A crossed-beam study of the state-resolved dynamics of CH( X sup 2. Pi. ) + D sub 2. II. The isotopic exchange channel

The state-to-state integral cross sections for the isotopic exchange reaction of CH({ital X} {sup 2}{Pi}) with D{sub 2} to produce CD({ital X} {sup 2}{Pi}) have been measured in a crossed-beam apparatus by the laser-induced fluorescence method. Two types of measurements were performed: (1) the translational energy dependence of an individual quantum state of the product and (2) the state distribution of the products at fixed and well-defined translational energy. To understand some of the finer details of the reaction dynamics similar experiments were also carried out on the reaction CD({ital X} {sup 2}{Pi}) with H{sub 2} to give CH({ital X} {sup 2}{Pi}). For the isotopic exchange channel, the cross section decreased rapidly with increasing translational energy, signifying a complex formation reaction mechanism. The CD(CH) product rotational level distributions are substantially colder than a statistical expectation and are interpreted as the result of a multiple-impact collision between the receding products in the exit channel. A novel frequency-locking mechanism, similar to that proposed to understand the dynamics of inelastic collisions between CH and D{sub 2} (the preceding paper), is suggested to explain anomalous peaks in the product rotational level distribution for the isotopic exchange channel. However, the fine-structure state distributions for the reactive process, which are very different from that for the inelastic channel, cannot be rationalized by conventional interpretation. Some basic concepts about fine-structure selectivity in chemical reactions have been developed. With these concepts, it is conjectured that the fine-structure state distributions of open-shell molecules arising from reactive encounters could provide a fingerprint'' of the electronic wave function at the transition state.