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  • 1. Ernst, Hanna A.
    et al.
    Wolf, Thomas J. A.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. National Research Council of Canada, Canada.
    González-García, Núria
    Boguslavskiy, Andrey E.
    Stolow, Albert
    Olzmann, Matthias
    Unterreiner, Andreas-Neil
    Ultrafast Dynamics of o-Nitrophenol: An Experimental and Theoretical Study2015In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 119, no 35, p. 9225-9235Article in journal (Refereed)
    Abstract [en]

    The photolysis of o-nitrophenol (o-NP), a typical push pull molecule, is of current interest in atmospheric chemistry as a possible source of nitrous acid (HONO). To characterize the largely unknown photolysis mechanism, the dynamics of the lowest lying excited singlet state (S-1) of o-NP was investigated by means of femtosecond transient absorption spectroscopy in solution, time-resolved photoelectron spectroscopy (TRPES) in the gas phase and quantum chemical calculations. Evidence of the unstable aci-nitro isomer is provided both in the liquid and in the gas phase. Our results indicate that the Si state displays strong charge transfer character, which triggers excited state proton transfer from the OH to the NO2 group as evidenced by a temporal shift of 20 fs of the onset of the photoelectron spectrum. The proton transfer itself is found to be coupled to an out-of-plane rotation of the newly formed HONO group, finally leading to a conical intersection between Si and the ground state So. In solution, return to So within 0.2-0.3 ps was monitored by stimulated emission. As a competitive relaxation channel, ultrafast intersystem crossing to the upper triplet manifold on a subpicosecond time scale occurs both in solution and in the gas phase. Due to the ultrafast singlet dynamics, we conclude that the much discussed HONO split-off is likely to take place in the triplet manifold.

  • 2.
    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.

  • 3.
    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.

  • 4.
    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.

  • 5. Glover, William J.
    et al.
    Mori, Toshifumi
    Schuurman, Michael S.
    Boguslavskiy, Andrey E.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. National Research Council Canada, Canada.
    Stolow, Albert
    Martinez, Todd J.
    Excited state non-adiabatic dynamics of the smallest polyene, trans 1,3-butadiene. II. Ab initio multiple spawning simulations2018In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 148, no 16, article id 164303Article in journal (Refereed)
    Abstract [en]

    The excited state non-adiabatic dynamics of the smallest polyene, trans 1,3-butadiene (BD), has long been the subject of controversy due to its strong coupling, ultrafast time scales and the difficulties that theory faces in describing the relevant electronic states in a balanced fashion. Here we apply Ab Initio Multiple Spawning (AIMS) using state-averaged complete active space multistate second order perturbation theory [SA-3-CAS(4/4)-MSPT2] which describes both static and dynamic electron correlation effects, providing a balanced description of both the initially prepared bright 1(1)B(u) (pi pi*) state and non-adiabatically coupled dark 2(1)A(g) state of BD. Importantly, AIMS allows for on-the-fly calculations of experimental observables. We validate our approach by directly simulating the time resolved photoelectron-photoion coincidence spectroscopy results presented in Paper I [A. E. Boguslavskiy et al., J. Chem. Phys. 148, 164302 (2018)], demonstrating excellent agreement with experiment. Our simulations reveal that the initial excitation to the 11Bu state rapidly evolves via wavepacket dynamics that follow both bright-and dark-state pathways as well as mixtures of these. In order to test the sensitivity of the AIMS results to the relative ordering of states, we considered two hypothetical scenarios biased toward either the bright B-1(u) or the dark 2(1)A(g) state. In contrast with AIMS/SA-3-CAS(4/4)-MSPT2 simulations, neither of these scenarios yields favorable agreement with experiment. Thus, we conclude that the excited state non-adiabatic dynamics in BD involves both of these ultrafast pathways.

