bluuebunny/crossref_metadata_2025 huggingface dataset:
Classical calculations of NH3 and H2O rotational excitation in energetic collisions with atomic oxygen
Authors: Kolb C. E. , Elgin J. B.
Published: February 2003

Classical, rigid rotor rotational excitation probabilities have been calculated for a symmetric top, NH3, and an asymmetric rotor, H2O, undergoing high energy collisions with atomic oxygen. A Monte Carlo procedure was utilized to determine both the probability distributions for total translational to rotational energy transfer and the resulting distributions of rotational angular momenta. Intermolecular potentials constructed from interpenetrating hard spheres were employed to obtain results applicable to high energy (?1 eV) atom/rigid rotor collisions.
Comparison of information theoretic and collision-dynamics calculations of vibrational transition probabilities
Authors: Nanbu K.
Published: February 2003

Multiquantum vibrational transition probabilities for the collision of an atom with a harmonic oscillator are calculated by using the recently introduced information theoretic analysis. The sum rule in the information theoretic analysis is determined by use of the result from a semiclassical treatment of the collinear collision [A+BC(I) →A+BC(n)] with exponential repulsive potential. We conclude from an extensive calculation that the information theoretic probabilities for upward transition (I<n) show a good qualitative agreement with the collision-dynamics ones when the conditions (2?ω/C)1/2L≲1 and mAmC/mB(mA+mB+mC) ≪1 hold, where ? is the reduced mass of the colliding system, ω is the angular frequency of the oscillator, L is the steepness constant of the potential, and mi is the mass of atom i.
Electron spin resonance study of octane radical cations in irradiated pentane matrices
Authors: Nauwelaerts Frans , Lemahieu Maria , Ceulemans Jan
Published: February 2003

ESR spectra of octane radical cations have been obtained by irradiating pentane matrices, containing octane and 2-chloropentane as additives, and by taking the difference spectrum by computer between the spectra before and after (selective) removal of the octane radical cations by illumination. The spectrum of octane radical cations consists of a single unresolved line; the width of the line is mainly due to unresolved Fermi-contact hyperfine interaction with hydrogen (or deuterium) nuclei of the radical cation and of adjacent matrix molecules. Fermi-contact hyperfine interaction with hydrogen (or deuterium) nuclei of the matrix is attributed to the formation of a charge-transfer complex between octane radical cations and matrix molecules.
Rotational magnetic moment spectrum of SF6
Authors: Ozier I. , Yi P. N. , Ramsey N. F.
Published: February 2003

The rotational magnetic moment spectrum of SF6 has been studied by the molecular beam magnetic resonance method. The transitions observed involve the reorientation of the rotational angular momentum relative to an external magnetic field in the high field limit. The magnitude and sign of the rotational g-factor gJ and of the average fluorine spin rotation constant ca have been determined. The results are gJ=−(0.03732±0.00005) nm and ca=−(5.27±0.40) kHz. The applications of these results to the paramagnetic part of the average nuclear shielding constant, to the paramagnetic part of the molecular magnetic susceptibility, to the electronic term in gJ, and to fluorine spin–lattice relaxation experiments in SF6 are discussed.
Electron-gas model for open shell–closed shell interactions. II. Application to the alkali monoxides
Authors: Clugston Michael J. , Gordon Roy G.
Published: February 2003

In the preceding paper, we have extended the electron-gas method of calculating interatomic forces to the case of an open shell atom interacting with a closed shell atom. We here apply this theory to the calculation of the bond distance, dissociation energy, and vibrational frequency for the Π states of the alkali monoxides. We obtain better than 10% agreement with the data, which is either from experiment or from other a priori calculations. We comment on the electronic reasons behind our calculated decreased separation in total energy between the Σ and Π states on going from LiO through RbO. Experimentally, the Π state lies lower for LiO while the Σ state is lower for RbO. We argue that we do not predict this change because our Σ states are inaccurate by a few tenths of an electron volt, owing to a small degree of covalent bonding not calculable using electron-gas methods. As in the preceding paper, we conclude that in these systems the electron-gas method will be useful for the calculation of Π states but will not be reliable for Σ states. The Π states involve mainly closed shells in the region of overlap, and this explains the success of our closed shell model in describing these states.
Rotating waves as asymptotic solutions of a model chemical reaction
Authors: Erneux Th. , Herschkowitz-Kaufman M.
Published: February 2003

Numerical simulations on a model chemical reaction are reported. Wavelike activity, including rotating waves in two dimensions, is shown to arise as a result of an instability triggered by diffusion.
Charge transfer cross sections in argon ion–diatomic molecule collisions
Authors: Hedrick A. F. , Moran T. F. , McCann K. J. , Flannery M. R.
Published: February 2003

