Separated projectile and targetK x-ray production in symmetric heavy ion collisions as a function of the target thickness

Cross sections for projectile and targetK x-ray emission have been measured as a function of the target thickness for the symmetric systems Ni-Ni, Cu-Cu, Nb-Nb, Ag-Ag at energies between 75 and 105 MeV. The projectileK x-rays were separated from the target ones by using the Doppler shift. K_α and K_β energy shifts and K_β to K_α intensity ratios were also determined and used to calculateM- andL-ionization and the correspondingK-fluorescence yield for both collision partners. At non vanishing small target thicknesses, the targetK-vacancy production cross section is generally larger than that of the projectile. By analysing the target thickness dependence of the cross sections with a two component model which takes also into account the evolution of the projectileM-,L- andK-shell population inside the solid, targetK-vacancy sharing and cross sections forKK electron capture in symmetric systems could be determined. These results are in good agreement with molecular orbital model predictions.     

K-shell ionization in 7.5- and 8.6-MeV/a.m.u. U+U collisions at very small impact parameters

K-vacancy production probabilities in elastic 7.5- and 8.6-MeV/a.m.u. U+U collisions are reported for impact parameters less than 15 fm. In 7.5-MeV/a.m.u. U+U collisions the ionization probability rises above the trend indicated by larger impact parameter measurements, increasing to 1.8 vacancies per collision at the smallest impact parameter. The measured probabilities for 8.6-MeV/a.m.u. collisions increase to a maximum value of slightly less than 2 vacancies per collision at the smallest impact parameters. The data is compared to previous results and existing theory.     

Measurement of impact parameter dependent probabilities and total cross sections for targetK-shell ionization by He+ ions

We have measured the impact parameter dependent K-shell ionization probabilitiesP _K (b) of Ca, Cr and Cu from collisions with 4.04 MeV He⁺ ions by particle-K X ray coincidences. A dependence on the target thickness was investigated to study a possible influence of multiple collisions onP _K (b). We measured the total cross section σ _K forK vacancy production simultaneously withP _K (b) and σ _K agrees within 30% with the integratedP _K (b). A comparison of theP _K (b) and σ _K with SCA calculations of Trautmann et al. using RHFS wavefunctions for united atom and separated atom states is discussed.     

Electron-impact excitation of the 3s'3P° and 2s2p53P° autoionizing states of atomic oxygen

The excitation of the 3s''³P° and 2s2p⁵³P° autoionizing states of OI by electron impact on atomic oxygen has been studied. Absolute cross section values from threshold to 300 eV have been obtained for the 3s''³P° state. Limited emission cross section data for the 2s2p⁵³P° and other partially radiating autoionizing states were also obtained at an impact energy of 100 eV. These results suggest that the excitation of the OI autoionizing states account for ≈33% of the total O⁺ total ionization cross section at 40 eV under optically thin conditions and that the cross section ratio, σ(⁴S)/σ(²D + ²P), has been underestimated substantially in earlier theoretical work. Electron-impact excitation of the 3s''³P° and 2s2p⁵³P° states in an optically thick medium enhances the effective total ionization cross section for atomic oxygen by ≈15% and increases the specific O⁺(⁴S) production rate by ≈25% with important implications for O⁺ ion chemistry in gaseous discharges and planetary ionospheres.     

Target-thickness dependence of inner shell excitation in 1.4 MeV/u Pb-Pb collisions

For 1.4 MeV/u Pb³⁰⁺ → Pb collisions the impact parameter dependence of theK andL x-ray emission is measured for two thin target foils (30 and 300 µg/cm²). Unexpectedly, theK excitation probability at small impact parameters (<300 fm) is smaller for the thicker foil. We attempt to explain this behaviour by a blocking of exit channels for direct excitation caused by steady state projectile excitation in the thicker target.     

