A NEXAFS AND UPS STUDY OF THE ADSORPTION OF TETRACHLOROETHYLENE, TRICHLOROETHYLENE, ISO-DICHLOROETHYLENE, CIS-DICHLOROETHYLENE, AND TRANS-DICHLOROETHYLENE ON PLATINUM SURFACES AT 95-K - MULTILAYERS AND MONOLAYERS

UPS spectra of multilayers and monolayers of chlorinated ethylene molecules condensed or adsorbed on Pt(111) and Pt(110)(1 x 2) surfaces at 95 K are in close agreement. It shows that adsorption of these molecules is not dissociative and that they are weakly bonded to the surfaces in the monolayer range. NEXAFS has also been used in the case of Pt(111) samples. The multilayer spectra compare well with the analogous spectra of the fluorinated molecules, and the spacings between the various C1s levels agree with the corresponding values for the pi* transitions. This shows that these resonances involve C1s core level transitions to unoccupied pi* levels, which are located at almost the same energy in the excited state. The polarization dependence of the synchrotron light indicates that for monolayers the C=C axis and molecular planes are parallel to the surface. The pi* resonance widths confirm that the adsorption of all chlorinated molecules is not dissociative at low temperatures, as suggested from the catalytic destruction of unsaturated chlorinated C2 compounds which occurs only at high temperatures. Above 120-130 K and under vacuum, desorption depletes the surface without conversion to a more firmly bonded species, a process occurring under catalytic conditions.