bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociative ionization of molecules by electron impact. II. Kinetic energy and angular distributions of N+ and N++ ions from N2
Authors: Deleanu Liliana , Stockdale J. A. D.
Published: February 2003

An apparatus containing crossed molecular and pulsed electron beams has been used to obtain distributions in kinetic energy and angle of fast (≳1.5 eV) N+ and N++ ions produced through dissociative ionization of N2 by impact of electrons with energies from threshold to 300 eV. Previously unreported structure is found in both N+ and N++ kinetic energy spectra. The angular distributions of N+ (energy resolved) and N++ (total) ions are found to be substantially isotropic for electron beam–ion detector angles from 30°–110°.
Study of the structure of molecular complexes. IX. The Hartree–Fock energy surface for the H2O–Li–F complex
Authors: Kress James W. , Clementi E. , Kozak John J. , Schwartz Maurice E.
Published: February 2003

A large number of geometrical configurations (250) are computed with a large Gaussian basis set in the Hartree–Fock approximation for the H2O–Li–F complex. The many-dimensional potential energy surface has been sampled by keeping the molecule of water at a fixed position and by allowing the lithium and the fluorine to assume many positions in space. Because of the symmetry (C2v) of the water molecule, the 250 computations correspond to a sampling of about 600 configurations. The sampling includes a few highly repulsive configurations (up to about 300 kcal/mole in repulsion); the remaining points are either in the strongly attractive regions or in the weakly attractive regions of the surface. The stabilization energy of the complex reveals the existence of at least three possible structures: the Li–F–H2O structure (with C2v symmetry), with a stabilization energy (relative to H2O, F−, and Li+) of about −186 kcal/mole; a second Li–F–H2O structure with the fluorine forming a hydrogen bond (with one of the H–O groups of the water molecule), with a stabilization energy of about −191 kcal/mole; and a third structure, H2O–Li–F (with C2v symmetry), with a stabilization energy of about −201 kcal/mole. The main goal of this work is not the determination of the structures of the H2O–Li–F complex, but the construction of a reliable potential to be used in the study of the structure and properties of ionic solutions. For this reason, the computed Hartree–Fock energies have been accurately fitted with a simple analytical expression. In addition, the Hartree–Fock energy for the complex has been analyzed by partitioning it into two-body interaction energies (Li–F, Li+–H2O, and F−–H2O) and into three-body potential-energy terms. Finally, a second type of partitioning, the so-called bond energy analysis, has been presented and discussed.
High resolution photoionization study of ion-pair formation in H2, HD, and D2
Authors: Chupka W. A. , Dehmer P. M. , Jivery W. T.
Published: February 2003

A new photoionization mass spectrometer which is significantly superior to previous instruments in both wavelength resolution and ion intensity is described. It has been used in the study of photon-induced ion-pair formation at 78 °K in para-H2, ordinary H2 and D2 (the ortho-para equilibrium mixtures), and HD in the wavelength region from 718 to 700 Å, at a wavelength resolution (FWHM) of 0.035 Å for para-H2, 0.07 Å for ordinary H2 and D2, and 0.15 Å for HD. The threshold for formation of ion pairs from para-H2 occurs at 715.753+0.046−0.008 Å (17.3223+0.0002−0.0011 eV) and this value together with the accurately known values of the dissociation energy of H2 and the ionization potential of atomic hydrogen yields a value of the electron affinity of the hydrogen atom of 0.7542 −0.0004 +0.0013 eV in excellent agreement with the theoretical value of 0.75421 eV calculated by Pekeris. The observed thresholds for ion-pair formation in D2 and HD are also in excellent agreement with the calculated positions. For para-H2 the region within 0.5 Å of the threshold has a continuumlike appearance contains window resonances which are interpreted as predissociating members of a Rydberg series converging to H+2(v=9, N=2). The continuum results from closely spaced predissociating Rydberg states converging to H+2(v=9 N=0) which were not resolved in the present experiment. A number of other tentative Rydberg state assignments were made for para-H2 based on the results of an analysis using multichannel quantum defect theory. No Rydberg state identifications were attempted for the other systems due to the greatly increased complexity of their spectra. For HD which can dissociate to either H++D− or H−+D+, the H−/D− ratio is ∼2 just above threshold and appears to decrease with decreasing wavelength; however, this is due, at least in part, to discrimination by the detection system against the more energetic H− ions. Some conclusions and inferences regarding the mechanism for predissociation in H2 and HD are discussed, particularly the possible importance of the 4fσ 1Σ+u state.
Molecular reorientation and boron nuclear quadrupole interaction in crystalline o
r
t
h
o-carborane
Authors: Leffler A. J. , Alexander M. N. , Sagalyn P. L. , Walker N.
Published: February 2003

