Electron and photon irradiation of benzene and chlorobenzene on Si(1 1 1) 7 × 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 × 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-σ-bound, S, state to a more-strongly held single-σ-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 σphat(193 nm)=1.5×10−21 cm2, and that for ClPh at short and long wavelength was σphat(193 nm)=2.2×10−21 cm2 and σphat(532 nm)=3.1×10−23 cm2. hlorination reaction: Under photon irradiation the chlorination reaction ClPh(ad)/Si(1 1 1) → Cl–Si took place with cross-section σphrxn(193 nm)=2.2×10−22 cm2 and σphrxn(532 nm)=1.5×10−24 cm2. 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 ∼3× more probable than U → F.