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  • 1. Andersson, Renée
    et al.
    Kadi, Malin
    Davidsson, Jan
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Photoionisation of molecular wavepackets – the NaK(C1Σ+) case2002In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 352, no 1-2, p. 106-112Article in journal (Refereed)
  • 2.
    Boscolo Bibi, Sara
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    El-Zohry, Ahmed M.
    Stockholm University, Faculty of Science, Department of Physics.
    Davies, Bernadette
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Stockholm University, Faculty of Science, Department of Physics.
    Grigorev, Vladimir
    Stockholm University, Faculty of Science, Department of Physics.
    Goodwin, Christopher M.
    Stockholm University, Faculty of Science, Department of Physics.
    Lömker, Patrick
    Stockholm University, Faculty of Science, Department of Physics.
    Holm, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Ali-Löytty, Harri
    Garcia-Martinez, Fernando
    Schlueter, Christoph
    Soldemo, Markus
    Stockholm University, Faculty of Science, Department of Physics.
    Koroidov, Sergey
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Multi-spectroscopic study of electrochemically-formed oxide-derived gold electrodes2024In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 26, no 3, p. 2332-2340Article in journal (Refereed)
    Abstract [en]

    Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H3PO4, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au2O3 or Au(OH)3. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

  • 3.
    Elshakre, M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Storchi, L.
    Kloda, Tomasz
    Stockholm University, Faculty of Science, Department of Physics.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Heijkenskjold, F.
    Gengelbach, A.
    Karlsson, L.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Tarantelli, F.
    Feifel, R.
    A photoelectron and double photoionization study of the valence electronic structure of 1,4-bromofluorobenzene2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, no 18, p. 184302-Article in journal (Refereed)
    Abstract [en]

    Conventional photoelectron and time-of-flight photoelectron-photoelectron coincidence (TOF-PEPECO) spectra have been measured for the outer valence region of the 1,4-bromofluorobenzene molecule. The photoelectron spectra were recorded using Hela radiation from a resonance Source, and the TOF-PEPECO spectra were recorded using HeII alpha radiation from a pulsed resonance source. The former provide energies of the cationic states and the latter of the dicationic states. The spectra are adequately interpreted with the aid of accurate Green's function calculations, showing very significant correlation effects. The lowest double ionization energy is found at 23.45 eV associated with the (4b(1))X-2 (1)A(1) dicationic state.

  • 4. Gador, Niklas
    et al.
    Zhang, Bo
    Andersson, Renée
    Johansson, Pia
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Coherent multichannel nonadiabatic dynamics and parallel excitation pathways in the blue-violet absorption band of Rb22002In: Chemical Physics Letters, Vol. 368, p. 202-208Article in journal (Refereed)
    Abstract
  • 5. Gador, Niklas
    et al.
    Zhang, Bo
    Andersson, Renée
    Johansson, Pia
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Coherent multichannel nonadiabatic dynamics and parallel excitation pathways in the blue-violet absorption band of Rb22003In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 368, no 1-2, p. 202-208Article in journal (Refereed)
  • 6. Gador, Niklas
    et al.
    Zhang, Bo
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Molecular rotation–vibration coupling by quantum wavepacket interference at potential energy curve crossings2005In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 412, no 4-6, p. 386-390Article in journal (Refereed)
  • 7. Gador, Niklas
    et al.
    Zhang, Bo
    Karlsson, Hans O.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Dynamical interference structures in fully coupled bound-bound state quantum wave-packet dynamics2004In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, no 3, article id 033418Article in journal (Refereed)
  • 8.
    Geng, Ting
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ehrmaier, Johannes
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. University of Copenhagen, Denmark.
    Richings, Gareth W.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Worth, Graham
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Time-Resolved Photoelectron Spectroscopy Studies of Isoxazole and Oxazole2020In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 124, no 20, p. 3984-3992Article in journal (Refereed)
    Abstract [en]

