Chemical Binding Effects Research Articles

Corrections to structural parameters for the chemical binding effect as determined by gas electron diffraction: CO 2

Corrections to structural parameters for the chemical binding effect as determined by gas electron diffraction: CO 2

Stopping cross-sections of CH4, C2H4, C3H6 and C4H10 for alpha particles in the energy region 0.3 to 5.5 MeV

Comparison of recent stopping power measurements in gases still shows significant discrepancies between different authors. As part of a continuing study of chemical binding and phase effects on the stopping power of heavy charged particles, measurements have been made on a number of hydrocarbon gases, using 241Am alpha particles. Stopping cross-sections are presented for CH4, C2H4, C3H6 and C4H10 for alpha energies in the range 0.3 to 5.5 MeV. These are compared with other published results, suggesting that the absolute uncertainties are no more than 3.5%, except at the very lowest energies. The comparisons are discussed, in particular for C3H6, for which two recent sets of measurements had shown significant differences.

High-energy electron scattering by diborane

Differential cross sections have been measured for high-energy electron scattering from diborane. The effect of chemical binding was seen by comparing experiment with the calculation for an independent-atom model. An ab initio molecular calculation of the elastic scattering was in good agreement with the experiment, and thus the proportion of the electron distribution involved in the chemical binding was estimated.

Chemical binding effects on resonance line shape and neutron capture rates

Calculated capture rates and Doppler coefficients in uranium compounds do not agree with experimental results. The difference can be due to: (1) errors in the resonance parameters and/or that (2) the Maxwell gas treatment, which considers nuclei free when in fact they are bound in a crystalline structure, is inadequate. The purpose of the present work is to introduce models which account for chemical binding in the resonance treatment and to determine if such models are capable of removing the discrepancy. Both harmonic and damped vibrational models of chemical binding were considered. In the harmonic model the vibrational amplitude of an atom in a crystal lattice is independent of time. In the damped vibrational model this amplitude is allowed to decay in time. It was found that the line shape obtained with the harmonic approximation relative to the free-gas, Maxwell model, is shorter at the peak and broader in the wings. This leads to less resonance self shielding and yields a slight increase in the discrepancy between theory and experiments (1–3%). The line shape obtained with the damped vibrational model is taller at the peak and narrower in the wings than that obtained with the Maxwell model. This leads to more resonance self shielding and yields a slight decrease in the discrepancy between theory and experiment (1–3%). This decrease, however, is not enough to remove all the discrepancy (∼10%). Further evaluation of the magnitude of the damping coefficient is required.

Electron distribution in water by high‐energy electron scattering

AbstractThe differential cross‐sections for high‐energy electrons scattered from water have been measured over a wide range of momentum transfers. The effect of chemical binding was seen from the comparison between the experiment and the calculation for an independent‐atom model. The ab initio calculation using SCF MO was carried out with respect to the elastic scattering. It was in a good agreement with the experiment and thus a reliable electron distribution in water was obtained.

Neutron Wave Propagation in Heavy Water

We report here some results of our study of neutron wave propagation in heavy water using the scattering kernel proposed by Bansal et al. Calculations have been made both by including and by neglecting the contribution of molecular modes. We find better agreement with the experimental results of Perez et al. when contribution of molecular modes is included in the scattering kernel. This study also reveals the importance of including the chemical binding effects in the scattering kernel.

Electron scattering differential cross sections for C 2H 2, C 2H 4, and C 2H 6 by gas phase electron diffraction - binding effects

Experimental differential cross sections for 40 keV electrons scattered by C 2H 2, C 2H 4 and C 2H 6 molecules were measured using the gas electron diffraction method in the range of the scattering variable s from s = 1 A −1 to s = 30 A −1. The differential cross sections for neon were also measured and compared with calculated differential cross sections to calibrate the diffractograph. Experimental differential cross sections show significant deviations with respect to theoretical differential cross sections calculated from the Debye-Ehrenfest model, mainly in the range of small scattering angles. The observed differences are connected to chemical binding effects. From the experimental data, an estimation of the binding energy was carried out. The deduced values: −0.58 ± 0.20 au for C 2H 2, −0.94 ± 0.30 au for C 2H 4 and −1.23 ± 0.40 au for C 2H 6 are in agreement with those obtained by thermochemical methods.

A photoelectron spectroscopic study of the adsorption of CO and CO2 on platinum

The adsorption of CO and CO2 on platinum has been studied by UV photoelectron spectroscopy using both He I and He II radiation. The modulation of the intensity of the spectral features observed for adsorbed CO as the photon energy is changed is used to assign the observed levels. The results are in reasonable agreement with recent theoretical and experimental work. The levels are observed to shift by different amounts compared to gas phase CO because of chemical binding effects. The adsorption of CO2 produces spectral features that are shifted by the same amount compared to gas phase CO2. This, together with the absence of any localized attenuation of the platinum valence band and the low heat of adsorption, indicates that CO2 is physisorbed on platinum.

