Abstract
The problem of the formation cross section of a meso-molecule inside a normal molecule is shown to be an example of the applicability of a low-energy version of the impulse approximation, related to the neutron scattering theories of Fermi (1936) and Lamb (1939). The key property is the strong short-range nature of the meso-molecular potential. Previous theories are identified within this approach. Attention is given to the relation between formation and decay widths of the resonant states.