Charge-transfer cross sections have been examined for Ar+–H2, N2, O2, CO, and NO collisions, in the energy range 0.5–3.0 keV, using time-of-flight techniques in which fast neutral Ar products from the electron transfer processes are measured. Cross sections in Ar+–H2 and N2 systems are comparatively large and decrease with ion kinetic energy while the Ar+–O2, CO, and NO cross sections increase with incident ion kinetic energy. The Ar+ ion beam was composed of 2P3/2 and 2P1/2 states with no evidence of long-lived, electronically excited states participating in these reactions. A multistate impact parameter treatment of atomic ion–diatomic molecule collisions is presented and applied to the Ar+(2P3/2, 2P1/2)−H2(X 1Σ+g, v0′=0) system. The resulting coupled differential equations have been solved numerically, with convergence of the cross sections achieved by inclusion of a comparatively large number of possible product channels in the wavefunction expansion of the system. At low ion kinetic energies both the energy defects and vibrational overlaps determine the population of the product vibrational states while at high kinetic energies the vibrational overlaps control the product-ion vibrational-state distribution. Small-angle scattering dominates the charge-transfer reactions with forward inelastic processes becoming more intense as the ion kinetic energy is increased.
Electronic structure of the fluorobenzenes, ethylene, and tetrafluoroethylene
Authors: Duke C. B. , Yip K. L. , Ceasar G. P. , Potts A. W. , Streets D. G.
Published: February 2003

The electronic structures of C6H6-nFn, 1⩽n⩽6, C2H4, and C2F4 are evaluated by extending the new ’’spectroscopic’’ self-consistent-field CNDO/S2 molecular orbital model to include a parameterization of fluorine. As in our earlier studies of conjugated hydrocarbons, the CNDO/S2 model provides an adequate description of photoelectron spectra associated with states of ionization potential I≲13 eV. The intensities of vibrational progressions evident in the photoelectron spectra are analyzed to extract the linear coupling constants of electrons in the various molecular orbitals to the totally symmetric (a1g) molecular vibrations and, when allowed by symmetry, to e2g molecular vibrations. These coupling constants differ substantially from those obtained for benzene and deuterobenzene. Their values reflect the consequences of the heavy fluorine mass and the incompressibility of the charge density on the fluorine substituent. They lead, moreover, to explicit predictions of increased insulating behavior for fluorobenzenes relative to benzene in the solid state. Analyses of the ultraviolet absorption spectra of the fluorobenzenes are less satisfactory than in the previously considered cases of conjugated hydrocarbons. The fluorine substituents produce ionic virtual orbitals whose greater diffuseness relative to the occupied orbitals must be incorporated into the model in order to achieve a quantitative (ΔE⩽0.1 eV) description of all of the fluorobenzene ultraviolet absorption spectra. The singlet absorption spectrum of C2H4, however, is described quantitatively; a surprising result considering the simplicity of the CNDO/S2 model.
ENDOR study of the 3αH radical in deoxycytidine 5′-phosphate⋅H2O
Authors: Close David M. , Fouse George W. , Bernhard William A.
Published: February 2003

The dominant radical observed at room temperature in x-irradiated deoxycytidine 5′-phosphate monohydrate (5′dCMP) shows anisotropic hyperfine coupling to three α hydrogens. The principal values of these three couplings, determined by an ENDOR experiment, are α (1):−18.77, −42.96, −71.16 MHz; α (2):−19.15, −44.24, −67.18 MHz; and α (3):−11.69, −25.77, and −34.93 MHz. Detailed SCF MO calculations at the INDO level have been performed to show that the 3αH radical has the general structure of H1Ċ (H2) –CX=C(H3)Y. The relationship of the 3αH radical observed in 5′dCMP, to similar radicals observed in cytidine and 3′CMP, is discussed. A reaction scheme is proposed to show how the 3αH radical could be formed in the C5'–C3' region of the ribose moiety.
Uncertainty in the inelastic resonant scattering assisted by phonons
Authors: García N. , García-Sanz J. , Solana J.
Published: February 2003

We have analyzed the inelastic minima observed in new results of He atoms scattered from LiF(001) surfaces. This is done considering bound state resonance processes assisted by phonons. The analysis presents large uncertainties. In the range of uncertainty, we find two ’’possible’’ bands associated with the vibrations of F− and Li+, respectively. Many more experimental data are necessary to confirm the existence of these processes.
Generalized van der Waals theory of nematic liquid crystals: An alternative formulation
Authors: Cotter Martha A.
Published: February 2003

An alternative formulation of the generalized van der Waals theory of nematic liquid crystals is presented and discussed briefly. The derivation is considerably more straightforward than that of Gelbart and Baron and is closely analogous to the usual derivation of the van der Waals equation of state.
Uranium hexafluoride negative ions from surface reactions involving uranium and fluorine
Authors: Compton R. N. , Reinhardt P. W. , Garrett W. R.
Published: February 2003

Negative ion yield has been measured by reacting fluorine gas with a 30 mm diameter uranium wire. The negative ions emitted from the surface were mass analyzed in a time-of-flight mass spectrometer. (AIP)
On the formation of positive and negative ions in gaseous UF6
Authors: Compton R. N.
Published: February 2003

Total cross sections for the formation of positive ions by electron impact on UF6 have been determined from threshold to 1 keV. A trapped electron apparatus is used to determine cross sections for attachment of electrons to UF6 from 1 to 20 eV. Maxima are observed at 2.15±0.05 eV and ∼7 eV. The cross section at 2.15 eV is 2.7±1 Å2. Our previous studies showed that UF−5 also peaks at this energy. The negative ion products resulting from collisions between orthogonal crossed beams of alkali atoms (Na, K, Cs) with UF6 are also investigated and the ions observed are UF−6, UF−5, and F− in order of increasing endothermicity for the reactions. UF−6 ions are observed to occur at approximately zero energy for all alkali reactions. From these observations, the electron affinity of UF6 is estimated to be greater than or equal to 5.1 eV. UF−6 ions are also detected when only a thermal alkali beam enters the reaction region of the mass spectrometer. Although the possibility of a surface reaction is not entirely ruled out, evidence is presented for a two-body collisional ionization reaction between thermal alkali atoms and UF6. The threshold for the production of F− gives a bond dissociation energy, D (UF5–F) =3.0±0.2 eV, in good agreement with the unpublished value of Hildenbrand. Using this bond dissociation energy, together with the threshold for forming UF−5, we calculate the electron affinity of UF5 to be 4±0.4 eV. This value is derived assuming that the final products are A++F+UF5− (A=alkali atom).
Determination of the pair polarizability tensor for the Ne diatom
Authors: Kress James W. , Kozak John J.
Published: February 2003