Measurements of Dissociation Constants of Carbonic Acid in Synthetic Seawater by Means of a Cell Without Liquid Junction

Using the “total hydrogen ion concentration scale”, the first, K ₁, and second, K ₂, stoichiometric dissociation constants of carbonic acid have been determined in synthetic seawater for temperature and salinity ranges of 0–30 °C and 1.5–40, respectively. The values of the K ₁ have been determined potentiometrically by means of a cell without liquid junction, composed using a pH-glass electrode and silver–silver chloride electrode. This cell was equilibrated with CO₂ at 1 atm total pressure. The same cell was used for the determination of K ₂. Dissolved inorganic carbon was measured by the coulometric method, total alkalinity was known from the preparation of the synthetic seawater, and pH measurements provided the data required for the calculations of K ₂. Estimated precision is about ±0.003 pK and uncertainty less than 0.01 pK for K ₁ and twice that for K ₂. Our K ₁ data agree with “best” published data within ±0.01 for log₁₀ K ₁ in the 30–40 salinity range obtained for natural seawater, but become progressively higher as the salinity decreases. Our results for the second stoichiometric dissociation constants agree within ±0.015 for log₁₀ K ₂ in the 30–40 salinity range with available “best” published data for natural seawater, but become progressively lower as the salinity decreases. Since constants obtained in this paper are reliable for the high salinity range (25–40) and strive to the thermodynamic constants at 0 salinity, they are recommended for study of carbonate system in estuaries.     

A Correlation Between Solvatochromic Solvent Polarity Parameters and the Ionization Constants of Various Phenols in 1,4-Dioxane–Water Mixtures

Precise thermodynamic ionization constants K for 3-nitrophenol, 3,4-dichlorophenol, and 4-cyanophenol have been obtained in 1,4-dioxane-water mixtures (0–70% volume fraction in dioxane) at 25°C using a potentiometric method. The same information for another twelve cationic, neutral, and anionic phenols were taken from the literature. Three different methods were used to study the effects of the solvents on the ionization constants: one involves a single polarity parameter, E _T(30); the next involves the Kamlet–Taft multiparametric method; and the last involves the preferential solvation model. The pK values follow the preferential solvation model, but the parameters obtained are highly correlated. Using the data for the phenol molecule as reference, a linear correlation between ΔpK and E _T(30) has been used to develop a new method of obtaining pK values for any binary solvent composition, with only the pK in water known. The pK(s) values correlate with the molecular parameters for the dipolarity/polarizability of the solvent π* and its hydrogen-bond donor ability α. The preferential solvation parameter, f _12/1, correlates with the parameter for the hydrogen-bond donor ability of the solvent. All the phenols follow Hammett's equation and the reaction constants have been calculated for the different water–dioxane mixtures.     

Measurement of L x-ray production cross sections by 60 keV photons and average L shell fluorescence yields of lanthanides

L x-ray production cross sections have been measured for lanthanides with 60 keV. The measured L x-ray production cross section values for the lanthanides are in good agreement with the theoretical ones evaluated using L _i subshell fluorescence yields ω_i, Coster-Kronig transition probabilities ? _ij based on the Relativistic-Hartree-Slater theory, K to L _i subshell vacancy transfer probabilities n _KLi, fractions of the ratiative width of the subshell F _ny and L _i subshell photoionisation cross section σ_Pi. The average L shell fluorescence yields ω_L have also been derived using the presently measured total L x-ray production cross section values and the theoretical K to L shell vacancy transfer probabilities. These results are compared with theoretically predicted values.     

Effect of temperature on O⊘ reactions and equilibria: A pulse radiolysis study

The pK_a of the hydroxyl radical was measured over the 20–80°C temperature range. At 20°C, the pK_a was 11.84 and fell to 10.81 at 80°C. The dissociation constant for the ozonide anion (O⊘₃⇌O₂+O⊘) was found to be 5.5 × 10^-7 mol dm^-3 at 20°C and 46.2 × 10^-7 mol dm^-3 at 70°C. The rate constants and activation energies for the reaction of O⊘ and OH with 2-propanol, methanol and 3-hexene-1,6-dicarboxylate ions have also been measured.     

The relationship between the association constants of phenol with ethers and those of tricholoroacetic acid with ethers : Estimation of association constants of trichloroacetic acid with oxirane

The association constants, K_p, of phenol with ethers (tetrahydrofuran, tetrahydropyran dibutyl ether, dipropyl ether, 1,4-dioxane) and those , K_T, of trichloroacetic acid with ethers were measured in CCl₄ over a temparature range 20°–40° using near IR spectra. A linear relationship between K_p and K_T was found. On the basis of this realtionship the association constants of trichloroacetic acid with propylene oxide were estimated from those of phenol with propylene oxide.     

