Photolysis of adsorbed polyatomic molecules on dielectric surfaces: General mechanisms

Abstract

The photochemistry of adsorbed molecules critically differs from gaseous processes due to very fast vibrational relaxation in adsorbates. The adlayer photolysis is only possible via repulsive states, but their number is limited by avoided crossing. Pure repulsive interactions often occurred only at long bonds near dissociation limits. Such regions cannot be populated by the absorption of molecules in the equilibrium ground state. A general way to reach them consists of photoexcitation of a molecule to some high vibrational level, stretching the bond in the course of its vibration, and transition via photon emission. The proposed mechanism was performed using a single UV laser. Various bonds in numerous adsorbates were exclusively ruptured in this way. Our experimental results for adsorbates were compared with published data on gaseous photolysis. This made it possible to determine the differences between surface and gaseous photochemistry, as well as distinctive features of the proposed mechanism.