Physica B 292 (2000) 59}70 Excitons, electron center di!usion and adsorptivity of atomic H on LiH (0 0 1) surface: Ab initio study A.S. Shalabi *, A.M. El-Mahdy, M.A. Kamel, H.Y. Ammar Chemistry Department, Faculty of Sciences, Benha University, Benha, Egypt Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Received 23 November 1999; received in revised form 9 March 2000; accepted 9 March 2000 Abstract An attempt has been made to examine the bulk and surface properties of exciton bands near F>, F and F\ centers (a, b and c bands), di!usion of electron centers (F>, F and F\) and adsorptivity of atomic H over the undefected and defected (F>, F and F\) surfaces of LiH using an ab initio embedded cluster method at the Hartree}Fock approximation and Moller}Plesset second-order perturbation correction. The results con"rm the exclusive dependence of the exciton bands on the type of the electron center. The activation energy for bulk di!usion increases monotonically in the series F>PFPF\. Bulk and surface relaxation e!ects are more important for F> than for F and F\ centers. The introduction of F or F\ center changes the nature of adsorption from physisorption to chemisorption. The introduction of F\ center changes the nature of LiH surface from an insulating surface to a semiconducting surface. As F and F\ centers are introduced, the HOMO and LUMO levels of the substrate shift to higher energies and the band gaps become narrower. These changes in the electronic structure make charge transfer between adsorbate and substrate energy levels and spin pairing with F center more facile in the course of adsorbate}substrate interactions. 2000 Elsevier Science B.V. All rights reserved. Keywords: Excitons; F>, F and F\ di!usion; H/Li (0 0 1) adsorptivity; Ab initio calculations 1. Introduction A non-traditional approach to understand the host dependence of band gaps is to start with a model for the host absorption. A complete treatment would involve the theories of excitons [1}3] and defects [4}8] which take into account the band structure of alkali halides. This would be a major undertaking and well beyond our present goal. We therefore use the simple electron-transfer model of * Corresponding author. Tel.: #20-13-22578; fax: #20-24188738. the fundamental optical absorption of ionic solids developed by Hilsch and Pohl [9,10]. This model treats ionic solids in the extreme tight-binding approximation and, in its simplest form, explains the fundamental optical absorption as the transfer of an electron from a negative ion to a neighboring positive-ion. In more sophisticated treatments, the "nal state is taken to be a symmetrized linear combination of positive-ion states [11]. F and F\ centers are well-de"ned defects in ionic crystal, but F> could not have an absorption band in the same way that we see absorption for the F and F\ centers. However, it has been shown clearly that 0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 4 8 7 - 7 60 A.S. Shalabi et al. / Physica B 292 (2000) 59}70 an ultraviolet absorption band called a band arises from the vacancy [12]. In KI, the band was found at 2380 As , which is close to the "rst-exciton peak at 2130 As . This has led to the interpretation of the band as arising from the formation of an exciton in the vicinity of a vacancy. Bassani and Inchauspe [13] were able to show a reasonable agreement between calculations based on Hilsch and Pohl model of exciton and experiment. If the exciton is formed farther from the vacancy, it will be perturbed less and will therefore be closer to the fundamental exciton energy. The existence of the a band is very useful in unraveling the motion and capture of electrons in alkali halides as well as other processes involving impurities, di!usion of ions or electron transfer. An exciton formed near an F center gives rise to an ultraviolet absorption called the b band. The b band may be used to measure the presence of F centers, but it is usually more convenient to measure the F band itself. It seems likely that all centers have perturbed excitons formed near them. In cases other than a and b bands, these have not been extensively studied experimentally since other properties of the centers are usually easier to