bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical study of ion-defect clustering in CaF2:Er3+
Authors: Fenn John B. , Wright John C. , Fong Francis K.
Published: February 2004

The effect of ion-defect clustering in CaF2:Er3+ has been investigated by means of fluorescence spectroscopy and lifetime measurements. The fluorescence spectra of CaF2:Er3+ exhibit a sensitive dependence on the dopant concentration and on the temperature history of the crystal. At low concentrations (≲ 0.01 mole%), only fluorescence lines arising from single Er3+–F− interstitial pairs are observed. As the concentration increases, a large number of new lines are observed. From the explicit dependence of the line intensity on the dopant concentration, the new lines are attributed to the dimerization of the Er3+–F− interstitial pairs. At higher concentrations (> 0.5 mole%), the total observed fluorescence intensity departs dramatically from a linear increase with concentration, indicative of the formation of high-order Er3+–F− interstitial clusters. The dissociation of the dimeric clusters to single pairs occurs readily at ≳ 1200 K. The present findings help to clarify the origin of the frequently observed ``cubic'' site symmetry for the trivalent cation in fluorite crystals.
Vibrational spectra of solid fluorine
Authors: Niemczyk T. M. , Getty R. R. , Leroi G. E.
Published: February 2004

Raman and far infrared spectra of crystalline fluorine have been obtained in both solid phases. As expected for the disordered higher temperature form, only the Raman-active stretching fundamental is observed for β-F2. Near ∼ 20 °K α-F2 exhibits the following spectral features (in cm−1): Raman — ν(1 – 0) 985; ν(2 – 0) 1764; ν(libration) 44, 55, 77, (93?); infrared — ν(translation) 28, 42.5, 80. The predictions of a preliminary calculation of the optically active lattice frequencies for the low temperature phase are in poor agreement with the observed lattice frequencies, suggesting that an improved model will be required to help distinguish the correct α-F2 crystal structure. Solid fluorine more closely resembles oxygen that it does the other halogens. In particular, the intermolecular forces are extremely weak, as exemplified by the small shifts of the internal frequencies from their gas phase values, the absence of observable factor group splitting of the fundamental and the overtone, and the relatively low values of the external (lattice) frequencies.
Phase transitions and end effects in models of biopolymers
Authors: Nordholm K. Sture J. , Rice Stuart A.
Published: February 2004

We investigate, using grand ensemble methods in the thermodynamic limit, the properties of three models simulating the behavior of polypeptides and perfect and imperfect matching polynucleotides. The polypeptide model is found to have no phase transition or significant end effects, while the polynucleotide models may have both. We obtain the presence and order of the phase transition as a function of the parameters of the models. The thermodynamic phase transition which is related to the melting of helical sequences in the long molecule is shown to be accompanied by a structural phase transition consisting in the sudden unwinding of the two strands of the polynucleotide from the ends.
Temperature dependence of the rate constants for the third order reactions: O2++2O2=O4++O2 and O4++2O2=O6++O2
Authors: Payzant J. D. , Cunningham A. J. , Kebarle P.
Published: February 2004

The temperature dependence of the rate constants for the reactions: O2++2O2=O4++O2 (1), O4++2O2=O6++O2 (2), were measured in a pulsed electron beam mass spectrometer in the temperature range 80–300 °K. The values k1 = 2.6 × 10−30 and k2 = 7 × 10−32 cm6 molecule−2 · sec−1 were obtained for 300 °K. Both reactions were shown to be third order. The temperature dependence fitted the relationship k = const T−n; n = 3.2 for Reaction (1) and 5.1 for Reaction (2). The equilibrium constants K1 and K2 could be measured over most of the experimental range. Van't Hoff plots of the equilibrium constants led to an approximate confirmation of ΔH1 and ΔH2 determined earlier by Conway.
Dynamics of the C+–H2 reaction
Authors: Mahan Bruce H. , Sloane Thompsom M.
Published: February 2004

The reaction of C+(H2, H)CH+ and its isotopic variants have been investigated by measuring the velocity vector distribution of product ions formed when a collimated, energy selected beam of C+(2P(1/2)) impinges on a target gas. At relative energies below 4 eV, the reaction product velocity distribution shows a high, but not perfect, degree of symmetry in the barycentric system. The nonreactively scattered C+ shows a very inelastic component in addition to elastic features. The form of the distributions suggests that in many collisions, the system passes through configurations in which all three atoms interact strongly for times which are of the order of one rotational period. At higher relative energies, the product distributions are consistent with a direct interaction mechanism, but indications of strong three-body interactions remain. The behavior is consistent with the electronic state correlation diagram for the system, which shows that the deep (4 eV) potential well of symmetric CH2+ is accessible to the reactants and products.
On the theory of fluorescence decay experiments. I. Nonrandom distortions
Authors: Isenberg Irvin
Published: February 2004

A number of significant nonrandom instrumental errors in fluorescence time decay experiments may be automatically corrected by an analytical procedure that is called moment index displacement (MD). An MD of order n is defined as the process of calculating the decay constants from the set [Gn + 1, Gn + 2,..., Gn + 2ν] instead of [G1, G2,..., G2ν]. An MD of any order will automatically yield correct decay parameters for any size of light scattering error. Discrepancies between the zero point of the measured exciting light and the measured fluorescence are also corrected by MD, as are errors due to lamp drift.
Magnetic dipole moment integrals over Slater orbitals
Authors: Ichimura Hideo , Rauk Arvi
Published: February 2004