  • 6. Liang, Yu
    et al.
    Bradler, Maximilian
    Klinger, Melanie
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Balaban, Mihaela Carmen
    Balaban, Teodor Silviu
    Riedle, Eberhard
    Unterreiner, Andreas-Neil
    Ultrafast Dynamics of meso-Tetraphenylmetalloporphyrins: The Role of Dark States2013In: ChemPlusChem, E-ISSN 2192-6506, Vol. 78, no 10, p. 1244-1251Article in journal (Refereed)
    Abstract [en]

    Studying the relaxation pathways of porphyrins and related structures upon light absorption is crucial to understand the fundamental processes of light harvesting in biosystems and many applications. Herein, we show by means of transient absorption studies, following Q- and Soret-band excitation, and abinitio calculations on meso-tetraphenylporphyrinato magnesium(II) (MgTPP) and meso-tetraphenylporphyrinato cadmium(II) (CdTPP) that electronic relaxation following Soret-band excitation of porphyrins with a heavy central atom is mediated by a hitherto disregarded dark state. This accounts for an increased rate of internal conversion. The dark state originates from an orbital localized at the central nitrogen atoms and its energy continuously decreases along the series from magnesium to zinc to cadmium to below 2.75eV for CdTPP dissolved in tetrahydrofuran. Furthermore, we are able to directly trace fast intersystem crossing in the cadmium derivative, which takes place within (110 +/- 20)ps.

  • 7. Liang, Yu
    et al.
    Klinger, Melanie
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Unterreiner, Andreas-Neil
    On the origin of high transient anisotropies: An exemplification in a Cd-porphyrin2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 139, no 22, p. 224309-Article in journal (Refereed)
    Abstract [en]

    Transient anisotropy is a widely used spectroscopic method to access the polarization dynamics of a molecular sample. In this contribution, we present results on 5,10,15,20-tetraphenyl-porphyrinato cadmium (II) in tetrahydrofuran which exhibits values exceeding the typical range between 0.4 and -0.2 in dependence of the probe wavelength. These findings are explained by varying contributions from excited state absorption and ground state bleaching/stimulated emission. Model calculations show that time zero values and time decays are complex values that often do not correlate with the underlying physical processes. As a consequence, the interpretation of anisotropy experiments necessitates extreme care.

  • 8. 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.

  • 9. 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.

  • 10.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Munich.
    Boguslavskiy, Andrei E.
    Schuurman, Michael S.
    Brogaard, Rasmus Y.
    Unterreiner, Andreas N.
    Wrona-Piotrowicz, Anna
    Werstiuk, Nick H.
    Stolow, Albert
    Substituent Effects on Dynamics at Conical Intersections: Cycloheptatrienes2013In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 117, no 40, p. 10239-10247Article in journal (Refereed)
    Abstract [en]

    Using selective methyl substitution, we study the effects of vibrational dynamics at conical intersections in unsaturated hydrocarbons. Here, we investigate the excited state nonadiabatic dynamics of cycloheptatriene (CHT) and its relation to dynamics in other polyenes by comparing CHT with 7-methyl CHT, 7-ethyl CHT, and perdeuterated CHT using time-resolved photoelectron spectroscopy and photoelectron anisotropy. Our results suggest that, upon pi pi*-excitation to the bright 2A state, we observe an early intersection with the dark 2A' state close to the Franck-Condon region with evidence of wavepacket bifurcation. This indicates that the wavepacket evolves on both states, likely along a planarization coordinate, with the majority of the flux undergoing nonadiabatic transition via conical intersections within 100 fs following light absorption. In CHT, large amplitude motion along the planarization coordinate improves the intra-ring pi-overlap, yielding a delocalized electronic density. However, substitutions in 7 position, chosen to modify the inertia of the planarization motion, did not markedly alter the first step in the sequential kinetic scheme. This suggests that there is a crossing of potential energy surfaces before planarization is achieved and, thus, nonadiabatic transition likely takes place far away from a local minimum.

  • 11.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. National Research Council Canada.
    Boguslavskiy, Andrey E.
    Anisotropy in Time-Resolved Photoelectron Spectroscopy in the Gas Phase2017In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 121, no 50, p. 9612-9618Article in journal (Refereed)
    Abstract [en]

    Transient absorption anisotropy is a well-established technique in time-resolved liquid phase spectroscopy. Here, we show how the technique is applied in the gas phase for time-resolved photoelectron spectroscopy and what type of additional information can be obtained as compared to other techniques. We exemplify its use by presenting results on rotational revivals in pyrazine after excitation at 324 nm and provide new insights into two recent experiments: (i) the difference between Rydberg and valence state excitation after one- and two photon absorption in butadiene and (ii) excitation to the two lowest lying vibronic modes of the degenerate pi 3p Rydberg state in 1-azabicyclo[2.2.0]octane. Going forward, we expect the technique to be used on a regular basis, especially with the advent of high harmonic probe sources and liquid beam setups where other techniques to extract polarization-dependent information such as velocity map imaging cannot easily be applied.

