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Öberg, Henrik
Publications (10 of 16) Show all publications
Nilsson, A., LaRue, J., Öberg, H., Ogasawara, H., Dell'Angela, M., Beye, M., . . . Pettersson, L. G. M. (2017). Catalysis in real time using X-ray lasers. Chemical Physics Letters, 675, 145-173
Open this publication in new window or tab >>Catalysis in real time using X-ray lasers
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2017 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 675, p. 145-173Article in journal (Refereed) Published
Abstract [en]

We describe how the unique temporal and spectral characteristics of X-ray free-electron lasers (XFEL) can be utilized to follow chemical transformations in heterogeneous catalysis in real time. We highlight the systematic study of CO oxidation on Ru(0001), which we initiate either using a femtosecond pulse from an optical laser or by activating only the oxygen atoms using a THz pulse. We find that CO is promoted into an entropy-controlled precursor state prior to desorbing when the surface is heated in the absence of oxygen, whereas in the presence of oxygen, CO desorbs directly into the gas phase. We monitor the activation of atomic oxygen explicitly by the reduced split between bonding and antibonding orbitals as the oxygen comes out of the strongly bound hollow position. Applying these novel XFEL techniques to the full oxidation reaction resulted in the surprising observation of a significant fraction of the reactants at the transition state through the electronic signature of the new bond formation.

National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:su:diva-143458 (URN)10.1016/j.cplett.2017.02.018 (DOI)000400200800025 ()
Available from: 2017-06-07 Created: 2017-06-07 Last updated: 2022-03-23Bibliographically approved
Beye, M., Öberg, H., Xin, H., Dakovski, G. L., Dell'Angela, M., Föhlisch, A., . . . Wurth, W. (2016). Chemical Bond Activation Observed with an X-ray Laser. Journal of Physical Chemistry Letters, 7(18), 3647-3651
Open this publication in new window or tab >>Chemical Bond Activation Observed with an X-ray Laser
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2016 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, no 18, p. 3647-3651Article in journal (Refereed) Published
Abstract [en]

The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.

National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:su:diva-135185 (URN)10.1021/acs.jpclett.6b01543 (DOI)000383641800019 ()27584914 (PubMedID)2-s2.0-84987786651 (Scopus ID)
Available from: 2016-11-21 Created: 2016-11-01 Last updated: 2022-10-17Bibliographically approved
Gladh, J., Öberg, H., Pettersson, L. G. M. & Öström, H. (2015). Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy. Surface Science, 633, 77-81
Open this publication in new window or tab >>Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy
2015 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 633, p. 77-81Article in journal (Refereed) Published
Abstract [en]

We demonstrate that temperature-programmed vibrational sum-frequency generation (SFG) spectroscopy has a unique sensitivity to certain adsorbate overlayer structural transitions. In the CO stretching vibration of co-adsorbed CO/O(2x1)/Ru(0001) we observe pronounced dips in the spectral intensity as the adsorbate overlayer undergoes structural transitions with temperature. Combining with temperature-programmed desorption (TPD) a more complete picture of temperature-dependent structural transitions is obtained. We extract kinetic parameters from the SFG data and obtain good agreement with TPD when both techniques see the same transition. Infrared-infrared visible SFG is used to determine changes in inter-adsorbate coupling that allow us to experimentally assign the structural transitions. Furthermore, density functional theory calculations of the proposed structures and energetics are performed to verify the experimental assignments.

Keywords
Vibrational spectroscopy, Sum-frequency generation, Surface science, Density functional theory calculations
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-114344 (URN)10.1016/j.susc.2014.11.006 (DOI)000348336000011 ()2-s2.0-84919935210 (Scopus ID)
Note

AuthorCount:4;

Available from: 2015-03-09 Created: 2015-03-02 Last updated: 2022-10-14Bibliographically approved
Öberg, H., Gladh, J., Marks, K., Ogasawara, H., Nilsson, A., Pettersson, L. G. M. & Östrom, H. (2015). Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001). Journal of Chemical Physics, 143(7), Article ID 074701.
Open this publication in new window or tab >>Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)
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2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 7, article id 074701Article in journal (Refereed) Published
Abstract [en]

We studied CO oxidation on Ru(0001) induced by 400 nm and 800 nm femtosecond laser pulses where we find a branching ratio between CO oxidation and desorption of 1: 9 and 1: 31, respectively, showing higher selectivity towards CO oxidation for the shorter wavelength excitation. Activation energies computed with density functional theory show discrepancies with values extracted from the experiments, indicating both a mixture between different adsorbed phases and importance of non-adiabatic effects on the effective barrier for oxidation. We simulated the reactions using kinetic modeling based on the two-temperature model of laser-induced energy transfer in the substrate combined with a friction model for the coupling to adsorbate vibrations. This model gives an overall good agreement with experiment except for the substantial difference in yield ratio between CO oxidation and desorption at 400 nm and 800 nm. However, including also the initial, non-thermal effect of electrons transiently excited into antibonding states of the O-Ru bond yielded good agreement with all experimental results.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-121519 (URN)10.1063/1.4928646 (DOI)000360440400037 ()2-s2.0-84939865467 (Scopus ID)
Available from: 2015-10-09 Created: 2015-10-05 Last updated: 2022-10-14Bibliographically approved
Öberg, H., Gladh, J., Dell'Angela, M., Anniyev, T., Beye, M., Coffee, R., . . . Pettersson, L. G. (2015). Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser: DFT prediction and X-ray confirmation of a precursor state. Surface Science, 640, 80-88
Open this publication in new window or tab >>Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser: DFT prediction and X-ray confirmation of a precursor state
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2015 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 640, p. 80-88Article in journal (Refereed) Published
Abstract [en]

We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate.

