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Photochemical studies with the gaseous flash discharge. I 391 known photochemical reactions was profoundly changed at high intensities, and it was therefore not possible to use these reactions as a measure of intensity. A few measurements using the uranyl-oxalate actinometer were reported, but the interpretation was necessarily provisional in the absence of any information on the behaviour of this actinometer at high intensities. This part is concerned with an investigation of the properties of the uranyl-oxalate actinometer up to the highest available intensities. Part II describes a study of the properties of the flash discharge in the ultra-violet region using both the uranyl-oxalate actinometer and photographic methods.
392 M. I. Christie and G. Porter the ideal of a point source and parallel beam cannot be attained simultaneously with the high light output required, and some form of compromise was necessary. The shape of the lamp and the reflector were therefore chosen to give an approxi mation to a parallel beam with a minimum loss of radiated energy. Constructional details were as described in a previous paper (Porter 1950a). In order that the lamp could be pumped out and refilled situ it was connected to a vacuum system containing the usual pumps, a mercury manometer, a spinning McLeod gauge, a collapsed spoon gauge and krypton supply.
Photochemical studies with the gaseous flash . I 393 Since the extinction coefficients of the uranyl-oxalate system vary rapidly with wave-length in the region 2000 to 4900A it was essential to reduce the intensity of the flash by strictly neutral methods so that the relative distribution of energy with wave-length was unaltered. The methods used were: (1) calibrated screen and (2) distance effect.
394 M. I. Christie and G. Porter (2) Table 1 shows that the decomposition per flash is independent of the number of flashes, confirming that the effective light absorbed is independent of oxalate decomposition (to 40%). In this respect, therefore, the photolysis at flash intensity is the same as that at ordinary intensities.
396 M. I. Christie and G. Porter effective in decomposing uranyl oxalate was computed by graphical integration of the expression /•4900 / /*4900 AX,dQA/ dA, J 2000 / 2000 where Ax is the percentage absorption in 2-54 cm of light of wave-length A (obtained as described) and Qx is the quanta emitted at wave-length A (relative values obtained from the distribution with wave-length (part II)). A was found to be 36 %. This value is approximate only, since the values of Ax and conse quently A refer to a parallel beam of light passing normally through the cell.