘXR of a number of piezoelectric semiconductors with chalcopyrite structure have been determined at room temperature by x-ray diffraction. A new series of heat capacity measurements over the temperature range 1.2–40 °K, for four chalcopyrites, give ΘD values at 0 °K. A common proportionality is found between each of these ΘXR and ΘD values and microhardness, and also melting point: Acceptable reproducibility is given for the AIBIIICVI2 chalcopyrites and, separately, the AIIBIVCV2 compounds. Values of Θ are predicted for 15 additional chalcopyrites. Excellent proportionality between Θelastic and compressibility is found for the europium chalcogenides, based on the data of Shapira and Reed.
Path integral representation of the reaction rate constant in quantum mechanical transition state theory
Authors: Miller William H.
Published: February 2003

Feynman’s path integral representation of the Boltzmann operator e−βH is used to express the rate constant of a recently formulated quantum mechanical version of transition state theory. By evaluating the path integral in two separate stages, one is able to interpret the result as a generalization of a model suggested several years ago by Johnston and Rapp for handling the nonseparable aspect of tunneling in transition state theory. A Fourier series expansion of the path integral is also developed, and this approach has promise for direct numerical evaluation of the quantum rate expression.
Continuum radiation and potentials of Na–noble gas molecules
Authors: York George , Scheps Richard , Gallagher Alan
Published: February 2003

The normalized emission spectra resulting from the perturbation of Na atoms by 1–1000 Torr of noble gas have been measured in the range 10–150 nm about the 589 nm resonance line. This spectrum, due to the A–X and B–X transitions of the Na–noble gas molecules, was measured with 1.5 nm resolution. The Na, in a cell at a temperature of 417±5 °K, was optically pumped to the 3 2P1/2 and 3 2P3/2 states by resonance radiation. The resulting emission was measured at several Na densities to obtain the normalized emission spectra in the limit of zero radiation entrapment. Using the Franck–Condon principle, or equivalently the quasistatic theory of line broadening, and density factors, the A 2Π1/2,3/2 and X 2Σ1/2 state potentials for NaAr, NaKr, and NaXe have been obtained from the pressure dependence of the spectra. Continuum absorption and emission coefficients for these molecules, obtained by application of the same theories, are also presented. The analysis identifies the low pressure limit of the spectrum as due to free collision states above the angular momentum barrier. The population factors corresponding to quasibound, orbiting resonances are evaluated and used in this analysis.
Extrinsic photoemission and diffusion of ’’free’’ excitons in solid xenon
Authors: Ophir Zohar , Schwentner Nikolaus , Raz Baruch , Skibowski Michael , Jortner Joshua
Published: February 2003

Extrinsic photoemission from pure solid rare gases at energies below the direct photoemission threshold is assigned to the diffusion of ’’free’’ mobile excitons (prior to exciton trapping) to the gold (emitter) substrate followed by exciton-enhanced electron ejection from the electrode. From a semiquantitative study of the extrinsic photoemission yield of solid Xe as a function of the film thickness we get the diffusion length l?300 Å, for exciton migration on the time scale of 10−11–10−12 sec.
Elastic and rotationally inelastic H–H2 and H–D2 collisions
Authors: McGuire Paul , Krüger Heinz
Published: February 2003

The coupled-states approximation in the body-fixed coordinate system is applied to the study of elastic and rotationally inelastic H–H2 and H–D2 collisions. For both systems, integral and differential cross sections are computed in the collision energy range 0.5–1.5 eV. The isotope effect in the integral cross sections for j=0→j ′ (j ′=0,2,4,6) is found to be more pronounced for increasing j ′. In addition, as the ratio of rotational energy transferred to kinetic energy, i.e., ΔE/E, decreases, the differential cross sections shift from backward to forward peaked.
Spin density distributions in primary oxidation and reduction products of irradiated nucleic acid constituents
Authors: Adams Stephen M. , Box Harold C.
Published: February 2003

Molecular orbital calculations of spin density at the INDO level of approximation have been performed for the primary oxidation and reduction products of irradiated nucleic acid constituents and for some related molecules. For the pyrimidine anions INDO spin densities are in qualitative agreement with experimentally obtained spin densities. INDO spin densities for certain of the neutral oxidation products do not agree with experimental results.
Measured cross section for the formation of H+2 by fast proton impact on methane
Authors: Cook C. J. , Smyth N. R. A. , Heinz O.
Published: February 2003

The classical theory of ion–molecule rearrangement collisions proposed by Bates, Cook, and Smith for the capture cross section for the formation of H+2 in the reaction H++CH4→CH3+H+2 was confirmed. Measurements at incident proton energies of 70, 85, 100, and 150 eV gave cross sections peaking sharply at scattering angles (laboratory coordinates) of about 46° as predicted. Typical values of the total cross section are 2.0×10−21 cm2 at 70 eV and 7.6×10−22 cm2 at 100 eV, a factor of 30 less than the theoretical values, which are believed to represent upper limits to the cross section. The asymmetrical shape of the differential cross section is consistent with that predicted by a classical construction method described by Gillen, Mahan, and Winn.
Stark energies for molecules with vibrationally induced dipole moments
Authors: Muenter Annabel A. , Dyke T. R.
Published: February 2003