General expressions for all magnetic dipole moment integrals over Slater-type orbitals are presented in a form suitable for ready computation. The two-center integrals are expressed concisely in terms of the C functions introduced by Ruedenberg, Roothaan, and Jaunzemis.
Selection rules for the intermolecular enhancement of spin forbidden transitions in molecular oxygen
Authors: Long Charles , Kearns David R.
Published: February 2004

An experimental examination of the spin forbidden transitions of molecular oxygen dissolved in various solvents and in the gas phase reveals the following selection rules governing the intermolecular enhancement of these transitions: (i) Nonheavy atom solvents selectively enhance the 1Δg← 3Σg− transition (factor of 1000) but not the 1Σg+← 3Σg− transition; (ii) the bimolecular oxygen enhancement of the 1Δg← 3Σg−(0,0) transition is selectively suppressed in solution by a factor of ∼ 3; (iii) heavy atom solvents enhance the 1Σg+← 3Σg− transition (factor of 10 in iodine containing solvents) but not the 1Δg← 3Σg− transition. The possible origins of these different selection rules are discussed in terms of previous theoretical treatments of the interaction of oxygen with electronically excited molecules.
Characteristic energy losses by slow electron impact on thin-film alkanes at 77 °K
Authors: Hiraoka Kenzo , Hamill William H.
Published: February 2004

Characteristic energy losses by slow electron impact have been measured on thin films of several alkanes at 77 °K. The cross section for the lowest loss at [inverted lazy s] 0.6 eV increases tenfold in the series n -pentane to n -nonane with increasing carbon number. For a given carbon number it decreases markedly with branching. Losses at [inverted lazy s] 3, [inverted lazy s] 6, [inverted lazy s] 9, and [inverted lazy s] 22 eV are common to all alkanes with no strong dependence on molecular size or configuration. The lowest losses may be due to temporary negative ion states or to pre-existing trapping potentials. Losses at [inverted lazy s] 3 eV may be due to the highest temporary negative ions or to triplets. If the former is correct, then [inverted lazy s] 6 eV losses can be attributed to triplets. The [inverted lazy s] 9 eV losses are due to upper singlet states, and the [inverted lazy s] 22 eV losses may be due to collective excitations.
Fermi-contact hyperfine interaction and the electron spin g factor in H2 d(3p) 3Πu
Authors: Freund Robert S. , Miller Terry A.
Published: February 2004

Microwave optical magnetic resonance induced by electrons (MOMRIE) experiments have been continued on the d(3p) 3Πu states of ortho- and para-H2. Transitions that would at the high magnetic field limit be forbidden have been observed at low field (∼ 2 kG). Four such transitions in ortho-H2 and two in para-H2 have been accurately measured. Previously MOMRIE transitions within these states of H2 had been observed in the 12–15 kG range where selection rules permit only transitions which are insensitive to the electron spin g factor gs and the Fermi-contact hyperfine interaction aF. The newly observed transitions are sensitive to these parameters and when combined with previous high field data determine gs in the ν = 0–3 vibrational levels of ortho- and para-H2 and aF in the ν = 0–3 vibrational levels of ortho-H2. The values of aF are comparable to those for the c(2p) 3Πu state of H2 and the X 2Σg+ state of H2+, but interestingly (for a given ν) are larger than either value. The values of gs are found to be close to the free electron spin g factor for all the measured vibrational levels of ortho- and para-H2. However, they show a slight deviation to the high side of the free electron value, comparable to the experimental error.
Photoionization of atomic oxygen from 920 to 650 Å
Authors: Dehmer P. M. , Berkowitz J. , Chupka W. A.
Published: February 2004

Atomic oxygen was produced in a low pressure gas discharge and was equilibrated in a small chamber to eliminate electronically excited species prior to the formation of an atomic beam which was photoionized. Relative photoionization cross sections were measured from 920 to 650 Å at a wavelength resolution (FWHM) of 0.42 Å. From the ratio of threshold intensities corresponding to ionization from the 3P 2, 3P 1, and 3P 0 states of OI, the beam temperature was estimated to be 375 ± 25°K. Autoionization structure was observed corresponding to all of the transitions seen in absorption by Huffman, Larrabee, and Tanaka, including those transitions for which autoionization is forbidden on the basis of Russell-Saunders coupling. The seven lowest energy autoionizing lines were measured at a wavelength resolution (FWHM) of 0.16 Å. The distribution of intensities within a multiplet corresponded to the theoretical intensities predicted for an approximately 375°K beam in all but two cases. These notable exceptions were the two most intense autoionization peaks in the spectra, both of which were forbidden in pure Russell-Saunders coupling. The anomalous intensity distributions may be attributed to competition from emission which preferentially robs intensity from selected transitions within a multiplet, to nonstatistical autoionization probabilities, or to perturbations which alter the relative absorption intensities within a multiplet.
Magnetothermodynamics of antiferromagnetic, ferroelectric, ferroelastic β-Gd2(MoO4)3. IV Thermodynamic temperature and other properties without heat introduction below 0.5°K. Fields to 10 kG along the c+ crystal axis
Authors: Fisher R. A. , Hornung E. W. , Brodale G. E. , Giauque W. F.
Published: February 2004

