Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy.

UV pump-extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C-Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C-Br fission along an approximately CS symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C3v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage

New insights into the laser-assisted photoelectric effect from solid-state surfaces

Photoemission from a solid surface provides a wealth of information about the electronic structure of the surface and its dynamic evolution. Ultrafast pump-probe experiments are particularly useful to study the dynamic interactions of photons with surfaces as well as the ensuing electron dynamics induced by these interactions. Time-resolved laser-assisted photoemission (tr-LAPE) from surfaces is a novel technique to gain deeper understanding of the fundamentals underlying the photoemission process. Here, we present the results of a femtosecond time-resolved soft X-ray photoelectron spectroscopy experiment on two different metal surfaces conducted at the X-ray Free-Electron Laser FLASH in Hamburg. We study photoemission from the W 4f and Pt 4f core levels using ultrashort soft X-ray pulses in combination with synchronized infrared (IR) laser pulses. When both pulses overlap in time and space, laser-assisted photoemission results in the formation of a series of sidebands that reflect the dynamics of the laser-surface interaction. We demonstrate a qualitatively new level of sideband generation up to the sixth order and a surprising material dependence of the number of sidebands that has so far not been predicted by theory. We provide a semi-quantitative explanation of this phenomenon based on the different dynamic dielectric responses of the two materials. Our results advance the understanding of the LAPE process and reveal new details of the IR field present in the surface region, which is determined by the dynamic interplay between the IR laser field and the dielectric response of the metal surfaces.Comment: 18 pages, 3 figure

Shapes of rotating normal fluid 3He versus superfluid 4He droplets in molecular beams

Previous single-pulse extreme ultraviolet and X-ray coherent diffraction studies revealed that superfluid 4He droplets obtained in free jet expansion acquire sizable angular momentum, resulting in significant centrifugal distortion. Similar experiments with normal fluid 3He droplets may help elucidating the origin of the of the large degree of rotational excitation and highlight similarities and differences of dynamics in normal and superfluid droplets. Here, we present the first comparison of the shapes of isolated 3He and 4He droplets following expansion of the corresponding fluids in vacuum at temperatures as low as ~ 2 K. Large 3He and 4He droplets with average radii of ~160 nm and ~350 nm, respectively, were produced. We find that the majority of the 3He droplets in the beam correspond to rotating oblate spheroids with reduced average angular momentum ($\Lambda$) and reduced angular velocities ($\Omega$) similar to that of 4He droplets. Given the different physical nature of 3He and 4He, this similarity in $\Lambda$ and $\Omega$ may be surprising and suggest that similar mechanisms induce rotation regardless of the isotope. We hypothesized that the observed distribution of droplet sizes and angular momenta stem from processes in the dense region close to the nozzle. In this region, the significant velocity spread and collisions between the droplets induce excessive rotation followed by droplet fission. The process may repeat itself several times before the droplets enter the collision-fee high vacuum region further downstream.Comment: 29 pages, 6 figure

Femtosecond time-resolved two-photon photoemission studies of ultrafast carrier relaxation in Cu_2O photoelectrodes

Cuprous oxide (Cu_2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu_2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu_2O surface as well as the interface between Cu_2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu_2O and provide guidance for future material development

Elektronische Struktur und Elektrontransferdynamiken an Farbstoff-Halbleiter Grenzflächen untersucht mittels zeitaufgelöster Photoelektronenspektroskopie im XUV Bereich