For the various classes of molecules capable of having vibrationally induced electric dipole moments, explicit expressions for the resultant Stark energies are derived. In addition, the change in the vibrationally induced Stark effect with isotopic substitution of the B atoms in a tetrahedral AB4 molecule is considered.
ESR spectrum and structure of BrF6
Authors: Nishikida Koichi , Williams Ffrancon , Mamantov Gleb , Smyrl Norman
Published: February 2003

The BrF6 radical has been obtained by γ irradiation of a mixture of BrF5 in SF6. The ESR spectrum was obtained and used to confirm the identity of the radical and to obtain hyperfine coupling constants. (AIP)
Close coupled calculations of electron–hydrogen atom scattering using a noniterative integral equation technique [J. Chem. Phys. 57, 4770 (1972)]
Authors: Knirk Dwayne L. , Hayes Edward F.
Published: February 2003

Some comments are made upon the origin and implementation of the equations for electron–hydrogen atom scattering presented in the earlier paper by Kirk and Hayes, J. Chem. Phys. 57, 4770 (1972). (AIP)
Electric field gradients in CuBr2
Authors: Liu Y. S.
Published: February 2003

The electric field gradients at both the Br− site and the Cu2+ site in a single crystal of CuBr2 have been calculated using a point charge model and taking the effect of chemical bondings into consideration. Comparison with experimental data in NQR has been made and the value 〈r−3〉3d of the electrons of Cu2+, as well as the shielding effect of the inner electron core are discussed. The ionicity is also deduced.
On the display of basic properties of the molecular electronic wavefunctions in the (e,2e) quasielastic knockout experiments
Authors: Levin V. G. , Neudatchin V. G. , Pavlitchenkov A. V. , Smirnov Yu. F.
Published: February 2003

Some theoretical aspects of quasielastic knockout of an electron by a fast electron from molecules are discussed and a few general formulas are given. Angular correlation curves of two final electrons are calculated for loosely bound electron orbitals in molecules Li2, N2, F2, BH, CH, LiH, and HF to illuminate the influence of both hybridization and kind of of bonding (ionic, covalent) on (e,2e) form factors.
Studies of inelastic molecular collisions using impact parameter methods. III. Line shape functions
Authors: Fisanick-Englot Georgia , Rabitz Herschel
Published: February 2003

Extension of impact parameter methods within an effective Hamiltonian framework is made to yield generalized cross sections or transition rates such as those encountered in spectral line broadening. An exact impact parameter expression is derived for the diagonal elements of the generalized cross sections. Approximations to the exact impact parameter expression are discussed, and it is shown that they involve two stages: (1) approximation of the S-matrix elements and (2) replacement of a sharply peaked nonlocal impact parameter kernel by a delta function. The special case of a high-energy exponential impact parameter form is used for illustration.
Phase equilibrium isotope effects in molecular solids and liquids. Vapor pressures of the isotopic nitrous oxide molecules
Authors: Yato Yumio , Lee Myung W. , Bigeleisen Jacob
Published: February 2003

The vapor pressures of three isotopic samples of N2O enriched in 15N and 18O have been compared with normal N2O over the temperature range 143–219 ° K. Over the temperature range studied in the solid and in the liquid in the neighborhood of the triple point, the vapor pressures are in the sequence 14N216O≳ 14N15N16O≳15N14N16O≳ 14N218O. The crossover phenomena have been observed for 15N14N16O/14N218O at 207 °K and for 14N15N16O/14N216O at 215 °K. The experimental data confirm hindered translation in both the solid and the liquid as well as the potential energy coupling of the translation and rotation found in CO2. Shifts in the internal stretching and bending modes on condensation lead to significant differences in the vapor pressures of the isomers 15N14N16O and 14N15N16O. The latter has an abnormally large change in vapor pressure on melting compared with the other isotopic N2O molecules studied. This is associated with the coupling of the internal bend coordinate with the translation in the solid, which vanishes in the liquid. The isotope effect in the high temperature liquid is explained in terms of an anharmonicity of the external motions in addition to the temperature dependence of the stretching force constant. Model calculations of the logarithm of the reduced partition function ratios of the solid and liquid ln(fc/fg), are made in an anharmonic version of the Stern–Van Hook–Wolfsberg cell model. This model provides a good representation of the experimentally determined ln(fc/fg) for solid and liquid N2O.
Fluorescence and energy transfer near interfaces: The complete and quantitative description of the Eu+3/mirror systems
Authors: Chance R. R. , Miller A. H. , Prock A. , Silbey R.
Published: February 2003

