The absorption spectrum of triplet pentacene in benzene and cyclohexane was determined at 50°C and assignments of the peaks were made. A kinetic study of the triplet absorption showed that the triplet decays by three competing processes : (1) a temperature-independent (between -96 and + 50°C) radiationless transition to the ground state, (2) a triplet-triplet quenching process of low efficiency, and (3) a reversible reaction with ground-state pentacene producing a transient compound assumed to be a pentacene dimer. 1937

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The methanol photodecomposition quantum yield increases with the addition of different foreign gases (alkanes, rare gases, C 0 2 , SF6). Whereas the methanol absorption spectrum presents a continuum in the 1850 8, region, the added gas effect implies that collisions are involved in reaching the dissociative state. This apparent contradiction between photochemical and spectroscopic data leads to the postulate of the existence of a collision-induced predissociation. The efficiency of the different gases in promoting the predissociation correlates with their polarizability. In absence of spin-orbit coupling, the collision-induced predissociationc an be explained on the basis of a van der Wads interaction. With Kr and Xe as added gases, spin-orbit coupling is an additional dissociative channel.

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The properties of thin gelatin films sensitized with a dye (methylene blue) when used as a holographic material are discussed. On illumination the excited dye molecule changes its structure giving a colorless molecule. The higher rate of bleaching and therefore the higher diffraction efficiency of the developed phase gratings are obtained in an oxygen-free atmosphere. Furthermore, the light absorption probability of the dye molecules depends on its position with respect to the electric vector of the light, with the result that the exposed part of the dye becomes dichroic when illuminated with linearly polarized light. This result allows the use of this material for polarizing holography.