Study Of Photochemical Reactions Research Articles

Low-temperature spectroscopy of organic molecules in solids by photochemical hole burning

Characteristic features of photochemical hole-burning (PHB) in the impurity spectra of low-temperature solids and PHB applications in molecular spectroscopy are considered. The evolution of a no-phonon hole and a phonon sidehole in excitation and fluorescence spectra is analysed on the basis of model calculations. Some more complex models for PHB are considered, which take into account reverse reactions, the optical thickness of the sample, the inhomogeneous dispersion of both homogeneous linewidths and transition energies of the photoproduct and quasi-static impurity-impurity interactions. The effects of PHB on fluorescence line narrowing are discussed. By PHB homogeneous linewidths of purely electronic and vibronic no-phonon lines in the spectra of some porphine and phthalocyanine derivatives in various matrices are measured and their temperature dependence is studied. The latter is found to be essentially different in polycrystalline and glassy matrices. Line-broadening mechanisms are discussed. By PHB the existence of an inhomogeneous distribution of vibrational frequencies in molecular impurities is established. The applications of PHB in the studies of photochemical reactions in solid solutions of phthalocyanine derivatives and tetracene are regarded and the mechanisms of these reactions are discussed. The results obtained by PHB for chlorophyll and its derivatives are presented.

Edmund John Bowen, 29 April 1898 - 19 November 1980

Edmund John Bowen, known to everyone as Ted Bowen, died on 19 November 1980 after a short illness. He was a pioneer in the study of photochemical reactions and a major contributor to our know ledge of the production and quenching of fluorescence, but his influence goes far beyond his published scientific work. There can be few of his pupils, colleagues and other friends who have not benefited repeatedly from his unselfish and enthusiastic help and advice, and his ability to explain abstruse theories in simple terms has helped the understanding of several generations of scientists.

The study of photochemical reactions in polyoximurethane

The irradiation of polyoximurethane at 77K yields only iminoxy radicals. The gaseous products are CO and CO2. The mechanism of photodegradation, including 2 initial processes of N-O and C-O bond scission is discussed. The ratio between the initial processes depends on the wavelength of the light used. Only the C-O bond cleavage results in polymer degradation.

Studies of Photochemical Reactions Using Organic Photosensitizers. III. Direct Photolysis and Biacetyl-photosensitized Decomposition of Azopropanes

Abstract The direct photolyses in the two wavelength regions and the biacetyl-photosensitized decomposition have been studied for l,l′-azo-n-propane and 2,2′-azo-isopropane. The value of kd/kc, the ratio of the disproportionation rate to the combination rate of free radicals, has been found to be slighlty dependent on the exciting wavelength as for n-propyl and isopropyl radicals. The higher the exciting energies, the larger the ratio kd/kc. From the quenching study of biacetyl phosphorescence, it was found that the energy of the excited biacetyl molecule was transferred to the azopropane molecule at every collision.

The use of semiconductor electrodes in the study of photochemical reactions

AbstractSemiconductor electrodes can act either as donors or acceptors for electron exchange with excited molecules in the electrolyte. Unlike at metal surfaces, reaction with both the ground level and the excited level will not occur simultaneously provided that the semiconductor has a sufficiently large band gap. It is therefore possible to observe the injection of electrons or of holes into the semiconductor. The photocurrents produced in this way can be used to investigate the photochemical behaviour of the excited molecule. A series of examples is given in which the injected photocurrents are increased or decreased by simultaneously occurring reactions between the excited molecules and other species in the electrolyte. The interpretation of the observed effects is compatible with the notions concerning analogous homogeneous photochemical reactions.

A Stable, High-Intensity Vacuum Ultraviolet Source

A stable, high-intensity resonance lamp operated by a 2450 Mc/sec microwave power source has been developed which is of sufficient intensity and stability to be useful for the kinetic study of photochemical reactions. This light source has maintained a constant output (approximately 2 × 1017 quanta/sec for krypton) for times ranging from 80 h to 120 h. The optimum pressure of krypton in the lamp is 0.28 torr.

Further studies of photo-chemical reactions in living human eyes

The method of rapid fundus reflectometry was applied to a detailed study of light-sensitive reactions in foveal and, mainly, extra-foveal parts of the human retina. The validity of the Bunsen-Roscoe law was found to be limited. At very high intensities, changes were observed which may be due to transient retinal oedema. Photo-labile material other than rod visual purple was easily detected: comparison with results obtained for the fovea suggests that the cone-pigments, primochrome and secundochrome, are present also in extra-foveal regions. Both bleaching rates and the amount of pigment bleached support the view that the cone inner limb has the ability to change the vergence of the incident light.