The pair polarizability tensor A for the neon diatom Ne2 has been determined as a function of the Ne–Ne separation using an extended Gaussian basis set and the finite perturbation coupled Hartree–Fock technique. Our results are compared with calculations reported previously on the Ne diatom (the earlier calculations were based on a density functional approach), and are found to differ significantly in the estimate of the anisotropy in the tensor A.
Proton affinities and cluster ion stabilities in CO2 and CS2. Applications in Martian ionospheric chemistry
Authors: Meot-Ner (Mautner) M. , Field F. H.
Published: February 2003

Thermodynamic values for the reaction CO2H++CH4?CH5+ +CO2 were measured by pulsed high pressure mass spectrometry as ΔH°=−2.8 kcal/mole, ΔS°=−2.4 e.u.; for CS2H+ +H2O?H3O++CS2 as ΔH°=−2.3 kcal/mole, ΔS°=−7.7 e.u.; and for CS2H++H2S?H3S++CS2 as ΔH°=−7.6 kcal/mole, ΔS°=−9.6 e.u. The proton affinities of CO2 and CS2 are found to be 124.0 and 166.6 kcal/mole, respectively. The large entropy changes in the reactions involving CS2 are explained by a nonlinear CS2H+ ion. Stabilities of cluster ions, i.e., −ΔH° for the ion–molecule association reactions leading to the cluster ions were measured (in kcal/mole) as follows: For radical ions: CO2+⋅CO2, 16.2; CO2+⋅2CO2, 6.0; CS2+⋅CS2, 21.9; S2+⋅CS2, 21.9. For protonated, even-electron ions: CO2H+⋅CO2, 20.1; CS2H+CS2, 11.1; H3O+⋅CO2, 14.4. The stability of CO2H+⋅CO2 is unusually high for a small protonated cluster ion and indicates significant hydrogen bonding in this ion. Entropies of association were found in the range −17 to −26 e.u. Limits for the stabilities of O2+⋅nCO2 (n=1–3) were also established. The application of these results in Martian ionospheric chemistry is discussed.
Effusion studies of the thermal decomposition of magnesium and calcium sulfates
Authors: Lau K. H. , Cubicciotti D. , Hildenbrand D. L.
Published: February 2003

The torsion-effusion method, with simultaneous torque angle and weight loss measurements, was used to study the thermal decomposition of MgSO4 and CaSO4. The initial step in the decomposition/vaporization process appears to be the evolution of an SO3 molecule from the lattice, followed by conversion to SO2 and O2 on the surface. Measurements of decomposition pressure as a function of orifice area show that both steps are kinetically hindered. The experimental results can be correlated very satisfactorily by the Whitman–Motzfeldt model, so that reliable equilibrium data can be derived despite large degrees of undersaturation of the vapors. With MgSO4 it was possible to evaluate both the SO3 and SO2+O2 equilibrium pressures, the latter by addition of a small amount of Fe2O3 catalyst to the sample. Analysis of the derived equilibrium pressures yields values of −310.3±0.5 and −344.8±0.5 kcal/mol for the standard enthalpies of formation of MgSO4(s) and CaSO4(s), respectively, at 298 K.
Detection and assignment of the ’’phantom’’ photochemical singlet of t
r
a
n
s-stilbene by two-photon excitation
Authors: Stachelek Thomas M. , Pazoha Thomas A. , McClain W. M. , Drucker Ronald P.
Published: February 2003

We have made a complete polarization study of the two-identical-photon excitation (TIPE) spectrum of stilbene in solution in the energy region 34 400 to 45 000 cm−1, which shows a strongly allowed two-photon transition with a maximum at 41 000 cm−1. The upper state of this transition is of vibronic symmetry Ag. We believe that the electronic part of the upper state is also of Ag symmetry and that it is identifiable with the ’’phantom’’ singlet state postulated by Saltier and by Orlando and Siebrand as the major pathway for cis–trans photoisomerization of this molecule.
Thomas–Fermi–Dirac interaction potentials for rare gas systems
Authors: Yonei K. , Goodisman J.
Published: February 2003

The energy components and the Thomas–Fermi–Dirac interatomic interaction potentials of the Ar–Ar, Kr–Kr, and Xe–Xe systems are calculated for several internuclear distances, solving exactly the Thomas–Fermi–Dirac equation for the diatomic systems. Comparisons are made to results from other density functional approaches. The dependence of the energy components on the nuclear charge is discussed, and it is suggested that (EαTFD −EαTF)/EαTF, the relative deviation of a TFD energy component from that calculated from the Thomas–Fermi density, is proportional to Z−2/3. A possible analytic form of the interatomic potential is proposed.
Master equation theory for steady-state chemical reactions: Dissociation of diatomic molecules in gases
Authors: McCoy B. J. , Carbonell R. G.
Published: February 2003

