cage effect

When in a condensed phase, or in a dense gas, reactant molecules come together, or species are formed in proximity to one another, and are caged in by surrounding molecules, they may undergo a set of collisions known as an encounter; the term 'cage effect' is then applied.
The cage effect is also known as the Franck–Rabinowitch effect.
See also: cage.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 155