Low-temperature adsorption states of benzene on Si(111)7x7 studied by synchrotron-radiation photoemission

The benzene adsorption on Si(111)7 x 7 was investigated by synchrotron-radi ation valence-band photoemission in the temperature range 30-300 K. We foun d that when benzene is adsorbed on Si(111)7 x 7 at low temperature there is a one-to-one correspondence between the adsorbed and the gas-phase feature s. Furhermore, the interaction of benzene with the surface states is very w eak, hinting at a physisorption process. Upon temperature increase, the ads orption features change gradually, but significantly. In particular, we obs erve a splitting of the features related to the degenerate orbitals, indica ting that the benzene bond to the silicon surfaces is turning into chemisor ption. This is the first evidence by photoemission spectroscopy that such a transition occurs, to our knowledge. Furthermore, the spectrum we obtain u pon transition from the physisorbed to the chemisorbed state corresponds to the one obtained by dosing benzene at room temperature, thus showing that the chemisorption state does not depend on the way it is achieved. Upon exp osure to a benzene multilayer we observe a direct transition from multilaye r to chemisorbed state without intermediate transition to a physisorbed sta te, thus indicating that the multilayer desorption and the physisorption-to -chemisorption transition occur simultaneously. The adsorption of a few ben zene overlayers on Si(111)7 x 7 at low temperature allowed us to monitor si multaneously both the first adlayer (hence the physisorption-to-chemisorpti on transition) and the overlayer features (hence the multilayer desorption) , confirming that the physisorption-to-chemisorption transition and multila yer desorption are competing processes, and indicating a way to study subst rate-adsorbate systems.