| 1. |
|
|
| 2. |
- Brumboiu, Iulia Emilia, et al.
(författare)
-
Core-hole delocalization for modeling x-ray spectroscopies : A cautionary tale
- 2022
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 156:21
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of core-hole delocalization for x-ray photoelectron, x-ray absorption, and x-ray emission spectrum calculations is investigated in detail using approaches including response theory, transition-potential methods, and ground state schemes. The question of a localized/delocalized vacancy is relevant for systems with symmetrically equivalent atoms, as well as near-degeneracies that can distribute the core orbitals over several atoms. We show that the issues relating to core-hole delocalization are present for calculations considering explicit core-hole states, e.g., when using a core-excited or core-ionized reference state or for fractional occupation numbers. As electron correlation eventually alleviates the issues, but even when using coupled-cluster single-double and perturbative triple, there is a notable discrepancy between core-ionization energies obtained with localized and delocalized core-holes (0.5 eV for the carbon K-edge). Within density functional theory, the discrepancy correlates with the exchange interaction involving the core orbitals of the same spin symmetry as the delocalized core-hole. The use of a localized core-hole allows for a reasonably good inclusion of relaxation at a lower level of theory, whereas the proper symmetry solution involving a delocalized core-hole requires higher levels of theory to account for the correlation effects involved in orbital relaxation. For linear response methods, we further show that if x-ray absorption spectra are modeled by considering symmetry-unique sets of atoms, care has to be taken such that there are no delocalizations of the core orbitals, which would otherwise introduce shifts in absolute energies and relative features.
|
|
| 3. |
- Brumboiu, Iulia Emilia, 1987-, et al.
(författare)
-
Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine
- 2014
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 118:5, s. 927-932
-
Tidskriftsartikel (refereegranskat)abstract
- To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results, three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b2g1eg3a1g1b1g0 electronic configuration.
|
|
| 4. |
- Brumboiu, Iulia Emilia, et al.
(författare)
-
Influence of Electron Correlation on the Electronic Structure and Magnetism of Transition-Metal Phthalocyanines
- 2016
-
Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 12:4, s. 1772-1785
-
Tidskriftsartikel (refereegranskat)abstract
- There exists an extensive literature on the electronic structure of transition-metal phthalocyanines (TMPcs), either as single molecules or adsorbed on surfaces, where explicit intra-atomic Coulomb interactions of the strongly correlated orbitals are included in the form of a Hubbard U term. The choice of U is, to a large extent, based solely on previous values reported in the literature for similar systems. Here, we provide a systematic analysis of the influence of electron correlation on the electronic structure and magnetism of several TMPcs (MnPc, FePc, CoPc, NiPc, and CuPc). By comparing calculated results to valence-band photoelectron spectroscopy measurements, and by determining the Hubbard term from linear response, we show that the choice of U is not as straightforward and can be different for each different TMPc. This, in turn, highlights the importance of individually estimating the value of U for each system before performing any further analysis and shows how this value can influence the final results.
|
|
| 5. |
- Brumboiu, Iulia Emilia
(författare)
-
Molecules and Light : A Journey into the World of Theoretical Spectroscopy
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two of the main technological challenges of the century are the production of clean energy, on the one hand, and the development of new materials for electronic and spintronic applications that could increase the speed and the storage capacity of regular electronic devices, on the other hand. Organic materials, including fullerenes, organic polymers and organic molecules with metal centres are promising candidates for low-cost, flexible and clean technologies that can address these challenges. A thorough description of the electronic properties of such materials is, therefore, crucial. The interaction of electromagnetic radiation with the molecule can provide the needed insight into the electronic and vibrational levels and on possible chemical interactions. In order to explain and interpret experimentally measured spectra, a good theoretical description of the particular spectroscopy is necessary. Within density functional theory (DFT), the current thesis discusses the theoretical tools used to describe the spectroscopic properties of molecules with emphasis on two classes of organic materials for photovoltaics, molecular electronics and spintronics. Specifically, the stability of the fullerene derivative PC60BM is investigated in connection with its use as an electron acceptor in organic solar cells and the valence band electronic structure of several transition metal phthalocyanines is studied for their possible application in electronics and spintronics. The spectroscopies discussed in the current work are: the photoelectron spectroscopy of the valence band, X-ray photoelectron spectroscopy of the core levels, near-edge X-ray absorption fine structure, Infrared and Raman vibrational spectroscopies
|
|
| 6. |
- Brumboiu, Iulia Emilia, et al.