observe. However, theoretically, an exciton formed near an F\ center may give rise to an ultraviolet absorption that we may call c band. One aim of this paper is therefore to report, to our knowledge, the "rst exciton band calculations near F>, F and F\ centers in LiH. Investigation of mass transport in alkali hydrides are related to their functional properties and longterm stability [14]. Very often ion di!usion is associated with point defects. The di!usion of electron centers F (one electron trapped by an anion vacancy) and F\ (two electrons trapped by an anion vacancy) is practically not studied [15]. At the same time, the extensive data on F center di!usion in alkali halides is based on the mobility measurements under electrolytic coloration. The studies of F> (anion vacancy) di!usion are based on the ionic conductivity and radioactive-tracer di!usion measurements. The signi"cant e!ect found is that F> centers have activation enthalpies typically about half of those of the F centers [16]. To our knowledge theoretical and experimental information on electron center di!usion in alkali hydrides are scarce, so that the second aim of this paper is to examine the di!usion of the given hole centers in the bulk and at the surface of LiH. Theoretical and experimental studies of adsorbate}substrate interactions have become of increasing importance [17,18]. This is due to the fact that they are related to a variety of technologically signi"cant processes, not least of which are catalysts, corrosion and gas sensors. On the other hand, smooth surfaces can have point defects or steps, which can locally strongly modify the adsorbate}substrate interactions. The chief problem in studying adsorbate}substrate interactions computationally is the treatment of the extended surface when examining a localized phenomenon like chemisorption [19]. For simple systems such as atoms or small molecules interacting on surfaces, it can be feasible to use an extended two-dimensional periodic systems, to study an ordered overlayer of adsorbate on the surface. Such examinations have sometimes used slab calculations [20], although more recently surface embedding is providing a promising route forward [21]. Several theoretical studies have been done to simulate adsorption of simple systems on ionic surface [22}24]. For oxide surfaces, these studies highlight the formation of metal}oxygen bonding and antibonding states, the latter being either completely or partially "lled. Thus the interactions are considered to be mainly of a chemical nature. This is a reasonable assumption for relatively reactive surfaces such as those of many metal oxides. However, other ionic surfaces are known to be highly stable and the nature of the metal-surface (1 0 0) bond is not so clear. The second aim of this paper is, therefore, to report results of SCF and MP2 calculations on an electronically inert (0 0 1) surface of an insulator LiH. The e!ects of introducing the surface F>, F and F\ on modifying the nature of adsorbate}substrate interactions were then examined. We have not been able to "nd any experimental data on the characteristics of hydrogen adatoms on lithium-hydride surface, hence our results serve as theoretical predictions. 2. Methods and calculations The optimal way to represent the extended lattice appears to be to choose point ions which A.S. Shalabi et al. / Physica B 292 (2000) 59}70 correctly mimic the Madelung potential and its gradient at the `active site(s)a for chemisorption [25], or alternatively, for surfaces by a Parry summation [26,27], and the gradient can be determined by its slope as a probe is removed away from the `active site(s)a. This termination of the cluster by point ions has signi"cant advantages in that it adds little to the computational cost of the calculation; only additional one-electron integrals need to be calculated [19]. To represent the extended crystal properly, some care needs to be taken in choosing the charges of the point ions, according to the conditions outlined by Harris [28]. For a bulk crystal, the criteria are that there must be no net charge, no net dipole and no net quadruple in the cluster. For a surface, there is a small dipole which is induced by surface rumpling; so the