Quantum theory of the photodissociation of IBr adsorbed on an MgO(001) surface

The photodissociation of an isolated IBr molecule adsorbed on an MgO(001) surface is studied theoretically. The calculations correspond to an excitation into the repulsive (Y0⁺)/quasibound (B ³Π₀₊) electronic state manifold, which may lead to the production of excited state or ground state bromine atoms. Using a quantum scattering method, we calculate the photoabsorption line shape for this process and the [Br]/[Br*] branching ratio as a function of photoexcitation wavelength. In the quantum calculations the IBr stretching was treated exactly, the in-plane librational mode was treated in the sudden approximation, and all other adsorbate and crystal modes were frozen. In addition, we studied the photodissociation process classically in order to explore the validity of freezing most of the modes. In the quantum calculations it was found that the width and intensities of the structured part of the absorption profile were greatly increased compared with the gas-phase photodissociation process. This was attributed to the stabilization of both electronic states by the molecule/surface interactions. The classical results showed that at least semiquantitatively, the crystal modes are unlikely to affect the process on the timescale pertinent to the calculated line shape.