Searcharxiv⌕ Search

arXiv subjects

Michael Martins

Publications and source records attributed to Michael Martins.

11 recordsLinked to original sources
arXiv2024

High-Resolution X-Ray Spectroscopy of Interstellar Iron Toward Cygnus X-1 and GX 339-4

We present a high-resolution spectral study of Fe L-shell extinction by the diffuse interstellar medium (ISM) in the direction of the X-ray binaries Cygnus X-1 and GX 339-4, using the XMM-Newton reflection grating spectrometer. The majority of interstellar Fe is suspected to condense into dust grains in the diffuse ISM, but the compounds formed from this process are unknown. Here, we use the laboratory cross sections from Kortright & Kim (2000) and Lee et al. (2009) to model the absorption and scattering profiles of metallic Fe, and the crystalline compounds fayalite (Fe$_2$SiO$_4$), ferrous sulfate (FeSO$_4$), hematite ($α$-Fe$_2$O$_3$), and lepidocrocite ($γ$-FeOOH), which have oxidation states ranging from Fe$^{0}$ to Fe$^{3+}$. We find that the observed Fe L-shell features are systematically offset in energy from the laboratory measurements. An examination of over two dozen published measurements of Fe L-shell absorption finds a 1-2 eV scatter in energy positions of the L-shell features. Motivated by this, we fit for the best energy-scale shift simultaneously with the fine structure of the Fe L-shell extinction cross sections. Hematite and lepidocrocite provide the best fits ($\approx +1.1$ eV shift), followed by fayalite ($\approx +1.8$ eV shift). However, fayalite is disfavored, based on the implied abundances and knowledge of ISM silicates gained by infrared astronomical observations and meteoritic studies. We conclude that iron oxides in the Fe$^{3+}$ oxidation state are good candidates for Fe-bearing dust. To verify this, new absolute photoabsorption measurements are needed on an energy scale accurate to better than 0.2 eV.

astro-ph.GA↗
arXiv2023

Evidence for Conical Magnetic Structure in M-type BaFe12O19 Hexaferrite: A Combined Single-Crystal XMCD and Neutron Diffraction Study

The magnetic ground state of BaFe12O19 (BFO) was investigated using X-ray absorption at 1.2 K and 1.5 K, respectively. The XMCD measurements on single-crystals of BFO in grazing incidence geometry reveal the canting of the spins away from the c-axis of the hexagonal unit cell. Single-crystal neutron diffraction studies reveal magnetic satellite peaks along the 00l reciprocal lattice row around the forbidden l = 2n +/- 1 positions confirming conical-type magnetic structure in the ground state of BFO. The observation of the conical magnetic structure of BFO opens the possibility of type-II multiferroicity in undoped BFO also.

cond-mat.mtrl-sci↗
arXiv2022

Climbing the N-shell resonance ladder of xenon

The dependency on the excitation energy of ultrafast multi-photon ionization of xenon by intense, short extreme ultraviolet pulses (XUV) was investigated in the vicinity of the 4$d$ 'giant' resonance using ion time-of-flight spectroscopy. The yields of the high charge states of xenon show strong variations with the excitation energy. With reference to simulated absorption spectra, we can link the photon energy dependency to resonance structures of single-electron excitations mainly in the xenon N-shell and purely sequential multi-photon absorption.

physics.atom-ph↗
arXiv2022

Electronic Quantum Coherence in Glycine Molecules Probed with Ultrashort X-ray Pulses in Real Time

Quantum coherence between electronic states of a photoionized molecule and the resulting process of ultrafast electron-hole migration have been put forward as a possible quantum mechanism of charge-directed reactivity governing the photoionization-induced molecular decomposition. Attosecond experiments based on the indirect (fragment ion-based) characterization of the proposed electronic phenomena suggest that the photoionization-induced electronic coherence can survive for tens of femtoseconds, while some theoretical studies predict much faster decay of the coherence due to the quantum uncertainty in the nuclear positions and the nuclear-motion effects. The open questions are: do long-lived electronic quantum coherences exist in complex molecules and can they be probed directly, i.e. via electronic observables? Here, we use x-rays both to create and to directly probe quantum coherence in the photoionized amino acid glycine. The outgoing photoelectron wave leaves behind a positively charged ion that is in a coherent superposition of quantum mechanical eigenstates lying within the ionizing pulse spectral bandwidth. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay and by the photoelectron emission from sequential double photoionization. Sinusoidal temporal modulation of the detected signal at early times (0 - 25 fs) is observed in both measurements. Advanced ab initio many-electron simulations, taking into account the quantum uncertainty in the nuclear positions, allow us to explain the first 25 fs of the detected coherent quantum evolution in terms of the electronic coherence.

physics.chem-ph↗
arXiv2021

Magnetic ordering in metal-free radical thin films

Molecular systems are materials that intersect with many different promising fields such as organic/molecular electronics and spintronics, organic magnetism and quantum computing1-7. Particularly, magnetism in organic materials is very intriguing: the possibility to realize long-range magnetic order in completely metal-free systems means that magnetic moments are coupled to useful properties of organic materials, such as optical transparency, low-cost fabrication, and flexible chemical design. Magnetic ordering in light elements, such as nitrogen and carbon, has been studied in magnetic-edged graphene nanoribbons8 and bilayers9, and polymers10 while in organic thin films most of the investigations show this effect as due to the proximity of light atoms to heavy metals, impurities, or vacancies11. Purely organic radicals are molecules that carry one unpaired electron giving rise to a permanent magnetic moment, in the complete absence of metal ions.12-14 Inspired by their tremendous potential, here we investigate thin films of an exceptionally chemically stable Blatter radical derivative15 by using X-ray magnetic circular dichroism (XMCD)16-18. Here we observe XMCD at the nitrogen K-edge. Our results show a magnetic ordering different than in the single crystals and calculations indicate, although weak, a long-range intermolecular coupling. We anticipate our work to be a starting point for investigating and modelling magnetic behaviour in purely organic thin films. The tuning of the magnetic properties by the molecular arrangement in organic films is an exciting perspective towards revealing new properties and applications.

