bluuebunny/crossref_metadata_2025 huggingface dataset:
Crossed beam studies of the hydrogen exchange reaction: The reaction of H and D atoms with T2 molecules
Authors: Kwei George H. , Lo Vincent W. S.
Published: February 2003

The dynamics of the hydrogen exchange reaction has been studied in crossed molecular beam experiments using a tritium atom detection scheme. The selectivity and the cumulative feature of T-atom collection provide adequate signal levels and the mass changes introduced by the labeling provide favorable kinematics which allow both a single-valued laboratory↔c.m. transformation and a more efficient laboratory→c.m. conversion of the collision energy. Measured angular distributions of reactively scattered T atoms for the H+T2 and D+T2 systems show that: (1) the product molecules are scattered backwards with respect to the incoming atomic beam implying that these reactions proceed via a direct rebound mechanism, (2) the laboratory angular distributions are quite broad [ranging from ∼70 ° to ∼125 ° FWHM for D(2120 K)+T2 and H(2600 K)+T2, respectively], but agree reasonably well with theoretical predictions if the latter are properly averaged over collision energies; the observed increase in widths of the angular distributions with increasing collision energy appears to be a general property of impulsive collisions involving repulsive barriers, (3) no information is available for the recoil velocity distributions, but the condition E=E′ used in the c.m.→laboratory transformations produce laboratory distributions that provide equally good fits over both branches of the distribution, and (4) under identical conditions, the product T-atom intensity for the H+T2 system is higher than that for the D+T2 system, this difference arises primarily from the different collision energy ranges sampled in the experiments; with a higher temperature H-atom beam, the signal increases substantially, but a quantitative measure of the increase in cross section is precluded by the uncertainty in the H-atom flux. These results are compared with a previous crossed beam study of the D+H2 system and with predictions from various theoretical studies.
Approximate exchange energy as a functional of the electron density. Light atoms
Authors: Berrondo M. , Flores-Riveros A.
Published: February 2003

We derive an expression for the exchange energy integral in terms of the density for the case of light atoms containing s and p electrons. We start from an approximate functional form of the density matrix for an electron cloud in the presence of an attractive nucleus. An important restriction to consider is the Pauli principle. A correction factor is included to account for the multipole expansion in an average way. The results we obtain, both for closed and open shells, are within a few percent of the exact exchange integral.
Influence of the solvent on the conformation of a chain molecule
Authors: Sablon M. , Bellemans A. , Orban J.
Published: February 2003

Molecular dynamics results are reported for a simple two-dimensional model; a flexible trimer consisting of three hard disks and 61 hard disks of the same type enclosed in a square box. The results of these simulation indicate that solvent plays on important role in determining chain conformations.(AIP)
Near infrared photoelectrochemical responses of copper phthalocyanine thin film electrodes
Authors: Minami Nobutsugu
Published: February 2003

The near infrared photocurrent of copper phthalocyanine CuPc thin film electrodes in contact with electrolyte solutions is reported. The significance of the NIR photocurrent in the field of dye photo chemistry is discussed. (AIP)
Dissociative excitation of HgBr2 in collisions with a beam of metastable nitrogen
Authors: Fahey D. W. , Schearer L. D.
Published: February 2003

The observations of HgBr (B−X) fluorescence resulting from collisions of HgBr2 with a vibrationally excited beam of N2 molecules in A 3Σu+ electronic state is reported. (AIP)
Photon correlation spectroscopy of polydisperse systems
Authors: Briggs J. , Nicoli D. F.
Published: February 2003

Using the method of cumulants, we investigate the distribution of diffusion coefficients in polydisperse systems. We consider the intensity weighting for distributions of spherical particles having the same polarizability. Results of computer simulations are presented for a variety of polydisperse situations. Also, comparisons are made between predicted and measure results for mixtures of Dow latex spheres.
Ferroelectric and antiferroelectric dynamics of pseudo-one-dimensional CsD2PO4
Authors: Blinc R. , Barreto F. C. Sa
Published: February 2003

A simple model is proposed for the pseudo-one-dimensional (1d) ferroelectric and antiferroelectric phase transitions in CsD2PO4 which is based on the behavior of spin 1/2 Ising chains in a longitudinal molecular field. The observed phase diagram can be reproduced if one assumes that the coupling constant beween ’’unlike’’ chains changes its sign when the hydrostatic pressure exceeds a critical value. In addition to the usual ’’critical’’ slowing down of the order parameter fluctuations an ’’anisotropy’’ slowing down is predicted which may explain the fact that the soft modes in 1d CsH2PO4 and PbHPO4 are much slower than in 3d KH2PO4 and KD2PO4.
Dipolar relaxation and nuclear Overhauser effects in nonrigid molecules: The effect of fluctuating internuclear distances
Authors: Tropp James
Published: February 2003

A simple treatment of the dipolar contribution to nuclear magnetic relaxation is developed for molecules in which conformational fluctuations modulate the relevant internuclear distances. Expressions are given for AX system spectral densities using a general model in which conformational fluctuations occur as random jumps among discrete conformations, while the molecule as a whole undergoes rotational diffusion as either a spherical or a symmetric top. Approximations valid for proton spin systems are given for cases in which the jumping rates are either fast or slow compared to rotational diffusion; the results are independent of the jumping rates. Similar results are obtained for cross-correlation spectral densities. For complex spin systems, e.g., An1Mn2Xn3..., the cross-relaxation constants σij which couple pairs of magnetizations, depend only upon autocorrelation, spectral densities and are thus easily obtained from the AX system results. Measurement of σij by time resolved Overhauser effect experiments is discussed with special attention to the problems raised by dipolar cross-correlation and spin diffusion. Numerical calculations of σij are given for several geometries describing dipolar relaxation of a rotating methyl proton, and a fixed nonmethyl proton in a slowly tumbling macromolecule.
Generalized perturbed Hartree–Fock theory
Authors: Rossky Peter J. , Karplus Martin
Published: February 2003

