No abstract

Experiments are performed to compare the ablation behavior in the IR and UV spectral regions of a doped standard polymer, PMMA, and a specially tailored photopolymer, i.e., a triazene copolyester, to elucidate the underlying mechanisms. The results are discussed in light of current theories about photochemical and photothermal pathways of ablation. Further experiments are performed with nanosecond and picosecond pulses to study the impact of pulse length on the material. From the failure to induce ablation in the IR by doping the specialty polymer with an optical molecular heater we conclude that etching in the UV of this compound is mainly governed by a photochemical process. This result is contrasted by successful ablation of doped PMMA in the IR via a thermal unzipping mechanism. With respect to practical applications, the results show convincingly that the presence of an absorbing chromophore in the polymer is a prerequisite for achieving high-resolution structuring, since molecular absorption is required for an efficient distribution of incorporated photonic energy.

The interaction of unexcited oxygen molecules with clean GaN{0001}-1×1 surfaces was investigated using X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Clean surfaces were prepared by a HF dip followed either by desorption of Ga films deposited at room temperature or by nitrogen-ion bombardment and annealing. During exposures in the range from 0.3 up to 10 15 L-O2 any excitations of the oxygen were avoided. Oxygen coverages determined from the XPS and the AES data differ by a factor of two. The larger XPS-derived coverages are considered to be more reliable since the AES signals decayed during data recording. The oxygen uptake takes place in two consecutive stages. The first one is identified as dissociative chemisorption and the second one is tentatively attributed to field-assisted diffusion by the Mott-Cabrera mechanism. The dissociative chemisorption is characterized by an initial sticking coefficient of 0.12 ± 0.08 and a saturation coverage of 0.79 ± 0.1 monolayers that is reached after exposures of 10 3 L-O2. The second mechanism sets in at exposures to 10 8 L-O2 but reaches no saturation even with the largest doses applied.

The technique of forced Rayleigh scattering (FRS) was used to obtain information on the microstructure of two surfactant systems:  i.e. bis(2-ethylhexyl) sulfosuccinate (AOT)/octanol/water and tetradecyldimethylammonium oxide (TDMAO)/tetradecyltrimethylammonium bromide (TTMAB)/hexanol/water. Diffusion coefficients of the dye molecules congo red and methyl red, and of surfactant aggregates labeled with these dye probes, have been determined. In the L2 (microemulsion) phase of AOT/octanol/water stained with congo red, diffusion coefficients have been found to decrease with increasing water content. In contrast, at the transition from the L2 to the cubic phase, a continuous increase was observed. Although submicroscopically bicontinuous structures may exist in the microemulsion, the interconnectivity of the rodlike aggregates is not sufficient to enable free diffusion of the water soluble dye over the distance range investigated by FRS. Thus the determined diffusion coefficients reflect the motion of dynamic structural units. The situation is different in the cubic phase:  here the long range order makes it possible for the dye molecules to diffuse in the water channels. Compared to unhindered motion in aqueous solution, the diffusion coefficient is reduced to about 20% by steric obstruction effects. The continuous increase of the diffusion coefficient at the phase transition reflects the close structural relationship between the two phases near the phase boundary, whilst there is a remarkable difference in the L2 phase further away. For the system TDMAO/TTMAB/hexanol/water the transition from the micellar L1 to the vesicular Lα phase with increasing alcohol content has been investigated. The lipophilic dye methyl red is anchored in the surfactant layers, as is verified by polarization microscopy. In the L1 phase there is a transition from spherical to rodlike micelles. Self-diffusion coefficients of these spheres and rods have been measured, and the results have been compared to theoretical values, calculated by the model of Edwards and Doi for concentrated solutions of rodlike molecules. In the Lα phase the vesicles are densely packed, so that the diffusion coefficients originate from the motion of the dye probe within the surfactant bilayers and from an exchange between different bilayers. In complementary experiments, the mobility of the vesicles in an electric field has been probed.