bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational anisotropy and solvent–fluorophore bonds: An investigation by differential polarized phase fluorometry
Authors: Mantulin William W. , Weber Gregorio
Published: February 2003

Differential polarized phase fluorometry has been employed in a comparative study of unsubstituted aromatic hydrocarbons, anthracene, perylene, chrysene, and molecules with similar aromatic skeleton but with several groups that can form hydrogen bonds with the solvent. Strongly anisotropic rotations are demonstrated in the former cases by values of the maximum differential tangent 15–25% smaller than the values characteristic of spherical rotations. On the other hand, molecules having two or more groups that form hydrogen bonds with the solvent show values of the differential tangent that agree with the isotropic rotation value within 1%–2%. In these latter cases the apparent molar volumes computed by Einstein’s equation are close to those expected from the molecular mass, but in the unsubstituted aromatics they are anomalously small, in some cases only one-fifth of the volume expected. By comparison with the rotational models discussed in the previous paper it is concluded that the anisotropic rotations of the unsubstituted aromatic hydrocarbons are best represented by a prolate inertial ellipsoid carrying the transition moments in an equatorial plane and with an axial rotational rate faster than the equatorial rates by a factor of 10–30.
Charge transfer excitation channels in positive ion–N2O collisions at low energy
Authors: Bearman G. H. , Harris H. H. , Leventhal J. J.
Published: February 2003

Competition between various charge transfer excitation channels has been investigated for collisions between a number of positive ions and N2O molecules over the kinetic energy range 20–1000 eV. Formation of excited products in these collisions was investigated by observing the luminescence emanating from the collision cell of an ion beam apparatus. It was found that all singly charged projectiles except He+ and Ne+ produced spectra dominated by N2O+ (?→?) transitions; impact of He+ and Ne+ yielded almost equal amounts of N2O+(?) and N2+(B). Only low yields of other possible excited species were observed. It is concluded that conversion of kinetic energy to internal energy is of little importance in collisions between these positive ions and N2O molecules, and that the excited species produced depend almost entirely on reaction energetics.
Classical trajectory study of the effect of vibrational energy on the reaction of molecular hydrogen with atomic oxygen
Authors: Johnson B. R. , Winter N. W.
Published: February 2003

The dynamics of the reaction O+H2(v) →OH+H is studied by means of three dimensional classical trajectory calculations on an LEPS potential energy surface. Rate constants are calculated for the two cases in which the H2 molecule is initially in the v=0 and v=1 vibrational state. In the temperature range 298–1000 °K these rates are fit very well by the formulas (cm3 molecule−1 sec−1) k=2.81T×10−14 exp(−4279/T) and k=4.65T×10−14 exp(−1868/T). The calculated value of k at 300 °K is 2.8×10−14 cm3 molecule−1 sec−1 which is below the upper bound established by Birely [J. H. Birely, J.V.V. Kasper, F. Hai, and L. A. Darnton, Chem. Phys. Lett. 31, 220 (1975)]. The branching ratio Γ, defined as the ratio of the rates for populating the v′=1 and v′=0 state of OH when H2 is initially in the v=1 state is also calculated and fit by the expression Γ=2.3 exp(196/T). The value at 300 °K is 4.4.
Rotational alignment in inelastic collisions
Authors: Alexander Millard H. , Dagdigian Paul J.
Published: February 2003

Using recently developed expressions for fully state-selected atom–molecule rotationally inelastic cross sections, we determine the degree of rotational alignment under l-dominant (weak coupling) and potential dominant (strong coupling) conditions. In the former limit, expected to be appropriate whenever strong long-range forces are present, the alignment effects are contained in form factors which are products of Wigner 3j symbols. The largest cross section is predicted to correspond to the case where both the initial and final rotational angular momentum vectors lie perpendicular to the initial relative velocity vector. Also, in the l-dominant limit the degree of alignment of the inelastically scattered molecules is predicted to be independent of the initial degree of rotational alignment. Under extreme potential dominant conditions, which we simulate with a statistical description of the inelastic T matrix, the final mj-state distribution will be uniform. In molecular beam experiments where fluorescence emission is used to monitor the occurrence of rotationally inelastic collisions, the predicted l-dominant alignment effects should be easily detectable by observing the linear polarization of the absorbed and emitted radiation.
A kinetic theory of quantum state diffusion
Authors: Monchick L. , Hunter L. W.
Published: February 2003

A kinetic theory is formulated for quantum state diffusion in a simple polyatomic gas not at equilibrium. The analogs of the Stefan–Maxwell equations are derived in two representations: in the first, a basis set for expanding the density matrix is chosen that is closest to the phenomenological description of quantum state diffusion and yields easily recognizable generalizations of the binary diffusion coefficients, now tensors, as well as source–sink terms, which is a new feature. In the second, a rotationally adapted basis set is used. Finally, the effect of exchange symmetry is discussed briefly.
Isotope separation of oxygen-17, oxygen-18, carbon-13, and deuterium by ion laser induced formaldehyde photopredissociation
Authors: Marling Jack
Published: February 2003