    The excited state relaxation pathways of isoxazole and oxazole upon excitation with UV-light were investigated by nonadiabatic ab initio dynamics simulations and time-resolved photoelectron spectroscopy. Excitation of the bright ππ*-state of isoxazole predominantly leads to ring-opening dynamics. Both the initially excited ππ*-state and the dissociative πσ*-state offer a combined barrier-free reaction pathway, such that ring-opening, defined as a distance of more than 2 Å between two neighboring atoms, occurs within 45 fs. For oxazole, in contrast, the excited state dynamics is about twice as slow (85 fs) and the quantum yield for ring-opening is lower. This is caused by a small barrier between the ππ*-state and the πσ*-state along the reaction path, which suppresses direct ring-opening. Theoretical findings are consistent with the measured time-resolved photoelectron spectra, confirming the timescales and the quantum yields for the ring-opening channel. The results indicate that a combination of time-resolved photoelectron spectroscopy and excited state dynamics simulations can explain the dominant reaction pathways for this class of molecules. As a general rule, we suggest that the antibonding σ*-orbital located between the oxygen atom and a neighboring atom of a five-membered heterocyclic system provides a driving force for ring-opening reactions, which is modified by the presence and position of additional nitrogen atoms.

  • 9.
    Geng, Ting
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Galván, Ignacio F.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Lindh, Roland
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Time-resolved photoelectron spectroscopy studies on pyrazole and several methylated derivativesManuscript (preprint) (Other academic)
    Abstract [en]

    We present femtosecond time-resolved photoelectron spectra and ab initio studies on pyrazole andits methylated derivatives 1-, 3-, and 5-methylpyrazole. Excitation at 200 nm populates both the two lowest lying states, a 1ππ* state and a mixed 1πσ*/1π3s Rydberg state, from where three relaxation channels are observed: ring puckering, N-H bond cleavage, and ring opening via N-N bond breaking. The N-N bond breaking channel is the fastest process, occurring within one vibrational cycle of the Franc Condon active ring stretching mode. N-H bond cleavage is observed to be aminor channel, and occurs upon direct excitation to the mixed π3s/πσ* state. Finally, ring puckering occurs after a timescale of a few hundred fs because the molecules need time to find the gradient towards this conical intersection. However, this channel is accessed if the initially triggered processes are not successful. The quantum yields of the different channels were found to be very sensitive of the relative positioning of the excited states. In pyrazole and 5-methylpyrazole, N-Nbond cleavage dominates. In 1-, and 3-methylpyrazole, while the 1ππ* state drops in energy the dissociating 1πσ* valence state does not, and this leads to an increased barrier towards ring cleavage and a decreased the quantum yield for N-N bond cleavage. Upon excitation at 267 nm of 1- and 3-methylpyrazole, ring puckering is the only available pathway.

  • 10.
    Geng, Ting
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Neville, Simon P.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 14, article id 144307Article in journal (Refereed)
    Abstract [en]

    The involvement of intermediate Rydberg states in the relaxation dynamics of small organic molecules which, after excitation to the valence manifold, also return to the valence manifold is rarely observed. We report here that such a transiently populated Rydberg state may offer the possibility to modify the outcome of a photochemical reaction. In a time resolved photoelectron study on pyrrole and its methylated derivatives, N-methyl pyrrole and 2,5-dimethyl pyrrole, 6.2 eV photons (200 nm) are used to excite these molecules into a bright pi pi* state. In each case, a pi 3p-Rydberg state, either the B-1(pi 3p(y)) or the A(2)(pi 3p(z)) state, is populated within 20-50 fs after excitation. The wavepacket then proceeds to the lower lying A(2)(pi sigma*) state within a further 20 fs, at which point two competing reaction channels can be accessed: prompt N-H (N-CH3) bond cleavage or return to the ground state via a conical intersection accessed after ring puckering, the latter of which is predicted to require an additional 100-160 fs depending on the molecule.

  • 11.
    Gladh, Jörgen
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)2013In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 615, p. 65-71Article in journal (Refereed)
    Abstract [en]

    We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3-4 higher desorption yield at comparable fluence when desorption is induced via 400 nm light, compared to 800 nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.