Using our previous magnetothermodynamic data on electrically polarized β-Gd2(MoO4)3 with high fields along the c axis [J. Chem. Phys. 56, 193 (1972)] above 0.5°K as a basis, measurements of magnetic moment on 62 demagnetization-remagnetization isentropes were used to extend the properties to the region below 0.5°K. The isentropic change in enthalpy H was evaluated by means of the magnetic work (d H = − M d H)s. The thermodynamic temperature was obtained from T=(∂ H/∂ S)H. Thermodynamically correlated values for entropy, enthalpy, magnetic moment, internal energy, and heat capacity are tabulated over the temperature range 0.1–0.5°K and at fields of 0–10 000 G. At fields below 1615 G the substance remained in an essentially antiferromagnetic condition. At limiting low temperatures and fields of 0, 500, and 1000 G, the enthalpy varied as the 4/3 power of the entropy (C ∝ T3). Above 1615 G there was a region in which the classical dipole field balanced field increases as the transition continued gradually and reversibly toward saturation paramagnetism without abrupt spin reorientation (i.e., no irreversible ``spin flop''). In the region from about 1.7–2.9 kG, the limiting value of enthalpy varied approximately as the 6/5 power of the entropy. This indicates that the isoerstedic heat capacity C H = 5S = constT5. It is pointed out that this implies five active degrees of freedom for the dipoles at the limiting low temperatures. Also, in the above region (∂M/∂ S)H ≈ 0, while S, T, and CH were essentially independent of field.
High temperature negative ions: Electron impact of As4 vapor
Authors: Bennett S. L. , Margrave J. L. , Franklin J. L. , Hudson J. E.
Published: February 2004

Appearance potentials and ion translational energies have been measured for the negative ions As−, As2−, and As3− formed by dissociative resonance capture of As4. The following thermochemical values (all in kilocalorie/mole) have been obtained: Δ Hf,298o(As2−)=43.1± 4.5; Δ Hf,298o(As3−)=38.3± 4.3; E.A.(As2)=2.4± 4.1; E.A.(As3)=19.3± 8.2 and Δ Hf,298o(As3)=57.6± 3.9. The measurement of the increase in translational energy through resonance gave α = 0.43 for the three processes studied.
Vibrational relaxation in polyatomic molecules: SF6
Authors: Breshears W. D. , Blair L. S.
Published: February 2004

Vibrational relaxation in mixtures of 5%, 10%, and 25% SF6 in Ar has been studied behind incident shock waves over the temperature range 450–1050 K by time-resolved measurement of postshock density gradients using a laser beam deflection technique. Short extrapolation of the results obtained yields excellent agreement with ultrasonic data in the range 200–420 K. Contrary to original reports, results from infrared fluorescence and double resonance measurements at 300 K are shown to be in disagreement with the ultrasonic data. A discussion of relaxation behavior in polyatomic molecules under differing conditions of departure from equilibrium shows that such a discrepancy is to be expected in general.
Multiple site effects on the Raman spectra of RbNO3 and CsNO3
Authors: Brooker M. H.
Published: February 2004

Multiple band structure in the symmetric stretching region of the Raman spectra of RbNO3 IV and CsNO3 II confirms the presence of NO3− ions on three distinct crystallographic C1 sites and supports the proposed C32 or C33 space group. Assignments were confirmed by 18O-isotopic dilution studies.
Excitation of metastable N2(A 3Σu+) vibrational levels by electron impact
Authors: Borst Walter L. , Chang Sue L.
Published: February 2004

Absolute cross sections for the excitation of individual vibrational levels of the metastable A 3Σu+ state of nitrogen by electron impact were calculated using recent experimental data for the direct excitation of the A 3Σu+ state. The calculations include direct excitation as well as fast cascade contributions from the B 3Πg and C 3Πu states of N2. The data input to the calculations consists of cross sections for direct excitation of the entire A, B, and C states, Franck-Condon factors for the transition from the N2(X 1Σg+,  v = 0) ground state to the vibrational levels of the A, B, and C states, and transition probabilities for the transitions C → B and B → A. The results obtained consist of graphs displaying the direct, cascade, and total cross sections of the A state both as a function of electron energy as well as vibrational quantum number. Electron energies range from threshold near 6 eV to 45 eV and vibrational quantum numbers range from ν = 0 to ν = 22. The results show that the population rates for the lowest vibrational levels of the A state are dominated by cascade, whereas the higher vibrational levels are directly excited from the N2 ground state. In general, the lowest vibrational levels of the A state are most efficiently populated by monoenergetic electrons and the nonmonoenergetic electron fluxes which are encountered in the aurora. Applications of the present results to processes in the upper atmosphere are briefly discussed.
Electron spin resonance of chlorine dioxide in inert matrices at 4.2°K
Authors: McDowell C. A. , Raghunathan P. , Tait J. C.
Published: February 2004

The electron paramagnetic resonance spectrum of ClO2 trapped in neon, argon, and krypton matrices has been investigated at 4.2°K. The linewidths of the polycrystalline spectra were narrow enough so that forbidden transitions due to the nuclear Zeeman and nuclear quadrupole interactions could be resolved. The g, A, and Q tensors for each matrix were determined by comparison between experimental and computer simulated spectra. Partial orientation has been observed in all three matrices; the preferred orientation is with the molecular plane parallel to the deposition surface. Upon rotating the magnetic field to a position where it is either parallel or perpendicular to the deposition plane, marked differences in line intensities were apparent and an estimate of an orientation distribution function was made from comparison with simulated spectra. Annealing the argon matrix led to the appearance of a second trapping site with slightly different hyperfine and g tensors. This site is thought to be a substitutional one. The shifts of the hyperfine and g tensors in the three matrices have been qualitatively discussed in terms of van der Waals and Pauli interaction forces.
The Dufour effect
Authors: Ingle Sara E. , Horne Frederick H.
Published: February 2004

