| 1. |
- Ali-Löytty, Harri, et al.
(författare)
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Chemical Dissolution of Pt(111) during Potential Cycling under Negative pH Conditions Studied by Operando X-ray Photoelectron Spectroscopy
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:41, s. 25128-25134
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Tidskriftsartikel (refereegranskat)abstract
- Dissolution of a platinum catalyst is a major degradation mechanism of fuel cells, but the exact reaction mechanism has remained unclear. Here, electrochemical ambient pressure X-ray photoelectron spectroscopy (EC-APXPS) was utilized to provide direct information on chemical species on a single-crystal Pt(111) electrode under extremely low pH conditions. Measurements were conducted using a novel condensed electrolyte film electrochemical cell applying work function measurement as a loss-free probe for electrochemical potential. We show that platinum can dissolve chemically as Pt2+ ion during potential cycling and redeposit as Pt2+ at the onset potential for cathodic reactions. The dissolution of Pt does not require electrochemical oxidation via oxide place exchange. In contrast, the adsorption of oxygenated species (OH* or O*) at the onset potential for anodic reactions is a sufficient prerequisite to the dissolution. These results provide new insight into the degradation mechanism of Pt under extremely low pH conditions, predicted by the Pourbaix diagram, having practical applications to the durability of Pt-based catalysts in electrochemical energy conversion devices.
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| 2. |
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| 3. |
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| 4. |
- Andersson, Klas J., et al.
(författare)
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Preparation, Structure, and Orientation of Pyrite FeS2{100} Surfaces : Anisotropy, Sulfur Monomers, Dimer Vacancies, and a Possible FeS Surface Phase
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:38, s. 21896-21903
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Tidskriftsartikel (refereegranskat)abstract
- Sulfur dimer (S-2(2-)) terminated pyrite FeS2{100} surfaces with a low energy electron diffraction (LEED) pattern of 2 x 1 symmetry are reported. The 2 X 1 symmetry correlates with the orientation of the anisotropic surface structure and external symmetry of macroscopic striations on the pyrite cube face. The basic condition to form these surfaces is a mild 200 V Ne+ sputter-cleaning procedure followed by a 570 K anneal of the sample in a 10(-7) Ton S-2(g) atmosphere. Controlled amounts of surface sulfur monomers (S2-) can be introduced by mild sputtering of the sulfur dimer terminated surfaces. At low monomer concentrations the surface displays the same characteristic 1 x 1 LEED pattern as that for fracture-generated surfaces. With increasing sulfur depletion, a (1/ root 2 x 1/ root 2)R45 degrees LEED pattern emerges, and soft X-ray photoelectron spectroscopy (XPS) results show a sulfur dinner deficient near-surface region and a new high binding energy sulfur spectral component suggesting the presence of local coordination environments where sulfur monomers are coordinated by four Fe ions compared to three as in the pyrite structure. The plausible formation of a defective FeS-like surface phase where monomeric sulfurs are coordinated by four Fe ions, and bond counting energetics favoring surface sulfur monomer recombination around Fe vacancy sites on pyrite FeS2{100}, both imply surface sulfur dimer vacancy sites with unique adsorption and reactivity properties. Taken together, our results suggest a very rich and dynamic defect structural landscape at pyrite FeS2{100} surfaces with direct implications for its surface chemical activity.
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| 5. |
- Anniyev, Toyli, et al.
(författare)
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Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts
- 2010
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 12:21, s. 5694-5700
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for selectively probing the platinum partial d-density of states (DOS) in a Pt-Cu nanoparticle catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR). The information about occupied Pt d-band states was complemented by Pt L-2-edge X-ray absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to higher binding energy compared to pure platinum. The effect of this electronic structure perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy and HAXPES allows for an experimental derivation of the valence electronic structure in an element-specific way for 5d metal catalysts.
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| 6. |
- Beye, Martin, et al.
(författare)
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Chemical Bond Activation Observed with an X-ray Laser
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:18, s. 3647-3651
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Tidskriftsartikel (refereegranskat)abstract
- The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.
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| 7. |
- Casalongue, Hernan G. Sanchez, et al.