  • 12. Hohmann, Lea
    et al.
    Marks, Kess
    Chien, Tzu-En
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Muntwiler, Matthias
    Engvall, Klas
    Göthelid, Mats
    Harding, Dan J.
    Effect of Coadsorbed Sulfur on the Dehydrogenation of Naphthalene on Ni(111)2024In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 128, no 1, p. 67-76Article in journal (Refereed)
    Abstract [en]

    There are several difficulties when experimentally determined reaction mechanisms are applied from model systems to real catalysis. Besides the infamous pressure and material gaps, it is sometimes necessary to consider impurities in the real reactant feedstock that can act as promoters or catalyst poisons and alter the reaction path. In this study, the effect of sulfur on the dehydrogenation of naphthalene on Ni(111) is investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. Sulfur induces a (5√3 × 2) surface reconstruction, as previously reported in the literature. The sulfur does not have a strong effect on the dehydrogenation temperature of naphthalene. However, the presence of sulfur leads to a preferred formation of carbidic over graphitic carbon and a strong inhibition of carbon diffusion into the nickel bulk, which is one of the steps of destructive whisker carbon formation described in the catalysis literature.

  • 13.
    Johansson, Pia
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mazets, I. E.
    Vasyutinskii, O. S.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Isotropic quantum beats: I. Pulsed pump-probe detection2004In: European Physical Journal D: Atomic, Molecular and Optical Physics, ISSN 1434-6060, E-ISSN 1434-6079, Vol. 31, no 1, p. 121-129Article in journal (Refereed)
  • 14.
    Johansson, Pia
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mazets, Igor E
    Vasyutinskii, Oleg S
    Hansson, Tony
    Isotropic quantum beats: I. Pulsed pump-probe detection2004In: The European Physical Journal D, Vol. 31, p. 121-129Article in journal (Refereed)
    Abstract
  • 15.
    Kloda, Tomasz
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Matsuda, Akitaka
    Karlsson, Hans O.
    Elshakre, Mohamed
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Eland, John H. D.
    Feifel, Raimund
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Strong-field photoionization of O-2 at intermediate light intensity2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 82, no 3, p. 33431-Article in journal (Refereed)
    Abstract [en]

    We investigated by electron spectroscopy the strong-field multiphoton ionization of O-2 molecules with ultrashort laser pulses in the intensity range between the multiphoton and tunneling regimes. The ionization proceeds by at least three different mechanisms, in addition to the eight- and nine-photon nonresonant pathways. Transient multiphoton resonances with vibrational Rydberg levels give rise to direct Freeman-type peaks with sublaser linewidth and spin-orbit splitting. Some resonance levels actually become populated and yield extremely narrow lines because of postpulse vibrational autoionization. When the lowest photon order resonance channel for the Rydberg states is closed, a third contribution becomes dominant with a main peak at 0.4 eV that shares its main properties with the recently discovered universal low-energy structure in the electron spectra of atoms and molecules [C. I. Blaga et al., Nat. Phys. 5, 335 (2009); W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)]. The variation of the Freeman resonance spectrum with the laser peak intensity is well correlated with the vibronic Franck-Condon factors for the overlap of the intermediate Rydberg state with the O-2 ground state. Accordingly, the Freeman peaks could be unambiguously assigned to individual vibronic multiphoton resonances, and the disappearance of the Freeman resonances at a certain laser intensity could be explained. The population of the autoionizing Rydberg states could be assigned similarly to such vibronic resonances.

