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One-dimensional cuts through multidimensional potential energy surfaces by tunable X-rays
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
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2018 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The concept of the potential-energy surface (PES) and directional reaction coordinates is the backbone of ourdescription of chemical reaction mechanisms. Although the eigenenergies of the nuclear Hamiltonian uniquely link a PES to its spectrum, this information is in general experimentally inaccessible in large polyatomic systems. This is due to (near) degenerate rovibrational levels across the parameter space of all degrees of freedom, which effectively forms a pseudospectrum given by the centers of gravity of groups of close-lying vibrational levels. We show here that resonant inelastic x-ray scattering (RIXS) constitutes an ideal probe for revealing one-dimensional cuts through the ground-state PES of molecular systems, even far away from the equilibrium geometry, where the independent-mode picture is broken. We strictly link the center of gravity of close-lying vibrational peaks in RIXS to a pseudospectrum which is shown to coincide with the eigenvalues of an effective one-dimensional Hamiltonian along the propagation coordinate of the core-excited wave packet. This concept, combined with directional and site selectivity of the core-excited states, allows us to experimentally extract cuts through the ground-state PES along three complementary directions for the showcase H2O molecule.

Place, publisher, year, edition, pages
2018.
National Category
Theoretical Chemistry Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-227948OAI: oai:DiVA.org:kth-227948DiVA, id: diva2:1205796
Note

QC 20180515

Available from: 2018-05-15 Created: 2018-05-15 Last updated: 2018-05-15Bibliographically approved
In thesis
1. Quantum Nuclear Dynamics in Resonant X-ray Scattering of Gas-Phase and Liquid Systems
Open this publication in new window or tab >>Quantum Nuclear Dynamics in Resonant X-ray Scattering of Gas-Phase and Liquid Systems
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the role of the nuclear degrees of freedom in X-ray induced molecular processes. An important part of it is devoted to establishing theoretical principles to model and interpret high-resolution resonant X-ray scattering experiments in gases and liquids. Our investigations address the resonant inelastic x-ray scattering (RIXS) of H2O(g), H2O(l) and CH3OH(g) and Auger emission induced by hard X-rays in CO(g). The simulations for gas-phase systems are based on a multi-mode wave packet formalism and on potential energy surfaces computed with multi-configurational approaches.

For liquid systems, we propose a classical/quantum formalism for simulating RIXS based on a combination of ab initio molecular dynamics, density functional theory calculations and quantum nuclear wave packet propagation. The developed model is able to reproduce the experimental observation of shortening of the vibrational progression in H2O(l).

We show that electronically-elastic RIXS has an intrinsic capability to map the potential energy surface and to carry out vibrational analysis of the electronic ground state in free molecules as well as liquids. For gas-phase water, we see that the landscape of different core-excited states cause the nuclear wave packet to be localized along specific directions thus allowing to reconstruct one-dimensional potential energy curves. For liquid water, we propose a model for deriving, from experiment, confidence intervals for the molecular potential energy curves along the OH bonds, which are determined by the local arrangement of the hydrogen bond network.

We also investigate the role of ultra-fast rotations induced by photoionization by hard X-rays. In this case, the ejection of a fast photoelectron results in an ultra-fast rotational motion of the molecule, which combined with the anisotropy of the Auger process causes the spectral profile to be split due to a dynamical Doppler effect.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 88
Series
TRITA-CBH-FOU ; 2018:24
Keywords
resonant inelastic X-ray scattering, X-ray absorption, water, methanol, CO, rotational doppler effect, recoil, wave packet, non-Franck-Condon effect, ultra-fast molecular dissociation, potential energy surface, hydrogen bond, liquid
National Category
Theoretical Chemistry Physical Sciences Atom and Molecular Physics and Optics
Research subject
Theoretical Chemistry and Biology
Identifiers
urn:nbn:se:kth:diva-227962 (URN)978-91-7729-806-9 (ISBN)
Public defence
2018-06-12, FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20180515

Available from: 2018-05-15 Created: 2018-05-15 Last updated: 2018-05-16Bibliographically approved

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