The classical electromagnetic description of fluorescent emission and energy transfer in the Eu+3/mirror systems is shown to be in quantitative agreement with the results of eight experimental systems studied by the fatty acid monolayer assembly technique. The emitter lifetime measured as a function of distance from the mirror(s) is found to be consistent with an isotropic spatial orientation for the emitter in all cases; furthermore, it is shown to be exclusively so in most of these cases. Both the quantum yield and the radiative lifetime of the luminescent state of the Eu+3 ion are determined by theoretical fits to the data. Whereas the quantum yield spans a range of 0.69–0.86, the radiative lifetime of the electric dipole transition at 612 nm is nearly constant, as required, with a value of 803±29 μsec. Both the quantitative agreement between theory and experiment and the consistency among the eight experimental systems in predicting the radiative lifetime provide a clear demonstration of the utility of the fatty acid monlayer assembly technique as a method of measurement of emission properties of lunimescent systems and of dielectric properties of surfaces.
A search for coherent features in the angular dependence of the 10 eV and 14 eV energy loss lines in CCl4 using 25 keV electrons
Authors: Lee J. S. , Wong T. C. , Bonham R. A.
Published: February 2003

A reinvestigation of the angular dependence of the 10 eV and 14 eV energy loss lines in CCl4 has shown that the coherent features most easily observed are due to intermolecular multiple scattering. The high sensitivity of these lines to multiple scattering and their observed angular dependence strongly argues that both transitions must be dipole allowed. From this study it is concluded that previously predicted coherent structure in the angular dependence must be of a smaller magnitude than heretofore suspected.
Molecular relaxation processes of some halonaphthalenes in a polystyrene matrix
Authors: Tay S. P. , Walker S.
Published: February 2003

The dielectric absorption of eight halonaphthalenes in a polystyrene matrix has been measured in the frequency range of 102–105 Hz and in two cases also in the range of 2.2×104 to 5×107 Hz and the enthalpy of activation for the molecular relaxation process determined by using the Eyring rate expression. The relaxation time and enthalpy of activation vary as the inclination of the dipole to the principal axes is altered. In fact, the relaxation time is appreciably longer for the 2- than the 1-halonaphthalenes while the enthalpies of activation of the latter range between 18–22 kJ mol−1 and for the former 23–41 kJ mol−1. The enthalpy variation between the 1- and 2-halonaphthalenes is to be contrasted with the behavior of quinoline and isoquinoline in Nujol solution where the solute molecules have virtually identical enthalpies of molecular reorientation.
Mössbauer study of cytochrome c3
Authors: Ono Kazuo , Kimura Keisaku , Yagi Tatsuhiko , Inokuchi Hiroo
Published: February 2003

Oxidized cytochrome c3 enriched with 57Fe heme has been investigated using the Mössbauer effect of 57Fe. The isomer shift, quadrupole splitting, and magnetic hyperfine interaction were determined at 300, 77, and 4.2 K. The electronic state is low spin ferric state and is discussed in connection with ESR data. At 4.2 K, the heme–heme interaction was observed to be more than seven times stronger than expected. A strongly reduced relaxation was also observed in the spectra obtained in applied magnetic fields of a few hundred oersteds. This shows that the neighboring ferric ions are completely unequivalent with respect to orientations of the g tensor axes.
Observation of a new electronic transition in N2 at 31.4 eV by means of high energy electron impact spectroscopy
Authors: Lee J. S. , Wong T. C. , Bonham R. A.
Published: February 2003

A new transition in molecular nitrogen at an energy loss of 31.4±0.5 eV has been observed by high energy electron impact spectroscopy at momentum transfers larger than 2 a.u. The absolute generalized oscillator strength was measured over the momentum transfer range 0.5?K2?5.08 a.u. using 25 and 33 keV electrons. The observed angular behavior of this line suggests that it may be due to a dipole forbidden quadrupole allowed process. The term symbol 1Πg with the electronic configuration (2sσg)−1 (2pπg)1 is tentatively assigned to this transition and it is suggested that this state may decay into the D 2Πg state of N+2 by Auger cascade.
A wide-line NMR study of reorientation of some spherical-top molecules enclathrated in water
Authors: Garg S. K. , Gough S. R. , Davidson D. W.
Published: February 2003

NMR spectra at temperatures down to 1.8 K are given for the 19F resonance of SF6 and CF4 in H2O and D2O clathrates and for the 1H resonance in CH4–D2O and CH4–tetrahydrofuran-d8–D2O clathrates. The second moments correspond effectively to isotropic reorientation of encaged SF6, CF4, and CH4 molecules at temperatures above 13, 22, and 4 K, respectively. The CH4 spectra are only slightly broadened at 1.8 K. For SF6 and CF4 a low-temperature transition in second moment is characterized by the superposition on the rigid lattice band of a narrow component whose intensity increases progressively with rise of temperature. This ’’apparent phase-change effect,’’ after Resing, is attributed to a very broad distribution of reorientational correlation times, here associated with orientational disorder of the water molecules of the lattice. The behavior during the transition agrees with a model which assumes a Gaussian distribution of activation energies about a mean value of 207 cal/mol for SF6 and 360 cal/mol for CF4.
De-excitation of the Ar 3P2, 3P1, and 1P1 states in pure argon and in Ar/H2 or Ar/CO mixtures
Authors: Bourene M. , Dutuit O. , Le Calve J.
Published: February 2003

