bluuebunny/crossref_metadata_2025 huggingface dataset:
H2–H fragmentation resulting from collisions of 10 keV H+3 on H2
Authors: Vogler M. , Meierjohann B.
Published: February 2003

H2–H fragmentation resulting from charge transfer collisions of 10 keV H+3 ions on H2 was studied using a flight-time-difference method. The gas pressure in the ion source was varied in order to get H+3 ions of different vibrational excitation. The measured energy and angle distributions of the dissociation fragments resulting from both weakly and highly excited H+3 ions show a broad continuum of translational energies in the center-of-mass frame of the projectile. It can be explained by dissociation of H3 in its electronic ground state. In addition, the translational energy spectrum belonging to weakly excited H+3 ions reveals some structure which points to a predissociation of the H3 molecule.
Frequency-dependent multipole polarizabilities. II. Application of a reduced Green’s function formulation of TDHF theory to the helium isoelectronic sequence
Authors: McDowell Keith , Lewis Lynn
Published: February 2003

A reduced Green’s function formulation of time-dependent Hartree–Fock theory is developed using diagrammatic techniques. The resulting integral equations are transformed using a power moment approach. An asymptotic ansatz of the form exp{−[−2(εi ∓ω)]1/2r}am† polynomial is used to fit the radial functions φ±l(r). From these functions, the frequency-dependent dipole, quadrupole, and octapole polarizabilities of the helium isoelectronic sequence for Z=2–10 are calculated. Results are reported for the static case. It is found that the poles of the polarizabilities which correspond to TDHF excitation energies from the ground state can be accurately determined for highly excited states. These excitation energies are reported and are related to a Rydberg formula. The resulting quantum defects are also reported.
Analysis of aggregate optical spectra using moments. Application to the purple membrane of halobacterium halobium
Authors: Hemenger R. P.
Published: February 2003

The problem of extracting structural information from the optical spectra of aggregates of molecules interacting through their electronic transitions is studied. One serious difficulty common to all approaches to this problem is that of properly taking into account the effects of molecular vibrations. A series of exact relations derived previously which are correct with regard to molecular vibrations provide a number of independent, explicit connections between aggregate geometrical parameters and moments of experimental spectra. It is shown that, by applying these moment relations to the optical absorption and circular dichroism spectra of simple aggregates, a complete set of equations can be found, i.e., enough equations can be found to solve for all of the geometrical parameters which enter into the expressions for absorption and circular dichroism spectra. This procedure is applied in some detail to the purple membrane of Halobacterium halobium. The results are completely consistent with what is known about its structure.
The diffusion of an atom on a solid surface
Authors: Efrima Shlomo , Metiu Horia
Published: February 2003

A microscopic model for the diffusional motion of an atom chemisorbed on a solid surface is presented. The atom located at a given site is described by a set of states which can be divided into two classes. One class is composed of states whose energy is located below the intersite barrier. A particle described by one of these states can go to a neighboring site only through tunneling. The second class of states have higher energy than the intersite barrier. They are delocalized and an atom occupying such a state can change its site with relative ease. The vibrational motion of the lattice can transfer energy to or take it from the particle and it can also change the intersite transfer probability. As a result the particle may undergo three types of processes: 1) change state but not site (vertical transitions); 2) change site and state (oblique transitions); or 3) change site but not energy (horizontal transitions). The theory derives a kinetic equation for the probability that at a given time the particle is located at a given site and has a specified energy. This is then used to compute the mean square displacement of the particle and its diffusion coefficient. All these quantitities are expresed in terms of rate coefficients for horizontal, oblique, and vertical transitions. Microscopic expressions are provided for these quantities as a function of the particle phonon coupling, intersite transfer probability, particle energies, and lattice properties. The temperature dependence of the rate coefficients is analyzed numerically. If tunneling is minimal and a change of site can occur only after the phonon transfer promotes the particle in a state located above the intersite barrier, an Arrhenius temperature dependence of the diffusion coefficient is observed. The form of the functional dependence of the pre-exponential factor on temperature changes with anharmonicity, Einstein frequency, and phonon particle coupling. If the lattice is harmonic the activation energy is equal to the intersite barrier plus the energy of the distortion of the lattice upon chemisorption. If anharmonicity is important the activation energy is close or equal to the intersite barrier. If tunneling is important non-Arrhenius temperature dependence may be observed.
A reinvestigation of the Raman spectrum of water
Authors: Moskovits M. , Michaelian K. H.
Published: February 2003

The Raman spectrum of liquid water has been reinvestigated using a technique which automatically records the depolarization ratio along with the polarized Raman spectrum. The exceptionally accurate depolarization ratios thus obtained, as well as the isotropic and the anisotropic spectra of water, indicate that the libration region extends to higher frequencies than previously thought and that a new polarized band originating from restricted translation occurs near 290 cm−1. The translational spectrum is in good agreement with that calculated by Weres and Rice.
Three and four center elimination of HCl in the multiphoton dissociation of halogenated hydrocarbons
Authors: Sudbo/ Aa. S. , Schulz P. A. , Shen Y. R. , Lee Y. T.
Published: February 2003