criteria of no net dipole do not hold rigorously [19]. The choice of the appropriate changes for the point ions has been discussed for an FCC structure like MgO [29]. Early studies by Kunz and co-workers [30] and by Clobourn and Mackrodt [31] used clusters which were terminated by full ionic charges. One of the aspects of these calculations (which is most surprising) is that very small clusters * sometimes a single surface ion * can be adequate to represent surface reactivity. This is a consequence of the high degree of localisation of the electrons on the ions, and would not hold for materials with any appreciable degree of covalence. To simulate the LiH crystal, we follow a procedure previously reported for MgO [32] and LiH [33] crystals. A "nite crystal of 288 point charges was constructed. The Coulomb potential along the X- and >-axis of this crystal is zero by symmetry as in the host crystal, Fig. 1. The charges on the outer shells listed in Table 1 were modi"ed to make the Coulomb potential at the four central sites equal to the Madelung potential of the host crystal and to make the eight points with coordinates (0, $R, $R) and ($R, 0, $R), where R is half the lattice distance, which for LiH is 2.04 As , equal to zero as it should be in the host crystal. With these charges, 0.409283 and 0.800909, the Coulomb potential in the region occupied by the central ions is very close to that in the unit cell of the host crystal. All charged centers with Cartesian coordinates $X, 61 Fig. 1. Representation of the Z"0 plane of the lattice used in the calculations. $> and Z"2R, 4R, 6R and 8R were then removed to generate a surface of 176-charged centers occupying the three-dimensional space $X, $> and !Z"0, 2R, 4R, 6R and 8R. The coordinates of these charged centers are given in Table 1. Quantum clusters were then embedded within the central region of the crystal bulk or surface. All the electrons of the embedded clusters were included in the Hamiltonian of ab initio calculations. Other crystal sites entered the Hamiltonian as point charges. The adsorption energy E of the adatom on the sub strate surface was calculated from the relation E "E !E !E . The terms appearing on the right-hand side are the total energies of the complex (adsorbate#substrate), the adsorbate (free hydrogen atom) and the substrate (undefected or defected), obtained from three independent calculations using the same supercell. The negative adsorption energy E . in dicates that the bound adsorbate is thermally stable. The Hartree}Fock and electron correlation calculations were carried out using the Gaussian 6}31##g(d, p) internal basis. This basis puts diffuse and polarization functions to both heavy atoms and hydrogens. For an ionic cluster such as 62 A.S. Shalabi et al. / Physica B 292 (2000) 59}70 Table 1 Speci"cation of the "nite lattice used for crystal bulk and surface calculations. R is half the lattice distance, which for LiH is 2.04 As and r is the distance of the appropriate shell from the center of the lattice R/r Coordinates/R "X", ">", "Z" No. of centers Coordinates/R "X", ">", !Z No. of centers Charge "q" 2 6 10 14 18 22 26 26 30 34 34 38 38 42 46 50 50 50 54 54 58 66 54 62 66 82 86 110 112 310 312 114 332 510 314 512 334 530 532 116 514 316 550 534 710 552 336 730 554 712 732 118 910 912 4 8 8 16 8 8 8 16 16 8 8 16 8 16 16 4 16 8 8 8 8 8 16 16 8 8 16 110 112 310 312 114 332 510 314 512 334 530 532 116 514 316 550 534 710 552 336 730 554 712 732 118 910 912 4 4 8 8 4 4 8 8 8 4 8 8 4 8 8 4 8 8 4 4 8 4 8 8 4 8 8 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.409283 0.409283 0.800909 0.800909 0.800909 Crystal bulk. Crystal surface. Li H , with 40 interacting electrons, there are 250 basis functions and 400 primitive Gaussians. As shown, the basis set is large enough so that the basis set superposition error BSSE is minimized even at the correlated level [33]. The present quantum mechanical calculations were carried out using Gaussian 98 system [34]. 