Electron excitation, in solids

The complexity of the spectra and the difficult interpretation due to the low sensitivity to surface defects, makes a theoretical support highly desirable in order to provide a firm attribution of the observed bands. The calculation of electronic excitations in solids is very challenging, and only recently accurate configuration interaction (Cl) calculations have been reported on this problem [28,29], To this end, finite clusters embedded in ECP and point charges have been used. A calibration of the accuracy of the results, possible for bulk F and F transitions where assignments are unambiguous, have shown that Cl calculations tend to overestimate the optical transitions of F centers in MgO bulk by about 15% because of limitations in the size of the basis set [28]. 

The EELS results provide reliable information about the density of electrons in the valence band of solids, electron excitations in solids, and volume density of the material. The EELS spectrum was obtained from the spectrum of the Is carbon photoemission line corresponding to the electron energy of about 1 keV. We have followed the procedure described in ref. [15] and removed the elastic Is line to obtain the loss function in order to evaluate the energy losses as shown in Figures 11.9 and 11.10(a). 

Although some optical techniques, such as soft X-ray absorption and optical reflectance measurements, provide comparative information about solids with higher energy resolution, EELS enjoys several unique advantages over optical spectroscopies. First of all, unlike optical reflectance measurements which are sensitive to the surface condition of the sample, the transmitted EELS represents the bulk properties of the material. Secondly, EELS spectra can be measured with q along specific controllable directions and thus, can be used to study the dispersion of plasmons, excitons, and other excitations [8.1-8.5]. Such experiments offer both dynamics as well as symmetry information about the electronic excitations in solids. In addition, the capability to probe the electronic structure at finite momentum-transfer also allows one to investigate the excited monopole or quadrupole transitions, which cannot be directly observed by conventional optical techniques limited by the dipole selection rule. 

The radiation from radionuclides interacts with matter and causes ionization, excitation, or chemical changes. These effects are utilized in the methods of radiation detection and measurement. Among them the most commonly used effects are the ionization in gases, the interaction of radiation with semiconducting materials, the orbital electron excitation in solids and liquids, and the specific chemical reactions in sensitive emulsions. 

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

The classical theory for electronic conduction in solids was developed by Drude in 1900. This theory has since been reinterpreted to explain why all contributions to the conductivity are made by electrons which can be excited into unoccupied states (Pauli principle) and why electrons moving through a perfectly periodic lattice are not scattered (wave-particle duality in quantum mechanics). Because of the wavelike character of an electron in quantum mechanics, the electron is subject to diffraction by the periodic array, yielding diffraction maxima in certain crystalline directions and diffraction minima in other directions. Although the periodic lattice does not scattei the elections, it nevertheless modifies the mobility of the electrons. The cyclotron resonance technique is used in making detailed investigations in this field. 

In this chapter we will consider the techniques developed to detect and quantitatively measure how much ionization and/or excitation is caused by different nuclear radiations. As all radiation creates ionization and/or excitation, we will separate the discussion of detection methods according to the general techniques used to collect and amplify the results of the interaction of the primary radiation with matter rather than by the type of radiation. These detection methods can be classified as (a) collection of the ionization produced in a gas or solid, (b) detection of secondary electronic excitation in a solid or liquid scintillator, or (c) detection of specific chemical changes induced in sensitive emulsions. 

Rare-gas solids (RGS), or atomic cryocrystals, are the model systems in physics and chemistry of solids, and enormous amount of information about electronic excitations in RGS has been documented in several books [2-5]... 

R. Rohlfing, S. G. Louie, Quasiparticle and optical excitations in solids and clusters, in A. Gonis, N. Kioussis, N. Ciftan (Eds.), Electron Correlations and Materials Properties, Kluwer Academic/Plenum Publishers, New York, 1999, pp. 309-328. 

An even longer-range transfer, showing a 1/r3 dependence, may occur in crystals, solid solutions, and some fluids, as a result of exciton migration. The concept of the exciton was introduced by Frenckel to interpret certain crystal spectra an electron-hole pair was looked upon as an entity that could move about the crystal as a result of interactions between lattice sites. For the present purposes, the electronic excitation in an irradiated species can be regarded as an exciton that is free to wander over a considerable number of lattice sites. 

The pump-probe pulses are obtained by splitting a femtosecond pulse into two equal pulses for one-color experiments, or by frequency converting a part of the output to the ultraviolet region for bichromatic measurements. The relative time delay of the two pulses is adjusted by a computer-controlled stepping motor. Petek and coworkers have developed interferometric time-resolved 2PPE spectroscopy in which the delay time of the pulses is controlled by a piezo stage with a resolution of 50 attoseconds [14]. This set-up made it possible to probe decoherence times of electronic excitations at solid surfaces. 

Fig. 10. Transfer of electronic-excitation energy in the photosynthetic unit of purpie bacteria. The figure shows a funneling of excitation energy from different energy levels (left scale) toward the reaction center. The vertical dashed arrows indicate intracomplex transfers, and the slanted solid arrows indicate intercomplex transfers. Note that LH1 exists in all purple bacteria while LH2 exists in most and LH3 exists only in certain species. Figure source Hu, Ritz, Damjanovic and Schulten (1997) Pigment organization and transfer of electronic excitation in the photosynthetic unit of purple bacteria. J Phys Chem 101 3859.

Knoester, J. and Agranovich, V. M. (2003). Frenkel and charge-transfer excitons in organic solids. In Agranovich, V. M. and Bassani, G. F. (Eds.), Electronic Excitations in Organic Based Nanostructures. Thin Films and Nanostructures 31. Elsevier Academic Press, Amsterdam, pp. 1-96. 

Photoemission spectroscopy applied to chemistry and electronic properties studies is a fairly recent development. The x-ray photoemission spectroscopy (XPS) technique was developed, primarily to be a chemical analysis tool (1). In particular it was observed that the absolute binding energies of the atomic-like electron core levels are dependent on the chemical state of the atom under study. This observation led to the widespread use of XPS for basic and applied chemistry studies. Many studies were also undertaken to better understand the physics of the various excitation processes involved. Consequently, XPS has become a powerful tool for studying electronic structure of the outer electron states in solids. 

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