Three and four center unimolecular elimination reactions of HCl have been investigated for CHF2Cl, CHFCl2, CH3CCl3, CH3CF2Cl, and CHClCF2 in a molecular beam experiment using infrared multiphoton absorption to energize the molecule. The translational energy distributions obtained in this work show that the average translational energy released to the fragments is around 8–12 kcal/mole, except for the three center elimination reaction from CHClCF2, which gives a value of 1 kcal/mole. In four center eliminations, the translational energy released is less than 20% of the potential energy barrier of the back reaction. This is somewhat less than previous indications that approximately 30% of the potential energy barrier of the exit channel in four center reactions should be released into translation.
The glass transition of atomic glasses
Authors: Hudson Suzanne , Andersen Hans C.
Published: February 2003

The nature of the glass transition in liquids composed of monatomic species is discussed using a hard sphere model. Recent molecular dynamics calculations by Woodcock [J. Chem. Soc. Faraday II 72, 1667 (1976)] indicate that a hard sphere fluid undergoes a glass transition when it is compressed to a high enough density. This calculation provides one of several independent methods of estimating the packing fraction at the glass transition, ηg, for hard spheres. The estimated value of ηg for hard spheres, 0.533 ±0.014, is substantially lower than 0.637, the packing fraction of dense random packed hard spheres. The effective ηg for a number of atomic glass forming liquids with continuous interatomic potentials also are estimated, and are found to lie in or near the range of estimated values of ηg for hard spheres. The liquids considered are a Lennard-Jones fluid for which the liquid–glass transition has been studied by a molecular dynamics calculation [A. Rahman, M. Mandell, and J. McTague, J. Chem. Phys. 64, 1564 (1976)], and several real glass forming metal–metalloid alloys. The similarity of ηg for the hard sphere fluid, for the Lennard-Jones fluid, and for metal alloys suggest that it is the short ranged repulsive forces acting between atoms which are responsible for the glass transition. These results also suggest that in general, ηg=0.53±0.02 for atomic liquids. This provides a criterion for predicting the glass transition temperatures for materials which have not yet been observed in the glassy state.
Collision-induced energy transfer in the B 3Σu− state of diatomic sulfur
Authors: Caughey Thomas A. , Crosley David R.
Published: February 2003

Rates for collision-induced vibrational and rotational energy transfer within the B 3Σu− state of diatomic sulfur are reported. The S2 was initially excited to v′=4, N′=40, J′=41 by absorption of a Zn atomic line. Fluorescence measurements, made as a function of foreign gas pressure, were used to determine the rates. The collision partners investigated were the five rare gases, H2, and N2. In addition, the rate of quenching of S2 (B, v′=4) by S2 (X) is reported.
A new method for solving the coupled equations in the adiabatic representation
Authors: Babamov Vasil K.
Published: February 2003

A new method for solving the coupled equations in the adiabatic representation which arise in quantum mechanical treatment of atomic and molecular collisions is developed. The solutions of the uncoupled adiabatic equations, obtained from the coupled equations in the adiabatic representation by neglecting the coupling terms, are used to derive a set of new exact equations for the purely inelastic effects of the collision. The solutions of the new equations are practically nonoscillatory and can be solved numerically with little computational effort.
Nuclear magnetic resonance spectroscopy in cholesteric liquid crystals .II. The blue phase
Authors: Collings Peter J. , McColl James R.
Published: February 2003

NMR spectroscopy is utilized to study the little understood ’’blue phase’’ of the cholesteric liquid crystals cholesteryl m-fluorobenzoate and cholesteryl decanoate. The NMR spectra in the ordinary cholesteric phase has both structure and width; these features are reduced in the blue phase to the point where they resemble isotropic phase spectra. The presence of a wider than isotropic NMR line, however, indicates the presence of long-range orientational order and provides data which place constraints on the possible structure of the blue phase. In addition, the results show considerable thermal hysteresis for the blue-cholesteric transition in cholesteryl decanoate.
Theoretical studies of hydrogen rare gas complexes. III. ArnH and ArnH+ clusters
Authors: Matcha Robert L. , Milleur Mac B.
Published: February 2003

The structures and energies of excited and ground states of ArnH and ArnH+ clusters (n=1,2) are investigated in the LCAO–SCF–MO approximation. Excited neutral states are found to be binding with binding energies close to the corresponding ionic cluster ArnH+. Vibrational modes are calculated for bound electronic states.
Spin densities in LiH+
Authors: Tortorelli James J. , Harriman John E.
Published: February 2003

Electronic energies and spin density distributions for LiH+ have been determined by spin-polarized self-consistent-field and full configuration interaction calculations with several basis sets. For the 2Σ ground state Re, De ,the dipole moment, and vibrational level spacings have been estimated. The lowest 2Π state is purely repulsive, but the vertical excitation energy to it from the rovibronic ground state has been estimated. Spin densities have been investigated at the nuclei and as distributions in space. Direct and spin-polarization contributions are discussed. Spin densities at the nuclei are found to be sensitive to inclusion in the basis set of very compact functions, and to correlation.
Chemiluminescent reactions of sulfur atoms and oxygen atoms in solid argon matrices. SO chemiluminescence
Authors: Tevault David E. , Smardzewski Richard R.
Published: February 2003

