Question

List and briefly explain what are the three decay paths of an electronically excited molecule to

its electronic ground state. Please explain and show all work.

Answer 1

Excited states have limited life times (typically in the order of a nanosecond) and they can decay via several modes. Sometimes the excited state is so weakly bound that it will dissociate. For example, ozone undergoes a photodissociation to O₂ and atomic oxygen after absorbing a photon of ultraviolet light.

A second possibility is that the excited state returns to the ground state without emitting a photon. Such radiationless decay occurs readily when the excited state and the ground state potential energy curves meet via a conical intersection. For example, the excited states of DNA bases in Watson-Crick base paired geometry are very short-lived thanks to efficient radiationless decay; this protects DNA from photochemical damage.

The third possibility of return to the ground state is via the emission of photon. Such radiative decay is commonly called fluorescence.