Four-center reactions: A quantal model for H-4

We develop a quantal model for studying four-center reactions, A(2)+B-2-->2AB, and collision induced dissociation A(2)+B-2-->A+B-2+A. The method involves using hyperspherical coordinates to describe vibrations of the A(2) and B-2 bonds and a global vibration and rotation of the exchange products. Application to the H-4 system is presented, using a realistic potential energy surface. The reaction goes through a four-center linear transition state located just above the dissociation threshold. In the energy range studied (5-5.5 eV), collision induced dissociation competes with the four-center reaction and is the dominant process. It is found that vibrational energy, originally deposited in one of the diatomic partners, is much more efficient than translational energy in promoting reaction. Vibrational and rotational final distributions show that the products are internally hot. This simple quantal model, yet very demanding computationally, illustrates in detail many features of the H-4 dynamics above the dissociation threshold, and could serve to study other four center reactions with trapezoidal or linear transition states. (C) 1996 American Institute of Physics.