Ultrafast changes in the crystal structure of GaAs induced by intense femtosecond laser pulses are detected and investigated. Atomic force microscopy and Raman microprobe analysis of the laser-treated area show centrosymmetric (disordered) features which are different from the original zinc-blend structure of the GaAs lattice. The frozen-in structure shows evidence for a special heat transfer from the laser-induced crater to the boundary, namely the heat has been transferred ballistically by a high-density electron-hole plasma.

Picosecond pulsed laser irradiation of several metallic photocathodes (W, Ta, Al and Au) was carried out along with work function measurements during the irradiation process, work function measurements were performed before irradiation and after different irradiation periods. Metals used in this study behaved differently according to their affinity towards oxygen. Most of these metal surfaces were covered with a native oxide layer, except gold. Photolythic or thermal processes could occur depending on the laser wavelength, in the former case, the native oxide layer was removed pulse by pulse while in the latter, surface oxidation was enhanced. For the tungsten and tantalum samples, the kinetics of oxide removal occurred in distinct stages relative to the native oxide multilayer structure. In the case of the Au sample, no native oxide layer formed on its surface (noble metal) and the laser irradiation induced a surface smoothing only. A linear increase of the photoelectric signal was observed during laser irradiation of the aluminium sample probably due to the partial surface oxide removal and the sub-surface oxide incorporation. The presence of various solid phases of the oxide layer could be determined from pulsed laser irradiation measurements.

By using laser-induced photoemission and complementary in situ monitoring of photoelectric response spectra, damage threshold values have been determined for three wavelengths (213, 266 and 532nm) of a pulsed picosecond Nd-YAG laser. The photoelectric signal is extremely sensitive on the surface state of the diamond-like carbon (DLC) film, therefore laser-induced surface transformations could be observed. Measuring the photoelectric signal and the charge of ionized species stemming from ablation processes, laser damage threshold values could be determined in situ with a very high sensitivity surpassing usual optical methods. Results were analyzed in terms of applied power density and number of shots, and structural changes were characterized by ex situ Raman measurements before and after illumination.

In this work, we correlate the structural properties of DLC films with their photoelectric properties. DLC layers are deposited on silicon wafers by RF plasma-assisted chemical vapor deposition (RF PACVD). The films are irradiated with picosecond Nd3+-YAG laser pulses at different wavelengths (213 and 266 nm) and energies (few μJ to mJ). Photoelectric sensitivity values are reported and compared with those of metals. The DLC coatings are characterized before and after irradiation by IR spectroscopy, micro-Raman spectroscopy and ellipsometry in order to observe laser-induced structural modifications of the layers. In situ measurements of work function variations at the surface induced by laser pulses are also reported.

K+-implanted W samples with various implantation depths were investigated. Generally, implantation of alkali ions gives rise to two competitive effects: it lowers the surface work function, however it enhances surface oxidation too which in turn leads to a slight work function increase. In opposite to alkali overlayers, implanted species confined within a `metallic cage' resist the applied laser irradiation and alkali removal occurs only to a small extent. Measurements of laser-induced photoelectric charge pulses indicate a variation of the photoemission yield as a function of surface oxide thickness. Moreover, following the charge pulse evolution over a longer period (2–3 h), one finds that the laser-induced oxide removal characteristics depends on the implantation parameters as well. Results are compared to those obtained for pure, non-implanted W and the mechanisms responsible for the work function lowering are discussed.

Results of UV (308 nm) laser pulse induced dry etching with subsequent Pd deposition from a PdCl2 solution (acid base with pH=1) on polyimide surface are reported. The surface roughness has been determined before and after illumination. The fractal-based examination techniques based on the area–perimeter and the structure function methods. It could be concluded that the preetching of the polyimide surface significantly enhances the flux of Pd atoms deposited onto the surface, which is a power function of the number of shots (30 mJ/cm2 at 20 ns pulse duration) with a power coefficient close to 0.5. It has been observed that the seeding process started simultaneously at a number of places resulting in island-like deposits. Fractal characterisation of surfaces resulted in a fractal dimension changing between 2.28 and 2.71 carried out with the methods outlined earlier. It might be concluded that the seeding process occurs at atomic scale, but it exhibits a very strong trend towards the formation of aggregates and/or cluster-type structures.

Photocurrent measurements yielded new data, which were used to determine the surface work function of a native oxide-covered tungsten photocathode. The photoemission was generated by a continuous UV source, The surface work function has been measured at various stages of a pulsed UV laser-induced oxide removal in order to characterize the process. Since the measured surface work function can be correlated with the surface coverage, along with the measurements of charge of laser-induced photoelectrons a description of the laser-oxide interaction is presented. A fully computer-controlled experimental setup is demonstrated, achieving photoelectric measurements at the spot of laser illumination. This apparatus enables us to follow the work function temporal dependence during laser illumination, thus giving a detailed description for the surface coverage dynamics. (C) 1999 Elsevier Science B.V. All rights reserved.

