Real time XPS investigation of the impact energy dependence of C2H4 adsorption on Ag(100)

We investigate ethylene adsorption on clean and oxygen-covered Ag(100) dosing gas-phase molecules at selected kinetic energies E-i by a supersonic molecular beam and by performing time-resolved x-ray photoemission measurements at the SuperESCA beamline of Elettra. In accord with a previous study [Chem. Phys. Lett. 331, 177 (2000)] we find that at low crystal temperatures T ethylene can access a variety of adsorption states. In particular, metastable physisorption is observed for all E-i. Chemisorption into the pi-bonded state occurs only for ethylene seeded in He (E-i=0.36 eV) and for T<110 K for the clean surface, while it occurs readily when the surface is precovered with oxygen, the barrier to adsorption being removed. Apart from these species the data show the existence of a further, very weakly bound, state desorbing at T=110 K which was not reported previously, to our knowledge.