A steady-state form of the master equation is solved to determine rate constants for dissociation of diatomic molecules. A truncated Morse oscillator model describes the vibrational energies and the (SSH) transition probabilities. Single-quantum vibrational–translational energy exchanges are shown to dominate the rate of dissociation. The theory provides order of magnitude agreement with experimental data for H2, N2, O2, NO, F2, Cl2, Br2, and I2. Anharmonicity of the potential contributes to the lowering of the energy of activation below the bond dissociation energy. Deviations from the Boltzmann distribution cause the energy of activation to decrease at higher temperatures.
Quantitative determination of water coverage on KCl(001) by secondary ion mass spectroscopy
Authors: Kaarmann H. , Hoinkes H. , Wilsch H.
Published: February 2003

KCl(001) cleavage planes below room temperature physisorb water, detectable by secondary ion mass spectroscopy (SIMS). Adsorption isobars and isotherms on that single cyrstal plane were measured with static SIMS and used as a monitor of surface coverage. Using the Langmuir model for the physisorption of H2O to KCl(001), a quantitative determination of relative surface coverage is achieved. Coverages as low as 10−5 of a monolayer could be measured. At a surface temperature TSF=150 K and a partial pressure of water pH2O=1×10−8 Torr, the coverage is one ordered monolayer. The adsorption energy of H2O on KCl(001) is (430±30) meV.
Hindered rotational energy levels of a tetrahedron in an orthorhombic crystalline field
Authors: Smith David
Published: February 2003

The hindered rotational energy levels for a tetrahedron in an orthorhombic crystalline field have been computed. The symmetry group of the Hamiltonian is ?×C2, where ? is the tetrahedral group of rotations about body-fixed axes and C2 represents 180° rotations about the space-fixed axes. The rotational heat capacity of the ammonium ion has been computed using this model. For low barriers to rotation the maximum in the rotational heat capacity is very broad.
Further studies of 4He–H2 vibrational relaxation
Authors: Alexander Millard H.
Published: February 2003

We report coupled-states cross sections for the ro–vibrational relaxation of the v=1, j=0,2,4, and 6 levels of p H2 in collision with 4He. The correlated potential surface of Raczkowski was used. For a given translational energy the total de-excitation cross sections are enhanced by initial rotation. Also, a higher degree of initial rotational excitation results in a more sharply peaked distribution of final rotational states. The qualitative energy dependence of the computed cross sections resembles that predicted by earlier three-dimensional scattering calculations. The addition of the P4(cosϑ) anisotropy into the interaction potential leads to a significant change in the vibrationally inelastic cross sections. The theoretical relaxation rates parallel the experimental values but are about five times smaller.
The generation, diffusion, spin motion, and recombination of radical pairs in solution in the nanosecond time domain
Authors: Schulten Z. , Schulten K.
Published: February 2003

Pairs of radical ions generated in polar solvents by photoinduced electron transfer either recombine within a few nanoseconds to singlet and triplet products or separate. On the basis of recent time-resolved observations of a magnetic field dependence of the pair recombination a theoretical description of this process is provided. The description, similar to the radical pair theory of CIDNP and CIDEP, is founded on a coherent spin motion superimposed on the diffusive motion of the radicals. The spin motion is induced by the hyperfine coupling between electron and nuclear spins and can be modulated by low (0–200 G) magnetic fields. The spin-selective recombination of radicals is accounted for by a Feshbach optical potential. The diffusion process described by a Smoluchowski operator depends sensitively on the solvent properties. For the case of free Brownian motion, simple analytical expressions for the time- and magnetic-field-dependent recombination yields are derived. For the Brownian motion of oppositely charged radical ions a differential–difference approximation is used to demonstrate the dependence of the recombination yields on the viscosity and polarity of the solvent medium as well as on the strength of the hyperfine coupling and on the rate of the electron back transfer.
Continuum limit of the rotational isomeric state model: Higher moments of the monomer–monomer distribution function
Authors: Baram Asher , Gelbart William M.
Published: February 2003

In an earlier paper we established anomalies in the angular moments 〈PJ(cosϑn) 〉 of the monomer–monomer distribution function for flexible polymers. (PJ is the Jth Legendre polynomial, ϑn is the angle between the first and nth monomers, and the brackets denote a thermal average). We show here how these anomalies arise from the tetrahedral symmetry of the three-state rotational isomer model and how they disappear in the continuum limit of torsional conformations. We also relate the averages 〈rN2p〉 of the first few powers of the end-to-end distance rN to the angular moments and evaluate them in the continuum limit. It is concluded that the eighth and higher radial moments contain spurious contributions when calculated within the usual three-state model.
Ground state relaxation measurements by laser-induced depopulation
Authors: Feinberg R. , Teets R. E. , Rubbmark J. , Schawlow A. L.
Published: February 2003

We have directly measured the repopulation rate for an individual rotational level in the electronic ground state of a homonuclear molecule. A pulsed dye laser was used to depopulate (v″, J″) level of Na2 by pumping a transition of the form (v′, J′) ← (v″, J″) in the B1Πu-X1Σ+g blue–green band system. The (v″, J″) state was repopulated by collisions with other sodium species and with an argon buffer gas. The transmission of a second, delayed, pulsed dye laser was monitored, serving as a probe of the (v″, J″) state’s population at various delay times. The argon pressure was varied and the procedure repeated. The Na2–argon collision cross section may also be determined by this method.
Beam maser double resonance measurements on HDO
Authors: Kukolich S. G.
Published: February 2003

