Ultrashort and metastable doping of the ZnO surface by photoexcited defects

Shallow donors in semiconductors are known to form impurity bands that induce metallic conduction at sufficient doping densities. The perhaps most direct analogy to such doping in optically excited semiconductors is the photoexcitation of deep electronic defect or dopant levels creating defect excitons (DX) which may act like shallow donors. In this work, we use time- and angle-resolved photoelectron spectroscopy to observe and characterize DX at the surface of ZnO. The DX are created on a femtosecond timescale upon photoexcitation and have a spatial extend of few nanometers that is confined to the ZnO surface. The localized electronic levels lie at 150 meV below the Fermi energy, very similar to the shallow donor states induced by hydrogen doping [Deinert et al, Phys. Rev. B, 91, 235313 (2015)]. The transient dopants exhibit a multi-step decay ranging from hundred’s of picoseconds to 77 μs and even longer. By enhancing the DX density, a Mott transition occurs, enabling the ultrafast metallization of the ZnO surface, which we described previously [Gierster et al. Nat. Commun. 12, 978 (2021)]. Depending on the defect density, the duration of the photoinduced metallization ranges from picoseconds to μs and longer, corresponding to the decay dynamics of the DX. The metastable lifetime of the DX is consistent with the observation of persistent photoconductivity (PPC) in ZnO reported in literature [Madel et al., J. Appl. Phys. 121, 124301 (2017)]. In agreement with theory on PPC [Lany and Zunger, Phys. Rev. B 72, 035215 (2005)], the deep defects are attributed to oxygen vacancies due to their energetic position in the band gap and their formation by surface photolysis upon UV illumination. We show that the photoexcitation of these defects is analogous to chemical doping and enables the transient control of material properties such as the electrical conductivity from ultrafast to metastable timescales. The same mechanism should be at play in other semiconductor compounds with deep defects

Ultrafast dynamics in solids probed by femtosecond time-resolved broadband electronic sum frequency generation

Time-resolved sum frequency generation is an established tool to investigate the ultrafast vibrational dynamics with surface and interface specificity, which can be extended to the regime of electronic transitions using a white light continuum as demonstrated previously by studies of liquid interfaces. We expand this technique to the investigation of solid single crystal samples. In particular, we demonstrate the potential of electronic sum frequency generation by probing the non-equilibrium dynamics at excitonic resonances in ZnO with a sensitivity as small as 0.6% and with a time resolution of 160 fs

Theory of dark resonances for alkali vapors in a buffer-gas cell

We develop an analytical theory of dark resonances that accounts for the full atomic-level structure, as well as all field-induced effects such as coherence preparation, optical pumping, ac Stark shifts, and power broadening. The analysis uses a model based on relaxation constants that assumes the total collisional depolarization of the excited state. A good qualitative agreement with experiments for Cs in Ne is obtained.Comment: 16 pages; 7 figures; revtex4. Accepted for publication in PR

Pseudoheterodyne near-field imaging at kHz repetition rates via quadrature-assisted discrete demodulation

Scattering-type scanning near-field optical microscopy enables the measurement of optical constants of a surface beyond the diffraction limit. Its compatibility with pulsed sources is hampered by the requirement of a high-repetition rate imposed by lock-in detection. We describe a sampling method, called quadrature-assisted discrete (quad) demodulation, which circumvents this constraint. Quad demodulation operates by measuring the optical signal and the modulation phases for each individual light pulse. This method retrieves the near-field signal in the pseudoheterodyne mode, as proven by retraction curves and near-field images. Measurement of the near-field using a pulsed femtosecond amplifier and quad demodulation is in agreement with results obtained using a CW laser and the standard lock-in detection method

Bee poisoning incidents in Germany in spring 2008 caused by abrasion of active substance from treated seeds during sowing of maize

contribution to session V Honey bee poisoning incidents and monitoring schemes In spring 2008 a high number of bee poisoning incidents was recorded during sowing of maize in the Upper Rhine valley and in South Bavaria near Passau. More than 11.500 honey bee colonies from about 700 beekeepers in the Upper Rhine valley showed symptoms of insecticide poisoning. The reason for the poisoning was the abrasion of dust from maize seeds treated with the insecticide Poncho Pro (a.s. clothianidin) during the sowing process and blowing out of this dust containing the active substance into the environment with pneumatic sowing machines, resulting in contamination of nectar and pollen. The poisonings occurred in areas in southern Germany in which an eradication program for the quarantine pest Diabrotica virgifera virgifera was active and where clothianidin was used at a high rate (125 g a.s. /ha) on a large scale. An exceptionally high amount of dust of up to 80 g per 100.000 kernels of maize was detected in some of the maize seed batches. The chemical analysis of dust, plant samples, bee samples, fresh pollen and bee bread confirmed the poisoning by clothianidin originating from treated maize seeds. No correlation with any bee pathogens was detected. Keywords: seed treatment, drilling machines, neonicotinoid, clothianidin, dust, maize, drift, bee poisoning, honey bee

Ultrafast evolution of the complex dielectric function of monolayer WS₂ after photoexcitation

