Retarding potential method

D) LOW-ENERGY ELECTRON BEAM (RETARDING-POTENTIAL) METHODS... 

A low-energy electron beam can also be obtained using a field emission tip and used in the field emission retarding-potential method. This combination provides an absolute measure of the sample work fiinction and the resolution is excellent [52]. 

It is desirable to measure the ratio m e the appearance potential, and kinetic energy in the same experiment, but there are considerable difficulties in doing this, and usually in the mass spectrographic experiment only the first two of these have been measured, while the retarding potential method of Lozier measured the second and third. 

There are many ways to measure the work function or work function change which can be generally classified as electron emission methods (thermionic emission, field emission and photoelectron emission), low-energy electron beam (retarding-potential) methods and capacitance methods [M, 51 and 52]. Absolute work function values can be measured using emission methods while the other techniques measure only work-function changes. 

The following tables summarize the results of experimental work function data. Tjub is the substrate temperature in K during deposition. RT denotes room temperature. The methods of work function measurement are VC Vibrating capacitor method RP Retarding potential method SEC(E) and SEC(P) Electron- and photon-induced secondary electron method respectively FE Field emission method. A( )min is the work function change at the minimum, A( )ML is the work function change at one saturated monolayer, fig is the initial dipole moment. Where appears in the table, a minimum was not observed. The tables are adapted and updated from [89A1],... 

Figure Bl.26.23. Current-voltage curves observed in the retarding potential difference method of work-fimction Miboxfmeasurement] (Hudson J B 1992 Surface Science (Stoneham, MA Butterworth-Heinemaim)).

Fig. 6. Potential distribution and electron kinetic energy functions in a simple ion source using the R.P.D. method (see text) the effect of a variable retarding potential is shown schematically.

Figure 2. Negative ion autodetachment lifetime (9) and negative ion current (measured without the retarding potential difference method) (O) for Ct(CN)T as a function of electron energy (VI)...

D N2) was determined as 9 79, 7 90, 7 42, 6 23, or 5-76 eV according to the assumed states of excitation of the nitrogen ion and the nitrogen atom produced. Spectroscopically obtained values for Z)(N2) are 9 76 or 7 38 eV, depending on the assumptions made. The retarding potential and appearance potential measurement alone is satisfactory for the interpretation of electron impact processes in homonuclear diatomic molecules, where there can be no doubt about the mass number of the ions. Possible confusion for heteronuclear diatomic molecules is not likely to be very great, but the method by itself is clearly inapplicable to dissociative ionization processes in polyatomic molecules, where the number of possible products is large. 

Earlier estimates of ionization potentials obtained with conventional mass spectrometric ion sources were beset with difficulties caused by the relatively wide energy spread (0.5—1 eV) in the thermally produced electron beam. Refinements of the electron-impact method, such as use of an energy-resolved electron beam from a double-hemispherical electrostatic monochromator18 or the retarding potential difference (RPD) method19 afforded values which are in good agreement with photoionization and spectroscopic data. 

Selection of Fire Retardants. The choice of flame retardants depends on the nature of the polymer, the method of processing, the proposed service conditions, and economic considerations. Although the processing, service, and economic factors are impor.pa tant, the flame-retardancy potential of an additive is of primary importance, and this factor can be readily evaluated by thermal analysis. 

C.E. Melton and W. H. Hamill, Appearance potentials by the retarding potential-difference method for secondary ions produced by excited-neutral, excited ion-neutral, and ion-neutral reactions, J. Chem. Phys. 41, 1469-1474 (1964). 

Restricted Hartree-Fock method (RHI, 408 109,499 retarded potential,... 

Basically, there are three ways of measuring the kinetic energy of electrons. The first is to measure the time needed to traverse a known distance. In the second method a retarding potential is applied to the electrons to be analysed. The third method is based on the deflection of the electrons in an electrostatic or magnetic field. 

The use of flame retardants came about because of concern over the flammabiUty of synthetic polymers (plastics). A simple method of assessing the potential contribution of polymers to a fire is to examine the heats of combustion, which for common polymers vary by only about a factor of two (1). Heats of combustion correlate with the chemical nature of a polymer whether the polymer is synthetic or natural. Concern over flammabiUty should arise via a proper risk assessment which takes into account not only the flammabiUty of the material, but also the environment in which it is used. 

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