Dissociation and predissociation

Another fate for an electronically excited molecule is dissociation, the breaking of bonds (Fig. 14.26). The onset of dissociation can be detected in an absorption spectrum by seeing that the vibrational structure of a band terminates at a certain energy. Absorption occurs in a continuous band above this dissociation limit because the final state is an unquantized translational motion of the fragments. Locating the dis sociation limit is a valuable way of determining the bond dissociation energy. In some cases, the vibrational structure disappears but resumes at higher photon energies. This predissociation can be interpreted in terms of the molecular potential energy curves shown in Fig. 14.27. When a molecule is excited to a vibrational level, its electrons may undergo a redistribution that results in it undergoing an internal conversion, a radiationless conversion to another state of the same multiplicity. An internal conversion occurs most readily at the point of intersection of the two molecular potential energy curves, because there the nuclear geometries of the two states are the same. The state into which the molecule converts may be dissociative, so the states near the intersection have a finite lifetime, and hence their energies are imprecisely defined. As a result, the absorption spectrum is blurred in the vicinity of the intersection. When the incoming photon brings enough energy to excite the molecule

Fig. 14.26 When absorption occurs to unbound states of the upper electronic state, the molecule dissociates and the absorption is a continuum. Below the dissociation limit the electronic spectrum shows a normal vibrational structure.

Fig. 14.27 When a dissociative state crosses a bound state, as in the upper part of the illustration, molecules excited to levels near the crossing may dissociate. This process is called predissociation, and is detected in the spectrum as a loss of vibrational structure that resumes at higher frequencies.

to a vibrational level high above the intersection, the internal conversion does not occur (the nuclei are unlikely to have the same geometry). Consequently, the levels resume their well-defined, vibrational character with correspondingly well-defined energies, and the line structure resumes on the high-frequency side of the blurred region.

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