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-radiation valence-band photoemission in the temperature range 30-300 K. We found 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 features. Furhermore, the interaction of benzene with the surface states is very weak, hinting at a physisorption process. Upon temperature increase, the adsorption features change gradually, but significantly. In particular, we observe a splitting of the features related to the degenerate orbitals, indicating that the benzene bond to the silicon surfaces is turning into chemisorption. This is the first evidence by photoemission spectroscopy that such a transition occurs, to our knowledge. Furthermore, the spectrum we obtain upon 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 exposure to a benzene multilayer we observe a direct transition from multilayer to chemisorbed state without intermediate transition to a physisorbed state, thus indicating that the multilayer desorption and the physisorption-to-chemisorption transition occur simultaneously. The adsorption of a few benzene overlayers on Si(111)7 x 7 at low temperature allowed us to monitor simultaneously both the first adlayer (hence the physisorption-to-chemisorption transition) and the overlayer features (hence the multilayer desorption), confirming that the physisorption-to-chemisorption transition and multilayer desorption are competing processes, and indicating a way to study substrate-adsorbate systems.