Department of Physical Chemistry

A systematic experimental study applying nanosecond laser pulses with 532 nm radiation to a specially selected set of historical paper samples exhibiting a broad range of composition and origin was undertaken to develop a quantitative irradiation dose model. This allows the optimisation of laser pulse number and fluence in order to maximise cleaning efficiency while excluding substrate modifications such as yellowing or destruction.

The laser-induced deformation of a typical commercial cantilever commonly used for scanning near-field optical microscopes was investigated by means of a software package based on the finite element method. The thermo-mechanical behaviour of such a cantilever whose tip was irradiated by a laser beam was calculated in the temperature regime between room temperature and 850 K. The spatial tip displacement was simulated at timescales <0.1 ms, since feedback-based constant force measurements exhibit reaction times in this range. It could be shown that in addition to former tip-based thermal expansion calculations the cantilever deformation is already significant at moderate temperatures, particularly when a reflective coating is present. The experimental and calculated results suggest that tip scanning in cantilever-based scanning probe microscopes for laser-based surface modification applications should be performed in thermal equilibrium.

PAPER Kautek et al. Optical near-fi eld excitation at commercial scanning probe microscopy tips: a theoretical and experimental investigation A systematic study of the influence of the excitation angle, the light polarization and the coating thickness of commercial SPM tips on the field enhancement in an apertureless scanning near-field optical microscope is presented. A new method to optimize the alignment of the electric field vector along the major tip axis by measuring the resonance frequency was developed. The simulations were performed with a MNPBEM toolbox based on the Boundary Element Method (BEM). The influence of the coating thickness was investigated for the first time. Coatings below 40 nm showed a drastic influence both on the resonance wavelength and the enhancement. A shift to higher angles of incidence for the maximum enhancement could be observed for greater tip radii.

Galvanic interaction between carbon steel and six galvanic coating materials including SIGAL®-aluminum, cadmium, zinc and duplex combinations with copper, nickel and tin has been studied by potentiodynamic techniques and continuous monitoring of the galvanic current. Results for 12 galvanic couples are reported here for immersion in argon and air saturated sulfate and chloride solutions of various acidity. Steel can cathodically be protected not only by cadmium and zinc, but also by SIGAL®-aluminum, in moist urban, industrial, and marine atmospheres. Ultrathin intermediate layers in duplex coatings, of e.g. nickel or tin, have practically negligible influence on the aluminum top layer and the steel base.

We present an Yb-fiber oscillator with an all-polarization-maintaining cavity with a higher-order-mode fiber for dispersion compensation. The polarization maintaining higher order mode fiber introduces not only negative second order dispersion but also negative third order dispersion in the cavity, in contrast to dispersion compensation schemes used in previous demonstrations of all-polarization maintaining Yb-fiber oscillators. The performance of the saturable absorber mirror modelocked oscillator, that employs a free space scheme for coupling onto the saturable absorber mirror and output coupling, was investigated for different settings of the intracavity dispersion. When the cavity is operated with close to zero net dispersion, highly stable 0.5-nJ pulses externally compressed to sub-100-fs are generated. These are to our knowledge the shortest pulses generated from an all-polarization-maintaining Yb-fiber oscillator. The spectral phase of the output pulses is well behaved and can be compensated such that wing-free Fourier transform limited pulses can be obtained. Further reduction of the net intracavity third order dispersion will allow generating broader output spectra and consequently shorter pulses, without sacrificing pulse fidelity.

Laser spot size and pulse number are two major parameters influencing the ablation of solids. The extended defect model describes the dependence of the threshold fluence on the basis of high and low density defects. This model was successfully applied to silicon and stainless steel. It is demonstrated that heat accumulation cannot describe the experimental results.

The spot size dependence of pulse laser-induced ablation thresholds of solid materials cannot be satisfactory described by the two existing quantitative models based on defect densities and on heat accumulation. In the present study, the heat accumulation model was amended but still yielded results in contradiction to experimental observations. The existing defect model was extended to account for incubation where optically active high-density defects with a separation below the radiation wavelength are generated. The reduction of the threshold for large beam radii could be ascribed to the laser spot covering a finite number of optically active low-density defects (LDD) embedded in the matrix material. This new generic model combining the spot size and pulse number dependence of femtosecond pulse laser-induced ablation thresholds was demonstrated with high-impact polystyrene. The average distance of the optically active LDD obtained from the generic model could be confirmed by scanning electron microscopy.

