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Ultrafast nonadiabatic dynamics probed by nitrogen K-edge absorption spectroscopy
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0003-1058-2588
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2020 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 22, no 5, p. 2667-2676Article in journal (Refereed) Published
Abstract [en]

The emergence of X-ray free electron lasers (X-FELs) has made it possible to probe structural dynamics on the femtosecond timescale. This extension of experimental capabilities also calls for a simultaneous development in theory to help interpret the underlying structure and dynamics encoded within the experimental observable. In the ultrafast regime this often requires a time-dependent theoretical treatment that describes nuclear dynamics beyond the Born–Oppenheimer approximation. In this work, we perform quantum dynamics simulations based upon time-evolving Gaussian basis functions (GBFs) and simulate the ultrafast X-ray Absorption Near-Edge Structure (XANES) spectra of photoexcited pyrazine including two strongly coupled electronically excited states and four normal mode degrees of freedom. Two methods to simulate the excited state XANES spectra are applied, the first is based upon the multi-configurational second order perturbation theory restricted active space (RASPT2) method and the second uses a combination of the maximum overlap method (MOM) and time-dependent density functional theory (TDDFT). We demonstrate that despite the simplicity of the MOM/TDDFT method, it captures several qualitative features of the RASPT2 simulations at much reduced computational effort. However, features such as the conical intersection are a particular exception as they require a multi-configurational treatment. For the nuclear dynamics, we demonstrate that even a small number of GBFs can provide reasonable description of the spectroscopic observable. This work provides perspectives for computationally efficient approaches important for addressing larger systems.

Place, publisher, year, edition, pages
2020. Vol. 22, no 5, p. 2667-2676
National Category
Atom and Molecular Physics and Optics
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-179755DOI: 10.1039/C9CP03019KOAI: oai:DiVA.org:su-179755DiVA, id: diva2:1412240
Funder
Swedish Research Council, 2015-03956Swedish National Infrastructure for Computing (SNIC), SNIC2018-1-4Available from: 2020-03-05 Created: 2020-03-05 Last updated: 2020-03-13Bibliographically approved
In thesis
1. Fingerprints of light-induced molecular transients: from quantum chemical models of ultrafast x-ray spectroscopy
Open this publication in new window or tab >>Fingerprints of light-induced molecular transients: from quantum chemical models of ultrafast x-ray spectroscopy
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Absorption of sunlight generates renewable electricity and powers the growth of plants, but also causes severe damage both to synthetic materials and biological tissue. The wildly varying outcomes of these light-induced processes are ultimately determined by much slighter differences in their underlying reaction pathways, induced by the transient properties of short-lived and miniscule molecules; a powerful approach to their detection and characterization is offered by ultrafast x-ray spectroscopy, with identification of spectral fingerprints and further guidance from quantum chemical models.

This thesis contains the computational half of three experimentally joint projects that push the limits for detection of electronic, spin and structural dynamics of small molecular systems in solution. A wide selection of theoretical frameworks are combined to model various aspects of the measurements: from multi-configurational descriptions of non-adiabatic couplings in the photo-dynamics and multi-electron transitions in the x-ray spectroscopy, to affordable simulations of extensive aqueous solutions by density functional theory and classical mechanics.

Applied to experimental data, the presented quantum chemical results allowed in particular to: simultaneously identify molecular forms and electronic states of aqueous 2-thiopyridone, to determine a detailed pathway for its excited-state proton-transfer; characterize the charge-transfer state of aqueous ferricyanide, to extend well-known concepts from steady-state spectroscopy into the ultrafast domain; establish the newly implemented framework of multi-configurational Dyson orbitals, as a powerful tool for simulation of photoelectron spectroscopy.

A number of computational predictions are additionally presented for hitherto-unexplored experimental regions, which may help to guide and optimize future measurements.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2020. p. 64
Keywords
time-resolved x-ray spectroscopy, quantum chemistry, electronic structure, multi-configurational self-consistent field, density functional theory, molecular dynamics, Born-Oppenheimer dynamics, non-adiabatic dynamics, proton-transfer, charge-transfer, solvatization, Dyson orbital
National Category
Atom and Molecular Physics and Optics
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-179758 (URN)978-91-7911-052-9 (ISBN)978-91-7911-053-6 (ISBN)
Public defence
2020-04-22, FA32, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2015-03956
Available from: 2020-03-30 Created: 2020-03-09 Last updated: 2020-03-30Bibliographically approved

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