Four-center reactions: A quantal model for H4

Abstract

We develop a quantal model for studying four-center reactions, A2+B2→2AB, and collision induced dissociation A2+B2→A+B2+A. The method involves using hyperspherical coordinates to describe vibrations of the A2 and B2 bonds and a global vibration and rotation of the exchange products. Application to the H4 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 H4 dynamics above the dissociation threshold, and could serve to study other four center reactions with trapezoidal or linear transition states.