69 research outputs found
Cluster emission under femtosecond laser ablation of silicon
Rich populations of clusters have been observed after femtosecond laser
ablation of bulk silicon in vacuum. Size and velocity distributions of the
clusters as well as their charge states have been analyzed by reflectron
time-of-flight mass spectrometry. An efficient emission of both neutral silicon
clusters Sin (up to n = 6) and their cations Sin+ (up to n = 10) has been
observed. The clusters are formed even at very low laser fluences, below
ablation threshold, and their relative yield increases with fluence. We show
the dependencies of the cluster yield as well as the expansion dynamics on both
laser wavelength and laser fluence. The mechanisms of the cluster formation are
discussed
Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling
We present experimental and modeling studies of UV nanosecond pulsed laser
desorption and ablation of (111) bulk silicon. The results involve a new
approach to the analysis of plume formation dynamics under high-energy photon
irradiation of the semiconductor surface. Non-thermal, photo-induced desorption
has been observed at low laser fluence, well below the melting threshold. Under
ablation conditions, the non-thermal ions have also a high concentration. The
origin of these ions is discussed on the basis of electronic excitation of Si
surface states associated with the Coulomb explosion mechanism. We present a
model describing dynamics of silicon target excitation, heating and
harge-carrier transport
Synthesis and Laser Processing of ZnO Nanocrystalline Thin Films
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 SiO2 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 post-growth 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
Laser ablation synthesis of zinc oxide clusters: a new family of fullerenes?
Positively charged zinc oxide clusters ZnnOm (up to n = 16, m <= n) of
various stoichiometry were synthesized in the gas phase by excimer ArF laser
ablation of a ZnO target and investigated using time-of-flight mass
spectrometry. Depending on ablation conditions, either metal rich or
stoichiometric clusters dominate in the mass spectrum. When the irradiated
target surface is fairly fresh, the most abundant clusters are metal rich with
Zn(n+1)On and Zn(n+3)On being the major series. The stoichiometric clusters are
observed with an etched ablated surface. The magic numbers at n = 9, 11 and 15
in mass spectra of (ZnO)n clusters indicate that the clusters have hollow
spheroid structures related to fullerenes. A local abundance minimum at n = 13
provides an additional evidence for the presence in the ablation plume of
fullerene-like (ZnO)n clusters
Interaction des photons UV avec le silicium massif et nanocristallin
In FrenchInternational audienceThe studies of interaction of the UV photons with bulk and nanocristalline silicon by time-of-flight (TOF) mass-spectrometry allowed to reveal two populations of Si ion monomers. The first rapid population ejected even at low laser fluences is attributed to Colombian repulsion between the surface charges which are induced by emission of the photoelectrons. When the laser fluence increases, the second slow ion population appears. This population originates from the thermal processes in the irradiated material. The original result of this work is the observation of the non-thermal ion population which represents a considerable part of the laser-induced plume.L'étude de l'interaction entre photons UV et le silicium massif et nanocristallin par spectrométrie de masse à temps de vol permet de distinguer deux populations de monomères ioniques Si+. Une population rapide est éjectée dès les faibles fluences laser, dont l'origine est attribuée aux répulsions Coulombiennes entre les charges de surface créées par l'émission de photo-électrons. Alors que la fluence d'irradiation est augmentée, une population plus lente apparaît issue de phénomènes thermiques initiés dans le matériau irradié. La mise en évidence d'une population ionique non thermique qui constitue une part importante des ions présents dans la plume laser est un résultat original de cette étude
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