3. Results and discussion The host dependence of band gaps and exciton bands on the type of the defect in the bulk and at the surface of LiH is given in Table 2. The band gap was calculated as the di!erence between the valence and conduction bands, and the exciton band was calculated as the di!erence in Coulomb potentials attributed to the transfer of an electron from a negative ion to a neighboring positive ion, both placed in the deep Coulomb "eld of the simulated crystal. The calculations were carried out for the four central ions surrounded by the nearest-neighbors, Li H in the bulk and Li H at the surface. All ion clusters were embedded in point charges as de"ned in Table 1, and the ions as well as the point charges were included in the Hamiltonian of the ab initio calculations. The results given in Table 2 emphasize the exclusive dependence of energies on the type of the defect in a manner analogous to Glasner}Tompkins relation [35] in which the di!erence between the "rst-exciton absorption energy and the F-band A.S. Shalabi et al. / Physica B 292 (2000) 59}70 63 Table 2 Bulk and surface band gaps E and exciton bands E in LiH E /eV Bulk H\ Li H F> Li H F Li H F\ Li H Surface H\ Li H F> Li H F Li H F\ Li H 9.86 9.52 9.49 5.85 5.93 1.08 1.39 e-H\ Li H e-F> Li H (a) 7.92 9.56 9.55 4.14 4.22 0.34 0.37 e-H\ Li H e-F> Li H (a) e-F Li H (b) e-F\ Li H (c) e-F Li H (b) e-F\ Li H (c) E /eV E !E 17.32 17.97 15.5 16.89 17.16 15.78 17.35 7.46 8.45 6.01 11.04 11.23 14.70 15.96 17.42 18.06 15.64 16.89 16.96 16.02 16.53 9.50 8.50 6.09 12.75 12.74 15.68 16.16 Basis functions are added to the F>, F and F\ centers. energy in alkali halides depends almost exclusively on the negative ion species. The validity of the present calculations of band gaps and exciton bands has been veri"ed with the Mollwo}Ivey relation [36,37] for the F center, and Hilsch}Pohl model [9,10] for the exciton band of the undefected crystal as well as with experiment [38]. For F center, the Mollwo}Ivey value is 4.76 eV and the experimental value is 5.0 eV. For exciton band, the Hilsch}Pohl model value is 17.17 eV. While the present exciton band of 17.32 eV in e}H\ Li H compares well with the empirical value 17.17 eV of Hilsch and Pohl model, the present band gap of 5.85 eV in F-Li H does not compare well with the experimental value 5.0 eV. We may note that a band gap of 4.82 eV was calculated for F-Li cluster with vacancy centered functions [39]. We therefore added basis functions to the defect centers and recalculated the corresponding band gaps and exciton bands. As shown in Table 2, while the addition of basis functions to the F center of the bulk FLi H cluster did not enhance the band gap value (5.93 eV) relative to the experimental value (5.0 eV), the addition of basis functions to the F center of the surface Fli H cluster slightly en hanced the band gap value (4.22 eV) relative to the experimental value. Moreover, the addition of basis functions reversed the trends of E values, but did not change the trends of E or E }E values (Fig. 2). However, the present values of F band gaps are more consistent with the fact that the gap between occupied and unoccupied states in the Hartree} Fock scheme overestimates the transition energy. Moreover, the shown di!erence between bulk and surface band gaps and exciton bands must be attributed to the di!erence in crystalline potentials. Since an electron center appears as a charged defect of quantum mechanical nature, it is very likely to be associated with local distortions in the lattice. We have "rst determined the optimal-relaxation modes corresponding to simultaneous inward}outward displacements of the nearestneighbor cations to the defect center. Bulk and surface optimal-relaxation modes were found to be represented by simultaneous outward displacements of the nearest-neighbor cations to the defect and are given schematically in Fig. 3. Secondly, the nearest-neighbor cations to an electron center namely F>Li , FLi and F\Li in the bulk and F>Li , FLi and F\Li at the surface were simul taneously displaced outwards by 10% of the lattice interionic distance to estimate its contribution to energy lowering. Since the ion cluster model represented by an electron center surrounded by only 64 A.S. Shalabi et al. / Physica B 292 (2000) 59}70 Fig. 2. A model of creation of an exciton in which the arrow represents the transfer of an electron. eH\ Li H and eH\Li H represent exciton in the bulk and the surface of the perfect