Effect of reagent translation on the dynamics of the exothermic reaction Ba+HF

Abstract

The Ba+HF (v=0)→BaF+H reaction has been studied as a function of relative collision energy (3–13 kcal/mole) using a crossed beam geometry in which a seeded HF beam intersects a thermal Ba beam. The vibrational and rotational distributions of the BaF product are determined from computer simulations of its excitation spectrum. The reaction cross section is found to have a low threshold (∼1 kcal/mole). With increasing collision energy the cross section increases to a maximum in the range of 6–8 kcal/mole. An upper bound of 15 Ų is placed on the absolute value of the reaction cross section. The fraction f ′ of energy appearing in translation, rotation, or vibration of the products is roughly constant over the range of collision energies studied with nearly half going into product translation and the remainder being divided nearly equally between product rotation and vibration. However, while rot≳ increases slowly with collision energy, vib≳ first rises then falls and trans≳ first falls then rises, the crossover occuring at collision energies for which the reaction cross section reaches its maximum. The fractional energy disposal in the different product modes is qualitatively consistent with the calculations of phase space theory, but the detailed behavior cannot be matched. However, the product rotational distribution observed for each vibrational level agrees closely with the predictions of this model, which is a consequence of the kinematic constraint for this mass combination. The Ba+HF reaction as a function of collision energy shows both similarities and differences with the corresponding studies for Ba+HCl and Ba+HBr. Possible explanations for this contrasting behavior are presented.