Electromagnetic radiation excited species

The absorption of electromagnetic radiation by molecular species in solution in the UV/visible region is followed by relaxation from excited electronic states to the ground state mostly by a combination of radiationless processes. Vibrational relaxation, where the excess energy is rapidly dis-... 

Photoredox processes include both photoreduction and photo-oxidation of the excited species. An electron transfer that results from an electronic state produced by the resonant interaction of electromagnetic radiation with matter is called photoinduced electron transfer (PET) [30-32]. This can be done by either a direct or a photosensitized process (see below and Figure 6.6). 

Electromagnetic radiation, besides being a probe of surface structure, can excite electrons in the species in solution (especially in organic compounds) or in the electrode itself (especially in semiconductor electrodes). This photon excitation can lead to electron transfer between electrode and solution. The study of these phenomena is photoelectrochemistry and can be very important in conversion of solar energy into electricity in order to convert substances (photoelectrolysis). 

A molecule possesses different energy states rotational, vibrational, vibrational-rotational, translational, and electronic. Different types of electromagnetic radiation are capable of transforming the normal states in molecules to produce altered states. For example, UV and visible light normally produce electronic transitions from the ground state to excited states. Once the absorbing species (D) is in an excited state (D ), it can undergo a variety of processes ... 

Photoallergic responses to sunscreen products represent an area where chemical components not known to produce dermal allergic responses are activated by solar radiation to produce ACD. I45 46 It is hypothesized here that in these instances, electromagnetic radiation may activate the skin to adsorb and/or absorb species that do not so react without the incident radiation. Alternatively, the molecular species themselves are excited by the radiation to molecular states that make them more reactive to skin. Chemicals exhibiting photoallergic contact dermatitis include oxybenzone, butyl methoxy dibenzoylmethane, methoxycinnamate, and benzophenone—all molecules that are readily photoexcitedJ45 ... 

Fluorescence is the emission of electromagnetic radiation by a species that has absorbed exciting radiation from an outside source. Intensity of emitted (fluorescent) light is directly proportional to the concentration of the excited species (see Chapter 3). 

When the sample is stimulated hy application of an external electromagnetic radiation source, several processes are possible. For example, the radiation can be scattered or reflected. What is important to us is that some of the incident radiation can be absorbed and thus promote some of the analyte species to an excited state, as shown in Figure 24-5. In absorption spectroscopy, we measure the amount of light absorbed as a function of wavelength. This can give both qualitative and quantitative information about the sample. In photoluminescence spectroscopy (Figure 24-6), the emission of photons is measured after absorption. The most important forms of photoluminescence for analytical purposes are fluorescence and phosphorescence spectroscopy. 

Fluorescence and phosphorescence are analytically important emission processes in which atoms or molecules are excited by the absorption of a beam of electromagnetic radiation. The excited species then relax to the ground state, giving up their excess energy as photons. Fluorescence takes place much more rapidly than phosphorescence and is generally complete in about 10 s (or less) from the time of excitation. Phosphorescence emission may extend for minutes or even hours after irradiation has ceased. Because fluorescence is considerably more important than phosphorescence in analytical chemistry, our discussions focus mostly on fluorescence. 

Absorption of electromagnetic radiation Processes in which radiation causes transitions in atoms and molecules to excited states the absorbed energy is lost, usually as heat, as the excited species return to their ground states. 

Emission spectrum The collection of spectral lines or bands that are observed when species in excited states relax by giving off their excess energy as electromagnetic radiation. 

While such radiation may be a component of the radiation resulting from electron excitations, radioactive decay, or nuclear fission processes, the consequences for azides are usually mild and non-self-sustaining. When intense beams and the full spectrum of species associated with nuclear and electromagnetic radiation are considered, more spectacular effects can be observed, and it is these which take up most of the discussion here. 

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