Film formation of decanethiol [CH3(CH2)9SH], didecyl disulfide [CH3(CH2)9S-S(CH2)9CH3], and didecyl sulfide [CH3(CH2)9-S-(CH2)9CH3] on polycrystalline gold from ethanol solutions was studied in situ by second harmonic generation. The completed films were characterized by X-ray photoelectron spectroscopy and voltammetry. Film formation is described by Langmuir adsorption kinetics and is found to be fastest for the thiol. Referenced to the number of thiolate units the disulfide adsorbs about 40% more slowly. In sharp contrast, the rate of alkyl sulfide adsorption is smaller by more than 3 orders of magnitude. Films formed from the thiol and disulfide are indistinguishable and virtually free of contaminations. In contrast, the maximum coverage achieved with the sulfide is considerably lower and substantial amounts of contamination remain on the substrate. Differences between thiol/disulfide films and sulfide films are also indicated by their electrochemical behavior.
We consider the behaviour of open quantum systems in dependence on the coupling to one decay channel by introducing the coupling parameter α being proportional to the average degree of overlapping. Under critical conditions, a reorganization of the spectrum takes place which creates a bifurcation of the time scales with respect to the lifetimes of the resonance states. We derive analytically the conditions under which the reorganization process can be understood as a second-order phase transition and illustrate our results by numerical investigations. The conditions are fulfilled e.g. for a picket fence with equal coupling of the states to the continuum. Energy dependencies within the system are included. We consider also the generic case of an unfolded Gaussian Orthogonal Ensemble. In all these cases, the reorganization of the spectrum occurs at the critical value α crit of the control parameter globally over the whole energy range of the spectrum. All states act cooperatively.
During the last 10 years various types of soft x-ray monochromators have been developed, which are optically based on the plane grating focusing condition introduced in 1980. These instruments as well as those using the original 1980 optical configuration are reviewed and compared to the other type of high performance soft x-ray grating monochromator, the Rowland circle based spherical grating monochromator (SGM). Performance data of a plane grating monochromator (HE-PGM3), which was recently commissioned at BESSY and which offers a broad spectral range (40-2000 eV) and very high spectral resolution (up to EIAE-10 000) are given in more detail. The performance of grating and crystal monochromators is compared in the l-2 keV photon energy range.
We report additional rich fine structures in high-resolution near-edge x-ray-absorption fine structure (NEXAFS) spectra of large organic molecules using NTCDA on Ag(111) as an example. These fine structures are completely interpreted as vibronic coupling to electronic core excitations. The coupling is mode selective; predominantly one vibronic mode couples to each excitation. The fit results suggest the occurrence of a Davydov splitting, first observed for core excitons. Morphological differences substantially influence the electron-vibron coupling, indicating a strong intermolecular interaction. Thus NEXAFS becomes a more subtle probe for organic solids.
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