We investigate the importance of the far red wing of the Lya line of hydrogen in the atmospheres of cool white dwarfs of pure hydrogen composition. We find that this absorption process dominates all important sources of opacity in the blue part of the optical spectrum of these stars. Our successful fits to the spectra of cool DA/DC white dwarfs indicate that the far red wing of the Lya line is the source of opacity that had been missing in the models. The observed sequence of cool white dwarfs in color-color diagrams is very well reproduced by our new pure hydrogen atmosphere models, suggesting that the atmospheric composition of the coolest DC white dwarfs must be revisited.

We present a new set of cooling models and isochrones for both H-and Heatmosphere white dwarfs, incorporating accurate boundary conditions from detailed model atmosphere calculations, and carbon-oxygen chemical abundance profiles based on updated stellar evolution calculations from the BaSTI stellar evolution archive -a theoretical data center for the Virtual Observatory. We discuss and quantify the uncertainties in the cooling times predicted by the models, arising from the treatment of mixing during the central H-and He-burning phases, number of thermal pulses experienced by the progenitors, progenitor metallicity and the 12 C(α, γ) 16 O reaction rate. The largest sources of uncertainty turn out to be related to the treatment of convection during the last stages of the progenitor central He-burning phase, and the 12 C(α, γ) 16 O reaction rate.We compare our new models to previous calculations performed with the same stellar evolution code, and discuss their application to the estimate of the age of the solar neighborhood, and the interpretation of the observed number ratios between H-and He-atmosphere white dwarfs. The new white dwarf sequences and an extensive set of white dwarf isochrones that cover a large range of ages and progenitor metallicities are made publicly available at the official BaSTI website.

A comprehensive phase diagram of lowest-energy structures and compositions of the rutile TiO2(110) surface in equilibrium with a surrounding gas phase at finite temperatures and pressures has been determined using density functional theory in combination with a thermodynamic formalism. The exchange of oxygen, hydrogen, and water molecules with the gas phase is considered. Particular attention is given to the convergence of all calculations with respect to lateral system size and slab thickness. In addition, the reliability of semilocal density functionals to describing the energetics of the reduced surfaces is critically evaluated. For ambient conditions the surface is found to be fully covered by molecularly adsorbed water. At low coverages, in the limit of single, isolated water molecules, molecular and dissociative adsorption become energetically degenerate. Oxygen vacancies form in strongly reducing, oxygen-poor environments. However, already at slightly more moderate conditions it is shown that removing full TiO2 units from the surface is thermodynamically preferred. In agreement with recent experimental observations it is furthermore confirmed that even under extremely hydrogen-rich environments the surface cannot be fully hydroxylated, but only a maximum coverage with hydrogen of about 0.6-0.7 monolayer can be reached. Finally, calculations of migration paths strongly suggest that hydrogen prefers to diffuse into the bulk over desorbing from the surface into the gas phase.

The dynamics of an F-center created by an oxygen vacancy on the TiO2(110) rutile surface has been investigated using ab initio molecular dynamics. These simulations uncover a truly complex, time-dependent behavior of fluctuating electron localization topologies in the vicinity of the oxygen vacancy. Although the two excess electrons are found to populate preferentially the second subsurface layer, they occasionally visit surface sites and also the third subsurface layer. This dynamical behavior of the excess charge explains hitherto conflicting interpretations of both theoretical findings and experimental data.PACS numbers: 71.15. Pd, 73.20.At, 82.65.+r, 73.20.Jc Titanium dioxide (TiO 2 ) is one of the most thoroughly investigated metal oxides, due to its broad range of uses in several key technologies including heterogeneous catalysis, pigment materials, photocatalysis, and energy production, to name but a few [1][2][3]. It is well known that bulk and surface defects govern the properties of titania, and are thus of fundamental importance in virtually all its applications [4][5][6]. The most common point defects on the TiO 2 (110) rutile surface are oxygen vacancies (O v ) in the two-fold coordinated O rows and Ti interstitials [7,8]. In particular, removal of an O atom gives rise to two excess electrons and the appearance of new electronic states in the band gap at about 0.7-0.9 eV below the conduction band edge creating an F-center [9][10][11]. Although the two excess electrons can in principle be localized on any Ti atom, they are believed to preferentially occupy specific Ti-3d orbitals, thus formally creating Ti 3+ sites [10,12]. In stark contrast, recent experiments [13] suggest a qualitatively different viewpoint: charge localization is found to be more disperse, with the excess electrons being shared by several surface and subsurface Ti ions. Furthermore, STM/STS experiments have revealed charge delocalization involving more than ten Ti sites [14].Unfortunately, different computational methods yield conflicting results [11]. Local/semilocal density functionals (LDA/GGA) predict a rather delocalized defect level for O vacancies on TiO 2 (110) with an energy right at the bottom of the conduction band [11]. However, it is well known that such functionals bias against localization on strongly correlated d-states, and hence alternative methodologies are welcome. Recent studies of defective TiO 2 surfaces [15-21] have focused on "pragmatic and practical" correction schemes using hybrid functionals or a Hubbard correction. Although both schemes yield the expected gap states, they each predict vastly different localization topologies of the excess charge.Using B3LYP on a c(4×2) slab with an O vacancy, the defect charge is found to be localized on d -orbitals of two surface Ti atoms [15]. In particular, one unpaired electron is found on the under-coordinated Ti(11) site, while the other moves to an adjacent five-fold coordinated Ti 5c atom, such as Ti (7); see Fig. 1 for our site labeling scheme. By contrast,...