The conventional coupled perturbed Hartree–Fock theory is generalized to allow for an arbitrary independent-particle zero-order Hamiltonian and arbitrary perturbed Hartree–Fock Hamiltonian. The perturbation series for the resulting (unrestricted) Hartree–Fock wave function, density matrices, and energy are developed in diagrammatic form. A linked cluster expansion for the Hartree–Fock wave function is derived. It is shown that any expectation value for the perturbed Hartree–Fock state is represented by a precise subset of the diagrams contributing to the perturbation result for the exact perturbed electronic Hamiltonian but that this is not true for the wave funtion. Explicit results are given for the wave function through third order and the energy through fourth order. Further, general topological rules are presented which define the Hartree–Fock diagrams through all orders, and allow generalization of the formalism to particular applications. By comparison of the present results with those for the exact perturbation theory, it is shown that the subset of topologies which enter the Hartree–Fock perturbation expansion always correspond to contributions in which the electron–electron potential acts between uncorrelated one-electron terms, a result consistent with the usual interpretation of the Hartree–Fock approximation.
Hexadecapolar U-branch transitions in the infrared fundamental band of gaseous hydrogen at 77 K
Authors: Reddy S. Paddi , Sen A. , Prasad R. D. G.
Published: February 2003

U-branch transitions corresponding to the rotational selection rule ΔJ=+4 in the collision-induced fundamental band of normal H2 at 77 K have been investigated for gas densities in the range 300–520 amagat with a 2 m absorption cell. These transitions, arising from the hexadecapolar induction mechanism in the colliding pairs of molecules, occur in the high wave number tail of the overlap and quadrupolar transitions of the band and include primarily single transitions U1(J) and double transitions Q1(J)+U0(J), J=0 and 1, at 77 K. The experimental absorption profiles were analyzed by assuming a new expression for the contribution of the total wing of the overlap and quadrupolar transitions and a dispersion type function as well as a Levine–Birnbaum function for the U transitions, and the characteristic half-width parameters δu of the dispersion function and δ′u of the Levine–Birnbaum function have been obtained. The experimental value of the binary absorption coefficient of the U1(1) transition, derived from the profile analysis, is 2.40×10−10 cm−1 amagat−2, which is in good agreement with the theoretical value.
Analytic estimation of almost resonant molecular energy transfer due to multipolar potentials. V
V processes involving CO2
Authors: Pack Russell T.
Published: February 2003

A quantum mechanical approach to almost resonant energy transfer by multipolar potentials in molecular collisions is presented. A distorted wave formalism is used, so that most of the limitations of earlier work are retained, but a natural cutoff for the Born approximation is derived which is smooth and physically reasonable. Analytic expressions for the differential and integral cross sections are obtained without a partial wave expansion. Several almost resonant VV energy transfer processes are used as examples, and predictions are given for processes involving CO2(00°2).
Chemi-ionization in the metastable neon–metastable argon system
Authors: Neynaber R. N. , Tang S. Y.
Published: February 2003

Studies were made by a merging-beams technique of the associative ionization (AI) reaction (1) Ne*+Ar*→NeAr++e and the Penning ionization (PI) reactions (2) Ne*+Ar*→Ne+Ar++e and (3) Ne*+Ar*→Ne++Ar+e. The relative kinetic energy of the reactants was varied from 0.01 to 10 eV. The Ne* and Ar* each represents a composite of the metastable 0P2,0 states. There is a complication in the present investigation which arises because AI and PI occur in collisions of Ne* with ground-state Ar. Since the reactant beams consist of metastable as well as ground-state species, the measurements are composites of chemi-ionization in both the Ne*–Ar* and Ne*–Ar systems. Information on AI and PI for the Ne*–Ar* system is obtained by subtracting from these composite measurements known contributions of the Ne*–Ar system. From such information it appears that the molecular states of the reactants are different for reactions (2) and (3).
The reaction X+Cl2→XCl+Cl (X=Mu,H,D). I. A new inversion procedure for obtaining energy surfaces from experimental detailed and total rate coefficient data
Authors: Connor J. N. L. , Jakubetz W. , Manz J. , Whitehead J. C.
Published: February 2003

