Abstract: In a beam-beam scattering experiment and under the single-collision condition the two channel reaction

\mathrmBa+\mathrmCH_2 \mathrmClBr \rightarrow\left\\beginarrayl
\mathrmBaCl+\mathrmCH_2 \mathrmBr （a） \\
\mathrmBaBr+\mathrmCH_2 \mathrmCl （b）
\endarray\right.

has been studied using laser induced fluorescence(LIF). The ro-vibrational distributions as well as branching ratio of BaCl and BaBr has been obtained by means of LIF spectrum simulation. The product energy partitions of BaCl and BaBr are \mathrmf_\mathrmv(\mathrmBaCl)=21 \% 、 \mathrmf_\mathrmv(\mathrmBaBr)=23 \%, f_R(B a C l)=9 \% 、 f_R(B a B r)=17 \% respectively and the branching ratio is P(a) / P(b)=0.39. It is found that (1) besides the dynamic constraints for the vibrational and rotational excitations of the products there is another independent dynamic constraint for the branching ratio; (2)the co-linear intermediate configuration in channel (a) produces BaCl mainly in lower rotational levers and the higher rotational excited BaBr are produced mostly through a bent intermediate configuration in channel (b); (3) the vibrational distributions of BaCl and BaBr are affected by one and two dynamic constraints respectively. It implies that the different mechanisms may be caused by the different configuration change of \mathrmCH_2 \mathrmClBr during the reaction.