The ultrafast dynamics of photo excited Ruthenium transition metal complexes and the heterogeneous electron transfer (HET) to nanostructured semiconductor interfaces are investigated by means of time-resolved XUV photoelectron spectroscopy (PES). The process of high-order harmonic generation is utilized to achieve the necessary photon energy of up to 40 eV and a temporal resolution below 100 fs. The capabilities of PES are systematically studied and its advantages compared to other spectroscopic pump-probe methods are demonstrated. At the interface between the N719 ruthenium dye complex and the commonly used TiO2 substrate the application of PES allowed to determine for the first time directly the absolute binding energies of the excited states involved in the electron dynamics. The transient signal could be decomposed into the particular contributions. The found energetic structure gives rise to a strong driving force for the injection from the singlet 1MLCT state and a slow electron transfer from the triplet 3MLCT state, the latter being possible due to a partial overlap of the triplet state band of N719 and the conduction band of TiO2. The lifetimes inferred in this study of (20 ± 10) fs for the 1MLCT and the 3MLCT yielding a slow electron transfer on the picosecond time scale are in excellent agreement with the values reported in literature. Based on these findings, the interfacial charge transfer from N3 dye molecules into ZnO thin films is analyzed. Previous investigations found that the HET occurs on a generally much slower picosecond timescale related to their TiO2 counterparts. A comparative study between both interfaces reveals direct evidence for the formation of an interfacial electronic state at ZnO being the origin of the delayed release of free charge carriers into the conduction band. It is further highlighted that ZnO bulk properties such as the conduction band density of states and the energetic band alignment are of minor importance for the electron transfer process whereas the specific dye- ZnO interaction is argued to play a key role in the HET. In the last part, the ultrafast excited states properties of [Ru(bpy)3]2+ dissolved in ionic liquids (ILs) are investigated. The focus is laid on the general applicability of ILs as solvents with regard to time resolved PES experiments. This material class is known to exhibit extremely low vapor pressures being suitable if UHV conditions are required. It is demonstrated that ILs are beneficial if the frontier molecular orbitals and dynamics of different solutes needs to be addressed. From steady state spectra the HOMO band of the [Ru(bpy)3]2+ molecule, comprised of the Ru t2g orbital, can be clearly distinguished from the solvent contribution. Its absolute binding energy is found to be identical to results obtained from liquid water microjet experiments. From a global fit analysis of the transient signals the binding energies of the initially populated 1MLCT state and the thermally relaxed 3MLCT are determined. The results in terms of a kinetic three state model which includes the intersystem crossing from the 1MLCT to the 3MLCT state as well as a state to account for the intra-molecular vibrational relaxation within the triplet configuration are in good agreement with literature values obtained in aqueous solution.Diese Arbeit untersucht die Ladungsträgerdynamiken von optisch angeregten Ruthenium Übergangsmetallkomplexen chemisch gebunden auf der Oberfläche von nanostrukturierten Halbleitern mittels zeitaufgelöster Photoelektronen Spektroskopie (PES). Die dazu benötigten ultrakurzen Laserpulse im extremultravioletten Spektralbereich (XUV) werden durch den Prozess der Erzeugung von Hohen Harmonischen bereitgestellt. Die besondere Eignung der zeitaufgelösten PES im Bereich von Farbstoff-Halbleiter Grenzflächen sowie die Vorteile gegenüber anderen spektroskopischen Methoden werden aufgezeigt. Unter Verwendung von PES konnten erstmalig an der Grenzfläche zwischen dem Farbstoffkomplex N719 und dem Halbleitersubstrat TiO2 direkt die absoluten Bindungsenergien der am Ladungstransfer beteiligten angeregten Zustände bestimmt werden. Die Positionen der Energieniveaus des Singulett 1MLCT Zustandes wurden mit 0.7 eV über sowie die des Triplett 3MLCT Zustandes mit 0.2 eV unter dem Leitungsbandminimum gefunden. Dies bedeutet für den erstgenannten Zustand eine große Triebkraft für den Ladungstransfer und stellt die Hauptursache für die ultrakurzen Lebenszeiten im Bereich von (20±10) fs dar. Im Gegensatz dazu ist eine Ladungsträgerinjektion vom tiefer liegenden Triplett Zustand nur aufgrund einer partiellen Überlappung mit dem Leitungsband des Halbleiters möglich und erklärt die in diesem Fall gefundenen deutlich langsameren Injektionszeiten im Pikosekundenbereich. Unter Zuhilfenahme der bisherigen Erkenntnisse auf der TiO2 Oberfläche wurde nachfolgend der Ladungstransfer an ZnO Schichten untersucht. Hier ergaben frühere Untersuchungen, dass die Elektroneninjektion vorwiegend im Pikosekundenbereich stattfindet. Die Ursache konnte in einer vergleichenden Studie in der Bildung eines Grenzflächenzustandes zwischen Farbstoff und Halbleiter gefunden werden. Dadurch stehen diese nicht mehr unmittelbar als freie Ladungsträger zur Verfügung und es kommt zu einer erhöhten Rekombination in den Grundzustand. Dabei ist besonders hervorzuheben, dass die Stoffeigenschaften von ZnO wie z.B. die Zustandsdichte im Leitungsband sowie dessen relative Lage zu den ladungsträgerinjizierenden Zuständen des Farbstoffmoleküls nur eine untergeordnete Rolle spielen und vielmehr die Wechselwirkung zwischen Farbstoff und Halbleiter von zentraler Bedeutung ist. Abschließend wurde die Eignung von Ionischen Flüssigkeiten (IL) in Bezug auf die Anwendung im Ultrahochvakuum untersucht. Dabei wurde überprüft, ob die ultraschnellen Ladungsträgerdynamiken des in IL gelösten Farbstoffmoleküls [Ru(bpy)3]2+ gezielt adressiert werden können. Es zeigt sich dabei, dass aufgrund der positiven Eigenschaften des Lösungsmittels eine klare Differenzierung zwischen den Valenzbändern von Farbstoff und Lösungsmittel möglich ist. Weiterführende Untersuchen von Ladungstransferprozessen und Farbstoff Halbleiter System in flüssiger Umgebung werden diskutiert

The association between cannabis use, mental illness and suicidal behaviour: what is the role of hopelessness

Cannabis is one of the most common illegal psychoactive substance used in European countries, in particular among adolescents and young adults (1). It has been estimated that almost 55% of adolescents aged 15–19 years have used cannabis at least once in their lifetime (2), while past year use is reported by approximately 30% of 15–17 year olds and over 47% of those aged 18–19 years (3). Cannabis use has been associated with several adverse life outcomes including unemployment, legal problems, dependence, early school leaving, increased risk of developing both psychotic and affective disorders (3, 4) together with brain structural and functional abnormalities (5, 6). An association between cannabis use, psychiatric disorders and suicidal behavior has also frequently been reported, although the exact nature of this link is still poorly understood (4)

The Role of White Matter Abnormalities in Treatment-Resistant Depression: a Systematic Review.

Background: Patients with treatment-resistant depression (TRD) commonly report significant disability, together with an increased risk of functional impairment. Neuroimaging technology has been used to investigate the neuropathology of this complex illness, but it is still quite unknown whether abnormalities in the integrity of the white matter (WM) of specific brain areas may be considered as trait markers of TRD. Methods: Electronic databases were searched from 1980 to 2013. Nine studies - comprising a total of 228 subjects and 171 controls - fulfilled our inclusion criteria and were included in the present overview. Results: Several cross-sectional studies showed the association between WM abnormalities and TRD. According to the selected studies, sub-callosal cingulated cortex (SCC) WM abnormalities were largely implicated in the pathogenesis of both major depressive disorder (MDD) and TRD. However, alterations in cortical-limbic or cortical-subcortical circuits, particularly the left middle frontal gyrus (which is thought to have a major role in emotional regulation) may also be involved in the pathophysiology of TRD. Conclusions: TRD may be related to the presence of specific brain microstructural WM abnormalities. WM abnormalities of specific brain regions such as SCC, may have a major involvement in the pathogenesis of TRD