(författare)
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Operando Characterization of an Amorphous Molybdenum Sulfide Nanoparticle Catalyst during the Hydrogen Evolution Reaction
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:50, s. 29252-29259
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Tidskriftsartikel (refereegranskat)abstract
- Molybdenum sulfide structures, particularly amorphous MoS3 nanoparticles, are promising materials in the search for cost-effective and scalable water-splitting catalysts. Ex situ observations show that the nanoparticles exhibit a composition change from MoS3 to defective MoS2 when subjected to hydrogen evolution reaction (HER) conditions, raising questions regarding the active surface sites taking part in the reaction. We tracked the in situ transformation of amorphous MoS3 nanoparticles under HER conditions through ambient pressure X-ray photoelectron spectroscopy and performed density functional theory studies of model MoSx systems. We demonstrate that, under operating conditions, surface sites are converted from MoS3 to MoS2 in a gradual manner and that the electrolytic current densities are proportional to the extent of the transformation. We also posit that it is the MoS2 edge-like sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS2 edge-like sites after the reduction of MoS3 sites.
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| 8. |
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| 9. |
- Diesen, Elias, et al.
(författare)
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Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy
- 2021
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 127:1
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Tidskriftsartikel (refereegranskat)abstract
- We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.
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| 10. |
- Eilert, André, et al.
(författare)
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Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction
- 2017
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:1, s. 285-290
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Tidskriftsartikel (refereegranskat)abstract
- Copper electrocatalysts derived from an oxide have shown extraordinary electrochemical properties for the carbon dioxide reduction reaction (CO2RR). Using in situ ambient pressure X-ray photoelectron spectroscopy and quasi in situ electron energy loss spectroscopy in a transmission electron microscope, we show that there is a substantial amount of residual oxygen in nanostructured, oxide-derived copper electrocatalysts but no residual copper oxide. On the basis of these findings in combination with density functional theory simulations, we propose that residual subsurface oxygen changes the electronic structure of the catalyst and creates sites with higher carbon monoxide binding energy. If such sites are stable under the strongly reducing conditions found in CO2RR, these findings would explain the high efficiencies of oxide-derived copper in reducing carbon dioxide to multicarbon compounds such as ethylene.
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| 11. |
- Friebel, Daniel, et al.
(författare)
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In situ x-ray probing reveals the importance of surface platinum oxide formation in fuel cell catalysis
- 2011
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 13:1, s. 262-266
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Tidskriftsartikel (refereegranskat)abstract
- In situ X-ray absorption spectroscopy (XAS) at the Pt L3 edge is a useful probe for Pt–O interactions at polymer electrolyte membrane fuel cell (PEMFC) cathodes. We show that XAS using the high energy resolution fluorescence detection (HERFD) mode, applied to a well-defined monolayer Pt/Rh(111) sample where the bulk penetrating hard X-rays probe only surface Pt atoms, provides a unique sensitivity to structure and chemical bonding at the Pt-electrolyte interface. Ab initio multiple-scattering calculations using the FEFF code and complementary extended X-ray absorption fine structure (EXAFS) results indicate that the commonly observed large increase of the white-line at high electrochemical potentials on PEMFC cathodes originates from platinum oxide formation, whereas previously proposed chemisorbed oxygen-containing species merely give rise to subtle spectral changes.
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| 12. |
- Furukawa, Masashi, et al.
(författare)
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Geometrical characterization of adenine and guanine on Cu(110) by NEXAFS, XPS, and DFT calculation
- 2007
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:23, s. 5433-5440
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption of purine DNA bases (guanine and adenine) on Cu(110) was studied by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine-structure spectroscopy (NEXAFS), and density-functional theory (DFT) calculation. At coverages near 0.2 monolayers, Angular-resolved NEXAFS analysis revealed that adenine adsorbates lie almost flat and that guanine adsorbates are tilted up on the surface with the purine ring parallel to the atom rows of Cu(110). Referring to the previous studies on pyrimidine DNA bases [M. Furukawa, H. Fujisawa, S. Katano, H. Ogasawara, Y. Kim, T. Komeda, A. Nilsson, M. Kawai, Surf. Sci. 532-535 (2003) 261], the isomerization of DNA bases on Cu(110) was found to play an important role in the adsorption geometry. Guanine, thymine and cytosine adsorption have an amine-type nitrogen next to a carbonyl group, which is dehydrogenated into imine nitrogen on Cu(110). These bases are bonded by the inherent portion of -NH-CO- altered by conversion into enolic form and dehydrogenation. Adenine contains no CO group and is bonded to Cu(110) by participation of the inherent amine parts, resulting in nearly flatly-lying position.
