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Multimode resonant X-ray scattering of free molecules
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Siberian Federal University.ORCID iD: 0000-0003-3754-6763
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on the role that nuclear dynamics plays in the formation of X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra of multimode free molecules. A combined approach based on ab initio electronic structure methods and quantum nuclear wave packet dynamics is applied to two systems -- water and methanol in the gas phase. An IR-pump – X-ray-probe spectroscopy of vibrationally excited water and its isotope substitutions is employed to explore different vibrational progressions of the final electronic state due to a spatial filtration of the vibrations in the core-excited state and selection rules. It was demonstrated the possibility to use RIXS as a tool to study X-ray absorption from a selected vibrational level of the ground state. IR-pump – X-ray-probe spectroscopy applied to the HDO molecule sheds light on the old classical problem of wave function collapse: we demonstrate numerically the gradual collapse of the initially localised vibrational wave function in the HDO molecule. It is also explained the dynamical nature of the splitting of the 1b1 peak in the RIXS spectrum of H2O, HDO and D2O molecules. This splitting is referred to close-lying molecular and atomic-like peaks. In order to study the methanol molecule a special theoretical tool for studies of multimode molecules has been developed. This approach combines the advantages of the quantum wave packet technique for simulations of the dynamics in dissociative states with the efficiency of the Franck-Condon method for computing transitions between bound states. It is shown that the multimode nuclear dynamics plays an important role in XAS and RIXS spectra of methanol. The XAS and RIXS spectra formation was explained taking into account different dynamics in different core-excited potential energy surfaces, as well as the entanglement of vibrational modes by anharmonicity and by the life-time vibrational interference.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2018. , p. 73
Series
TRITA-CBH-FOU ; 2018:23
Keywords [en]
X-ray absorption spectroscopy, core-excited state, nuclear dynamic, wave-packet, resonant inelastic X-ray scattering, Franck-Condon, water, methanol, water isotopomer, pump-probe, quantum chemistry, ultrafast dissociation
National Category
Atom and Molecular Physics and Optics
Research subject
Chemistry; Physics
Identifiers
URN: urn:nbn:se:kth:diva-227971ISBN: 978-91-7729-800-7 (print)OAI: oai:DiVA.org:kth-227971DiVA, id: diva2:1206067
Public defence
2018-06-11, FA32, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20180516

Available from: 2018-05-16 Created: 2018-05-15 Last updated: 2018-05-16Bibliographically approved
List of papers
1. Gradual collapse of nuclear wave functions regulated by frequency tuned X-ray scattering
Open this publication in new window or tab >>Gradual collapse of nuclear wave functions regulated by frequency tuned X-ray scattering
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, no 43891Article in journal (Refereed) Published
Abstract [en]

As is well established, the symmetry breaking by isotope substitution in the water molecule results in localisation of the vibrations along one of the two bonds in the ground state. In this study we find that this localisation may be broken in excited electronic states. Contrary to the ground state, the stretching vibrations of HDO are delocalised in the bound core-excited state in spite of the mass difference between hydrogen and deuterium. The reason for this effect can be traced to the narrow “canyon-like” shape of the potential of the state along the symmetric stretching mode, which dominates over the localisation mass-difference effect. In contrast, the localisation of nuclear motion to one of the HDO bonds is preserved in the dissociative core-excited state . The dynamics of the delocalisation of nuclear motion in these core-excited states is studied using resonant inelastic X-ray scattering of the vibrationally excited HDO molecule. The results shed light on the process of a wave function collapse. After core-excitation into the state of HDO the initial wave packet collapses gradually, rather than instantaneously, to a single vibrational eigenstate.

Place, publisher, year, edition, pages
London: Nature Publishing Group, 2017
Keywords
Atomic and molecular interactions with photons
National Category
Natural Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-204921 (URN)10.1038/srep43891 (DOI)000395655000001 ()28266586 (PubMedID)2-s2.0-85014777715 (Scopus ID)
Funder
Swedish Research CouncilCarl Tryggers foundation Knut and Alice Wallenberg Foundation, 2013.0020
Note

QC 20170419

Available from: 2017-04-04 Created: 2017-04-04 Last updated: 2018-05-16Bibliographically approved
2. Infrared-pump–x-ray-probe spectroscopy of vibrationally excited molecules
Open this publication in new window or tab >>Infrared-pump–x-ray-probe spectroscopy of vibrationally excited molecules
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2017 (English)In: Physical Review A, ISSN 2469-9926, Vol. 95, no 4, article id 042502Article in journal (Refereed) Published
Abstract [en]

