Photolysis of CH3I on Cu(110) at 337 nm: Direct and charge-transfer photodissociation

Near-UV photodissociation of oriented CH3I adsorbed on Cu(110)–I

Methyl iodide adsorbed on a Cu(110)-I surface has been found to be highly orientationally ordered. We have exploited this orientation to select different CH(3)I excited states for photodissociation by using polarized near-UV light at wavelengths of 308, 248, and 222 nm. Using p-polarized light at all three wavelengths, we find that dissociation proceeds largely via the (3)Q(0) state, consistent with the picture from gas-phase photolysis. In contrast, using s-polarized light we find contributions from the (3)Q(1) state at lambda=308 nm, the (1)Q(1) state at lambda=248 nm, and the (E,1) state at lambda=222 nm-the latter being a state that has not been implicated in gas-phase studies of CH(3)I A-band photolysis. We also note the contribution to surface photodissociation from low-energy photoelectrons causing dissociative electron attachment to adsorbed CH(3)I and have identified the promotion of direct photodissociation pathways during lambda=308 nm photolysis.

Thermal and Photochemistry of tert-Butyl Iodide on Rutile TiO2(110)

The thermal and photochemistry of tert-butyl iodide (t-buI) dosed at 100 K on rutile TiO2(110) has been studied using isothermal and temperature programmed desorption mass spectrometry. Nondissociative adsorption and desorption dominate the thermal behavior with dose-dependent t-buI desorption peaks at nominally 220 and 150 K. Ultraviolet photochemistry occurs readily, but the behavior of submonolayers and multilayers differ qualitatively. Ejection of t-buI and i-C4H8 dominate during submonolayer photolysis at 100 K. Multilayer photolysis results are also dominated by ejection during irradiation, but the t-buI component is strongly suppressed, and the maximum rates of i-C4H8 and HI ejection did not occur at the outset. A mechanistic model capturing the observations involves both direct and substrate-mediated electronic excitation of t-buI. According to this model, ejection of t-buI occurs only from transient substrate-mediated formation of anionic t-buI. For either excitation path, the C-I dissociation probability is significant, and the excited tert-butyl product rearranges readily to form i-C4H8 that is ejected. For any local region where there is multilayer coverage, products formed at the adsorbate-vacuum interface are ejected promptly, but products formed within the multilayer are trapped. Thus, ejection of t-buI is suppressed, and trapped primary photodissociation products, tert-butyl and I, react to either reform t-buI or rearrange to i-C4H8 and HI. The latter two products remain trapped and are subsequently induced to desorb by acquisition of momentum from collisions with subsequently formed translationally excited photodissociation products.

Surface-Aligned Ion−Molecule Reaction on the Surface of a Molecular Crystal CD3+ + CD3I → C2D5+ + DI

An ion-molecule reaction has been observed from a condensed molecular crystal of CD(3)I using the time-of-flight electron-stimulated desorption ion angular distribution technique. The CD(3)I multilayer is produced by growth on an ordered substrate. The reaction occurs between CD(3)(+) ions produced by electron-stimulated desorption and neighbor CD(3)I molecules in the topmost layer of the molecular crystal of CD(3)I, forming product C(2)D(5)(+) ions whose desorption dynamics have been measured. The normal momentum of the product ion is close to that of the reactant ion, suggesting that the reaction is dominated by a two-body collision, i.e., the momentum of the reactant CD(3)(+) ion governs the momentum of the product C(2)D(5)(+) ion. The ion-molecule reaction is of high cross section since the C(2)D(5)(+) yield is comparable to the CD(3)(+) yield. It is found that the yield and directionality of the emission of the C(2)D(5)(+) product ion is governed by the molecular order that is characteristic of the molecular crystal of CD(3)I. Destroying or modifying this order by using a spacer layer of H(2)O diminishes the C(2)D(5)(+) product ion yield relative to the reactant CD(3)(+) yield and broadens the ion emission directions.