crystal. e-F> Li H (a) and e-F> Li H (a) represent excitons near F> centers in the bulk and the surface (a-bands). e-FLi H (b) and e-FLi H (b) represent exitons near F centers in the bulk and the surface (b-bands). e F\ Li H (c) and e-F\Li H (c) represent excitons near F\ centers in the bulk and the surface (c-bands). nearest-neighbor cations sees only point charges at the cluster boundaries, the calculations were repeated taking into account the nearest neighbor cations to defect sites in F>Li H , FLi H , F\Li H , F>Li H , FLi H and F\Li H to determine the e!ects of replacing the next nearestneighbor point charges by real ions. Energy changes attributed to simultaneous outward displacements of nearest-neighbor cations to a defect site by 10% of the lattice-interionic distance in the bulk and at the surface of LiH and the e!ects of replacing the next nearest-neighbor point charges by real ions are given in Table 3. The calculated energy changes show that the bulk and surface relaxation e!ects are more important for F> than for F and F\, the e!ects of replacing the next nearest-neighbor point charges by real ions are more important in the bulk than on the surface. On the other hand, Fig. 3. Z"0 plane representation of the optimal relaxation mode. Table 3 Energy changes attributed to simultaneous outward displacments of nearest-neighbor cations to a defect site by 10% and the e!ects of replacing the next nearest-neighbor point charges by real ions. Energies are given in eV Bulk F> Li F Li F\ Li F> Li H F Li H F\ Li H 3.40 1.02 2.00 3.21 1.90 1.57 Surface F> Li F Li F\ Li F> Li H F Li H F\ Li H 3.01 1.14 2.24 2.83 0.80 2.12 the energy changes are heavily dependent on the defect type and crystalline environment and are mainly attributed to the signi"cant changes in Coulombic interactions between a defect center and its surrounding crystalline potential. We have considered the di!usion of F>, F and F\ centers along the 11 1 02 axis to the next nearest-neighbor anions. The di!usion path D has been divided into six steps, D"0.0}0.5. D"0.0 represents the original defect con"guration and D"0.5 A.S. Shalabi et al. / Physica B 292 (2000) 59}70 represents the next nearest-neighbor anion displaced midway along the di!usion path, Fig. 4. The total energies of defect con"guration are given as a function of the di!usion path D in Fig. 5. As shown in Fig. 5, both of the equilibrium and saddle-point con"gurations are sensitive to the defect type (F>, F and F\), crystalline "eld (bulk or surface), the level of calculations (SCF or MP2) and the outward relaxation of the nearest-neighbor ions (to the two anion sites at the 11 1 02 direction) by 5% of the internuclear separation. The activation energy of the defect-di!usion hops was calculated as the di!erence in the total energies of these two con"gurations (equilibrium and saddle point). Within the bulk crystalline "eld, these were calculated to be 0.47, 2.35 and 1.68 eV at the SCF level, and 0.46, 2.24 and 2.31 eV at the MP2 level for F>, F and F\, respectively. Within the surface crystalline potential, activation energies were calculated to be 0.38, 1.37 and 0.38 eV at the SCF level, and 0.36, 1.41 and 0.36 eV at the MP2 level for F>, F and F\, respectively, as shown. While the activation energy for bulk di!usion increases monotonically in the series F>PFPF\, the activation energies for surface di!usion of F> and F are close and increase in the direction F>" F\PF. The contribution of MP2 correction to the activation energy was marginal and did not alter the predicted orders. On the other hand, while the migration energies for the positively and negatively charged defects, F> and F\, are small (0.36 eV) and Fig. 4. Z"0 plane representation of electron center di!usion hops. 65 close and that for the neutral F center is much larger (1.41 eV) for the unrelaxed surface, the migration energies for the F>, F and F\ defects are quite close (0.442, 0.441 and 0.397 eV) for the relaxed surface. The importance of surface relaxation is therefore con"rmed and the disturbed trends of F>, F and F\ surface migration energies relative to the bulk could be attributed to the non-homogeneous surface potentials where each ion is a!ected by the potentials of only "ve