High resolution measurements of hyperfine structure on the 220–221 transition of HDO were made with a two-cavity beam maser using the Ramsey method and using a new double resonance technique. Resonances for magnetic dipole transitions between hyperfine sublevels of the rotational states were detected by observing intensity changes in the two-cavity interference pattern for microwave rotational transitions. The linewidth (fwhm) for the magnetic dipole transitions was 1 kHz. This linewidth allows better effective resolution than the two-cavity Ramsey resonance when nozzle beams are used. Deuterium quadrupole coupling and spin–rotation interaction strengths were obtained. This double resonance technique should be useful for other small molecules, particularly for those cases where the data cannot be obtained from the microwave transitions.
Experimental evidence on the source of negative peaks in coherent anti-Stokes Raman spectra
Authors: Carreira L. A. , Goss L. P. , Malloy Thomas B.
Published: February 2003

Under certain conditions, when the laser pump frequency approaches an electronic transition frequency, negative peaks have been observed in coherent anti-Stokes Raman spectra. Two conflicting explanations of this phenomenon have been offered. We have noted that these two explanations lead to a different dependence of the intensity of the negative peaks on the power in the pump beam. We have experimentally determined this power dependence in the case of a 5×10−3M solution of N, N-diethyl-p-nitrosoaniline in benzene and found that the intensity of the negative peaks varies with the square of the power in the pump beam. This result is consistent with the explanation that the negative peaks arise from a cross term between the background susceptibility and the imaginary part of the Raman susceptibility and not from the inverse Raman effect.
Gaussian basis sets for molecular calculations. The representation of 3d orbitals in transition-metal atoms
Authors: Hay P. Jeffrey
Published: February 2003

Augmented (4d) and (5d) Gaussian basis sets are presented which provide a balanced description of the 4s23dn−2, 4s3dn−1, and 3dn configurations of transition−metal atoms. When compared to accurate Hartree–Fock calculations, the (4d) and (5d) expansions in the literature, which have been optimized for the 4s23dn−2 configuration, can lead to errors of several eV for states of the latter configurations where the 3d orbitals become more diffuse. The augmented sets reported here consist of a single diffuse basis function added to the original sets, where the orbital exponent of the added function was optimized for the 3dn configuration. Basis sets and contraction schemes are presented for the atoms Sc through Cu.
The arbitrary dynamic behavior of open chemical reaction systems
Authors: Perelson Alan S. , Wallwork David
Published: February 2003

We show that if the kinetic equations describing a set of chemical reactions occurring in a one-phase closed system are only required to obey a very general set of postulates (mass conserved, concentrations nonnegative, vector fields which are continuous with continuous derivatives, existence of equilibrium, stability of equilibrium, existence of potential functions which are extremal at equilibrium, detailed balance, law of definite proportions, and a dissipation inequality equivalent to the second law of thermodynamics), then when coupled to input and output flows the kinetic equations are compatible with any local dynamical behavior consistent with the dimension of the chemical state space. From this one can draw one of two conclusions: either open chemical systems are capable of exhibiting any behavior or that there exist additional restrictions on the types of equations that can be used to describe chemical reaction systems.
Hydrodynamic thickness of adsorbed polymers
Authors: Varoqui R. , Dejardin P.
Published: February 2003

Relationships between the molecular dimensions of polymers adsorbed on solid surfaces and the ’’hydrodynamic thickness’’ obtained by viscosimetry or flow rates through capillaries are derived by solving the corresponding Navier–Stokes equation. An exponential density distribution of polymer segments in the vicinity of the adsorbing interface was used. The theoretical results are discussed in connection with the experimental data of the literature.
Electron transfer at sensitized TiO2 electrodes
Authors: Spitler Mark T. , Calvin Melvin
Published: February 2003

Electron transfer from the excited state of tetra-iodo, tetra-chloro fluorescein (rose bengal) to the conduction band of TiO2 has been studied through photoelectrochemical techniques. The measured transfer rate was correlated with information from absorbance and adsorption measurements of this dye molecule on the (001) surface of the single crystals used as electrodes. The quantum efficiency for the photoinjection of electrons was determined to be 4.0×10−3 independent of the pH of the electrolyte and the dye surface concentration. With these data, an argument is given supporting the existence of excitation transfer among the dye molecules on the surface. A temperature study of the electron transfer efficiency yielded an activation enthalpy of 3.2±0.3 kcal/mole with a pre-exponential factor of 1.0. A heat of adsorption of 7.6±0.5 kcal/mole for this dye on TiO2 single crystals was derived from analysis of adsorption isotherms measured at 21.5 and 36.5°C. In addition to the photo-oxidation process, a photoreduction was also observed upon cathodic polarization of the semiconductor. Action spectra revealed participation of solution dye molecules in this reaction; the reduction rate is dependent upon oxygen concentration in solution. It was concluded that the oxidized dye in solution accepts an electron from the TiO2 to generate this current. This photoreduction was found with triphenylmethane and thiazine dyes as well as with other fluorescein derivatives.
Application of complementary variational principles to the Kirkwood equation for hard spheres at very high densities
Authors: Co King U. , Kozak John J. , Luks Kraemer D.
Published: February 2003