The de-excitation of the 3P2, 3P1, and 1P1 states of argon was studied in pure argon between 10 and 200 torr and in Ar+CO and Ar+H2 mixtures. These states, which were followed by absorption spectroscopy, are populated after excitation of the gas by a short (20 ns) pulse of 500 keV electrons. The three body rate constants k2, which were not considered in previous low pressure studies, were measured for the two resonant states 3P1(k2=1.7×10−32 cm6 s−1) and 1P1 (k2=0.9×10−32 cm6 s−1). For the metastable 3P2, the measured value of k2 (1.6×10−32 cm6 s−1) is in good agreement with those found in the literature. However, our two body rate constant k1 for the 3P2 state is about ten times higher than values obtained previously from low pressure measurements. The rate constants for the quenching by CO and H2 were measured for the metastable state 3P2 (1.85 and 10.5×10−11 cm3 s−1) and for the resonant states 3P1 (4.5 and 20×10−11 cm3 s−1) and 1P1 (8.5 and 33×10−11 cm3 s−1).
Vacuum ultraviolet absorption spectra of solid N2O and CO2 at 53 K
Authors: Monahan Kevin M. , Walker William C.
Published: February 2003

Photoabsorption spectra of thin films of solid N2O and CO2 were obtained in the 105–240 nm wavelength region. The gas phase absorption continua were shifted by as much as 0.53 eV in the solid. Dipole forbidden transitions such as N2O(1Δ←1Σ+), CO2(1Δu←1Σ+g), and CO2(1Πg←1Σ+g) were intensified. The spin forbidden N2O(3Σ+←1Σ+) and N2O(3Δ←1Σ+) transitions were not detected even in 30 μm thick films. Dipole allowed transitions such as N2O(1Π←1Σ+), N2O (1Σ+←1Σ+), CO2(1Σ+u←1Σ+g), and CO2 (1Πu←1Σ+g) became diffuse. Just below the CO2 (1Σ+u←1Σ+g) absorption edge, an exciton peak was observed at 10.73 eV.
Stability and structure of H3CO+ formed from COH++H2 at low temperature
Authors: Hiraoka Kenzo , Kebarle Paul
Published: February 2003

The gas phase equilibrium (1) HCO++H2 = H3CO+ was studied in the temperature range −100 to −165 °C with a pulsed electron beam high pressure ion source mass spectrometer. Van’t Hoff plots of K1 led to ΔH °1 = −3.9 kcal/mole and ΔS °1 = −20.5 e.u. (stand. state 1 atm). This ΔH1 and standard thermochemical data lead to ΔHf(H3CO+) = 196.8 kcal/mole. Thus the H3CO+ formed by (1) is not protonated formaldehyde H2COH+ whose ΔHf = 169.8 kcal/mole. The reactivity of H3CO+ observed in ion molecule reactions and the theoretically predicted charge distribution in HCO+ suggest that the structure of H3CO+ is an HCO+ in which the partially positive hydrogen forms a weak three center bond with an H2. This bond is similar to, but weaker than the bond occurring between H3+ and H2 in the H5+ molecule.
Kinetic processes determining the time dependence of vuv emission in He
Authors: Payne M. G. , Klots Cornelius E. , Hurst G. S.
Published: February 2003

We present an interpretation of the data by Bartell, Hurst, and Wagner (BHW) on the time resolved vuv emission from He excited by pulses of fast charged particles. Here we argue that the very large term, linear in pressure, observed in the decay rate of the He(2 1P) population represents He(2 1P) + He(11S) →He(2 1S) + He(1 1S) and that the dominant mechanism for this reaction is strong rotational coupling of the adiabatic states 1Πg(1 1S+2 1P) and 1Σ+g(1 1S+2 1S) in the region of internuclear separation near the potential curve crossing at R?2.04 Å. The accurate prediction of the reaction rate, together with the fact that no other singlet states appear accessible at room temperature, leads to the conclusion that in nearly pure He most of the large He(2 1P) population is converted to He(2 1S) either by radiative transitions, the two-body process described above, or by the related predissociation of B 1Πg(1 1S+2 1P) when the molecule is formed in a high vibrational state. Thus, most of the continuum radiation comes from the A 1Σ+u and possibly the D 1Σ+u molecules, and the time dependence of radiation from the A 1Σ+u molecule is rate-limited by decay of He(2 1S) by various collisional processes.
A Hartree–Fock interaction potential between a rigid asymmetric top and a spherical atom: (H2CO,He)
Authors: Garrison Barbara J. , Lester William A. , Schaefer Henry F.
Published: February 2003

A basis set of better than ’’triple zeta plus polarization’’ quality is used to compute the interaction potential between a rigid H2CO(1A1) and He(1S). The highly anisotropic energy surface has a slight attraction arising from induction effects at intermolecular separations around 9 a.u. It is fitted to a spherical harmonic expansion to facilitate scattering applications.
The electronic structure of the vanadate and chromate ions as calculated by the MS Xα method
Authors: Gubanov V. A. , Weber J. , Connolly J. W. D.
Published: February 2003

