Laser assisted associative desorption of N2 and CO from Ru(0001)

Abstract

An experimental technique, laser assisted associative desorption (LAAD), is described for determining adiabatic barriers to activated dissociation at the gas-surface interface, as well as some aspects of the dynamics of associative desorption. The basis of this technique is to use a laser induced temperature jump (T-jump) at the surface to induce associative desorption and to measure the translational energy distribution of the desorbing molecules. The highest translational energies observed in desorption are a lower bound to the adiabatic barrier and the shapes of the translational energy distributions provide information on the dynamics. Implementation of the experimental technique is described in detail and unique advantages and possible limitations of the technique are discussed. The application of this technique to very high barrier surface processes is described; associative desorption of N2 from Ru(0001) and CO formed by C+O and C2+O on Ru(0001). N2 barriers to dissociation increases strongly with N coverage and co-adsorbed O, in good agreement with DFT calculations. No isotope effects are seen in the associative desorption, indicating that tunneling is not important. The full energy distributions suggest that very large energy loss to the lattice occurs after recombination at the high barrier and prior to N2 desorption into the gas phase. The mechanism for this remarkably large energy loss is not well understood, but is likely to be general for other high barrier associative desorption reactions. CO associatively desorbs nearly thermally from both C+O and C2+O associative reactions. It is argued that this is due to large energy loss for this system as well, followed by indirect scattering in the deep CO molecular well before final exit into the gas phase.