Precursor Mediated and Direct Adsorption: Two Channels Leading to Chemisorption

Abstract

The preceding papers by WEINBERG and by AUERBACH and RETTNER have addressed the question of how important precursors are in kinetic processes on surfaces from different perspectives. Both papers have considered the evidence for the existence of precursors and all authors agree that precursors to adsorption have been clearly identified under certain conditions. The clearest proof of existence of the precursor is its spectroscopic identification, and numerous studies [1–4] carried out at very low temperatures have obtained “fingerprint” spectra or other electronic features which can unambiguously be identified with extrinsic and intrinsic precursors. However, as WEINBERG points out, molecularly chemisorbed species can also be regarded as precursors to dissociatively adsorbed species. They can be isolated at much higher temperatures since the energies of adsorption, desorption and migration are scaled to higher values.