Photoinduced charge transfer reactions at surfaces: CF3I on Ag(111)

Abstract

The surface photochemistry of submonolayer to multilayer amounts of CF3I, adsorbed on Ag(111) at 95 K, has been studied using 248 and 193 nm pulsed laser excitation. For low doses, there is some thermally activated dissociation, limited to 30% of the first monolayer, to form adsorbed CF3 and I. The remaining CF3I adsorbs molecularly. Neither CF3 nor I is photoactive, but adsorbed CF3I is photodissociated, by C–I bond cleavage, at both 248 and 193 nm. A fraction of the resulting CF3 and I desorbs during photolysis; the remainder is retained as chemisorbed CF3 and I. The former processes were probed using time-of-flight and Fourier transform mass spectrometry. The retained products were detected by post-irradiation temperature programmed desorption and Auger electron spectroscopy. The photochemistry varied with wavelength and coverage. Regarding the mechanism, for both 193 and 248 nm, there is good evidence that both submonolayer and multilayer CF3I molecules absorb photons and dissociate into CF3 and I, i.e., direct photodissociation. There is evidence, based on time-of-flight distributions of CF3 photofragments, that I(2p1/2), electronically excited I, is produced at both wavelengths, while ground state atomic iodine is produced only at 248 nm. At both 193 and 248 nm, and for coverages up to three monolayers, there is also evidence for a charge transfer process involving hot electrons produced by photon absorption in Ag(111), i.e., substrate mediated photodissociation. These hot carriers attach to CF3I, and the resulting anion dissociates into CF3 and I−. The latter is detected by Fourier transform mass spectrometry and the former as a low-velocity component in time-of-flight mass spectrometry. At 193, but not 248 nm, there is evidence for a second, substrate independent, charge transfer process also leading to CF3 and I−. For coverages exceeding ten monolayers, approximately 80% of the reaction was through this channel, the remaining 20% occurring through the direct photodissociation channel. To account for this second charge transfer channel, photoinduced intermolecular charge transfer is proposed.