Near ultraviolet photopredissociation of formaldehyde has been verified as a highly selective method for enrichment of the rare stable isotopes of oxygen, hydrogen, and carbon contained in this molecule. High resolution absorption spectra of rare-isotope-substituted fomaldehyde show coincidences to within the Doppler profile with strong ion laser transitions in Xeiii at 345 nm and Neii at 332 and 338 nm. By tuning each ion laser transition with an intracavity etalon, optimum enrichment of each isotope was selected by monitoring formaldehyde fluorescence. Oxygen-18 was enriched up to ninefold using natural neon lasing at 332.374 nm, while substituting Ne-22 permitted up to ninefold O-17 enrichment and 33-fold C-13 enrichment at 332.371 nm. Use of D2CO instead of H2CO permitted 44-fold O-18 and 27-fold O-17 enrichment. Hydrogen photoproducts showed 24-fold HD enrichment at 337.822 nm, 60-fold HD enrichment at 345.426 nm, and 180-fold D2 enrichment at 332.377 nm. Single vibronic level photolysis of HDCO shows greater than 90% molecular photoproducts at wavelengths longer than 330 nm and a sudden rise to greater than 50% radical photoproducts below 320 nm. Photopredissociation of D2CO at 332 nm yields less than 1% radical photoproducts. Isotope scrambling in HDCO is about 40% at 325 nm, but drops to less than 6% at 338 nm. Isotope scrambling is 24% for both C-13 and O-18 at 332 nm. Enrichment of deuterium, carbon-13, and oxygen-18 was carried out using formaldehyde with initial isotope composition ranging from natural abundance to 50% to yield photoproduct with up to 88% isotopic purity.
NMR of oriented CDH2I and symmetry related isotope effects
Authors: Wooten Jan B. , Jacobus John , Savitsky G. B. , Beyerlein A. L.
Published: February 2003

Measurements of the orientation parameters of a 13C enriched sample of CDH2I in nematic solvent employing 1H, 2H, and 13C NMR are reported. These parameters denoted as SCH, SCD, SHH, and SHD, obey the following relation which assumes mirror plane symmetry: SCD=−2cosα (SHH+2SHD)−2SCH, rather than the following relations which assume C3 rotational symmetry: SCD=SCH=−2cosα SHH=−2cosαSHD, where α equals the HCH or HCD bond angle. Consequently it is concluded that isotope effects on the orientation parameters, such as SCH≠SCD, reported earlier [J. B. Wooten, A. L. Beyerlein, J. Jacobus, G. B. Savitsky, J. Chem. Phys. 65, 2476 (1976)] result from a change in symmetry upon deuteration. The value of α=112.67° that best describes the reported data on CDH2I and CH3I in nematic phase differs from the gas phase microwave value 111.6° suggesting bond angle distortions in the nematic phase. Thus the preferable method of obtaining SCD for quadrupole coupling constant determinations of the CD bond is from the 13CD dipolar coupling and one molecular parameter, the CD bond length, which is not sensitive to molecular distortions. The quadrupole coupling constant measured in this work for CDH2I using SCD obtained from 13CD dipolar couplings is 163.1±3.8 kHz.
Quantum oscillations in the transition probability for rotational excitation of H2 by H+ impact at 4.67 eV
Authors: McGuire P. , Schmidt H. , Hermann V. , Linder F.
Published: February 2003

A quantitative comparison for quantum oscillating in the transition probability of individual rotations states for the H+−H2 system has been achieved. The present state of refinement in both theoretical and experimental techniques is discussed.(AIP)
Condensation of sulfur hexafluoride in steady supersonic nozzle flow
Authors: Wu Benjamin J. C. , Wegener Peter P. , Stein Gilbert D.
Published: February 2003

Condensation of SF6 carried in argon by homogeneous nucleation has been studied experimentally in two continuously operating supersonic wind tunnels. Condensation onset was detected by Rayleigh light scattering. Calibration of the nozzles by static pressure measurements, together with a solution of equations of motion allowing for the caloric nonideality of the vapor, permitted the determination of the thermodynamic state at the observed onset of condensation. The experiments covered conditions of condensation onset in a range 77<T<113 K, and 0.09<pSF6<4.6 torr, well below the triple point of SF6. The adiabatic supercooling ranged from 38 to 45 C and the results agree qualitatively with the classical theory of homogeneous nucleation.
A new method for chemical instability with diffusion
Authors: Kawasaki Kyozi , Kim Shoon K.
Published: February 2003

A method is presented by which the solutions of a class of nonlinear rate equations describing chemical reactions with instability and diffusion are related to the solutions of the same equations without diffusion in a certain long time and weakly nonlinear regime. The method allows us to treat diffusion and nonlinearity separately in a clean-cut way.
Valence electron momentum distributions for acetylene
Authors: Coplan M. A. , Moore J. H. , Tossell J. A.
Published: February 2003

The momentum distributions of valence electrons for acetylene molecules were determined by the (e, 2e) technique. This technique involves knock-out of each electron of interest by electron impact with complete determination of the momenta of the incident and out going electrons.(AIP)
Theory of rare gas–group VI 1S–1D collision-induced transitions
Authors: Julienne Paul S.
Published: February 2003

The collision-induced 1S–1D emission of a Group VI metastable 1S atom in the presence of a rare gas background is investigated theoretically. Approximate model potentials are constructed for the Group VI 1S–rare gas interaction based on the mixed rare gas ground state potentials. The model is supplemented by ab initio calculations for ArO and by the RKR potential for XeO. Long range perturbation theory is used for the induced dipole transition moment, supplemented at short range by the available ab initio calculations on Ar, Kr, and Xe oxides. The model gives a satisfactory account of the measured emission coefficients for the rare gas oxides and sulfides and predicts emission coefficients for the rare gas selenides.
G,K double-minimum excited state of molecular hydrogen and its isotopes
Authors: Wicke Brian G.
Published: February 2003