Appearance potential spectrum of barium

When the conventional tungsten filament in an appearance potential spectrometer was replaced by an oxide coated filament, barium contamination was detected on the iron specimen being examined. The spectrum obtained showed prominent M structure of barium in addition to weak iron L structure. Even after the work function correction is applied, the barium edges are shifted by several eV from the respective energy levels quoted in the literature. This is presumably the effect of chemical binding of the barium atoms on the surface. The iron and the barium spectra showed slight changes in structure when the specimen was heated in situ.

Bethe surface, elastic and inelastic differential cross sections, Compton profile, and binding effects for H2 obtained by electron scattering with 25 keV incident electrons

Electron impact spectra for H2 have been obtained at scattering angles of 1°, 1.5°, 2°, 3°, 4°, 5°, 7°, and 10° using 25 keV incident electrons. The measured intensities were converted to generalized oscillator strengths and placed on an absolute scale at each scattering angle by the use of the Bethe sum rule for the generalized oscillator strength distribution. The cross section differential with respect to both solid angle and energy loss of the scattered electron was corrected for relativistic and exchange effects and integrated over energy loss to obtain the inelastic differential cross sections. In addition the elastic cross section differential with respect to solid angle was measured. The results are all in excellent agreement with theoretical calculations. The total elastic cross section was determined using additional data from another source. The Compton profile was determined from the 7° scattering data and was found to agree well with the previous x-ray results. Consistent generalized optical sums (2,1, − 1, − 2) and optical sum inequalities for the generalized oscillator strength were also obtained. In addition inelastic scattering factors were computed from the (−1) optical sum and were found to agree well with available theory. Chemical binding effects were explored using electron density difference functions obtained from the differential cross sections. For the first time the intensity difference functions for the elastic and inelastic scattering were determined separately. It is argued that the experimental method used in this study to obtain the intensity difference functions represents the best approach so far developed for obtaining direct information about molecular real space charge densities.

On the Complete Eigenvalue Spectrum for the Neutron Wave Problem in Multigroup Diffusion Theory

We report here the complete eigenvalue structure and the corresponding eigenfunctions of the neutron transport operator for the neutron wave propagation through water and heavy water. The calculations have been performed in multigroup diffusion theory approximation using realistic scattering kernels. The results are found to be rather insensitive to the chemical binding effects. The bounds on the eigenvalues have also been discussed and the results corresponding to the fundamental mode eigenvalue have been compared with the experimental data for heavy water. Our results show that there exists only one resolvable mode for water and four resolvable modes for heavy water.

A High Precision Electron Diffraction Unit for Gases

An electron diffraction unit using counting techniques has been built which makes it possible to record relative total (elastic and inelastic) differential scattering cross sections directly. Features of the unit which deviate from those in conventional units (which incorporate a rotating sector and photographic plates) are described in detail. The possible uses and limitations of the new instrument are discussed. The total scattered intensities in the range 1.5≤s(Å−1)≤7.2 with 40 keV electrons from N2O and CO2 are presented as examples. The results are compared to the predictions of the independent atom model and differences due to chemical binding and electronic correlation effects are observed.

Measurement of Neutron Slowing Down Time in Light Water, Ice, Paraffin and Santowax

Measurements of the neutron slowing down times in light water, ice, paraffin and santowax have been made by pulsed neutron technique. Bursts of D-T neutrons of 0.1μsee width were generated in moderator cubes of 40×40×40 cm3. The slowing down neutrons were detected by bare as well as energy-selective filter-covered BF3 counters, and analyzed with a 256-channel time analyzer. Slowing down times in the moderator were determined by interpreting the increment of the difference of events between the two counters as attributable to the fraction of the neutrons slowing down at the time of measurement below the cut-off energy of the filter. The measured slowing down times below 0.63 and 0.43 eV agreed well with theoretical values on the 0°K free gas model. On the other hand, the measured values below 0.20 eV were found to be appreciably greater than the theoretical, which would appear to indicate that he effect of thermal agitation and chemical binding come into play at this range of energy.

The determination of chemical binding effects in N2 by electron scattering

The total differential (elastic plus inelastic) cross section has been measured for electron scattering by N 2 with 40 keV electrons. Data was obtained over the range 1.5 ⩽ s ⩽ 12Å −1 ( s = (4π/λsin 1 2 θ). The results are in excellent agreement with previous photographic measurements and conclusively show that previously observed maxima and minima in the intensity difference curve between experiment and independent atom model theory are the result of chemical binding. The binding energy was also estimated and was found to be too large by some 10 eV.