  • 16.
    Koroidov, Sergey
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Winiwarter, Anna
    Diaz-Morales, Oscar
    Stockholm University, Faculty of Science, Department of Physics.
    Görlin, Mikaela
    Stockholm University, Faculty of Science, Department of Physics.
    Halldin Stenlid, Joakim
    Stockholm University, Faculty of Science, Department of Physics.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Börner, Mia
    Stockholm University, Faculty of Science, Department of Physics.
    Goodwin, Christopher Matthew
    Stockholm University, Faculty of Science, Department of Physics.
    Soldemo, Markus
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars Gunnar Moody
    Stockholm University, Faculty of Science, Department of Physics.
    Rossmeisl, Jan
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Chorkendorff, Ib
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Chemisorbed oxygen or surface oxides steer the selectivity in Pd electrocatalytic propene oxidation observed by operando Pd L-edge X-ray absorption spectroscopy2021In: Catalysis Science & Technology, ISSN 2044-4753, E-ISSN 2044-4761, Vol. 11, no 10, p. 3347-3352Article in journal (Refereed)
    Abstract [en]

    Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L-edge X-ray absorption spectroscopy (XAS) as a sensitive probe to elucidate surface processes occurring during catalysis. Together with ab initio multiple-scattering calculations, our XAS results enable assignment of characteristic changes of the Pd L-edge intensity and energy position in terms of a mechanistic understanding of the selective oxidation of propene. The results, supported by electrochemical density functional theory DFT simulations, show that in the potential range of 0.8–1.0 V vs. the reversible hydrogen electrode (RHE), selective oxidation of propene to acrolein and acrylic acid occurs on the metallic Pd surface. These reactions are proposed to proceed via the Langmuir–Hinshelwood mechanism. In contrast, for the potential range of 1.1–1.3 V vs. RHE, selective oxidation of propene to propylene glycol takes place on a Pd oxide surface.

  • 17. MacDonell, Ryan J.
    et al.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Geng, Ting
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Feifel, Raimund
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Schuurman, Michael S.
    Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation2016In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 145, no 11, article id 114306Article in journal (Refereed)
    Abstract [en]

    We report a joint experimental and theoretical study on the photoinitiated ultrafast dynamics of acrylonitrile (AN) and two methylated analogs: crotonitrile (CrN) and methacrylonitrile (MeAN). Time-resolved photoelectron spectroscopy (TRPES) and ab initio simulation are employed to discern the conical intersection mediated vibronic dynamics leading to relaxation to the ground electronic state. Each molecule is pumped with a femtosecond pulse at 200 nm and the ensuing wavepackets are probed by means of one and two photon ionization at 267 nm. The predominant vibrational motions involved in the de-excitation process, determined by ab initio trajectory simulations, are an initial twisting about the C=C axis followed by pyramidalization at a carbon atom. The decay of the time-resolved photoelectron signal for each molecule is characterized by exponential decay lifetimes for the passage back to the ground state of 60 +/- 10, 86 +/- 11, and 97 +/- 9 fs for AN, CrN, and MeAN, respectively. As these results show, the excited state dynamics are sensitive to the choice of methylation site and the explanation for the observed trend may be found in the trajectory simulations. Specifically, since the pyramidalization motion leading to the conical intersection with the ground state is accompanied by the development of a partial negative charge at the central atom of the pyramidal group, the electron donation of the cyano group ensures that this occurs exclusively at the medial carbon atom. In this way, the donated electron density from the cyano group directs the wavepacket to a particular region of the intersection seam. The excellent agreement between the experimental and simulated TRPES spectra, the latter determined by employing trajectory simulations, demonstrates that this mechanistic picture is consistent with the spectroscopic results.

  • 18.
    Marks, Kess
    et al.
    Stockholm University, Faculty of Science, Department of Physics. KTH Royal Institute of Technology, Sweden.
    Erbing, Axel
    Stockholm University, Faculty of Science, Department of Physics.
    Hohmann, Lea
    Chien, Tzu-En
    Yazdi, Milad Ghadami
    Muntwiler, Matthias
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Engvall, Klas
    Harding, Dan J.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Gothelid, Mats
    Naphthalene Dehydrogenation on Ni(111) in the Presence of Chemisorbed Oxygen and Nickel Oxide2024In: Catalysts, E-ISSN 2073-4344, Vol. 14, no 2, article id 124Article in journal (Refereed)
    Abstract [en]

