viii Contents 12.3 SIGMATROPIC REACTIONS 657 PROBLEMS 677 PHOTOCHEMISTRY 681 13.1 LIGHT ABSORPTION 681 13.2 UNIMOLECULAR PHOTOPHYSICAL PROCESSES 687 13.3 BIMOLECULAR PHOTOPHYSICAL PROCESSES 693 13.4 PHOTOCHEMICAL REACTIONS 706 PROBLEMS 729 Index 731