Photochemistry absorption

When electromagnetic radiation and atomic or molecular systems interact, there are three distinct processes which are relevant to photochemistry absorption, stimulated emission and spontaneous emission. These were treated theoretically by Einstein and are shown diagramaticaUy for a system involving two states, 1 and 2, in Fig. 1.19. 

Contradictory evidence regarding the reaction to fonn 8 and 9 from 7 led the researchers to use TREPR to investigate the photochemistry of DMPA. Figure B1.16.15A shows the TREPR spectrum ofthis system at 0.7 ps after the laser flash. Radicals 6, 7 and 8 are all present. At 2.54 ps, only 7 can be seen, as shown in figure B1.16.15B. All radicals in this system exliibit an emissive triplet mechanism. After completing a laser flash intensity sPidy, the researchers concluded that production of 8 from 7 occurs upon absorption of a second photon and not tiiemially as some had previously believed. 

In photochemistry, we are interested in the system dynamics after the interaction of a molecule with light. The absorption specbum of a molecule is thus of primary interest which, as will be shown here, can be related to the nuclear motion after excitation by tbe capture of a photon. Experimentally, the spectrum is given by the Beer-Lambert law... 

The physical properties of the xanthene type dye stmcture in general have been considered. For example, the aggregation phenomena of xanthene dyes has been reviewed (3), as has then photochemistry (4), electron transfer (5), triplet absorption spectra (6), and photodegradation (7). For the fluoresceins in particular, spectral properties and photochemistry have been reviewed (8), and the photochemistry of rhodamines has been investigated (9). 

Laser Photochemistry. Photochemical appHcations of lasers generally employ tunable lasers which can be tuned to a specific absorption resonance of an atom or molecule (see Photochemical technology). Examples include the tunable dye laser in the ultraviolet, visible, and near-infrared portions of the spectmm the titanium-doped sapphire, Tfsapphire, laser in the visible and near infrared optical parametric oscillators in the visible and infrared and Line-tunable carbon dioxide lasers, which can be tuned with a wavelength-selective element to any of a large number of closely spaced lines in the infrared near 10 ]lni. 

Another area of research ia laser photochemistry is the dissociation of molecular species by absorption of many photons (105). The dissociation energy of many molecules is around 4.8 x 10 J (3 eV). If one uses an iafrared laser with a photon energy around 1.6 x 10 ° J (0.1 eV), about 30 photons would have to be absorbed to produce dissociation (Eig. 17). The curve shows the molecular binding energy for a polyatomic molecule as a function of interatomic distance. The horizontal lines iadicate bound excited states of the molecule. These are the vibrational states of the molecule. Eor... 

Lasers can be coupled efficiently to fiber optic devices to deHver intense monochromatic light precisely to the desired region of the body, including internal organs (see Fiber optics). As in other cases of laser-induced photochemistry, biphotonic effects may be important (87). Lasers also offer the advantage of being able to concentrate the incident energy in a spectral bandpass matched to the absorption band of the sensitizer. 

While di-i-butyl (34) and dicumyl hyponitrites (35) have proved convenient sources of Tbutoxy and cumyloxy radicals respectively in the laboratory,71 72 115"117 the utilization of hyponitrites as initiators of polymerization has been limited by difficulties in synthesis and commercial availability. Dialkyl hyponitrites (16) show only weak absorption at A>290 ntn and their photochemistry is largely a neglected area. The triplet sensitized decomposition of these materials has been investigated by Mendenhall et a .11 s... 

There is a multiplicity of pathways for thermal dediazoniations. An analogous situation is to be expected for photochemical dediazoniations. Based on the general experience that light-sensitive reactions often involve free radical intermediates, it was commonly assumed that all photolytic dediazoniations are free radical reactions. Horner and Stohr s results (1952), mentioned above, could lead to such a conclusion. More sophisticated methods of photochemistry also began to be applied to investigations on arenediazonium salts, e. g., the study of photolyses by irradiation at an absorption maximum of the diazonium ion using broad-band or monochromatic radiation. This technique was advocated by Sukigahara and Kikuchi (1967 a, 1967 b,... 

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

Finally a few sentences are deserved for the vast area of DNA photochemistry. Thymine dimerization is the most common photochemical reaction with the quantum yield of formation in isolated DNA of all-thymine oligodeoxynucleotides 2-3% [3], Furthermore, a recent study based on femtosecond time-resolved transient absorption spectroscopy showed that thymine dimers are formed in less than 1 ps when the strand has an appropriate conformation [258], The low quantum yield of the reaction in regular DNA is suggested to be due to the infrequency of these appropriate reactive conformations. 

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