Molecular reorientation in crystalline ortho-carborane has been studied by 1H, 10B, and 11B nuclear magnetic resonance (NMR) at several frequencies, in the temperature interval 155–370 K. The phase transition originally found by Baughman has been determined in our measurements to occur at 274±2 K. o-carborane reorientation in the high temperature phase has been found by 1H and 11B T1 measurements to have an activation energy EA=5.8±1.3 kJ/mole. Pulse and cw NMR data are consistent with a picture of the high temperature phase in which o-carborane molecules undergo rapid isotropic reorientation. NMR data for the low temperature phase demonstrate the reorientation must be anisotropic. The boron lines are quadrupole-split. The width and shape of the 11B central line are functions of NMR frequency and temperature. At low temperature and low frequency the 11B line is broad and asymmetric. Evidence indicates the width of this line is due mainly to lifetime broadening. 1H and 11B T1 data in the high temperature phase, and a theory of the frequency dependence of the 11B central line in the low temperature phase, yield an ’’average’’ 11B quadrupolar coupling constant (e2Qq/h¯) =1.3±0.3 MHz. The 1H spin–lattice relaxation time in the low temperature phase is an asymmetric function of inverse temperature. The apparent activation energy for temperatures above the T1 minimum is EA=33.5±1.8 kJ/mole at NMR frequencies of 10 and 20 MHz. Below the T1 minimum, EA=14.0±1.1 kJ/mole at 10 MHz, and EA=18.9±0.7 kJ/mole at 20 MHz.
Vacuum ultraviolet fluorescence from photodissociation fragments of CO and CO2
Authors: Lee L. C. , Carlson R. W. , Judge D. L. , Ogawa M.
Published: February 2003

The vacuum ultraviolet fluorescence from photofragments of CO and CO2 has been studied and the cross sections for the production of fluorescence have been measured. The fragments were produced by interactions of the gases with synchrotron radiation in the wavelength region λλ175–800 Å. The dependence of the fluorescence intensity on gas pressure was studied and at high pressure additional fluorescence induced by photoelectrons was observed. Discrete features were observed in CO in the primary photon wavelength region λλ570–610 Å. Similar features were observed in CO2 in the λλ690–800 Å region corresponding to Rydberg states converging to the CO+2 (A 2Π) and CO+2 (B 2Σ+) states. The observed structure results from interactions of Rydberg states with dissociation continua. At shorter primary photon wavelengths the cross sections for the production of fluorescence from photofragments of CO decrease monotonically from 3.6 Mb at λ480 Å to 1.2 Mb at λ180 Å. In contrast with this, two broad bands were observed in the fluorescence from photofragments of CO2 with peak cross sections of 0.73 Mb at λ230 Å. The photodissociation and dissociative ionization processes that produce the observed results are discussed.
Raman scattering in hydrogen halide gases
Authors: Cherlow J. M. , Hyatt H. A. , Porto S. P. S.
Published: February 2003

We have measured the Q branch vibrational Raman cross sections of the hydrogen halide gases HCl, HBr, and HI relative to that of N2. We also measured the variation of the HI cross section as a function of the incident laser frequency. The absence of preresonance behavior in HI suggests that the virtual states primarily responsible for Raman scattering lie near or above the ionization potential. This in turn implies that the variation of the Raman cross section among the hydrogen halide molecules results primarily from a variation in the magnitude of the dipole matrix elements rather than from a variation in the energy of the virtual states involved.
Intramolecular excimer formation in 1,3-bis(N-carbazolyl)propane: Solvent viscosity effects
Authors: Johnson G. E.
Published: February 2003

Rate constants for intramolecular excimer formation and dissociation in 1,3-bis(N-carbozolyl)propane have been measured in a number of different solvents using time resolved fluorescence decay techniques. It is found that within homologous series of solvents both rate constants decrease progressively with increasing viscosity and obey a relationship originating with the concept of free volume requirements for intramolecular conformational change. Among a group of chemically and structurally different solvents both rate constants are observed to vary randomly with the macroscopic solvent viscosity indicating that other solvent properties are also important. The results are discussed in terms of a qualitative model in which the microstructure of the solvent shell surround the interacting chromophores, as determined by the degree of solvation, ultimately determines the ease with which this intramolecular photophysical process can occur.
Kinetic theory of thermal transpiration and mechanocaloric effect. II
Authors: Loyalka S. K.
Published: February 2003

The problem of thermal transpiration between parallel plates and in a cylindrical capillary is considered by using the Maxwellian diffuse-specular reflection at the surface. The relevant integral equations are solved by a simple numerical scheme and results for both the Poiseuille flow and the thermal creep flow rate are obtained. Some recently reported variational results for the Poiseuille flow appear to be inaccurate (off by about 10%–15%), and there is a need for further refinement in the variational calculations. The effect of partial accommodation is found more significant in the free molecule region than in the near continuum region. Thus to minimize the surface effects, the experiments on the thermal conductivity measurements via thermal transpiration would best be conducted in the near continuum region.
The measurement of T1 and T2 for the ammonia inversion doublets in the ground vibrational state
Authors: Hoke W. , Ekkers J. , Flygare W. H.
Published: February 2003

The measurement of T1 and T2 in the ground vibrational state inversion levels of 15NH3 is reported for several values of J and K. Newly developed transient experiments are used to measure, independently, T1 and T2. (AIP)
Bound-state energies of the exponentially screened static and cosine Coulombic potentials
Authors: Bessis N. , Bessis G. , Corbel G. , Dakhel B.
Published: February 2003

Bound state energies of the exponentially screened Coulomb potentials: V (r) =−Ze−αr cosεαr/r, static (ε=0) and oscillatory (ε=1), are obtained by a perturbation calculation on the basis of the Hulthén functions. Closed form expressions of the perturbative matrix elements as well as of the Hulthén functions, for ’’S’’ states and for l≠0 states (with an approximated rotational term), are obtained by the ladder operator method. When varying the values of the Hulthén parameter, the first order perturbed eigenvalues of the screened potentials are found to be close to the exact values. New results (l≠0) are given for the cosine case.
Modification of Glauber theory for dynamic scattering of electrons by polyatomic molecules
Authors: Bartell L. S.
Published: February 2003

