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Charge-collector probe measurements have been performed to elucidate ion acceleration in laser-induced plasma plumes over a range of laser fluences important for pulsed laser deposition. The fundamental (1064 nm) or second (532 nm) harmonics of a Nd:YAG laser were used for ablation. The evolution of the time-of-flight ion signal from single-peaked to double-peaked and again to single-peaked with increasing laser fluence in the range of 2-25 J/cm(2) has been followed. The analysis of the ion velocity distributions shows that increasing laser fluence results in the appearance of a portion of accelerated ions that can be recognized as an additional fast peak in the time-of-flight distribution. The dependencies of the ion signal on the target-to-collector distance, the background pressure, and the wavelength of laser radiation have been studied. The results are discussed from the viewpoint of the generation of a self-consistent ambipolar electric field (so-called double layer). The observed ion acceleration suggests that formation in the plume of a high-energetic electron tail due to absorption of laser radiation is responsible for the development of a double layer.

The interaction between laser-induced plasma plume and gas background during film deposition is studied theoretically. The gas dynamic model has been developed to describe the temporal and spatial evolution of the plume expanding into a gas under a pressure of typically a few tens of pascals. The model is based upon the generation of a spherical plasma cloud whose expansion is described in a two-temperature approximation by use of the Euler equations. The dynamics of laser ablation of YBa2Cu3O7-x superconductor in an oxygen environment have been analysed using this model. The calculations show that the plume does not stop upon reaching the maximum propagation distance but moves repeatedly back and forth up to 200 mu s after ablation. Two types of shock waves determine the interaction dynamics: series of primary (external) waves which propagate through the ambient gas, and a secondary shock wave which is formed in the plume region and executes a periodic motion. The conversion of cloud ionization energy into gas dynamic flow during expansion approximately doubles the kinetic energy of the plasma, but has no significant effect on the plume propagation distance. The results are in quantitative agreement with the previously observed oscillation behaviour of the plume.

The expansion of a laser-ablation plume into different ambient gases is investigated theoretically using a two-fluid gas-dynamic model and experimentally with time-of-flight mass spectrometry. Both calculations and measurements, performed for laser ablation of Cu in oxygen or noble gases, reveal an oscillatory behaviour of plume expansion dynamics which is strongly dependent on the molecular weight of the ambient gas. Simple gas-dynamic considerations based on the analogy between an ablation plume and a supersonic underexpanded gaseous jet are found to explain a number of the effects of the interaction between the plume and the background gas. The effect of plume focusing observed previously at fairly high pressures of various ambient gases is reasonably described using the phenomenology of the underexpanded jet. The analogy also predicts vortex formation at the plume periphery.

Photoelectron spectra of C 60 and C 70 , ionized with ultrashort laser pulses, are measured with a momentum-mapimaging electron spectrometer. Above the photon energy, 1.6 eV, the spectra are essentially structureless and well described by Boltzmann distributions, with temperatures on the order of 10 4 K. This result is similar to previous results for C 60 using a time-of-flight electron spectrometer, which are confirmed in this study. Comparisons of electron energy distributions recorded for identical laser intensities but for different pulse durations demonstrate unambiguously that a significant fraction of the electrons are emitted in quasithermal processes and results argue strongly against a field ionization or direct multiphoton ionization mechanism. For electron energies above the photon energy, which account for about half the intensity, the whole signal is consistent with this quasithermal emission.

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We present the results of experiments on synthesis of ZnO nanoclusters by reactive pulsed laser deposition (PLD). The nanoclusters were formed and crystallized in the gas phase and deposited on SiO 2 substrates. The nanostructured films were characterized by conventional photoluminescence (PL). The PL spectra consist of a narrow UV excitonic band and a broad visible band related to defects in the film. The film preparation conditions such as the substrate temperature, ambient gas nature and pressure, were optimized in order to increase the intensity of excitonic emission and prevent the formation of defects. A postgrowth annealing by UV laser radiation improved the optical quality of the deposited films.The photoluminescence intensity was found to be dependent significantly on the laser fluence and on the number of shots per site. The nature of the defects responsible for the observed luminescence in a visible range is discussed.