Submonolayer sensitive adsorption study of trichloroethene on single crystal surfaces by means of MIES, UPS and TPD

Abstract

By means of ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES), combined with temperature programmed desorption (TPD) experiments, the coverage dependent adsorption properties of trichloroethene (TCE) on Mo(100), Mo(112) and Pt(111) support were probed and are compared to findings on MgO(100). Peaks in the PE spectra corresponding to different MOs of the TCE molecule were fitted and their respective energy positions could be determined. Comparison with the reported IP energies of the gaseous TCE shows a uniform shift with coverage (save for small deviations on Pt for the submonolayer regime) of all orbital energies for the adsorbed molecule on each studied surface; evidence of a weak interaction of the molecule with the substrate. Additionally, the relative peak areas of the TCE MOs were investigated and showed signal saturation depending on the probe depth of the respective spectroscopic technique. The acquired TPD spectra are in accordance with weakly interacting TCE, which physisorbs on each substrate and condenses into multilayers without geometric reorganization of the molecule. The presented electron emission (EE) data demonstrates the detection limit of our UPS and MIES setup: At best UPS is sensitive to 0.23ML of TCE (detection of peak energy position and intensity of all five TCE MOs), while the MIES data still monitors all spectral features from TCE at 0.13ML (corresponding 1.1×1014TCEcm−2 for the less sensitive cases). Concerning peak energies only, the detection limit can in the best case be further extended down to 0.02 molecules per adsorption site (equal to θ=13%ML). The achieved (TCE) sensitivity is superior to NEXAFS and UPS data available in the literature (detection limit θ=1ML).