The two general paradigms of
dissociation dynamics are direct dissociation (DD) and
precursor mediated dissociation (PD).
The two processes occur on very different timescales and their experimental
signatures and theoretical treatments are quite different. The "definitions" of
the processes are most easily understood when they are side-by-side.
| Direct Dissociation | Precursor Mediated Dissociation | |
|---|---|---|
|
The reaction proceeds from molecule to separated fragments smoothly from start
to finish without stopping, i.e. there is a gradual uninterrupted evolution
from molecule to separated atoms. Here is an illustration (Animated GIF - 6.4 kBytes) Or a frames version (better for PC's) |
The molecule is trapped and bounces around intact in some intermediate state before dissociating. This
intermediate state could be, for instance, a negative molecular ion. Here is an illustration (Animated GIF - 12.1 kBytes) Or a frames version (better for PC's) |
Of course this clear division is an idealisation and in reality there are many possibilities between purely direct behaviour and total trapping.
As a working definition let's call all dissociation reactions involving
intermediate steps precursor mediated, even when the precursor is chemically
identical to the incident molecule. This definition then includes physisorption and trapping in
dynamic precursors
due to excitation of the internal molecular states (i.e. rotations and
vibrations) as the adsorption bond is formed.
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©G.R. Darling