Energy levels emission/absorption spectroscopy

Molecular Energy Levels and Absorption and Emission Spectroscopy... 

Atomic and Molecular Energy Levels. Absorption and emission of electromagnetic radiation can occur by any of several mechanisms. Those important in spectroscopy are resonant interactions in which the photon energy matches the energy difference between discrete stationary energy states (eigenstates) of an atomic or molecular system = hv. This is known as the Bohr frequency condition. Transitions between... 

Luminescence spectroscopy involves three related optical methods fluorescence, phosphorescence, and chemiluminescene. These methods utilize excited molecules of an analyte to give a species whose emission spectrum can provide information about the molecule. In fluorescence, atoms can be excited to a higher energy level by the absorption of photons of radiation. Some features of luminescence methods are increased sensitivity (in the order of three magnitudes smaller than absorption spectroscopy), larger linear range of concentration, and method selectivity (Parsons 1982). 

Investigation of atomic spectra yields atomic energy levels. An important chemical application of atomic spectroscopy is in elemental analysis. Atomic absorption spectroscopy and emission spectroscopy are used for rapid, accurate quantitative analysis of most metals and some nonmetals, and have replaced the older, wet methods of analysis in many applications. One compares the intensity of a spectral line of the element being analyzed with a standard line of known intensity. In atomic absorption spectroscopy, a flame is used to vaporize the sample in emission spectroscopy, one passes a powerful electric discharge through the sample or uses a flame to produce the spectrum. Atomic spectroscopy is used clinically in the determination of Ca, Mg, K, Na, and Pb in blood samples. For details, see Robinson. 

The use of tunable lasers as sources in electronic absorption and emission spectroscopy has made possible a very considerable increase in resolution and precision. Electronic spectra are often difficult to analyze because of the many transitions involved. However, with a tunable laser source, one can tune the laser frequency to a specific absorption frequency of the molecule under study and thus populate a single excited electronic vibration-rotation energy level the resulting fluorescence emission spectrum is then simple, and easy to analyze. 

The relation AE = hv allows the separations between the energy levels of a system to be determined by measurement of the frequencies of light absorbed or emitted by the system, giving rise to absorption and emission spectroscopy. The techniques used in spectroscopy vary with the region of the electromagnetic spectrum. In the following paragraphs, we survey the various kinds of spectroscopy. 

If high temperatures eventually lead to an almost equal population of the ground and excited states of spectroscopically active structure elements, their absorption and emission may be quite weak, particularly if relaxation processes between these states are slow. The spectroscopic methods covered in Table 16-1 are numerous and not equally suited for the study of solid state kinetics. The number of methods increases considerably if we include particle radiation (electrons, neutrons, protons, atoms, or ions). We note that the output radiation is not necessarily of the same type as the input radiation (e.g., in photoelectron spectroscopy). Therefore, we have to restrict this discussion to some relevant methods and examples which demonstrate the applicability of in-situ spectroscopy to kinetic investigations at high temperature. Let us begin with nuclear spectroscopies in which nuclear energy levels are probed. Later we will turn to those methods in which electronic states are involved (e.g., UV, VIS, and IR spectroscopies). 

The properties of absorption and luminescence emissions of atoms are important in analytical techniques as well as in spectroscopy in general. The absorption and emission spectra of atoms are line spectra which provide the unmistakable fingerprint of each element, and this is used in the analytical technique known as atomic absorption spectroscopy for example. Although the energy levels of atoms are shown as simple lines in a qualitative picture such as that of Figure 3.3, the absorption and emission lines which correspond to transitions between these levels are not infinitely narrow (that is, absolutely monochromatic) because of several effects. 

The Spectroscopy of Proton Transfer and the Forster Cycle. The absorption and emission spectra of the acid and base forms of a molecule are of course different, since these forms represent distinct chemical species. The excitation energies (e.g. S0- St) are however related according to a very simple scheme of energy levels known as the Forster cycle this is shown in Figure 4.47(a). 

Most other forms of spectroscopy do not involve emission of extra particles such as electrons, but the straightforward absorption or emission of photons. These processes increase or decrease the energy of an atom ex molecule, by an amount equal to the photon energy. The results all reinforce the conclusion of photoelectron spectroscopy that only discrete energy levels occur (see Fig. 1.12). For example, the line spectra of atoms, known since the early nineteenth century, only contain lines at certain well-defined wavelengths. The quantization of energy, not only in electromagnetic radiation but in material systems, is an inescapable conclusion rtf spectroscopy. 

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