Using Glover and Weinhold’s6 double minimum potential for the second excited 1Σ+g state of hydrogen, calculations are performed for the vibrational levels of H2, HD and D2. Vibrational eigenvalues and the rotational constants are tabulated. (AIP)
The deuteron quadrupole coupling constant in CD3F
Authors: Snyder Lawrence C.
Published: February 2003

The deuteron quadrupole coupling constant of CD3F deduced from NMR in liquid crystalline solvents is χCD=161 kHz if one assumes the asymmetry parameter to be negative with the value η=−0.109 computed from an ab initio wavefunction. The assumption by Bhattacharyya and Dailey of η=+0.03 implied χCD=133 kHz, which is much smaller than χCD=202 kHz computed from an ab initio wavefunction in a ’’double-zeta’’ Gaussian basis with polarization functions.
Hydrodynamic screening and viscous drag at finite concentration
Authors: Adelman S. A.
Published: February 2003

The concentration dependence of steady linearized viscous flow in a suspension of spheres of number density ρ0 is studied using a new formulation of the ideas of hydrodynamic screening [cf. K. F. Freed and S. F. Edwards, J. Chem. Phys. 61, 3626 (1974)]. A concentration dependent effective pure fluid Navier–Stokes equation is derived via a mean field treatment similar in spirit to Debye–Hückel theory. The effective Navier–Stokes equation is shown to contain concentration dependence through an inverse screening length κ and a renormalized viscosity η[ρ0]. The quantities κ and η[ρ0] are expressed in terms of the zero frequency and small wavevector limit of the particle current autocorrelation function. The Green’s function of the effective Navier–Stokes equation (screened Oseen tensor) is determined for the case of incompressible flow. The velocity field v(r) for steady, incompressible flow past a sphere of radius R is then calculated for both slip and stick boundary conditions. From v(r), the concentration dependent drag coefficient ζ[ρ0] for a sphere of radius R is determined in terms of κ and η[ρ0]. The result gives a generalization of Stokes’ law to finite concentration suspensions. Finally, two simple approximation treatments of the particle current autocorrelation function are proposed and the forms they yield for η[ρ0] and κ are discussed.
Orientational distributions in partially ordered solids as determined from NMR and ESR line shapes
Authors: Hentschel R. , Schlitter J. , Sillescu H. , Spiess H. W.
Published: February 2003

The orientational distribution in partially ordered solids, drawn polymers, etc. is related with NMR and ESR line shapes governed by anisotropic shift, dipolar, or quadrupolar couplings. In principle, the theory yields the complete orientational distribution by numerical deconvolution or spectral fitting procedures. Furthermore, the line shape can be decomposed into subspectra that correspond to the moments of the orientational distribution. The theory is applied to 2D-NMR line shapes in partially ordered solid benzene, and to ESR nitroxide spin label spectra.
Molecular collisions and depolarization of emission from I2 in the gas phase excited by circularly polarized light
Authors: Katô Hajime
Published: February 2003

Molecular iodine in the gas phase is excited selectively to a single rotational–vibrational level of electronic state 3ΠO +u(B) by a circularly polarized single mode tunable dye laser. By tuning the excitation frequency we have studied the dependence of polarization of the emission and the intensity asymmetries for the rotationally inelastic transitions J′→J′+δJ′ and J′→J′−δJ′ on the quantum numbers of J′ and v′. A theoretical study on the collision between two diatomic molecules was done without neglecting spatial orientation effects. The theory accounts for the high polarization in both the resonance emission and the emissions from rotational and rotational–vibrational energy transferred states, especially for high J′ states.
Steady state dissociation of shock heated HF, HCl, and CO in excess Ar
Authors: Ramakrishna M. , Babu S. V.
Published: February 2003

The steady state dissociation rate constants of shock heated HF,HCl, and CO in the presence of large excess of Ar are calculated, assuming that dissociation can occur from all the vibrational levels, at several temperatures and compared with experimental results. Both the truncated harmonic oscillator (THO) model and Morse oscillator (MO) model are considered. Both the models fit the dissociation rates satisfactorily in the temperature ranges considered. The results are in accord with a ’’weak bias’’ mechanism and large depletion in the higher vibrational levels.
Charge exchange and chemical reaction in the H2++H2 system. I. Characterization of the potential energy surfaces and nonadiabatic regions
Authors: Stine J. R. , Muckerman J. T.
Published: February 2003

Potential energy surfaces for the H+4 system are calculated by the valence bond diatomics-in-molecules method in the zero overlap-of-atomic-orbitals approximation. The adiabatic potential energy surfaces are obtained by the diagonalization of an 8×8 Hamiltonian matrix and are ideally suited for classical trajectory studies involving electronic transitions. The ground state surface of H+4 is discussed and particular emphasis is given to those regions of configuration space for which this surface avoids an intersection with that of the first excited electronic state.
Dynamics of stiff polymer chains. V. Interaction between local and global modes
Authors: Fixman M. , Evans G. T.
Published: February 2003