The theory of complementary variational principles for a nonlinear integral equation with symmetric positive-definite kernel is applied to the Kirkwood equation (under the superposition approximation) for g(2)(x) for a system of hard spheres at very high densities. At all λ values beyond the lower bound on the limit of stability of the dense fluid regime, variational analysis of the approximate analytical g(2)(x) functions suggests infinite periodicity in g(2)(x). This behavior is in agreement with an earlier study of high density solutions to the Kirkwood equation for hard spheres, where the solutions were obtained by iterative techniques.
Theoretical study of molecular dipole moment functions. I. The X 1Σ+ state of CO
Authors: Kirby-Docken K. , Liu B.
Published: February 2003

Approximate dipole moment functions for the X 1Σ+ state of CO have been calculated using several configuration interaction wavefunctions. The best theoretical results obtained for the dipole derivatives at Re and vibrational transition matrix elements are found to be in quantitative agreement with available experimental data. The theoretical dipole moment functions, in contrast to empirical functions, possess correct long range behavior. Through a detailed comparison of different approximate wavefunctions, useful information has been obtained on the relative importance of different types of correlation effects on the accurate determination of molecular dipole moment functions.
The formation of the LiH molecule on polycrystalline molybdenum
Authors: Wu C. H.
Published: February 2003

The LiH molecule has been observed by desorption when Li(g) and H2(g) permeate through a molybdenum membrane. Li(g) and LiH(g) have the same time constant, and their heats of desorption are 22 kcal/mole. The ratio of the partial pressures of LiH(g) to Li(g) in the desorption is about 140 times higher than that in equilibrium: (pLiH/pLi)desorption[(pLiH/pLi) equilibrium]−1=140. The rate of formation of LiH on polycrystalline tantalum is at least 10 times smaller than that on molybdenum; on gold no formation of this molecule could be observed. It seems that molybdenum is a good catalyst for the formation of the LiH molecule. Further, the possible mechanisms for this formation have been discussed.
A neutron diffraction study of squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione)
Authors: Semmingsen D. , Hollander F. J. , Koetzle T. F.
Published: February 2003

A neutron diffraction study of squaric acid (C4H2O4; 3,4-dihydroxy-3-cyclobutene-1,2-dione) at 25°C has been carried out. Space group P21/m, a=6.143(2), b=5.286(2), c=6.148(2) Å, β=89.96(2) °, Z=2. The structure has been refined with 67 variable parameters on 1026 reflections, comprising one hemisphere of the reciprocal lattice to yield a conventional crystallographic R value of 0.025. An anisotropic extinction correction was applied, and 66 reflections severely effected by extinction were not used in the final cycles of refinement. The squaric acid molecules are linked by strong hydrogen bonds to form infinite planar layers in the crystallographic mirror planes. The hydrogen bond lengths are H⋅⋅⋅O 1.034 and O⋅⋅⋅O 2.554 Å, the O−H⋅⋅⋅O angle is 177.5°. Average intramolecular bond distances are C–O 1.288, C=O 1.229, C–C 1.500 and 1.462, and C=C 1.414 Å.
An EPR and ENDOR study of the low frequency fluctuations and cluster formation in the hydrogen bonded ferroelectrics KH2PO4 and KD2PO4 and the antiferroelectrics NH4H2PO4 and ND4D2PO4
Authors: Dalal N. S. , Hebden J. A. , Kennedy D. E. , McDowell C. A.
Published: February 2003

EPR, ENDOR, and x-ray photoelectron (ESCA) spectroscopy are used here to characterize AsO4−4 and SeO3−4 radicals in KH2PO4, KD2PO4, NH4H2PO4, and ND4D2PO4 in their paraelectric and ferroelectric–antiferroelectric phases. Temperature dependence of the 75As or 77Se hyperfine structure and of the proton superhyperfine structure was used as a microscopic probe for studying low frequency (108–1011 Hz) fluctuations near the phase transitions. Above a certain temperature T*≳Tc, the EPR spectra show (the expected) axial symmetry. Below T*, the spectra exhibit the symmetry of either the ferroelectric or the antiferroelectric phase, and thus provide evidence for the formation of polarized clusters. From an analysis of the temperature dependence of the observed line shapes we deduce that the motion of the hydrogen bond protons is strongly coupled to that of the heavier ions; and the width of this mode is in the megahertz range around T*. This conclusion has been reached by using both probes in the phosphates. For arsenates, however, the SeO3−4 could not be stabilized.
Medium cluster model for vapor pressure isotope effects
Authors: Pollin J. S. , Ishida Takanobu
Published: February 2003

A new model for liquids has been developed which explicitly takes intermolecular interactions into account. The model considers the condensed phase as being represented by a cluster of regularly arranged molecules consisting of a central molecule and a variable number of molecules in the first coordination shell. The methods of normal coordinate analysis are used to determine the modes of vibration of the condensed phase cluster, and the isotopic reduced partition function ratio is calculated by using an appropriate number of normal frequencies of the modes attributable to motions of the central molecule. Clusters of three, seven, nine, and 13 methane molecules in three different mutual molecular orientations have been tested for 13C/12C and D/H substitutions in liquid methane. In the case of methane isotopes, the medium cluster calculations have yielded better agreement with experimental values for the first quantum correction term and the zero-point energy term of the Bigeleisen formula without significantly increasing the number of force constants beyond those needed for simple cell model calculations.
Studies of Rydberg state reactions
Authors: Klots Cornelius E.
Published: February 2003