Energy loss of charged particles in matter

A new method of determining the differential energy loss of charged excited nuclei in matter is described. The method is based on the velocity dependence of the Doppler shift of the gamma quanta emitted by moving nuclei. No theoretical assumptions concerning the velocity dependence of atomic or nuclear collisions are necessary. The method and mathematical analysis, described in detail, are applied to the energy loss of Li ions emitting gamma quanta of 477 keV in the kinetic energy range between 100 and 800 keV. It is found that the energy loss of Li ions is linearly proportional to the velocity within 2% for several substances ofZ=1 to 74. The small limits of error, which can be obtained, allow an application of this method to questions as e.g. theZ-dependence of the stopping power on the nuclear charge of the stopping material, chemical binding effects, the time dependence of the adjustment of the equilibrium charge of the projectile after nuclear reactions, or the determination of nuclear angular correlations.

Measurement of Neutron Slowing Down Time in Light Water, Ice, Paraffin and Santowax

Measurements of the neutron slowing down times in light water, ice, paraffin and santowax have been made by pulsed neutron technique. Bursts of D-T neutrons of 0.1μsee width were generated in moderator cubes of 40×40×40 cm3. The slowing down neutrons were detected by bare as well as energy-selective filter-covered BF3 counters, and analyzed with a 256-channel time analyzer. Slowing down times in the moderator were determined by interpreting the increment of the difference of events between the two counters as attributable to the fraction of the neutrons slowing down at the time of measurement below the cut-off energy of the filter. The measured slowing down times below 0.63 and 0.43 eV agreed well with theoretical values on the 0°K free gas model. On the other hand, the measured values below 0.20 eV were found to be appreciably greater than the theoretical, which would appear to indicate that he effect of thermal agitation and chemical binding come into play at this range of energy.

Measurements of the Neutron Slowing Down Times in Finite Graphite Systems by the Resonance Filter Method

To shed further light on the neutron slowing down process, slowing down times were measured in graphite systems by the resonance filter method. The distribution of the slowing down time from 14MeV to a few energy points from thermal equilibrium was obtained in the form of differences of time response between compensation-filter covered and resonance-filter covered BF3 counters. From the time distributions thus determined were derived the most probable slowing down time t max and the average slowing down time <t>, which were compared with the theoretical values calculated with the use of various scattering kernels. The effects of finite medium and space dependence were also evaluated. The values of t max obtained were 9.5±0.5, 10.5±0.5, 20±1, 55±4, 71±4 and 101±8μs for 5.2, 4.91, 1.46, 0.28, 0.2 and 0.11 eV, respectively. Of these values, those over 1.46 eV show good agreement with the theoretical values by 0°K free gas kernel, while those under 0.28 eV agree well with the values by Williams' model, in which the effects of chemical binding and thermal vibration are considered.

A mathematical model of the equilibrium distribution of chemical complexes and the biological effects of chemical binding.

A general equation is derived describing the concentration of all possible complexes of a central molecule with a set of ligands bound to the central molecule. This deduction allows the reaction rate constants for the binding of a given molecule to the central molecule to depend on the species of molecules already bound and the location of the molecules already bound. The model thus allows for structural alteration of the central molecule by binding. Functions describing the concentration dependence of any effect whatever depending on the distribution of complexes are deduced. Possible applications and methods of application are indicated.

Measurement of Neutron Slowing Down Time in Graphite

A series of pulsed neutron experiments was performed to investigate the chemical binding effect on the neutron slowing down time. Bursts of D-T neutrons of 1 μsec width were generated in a hexagonal prism with 240 cm high with 120 cm flanks, made of reactor grade graphite with a density of 1.54. The slowing down neutrons were detected by bare as well as energy-selective filter-covered BF3 counters, and analyzed with a 256 channel time analyzer. Slowing down times in graphite were determined by interpreting the increment of the difference of events between the two counters to be due to the contribution made by the fraction of the slowing down neutrons at the time of measurement that was below the cut-off energy of the filter. The results of measurements showed good agreement with calculation based on the crystal model.

Diffusion length calculations in water

An extensive series of calculations have been performed to determine the asymptotic neutron energy spectra and diffusion lengths in water poisoned by absorbers like boron, cadmium and gadolinium at different concentrations. The effect of chemical binding has been studied by comparing the results obtained by using the well-known Nelkin model of water with the free-gas model for hydrogen. Diffusion theory has been used in making the calculations; however, to test the validity of the diffusion approximation, in a few cases we have also made some transport theory calculations and compared the two results. We find that even for fairly large absorptions (i.e. for κ Σ min ≈ 0·7–0·8 ) diffusion theory gives results which compare well with those obtained by using transport theory.