Relaxation spectra are analyzed for vector and tensor modes of a polymer chain with rigidly constrained bond lengths and angles in dilute solution. Illustrative approximate calculations are made for a chain with independent bond rotational potentials, and the constraints are found to reduce the relaxation rates of local modes to values that overlap the rate spectrum of some global modes significant to mechanical shear response. We conjecture that mixing of local and global modes occurs in the region of overlap, and that as a consequence of mixing a gap develops in the spectrum of relaxation rates. Pending a quantitative study of mixing, a crude approximation is made that the global modes with small relaxation rates retain a Rouse–Zimm form, while modes in the region of overlap have rates elevated significantly above the frequencies used in mechanical shear experiments. Then numerical values of the enhanced rates are unnecessary for the calculation of the complex dynamic viscosity. The latter is given by a formula of the Rouse–Zimm type, truncated at a specified mode number κD, and enhanced by a frequency independent real viscosity, the latter calculable Rouse–Zimm (low frequency) relaxation rates, and the cutoff κD. With κD determined from the experimental dielectric relaxation rate of poly (p-chlorostyrene), the theoretical complex dynamic viscosity of polystyrene was calculated and found to be in good agreement with experimental values.
Self-consistent treatment of the internal energy, chemical potential, and critical constants of liquid alkali metals and selected alloys
Authors: Leribaux Henri R. , Engel Anthony W.
Published: February 2003

A systematic investigation is made of the consequences of the pseudopotential theory of metals on the thermodynamic properties of metallic liquids, including mixtures. We obtain expressions for the internal energy, the Gibbs free energy, and excess chemical potentials of general metallic mixtures with any number of constituents. We recommend on this basis optimum pseudopotentials for all liquid alkali metals. We obtain good agreement for the excess chemical potental of lithium in selected alloys. Finally, on the basis of metallic theory, we obtain a new scaling law for the critical constants of liquid metals which seems to be well satisfied by the metals of the groups I and IIIa, except perhaps for lithium and thallium.
Dynamics of translations and rotations in molecular crystals
Authors: Michel K. H. , Naudts J.
Published: February 2003

The dynamics of rotations and translations in molecular crystals is derived from a microscopic Hamiltonian. The dynamical variables are the lattice displacements and their conjugate momenta for the translations and, in the case of orientationally disordered phases, the appropriate symmetry-adapted functions for the rotations. Coupled dynamical equations for the oscillatory lattice displacements and the orientational molecular relaxation are obtained. The coupling between these equations is expressed in terms of static susceptibilities which are calculated from the Hamiltonian. The coherent inelastic neutron scattering law, including interference between translations and rotations, is derived. Our results are relevant for inelastic neutron, Brillouin, and Raman scattering experiments. As a concrete example we refer to KCN.
Raman and infrared frequency shifts proceeding from ionization of perhalo-p-benzoquinones to radical anions
Authors: Girlando Alberto , Zanon Ines , Bozio Renato , Pecile Cesare
Published: February 2003

The vibrational assignment of the fundamental modes of 2,3,5,6-tetrachloro- and 2,3,5,6-tetrabromo-p-benzoquinone (chloranil and bromanil, respectively) radical anions is reported and compared with that of the neutral molecules. For this purpose the single crystal Raman and infrared spectra of bromanil are also given and interpreted. The absence of the vibronic effects present in the infrared spectra of some free radical salts is demonstrated, and the vibrational spectra of the potassium salts of both radical ions are shown to be interpretable in terms of substantially unperturbed molecular structures even in the case of crystalline samples. The vibrational assignments and their normal coordinate analysis allow one to obtain most of the relevant information on the frequency shifts proceeding from the addition of an extra electron to the perhalo-p-benzoquinone structure.
Negative ion–molecule reactions of ozone and their implications on the thermochemistry of O3−
Authors: Lifshitz C. , Wu R. L. C. , Tiernan T. O. , Terwilliger D. T.
Published: February 2003

An in-line double mass spectrometer has been employed to determine reaction rate coefficients and excitation functions for several types of negative ion reactions involving ozone. The interactions studied include electron transfer reactions, such as, M−+O3→M+O3− (where M−=O−, OH−, F−, Cl−, Br−, I−, S−, SH−, Cl−2, C2H−, NO2−, and CO3−) and particle transfer reactions, such as MO−+O2→M+O3− (where MO−=O2−, NO2−, NO3−, CO3−). Translational energy thresholds have been determined for those reactions which are endothermic by applying exact Doppler corrections for the thermal motion of the neutral as well as corrections for the translational energy distribution of the projecticle ions. These experiments place a lower limit of 2.26+0.04−0.06 eV on the electron affinity of ozone. This value is in excellent agreement with the value computed from the bond dissociation energy of O3− in its most stable configuration, D00(O−–O2) =1.80 eV, as deduced from measurements of the translational energy thresholds for the collisional dissociation process, O−3+M→O−+O2+M, where M=He, Ar. Further implications of these experiments with respect to the structure, thermochemistry, and excited states of O−3 are discussed.
Preresonance enhancement of the coherent anti-Stokes Raman spectra of fluorescent compounds
Authors: Carreira L. A. , Goss L. P. , Malloy Thomas G.
Published: February 2003

Preresonance excitation profiles of β-naphthol and the β-naphtholate ion have been determined by coherent anit-Stokes Raman spectroscopy (CARS). The fluorescence rejection of the CARS technique allowed Raman spectra to be obtained in a region where fluorescence would obscure the spontaneous Raman signal. A method of taking the ratio of the Raman resonant CARS signal to the background signal allowed for a more convenient and reliable method of generating CARS excitation profiles. The observed excitation profiles are in excellent agreement with those predicted using a single line approximation.
Diffusion-controlled reaction rate to a buried active site
Authors: Samson René , Deutch J. M.
Published: February 2003