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| 13. |
- Kaya, Sarp, et al.
(författare)
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Highly Compressed Two-Dimensional Form of Water at Ambient Conditions
- 2013
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- The structure of thin-film water on a BaF2(111) surface under ambient conditions was studied using x-ray absorption spectroscopy from ambient to supercooled temperatures at relative humidity up to 95%. No hexagonal ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The oxygen K-edge x-ray absorption spectrum of liquid thin-film water on BaF2 exhibits, at all temperatures, a strong resemblance to that of high-density phases for which the observed spectroscopic features correlate linearly with the density. Surprisingly, the highly compressed, high-density thin-film liquid water is found to be stable from ambient (300 K) to supercooled (259 K) temperatures, although a lower-density liquid would be expected at supercooled conditions. Molecular dynamics simulations indicate that the first layer water on BaF2(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.
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| 14. |
- Ketteler, Guido, et al.
(författare)
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The nature of water nucleation sites on TiO2(110) surfaces revealed by ambient pressure X-ray photoelectron spectroscopy
- 2007
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 111:23, s. 8278-8282
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Tidskriftsartikel (refereegranskat)abstract
- X-ray photoelectron spectroscopy at ambient conditions of pressure (up to 1.5 Torr) and temperature (265 K < T < 800 K) was used to study the adsorption of water on rutile TiO2(110) under conditions of thermodynamic equilibrium. It was found that OH groups in bridging positions, normally present in small amounts due to residual O-vacancies, act as nucleation sites for subsequent water adsorption. The adsorption enthalpy of water binding to these sites is similar to 70 kJ/mol, much stronger than that in the bulk liquid (45 kJ/mol). A model is proposed that relates the structure of the oxide surface to its hydrophilic character.
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| 15. |
- Kim, Kyung Hwan, et al.
(författare)
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Anisotropic X-Ray Scattering of Transiently Oriented Water
- 2020
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Ingår i: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 125:7
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Tidskriftsartikel (refereegranskat)abstract
- We study the structural dynamics of liquid water by time-resolved anisotropic x-ray scattering under the optical Kerr effect condition. In this way, we can separate the anisotropic scattering decay of 160 fs from the delayed temperature increase of similar to 0.1 K occurring at 1 ps and quantify transient changes in the O-O pair distribution function. Polarizable molecular dynamics simulations reproduce well the experiment, indicating transient alignment of molecules along the electric field, which shortens the nearest-neighbor distances. In addition, analysis of the simulated water local structure provides evidence that two hypothesized fluctuating water configurations exhibit different polarizability.
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| 16. |
- Kim, Kyung Hwan, et al.
(författare)
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Temperature-Independent Nuclear Quantum Effects on the Structure of Water
- 2017
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 119:7
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Tidskriftsartikel (refereegranskat)abstract
- Nuclear quantum effects (NQEs) have a significant influence on the hydrogen bonds in water and aqueous solutions and have thus been the topic of extensive studies. However, the microscopic origin and the corresponding temperature dependence of NQEs have been elusive and still remain the subject of ongoing discussion. Previous x-ray scattering investigations indicate that NQEs on the structure of water exhibit significant temperature dependence [Phys. Rev. Lett. 94, 047801 (2005)]. Here, by performing wide-angle x-ray scattering of H2O and D2O droplets at temperatures from 275 K down to 240 K, we determine the temperature dependence of NQEs on the structure of water down to the deeply supercooled regime. The data reveal that the magnitude of NQEs on the structure of water is temperature independent, as the structure factor of D2O is similar to H2O if the temperature is shifted by a constant 5 K, valid from ambient conditions to the deeply supercooled regime. Analysis of the accelerated growth of tetrahedral structures in supercooled H2O and D2O also shows similar behavior with a clear 5 K shift. The results indicate a constant compensation between NQEs delocalizing the proton in the librational motion away from the bond and in the OH stretch vibrational modes along the bond. This is consistent with the fact that only the vibrational ground state is populated at ambient and supercooled conditions.
