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 mole cules 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 molecul es 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(1 11) samples. The multilayer spectra compare well with the analogous sp ectra of the fluorinated molecules, and the spacings between the vario us C1s levels agree with the corresponding values for the pi transiti ons. This shows that these resonances involve C1s core level transitio ns to unoccupied pi levels, which are located at almost the same ener gy in the excited state. The polarization dependence of the synchrotro n light indicates that for monolayers the C=C axis and molecular plane s are parallel to the surface. The pi resonance widths confirm that t he adsorption of all chlorinated molecules is not dissociative at low temperatures, as suggested from the catalytic destruction of unsaturat ed chlorinated C2 compounds which occurs only at high temperatures. Ab ove 120-130 K and under vacuum, desorption depletes the surface withou t conversion to a more firmly bonded species, a process occurring unde r catalytic conditions.