Redistribution of radiation

Content. After a brief overview of molecular collisions and interactions, dipole radiation, and instrumentation (Chapter 2), we consider examples of measured collision-induced spectra, from the simplest systems (rare gas mixtures at low density) to the more complex molecular systems. Chapter 3 reviews the measurements. It is divided into three parts translational, rototranslational and rotovibrational induced spectra. Each of these considers the binary and ternary spectra, and van der Waals molecules we also take a brief look at the spectra of dense systems (liquids and solids). Once the experimental evidence is collected and understood in terms of simple models, a more theoretical approach is chosen for the discussion of induced dipole moments (Chapter 4) and the spectra (Chapters 5 and 6). Chapters 3 through 6 are the backbone of the book. Related topics, such as redistribution of radiation, electronic collision-induced absorption and emission, etc., and applications are considered in Chapter 7. 

Collisional redistribution of radiation. A system A + B of two atoms /molecules may be excited by absorption of an off-resonant photon, in the far wing of the (collisionally) broadened resonance line of species A. One may then study the radiation that has been redistributed into the resonance line - a process that may be considered the inverse of pressure-broadened emission. Interesting polarization studies provide additional insights into the intermolecular interactions [118, 388]. 

K. Burnett and J. Cooper. Collisional redistribution of radiation II. The effects of degeneracy on the equations of motion for the density matrix. Phys. Rev. A, 22 2027-2043... 

Collisional Redistribution of Radiation Collision Induced Fluorescence... 

Electronic absorption spectra are produced when electromagnetic radiation promotes the ions from their ground state to excited states. For the lanthanides the most common of such transitions involve excited states which are either components of the ground term or else belong to excited terms which arise from the same 4f" configuration as the ground term. In either case the transitions therefore involve only a redistribution of electrons within the 4f orbitals (i.e. f—>f transitions) and so are orbitally forbidden just like d—>d transitions. In the case of the latter the rule is partially relaxed by a mechanism which depends on the effect of the crystal field in distorting the symmetry of the metal ion. However, it has already been pointed out that crystal field effects are very much smaller in the case of ions and they... 

The depth resolution (i.e. the ability to discriminate between atoms in adjacent thin layers) is limited by the primary beam causing redistribution of target atoms prior to their emission as ions, and to segregation and radiation-enhanced diffusion processes. The local topography can also lead to a loss of depth resolution with sputter depth. 

The emission of radiation due to redistribution of electrons among the permitted molecular energy levels follows the same pattern as with atomic emissions. 

The mechanisms of interaction between fluorouracil and radiation are not clearly understood. Different hypotheses have been postulated to explain the synergistic or potentiated effect of 5-FU with radiation including redistribution of cells to a more radiosensitive cell cycle phase, deranged pyrimidine pools with reduced capacity for repair of DNA damage, and activation of apoptosis. The effect of 5-FU on radiation damage also appears to vary in different cell lines, thus complicating the extrapolation of laboratory results into clinical practice. 

These same authors also report a dose-dependent increase in the apoptotic rate after the administration of gemcitabine (33), which they believe correlates with the elimination of the more radioresistant S phase population of cells and redistribution of the remaining cells into more radiosensitive compartments of the cell cycle. They also report in another study that reoxygenation of the resistant hypoxic fraction of tumor cells is also a mechanism for the action of gemcitabine (34). Therefore, elimination of these S phase tumor cells may aid the radiation response by not only causing cell cycle synchronization but also by leading to reoxygenation of hypoxic cells. 

The dose of radiation delivered by an internally deposited radionuclide depends on the quantity of radioactive material residing in situ. This quantity decreases as a function of the physical half-life of the radionuclide and the rate at which the element is redistributed or excreted (i.e., its biological half-life). Because the physical half-life is known precisely and the biological half-life can be characterized within limits for most radionuclides, the dose to a tissue that will ultimately be delivered by a given concentration of a radionuclide deposited therein can be predicted to a first approximation. The collective dose to a population that will be delivered by the radionuclide—the so-called collective dose commitment—serves as the basis for assessing the relevant long-term health effects of the nuclide. 

However, the spatial inhomogeneity in the distribution of reagents is not the only reason why the radiolysis of substances in the condensed state is different from that of gases. As we have already mentioned in Section VIII, as we pass from the gaseous state to the condensed one, at the primary stage of radiolysis we already observe a redistribution of yields of primary active particles (resulting in the increase of the yield of ionized states). Also different are the subsequent relaxation processes, as well as the processes of decay of excited and ionized states.354 Another specific feature of processes in a condensed medium is the cage effect, which slows down the decay of a molecule into radicals.355 Finally, the formation of solvated electrons is also a characteristic feature of radiation-chemical processes in liquids.356... 

Entropy production during chemical change has been interpreted [7] as the result of resistance, experienced by electrons, accelerated in the vacuum. The concept is illustrated by the initiation of chemical interaction in a sample of identical atoms subject to uniform compression. Reaction commences when the atoms, compacted into a symmetrical array, are further activated into the valence state as each atom releases an electron. The quantum potentials of individual atoms coalesce spontaneously into a common potential field of non-local intramolecular interaction. The redistribution of valence electrons from an atomic to a metallic stationary state lowers the potential energy, apparently without loss. However, the release of excess energy, amounting to Au = fivai — fimet per atom, into the environment, requires the acceleration of electronic charge from a state of rest, and is subject to radiation damping [99],... 

The formation of radiation defects under irradiation of the fullerene films by the bombarding particles leads to the essential modification of electronic subsystem, which determines their optical and electrophysical properties. However, the mechanisms of radiation defect formation with the use of different types of irradiation and dose load, and also the nature of a change in the electronic properties in this case are studied insufficiently. It is necessary to note that in the case of the condensed state of fullerenes not only the radiation damages of the molecular polyhedrons, which by themselves influence the redistribution of... 

The concerns for changes in atmospheric ozone can be divided into two major categories changes in total column of ozone, and changes in the concentrations at particular altitudes. The penetration of ultraviolet radiation to the surface of the earth is determined almost entirely by the total amount of ozone in the atmospheric column, with very litde dependence on the altitude distribution of this ozone. However, if the prime concern is with processes such as the conversion of ultraviolet energy into heat after absorption by ozone (i.e. with the temperature structure of the stratosphere), then a redistribution of ozone to different altitudes is extremely important. 

While there was considerable confusion in the earliest experiments, it appears that most researchers had realized that radiation chemistry depended on the deposition of the energy in the solvent or dominant species and then a redistribution of the energy from the solvent. The exact nature of the early events where energy was deposited in the solvent (or dominant material) and then transferred to the compounds of interest was not known, but the similar products that one would get with different ratios of gases were strong indicators of the role of energy deposition in the solvent. 

It is quite clear from the above that noticeable hopping conductivity can exist when there is a large density of localized states (traps for electrons) only, which can be provided by initial structure defects or by radiation damages. As far as hopping parameters (and possible free electron concentration) do depend on temperature to a large extent, the temperature increase will lead to a redistribution of internal electric fields and currents. The results of some pertinent experiments are presented below. 

More recently (1965) Ryskin, Tkachuk and Tolstoi (30) measured the relaxation time t of a large number of platinocyanides and found t to be of the order of 10 to 10 sec. They also noted that the independence of the luminescent spectrum with regard to the exciting radiation shows that the redistribution of the electrons on the excited levels responsible... 

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