ON THE USE OF A BORN-OPPENHEIMER TYPE SEPARATION IN THE TREATMENT OF THE DYNAMICS OF MOLECULAR-COLLISIONS

The Born-Oppenheimer separation, commonly invoked in electronic/nuclea r motions, is extended to separate the motions of light and heavy nucl ei in the dynamics of molecular collisions. The method is applied to v arious heavy-light + heavy (hl + h) systems where the light atom is ei ther H or D. The hydrogenic wavefunctions, which describe the motion o f the light nucleus for clamped heavy nuclei, are found to have bondin g, anti-bonding or non-bonding properties depending on the localisatio n of the light nucleus. The Born-Oppenheimer approximation is valid fo r quasi-diabatic states which maintain the localisation property, when the distance between the heavy nuclei changes. A small number of thes e states support quasi-bound states of the triatomic systems, which ap pear as resonances in the dynamics. The quasi-diabatic states can be u sed to explain the dynamics of light atom exchange in hi + h collision s and of dissociative photodetachment of the (hlh)(-) anion. The metho d is shown to give very accurate results and to allow for a clear unde rstanding of the dynamics. (C) 1998 Elsevier Science B.V.