    Catalyst passivation through carbon poisoning is a common and costly problem as it reduces the lifetime and performance of the catalyst. Adding oxygen to the feed stream could reduce poisoning but may also affect the activity negatively. We have studied the dehydrogenation, decomposition, and desorption of naphthalene co-adsorbed with oxygen on Ni(111) by combining temperature-programmed desorption (TPD), sum frequency generation spectroscopy (SFG), photoelectron spectroscopy (PES), and density functional theory (DFT). Chemisorbed oxygen reduces the sticking of naphthalene and shifts H2 production and desorption to higher temperatures by blocking active Ni sites. Oxygen increases the production of CO and reduces carbon residues on the surface. Chemisorbed oxygen is readily removed when naphthalene is decomposed. Oxide passivates the surface and reduces the sticking coefficient. But it also increases the production of CO dramatically and reduces the carbon residues. Ni2O3 is more active than NiO.

  • 19.
    Marks, Kess
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Yazdi, Milad Ghadami
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Engvall, Klas
    Harding, Dan J.
    Göthelid, Mats
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Adsorption and decomposition of naphthalene on oxygen pre-covered Ni(111)Manuscript (preprint) (Other academic)
  • 20.
    Marks, Kess
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Yazdi, Milad Ghadami
    Piskorz, Witold
    Simonov, Konstantin
    Stefanuik, Robert
    Sostina, Daria
    Guarnaccio, Ambra
    Ovsyannikov, Ruslan
    Giangrisostomi, Erika
    Sassa, Yasmine
    Bachellier, Nicolas
    Muntwiler, Matthias
    Johansson, Fredrik O. L.
    Lindblad, Andreas
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Kotarba, Andrzej
    Engvall, Klas
    Göthelid, Mats
    Harding, Dan J.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111)2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 150, no 24, article id 244704Article in journal (Refereed)
    Abstract [en]

    The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H-2 gas, as well as the formation of carbidic and graphitic surface carbon. Published under license by AIP Publishing.

  • 21. Oesterling, Sven
    et al.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Geng, Ting
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    de Vivie-Riedle, Regina
    Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: a combined theoretical and experimental approach2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, p. 2025-2035Article in journal (Refereed)
    Abstract [en]

    For the series furan, furfural and β-furfural we investigated the effect of substituents and their positioning on the photoinduced relaxation dynamics in a combined theoretical and experimental approach. Using time resolved photoelectron spectroscopy with a high intensity probe pulse, we can, for the first time, follow the whole deactivation process of furan through a two photon probe signal. Using the extended 2-electron 2-orbital model [Nenov et al., J. Chem. Phys., 2011, 135, 034304] we explain the formation of one central conical intersection and predict the influence of the aldehyde group of the derivatives on its geometry. This, as well as the relaxation mechanisms from photoexcitation to the final outcome was investigated using a variety of theoretical methods. Complete active space self consistent field was used for on-the-fly calculations while complete active space perturbation theory and coupled cluster theory were used to accurately describe critical configurations. Experiment and theory show the relaxation dynamics of furfural and β-furfural to be slowed down, and together they disclose an additional deactivation pathway, which is attributed to the nO lonepair state introduced with the aldehyde group.

  • 22.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Copenhagen, Denmark.
    Galiana, Joachim
    Geng, Ting
    Stockholm Univ, AlbaNova Univ Ctr, Dept Phys, S-10691 Stockholm, Sweden.
    Larsson, Tobias L.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Galvan, Ignacio Fdez.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Vacher, Morgane
    Competition between ring-puckering and ring-opening excited state reactions exemplified on 5H-furan-2-one and derivatives2020In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 152, no 6, article id 064301Article in journal (Refereed)
    Abstract [en]

    The influence of ring-puckering on the light-induced ring-opening dynamics of heterocyclic compounds was studied on the sample 5-membered ring molecules gamma-valerolactone and 5H-furan-2-one using time-resolved photoelectron spectroscopy and ab initio molecular dynamics simulations. In gamma-valerolactone, ring-puckering is not a viable relaxation channel and the only available reaction pathway is ring-opening, which occurs within one vibrational period along the C-O bond. In 5H-furan-2-one, the C = C double bond in the ring allows for ring-puckering which slows down the ring-opening process by about 150 fs while only marginally reducing its quantum yield. This demonstrates that ring-puckering is an ultrafast process, which is directly accessible upon excitation and which spreads the excited state wave packet quickly enough to influence even the outcome of an otherwise expectedly direct ring-opening reaction.