A new inversion method has been developed which uses detailed vibrotational and total rate coefficient data in order to obtain the potential energy surface for a chemical reaction. The method is applied to the reaction X+Cl2→XCl+Cl (X=Mu,H,D). The philosophy of the method is to separate the dynamical effects due to the collinear and the noncollinear parts of the potential surface, which are then treated independently, and to reduce a large amount of experimental data to a few informative quantities. These are then related to a small number of potential surface parameters. This compaction of data is carried out in an iterative scheme starting from a potential surface assumed to be sufficiently similar to the correct one. In the present case, the collinear part of the potential surface is constrained to be of the extended LEPS variety with correct asymptotic properties and two adjustable Sato parameters. Information theoretic techniques are used to obtain the fraction of reactive reagents and then the vibrotational product distribution for ground state reagents P(J′,v′ ‖ v=0) in a thermal reactant distribution. Next, these three dimensional P(J′,v′ ‖ v=0) are projected onto the corresponding collinear vibrational distribution PC(v′ ‖ v=0). This distribution is then further reduced to its most informative moment 〈fv′〉C to 𝒜⊥, the attractivity of the potential surface. An estimate of the barrier height Eb of X+Cl2 is made from the isotopic ratios of thermal rate coefficients, which are assumed to be dominated by collinear potential surface properties. We thus compact the original experimental data into two parameters 𝒜⊥ and Eb which determine the Sato parameters characterizing the collinear part of the potential surface. With Eb=1.5 kcal mol−1, the collinear part of the extended LEPS surface which best reproduces 〈fv′〉C for the H+Cl2 and D+Cl2 reactions has Sato parameters of S(XCl)=0.067 and S(Cl2)=−0.113. We have not explicitly derived the noncollinear part of the potential surface due to the present unavailability of simple parametrized models for the angular behavior.
Pyroelectric Ba(NO2)2⋅H2O: Room temperature crystal structure
Authors: Abrahams S. C. , Bernstein J. L. , Liminga R.
Published: February 2003

Barium nitrite monohydrate, Ba(NO2)2⋅H2O, is strongly pyroelectric at room temperature and crystallizes in the hexagonal system with space group P65, or the enantiomorphous P61, and with six formulas in the unit cell. The lattice constants at 298 K are a=7.074 90±0.000 03 and c=17.890 87±0.000 12 Å (λCuKα1=1.540598 Å). The integrated intensities of most reflections within a quadrant of reciprocal space having radius (sinϑ)/λ?1.15 or 1.08 Å−1 were measured on two different crystals, each with different diffractometer, resulting in 3218 Fmeas for one set and 3671 Fmeas for the second set of independent structure factors. The crystal structure was solved from Patterson and Fourier series, and refined by the method of least squares. The final agreement factor R=0.026 for the first set, 0.018 for the second set of Fmeas. The two sets of Fmeas and the derived structural parameters are compared using normal probability plot analysis. The two NO−2 ions have a mean N–O distance of 1.246±0.002 Å and O–N–O angle of 114.2±0.2°. The Ba2+ ion is coordinated by nine oxygen and one nitrogen atoms at an average distance of 2.899 Å. The water molecule has a weighted mean O–H distance of 1.04±0.03 Å and H–O–H angle of 102.7°±4.3°: it is hydrogen bonded to two independent NO−2 ions by an O–H⋅⋅⋅O bond of 2.878 Å and an O–H⋅⋅⋅N bond of 2.951 Å.
Rotational tunneling in methane (CD4): Isotope effect
Authors: Press W. , Prager M. , Heidemann A.
Published: February 2003

Well-defined tunneling lines in the μeV range have been observed in CD4 at T=4 K by high resolution neutron scattering. The observed change of tunneling energies by about a factor 50 upon deuteration is in good accord with a theoretical estimate. The symmetry of the neutron scattering operator implies a selection rule.
Primitive model electrolytes. II. The symmetrical electrolyte
Authors: Valleau John P. , Cohen L. Kenneth , Card Damon N.
Published: February 2003

Canonical Monte Carlo results are reported for the 2:2 aqueous electrolyte in the primitive model. The various theories of dilute coulombic systems are compared with canonical and grand canonical Monte Carlo data on the thermodynamics and structure of this system: HNC, BGY, MPB, and ORPA+B2 are comparably successful, most other theories less so. The MC results show that there are many triple ions or larger clusters even at low concentrations. The problem of extrapolating experimental thermodynamic data for 2:2 electrolytes is discussed.
Numerical study of intramolecular vibrational energy transfer: Quantal, classical, and statistical behavior
Authors: Kay Kenneth G.
Published: February 2003

The dynamics of a system of two coupled, one-dimensional, nonlinear oscillators is studied both quantum mechanically and classically. The quantal and classical dynamics substantially agree in most cases including some in which the maximal Lyapunov number indicates the classical behavior is stochastic. An exception occurs when one oscillator is prepared in a highly excited state. In such a case, the classical behavior is stochastic and significantly more ergodic than the quantal behavior. The source of the reduced quantal ergodicity is identified as failure of the classical approximation for narrow nonlinear resonances. Model calculations suggest that layers of reduced ergodicity similar to the one in the present system may be prevalent in real molecules. Quantal and classical statistical calculations are performed for comparison with the dynamical results. The quantal statistical calculations are based on a definition of quantal mixing which is discussed in some detail. The computed quantal statistics agree well with the corresponding classical statistics. However, the statistics do not agree very well with the dynamics for either the quantal or the classical case. The implications of these results for classical treatments of unimolecular dynamics, statistical intramolecular behavior, and the RRKM theory are discussed.
Statistical mechanics of helical wormlike chains. XI. Anisotropic light scattering near the rod limit
Authors: Fujii Motoharu , Yamakawa Hiromi
Published: February 2003

Analytical expressions for anisotropic components of the scattered intensity for the helical wormlike chain, including the ordinary wormlike chain as a special case, near the rod limit are derived by an application of the so-called ε method of Part VI to the general equations of Part IX. Evaluation is carried out of the polarized (Vv), depolarized (Hv), and horizontal–horizontal (Hh) components. Because of their complexity, the derived equations cannot be used for the determination of the mean-square radius of gyration. However, the numerical analysis shows that this quantity for very stiff polymers (not rigid rods) may be determined by analyzing experimental data for the Vv component by the use of the simple equation for rods.
Vibrational level populations in the autoionization of oxygen
Authors: Eland J. H. D.
Published: February 2003