Using the MSXα method, calculations of the electronic structure of the tetrahedral VO3−4 and CrO2−4 complexes have been carried out. The choice of the MSXα parameters and their influence on the results is discussed. The calculations are done in such a way so as to allow a full comparison with the corresponding results obtained by Johnson for the MnO−4 complex. The calculated electronic structures of the vanadate and the chromate ions are compared with the x-ray emission spectra and a new assignment of the spectral bands is proposed. The transition state procedure is used to calculate both ionization energies and optical transition energies. The results agree well with the observed ESCA and optical spectra, the agreement being better than that previously obtained from ab initio and semiempirical calculations.
Continuous emissions of argon and krypton in the near ultraviolet
Authors: Birot A. , Brunet H. , Galy J. , Millet P. , Teyssier J. L.
Published: February 2003

The spectroscopic and kinetic study of argon and krypton luminescence is carried out using two complementary methods. Over the range of wavelengths from 200 to 800 nm, the optical spectrum of these gases, determined by means of a single photoelectron spectrophotometer, shows the presence of atomic lines and continuums. In the case of argon, between 400 and 500 nm, a weak continuum appears. This continuum had never been observed under similar experimental conditions. The decay of the luminosity is reconstituted by means of the time-amplitude conversion method. The decrease of light of continuums is described by a difference of exponential terms. The variation of time constants with pressure allows the kinetic scheme which describes the mechanisms of the postluminescence to be deduced. The study of the continuums has confirmed the formation of diatomic molecules of rare gases as the result of triple collisions. The kinetic scheme is as follows: Al→k1 Al′+hν1, Al+2A→k3 A 2+A, and Al2→k′1 Al2+hν0 (continuum). The molecular states created have a very short lifetime (k′−11 ∼ 5 ns) compared with those of the lowest radiative states (3–5 μs). The rate constant k3 is very important (k3 ∼ 10−30 cm6s−1). The atomic states responsible for the creation of dimers certainly belong to the configurations 3p5 5p in argon and 4p5 6p in krypton.
Vibrational relaxation of HCl(v=1) in the presence of several diatomic molecules at 295–750 °K
Authors: Bott J. F. , Cohen N.
Published: February 2003

The relaxation rates of HCl(v=1) by HF, HBr, H2, D2, N2, and Cl2 have been measured at 295 to 700 °K in shock tube laser-induced fluorescence experiments.
Combinatorial and topological methods in nonlinear chemical kinetics
Authors: Glass Leon
Published: February 2003

Combinatorial and topological techniques are developed to classify nonlinear chemical reaction networks in terms of their qualitative dynamics. A class of N coupled equations, based on a hypothesis concerning biological control by Monod and Jacob is derived. Transitions between volumes in concentration space for these equations are represented as directed edges on N cubes (hypercubes in N dimensions). A classification of the resulting state transition diagrams for N=2,3 is given. A version of a topological theorem by Poincaré and Hopf is derived which is appropriate for application to chemical systems. This theorem is used to predict the existence of critical points in continuous nonlinear equations with oscillation and bistability on the basis of their state transition diagrams. A large number of nonlinear kinetic equations proposed in previous studies by other authors are classified in terms of their state transition diagrams.
Two-photon circular dichroism
Authors: Power E. A.
Published: February 2003

Optically active molecules will show differential absorption, as between opposite handedness of the radiation, for two-photon absorption. A theoretical estimate is made of the expected ratio of the differential rate to the total rate and an order of magnitude of this ratio for a simple helical model is found.
Low frequency depolarized light scattering in the VH geometry from quinoline
Authors: Rouch Jacques , Chabrat Jean Pierre , Letamendia Louis , Vaucamps Claude , Gershon N. D.
Published: February 2003

Low frequency depolarized light scattering spectra in the VH geometry are measured in liquid and supercooled quinoline from 37 to −35 °C and compared with the theory of Gershon and Oppenheim. The spectra are separated into contributions of different modes which can be dissipative and/or vibrating according to Gershon and Oppenheim’s results, and it is shown that a central dip does not necessarily mean dissipative modes, e.g., at 2 °C. Shear viscosities deduced from the spectra are close to values measured macroscopically. Orientational relaxation times obtained are exponential vs 1/T over the temperature range measured, and the coupling parameter between the orientational and the linear momentum densities is about proportional to the viscosity width.
The temperature dependence of the HF vibrational energy distribution in the insertion–elimination reactions between NH and fluoromethanes
Authors: Poole Philip R. , Pimentel George C.
Published: February 2003