In its original form, Glauber scattering theory achieves a considerable simplification by neglecting several factors of modest consequence in atomic scattering but of importance in molecular scattering. These include, in particular, the spread of the shadow cast by a scatterer as a wave propagates downstream in the target. Although such effects do not matter in the original range of validity of the theory, the characteristic distances in molecules restrict the unmodified theory to an excessively small angular range. The present treatment introduces phase factors to take into account the distribution of scatterers in the incident direction and the propagation of the scattered waves in the target. This appreciably extends the formal range of applicability.
Energy transfer in one-dimensional molecular crystals: Direct and indirect energy exchange in the non-Boltzmann regime
Authors: Lewellyn M. T. , Zewail A. H. , Harris C. B.
Published: February 2003

Direct energy transfer between localized states via virtual coupling with the host states in one-dimensional molecular solids is demonstrated experimentally using optical and optically detected electron spin coherence techniques. The relative importance of direct transfer versus indirect transfer (which is characterized by the phonon-assisted promotion of the localized state to the band with the subsequent radiationless decay into a mobile exciton followed by retrapping) has been determined in isotopically mixed 1,2,4,5-tetrachlorobenzene crystals over a temperature range where the thermal energy of the lattice is insufficient to establish Boltzmann equilibria between the localized and delocalized states (non-Boltzmann regime). The results demonstrate that the transfer mechanisms between trap and host states are very sensitive to the trap concentration and the temperature of the lattice, and that direct exchange is dominant at high trap concentrations. Finally, the experimental results are compared with the theoretical expectations for excitation yields and trap-to-trap distances in the isotopically mixed crystals.
Metal-isotope measurements of iron pentacarbonyl
Authors: Hutchinson B. , Hance R. L. , Bernard B. B. , Hoffbauer M.
Published: February 2003

The infrared spectra of 54Fe(CO)5 and natural abundant Fe(CO)5 have been measured and compared in the 350 to 4000 cm−1 region. These measured shifts are compared to shifts calculated from three force fields.
Theory of random walks with limited order of non-self-intersections used to simulate macromolecules
Authors: Wall Frederick T.
Published: February 2003

A general theory of random walks subject to partial elimination of self-intersections has been carried out. Specifically, the theory applies to so-called self-avoiding walks of order p, meaning that loops of p or less are forbidden while larger loops are allowed. All walks, used to represent macromolecules, are categorized by the configurations of their last p−1 steps. The theory deals with the successive addition of steps through use of transition matrices, together with their eigenvectors and eigenvalues, taking cognizance of the effect of symmetry. An exact solution is derived in general form and it is found that the mean square end-to-end separation of the walks becomes asymptotically linear in n, the number of steps. The slope of the asymptotic line increases with the order p, presumably without limit. All classes of walks, characterized by their different end configurations, asymptotically exhibit similar linear behavior with the same slope but with different intercepts.
Laser photoluminescence of TiO in Ne at 4 °K
Authors: Brom J. M. , Broida H. P.
Published: February 2003

Pseudo-gas-phase photoluminescence spectra of TiO isolated in Ne matrices at 4 °K have been obtained. Photoluminescence is induced by laser excitation from the X 3Δ ground state to B 3Π and C 3Δ electronic levels using a cw tunable dye laser and a cw argon ion laser, respectively. Red emission from B 3Π vibrational levels terminating in ground state levels is observed. In addition, the B 3Π (v=0) and B 3Π (v=1) levels have been found to undergo matrix-induced intersystem relaxation into the b 1Π state. Near-infrared emission between excited states in the singlet manifold of TiO has been observed and identified as belonging to the δ (b 1Π→a 1Δ) and the φ (b 1Π→d 1Σ+) band systems. Observed δ and φ emission bands are uniformly shifted by −93 cm−1 and +73 cm−1, respectively, from observed or predicted gas phase bandheads. The line shape of the δ and φ emission in the solid is that of a sharp zero-phonon line accompanied by a broad multiphonon sideband. An additional sharp zero-phonon emission system, here labeled the ζ system, is found to overlap several broad multiphonon bands of the red γ′ (B 3Π→X 3Δ) emission. Evidence obtained supports the tentative conclusion that the ζ system is due to intercombination b 1Π→X 3Δ emission. From the assigned intercombination transition the energy of the low-lying a 1Δ state is determined to be 3500 cm−1 above the X 3Δ ground state. The 3500 cm−1 value for the relative energy between the isoconfigurational a 1Δ and X 3Δ states is in qualitative agreement with theoretical expectations based on consideration of the bonding in the TiO molecule.
Low-frequency vibrational spectra and ring puckering of cyclopentene-d8
Authors: Villarreal J. R. , Bauman L. E. , Laane J. , Harris W. C. , Bush S. F.
Published: February 2003

More than 25 ring-puckering transitions have been observed in the vapor-phase Raman and far-infrared spectra of cyclopentene-d8. The ring-puckering potential energy function in reduced form was determined to be V=18.20(Z4−6.88 Z2) cm−1 which represents a barrier to inversion of 215 cm−1 (0.61 kcal/mole). The barrier differs from that of the undeuterated cyclopentene by 17 cm−1 owing to mixing of other motions (presumably CH2 rockings). The equilibrium value of the molecule is calculated to have a dihedral angle of 26°.
Cross section for single and double ionization of N2 and O2 molecules by electron impact from threshold up to 170 eV
Authors: Märk T. D.
Published: February 2003

