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THIRD REPORT OF THE COMMITTEE ON PHOTO- CHEMISTRY,1 NATIONAL RESEARCH COUNCIL INTRODUCTION HUGH S. TAYLOR Department of Chemistry, Princeton University, Princeton, New Jersey Received May 25, 1988 The First Report of the Committee on Photochemistry presented six papers dealing with the quantitative technique of photoreactions, the clas- sical point of view with respect to these processes, and the relation between the physical concept of quantized absorption and the chemical processes which succeed such absorption. The Second Report summarized the researches of the physicist with respect to the absorption process and their implications in the primary absorption process in photochemistry, and gave some examples of the secondary processes consequent upon such in- itial processes of absorption. The discussion was necessarily conned to gaseous systems since, at that time, knowledge of the nature of the indivi- dual steps in condensed systems was less certain. In the seven years that have elapsed since the previous report there have been a large number of contributions to the subject and the technique of photochemistry. These have served to broaden the bases upon which the science rests, to add to our techniques, to increase the quantitative nature of our knowledge, to permit an extension of our investigations to the more complex condensed systems, and to make possible a more scientific pres- entation of important problems in the field of applied photochemistry. Some phases of this activity are summarized in the present report, in which some eleven papers dealing with various topics are assembled. Professor Mantels reviews some of the contributions to experimental technique (page 701) and has prepared with the assistance of the Committee a critical table of quantum yields (page 713~. Professor Rollefson has summarized the more recent aspects of the primary absorption process (page 733~. In the analysis of reactions subsequent to the primary process, Professor Dickin- ~ This Committee of the National Research Council, Division of Chemistry and Chemical Technology, is composed (1937-38) of the following members: Hugh S. Taylor, Chairman, Wilder D. Bancroft, Farrington Daniels, Roscoe G. Dickinson, Philip A. Leighton, Samuel C. Lind, George K. Rollefson. Contributions to this report have also been kindly prepared by Gertrud Kornfeld and Winston M. Manning. ~

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2 HUGH S. TAYLOR son has discussed the correlation of photoprocesses in the gaseous phase with those in solutions (page 739~. Recent researches into a variety of photoprocesses have considerably increased our knowledge of the secondary processes in which free radicals are produced in the primary process. From this standpoint Professor Leighton has summarized the researches on aldehyde and ketone photolysis (page 749), and the writer has dealt with the reactions in alkyl iodides, metal alkyls, mercury-photosensitized hydro- genations and decompositions of hydrocarbons, and the photolysis of azo compounds (page 763~. Professor Rollefson has dealt generally with the problem of the evaluation of specific reaction rate constants (page 773), and the writer has summarized present knowledge in the case of ammonia decomposition (page 783) and the reactions involving the photochlorination of hydrogen and of carbon monoxide (page 789~. Two final contributions in the domain of applied photochemistry, that of Dr. Gertrud Kornfeld on "The Action of Optical Sensitizers on the Photographic Plate" (page 795) and of Dr. W. M. Manning on "Photosynthesis" (page 815), complete the series of contributions. It is not intended that this report shall represent an exhaustive summary of the photochemical work which has accumulated since the issue of the preceding report. Many publications in the field have not been covered, even though the papers here reviewed exceed six hundred. Other topics undoubtedly merit attention equal to that given to the subjects here dis- cussed. That this is so indicates both a healthy state of photochemical science and a continuing task for the Committee. .