The reaction mechanisms of high Rydberg and metastable states of He are studied by mass spectroscopy. It is concluded that the former type of reaction is basically a charge-transfer type dictated by the initial electron attachment.(AIP)
Further comment on sudden approximations in rotationally inelastic scattering
Authors: Pack Russell T
Published: February 2003

The quantum-mechanical infinite order sudden (IOS) approximation is further discussed in relation to rotationally inelastic scattering in molecular collisions.(AIP)
Localized and delocalized core holes and their interrelation
Authors: Cederbaum L. S. , Domcke W.
Published: February 2003

When there are several equivalent atomic sites over which a core hole may be delocalized the question arises whether the core hole should be thought of as localized or delocalized. The problem is analyzed in terms of relaxation and correlation effects using the Green’s function approach. It is shown that the relaxation energy of a localized core hole is equal to the sum of the relaxation energy and the change of correlation energy due to relaxation of a delocalized core hole. When delocalized (symmetry adapted) orbitals are used, both relaxation and correlation effects are important and of the same order of magnitude. When localized (nonsymmetry adapted) orbitals are used, the correlation effects are much smaller than the relaxation effects. This explains the good results obtained with ΔSCF calculations employing localized orbitals.
Impact parameter approach to three-dimensional vibrationally inelastic ion (atom) –molecule collisions: Application to H++H2
Authors: Krüger H. , Schinke R.
Published: February 2003

Application of the straight-line impact parameter method is made to scattering of ions (atoms) on molecules at energies Ec.m.≳10 eV. Two different quantum versions of a DECENT-like model are applied to calculate integral and differential excitation cross sections for H++H2 scattering on the lowest diabatic singlet surface. We find good agreement with experimental data and with other theoretical results.
A bimolecular mechanism for ketene photodissociation in the near ultraviolet
Authors: Hase William L. , Kelley Patrick M.
Published: February 2003

The photochemistry of ketene has been studied at 3130, 3340, and 3660 Å. Quantum yields for carbon monoxide, ethylene, ethane, and acetylene were measured versus pressure. As p→0 the carbon monoxide quantum yield approaches 2 at each of the three wavelengths. Ethane and acetylene exhibit linear Stern–Volmer plots at 3130 and 3340 Å. However, at 3660 Å no acetylene was detected and ethane is formed in trace amounts. The above measurements are interpreted in terms of a mechanism which involves adduct formation between the 1A1, and 1A2(1A″) electronic states of ketene.
V–V energy transfer in H2-additive gas mixtures using a stimulated Raman vibrational fluorescence technique
Authors: Miller Richard G. , Hancock J. K.
Published: February 2003

We report H2 V→V transfer rates to a number of simple molecules using a stimulated Raman vibrational fluorescence technique. The additive gases and respective rate constants (sec−1 torr−1) are HCl(1510±210), DCl(689±30), CO2(497±30), N2O(462±14), HBr(224±18), DBr(212±36), NO(42±17), 12CO(12.3±9.5), and 13CO(9.7±1.9). The ease of experimental data collection, even at total pressures as low as 70 torr, shows the utility and general application of this approach in the excitation of Raman active vibrations in simple systems.
Transferability of electron pair wavefunctions. IV. Recreation of the hydrogen peroxide hindered rotation potential
Authors: Howard Robert E. , Levy Mel , Shull Harrison , Hagstrom Stanley
Published: February 2003

Transferability of strongly orthogonal geminals is used to reproduce the hindered rotation potential of hydrogen peroxide. Geminals optimized at one conformation are transferred via a truncation–rotation–reorthogonalization process to a variety of other conformations about the curve. In almost every case, the optimized cis barrier is reproduced to within 10% and the spectrum of torsional levels within the curve constructed by transfer closely resembles that within the optimized curve. Slight errors arising from restricted geometry and from truncation and reorthogonalization of the wavefunction are discussed and assessed.
The generalized cumulant expansion approach to stochastic reductions in molecular collision dynamics: Applications to collisional energy transfer
Authors: Schatz George C.
Published: February 2003

A generalized cumulant expansion method is developed for applying stochastic reductions to molecular collision processes. We begin by introducing an approximate partitioning of a collision system into two subsystems Sr and Si which are assumed to be weakly correlated. Cumulant expansion methods are then used to simultaneously perform a stochastic reduction over Si, and a projection of the diagonal elements of the reduced density matrix for Sr thereby leading to a Pauli master equation describing Sr. We then apply this general equation using an impulse approximation partitioning to problems in inelastic V–T and R–T scattering. For He+H2 vibrationally inelastic collisions, the stochastic theory predicts low order moments and some probabilities in very good agreement with exact quantum results. In applications to He+H2 rigid rotor scattering, integral cross sections and opacity functions within 10%–30% of exact results are obtained at most energies.
Evaluation of the interatomic potential and the interaction dipole moment of rare gas mixtures from their ir spectra
Authors: Bar-Ziv Ezra
Published: February 2003

The interatomic potential and the interaction dipole moment of the rare gases are evaluated directly from their infrared spectra using Laplace transforms. A comparison is made between the calculated dipole moment and the potential and those from the literature.
Transport processes in compressed liquid methanol
Authors: Jonas J. , Akai J. A.
Published: February 2003