Transition metal dichalcogenides emerged as ideal materials for the investigation of exciton physics. Retrieving the excitonic signature in optical spectra, and tracking their time evolution upon photoexcitation requires appropriate analysis procedures, particularly when comparing different measurements, experimental techniques, samples, and substrates. In this work, we investigate the ultrafast time evolution of the exciton resonance of a monolayer of WS2 deposited on fused silica and Si/SiO2, and using two different measurement techniques: time-resolved reflectance and transmittance contrast. By modelling the dielectric function of the exciton with a Lorentz oscillator, using a Fresnell equations formalism, we derive analytical expressions of the exciton lineshape in both cases. The 2D linearized model introduced by Li et al. [Y. Li and T. F. Heinz, 2D Mater., 2018, 5, 025021] is used for the transmittance of the transparent substrate and a Fresnel transfer matrix method [O. Stenzel, The Physics of Thin Film Optical Spectra, Springer Series in Surface Science, 2016] is used to derive the reflectance in the case of the layered Si/SiO2 substrate. By fitting two models to the time-dependent optical spectra, we extract and quantify the time evolution of the parameter describing the excitonic resonance. We find a remarkable agreement between the extracted dynamics from both experiments despite the different side conditions, showing the equivalence and reliability of the two analysis methods in use. With this work, we pave the way to the resilient comparison of the exciton dynamics from different samples, measurements technique and substrates

Locating the Nordstream explosions without a velocity model using polarization analysis

The seismic events that preceded the leaks in the Nordstream pipelines in the Baltic Sea have been interpreted as explosions on the seabed, most likely man-made. We use a polarization-based location method initially developed for marsquakes to locate the source region without a subsurface velocity model. We show that the 2 largest seismic events can be unambiguously attributed to the methane plumes observed on the sea surface. The two largest events can be located with this method, using 4 and 5 stations located around the source, with location uncertainties of 30km and 10x60km. We can further show that both events emitted seismic energy for at least ten minutes after the initial explosion, indicative of resonances in the water column or the depressurizing pipeline.Comment: 6 pages, 2 figures, submitted as fast report to Seismic

Uncovering the (un-)occupied electronic structure of a buried hybrid interface

The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of p-quinquephenyl pyridine (5P-Py) adsorbed on ZnO(10-10). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV and causes an occupied in-gap state (IGS) directly below the Fermi level $E_\text{F}$. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1 eV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2 eV above $E_\text{F}$, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces

O₂ reduction at a DMSO/Cu(111) model battery interface

In order to develop a better understanding of electrochemical $\mathrm{O_2}$reduction in non-aqueous solvents, we apply two-photon photoelectronspectroscopy to probe the dynamics of $\mathrm{O_2}$ reduction at aDMSO/Cu(111) model battery interface. By analyzing the temporal evolution ofthe photoemission signal, we observe the formation of $\mathrm{O_2^-}$ from atrapped electron state at the DMSO/vacuum interface. We find the verticalbinding energy of $\mathrm{O_2^-}$ to be 3.80 $\pm$ 0.05 eV, in good agreementwith previous results from electrochemical measurements, but with improvedaccuracy, potentially serving as a basis for future calculations on thekinetics of electron transfer at electrode interfaces. Modelling the$\mathrm{O_2}$ diffusion through the DMSO layer enables us to quantify theactivation energy of diffusion (31 $\pm$ 6 meV), the diffusion constant (1$\pm$ 1$\cdot 10^{-8}$ cm$^2$/s), and the reaction quenching distance forelectron transfer to $\mathrm{O_2}$ in DMSO (12.4 $\pm$ 0.4 \unicode{x212B}),a critical value for evaluating possible mechanisms for electrochemical sidereactions. These results ultimately will inform the development andoptimization of metal-air batteries in non-aqueous solvents.<br

Determination of the electron's solvation site on D₂O/Cu(111) using Xe overlayers and femtosecond photoelectron spectroscopy

We investigate the binding site of solvated electrons in amorphous D2O clusters and D2O wetting layers adsorbed on Cu(111) by means of two-photon photoelectron (2PPE) spectroscopy. On the basis of different interactions of bulk- or surface-bound solvated electrons with rare gas atoms, titration experiments using Xe overlayers reveal the location of the electron solvation sites. In the case of flat clusters with a height of 2-4 bilayers adsorbed on Cu(111), solvated electrons are found to reside at the ice - vacuum interface, whereas a bulk character is determined for solvated electrons in wetting layers. Furthermore, time-resolved experiments are performed to determine the origin of the transition between these different solvation sites with increasing D2O coverage. We employ an empirical model calculation to analyse the rate of electron transfer back to the substrate and the energetic stabilization of the solvated electrons, which allows further insight into the binding site for clusters. We find that the solvated electrons reside at the edges of the clusters. Therefore, we attribute the transition from surface- to bulk-solvation to the coalescence of the clusters to a closed ice film occurring at a nominal coverage of 2-3 BL, while the distance of the binding sites to the metal-ice interface is maintained