  • 12.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. National Research Council Canada.
    Boguslavskiy, Andrey E.
    Stolow, Albert
    Two-Photon Excited State Dynamics of Dark Valence, Rydberg, and Superexcited States in 1,3-Butadiene2014In: The Journal of Physical Chemistry Letters, E-ISSN 1948-7185, Vol. 5, no 3, p. 560-565Article in journal (Refereed)
    Abstract [en]

    Two-photon absorption in systems with parity permits access to states that cannot be prepared by one-photon absorption. Here we present the first time-resolved photoelectron spectroscopy study using this technique, applied to 1,3-butadiene, in which we investigated the dynamics of its dark valence, Rydberg, and superexcited states. The dark valence state dynamics are accessed via the Rydberg manifold, excited by two photons of 400 nm. We find that the 'dark' 2(1)A(g) state populated in this manner has a much longer lifetime than when accesses via the 1(1)B(u) 'bright' valence state when populated by one photon of 200 nm. In addition, we compared the dynamics of the 3s pi- and 3d pi-Rydberg states. These Rydberg states relax to the valence manifold on a subpicosecond time scale, with the 3s pi-Rydberg state decay rate being larger due to a stronger valence-Rydberg mixing. Finally, we investigated superexcited valence states that fragment or autoionize within 200 fs, likely without involving Rydberg states.

  • 13.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Munich .
    Broman, Sören L.
    Petersen, Michael A.
    Khakhulin, Dmitry V.
    Brogaard, Rasmus Y.
    Nielsen, Mogens Brondsted
    Boguslavskiy, Andrey E.
    Stolow, Albert
    Solling, Theis I.
    On the Condensed Phase Ring-Closure of Vinylheptafulvalene and Ring-Opening of Gaseous Dihydroazulene2013In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 117, no 16, p. 3340-3347Article in journal (Refereed)
    Abstract [en]

    Dihydroazulenes are interesting because of their photoswitching behavior. While the ring-opening to vinylheptafulvalene (VHF) is light induced, the back reaction is known to proceed thermally. In the present paper, we show the first gas phase study of the ring-opening reaction of 2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (Ph-DHA) by means of time-resolved photoelectron spectroscopy which permits us to follow the ring-opening process. Moreover, we investigated s-trans-Ph-VHF in a series of transient absorption experiments, supported by ab initio computations, to understand the origin of the absence of light-induced ring-closure. The transient absorption results show a biexponential decay governed by a hitherto unknown state. This state is accessed within 1-2 ps and return to the ground state is probably driven through a cis-trans isomerization about the exocyclic C-1=C-2 double bond. The rapid decrease in potential energy disfavors internal rotation to s-cis-Ph-VHF, the structure that would precede the ring-closure reaction.

  • 14.
    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.

  • 15.
    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.

  • 16.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Josefsson, Ida
    Stockholm University, Faculty of Science, Department of Physics.
    Geng, Ting
    Stockholm University, Faculty of Science, Department of Physics.
    Richter, Robert
    Sa'adeh, Hanan
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Mucke, Melanie
    Dissociation kinetics of excited ions: PEPICO measurements of Os-3(CO)(12) - The 7-35 eV single ionization binding energy region2018In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 148, no 8, article id 084301Article in journal (Refereed)
    Abstract [en]

    In this article, we study the photoinduced dissociation pathways of a metallocarbonyl, Os-3(CO)(12), in particular the consecutive loss of CO groups. To do so, we performed photoelectron-photoion coincidence (PEPICO) measurements in the single ionization binding energy region from 7 to 35 eV using 45-eV photons. Zero-energy ion appearance energies for the dissociation steps were extracted by modeling the PEPICO data using the statistical adiabatic channel model. Upon ionization to the excited ionic states above 13 eV binding energy, non-statistical behaviorwas observed and assigned to prompt CO loss. Double ionization was found to be dominated by the knockout process with an onset of 20.9 similar to 0.4 eV. The oscillator strength is significantly larger for energies above 26.6 similar to 0.4 eV, corresponding to one electron being ejected from the Os3 center and one from the CO ligands. The cross section for double ionization was found to increase linearly up to 35 eV ionization energy, at which 40% of the generated ions are doubly charged.