Absolute ionization cross sections have been measured for the processes N2+e→N2++2e, N2+e→N2+++3e, O2+e→O2++2e, and O2+e→O2+++3e as a function of electron energy up to 170 eV in a crossed molecule–electron beam apparatus. The ionization cross section curve for producing the singly charged parent ions N2+ and O2+ is found to have at least one inflection of the type first observed by Lawrence for mercury. Using nth root extrapolation the following minimum ionization potentials were derived from the low energy cross section behavior: N2+(X 2Σg+) =15.6±0.1 eV, N2++(3Πu) =42.9±0.3 eV, O2+(X 2Πg) =12.1±0.1 eV, and O2++(X 1Σg+) =35.6±0.3 eV. From distinct breaks in the single ionization cross section curve the following excited states were observed; N2+(B 2Σ+u) =18.8±0.2 eV, O2+(a 4Πu) =16.0±0.2 eV, and O+2(2Σ−g) =21.0±0.2 eV. In the double ionization cases, a square law was found from threshold up to some 20 eV above threshold. All results are compared with previous measurements if available.
Nonadiabatic effects in the collision of F(2P) with H2(1Σ+g). I. SCF interaction potentials for the 1 2A′, 2 2A′, and 2A″ states in the reactant region
Authors: Rebentrost Frank , Lester William A.
Published: February 2003

Interaction potentials for the 1 2A′, 2 2A′, and 2A″ states of the F+H2 system are computed in the self-consistent field (SCF) approximation for a range of geometries suitable for the study of nonadiabatic interactions at thermal energies. Notable features found for the energy surfaces are (a) dominance at long range of the quadrupole–quadrupole interaction, (b) strong repulsive forces at small F–H2 separations for the excited-state surfaces, and (c) a pseudocrossing region with a conical intersection for the collinear nuclear arrangement at R (F–H2) =5.2 a.u.
Experimental dipole moment function and calculated radiative lifetimes for vibrational transitions in carbon monoxide a 3Π
Authors: Wicke Brian G. , Klemperer William
Published: February 2003

We have used precision Stark effect data from a previously reported molecular beam electric resonance study of CO a 3Π to determine the dipole moment operator as a function of the internuclear coordinate R for this state. The resulting experimental dipole moment function is well determined in the range 1.10 Å⩽R⩽1.35 Å, corresponding to the region sampled by the v=0–3 vibrational levels used for this analysis. With this dipole moment function we then calculate the spontaneous vibrational transition probabilities for the lower vibrational levels within CO a 3Π. We obtain a lifetime for CO a 3Π v=1→v=0 transition of 17 msec, approximately half the corresponding v=1→v=0 lifetime observed for ground state CO X 1Σ+. It is shown that vibrational radiative lifetimes are quite comparable to electronic radiative lifetimes in CO a 3Π.
Steric hindrance in the gas phase singlet electronic energy transfer: From naphthalene to the t
r
a
n
s-azobutane isomers
Authors: Loper Gary L. , Lee Edward K. C.
Published: February 2003

The singlet intermolecular energy transfer rates from naphthalene (S1, ∼91 kcal/mol) to the various trans-azobutane isomers have been measured in the gas phase. The first excited singlet states of these azobutane isomers lie below ∼70 kcal/mol. The rate constants for singlet collisional energy transfer (kc) are 30.3±2.6, 18.9±0.9, 12.6±1.0, and 3.2±0.3 in units of 1010 M−1⋅sec−1, for the accepter series azo-n-butane, azoisobutane, azo-sec-butane, and azo-tert-butane, respectively. The tenfold decrease in the rates of energy transfer through this series have been attributed to increasing steric inhibition of the electron exchange contribution to singlet–singlet electronic energy transfer.
Generalized x-ray scattering factors in diatomic molecules
Authors: Stewart Robert F. , Bentley John , Goodman Bernard
Published: February 2003

Generalized x-ray scattering factors for atoms (pseudoatoms) in diatomic molecules are determined from a finite multipole expansion of the charge density about each nucleus. The Fourier–Bessel coefficients of the pseudoatom radial density functions are determined by a least squares fit to the molecular form factor. All molecular one-center averages of the form 〈g (ra) Pj(cosϑa) 〉 are correctly given by the pseudoatom superposition whenever j?J, where J is the highest order multipole included in the a pseudoatom, regardless of the highest multipole order K for the b pseudoatom. To illustrate this property, a number of relationships between moments of the molecular charge distribution and of the pseudoatom are given. In addition, a sum rule relating the molecular form factor to the expectation values 〈ra−(j+1)Pj(cosϑa) 〉 and 〈rb−(k+1)Pk(cosϑb) 〉 is derived. For H2 the theoretical, coherent x-ray scattering intensity is reproduced to about 1% for J=K=1 and to about 0.1% for J=K=2.
Diatomic generalized x-ray scattering factors: Results from Hartree–Fock electron density functions
Authors: Bentley John , Stewart Robert F.
Published: February 2003