Equations for the Dufour effect, which is the development of a temperature gradient due to diffusion, have been solved for geometrically well-defined cells which have either all walls adiabatic or adiabatic lateral walls and diathermic ends. Two self-consistent, well-ordered perturbation schemes have been used, and heat of mixing, variability of all properties, and the barycentric velocity are included explicitly. For typical nonelectrolytes, the temperature difference produced by the Dufour effect could be as large as 0.2 deg for a diffusing mixture with initial mass fraction difference of 0.8. These results can be used (1) to design experiments to test the Onsager heat-matter reciprocal relation and (2) to avoid undesired temperature variations in diffusion experiments.
Role of sulfur d orbitals in isothiocyanic acid and an electronic-structure comparison with isocyanic acid
Authors: Howell James M. , Absar Ilyas , Van Wazer John R.
Published: February 2004

The electronic structure of isothiocyanic acid, HNCS, was determined by an ab initio LCAO-MO-SCF calculation. Isocyanic acid, HNCO, was also calculated and the two molecules are compared. The basis set consisted of uncontracted, atom-optimized Gaussian functions, with nine s plus five p exponents to describe the sulfur and five s plus two p apiece for the carbon, nitrogen, and oxygen. The hydrogen was represented by three s-type functions. An additional calculation of HNCS was carried out by adding a d set to the description of the sulfur. The molecular orbitals of HNCS are analyzed using simple perturbation theory applied to the localized valence-bond functions and the SCF orbitals are displayed using planar electron-density plots. The electronic structure of HNCS is compared with that of HNCO and it is concluded that (1) the π system in HNCS involves a nitrogen lone pair stabilized by a higher-lying C–S π bond while the π system of HNCO consists of a C–O π bond stabilized by the higher-energy nitrogen lone pair and (2) the d orbitals of sulfur are accepting density in a σ rather than a π fashion.
ESR spectrum and the theory of zero field splitting for the exchange coupled Cu2Cl62− dimers in φ4AsCuCl3
Authors: Chow C. , Willett R. D.
Published: February 2004

Electron paramagnetic resonance has been observed in φ4AsCuCl3 at room temperature. The spectrum is characteristic of a spin one system. Spin Hamiltonian parameters are gx = 2.08, gy = 2.11, gz = 2.32, D = −0.111 cm−1, and E = 0.009 cm−1. The principal axes of the g and D tensor are coincident. The rhombic spin Hamiltonian is a reflection of the distorted tetrahedral environment of the Cu2+ ions. An expression for the angular dependence of the D tensor components with arbitrary field rotation is developed. The usual method of obtaining the principal axes is discussed. The results from magnetic data compare favorably with the predictions based on structural analysis. The spin Hamiltonian can be rationalized on the basis of a chloride bridged binuclear species. Low temperature spectra show profound increase in intensity but little or no change in line positions. The Cu2+ ions are pairwise coupled to give a triplet ground state and a singlet excited state at 36 cm−1.
Diatomic interhalogens: Systematics and implications of spectroscopic interatomic potentials and curve crossings
Authors: Child M. S. , Bernstein R. B.
Published: February 2004

Spectroscopically derived potential curves for the low-lying excited states of the homonuclear and heteronuclear diatomic interhalogens are systematized by taking cognizance of the spin-orbit state of the dissociation products. Observed regularities lead to the prediction of a bound B (3Π0+) state of F2 with r′e ≃ 1.9 Å, D′e ≃ 3300 cm−1. Several of the heteronuclear interhalogens show evidence of a strong interaction between the B (3Π0+) state and a repulsive Y (O+) state, leading to the formation of a new B′(O+) state. An analysis of these interactions has led to the construction of sets of diabatic potentials consistent with the best estimated adiabatic curves and thus to the determination of the crossing points rx and interaction strengths V12(rx). The patterns of V12(rx) values suggest that they depend mainly upon the spin-orbit coupling constant of the excited atomic dissociation product. Reported also are repulsive segments of potential curves deduced for the Y (O+) states for a series of 1Π states and a few higher-lying (Z) states, the latter two series from analysis of existing continuum absorption data. Repulsive interactions in all four families of states studied [3Π0+, 1Π1, Y(O+), and Z] show clear regularities governed by steric effects. Implications of the present results with respect to the theory of interatomic forces, predissociation phenomena, and the scattering of ground state halogen atoms are discussed.
Even-tempered atomic orbitals. II. Atomic SCF wavefunctions in terms of even-tempered exponential bases
Authors: Raffenetti Richard C.
Published: February 2004

Atomic self-consistent-field calculations in terms of even-tempered exponential bases are presented. Accuracy with respect to the Hartree-Fock limit is examined as a function of basis size. The restrictions which distinguish even-tempered bases from the usual Slater-type bases, and enhance their utility, are found to have no deleterious effect on the performance in terms of obtainable results. Optimal bases for many atoms are determined and tabulated for use in molecular computation.
Even-tempered atomic orbitals. III. Economic deployment of Gaussian primitives in expanding atomic SCF orbitals
Authors: Bardo Richard D. , Ruedenberg Klaus
Published: February 2004

