Collisional activation and reaction

In a true unimolecular reaction, the energized molecules are formed by absorption of electromagnetic radiation see Section 7.2.2. In an apparent unimolecular reaction, the first step is the formation of the energized molecules by collisions with other molecules. We consider in the following subsection the interplay between the formation of energized molecules—by bimolecular collision—and their subsequent reaction. 

The energy needed for activation of a molecule can be provided by collisions with 

Activation by inelastic collisions is also called thermal activation. After a large number of collisions, a distribution over internal (rotational and vibrational) states will be established as given by the Boltzmann distribution at the given temperature. 

The activation and its interplay with chemical reaction is often described by the so-called (generalized) Hinshelwood Lindemann mechanism  

The apparent unimolecular rate constant kuni may be written 

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