arXiv subjects

Frederico Alves Lima

Publications and source records attributed to Frederico Alves Lima.

3 recordsLinked to original sources

arXiv2026

Spin waves excited by hard x-ray transient gratings

Recent progress in ultrafast x-ray sources helped establish x-rays as an important tool for probing lattice and magnetic dynamics initiated by femtosecond optical pulses. Here, we explore the potential of ultrashort hard x-ray pulses for driving magnetic dynamics. We use a transient grating technique in which a spatially periodic x-ray excitation pattern gives rise to material excitations at a well-defined wave vector, whose dynamics are monitored via diffraction of an optical probe pulse. The excitation of a ferrimagnetic gadolinium bismuth iron garnet film placed in an external tilted magnetic field by x-rays at the Gd L3 edge results in both magnetic and non-magnetic transient gratings whose contributions to the diffracted signal are separated by polarization analysis. We observe the magnetization precession at both longitudinal acoustic and spin wave frequencies. An analysis with the Landau-Lifshitz-Gilbert equation indicates that the magnetization precession is driven by strain resulting from thermal expansion induced by absorbed x-rays. The results establish x-ray transient gratings as a tool for driving coherent phonons and magnons, with the potential of accessing wave vectors across the entire Brillouin zone.

physics.optics↗

arXiv2023

Ultrafast two-colour X-ray emission spectroscopy reveals excited state landscape in a base metal dyad

Effective photoinduced charge transfer makes molecular bimetallic assemblies attractive for applications as active light induced proton reduction systems. For a more sustainable future, development of competitive base metal dyads is mandatory. However, the electron transfer mechanisms from the photosensitizer to the proton reduction catalyst in base metal dyads remain so far unexplored. We study a Fe-Co dyad that exhibits photocatalytic H2 production activity using femtosecond X-ray emission spectroscopy, complemented by ultrafast optical spectroscopy and theoretical time-dependent DFT calculations, to understand the electronic and structural dynamics after photoexcitation and during the subsequent charge transfer process from the FeII photosensitizer to the cobaloxime catalyst. Using this novel approach, the simultaneous measurement of the transient Kalpha X-ray emission at the iron and cobalt K-edges in a two-colour experiment is enabled making it possible to correlate the excited state dynamics to the electron transfer processes. The methodology, therefore, provides a clear and direct spectroscopic evidence of the Fe->Co electron transfer responsible for the proton reduction activity.

physics.chem-ph↗

arXiv2022

Classical, semi-classical and quantum optical models for x-ray planar cavity with electronic resonance

Here two theoretical models of semi-classical matrix and quantum Green's function are developed for the system of x-ray thin-film planar cavity with inner-shell electronic resonances. The semi-classical model is based on the matrix formalism to treat each layer as the propagating matrix. The crucial idea is to expand the propagating matrix of the resonant atomic layer under ultrathin-film approximation, then derive the analytical expression of the spectral observation, i.e, the cavity reflectance. The typical cavity effects of cavity enhanced decay rate, cavity induced energy shift and the Fano interference which were observed in the recent experiments could be phenomenologically interpreted. The second quantum model employs the analytical Green's function to solve the cavity system. The system Hamiltonian and the effective energy-level are derived. The effective energy-level scheme indicates that the cavity effect acts on the regulation of the intermediated core-hole state. To test the validity of the semi-classical matrix and quantum Green's function models, the classical Parratt's formalism and the dispersion correction of the atomic refractive index are also recalled. Very good agreements in reflectivity spectra between semi-classical and quantum models with the Parratt's results are observed. The present semi-classical matrix and quantum Green's function models will be useful to predict the new phenomena and optimize the cavity structures for future experiments and promote the emerging of quantum optical effects with modern x-ray spectroscopy techniques.

quant-ph↗