Ethylene adsorption on Ge(100)-(2x1): A combined angle-resolved photoemission and thermal desorption spectroscoipy study

摘要

Ethylene adsorption on vicinal, single-domain Ge(100)-(2 * 1) has been investigated by thennal desorption spectroscopy (TPD) and angle-resolved photoemission (ARUPS) using linearly polarized synchrotron radiation. Thermal desorption experiments show that chemisorbed C2H4 desorbs from Ge(IOO) nondissociatively around 393 K with a high temperature shoulder which is tentatively assigned to step site desorption. The ethylene saturation coverage is strongly temperature dependent. Adsorption at 90 K saturates at 0.38 monolayer (ML), whereas adsorption at 170 K leads to a saturation coverage of approximately 1 ML. This behavior is explained by an adsorption barrier for coverages exceeding 0.38 ML. ARUP spectra for a dilute and the saturated ethylene monolayer reveal clear differences. Using photoeinission selection rules a highly ( C2u) symmetric adsorption geometry with a C-C bond axis parallel to the Ge-Ge dimmer axis is found for the dilute layer; whereas a reduced C2 adsorption symmetry is found for the saturated ethylene layer. The comparison of photoemission spectra for C_2H_4 on Ge(IOO) and Si(IOO) shows that C2H4 is di-u bound to the dangling bonds of a single Ge-Ge dimmer. For two molecular orbitals, 1 b3u and 1 b2g , one-dimensional band structures with dispersion widths of 0.5 and 0.39 eV, respectively, along the Ge-Ge dimmer rows are found which present a straightforward explanation for the observed symmetry reduction and adsorption behavior.