A low symmetry adsorption state of benzene on Si(111)7x7 studied by photoemission and photodesorption

Benzene adsorption on Si(111)7 x 7 has been studied by valence band and Si2p photoemission and photodesorption. We hypothesize a molecular adsorption of benzene on Si(111)7 x 7, involving simultaneously an adatom and a rest atom in the adsorption process. The assignment of the benzene adsorption features has been made by comparison with gas-phase benzene and with ARUPS spectra of benzene on Si(100)2 x 1. We find that the degenerate orbitals of the e type lose their degeneracy with decrease of symmetry. We suppose that two opposite carbon atoms are involved in the adsorption process, leading to a partial rehybridization of the two carbon atoms from sp(2) to sp(3) that makes the adsorbate state intermediate between benzene and 1,4 cyclohexadiene. We also observed a high desorption rate of benzene from the silicon surface that shows a weak benzene-silicon bond. D+ is the only ion produced in the Si2p energy range, due to the enhanced production of secondary electrons in correspondence of the ionization threshold. (C) 1998 Elsevier Science B.V. All rights reserved.