An approximate expression is presented for the rate coefficient k to an embedded active site. The site is located at a depth (sR) in a hollow channel whose sides are located at the polar angle ϑ0, in a sphere of radius R. The result is k (ϑ0,s) = (2πDR)[1−cosϑ0][(1−s)/s +η (cosϑ0)]−1, where D is the diffusion coefficient and η (x) is given as a series of Legendre polynomials.
On the possibility of direct solution of the classical Liouville equation for inelastic molecular collisions; the reduced Liouville equation
Authors: Miller William H. , Skuse Brian M.
Published: February 2003

Starting with the ordinary classical Liouville equation for the time evolution of phase space distribution functions, total energy conservation is used to obtain a reduced Liouville equation which determines the steady-state (i.e., time independent) reduced distribution function on the ’’energy shell’’ in phase space. Boundary conditions which correspond to time-independent scattering theory are easily imposed, and one sees clearly how to extract the time-independent transition probability matrix for a given total energy; this is the classical analog of the time-independent on-shell S matrix of quantum scattering theory. The reduced Liouville equation is of a form that is amenable to direct numerical solution, and ways for approaching this are described. A particular approximate version of the reduced Liouville equation is seen to be equivalent to a recently proposed stochastic model.
Physical studies of nematic azoxybenzenes. I. Magnetic susceptibilities and the order parameter
Authors: de Jeu W. H. , Claassen W. A. P.
Published: February 2003

The anisotropy of the magnetic susceptibility and the density of the series of nematic p,p′-di-n-alkyl and p,p′-di-n-alkoxy azoxybenzenes are reported as a function of temperature. Assuming that the molecules are effectively axially symmetric the anisotropy of the susceptibility is a direct measure of the nematic order parameter S. The results for S agree reasonably well with the predictions from Maier and Saupe’s theory of the nematic phase. With increasing chain length both the order parameter and the relative volume change at the nematic–isotropic transition alternate parallel to the nematic–isotropic transition temperature and the transition entropy.
Physical studies of nematic azoxybenzenes. II. Refractive indices and the internal field
Authors: de Jeu W. H. , Bordewijk P.
Published: February 2003

The ordinary refractive index and the birefringence are reported for two homologous series of nematic liquid crystals, the p,p′-di-n-alkyl and p,p′-di-n-alkoxy azoxybenzenes. It is found that the high-frequency dielectric anisotropy is proportional to the anisotropy of the magnetic susceptibility. For axially symmetric molecules this proportionality allows a simplification of the intricate problem of the internal field, which can be taken as independent of the anisotropy of the surroundings of a molecule. Representing a molecule by a homogeneously polarizable spheroid, an equation is derived which relates the anisotropic high-frequency dielectric permittivity to the anisotropic molecular polarizability, and which gives consistent results for both the solid state and the nematic phase.
Dependence of orthopositronium annihilation rates on density fluctuations in methane gas
Authors: McNutt J. D. , Sharma S. C.
Published: February 2003

Recent experiments have shown that the annihilation rate of positrons which form orthopositronium in methane gas varies linearly with gas density at low densities, but significantly deviates from linearity at high densities. We present evidence that this annihilation behavior in methane results from density fluctuations that are characteristic of an imperfect gas.
Quantum mechanical theory of collisional ionization in the presence of intense laser radiation
Authors: Bellum John C. , George Thomas F.
Published: February 2003

We present a quantum-mechanical formalism for treating ionizing collisions occurring in the presence of an intense laser field. The theory rigorously takes into account both the intense laser radiation and the internal electronic continuum states associated with the emitted electrons. We accomplish this essentially by combining discretization techniques, used in a recent study of fieldfree collisional ionization, with expansions in terms of so-called electronic-field representations for the quasi-molecule-plus-photon system. This leads to a coupled-channel description of the heavy-particle dynamics which involves effective electronic-field potential surfaces and continua. We also discuss qualitatively characteristic features of ionizing collisions accompanied by intense lasers, drawing comparisons with their fieldfree counterparts. Our remarks are designed to encourage experimental investigation of collisional ionization in the presence of intense lasers, and to stimulate further theoretical work. Because the electronic continuum meets requirements of exact energy resonance for absorption of a photon over large ranges of the internuclear separations, collisional ionization in an intense field should occur much more readily than other field-influenced inelastic collisions, in which photon absorption is resonant only near potential surface pseudocrossings. We therefore suggest laser-influenced ionizing collisions as very good candidates for experimental verification of the effects of intense laser radiation on inelastic collisional processes. We describe the anticipated behavior of the energy distribution of electrons emitted due to radiative coupling. Our comments are based on some physically reasonable assumptions about the electronic transition dipole matrix elements between discrete and continuum electronic states. Actual calculations of such matrix elements involve special electronic structure considerations, and these are outlined in some detail in an Appendix.
On the theory of chemically induced electron polarization (CIDEP): Vector model and an asymptotic solution
Authors: Monchick L. , Adrian F. J.
Published: February 2003

It is shown that the stochastic–Liouville model of CIDEP can be cast into the form of a ’’Bloch-type’’ equation with diffusion. This leads to a generalized vector model of the radical pair mechanism of chemically induced magnetic polarization, which gives a clear picture of the qualitative features of both CIDNP and CIDEP. For the case of simple Brownian motion of the two radicals the differential form of the stochastic–Liouville equations of CIDEP is readily converted into a single integral equation. In the limit of slow singlet–triplet mixing by the magnetic spin Hamiltonian, and with an exchange interaction that decays exponentially with radical separation, this integral equation may be solved exactly. The electron polarization then takes the form of a superposition of an infinite number of ’’contact exchange modes’’.
The interaction of charged disks in a dielectric medium
Authors: Adamson Mary E. , Pedersen Lee G.
Published: February 2003