cond-mat.mtrl-sci↗
arXiv2020

Near L-Edge Single and Multiple Photoionization of Doubly Charged Iron Ions

Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for single and up to five-fold photoionization of Fe$^{2+}$ ions in the energy range 690--920 eV. This range contains thresholds and resonances associated with ionization and excitation of $2p$ and $2s$ electrons. Calculations were performed to simulate the total absorption spectra. The theoretical results show very good agreement with the experimental data, if overall energy shifts of up to 2.5 eV are applied to the calculated resonance positions and assumptions are made about the initial experimental population of the various levels of the Fe$^{2+}$([Ar]$3d^6$) ground configuration. Furthermore, we performed extensive calculations of the Auger cascades that result when an electron is removed from the $2p$ subshell of Fe$^{2+}$. These computations lead to a better agreement with the measured product-charge-state distributions as compared to earlier work. We conclude that the $L$-shell absorption features of low-charged iron ions are useful for identifying gas-phase iron in the interstellar medium and for discriminating against the various forms of condensed-phase iron bound to composite interstellar dust grains.

physics.atom-ph↗
arXiv2019

Near L-edge photoionization of triply charged iron ions

Relative cross sections for $m$-fold photoionization ($m=1,\ldots,5$) of Fe$^{3+}$ by single photon absorption were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source operated at DESY in Hamburg, Germany. The photon energies used spanned the range of $680-950\,\mathrm{eV}$, covering both the photoexcitation resonances from the $2p$ and $2s$ shells as well as the direct ionization from both shells. Multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations were performed to simulate the total photoexcitation spectra. Good agreement was found with the experimental results. These computations helped to assign several strong resonance features to specific transitions. We also carried out Hartree-Fock calculations with relativistic extensions taking into account both photoexcitation and photoionization. Furthermore, we performed extensive MCDHF calculations of the Auger cascades that result when an electron is removed from the $2p$ and $2s$ shells of Fe$^{3+}$. Our theoretically predicted charge-state fractions are in good agreement with the experimental results, representing a substantial improvement over previous theoretical calculations. The main reason for the disagreement with the previous calculations is their lack of inclusion of slow Auger decays of several configurations that can only proceed when accompanied by de-excitation of two electrons. In such cases, this additional shake-down transition of a (sub-)valence electron is required to gain the necessary energy for the release of the Auger electron.

physics.atom-ph↗
arXiv2017

Near L-Edge Single and Multiple Photoionization of Singly Charged Iron Ions

Absolute cross sections for m-fold photoionization (m=1,...,6) of Fe+ by a single photon were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680-920 eV which covers the photoionization resonances associated with 2p and 2s excitation to higher atomic shells as well as the thresholds for 2p and 2s ionization. The corresponding resonance positions were measured with an uncertainty of +- 0.2 eV. The cross section for Fe+ photoabsorption is derived as the sum of the individually measured cross-sections for m-fold ionization. Calculations of the Fe+ absorption cross sections have been carried out using two different theoretical approaches, Hartree-Fock including relativistic extensions and fully relativistic Multi-Configuration Dirac Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross sections are in good agreement with each other and with the experimental results. In addition, the complex deexcitation cascades after the creation of inner-shell holes in the Fe+ ion have been tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium.

astro-ph.IM↗
arXiv2016

Ultrafast charge redistribution in small iodine containing molecules

The competition between intra molecular charge redistribution and fragmentation has been studied in small molecules containing iodine by using intense ultrashort pulses in the extreme ultraviolet regime (XUV). We show that after an element specific inner-shell photoionization of diiodomethane (CH$_2$I$_2$) and iodomethane (CH$_3$I), the induced positive charge is redistributed with a significantly different efficiency. Therefore, we analyze ion time-of-flight data obtained from XUV-pump XUV-probe experiments at the Free Electron Laser in Hamburg (FLASH). Theoretical considerations on the basis of ab initio electronic structure calculations including correlations relate this effect to a strongly molecule specific, purely electronic charge redistribution process that takes place directly after photoionization causing a distribution of the induced positive charge predominantly on the atoms which exhibit the lowest atomic ionization potential, i.e, in the molecules considered, the iodine atom(s). As a result of the very different initial charge distributions, the fragmentation timescales of the two molecules experimentally observed are strikingly different.

physics.chem-ph↗
arXiv2006

Non-collinear magnetism of Cr nanostructures on Fe$_{3ML}$/Cu(001): first--principles and experimental investigations

A combined experimental, using X-ray Magnetic Circular Dichroism and theoretical investigation, using full-potential Korringa-Kohn-Rostoker (KKR) Green function method, is carried out to study the spin structure of small magnetic Cr adatom--clusters on the surface of 3 monolayers of $fcc$ Fe deposited on Cu(001). The exchange interaction between the different Cr adatoms as well as between the Cr atoms and the Fe atoms is of antiferromagnetic nature and of comparable magnitude, leading due to frustration to complex non-collinear magnetic configurations. The presence of non-collinear magnetic coupling obtained by {\it ab initio} calculations is confirmed by the experimental results.

cond-mat.mtrl-sci↗