On the validity of the energy sudden approximation

This paper contains an examination of the conditions under which the energy sudden (ES) approximation may be expected to be valid. Our approach involves using dimensional analysis to identify (dimensionless) quantities which control energy suddenness and in this fashion three sets of ES criteria emerge. One involves the relative kinetic energy between collision partners and the energy spacing of the internal states of interest; another the strength of the coupling interaction and the same spacing; and a third involves the masses of the colliding molecules and component atoms. We discuss the relationship between these conditions and the justifications given by earlier workers for adopting the ES approximation and then the mass conditions in particular are used as the basis for certain broad statements concerning the applicability of the ES method within nonreactive diatom-diatom and reactive atom-diatom collisions. Finally, a number of avenues for further development of this work are discussed.