When dilute argon matrices containing H2S+O3, OCS+O3 and H2S+NO2 molecules are exposed to ultraviolet radiation and subsequently allowed to warm from 8 to ∼20°K, several intense visible emissions with extensive vibrational structure appear. Besides those emissions of O2, S2, and SO2, two new luminescent systems are evident. Extensive oxygen-18 and deuterium isotopic studies demonstrated the absence of hydrogen and the presence of a single oxygen atom in the emitting species of both systems, while matrix concentration studies established that the excited state species was an intermediate in SO2 formation, presumably molecular SO. The more intense of the two systems is located between 490–870 nm and contains up to ten members. The low values calculated for T0=22 543 cm−1 and ωe″=1130.7 cm−1 suggest that this transition is most likely SO (c1Σ−→ a 1Δ). A second, weaker emission system of SO with approximately eight members was found between 385–600 nm. T0 for this latter system was estimated at 28 400±1150 cm−1, the larger uncertainty being due to the weakness of the system and overlap with SO2 chemiluminescence. The magnitude of T0, as well as the value calculated for ωe″=1157.9 cm−1, is consistent with the assignment of this transition to the SO(A′ 3Δ→X 3Σ−) system which has recently been reported for S2 and O2 chemiluminescence. Ground state oxygen atoms O(3P) were observed to react with H2S in cryogenic matrices to produce only SO2 chemiluminescence, while no emissions were observed in similar experiments with OCS.
Debye–Hückel theory of model polyelectrolytes
Authors: Soumpasis Dikeos
Published: February 2003

We use the superposition principle and suitable expansions to derive the exact Coulomb and Debye–Hückel screened potentials of certain discrete, periodic, charge distribution modeling real extended polyions. Using these potentials, we extend the range of validity of the Debye–Hückel condensation theory of Manning to higher mobile ion concentrations where consideration of ionic size and the specific polyion structure leads to explicit corrections of the limiting law results. It is found that, in the limit of infinite dilution and as far as colligative properties are concerned, all extended polyions of interest are satisfactorily modeled by the simple continuously charged straight line picture introduced by Manning. This should not be the case in applications, where the polyion potential enters directly (e.g., NMR spectroscopy). The range of the validity of the Debye–Hückel condensation approach is estimated through comparison with some typical experimental data.
Convergence properties of the effective Hamiltonian for the excitation energies in closed-shell systems
Authors: Sano Tatsuji , I’Haya Y. J.
Published: February 2003

Methods for calculations of the effective Hamiltonian for particle–hole excitations of closed shell molecular systems, based on the linked-diagram expansions of the Rayleigh–Schrödinger (the folded diagram expansion) and Brillouin–Wigner (the Bloch–Horowitz diagram expansion) perturbation schemes for a quasidegenerate multiconfigurational model space, are studied. The convergence properties of these perturbative methods up to third order, in particular paying attention to the presence of a crossing of levels, are numerically studied for excitation energies of a π-electron system of trans-butandiene. The Padé approximant consistent with the continued fraction approach is applied to the folded diagram expansion treatment. The results of the low-order Padé approximants are similar to those of the Bloch–Horowitz diagram expansions. The singularity of the [2/1] Padé approximant is not necessarily related to the crossing of levels. A strong energy dependence of the effective interaction is shown to be attributed to the crossing of levels.
Radiation induced free radical formation in sulfur-substituted purines. 6-Mercaptopurine at 77 K
Authors: Sagstuen Einar
Published: February 2003

Single crystals of the sulfur-containing purine base derivative 6-mercaptopurine have been irradiated by 4.0 MeV electrons and studied by electron spin resonance techniques at 77 K. The two main resonances observed, R1 and R3, were analyzed and identified. The R1 resonance exhibited a large g-value variation. The g-tensor principal values are 2.173, 2.002, and 1.989, and the resonance is ascribed to a thiyl radical formed by the abstraction of a hydrogen atom from N7. The R3 resonance showed an almost isotropic 1:2:1 triplet hyperfine structure with a splitting value of 38.5 G. The g-tensor principal values are 2.0138, 2.0065, and 2.0020. This resonance is assigned to a radical formed by hydrogen addition at the C8 position. The radical identifications were arrived at by a combination of the ESR data and results from molecular orbital calculations.
The electronic structure and conductivity of tetrathiotetracene, tetrathionaphthalene, and tetraselenotetracene studied by ESCA
Authors: Riga Joseph , Verbist Jacques J. , Wudl Fred , Kruger Albert
Published: February 2003

High resolution x-ray photoelectron spectra are presented for tetrathiotetracene, tetrathionaphthalene, and tetraselenotetracene. C1s and S2p core levels and their shake-up satellites, as well as valence band spectra, are related with molecular geometry to give a detailed picture of electronic structure and bonding. The model is applied to the semiconducting properties of these solids compared to those of their complexes with electron acceptors, and those of the corresponding acenes, shedding light on the particular role of the disulfide (or diselenide) groups.
Depolarized Rayleigh spectroscopy in the n-alkanes
Authors: Patterson G. D. , Lindsey C. P. , Alms G. R.
Published: February 2003

