Low-Temperature STM and UPS Study of Adsorption States of 1,4-Cyclohexadiene on Si(100)c(4×2)

The adsorption states of 1,4-cyclohexadiene on Si(100) were studied with low-temperature scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We newly observed two kinds of molecular images by STM, which clearly differed from the previous STM results obtained at room temperature. Molecular protrusion was observed between the down Si dimer atoms of the neighboring dimer rows on Si(100)c(4×2) at 80 K. After heating the substrate, this species converted to a new species that was located on the Si dimer. The UPS results indicate that adsorbed 1,4-cyclohexadiene has two π bonds at 100 K. After annealing at 140 K, one of the two π bonds reacts with the dimer. Thus, we conclude that 1,4-cyclohexadiene is adsorbed on Si(100)c(4×2) as a π-complex species at ∼100 K and that the π-complex species is transformed to a di-σ species after the substrate is heated beyond 140 K. The present results agree with a recent report by Kato et al. [J. Phys. Chem. C 2007, 111, 2557], and we must rev...