Autoionization from the I and I′ resonance series of O2 has been reexamined using high resolution for photons and electrons. Photoelectron spectra on the resonances are predicted from the upper potential energy surface parameters and photoionization yield curve alone, without reference to the uncertain line shapes and widths. Bond lengths are determined as 1.370±.005 Å in I and 1.380±.005 Å in I′ of O2.
The dual melting curves and metastability of carbon tetrachloride
Authors: Bean Vern E. , Wood Sharrill D.
Published: February 2003

Carbon tetrachloride has three known solid phases at atmospheric pressure: Ia (face-centered cubic), Ib (rhombohedral), and II (monoclinic). Both Ia and Ib melt directly at temperatures some 5 K apart. These phase changes have been traced as a function of hydrostatic pressure up to 350 MPa. Between atmospheric pressure and 100 MPa, CCl4 has dual melting curves; one for Ia, and a few degrees higher, one for Ib. The two curves diverge with increasing pressure. Above 100 MPa it was not possible to detect the Ia phase. There appears to be no Ia–Ib-liquid triple point. The metastability associated with these phases is discussed.
The absolute infrared intensities of propyne-d and propyne-d3
Authors: Bode J. H. G. , Smit W. M. A. , Visser T. , Verkruijsse H. D.
Published: February 2003

The integrated infrared intensities of the fundamental modes of propyne-d0 and propyne-d3 have been measured by the Wilson– Wells–Penner–Weber method, using nitrogen as broadening gas at a pressure of 60 atm. From the measured intensities eight sets of ∂μ/∂S values were obtained using an iterative least-squares fitting procedure. The eight sets were left after fixing the signs of six ∂μ/∂Si parameters using CNDO and ab initio results and known values for related molecules. Four of the remaining sets were discarded after prediction of the propyne-d1 intensities. Using additional arguments from Coriolis interactions, predictive abilities and further comparison with related molecules one of the remaining four sets could be selected as the more preferable one. The final set ∂μ/∂Si values and the resulting calculated intensities are discussed and compared with the results of Kondo and Koga. The present sign combination is in full agreement with the ones selected by Kondo and Koga. Also the observed intensities, the ∂μ/∂Si values and the calculated intensities are in close accordance.
Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect
Authors: Litfin G. , Pollock C. R. , Curl R. F. , Tittel F. K.
Published: February 2003

The use of magnetic rotation spectroscopy (Faraday effect) to reduce the effects of source noise and improve sensitivity of spectroscopy with color center lasers has been analyzed theoretically and tested experimentally on the vibrational overtone band of NO. Sensitivity improvement of a factor of 30 compared with simple Zeeman modulation has been demonstrated. As an example of this technique, the first observation of the fundamental vibrational band of the OH radical in absorption is reported.
Laser induced flame chemistry of Li (2 2P1/2,3/2) and Na (3 2P1/2,3/2). Implications for other saturated mode measurements
Authors: Muller C. H. , Schofield K. , Steinberg M.
Published: February 2003

Saturated laser fluorescence measurements of sodium or lithium in a series of fuel rich, atmospheric pressure H2/O2/N2 flames at 1700–2200 K indicate induced chemical interactions between the excited 2P1/2,3/2 states of the metals and the H2O or H2 flame constituents. A steady state redistribution occurs among the metal’s elemental, hydroxide and hydride forms within the initial fraction of the μs laser pulse duration. A saturated absorption model incorporating these chemical effects illustrates the significant depletion of the free atom concentrations under these conditions and explains previous discrepancies between such measurements and conventional absorption experiments. Estimates of the rates of the reactions between the 2P1/2,3/2 states of sodium or lithium with H2O or H2 indicate that they proceed predominantly via the nonadiabatic physical relaxation channel. For sodium the two chemical channels are relatively inefficient constituting only about 2% and 0.5% of the total interaction cross section with H2O and H2, respectively. Even so, they are still sufficient to drain off a reasonable fraction of the free atoms into these molecular metastable sinks. No evidence of contributions from laser enhanced ionization was noted. Applications of the saturated laser fluorescence technique as a general combustion species monitor must necessarily be restricted to the use of nanosecond pulse length lasers in order to circumvent these potential chemical relaxations.
Alkali halide photofragment spectra. III. Alkali chloride bond energies and excited state symmetries at 266 nm
Authors: Su Tzu-Min R. , Riley Stephen J.
Published: February 2003

Photofragment spectroscopy studies of the four heavier alkali chlorides at 266 nm are reported. Alkali atom recoil time-of-flight distributions provide indirect evidence for production of both ground 2P3/2 and excited 2P1/2 Cl atoms, but quantitative branching ratios are only approximate. The following bond dissociation energies (D00, kcal mole−1, all uncertainties ±2 kcal mole−1) are determined: NaCl=97.6; KCl=102.6; RbCl=101.3; CsCl=104.4. From recoil angular distributions the percentage parallel transition intensities to the unresolved excited states are calculated to be: NaCl=16±2; KCl=26±5; RbCl=32±1; CsCl=25±6.
Electrical conduction in ammonium and alkali metal perchlorates
Authors: Rajeshwar K. , Nottenburg R. , Verneker V. R. Pai , DuBow J.
Published: February 2003

