Atomic effects in solids

There are many connections between the physics of free atoms and that of solids which have been noted, in passing, several times already in the present volume. One should add that many-body theory and, especially, the concept of excitations as quasiparticles in free atoms, owe much to the theory of excitations in solids [590]. 

The crudest model for a one-dimensional solid is that of atoms brought together in a regular line, as would occur in a highly simplified ideal crystal. When this is done, the electrons from any given subshell are treated 

differentiating each equation and multiplying by the other 

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However, solids do not always behave according to the idealised band model. When photoabsorption spectra are studied in the X-ray range, it can happen that the observations reveal quasiatomic properties only slightly modified by solid state effects. These atomic effects in solids must be distinguished from purely solid state properties, but are also of intrinsic interest once properly understood, they provide an alternative route to the study of atoms in condensed matter. 

When Max Planck wrote his remarkable paper of 1901, and introduced what Stehle (1994) calls his time bomb of an equation, e = / v , it took a number of years before anyone seriously paid attention to the revolutionary concept of the quantisation of energy the response was as sluggish as that, a few years later, whieh greeted X-ray diffraction from crystals. It was not until Einstein, in 1905, used Planck s concepts to interpret the photoelectric effect (the work for which Einstein was actually awarded his Nobel Prize) that physicists began to sit up and take notice. Niels Bohr s thesis of 1911 which introduced the concept of the quantisation of electronic energy levels in the free atom, though in a purely empirical manner, did not consider the behaviour of atoms assembled in solids. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

That steps involving atomic or molecular motion can be rate determining, even in fluids, is well known through diffusion limited reaction rates and the solvent cage effect. In solids, motion more subtle than translational diffusion can be influential, and cases of rotational diffusion control are familiar [7],... 

In the atomic context the need for relativistic corrections to Exc[n] is obvious and has led to the development of the relativistic LDA (RLDA) [5,6,24]. On the basis of RLDA calculations for metallic Au and Pt, MacDonald et al. [25,26] have concluded that in solids relativistic contributions to Exc[n] can produce small but significant modifications of measurable quantities, as eg. the Fermi surface area. On the other hand, it has been shown [7] that the RLDA suffers from several shortcomings, eg. from a drastic overestimation of transverse exchange contributions, thus making the RLDA a less reliable tool than its nonrelativistic counterpart. As relativistic corrections are clearly misrepresented by the RLDA, it seems worthwhile to reinvestigate the role of relativistic arc-effects in solids on the basis of a more accurate form for Exc[n. ... 

G. J. Dienes, G. H. Vineyard, Radiation Effects in Solids, Interscience, London, 1957 A. J. Swallow, Radiation Chemistry of Organic Compounds, Pergamon, Oxford, 1960 A. Charlesby, Atomic Radiation and Polymers, Pergamon, Oxford, 1960... 

In this chapter the solid state extrusion of different grades of polyethylene is discussed. The term coprdyetfaylene stands as weU for short and long branched PE as for the nearly alternating 1 1 oipolymer poly(ethjdene-aj-chlorotrifluoroethylene) (PECTFE). It is well known that even HDPE cxsitains a certain amount of ort branches. Therefore, it isi of interest to note that already one butyl side group per thousand main diain carboni atoms effects the solid state extrusion properties of PE remarkably. 

Many-atom interactions in solids by V. Heine, I. J. Robertson and M. C. Payne, Phil. Trans. Roy. Soc. Land., A334, 393 (1991) gives a thoroughly thoughtful overview of the difficulties that must be faced in trying to create effective theories of interatomic interactions in which explicit reference to the electronic degrees of freedom has been renounced. 

Finally, a subject of fundamental importance in atomic physics is the study of how electronic properties are modified by the atomic environment, as in molecules or in the solid state. New situations have been found at the frontier between atomic physics, molecular physics and the physics of condensed matter. This area has grown considerably with the discovery of giant resonances which, though atomic in origin, were first observed in the soft X-ray spectra of solids. Since then, resonant photoemission has become a well-established experimental technique in solid state physics, and valence fluctuations and intermediate valence effects in solids have been shown to involve localised orbitals which are partly atomic in character. 

Since the barrier heights between the potential minima can be readily changed, diffusion process can be studied in detail by optical spectroscopy. Non-occupied minima correspond to vacancies in solid crystals and their influence on the interaction between neighbouring atoms gives information on vacancy effects in solids. 

Other fields benefited, too, from the strong neutron fluxes now accessible for irradiations. New phenomena were observed, such as the annealing of hot-atom effects in inorganic solids by postirradiation heating, leading to an increased retention in the form of the original species (Green and Maddock 1949 Rieder et al. 1950). A new and powerful tool was discovered by Rudolph Mossbauer in 1958 recoilless emission ofy rays with natural or nearly natural line width from nuclei embedded in a solid lattice (Mossbauer 1958, 1962). Resonant absorption of such emissions, e.g., the 14.4 keV transition of Fe fed in the decay of Co (T1/2 = 270 d), provides... 

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