The influence of a water solvent on the optical absorption properties of alizarin is investigated using time-dependent density functional theory (TDDFT). The solvent is modeled at two different levels of theory: a structureless dielectric medium, using a polarizable continuum model, and the explicit inclusion of water molecules which are treated at the same level of theory as the solute. Thermal effects on the photoabsorption spectra are included by combining TDDFT with first principle molecular dynamics. The effect of molecular distortions on the excitation energies and oscillator strengths is analyzed, and a strong correlation between particular structural and optical properties is found.

We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co3O4). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O 1s to O 2p states hybridized with the unoccupied 3d states of cobalt atoms. Also, since Co3O4 contains two types of Co atoms, i.e., Co3+ and Co2+, we find that contribution of Co2+ ions to the pre-edge peak is solely due to single spin-polarized t2g orbitals (dxz, dyz, and dxy) while that of Co3+ ions is due to spin-up and spin-down polarized eg orbitals (dx2−y2 and dz2). Furthermore, we deduce the magnetic moments on the Co3+ and Co2+ to be zero and 3.00 μB respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co3O4 consists of antiferromagnetically ordered Co2+ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins.

No abstract