nearest-neighbors. Moreover, the nature of interaction between an anion vacancy or its trapped electron(s) and the surrounding crystalline "eld is expected to control the order of defect migration energy. The interaction of hydrogen with solid surface underpins a number of catalytic processes. It is important in its own right as well as being exemplar of a simple model system. Kunz and co-workers used a small point ion block with fully ionic charges to represent the environment, treating H chemisorption at defects like anion vacancies [40] and cation vacancies [41]. Pople et al. [42] have investigated the chemisorption of H on the V center, assuming a symmetric reaction path in which the hydrogen approaches parallel to the surface. More recently, Kobayashi et al. [43] carried out ab initio calculations on the adsorption of hydrogen molecule onto MgO (1 0 0) surface. Guo and Bruch [44] examined the electrostatic energy in the adsorption of monatomic H and He on MgO (0 0 1) and LiF (0 0 1). Kobayashi et al. [45] carried out density functional calculations on the dissociative adsorption of hydrogen molecule on MgO surfaces. In general, adsorbate}substrate interactions result from the tendency of the adsorbate valence electrons to hybridize with the available substrate electronic states. This hybridization can be expected to have a major role if there exists a small energy gap between the adsorbate and substrate electronic states, or if the adsorbate has an open-shell electronic structure. We discuss our results of adsorbate}substrate interactions by studying the di!usion characteristics of a single H atom on the undefected and defected (F>, F and F\) surfaces of LiH. The ion clusters, H\Li H , F>Li H , FLi H and F\Li H were embedded in three dimensional array of point charges ($X, $> and !Z) as 66 A.S. Shalabi et al. / Physica B 292 (2000) 59}70 Fig. 5. Total energies of F>, F and F\con"gurations as a function in the di!usion path D. described in the methods sections and the point charges as well as the real ions were then included in the Hamiltonian of ab intio calculations. Fig. 6 shows schematically the 3-D unit cells and the path along which we have optimized the adatom}surface distance at selected substrate locations D. Optimal adsorption energies E /E and heights R /As for the di!usion of atomic H over the undefected and defected (F>, F and F\) surfaces of LiH from the top of H\ (D"0.0) to the top of the nearest neighbors H, F>, F or F\ (D"1.0) are represented graphically in Fig. 7. De"ning the optimal adsorption site as the substrate location D at which the strongest adsorbate}substrate interaction occurs, the optimal adsorption site of the undefected surface was found to be on the top of the substrate H\ ion, 3.5 As above the substrate plane in the SCF calculations and 3.3 As in the MP2 calculations. The optimal adsorption sites of the F> surface were found to be on the top of a substrate location D"0.5, 1.02 As above the substrate plane in the SCF calculations and D"0.3, 0.51 As Fig. 6. 3-D representation of atomic hydrogen di!usion over the undefected H\ Li H and defected F> Li H , F Li H and F\ Li H surfaces of LiH. above the substrate plane in the MP2 calculations. An optimal adsorption site for both of F and F\ surfaces was observed in the substrate plane at location D"1.0, in the SCF and MP2 calculations. A.S. Shalabi et al. / Physica B 292 (2000) 59}70 The contribution of electron-correlation correction to adsorbate}substrate interactions decreases monotonically in the series F>PFPF\. The near 67 coincidence of SCF and MP2 curves for F\ surface may imply that repulsive interactions play a dominant role. The results show that the optimal Fig. 7. The adsorptivity of atomic H over the undefected H\ Li H and defected F> Li H , F Li H and F\ Li H surfaces of LiH as a function in the di!usion path D. 68 A.S. Shalabi et al. / Physica B 292 (2000) 59}70 adsorption energies have been enhanced by 0.138, 3.29 and 5.11 eV at the SCF level and by 0.44, 3.86 and 5.1 eV at the MP2 level under the e!ect of introducing F>, F and F\ centers respectively. A strong sign for hybridization or chemical bond formation between adsorbate and substrate electronic states of F and F\ surfaces is