Depolarized Rayleigh spectra have been obtained for n-dodecane, n-hexadecane, n-docosane, n-octacosane, n-hexatriacontane, and Polywax 1000, a linear polyethylene of nominal molecular weight 1000, as a function of temperature in the melt. Relaxation times determined from the spectral linewidths have been analyzed in terms of the reorientational motions of the molecules. For the lower n-alkanes, the average relaxation time is determined primarily by overall molecular reorientation. For the higher n-alkanes and Polywax 1000, there is definite evidence that intramolecular conformation changes are contributing to the linewidths.
Magnetic transitions in the initial-llabeled interpretation of the CS approximation. Computations for He+HCl
Authors: Monchick L. , Kouri D. J.
Published: February 2003

Calculations of Arthurs–Dalgarno space frame differential and integral cross sections for intermultiplet transitions, quantized with respect to the incident rotational angular momentum, are carried out for He + HCl collisions using the initial-l labeled version of the coupled states (CS-li) method given recently by Khare. Comparisons with accurate close coupling results of Monchick are made and the CS-li results for integral polarization cross sections are found to be at least semiquantitative, with the best agreement occurring for inelastic j0→j, and for m0≠m collisions. Elastic depolarization differential cross sections 𝒥m(dσ/dΩ)(j0m0→jm) are most accurate in the forward and backward directions, while the inelastic cross sections 𝒥m(dσ/dΩ)(j0m0→jm) are accurate only in the backward direction. In the midangular region, the magnitudes of the depolarization cross sections are inverted so that those starting from m0=0 are too large, and those with increasing initial m0 are too small.
A structural description of liquid particle dispersions: Ultracentrifugation and small angle neutron scattering studies of microemulsions
Authors: Dvolaitzky M. , Guyot M. , Lagües M. , Le Pesant J. P. , Ober R. , Sauterey C. , Taupin C.
Published: February 2003

Microemulsions are colloidal systems which are made by a dispersion of minute droplets (a few hundred Å in diameter) of one component (e.g. water) in a continuous medium formed by another immiscible component (e.g. oil); the droplets are surrounded by a mixed film of surfactants. New information has been obtained about the distribution of the various components in the elementary droplet. The hydrodynamical interface is shown to contain an important amount of the continuous phase and a quantity of alcohol which increases with the amount of solubilized water. In contrast, the corresponding variation of the area per polar head of the surfactant molecule is relatively small. Small angle neutron scattering indicates that a portion of the interfacial film, 9 Å thick, is not penetrated by the oily phase. The droplets are weakly correlated.
Light scattering studies of orientational fluctuations of CS2
Authors: Higashigaki Y. , Whittenburg S. L. , Wang C. H.
Published: February 2003

We have measured the depolarized Rayleigh relaxation time of CS2 as a function of temperature and concentration in an optically isotropic solvent CCl4. The concentration dependence of the relaxation times indicates the importance of orientational pair correlation in CS2. The orientational pair correlation factor is found to depend on concentration at high CS2 concentration, as well as viscosity and temperature, in contrast to what is observed in other small molecules. The single particle reorientation times were obtained by extrapolating from the depolarized Rayleigh relaxation times to infinite dilution. These were compared with the predictions of the hydrodynamic slip and stick models for rotational diffusion. As is usual for small molecules, the slip boundary condition better approximates the experimental results. The single particle reorientation times were also compared with the correlation times obtained by 13C NMR and 33S NQR measurements. They were found to be in good agreement.
Study of the excited-state Raman process in optically pumped ruby
Authors: Halperin B. , Koningstein J. A.
Published: February 2003

Raman-scattering transitions between the excited states Ē (2E) and 2Ā (2E) of Cr3+ in αAl2O3 are studied, with the laser radiation (λ=514.5 nm) simultaneously serving as an optical pump and a Raman probe. This gives rise to a quadratic dependence of the Raman signal on the intensity of the incident radiation at low laser powers, which tends to become linear at higher power levels due to the onset of saturation effects. An analysis is made of the tensorial symmetry properties of the scattering, based on group-theoretical selection rules as well as on a perturbative expansion in the spin–orbit coupling parameter. In comparing the observed polarized spectra with the results of the theoretical analysis, proper account is taken of the dichroic features of the broad absorption bands in ruby, which affect the pumping process and the accompanying population in the excited state. An alternative method for measuring any asymmetry in the scattering tensor is described and employed, making use of the relation between Stokes and anti-Stokes Raman intensities for a fixed scattering geometry.
The electronic density of bulk and surface site atoms of diamond from a
b i
n
i
t
i
o model calculations
Authors: Fink William H.
Published: February 2003

A b initio calculations on five, four, and three atom clusters of a basic tetrahedral arrangement of carbon atoms in a Gaussian lobe function basis are reported. All calculations are in a single-determinant restricted Hartree–Fock framework. In addition to the simple SCF calculations, results of a model calculation which simulates the presence of the remaining atoms of the bulk are presented. In the case of the five atom cluster this simulation is via an iterative projection of the central atom electronic density onto the peripheral atoms. For the four and three atom clusters the result of the five atom calculation is fixed on the atoms of the cluster which are to represent the interior of the crystal.
Orientational correlation times of nonrigid molecules. II. Tensor correlations
Authors: Evans Glenn T.
Published: February 2003