Thermodynamics of the second-stage dissociation of n-(2-acetamido)iminodiacetic acid in water from 5 to 55°c

The thermodynamic second dissociation constants of the protonated form of N-(2-acetamido)iminodiacetic acid were determined at 12 temperatures from 5–55°C by measurement of the electromotive force using a cell without liquid junction, with hydrogen and silver—silver bromide electrodes. At 25°C, pK₂is 6.844. The standard changes in Gibbs energy, enthalpy, entropy and heat capacity were derived from the change of the pK₂ values with temperature. At 25°C, ΔG° = 9335 cal mol^-1, ΔH° = 2928 cal mol^-1, ΔS^o = -21.5 cal K^-1 mol^-1, and ΔC°_p = -34 cal K^-1 mol^-1. The results are interpreted and compared with those of structurally related compounds.     

Search for interference structures in the quasimolecular X-ray-andK-vacancy production in close collisions of 462 MeV hydrogen-like Kr ions on Mo solid targets

The impact parameter dependence of the emission of quasimolecular radiation and of quasiresonantK?K transfer has been investigated for collisions of 462 MeV Kr³⁵⁺?Mo solid target. The results are compared with calculations based on the MO model for the radiative decay of 1sσ vacancies and nonradiative couplings of the 1sσ and 2pσ level. The data are discussed with special emphasis of the influence of the solid target. Furthermore the possible influence of nuclear reactions on the MO-spectrum is considered.     

Amidines-XVIII: Tautomeric equilibria and pka values of N,N'-disubstituted amidines. Substituent effects

The influence of substitution of amidine group on tautomeric equilibria constants and basicities is discussed. Equations based on correlation analysis methods are derived enabling predictions of both, microscopic pK_a, values of individual tautomers, measured macroscopic pK_a values of the tautomeric mixture, as well as the tautomeric equilibrium constant (as pK_T). It is shown that pK_arn values of unsymmetrically N,N'-disubstituted amidines should obey a non-linear relation with σ° constants, and only for symmetrically N,N'-disubstituted amidines obey the linear Hammett equation. Tautomeric equilibrium constants of N,N'-disubstituted amidines correlate withσ° substituent constants. The methods of prediction of pK_a value of both tautomers and pK_T value are proposed.Derived relations are applied to the series of N,N'-diphenylacetamidines and benzamidines.     

Coherence parameters for He+(2p) capture and H(2p) excitation in He2+-H(1s) collisions

Semiclassical coupled channel calculations have been carried out for the collision system He²⁺-H(1s) in the velocity range 0.15–3.0 a.u. (impact energies 0.5–225 keV/amu) in order to study capture probabilities and alignment and orientation parameters for the dominant He⁺(n=2) channels. A 14-state AO basis set calculation has been combined with an analytical treatment of the asymptotic collision region. For impact velocities about and abovev=0.6 a.u. a strong propensity for resonance capture into an oriented He⁺(2p) state with the same sense of rotation as the collisional rotation of the internuclear axis is predicted together with a very smooth behaviour of the alignment angle as function of impact parameter. Eikonal method calculations of differential capture cross sections predict that the left/right orientation asymmetry will prevail in differential scattering experiments. The resulting total cross sections for capture into specificnl-substates (n=2, 3) and the total light polarisation parameter for He⁺(2p) capture compare well with previous work. Finally we report H(2s,2p) excitation cross sections, probabilties and H(2p) alignment and orientation parameters, following the established propensity rule for orientation in H(2p) excitation.     

Can multiple structures in positron spectra be caused by atomic effects?

In recent experiments at least two structures have been detected in positron spectra from subcritical (total chargeZ ≦ 174) heavy-ion collisions at bombarding energiesE _Lab=5.7?5.9 MeV/n. The origin of these structures is unexplained up to now. Atomic effects have already been discussed as a possible source in a previous paper, where a schematic ansatz for the electronic transition matrix elements was employed. Meanwhile numerical calculations forK-vacancy formation also exhibited oscillations in the impact parameter dependence when using this ansatz. In this paper we report on a scaling law describing these oscillations. As a consequence certain subsidiary conditions have to be imposed on our schematic ansatz, which in turn yield a new understanding on eligible atomic sources for multiple structures in the emission spectra of positrons.     