Expansion of atomic self-consistent-field orbitals in terms of Gaussian-type primitive AO's calls for the most economic deployment and efficient use of all available basis functions to avoid computational waste. In even-tempered representations maximal use of all primitives in representing all SCFAO's is ensured. Here, the optimal number of primitives per individual SCFAO, consistent with a given total number of primitives is investigated. Maximal effectiveness is found to occur when the following conditions prevail: (1) The number of primitives per SCFAO is approximately the same for all SCFAO's, say x; (2) the number of primitives needed to express the set of SCFAO's with angular-momentum quantum number l in atom A is approximately given by Nl(A) = x[a + byl(A)] where a and b are constants and yl(A) is the number of different SCFAO's with quantum number l in atom A. Thus the efficiency rapidly increases with yl(A). Explicit results and optimal expansions based on 3–14 primitives are discussed for all atoms up to krypton. The achieved systematization permits the unambiguous selection of economic even-tempered expansions for molecular calculations.
Even-tempered atomic orbitals. IV. Atomic orbital bases with pseudoscaling capability for molecular calculations
Authors: Bardo Richard D. , Ruedenberg Klaus
Published: February 2004

Even-tempered atomic orbital bases are formulated which have the property that atomic orbital scaling can be closely simulated through variation of linear expansion coefficients. This ``adaptation to pseudoscaling'' involves two types of adjustments: (1) Optimal orbital exponents and basis sizes are determined for the even-tempered exponential or Gaussian primitives and (2) optimal linear combinations of even-tempered Gaussian primitives are found to serve as a ``reduced basis of contracted functions.'' The efficiency of the method is discussed.
Reactions of H*(22P–22S) + H2 and their isotopes
Authors: Van Volkenburgh Gregg , Carrington Tucker , Young Robert A.
Published: February 2004

The four possible H- and D-substituted reactions in the title have been studied at room temperature using an optically thin flow system. An experimental cross section for total collisional deactivation in each reaction has been measured by analyzing the quenching of Lyman-α fluorescence; the results cover a range of 84–91 Å2, with an individual uncertainty of 5%. An experimental cross section for associative ionization has also been measured for each reaction using ion collection techniques; these results cover a range of 0.82–1.10 Å2±5%. For the reactants H*(n=2)+D2, the experimental cross section for producing D*(n=2)+HD is 0.28 Å2±25%. It has been measured by analyzing H and D Lyman-α fluorescence signals which are observed simultaneously. The product channels observed and the other exothermic channels (for example HDvib+D, and H+D+D) are discussed in terms of intersections of qualitative potential energy surfaces, and it is suggested that nearly all quenching produces three ground state atoms. Collisional interconversion of the 22P and 22S states of H*(n=2) and D*(n=2) is also discussed. Because of this process, each of the experimental results is related to a pair of reactions-one involving the 22P state and the other the 22S state.
Iterative unfolding of intensity data, with application to molecular beam scattering
Authors: Siska P. E.
Published: February 2004

An efficient iterative technique is presented for solving the Fredholm integral equation of the first kind, h(x) = ∫abdx′ g(x, x′)f(x′), which occurs in various forms in experimental intensity measurements; h is the measured intensity distribution, f is the ideal distribution, and g is a bandpass function representing apparatus resolution. The scheme is based on the equations f0(x) = h(x) and fn+1(x) = fn(x)[h(x)/∫dx′g(x, x′)fn(x′)] and is shown to result in a nonnegative distribution free of diffraction effects. The method is easily extended to multidimensional abscissas x and multidimensional integrals, and an application to molecular beam data reduction for doubly differential reaction cross sections is presented.
Separation of isotopically substituted liquids in the thermal diffusion column
Authors: Rutherford W. M.
Published: February 2004

Isotopic separation of several liquid compounds was studied experimentally in a precisely constructed, 60-cm thermal diffusion column at wall spacings of 0.254 and 0.383 mm. The systems studied were benzene, cyclohexane, carbon disulfide, 1-chloropropane, and bromoethane. The measurements for benzene and cyclohexane included the C6H6–C6D6 and the C6H12–C6D12 pairs. The experiments were done under both static and flowing conditions in order to get experimental values of the initial transport coefficient and the remixing coefficient of the column for each system. The experimentally determined remixing coefficients were found to agree with those predicted from the theory of the thermal diffusion column. Experimental values of the thermal diffusion factor were obtained from the initial transport coefficients. A simple mass-dependent relationship was found to be inadequate to describe the results even for different isotopic substitutions within the same compound. Results for the C6H6–C6D6 pair were found to agree quite well with recalculated results based on the early work of Korsching.
Energy distribution of photoelectrons emitted by solutions: Theoretical analysis
Authors: Nemec Ladislav
Published: February 2004

A general equation is derived for energy distribution curves (EDC) of photoelectrons emitted in vacuum by solutions of an emitter dissolved in a liquid. The following features are assumed: quasifree electrons, upon generation by photoionization, are distributed according to their kinetic energy; scattering of quasifree electrons in the liquid entails a loss of kinetic energy; the threshold function for the liquid-vacuum interfacial barrier is not considered. Some properties of EDC's are deduced, and explict forms of the EDC equation are examined for a Gaussian distribution of quasifree electrons upon generation and for two specific energy loss laws: constant loss in time and loss inversely proportional to the speed of quasifree electrons. Analysis based on superposition of shifted and normalized experimental EDC's is also examined.
Vibrational deactivation of O3(101) molecules in gas mixtures
Authors: Rosen David I. , Cool Terrill A.
Published: February 2004

