257 nm photoinduced chemistry of methyl iodide adsorbed on MgO(100)

Abstract

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.

Surface‐Aligned Femtochemistry: Real‐Time Dynamics of Photoinduced I2 Formation from CD3I on MgO(100)

“…[13,14] On magnesia ultrathin films on Mo(100), a similar orientation was reported previously, with the methyl group heading toward the substrate surface (cf. Figure 1 a).…”

“…Interestingly, this also provides a new mechanism for the previously observed pronounced and puzzling I* quenching in surface photodissociation experiments. [14] In conclusion, for the first time, rich details of bimolecular surface photoreaction dynamics have been probed in real time by mass spectrometric detection of intermediates and products in a pump-probe scheme. The influence of the insulating surface on the reaction dynamics manifests itself in: i) the alignment of the reactants prior to photoreaction, ii) the trapping of the dissociative transition state preventing direct decomposition as in the gas phase, and in this way iii) enhancing the possibility for a non-adiabatic transition of the wave packet through the conical intersection; in iv) providing an additional energy dissipation pathway for the bimolecular reaction to proceed, and in v) breaking the molecular symmetry of the I 2 (B) state to enable predissociation, thus opening a new channel for I* de-excitation at the surface.…”

Extended BEG model of monhalogenated methanes physisorbed on ionic crystals

“…CH 3 I was inferred to adsorb perpendicular to the substrate surface on MgO, with a parallel alignment of the adsorbate molecules at low coverage transitioning to islands of adsorbate preferring an anti-parallel alignment as the coverage increases, in agreement with our model. [40,41] They do not observe a transition back to parallel alignment we see in our model. [40,41] TPD and TOF mass spectroscopy measurements of CH 3 I…”

“…[40,41] They do not observe a transition back to parallel alignment we see in our model. [40,41] TPD and TOF mass spectroscopy measurements of CH 3 I…”

“…[39] Several methyl halides adsorbed on both MgO and NaCl have been studied using temperatureprogrammed desorption (TPD) and time of flight (TOF) mass spectroscopy. [35,40,41] These measurements were used to infer the orientational ordering of the adsorbed layer. CH 3 I was inferred to adsorb perpendicular to the substrate surface on MgO, with a parallel alignment of the adsorbate molecules at low coverage transitioning to islands of adsorbate preferring an anti-parallel alignment as the coverage increases, in agreement with our model.…”