Date and Time
Location
Abstract
I will discuss a number of femtosecond pump-probe experiments of transition metal oxides, starting with the
ground state x-ray absorption spectra (XAS). There is no unied theory for the interpretation of XAS spectra and
one-particle excitation models are used for excitations of 1s electrons, for example oxygen K edges [1], while
multiplet-based model Hamiltonians are used for excitations of 2p electrons [2,3]. It will be shown that the
crystal eld splitting is reduced by ~20% both in oxygen K edges and metal L edges.
Femtosecond pump-probe XAS spectra compare the ground state (laser-off) spectrum with the excited state
(laser-on) spectrum. From the transient spectrum the excited state spectrum at a certain time delay can be
approximated. Using TiO2 as example, the changes in the electronic structure will be described [5]: The T2g
peaks shifts to lower energy and the eg peak shifts more to lower energy, thereby decreasing the crystal eld
splitting in the optically excited state. In the excited state there is less titanium character in the valence band,
which shifts the oxygen spectra down in energy and the titanium spectra up in energy. This model nicely
explains the initial (~50fs) reaction to the optical excitation. Examples are shown for TiO2 [4], Fe2O3 [5] and
LaFeO3 [6].
I will end with some remarks on the expected femtosecond pump-probe 2p3d resonant inelastic x-ray
scattering spectra of transition metal oxides [7,8], both regarding electronic, magnon and phonon excitations.
[1] F.M.F. de Groot et al. Phys Rev B 40, 5715 (1989)
[2] F.M.F. de Groot et al. Phys Rev B 42, 5459 (1990)
[3] Core Level Spectroscopy of Solids Frank de Groot and Akio Kotani (Taylor & Francis CRC press, 2008)
[4] S. Park et al. Nature Communications 13, 2531 (2022)
[5] Y. Uemura et al. J Phys Chem Lett 13, 4207 (2022)
[6] M. Lazemi et al. Advanced Materials 2502932 (2025)
[7] F.M.F. de Groot et al. Phys Rev B 57, 14584 (1998)
[8] F.M.F. de Groot et al. Nature Rev Meth Primer 4, 45 (2024)
Links
Poster
