Impact measurements, experimental methods

Quantum mechanical and selected semiclassical and semiempirical methods for the calculation of electron impact ionization cross sections are described and their successes and limitations noted. Experimental methods for the measurement of absolute and relative ionization cross sections are also described in some detail. Four theoretical methods, one quantum mechanical and three semiclassical, have been used to calculate cross sections for the total ionization of the inert gases and small molecules and the results compared with experimental measurements reported in the literature. Two of the theoretical methods, one quantum mechanical and one semiclassical, have been applied to the calculation of orientation-dependent electron impact ionization cross sections and the results compared with recent experiments. 

For conditions of constant craze thickening rate, agreement between prediction and experiment is already very encouraging, particularly at higher speeds. There are still refinements to be made to the experimental method, and drop-weight striker speeds of less than 0.5 m/s will remain inherently difficult to control. Nevertheless, the Full Notch Impact test provides a promising direct method for the measurement of cohesive properties in tough polymers. 

The measurement of water solubility, water-octanol and water-soil partition coefficients presented several unique challenges. Novel experimental methods and data are reported for these physical constants. In some cases, experimental data is at odds with earlier published values. Comprehensive comparison of all available estimated, calculated, and experimental data are presented. The new values impact the calculation of the mobility of dioxin in soil, as well as other distribution properties. 

The principal experimental methods for measuring bond dissociation energies in polyatomic molecules have been described in detail by Cottrell (35). Two methods in particular have proved fruitful in recent years, namely the toluene carrier gas technique introduced by Szwarc (167) to investigate the unimolecular kinetics of pyrolytic decomposition reactions, and the electron impact method, as applied by Stevenson (165) and widely used since by several investigators. These methods do not usually give very sharp D values, and the number of well-authenticated ( 2... 

In 1935, Frumkin and M. A. Proskumin [11] used ac measurements to measure the electric double layer capacitance. This permitted empirical values of the double layer capacitance to be compared rigorously with theory for the first time. The authors were successful because they carried out their experiments under conditions of exceptional cleanliness, such as glassware that had been acid-cleaned and water that had been multiply distilled. This work had an enormous impact internationally, because it demonstrated how reproducible measurements of the electrical double layer could be obtained. Indeed, Frumkin s experimental methods soon became legendary among western scientists. From that time forward, the ac method became the standard method of measuring interfacial capacitance. 

The paper discusses the application of dynamic indentation method and apparatus for the evaluation of viscoelastic properties of polymeric materials. The three-element model of viscoelastic material has been used to calculate the rigidity and the viscosity. Using a measurements of the indentation as a function of a current velocity change on impact with the material under test, the contact force and the displacement diagrams as a function of time are plotted. Experimental results of the testing of polyvinyl chloride cable coating by dynamic indentation method and data of the static tensile test are presented. 

Storer model used in this theory enables us to describe classically the spectral collapse of the Q-branch for any strength of collisions. The theory generates the canonical relation between the width of the Raman spectrum and the rate of rotational relaxation measured by NMR or acoustic methods. At medium pressures the impact theory overlaps with the non-model perturbation theory which extends the relation to the region where the binary approximation is invalid. The employment of this relation has become a routine procedure which puts in order numerous experimental data from different methods. At low densities it permits us to estimate, roughly, the strength of collisions. 

The interpretation of previous attempts at measuring the impact of metals on microbially mediated processes has been hindered by the use of a wide range of experimental conditions and measurements. Already, a shift from studies based on total metal concentration to those based on bioavailable metal concentrations has occurred. The next step will entail accurately predicting and measuring metal speciation patterns in order to identify microbial responses to metal speciation. Only then will it be possible to develop more effective methods to quantify and mitigate deleterious effects of metals on the myriad processes that microbes mediate in the environment. 

Theoretical models of the electron impact ionization process have focused on the calculation of the ionization cross section and its energy dependence they are divided into quantum, semiclassical and semiempirical. Methods for the calculation of the ionization cross section and experimental techniques developed for the measurement of absolute ionization cross sections will be described in more detail below. Cross sections calculated using the semiempirical additivity method developed by Deutsch and Mark (DM) and their coworkers,12-14 the binary-encounter-Bethe (BEB) method of Kim and Rudd,15 16 and the electrostatic model (EM) developed by Vallance, Harland, and Maclagan17,18 are compared to each other and to experimental data. 

This expression reproduces the experimentally measured ionization efficiency curves surprisingly well, considering the simplicity of the model on which it is based. There is a discontinuity in the function at the maximum (when X = Xmax) but this affects only a small region of the ionization efficiency curve, and satisfactory values of the cross section are still obtained over this region. A great advantage of this method is that it is very simple to apply, depending on only three parameters the molecular polarizability volume, the ionization potential, and the maximum electron impact ionization cross section. These can be measured or calculated values (from the ab initio EM method described above, for example). 

Measurements by electron impact methods have led to experimental stabilization energies of a number of alkylcarbonium ions. Muller and Mulliken (1958) compared these with calculated values obtained by a procedure based on the LCAO-MO approximation, the large stabilization energies found for carbonium ions being attributed to the combined effects of hyperconjugation and charge redistribution (Table 1). 

Rotational excitation. The rotational excitation of molecules by electron impact has been studied less experimentally. At present there are no direct methods of measuring the cross sections of rotational transitions, and the latter are found by measuring the mobility of electrons in gases.228 So the available data are mostly theoretical. A favorable circumstance here is the fact that we are able to obtain sufficiently accurate analytical formulas for the cross sections of rotational excitation that do not require detailed knowledge of the molecule s structure.223... 

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