The photolysis of hydrazoic acid, HN3, in the presence of fluorinated methanes and argon produces hydrofluoric acid, HF, through imine (NH, probably a 1Δ) insertion–elimination reactions, but chemical laser emission has not been observed. Nevertheless, by the tandem laser, zero-gain temperature method, the HF vibrational population ratios have been measured as a function of temperature (225–400 °K) for CF4, CF3H, CF2H2, and CFH3. Part of the HF is evidently produced by NH(a 1Δ) insertion into the C–H bond with subsequent HF elimination. However, there is an unexpected, competing reaction that involves the C–F bonds, either NH insertion–elimination or hot H atom abstraction to form HF. The HF produced from CF4 shows a simple, Arrhenius type temperature dependence fitted by the expression N1/N0=0.115 exp(950 cal/RT). The N1/N0 ratios for CF3H and CF2H2 can be correlated with those of CF4 by assuming competing C–F and C–H bond reactions weighted by the relative number of each type of bond. With this interpretation, the C–H bond insertion–elimination produces HF with the temperature dependence N1/N0=1.52 exp(−1240 cal/RT). Consistent with all earlier chemical lasers based upon elimination reactions, both the N2/N1 and N1/N0 ratios are below unity. At room temperature, the NH insertion into the C–H bond gives N1/N0=0.2, the lowest population ratio yet measured for an elimination reaction and apparently HF(v=0) is produced with a lower activation energy than that for HF(v=1).
Chemiluminescence of AlO
Authors: Rosenwaks S. , Steele R. E. , Broida H. P.
Published: February 2003

Ultraviolet, visible, and near ir emission has been observed from flames produced by reactions of Al atoms with various oxidants. Spectra of the flames and photon yields for the reactions were measured. Emission from the A, B, and C states to the ground state of AlO has been identified. A continuumlike emission between 280 and 750 nm also has been observed; it is suggested that this emission is not from diatomic aluminum oxide. Photon yields obtained are 3% for Al + microwave discharged O2, 2% for O3, 0.4% for N2O, and less than 0.005% for O2, CO2, NO2, NO, and CO.
Differential cross sections for elastic electron scattering. III. The C2H2, C2H4, and C2H6 results
Authors: Fink M. , Jost K. , Herrmann D.
Published: February 2003

Relative elastic differential cross sections for electrons scattered from C2H6, C2H4, and C2H2 have been measured for angles between 3 and 130 deg and electron energies between 100 and 1000 eV. The data agree very well, for angles larger than 40 deg, with calculations in the independent atom model approximation and partial wave atomic scattering factors. The results show strong charge cloud polarization effects at small angles and low energies. At higher energies the molecular binding effects cause the cross sections to fall below the calculated results.
Raman spectra and defect fluorescence of anthracene and naphthalene crystals at high pressures and low temperatures
Authors: Nicol Malcolm , Vernon Mary , Woo John Tak
Published: February 2003

Raman spectra of the lattice modes of crystalline anthracene and naphthalene have been measured at pressures to 36 kbar at several temperatures from 450 K to below 30 K. The frequencies of these lattice modes are very sensitive to pressure, and at room temperature, most modes double in frequency between 1 bar and 30 kbar. At pressures less than 10 kbar, the vibrational modes also are very sensitive to temperature at constant pressure. The Raman shifts of the lattice modes of anthracene increase significantly (10–20 cm−1) upon isobaric cooling from room to liquid helium temperatures at pressures below 10 kbar; at higher pressures, the Raman shifts of these modes change by less than 5 cm−1 upon cooling. For naphthalene, the smaller temperature dependence was observed even at 5 kbar, the lowest pressure studied. The naphthalene results agree well with observations to 10 kbar reported by Dows et al.; however, the anthracene results more closely correspond to the observations reported by Wong and Whalley, whose measurements also extend to pressures greater than 10 kbar. No evidence of a phase transformation to naphthalene II was observed even when a crystal had been maintained at 30 kbar and 425 K for more than 24 h. At pressures greater than 17 kbar for anthracene and 30 kbar for naphthalene, an excimerlike fluorescence of defects in the melt-grown crystals was directly excited by the argon laser at 514.5 nm and dominated the Raman spectra. The anthracene defect fluorescence could be excited with 457.9 nm radiation at all pressures and changed from a banded to a predominately broad structureless spectrum at less than 1 kbar; however, Raman spectra of anthracene at high pressures excited with 632.9 nm were characteristic of a compressed atmospheric-pressure structure. No additional features characteristic of a dimer defect could be detected in the Raman spectra of anthracene crystals that had been held at 30 kbar and irradiated with ∼100 mW of 363.8 nm radiation for as long as 8 h.
Exchange symmetry and the quantum Boltzmann equations
Authors: Hunter L. W.
Published: February 2003

The quantum Boltzmann equations in the kinetic theory of a dilute gas are based on an assumed relation between the first and second order distribution functions. As usually formulated, this relation is inconsistent with exchange symmetry requirements. This paper discusses the changes which occur in the quantum Boltzmann equations and their collision integrals when the relation is modified to be consistent with the symmetry requirements for exchanges of whole molecules.
Electron impact excitation of the 1 1S→2 3S transition in helium; differential cross-section determinations at high incident kinetic energies
Authors: Dillon Michael A.
Published: February 2003

Differential cross sections have been determined to within ±8% accuracy for the 1 1S→2 3S transition in helium at 200, 300, 400, and 500 eV incident electron energies and scattering angles out to 35°. Relative measurements were made absolute by both direct and indirect normalization to highly precise elastic differential cross sections. Serious deviations from the predictions of first-order plane wave theory are found throughout the range of experimental parameters employed in the present work. Further consideration suggests that such discrepancies persist to incident energies as high as 1000 eV raising serious doubts about the validity of the Born–Oppenheimer approximation.
Hyperfine couplings in primary radiation products
Authors: Box Harold C. , Freund Harold G. , Lilga Kenneth T. , Budzinski Edwin E.
Published: February 2003

