The vibrational structure of benzene adsorbed on Si(001)

High resolution electron energy loss spectroscopy (HREELS) measurements and density functional model cluster calculations are presented to clarify the vibrational structure of the adsorption system C6H6/Si(001). All vibration al modes of the adsorption complex, which previously was identified to exhi bit a cyclohexadiene-like structure, have been calculated and characterized according to the motion of the different atoms of the adsorption complex. Special emphasis is placed on the low-frequency modes. The coupling between the adsorbate and the substrate modes is analyzed with the help of a model that represents various limiting situations. Different coupling variants a re found to apply to different collective modes of the adsorbate. The A(1) and B-1 modes can be described rather well by a model that only encompasses the adsorbate and the Si dimer underneath; for the A(2) and B-2 modes a fr ozen substrate description of the adsorption complex is more appropriate. ( C) 2000 American Institute of Physics. [S0021-9606(00)70505-8].