Generalized x-ray scattering factors based on multipole representation of the atoms have been calculated from Hartree–Fock Roothaan one-electron molecular density functions of first row diatomic hydrides (BH to FH) and the 14 electron series BF, N2, (1Σ)CO, and (3Π)CO. The effect of different truncation of the mutipole expansions is discussed for NH. Results for the other molecules from expansions up to quadrupolar terms on each center are discussed. The monopole scattering factors are virtually the same as Hartree–Fock atomic form factors except at low sinϑ/λ values. The dipole and quadrupole scattering factors are rather specific to the particular molecule for the valence region, but core polarization features are similar. Static charge properties of the molecule are accurately given by the generalized scattering factors. However, charge properties associated with the individual pseudoatoms are strongly dependent on truncation of the multipole expansion. The coherent x-ray scattering intensities of the molecule can be calculated from the generalized x-ray scattering factors with a maximum relative error no larger than 4×10−5.
Thermomagnetic torque in hydrogen isotopes
Authors: Cramer J. A.
Published: February 2003

The thermomagnetic torque has been measured in parahydrogen and ortho and normal deuterium for pressures from 0.10 to 2.0 torr and temperatures from 100 to 370 K. Since the torque depends on the precession of the molecular rotational magnetic moment around the field direction, coupling of the molecular nuclear spin to the rotational moment can affect the torque. Evidence of spin coupling effects is found for the torque in both deuterium modifications. In para hydrogen the torque at all temperatures and pressures exhibits behavior expected of a gas of zero nuclear spin molecules. Additionally, earlier data for hydrogen deuteride and for normal hydrogen from 105 to 374 K are evaluated and discussed. The high pressure limiting values of torque peak heights and positions for all these gases are compared with theory.
Chemiluminescence studies. IV. Pressure-dependent photon yields for Ba, Sm, and Eu reactions with N2O, O3, O2, F2, and NF3
Authors: Eckstrom D. J. , Edelstein S. A. , Huestis D. L. , Perry B. E. , Benson S. W.
Published: February 2003

Chemiluminescence spectra and photon yields are presented for reactions of Ba, Sm, and Eu with N2O, O3, O2, F2, and NF3 for the pressure range 0.5–20 torr. Peak yields range from 2.5% to 70%, with the reactions of Sm with NF3 and F2 having the highest yields. These latter reactions put from 12% to 16% of their available thermal energy into luminous output. The pressure dependence of the photon yields deduced from these and other recent measurements suggests that the initial exothermic reaction primarily populates high vibrational levels of the ground electronic state of the newly formed diatomic. Radiating states would then be populated mainly by collision-induced vibrational-to-electronic internal conversion.
Rotational friction coefficients for ellipsoids and chemical molecules with the slip boundary condition
Authors: Youngren G. K. , Acrivos A.
Published: February 2003

Friction coefficients are calculated numerically for ellipsoids rotating about their principal axes and for a benzene molecule rotating normal to its axis of symmetry under conditions of creeping flow and slip boundary conditions. It has been shown previously, that if a benzene molecule is approximated by an oblate spheroid, the predicted and experimental friction coefficients differ by more than 40%. The present study employs a more realistic shape for benzene and finds a difference of 10%, which is within the limits of the numerical and experimental uncertainty.
Direct vibrational excitation of ions and molecules via charge exchange in N+2–N2, O+2–O2, CO+–CO, and NO+–NO collisions
Authors: Moran T. F. , McCann K. J. , Flannery M. R.
Published: February 2003

Differential and integral cross sections for direct vibrational excitation in symmetric N+2(X 2Σ+g, υ′0) –N2(X 1Σ+g, υ″0), O+2(X 2Πg, υ′0) –O2(X 3Σ−g, υ″0), CO(X 2Σ+, υ′0) –CO(X 1Σ+, υ″0) and NO+(X 1Σ+, υ′0) –NO(X 2Πr, υ″0) collisions have been determined for ions with 0.004 to 2.2 keV kinetic energies. Inelastic channels in the individual symmetric ion–molecule systems are strongly coupled to each other and to the energy resonant charge exchange channels. Differential cross sections for excitation of individual inelastic product channels have been computed as a function of scattering angle using a multistate impact parameter description of the collision. Small angle scattering dominates and becomes more concentrated in the forward direction with increasing ion kinetic energy. Multistate integral cross sections for individual channels and integral cross sections summed over all product channels are presented as a function of both ion kinetic energy and reactant ion vibrational state.
Vibrational relaxation in He-H2 in the high energy regime
Authors: Novak P. , Wartell M. A.
Published: February 2003

Semiclassical calculations of translation–vibration energy transfer in the He–H2 system have been carried out and compared with recent experimental measurements on the system. (AIP)
Dissociation of H+3 to form metastable hydrogen
Authors: Thomas E. W. , Fitzwilson R. L. , Sauers I. , Ford J. C.
Published: February 2003

The differential cross section for the formation of metastable hydrogen atoms from H+3 is measured for the system of H+3 beam scattering on a gas target of H2, Ar, and N2. (AIP)
Brillouin scattering by molecular crystals: s-trichlorobenzene
Authors: Swanson Douglas , Brunel Louis-Claude , Dows David A.
Published: February 2003