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| 17. |
- LaRue, Jerry L., et al.
(författare)
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THz-Pulse-Induced Selective Catalytic CO Oxidation on Ru
- 2015
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 115:3
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the use of intense, quasi-half-cycle THz pulses, with an associated electric field component comparable to intramolecular electric fields, to direct the reaction coordinate of a chemical reaction by stimulating the nuclear motions of the reactants. Using a strong electric field from a THz pulse generated via coherent transition radiation from an ultrashort electron bunch, we present evidence that CO oxidation on Ru(0001) is selectively induced, while not promoting the thermally induced CO desorption process. The reaction is initiated by the motion of the O atoms on the surface driven by the electric field component of the THz pulse, rather than thermal heating of the surface.
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| 18. |
- LaRue, Jerry, et al.
(författare)
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Symmetry-resolved CO desorption and oxidation dynamics on O/Ru(0001) probed at the C K-edge by ultrafast x-ray spectroscopy
- 2022
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 157:16
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Tidskriftsartikel (refereegranskat)abstract
- We report on carbon monoxide desorption and oxidation induced by 400 nm femtosecond laser excitation on the O/Ru(0001) surface probed by time-resolved x-ray absorption spectroscopy (TR-XAS) at the carbon K-edge. The experiments were performed under constant background pressures of CO (6 × 10−8 Torr) and O2 (3 × 10−8 Torr). Under these conditions, we detect two transient CO species with narrow 2π* peaks, suggesting little 2π* interaction with the surface. Based on polarization measurements, we find that these two species have opposing orientations: (1) CO favoring a more perpendicular orientation and (2) CO favoring a more parallel orientation with respect to the surface. We also directly detect gas-phase CO2 using a mass spectrometer and observe weak signatures of bent adsorbed CO2 at slightly higher x-ray energies than the 2π* region. These results are compared to previously reported TR-XAS results at the O K-edge, where the CO background pressure was three times lower (2 × 10−8 Torr) while maintaining the same O2 pressure. At the lower CO pressure, in the CO 2π* region, we observed adsorbed CO and a distribution of OC–O bond lengths close to the CO oxidation transition state, with little indication of gas-like CO. The shift toward “gas-like” CO species may be explained by the higher CO exposure, which blocks O adsorption, decreasing O coverage and increasing CO coverage. These effects decrease the CO desorption barrier through dipole–dipole interaction while simultaneously increasing the CO oxidation barrier.
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| 19. |
- Miller, Daniel J., et al.
(författare)
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Low O-2 dissociation barrier on Pt(111) due to adsorbate-adsorbate interactions
- 2010
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics. - 0021-9606 .- 1089-7690. ; 133:22, s. 224701-
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Tidskriftsartikel (refereegranskat)abstract
- O2 dissociation on Pt(111) has been followed at low and saturation coverage using temperature-programmed x-ray photoelectron spectroscopy and simulated with mean-field kinetic modeling, yielding dissociation (Ea) and desorption (Ed) barriers of 0.32 and 0.36 eV, respectively. DFT calculations show that Ea is strongly influenced by the O–O interatomic potential in the atomic final state: of the supercells considered, that which maximizes attractive third-nearest-neighbor interactions in the atomic final state yields both the lowest computed dissociation barrier (0.24 eV) and the best agreement with experiment. It is proposed that the effect of adsorbate-adsorbate interactions must be considered when modeling catalytic processes involving dissociative steps.
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| 20. |
- Nikitin, Anton, et al.
(författare)
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Hydrogen storage in carbon nanotubes through the formation of stable C-H bonds
- 2008
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 8:1, s. 162-167
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Tidskriftsartikel (refereegranskat)abstract
- To determine if carbon-based materials can be used for hydrogen storage, we have studied hydrogen chemisorption in single-walled carbon nanotubes. Using atomic hydrogen as the hydrogenation agent, we demonstrated that maximal degree of nanotube hydrogenation depends on the nanotube diameter, and for the diameter values around 2.0 nm nanotube-hydrogen complexes with close to 100% hydrogenation exist and are stable at room temperature. This means that specific carbon nanotubes can have a hydrogen storage capacity of more than 7 wt % through the formation of reversible C-H bonds.