Four electronic problems of importance to biological calls and of increasing complexity are solved: the charged finite disk in (a) a nonionic dielectric and (b) in an ionic dielectric, and two separated parallel charged disks in (c) a nonionic dielectric and (d) in an ionic dielectric. Electrostatic interaction energies and pressures are computed for the two disk problem.
Model potential for reactions with solid surfaces: H2 +Fe(001)
Authors: Wolken George
Published: February 2003

A semiempirical potential used to model catalytic reactions occurring on solid surfaces as well as gas–solid energy transfer has been generalized to include the possibility of removal of atoms from the surface. Hence, the model is appropriate for future dynamical studies of combustion of solids, erosion, etc. A comparison is made between the present model and a pairwise additive potential for molecular hydrogen interacting with an iron surface. Unlike the pairwise additive approximation, the present semiempirical potential is capable of generating the experimental activation barrier to adsorption and the absence of molecular adsorption.
Deactivation of N2 + (A 2Πu) at above thermal energies
Authors: Sheridan J. R. , Merlo T. A. , Enzweiler J.
Published: February 2003

Measured cross sections for deactivation of the A 2Πu states of N+2 in O2, H2, and He are presented for the energy range 6.5 to 12 keV. Deactivation cross sections are determined by fitting a three-parameter ’’growth’’ equation for excited states to the measured variation of the population of excited states in a fast molecular beam with the pressure of the target gas through which it passed. The cross sections are significantly larger for the molecular targets and the cross sections for deactivation in O2 have the same magnitude (about 2×10−14 cm2) as those measured by other investigators at thermal energies.
Transition operators for atom–atom potentials: The Hilbert–Schmidt expansion
Authors: Olson John A. , Micha David A.
Published: February 2003

Transition operators 〈p‖t (ε) ‖p′〉 with p≠p′≠√2mε, required in collisions of atoms with bound atoms, are investigated and calculated within the Hilbert–Schmidt expansion. We have expanded in the eigenvalues and eigenfunctions of the kernel of the Lippmann–Schwinger equation, to obtain t operators for a Morse potential describing the ground 1Σ potential of H2, and for a repulsive Hulthén potential representing the lowest 3Σ potential of H2. Studies made at negative energies ε, required for inelastic and reactive processes below the dissociation threshold of the target, show that the expansion for the Morse potential is rapidly convergent, and that the relative magnitude of the + operator for the Hulthén potential is at least 1 order of magnitude smaller. Results are presented for momenta and energies appropriate to reactive collisions at energies just above the saddle-point energy of the H3 potential surface.
The 147-nm photolysis of hexafluoroacetone
Authors: Perkins G. G. A. , Austin E. R. , Lampe F. W.
Published: February 2003

The photochemical decomposition of (CF3)2CO by light of 147-nm wavelength has been studied by monitoring mass spectrometrically the formation of C2F6. Addition of NO results in the formation of CF3NO and (CF3)2NOCF3 with complete suppression of C2F6, showing that formation of CF3 radicals is the only significant primary dissociation. The quantum yield of (CF3)2CO increases from a low-pressure limit of 0.45 to 1.55 at about 10 torr and appears to have a high-pressure limit of 2. This rather unusual pressure dependence is opposite to that generally observed in photochemical reactions and in particular to that found in the (CF3)2CO photolysis with light of wavelength greater than 260 nm. A mechanism involving activation of a second molecule of (CF3)2CO by energy transfer in a collision with a photoexcited molecule is proposed to explain the experimental results.
Collisional ionization between alkali atoms and some methane derivatives: Electron affinities for CH3NO2, CF3I, and CF3Br
Authors: Compton R. N. , Reinhardt P. W. , Cooper C. D.
Published: February 2003

The negative ion products resulting from collisions between orthogonal beams of alkali atoms (Na, K, Cs) and the methane derivatives CH3CN, CH3NO2, CF3Br, and CF3I have been studied in the energy range from reaction thresholds to ∼40 eV (LAB). Stable negative ions with masses corresponding to the last three molecules were detected and the following electron affinities are derived from measurements of the energy threshold for the ion pair production reactions: E.A.(CH3NO2) =0.44+0.1−0.2 eV; E.A.(CF3Br) =0.91±0.2 eV, and E.A.(CF3I) =1.57±0.2 eV. From measurements of the difference between the energy threshold for the appearance of various fragment ions and the parent ion, the following bond dissociation energies are deduced: D(CH3–NO−2) =0.56±0.2 eV; D(CF3–Br−) =0.54±0.2 eV and D(CF3–I−) =0.32±0.2 eV. An argument is presented which adds further strength to the suggestions of Williams et al. and Jordan and Wendoloski that electron binding to CH3CN is dominated by the dipole field.
Crossed molecular beams kinetics: SnO recoil velocity spectra from Sn+O2
Authors: Freedman Andrew , Behrens Richard , Parr Timothy P. , Herm Ronald R.
Published: February 2003

