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Optically dark excitons determine a wide range of properties of photoexcited semiconductors yet are hard to access via conventional spectroscopies. Here, we develop a time-resolved ultrafast photocurrent technique (trPC) to probe the formation dynamics of optically dark excitons. The nonlinear nature of the trPC makes it particularly sensitive to the formation of excitons occurring at the femtosecond timescale after the excitation. As proof of principle, we extract the interlayer exciton formation time 0.4~ps at 160 $\mu$J/cm$^2$ fluence in a MoS$_2$/MoSe$_2$ heterostructure and show that this time decreases with fluence. In addition, our approach provides access to the dynamics of carriers and their interlayer transport. Overall, our work establishes trPC as a technique to study dark excitons in various systems that are hard to probe by other approaches.

The coupled spin and valley degrees of freedom in transition metal dichalcogenides (TMDs) are considered a promising platform for information processing. Here, we use a TMD heterostructure MoS2–MoSe2 to study optical pumping of spin/valley polarized carriers across the interface and to elucidate the mechanisms governing their subsequent relaxation. By applying time-resolved Kerr and reflectivity spectroscopies, we find that the photoexcited carriers conserve their spin for both tunneling directions across the interface. Following this, we measure dramatically different spin/valley depolarization rates for electrons and holes, ∼30 and

The coupled spin and valley degrees of freedom in transition metal dichalcogenides (TMDs) are considered a promising platform for information processing. Here, we use a TMD heterostructure ${\text{MoS}_{2}-\text{MoSe}_{2}}$ to study optical pumping of spin/valley polarized carriers across the interface and to elucidate the mechanisms governing their subsequent relaxation. By applying time-resolved Kerr and reflectivity spectroscopies, we find that the photoexcited carriers conserve their spin for both tunneling directions across the interface. Following this, we measure dramatically different spin/valley depolarization rates for electrons and holes, $\sim 30\,{\text{ns}}^{-1}$ and $< 1\,{\text{ns}}^{-1}$, respectively and show that this difference relates to the disparity in the spin-orbit splitting in conduction and valence bands of TMDs. Our work provides insights into the spin/valley dynamics of free carriers unaffected by complex excitonic processes and establishes TMD heterostructures as generators of spin currents in spin/valleytronic devices., 15 + 6 pages; 4 + 3 figures

Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4 fs electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to orders of magnitude compared to equilibrium., added reference 24, clarified the meaning of photo-induced, emphasized that XMCD probes the magnetic moment localized at 4f electrons

Titel, Kurzfassung und Inhaltsverzeichnis 1\. Einleitung 1 2\. Grundlagen 7 3\. Experiment 49 4\. Elektronensolvatisierung in amorphen Eisschichten 83 5\. Delokalisierte Zustände in kristallinen Eisschichten 113 6\. Elektroneneinfang in kristallinen Eisschichten 125 7\. Zusammenfassung und Ausblick 157 A Bildladungszustände vor der Ru(001)-Oberfläche 161 Literaturverzeichnis 167 Apparative Komponenten, Publikationen, Danksagung 183, Mittels zeit- und winkelaufgelöster Zwei-Photonen-Photoemissionsspektroskopie (2PPE) ist die Dynamik von Überschusselektronen in molekular dünnen Eisschichten auf Metalloberflächen untersucht worden. Ziel dieser Arbeit ist es, zum Verständnis fundamentaler Wechselwirkungsmechanismen zwischen einem Elektron und einer molekularen Umgebung beizutragen. Insbesondere wird der Einfluss der Struktur auf die Elektronendynamik aufgezeigt. Es wurden unterschiedliche Lokalisierungsphänomene in amorphen und in kristallinen Eisschichten gefunden. In amorphen Schichten findet eine Solvatisierung lokalisierter Zustände am Leitungsbandboden der Eisschicht statt. Der Solvatisierungsprozess äußert sich einerseits in einer kontinuierlichen energetischen Verschiebung des elektronischen Zustands, die auf der Cu(111)-Oberfläche über 1.5 ps verfolgt werden konnte. Darüber hinaus ist die Stabilisierung mit einer zunehmenden räumlichen Einschnürung der Wellenfunktion verknüpft, die sich in einer Verbreiterung der Impulsverteilung parallel zur Oberfläche und in einer Verringerung der Rücktransferrate solvatisierter Elektronen ins Metallsubstrat äußert. Die Solvatisierungsdynamik zeigt eine ausgeprägte Bedeckungsabhängigkeit, die darauf zurückgeführt wird, dass bei Bedeckungen, The dynamics of excess electrons in molecular thin ice films on metal surfaces have been investigated using time- and angle-resolved two-photon- photoemission spectroscopy (2PPE). The work aims at a fundamental understanding of interaction mechanisms between an electron and a molecular environment and focuses on the correlation of structure and electron dynamics. For amorphous and crystalline ice layers different phenomena of electron localization are found. In the amorphous case the ultrafast solvation dynamics of localized electrons which starts at the bottom of the conduction band is studied in the time domain. The solvation process manifests itself as a continuous energetic shift of the electronic state, which can be followed over 1.5 ps for D2O on a Cu(111) substrate. Furthermore this energetic stabilization is accompanied by an increasing spatial constriction of the electron wave function, leading to a broadening of the momentum distribution parallel to the surface and to a decrease of the back transfer rate of solvated electrons. The electron solvation dynamics show a pronounced coverage dependence. At coverages