摘要：

Radiationless transitions consist of internal conversion, a spin-allowed electronci relaxation, and intersystem crossing, a spin-forbidden electronic relaxation. The historical development of radiationless transitions will be briefly reviewed.I will then discuss the importence of Duschinsky effect on the radiationless transitions of intermediate size molecules like ethylene. Ab initio calculations of the internal conversion in ethylene will be demonstrated. Sub-picosecond internal conversions have been observed to take place in the photosynthetic bacterial reaction centers. In this case,vibrational relaxation is not faster than internal conversion and thus "temperature" has no meaning in this case. I will show how to theoretically treat these ultrafast nonradiative processes.

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