The characteristics of proton couplings in primary radiation products in irradiated protein and nucleic acid constituents are described. The range of β proton couplings in carboxylic acid anions is shown to be much smaller than for typical β couplings in π-electron radicals. A close correlation between σ proton coupling tensors in conjugated oxidation and reduction products and principal axes derived from crystal structure is demonstrated. Proton hyperfine coupling tensors in the primary oxidation product of aminoisobutyric acid and the 13C hyperfine coupling tensor in the primary reduction product are reported.
Selective laser excitation of charge compensated sites in CaF2:Er3+
Authors: Tallant David R. , Wright John C.
Published: February 2003

A technique of selective laser excitation has been developed and used to study the optical spectrum of the charge compensated sites in CaF2:Er3+. The technique allows the unambiguous interpretation and classification of the spectral lines observed from crystals with multiple sites. The absorption lines of specific sites have been excited with a narrow band dye laser. Nonresonant energy transfer between sites has not been observed and fluorescence from a single site can be obtained if no overlap occurs between absorption lines of different sites. The fluorescence transitions of CaF2:Er3 have been classified into two sites having an Er3+ ion and fluoride interstitial compensation and 16 sites having more than one Er3+ ion. The single-pair sites correspond to those previously identified with C4v and C3v site symmetry. The identification of the remaining sites with clusters containing more than one Er3+ ion is confirmed by the observation of ion–ion interactions, including up-conversion, cross relaxation, and pair transfer phenomena. No evidence was found for ion–ion interactions involving the identified single-pair sites. We have also succeeded in distinguishing the 16 multiple pair sites into 11 sites with similar crystal field splittings, four sites exhibiting efficient energy transfer between spectroscopically differentiable Er3+ ions with the site, and one site having spectral features similar to those of the single-pair site of C3v symmetry. The low dopant concentrations at which sites of multiple Er3+ ions are formed will affect theoretical interpretations of previous experiments.
Intermolecular contributions to the isotope effect on excited state lifetimes: Benzene
Authors: Tudron Frank B. , van Pruyssen Jean M. , Colson Steven D.
Published: February 2003

The phosphorescence lifetimes of several deuterium isotopes of benzene are measured. The lifetimes of C6H5D and p-C6H4D2 are found to depend upon the orientation of the molecule in the site field for C6D6, sym-C6H3D3, and borazine host crystals. This variation is measurable because the six or three possible orientations of the respective isotopic guests group into two energetically different sets of sites. Measurements of the lifetimes of each component of the resulting spectral doublets show an orientational difference in their lifetimes that is a significant fraction of the difference between the lifetimes of C6H6 and C6D6 in the borazine and, presumably, the benzene host crystals. In fact, the lifetimes of C6H6, C6H5D, p-C6H4D2, sym-C6H3D3, and C6D6 in borazine can all be fit by a model which assumes that the isotope effect is entirely of intermolecular origin. It was found that one orientation gave a much larger isotope effect than the other. None of the existing theoretical models for interpreting isotope effects on excited state lifetimes is consistent with these data. Thus, much of the work on benzene which attempts to interpret changes in the phosphorescence decay rates in terms of basically intramolecular theoretical models must be reconsidered. Arguments are given which suggest that at least part, if not all, of the isotope effect observed in the benzene and borazine hosts is in the radiative decay channel.
The low-energy singlet states of a simple merocyanine dye crystal
Authors: Walker E. I. P. , Marchetti A. P. , Salzberg C. D.
Published: February 2003

Spectrophotometric reflectance measurements have been performed on microcrystals of a simple merocyanine dye, 3-ethyl-5-[2- (1,3-dithiolanylidene)] rhodanine. Using the Kramers–Kronig transformation, the optical constants for various directions of light propagation and polarization were obtained from the reflectance data. The crystalline spectra are consistent with the predictions of simple Davydov theory. Measurements on a thick crystal at 77 °K show that the lower-lying forbidden factor group states of the strong molecular transition are at least 103 times weaker than the allowed states. The molecular transition directions, determined from polarization ratios, are also given.
Reactions of solvated electrons with solutes at high concentrations. A laser photolysis and pulse radiolysis study
Authors: Bromberg A. , Thomas J. K.
Published: February 2003

Solvated electrons were produced in several liquids by laser photoionization of aromatic solutes, and by pulse radiolysis. The yields of solvated electrons were decreased by high concentrations (0.1–1.0M) of phenol, benzene, and ethyl acetate; the effectiveness of the solutes in reducing the e−s yields was greater in the photolysis experiments than in the corresponding pulse radiolysis experiments. The data were, however, identical with light of wavelength 3471 or 2650 Å, from ruby and neodymium lasers, respectively. The data are used to discuss several theories. Solvated electrons in alcohols were photobleached with red light (λ=6942 Å) giving H atoms. The quantum yield for the process was found to be low.
Inelastic collisions between an atom and a diatomic molecule. II. H+H2 rotational excitation
Authors: Choi B. H. , Tang K. T.
Published: February 2003

