Elementary relaxation processes investigated by femtosecond photoelectron spectroscopy of two‐dimensional materials

Elementary scattering processes in solid matter occur on ultrafast timescales and photoelectron spectroscopy in the time domain represents an excellent tool for their analysis. Conventional photoemission accesses binding energies of electronic states and their momentum dispersion. The use of femtosecond laser pulses in pump‐probe experiments allows obtaining direct insights to the energy and momentum dependence of ultrafast dynamics. This article introduces the elementary interaction processes and emphasizes recent work performed in this rapidly developing field. Decay processes in the low excitation limit are addressed, where electrons decay according to their interaction with carriers in equilibrium. Here, hot electron relaxation in epitaxial metallic film is reviewed. In the limit of an intense optical excitation, scattering of the excited electrons among each other establishes a non‐equilibrium state. Results on charge‐density wave materials and the effect of coherent nuclear motion on the electronic structure, which can break low symmetry ground states, are discussed. Figure reprinted with permission from [71].     

Bond polarization and image-potential screening in adsorbed C6F6 on Cu(1 1 1)

The orientation of hexafluorobenzene (C₆F₆) on the Cu(1 1 1) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C₆F₆ differ significantly in comparison to its hydrocarbon analog C₆H₆. C₆F₆ is found to adsorb on Cu(1 1 1) with the ring plane parallel to the surface for coverages below 10 ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2 ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate.     

Femtosecond dynamics of the collinear-to-spiral antiferromagnetic phase transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.     

A time-of-flight spectrometer for angle-resolved detection of low energy electrons in two dimensions

We have developed a time-of-flight photoelectron spectrometer that simultaneously analyzes low energy electrons photoemitted from solid surfaces in an energy- and angle-resolved manner. To achieve this, a field free drift tube with an acceptance angle of 22° is combined with two-dimensional position-sensitive detection of photoelectrons, which is realized by a microchannel plate stack and a delay-line anode for position encoding. Here, we present the design considerations and principles of operation including analysis of multiple events per light pulse. The performance of the spectrometer is demonstrated by photoemission from a Cu(111) single crystalline surface by UV femtosecond laser pulses at 6.2 eV photon energy. PACS 71.20.-b; 73.20.At; 78.47.J-; 79.60.-i     

Eta 1 and eta 2 Coordination of 1-amino-closo-dodecaborate

The reaction of the sodium salt of 1-amino-closo-dodecaborate [Na]2[NH2-B12H11] ([Na]2[1]) with [Au(PPh3)Cl] and [Ni(THF)2(Br)2] led to eta 1(N) coordination of 1in [Na][Au(PPh3)(NH2-B12H11)] (2) and [Na]6[Ni(NH2-B12H11)4] (3), respectively. Furthermore, eta 2(N,BH) coordination of was found in [MePPh3][Rh(PPh3)2(NH2-B12H11)] (4), which was synthesized by the reaction of [MePPh3][Na][1] with [Rh(PPh3)3Cl]. All compounds were characterized by single crystal X-ray diffraction and heteronuclear NMR spectroscopy.