ASSESSING THE ACCURACY OF THE VIBRATIONAL SUDDEN APPROXIMATION IN COLLINEAR ATOM PLUS TRIATOMIC MOLECULE COLLISIONS

Exact and vibrational sudden approximation (VSA) transition probabilit ies for He + HI2, BeI2, BeBr2 and CO2 at 0.5 to 3.0 eV are calculated and compared. The agreement between the VSA and the exact results for the excitation of the symmetric stretch is excellent for HI2 (even for large changes in the vibrational quantum number), degrades slightly o n going to BeI2 and again to BeBr2 and becomes unacceptable for the CO 2 case. These trends are in accord with previous validity criteria. Th e agreement for the excitation of the asymmetric stretch is only quali tatively correct.