Elektrochemie/Halbleiter/Oberflachenerscheinungen/Photoelektrochemie/Sonnenenergie Semiconducting transitio¡r metal dichalcogenides rvith layer structure belong to the ntost stable electrocle ntaterials in electrochemical solar cells. The photoelectrochemical reactions horvever, are complicated by the surface arrisotropy. Results of photoelectrochemical cells with n-type MoSe2 and WSe2 demonstrate the inìportance of surlace morphology for the energy conversion efficiency.-The optimal energy conversion yield of these layered semiconductors has turned out being lower than theoretically expectecl from their band gaps. lt is shown that this is connected with the existence of an indirect bancl gap which restricts light absorption in a considerable energy quantum range above the band gap. Besides this, surface defects and inhomogeneities reduce the quantum yield by increased surface recombination.-Photoelectrochenrical hydrogen evolution has been investigated with p-type WSe2. lt is shown that a considerable energy gain can be reached by the catalytic action of platinum deposits on the surface of the elecrrocle. This is explained in terms of surface state caralysis. Halbleitencle Übergangsmetalldichalkogenide nrit Schichtgitterstruktur gehören zu den beständigsten Elektrodenmaterialien lùr elekrro-chemische Solarzellen. Photoelektrochemische Reaktionen werden jedoch durch die Oberflächenanisotropie kompliziert. Ergebnisse von photoelektrochemischen Zellen mit n-leitendenl I\'loSe, und WSe2 zeigen die Bedeutung der Oberflächenmorphologie für den Wirkungs-grad der Energieumwandlung.-Der optimale Energieumrvandlungswirkungsgrad dieser Schichtgirterhalbleiter erwies sich als geringer, als man gemâß der Bandlücke theoretisch er\varten konnte. Es wird gezeigt, daß dies mit der Existenz einer indirekten Bancllücke in Zusammenhang steht, welche eine sehr schrvache Lichtabsorption in einem beträchtlichen Energiebereich oberhalb der Bandkante bedingt. Außerdem verringern Oberflächendcfekte und lnhomogenìtäten durch Oberflächenrekombination die euantenausbeute.-Die photoelektrochemische Wasserstoflentwicklung wurde an pleitendem WSe2 unrersucht. Es wird gezeigt, daß ein beträchtlicher Energie-gervinn durch die katalytische Wirkung von an der Elektrodenoberfläche abgeschiedenem Platin erreicht rverden kann. Dies wird anhand von Begriffen der Grenzflächenkatalyse erklârt.

A model of the impedance of porous electrodes was derived. The model generalized the de Levie model in so far that an in-homogeneous inside electrode surface is assumed. Then the charge transfer resistance and the double-layer capacity were assumed to be distributed. The width of the distribution is characterized by a distribution parameter. The inhomogeneities led to characteristic deviations of the Nyquist impedance plots compared to those for homogeneous inside electrode surfaces: the slope angle of the straight line for higher frequencies was less than p=4, the slope angle of p=2 of the straight line obtained for double-layer charging only and lower frequencies decreased and the semicircle behaviour obtained for double-layer charging and a Faradaic reaction in parallel were replaced by a depressed semicircle. The model allows the determination of the following parameters: the total effective double-layer capacity, the total effective charge transfer resistance, the distribution parameter and the total resistance of the elec-trolyte inside the pores. The impedance of porous zinc electrodes in weakly alkaline electrolyte can be theoretically well described with the new model. A scanning force microscopy 2-D-image of the morphology of the zinc electrode was performed and discussed.

In situ reflection absorption FTIR spectroscopy (in situ FTIR) and small spot X-ray electron emission spectroscopy (XPS) of emersed electrodes were used to examine the electrochemical double layer on glassy carbon (GC) and the anodic storage reactions of the zinc bromine battery which are the formation of a non-aqueous N-methyl-ethyl-pyrrolidinium (MEP +) and/or N-methyl-ethyl-morpholinium (MEM +) polybromide phase. Oxidative conversion of the GC surface to COH, CO and COO − is observed during potential cycling between the hydrogen and oxygen evolution. The extrapolation of the intensity of the bending vibration signal due to adsorbed water allowed the determination of the point of zero charge (pzc). The pzc of GC is more than 1 V more negative than that of gold in the presence of MEP-Br or MEM-Br. Above the pzc, up to the bromide oxidation potential, MEM + shows a much stronger chemical affinity to GC than MEP + because the MEM morpholinium bridge oxygen shows a stronger interaction with the oxidised carbon surface species than the pure CH-structure of the pyrrolidinium species. This study suggests that the battery charging reaction involves mainly a heterogeneous electron transfer from Br − followed by a homogeneous chemical reaction leading to MEP-polybromide, whereas the conversion of specifically adsorbed MEM-Br ion pairs to polybromide is a slow process.