The laser excited fluorescence method has been employed to determine rate constants for vibrational relaxation of O3(101) molecules by O3, O2, CO2, H2, D2, CH4, N2, He, and Ar collision partners at 298°K. Excitation of the O3(001) level by absorption of laser transitions on the CO2 9.5 μ band have permitted the first measurements of vibrational relaxation in O3.
Crossed molecular beams study of the reaction F + {C2H2Cl2} → Cl + {C2H2ClF}
Authors: Shobatake Kosuke , Lee Yuan Tseh , Rice Stuart A.
Published: February 2004

We report studies of the reactions of F atoms with cis-1,2-, trans-1, 2- and 1, 1-dichloroethylene by the method of crossed molecular beams. We find that for the products of Cl emission the recoil energy distributions, deduced from product laboratory angular distributions, are consistent with existence of a reaction complex in which almost all vibrations participate in the partitioning of energy.
Monte Carlo studies of lattice-model polymer chains. III. Relaxation of Rouse coordinates
Authors: Verdier Peter H.
Published: February 2004

The relaxation of the seven lowest Rouse coordinates for simple lattice models of polymer chains of up to 64 beads, with and without excluded volume, is studied by simulation on a digital computer. The similarity between the relaxation of the lattice-model chains without excluded volume and that of a statistical-bead model, noted in previous studies of end-to-end length, is confirmed and examined in greater detail. The effect of excluded volume in slowing down the relaxation of the Rouse coordinates is examined, and a simple picture is suggested which accounts qualitatively for the results obtained. The nonnormal coordinate nature of the Rouse coordinates for chains with excluded volume is demonstrated by their nonexponential autocorrelation functions. However, the results suggest that for each chain length, there is a unique longest internal relaxation time, corresponding to an internal coordinate closely resembling the lowest Rouse coordinate.
V-V and V-T/R energy transfer studies of C2H4 by infrared laser double resonance
Authors: Yuan Raymond C. L. , Preses Jack M. , Flynn George W. , Ronn A. M.
Published: February 2004

Multiple exponential decay of the ν7, ν = 1 state of ethylene has been observed using laser double resonance. The slower measured rate is in excellent agreement with the results of fluorescence experiments and is consistent with a mechanism involving vibration-translation/rotation energy transfer in C2H4. The fast measured rate appears to arise from equilibration of the ν7, ν = 1 state with nearby excited vibrational states. Large differences in rates have been observed for this fast process during C2H4–C2H4 collisions versus C2H4-rare gas collisions. The temperature dependence of the slow decay rate has been measured in ethylene-rare gas mixtures and the deactivation probabilities found to be in general agreement with predictions based on Landau-Teller theory.
Rare-earth sulfides at high temperature. I. On the existence of equilibrium in Knudsen cells
Authors: Wu Richard L. C. , Gilles Paul W.
Published: February 2004

Two different approaches have been used to investigate whether gaseous equilibrium exists above solid mixtures and to study whether measurements of equilibrium constants can be accomplished inside a Knudsen cell. Both were mass spectrometric studies of isomolecular exchange reactions of rare-earth sulfides. The first was a study of a single reaction generated in different chemical systems containing different condensed reacting phases and thus yielding different sulfur chemical potentials. The second was a study of a reaction in the presence and absence of a third condensed substance. The results demonstrated that gaseous equilibrium was achieved in the cell for these particular systems. The opinion is expressed that similar studies should be performed on chemical systems being used for studies of isomolecular exchange reactions. In order that measurements on such reactions can be reliably made, the solids should have comparable volatility, and the gases should have comparable dissociation energies.
Effect of thermal energy on dissociative charge transfer
Authors: Lifshitz C. , Tiernan T. O.
Published: February 2004

Dissociations of the molecular ions of neopentane, 2-methylpentane, 2,3-dimethyl butane and n-octane, formed in charge-exchange collisions with Hg+, were studied using an in-line tandem mass spectrometer. The effect of collision chamber temperature upon the observed dissociations was examined. The results are in good agreement with previous calculations, based on the quasiequilibrium theory (QET) for unimolecular reactions and on the alkane photoionization data.
WKB approximations in inelastic collision theory
Authors: So/rensen Gert Billing
Published: February 2004

We have investigated the WKB approximation in inelastic scattering theory. Numerical results are used to comment on some approximations frequently used when deriving the classical trajectory equations or simplified expressions for the transition probabilities. We have furthermore shown that a simple choice of a mean trajectory in the classical trajectory equations gives very good results.
Light emission of electron impact excited hydrogen molecules and the dependence of electronic transition moment on internuclear distance
Authors: Schmoranzer H. , Geiger J.
Published: February 2004

By studying the intensity of light emitted by hydrogen molecules, which have been excited by keV electrons, the dependence of the electronic transition moment on internuclear distance can be determined over a wide region. Experimentally it was found for the Werner band system that the quantity ``transition energy times electronic transition moment'' is a constant between 0.6 and 1.6 Å as predicted by theory. Several vibrational bands exhibit strong intensity perturbations, especially the 3 → ν″ progression.
Physical adsorption of argon on boron nitride. A two-surface analysis of high-temperature adsorption data
Authors: Thomas H. E. , Ramsey R. N. , Pierotti R. A.
Published: February 2004

