Electron and photon irradiation of benzene and chlorobenzene on Si(111) 7 x 7

We describe an attachment reaction and a chlorination reaction due to electron or photon irradiation of benzene (Bz) and chlorobenzene (ClPh), adsorbed on a Si(1 1 1) 7 x 7 surface at room temperature using scanning tunneling microscopy. Radiation-induced attachment: In this and a subsequent theoretical paper we give evidence that benzene or chlorobenzene can be converted from a strained di-sigma-bound, S, state to a more-strongly held single-sigma-bound state, B. This counter-intuitive finding stems from the fact that the weakly-bound state has a cyclohexadiene ring, and the strongly-bound state has the additional resonance energy of a fully aromatic ring. The measured cross-section for photoattachment of Bz was cap sigma(at)(ph)(193 nm) = 1.5 x 10(-21) cm(2), and that for ClPh at short and long wavelength was sigma(at)(ph)(193 nm) =2.2 x 10(-21) cm(2) and sigma(at)(ph)(532 nm) = 3.1 x 10(-23) cm(2). Photochlorination reaction: Under photon irradiation the chlorination reaction ClPh(ad)/Si(1 1 1) --> Cl-Si took place with cross-section sigma(rxn)(ph)(193 nm) = 2.2 x 10(-22) cm(2) and sigma(rxn)(ph)(532 nm) = 1.5 x 10(-24) cm(2) . The mechanism of reaction of Cl is examined in the DFT approximation, yielding an upper limit of 1.9 eV for the thermal activation energy in localized atomic reaction. There was a marked propensity for the adsorbate to assemble in threes on each half-unit-cell at increased coverage. Photodesorption and photodiffusion from the faulted half (F) to the unfaulted half (U) of a unit cell gave evidence of being similar to3x more probable than U --> F. (C) 2003 Elsevier B.V. All rights reserved.