Keywords
CO desorption, Potential of mean force, Two-temperature model, Pump-probe, X-ray spectroscopy, Density functional theory
National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:su:diva-120168 (URN)10.1016/j.susc.2015.03.011 (DOI)000359167800012 ()2-s2.0-84946499100 (Scopus ID)
Available from: 2015-09-07 Created: 2015-09-02 Last updated: 2022-10-14Bibliographically approved
Östrom, H., Öberg, H., Xin, H., Larue, J., Beye, M., Dell'Angela, M., . . . Nilsson, A. (2015). Probing the transition state region in catalytic CO oxidation on Ru. Science, 347(6225), 978-982
Open this publication in new window or tab >>Probing the transition state region in catalytic CO oxidation on Ru
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2015 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 347, no 6225, p. 978-982Article in journal (Refereed) Published
Abstract [en]

Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the OK-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC-O bond lengths close to the transition state (TS). After 1 ps, 10% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-115673 (URN)10.1126/science.1261747 (DOI)000349958900034 ()25722407 (PubMedID)2-s2.0-84923862250 (Scopus ID)
Note

AuthorCount:30;

Available from: 2015-04-01 Created: 2015-03-27 Last updated: 2022-10-14Bibliographically approved
Xin, H., LaRue, J., Öberg, H., Beye, M., Dell'Angela, M., Turner, J. J., . . . Abild-Pedersen, E. (2015). Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations. Physical Review Letters, 114(15), Article ID 156101.
Open this publication in new window or tab >>Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations
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2015 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 15, article id 156101Article in journal (Refereed) Published
Abstract [en]

We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5 sigma and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-117452 (URN)10.1103/PhysRevLett.114.156101 (DOI)000352990700006 ()25933322 (PubMedID)2-s2.0-84929590594 (Scopus ID)
Note

AuthorCount:25;

Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2022-10-17Bibliographically approved
Öberg, H. (2014). Surface reactions and chemical bonding in heterogeneous catalysis. (Doctoral dissertation). Stockholm: Department of Physics, Stockholm University
Open this publication in new window or tab >>Surface reactions and chemical bonding in heterogeneous catalysis
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis summarizes studies which focus on addressing, using both theoretical and experimental methods, fundamental questions about surface phenomena, such as chemical reactions and bonding, related to processes in heterogeneous catalysis. The main focus is on the theoretical approach and this aspect of the results. The included articles are collected into three categories of which the first contains detailed studies of model systems in heterogeneous catalysis. For example, the trimerization of acetylene adsorbed on Cu(110) is measured using vibrational spectroscopy and modeled within the framework of Density Functional Theory (DFT) and quantitative agreement of the reaction barriers is obtained. In the second category, aspects of fuel cell catalysis are discussed. O2 dissociation is rate-limiting for the reduction of oxygen (ORR) under certain conditions and we find that adsorbate-adsorbate interactions are decisive when modeling this reaction step. Oxidation of Pt(111) (Pt is the electrocatalyst), which may alter the overall activity of the catalyst, is found to start via a PtO-like surface oxide while formation of α-PtO2 trilayers precedes bulk oxidation. When considering alternative catalyst materials for the ORR, their stability needs to be investigated in detail under realistic conditions. The Pt/Cu(111) skin alloy offers a promising candidate but segregation of Cu atoms to the surface is induced by O adsorption. This is confirmed by modeling oxygen x-ray emission (XES) and absorption spectra of the segregated system and near-perfect agreement with experiment is obtained when vibrational interference effects are included in the computed XES. The last category shows results from femtosecond laser measurements of processes involving CO on Ru(0001). Using free-electron x-ray laser experiments a precursor state to desorption is detected and also found in simulations if van der Waals effects are included. Resonant XES can be used to distinguish two different species of CO on the surface; vibrationally hot, chemisorbed CO and CO in the precursor state. Laser-induced CO oxidation on Ru(0001) is modeled and three competing mechanisms are found. Kinetic modeling reproduces the experiment qualitatively.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2014. p. 66
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-102323 (URN)978-91-7447-893-8 (ISBN)
Public defence
2014-05-12, sal FP41, AlbaNova universitetscentrum, Roslagstullsbacken 33, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 8: Manuscript.

Available from: 2014-04-16 Created: 2014-04-01 Last updated: 2022-02-23Bibliographically approved
Dell'Angela, M., Anniyev, T., Beye, M., Coffee, R., Foehlisch, A., Gladh, J., . . . Nilsson, A. (2013). Real-Time Observation of Surface Bond Breaking with an X-ray Laser. Science, 339(6125), 1302-1305
Open this publication in new window or tab >>Real-Time Observation of Surface Bond Breaking with an X-ray Laser
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2013 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 339, no 6125, p. 1302-1305Article in journal (Refereed) Published
Abstract [en]

We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-89556 (URN)10.1126/science.1231711 (DOI)000316053400033 ()2-s2.0-84874983375 (Scopus ID)
Funder
Swedish Research Council
Note

AuthorCount:24;

Available from: 2013-05-02 Created: 2013-04-29 Last updated: 2022-10-07Bibliographically approved
Beye, M., Anniyev, T., Coffee, R., Dell'Angela, M., Foehlisch, A., Gladh, J., . . . Öström, H. (2013). Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001). Physical Review Letters, 110(18), Article ID 186101.
Open this publication in new window or tab >>Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)
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2013 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 18, article id 186101Article in journal (Refereed) Published
Abstract [en]

We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-91297 (URN)10.1103/PhysRevLett.110.186101 (DOI)000319019300011 ()23683223 (PubMedID)2-s2.0-84876982765 (Scopus ID)
Funder
Swedish Research Council
Note

AuthorCount:23;

Available from: 2013-06-27 Created: 2013-06-24 Last updated: 2022-10-07Bibliographically approved
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