A study has been made of the low-coverage physical adsorption of argon on hexagonal boron nitride using a precision volumetric apparatus. Isotherms are reported at six temperatures in the range 198–273°K at surface coverages of less than about 3% of the BET monolayer volume. Electron micrographs indicate that two crystalline surfaces are exposed, corresponding to the basal plane and crystal edges. The isotherm data exhibit characteristics of adsorption on a slightly heterogeneous surface and are best-fitted to an adsorption model in which a two-energy surface is assumed. Adsorption is assumed to obey Henry's law on one surface and a submonolayer model isotherm on the other. Various models are used and compared and the data analyzed to yield values for the gas-solid interaction energy and surface area for the two surfaces. Estimates are also given for the interaction energy of neon, krypton, and xenon with boron nitride. Comparisons are made between theoretical and experimental values for the interaction energy of the inert gases with graphite and boron nitride.
Mechanism of CO fourth positive v u v chemiluminescence in the atomic oxygen reaction with acetylene. Production of C(3P, 1D)
Authors: Fontijn Arthur , Johnson Stanley E.
Published: February 2004

CO(A 1Π−X 1Σ) emission from the O/C2H2 reaction has been studied in a room temperature discharge flow system at pressures from 0.6 to 10 torr. Spectrometric measurements showed that CO(A 1Π) excitation extends to at least ν′ = 12, with anomalously large populations of the ν′ = 1, 4 and 6 levels. The overall emission intensity increases with increasing [Ar], [N2], and [He]. The presence of C(3P and 1D) atoms has been observed via resonance fluorescence. The probable CO(A 1Π)-formation mechanism is O + C2O → CO* + CO, followed by collision-induced cross relaxation: CO* + M → CO(A 1Π) + M, where CO* is identified as a CO state having a potential energy curve which overlaps that of CO(A 1Π), i.e., the d3Δ, e3Σ, a′3Σ and/or D1Δ states. The hypothetical reaction C + O + M → CO(A 1Π) + M does not populate CO(A 1Π) to a significant degree.
Electronic structure and optical index damage of iron-doped lithium niobate
Authors: Clark M. G. , DiSalvo F. J. , Glass A. M. , Peterson G. E.
Published: February 2004

The properties of Fe2+, Fe3+, and other 3dn ionic impurities in the LiNbO3 structure are investigated with a view to elucidating the mechanism of laser-induced reversible refractive index damage. Polarized optical absorption spectra of Fe2+ in LiNbO3 and LiTaO3 are reported and interpreted. The 2.66 eV band responsible for laser damage in Fe-doped LiNbO3 is identified as Fe2+ → Nb5+ intervalence transfer. The Fe d ε level lies in the 3.7 eV band gap at about 0.6 eV below the conduction band, and the activation energy for thermal Fe2+ → Nb5+ electron transfer is estimated to be about 0.9 eV, in reasonable agreement with the value of 1.3 ± 0.2 eV observed for thermal bleaching of Fe2+ centers in x-irradiated LiNbO3. Particular advantages of intervalence transfer as a mechanism for initiating optical index damage are noted. The observed electric field gradients in pure LiNbO3 and LiTaO3 are related to the B20 ligand-field parameter of a 3dn (n ≠ 5) impurity ion. Comparison with experimental estimates of B20 may provide an indication of the site of the impurity ion, as in the case of Cr3+ which appears to favor the Nb/Ta site. Magnetic susceptibility measurements in the range 4.2–300°K are reported for oxidized and reduced Fe-doped LiNbO3. It is demonstrated that susceptibility studies may be used to estimate the concentrations of Fe2+ and Fe3+ in LiNbO3, and that in reduced LiNbO3 the Nb d ε electrons show Pauli paramagnetism characteristic of metallic behavior. It is argued that the system will be nonmetallic at the lower conduction electron concentrations encountered during optical damaging and bleaching of unreduced crystals, and that low electron mobility may be a significant factor in the thermal processes observed in optically damaged LiNbO3.
Ab initio studies of interoxygen bonding in O2, HO2, H2O2, O3, HO3, and H2O3
Authors: Blint Richard J. , Newton Marshall D.
Published: February 2004

The nature of the interoxygen bonding in O2, HO2, H2O2, O3, HO3, and H2O3 has been investigated on the basis of ab initio (LCAO-MO) and experimental force constants, bond lengths, and energies. The fact that small basis sets appear to give the OO force constants for H2O2 and HO2 in the order opposite to that observed prompted an analysis of the sensitivity of the above properties with respect to various types of contracted Gaussian-type orbitals (CGTOs). A large basis set of s and p CTGOs, [43/2] is found to give proper qualitative account of OO bond lengths and force constants. However, polarization functions (3d functions on oxygen) are necessary for accurate calculation of relative bond energies in H2O2 and HO2. The species HO3 is estimated to be [inverted lazy s] 15 kcal/mole unstable with respect to O2 + OH, in agreement with empirical estimates, thus making unlikely its potential role as a reaction intermediate. Hydrogen trioxide (H2O3) is calculated to have OO bonds slightly shorter ([inverted lazy s] 1.44 Å) than in H2O2 (1.48 Å), but with OO force constants similar in magnitude to H2O2, and a large OO stretching interaction force constant. The latter fact, in conjunction with spectral data from analogous systems, would be expected to make the symmetric stretch frequency in H2O3 larger than that for the antisymmetric mode, and this conclusion is discussed in the context of recent experimental data of Giguère et al., which is attributed to H2O3 and H2O4. The preferred anti conformation (C2 symmetry) of the OH bonds in H2O3 is noted as being potentially relevant to the conformation of intermediates in the ozonolysis of olefins. HO2 is calculated to have a very low lying (< 1 eV) excited state (2A′), as suggested by other workers, for which we estimate an OO bond length of 1.48 Å. Variations in OO bonding strength are analyzed in terms of Pauling bond orders and π-electron density matrix elements. Force constants are not always found to vary monotonically with bond length, and the distinction between symmetrized and unsymmetrized force constants is emphasized in this connection.
Fully quantum study of near resonant D2-D2 vibrational energy transfer
Authors: Alexander Millard H.
Published: February 2004

