Abstract: Under the reaction of a heavy atom Awith a heavy atom B-light atom Cmolecule, information on the reactive impact parameters can be obtained. On the basis of previous work, we report the impact parameter and relative velocity dependence of chemical reactions. The state-to-state reaction, Ba(¹S₀)+HI(X¹Σ⁺, v, J)BaI(X²Σ⁺, v, J)+H(²S_1/2), was performed under crossed beam conditions using high resolution, high stable laser spectroscopy to determine vibrational and rotational resolved population distributions of BaIin relative velocity(v_r) range of 850 m/s to 1300 m/s. Velocity distributions of the reagents were measured using Doppler and Doppler-free spectroscopy for Ba and time-of-flight method for the supersonic HIbeam. The results, combined with previous experimental studies on this reaction system, show a remarkable collision energy dependence. Between low and high collision energies, a transition occurs in the intensity, width, and peak location of the product vibrational and rotational population distributions. The rotational distributions of the crossed-beam experiments are extremely narrow but broaden at lower collision energies. As the collision energy is increased above 20.9 kJ/mol, the BaIrotational excitation is very near the energetic limit, and the maximum for the BaI(v=0) rotational population distribution moves from J=415.5 to J=538.5. In contrast, below the transition onset, the maximum remains unchanged around J=420.5. Morever, the peaks of the BaI(v=1) and BaI(v=2) rotational distributions appear at successively lower J values, as expected from energy conservation arguments. Detailed analysis of the collision energy dependence of the specific opacity functions offers insight into the role of conservation of energy and angular momentum in influencing this reaction. At low collision energies, the maximum reactive impact parameter, b_max, is determined by an angular momentum(centrifugal) barrier. At collision energies larger than_20.9 kJ/mol, conservation of energy dictates the value of b_max. These two processes are identified as the mechanisms that control the Ba+HIreaction cross section. The transition between the two mechanisms provides an interpretation for the bimodal character of the BaIproduct internal-state distribution. When v_r changes from 850 m/s to 1300 m/s, the maximum impact parameter(b_max) of BaI(v=0) changes from 0.460 nm to 0.390 nm. The shape of the derived specific opacity function P_v(b)(truncated Gaussion function) changes with v_r. The relative cross-reaction appears to vary with reagent relative velocity v_r, the cross-section increases with v_r and reaches a maximum at 1100 m/s. Then, the cross-section shows a more pronounced decrease at large collision energies. The behavior for low relative velocity indicates the existence of a reaction energy barrier.

Key words: Impact parameter, Collision energy, Statetostate reaction, Specific opacity function