Our latest research delves into the intricate world of transient X-ray photoelectron spectra (XPS) to better understand the photoactivated dynamics in molecules and materials. XPS is a powerful technique that allows us to probe the electronic structure of substances, and simulating these spectra can bridge the gap between theoretical predictions and experimental observations. Our study focuses on the photodissociation of iron carbonyl complexes, specifically Fe(CO)₅ and its fragments Fe(CO)₄ and Fe(CO)₃, using the advanced multireference algebraic diagrammatic construction theory (MR-ADC).
The Challenge of Simulating XPS
Simulating XPS, especially along a photochemical reaction pathway, is formidable. It demands an accurate description of the electronic structure, incorporating several complex factors:
- Core-hole screening: How the remaining electrons reorganize themselves around a missing electron in a core orbital.
- Orbital relaxation: Changes in the orbitals’ shapes and energies after ionization.
- Electron correlation: The interactions between electrons that are not captured by a simple mean-field approximation.
- Spin-orbit coupling: The interaction between an electron’s spin and its orbital motion is particularly significant in heavy elements like iron.
Methodology: MR-ADC
We employed MR-ADC, a sophisticated computational method, to tackle these challenges. MR-ADC is particularly well-suited for studying systems where electron correlation is crucial. It allows for a detailed examination of core-ionized states and their corresponding XPS spectra. Our study investigated the Fe 3p and CO 3s XPS spectra of Fe(CO)₅ and its photodissociation products after excitation with 266 nm light.
Key Findings
Our simulations provided several insightful results:
- Agreement with Experimental Data: The transient Fe 3p and CO 3s XPS spectra we calculated showed a good match with the experimental measurements reported by Leitner et al. This validation is crucial as it underscores MR-ADC’s reliability in capturing these complex systems’ essential physics.
- Core-Hole Screening and Chemical Shifts: While core-hole screening effects contribute to the observed shifts in the XPS spectra, they alone do not account for the large shifts observed experimentally. We found that spin-orbit coupling and ligand-field splitting significantly contribute to these shifts.
- Importance of High-Order Correlation Effects: Our results highlight that high-order electron correlation effects are essential for accurately simulating the XPS spectra of transition metal complexes. Single-reference methods fail to capture the nuanced electronic interactions in these systems.
Implications and Future Work
Our findings suggest that accurately interpreting the transient XPS spectra of transition metal compounds requires considering multiple electronic structure effects beyond core-hole screening. Spin-orbit coupling and ligand-field interactions are crucial and must be incorporated into theoretical models.
This study marks the first use of MR-ADC to investigate the XPS spectra of iron carbonyl complexes. The method’s ability to handle strong electron correlation in specific orbitals while using large basis sets is particularly advantageous. Our results confirm that the MR-ADC(2)-X method yields highly accurate spectra, which can be further refined by expanding the active space in calculations.
Moving Forward
Looking ahead, our group is working on enhancing MR-ADC by incorporating higher-order treatments of spin-orbit coupling and including vibrational effects. These advancements will further improve the accuracy of XPS simulations, providing deeper insights into the electronic dynamics of complex molecular systems.
Conclusion
Our research underscores the importance of advanced computational methods like MR-ADC in bridging the gap between theory and experiment in studying XPS. We can better understand the underlying electronic dynamics and interactions by accurately simulating the transient XPS spectra of iron carbonyl complexes. This work paves the way for more detailed investigations into the photoactivated processes in other transition metal compounds, potentially leading to new discoveries in materials science and photochemistry.
Gaba, N. P., de Moura, C. E. V., Majumder, R. & Sokolov, A. Yu. Simulating transient X-ray photoelectron spectra of Fe(CO) 5 and its photodissociation products with multireference algebraic diagrammatic construction theory. Phys. Chem. Chem. Phys. 26, 15927–15938 (2024). [Link]