  • 23.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Geng, Ting
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    The ring-opening channel and the influence of Rydberg states on the excited state dynamics of furan and its derivatives2018In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 149, no 8, article id 084303Article in journal (Refereed)
    Abstract [en]

    One important relaxation pathway for photo-excited five-membered heterocyclic organic molecules is ring-opening via a dissociative pi sigma* state. In this study, we investigate the influence of this pathway in furan and several hydrogenated and methylated derivatives by combining time-resolved photoelectron spectroscopy with time-dependent density functional theory and coupled cluster calculations. We find strong experimental evidence that the ring-opening channel is the major relaxation channel in furan, 2,3-dihydrofuran, and 2-methylfuran (2-MF). In 2,5-dimethylfuran (25-DMF), however, we observe that the molecules relax either via a pi 3s Rydberg state or through a direct return to the ground state by undergoing ring-puckering motions. From the supporting calculations, for 2-MF and 25-DMF, we predict that there is strong mixing between the pi sigma* state and the pi 3s Rydberg state along the ring opening pathway. However, in 25-DMF, no crossing between the pi sigma*/pi 3s state and the initially excited pi pi* state can be found along the ring opening coordinate, effectively blocking this channel.

  • 24.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Geng, Ting
    Stockholm University, Faculty of Science, Department of Physics.
    Thompson, Travis
    Baluyot, Noel
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Tapavicza, Enrico
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Cyclohexadiene Revisited: A Time-Resolved Photoelectron Spectroscopy and ab Initio Study2016In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 120, no 15, p. 2320-2329Article in journal (Refereed)
    Abstract [en]

    We have reinvestigated the excited state dynamics of cyclohexa-1,3-diene (CHD) with time-resolved photoelectron spectroscopy and fewest switches surface hopping molecular dynamics based on linear response time dependent density functional theory after excitation to the lowest lying pi pi* (1B) state. The combination of both theory and experiment revealed several new results: First, the dynamics progress on one single excited state surface. After an incubation time of 35 +/- 10 fs on the excited state, the dynamics proceed to the ground state in an additional 60 +/- 10 fs, either via a conrotatory ring-opening to hexatriene or back to the CHD ground state. Moreover, ring-opening predominantly occurs when the wavepacket crosses the region of strong nonadiabatic coupling with a positive velocity in the bond alternation coordinate. After 100 fs, trajectories remaining in the excited state must return to the CHD ground state. This extra time delay induces a revival of the photoelectron signal and is an experimental confirmation of the previously formulated model of two parallel reaction channels with distinct time constants. Finally, our simulations suggest that after the initially formed cis-Z-cis HT rotamer the trans-Z-trans isomer is formed, before the thermodynamical equilibrium of three possible rotamers is reached after 1 ps.

  • 25.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Stenrup, M.
    Geng, Ting
    Stockholm University, Faculty of Science, Department of Physics.
    Lindh, R.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Feifel, R.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Influence of Alkoxy Groups on the Photoinduced Dynamics of Organic Molecules Exemplified on Alkyl Vinyl Ethers2015In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 119, no 45, p. 11105-11112Article in journal (Refereed)
    Abstract [en]

    A series of different alkyl vinyl ethers is investigated to decipher the possible reaction channels upon photoexcitation to the pi 3s-Rydberg and the pi pi*-valence state at 200 nm using time-resolved photoelectron spectroscopy and on-the-fly time-dependent density functional theory dynamics simulations. The results indicate two possible relaxation pathways: (1) a radiationless decay through the pi pi*-state back to the ground state via torsion of the C=C double bond, in accordance with the dynamics found in ethylene; and (2) a fast dissociation of the C-O bond between the alkyl and the vinoxy group in the pi sigma*-state. The latter state can be accessed only after excitation to the pi 3s-Rydberg state (quantum yield of similar to 50% according to the dynamics simulations). Additionally, the excited state barrier leading to formation of a vinyl radical was found to be too high to be crossed. These results indicate that the dynamics of ethers crucially depend on the excitation wavelength and that the pi sigma*-state constitutes an important competitive reaction channel that leads to dissociation of the molecules.