  • 17.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Josefsson, Ida
    Stockholm University, Faculty of Science, Department of Physics.
    Richter, Robert
    Prince, Kevin C.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Mucke, Melanie
    Ionization and photofragmentation of Ru-3(CO)(12) and Os-3(CO)(12)2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 15, article id 154305Article in journal (Refereed)
    Abstract [en]

    In this paper, we use a combination of photoelectron spectroscopy, mass spectrometry, and density functional theory calculations to get a detailed understanding of valence single and double ionization and the subsequent dissociation processes. This is exemplified on benchmark systems, trimetallo-dodecacarbonyls M-3(CO)(12) with M = Ru, Os, where the energy remaining in the molecule after photoionization can be retrieved by measuring the degree of fragmentation of the molecular ion. The intensity of different mass peaks can thus be directly related to ionization cross sections obtained by photoelectron spectroscopy. We find that the M-CO dissociation energy rises as the number of CO ligands decreases due to dissociation. Moreover, ionization of the CO ligands has a higher cross section than that of the metal center for both single and double ionization. After advanced fragmentation, a CO bond can break and the carbon atom remains bonded to the metal core. In addition, we found that the valence ionization cross sections of M-3(CO)(12) are maximal at about 40 eV photon energy thus showing a more pronounced shape resonance than Ru and Os-complexes with a single metal atom center. Finally, an np. nd giant resonance absorption causes a significant increase of the ionization cross section above 50 eV for Ru-3(CO)(12).

  • 18.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Copenhagen, Denmark.
    Larsen, M. A. B.
    Skov, A. B.
    Liisberg, M. B.
    Geng, Ting
    Stockholm University, Faculty of Science, Department of Physics.
    Sølling, T. I.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Time-Resolved Photoelectron Studies of Thiophene and 2,5-Dimethylthiophene2018In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 122, no 45, p. 8809-8818Article in journal (Refereed)
    Abstract [en]

    The photoinduced dynamics of thiophene and 2,S-dimethylthiophene (2,5-DMT) were investigated upon excitation at 200 and 255 nm (2,5-DMT only) using time-resolved photoelectron spectroscopy and compared with results from ab initio coupled cluster calculations. For thiophene, depopulation of the initially excited B 2 (pi(3)pi(4)*) state to the lower-lying A(1) (pi(3)pi(4)*) state occurs within 25 +/- 20 fs, with a subsequent bifurcation into a ring-puckering channel and a ring-opening channel with lifetimes of 80 +/- 20 and 450 +/- 50 fs, respectively. For 2,5-DMT, the dynamics following excitation at 200 nm is described by a monoexponential decay with a time constant of 120 +/- 20 fs, while that following excitation at 255 nm is best fit by a biexponential decay with time constants of 115 +/- 20 fs and 15 +/- 3 ps, respectively. The fast signal observed after excitation of 2,5-DMT is assigned to the ring-opening channel, which is favored with respect to thiophene due to a lower excited-state barrier along the ring-opening coordinate and an increased inertia toward the ring-puckering channel. Coupled cluster calculations have been undertaken to compare the relaxation dynamics of thiophene to thiazole and isothiazole. For the latter two molecules, we find a strong gradient along the ring-opening coordinate in the Franck-Condon region of the initially populated pi pi* state and predict that ring-opening is the dominating relaxation channel after photoexcitation. We use the extracted information for a comparison of the thiophene dynamics with the light-induced processes observed in other five-membered heterocyclic molecules.

  • 19.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Munich, Germany.
    Schuurman, Michael S.
    Wu, Guorong
    Lang, Peter
    Mucke, Melanie
    Feifel, Raimund
    Stolow, Albert
    Internal Conversion versus Intersystem Crossing: What Drives the Gas Phase Dynamics of Cyclic alpha,beta-Enones?2014In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 118, no 12, p. 2279-2287Article in journal (Refereed)
    Abstract [en]

