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Experimental observations of the angular distribution of droplets during laser ablation and deposition of Al thin films are presented and discussed. The experimental results, obtained by simply moving the laser spot position with respect to the rotation axis of the target, allow clarification of the unexpected symmetric double peaked angular droplet distribution on the films. These results provide direct evidence that a laser fluence threshold exists, beyond which droplets are generated from a melt displacement and ejection mechanism rather than from a phase explosion. The main directions of particulate ejection are related to the particular geometry of the laser generated tracks, whose profiles depend on the relative position of the incident beam with respect to the rotation axis of the target. [ABSTRACT FROM AUTHOR]

A 250-TW laser system (FLAME - Frascati laser for acceleration and multidisciplinary experiments) is now in its commissioning phase in a new laboratory at LNF-INFN in the framework of the PLASMONX (Plasma acceleration and monochromatic X-ray generation) project. The laser will deliver<25 fs duration pulses with an energy up to 6 J, at a 10 Hz repetition rate. An ad hoc target area has also been designed and is currently being set up, allowing the first test experiments of electron laser wakefield acceleration to be carried out over the next few months in a safe, radiation-protected environment. An overview of the main features of the laser system and target area is given, along with a survey of the design and set-up of the self-injection test experiment, which is expected to reach the production of sub-GeV electron bunches. [ABSTRACT FROM AUTHOR]

We report the results of preliminary tests on Mg film photocathodes deposited by pulsed laser ablation deposition on various substrates. A comparison with other deposition techniques is presented. We illustrate briefly the technique of deposition of thick films (up to several microns). Scanning electron microscope analysis of the films before and after laser cleaning has been performed to characterize the changes occurred on the surface. Measurements of the distribution and integrated value of quantum efficiency over the illuminated surface are reported. Plans for the deposition on the back flange of an RF gun are illustrated. [ABSTRACT FROM AUTHOR]

The angular distribution of the ablated material was studied during sub-ps Si laser ablation deposition using a special hemicylindrical substrate holder and different laser fluences ranging between 0.4 and 1.7 J/cm2. Scanning electron microscopy analysis of the deposited films showed that, independent of the fluence, the distribution of the deposited droplets presents two maxima. The first maximum corresponds to the average plume deflection angle value due to the local surface orientation produced by the preferential etching process. The second maximum is observed approximately at 45° with respect to the normal of the target surface, and is related to the phase explosion products that expand along the incident laser beam direction. The investigation of the twofold distribution of the sub-μm size deposited droplets is important to improve the quality of the deposited coatings. [ABSTRACT FROM AUTHOR]

Abstract: Mg films deposited by pulsed laser ablation on a Cu substrate were tested as photocathodes in a DC diode. They showed features of robustness and a quantum efficiency of at low accelerating field when illuminated by 266nm light at normal incidence. A graphite protective layer of about 20nm thickness deposited by laser ablation just after the Mg deposition made the cathode stable and of easy handling. The cathodes could stay for several weeks in air before being inserted in the test chamber. The preparation procedure and the tests are presented. [Copyright &y& Elsevier]

Abstract: Nanostructured tin oxide films were deposited on Si(100) substrates at room temperature using the PLAD method. Depositions were achieved by using a XeCl* laser, with a fluence of 10J/cm2 to ablate a SnO2 target either in vacuum or in an atmosphere containing a relatively low partial pressure (10−2 to 100Pa) of O2. A range of spectroscopic diffraction and real space imaging techniques, SEM, EDS, XRD, and XPS were used in order to characterize the surface morphology, structure, and composition of films. XPS results indicate that the film deposited in vacuum comprises a mixture of Sn, SnO and SnO2. In contrast, films deposited in oxygen comprise only SnO and SnO2. This indicates that the deposition of tin oxide films in relatively low partial pressures of O2 prevents the accumulation of unreacted tin metal within the resulting film. SEM studies reveal a sub-micron grain size structure, while XRD analyses indicate that the samples are crystalline, with no evidence for the presence of amorphous material. In this paper we discuss the significance and implications of these findings in terms of the optimization of tin oxide films for use in gas sensing devices and other technological applications. [Copyright &y& Elsevier]

The growth of thin films by pulsed-laser deposition involves extremely complex physical processes. The study of different aspects of the basic mechanisms of the laser ablation is still in progress, with special emphasis on the modelling of the plume deflection effect and the structure formation on the irradiated surface. In this work, the initial stages of the laser ablation process have been investigated considering the surface roughness and the continuous morphological changes of the surface produced by laser irradiation. Assuming a thermal ablation model, a computational approach of the structure formation on the irradiated surface by finite elements using ansys® (6.1) has been developed. Complementary, different ablation experiments on silicon wafers using a XeCl excimer laser (308 nm) impinging at 45° with respect to the target were carried out. Scanning electron microscope analyses were performed to study the morphological changes of the Si surface. The validity of the model and the agreement of the numerical results are discussed. [Copyright &y& Elsevier]