Laser-induced surface damage introduces a dramatic change in the photoelectric properties of metallic and semiconductor substrates. Hence, by varying the applied laser intensity, ultra-short- pulsed laser-induced photoemission (in the mono- or multiphotonic regime according to the applied wavelength) can be used to monitor in situ surface structural changes. Results for a model system (thin diamond-like carbon (DLC) layer on an Si carrier) are presented, where, after ablation of the DLC layer, we observed a rapid increase in the photoelectric contribution from the underlying substrate. High-sensitivity measurements are presented for Au and W substrates where due to laser- induced damage within the escape depth of electrons and because of the apparition of ionized species upon ablation, the measured charge can indicate the onset of surface damage. Results are presented for various wavelengths (213 nm, 266 nm and 532 nm) and discussed in the light of previous results of threshold measurements obtained by optical methods.

In-situ monitoring of the vacuum chamber pressure during laser-induced oxide desorption on W surfaces provides a view on the desorption process. Sudden pressure variations suggest a change in the rate of desorption. During the desorption experiment, due to the incident pulsed UV laser light, photoelectrons are ejected from the sample surface. Evaluating the temporal variation of the measured charge shows a good agreement with the obtained pressure measurements. These results are attributed to the specific oxide structure on the W consisting of various tungsten oxides. (C) 1999 Elsevier Science B.V. All rights reserved.

In the past years image analysis has gained great relevancein several fields of scientific application. With these systems an affordable price might be limiting factor for small-scale research projects or educational purposes. The system presented here provides an excellent performance-price ratio and is thus an excellent choice for low-budget applicetions. The system is described and its capabilities are demonstrated in a scientific example.

The fifth harmonic (213 nm) of a picosecond Nd3+ :YAG laser was used to remove small fractions of the covering oxide overlayer from polycrystalline metallic photocathodes. The evolution of the photoemitted charge (high-density electron pulses) was followed along with the surface reflectivity change, and they were interpreted in terms of the work function change of the surface. The careful choice of the laser fluence enables us to investigate the cleaning kinetics within a relatively narrow coverage range. Moreover, after recontaminating the surface at different pressures, we observe distinct kinetics as well. This latter effect is explained by the various rates of oxygen incorporation into the oxide lattice, resulting in a looser, rougher surface for high pressures and a more compact oxide for low pressures.

A native oxide film on a metallic surface corresponds generally to a multilayer. We describe a method using a number of picosecond laser pulses at 213 nm to achieve partial removal of the oxide overlayer on a polycrystalline W substrate under UHV conditions. The process of oxide removal was monitored by measuring the charge of emitted photoelectrons by using an appropriate anode (collector)–cathode (target) setup, and simultaneously, the surface reflectivity for each pulse. The temporal evolution of the photoelectric charge shows – as expected – an increasing tendency due to the lowering of the surface work function, but moreover – under certain experimental conditions – it shows the presence of several kinks, which are interpreted as a sign of changes in the surface morphology. The results are discussed within the frame of a model describing mono/multilayer desorption phenomena.

Polycrystalline metallic photocathodes with a covering native oxide layer were exposed to several wavelengths (213, 266 and 355 nm) of a picosecond Nd 3+ -YAG laser. Laser illumination at each wavelength was used to induce oxide film removal and a measurable photoelectric response simultaneously. The change of the photoelectric signal was monitored throughout the irradiation process. The evolution of the photoelectric signal was investigated as a function of the applied wavelength and interpreted in terms of possible surface structural changes.

Experimental results of laser assisted chemical vapor deposition of nickel from Ni(CO)4 and theoretical treatment of deposition process are presented. The nickel deposition has ben realized by scanning of Ar+ laser beam (100 - 400 mW, (lambda) equals 515 nm and 488 nm) on Si surfaces in atmosphere of Ni(CO)4 with 0.2 - 2.0 mbars with scanning speeds of 20 - 700 micrometers /s. As a result homogeneous Ni lines on Si have been deposited with a typical volumetric growth rate of 250 micrometers 3/s and widths of 10 - 20 micrometers and thickness of 0.2 - 0.5 micrometers . The electrical resistivity of lines deposited was cca 7 (mu) (Omega) cm. The theoretical treatment includes computations of the temperature distribution in both gas- phase and solid substrate. The reaction rate is computed on base of local concentration and local temperatures, within the frame of finite element methods using triangles as a base of computing.

Poly (tetrafluorethylene) and polyimide samples were irradiated by a pulsed laser source at 308 nm and the resulting surface morphology was investigated. The photoablated surfaces show a strong dependence on the optical and structural parameters of the polymers. The roughness of the fractal surfaces has been characterized by means of calculating their fractal dimensions and the results are interpreted as a function of the polymer physical parameters.