Internal Energy of Reaction Products by Velocity Analysis. I. Scattered KBr* from the Crossed Molecular Beam Reaction K+HBr

The velocity distribution of excited KBr* formed in reactive collision between crossed beams of velocity-selected K and thermal HBr has been measured as a function of laboratory scattering angle for two different initial relative kinetic energies, E. The most probable value of the recoil velocity of the KBr* yields the final relative kinetic energy E′ and thus the value of the most probable ``collision-exothermicity'' Q ≡ E′ − E = ΔD0° − ΔEint, where ΔEint is the most probable value of the internal energy change. Results of a series of experiments at E = 1.9 kcal/mole gave Q = 0.5±0.5 kcal/mole (out of an a priori range from − 1.9 to +5.1 kcal/mole) and thus the most probable internal (excitation) energy for the KBr: E*KBr = 4.6±0.7 kcal/mole. The observed angular and velocity distributions of KBr imply that the reactive scattering in the center-of-mass (c.m.) system is predominantly confined to low angles (i.e., ``forward hemisphere,'' with the KBr* product preferring the direction of the incident K in the c.m.). Less extensive experiments at E = 3.4 kcal/mole served to confirm these conclusions.