Photons electron bombardment

UHV techniques are usually classified in terms of the electron/photon method, as is shown in Table 2.3 which lists the common electron bombardment and emission techniques that have been employed in electrochemical studies. A detailed description of UHV surface analysis techniques is beyond the scope of this book there are many excellent reference texts that can be consulted for this purpose (see further reading list). It is sufficient to note that methods involving electron bombardment or emission are inherently surface-sensitive as a result of the low pathlength, or escape depth, of electrons in condensed media. In addition, Table 2.3 briefly describes the type of information each method provides. 

Modem instrumentation has improved substantially in recent years, which has enabled the measurement of XPS spectra of superior resolution necessary to reveal the small BE shifts present in highly covalent compounds such as those studied here. In a laboratory-based photoelectron spectrometer, a radiation source generates photons that bombard the sample, ejecting photoelectrons from the surface that are transported within a vacuum chamber to a detector (Fig. 2). The vacuum chamber is required to minimize the loss of electrons by absorption in air and, if a very high quality vacuum environment is provided (as is the case with modem instruments), the surface contamination is minimized so that the properties of the bulk material are more readily determined. 

High-energy radiation may be classified into photon and particulate radiation. Gamma radiation is utilized for fundamental studies and for low-dose rate irradiations with deep penetration. Radioactive isotopes, particularly cobalt-60, produced by neutron irradiation of naturally occurring cobalt-59 in a nuclear reactor, and caesium-137, which is a fission product of uranium-235, are the main sources of gamma radiation. X-radiation, of lower energy, is produced by electron bombardment of suitable metal targets with electron beams, or in a... 

A deuterium arc lamp has two electrodes, bathed in an atmosphere of deuterium, between which a metallic screen pierced with a hole of 1 mm in diameter is placed. The discharge current creates an intense arc at the level of this hole, which is close to the anode. Under electron bombardment, deuterium molecules dissociate, emitting a continuum of photons over the range of 160 to 400 nm (Fig. 11.9). 

The principal impediment to effective process design and analysis is the limited understanding of synergistic effects due to ion, photon, and electron bombardment of solid surfaces during etching and deposition. Fundamental relationships must be established between the gas-phase chemistry the surface chemistry as modified by radiation and etch profiles, rates, selec-tivities, and film properties. 

Electron spectroscopy is the study of electrons emitted when matter is irradiated with photons or bombarded with particles. In photoelectron spectroscopy (PES), a monochromatic source of irradiation is used and the kinetic energies of directly ejected (primary) electrons are analysed by means of an electron spectrometer, i.e. an intensity... 

In XPS a vacancy is created in an electronic level close to the nucleus by photon bombardment or, in certain cases, by electron bombardment. It is probable that this vacancy is filled by an electron coming from a higher electronic level further from the nucleus (Fig. 12.11). The excess energy... 

Photons of X-radiation are emitted from the specimen under electron bombardment. They are characteristic for elements and could be used to determine the element distribution on the surface of the sample under investigation. 

Electron spectroscopy involves the detection of electrons escaping from a catalyst surface under photon or electron bombardment. The conventional applications of these techniques therefore require the specimen to be situated in a vacuum ofl0 Pa(s 10 Torr) or even lower pressure. The catalytic reaction thus has to be interrupted prior to spectroscopic analysis, so the information is confined to stable, that is, evacuation-resistant adsorbed layers present on the catalyst after interruption of the FT synthesis. 

These techniques often involve photon and electron bombardment and emission, with a vast number of possibilities. A multiple of methods and acronyms have appeared, bringing confusion for the uninitiated. Clean well-characterized surfaces are studied under conditions of ultrahigh vacuum to preserve purity. Research of this type is now beginning to make... 

X-ray photons arc yet a third product of electron bombardment of a solid. Both characteristic line spectra and an X-ray continuum are produced. This radiation serves as the basis for the electron niicroprobe for X-ray fluorescence analysis of SHM images. 

The prineiple of the hybrid PMT is also used in electron-bombarded (EBD) CCDs. In these deviees the diode is replaced with a CCD image sensor. Electron multiplication results in a eonsiderable gain effect so that EBD-CCDs are able to deteet single photons. However, the quantum efficiency of the photocathode is lower than the optieal quantum effieiency of a CCD sensor. If an acquisition time of a few ten seconds can is acceptable it is often better to deteet the photons directly in a cooled CCD. 

Surface reactions are extremely important. They are promoted by bombardment with electron ions, and photons. Ions and electrons had been taken into account in the first models of plasma polymerization, e.g., by Williams and Hayes [23], who worked with a 10-kHz frequency applied to parallel plate electrodes. They proposed that the monomer is adsorbed on the surface of the electrodes. By ion and electron bombardment from the discharge, a part of the monomer is converted into surface... 

There are always four stages in any scheme of x-ray emission analysis. The excitation of characteristic radiation from the specimen by bombardment with high-energy photons, electrons, protons, etc. the selection of a characteristic emission line from the element in question by means of a wavelength or energy-dispersive spectrometer the detection and integration of the characteristic photons to give a measure of characteristic emission line intensity and finally, the conversion of the characteristic emission line intensity to elemental concentration by use of a suitable calibration procedure. 

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