Energy levels, spectral studies

The set of energy levels associated with a particular substance is a unique characteristic of that substance and determines the frequencies at which electromagnetic radiation can be absorbed or emitted. Qualitative information regarding the composition and structure of a sample is obtained through a study of the positions and relative intensities of spectral lines or bands. Quantitative analysis is possible because of the direct proportionality between the intensity of a particular line or band and the number of atoms or molecules undergoing the transition. The various spectrometric techniques commonly used for analytical purposes and the type of information they provide are given in Table 7.1. 

Both the semiquantitative and quantitative types of calculation yield energy-level schemes from which, in principle, spectral transition energies may be calculated. Comparison of these schemes with observed electronic spectra should enable an assessment of their relative values to be made however, the spectral data on organometallic molecules are very limited and often qualitative. There are few accurate studies involving polarization data and solvent shifts from which the nature of the observed bands can be classified and since, in most cases, the theoretical schemes of bond levels yield an embarrassing number of transitions, this lack of assignment is serious. 

For a same molecular ratio of aqueous NaY solutions (Y = OH, Cl), experimental data underlines specific effects of nascent OH radicals on transient UV and near-IR electronic configurations. Complex investigations of PHET reactions in the polarization CTTS well of aqueous CT and OH ions are in progress. We should wonder whether a change in the size of ionic radius (OH -1.76 A vs Cl" 2.35 A) or in the separation of the energy levels influence early branchings of ultrafast electronic trajectories. A key point of these studies is that the spectroscopic predictions of computed model-dependent analysis are compared to a direct identification of transient spectral bands, using a cooled Optical Multichannel Analyzer... 

In this section we shall particularly study the dynamic properties of a core hole in terms of its self-energy and spectral function15 19,23,27 32). This is a kind of model problem because one does not discuss by which physical mechanism the core hole is created. The hole is simply created in the system at a specific instant of time and destroyed at a later time. By studying the development of the core hole during this interval one gets a picture of how the core level strength becomes distributed over the various possible levels of the ionic system. Nevertheless, since the creation of the core hole is sudden, the resulting spectral function is very closely connected to the X-ray photoelectron spectrum (XPS) as already briefly discussed in Sect. 2.2, Eq. (6). 

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