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| 21. |
- Nordlund, Dennis, et al.
(författare)
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Probing the Electron Delocalization in Liquid Water and Ice at Attosecond Time Scales
- 2007
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Ingår i: Physical Review Letters. - : American Physical Society. - 1079-7114 .- 0031-9007. ; 99, s. 217406-
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Tidskriftsartikel (refereegranskat)abstract
- We determine electron delocalization rates in liquid water and ice using core-hole decay spectroscopy. The hydrogen-bonded network delocalizes the electrons in less than 500 as. Broken or weak hydrogen bonds—in the liquid or at the surface of ice—provide states where the electron remains localized longer than 20 fs. These asymmetrically bonded water species provide electron traps, acting as a strong precursor channel to the hydrated electron.
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| 22. |
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| 23. |
- Ogasawara, Hirohito, et al.
(författare)
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Operando X-Ray Photoelectron Spectroscopy Studies of Aqueous Electrocatalytic Systems
- 2016
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Ingår i: Topics in catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 59:5-7, s. 439-447
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Tidskriftsartikel (refereegranskat)abstract
- Development of efficient fuel cell and electrochemical cell devices to retrieve energy in a renewable manner lies in the molecular level understanding of the conversion processes taking place at surfaces and interfaces. These processes involve complicated bond breaking and formation at the surfaces as well as charge transfer through interfaces which are challenging to track under operational conditions. We address the nature of these interfacial processes using ambient pressure X-ray photoelectron spectroscopy by leveraging both its chemical and surface sensitivity. Herein, we give several examples of fuel cell and electrolysis reactions to demonstrate the importance of probing the surface under operating conditions. Oxygen reduction reaction taking place on the platinum cathode in proton exchange membrane fuel cells, water splitting reactions including oxygen evolution reaction over IrO2 and hydrogen evolution reaction over MoSx reveal that different species dominate on the surface under different operational conditions and surface activities are directly related to the stabilities of those intermediate species and possible structural rearrangements of the catalyst material.
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| 24. |
- Ogasawara, Hirohito, et al.
(författare)
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X-ray free electron laser studies of electron and phonon dynamics of graphene adsorbed on copper
- 2023
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Ingår i: Physical Review Materials. - 2475-9953. ; 7:2
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Tidskriftsartikel (refereegranskat)abstract
- We report optical pumping and x-ray absorption spectroscopy experiments at the Pohang Accelerator Laboratory free electron laser that probes the electron dynamics of a graphene monolayer adsorbed on copper in the femtosecond regime. By analyzing the results with ab initio theory we infer that the excitation of graphene is dominated by indirect excitation from hot electron-hole pairs created in the copper by the optical laser pulse. However, once the excitation is created in graphene, its decay follows a similar path as in many previous studies of graphene adsorbed on semiconductors, i.e., rapid excitation of strongly coupled optical phonons and eventual thermalization. It is likely that the lifetime of the hot electron-hole pairs in copper governs the lifetime of the electronic excitation of the graphene.
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| 25. |
- Perakis, Fivos, et al.
(författare)
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Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of liquid water feature a variety of time scales, ranging from extremely fast ballistic-like thermal motion, to slower molecular diffusion and hydrogen-bond rearrangements. Here, we utilize coherent X-ray pulses to investigate the sub-100 fs equilibrium dynamics of water from ambient conditions down to supercooled temperatures. This novel approach utilizes the inherent capability of X-ray speckle visibility spectroscopy to measure equilibrium intermolecular dynamics with lengthscale selectivity, by measuring oxygen motion in momentum space. The observed decay of the speckle contrast at the first diffraction peak, which reflects tetrahedral coordination, is attributed to motion on a molecular scale within the first 120 fs. Through comparison with molecular dynamics simulations, we conclude that the slowing down upon cooling from 328 K down to 253 K is not due to simple thermal ballistic-like motion, but that cage effects play an important role even on timescales over 25 fs due to hydrogen-bonding.
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