We have carried out a detailed theoretical study of the 02–11 and 03–12 near-resonant vibrational energy transfer processes which can occur during the collision of two D2 molecules. Assuming a collinear geometry, we have solved the collision dynamics fully quantum-mechanically, using a modified version of the Gordon program. We have investigated three different model potentials and one full ab initio potential surface. Both at high and low collision energies, the V–V transition probabilities are extremely sensitive to the particular potential surface used. We further show that nearest-atom model potentials greatly overestimate transition probabilities at high energies. Also of interest is the result that at moderate to high energies near-resonant V–V transfer probabilities are largely insensitive to the choice of a harmonic or Morse oscillator description of the two colliding molecules. Finally we show that the simple two-state approximation appears to be remarkably accurate over a wide range of energies.
Proton spin echo study of spin-spin relaxation in liquid crystals
Authors: Dong Ronald Y. , Wiszniewska M. , Tomchuk E. , Bock E.
Published: February 2004

Proton spin-spin relaxation times ``T2'' were measured as a function of temperature in the nematic phase of p-methoxybenzylidene-p-n-butylaniline and in the nematic and smectic C phases of p-n-heptyloxyazobenzene using the 90°–45° echo. By comparing ``T2'' in these two liquid crystals, we infer that short range smectic order exists even 10° above the nematic-smectic C phase transition in p-n-heptyloxyazobenzene. The angular dependence in ``T2'' was also determined in the smectic C phase at two different temperatures.
Dissipative structures in chemical oscillations with concentrations-dependent frequency
Authors: Smoes Marie-Luce , Dreitlein Joseph
Published: February 2004

A general model for the propagation of chemical waves is proposed for a given class of chemical oscillations. This model gives a satisfactory qualitative account of the dissipative structures observed in the Belousov-Zhabotinskii reaction. The generality of this model makes it useful for predicting the possibility of chemical waves when the experimentally observed oscillations have a concentration-dependent frequency.
Classical trajectory study of the K + CH3I reaction
Authors: LaBudde R. A. , Kuntz P. J. , Bernstein R. B. , Levine R. D.
Published: February 2004

Classical trajectory calculations have been carried out in an attempt to simulate the reactive scattering of K by RI (R≡CH3). Several semiempirical three-body potential surfaces have been explored. An electron-jump surface of the covalent-ionic type, with considerable ``repulsive energy release,'' accounts for most of the gross features of the scattering behavior. In particular, it has provided an understanding of the maximum in the reactive cross section σR (E) for the exchange reaction, which can be attributed to recrossing from the ionic to the covalent region of the potential. The present calculations also predict the onset (above 2.5 eV) of collision-induced dissociation (CID), and show the conservation of total reaction cross section as CID takes over from the exchange reaction.
Classical trajectory study of exothermic ion-molecule reactions
Authors: Kuntz P. J. , Roach A. C.
Published: February 2004

Classical trajectory studies of reactions A+ + BC → AB+ + C (BC = D2 or HD) over a wide range of collision energy E and on a number of exothermic potential energy surfaces show the dependence of the average translational exoergicity on reactant and product masses and on E to be nearly surface independent and of a form agreeing both with experiment for A = Ar and with the predictions of a number of direct interaction models. It is argued that this behavior may be a consequence of the relative lightness of BC and is not a sensitive indicator of the collision dynamics. Forward peaked angular distributions are observed on all surfaces at all energies, and at high energy a migration mechanism is demonstrated to contribute to the simultaneous occurrence of forward scattering and large momentum transfer to the product atom. There is no tendency for the product translational energy distribution to narrow at high energy, as would be expected if a stripping mechanism were approached.
Electronic states of imino radicals formed from the vacuum-ultraviolet photolysis of ethylenimine
Authors: Kawasaki Masahiro , Iwasaki Masahiro , Tanaka Ikuzo
Published: February 2004

The direct and sensitized photolyses of ehtylenimine (EI) vapor have been investigated at Kr and Xe resonance lines. The fluorescence of NH(c 1Π) or NH(A 3Π) resulting from the scission of EI is observed; EI + h ν(123.6 nm) → NH(c 1Π); EI + Kr*(3P1) → NH(c 1Π) and NH(A 3Π); EI + h ν(147.0 nm) → no emission; and EI + Xe*(3P1) → NH(A 3Π). It is suggested that the decomposition in the direct photolysis is in accordance with the spin conversation rule since NH(A 3Π) observed in the sensitized photolysis is not detected in the direct photolysis. In addition, an approximate application of the theory of unimolecular reaction to the decomposition of excited ethylene has been made in the direct photolysis of EI at Xe and Kr resonance lines indicating spin conservation in the direct photolysis, using the emiprical equation proposed by Hampson and McNesby.