Embedded-Cluster Study of Hydrogen Interaction with an Oxygen Vacancy at the Magnesium Oxide Surface Annalisa D’Ercole, Elio Giamello, and Cesare Pisani* Dipartimento di Chimica IFM, UniVersita ` di Torino, and Unita ` INFM di Torino, Via Giuria 5, 1-10125 Torino, Italy Lars Ojama 1 e Department of Physical Chemistry, Arrhenius Laboratory, Stockholm UniVersity, 10691 Stockholm, Sweden ReceiVed: January 8, 1999 An embedded-cluster Hartree-Fock approximation is adopted for simulating the formation of F s (H) color centers at the (001) surface of magnesium oxide. This process is assumed to take place in two steps at an isolated surface anion vacancy: first, a hydrogen molecule is adsorbed dissociatively at the defect; second, following UV irradiation, a neutral hydrogen atom is removed and an electron remains trapped at the vacancy with a hydroxyl group nearby. According to the present calculations, the activation energy for the dissociation is appreciable (about 25 kcal/mol) and the products (a proton bound to a low-coordinated oxygen and a hydride ion above the vacancy) are considerably less stable than the reactants. The excitation of the adsorbed species owing to the UV irradiation is simulated by considering a singlet-triplet transition of the hydride- vacancy complex, which then dissociates into an H atom and a trapped lone electron. The electronic structure and the EPR parameters of the resulting paramagnetic state are explored. The theoretical results agree in many respects with the experimental data as concerns one of the forms of heterolitically dissociated hydrogen which are found at the defective MgO surface. However, from the viewpoint of the energetics, this model is untenable because that species is known to form irreversibly at room temperature with low activation energy. 1. Introduction In spite of its long history, the problem of the characterization of the surface of MgO is still a subject of active research. Important experimental information is currently obtained through the combined use of ultraviolet (UV), infrared (IR), and electron paramagnetic resonance (EPR) spectroscopies, both in the absence and in the presence of an adsorbed species. In the case of hydrogen adsorption, after the first observation of the unusual phenomenon of heterolytic splitting at the MgO surface, 1 subsequent detailed studies have shown that two different families of dissociated hydrogen are formed, to be indicated in the following as R and . The first corresponds to an H + -H - pair weakly bound at the surface (its concentration is pressure- dependent and vanishes upon evacuation) and the second to an analogous pair which however is irreversibly held at the surface at room temperature. 2,3 Both species have been carefully characterized by IR spectroscopy: the H + species is stabilized at the surface as a hydroxyl group, while H - is adsorbed at cationic sites. It is worth noting that in the R family the hydride appears as a linearly adsorbed species, while the  hydride is in a bridged configuration. For the formulation of suitable models of such kind of experimental findings, theoretical calculations are becoming an important complementary tool. In a recent paper which summarizes the experience of many years of research in this field and utilizes new data obtained with all the above-mentioned tools, one of us (E.G.) has formulated a novel model for the F s + (H) color center, an EPR active species which is known to be formed at the surface of fully dehydrated MgO in the presence of hydrogen and after UV activation. 3 According to this hypothesis, to be cited in the following as the vertex model, the trapped electron is localized at a site formed by three magnesium cations at a micro(111) face of the sample (in the limiting case, at the vertex of a one- layer terrace on the (100) face). It contrasts with the more traditional one, formulated long ago by Tench 4,5 and re-explored recently by several authors, 6-10 according to which the electron trap consists of an oxygen vacancy at the (001) face. The divacancy model proposed by Lunsford 11,12 is still another possibility which has been the object of recent work. 13 With respect to these older hypotheses, the vertex model appears in better agreement with the experimental evidence concerning the surface chemistry of hydrogen on MgO and some features of the color center, in particular as concerns the modification of the EPR spectrum after oxygen adsorption. Furthermore, it does not require the energetically expensive formation of an oxygen vacancy at the flat (001) surface. An important aspect of the analysis carried out in that study is the recognition that the formation of the F s + (H) center upon UV activation takes place in parallel with the disappearance of the hydride species, especially with that belonging to the R family; the hydride is therefore considered to be the source of the trapped electron. In the present paper we consider, from a theoretical viewpoint, the problem of the formation of the F s + (H) center. In order to keep the computational burden within reasonable limits, we take a step back and treat the case where hydrogen is adsorbed at an isolated surface anion vacancy. From the vertex model, however, we take the suggestion that a crucial role in the formation of the active species is played by a pre-existing hydride ion. We then consider explicitly the process of the heterolytic dissociation of the molecule at the vacancy, to form the precursor state of 3872 J. Phys. Chem. B 1999, 103, 3872-3876 10.1021/jp990117d CCC: $18.00 © 1999 American Chemical Society Published on Web 04/27/1999