The adsorption structures and thermal desorption behavior of C2H2 on Ge(100) were studied in ultrahigh vacuum by scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). The STM investigation revealed that, at low coverage, C2H2 initially adsorbs onto the Ge(100) surface with two types of configurations: (i) a di-sigma configuration on top of a single Ge-Ge dimer (on-top) and (ii) a tetra-sigma configuration parallel to the dimer axes, bridging two neighboring Ge dimers (p-bridge). TPD measurements show that chemisorbed C2H2 desorbs from Ge(100) nondissociatively with two different desorption features, denoted as alpha (520 K) and beta (560 K). In addition, it was found that the desorption of C2H2 follows first order kinetics for both states and that the desorption energies of the alpha (520 K) and beta (560 K) states are 1.3 and 1.4 eV, respectively. STM studies of the adsorption of C2H2 at various Ge surface temperatures indicate that the alpha and beta features correspond to the on-top and p-bridge configurations, respectively.(C) 2002 American Institute of Physics.