REVERSIBLE WORK TRANSITION-STATE THEORY - APPLICATION TO DISSOCIATIVEADSORPTION OF HYDROGEN

A practical method for finding free energy barriers for transitions in high-dimensional classical and quantum systems is presented and used to calculate the dissociative sticking probability of H-2 On a metal s urface within the transition state theory, The reversible work involve d in shifting the system confined to a hyperplane from the reactant re gion towards products is evaluated directly. Quantum mechanical degree s of freedom are included by using Feynman path integrals with the hyp erplane constraint applied to the centroid of the cyclic paths. An opt imal dividing surface for the rate estimated by the transition state t heory is identified naturally in the course of the reversible work eva luation. The free energy barrier is determined relative to the reactan t state directly so that an estimate of the transition rate can be obt ained without requiring a solvable reference model for the transition state. The method has been applied to calculations of the sticking pro bability of a thermalized hydrogen gas on a Cu(110) surface. The two h ydrogen atoms and eight surface Cu atoms were included quantum mechani cally and over two hundred atoms in the Cu crystal where included clas sically. The activation energy for adsorption and desorption was deter mined and found to be significantly lowered by tunneling at low temper ature. The calculated values agree quite well with experimental estima tes for adsorption and desorption. Dynamical corrections to the classi cal transition state theory rate estimate were evaluated and found to be small.