  • 26.
    Viarbitskaya, Sviatlana
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kapshai, Valery
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Size dependence of second-harmonic generation at the surface of microspheres2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 5, p. 53850-Article in journal (Refereed)
    Abstract [en]

    The resonance-enhanced surface second-harmonic generation (SHG) from a suspension of polystyrene microspheres was investigated as a function of particle size in a range of the order of the fundamental wavelength for two different second-harmonic-enhancing dyes-malachite green and pyridine 1. The two dyes gave the same strongly modulated pattern of the forward second-harmonic scattering efficiency. Direct comparison to the nonlinear Rayleigh-Gans-Debye (NLRGD) and nonlinear Wentzel-Kramers-Brillouin (NLWKB) model predictions showed that the NLWKB model reproduces the overall trend in the size dependence but fails with respect to the strong modulations. The standard NLRGD model was found to fail altogether in the present particle size range, which was well beyond the observed upper particle size for which the NLRGD and NLWKB models give comparable results. A generalization of the NLRGD model to allow for dispersion and to use the particle refractive indices instead of those of the surrounding medium extended its applicability range by almost an order of magnitude in particle size. There is a pronounced maximal SHG efficiency for particles with a radius that is close to the fundamental wavelength inside the particle. The optically soft particle approximation is inadequate to describe the SHG in this particle size range, as refraction and reflection of the waves at the particle surface have a decisive influence. Dispersion of the media plays a negligible role for particle sizes up to about twice the optimal one for SHG.

  • 27.
    Wang, Dong
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Larson, Åsa
    Stockholm University, Faculty of Science, Department of Physics.
    Karlsson, Hans O.
    Larson, Jonas
    Quantum interference structures in trapped ion dynamics beyond the Lamb-Dicke and rotating wave approximations2008In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 77, no 5Article in journal (Refereed)
    Abstract [en]

    We apply wave-packet methods to study an ion-trap system imposing neither the rotating wave nor the Lamb-Dicke approximations. By this approach we show the existence of states with restricted phase-space evolution as a genuine consequence of quantum interference between wave-packet fractions. A particular instance of such a state oscillates between maximal entanglement and pure disentanglement between the constitute subsystems, where the characteristic crossover time is very rapid. Over longer time periods the dynamics of these states exhibits collapse-revival patterns with well-resolved fractional revivals in autocorrelation, inversion, and entanglement.

  • 28.
    Wang, Dong
    et al.
    Stockholm University, Faculty of Science.
    Larson, Åsa
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Karlsson, Hans O.
    Molecular quantum wave-packet splitting and revivals in shared phase space2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 79, no 2, p. 23402-Article in journal (Refereed)
    Abstract [en]

    The evolution of a molecular wave packet created by an ultrashort laser pulse in a system of two coupled bound states is investigated by quantum dynamics calculations and semiclassical theory. Under suitable dynamical quantum interference conditions, the wave packet may be split into two separable fractions that move in different but partially overlapping regions of the energetically available phase space. Each wave-packet part can be individually addressed in the divided parts of the molecular phase space, and they are shown to go through separate long-term collapse-revival cycles analogous to those of wave packets moving in single anharmonic potentials. In a pump-probe scheme, the dynamics of the system would look very different depending on what internuclear distances are probed. The regular dynamics observable in the separated parts of the phase space takes on a quite irregular appearance in the regions that are shared by the wave-packet components. The wave-packet regularity is shown to depend sensitively on the pump pulse wavelength, which is a reflection of the energy range over which the quantum interference conditions are maintained. These conditions, as well as the wave-packet fraction revival times, are well reproduced by semiclassical theory.