Experimental results on the temperature and frequency dependence of the ac electrical conductivity of rubidium perchlorate (RbP) and cesium perchlorate (CsP) are presented. Studies were carried out in the temperature range from ambient to ∼300 °C and at frequencies ranging from 50 Hz to 1 kHz. Both nominally pure compounds and those doped with CrO42−, SO42−, and Mn2+ were studied. The electrical conduction behavior of these materials is compared with that previously observed in this laboratory for potassium perchlorate (KP) with a view to highlight some common features. Results previously reported in the literature on an analogous material—ammonium perchlorate (AP)—are critically examined in light of the present data. A unified defect-structure model is finally presented. This model emphasizes similarities in the electrical conduction behavior of the above compounds. More importantly, the experimental results on AP seem to be more consistent with this model than with the currently accepted proton transfer mechanism.
Coexistence curve of polystyrene in methylcyclohexane. I. Range of simple scaling and critical exponents
Authors: Dobashi T. , Nakata M. , Kaneko M.
Published: February 2003

We have measured coexistence curves for the system polystyrene–methylcyclohexane with varying molecular weight from Mw=1.02×104 to 71.9×104. In the temperature range ε=(Tc−T)/Tc≳0.03, a deviation from simple scaling was observed for systems with Mw=10.9∼71.9×104. The effect of correction terms on simple scaling was negligibly small for systems with Mw=1.02∼4.64×104. This finding is compatible with molecular weight dependence of a critical value of ε for the validity of the Landau theory. In the appropriate temperature range, where a contribution from the correction terms is negligible, an analysis by simple scaling gives the exponent β=0.332±0.001 independent of molecular weight. The critical exponent for the diameter is also independent of molecular weight and determined as 0.858±0.005, which is consistent with results of recent specific heat measurements.
Coexistence curve of polystyrene in methylcyclohexane. II. Comparison of coexistence curves observed and calculated from classical free energy
Authors: Dobashi T. , Nakata M. , Kaneko M.
Published: February 2003

Comparison of coexistence curves observed and calculated from the classical free energy was made for the systems polystyrene–methylcyclohexane and polystyrene–cyclohexane. Classical free energy was determined from the molecular weight dependence of the critical point. At low molecular weight, the numerically calculated coexistence curve was well represented by the leading term with the exponent β=1/2. The shape of the calculated coexistence curve is very different from the observed one because of the comparable values of the coefficient and different values of the exponent β observed and calculated. At high molecular weight, the entire shape of the observed coexistence curve is fairly well described by the calculated one. This fair description was caused by the large value of the calculated coefficient compared with the observed one and the narrow range of simple scaling. The numerically calculated coexistence curve was found to deviate from the leading term with β=1/2 at high molecular weight. Both in diameter and concentration difference, a transition from the critical region to the classical region was observed for the system polystyrene–cyclohexane with molecular weight Mw=156×104.
Photodissociative production of O(1S) from N2O and S(1S) from OCS in rare gas matrices
Authors: Taylor R. V. , Walker W. C. , Monahan K. M. , Rehn V.
Published: February 2003

Photoluminescence excitation spectra of Ar:N2O (1%), Ar:OCS (1%), and Kr:OCS (1%) were obtained in the region 180–104 nm, using UV synchrotron radiation at SSRL. Photodissociation energy thresholds in the matrix were found to be: 8.06 eV for N2O→N2(X 1Σ+)+0(1S) and 7.17 eV for OCS→CO (X 1Σ+)+S(1S). Our results show several effects due to interactions between the matrix and the triatomic molecule: (1) differential blue shifts of the 1Π←1Σ+ and 1Σ+←1Σ+ transitions, (2) a greatly enhanced predissociation rate from 1Π to 1Σ+, (3) matrix dependent energy shifts of a particular Rydberg state, (4) nonradiative energy transfer between the matrix and the triatomic, involving either self-trapped excitons and a Foster–Dexter mechanism or free excitons and a harpooning mechanism. To our knowledge, this is the first observation of the free exciton–harpooning mechanism in a rare gas matrix.
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o study of the vibrational dependence of hyperfine coupling constants in the methyl, silyl, and formaldehyde anion radicals
Authors: Ellinger Y. , Pauzat F. , Barone V. , Douady J. , Subra R.
Published: February 2003

The influence of vibrational effects on the isotropic hyperfine coupling constants is studied by ab initio methods for the CH3, SiH3, and H2CO− radicals. The calculations are carried out using double-zeta plus polarization basis sets of contracted Gaussian orbitals. A spin-restricted SCF plus perturbative configuration interaction method including all spin adapted configurations with three uncoupled electrons is employed. The perturbation treatment is extended to second order which yields a definitive improvement of the results over previous first order calculations. A quantum mechanical treatment of the vibrational problem shows that the influence of the nuclear motion depends on the shape of the potential. The vibrational effects are found necessary to account for the experimental findings and the overall results are in very good agreement with experiment. It is found that the usual classical approach fails to reproduce the correct behavior of the splittings, especially for low temperature experiments.
Vibrational study of phase transition of solid malononitrile
Authors: Le Calvé N. , Pasquier B. , Novak A.
Published: February 2003