therefore expected, mainly because of spin pairing (for F surface) and because of the small energy gap between adsorbate and substrate electronic states (for F\ surface). Vidali et al. [46] reported 17.8 meV and 2.7 As for adsorption energy and height in H/LiF(0 0 1) system. This result may be compared with the present 8.5 meV and 3.3 As for adsorption energy and height calculated for the adsorption of H at the undefected surface in the MP2 calculations at the optimal substrate location D"0.0. The basic difference between adsorption energies and heights in the two systems is attributed to the very di!used nature of surface hydrides due to the large fractional excess of the negative charge. We may note that the electrostatic "elds near the surface of an ionic crystal are large enough [47}52] that the polarization energy is a major component of the adsorption energy for many physisorbed species. However, the "elds are so nonuniform [53,54] that the dipole polarizability gives an inaccurate account of the polarization energy. Guo and Bruch [44] calculated the electrostatic polarization energy for H and He on LiF and MgO surfaces and reported that it contributes at the 10% level for atomic hydrogen. They also reported: The trend is that the electrostatic terms will be signi"cant for more polarizable inert gases such as Kr and Xe. Calculating the polarization energy to within 25% for such adsorbates will require inclusion of many multipole con"gurations. Binding of atomic H at H\ is favored over binding at the substrate location D"0.5 of the undefected surface, and binding at the substrate location D"0.3 or 0.5 is strongly favored over binding at the other substrate locations of F> surface. For F and F\ surfaces, binding at the anion vacancy is also favored over binding at the other substrate locations. These preferential bindings suggest the possibility of atomic H di!usion over the surface. To examine this probability, we have calculated the Table 4 HOMO, LUMO and HOMO}LUMO energies of F>, F and F\ surfaces of LiH. Energies are given in Hartrees F> Li H F Li H F\ Li H HOMO LUMO HOMO}LUMO !0.45623 !0.45669 !0.19055 !0.19269 !0.04952 !0.18675 !0.16367 !0.16504 !0.06548 !0.06478 !0.03366 0.00779 !0.29256 !0.29165 !0.12507 !0.12791 !0.01586 !0.19454 Basis functions are added to the F>, F and F\ centers largest variations in the correlated adsorption energies in Fig. 7. These were calculated to be 1.8 meV for the undefected surface and 51.7 meV, 2.94 and 4.5 eV for the F>, F and F\ defected surfaces respectively. These results show that the activation barriers for the mobility of atomic H over the defected surfaces increase nearly exponentially in the series F>PFPF\. However, in reality, the adatom di!usion on these surfaces may include more complicated mechanisms, such as atomic exchange mechanisms, which we have not attempted to examine here. To investigate the di!erences in adsorption among F>, F and F\ surfaces, the local densities of state (LDOS) have been evaluated. The band gap as the di!erence between HOMO and LUMO of F>, F and F\ surfaces are given in Table 4. The band gaps were reduced by 4.56 eV for F and by 2.97 eV for F\ surface as a consequence of the HOMO and LUMO shifting to higher energies. The energy levels of the atomic hydrogen occupied and unoccupied AO's were calculated to be !13.57 and 2.63 eV respectively. Since charge transfer occurs from the hydrogen occupied AO to the surface unoccupied or singly occupied MO's (donation) and from the surface occupied or singly occupied MO's to the hydrogen unoccupied AO (back-donation), the reported change in the electronic structure makes charge transfer and spin pairing (with F surface) more facile in the course of adsorbate}substrate interactions. This is supported by our previous observation that adsorptivity was drastically increased over F and F\ surfaces. A.S. Shalabi et al. / Physica B 292 (2000) 59}70 4. Conclusions Some important properties of LiH-ionic crystal such as excitons, band gaps, activation energies of defect di!usion and adsorbate}substrate interactions can be adequately described using ab initio methods of molecular electronic structure calculations. 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