Second rank orientational correlation times of two and three bond chain molecules are calculated by means of a Mori formulation of the Fixman equations of motion of a stiff chain. The correlation times correspond to five tensor modes which sample the orverall tumbling of the asymmetric top molecules as well as the fluctuations in the torsional degree of freedom (for the three bond chain). Flexible three bond chains with 60° and 70° bond angle supplements have correlations times which satisfy an inequality τD<τ1<τ2, τR<τE where τD is associated with the tensor (b1−b2) (b1−b2), τ1 with b1b1, τ2 with b2b2, τR with the Rayleigh scattering mode b1b1+b2b2+b3b3, and τE with the tensor constructed from the end to end vector b1+b2+b3. The ratio of correlation times of first to second rank tensors is roughly 1–3 for the D mode, 3 for the E mode, and 3–5 for 1 and 2 modes.
F and G sum rules as error indicators of experimental vibrational intensity data: The CHxD3−xBr molecules
Authors: Bruns Roy E. , de Barros Neto B.
Published: February 2003

Using the CHxD3−xBr molecules as examples the F and G sum rules are shown to be useful indicators of errors in gas-phase fundamental vibrational intensity data. The recently measured intensities for CH3Br, CH2DBr, and CHD2Br are shown to be nicely consistent with the sum rule restrictions although the CD3Br data apparently contain considerable experimental error. Diagnostic capabilities of these sum rules for localizing the sources of the experimental error are discussed.
Chemiluminescence of SO (? 1Σ −→ ? 1Δ) in solid argon
Authors: Lee Yuan-Pern , Pimentel George C.
Published: February 2003

Matrix samples of OCS/O2/Ar (1/1/200), OCS/NO2/Ar (1/1/200), and SO2/Ar (1/100) were photolyzed at 10 °K. Subsequent slow warming produced previously observed thermoluminescence due to S2 and SO2. In addition, a new progression was observed between 5200 and 7700 Å, and, for OCS/O2/Ar samples, a broad emission peak at 7816 Å. With 18O2, the bands in the progression displayed isotopic shifts that permit deduction of ν00=21 363 cm−1, ωe′′=1097 cm−1, and ωexe′′=7 cm−1. By analogy to the energy levels of O2 and S2, it is possible to assign the progression to the ? 1Σ−→? 1Δ transition of sulfur monoxide SO. This is the first observation of this transition of SO, and it provides the first spectroscopic information about the ? 1Σ− state. The 7816 Å peak is attributed to emission by sulfur atom 1S→1D. The excitation of sulfur atoms may occur through energy transfer; if so, some of this transfer is from the chemically excited SO2* (? 3B1) molecule.
Electron transfer in collisions between atomic ions and rare-gas atoms for primary-ion energies below 200 eV. II
Authors: Maier William B.
Published: February 2003

Cross sections have been measured for electron transfer from rare-gas atoms to H+, He+, Ne+, Ar+, and Kr+, for reactant kinetic energies E0 in the center-of-mass between 0.4–120 eV, and for ground-state reactants. These cross sections appear to have thresholds. The magnitudes of the cross sections and the apparent threshold energies depend systematically on the identities of the primary ion and target atoms. These reactions appear to leave the products in excited states. The secondary ions normally pick up considerable momentum from the primary ion and are produced with a distrubition in their kinetic energy. Interesting structure is found in the cross section for Kr++Ar→Ar++⋅⋅⋅. At low E0, excited primary ions are found to yield very different cross sections than unexcited primary ions. It is suggested herein that an adequate theoretical description of these findings may require fairly accurate potential energy curves for the reactant and product systems.
Resonance CARS line shapes: Excited state parameters for flavin adenine dinucleotide
Authors: Dutta Prabir K. , Spiro Thomas G.
Published: February 2003

CARS line shapes have been measured as a function of excitation frequency for two vibrational modes, 1635 and 601 cm−1 of FAD. They are satisfactorily reproduced, over the visible absorption band, using reasonable parameters in a simple scattering model. Line shape variations from positive Lorentzian through dispersive to negative Lorentzian reflect the frequency dependence of the real and imaginary part of the Raman susceptibility when the background contribution is relatively large. The frequencies at which these features are found depend on the electronic transition frequency and bandwidth, as well as on the vibrational frequency. A method is given for determining the vibrational frequency from CARS bands with non-Lorentzian lines shapes.
Bimolecular and ’’three-body’’ quenching of resonance state argon atoms by nitrogen at argon pressures in the 200–700 torr region
Authors: Firestone R. F. , Chen Mon-Chao
Published: February 2003

Measurements of effects of the partial pressures of N2 and Ar on the decay rates of Ar(1P1) and Ar(3P1) atoms in Ar/N2 mixtures at PAr from 200 to 700 torr at 296 K indicate that the quenching efficiency of N2 increases with increasing argon pressure. It is suggested that quenching of transient, dissociative, vibrationally excited argon excimers by N2 is responsible for this observation. Rate constants for direct bimolecular quenching of Ar(1P1) and Ar(3P1) by N2 are 2.07 (±0.09)10−11 and 1.29(±0.14)10−11 cm3 molecule−1 s−1, respectively. Rate constants for quenching of dissociative, vibrationally excited excimer species associated with Ar(1P1) and Ar(3P1) decay channels are estimated to be equal to 3.3×10−9 and 2.4×10−9 cm3 molecule−1 s−1, respectively.
Study of molecular motion in modification C of the n-alkane n-C33H68 by means of incoherent quasielastic neutron scattering
Authors: Ewen B. , Richter D.
Published: February 2003