The Raman and Brillouin scattering spectra of single crystal s-trichlorobenzene at room temperature, and the Raman spectrum of a polycrystalline sample at low temperatures, have been obtained. Seventeen of the 21 predicted lattice vibrations have been observed in the Raman spectrum. Brillouin spectra at various angles of scattering and for various crystal orientations have yielded sound velocities for 11 modes of propagation. The nine independent elastic constants for the orthorhombic crystal have been evaluated. The dynamic and static properties of the crystal lattice have been calculated using an atom–atom intermolecular potential model with parameters derived by Bonadeo. Agreement with the Raman lattice frequencies is to about 7%, with the observed sound velocities to about 9%. The atom–atom potential is capable of predicting dynamic and static properties of the lattice with reasonable accuracy, but is apparently not completely adequate.
Absolute rate constant determinations for the deactivation of O(1D) by time resolved decay of O(1D) →O(3P) emission
Authors: Davidson J. A. , Sadowski C. M. , Schiff H. I. , Streit G. E. , Howard Carleton J. , Jennings D. A. , Schmeltekopf A. L.
Published: February 2003

Absolute rate constants for the deactivation of O(1D) atoms by some atmospheric gases have been determined by observing the time resolved emission of O(1D) at 630 nm. O(1D) atoms were produced by the dissociation of ozone via repetitive laser pulses at 266 nm. Absolute rate constants for the relaxation of O(1D) (×10−10 cm3 molecule−1⋅s−1) by N2(0.30±0.01), O2(0.41±0.05), CO2(1.2±0.09), O3(2.4±0.1), H2(1.3±0.05), D2(1.3±0.05), CH4(1.3±0.3), HCl(1.4±0.3), NH3 (3.4±0.3), H2O(2.1±1.0), N2O(1.4±0.1), and Ne (<0.0013) are reported at 298 K. The results obtained are compared with previous relative and absolute measurements reported in the literature.
The mutual diffusion study in the systems containing benzene or benzene-d6 at 25 °
Authors: Albright John G. , Aoyagi Keichi
Published: February 2003

The mutual diffusion coefficients of benzene and benzene-d6 were measured at low solute concentrations in each of the following solvents: n-hexane, cyclohexane, dodecane, and hexadecane. The observed mutual diffusion coefficients for the systems containing benzene-d6 were consistently lower than those observed for the corresponding systems containing benzene. The results of this investigation show that a measurable isotope effect exists in liquid diffusion and that the magnitude of the effect depends on the physiochemical properties of the solvents. The magnitude appears to be more closely related to the viscosity of the solvents than to the molecular weights of the solvents.
Violations of Kleinman symmetry in nonlinear optics: The forbidden coefficient of α-quartz
Authors: Crane G. R. , Bergman J. G.
Published: February 2003

The magnitude, sign, and dispersion of the Kleinman forbidden nonlinear optical coefficient d14 for α-quartz has been measured via second harmonic generation. The results, relative to d11 (α-SiO2), are +0.009 and +0.026 for λ0=1.064 μ and 0.532 μ, respectively. The dispersion of the ratio d14/d11 qualitatively agrees with a recent theoretical prediction that d14/d11∝ω2. A microscopic model in terms of bonds and electron lone pairs of symmetry C∞ shows the Si→O bond polarizability β14 to be ten times smaller than the corresponding polarizability found for I→O and Te→O bonds.
Stochastic theory of the kinetics of phase transitions
Authors: Metiu Horia , Kitahara Kazuo , Ross John
Published: February 2003

We present a stochastic theory of the kinetics of phase transitions in univariant, nonuniform systems. We assume a master equation and a relation of the transition probability to the free energy [J. S. Langer, Ann. Phys. (N.Y) 65, 53 (1971)]. The free energy is taken to be of the Cahn–Hilliard form. By means of path integral methods we obtain a formal solution from which we derive a deterministic differential equation for the most probable variation of the density distribution in time. This equation is of the Landau–Ginzburg type. We show the existence of a Liapunoff function for this kinetic equation, which is then used to derive simply the classical result of nucleation theory for the critical radius of a droplet. Next we show from the kinetic equation that if the structure of the free energy is such that a phase transition occurs and metastable states are possible, then the kinetics of the decay to the stable and metastable states is nonlinear, of the cubic type. Finally we derive from the kinetic equation solutions for solitary waves which describe the motion of the interface in evaporation and condensation.
Electron energy loss measurements on pyrene (C16H10) vapor
Authors: Venghaus H. , Hinz H.-J.
Published: February 2003

Electron energy loss spectra from 0 to 25 eV were measured with 50 keV electrons on pyrene vapor. The results indicate the existence of five strong excitations between 3.9 and 7.0 eV, identified as π–π* excitations. Four of them exhibit vibrational sublevels with 0.17 eV energy separation. Less pronounced maxima of the energy loss function are observed between 8 and 11 eV, which cannot be interpreted conclusively. Broader structures between 11 and 18 eV are attributed to excitations involving σ states. The present results are related to absorption measurements on pyrene in solution and to investigations of pyrene single crystals. Values for the solution and crystal shift of the different π–π* excitations are obtained (∼−0.2 eV).
Effects of partial cross sections on the energy distribution of slow secondary electrons
Authors: Tuckwell Henry C. , Kim Yong-Ki
Published: February 2003

The shapes of the energy distribution of secondary electrons can be predicted for fast incident charged particles by using dipole oscillator strengths in conjunction with the Born formula. Target atoms or molecules may have one or more ionization potentials close to the first one and the corresponding partial photoionization cross sections are sometimes large. Allowance must then be made for the differences in kinetic energies of photoelectrons which leave alternative ionic states. This produces a significant change in the expected secondary electron spectrum at low energies. The photoionization data for NO are analyzed and the predicted shape of the secondary electron energy distribution agrees well with that of the experimental distribution at those secondary electron energies where a comparison is possible. Predicted low energy results from a preliminary analysis for N2 are also reported.
Recombination of iodine atoms by flash photolysis over a wide temperature range. VII. Recombination between 206 and 300 °K
Authors: Chang H. W. , Burns George
Published: February 2003