Laboratory (LAB) angular and time-of-flight (TOF) recoil velocity spectra of SnO product produced by crossing a thermal Sn beam with an O2 nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer–quadrupole mass filter ’’universal’’ detector. Product center-of-mass (c.m.) recoil angle and energy distributions have been fit to the data by convoluting the c.m.→LAB transformation over the measured speed distributions in both beams and the instrumental TOF resolution. The results establish that the Sn+O2 bimolecular reaction proceeds via formation of an osculating or long-lived SnO2 collision intermediate. In addition, comparison of observed LAB product signals have established approximate relative total cross sections for reaction of Sn with O2, CO2, SO2, N2O, CH3OH, and CH3NO2.
Production of Cl−⋅HCl cluster ions in HCl–N2 mixtures
Authors: Corbin R. J. , Nygaard Kaare J. , Snow W. R. , Schearer Laird D.
Published: February 2003

A drift-tube–mass-spectrometer apparatus has been used to study the production of Cl−⋅HCl cluster molecules in HCl–N2 mixtures. The concentration of HCl was 730 ppm and the total pressure ranged from 0.1–5 Torr. The primary ion C1− is produced mainly by dissociative attachment of HCl and is converted to a cluster ion in a three-body collision. The rate of formation of the cluster ion is k2?10−25 cm6/sec at E/N=12 Td.
’’Effective’’ site symmetry in molecular crystals. Orientational energies of dilute guest probes in isotopically mixed, partially deuterated ethylene crystals
Authors: Whitfield R. G. , Leroi G. E.
Published: February 2003

The midinfrared and Raman spectra of dilute mixed crystals of the partially deuterated ethylenes (C2H3D; cis-, trans- and 1,1-C2H2D2; C2HD3) in C2H4 and in one another have been obtained. The guest modes often exhibit multiplet structure arising from different relative orientations of the neighboring molecules. The number of energetically inequivalent orientations is diagnostic of the ’’effective’’ site symmetry of the host lattice (i.e., symmetry in the spectroscopic rather than the mathematical sense), which was found to be Ci for each of the partially deuterated ethylenes. Consistent with the predictions of the virtual crystal approximation, the number and energies of the orientational components of the guest molecules are independent of the isotopic nature of the host crystal, which indicates that the intermolecular force field is insensitive to the isotopic composition of the interacting ethylene molecules. The orientational effect can be used to distinguish among several possible effective site symmetries for any crystalline lattice, provided that the number of predicted orientational components differ.
Temperature dependence of the rates of reactions HgH+NO→HNO+Hg and HgH+M→Hg+H+M
Authors: Oka K. , Cvetanović R. J.
Published: February 2003

Hydrogen atoms and mercury hydride molecules act as two simultaneous precursors of the HNO (1A″) chemiluminescence observed in the gaseous mixtures of H2, NO, and Hg illuminated by the 253.7 nm radiation, the intensity of which is modulated at a low (10 Hz) and several higher (4–30 kHz) frequencies. In gaseous mixtures with an excess of isobutene added most of the hydrogen atoms are scavenged by the isobutene and HgH is then essentially the only precursor of the luminescence. The phase shift between the modulated 253.7 nm light and the chemiluminescence provides the rate of decay of HgH. The decay rate at 298 °K is mainly determined by the rate of the reaction HgH +NO→HNO+Hg. At higher temperatures (up to 473 °K) the rate of the thermal decomposition of HgH increases with temperature and is found to be proportional to the total pressure. The least-squares Arrhenius expressions for the rate constants (in 1.129 mole−1 sec−1) are: kHgH+NO= (1.63±0.02) ×1010 exp[(231.1±9.6)/RT] and kHgH+M= (5.3±.0) ×1010 exp[(−7492±178) RT]. The activation energies are expressed in cal mole−1. Since hydrogen is present in large excess M is mainly H2. The indicated uncertainties are the least-squares standard deviations.
Canonical transformation approach to the linear Jahn–Teller effect E–e
Authors: Barentzen H. , Polansky O. E.
Published: February 2003

An analytical approach is presented for the linear Jahn–Teller effect E–e. This approach yields analytical expressions for both the ground state and the lowest five excited states over the weak and intermediate coupling range (0?k2?3). The method is based on canonical transformations combined with perturbation theory. A comparison of our results with existing numerical solutions to this problem shows good agreement within the weak and intermediate coupling range. Ham’s reduction factors p and q as well as the oscillating behavior of the energy levels for large quantum number n are also discussed.
Classical trajectories using the full a
b i
n
i
t
i
o potential energy surface H−+CH4→CH4+H−
Authors: Leforestier Claude
Published: February 2003

The classical trajectory method has been coupled with the gradient evaluation of the ab initio potential energy with respect to nuclear coordinates to study molecular dynamics. Trajectories, starting from a known transition state, can be computed in the full space of Cartesian coordinates in order to provide information on the reaction dynamics. The nucleophilic substitution reaction H−+CH4→CH4+H− is treated as an example and is shown to occur via a nonadiabatic path where initial vibrational energy of the methane molecule is essential for reaction.
Melting curve of ice VII
Authors: Mishima O. , Endo S.
Published: February 2003

The change of melting temperature of ice VII with increasing pressure up to 150 kbar was obtained by using resistance increase of the manganin gauge that occurred contemporaneously in the specimen. The melting point vs pressure curve obtained can be well represented by the equation (Pm−22.1)/5.342= (Tm/355)5.220−1.
Single vibronic level photochemistry of formaldehydes in the ? 1A2 state: Radiative and nonradiative processes in H2CO, HDCO, and D2CO
Authors: Miller Richard G. , Lee Edward K. C.
Published: February 2003