By means of incoherent quasielastic neutron scattering experiments on uniaxially oriented n-C33H68 samples it was possible to give an experimental proof that molecular motion in modification C is composed of rotational and translational components. The analysis of the quasielastic spectra was performed comparing them to theoretical scattering laws, which were calculated on the basis of different models, compatible with the results of former x-ray, NMR and ir experiments. This procedure leads to the conclusion that translational jumps in the chain direction, the jump distance 1.27 Å, and rotational 180° jumps take place independently. At 67 °C the jump rates are: vt = (2.6±0.4) ×108 s−1 and vr= (5±2) ×107 s−1, respectively.
Deuterium isotope and molecular structure effects on charge trapping and scavenging in γ-irradiated glassy hydrocarbons
Authors: Wang H. Y. , Willard J. E.
Published: February 2003

An investigation of the cause of the higher (by a factor of ∼1.5) yields of trapped electrons [G (et−)] produced by the γ irradiation of 3-methylpentane-d14 (3MP-d14), methylcyclohexane-d14 (MCHx-d14), and 3-methylheptane-d18 (3MHp-d18) glasses at 77 K as compared to 3MP-h14, MCHx-h14, and 3MHp-h18 has provided new evidence and insight on such isotope effects and on the effects of molecular structure on G (e−t). The absolute reduction in G (et−) by a given concentration of the electron scanvengers SF6, CH3I, and biphenyl (φ2) is always greater in the deuterated than in the protiated form of 3MP or MCHx. G (φ−2) from the scavenging of e− by φ2 is higher in 3MP-d14 than 3MP-h14, in MCHx-d14 than in MCHx-h14 and in either form of 3MP than MCHx. The fractional decay rate of e−t is slower in deuterated than protiated matrices both at the glass transition temperature and below, and is faster than the fractional decay rate of trapped radicals. In the deuterated matrices, as in protiated matrices, the decay rates are reduced by positive charge scanvengers. G (TMPD+) from the scanvenging of positive charge by tetramethylparaphenylenediamine (TMPD) is the same, independent of deuteration, in 3MP-d14 and in 3MP-h14 glasses and varies with molecular structure in a manner which is uncorrelated with G (et−). The positive charge scavengers 2-methyl-1-pentene (2MP-1), triethylamine (TEA), and propene (C3H6 and C3D6) each cause an equal or greater absolute increase in G (e−t) in perdeuterated matrices than in the corresponding protiated matrices. They give different profiles of enhancement vs concentration in different alkanes. C3H6 and C3D6 produce different G (et−) enhancement profiles when compared in the same matrix. The optical spectra of e−t in deuterated and protiated alkanes and alkanes of different molecular structure are similar. The data indicate that the isotope effects and the effects of molecular structure on G (e−t) are not due to differences in the average trap depths or positive charge migration in the isotopic substances. They must be ascribed to either differences in electron migration distance at birth, or to spectroscopic state matching effects on the probability of e−t tunneling to cations, or both. The differences in G (e−t) enhancement by C3H6 and C3D6 in 3MP-h14 (and in 3MP-d14) seem to require the latter hypothesis. In all of this work the G (e−t) values have been determined spectrophotometrically, with ESR confirmation, using techniques designed to minimize uncertainties due to decay during and after irradiation.
Photoabsorption in carbon monoxide: Stieltjes–Tchebycheff calculations in the separated-channel static-exchange approximation
Authors: Padial N. , Csanak G. , McKoy B. V. , Langhoff P. W.
Published: February 2003

Theoretical investigations of total and partial-channel photoabsorption cross sections in carbon monoxide are reported employing the Stieltjes–Tchebycheff (S–T) technique and separated-channel static-exchange calculations. Pseudospectra of discrete transition frequencies and oscillator strengths appropriate for individual excitations of each of the six occupied molecular orbitals are constructed using Hartree–Fock core functions and normalizable Gaussian orbitals to describe the photoexcited and ejected electrons. Use of relatively large basis sets of compact and diffuse functions insures the presence of appropriate discrete Rydberg states in the calculations and provides sufficiently dense pseudospectra for the determination of convergent photoionization cross sections from the S–T technique. The calculated discrete vertical electronic excitation spectra are in very good agreement with measured band positions and intensities, and the partial-channel photoionization cross sections are in correspondingly good accord with recent electron–electron (e,2e) coincidence, synchrotron-radiation, and line-source branching-ratio measurements. Predicted resonance features in the X, B, O2s−1, and carbon K-shell channels are in particularly good agreement with the positions and intensities in the measured cross sections. A modest discrepancy between experiment and theory in the A-channel cross section is tentatively attributed to channel-coupling mechanisms associated with opening of the 1π shell. The total vertical electronic S–T photoionization cross section for parent-ion production is in excellent agreement with recent electron–ion coincidence measurements. Comparisons are made between ionization processes in carbon monoxide and in the previously studied nitrogen molecule, and similarities and differences in the respective cross sections are clarified in terms of conventional molecular-orbital theory.
Semiclassical theory of near-adiabatic photon-assisted molecular collisions
Authors: Copeland Drew A.
Published: February 2003

