The 257 nm photochemistry ofCD 3 I adsorbed on MgO( 1(0) has been investigated. The photo fragments ejected along the surface normal were probed using resonantly enhanced multi photon ionization spectroscopy coupled with time-of-flight mass spectrometry. Methyl radicals and atomic iodine fragments in both the ground Ie P 3/2 ) = I and spin-orbit excited lepl/ 2 ) =1* states were observed as direct photofragments produced at the surface. Laserinduced desorption of methyl iodide competes with dissociation at the surface. The total cross section for removal of adsorbed CD31 from the surface at 257 nm is comparable to the gas phase cross section. A significant portion of the atomic iodine fragments remain trapped at the surface with preferential trapping of 1*. Additionally, nonresonant I + and 1 2 + signals were detected and initially observed to increase as a function of the irradiation time and then decrease with further irradiation. This behavior is shown to be indicative of photoinduced production and photoinduced removal of a surface intermediate. A surface reaction mechanism and the existence of a common precursor resulting from the interaction of adsorbed atomic iodine with adsorbed methyl iodide or methyl iodide clusters is postulated.

Theoretical and experimental results are compared for the 257 nm photolysis of methyl iodide adsorbed on an Mg0(100) crystal. Molecular-dynamics calculations treat CH3I as a pseudodiatomic molecule and describe the geometry and the vibrational and librational frequencies of ground state CH3I on the surface of a solid at 125 K. The simulations modeled the photodissociation dynamics of the adsorbed species. The photoexcitation of CH3I at 257 nm is to the 3Qo state which is, in tum, coupled to the IQI state. The electronic surface coupling allows for two dissociation pathways, producing either ground-or excited-state iodine atoms in concert with ground-state methyl radicals. The 1*/1 branching ratio and the velocity and angular distributions of both photofragments are predicted by the theory. A comparison is made between these predictions and experimental observation of the 1*/1 branching ratio, the velocity distribution of the methyl fragment, and the internal state distribution of the methyl. A substantial lowering of the 1*/1 ratio as compared to data from the gas-phase photodissociation studies is both predicted by theory and seen experimentally. Theoretical simulations attribute this change to efficient trapping of the 1* photofragments by the surface. Further comparisons between the theoretical predictions and the experimental data support a model where the molecule is aligned perpendicular to the surface and the escape of iodine atoms from the surface following the photodissociation of adsorbed methyl iodide involves collisions with the surface. 5168

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Methyl rotation, vibration, and alignment from a multiphoton ionization study of the 266 nm photodissociation of methyl iodideMeasurements of the angular and polarization dependence of methyl photofragments produced as a result of 257-nm photodissociation of methyl iodide adsorbed on MgO( 1(0) are reported. At submonolayer coverage an adsorbate geometry is proposed where methyl iodide has its molecular axis aligned along the surface normal, iodine being directed towards the surface. This orientation is compatible with other experimental and theoretical data which is discussed.

Application of the 118 nm single photon laser ionization technique to a molecular beam epitaxy machine is used for the first time to demonstrate direct desorption of As atoms from Si(100). Both As2 and As are the desorbing species from 1 ML of arsenic on silicon above 1000 K. This is in contrast to previously reported models that considered only dimer desorption. With a continuous flux of As4, the scattered and desorbing arsenic species from Si(100) are examined as a function of surface temperature (650–1200 K). Atomic desorption is large, 75%±19%, above 1000 K, and complete conversion of As4 to As2 and As occurs at 1200 K. The species selectivity of laser ionization time-of-flight mass spectroscopy has broader implications for GaAs growth.