Acid-base equilibria in ternary water/methanol/dioxane solvent systems : Determination of pKa values of some aliphatic monocarboxylic acids

The thermodynamic acidity constants of n-butanoic, n-pentanoic, n-hexanoic, and n-heptanoic acids were determined at 25°C in ternary water/dioxane/methanol mixtures. The results obtained show that the composite medium effect, expressed by a parameter b = dpK′/du (u being a variable expressing the solvent composition), depends on the ratio of the organic co-solvent concentrations. In the ternary mixtures, superposition of the various effects detected in the corresponding binary solvents (water/dioxane and water/methanol) enables simple interpolation formulae to be used to estimate the pK_a values in solution with any ratio of the three solvents.     

Analytical applications of the reaction of thorium with benzenephosphonic acid

Benzenephosphonic acid quantitatively precipitates thorium as Th(C₆H₅PO₃)₂·3H₂O at pH values as low as 0.5. The compound may be dried at 140° to 180° C and weighed, as a gravimetric means of determining thorium. On ignition, Th (C₂H₅PO₃)₂ 3 H₂O undergoes decomposition at 240° to 300° C to form Th(C₆H₅PO₃)₂·2H₂O, at 450° to 650° C to form Th(HPO₄)₂·2H₂O and finally at 800° to 1000° C to form Th(HPO₄)₂. The latter compound is stable to 1200° C.Potentiometrically (pK₁' = 0.91, pK₂' = 6.41) and spectrophotometrically (pK₁' = 0.96, pK₂' = 6.51) determined pK' values are reported. Absorption spectra of C₆H₅PO₃H₂, C₆H₅PO₃H^- and C₆H₅PO₃^-2 are reported. The solubility of Th (C₆H₅PO₃)₂·3H₂O was studied as a function of pH and the average value of the solubility product (K_sp = 4s³) was found to be 3.24·10^-31.     

Measurement of K shell radiative transition probabilities and K, L2 and L3 shell/subshell fluorescence yields of some elements in the atomic number range 30 ≤ Z ≤ 40

K shell radiative transition probabilities and K, L₂ and L₃ shell/subshell fluorescence yields were measured using the K_β/K_α intensity ratio for seven elements in the atomic number range 30≤Z≤40 (Zn, As, Se, Rb, Sr, Y and Zr). The targets were irradiated with γ photons of energy 59.5 keV from Am-241. The obtained values were compared with the theoretical values. It was observed that the present values agree with previous theoretical results. The L₂ and L₃ subshell fluorescence yields were the first measured for the present elements.     

Solubility of metal oxides in molten equimolar KBr-NaBr mixture at 973 K

Solubility products (pK _s,MO, molality) are measured by potentiometric titration with a Pt(O₂)|ZrO₂(Y₂O₃) oxygen electrode in the molten KBr-NaBr equimolar mixture at 973 K for the following oxides: CaO (5.00 ± 0.3), MnO (7.85 ± 0.3), NiO (9.72 ± 0.04), PbO (5.20 ± 0.3), and SrO (3.81 ± 0.3). The correlation between pK _s,MeO and the polarization of the corresponding cations by Goldschmidt is obtained.     

Complex Formation Equilibria of Unusual Seven Coordinate Fe(III) Complexes with DNA Constituents

Seven-coordinate Fe(III) complexes [Fe(dapsox)(H₂O)₂]⁺, where [dapsox = 2,6-diacetylpyridine-bis(semioxamazide)] is an equatorial pentadentate ligand with five donor atoms (2O and 3N), were studied with regard to their acid–base properties and complex formation equilibria. Stability constants of the complexes and the pK _a values of the ligands were measured by potentiometric titration. The interaction of [Fe(dapsox)(H₂O)₂]⁺ with the DNA constituents, imidazole and methylamine·HCl were investigated at 25 °C and ionic strength 0.1 mol·dm^?3 NaNO₃. The hydrolysis constants of the [Fe(dapsox)(H₂O)₂]⁺ cation (pK _a1 = 5.94 and pK _a2 = 9.04), the induced ionization of the amide bond and the formation constants of the complexes formed in solution were calculated using the nonlinear least-squares program MINIQUAD-75. The stoichiometry and stability constants for the complexes formed are reported. The results show the formation of 1:1 and 1:2 complexes with DNA constituents supporting the hepta-coordination mode of Fe(III). The concentration distributions of the various complex species were evaluated as a function of pH. The thermodynamic parameters ΔH° and ΔS° calculated from the temperature dependence of the equilibrium constants were investigated for interaction of [Fe(dapsox)(H₂O)₂] with uridine.