We develop a theory of infrared (IR)-pump–x-ray-probe spectroscopy for molecular studies. We illustrate advantages of the proposed scheme by means of numerical simulations employing a vibrational wave packet technique applied to x-ray absorption and resonant inelastic x-ray scattering (RIXS) spectra of the water molecule vibrationally excited by a preceding IR field. The promotion of the vibrationally excited molecule to the dissociative 1a−114a1 and bound 1a−112b2 core-excited states with qualitatively different shapes of the potential energy surfaces creates nuclear wave packets localized along and between the OH bonds, respectively. The projection of these wave packets on the final vibrational states, governed by selection and propensity rules, results in spatial selectivity of RIXS sensitive to the initial vibrationally excited state, which makes it possible to probe selectively the ground state properties along different modes. In addition, we propose to use RIXS as a tool to study x-ray absorption from a selected vibrational level of the ground state when the spectral resolution is sufficiently high to resolve vibrational overtones. The proposed technique has potential applications for advanced mapping of multidimensional potential energy surfaces of ground and core-excited molecular states, for symmetry-resolved spectroscopy, and for steering chemical reactions.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-207761 (URN)10.1103/PhysRevA.95.042502 (DOI)000399378800004 ()2-s2.0-85017294332 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation, KAW-2013.0020Carl Tryggers foundation Swedish Research Council
Note

QC 20170602

Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2018-05-15Bibliographically approved
3. Ultrafast dissociation features in RIXS spectra of the water molecule
Open this publication in new window or tab >>Ultrafast dissociation features in RIXS spectra of the water molecule
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2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084Article in journal (Refereed) Published
Abstract [en]

In this combined theoretical and experimental study we report on an analysis of the resonant inelastic X-ray scattering spectra (RIXS) of gas phase water via the lowest dissociative core-excited state |1sO-14a11〉. We focus on the spectral feature near the dissociation limit of the electronic ground state. We show that the narrow atomic-like peak consists of the overlapping contribution from the RIXS channels back to the ground state and to the first valence excited state |1b1-14a11〉 of the molecule. The spectral feature has signatures of ultrafast dissociation (UFD) in the core-excited state, as we show by means of ab initio calculations and time-dependent nuclear wave packet simulations. We show that the electronically elastic RIXS channel gives substantial contribution to the atomic-like resonance due to the strong bond length dependence of the magnitude and orientation of the transition dipole moment. By studying the RIXS for an excitation energy scan over the core-excited state resonance, we can understand and single out the molecular and atomic-like contributions in the decay to the lowest valence-excited state. Our study is complemented by a theoretical discussion of RIXS in the case of the isotope substituted water (HDO and D2O) where the nuclear dynamics is significantly affected by the heavier fragments' mass.

Keywords
resonant inelastic X-ray scattering, water molecule, Ultrafast dissociation, valence excited state, core-excited state
National Category
Atom and Molecular Physics and Optics
Research subject
Chemistry; Physics
Identifiers
urn:nbn:se:kth:diva-227942 (URN)10.1039/C8CP01807C (DOI)000434246300010 ()2-s2.0-85048446772 (Scopus ID)
Note

QC 20180516

Available from: 2018-05-15 Created: 2018-05-15 Last updated: 2018-06-25Bibliographically approved
4. Resonant inelastic X-ray scattering and X-ray absorption of methanol at the near oxygen K-edge
Open this publication in new window or tab >>Resonant inelastic X-ray scattering and X-ray absorption of methanol at the near oxygen K-edge
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We report on a theoretical analysis of core-excitation spectra of gas and liquid phase methanol asobtained with use of X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering(RIXS). The electronic transitions are studied with complementary computational methods includ-ing strict and extended second-order algebraic diagrammatic construction (ADC(2) and ADC(2)-x),restricted active space second-order perturbation theory (RASPT2), and time-dependent densityfunctional theory (TDDFT)—providing a complete assignment of the near oxygen K-edge XAS.We show that multimode nuclear dynamics is of crucial importance for explaining the availableexperimental XAS and RIXS spectra. Multimode nuclear motions was considered in a developedmixed representation where dissociative states and highly excited vibrational modes are accuratelytreated with a time-dependent wave packet technique while the remaining active vibrational modesare described using Franck–Condon amplitudes. Particular attention is paid to the polarizationdependence of RIXS and the effects of the isotope substitution on the RIXS profile in the case ofdissociative core-excited states. Our approach predicts the splitting of the 2a RIXS peak to bedue to an interplay between molecular and atomic-like features arising in the course of transitionsbetween dissociative core- and valence-excited states. The dynamical nature of the splitting of the2a peak in RIXS of liquid methanol near pre-edge core excitation is shown. The theoretical resultsare in good agreement with available experimental data.

National Category
Atom and Molecular Physics and Optics Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-227941 (URN)
Note

QC 20180515

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

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