Recombination of iodine atoms was studied by flash photolysis of HI at 206 and 300 °K. The photolysis produced both H and I atoms. However, under present experimental conditions, the reaction H+HI→H2+I was completed within the duration of the flash, thus eliminating reactive hydrogen atoms and yielding additional quantities of iodine atoms. Condensation of solid I2 on cold walls of the reaction vessel was found to be much slower than iodine recombination reaction. For these reasons I atom recombination could be studied without interferance of side reactions. At 206 °K the recombination rate constants in He, Ar, and Xe were found to be (in 10−9l2 mole−2⋅sec−1) 2.8, 7.9, and 15.8, respectively, which agree well with the rate constants obtained earlier between 300 and 1200 °K [J. K. K. Ip and G. Burns, J. Chem. Phys. 56, 3155 (1972)]. Recombination rate constants in H2 and D2, 7.8×109 and 7.7×109 l2 mole−2⋅sec−1, respectively, agree reasonably well with earlier experimental results obtained at elevated temperatures [H. W. Chang and G. Burns, J. Chem. Phys. 59, 1043 (1973)]. It was found that the recombination rate constants of iodine atoms in HI can be expressed by the following equation, valid between 206 and 300 °K: logkHIr= 10.47−3.5log(T/300). This last reaction appears to occur via the radical–molecule complex mechanism. The intermediate radical in this reaction may be a relatively stable hydrogen bi-iodide, in which case the activation energy of the reaction IHI+I→IH+I2 should not be negligible. Alternatively, the intermediate could be an unstable I–IH radical, in which iodine atom may interact with both atoms of HI. However, in this case weak chemical forces involving interaction between two halogen atoms would predominate. At 487 nm iodine absorption coefficients ratio ε (206 °K)/ε (300 °K) is equal to 1.15.
Helium diffraction from tungsten (112)
Authors: Stoll A. George , Ehrhardt Jean-Jacques , Merrill Robert P.
Published: February 2003

The diffractive scattering of helium from W(112) in the [11̄0] azimuth is presented for incidence angles from 20° to 70°, surface temperatures from 1000 to 1600 K, and beam temperatures from 300 to 650 K. The first order beams are the only nonspecular features observed and their angular positions and relative intensities are reproduced well by semiclassical scattering calculations. Debye–Waller analysis of thermal attenuation gives a surface Debye temperature lower than the bulk, but larger than that derived from electron scattering.
Interactions in inorganic molecular crystals. Electronic spectra of ReF6 pure and mixed crystals
Authors: Bernstein E. R. , Meredith G. R.
Published: February 2003

Electronic absorption spectra of ReF6(5d1) are obtained for the pure crystal and UF6, MoF6, and WF6 mixed crystals. The observed transition arises from an intraconfigurational t2g→t2g promotion characterized Γ7g←Γ8g in 0h symmetry. The pure crystal is predicted to undergo a magnetic phase transition below ∼1.5 K. Pair spectra, two-molecule transitions, the ground state Jahn–Teller effect, linewidths, and vibrational assignments are discussed. The major mechanism for pair coupling and magnetic ordering is identified as superexchange through low lying delocalized charge transfer bands. These interactions are qualitatively discussed and energy localizing and delocalizing interactions are separated.
Surface defects and structural transitions in very low anchoring energy nematic thin films
Authors: Ryschenkow G. , Kleman M.
Published: February 2003

It has been discovered that the products of degradation of heated paper evaporated on glass constitute a low anchoring energy substrate for the nematic phase of MBBA (methoxy benzylidene butylaniline). The extrapolation length b=K/2Ws is of the order of 100 μm. In very thin specimens the anchoring is conical at low temperatures and becomes homeotropic at higher temperatures. A transition to (ordinary) strong anchoring conditions appears at still higher temperatures, presumably due to the appearance of a slight isotropic layer of MBBA at the interface. We have been interested in the study of defects typical of the weak anchoring phases, in specimens thinner than b. One observes the existence of Bloch walls. This typical defect allows for a direct measurement of the surface anchoring energy (Ws ∼ 10−4 erg/m2). Other defects are nuclei of strength ±1 (in repulsive or attractive interaction with the walls according to whether they are +1 or −1 nuclei), and reversing points which separate Bloch wall segments of opposite torsions. We demonstrate that Bloch walls constitute, in thin specimens (h<b) a favored configuration with respect to the more usual splitting in surface lines of these walls, and explain qualitatively the interactions between walls and nuclei. The stability of the different structural phases (conical and homeotropic) is related to the fact that at h<b the system is quasi-two-dimensional.
A semiclassical model for atomic scattering from solid surfaces—He and Ne scattering from W(112)
Authors: Masel R. I. , Merrill R. P. , Miller W. H.
Published: February 2003

