Recoiling atoms, range

The effect of recoil is three-fold firstly, the recoil atom is displaced from the site where it was located. It can thus be ejected directly into an adjacent phase. The displacement distance is approximately 40 nm, depending on the substrate (Harvey 1962) and is known as the range. It can be estimated using the following equation ... 

Q.) is based on the ejection of the recoiled particles out of the sample in the forward direction by an energetic heavy ion beam. The measured energy spectra of these recoiled atoms can be related to their concentration profiles. The use of range foil in front of the energy detector to permit selective absorption of the various recoils introduces a few limitations in the application of the technique, e.g. deterioration of the energy resolution and hence the depth resolution, the limitation on the accessible depth in the depth profile information, etc. Indeed, the practical utility of the experimental set-up is enormously reduced in the region where overlapping spectra of various atoms are difficult to separate. 

During the detection of ionization over the range of a-rays from polonium in a hydrogen atmosphere, abnormally rapid increases were observed when no solid window was present between the Po and the ionization chamber. This was not explainable by the volatility of Po nor to the transfer of Po together with the recoil atoms of Ra G. This was explained by the assumption that a hydride, H2P0 is formed, which diffuses into the ionization chamber. Such a compound is rapidly destroyed by a small concentration of air or through the action of a-rays. The formation of such a hydride would explain the very high absorption power of Pt and Pd for Po. 

Primary effects comprise recoil of the nucleus and excitation of the electron shell of the atom. The excitation may be due to recoil of the nucleus, change of atomic number Z or emission of electrons from the electron shell. Secondary effects and subsequent reactions depend on the chemical bonds and the state of matter. Chemical bonds may be broken by recoil or excitation. In gases and liquids mainly the bonds in the molecules are affected. The range of recoil atoms is relatively large in gases and relatively small in condensed phases (liquids and solids). Fragments of molecules are mobile in gases and liquids, whereas they may be immobilized in solids on interstitial sites or lattice defects and become mobile if the temperature is increased. 

Figure 9.12. Range of recoiling atoms of various mass numbers. 4 as a function of the recoil energy. (According to J. Alexander, M. F. Gazdik Phys. Rev. 120, 874 (I960)- B G Harvey Annu Rev Nucl. Sci. 10, 235 (I960).)...

The noble gas may escape from the solid by recoil or diffusion. The range R of recoiling atoms produced by a decay is about 100 pm in air and about 0.01 pm in solids. If R r, r being the radius of the grains or crystallites of the solid, only a small fraction of the recoiling atoms is able to escape from the solid ... 

The study of polyvalent recoiling atoms presents the opportunity of creating and studying a wide range of reactive radicals and ions at ambient temperatures in the presence of virtually any desired reaction partner. There are a number of examples of species such as methyne, CH (5) chloromethyne, CCl (6) sUylsilyene, SiHsSiH (7) and phase reaction products were obtained for the first time from recoil studies. 

Another possible geometry for ERDA is the transmission mode. In this case the sample must be thiimer than the range of the recoiled atom to be profiled, (its penetration depth in that material). The main advantage of transmission ERDA is an increased sensitivity (by up to two orders of magnitude) in comparison to conventional reflection-geometry ERDA. 

Direct interaction of recoil atoms or fragments, obtained in the C(y,n) C process with various Cg hydrocarbons, resulted in the formation of C-labelled methane, acetylene, ethane + ethylene, propane + propylene and l,3-butadiene Methane- and acetylene- have been produced in the radiochemical reaction between and liquid benzene, with the yield ratio (CH4/C2H2) being in the range of 0.02 to 0.03. 

Transmission ERD Transmission ERD uses a different geometry to standard ERD and requires the sample to be thinner than the range of the recoiled atoms. The beam is usually incident on the sample at or near normal incidence and recoils are detected using a detector placed directly behind it. It has the advantage of an improved sensitivity but the profiled depth is very limited. 

The use of radioactive techniques to determine surface excesses was further extended in a series of elegant works, where the limited range (-1000 A) of a-recoiling atoms in water was utilized. Surface excesses of Bi-212 ions were determined - e.g. coadsorbed with dodecylsulfate - by collecting and counting the recoiling Tl-208 atoms above the surface. 

There are two basic physical phenomena which govern atomic collisions in the keV range. First, repulsive interatomic interactions, described by the laws of classical mechanics, control the scattering and recoiling trajectories. Second, electronic transition probabilities, described by the laws of quantum mechanics, control the ion-surface charge exchange process. 

The major role of TOF-SARS and SARIS is as surface structure analysis teclmiques which are capable of probing the positions of all elements with an accuracy of <0.1 A. They are sensitive to short-range order, i.e. individual interatomic spacings that are <10 A. They provide a direct measure of the interatomic distances in the first and subsurface layers and a measure of surface periodicity in real space. One of its most important applications is the direct determination of hydrogen adsorption sites by recoiling spectrometry [12, 4T ]. Most other surface structure teclmiques do not detect hydrogen, with the possible exception of He atom scattering and vibrational spectroscopy. 

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