The Frontiers of Photochemistry

Photophysics and photochemistry are relatively young sciences, a real understanding of light-induced processes going back some 50 or 60 years. The development of quantum mechanics was an essential step, as classical physics cannot account for the properties of excited states of atoms and molecules. In the past 30 years the advent of new experimental techniques has given a major impetus to research in new areas of photochemistry, and these are the subject of this final chapter. It must of course be realized that these developments advance all the time, and that we talk here of a moving frontier, as it is in 1992. 

In the first place, we shall take a look at the recent advances in fast reaction photochemical kinetics and spectroscopy, in particular at picosecond laser flash photolysis and femtosecond observations. Next, photophysics and photochemistry in molecular beams will be considered. Here observations are made under single molecule-single photon conditions, and these experiments provide insight into the most fundamental unimolecular gas phase reactions. 

It would be presumptuous to attempt to forecast the future developments in this very wide field of light-induced processes which, as we have seen, covers photophysics, photochemistry and photobiology, with many practical applications. There is no set limit to knowledge, for we have learned since the last century that nature is infinitely more complex than we like to imagine. The advance of knowledge is then like an inflating balloon. As the content of knowledge increases with its volume, so the frontier with the unknown also increases as its surface. 

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Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the medium of chapters written by pioneers who are experts. The editorial policy has always been to solicit articles from scientists who have strong personal points of view, while encouraging critical discussions and evaluations of existing data. In no sense have the articles been simply literature surveys, although in some cases they may have also fulfilled that purpose. 

With the increased sophistication in experiment and interpretation since that time, photochemists have made substantial progress in achieving the fundamental objective of photochemistry elucidation of the detailed history of a molecule that absorbs radiation. The scope of this objective is so broad and the systems to be studied are so many that there is little danger of exhusting the subject. We hope that this series will reflect the frontiers of photochemistry as they develop in the future. 

This last problem is perhaps not strictly within the realm of photochemistry. It is however so important potentially that it cannot be overlooked on grounds of arbitrary separations between different branches of scientific research (perhaps the expression of interdisciplinary approach would best describe it). A few pages will therefore be devoted to the science and technology of artificially organized molecular systems such as monomolecular and multimolecular layers, micelles and spatially restricted environments like zeolites and since we reach here another of the frontiers of photochemistry, section 8.4 in the final chapter is devoted to these systems. 

Photochemistry has expanded enormously since those first days. A serious percentage of the papers in any single volume of the Journal of the American Chemical Society, for instance, can rightly fall in its purview. The emergence of the laser and the evolution of theoretical methods strongly influenced research. With new computational methodology almost no intermediate lives too short a time to be detected and its dynamics characterized. The fundamental objective of our field, elucidation of the history of a molecule that absorbs radiation, is now within reach in even the most complicated cases. We hope that the series continues to reflect the frontiers of photochemistry as it evolves into the future. 

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