A semiclassical formalism is presented which predicts specular atomic scattering from close-packed surfaces and either ’’rainbow’’ or diffractive scattering from strongly periodic surfaces. The intensities are a series of δ functions at the Laue conditions modulated by the product of the classical intensities and a structure factor accounting for interference among multiple trajectories within a unit cell. Narrow energy distributions in the incident beam and/or long wavelengths result in well-resolved diffracted beams, while broad distributions and/or small wavelengths result in coalesced beams which display only the ’’rainbowlike’’ intensity envelope. A simple elastic model using a 6–9 potential has been used to predict He and Ne scattering from W(112). For helium the peak positions and relative peak heights agree with the measured scattering essentially within experimental reproducibility, but the peak widths calculated with this elastic model are too narrow and the intensity maxima too large. Neon calculations reproduce qualitatively the rainbow patterns observed in the experiments but neither the intensities nor the positions of the rainbow features are correct, indicating the existence of large inelastic effects.
Rate constants for the reactions of OH with CH4 and fluorine, chlorine, and bromine substituted methanes at 296 K
Authors: Howard Carleton J. , Evenson K. M.
Published: February 2003

The absolute rate constants for the reactions of OH radicals with CH4 and fifteen fluorine, chlorine, and bromine substituted methane molecules have been measured using a discharge flow system and laser magnetic resonance detection of OH. Measurements were made at 296 K and at pressures between 107 and 1300 Pa. The results indicate that the reaction mechanism involves the abstraction of a hydrogen atom and formation of H2O and a methyl type radical product. Completely halogenated methane molecules are found to be relatively unreactive. Hydrogen containing molecules react with rate constants ranging from about 0.2 to 160×10−15 cm3/molecule⋅sec. The reactivity increases with decreasing carbon–hydrogen bond energies. Rough estimates are made of the Arrhenius parameters for the reactions.
Differential cross sections for elastic electron scattering. II. Charge cloud polarization in N2
Authors: Herrmann D. , Jost K. , Kessler J. , Fink M.
Published: February 2003

Relative elastic differential cross sections for electron scattering from N2 molecules have been measured with a crossed beam technique in order to investigate the influence of charge cloud polarization. The angular range studied was 3 to 135 deg and the electron energies were betwee 90 and 1000 eV. Comparison with theoretical calculations based on the independent atom model using partial wave atomic scattering factors shows strong deviations at small scattering angles. This discrepancy is discussed from the point of view of charge cloud polarization. It is found that calculations including an additional polarization potential improve the theoretical description of the experimental small angle data.
The velocity of sound and heat capacity in methane from near-critical to subcritical conditions and equation-of-state implications
Authors: Gammon B. E. , Douslin D. R.
Published: February 2003

Values of the velocity of sound and derived values of the constant volume heat capacity in methane were determined as functions of temperature and pressure along the phase boundaries, at 298.15 K, and at densities from 0–11.6 mol dm−3 for temperatures from 113.5–193.05 K. Relative values of the density were also determined. Velocity dispersion arising from vibrational relaxation, relaxation near the critical point, and relaxation near the triple point were characterized. The results were tested by use of extended thermodynamic scaling representations for the chemical potential μ. Within the limits of possible computational errors, the results indicate that along the critical isochore, (∂2μ/∂T2)p is finite, nonzero, and has a jump discontinuity at the critical temperature which implies that ε=2, (β+α) = (1/2), and α? (1/7), where α, β, and ε are scaling exponents.
Relative electronic transition moments for the e 3Σ−–a 3Π Herman System of CO
Authors: Slanger T. G. , Black G.
Published: February 2003

Excitation of the v′=4 level of the e 3Σ− state of CO by radiation from a sulfur lamp has made it possible to observe the v″=0–3 members of the v′=4 progression of the e 3Σ−–a 3Π transition. Relative electronic transition moments have been deduced from the observed intensities, and show only a small deviation from a constant value.
Correlations for interacting Brownian particles
Authors: Ackerson Bruce J.
Published: February 2003

The generalized Smoluchowski equation is employed to calculate the first two cumulants of decay of the intermediate scattering function for interacting colloidal particles. A model calculation is presented to investigate the behavior of the general form of the cumulants. The hydrodynamic interaction is examined and possible modifications of these results are discussed.
Theory of Fourier transform ion cyclotron resonance mass spectroscopy. I. Fundamental equations and low-pressure line shape
Authors: Comisarow Melvin B. , Marshall Alan G.
Published: February 2003

The fundamental relationships for linewidth and resolution for all forms of ion cyclotron resonance spectroscopy are derived. Mass resolution and frequency resolution are shown to be numerically identical in all forms of ion cyclotron resonance spectroscopy. Theoretical Fourier transform ion cyclotron resonance (FT–ICR) spectral line shape has been calculated for the low-pressure limit in which there are essentially no ion–molecule collisions during the observation period. Absorption, dispersion, and magnitude (absolute-value) line shapes are illustrated and discussed. FT–ICR linewidth and resolution are calculated as a function of ionic mass and charge, applied magnetic field strength, and data acquisition time, for various linewidth criteria, and the results are tabulated. FT–ICR linewidth and resolution are then expressed in terms of ionic mass, computer data storage size, and minimum specified ionic mass in the FT–ICR mass range, for various linewidth criteria, and the results are tabulated. Next, FT–ICR upper mass limit is calculated as a function either of ionic charge, applied magnetic field strength, and data acquisition time, or computer data storage size and minimum specified ionic mass in the FT–ICR mass range, and the results are tabulated for several linewidth criteria. For the same observation time and linewidth criterion, FT–ICR zero-pressure resolution is 47% better than with conventional ’’drift’’ cell or ’’trapped-ion’’ cell ICR detection. Finally, the theoretical basis for the FT–ICR spectral segment extraction technique is described, for use in enhancing FT–ICR resolution by several orders of magnitude.