  • 29.
    Wang, Dong
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Larson, Åsa
    Karlsson, Hans O.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Molecular quantum wavepacket revivals in coupled electronic states2007In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 449, no 4-6, p. 266-271Article in journal (Refereed)
  • 30. Yazdi, Milad Ghadami
    et al.
    Moud, Pouya. H.
    Marks, Kess
    Stockholm University, Faculty of Science, Department of Physics.
    Piskorz, Witold
    Östrom, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Kotarba, Andrzej
    Engvall, Klas
    Göthelid, Mats
    Naphthalene on Ni(111): Experimental and Theoretical Insights into Adsorption, Dehydrogenation, and Carbon Passivation2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 40, p. 22199-22207Article in journal (Refereed)
    Abstract [en]

    An attractive solution to mitigate tars and also to decompose lighter hydrocarbons in biomass gasification is secondary catalytic reforming, converting hydrocarbons to useful permanent gases. Albeit that it has been in use for a long time in fossil feedstock catalytic steam reforming, understanding of the catalytic processes is still limited. Naphthalene is typically present in the biomass gasification gas and to further understand the elementary steps of naphthalene transformation, we investigated the temperature dependent naphthalene adsorption, dehydrogenation and passivation on Ni(111). TPD (temperature-programmed desorption) and STM (scanning tunneling microscopy) in ultrahigh vacuum environment from 110 K up to 780 K, combined with DFT (density functional theory) were used in the study. Room temperature adsorption results in a flat naphthalene monolayer. DFT favors the dibridge[7] geometry but the potential energy surface is rather smooth and other adsorption geometries may coexist. DFT also reveals a pronounced dearomatization and charge transfer from the adsorbed molecule into the nickel surface. Dehydrogenation occurs in two steps, with two desorption peaks at approximately 450 and 600 K. The first step is due to partial dehydrogenation generating active hydrocarbon species that at higher temperatures migrates over the surface forming graphene. The graphene formation is accompanied by desorption of hydrogen in the high temperature TPD peak. The formation of graphene effectively passivates the surface both for hydrogen adsorption and naphthalene dissociation. In conclusion, the obtained results on the model naphthalene and Ni(111) system, provides insight into elementary steps of naphthalene adsorption, dehydrogenation, and carbon passivation, which may serve as a good starting point for rational design, development and optimization of the Ni catalyst surface, as well as process conditions, for the aromatic hydrocarbon reforming process.

  • 31. Zhang, Bo
    et al.
    Gador, Niklas
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Bound-Bound State Quantum Wave Packet Dynamics in the Intermediate Coupling Range: The A Σ u + 1 ( 0 u + ) and b Π u 3 ( 0 u + ) System in R b 22003In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 91, no 17, article id 173006Article in journal (Refereed)
  • 32.
    Öberg, Henrik
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nestsiarenka, Yuliya
    Stockholm University, Faculty of Science, Department of Physics.
    Matsuda, A.
    Stockholm University, Faculty of Science, Department of Physics.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Adsorption and Cyclotrimerization Kinetics of C2H2 at a Cu(110) Surface2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 17, p. 9550-9560Article in journal (Refereed)
    Abstract [en]

    The kinetics of acetylene adsorption and cyclotrimerization was studied by vibrational sum-frequency generation spectroscopy (SFG) and density functional theory (DFT) calculations. At low temperature, SFG shows two resonances corresponding to acetylene adsorbed in two different sites. Upon heating, two new vibrational resonances appear. We interpret these resonances as being due to C2H2 island formation and adsorbed C4H4, which is the intermediate in the subsequent cyclotrimerization reaction to form benzene. A kinetic model is applied, which allows determination of the relevant activation barriers. The barrier for C2H2 diffusion is determined to be 43 +/- 1 kJ/mol. The activation barrier for formation of the C4H4 intermediate is found to be 84 +/- 6 kJ/mol and the barrier for benzene formation 5 +/- 3 kJ/mol lower. Barriers to diffusion and formation of C4H4 and C6H6 obtained from DFT calculations are in quantitative agreement with the experiments once the locally high coverage in C2H2 islands is included.

1 - 32 of 32
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