Dissociation and Predissociation

If excitation transfers a molecule in a state containing more energy than the separated components (atoms, or molecules), dissociation takes place. Above the dissociation limit the energy is not quantized and energy continuum appears instead of discrete energy levels. The dissociative state is not stable, which means that the molecule disassembled into the separated components very quickly (approximately during the time of one vibration).

In some spectra the discrete energy level structure disappears already for energies which are below the dissociation limit but reappears again at some higher energies. This phenomenon is called predissociation.

Predissociation can be usually explained as a result of internal conversion from an excited state A into a dissociative state B. We assume relationship between the vibrational states closely above the level of intersection and the translational motion during dissociation. These circumstances allow a separation of a molecule via state B without supplying the energy of dissociation for state A. Note that, in the range beyond the blurred caused by predissociation, discrete lines are found. An interpretation would be that for higher vibrational states, a conversion towards state B is excluded again.

Sometimes, predissociation is a result of some additional external factors. For example, we can imagine a molecule to be pushed from state A to state B due to collision with another atom, or molecule. Alternatively, an external electric, or magnetic field can cause to the predissociation of an initially stable molecule. In such cases, we speak of collision- or field- induced predissociation.

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