(författare)
-
Photoelectron Spectroscopy of Molecules Beyond the Electric Dipole Approximation
- 2019
-
Ingår i: Journal of Chemical Theory and Computation. - : AMER CHEMICAL SOC. - 1549-9618 .- 1549-9626. ; 15:10, s. 5483-5494
-
Tidskriftsartikel (refereegranskat)abstract
- A methodology implemented to compute photoionization cross sections beyond the electric dipole approximation using Gaussian type orbitals for the initial state and plane waves for the final state is applied to molecules of various sizes. The molecular photoionization cross sections computed for valence molecular orbitals as a function of photon energy present oscillations due to the wave-like nature of both the outgoing photoelectron and of the incoming photon. These oscillations are damped by rotational and vibrational averaging or by performing a k-point summation for the solid state case. For core orbitals, the corrections introduced by going beyond the electric dipole approximation are comparable to the atomic case. For valence orbitals, nondipole corrections to the total photoinization cross sections can reach up to 20% at photon energies above 1 keV. The corrections to the differential cross sections calculated at the magic angle are larger, reaching values between 30% and 50% for all molecules included. Our findings demonstrate that photoelectron spectroscopy, especially angle-resolved, on, e.g., molecules and clusters on surfaces, using high photon energies, must be accompanied by theories that go beyond the electric dipole approximation.
|
|
| 7. |
|
|
| 8. |
- Brumboiu, Iulia Emilia, 1987-
(författare)
-
The Electronic Structure of Organic Molecular Materials : Theoretical and Spectroscopic Investigations
- 2014
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the present thesis the electronic properties of two organic molecules were studied by means of density functional theory (DFT) in connection to their possible applications in organic photovoltaics and molecular spintronics respectively.The first analysed system is the C60 derivative PCBM extensively used in polymer solar cells for the charge separation process. Since fullerenes have been shown to undergo modifications as a result of light exposure, investigating their electronic structure is the first step in elucidating the photodegradation process. The electronic excitations from core levels to unoccupied molecular orbitals reveal not only the empty level structure of the molecule, but provide additional information related to the chemical bonds involving a specific atom type. In this way, they represent a means of determining the chemical changes that the molecule might withstand. The electronic transitions from carbon 1s core levels to unoccupied states are explained for the unmodified PCBM by a joint theoretical (DFT) and experimental study using the near edge x-ray absorption fine structure (NEXAFS) spectroscopy.The second investigated system is the transition metal phthalocyanine with a manganese atom as the metal center. Manganese phthalocyanine (MnPc) is a single molecular magnet in which the spin switch process can be triggered by various methods. It has been shown, for instance, that the adsorption of hydrogen to the Mn center changes the spin state of the molecule from 3/2 to 1. More interestingly, the process is reversible and can be controlled, opening up the possibility of using MnPc as a quantum bit in magnetic memory devices. Up to this date, the d orbital occupation in MnPc has been under a long debate, both theoretical and experimental studies revealing different configurations. In this thesis the electronic structure of the phthalocyanine is thoroughly analysed by means of DFT and the calculated results are compared to photoelectron spectroscopy measurements. The combination of theoretical and experimental tools reveals that in gas phase at high temepratures the molecule exhibits a mixed electronic configuration. In this light, the possible control of the specific electronic state of the central metal represents an interesting prospect for molecular spintronics.
|
|
| 9. |
|
|
| 10. |
|
|
