INTRAMOLECULAR ENERGY-FLOW AND BOND-DISSOCIATION IN IODOACETYLENE ANDIODODIACETYLENE

Intermolecular and intramolecular energy flow and subsequent bond diss ociation in collinear collisions I-C=C-H+Ar and I-C=C=C=C-H+Ar have be en studied by classical trajectory techniques over the collision energ y range of 0 to 10 eV. When the molecule is initially in the ground st ate, the overall energy transfer in I=C=C-H+Ar is very small, but in I -C=C=C=C-H+Ar it is large. The collisionally perturbed C-H bond stores a large amount of-energy from translation for a brief period during t he early stage of collision and transfers most of it to the inner regi on of the molecule, specifically to the low frequency C-I vibration. T hus the high-frequency vibration of the perturbed C-H bond during the collision plays a crucial role in determining the extent of intramolec ular energy transfer and, in turn, C-I dissociation. But in nondissoci ative collisions, there is another series of the C-H vibration at the latter stage of collision, transferring energy back to translation. Th is study also considers collision-induced intramolecular energy flow i n the molecule with an initially excited C-H bond. The relaxation of t he low-lying C-H excitation is very slow on a nanosecond time scale. H owever, when the excitation is high, the vibrational frequency of the C-H bond is significantly weakened, thus becoming comparable to that o f the triple bond, in which case the isolating effect of the adjacent C=C bond is no longer important and intramolecular energy flow becomes efficient.