Ab Initio Calculations of XUV Ground and Excited States for First-Row Transition Metal Oxides

Abstract

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core−hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe−Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides�TiO 2 , α-Cr 2 O 3 , β-MnO 2 , α-Fe 2 O 3 , Co 3 O 4 , NiO, CuO, and ZnO�are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core−valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Coherent charge hopping suppresses photoexcited small polarons in ErFeO 3 by antiadiabatic formation mechanism

“…The core-level transition Hamiltonian for the Fe M 2,3 edge is dominated by angular momentum components (46). The increase and decrease in absorption in the XUV spectrum therefore do not directly relate to electron and hole energies but instead relate to the change in overall Fe site occupation (oxidation state), the presence of phonon modes, and other structural excitations such as small polarons (46,47). The spectral evolution exhibits two distinctive vectors and timescales within the 5-ps time frame, which can be confirmed by singular value decomposition (20,48) (see overlays in Fig.…”

Determining Quasi-Equilibrium Electron and Hole Distributions of Plasmonic Photocatalysts Using Photomodulated X-ray Absorption Spectroscopy

“…36 According to the PDOS in Figure 3a, the photomodulated Ti L 2,3 edge predominantly probes photoexcited electrons over holes through the Ti 3d states; however, because of the screening and angular momentum coupling matrix elements in the X-ray transition Hamiltonian, holes will still perturb the core-tovalence transition excitons. 37,38 This work approximates the nanoparticle's amorphous TiO 2 as purely anatase phase. Although 10−20 nm TiO 2 nanoparticles typically consist of a mixture of anatase and brookite, anatase is a slightly more stable phase, and this approximation reduces the otherwise insurmountable computational costs of excited-state X-ray BSE calculations for hundreds of atoms.…”

“…An energy-dependent broadening method of the predicted spectrum was used to replicate the experimental core-hole lifetime broadening, which has a 3:2 broadening ratio (L 2 :L 3 ) for the TiO 2 Ti L 2,3 edge (see Supporting Information). According to the PDOS in Figure a, the photomodulated Ti L 2,3 edge predominantly probes photoexcited electrons over holes through the Ti 3d states; however, because of the screening and angular momentum coupling matrix elements in the X-ray transition Hamiltonian, holes will still perturb the core-to-valence transition excitons. , …”

“…The X-ray absorption simulation package, Obtaining Core Excitations from the Ab initio electronic structure and the NIST BSE solver (OCEAN), was used to model the plasmonic and excited-state properties in TiO 2 . , The package’s workflow has been described previously. , A variable-cell relaxation of the TiO 2 anatase crystal structure was initially performed to define suitable cell parameters. As part of the workflow, Quantum ESPRESSO , performed density functional theory (DFT) to calculate the ground-state electronic structure using a plane-wave basis set and a 350 Ry cutoff energy.…”

“…The standard OCEAN code can interpret static lattice heating by rerunning the DFT and BSE calculations for lattice parameters that simulate an isotropic thermal lattice expansion. However, our previous reports discuss the modified BSE code that simulates excited-state electrons and holes. ,, The standard code is modified to output the band structure as a usable k -point mesh array with defined energy values for each k -point. This array is then evaluated and modified to include an excited-state carrier population through a state filling.…”