When energy is absorbed by a molecule at a particular bond by a process of transition to a repulsive energy level or a sufficiently high point on an attractive level, rupture always results within one vibration period. All other photodecomposition processes are properly included under the title of predissociation. Thus, rupture may also result at the locus of absorption as the result of either spontaneous or induced adiabatic transition from an attractive to a repulsive or ``weakly attractive'' level. These are the only mechanisms of predissociation in diatomic molecules. In polyatomic molecules, spontaneous or induced shifts of energy or rearrangements of groups, resulting in decomposition, may also occur. Illustrations of the various processes suggested are cited from the photochemistry of sulfur, iodine, ozone, and the aldehydes. In the case of formaldehyde it has been possible to distinguish potential hypersurfaces and to indicate two different types of predissociation: one in a single hypersurface and the other a spontaneous transition between two hypersurfaces. The method used is extended to other aldehydes. Suggestions are made as to the interpretation of the photochemistry of several other substances and as to several problems which require solution.
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