Adsorption and desorption of acetylene on Ge(100)

Abstract

The adsorption and thermal desorption of acetylene on Ge(100) have been studied in ultrahigh vacuum by Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED). Acetylene is found to chemisorb and to desorb molecularly on Ge(100) via a mobile precursor in a different way with Si(100) on which chemisorbed acetylene was found to undergo two thermal reactions; a minor pathway (molecular desorption) and a major pathway (dissociation of acetylene). The difference between activation energy for desorption from the precursor and that for reaction from the precursor into the chemisorbed state is found to be ($E_p$ - $E_c$) = 3.6 ± 0.23 kcal/mol. TPD measurements show that two molecular desorption features at all coverages, indicating that at least two adsorption states can be formed. For desorption kinetics, $C_2H_2$ desorption is first order reaction and the activation energies for desorption at saturation coverage are 30.6 kcal/mol for $α_1$ state and 32.9 kcal/mol for $α_2$ state whereas the energy for desorption of acetylene from Si(100) was found to be 46.1 ± 2.0 kcal/mol. The lower desorption activation energy allows $C_2H_2$ to desorb prior to significant dissociation which is a major pathway in desorption from Si(100). The angular distrbution of $C_2H_2$ desorption is peaked at surface normal. The results are discussed on the basis of bonding geometry. From the retention of 2$\times$1 LEED patterns as increasing the $C_2H_2$ coverage, it is also concluded that $C_2H_2$ on Ge(100) chemisorbs molecularly.