The application of the semiclassical impact parameter method to the study of photon-assisted electronic transitions in near-adiabatic reactions such as A+B*+h/ω→A*+B is discussed. Approximate criteria for which this method describes such a collision process are derived. Regarding the applied field classically, the semiclassical equations are systematically extracted from the more general quantum mechanical equations. In conjunction with this derivation, the method of averaging is used to obtain a semiclassical solution of the time-dependent Schrödinger equation describing the unperturbed relative motion of the collision partners.
Comment on: ’’Energy transfer cross sections in relation to generalized oscillator strengths for the donor–acceptor pair benzene–acetone’’
Authors: Klump Kirby N. , Lassettre Edwin N.
Published: February 2003

The energy transfer cross sections for benzene and acetone are calculated using the generalized oscillator strengths of ref 6. (AIP).
A conjectured interpretation of the OH stretching spectrum of low density amorphous solid water
Authors: Madden William G. , Bergren Michael S. , McGraw Robert , Rice Stuart A. , Sceats Mark G.
Published: February 2003

The model developed in the preceding paper for the vibrational spectrum of ice Ih in the OH stretching region is extended to the description of the spectrum of amorphous solid water. The effects of structural disorder on the spectrum are explicitly considered using a random network model for the structure of the amorphous solid. The differences between the vibrational spectrum of ice Ih and that of the amorphous solid are found to arise from the larger variation in oxygen–oxygen bond distances in the amorphous solid. The departure of the random network from the regular crystalline connectivity of the ice Ih lattice appears to have little effect on the spectrum.
H Lyman-α emission from photodissociation of H2O
Authors: Mentall J. E. , Möhlmann G. R. , Guyon P. M.
Published: February 2003

Photodissociation of water vapor to produce H(n=2) atoms was examined from threshold to 500 Å. The experimental threshold of 813.0 Å nearly coincides with the thermodynamic limit. Excitation cross sections rise slowly from threshold to a maximum value of (1.55±0.12) ×10−18 cm2 at ∼709 Å and decreases to a negligible value below 550 Å. Structure in the excitation function is attributed to predissociation of Rydberg series converging to the ? 2B2 state of H2O+.
The effect of three-body dispersion forces in liquids on solubilities and related functions
Authors: Goldman Saul
Published: February 2003

The Axilrod–Teller equation is used in the context of the Leonard–Henderson–Barker liquid state perturbation theory to determine the effect of triple-body dispersion foces on the Henry’s law constant, the heat of solution at infinite dilution, and the partial molar volume at infinite dilution (K, ΔH2-, and ?2-, respectively) for the following systems: He in Ar, Ne in Ar, and Kr in Ar. The pair potentials used came from molecular beam scattering and other data. For those systems where experimental data existed for comparison (He in Ar and Ne in Ar) excellent agreement with the data was found. The effect of including these three-body forces was to increase the magnitude of each of K, ΔH2-, and ?2- over the corresponding values obtained by using only two-body forces. The relative importance of the three-body to the two-body term increased in the sequence (He in Ar)  <  (Ne in Ar) < (Kr in Ar). Finally, it was found that the presence of a spherical solute in argon had the effect of enhancing the proportion of repulsive like argon triplets, relative to the proportion of repulsive triplets that occur in unperturbed pure argon.
Theoretical study of collinear Be+FH(v1) →BeF(v2) +H
Authors: Schor Heloiza , Chapman Sally , Green Sheldon , Zare Richard N.
Published: February 2003

The potential energy surface for collinear Be+FH→BeF+H has been studied at various levels of ab initio approximation. A final surface was obtained from a first order configuration interaction wavefunction, using the iterative natural orbital method and a medium-sized basis set of Slater atomic functions; this is expected to give a semiquantitative description of the reactive process. The exothermicity is computed to be 6 kcal/mole which can be compared with the best experimental value of 2±4 kcal/mole. The barrier height is predicted to be 28 kcal/mole at a geometry where both internuclear separations are extended by about 0.4 bohr from their asymptotic equilibrium values. This surface differs qualitatively from simple LEPS models. The curvature of the reaction path is much more abrupt, the atom effecting little distortion of the partner molecule until quite close approach in both entrance and exit channels. The surface was fit with bicubic splines and dynamics was studied by the quasiclassical trajectory method as a function of initial kinetic energy for the reactant initially in v1=0 and v1=1. The reaction probability and final energy distributions were found to depend sensitively and selectively on the initial kinetic and vibrational energy. Most of the available energy is channeled into product translation; for v1=0 at higher initital kinetic energies, less than 10% of the available energy becomes product vibration. Also, addition of reactant vibrational energy has a profound effect on reaction probability and final vibrational distributions. Examination of typical trajectories made it possible to identify the surface features responsible for the dynamical behavior. For comparison, calculations were also done on a LEPS surface constructed to have the same barrier position and height. Because the LEPS surface has a more gently curved reaction path, with better coupling of vibrational and translational energy, it results in less specific energy use and disposal. For example, 40%–50% of the available energy was channeled into product vibration on the LEPS surface, and addition of reactant vibrational energy effected only small changes in the dynamics. These results underline the dangers of using oversimplified potential surfaces in the study of reactive collision dynamics.
Phase-difference locking of coupled oscillating chemical systems
Authors: Fujii Hidehiko , Sawada Yasuji
Published: February 2003

