Radiation deactivation

Types of atomic fluorescence. The solid lines represent radiational processes and the dashed lines non-radiational processes. In the latter, a single-headed arrow represents non-radiational deactivation and a double-headed arrow a thermal activation process. The term anti-Stokes is used when the radiation emitted is of shorter wavelength, i.e. greater energy than that absorbed. 

In stepwise fluorescence, the upper levels of the exciting and the emitted lines are different. In the normal case, the excited atoms lose part of their energy by collisional deactivation (by collision with flame molecules) and then return to the original (usually ground) state by radiational deactivation. Sodium, for example, is excited at the 330.3-nm line and undergoes stepwise fluorescence to emit a line at... 

According to eqn [1], /p would also be increased by optimizing the experimental conditions to protect the populated triplet from radiationless deactivation by collisions with impurities of the solvent itself. Phosphorescence demands radiational deactivation of the triplet. Unfortunately, however, owing to its long lifetime, the triplet is prone to many deactivation processes possible in fluid solution at room temperature (where collisions are very probable). In fact, in such experimental conditions the value of... 

Atomic fluorescence is the process of radiational activation followed by radiational deactivation, unlike atomic emission, which depends on the collisional excitation of the spectral transition. For this, the ICP is used to produce a population of atoms in the ground state and a light source is required to provide excitation of the spectral transitions. Whereas a multitude of spectral lines from all the accompanying elements are emitted by the atomic emission process, the fluorescence spectrum is relatively simple, being confined principally to the resonance lines of the element used in the excitation source. 

Atomic fluorescence is the process of radiation activation followed by radiation deactivation, unlike atomic emission which depends on the collisional excitation of the... 

Figure 4.10 Singlet and triplet excited states. Jablonski energy-state diagram (schematic) a, absorption b, fluorescence c, non-radiational deactivation d, phosphorescence. For a more detailed version, see Figure 4.11, Section 4.4.2.1.

The 3 Pi/2, 3 P2/2 excited states involved in the sodium D lines are the lowest energy excited states of the atom. Consequently, in a discharge in the vapour at a pressure that is sufficiently high for collisional deactivation of excited states to occur readily, a majority of atoms find themselves in these states before emission of radiation has taken place. Therefore... 

Hydrosdylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro sdylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

Steinfeld et al. [133] demonstrated the technical feasibility of solar decomposition of methane using a reactor with a fluidized bed of catalyst particulates. Experimentation was conducted at the Paul Scherrer Institute (PSI, Switzerland) solar furnace delivering up to 15 kW with a peak concentration ratio of 3500 sun. A quartz reactor (diameter 2 cm) with a fluidized bed of Ni (90%)/Al2O3 catalyst and alumina grains was positioned in the focus of the solar furnace. The direct irradiation of the catalyst provided effective heat transfer to the reaction zone. The temperature was maintained below 577°C to prevent rapid deactivation of the catalyst. The outlet gas composition corresponded to 40% conversion of methane to H2 in a single pass. Concentrated solar radiation was used as a source of high-temperature process heat for the production of hydrogen and filamentous... 

Subsequent to the formation of a potentially chemiluminescent molecule in its lowest excited state, a series of events carries the molecule down to its ground electronic state. Thermal deactivation of the excited molecule causes the molecule to lose vibrational energy by inelastic collisions with the solvent this is known as thermal or vibrational relaxation. Certain molecules may return radia-tionlessly all the way to the ground electronic state in a process called internal conversion. Some molecules cannot return to the ground electronic state by internal conversion or vibrational relaxation. These molecules return to the ground excited state either by the direct emission of ultraviolet or visible radiation (fluorescence), or by intersystem crossing from the lowest excited singlet to the lowest triplet state. 

As illustrated in Fig. 7.15, the electromagnetic radiation measured in an XRF experiment is the result of one or more valence electrons filling the vacancy created by an initial photoionization where a core electron was ejected upon absorption of x-ray photons. The quantity of radiation from a certain level will be dependent on the relative efficiency of the radiationless and radiative deactivation processes, with this relative efficiency being denoted at the fluorescent yield. The fluorescent yield is defined as the number of x-ray photons emitted within a given series divided by the number of vacancies formed in the associated level within the same time period. 

AES quantifies discrete radiation that is emitted by an excited atom when it deactivates to the ground state. This energy of excitation is... 

Radiationless transitions (Chapter 5), where no emission of electromagnetic radiation accompanies the deactivation process. 

It is important to note fluorescence seldom results from absorption of ultraviolet radiation of wavelengths lower than 250 nm because such radiation is sufficiently energetic to cause deactivation of the excited states by predissociation or dissociation Fluorescence due to a a transitions is seldom observed instead, such... 

Siebbeles, L.D.A. Emmerichs, U. Hummel, A. Bakker, H.J. J. Chem. Phys. 1997,107, 9339. Dellonte, S. Barigelletti, F. Orlandi, G. Flamigni, L. Intramolecular Deactivation Processes and Energy Transfer Mecahanism in Liquid Alkanes Studied by N2 Laser Two Photon Excitation. Proceedings 5th Tihany Symposium on Radiation Chemistry. Dobo, J. Hedvig, P. Schiller, R., Eds. Akademiai Kiado Budapest, 1983 437 pp. 

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