The self-diffusion coefficient of liquid methanol–OD has been measured as a function of temperature and pressure over the density range ∼0.8–1.0 g cm−3 and the temperature range 223 – 323°K using the NMR spin-echo technique. The shear viscosities and densities have also been determined under the same experimental conditions. The diffusion data are interpreted in terms of a model based on the modified smooth hard sphere theory corrected for the effects of hydrogen bonding. The reduction of the smooth hard sphere self-diffusion coefficient DSHS as a result of hydrogen bonding is expressed in terms of the parameter αHB. The strongly temperature dependent parameter αHB correlates well with the extent of hydrogen bonding as estimated from the proton NMR chemical shift studies. The Stokes–Einstein relation is found to be valid at the molecular level for liquid CH3OD. Increases in both the density and temperature cause the value of Dησ/kT to a shift from the sticking boundary condition towards the slipping boundary condition.
New spectroscopic constants and RKR potential for the A 1Σu+ state of Na2
Authors: Kaminsky M. E.
Published: February 2003

Analysis of high vibrational level data obtained in modulated population spectroscopy leads to more accurate spectroscopic constants for the A 1Σu+ state of Na2. A new method of solving the RKR integrals leads to quick, accurate calculation of the potential.
The origin of Stark shifts and splittings in molecular crystal spectra. II. Exact solution for the point dipole approximation: Durene and naphthalene
Authors: Chen Freeman P. , Hanson David M. , Fox David
Published: February 2003

An exact analysis, based on the point dipole approximation, of first and second order Stark effects in mixed crystal spectra is generalized to the case of two or more molecules per unit cell. The theory accounts for the anisotropic crystal lattice and all guest–host interactions and polarizability differences. Results of representative calculations for durene and naphthalene host crystals are reported, but existing spectroscopic data cannot be reanalyzed because the effective polarizabilities of the guest molecules in the ground and excited states are completely unknown.
Studies on diamagnetic susceptibility of the C=O group in carboxylic acids and esters
Authors: Gupta R. R. , Jain S. K. , Ojha K. G.
Published: February 2003

Diamagnetic susceptibilities for carboxylic acids, acetates, propionates, and methyl esters are reported. Diamagnetic susceptibility contributions of methylene groups and oxygen atom as C=O in these series are reported. χ0 as C=O depends on the environments in which C=O is present.
Electron affinities of polar molecules
Authors: Crawford Oakley H. , Garrett W. R.
Published: February 2003

All molecules having dipole moments greater than 1.625 D (0.639 ea0) have positive electron affinities in the Born–Oppenheimer approximation (i.e., when the nuclei are stationary). However, when nuclear motion is treated exactly, the above sufficient condition for binding an extra electron is modified. We have determined the magnitudes of Born–Oppenheimer electron affinities which are required in order to insure that the negative ions of polar molecules (μ≳1.625 D) are still stable when the nuclei are free to move.
Variation of chemical shielding with internal coordinates. Applications to diatomic molecules
Authors: Jameson Cynthia J.
Published: February 2003

The derivatives of 19F and 13C chemical shielding with respect to internuclear distance in the following diatomic molecules: F2, ClF, and CO, have been obtained from experimentally observed variation of chemical shifts with temperature in the isolated molecules. A general theory for the temperature dependence of chemical shielding in isolated molecules is given. In all three diatomic molecules considered here, (dσ/dR)e was found to be negative.
The magnetic phase diagram and exchange parameters in NiCl2⋅2H2O
Authors: Swüste C. H. W. , Botterman A. C. , Millenaar J. , de Jonge W. J. M.
Published: February 2003

Magnetization and susceptibility measurements on NiCl2⋅2H2O have been performed for T=1−150 K. From the results it follows that NiCl2⋅2H2O orders antiferromagnetically at 7.2 K. The ordering consists of weakly coupled ferromagnetic chains. At T=6.3 K a zerofield reorientation of magnetic moments occurs which gives rise to a different kind of ordering. In the ordered state the magnetic phase diagram has been measured in three different directions in fields up to 120 kOe. From this complicated phase diagram, which at least can be qualitatively understood, fairly accurate estimates for the exchange constants and single ion anisotropy could be deduced.
On the use of body-fixed coordinates in classical scattering calculations: Planar trajectory approximation for rotational excitation
Authors: Pattengill M. D.
Published: February 2003

The use of body-fixed coordinates in classical scattering calculations is investigated. It is shown that while the exact body-fixed equations are cumbersome, they are amenable to the introduction of simplifying approximations. One such approximation, the assumption of planar trajectories, is investigated in detail and is shown to yield accurate cross sections for rotationally inelastic Ar–N2 scattering.
Formation of ensembles with constraints of coherence
Authors: Procaccia Itamar , Mukamel Shaul , Ross John
Published: February 2003

We discuss the construction of the ensemble density matrix for a system for which off-diagonal elements (coherences) are known and thus are imposed constraints. The density matrix is determined according to maximization of the entropy subject to the imposed constraints. In contrast to the specification of diagonal density matrix elements, the specification of coherences has a significant influence on the unspecified density matrix elements. If the ensemble so formed serves as an initial condition, then the subsequent temporal evolution is affected by the initial specification of the coherences. Connections with experiments are indicated.