Polarised electrons sources

This phenomenon, first pointed out by Fano [143] and called the Fano effect, has been used to produce spin-polarised electron sources [144]. Sources based on irradiating atoms are now mainly of academic importance, because solid state sources based on GaAs devices yield a higher current more simply. 

A polarised electron beam is made to strike a surface using an electron gun with a spin polarised electron source. The electrons reflected from the surface are energy analysed and then their spin polarisation is determined. This provides information regarding spin dependent scattering within the sample. 

Electrons from a polarised primary electron beam are directed on to a surface and the spin state of the electrons that are reflected from the surface is determined. Typically, a polarised electron source consists of a GaAs crystal upon which polarised light is projected. The emerging photoelectrons from the GaAs crystal have up to 50 % polarisation. The electrons that are detected are first passed through an electron energy analyser and then through a Mott detector to determine the spin state of the electrons. 

The performance characteristics of various sources of polarised electrons have been reviewed extensively (e.g. Kessler, 1985). These sources depend either on the spin dependence of the interaction in the process in which the free electrons are produced, or on the pre-polarisation of one element... 

Fig. 2.12. Schematic of a GaAs source of transversely-polarised electrons using photemission from a negative electron affinity GaAs crystal.

An electron source. or electron gun traditionally the most frequent is the triode gun comprising a tungsten wire cathode heated to around 2 700 K. The electrons emitted by the cathode are accelerated by the electric field set up between this negatively polarised filament and the earthed anode. This gun can provide an electron beam of 30 keV with a current density per unit solid angle (also called bright-... 

NLO phenomena result from the interaction between light and matter and, more precisely, between the polarisable electron density and the strong electric field associated with a very intense laser beam. They were experimentally observed firstly in 1961 just after the development of intense laser sources in particular by Kaiser and Garett for two-photon absorption and by Fra n ken for They can be divided in two... 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

Figure 10.7 illustrates the use of an external power supply to provide the cathodic polarisation of the structure. The circuit comprises the power source, an auxiliary or impressed current electrode, the corrosive solution, and the structure to be protected. The power source drives positive current from the impressed current electrode through the corrosive solution and onto the structure. The structure is thereby cathodically polarised (its potential is lowered) and the positive current returns through the circuit to the power supply. Thus to achieve cathodic protection the impressed current electrode and the structure must be in both electrolytic and electronic contact. 

We locate the EBIT source and calibration source inside the Rowland circle by design. Bragg diffraction angles of calibration lines are in the range 29-45° while the helium-like resonances are observed around 39°. The plane of crystal dispersion is parallel to the electron beam axis. The crystal acts as a polarizer at Bragg angles near 45° and radiation polarised perpendicular to the electron beam axis is the dominant diffracted component. 

X-band cavity changes the fluorescence polarisation, which can be detected. The main features of the apparatus are illustrated in figure 11.12. One of the advantages of excitation with an electron beam, rather than with conventional monochromatic or white light sources, is that transitions between electronic states of different spin multiplicity are allowed consequently both singlet and triplet excited states can be populated. 

Integral cross sections for selected electron-impact excitation and ionisation processes have been largely obtained by measuring optical excitation functions. These need to be corrected to a varying degree of accuracy for effects such as cascade contributions and photon polarisation. The details of the experimental procedures, sources of errors and data evaluation have been discussed by Heddle and Keesing (1968). 

The magnetic coil shown in figs. 2.15 and 2.16 was used to orient the electron polarisation vector P parallel to the axis of the analysing target—Mott detector system. The deflection system is part of the differential pumping stage which is necessary for the maintenance of the required ultra-high vacuum in the source chamber. 

Fig. 2-36. Scheme of laboratory installation for recording polarisation curves of electronic conductors 1- electronic conductor (mineral or metal) 2- solution of electrolyte A- current electrode B- auxiliary current electrode M- measuring electrode N- non-polarisable measuring (reference) electrode cp- potentiometer CS- electric current source I- ammeter (reproduced with permission from Putikov, 1993). 

The essence of the CPC method consists of recording and interpreting polarisation curves obtained when the polarisable electrode is an electron-conducting ore body (Fig. 2-39). One pole of the current source, electrode A, is connected to the ore body by means of a special device (e.g., in a borehole intersection through the ore body). The... 

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