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The 257 nm photodissociation dynamics of methyl iodide multilayers adsorbed on MgO(100) have been studied using resonantly enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) to detect both methyl and iodine photofragments. The photofragments result from direct photolysis of the methyl iodide adsorbate. Methyl fragments with a translational and internal energy content comparable to that found in the gas phase dissociation of the isolated molecule are produced. However, the measured I/I* branching ratio is modified in favor of ground state (I) iodine production. Collisionally slowed methyl fragments, characterized by a translational temperature close to that of the surface, are also observed. Iodine fragments with velocities in excess of the gas phase limit are produced as a result of collisional energy transfer between iodine and faster moving methyl photofragments. A small amount of laser induced photodesorption, leading to the production of molecular methyl iodide, was also detected. The observed photofragmentation dynamics can be accounted for based on the ordered antiparallel structure adopted by the adsorbate molecules that are preferentially aligned along the surface normal within the physisorbed layer. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ultraviolet (UV) photodissociation dynamics of methyl iodide at 333.45 nm has been studied using resonantly enhanced multiphoton ionization in conjunction with a time-of-flight mass spectrometer (REMPI-TOFMS). At this wavelength methyl iodide photolysis involves a transition with significant parallel and perpendicular character. Both ground (I) and spin–orbit (I*) excited iodine result from a parallel transition to the 3Q0 state. In contrast, the perpendicular transition, which most likely involves initial excitation to the 3Q1 state, leads exclusively to production of I. Based on data from this study, an upper limit for the conical intersection between the 3Q0 and 1Q1 potential surfaces is ∼30 500 cm-1. [ABSTRACT FROM AUTHOR]

Theoretical and experimental results are compared for the 257 nm photolysis of methyl iodide adsorbed on an MgO(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 3Q0 state which is, in turn, coupled to the 1Q1 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 I*/I 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 I*/I branching ratio, the velocity distribution of the methyl fragment, and the internal state distribution of the methyl. A substantial lowering of the I*/I 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 I* 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. [ABSTRACT FROM AUTHOR]

Cites the use of a single-stage time-of-flight mass spectrometer in conjunction with resonance enhanced multiphoton ionization to study the dynamics of surface photodissociation processes as well as methyl radicals produced from a continuous source. Energetic characteristics of methyl radicals produced; Experimental discrimination of photofragments produced.