Solid phases and phase transitions of crystals of malononitrile, CH2(CN)2, have been studied by Raman and infrared spectroscopy. Among twelve Raman active lattice modes of T′ phase a soft mode has been identified and assigned to a R′b libration. The T′–F–T transitions can be considered as second order and of displacive type since the soft mode frequency decreases continuously to zero at Tc when the transition temperature is approached from either above or below. Critical exponent β∼1/2 has been determined from the soft mode frequency versus Tc-T plot. Half-width of the soft mode band has also been studied as a function of temperature and relaxation times have been extracted from the corresponding curves. The stable low temperature phase L has been characterized spectroscopically: unlike T′, F, T phases it contains CH...N hydrogen bonds. The behavior of its lattice frequencies with temperature indicates that L–F phase change is a first order transition.
CN(A 2Πi→X 2Σ+) and CN(B 2Σ+→X 2Σ+) yields from HCN photodissociation
Authors: Lee L. C.
Published: February 2003

The cross sections for the production of CN(A 2Πi→X 2Σ+) and CN(B 2Σ+→X 2Σ+) emissions from HCN photodissociation are measured using synchrotron radiation in the 1050–1550 Å region. The photoabsorption cross section of HCN is also measured, and the quantum yields for the emissions are determined. The spectra for the photoabsorption and photoemission cross sections are interpreted with the known excited states of HCN. Three new excited states are found from the CN emission quantum yields. It is observed that the vibrational population of the CN(B 2Σ+) photofragments produced by photodissociating HCN at Rydberg states are more vibrationally excited than those produced at other dissociative states. The photodissociation processes for producing CN emissions at various excited states are discussed.
Lifetimes and transition moments among excited electronic states of BaO
Authors: Hsu Yen Chu , Hegemann Bruce , Pruett J. Gary
Published: February 2003

The lifetime of the BaO C 1Σ+ state is investigated for several vibrational and rotational levels at 1 Torr and under molecular beam conditions. A collision free radiative lifetime of 10.5±1 nsec is observed for the v=0 level. Dispersed fluorescence from v=0, 1, and 2 to various low lying electronic states is measured and used to obtain electronic dipole transition moments of : μ2C–X=9.9 D2, μ2C–A=16.8 D2, μ2C–A′=9.8 D2, μ2C–b=3.1 D2, μ2A–X=3.2 D2, μ2A′–X=0.14 D2.
Rotationally inelastic collisions of LiH with He. II. Theoretical treatment of the dynamics
Authors: Jendrek Eugene F. , Alexander Millard H.
Published: February 2003

Integral cross sections for the rotational excitation of LiH (j=0,1, and 2→j′) in collisions with He have been determined at collision energies of 0.2057, 0.3057, and 0.4057 eV. The rigid-rotor collision dynamics were solved within the coupled states (CS) approximation. The ab initio potential surface described in the preceding article was used. Sizeable inelastic flux occurs even for large changes in the rotational quantum number, which is probably a reflection of the extreme anisotropy in the potential surface. The dependence of the cross sections on j′, the final rotational quantum number, is not monotonic but displays a pronounced oscillatory structure which is a sensitive function of collision energy. Use of the infinite order sudden approximation results in a disappearance of this oscillatory structure as well as a pronounced lowering in the actual magnitudes of the inelastic cross sections. By contrast excellent agreement with the CS j=1,2→j′ cross sections is provided by sudden limit scaling relations with the CS j=0→j′ cross sections as input. Neither the exponential-gap nor power-law parametrizations provide an adequate fit to the CS integral cross sections. The rotationally inelastic cross sections obtained with an electron-gas potential surface are significantly smaller, because the surface is less repulsive. Thermally averaged cross sections are determined for comparison with the results of the experiment described in the following article.
Recoil 18F chemistry. XI. High pressure investigation of 1,1-difluoroethane
Authors: Manning Ronald G. , Root John W.
Published: February 2003

Nuclear recoil 18F reactions in CH3CHF2 have been investigated throughout the effective pressure range 0.3–190 atm. The principal reaction channel is F-to-HF abstraction for which the combined yield from quasithermal and energetic processes in the presence of 5 mole% H2S additive is 83.4%±0.2%. A reaction mechanism is proposed that involves the organic product forming channels F-for-F, F-for-αH, F-for-βH, F-for-ĊH3 and F-for-ĊHF2. The results are compared with those reported for the 18F+CH3CF3 system.
Convergence of a perturbation technique for evaluating isotopic partition function ratios
Authors: Harvie Charles E. , Bopp Philippe , Wolfsberg Max
Published: February 2003

Schwinger perturbation theory for partition functions is developed in a form which makes practical the numerical evaluation of higher order terms. The vibrational Hamiltonian of a molecule in the harmonic approximation, H=(1/2)∑(gijpipj+fijq iqj), has been partitioned into an unperturbed Hamiltonian (the diagonal part, terms with i≠j) and a perturbation term (the off-diagonal part, terms with i=j). The perturbation theory technique up to fourth order is used to calculate vibrational partition functions for a number of molecules. Vibrational partition functions so calculated are employed to calculate reduced partition functions and reduced isotopic partition function ratios. The results obtained by perturbation theory are compared with those of exact calculations carried out by actually obtaining the normal mode vibrational frequencies of the vibrational Hamiltonian. The previously observed good agreement, especially for the reduced isotopic partition function ratios, between perturbation theory in second order and exact calculations, becomes usually even better in fourth order perturbation theory.
Infrared photochemistry of ethylene clusters
Authors: Casassa Michael P. , Bomse David S. , Beauchamp J. L. , Janda Kenneth C.
Published: February 2003

