Laser assisted associative desorption of N-2 and CO from Ru(0001)

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 assoc iative desorption. The basis of this technique is to use a laser induced te mperature jump (T-jump) at the surface to induce associative desorption and to measure the translational energy distribution of the desorbing molecule s. The highest translational energies observed in desorption are a lower bo und to the adiabatic barrier and the shapes of the translational energy dis tributions provide information on the dynamics. Implementation of the exper imental technique is described in detail and unique advantages and possible limitations of the technique are discussed. The application of this techni que to very high barrier surface processes is described; associative desorp tion of N-2 from Ru(0001) and CO formed by C+O and C-2+O on Ru(0001). N-2 b arriers 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 latti ce occurs after recombination at the high barrier and prior to N-2 desorpti on 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 C-2+O associative reactions. It is argued that this is d ue to large energy loss for this system as well, followed by indirect scatt ering in the deep CO molecular well before final exit into the gas phase. ( C) 2001 American Institute of Physics.