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The alignment of electronic energy levels at interfaces strongly influences the electronic properties of multilayer systems like, e.g., organic thin films. One very powerful technique to investigate this level alignment and determine electronic binding energies is angular resolved photoelectron spectroscopy (ARPES). This method also allows assigning the spectroscopic resonances to molecular orbitals, since the angular distribution of the photoelectron yield can be understood as a fingerprint of the orbitals in momentum space. So far the possibilities of this emerging concept have only been demonstrated for low symmetric surfaces and adsorbates with only few molecular orientations.

This approach is extended to highly symmetric surfaces by studying the monolayer structure of the prototypical molecule 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) on the Ag(111) surface. This structure is particularly challenging since it contains two non-equivalent molecules per unit cell (Mol. A and B, see Figure) in six rotational domains, i.e., in total six differently oriented molecules of each type. However, the ARPES signal from both molecules could still be separated and an almost perfect match between the experimental data and the calculated moment space distributions was found for the highest occupied and the lowest unoccupied molecular orbital (HOMO and LUMO).

Furthermore, the contributions of both molecules to the ARPES data were analyzed regarding their binding energy by a two-dimensional fitting algorithm called “orbital tomography”. The result, (experimentally obtained) densities of states projected on molecules A and B (PDOS), turned out to agree very well with scanning tunnelling spectroscopy results reported earlier. The present findings clearly indicate that even for complex surface structures containing many differently oriented molecules the orbital tomography technique allows reliable investigations of the electronic structure of individual molecular species in thin organic films.

Electrical conduction along dislocations in semiconductors has attracted much attention both from fundamental and practical viewpoints. Fundamentally, such dislocations can be one-dimensional electronic systems and their conduction mechanism has already been investigated. On the other hand, the issue of the dislocation conduction should be practically important because it might degrade the performance of semiconductor devices. Here, local current conduction due to dislocations in GaN has been studied by scanning spreading resistance microscopy (SSRM). Fresh dislocations, induced by plastic deformation, were used to see their intrinsic properties.

The SSRM images showed many bright spots with high conductivity. The spots form chains along the slip direction and appear to be due to the edge dislocations introduced by deformation, which are terminated at the observed surface. Here, the line direction and Burgers vector of the dislocations are l = [0001] and b = (a/3) [1 10], resp. Previous studies have shown that grown-in screw dislocations with l = b = c[0001] are conductive but that grown-in edge dislocations with l = [0001] and b = (a/3) [1 10] are not, in contrast to our results. This apparent discrepancy should arise from the difference in the core structure between deformation-induced fresh dislocations and grown-in dislocations possibly decorated with native point defect or impurities. Theoretically, the electronic structures of the dislocations in GaN have been shown to change sensitively with the core structure, which should bring large conductivity differences. Local current-voltage spectra, measured at conductive spot positions, indicated a Frenkel-Poole mechanism for the conduction. These findings provide new insights into the issue of electrical conduction along dislocations in semiconductors.

Significant progress made in evaluating the density of active species used in medical applications of plasma physics could improve the accuracy of treatment: this article presents the first determination of the absolute density of active substances called radicals found in a state of matter known as plasma. These findings could have important implications for medicine - for example, for stimulating tissue regeneration, or to induce a targeted antiseptic effect in vivo without affecting neighbouring tissues.

Laser fluorescence spectroscopy (LIF) is the detection method used to estimate the density of radicals in the plasma, made of charged species, active molecules such as radicals and atoms. The authors have chosen to focus on OH radicals because they are one of the most important reactive species in plasma science due to their high level of oxidation. This means that chemical reactions with OH initiate the destruction of harmful components either in the human body or in nature such as carbon monoxide, volatile organic compounds and methane.The problem is that, up to now, laser-induced fluorescent (LIF) capability to measure the absolute density of radicals has been very limited because of issues with registering and analysing the fluorescence signal.

In this study, the authors present a simplified model which takes into account energy transfer stemming from the radicals’ vibrations. It can be used to analyse the LIF signal at regular atmospheric pressure. They then confirm the validity of their model experimentally, with a plasma jet made of Argon gas mixed with water molecules.