It was experimentally verified that the phase difference of two Belousov–Zhabotinskii chemical systems is locked when Δω/M is smaller than a critical value, where Δω is the difference of the natural limit cycle frequencies and M is the coupling constant of the two chemical systems. Above the critical value, the systems behave almost independently. A simple model is presented to explain the observed phenomena.
Semiempirical potential surfaces and dynamical considerations for collisions between alkali metals and molecular oxygen: Li+O2 and Na+O2
Authors: Alexander Millard H.
Published: February 2003

The intermediates in both the inelastic and reactive collisions of alkali metals with molecular oxygen are the M+O−2 ion pairs. We develop here analytic forms for the lowest energy diabatic potential energy surfaces of both A′ and A″ electronic symmetry for the Li+O−2 and Na+O2− systems. An exponential–rational approximant functional form, which is found to provide an excellent description of the ionic alkali monoxide potential curves, is adapted to the alkali dioxide ion pairs. The degree of polarization of the O−2 charge distribution is modeled from available theoretical data for the homologous LiF2 system. With only a few variable parameters it is possibile to fit extremely well those features of the M+O−2 surfaces which are presently known either from experiments or ab initio calculations. The topologies of the A′ and A″ surfaces are substantially dissimilar. The lower energy surface (A″) for both LiO2 and NaO2 possesses a pronounced minimum in isoceles triangular geometry, as expected from the matrix isolation experiments; whereas the A′ surface displays little variation with the MOO angle. The fitted ion pair surfaces are used to discuss certain expected features of the M+O2 collision dynamics. Also, we show how it may be possible to relate the coupling matrix elements for the MO2 ionic–covalent surface crossings to equivalent matrix elements for the alkali monoxide systems, which are significantly easier to determine.
Classical and near classical rates for bimolecular dissociation and exchange
Authors: Snider Neil
Published: February 2003

An expression based on semiclassical transition state theory was derived for the sum of bimolecular rate constants for dissociation and exchange in a three-center collinear system.
Semiclassical phase space representation of the density matrix
Authors: Snider Neil
Published: February 2003

An approximation to the Wigner phase space representation of the canonical ensemble density matrix was developed. The approximation has the advantage of being relatively easy to compute. It was tested on models with a single degree of freedom and found to give accurate results in cases where quantum corrections are small or only moderately large.
Spin polarization of charge-transfer triplet excitons in phenanthrene–PMDA single crystals
Authors: Keijzers C. P. , Duran J. , Haarer D.
Published: February 2003

The spin polarization of charge-transfer triplet excitons in phenanthrene–PMDA single crystals is investigated in detail. It is shown that the symmetry of the tensor which describes the population inversion of the spin levels is due to small nπ* and ππ* admixtures, localized on the PMDA molecule. These admixtures to the total charge–transfer triplet wavefunction are so small that they can be neglected for the calculation of the triplet energy levels (zero field splitting tensor), yet they dominate the intersystem crossing process and determine the symmetry of the observed spin polarization.
The observation of the excimer formation process in pyrene and perylene crystals using a picosecond ruby laser and streak camera
Authors: Kobayashi Takayoshi
Published: February 2003

The rise curves of fluorescence from the excimer excitons in pyrene and perylene crystals were observed with the use of the equipment for fluorescence lifetime measurements in the picosecond region. The time constants of excimer formation in pyrene and perylene crystals at 293 and 77 K were found to be considerably shorter than 20 ps. From this result and the previously reported hopping time of excitons in pyrene crystal (20 ps) at room temperature, it is concluded that the sum of the time constants for conformational change of an energy-donating pair and excimer-energy transfer is about 20 ps.
Exciton line shapes and migration with stochastic exciton lattice coupling
Authors: Blumen Alexander , Silbey Robert
Published: February 2003

In a recent article, Sumi [J. Chem. Phys. 67, 2943 (1977)] has discussed the optical line shape of an exciton interacting with phonons using a Gaussian Markov process for this interaction. By assuming that the correlation time of the process is nonzero, he was able to explore various limits of (motional) narrowing of the line. His analysis used a dynamic coherent potential approximation (CPA) in order to calculate the line shape. In the present paper, we derive these results in closed analytic form, without the CPA, by using standard analysis. Since our results agree with Sumi very closely, the present approach provides a simple way of understanding the underlying physics. In addition, we show how the exciton density of states enters in a simple way, and compare the hemicircular and the Lorentzian forms for this density of states.
Radiative and radiationless vibronic deactivation rates in selectively excited CO+
Authors: Bondybey V. E. , Miller Terry A.
Published: February 2003

A tunable dye laser has been used to selectively excite particular (v=0–3) vibrational levels of A 2Π CO+. The CO+ is produced in a He bath by Penning ionization of CO by He metastables. By studying the time resolved emission of CO+ from selected vibrational levels as a function of He pressure, values have been obtained for the radiative decay rates and the velocity averaged cross-sections for CO+ vibrational relaxation upon He collision. The cross sections are all rather large (≳0.1 Å2) and increase considerably with increasing v. Time resolved spectra of the vibrationally relaxed emission require the postulation of a three-level system to fit their form. It is proposed that both of these effects are consistent with a vibrational relaxation mechanism wherein the A 2Π CO+ vibrational levels relax via intermediate, highly excited, vibrational levels of the ground X 2Σ state of CO+.