We report a further theoretical investigation of a model surface-aligned photoreaction with a moving surface approximation. The reaction is initiated by the photodissociation of a well-aligned HBr adsorbed on the LiF(001) surface. The collision of the dissociating H fragment with a coadsorbed CO2 leads to the OH and CO products. In an earlier theoretical study with a static surface model, it has been shown that the reactivity can be significantly enhanced relative to the corresponding gas phase reaction for some adsorption alignments. In this work, we address the roles played by surface motion and temperature. Our results indicate that some (∼0.3 eV) energy can be lost to the surface either from the adsorbed HOCO complex or from one of the products when it collides with the surface during the final disintegration of the HOCO complex. However, the energy transfer has a minor effect on the reactivity. The final state distributions of the products are found to be similar to those produced with the static surface model. On the other hand, a significant temperature effect is predicted for one adsorbate configuration. Apparently, the lowering of temperature for a well-aligned system results in a more narrowly focused alignment and higher reactivity.
The 257 nm photodissociation dynamics of CH3I adsorbed on a MgO(001) surface is studied using classical molecular dynamics method. The substrate is modeled by a 6×6×3 slab of movable ions surrounded by a semi-infinite array of static ions. A single adsorbate molecule is aligned with the surface normal, the methyl end pointed either toward or away from the substrate. The system is equilibrated by using a Monte Carlo method to obtain the starting configuration. Fragment final state distributions are calculated for kinetic energy, angle of departure, and rovibrational states. Upon photodissociation of the adsorbate with the methyl end pointed toward the surface, the methyl fragments experienced vibrational cooling, in agreement with experimental results. Some rotational excitation is predicted for fragments produced from the methyl down orientation. The kinetic energy distributions of both the methyl and iodine fragments are qualitatively similar to those obtained by experiment. The results are compared with those obtained by the same model for CH3I adsorbed on LiF(001). Trapping of iodine atoms by the surface has also been investigated in this simulation.
We report a quasi-classical trajectory study of a chemical reaction between H and CO 2 at the LiF(001) surface. The reaction is initiated by photodissociation of well-aligned HBr(ad) at 193 nm, which produces a "hot" H atom directed toward a nearby CO 2 (ad). Single molecules of each reactant are placed on a static surface, and a full-dimensional HCO 2 potential derived from ab initio calculations is used. The adsorbate-substrate and the adsorbate-adsorbate potentials consist of both nonelectrostatic and electrostatic contributions. Several energetically favorable adsorption configurations are determined by a Monte Carlo method. Quasi-classical trajectories are calculated at 80 K for four different adsorption configurations. We find that the reactivity at some configurations is significantly enhanced compared with the corresponding gas-phase simulation. The calculated impact parameters and incident angles of the surface-aligned collisions indicate that the enhanced reactivity can be largely attributed to the closeness and alignment of the coadsorbates on the surface. Owing to the long-lived complex, product distributions, with the exception of a departure angle, show little memory with regard to the initial configuration and are similar to those obtained in the gas phase. A significant number of the unreacted hydrogen atoms retain sufficient energy to make subsequent reaction with other coadsorbates a possibility. We find evidence of several dynamic features pertinent to the use of the surface as a template for reactivity enhancement, including scattering at the surface, the squeezed atom effect, chattering, and caging.
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