Line of sight techniques: Providing an inventory of all species arriving at and departing from a surface

Abstract

Line of sight (LOS) techniques comprise those methods in which species emanating from a surface (atoms, molecules, and radicals) undergo just a single pass through the ionization volume of a mass spectrometer before being pumped. This is achieved by enclosing the mass spectrometer within a cryoshield fitted with appropriate apertures, such that line of sight is established only between a patch on the sample surface (≈7 mm diameter) and the ionization volume. All LOS techniques are free from extraneous signals and have approximately equal detection probabilities for all species. Line of sight temperature programmed desorption, sticking probability (LOSSP), and product desorption (LOSPD) provide powerful and reliable ways of studying all aspects of surface kinetics, by allowing an inventory of all species arriving at and departing from a surface, for any combination of partial pressures, surface temperature, surface composition, and surface structure. Here we illustrate LOSSP and LOSPD using the reactions of 1-bromo-2-chloroethane, BrCH2CH2Cl,(BCE) and iodotrifluoromethane, CF3I, on Cu(111). For BCE we show that there is a 1:1 correspondence of product ethene to reactant BCE during dissociative adsorption at T>253 K, and that the dissociative adsorption is nonactivated with a transition state 11±2.5 kJ mol−1 below zero (0=molecule at infinity). For CF3I dissociative adsorption occurs at room temperature with a sticking probability of 0.96±0.02 to produce CF2⋅ which can either desorb as gaseous CF2⋅ radicals (observed) or undergo a coupling reaction and then desorb as gaseous C2F4 (also observed). No other gas phase products were observed.