Infrared irradiation of ethylene clusters formed in supersonic molecular beams, using a low power cw CO2 laser, results in the photodissociation of a large fraction of the van der Waals molecules. Under such conditions, infrared absorption intensity exhibits first-order power dependence and is readily detected as loss in molecular beam intensity. Intramolecular energy transfer rates, determined by measuring spectral linewidths, are shown to vary with the vibrational mode initially excited. Ethulene clsuters containing one quantum of vibrational energy corresponding to the νn7 fundamental in the monomer (949 cm−1) have a vibrationally predissociative lifetime of 0.33 psec. In comparison, the relaxation rate of ethulene-d4 clusters with one quantum of exctation corresponding to the ν12 (1078 cm−1) mode of C2D4 is 0.7 psec.
Anomalies in the fractionation by chemical equilibrium of 18O/16O relative to 17O/16O
Authors: Skaron S. , Wolfsberg Max
Published: February 2003

A theoretical investigation of relative 18O.16O and 17O/16O isotope effects is reported in order to lest the geo-cosmochemical assumption that equilibrium processes lead to R values around 2. (AIP)
A
b i
n
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t
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o calculations of the a 3Π–b 3Σ− transition of CH+
Authors: Kusunoki Isao , Sakai Shogo , Kato Shigeki , Morokuma Keiji
Published: February 2003

A b initio calculations are reported for the potential energy curves spectroscopic constants and transition moments for a 3Π and b 3Σ− of CH+. (AIP)
Representation of potential energy curves for diatomic molecules
Authors: Mattera L. , Salvo C. , Terreni S. , Tommasini F.
Published: February 2003

Power series expansion for molecular potentials is given with improved convergence properties relative to the Dunham expansion. (AIP)
Erratum: Experimental confirmation of the Jahn–Teller distortion of CH4+ [J. Chem. Phys. 7
2, 1402 (1980)]
Authors: Gemmell Donald S. , Kanter Elliot P. , Pietsch Werner J.
Published: February 2003

We report measured energy and angular distributions for H+ and Cn+ (n=2,3,4) fragments resulting from the collisional dissociation of 194 keV/amu CHn+ (n=0–4) in thin carbon foils. From the systematic trends of these ’’Coulomb explosion’’ spectra, one can obtain qualitative information on the structures of these species. In particular, the series displays a narrowing of the carbon angular and energy widths as protons are symmetrically added around a central carbon atom and provide a ’’focusing’’ effect. Because of the Jahn–Teller distortion, the widths measured for CH4+ are dramatically increased.
Purification, growth, structure, optical and electrical properties of single crystals of the π-molecular complex of phenothiazine with pyromellitic dianhydride
Authors: Anthonj R. , Karl N. , Robertson Beverly E. , Stezowski John J.
Published: February 2003

Single crystals of the black phenothiazine : pyromellitic dianhydride (PTZ:PMDA) 1:1 donor–acceptor complex, have been grown by sublimation from the zone-refined components. The PTZ:PMDA complex crystalizes with P1̄ symmetry (Z=2), a=7.197(1), b=19.072(5), c=6.886(1) Å, α=84.79(1) °, β=72.98(1) °, γ=85.72(2) ° at T=23 °C. The crystal structure model was refined with 5214 data { (sin ϑ/λ)max=0.8071 Å−1} to give R=0.049 and Rw=0.089. The crystal packing consists of two polar endless ...DADA... stacks related to one another by 1̄. The packing is compared with that in the analogous anthracene : PMDA complex. The PTZ molecule displays a modest fold about the N...S axis (dihedral angle 176.4°), however, the deviation from planarity is small in comparison with that in PTZ crystals. The stack axis is nearly perpendicular to the molecular planes, consequently the CT-dichroism lies essentially in the principal axis system of the indicatrix. The absorption edge is not very sharp, even at 4.2 K; it is located at 900±25 nm. This value is compared to the ionization energy of PTZ and to the energy of the CT transition in anthracene : PMDA. No optical emission could be detected; the pathways of radiationless deactivation are discussed in terms of CT–vibron coupling. An intrinsic dark conductivity of σ∼10−12 Ω−1 cm−1 was found. It is consistent with the value of the band gap (1.7 eV) obtained both from the measured activation energy of the dark conductivity and from the estimated thermal dissociation of CT excitons. Possible implications of the polar stacking on the excitonic energy levels and energy transport are briefly discussed.
Wave functions and effective potentials of alkalilike ions. II. Sodiumlike ions
Authors: Daniele Rosalba
Published: February 2003

The previous effective potentials approach [J. Chem. Phys. 70, 3462 (1979)] is extended to calculate energy values, approximate wave functions, and effective potentials for the valence electron of the sodiumlike ionized atoms in the ns, np, and nd states with n=3,...,8.
Correlation function modeling of phosphine via the third-order memory function
Authors: Wilde Richard E. , Chang Ta-Chau
Published: February 2003

Vibrational correlation functions obtained from the isotropic Raman scattering of the P–H stretching vibration of liquid and solid PH3 have been modeled using a memory-function approach. The modulation of the frequency is fast at all temperatures and probably arises from the rotational motions of the molecules. It is suggested that Coriolis resonance contributes to the vibrational dephasing. Information about the molecular dynamics has been obtained out to 7 ps.
Quenching of electronically excited barium atoms, Ba(5d 3D
J)
Authors: Whitkop P. G. , Wiesenfeld J. R.
Published: February 2003

