Classical S Matrix for Linear Reactive Collisions of H+Cl2

Classical trajectories are computed for linear H+Cl2 collisions and used to construct the classical limit of the S matrix for reactive and nonreactive transitions between individual quantum states. An interesting feature of this system is that both ``direct'' and ``complex'' mechanisms participate in the collision dynamics. The two mechanisms contribute additively to individual S-matrix elements, and within a ``random phase approximation'' for the complex contribution it is seen that they also contribute additively to the transition probability. The complex contribution to a transition probability is strictly classical, but interference structure may be prominent in the direct contribution. Comparison is made with quantum calculations for the same potential and the agreement is rather poor, even at a fairly coarse level. The reason for this disagreement is not completely clear, but it may be connected with the fact that complex formation is prominent.