    We investigate the competition between intersystem crossing (ISC) and internal conversion (IC) as nonradiative relaxation pathways in cyclic alpha,beta-unsaturated enones following excitation to their lowest lying (1)pi pi* state, by means of time-resolved photoelectron spectroscopy and ab initio computation. Upon excitation, the (1)pi pi* state of 2-cyclopentenone decays to the lowest lying (1)pi pi* state within 120 +/- 20 fs. Within 1.2 +/- 0.2 ps, the molecule subsequently decays to the triplet manifold and the singlet ground state, with quantum yields of 0.35 and 0.65, respectively. The corresponding dynamics in modified derivatives, obtained by selective methylation, show a decrease in both IC and ISC rates, with the quantum yields of ISC varying between 0.35 and 0.08. The rapid rates of ISC are explained by a large spin orbit coupling of 45-60 cm(-1) over an extended region of near degeneracy between the singlet and triplet state. Furthermore, the rate of IC is depressed by the existence of a well-defined minimum on the (1)n pi* potential energy surface. The nonadiabatic pathways evinced by the present results highlight the fact that these molecular systems conceptually represent intermediate cases between ultrafast dynamics mediated by vibrational motions at conical intersections versus those by statistical decay mechanisms.

  • 20.
    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.

  • 21.
    Schalk, Oliver
    et al.
    Stockholm University, Faculty of Science, Department of Physics. National Research Council of Canada.
    Townsend, Dave
    Wolf, Thomas J. A.
    Holland, David M. P.
    Boguslavskiy, Andrey E.
    Szori, Milan
    Stolow, Albert
    Time-resolved photoelectron spectroscopy of nitrobenzene and its aldehydes2018In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 691, p. 379-387Article in journal (Refereed)
    Abstract [en]

    We report the first femtosecond time-resolved photoelectron spectroscopy study of 2-, 3-and 4-nitrobenzaldehyde (NBA) and nitrobenzene (NBE) in the gas phase upon excitation at 200 nm. In 3- and 4-NBA, the dynamics follow fast intersystem crossing within 1-2 picoseconds. In 2-NBA and NBE, the dynamics are faster (similar to 0.5 ps). 2-NBA undergoes hydrogen transfer similar to solution phase dynamics. NBE either releases NO2 in the excited state or converts internally back to the ground state. We discuss why these channels are suppressed in the other nitrobenzaldehydes.

  • 22. Sekikawa, Taro
    et al.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. Hokkaido University.
    Wu, Guorong
    Boguslavskiy, Andrey E.
    Stolow, Albert
    Initial Processes of Proton Transfer in Salicylideneaniline Studied by Time-Resolved Photoelectron Spectroscopy2013In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 117, no 14, p. 2971-2979Article in journal (Refereed)
    Abstract [en]

    Excited-state intramolecular proton transfer (ESIPT) in salicylideneaniline (SA) and selected derivatives substituted in the para position of the anilino group have been investigated by femtosecond time-resolved photoelectron spectroscopy (TRPES) and time-dependent density functional theory (TDDFT). SA has a twisted structure at the energetic minimum of the ground state, but ESIPT is assumed to take place through a planar structure, although this has not been fully established. The TRPES studies revealed that the excited-state dynamics within the S-1 band varied significantly with excitation wavelength. At finite temperatures, the ground state was found to sample a broad range of torsional angles, from planar to twisted. At lower photon energies (370 nm), only the planar ground-state molecules were excited, and the excited-state reaction took place within 50 fs. At higher energies (350 and 330 nm), predominantly twisted ground-state molecules were excited: they had to planarize before ESIPT could occur. This process was found to be slower in methylated SA but did not change significantly in the brominated and nitrated SAs. These substitution effects on the decay dynamics can be explained by modifications of the potential barriers, as predicted by the TDDFT calculations, and support the mechanism of a twisting motion of the anilino ring prior to ESIPT. The contribution of another pathway leading to internal conversion within the enol form was found to be minor at the excitation wavelengths considered here.

  • 23. Wolf, T. J. A.
    et al.
    Kuhlman, T. S.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Martinez, T. J.
    Moller, K. B.
    Stolow, A.
    Unterreiner, A. -N
    Hexamethylcyclopentadiene: time-resolved photoelectron spectroscopy and ab initio multiple spawning simulations2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 23, p. 11770-11779Article in journal (Refereed)
    Abstract [en]