The formulation and the numerical procedure developed in the preceding paper for solving close coupled differential equations in the study of scatterings of diatomic molecules by atoms are applied to rotational excitations of H+H2 collisions. Converged results are obtained from three different potential functions suggested for this system. In addition to some new results, all previously reported results except one are essentially reproduced. The present close coupling results are drastically different from those of Wolken, Miller, and Karplus obtained using an identical potential. Detailed comparisons between results of close coupling calculation and of the distorted wave Born approximation are made for all three potentials and some general features of DWBA results are discussed. Correlations between genuine differences in cross sections and interaction potentials are found through S-matrix elements which also serve as a test for the accuracy of the calculation.
Polarized fluorescence and the Hanle effect in NH2
Authors: Kroll M.
Published: February 2003

Polarized fluorescence spectra of gaseous NH2, produced in a flow cell and excited by a tunable cw dye laser, have been observed. The measured degree of polarization of fluorescence agrees qualitatively with the values predicted by a model which does not include fine or hyperfine effects. Transitions originating from collisionally populated levels have also been observed to be polarized. The Hanle effect has been measured in several rotational levels of the (0,10,0) Π vibronic state of the 2A1 electronic state and the present analysis of the data yields a value of ∼350 nsec for the radiative lifetime of these levels and indicates a decrease in the depolarization cross section with increasing rotational angular momentum.
Vibrational deactivation of HF(v=1) and DF(v=1) by H and D atoms
Authors: Heidner R. F. , Bott J. F.
Published: February 2003

The deactivation of HF(v=1) and DF(v=1) by H and D atoms was studied at 295 K by means of the techniques of laser-induced infrared fluorescence and isothermal calorimetry. The upper limits for the deduced rate coefficients are well-defined, but the lower limits are imprecise. The results of this study are compared with other experimental data and with trajectory calculations. Qualitative data are presented regarding the role of reactive, i.e., F-atom exchange, versus nonreactive collisions in the removal of the v=1 level. A verification of the method was made by determining a rate coefficient for H+HCl(v=1) that is in good agreement with a recent study.
Theoretical equations of state for a high temperature Lennard-Jones gas
Authors: Carley David D.
Published: February 2003

Radial distribution functions are computed for a high temperature Lennard-Jones gas using a parametric integral equation. The parameter is determined for each isotherm by adjustment to give good agreement in pressure with Monte Carlo computations. The pressure and energy equations of state are obtained from the radial distribution functions. For each isotherm studied (T*=5, 7, 10, 14, 20, 30, 50, 70, 100), a Padé approximant is used to represent the pressure and energy, respectively, as functions of density. The consistency of the equations of state as here computed is tested by means of the energy equation.
Kinetics of CH4 formation from the reaction of H with graphite
Authors: Gould Robert K.
Published: February 2003

Small ribbon-shaped samples of spectroscopic grade graphite heated to between 500 and 1250 K have been exposed to atomic hydrogen produced by thermal dissociation of H2 on hot W and the rate of CH4 evolution has been measured mass spectrometrically. H2 pressures were between 1×10−6 and 5×10−5 Torr and arrival rates of H at the graphite surface ranged from 1×1014 to 5×1015 atoms cm−2 sec−1. When a well outgassed graphite surface is initially exposed to H atoms, the rate of CH4 evolution, NM, peaks and then decays to a steady-state rate. A mechanism is proposed and examined which adequately explains the results observed. The mechanism involves the formation of a CH4-producing surface complex by the reaction of an incident H atom with chemisorbed H atoms on the graphite surface. Additional chemisorbed H rapidly reacts with this complex and CH4 desorbs. The decay in NM to a steady-state evolution rate is apparently caused by a surface reaction in which pairs of the surface complexes responsible for CH4 evolution form a stable site-blocking species.
A photoionization study of the formation of CO+2 by reaction of excited O+2 ions with CO
Authors: Ajello Joseph M.
Published: February 2003

The production of CO+2 by the ion–molecule reaction O+2+CO→CO+2+O has been investigated using a photoionization mass spectrometer. The photoionization efficiency for production of CO+2 by this reaction was measured from threshold at 924 Å (13.42 eV) to 650 Å (19.07 eV). The appearance potential corresponds to reaction of O+2(X 2Πg) ions formed in the v′=6 level. The high vibrational level ions of the ground ionic state are most likely produced by autoionization of O2. At wavelengths shorter than 760 Å there is a large increase in the reaction cross section associated with formation of ions in the a 4Πu state. The peak reaction cross section occurs at 720 Å.
Monte Carlo and perturbation calculations for the square well fluid: Dependence on the square well range
Authors: Rosenfeld Y. , Thieberger R.
Published: February 2003

Monte Carlo and perturbation calculations for the square well system in the fluid phase, were performed for two different range parameters (R = 1.2042, 1.3675), in the inverse temperature range 0 ? X (=ε/KBT) ? 1. First order perturbation theory reproduces the machine calculated results to a fair degree of accuracy. Comparison with the triangular well fluid shows that the square well potential is rather a severe test to the theory.