Radiation-initiated energy transfer

In the case of polarized, but otherwise incoherent statistical radiation, one finds a rate constant for radiative energy transfer between initial molecular quantum states i and final states f... 

Physical properties of carbon black-filled EPR and EPDM elastomers have been found to be comparable with the suUur-cured analogues [372]. Aromatic oils increase the optimum dose requirement for these compounds due to the reaction of the transient intermediates formed during radiolysis of the polymer with the oil as well as energy transfer which is particularly effective when the oil contains aromatic groups. The performance and oxidative stability of unfilled EPDM as well as its blend with PE [373], and the thermal stabdity and radiation-initiated oxidation of EPR compounds are reported by a number of workers [374,375]. 

Kerma (k)—A measure of the kinetic energy transferred from gamma rays or neutrons to a unit mass of absorbing medium in the initial collision between the radiation and the absorber atoms. The SI unit is J/kg. The special name of this unit is the rad (traditional system of units) or Gray (SI). 

The most probable fate of a photon with an energy higher than the binding energy of an encountered electron is photoelectric absorption, in which the photon transfers its energy to the electron and photon existence ends. As with ionization from any process, secondary radiations initiated by the photoelectron produce additional excitation of orbital electrons. 

It is convenient initially to classify elementary reactions either as energy-transfer-limited or chemical reaction-rate-limited processes. In the former class, the observed rate corresponds to the rate of energy transfer to or from a species either by intermolecular collisions or by radiation, or intramolecular-ly due to energy transfer between different degrees of freedom of a species. All thermally activated unimolecular reactions become energy-transfer-limited at high temperatures and low pressures, because the reactant can receive the necessary activation energy only by intennolecular collisions. 

In the consideration of excited but not ionized states we would like to make a distinction between primary excited states, those formed directly by the initial interaction of the molecule with the primary radiation or secondary electrons, and secondary excited states, those formed by primary energy transfer or by charge neutralization. 

A very important bimolecular deactivation process is the electronic energy transfer (ET). In this process, a molecule initially excited by absorption of radiation, transfers its excitation energy by nonradiative mechanism to another molecule which is transparent to this particular wavelength. The second molecule, thus excited, can undergo various photophysical and photochemical processes according to its own characteristics. 

Photoreactions of alkynes with compounds such as ketones, thioketones or nitrocompounds often involve initial excitation of the non-alkyne addend, and because of this they do not necessarily involve an electronically excited state of the alkyne. However, we have noted in previous sections that it is not always clear whether or not the alkyne is the first species to be excited, nor does it follow that electronically excited alkyne is not involved when the alkyne does not absorb the radiation—it may, for instance, be obtained by energy transfer or it may be involved as an exciplex. For this reason, and for completeness, the account in this section is included. 

Since the solvent molecules have a majority in the liquid scintillator, they are initially excited by the radiation energy. The 7t electron of the solvent molecule plays an important role in the process of energy transfer due to its active mobility. 

Here /g is the intensity of incident monochromatic radiation, I is the intensity of radiation at a distance I cm, and e is the decadic molar extinction coefficient of an absorbing species (concentration, c mole. 1 ). This law is strictly valid only if molecular interactions are unimportant at all concentrations. Deviations occur for a variety of reasons this means that the validity of the law should be checked under the particular experimental conditions. An initial determination of the absorption spectrum of the compound under investigation is obligatory. This produces immediate qualitative information, particularly about the usefulness of the source of radiation. Banded, diffuse or continuous spectra give direct information about the complexity and variety of primary processes that may occur. Further information will be gained from the effect of radical traps such as Oj or NO, and of various energy transfer agents. 

Both elastic and inelastic scattering can be understood in terms of energy transfer between photons and molecules as illustrated in Figure 9.6. As mentioned before, photons of electromagnetic radiation can excite molecular vibration to a higher level when vph = vvit,. If the excited vibration returns to its initial level (Figure 9.6a), there is no net energy transfer from photons to molecular vibration. Thus, the scattered photons from molecules will have the same frequency as the incident radiation, similar to elastic collisions between photons and molecules. If the... 

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