The lowest optically metastable excited state of barium, Ba(5d 3DJ), was populated by laser excitation of the 6p 1P01←6s 1S0 (λ=553.5 nm) transition followed by the spontaneous emission 6p 1P01→5d 3DJ (λ∼1130 nm). No evidence for the normally spin allowed transition 6p 1P01 →5d 1D2 (λ=1500 nm) was obtained. The deactivation of Ba  (5d3DJ) by Ba  (6s 1S0) is extremely efficient proceeding with a thermal cross section of 1.3±0.1 nm2 at ?730 K. This is in sensible accord with recent calculations of Ba2 potential curves.
Anisotropic rotational diffusion studied by passage saturation transfer electron paramagnetic resonance
Authors: Robinson Bruce H. , Dalton Larry R.
Published: February 2003

The stochastic Liouville equation for the spin density matrix is modified to consider the effects of Brownian anisotropic rotational diffusion upon electron paramagnetic resonance (EPR) and saturation transfer electron paramagnetic resonance (ST–EPR) spectra. Spectral shapes and the ST–EPR parameters L″/L, C′/C, and H″/H defined by Thomas, Dalton, and Hyde at X-band microwave frequencies [J. Chem. Phys. 65, 3006 (1976)] are examined and discussed in terms of the rotational times τ∥ and τ⊥ and in terms of other defined correlation times for systems characterized by magnetic tensors of axial symmetry and for systems characterized by nonaxially symmetric magnetic tensors. For nearly axially symmetric magnetic tensors, such as nitroxide spin labels studied employing 1–3 GHz microwaves, ST–EPR spectra for systems undergoing anisotropic rotational diffusion are virtually indistinguishable from spectra for systems characterized by isotropic diffusion. For nonaxially symmetric magnetic tensors, such as nitroxide spin labels studied employing 8–35 GHz microwaves, the high field region of the ST–EPR spectra, and hence the H″/H parameter, will be virtually indistinguishable from spectra, and parameter values, obtained for isotropic diffusion. On the other hand, the central spectral region at x-band microwave frequencies, and hence the C′/C parameter, is sensitive to the anisotropic diffusion model provided that a unique and static relationship exists between the magnetic and diffusion tensors. Random labeling or motion of the spin label relative to the biomolecule whose hydrodynamic properties are to be investigated will destroy spectral sensitivity to anisotropic motion. The sensitivity to anisotropic motion is enhanced in proceeding to 35 GHz with the increased sensitivity evident in the low field half of the EPR and ST–EPR spectra. The L″/L parameter is thus a meaningful indicator of anisotropic motion when compared with H″/H parameter analysis. However, consideration of spectral shapes suggests that the C′/C parameter definition is not meaningfully extended from 9.5 to 35 GHz. Alternative definitions of the L″/L and C′/C parameters are proposed for those microwave frequencies for which the electron Zeeman anisotropy is comparable to or greater than the electron–nitrogen nuclear hyperfine anisotropy.
Dielectric friction in the plasma, harmonic oscillator, and rigid dipole dielectrics
Authors: McMahon D. R. A.
Published: February 2003

It is shown that the dielectric friction effect derived by Fatuzzo and Mason and by Nee and Zwanzig for a rigid dipole dielectric is of the same form as that of a harmonic oscillator dielectric satisfying the usual Clausius–Mossotti type complex permittivity. A pseudo-oscillator model is proposed which describes as particular limiting cases the harmonic oscillator dielectric, the plasma, and the high frequency end of the complex permittivity of a dielectric of rigid but rotationally strongly damped dipoles. Dielectric friction is shown to provide the long range coupling required to produce the collective plasma oscillations of a plasma.
Alternative derivation of Manning’s limiting law for self-diffusion of small ions in polyelectrolyte solutions
Authors: Yoshida Nobuo
Published: February 2003

The self-diffusion of small ions in polyelectrolyte solutions with added simple salt is treated by means of a previous method based on the cell model with cylindrical symmetry. The electrostatic potential set up by the polyion is assumed to be small. A limiting law for the self-diffusion coefficient is thereby obtained which is simple to use. The limiting law is shown to be almost equivalent to that formulated by Manning from his infinite line charge model. For a salt-free polyelectrolyte solution, however, their predictions are somewhat different from that of a limiting law previously derived without recourse to the assumption of small potential. The implications of this difference are discussed.
A new minimal Gaussian basis set for molecular calculations
Authors: Tavouktsoglou A. N. , Huzinaga S.
Published: February 2003

A new method of obtaining three-term Gaussian expansions for use as basis sets is presented. The basis sets are given for the first row atoms and sulfur and nickel at their ground states. The expansions were obtained by a least squares fitting and atomic optimization. An application of these sets to atomic and molecular calculations is also made.
Friction and velocity in Kramers’ theory of chemical kinetics
Authors: Larson R. S. , Kostin M. D.
Published: February 2003

The Fokker–Planck equation for the phase space distribution function is studied as a means of calculating rate constants for chemical reactions. For the case of a symmetric double minimum potential and moderate friction, the eigenfunction of interest is found via a similarity transformation coupled with singular perturbation methods, and the corresponding eigenvalue is obtained by using a variational formula. The resulting expression for the rate constant is seen to be a generalization of the well known Kramers formula. Its range of validity is determined by solving the Fokker–Planck equation numerically, and it is found that the new formula is quite accurate if the barrier height and the friction constant are at least moderately large. For the case of low friction, exact numerical values of the rate constant are obtained and these are used to develop a semitheoretical formula which is valid over the entire range of friction constants. This then permits an evaluation of the range of applicability of the rate constant predicted by the transition state method.