    Progress in our understanding of ultrafast light-induced processes in molecules is best achieved through a close combination of experimental and theoretical approaches. Direct comparison is obtained if theory is able to directly reproduce experimental observables. Here, we present a joint approach comparing time-resolved photoelectron spectroscopy (TRPES) with ab initio multiple spawning (AIMS) simulations on the MS-MR-CASPT2 level of theory. We disentangle the relationship between two phenomena that dominate the immediate molecular response upon light absorption: a spectrally dependent delay of the photoelectron signal and an induction time prior to excited state depopulation in dynamics simulations. As a benchmark molecule, we have chosen hexamethylcyclopentadiene, which shows an unprecedentedly large spectral delay of (310 +/- 20) fs in TRPES experiments. For the dynamics simulations, methyl groups were replaced by hydrogen atoms having mass 15 and TRPES spectra were calculated. These showed an induction time of (108 +/- 10) fs which could directly be assigned to progress along a torsional mode leading to the intersection seam with the molecular ground state. In a stepladder-type approach, the close connection between the two phenomena could be elucidated, allowing for a comparison with other polyenes and supporting the general validity of this finding for their excited state dynamics. Thus, the combination of TRPES and AIMS proves to be a powerful tool for a thorough understanding of ultrafast excited state dynamics in polyenes.

  • 24. Wolf, T. J. A.
    et al.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. University of Munich.
    Radloff, R.
    Wu, G.
    Lang, P.
    Stolow, A.
    Unterreiner, A. -N
    Ultrafast photoinduced dynamics of halogenated cyclopentadienes: observation of geminate charge-transfer complexes in solution2013In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 18, p. 6673-6683Article in journal (Refereed)
    Abstract [en]

    The photoinduced dynamics of the fully halogenated cyclopentadienes C5Cl6 and C5Br6 have been investigated in solution and gas phase by femtosecond time-resolved spectroscopy. Both in solution and in gas phase, homolytic dissociation into a halogen radical and a C5X5 (X = Cl, Br) radical was observed. In liquid phase, solvent-dependent formation of charge transfer complexes between geminate radicals was observed for the first time. These complexes were found to be surprisingly stable and offered the opportunity to follow the dynamics of specific radical pairs. In the case of C5Cl6 in trichloroethanol, a reaction of the chlorine radical with molecules from the solvent cage was observed.

  • 25. Wu, Guorong
    et al.
    Neville, Simon P.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. National Research Council, Canada.
    Sekikawa, Taro
    Ashfold, Michael N. R.
    Worth, Graham A.
    Stolow, Albert
    Excited state non-adiabatic dynamics of N-methylpyrrole: A time-resolved photoelectron spectroscopy and quantum dynamics study2016In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 144, no 1, article id 014309Article in journal (Refereed)
    Abstract [en]

    The dynamics of N-methylpyrrole following excitation at wavelengths in the range 241.5-217.0 nm were studied using a combination of time-resolved photoelectron spectroscopy (TRPES), ab initio quantum dynamics calculations using the multi-layer multi-configurational time-dependent Hartree method, as well as high-level photoionization cross section calculations. Excitation at 241.5 and 236.2 nm results in population of the A(2)(pi sigma*) state, in agreement with previous studies. Excitation at 217.0 nm prepares the previously neglected B-1(pi 3p(y)) Rydberg state, followed by prompt internal conversion to the A(2)(pi sigma*) state. In contrast with the photoinduced dynamics of pyrrole, the lifetime of the wavepacket in the A(2)(pi sigma*) state was found to vary with excitation wavelength, decreasing by one order of magnitude upon tuning from 241.5 nm to 236.2 nm and by more than three orders of magnitude when excited at 217.0 nm. The order of magnitude difference in lifetimes measured at the longer excitation wavelengths is attributed to vibrational excitation in the A(2)(pi sigma*) state, facilitating wavepacket motion around the potential barrier in the N-CH3 dissociation coordinate.

  • 26. Wu, Guorong
    et al.
    Neville, Simon P.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. National Research Council Canada, Canada.
    Sekikawa, Taro
    Ashfold, Michael N. R.
    Worth, Graham A.
    Stolow, Albert
    Excited state non-adiabatic dynamics of pyrrole: A time-resolved photoelectron spectroscopy and quantum dynamics study2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 7, article id 74302Article in journal (Refereed)
    Abstract [en]

    The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A(2)(pi sigma*) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A(1)(pi pi*) and B-2(pi pi*) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B-1(pi sigma*) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B-1(pi sigma*) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A(2)(pi sigma*) state.

1 - 26 of 26
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