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Chemical physics

Ultrafast relaxation in water

Nature volume 402, pages 473–475 (1999)Cite this article

When a molecule is excited, where and how fast does its excess energy go? The answer to this question is a prerequisite for understanding and predicting the course of many chemical and biological processes. Chemical reactions can take place only when enough internal energy has accumulated in the molecule. The study of chemical reactions is therefore intimately connected with the study of energy-relaxation processes that compete with the chemical reaction channel. Intermolecular excitation transfer is one such process that has been studied for more than half a century. A paper by Woutersen and Bakker on page 507 of this issue1 may force us to re-examine some of our notions about this important relaxation pathway.

When an isolated atom is optically excited it can relax to the ground state only by emitting radiation. In large molecular systems many degrees of freedom compete for the excitation energy and the winner rarely takes all. After a molecule is excited in solution, much of its energy usually ends up as increased solvent thermal motion, and can be regarded as wasted. Chemical interest often lies in other relaxation channels, for example electron transfer or chemical bond breaking. But even pathways that eventually lead to wasted thermal energy, such as intermolecular excitation transfer, can be of great interest at intermediate timescales. Knowing how energy flows between different molecular modes en route to complete relaxation can suggest ways to direct it to more useful channels, similar to harnessing the water flow in a river for useful work.

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Figure 1: Intermolecular energy transfer.

References

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  1. School of Chemistry, Sackler Faculty of Science, Tel Aviv University, Tel Aviv, 69978, Israel

    Abraham Nitzan

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  1. Abraham Nitzan
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Correspondence to Abraham Nitzan.

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Nitzan, A. Ultrafast relaxation in water. Nature 402, 473–475 (1999). https://doi.org/10.1038/44976

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