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Timescales of photophysical processes

المؤلف:  Peter Atkins، Julio de Paula

المصدر:  ATKINS PHYSICAL CHEMISTRY

الجزء والصفحة:  ص845-847

2025-12-27

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Timescales of photophysical processes

Electronic transitions caused by absorption of ultraviolet and visible radiation occur within 10⁻¹⁶–10⁻¹⁵ s. We expect, then, that the upper limit for the rate constant of a first-order photochemical reaction is about 1016 s⁻¹. Fluorescence is slower than

absorption, with typical lifetimes of 10⁻¹²–10⁻⁶ s. Therefore, the excited singlet state can initiate very fast photochemical reactions in the femtosecond (10−15 s) to picosecond (10−12 s) timescale. Examples of such ultrafast reactions are the initial events of vision (Impact I14.1) and of photosynthesis. Typical intersystem crossing (ISC) and phosphorescence times for large organic molecules are 10⁻¹²–10⁻⁴ s and 10⁻⁶–10⁻¹ s, respectively. As a consequence, excited triplet states are photochemically important. Indeed, because phosphorescence decay is several orders of magnitude slower than most typical reactions, species in excited triplet states can undergo a very large number of collisions with other reactants before deactivation. The interplay between reaction rates and excited state lifetimes is a very important factor in the determination of the kinetic feasibility of a photochemical process.