Details of experimental data on the fluorescence quantum yields and fluorescence decay times of H2CO, HDCO, and D2CO are given for various single vibronic level (SVL) photoexcitations. These values were also measured at various pressures in order to obtain the ’’isolated’’ molecule values and the collisional relaxation data. Highly structured variation of the SVL radiative lifetimes (τR) and the SVL nonradiative lifetimes (τNR) observed in H2CO in particular shows that the out-of-plane bending mode (ν4) is the ’’inducing’’ mode for the radiative transitions and also the ’’promoting’’ mode for the nonradiative transitions. A test of the radiative and the radiationless theory of Fleming, Gijzeman, and Lin shows qualitative agreement with the experimental data presented herein.
On the orientational order in β-nitrogen
Authors: Press W. , Hüller A.
Published: February 2003

It is shown that a model with precessing molecules in β-N2 gives a rather misleading picture of the density distribution at a molecular site. An alternative approach based on an expansion into symmetry-adapted surface harmonics is presented.
Molecular beam scattering experiments on the abstraction and exchange reactions of deuterium atoms with the hydrogen halides HCl, HBr, and HI
Authors: Bauer Wolfgang , Rusin Lev. Y. , Toennies J. Peter
Published: February 2003

Molecular beam scattering experiments have been carried out on the abstraction and exchange reactions of deuterium atoms (T=2600 K) with the hydrogen halides HX(T=300 K) in the range of scattering angles: 0°⩽ϑcm⩽70° (ϑcm=0° is the direction of the incident D-atom beam). The apparatus employed a very sensitive electron bombardment detector with a sufficiently low H2 background to make possible the measurement of differential cross sections of about 0.1 Å2/sr for reactively scattered HD and H and nonreactively scattered D-atoms. The measured HD signal can be largely attributed to various background sources and only serves to establish a rough upper limit on the abstraction cross section in the angular range investigated. The H-atom signal was more intense. The observed angular distribution was forward peaked, and is attributed to the exchange reaction. The nonreactively scattered D-atom signal was used in conjunction with a recently reported effective spherically symmetric potential to provide an absolute calibration of the detector sensitivity. The measured integral cross sections for the exchange reactions are 2.3 Å2 (D+HCl), 1.3 Å2 (D+HBr) and 1.6 Å2 (D+HI) with an estimated error of about ±30%. The absolute cross sections and the H-atom angular distributions are consistent with the DX distributions measured by McDonald and Herschbach. Both experimental angular distributions are considerably narrower than those predicted by the recent classical trajectory calculations of Raff, Suzukawa, and Thompson. The implications of the new data for the activation energies for the exchange reactions are discussed.
Valence bond study of hyperfine interactions and structure of the noble gas monohalides
Authors: Adrian Frank J. , Jette A. Norman
Published: February 2003

A semiempirical valence bond (VB) wavefunction that includes correlation between electrons of opposite spin is used to calculate hyperfine structure constants (hfc) of the noble gas monohalide molecules (NX). This wavefunction has the form ψ =N (2Σ)[χ𝒜Φ (N⋅⋅⋅X)+(1−χ2)1/2 𝒜Φ (N+⋅⋅⋅X−)] where N and X denote the noble gas and halogen, respectively, and 𝒜 is the antisymmetrization operator. This antisymmetrization and the effects of interatomic electron correlation on open shell states are the major contributors to the isotropic hfc. These correlation effects are treated by perturbation theory calculations of the van der Waals polarization of X by N in the VB structure Φ (N⋅⋅⋅X) and the Coulomb polarization of N+ by X− in Φ (N+⋅⋅⋅X−). It is also important to consider both valence and inner shell s electrons, and to include intra-atomic electron correlation. The latter is treated by using neutral atom orbitals for N and X in Φ (N⋅⋅⋅X) and cation and anion orbitals for N+ and X−, respectively, in Φ (N+⋅⋅⋅X−). Comparison of experimental hfc with calculated values yields estimates of the charge distribution (χ) and internuclear distance (R). In this procedure it is important to consider spin–orbital contributions to the observed hfc. Results are for XeF, R=4.70a0, χ=0.80. For KrF, R=4.00a0, χ =0.98. For XeCl, R=6.50a0, χ=0.955. These values are consistent with the relative sizes and electronegativities of the constituent atoms.
Statistical mechanics of helical wormlike chains. VII. Continuous limits of discrete chains
Authors: Yamakawa Hiromi , Shimada Jiro
Published: February 2003

A discrete chain which tends to the helical wormlike chain in the continuous limit is studied. For this purpose, the physics of a generalized continuous chain is considered from the point of view of the theory of elasticity, and it is pointed out that the helical wormlike chain should be distinguished from the true Bugl–Fujita chain, which has the regular helix extreme but not the true wormlike case. Then, a hypothetical discrete chain corresponding to the generalized continuous chain is found. In particular, it is shown that the helical wormlike chain is a special limit of this discrete chain with coupled rotations of some kind, while the Bugl–Fujita chain is that of the chain with independent rotations. The derivation of the differential equation for the Green’s function for the generalized chain is discussed rather in detail in the Appendix.
Comment on ’’The structure of a hard sphere fluid in contact with a soft repulsive wall’’
Authors: Abraham Farid F. , Singh Y.
Published: February 2003

Recent speculative statements concerning the interfacial fluid density profiles of various liquid/solid interface systems are examined by using the Monte Carlo method to simulate the various profiles. (AIP)